fundamentals of soil science 8th edition - henry foth

FUNDAMENTALS OFSOIL SCIENCE

FUNDAMENTALS OFSOIL SCIENCE

EIGHTH EDITION

HENRY D. FOTHMichigan State University

JOHN WILEY & SONSNew York • Chichester • Brisbane • Toronto • Singapore

Cover PhotoSoil profile developed from glacio-fluvial sand in a balsam fir-black spruceforest in the Laurentian Highlands of Quebec, Canada. The soil is classified asa Spodosol (Orthod) in the United States and as a Humo-Ferric Podzol inCanada.

Copyright © 1943, 1951by Charles Ernest Millar and Lloyd M. Turk

Copyright © 1958, 1965, 1972, 1978, 1984, 1990, by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada.

Reproduction or translation of any part ofthis work beyond that permitted by Sections107 and 108 of the 1976 United States CopyrightAct without the permission of the copyrightowner is unlawful. Requests for permissionor further information should be addressed tothe Permissions Department, John Wiley & Sons.

Library of Congress Cataloging In Publication Data:

Foth, H. D.Fundamentals of soil science / Henry D. Foth.-8th ed.

p. cm.I ncludes bibliographical references.ISBN 0-471-52279-11. Soil science. I. Title.

S591.F68 1990631.4-dc20

90-33890CIP

Printed in the United States of America

1098765432

Printed and bound by the Arcata Graphics Company

The eighth edition is a major revision in whichthere has been careful revision of the topicscovered as well as changes in the depth of cover-age. Many new figures and tables are included.Summary statements are given at the ends of themore difficult sections within chapters, and asummary appears at the end of each chapter.Many nonagricultural examples are included toemphasize the importance of soil properties whensoils are used in engineering and urban settings.The topics relating to environmental quality arefound throughout the book to add interest to manychapters. Several examples of computer applica-tion are included.

The original Chapter 1, "Concepts of Soil," wassplit into two chapters. Each chapter emphasizesan important concept of soil-soil as a mediumfor plant growth and soil as a natural body. Topicscovered in Chapter 1 include the factors affectingplant growth, root growth and distribution, nutri-ent availability (including the roles of root inter-ception, mass flow and diffusion), and soil fertil-i ty and productivity. The importance of soils as asource of nutrients and water is stressed in Chap-ter 1 and elsewhere throughout the book. Chapter2 covers the basic soil formation processes ofhumification of organic matter, mineral weather-i ng, leaching, and translocation of colloids. Thei mportant theme is soil as a three-dimensionalbody that is dynamic and ever-changing. The con-cepts developed in the first two chapters are usedrepeatedly throughout the book.

The next five chapters relate to soil physicalproperties and water. The material on tillage andtraffic was expanded to reflect the increasing ef-fect of tillage and traffic on soils and plant growthand is considered in Chapter 4. The nature of soilwater is presented as a continuum of soil waterpotentials in Chapter 5. Darcy's law is developed

PREFACE

V

and water flow is discussed as a function of thehydraulic gradient and conductivity. Darcy's Lawis used in Chapter 6, "Soil Water Management,"in regard to water movement in infiltration, drain-age, and irrigation. Chapter 6 also covers dis-posal of sewage effluent in soils and prescriptionathletic turf (PAT) as an example of precisioncontrol of the water, air, and salt relationships insoils used for plant growth. "Soil Erosion," Chap-ter 7, has been slightly reorganized with greateremphasis on water and wind erosion processes.

Chapters 8 and 9, "Soil Ecology" and "Soil Or-ganic Matter," are complimentary chapters relat-ing to the biological aspects of soils. The kindsand nature of soil organisms and nutrient cyclingremain as the central themes of Chapter 8. Anexpanded section on the rhizosphere has beenincluded. The distinctions between labile and sta-ble organic matter and the interaction of organicmatter with the minerals (especially clays) arecentral themes of Chapter 9. Also, the concept ofcation exchange capacity is minimally developedin the coverage of the nature of soil organic matterin Chapter 9.

Chapter 10, "Soil Mineralogy," and Chapter 11,"Soil Chemistry", are complimentary chapters re-l ating to the mineralogical and chemical proper-ties of soils. The evolution theme included inChapter 2 is used to develop the concept ofchanging mineralogical and chemical propertieswith time. Soils are characterized as being mini-mally, moderately, and intensively weathered,and these distinctions are used in discussions ofsoil pH, liming, soil fertility and fertilizer use, soilgenesis, and land use.

Chapters 12 through 15 are concerned with thegeneral area of soil fertility and fertilizer use.Chapters 12 and 13 cover the macronutrients andmicronutrients plus toxic elements, respectively.

vi

Chapters 14 and 15 cover the nature of fertilizersand the evaluation of soil fertility and the use offertilizers, respectively. Greater stress has beenplaced on mass flow and diffusion in regard tonutrient uptake. The interaction of water and soilfertility is developed, and there is expanded cov-erage of soil fertility evaluation and the methodsused to formulate fertilizer recommendations.Recognition is made of the increasing frequencyof high soil test results and the implications forfertilizer use and environmental quality. Greatercoverage is given to animal manure as both asource of nutrients and a source of energy. Infor-mation on land application of sewage sludge andon sustainable agriculture has been added.Throughout these four chapters there is a greateremphasis on the importance of soil fertility andfertilizers and on the environmental aspects ofgrowing crops.

The next four chapters (Chapters 16, 17, 18, and19) relate to the areas of soil genesis, soil taxon-omy, soil geography and land use, and soil surveyand land use interpretations. In this edition, thesubjects of soil taxonomy (classification) and ofsoil survey and land use interpretations have re-ceived increased coverage in two small chapters.The emphasis in the soil geography and land usechapter is at the suborder level. References tol ower categories are few. Color photographs ofsoil profiles are shown in Color Plates 5 and 6. Noreference to Soil Taxonomy (USDA) is made until

PREFACE

taxonomy is covered in Chapter 17. This allows aconsideration of soil classification after soilproperties have been covered. This arrangementalso makes the book more desirable for use intwo-year agricultural technology programs andoverseas, in countries where Soil Taxonomy is notused.

The final chapter, "Land and the World FoodSupply," includes a section on the world graintrade and examines the importance of nonagro-nomic factors in the food-population problem.

Both English and metric units are used in themeasurement of crop yields, and for some otherparameters. Using both kinds of units should sat-isfy both United States and foreign readers.

Special thanks to Mary Foth for the artwork andto my late son-in-law, Nate Rufe, for photographiccontributions. Over the years, many colleagueshave responded to my queries to expand myknowledge and understanding. Others have pro-vided photographs. The reviewers also have pro-vided an invaluable service. To these persons, Iam grateful.

Finally, this book is a STORY about soil. Thestory reflects my love of the soil and my devotionto promoting the learning and understanding ofsoils for more than 40 years. I hope that all whoread this book will find it interesting as well asinformative.

Henry D. FothEast Lansing, Michigan

BRIEF CONTENTS

VII

CHAPTER 1SOIL AS A MEDIUM FORPLANT GROWTH 1FACTORS OF PLANT GROWTH 1Support for Plants 1Essential Nutrient Elements 2Water Requirement of Plants 3Oxygen Requirement of Plants 4Freedom from Inhibitory Factors 5PLANT ROOTS AND SOIL RELATIONS 5Development of Roots in Soils 5Extensiveness of Roots in Soils 7Extent of Root and Soil Contact 8Roles of Root Interception, Mass Flow,and Diffusion 8SOIL FERTILITY AND SOIL PRODUCTIVITY 9

CHAPTER 2SOIL AS A NATURAL BODY 11THE PARENT MATERIAL OF SOIL 12Bedrock Weathering and Formation ofParent Material 12Sediment Parent Materials 13SOIL FORMATION 13Soil-Forming Processes 14Formation of A and C Horizons 14Formation of B Horizons 14

The Bt Horizon 15The Bhs Horizon 17

Formation of E Horizons 17Formation of 0 Horizons 18SOILS AS NATURAL BODIES 18The Soil-Forming Factors 18Soil Bodies as Parts of Landscapes 19How Scientists Study Soils as Natural Bodies 19I mportance of Concept of Soil as Natural Body 20

DETAILED CONTENTS

ix

CHAPTER 3SOIL PHYSICAL PROPERTIES 22SOIL TEXTURE 22The Soil Separates 22Particle Size Analysis 24Soil Textural Classes 25Determining Texture by the Field Method 25Influence of Coarse Fregments onClass Names 26Texture and the Use of Soils 26SOIL STRUCTURE 27Importance of Structure 28Genesis and Types of Structure 28Grade and Class 29Managing Soil Structure 29SOIL CONSISTENCE 31Soil Consistence Terms 31DENSITY AND WEIGHT RELATIONSHIPS 32Particle Density and Bulk Density 32Weight of a Furrow-Slice of Soil 33Soil Weight on a Hectare Basis 34SOIL PORE SPACE AND POROSITY 34Determination of Porosity 34Effects of Texture and Structure on Porosity 35Porosity and Soil Aeration 35SOIL COLOR 36Determination of Soil Color 37Factors Affecting Soil Color 37Significance of Soil Color 37SOIL TEMPERATURE 38Heat Balance of Soils 38Location and Temperature 39Control of Soil Temperature 39Permafrost 40

CHAPTER 4TILLAGE AND TRAFFIC 42EFFECTS OF TILLAGE ON SOILS ANDPLANT GROWTH 42Management of Crop Residues 42

X

Tillage and Weed Control 43Effects of Tillage on Structure and Porosity 43Surface Soil Crusts 44Minimum and Zero Tillage Concepts 44Tilth and Tillage 45

TRAFFIC AND SOIL COMPACTION 46Compaction Layers 46Effects of Wheel Traffic on Soils and Crops 47Effects of Recreational Traffic 47Effects of Logging Traffic on Soils andTree Growth 48Controlled Traffic 49

FLOODING AND PUDDLING OF SOIL 50Effects of Flooding 50Effects of Puddling 50Oxygen Relationships in Flooded Soils 51

CHAPTER 5 ,/SOIL WATER 54SOIL WATER ENERGY CONTINUUM 54Adhesion Water 55Cohesion Water 55Gravitational Water 56Summary Statements 56

ENERGY AND PRESSURE RELATIONSHIPS 57Pressure Relationships in Saturated Soil 57Pressure Relationships in Unsaturated Soil 58

THE SOIL WATER POTENTIAL 59The Gravitational Potential 59The Matric Potential 59The Osmotic Potential 60Measurement and Expression ofWater Potentials 60

SOIL WATER MOVEMENT 61Water Movement in Saturated Soil 62Water Movement in Unsaturated Soil 63Water Movement in Stratified Soil 63Water Vapor Movement 66

PLANT AND SOIL WATER RELATIONS 66Available Water-Supplying Power of Soils 66Water Uptake from Soils by Roots 67Diurnal Pattern of Water Uptake 68Pattern of Water Removal from Soil 69Soil Water Potential Versus Plant Growth 69Role of Water Uptake for Nutrient Uptake 71

SOIL WATER REGIME 71

DETAILED CONTENTS

CHAPTER 6SOIL WATER MANAGEMENT 73WATER CONSERVATION 73Modifying the Infiltration Rate 73Summer Fallowing 75Saline Seep Due to Fallowing 76Effect of Fertilizers on Water Use Efficiency 77

SOIL DRAINAGE 78Water Table Depth Versus Air and Water Contentof Soil 79Benefits of Drainage 80Surface Drainage 80Subsurface Drainage 80Drainage in Soil of Container-Grown Plants 81

I RRIGATION 82Water Sources 82I mportant Properties of Irrigated Soils 82Water Application Methods 83

Flood Irrigation 83Furrow Irrigation 83Sprinkler Irrigation

83Subsurface Irrigation 85Drip Irrigation

85

Rate and Timing of Irrigation 85Water Quality 86

Total Salt Concentration 86Sodium Adsorption Ratio 86Boron Concentration 87Bicarbonate Concentration 87

Salt Accumulation and Plant Response 89Salinity Control and Leaching Requirement 89Effect of Irrigation on River Water Quality 93Nature and Management of Saline andSodic Soils 93

Saline Soils 93Sodic Soils 93Saline-Sodic Soils 94

WASTEWATER DISPOSAL 94Disposal of Septic Tank Effluent 94Land Disposal of Municipal Wastewater 96

PRESCRIPTION ATHLETIC TURF 97

CHAPTER 7SOIL EROSION 100WATER EROSION 100Predicting Erosion Rates on Agricultural Land 100R = The Rainfall Factor 101

K = The Soil Erodibility Factor 102LS = The Slope Length and SlopeGradient Factors 103C = The Cropping-Management Factor 104P = The Erosion Control Practice Factor 105Application of the Soil-Loss Equation 106The Soil Loss Tolerance Value 107Water Erosion on Urban Lands 108Water Erosion Costs

109

WIND EROSION 110Types of Wind Erosion 110Wind Erosion Equation 111Factors Affecting Wind Erosion 111Deep Plowing for Wind Erosion Control 113Wind Erosion Control on Organic Soils 113

CHAPTER 8SOIL ECOLOGY 115THE ECOSYSTEM 115Producers 115Consumers and Decomposers 116

MICROBIAL DECOMPOSERS 116General Features of Decomposers 116Bacteria 117Fungi 117Actinomycetes 118Vertical Distribution of Decomposers inthe Soil 119

SOIL ANIMALS 119Worms 120

Earthworms 120Nematodes 121

Arthropods 121Springtails

121Mites 122Millipedes and Centipedes 122White Grubs 122

Interdependence of Microbes and Animalsi n Decomposition 123

NUTRIENT CYCLING 123Nutrient Cycling Processes 123A Case Study of Nutrient Cycling 124Effect of Crop Harvesting on Nutrient Cycling 124

SOIL MICROBE AND ORGANISMI NTERACTIONS 125The Rhizosphere 125Disease 126

Mycorrhiza 126Nitrogen Fixation 127

SOIL ORGANISMS AND ENVIRONMENTALQUALITY 128Pesticide Degradation 128Oil and Natural Gas Decontamination 128

EARTH MOVING BY SOIL ANIMALS 130Earthworm Activity 130Ants and Termites 130Rodents 131

CHAPTER 9SOIL ORGANIC MATTER 133THE ORGANIC MATTER IN ECOSYSTEMS 133

DECOMPOSITION AND ACCUMULATION 133Decomposition of Plant Residues 134Labile Soil Organic Matter 134Stable Soil Organic Matter 135Decomposition Rates 136Properties of Stable Soil Organic Matter 136Protection of Organic Matter by Clay 137

ORGANIC SOILS 139Organic Soil Materials Defined 139Formation of Organic Soils 139Properties and Use 140Archaeological Interest 140

THE EQUILIBRIUM CONCEPT 141A Case Example 141Effects of Cultivation 142Maintenance of Organic Matter inCultivated Fields 143Effects of Green Manure 144

HORTICULTURAL USE OF ORGANIC MATTER 144Horticultural Peats 145Composts 145

CHAPTER 10SOIL MINERALOGY 148CHEMICAL AND MINERALOGICAL COMPOSITION OFTHE EARTH'S CRUST 148Chemical Composition of the Earth's Crust 148Mineralogical Composition of Rocks 149

XIDETAILED CONTENTS

xii

WEATHERING AND SOIL MINERALOGICALCOMPOSITION 149Weathering Processes 150Summary Statement 150Weathering Rate and Crystal Structure 151Mineralogical Composition Versus Soil Age 153Summary Statement 155

SOIL CLAY MINERALS 155Mica and Vermiculite 156Smectites 158Kaolinite 159Allophane and Halloysite 160Oxidic Clays 160Summary Statement 161

ION EXCHANGE SYSTEMS OF SOIL CLAYS 161Layer Silicate System 161Oxidic System 162Oxide-Coated Layer Silicate System 162

CHAPTER 11SOIL CHEMISTRY 164CHEMICAL COMPOSITION OF SOILS 164

ION EXCHANGE 165Nature of Cation Exchange 165Cation Exchange Capacity of Soils 166Cation Exchange Capacity Versus Soil pH 167Kinds and Amounts of Exchangeable Cations 168Exchangeable Cations as a Source of PlantNutrients 169Anion Exchange 169

SOIL pH 170Determination of Soil pH 170Sources of Alkalinity 170

Carbonate Hydrolysis 170Mineral Weathering 171

Sources of Acidity 171Development and Properties of Acid Soils 171

Role of Aluminum 172Moderately Versus IntensivelyWeathered Soils 173Role of Strong Acids 174Acid Rain Effects 174

Soil Buffer Capacity 174Summary Statement 176

SIGNIFICANCE OF SOIL pH 176Nutrient Availability and pH 177Effect of pH on Soil Organisms 178Toxicities in Acid Soils 178pH Preferences of Plants 178

DETAILED CONTENTS

MANAGEMENT OF SOIL pH 178Lime Requirement 179

Lime Requirement of IntensivelyWeathered Soils 180Lime Requirement of Minimally and ModeratelyWeathered Soils 180

The Liming Equation and Soil Buffering 181Some Considerations in Lime Use 182Management of Calcareous Soils 182Soil Acidulation 183

EFFECTS OF FLOODING ON CHEMICALPROPERTIES 183Dominant Oxidation and Reduction Reactions 183Effect on Soil pH 184

CHAPTER 12PLANT-SOIL MACRONUTRIENTRELATIONS 186DEFICIENCY SYMPTOMS 186

NITROGEN 186The Soil Nitrogen Cycle 187Dinitrogen Fixation 187

Symbiotic Legume Fixation 187Nonlegume Symbiotic Fixation 192Nonsymbiotic Nitrogen Fixation 192Summary Statement 192

Mineralization 192Nitrification 193I mmobilization 194

Carbon-Nitrogen Relationships 195Denitrification 195Human Intrusion in the Nitrogen Cycle 196Summary Statement on Nitrogen Cycle 197Plant Nitrogen Relations 197

PHOSPHORUS 197Soil Phosphorus Cycle 198Effect of pH on Phosphorus Availability 199Changes in Soil Phosphorus Over Time 199Plant Uptake of Soil Phosphorus 200Plant Phosphorus Relations 201

POTASSIUM 202Soil Potassium Cycle 202

Summary Statement 203Plant Uptake of Soil Potassium 204Plant Potassium Relations 205

CALCIUM AND MAGNESIUM 205Plant Calcium and Magnesium Relations 206Soil Magnesium and Grass Tetany 206

SULFUR 207

CHAPTER 13MICRONUTRIENTS ANDTOXIC ELEMENTS 210IRON AND MANGANESE 210Plant "Strategies" for Iron Uptake 211

COPPER AND ZINC 212Plant Copper and Zinc Relations 213

BORON 214

CHLORINE 214

MOLYBDENUM 214Plant and Animal Molybdenum Relations 215

COBALT 216

SELENIUM 217

POTENTIALLY TOXIC ELEMENTSFROM POLLUTION 217

RADIOACTIVE ELEMENTS 218

CHAPTER 14FERTILIZERS 221FERTILIZER TERMINOLOGY 221Grade and Ratio 221General Nature of Fertilizer Laws 222Types of Fertilizers 222

FERTILIZER MATERIALS 222Nitrogen Materials 222Phosphorus Materials 224Potassium Materials 226Micronutrient Materials 228

MIXED FERTILIZERS 228Granular Fertilizers 228Bulk Blended Fertilizers 229Fluid Fertilizers

229

NATURAL FERTILIZER MATERIALS 230

CHAPTER 15SOIL FERTILITY EVALUATION ANDFERTILIZER USE 232SOIL FERTILITY EVALUATION 232Plant Deficiency Symptoms 232Plant Tissue Tests 232Soil Tests 233Computerized Fertilizer Recommendations 235

APPLICATION AND USE OF FERTILIZERS 237Time of Application 238Methods of Fertilizer Placement 238Salinity and Acidity Effects 240

ANIMAL MANURES 241Manure Composition and Nutrient Value 241Nitrogen Volatilization Loss from Manure 242Manure as a Source of Energy 243

LAND APPLICATION OF SEWAGE SLUDGE 244Sludge as a Nutrient Source 244Heavy Metal Contamination 245

FERTILIZER USE AND ENVIRONMENTALQUALITY 246Phosphate Pollution 246Nitrate Pollution 246Nitrate Toxicity 247

SUSTAINABLE AGRICULTURE 247

CHAPTER 16SOIL GENESIS 250ROLE OF TIME IN SOIL GENESIS 250Case Study of Soil Genesis 250Time and Soil Development Sequences 252

ROLE OF PARENT MATERIAL IN SOIL GENESIS 253Consolidated Rock as a Source ofParent Material 253Soil Formation from Limestone Weathering 253Sediments as a Source of Parent Material 254

Gulf and Atlantic Coastal Plains 255Central Lowlands 256Interior Plains 258Basin and Range Region 258Volcanic Ash Sediments 258

Effect of Parent Material Properties onSoil Genesis 258Stratified Parent Materials 259Parent Material of Organic Soils 260

ROLE OF CLIMATE IN SOIL GENESIS 260Precipitation Effects 260Temperature Effects 262Climate Change and Soil Properties 263

ROLE OF ORGANISMS IN SOIL GENESIS 263Trees Versus Grass and OrganicMatter Content 263Vegetation Effects on Leaching and Eluviation 264Role of Animals in Soil Genesis 265

ROLE OF TOPOGRAPHY IN SOIL GENESIS 265Effect of Slope 265

DETAILED CONTENTS xiii

Effects of Water Tables and Drainage 266Topography, Parent Material, andTime Interactions 267Uniqueness of Soils Developed in AlluvialParent Material 267

HUMAN BEINGS AS A SOIL-FORMING FACTOR 269

CHAPTER 17SOIL TAXONOMY 271DIAGNOSTIC SURFACE HORIZONS 271Mollic Horizon 271Umbric and Ochric Horizons 272Histic Horizon 272Melanic Horizon 272Anthropic and Plaggen Horizons 273

DIAGNOSTIC SUBSURFACE HORIZONS 273Cambic Horizon 273Argillic and Natric Horizons 274Kandic Horizon 274Spodic Horizon 275Albic Horizon 275Oxic Horizon 275Calcic, Gypsic, and Salic Horizons 275Subordinate Distinctions of Horizons 276

SOIL MOISTURE REGIMES 276Aquic Moisture Regime 276Udic and Perudic Moisture Regime 276Ustic Moisture Regime 277Aridic Moisture Regime 277Xeric Moisture Regime 278

SOIL TEMPERATURE REGIMES 279

CATEGORIES OF SOIL TAXONOMY 279Soil Order 279Suborder and Great Group 282Subgroup, Family, and Series 283

AN EXAMPLE OF CLASSIFICATION:THE ABAC SOILS 283

THE PEDON 284

CHAPTER 18SOIL GEOGRAPHY AND LAND USE 285ALFISOLS 285Aqualfs 285Boralfs 286Udalfs 286Ustalfs 293Xeralfs 293

xiv DETAILED CONTENTS

ANDISOLS 294Genesis and Properties 294Suborders 294

ARIDISOLS 294Genesis and Properties 295Aridisol Suborders 295Land Use on Aridisols 296

ENTISOLS 297Aquents 297Fluvents 297Psamments 298Orthents 298

HISTOSOLS 299Histosol Suborders 299Land Use on Histosols 300

INCEPTISOLS 300Aquepts 300Ochrepts 301Umbrepts 301

MOLLISOLS 302Aquolls 302Borolls 303Ustolls and Udolls 303Xerolls 304

OXISOLS 304Oxisol Suborders 306Land Use on Udox Soils 306Land Use on Ustox Soils 307Extremely Weathered Oxisols 307Plinthite or Laterite 308

SPODOSOLS 308Spodosol Suborders 308Spodosol Properties and Land Use 309

ULTISOLS 311Ultisol Suborders 311Properties of Ultisols 311Land Use on Ultisols 312

VERTISOLS 312Vertisol Suborders 313Vertisol Genesis 313Vertisol Properties 314Land Use on Vertisols 315

CHAPTER 19SOIL SURVEYS AND LAND-USEINTERPRETATIONS 318MAKING A SOIL SURVEY 318Making a Soil Map 318

Writing the Soil Survey Report 320Using the Soil Survey Report 321

SOIL SURVEY INTERPRETATIONS ANDLAND-USE PLANNING 322Examples of Interpretative Land-Use Maps 322Land Capability Class Maps 323Computers and Soil Survey Interpretations 323

SOIL SURVEYS AND AGROTECHNOLOGYTRANSFER 324

CHAPTER 20LAND AND THE WORLDFOOD SUPPLY 326POPULATION AND FOOD TRENDS 326Development of Agriculture 326The Industrial Revolution 326Recent Trends in Food Production 327Recent Trends in Per Capita Cropland 328Summary Statement 329

POTENTIALLY AVAILABLE LAND ANDSOIL RESOURCES 329World's Potential Arable Land 329Limitations of World Soil Resources 332Summary Statement 332

DETAILED CONTENTS XV

FUTURE OUTLOOK 332Beyond Technology 333The World Grain Trade 333Population Control and Politics 334

APPENDIX ISOIL TEXTURE BY THEFIELD METHOD 337

APPENDIX IITYPES AND CLASSES OFSOIL STRUCTURE 339

APPENDIX IIIPREFIXES AND THEIR CONNOTATIONSFOR NAMES OF GREAT GROUPS IN THEU.S. SOIL CLASSIFICATION SYSTEM(SOIL TAXONOMY) 341

GLOSSARY 342INDEX 353

CHAPTER 1

SOIL AS A MEDIUM FORPLANT GROWTH

SOIL. Can you think of a substance that has hadmore meaning for humanity? The close bond thatancient civilizations had with the soil was ex-pressed by the writer of Genesis in these words:

And the Lord God formed Man of dust from theground.There has been, and is, a revereor soil. Someone has said thatman life is woven on earthen losmells of clay." Even today, mpeople are tillers of the soil anto produce their food and fiberof soil as a medium of plant gantiquity and remains as one tant concepts of soil today (se

FACTORS OF PLANT GROThe soil can be viewed as a mand organic particles of varyingtion in regard to plant growthcupy about 50 percent of the remaining soil volume, about space, composed of pores of vsizes. The pore spaces contain

serve as channels for the movement of air andwater. Pore spaces are used as runways for smallanimals and are avenues for the extension andgrowth of roots. Roots anchored in soil suppportplants and roots absorb water and nutrients. Forgood plant growth, the root-soil environmentshould be free of inhibitory factors. The threeessential things tha sorb from the soiland (1) wa mainly evaporatedfro aves, (2) nutrients for nutrition, and(3) or root respiration.

nce for the ground "the fabric of hu-oms; everywhere itost of the world'sd use simple tools
. Thus, the conceptrowth was born inof the most impor-e Figure 1.1).
WTHixture of mineral size and composi-. The particles oc-soil's volume. The50 percent, is porearying shapes and air and water and

1

SuOnproengroarePlbuoareelshigirrblesat

use are:m plant le oxygen f

pport for Plane of the most obvide support for

able growing plawn by hydropon commonly supp

ants growing in yancy of the wat

droughty and infe than support. Sh-yielding crops

igated. There are nature of the suburated soil close

t plants abter that is

tsvious functions of soil is toplants. Roots anchored in soilnts to remain upright. Plantsics (in liquid nutrient culture)orted on a wire framework.water are supported by theer. Some very sandy soils thatertile provide plants with littleuch soils, however, producewhen fertilized and frequentlysoils in which the impenetra-soil, or the presence of water-to the soil surface, cause shal-

z SOIL AS A MEDIUM FOR PLANT GROWTH

l ow rooting. Shallow-rooted trees are easily blownover by wind, resulting in windthrow.

Essential Nutrient ElementsPlants need certain essential nutrient elements tocomplete their life cycle. No other element cancompletely substitute for these elements. At least16 elements are currently considered essential forthe growth of most vascular plants. Carbon, hy-drogen, and oxygen are combined in photosyn-thetic reactions and are obtained from air andwater. These three elements compose 90 percentor more of the dry matter of plants. The remaining13 elements are obtained largely from the soil.Nitrogen (N), phosphorus (P), potassium (K), cal-cium Ca), magnesium (Mg), and sulfur (S) arerequired in relatively large amounts and are re-ferred to as the macronutrients. Elements re-quired in considerably smaller amount are calledthe micronutrients. They include boron (B),chlorine (Cl), copper (Cu), iron (Fe), manganese(Mn), molybdenum (Mo), and zinc (Zn). Cobalt(Co) is a micronutrient that is needed by only

FIGURE 1.1 Wheat harvestnear the India-Nepal border.About one half of the world'speople are farmers who areclosely tied to the land andmake their living producingcrops with simple tools.

some plants. Plants deficient in an essential ele-ment tend to exhibit symptoms that are unique forthat element, as shown in Figure 1.2.

More than 40 other elements have been found

FIGURE 1.2 Manganese deficiency symptoms onkidney beans. The youngest, or upper leaves, havelight-green or yellow-colored intervein areas anddark-green veins.

in plants. Some plants accumulate elements thatare not essential but increase growth or quality.The absorption of sodium (Na) by celery is anexample, and results in an improvement of flavor.Sodium can also be a substitute for part of thepotassium requirement of some plants, if po-tassium is in low supply. Silicon (Si) uptake mayincrease stem strength, disease resistance, andgrowth in grasses.

Most of the nutrients in soils exist in the miner-als and organic matter. Minerals are inorganicsubstances occurring naturally in the earth. Theyhave a consistent and distinctive set of physicalproperties and a chemical composition that canbe expressed by a formula. Quartz, a mineralcomposed of SiO2, is the principal constituent ofordinary sand. Calcite (CaCO 3) i s the primarymineral in limestone and chalk and is abundant ismany soils. Orthclase-feldspar (KAISi3 O8 ) is avery common soil mineral, which contains po-tassium. Many other minerals exist in soils be-cause soils are derived from rocks or materialscontaining a wide variety of minerals. Weatheringof minerals brings about their decomposition andthe production of ions that are released into thesoil water. Since silicon is not an essential ele-ment, the weathering of quartz does not supply anessential nutrient, plants do not depend on theseminerals for their oxygen. The weathering of cal-cite supplies calcium, as Cat+ , and the weather-ing of orthoclase releases potassium as K+.

The organic matter in soils consists of the re-cent remains of plants, microbes, and animalsand the resistant organic compounds resultingfrom the rotting or decomposition processes. De-composition of soil organic matter releases es-sential nutrient ions into the soil water where theions are available for another cycle of plantgrowth.

Available elements or nutrients are those nutri-ent ions or compounds that plants and microor-ganisms can absorb and utilize in their growth.Nutrients are generally absorbed by roots ascations and anions from the water in soils, or thesoil solution. The ions are electrically charged.

FACTORS OF PLANT GROWTH

3

Cations are positively charged ions such as Ca t

and K+ and anions are negatively charged ionssuch as NO3- (nitrate) and H 2 PO4

- (phosphate).The amount of cations absorbed by a plant isabout chemically equal to the amount of anionsabsorbed. Excess uptake of cations, however, re-sults in excretion of H+ and excess uptake ofanions results in excretion of OH- or HCO 3

- tomaintain electrical neutrality in roots and soil.The essential elements that are commonly ab-sorbed from soils by roots, together with theirchemical symbols and the uptake forms, are listedi n Table 1.1.

I n nature, plants accommodate themselves tothe supply of available nutrients. Seldom or rarelyis a soil capable of supplying enough of the essen-tial nutrients to produce high crop yields for anyreasonable period of time after natural or virginl ands are converted to cropland. Thus, the use ofanimal manures and other amendments to in-crease soil fertility (increase the amount of nutri-ent ions) are ancient soil management practices.

Water Requirement of PlantsA few hundred to a few thousand grams of waterare required to produce 1 gram of dry plant mate-rial. Approximately one percent of this water be-comes an integral part of the plant. The remainderof the water is lost through transpiration, the lossof water by evaporation from leaves. Atmosphericconditions, such as relative humidity and temper-ature, play a major role in determining howquickly water is transpired.

The growth of most economic crops will becurtailed when a shortage of water occurs, eventhough it may be temporary. Therefore, the soil'sability to hold water over time against gravity isimportant unless rainfall or irrigation is adequate.Conversely, when soils become water saturated,the water excludes air from the pore spaces andcreates an oxygen deficiency. The need for theremoval of excess water from soils is related to theneed for oxygen.

4

TABLE 1.1 Chemical Symbols and Common Formsof the Essential Elements Absorbed by Plant Rootsfrom Soils

FIGURE 1.3 The soil in which these tomato plantswere growing was saturated with water. The stopperat the bottom of the left container was immediatelyremoved, and excess water quickly drained away.The soil in the right container remained watersaturated and the plant became severely wiltedwithin 24 hours because of an oxygen deficiency.

SOIL AS A MEDIUM FOR PLANT GROWTH

Oxygen Requirement of PlantsRoots have openings that permit gas exchange.Oxygen from the atmosphere diffuses into the soiland is used by root cells for respiration. The car-bon dioxide produced by the respiration of roots,and microbes, diffuses through the soil porespace and exits into the atmosphere. Respirationreleases energy that plant cells need for synthesisand translocation of the organic compoundsneeded for growth. Frequently, the concentrationof nutrient ions in the soil solution is less than thatin roots cells. As a consequence, respirationenergy is also used for the active accumulation ofnutrient ions against a concentration gradient.

Some plants, such as water lilies and rice, cangrow in water-saturated soil because they havemorphological structures that permit the diffusionof atmospheric oxygen down to the roots. Suc-cessful production of most plants in water culturerequires aeration of the solution. Aerobic micro-organisms require molecular oxygen (O 2) and useoxygen from the soil atmosphere to decomposeorganic matter and convert unavailable nutrientsin organic matter into ionic forms that plants canreuse (nutrient cycling).

Great differences exist between plants in theirability to tolerate low oxygen levels in soils. Sensi-

FIGURE 1.4 Soil salinity (soluble salt) has seriouslyaffected the growth of sugar beets in the foregroundof this irrigated field.

tive plants may be wilted and/or killed as a resultof saturating the soil with water for a few hours, asshown in Figure 1.3. The wilting is believed toresult from a decrease in the permeability of theroots to water, which is a result of a disturbance ofmetabolic processes due to an oxygen deficiency.

Freedom from Inhibitory FactorsAbundant plant growth requires a soil environ-ment that is free of inhibitory factors such as toxicsubstances, disease organisms, impenetrablelayers, extremes in temperature and acidity orbasicity, or an excessive salt content, as shown inFigure 1.4.

PLANT ROOTS AND SOIL RELATIONSPlants utilize the plant growth factors in the soil byway of the roots. The density and distribution ofroots affect the amount of nutrients and water thatroots extract from soils. Perennials, such as oakand alfalfa, do not reestablish a completely newroot system each year, which gives them a distinctadvantage over annuals such as cotton or wheat.Root growth is also influenced by the soil environ-ment; consequently, root distribution and densityare a function of both the kind of plant and thenature of the root environment.

Development of Roots in SoilsA seed is a dormant plant. When placed in moist,warm soil, the seed absorbs water by osmosis andswells. Enzymes activate, and food reserves in theendosperm move to the embryo to be used ingermination. As food reserves are exhausted,green leaves develop and photosynthesis begins.The plant now is totally dependent on the sun forenergy and on the soil and atmosphere for nutri-ents and water. In a sense, this is a critical periodi n the life of a plant because the root system issmall. Continued development of the plant re-quires: (1) the production of food (carbohydrates,

PLANT ROOTS AND SOIL RELATIONS

5

etc.) in the shoot via photosynthesis and translo-cation of food downward for root growth, and(2) the absorption of water and nutrients by rootsand the upward translocation of water and nutri-ents to the shoot for growth.

After a root emerges from the seed, the root tipelongates by the division and elongation of cellsin the meristematic region of the root cap. Afterthe root cap invades the soil, it continues to elon-gate and permeate the soil by the continued divi-sion and elongation of cells. The passage of theroot tip through the soil leaves behind sections ofroot that mature and become "permanent" resi-dents of the soil.

As the plant continues to grow and roots elon-gate throughout the topsoil, root extension intothe subsoil is likely to occur. The subsoil environ-ment will be different in terms of the supply ofwater, nutrients, oxygen, and in other growth fac-tors. This causes roots at different locations in thesoil (topsoil versus subsoil) to perform differentfunctions or the same functions to varying de-grees. For example, most of the nitrogen willprobably be absorbed by roots from the topsoilbecause most of the organic matter is concen-trated there, and nitrate-nitrogen becomes avail-able by the decomposition of organic matter. Bycontrast, in soils with acid topsoils and alkalinesubsoils, deeply penetrating roots encounter agreat abundance of calcium in the subsoil. Underthese conditions, roots in an alkaline subsoil mayabsorb more calcium than roots in an acid top-soil. The topsoil frequently becomes depleted ofwater in dry periods, whereas an abundance ofwater still exists in the subsoil. This results in arelatively greater dependence on the subsoil forwater and nutrients. Subsequent rains that rewetthe topsoil cause a shift to greater dependence onthe topsoil for water and nutrients. Thus, the man-ner in which plants grow is complex and changescontinually throughout the growing season. Inthis regard, the plant may be defined as an inte-grator of a complex and ever changing set ofenvironmental conditions.

The root systems of some common agricultural

FIGURE 1.5 The tap root systems of two-week oldsoybean plants. Note the many fine roots branchingoff the tap roots and ramifying soil. (Scale on theright is in inches.)

crops were sampled by using a metal frame tocollect a 10-centimeter-thick slab of soil from thewall of a pit. The soil slab was cut into smallblocks and the roots were separated from the soilusing a stream of running water. Soybean plantswere found to have a tap root that grows directlydownward after germination, as shown in Figure1.5. The tap roots of the young soybean plants areseveral times longer than the tops or shoots. Lat-eral roots develop along the tap roots and spacethemselves uniformily throughout the soil occu-pied by roots. At maturity, soybean taproots willextend about 1 meter deep in permeable soilswith roots well distributed throughout the topsoiland subsoil. Alfalfa plants also have tap roots thatcommonly penetrate 2 to 3 meters deep; somehave been known to reach a depth of 7 meters.

Periodic sampling of corn (Zea maize) root


FIGURE 1.6 Four stages for the development of theshoots and roots of corn (Zea maize). Stage one(left) shows dominant downward and diagonal rootgrowth, stage two shows "filling" of the upper soillayer with roots, stage three shows rapid elongationof stem and deep root growth, and stage four (right)shows development of the ears (grain) and braceroot growth.

systems and shoots during the growing seasonrevealed a synchronization between root andshoot growth. Four major stages of developmentwere found. Corn has a fibrous root system, andearly root growth is mainly by development ofroots from the lower stem in a downward anddiagonal direction away from the base of the plant(stage one of Figure 1.6). The second stage of rootgrowth occurs when most of the leaves are de-veloping and lateral roots appear and "fill" orspace themselves uniformily in the upper 30 to 40centimeters of soil. Stage three is characterized byrapid elongation of the stem and extension ofroots to depths of 1 to 2 meters in the soil. Finally,during stage four, there is the production of theear or the grain. Then, brace roots develop fromthe lower nodes of the stem to provide anchorageof the plant so that they are not blown over by thewind. Brace roots branch profusely upon enteringthe soil and also function for water and nutrientuptake.

FIGURE 1.7 Roots of mature oat plants grown inrows 7 inches (18 cm) apart. The roots made up 13percent of the total plant weight. Note the uniform,lateral, distribution of roots between 3 and 24 inches(8 and 60 cm). Scale along the left is in inches.

Extensiveness of Roots in SoilRoots at plant maturity comprise about 10 percentof the entire mass of cereal plants, such as corn,wheat, and oats. The oat roots shown in Figure 1.7weighed 1,767 pounds per acre (1,979 kg/ha) andmade up 13 percent of the total plant weight. Fortrees, there is a relatively greater mass of roots ascompared with tops, commonly in the range of 15to 25 percent of the entire tree.

PLANT ROOTS AND SOIL RELATIONS 7

There is considerable uniformity in the lateraldistribution of roots in the root zone of manycrops (see Figure 1.7). This is explained on thebasis of two factors. First, there is a random distri-bution of pore spaces that are large enough forroot extension, because of soil cracks, channelsformed by earthworms, or channels left from pre-vious root growth. Second, as roots elongatethrough soil, they remove water and nutrients,which makes soil adjacent to roots a less favor-able environment for future root growth. Then,roots grow preferentially in areas of the soil de-void of roots and where the supply of water andnutrients is more favorable for root growth. Thisresults in a fairly uniform distribution of rootsthroughout the root zone unless there is some

barrier to root extension or roots encounter anunfavorable environment.

Most plant roots do not invade soil that is dry,nutrient deficient, extremely acid, or water satu-rated and lacking oxygen. The preferential devel-opment of yellow birch roots in loam soil, com-pared with sand soil, because of a more favorable

FIGURE 1.8 Development of the root system of ayellow birch seedling in sand and loam soil. Bothsoils had adequate supplies of water and oxygen,but, the loam soil was much more fertile. (AfterRedmond, 1954.)


combination of nutrients and water, is shown inFigure 1.8.

Extent of Root and Soil ContactA rye plant was grown in 1 cubic foot of soil for 4months at the University of Iowa by Dittmer (thisstudy is listed amoung the references at the end ofthe chapter). The root system was carefully re-moved from the soil by using a stream of runningwater, and the roots were counted and measuredfor size and length. The plant was found to havehundreds of kilometers (or miles) of roots. Basedon an assumed value for the surface area of thesoil, it was calculated that 1 percent or less of thesoil surface was in direct contact with roots.Through much of the soil in the root zone, thedistance between roots is approximately 1 cen-timeter. Thus, it is necessary for water and nutri-ent ions to move a short distance to root surfacesfor the effective absorption of the water and nutri-ents. The limited mobility of water and most of thenutrients in moist and well-aerated soil meansthat only the soil that is invaded by roots cancontribute significantly to the growth of plants.

TABLE 1.2 Relation Between Concentration of Ions in the Soil Solution andConcentration within the Corn Plant

Roles of Root Interception, Mass Flow,and DiffusionWater and nutrients are absorbed at sites locatedon or at the surface of roots. Elongating rootsdirectly encounter or intercept water and nutrientions, which appear at root surfaces in position forabsorption. This is root interception and accountsfor about 1 percent or less of the nutrients ab-sorbed. The amount intercepted is in proportionto the very limited amount of direct root and soilcontact.

Continued absorption of water adjacent to theroot creates a lower water content in the soil nearthe root surface than in the soil a short distanceaway. This difference in the water content be-tween the two points creates a water content gra-dient, which causes water to move slowly in thedirection of the root. Any nutrient ions in the waterare carried along by flow of the water to rootsurfaces where the water and nutrients are both inposition for absorption. Such movement of nutri-ents is called mass flow.

The greater the concentration of a nutrient inthe soil solution, the greater is the quantity of thenutrient moved to roots by mass flow. The range

Adapted from S. A. Barber, "A Diffusion and Mass Flow Concept of SoilNutrient Availability, "Soil Sci., 93:39-49, 1962.Used by permission of the author and The Williams and Wilkins Co., Baltimore.a Dry weight basis.

of concentration for some nutrients in soil water isgiven in Table 1.2. The calcium concentration(Table 1.2) ranges from 8 to 450 parts per million(ppm). For a concentration of only 8 ppm in soilwater, and 2,200 ppm of calcium in the plant, theplant would have to absorb 275 (2,200/8) timesmore water than the plant's dry weight to move thecalcium needed to the roots via mass flow. Statedin another way, if the transpiration ratio is 275(grams of water absorbed divided by grams ofplant growth) and the concentration of calcium inthe soil solution is 8 ppm, enough calcium will bemoved to root surfaces to supply the plant need.Because 8 ppm is a very low calcium concentra-tion in soil solutions, and transpiration ratios areusually greater than 275, mass flow generallymoves more calcium to root surfaces than plantsneed. In fact, calcium frequently accumulatesalong root surfaces because the amount moved tothe roots is greater than the amount of calciumthat roots absorb.

Other nutrients that tend to have a relativelylarge concentration in the soil solution, relative tothe concentration in the plant, are nitrogen, mag-nesium, and sulfur (see Table 1.2). This meansthat mass flow moves large amount of these nutri-ents to roots relative to plant needs.

Generally, mass flow moves only a smallamount of the phosphorus to plant roots. Thephosphorus concentration in the soil solution isusually very low. For a soil solution concentrationof 0.03 ppm and 2,000 ppm plant concentration,the transpiration ratio would need to be more than60,000. This illustration and others that could bedrawn from the data in Table 1.2, indicate thatsome other mechanism is needed to account forthe movement of some nutrients to root surfaces.This mechanism or process is known as diffusion.

Diffusion is the movement of nutrients in soilwater that results from a concentration gradient.Diffusion of ions occurs whether or not water ismoving. When an insufficient amount of nutrientsis moved to the root surface via mass flow, diffu-sion plays an important role. Whether or notplants will be supplied a sufficient amount of a

SOIL FERTILITY AND SOIL PRODUCTIVITY 9

nutrient also depends on the amount needed. Cal-cium is rarely deficient for plant growth, partiallybecause plants' needs are low. As a consequence,these needs are usually amply satisfied by themovement of calcium to roots by mass flow. Thesame is generally true for magnesium and sulfur.The concentration of nitrogen in the soil solutiontends to be higher than that for calcium, but be-cause of the high plant demand for nitrogen,about 20 percent of the nitrogen that plants ab-sorb is moved to root surfaces by diffusion. Diffu-sion is the most important means by which phos-phorus and potassium are transported to rootsurfaces, because of the combined effects of con-centration in the soil solution and plant demands.

Mass flow can move a large amount of nutrientsrapidly, whereas diffusion moves a small amountof nutrients very slowly. Mass flow and diffusionhave a limited ability to move phosphorus andpotassium to roots in order to satisfy the needs ofcrops, and this limitation partly explains why alarge amount of phosphorus and potassium isadded to soils in fertilizers. Conversely, the largeamounts of calcium and magnesium that aremoved to root surfaces, relative to crop plantneeds, account for the small amount of calciumand magnesium that is added to soils in fertilizers.

Summary StatementThe available nutrients and available water are thenutrients and water that roots can absorb. Theabsorption of nutrients and water by roots is de-pendent on the surface area-density (cm 2 /cm 3 ) ofroots. Mathematically:

uptake = availability x surface area-density

SOIL FERTILITY ANDSOIL PRODUCTIVITYSoil fertility is defined as the ability of a soil tosupply essential elements for plant growth with-out a toxic concentration of any element. Soil

10

fertility refers to only one aspect of plant growth-the adequacy, toxicity, and balance of plant nutri-ents. An assessment of soil fertility can be madewith a series of chemical tests.

Soil productivity is the soil's capacity to pro-duce a certain yield of crops or other plants withoptimum management. For example, the produc-tivity of a soil for cotton production is commonlyexpressed as kilos, or bales of cotton per acre, orhectare, when using an optimum managementsystem. The optimum managment system spec-i fies such factors as planting date, fertilization,irrigation schedule, tillage, cropping sequence,and pest control. Soil scientists determine soilproductivity ratings of soils for various crops bymeasuring yields (including tree growth or timberproduction) over a period of time for those pro-duction uses that are currently relevant. Includedi n the measurement of soil productivity are theinfluence of weather and the nature and aspect ofslope, which greatly affects water runoff and ero-sion. Thus, soil productivity is an expression of allthe factors, soil and nonsoil, that influence cropyields.

For a soil to produce high yields, it must befertile for the crops grown. It does not follow,however, that a fertile soil will produce highyields. High yields or high soil productivity de-pends on optimum managment systems. Manyfertile soils exist in arid regions but, within man-agement systems that do not include irrigation,these soils are unproductive for corn or rice.

SUMMARYThe concept of soil as a medium for plant growthi s an ancient concept and dates back to at leastthe beginning of agriculture. The concept empha-sizes the soil's role in the growth of plants. Impor-tant aspects of the soil as a medium for plantgrowth are: (1) the role of the soil in supplyingplants with growth factors, (2) the development


and distribution of roots in soils, and (3) themovement of nutrients, water, and air to root sur-faces for absorption. Soils are productive in termsof their ability to produce plants.

The concept of soil as a medium for plantgrowth views the soil as a material of fairly uni-form composition. This is entirely satisfactorywhen plants are grown in containers that containa soil mix. Plants found in fields and forests,however, are growing in soils that are not uniform.Differences in the properties between topsoil andsubsoil layers affect water and nutrient absorp-tion. It is natural for soils in fields and forests to becomposed of horizontal layers that have differentproperties, so it is also important that agricultur-i sts and foresters consider soils as natural bodies.

This concept is also useful for persons involved inthe building of engineering structures, solving en-vironment problems such as nitrate pollution ofgroundwater, and using the soil for waste dis-posal. The soil as a natural body is considered inthe next chapter.

REFERENCESBarber, S. A. 1962. "A Diffusion and Mass Flow Concept

of Soil Nutrient Availability." Soil Sci. 93:39-49.Dittmer, H. J. 1937. "A Quantitative Study of the Roots

and Root Hairs of a Winter Rye Plant." Am. Jour. Bot.24:417-420.

Foth, H. D. 1962. "Root and Top Growth of Corn."Agron. Jour. 54:49-52.

Foth, H. D., L. S. Robertson, and H. M. Brown. 1964."Effect of Row Spacing Distance on Oat Perfor-mance." Agron. Jour. 56:70-73.

Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. JohnWiley, New York.

Redmond, D. R. 1954. "Variations in Development ofYellow Birch Roots in Two Soil Types." ForestryChronicle. 30:401-406.

Simonson, R. W. 1968. "Concept of Soil." Adv . in Agron.20:1-47. Academic Press, New York.

Wadleigh, C. H. 1957. "Growth of Plants," in Soil, USDAYearbook of Agriculture. Washington, D.C.

One day a colleague asked me why the alfalfaplants on some research plots were growing sopoorly. A pit was dug in the field and a verticalsection of the soil was sampled by using a metalframe. The sample of soil that was collected was 5centimeters thick, 15 centimeters wide, and 75centimeters long. The soil was glued to a boardand a vacuum cleaner was used to remove loosesoil debris and expose the natural soil layers androots. Careful inspection revealed four soil layersas shown in Figure 2.1.

The upper layer, 9 inches (22 cm) thick, is theplow layer. It has a dark color and an organicmatter content larger than any of the other layers.Layer two, at the depth of 9 to 14 inches (22 to35 cm) differs from layer one by having a light-graycolor and a lower organic matter content. Bothlayers are porous and permeable for the move-ment of air and water and the elongation of roots.I n layer three, at a depth of 14 to 23 inches (35 to58 cm) many of the soil particles were arrangedinto blocklike aggregrates. When moist soil fromlayer three was pressed between the fingers, morestickiness was observed than in layers one andtwo, which meant that layer three had a greater

CHAPTER 2

SOIL AS A NATURAL BODY

11

clay content than the two upper layers. The rootspenetrated this layer with no difficulty, however.Below layer three, the alfalfa tap root encountereda layer (layer four) that was impenetrable (toocompact), with the root growing above it in alateral direction. From these observations it wasconcluded that the alfalfa grew poorly becausethe soil material below a depth of 58 centimeters:(1) created a barrier to deep root penetration,which resulted in a less than normal supply ofwater for plant growth during the summer, and(2) created a water-saturated zone above the thirdlayer that was deficient in oxygen during wet pe-riods in the spring. The fact that the soil occurrednaturally in a field raises such questions as: Whatkinds of layers do soils have naturally? How do thel ayers form? What are their properties? How dothese layers affect how soils are used? The an-swers to these questions require an understand-i ng that landscapes consist of three-dimensionalbodies composed of unique horizontal layers.These naturally occurring bodies are soils. A rec-ognition of the kinds of soil layers and theirproperties is required in order to use soils effec-tively for many different purposes.

1 2 SOIL AS A NATURAL BODY

FIGURE 2.1 This alfalfa taproot grew verticallydownward through the upper three layers. At a depthof 23 inches (58 cm), the taproot encountered animpenetrable layer (layer 4) and grew in a lateraldirection above the layer.

THE PARENT MATERIAL OF SOILSSoil formation, or the development of soils thatare natural bodies, includes two broad processes.

First is the formation of a parent material fromwhich the soil evolves and, second, the evolutionof soil layers, as shown in Figure 2.1. Approxi-mately 99 percent of the world's soils develop inmineral parent material that was or is derivedfrom the weathering of bedrock, and the rest de-velop in organic materials derived from plantgrowth and consisting of muck or peat.

Bedrock Weathering and Formation ofParent MaterialBedrock is not considered soil parent materialbecause soil layers do not form in it. Rather, theunconsolidated debris produced from the weath-ering of bedrock is soil parent material. Whenbedrock occurs at or near the land surface, theweathering of bedrock and the formation of par-ent material may occur simultaneously with theevolution of soil layers. This is shown in Figure2.2, where a single soil horizon, the topsoil layer,overlies the R layer, or bedrock. The topsoil layeri s about 12 inches (30 cm) thick and has evolvedslowly at a rate controlled by the rate of rockweathering. The formation of a centimeter of soili n hundreds of years is accurate for this exampleof soil formation.

Rates of parent material formation from the di-rect weathering of bedrock are highly variable. Aweakly cemented sandstone in a humid environ-ment might disintegrate at the rate of a centimeteri n 10 years and leave 1 centimeter of soil. Con-

FIGURE 2.2 Rock weathering and the formation ofthe topsoil layer are occurring simultaneously. Scaleis in feet.

versely, quartzite (metamorphosed sandstone)nearby might weather so slowly that any weath-ered material might be removed by water or winderosion. Soluble materials are removed duringlimestone weathering, leaving a residue of insolu-ble materials. Estimates indicate that it takes100,000 years to form a foot of residue from theweathering of limestone in a humid region. Wheresoils are underlain at shallow depths by bedrock,l oss of the soil by erosion produces serious con-sequences for the future management of the land.

Sediment Parent MaterialsWeathering and erosion are two companion andopposing processes. Much of the material lostfrom a soil by erosion is transported downslopeand deposited onto existing soils or is added tosome sediment at a lower elevation in the land-scape. This may include alluvial sediments alongstreams and rivers or marine sediments alongocean shorelines. Glaciation produced extensivesediments in the northern part of the northernhemisphere.

Four constrasting parent material-soil environ-ments are shown in Figure 2.3. Bare rock is ex-posed on the steep slopes near the mountaintops.Here, any weathered material is lost by erosionand no parent material or soil accumulates. Very

SOIL FORMATION 13

thick alluvial sediments occur in the valley. Verythick glacial deposits occur on the tree-coveredlateral moraine that is adjacent to the valley flooralong the left side. An intermediate thickness ofparent material occurs where trees are growingbelow the bare mountaintops and above the thickalluvial and moraine sediments. Most of theworld's soils have formed in sediments consistingof material that was produced by the weatheringof bedrock at one place and was transported anddeposited at another location. In thick sedimentsor parent materials, the formation of soil layers isnot limited by the rate of rock weathering, andseveral soil layers may form simultaneously.

SOIL FORMATIONSoil layers are approximately parallel to the landsurface and several layers may evolve simulta-neously over a period of time. The layers in a soilare genetically related; however, the layers differfrom each other in their physical, chemical, andbiological properties. In soil terminology, the lay-ers are called horizons. Because soils as naturalbodies are characterized by genetically developedhorizons, soil formation consists of the evolutionof soil horizons. A vertical exposure of a soil con-sisting of the horizons is a soil profile.

FIGURE 2.3 Four distinct soil-forming environments aredepicted in this landscape inthe Rocky Mountains, UnitedStates. On the highest andsteepest slopes, rock isexposed because anyweathered material is removedby erosion as fast as it forms.Thick alluvial sediments occuron the valley floor and on theforested lateral moraineadjacent to the valley flooralong the left side. Glacialdeposits of varying thicknessoverlying rock occur on theforested mountain slopes atintermediate elevations.

1 4

Soil-Forming ProcessesHorizonation (the formation of soil horizons) re-sults from the differential gains, losses, transfor-mations, and translocations that occur over timewithin various parts of a vertical section of theparent material. Examples of the major kindsof changes that occur to produce horizons are:(1) addition of organic matter from plant growth,mainly to the topsoil; (2) transformation repre-sented by the weathering of rocks and mineralsand the decomposition of organic matter; (3) lossof soluble components by water moving down-ward through soil carrying out soluble salts; and,(4) translocation represented by the movement ofsuspended mineral and organic particles from thetopsoil to the subsoil.

Formation of A and C HorizonsMany events, such as the deposition of volcanicash, formation of spoil banks during railroad con-struction, melting of glaciers and formation ofglacial sediments, or catastrophic flooding andformation of sediments have been dated quiteaccurately. By studying soils of varying age, soilscientists have reconstructed the kinds and thesequence of changes that occurred to producesoils.

Glacial sediments produced by continental andalpine glaciation are widespread in the northernhemisphere, and the approximate dates of theformation of glacial parent materials are known.After sediments have been produced near aretreating ice front, the temperature may becomefavorable for the invasion of plants. Their growthresults in the addition of organic matter, espe-cially the addition of organic matter at or near thesoil surface. Animals, bacteria, and fungi feed onthe organic materials produced by the plants, re-sulting in the loss of much carbon as carbondioxide. During digestion or decomposition offresh organic matter, however, a residual organicfraction is produced that is resistant to furtheralteration and accumulates in the soil. The resis-tant organic matter is called humus and theprocess is humification. The microorganisms and


animals feeding on the organic debris eventuallydie and thus contribute to the formation of hu-mus. Humus has a black or dark-brown color,which greatly affects the color of A horizons. Inareas in which there is abundant plant growth,only a few decades are required for a surface layerto acquire a dark color, due to the humificationand accumulation of organic matter, forming an Ahorizon.

The uppermost horizons shown in Figures 2.1and 2.2 are A horizons. The A horizon in Figure 2.1was converted into a plow layer by frequent plow-i ng and tillage. Such A horizons are called Aphorizons, the p indicating plowing or other distur-bance of the surface layer by cultivation, pastur-i ng, or similar uses. For practical purposes, thetopsoil in agricultural fields and gardens is synon-ymous with Ap horizon.

At this stage in soil evolution, it is likely that theupper part of the underlying parent material hasbeen slightly altered. This slightly altered upperpart of the parent material is the C horizon. Thesoil at this stage of evolution has two horizons-the A horizon and the underlying C horizon. Suchsoils are AC soils; the evolution of an AC soil isillustrated in Figure 2.4.

Formation of B HorizonsThe subsoil in an AC soil consists of the C horizonand, perhaps, the upper part of the parent mate-rial. Under favorable conditions, this subsoil layer

FIGURE 2.4 Sequential evolution of some soilhorizons in a sediment parent material.

FIGURE 2.5 A soil scientist observing soilproperties near the boundary between the A and Bhorizons in a soil with A, B, and C horizons. As rootsgrow downward, or as water percolates downward,they encounter a different environment in the A, B,and C horizons. (Photograph USDA.)

eventually develops a distinctive color and someother properties that distinguish it from the A hori-zon and underlying parent material, commonly ata depth of about 60 to 75 centimeters. This alteredsubsoil zone becomes a B horizon and developsas a layer sandwiched between the A and a newdeeper C horizon. At this point in soil evolution,i nsufficient time has elapsed for the B horizon tohave been significantly enriched with fine-sized(colloidal) particles, which have been translo-cated downward from the A horizon by percolat-i ng water. Such a weakly developed B horizon isgiven the symbol w (as in Bw), to indicate itsweakly developed character. A Bw horizon can bedistinguished from A and C horizons primarily bycolor, arrangement of soil particles, and an inter-mediate content of organic matter. A soil with A,B, and C horizons is shown in Figure 2.5.

During the early phases of soil evolution, thesoil formation processes progressively transformparent material into soil, and the soil increases inthickness. The evolution of a thin AC soil into athick ABwC soil is illustrated in Figure 2.4.

SOIL FORMATION 15

The Bt Horizon Soil parent materials frequentlycontain calcium carbonate (CaCO 3), or lime, andare alkaline. In the case of glacial parent materi-als, lime was incorporated into the ice when gla-ciers overrode limestone rocks. The subsequentmelting of the ice left a sediment that containslimestone particles. In humid regions, the limedissolves in percolating water and is removedfrom the soil, a process called leaching. Leachingeffects are progressive from the surface down-ward. The surface soil first becomes acid, andsubsequently leaching produces an acid subsoil.

An acid soil environment greatly stimulatesmineral weathering or the dissolution of mineralswith the formation of many ions. The reaction oforthoclase feldspar (KAISiO 3 ) with water and H+is as follows:

2 KAISiO3 + 9H 2O + 2H +(orthoclase)

= H4AI 2 Si 2 0 9 + 2K+ + 4H 4Si04(kaolinite)

(silicic acid)

The weathering reaction illustrates three impor-tant results of mineral weathering. First, clay parti-cles (fine-sized mineral particles) are formed-inthe example, kaolinite. In effect, soils are "clayfactories. Second, ions are released into the soilsolution, including nutrient ions such as K + .Third, other compounds (silicic acid) of varyingsolubility are formed and are subject to leachingand removal from the soil.

Clay formation results mainly from chemicalweathering. Time estimates for the formation of 1percent clay inn rock parent material range from500 to 10,000 years. Some weathered rocks withsmall areas in which minerals are being con-verted into clay are shown in Figure 2.6.

Many soil parent materials commonly containsome clay. Some of this clay, together with clayproduced by weathering during soil formation,tends to be slowly translocated downward fromthe A horizon to the B horizon by percolatingwater. When a significant increase in the claycontent of a Bw horizon occurs due to clay trans-location, a Bw horizon becomes a Bt horizon.

1 6

FIGURE 2.6 Weathering releases mineral grains inrocks and results in the formation of very fine-sizedparticles of clay, in this case, kaolinite.

Thin layers or films of clay can usually be ob-served along cracks and in pore spaces with a10-power hand lens. The process of accumulationof soil material into a horizon by movement out ofsome other horizon is illuviation. The t (as in Bt)refers to an illuvial accumulation of clay. The Bthorizon may be encountered when digging holesfor posts or trenching for laying undergroundpipes.

Alternating periods of wetting and drying seemnecessary for clay translocation. Some clay parti-


cles are believed to disperse when dry soil iswetted at the end of a dry season and the clayparticles migrate downward in percolating waterduring the wet season. When the downward per-colating water encounters dry soil, water is with-drawn into the surrounding dry soil, resulting inthe deposition of clay on the walls of pore spaces.Repeated cycles of wetting and drying build uplayers of oriented clay particles, which are calledclay skins.

Many studies of clay illuviation have beenmade. The studies provide evidence that thou-sands of years are needed to produce a significantincrease in the content of clay in B horizons. Anexample is the study of soils on the alluvialfloodplain and adjacent alluvial fans in the Cen-tral Valley of California. Here, increasing eleva-tion of land surfaces is associated with increasingage. The soils studied varied in age from 1,000 tomore than 100,000 years.

The results of the study are presented in Figure2.7. The Hanford soil developed on the floodplainis 1,000 years old; it shows no obvious evidence ofilluviation of clay. The 10,000-year-old Greenfieldsoil has about 1.4 times more clay in the subsoil(Bt horizon) than in the A horizon. Snelling soilsare 100,000 years old and contain 2.5 times moreclay in the Bt horizon than in the A horizon. The

FIGURE 2.7 Clay distributionas a function of time in soilsdeveloped from granitic parentmaterials in the Central Valleyof California. The Hanfordsoil, only 1,000 years old,does not have a Bt horizon.The other three soils have Bthorizons. The Bt horizon ofthe San Joaquin is a claypanthat inhibits roofs and thedownward percolation ofwater. (After Arkley, 1964.)

San Joaquin soil is 140,000 years old, and has 3.4times more clay in the horizon of maximum clayaccumulation as compared to the A horizon.

The three youngest soils (Hanford, Greenfield,and Snelling) are best suited for agriculture be-cause the subsoil horizons are permeable to wa-ter and air, and plant roots penetrate through the Bhorizons and into the C horizons. Conversely, thei mpermeable subsoil horizon in San Joaquin soilcauses shallow rooting. The root zone above theimpermeable horizon becomes water saturated inthe wet seasons. The soil is dry and droughty inthe dry season.

Water aquifiers underlie soils and varying thick-nesses of parent materials and rocks. Part of theprecipitation in humid regions migrates com-pletely through the soil and recharges underlyingaquifers. The development of water-impermeableclaypans over an extensive region results in lesswater recharge and greater water runoff. This hasoccurred near Stuttgart, Arkansas, where wellsused for the irrigation of rice have run dry becauseof the limited recharge of the aquifer.

The Bhs Horizon Many sand parent materialscontain very little clay, and almost no clay formsin them via weathering. As a consequence, clayilluviation is insignificant and Bt horizons do notevolve. Humus, however, reacts with oxides ofaluminum and/or iron to form complexes in theupper part of the soil. Where much water forleaching (percolation) is present, as in humidregions, these complexes are translocated down-ward in percolating water to form illuvial accumu-lations in the B horizon. The illuvial accumulationof humus and oxides of aluminum and/or iron inthe B horizon produces Bhs horizons. The h indi-cates the presence of an illuvial accumulation ofhumus and the s indicates the presence of illuvialoxides of aluminum and/or iron. The symbol s isderived from sesquioxides (such as Fe 2 O 3 andA1 2 O3). Bhs horizons are common in very sandysoils that are found in the forested areas of theeastern United States from Maine to Florida.

SOIL FORMATION 17

The high content of sand results in soils withlow fertility and low water-retention capacity(droughtiness).

Formation of E HorizonsThe downward translocation of colloids from theA horizon may result in the concentration of sandand silt-sized particles (particles larger than claysize) of quartz and other resistant minerals in theupper part of many soils. In soils with thin Ahorizons, a light-colored horizon may develop atthe boundary of the A and B horizons (see Figure2.4). This horizon, commonly grayish in color, isthe E horizon. The symbol E is derived from eluvi-ation, meaning, "washed-out." Both the A and Ehorizons are eluvial in a given soil. The mainfeature of the A horizon, however, is the presenceof organic matter and a dark color, whereas that ofthe E horizon is a light-gray color and having loworganic matter content and a concentration of siltand sand-sized particles of quartz and other resis-tant minerals.

The development of E horizons occurs morereadily in forest soils than in grassland soils, be-cause there is usually more eluviation in forestsoils, and the A horizon is typically much thinner.The development of E horizons occurs readily insoils with Bhs horizons, and the E horizons mayhave a white color (see soil on book cover).

A soil with A, E, Bt, and C horizons is shown inFigure 2.8. At this building site, the suitability ofthe soil for the successful operation of a septiceffluent disposal system depends on the rate atwhich water can move through the least per-meable horizon, in this example the Bt hori-zon. Thus, the value of rural land for homeconstruction beyond the limits of municipalsewage systems depends on the nature of thesubsoil horizons and their ability to allow forthe downward migration and disposal of sewageeffluent. Suitable sites for construction canbe identified by making a percolation test ofthose horizons through which effluent will bedisposed.

1 8

Formation of 0 HorizonsVegetation produced in the shallow waters oflakes and ponds may accumulate as sediments ofpeat and muck because of a lack of oxygen in thewater for their decomposition. These sedimentsare the parent material for organic soils. Organicsoils have 0 horizons; the O refers to soil layersdominated by organic material. In some cases,extreme wetness and acidity at the surface of thesoil produce conditions unfavorable for decom-position of organic matter. The result is the forma-tion of O horizons on the top of mineral soil hori-zons. Although a very small proportion of theworld's soils have O horizons, these soils arewidely scattered throughout the world.

SOILS AS NATURAL BODIESVarious factors contribute to making soils whatthey are. One of the most obvious is parent mate-rial. Soil formation, however, may result in manydifferent kinds of soils from a given parent mate-rial. Parent material and the other factors that areresponsible for the development of soil are thesoil-forming factors.


FIGURE 2.8 A soil with A, E,Bt, and C horizons thatformed in 10,000 years underforest vegetation. The amountof clay in the Bt horizon andpermeability of the Bt horizonto water determine thesuitability of this site for homeconstruction in a rural areawhere the septic tank effluentmust be disposed bypercolating through the soil.

The Soil-Forming FactorsFive soil-forming factors are generally recognized:parent material, organisms, climate, topography,

and time. It has been shown that Bt and Bhshorizon development is related to the clay andsand content within the parent material and/orthe amount of clay that is formed during soil evo-l ution.

Grass vegetation contributes to soils with thickA horizons because of the profuse growth of fineroots in the upper 30 to 40 centimeters of soil. Inforests, organic matter is added to soils mainly byleaves and wood that fall onto the soil surface.Small-animal activities contribute to some mixingof organic matter into and within the soil. As aresult, organic matter in forest soils tends to beincorporated into only a thin layer of soil, result-ing in thin A horizons.

The climate contributes to soil formationthrough its temperature and precipitation com-ponents. If parent materials are permanently fro-zen or dry, soils do not develop. Water is neededfor plant growth, for weathering, leaching, andtranslocation of clay, and so on. A warm, humidclimate promotes soil formation, whereas dryand/or cold climates inhibit it.

The topography refers to the general nature ofthe land surface. On slopes, the loss of water byrunoff and the removal of soil by erosion retardsoil formation. Areas that receive runoff watermay have greater plant growth and organic mattercontent, and more water may percolate throughthe soil.

The extent to which these factors operate isa function of the amount of time that has beenavailable for their operation. Thus, soil may be de-fined as:

unconsolidated material on the surface of theearth that has been subjected to and influencedby the genetic and environmental factors of par-ent material, climate, organisms, and topography,all acting over a period of time.

Soil Bodies as Parts of LandscapesAt any given location on the landscape, there is aparticular soil with a unique set of properties,including kinds and nature of the horizons. Soilproperties may remain fairly constant from thatlocation in all directions for some distance. Thearea in which soil properties remain reasonablyconstant is a soil body. Eventually, a significantchange will occur in one or more of the soil-forming factors and a different soil or soil bodywill be encountered.

Locally, changes in parent material and/or

FIGURE 2.9 A field or landscape containing black-colored soil in the lowest part of the landscape.This soil receives runoff water and erodedsediment from the light-colored soil on the slopingareas.

SOILS AS NATURAL BODIES

FIGURE 2.10 Soil scientists studying soils asnatural bodies in the field. Soils are exposed in thepit and soils data are displayed on charts forobservation and discussion.

slope (topography) account for the existence ofdifferent soil bodies in a given field, as shown inFigure 2.9. The dark-colored soil in the fore-ground receives runoff water from the adjacentslopes. The light-colored soil on the slopes devel-oped where water runoff and erosion occurred.Distinctly different management practices are re-quired to use effectively the poorly drained soil inthe foreground and the eroded soil on the slope.

The boundary between the two different soilssoils is easily seen. In many instances the bound-aries between soils require an inspection of thesoil, which is done by digging a pit or using a soilauger.

How Scientists Study Soils asNatural BodiesA particular soil, or soil body, occupies a particu-lar part of a landscape. To learn about such a soil,a pit is usually dug and the soil horizons aredescribed and sampled. Each horizon is de-scribed in terms of its thickness, color, arrange-ment of particles, clay content, abundance ofroots, presence or absence of lime, pH, and so on.Samples from each horizon are taken to the labo-ratory and are analyzed for their chemical, physi-

1 9

20

cal, and biological properties. These data are pre-sented in graphic form to show how various soilproperties remain the same or change from onehorizon to another (shown for clay content inFigure 2.7). Figure 2.10 shows soil scientistsstudying a soil in the field. Pertinent data arepresented by a researcher, using charts, and theproperties and genesis of the soil are discussed bythe group participitants.

I mportance of Concept of Soils asNatural BodiesThe nature and properties of the horizons in a soildetermine the soil's suitability for various uses. Touse soils prudently, an inventory of the soil'sproperties is needed to serve as the basis for mak-i ng predictions of soil behavior in various situa-tions. Soil maps, which show the location of thesoil bodies in an area, and written reports aboutsoil properties and predictions of soil behavior forvarious uses began in the United States in 1896. Bythe 1920s, soil maps were being used to plan thelocation and construction of highways in Michi-gan. Soil materials that are unstable must be re-


FIGURE 2.11 Muck (organic)soil layer being replaced withsand to increase the stability ofthe roadbed.

moved and replaced with material that can with-stand the pressures of vehicular traffic. The scenein Figure 2.11 is of section of road that was builtwithout removing a muck soil layer. The roadbecame unstable and the muck layer was eventu-ally removed and replaced with sand.

SUMMARYThe original source of all mineral soil parent ma-terial is rock weathering. Some soils have formeddirectly in the products of rock weathering at theirpresent location. In these instances, horizon for-mation may be limited by the rate of rock weather-i ng, and soil formation may be very slow. Mostsoils, however, have formed in sediments result-i ng from the erosion, movement, and depositionof material by glaciers, water, wind, and gravity.Soils that have formed in organic sediments areorganic soils.

The major soil-forming processes include:(1) humification and accumulation of organicmatter, (2) rock and mineral weathering, (3)l eaching of soluble materials, and (4) the eluvi-

ation and illuviation of colloidal particles. Theoperation of the soil formation processes overtime produces soil horizons as a result of differen-tial changes in one soil layer, as compared toanother.

The master soil horizons or layers include theO, A, E, B, C, and R horizons.

Different kinds of soil occur as a result of thei nteraction of the soil-forming factors: parent ma-terial, organisms, climate, topography, and time.

Landscapes are composed of three-dimen-sional bodies that have naturally (genetically) de-veloped horizons. These bodies are called soils.

Prudent use of soils depends on a recognitionof soil properties and predictions of soil behaviorunder various conditions.

REFERENCES

REFERENCESArkley, R. J. 1964. Soil Survey of the Eastern Stanislaus

Area, California. U.S.D.A. and Cal. Agr. Exp. Sta.Barshad, I. 1959. "Factors Affecting Clay Formation."

Clays and Clay Minerals. E. Ingerson (ed.), EarthSciences Monograph 2. Pergamon, New York.

Jenny, H. 1941. Factors of Soil Formation. McGraw-Hill,New York.

Jenny, H. 1980. The Soil Resource. Springer-Verlag,New York.

Simonson, R. W. 1957. "What Soils Are." USDA Agricul-tural Yearbook, Washington, D. C.

Simmonson, R. W. 1959. "Outline of Generalized The-ory of Soil Genesis." Soil Sci. Soc. Am. Proc. 23:161-164.

Twenhofel, W, H. 1939. "The Cost of Soil in Rock andTime." Am J. Sci. 237:771-780.

2 1

Physically, soils are composed of mineral andorganic particles of varying size. The particles arearranged in a matrix that results in about 50 per-cent pore space, which is occupied by water andair. This produces a three-phase system of solids,liquids, and gases. Essentially, all uses of soils are

greatly affected by certain physical properties.The physical properties considered in this chapterinclude: texture, structure, consistence, porosity,density, color, and temperature.

SOIL TEXTURE

The physical and chemical weathering of rocksand minerals results in a wide range in size ofparticles from stones, to gravel, to sand, to silt,and to very small clay particles. The particle-sizedistribution determines the soil's coarseness orfineness, or the soil's texture. Specifically, textureis the relative proportions of sand, silt, and clay ina soil.

The Soil SeparatesSoil separates are the size groups of mineral parti-cles less than 2 millimeters (mm) in diameter or

CHAPTER 3

SOIL PHYSICAL PROPERTIES

22

the size groups that are smaller than gravel. Thediameter and the number and surface area per

gram of the separates are given in Table 3.1.Sand is the 2.0 to 0.05 millimeter fraction and,

according to the United States Department of Agri-culture (USDA) system, the sand fraction is sub-divided into very fine, fine, medium, coarse, andvery coarse sand separates. Silt is the 0.05 to 0.002millimeter (2 microns) fraction. At the 0.05 milli-meter particle size separation, between sand andsilt, it is difficult to distinguish by feel the individ-ual particles. In general, if particles feel coarse orabrasive when rubbed between the fingers, the

particles are larger than silt size. Silt particles feelsmooth like powder. Neither sand nor silt is stickywhen wet. Sand and silt differ from each other onthe basis of size and may be composed of thesame minerals. For example, if sand particles aresmashed with a hammer and particles less than0.05 millimeters are formed, the sand has beenconverted into silt.

The sand and silt separates of many soils aredominated by quartz. There is usually a signifi-cant amount of weatherable minerals, such asfeldspar and mica, that weather slowly and re-

SOIL TEXTURE

TABLE 3.1 Some Characteristics of Soil Separates

a United States Department of Agriculture System.b I nternational Soil Science Society System.

The surface area of platy-shaped montmorillonite clay particles determined by the glycolretention method by Sor and Kemper. (See Soil Science Society of America Proceedings,Vol. 23, p. 106, 1959.) The number of particles per gram and surface area of silt and theother separates are based on the assumption that particles are spheres and the largestparticle size permissible for the separate.

lease ions that supply plant needs and recombineto form secondary minerals, such as clay. Thegreater specific surface (surface area per gram) ofsilt results in more rapid weathering of silt, com-pared with sand, and greater release of nutrientions. The result is a generally greater fertility insoil having a high silt content than in soils high insand content.

Clay particles have an effective diameter lessthan 0.002 millimeters (less than 2 microns). Theytend to be plate-shaped, rather than sperical, andvery small in size with a large surface area pergram, as shown in Table 3.1. Because the specificsurface of clay, is many times greater than that ofsand or silt, a gram of clay adsorbs much morewater than a gram of silt or sand, because wateradsorption is a function of surface area. The clayparticles shown in Figure 3.1 are magnified about30,000 times; their plate-shaped nature contrib-utes to very large specific surface. Films of waterbetween plate-shaped clay particles act as a lubri-cant to give clay its plasticity when wet. Con-versely, when soils high in clay are dried, there isan enormous area of contact between plate-shaped soil particles and great tendency for veryhard soil clods to form. Although the preceding

statements apply to most clay in soils, some soilclays have little tendency to show stickiness andexpand when wetted. The clay fraction usuallyhas a net negative charge. The negative chargeadsorbs nutrient cations, including Ca

2+'Mg2 +,

and K + , and retains them in available form for useby roots and microbes.

FIGURE 3.1 An electron micrograph of clayparticles magnified 35,000 times. The platy shape, orflatness, of the particles results in very high specificsurface. (Photograph courtesy Mineral IndustriesExperiment Station, Pennsylvania State University.)

23

SeparateDiameter,

mm'Diameter,

mmb

Number ofParticlesper Gram

SurfaceArea inI Gram,

cm2

Very coarse sand 2.00-1.00 90 11Coarse sand 1.00-0.50 2.00-0.20 720 23Medium sand 0.50-0.25 5,700 45Fine sand 0.25-0.10 0.20-0.02 46,000 91Very fine sand 0.10-0.05 722,000 227Silt 0.05-0.002 0.02-0.002 5,776,000 454Clay Below 0.002 Below 0.002 90,260,853,000 8,000,000`

24

Particle Size AnalysisSieves can be used to separate and determine thecontent of the relatively large particles of the sandand silt separates. Sieves, however, are unsatis-factory for the separation of the clay particles fromthe silt and sand. The hydrometer method is anempirical method that was devised for rapidlydetermining the content of sand, silt, and clay in asoil.

In the hydrometer method a sample (usually 50grams) of air-dry soil is mixed with a dispersingagent (such as a sodium pyrophosphate solution)for about 12 hours to promote dispersion. Then,the soil-water suspension is placed in a metal cupwith baffles on the inside, and stirred on a mixerfor several minutes to bring about separation ofthe sand, silt, and clay particles. The suspension

FIGURE 3.2 Inserting hydrometer for the 8-hourreading. The sand and silt have settled. The 8-hourreading measures grams of clay in suspension and isused to calculate the percentage of clay.


is poured into a specially designed cylinder, anddistilled water is added to bring the contents up tovolume.

The soil particles settle in the water at a speeddirectly related to the square of their diameter andinversely related to the viscosity of the water. Ahand stirrer is used to suspend the soil particlesthoroughly and the time is immediately noted. Aspecially designed hydrometer is carefully in-serted into the suspension and two hydrometerreadings are made. The sand settles in about 40seconds and a hydrometer reading taken at 40seconds determines the grams of silt and clayremaining in suspension. Subtraction of the 40-second reading from the sample weight gives thegrams of sand. After about 8 hours, most of the silthas settled, and a hydrometer reading taken at 8hours determines the grams of clay in the sample(see Figure 3.2). The silt is calculated by differ-ence: add the percentage of sand to the percent-age of clay and subtract from 100 percent.

PROBLEM: Calculate the percentage of sand,clay, and silt when the 40-second and 8-hour hy-drometer readings are 30 and 12, respectively;assume a 50 gram soil sample is used:

sample weight - 40-second reading x 100 = %sandsample weight

50 g - 30 gx 100 = 40% sand

509

8-hour readingx 100 = % clay

sample weight

12 g x 100 = 24% clay

50 g

1 00% - (40% + 24%) = 36% silt

After the hydrometer readings have been ob-tained, the soil-water mixture can be poured overa screen to recover the entire sand fraction. After itis dried, the sand can be sieved to obtain thevarious sand separates listed in Table 3.1.

SOIL TEXTURE

Soil Textural ClassesOnce the percentages of sand, silt, and clay havebeen determined, the soil can be placed in one of12 major textural classes. For a soil that contains15 percent clay, 65 percent sand, and 20 percentsilt, the logical question is: What is the texture ortextural class of the soil?

The texture of a soil is expressed with the use ofclass names, as shown in Figure 3.3. The sum ofthe percentages of sand, silt, and clay at any pointin the triangle is 100. Point A represents a soilcontaining 15 percent clay, 65 percent sand, and20 percent silt, resulting in a textural class nameof sandy loam. A soil containing equal amounts ofsand, silt, and clay is a clay loam. The area out-lined by the bold lines in the triangle defines agiven class. For example, a loam soil contains 7 to27 percent clay, 28 to 50 percent silt, and between22 and 52 percent sand. Soils in the loam classare influenced almost equally by all three sep-

25

FIGURE 3.3 The texturaltriangle shows the limits ofsand, silt, and clay contents ofthe various texture classes.Point A represents a soil thatcontains 65 percent sand, 15percent clay, and 20 percentsilt, and is a sandy loam.

arates-sand, silt, and clay. For sandy soils (sandand loamy sand), the properties and use of thesoil are influenced mainly by the sand content ofthe soil. For clays (sandy clay, clay, silty clay), theproperties and use of the soil are influencedmainly by the high clay content.

Determining Texture by theField MethodWhen soil scientists make a soil map, they use thefield method to determine the texture of the soilhorizons to distinguish between different soils.When investigating a land-use problem, the abilityto estimate soil texture on location is useful indiagnosing the problem and in formulating a so-lution.

A small quantity of soil is moistened with waterand kneaded to the consistency of putty to deter-mine how well the soil forms casts or ribbons

26

(plasticity). The kind of cast or ribbon formed isrelated to the clay content and is used to catego-rize soils as loams, clay loams, and clays. This isshown in Figure 3.4.

If a soil is a loam, and feels very gritty or sandy,it is a sandy loam. Smooth-feeling loams are highin silt content and are called silt loams. If thesample is intermediate, it is called a loam. Thesame applies to the clay loams and clays. Sandsare loose and incoherent and do not form rib-bons. More detailed specifications are given inAppendix 1.

I nfluence of Coarse Fragments onClass NamesSome soils contain significant amounts of graveland stones and other coarse fragments that arel arger than the size of sand grains. An appropriateadjective is added to the class name in thesecases. For example, a sandy loam in which 20 to50 percent of the volume is made up of gravel is agravelly sandy loam. Cobbly and stony are usedfor fragments 7.5 to 25 centimeters and more than25 centimeters in diameter, respectively. Rocki-ness expresses the amount of the land surfacethat is bedrock.

Texture and the Use of SoilsPlasticity, water permeability, ease of tillage,droughtiness, fertility, and productivity are all

FIGURE 3.4 Determination of texture by the fieldmethod. Loam soil on left forms a good cast whenmoist. Clay loam in center forms a ribbon that breakssomewhat easily. Clay on the right forms a longfexible ribbon.


FIGURE 3.5 Foundation soil with high clay contentexpanded and shrunk with wetting and drying,cracking the wall. (Photograph courtesy USDA.)

closely related to soil texture. Many clay soilsexpand and shrink with wetting and drying, caus-i ng cracks in walls and foundations of buildings,as shown in Figure 3.5. Conversely, many red-colored tropical soils have clay particles com-posed mainly of kaolinite and oxides of iron andalumimum. These particles have little capacity todevelop stickiness and to expand and contract onwetting and drying. Such soils can have a highclay content and show little evidence of stickinesswhen wet, or expansion and contraction with wet-ting and drying.

Many useful plant growth and soil texture rela-tionships have been established for certain geo-graphic areas. Maximum productivity for red pine(Pinus resinosa) occurs on coarser-textured soilthan for corn (Zea maize) in Michigan. The great-est growth of red pine occurs on soils with sandy

loam texture where the integrated effects of thenutrients, water, and aeration are the most desir-able. Loam soils have the greatest productivity forcorn. When soils are irrigated and highly fertil-ized, however, the highest yields of corn occur onthe sand soils. The world record corn yield, set in1977, occurred on a sandy soil that was fertilizedand irrigated.

The recommendations for reforestation in Ta-ble 3.2 assume a uniform soil texture exists withi ncreasing soil depth, but this is often not true inthe field. Species that are more demanding ofwater and nutrients can be planted if the underly-i ng soil horizons have a finer texture. In somei nstances an underlying soil layer inhibits rootpenetration, thus creating a shallow soil with lim-ited water and nutrient supplies.

Young soils tend to have a similar texture ineach horizon. As soils evolve and clay is translo-cated to the B horizon, a decrease in permeabilityof the subsoil occurs. Up to a certain point,however, an increase in the amount of clay in thesubsoil is desirable, because the amount of waterand nutrients stored in that zone can also be in-creased. By slightly reducing the rate of watermovement through the soil, fewer nutrients willbe lost by leaching. The accumulation of clay intime may become sufficient to restrict severely themovement of air and water and the penetration of

SOIL STRUCTURE

TABLE 3.2 Recommendations for Reforestation on Upland Soils inWisconsin in Relation to Soil Textureª

27

roots in subsoil Bt horizons. Water saturation ofthe soil above the Bt horizon may occur in wetseasons. Shallow rooting may result in a defi-ciency of water in dry seasons. A Bt horizon with ahigh clay content and very low water permeabilityis called a claypan. A claypan is a dense, compactlayer in the subsoil, having a much higher claycontent than the overlying material from which itis separated by a sharply defined boundary (seeFigure 3.6).

Claypan soils are well suited for flooded riceproduction. The claypan makes it possible tomaintain flooded soil conditions during the grow-ing season with a minimum amount of water.Much of the rice produced in the United States isproduced on claypan soils located on nearly levelland in Arkansas, Louisiana, and Texas. The othermajor kind of rice soil in the United States has verylimited water permeability because there is a claytexture in all horizons.

SOIL STRUCTURETexture is used in reference to the size of soilparticles, whereas structure is used in reference tothe arrangement of the soil particles. Sand, silt,and clay particles are typically arranged into sec-ondary particles called peds, or aggregates. The

ª Wilde, S. A., "The Significance of Soil Texture in Forestry, and Its Determination by aRapid Field Method," Journal of Forestry, 33:503-508, 1935. By permission Journal ofForestry.

Percent SiltPlus Clay Species Recommended

Less than 5 (Only for wind erosion control)5-10 Jack pine, Red cedar

10-15 Red pine, Scotch pine, Jack pine15-25 Increasing All pines25-35 demand for White pine, European larch, Yellow birch, White

water and elm, Red oak, Shagbark hickory, Black locustOver 35 nutrients White spruce, Norway spruce, White cedar,

White ash, Basswood, Hard maple, White oak,Black walnut

28

shape and size of the peds determine the soil'sstructure.

Importance of StructureStructure modifies the influence of texture withregard to water and air relationships and the easeof root penetration. The macroscopic size of mostpeds results in the existence of interped spacesthat are much larger than the spaces existing be-tween adjacent sand, silt, and clay particles. Notethe relatively large cracks between the structuralpeds in the claypan (Bt horizon) shown in Figure3.6. The peds are large and in the shape of blocksor prisms, as a result of drying and shrinkageupon exposure. When cracks are open, they areavenues for water movement. When this soil wets,however, the clay expands and the cracks be-tween the peds close, causing the claypan hori-zon to become impermeable. Root penetration


and the downward movement of water, however,are not inhibited in the B horizon of most soils.

Genesis and Types of StructureSoil peds are classified on the basis of shape. Thefour basic structural types are spheroid, platelike,blocklike, and prismlike. These shapes give riseto granular, platy, blocky, and prismatic types ofstructure. Columar structure is prismatic-shapedpeds with rounded caps.

Soil structure generally develops from materialthat is without structure. There are two structure-less conditions, the first of which is sands thatremain loose and incoherent. They are referred toas single grained. Second, materials with a signif-i cant clay content tend to be massive if they donot have a developed structure. Massive soil hasno observable aggregation or no definite and or-derly arrangement of natural lines of weakness.

FIGURE 3.6 A soil with astrongly developed Bt horizon;a claypan. On drying, cracksform between structural units.On wetting, the soil expands,the cracks close, and theclaypan becomesi mpermeable to water androots. (Photograph courtesySoil Conservation Service,USDA.)

Massive soil breaks up into random shaped clodsor chunks. Structure formation is illustrated inFigure 3.7.

Note in Figure 3.8 that each horizon of the soilhas a different structure. This is caused by differ-ent conditions for structure formation in each ho-rizon. The A horizon has the most abundant rootand small-animal activity and is subject to fre-quent cycles of wetting and drying. The structuretends to be granular (see Figure 3.8). The Bt hori-zon has more clay, less biotic activity, and isunder constant pressure because of the weight ofthe overlying soil. This horizon is more likely tocrack markedly on drying and to develop either ablocky or prismatic structure. The development ofstructure in the E horizon is frequently weak andthe structure hard to observe. In some soils the Ehorizon has a weakly developed platy structure(see Figure 3.8).

Grade and ClassComplete descriptions of soil structure include:(1) the type, which notes the shape and arrange-ment of peds; (2) the class, which indicates pedsize; and (3) the grade, which indicates the dis-tinctiveness of the peds. The class categories de-pend on the type. A table of the types and classesof soil structure is given in Appendix 2. Terms forgrade of structure are as follows:

SOIL STRUCTURE 29

0. Structureless-no observable aggregration orno definite and orderly arrangement of natu-ral lines of weakness. Massive, if coherent, asin clays. Single grained, if noncoherent, as insands.

1. Weak-poorly formed indistinctive peds,barely observable in place.

2. Moderate-well-formed distinctive peds,moderately durable and evident, but not dis-tinct in undisturbed soil.

3. Strong-durable peds that are quite evidenti n undisturbed soil, adhere weakly to one an-other, and become separated when the soil isdisturbed.

The sequence followed in combining the threeterms to form compound names is first the grade,then the class, and finally the type. An example ofa soil structural description is "strong finegranular."

Managing Soil StructurePractical management of soil structure is gener-ally restricted to the topsoil or plow layer in regardto use of soils for plant growth. A stable structureat the soil surface promotes more rapid infil-tration of water during storms and results in re-duced water runoff and soil erosion and greaterwater storage within the soil. The permanence ofpeds depends on their ability to retain their shape

FIGURE 3.7 Illustration ofthe formation of various typesof soil structure from single-grained and massive soilconditions. Structure types arebased on the shape ofaggregates.

30

-- 30-

- 60 --

-- 9O--

-- l2O--

FIGURE 3.8 Structure of the horizons of a forest soilwith a Bt horizon. Scale is in centimeters.


after being subjected to the disruptive effects ofraindrops and tillage. In addition, for peds to bepermanent, the particles in the peds must holdtogether when dry peds are wetted.

When dry soil is suddenly wetted, water movesi nto the peds from all directions and compressesthe air in the pore spaces. Peds unable to with-stand the pressure exerted by the entrapped airare disrupted and fall apart. Unstable peds at thesoil surface in cultivated fields are also brokenapart by the force of falling raindrops. Disruptionof peds produces smaller chunks and some indi-vidual particles, which float over the soil surfaceand seal off the surface to additional infiltratingwater. On drying, the immediate surface soil mayacquire a massive or structureless condition.

Researchers at the University of Wisconsinmeasured the development of peds in four soils inwhich they also determined the content of agentsknown to bind particles together and give pedsstability. These binding agents were microbialgum, organic carbon, iron oxide, and clay. Thedata in Table 3.3 show that microbial gum wasthe most important in promoting ped formation.Iron oxide, formed during weathering from min-erals that contain iron, was most important in onesoil and second most important when the foursoils were grouped together. Soil managementpractices that result in frequent additions of or-ganic matter to the soil in the form of plant resi-due, or manure, will tend to produce more micro-bial gum and increase ped formation and

Soil

Parr silt loamAlmena silt loamMiami silt loamKewaunee silt loamAll soils

SOIL CONSISTENCE 3 1

stability. Practices that result in keeping a vegeta-tive cover also reduce raindrop impact and thusreduce the disruption of peds.

SOIL CONSISTENCEConsistence is the resistance of the soil to defor-mation or rupture. It is determined by the cohe-sive and adhesive properties of the entire soilmass. Whereas structure deals with the shape,size, and distinctiveness of natural soil aggre-gates, consistence deals with the strength andnature of the forces between the sand, silt, andclay particles. Consistence is important for tillageand traffic considerations. Dune sand exhibitsminimal cohesive and adhesive properties, andbecause sand is easily deformed, vehicles caneasily get stuck in it. Clay soils can become stickywhen wet, and thus make hoeing or plowing dif-ficult.

Soil Consistence TermsConsistence is described for three moisture lev-els: wet, moist, and dry. A given soil may be stickywhen wet, firm when moist, and hard when dry. Apartial list of terms used to describe consistencei ncludes:

1. Wet soil-nonsticky, sticky, nonplastic,plastic

2. Moist soil-loose, friable, firm3. Dry soil-loose, soft, hard

TABLE 3.3 Order of Importance of Soil Constituents in Formation of Pedsover 0.5 mm in Diameter in the A Horizons of Four Wisconsin Soils

Order of Importance of Soil Constituents

Microbial gum > clay > iron oxide > organic carbonMicrobial gum > iron oxideMicrobial gum > iron oxide > organic carbonIron oxide > clay > microbial gumMicrobial gum > iron oxide > organic carbon > clay

Adapted from Chesters, G., O. J., Attoe, and O. N., Allen, "Soil Aggregation in Relation toVarious Soil Constituents," Soil Science Society of America Proceedings Vol. 21, 1957, p.276, by permission of the Soil Science Society of America.

32

Plastic soil is capable of being molded or de-formed continuously and permanently, by rela-tively moderate pressure, into various shapeswhen wet. Friable soils readily break apart and arenot sticky when moist.

Two additional consistence terms for specialsituations are cemented and indurated. Cementa-tion is caused by cementing agents such as cal-cium carbonate, silica, and oxides of iron andaluminum. Cemented horizons are not affected bywater content and limit root penetration. When acemented horizon is so hard that a sharp blow of ahammer is required to break the soil apart, the soilis considered to be indurated.

A silica cemented horizon is called a duripan.

Large rippers pulled by tractors are used to breakup duripans to increase rooting depth in soils.Sometimes a layer with an accumulation of car-bonates (a Ck horizon, for example) accumulatesso much calcium carbonate as to become ce-mented or indurated, and is transformed into apetrocalcic horizon. Petrocalcic horizons are alsocalled caliche. Cemented and indurated horizonstend to occur in soils of great age.

DENSITY AND WEIGHT RELATIONSHIPSTwo terms are used to express soil density. Parti-

cle density is the average density of the soil parti-cles, and bulk density is the density of the bulksoil in its natural state, including both the parti-cles and pore space.

Particle Density and Bulk DensityA soil is composed of mineral and organic par-ticles of varying composition and density.Feldspar minerals (including orthoclase) are themost common minerals in rocks of the earth'scrust and very common in soils. They have den-sities ranging from 2.56 g/cm 3 to 2.76 g/cm3 .Quartz is also a common soil mineral; it has adensity of 2.65 g/cm3 . Most mineral soils consistmainly of a wide variety of minerals and a smallamount of organic matter. The average particledensity for mineral soils is usually given as2.65 g/cm 3 .


Bulk samples of soil horizons are routinely col-lected in the field and are used for chemical anal-yses and for some physical analyses, such asparticle-size distribution. Soil cores are collectedto determine soil bulk density. The soil coresamples are collected to obtain a sample of soilthat has the undisturbed density and pore spacerepresentative of field conditions. Coring ma-chines are used to remove soil cores with a diam-eter of about 10 centimeters to a depth of a meteror more. The technique for hand sampling of indi-vidual soil horizons is shown in Figure 3.9. Care

FIGURE 3.9 Technique for obtaining soil cores thatare used to determine bulk density. The light-coloredmetal core fits into the core sampler just to the left.This unit is then driven into the soil in pile-driverfashion, using the handle and weight unit.

must be exercised in the collection of soil coresso that the natural structure is preserved. Anychange in structure, or compaction of soil, willalter the amount of pore space and, therefore, willalter the bulk density. The bulk density is the massper unit volume of oven dry soil, calculated asfollows:

bulk density =mass oven dry

volume

PROBLEM: If the soil in the core of Figure 3.9weighs 600 grams over dry and the core has avolume of 400 cm3 , calculate the bulk density.

bulk density = 400 0cm3 = 1.5 g/cm3

The bulk density of a soil is inversely related toporosity. Soils without structure, such as single-grained and massive soil, have a bulk density ofabout 1.6 to 1.7 g/cm 3 . Development of structureresults in the formation of pore spaces betweenpeds (interped spaces), resulting in an increase inthe porosity and a decrease in the bulk density. Avalue of 1.3 g/cm3 is considered typical of loamsurface soils that have granular structure. As theclay content of surface soils increases, theretends to be an increase in structural developmentand a decrease in the bulk density. As clay accu-mulates in B horizons, however, the clay fills ex-isting pore space, resulting in a decrease in porespace volume. As a result, the formation of Bthorizons is associated with an increase in bulkdensity.

The bulk densities of the various horizonsof a soil are given in Figure 3.10. In this soil theC horizon has the greatest bulk density,-1.7 g/cm3 . The C horizon is massive, or structurel-ess, and this is associated with high bulk densityand low porosity. Note that the Bt horizon is moredense than the A horizon and that there is aninverse relationship between bulk density and to-tal porosity (macropore space plus microporespace).

Organic soils have very low bulk densitycompared with mineral soils. Considerable varia-

DENSITY AND WEIGHT RELATIONSHIPS

FIGURE 3.10 Bulk density and pore space of thehorizons of a soil with A, E, Bt, and C horizons.(Data for Miami loam from Wascher, 1960.)

tion exists, depending on the nature of the or-ganic matter and the moisture content at the timeof sampling to determine bulk density. Bulk den-sities for organic soils commonly range from 0.1to 0.6 g/cm 3 .

Changes in soil porosity due to compaction arecommonly evaluated in terms of changes in bulkdensity.

Weight of a Furrow-Slice of SoilSoil erosion losses are commonly expressed astons of soil per acre. An average annual soil lossof 10 tons per acre is quite meaningless unless theweight of an inch of soil over 1 acre is known. Inthis instance, an erosion rate of 10 tons annuallywould result in the loss of a layer about 7 inchesthick, every 100 years.

The acre-furrow-slice has customarily beenconsidered to be a layer of soil about 7 inchesthick over 1 acre. An acre has 43,560 ft 2 . The

33

34

volume of soil in an acre-furrow-slice, therefore,is 25,410 ft3 to a depth of 7 inches. If the soil has abulk density of 1.3 g/cm3 , the soil has a density1.3 times that of water. Because a cubic foot ofwater weighs 62.4 pounds, a cubic foot of the soilwould weigh 81.1 (62.4 x 1.3) pounds. Theweight of a 7-inch-thick acre-furrow-slice of a soilwith a bulk density of 1.3 g/cm3 (81 lb/ft3) iscalculated as follows:

1 7/12 ft x 43 560 ft2 ] x [81.1 lb/ft 3 ]= 2,061,259 lb

This value-2,061,259-is usually rounded off as2,000,000 pounds as the weight of an acre-furrow-slice of typical oven-dry loamy soil.

Soil samples to assess the fertility of a soil areusually taken to the depth of the plow layer. Theresults are commonly reported as pounds of po-tassium, for example, per acre. Because it is usu-ally assumed that the plow layer of an acre weighs2 million pounds, the test results are sometimesreported as parts per 2 million (pp2m). For exam-ple, a soil test result for available potassium of 100pounds per acre (acre-furrow-slice) is sometimesgiven as 100 pp2m. In recent years there has beena tendency to plow deeper and, in many cases, thefurrow slice is considered to be 9 or 10 inchesthick and to weigh proportionally more.

PROBLEM: Calculate the weight of a 10-inch-thick acre-furrow-slice that has a bulk density of1.2 g/cm 3 .

[ 10/12 ft x 43 560 ft'] x [74.9 lb/ft 3 ]= 2,718,870 lb

Lime recommendations for increasing soil pH arebased on assumptions in regard to the weight ofthe soil volume that must be limed.

Soil Weight on a Hectare BasisA hectare is an area 100 meters square and con-tains 10,000 square meters (m2) or 100,000,000cm 2 . A 15 centimeter thick layer of soil over ahectare that has a bulk density of 1.3 g/cm 3 wouldweigh


100,000,000 cm 2 x 15 cm x 1.3 g/cm 3

= 1,950,000,000 g

The weight of a soil layer for a hectare that is 15centimeters thick is, therefore, 1,950,000 kg/ha.

SOIL PORE SPACE AND POROSITYThe fact that mineral soils have a particle densityand bulk density of about 2.65 and 1.3 g/cm3 ,respectively, means that soils have about 50 per-cent total pore space or porosity. In simple terms,rocks without pore space are broken down byweathering to form mineral soils that have about50 percent porosity. The pore spaces vary in size,and the size of pore spaces themselves can be asimportant as the total amount of pore space.

Determination of PorosityOven dry soil cores, which have been used todetermine bulk density, can be placed in a pan ofwater, allowed to saturate or satiate, and thenreweighed to obtain the data needed to calculatesoil porosity. When the soil is satiated, the porespace is filled with water, except for a smallamount of entrapped air. The volume of water in asatiated soil core approximates the amount ofpore space and is used to calculate the soil'sapproximate porosity. For example, if the soil in a400 cm2 core weighed 600 grams at oven dry, and800 grams at water satiation, the satiated soilwould contain 200 grams of water that occupies200 cubic centimeters of space (1 g of water has avolume of 1 cm3 ). Porosity is calculated asfollows:

cm3 pore spaceporosity = 3 X 100

cm soil volume

200 cm3x 100 = 50%

400 cm3

PROBLEM: A 500 cm3 oven dry core has a bulkdensity of 1.1 g/cm3 . The soil core is placed in apan of water and becomes water saturated. The

oven dry soil and water at saturation weigh 825grams. Calculate the total soil porosity.

Effects of Texture and Structureon PorositySpheres in closest packing result in a porosity of26 percent, and spheres in open packing have aporosity of 48 percent. Stated in another way, aball that just fits in a box occupies 52 percent ofthe volume of the box, whereas 48 percent of thevolume is empty space. These facts are true re-gardless of size of the spheres or balls. Single-grained sands have a porosity of about 40 percent.This suggests that the sand particles are not per-fect spheres and the sand particles are not in aperfect close packing arrangement. The low po-rosity of single-grained sands is related to theabsence of structure (peds) and, therefore, anabsence of interped spaces.

Fine-textured A horizons, or surface soils, havea wide range of particle sizes and shapes, and theparticles are usually arranged into peds. Thisresults in pore spaces within and betweenpeds. These A horizons with well-developed gran-ular structure may have as much as 60 per-cent porosity and bulk density values as low as1.0 g/cm3 . Fine-textured Bt horizons have a differ-ent structural condition and tend to have lessporosity and, consequently, a greater bulk densitythan fine-textured A horizons. This is consistentwith the filling of pore space by translocated clayand the effects of the weight of the overlying soil,which applies a pressure on the Bt horizon. Thepore spaces within the peds will generally besmaller than the pore spaces between the peds,resulting in a wide range in pore sizes.

SOIL PORE SPACE AND POROSITY 35

It has been pointed out that sand surface soilshave less porosity than clayey surface soils. Yet,our everyday experiences tell us that water movesmuch more rapidly through the sandy soil. Theexplanation of this apparent paradox lies in thepore size differences in the two soils. Sands con-tain mostly macropores that normally cannot re-tain water against gravity and are usually filledwith air. As a consequence, macropores have alsobeen called aeration pores. Since the porosity ofsands is composed mainly of macropores, sandstransmit water rapidly. Pores that are smallenough to retain water against gravity will remainwater filled after soil wetting by rain or irrigation,and are called capillary or micropores.

Because sands have little micropore space theyare unable to retain much water. Fine-texturedsoils tend to contain mainly micropores and thusare able to retain a lot of water but have littleability to transmit water rapidly. An example of thedistribution of micropore and macropore spacei n the various horizons of a soil profile is given inFigure 3.10. Note that the amounts of both totalporosity and macropore space are inversely re-l ated to the bulk density.

Porosity and Soil AerationThe atmosphere contains by volume nearly 79percent nitrogen, 21 percent oxygen, and 0.03percent carbon dioxide. Respiration of roots, andother organisms, consumes oxygen and producescarbon dioxide. As a result, soil air commonlycontains 10 to 100 times more carbon dioxide andslightly less oxygen than does the atmosphere(nitrogen remains about constant). Differences inthe pressures of the two gases are created beweenthe soil and atmosphere. This causes carbondioxide to diffuse out of the soil and oxygen todiffuse into the soil. Normally, this diffusion issufficiently rapid to prevent an oxygen deficiencyor a carbon dioxide toxicity for roots.

Although water movement through a uniformlyporous medium is greatly dependent on pore size,the movement and diffusion of gas are closely

36

correlated with total porosity. Gaseous diffusioni n soil, however, is also dependent on pore spacecontinuity. When oxygen is diffusing through amacropore and encounters a micropore that isfilled with water, the water-filled micropore actsas a barrier to further gas movement. Diffusion ofoxygen through the water barrier is essentiallyzero because oxygen diffusion through water isabout 10,000 times slower than through air.Clayey soils are particularly susceptible to poorsoil aeration when wet, because most of the porespace consists of micropores that may be filledwith water. Sands tend to have good aeration orgaseous diffusion because most of the porosity iscomposed of macropores. In general, a desirablesoil for plant growth has a total porosity of 50percent, which is one half macropore porosityand one half micropore porosity. Such a soil has agood balance between the retention of water forplant use and an oxygen supply for root res-piration.

Oxygen deficiencies are created when soils be-come water saturated or satiated. The wilting oftomato, due to water-saturated soil, is shown inFigure 1.3. Plants vary in their susceptibilty tooxygen deficiency. Tomatoes and peas are very


susceptible to oxygen deficiency and may bekilled when soils are water saturated for a fewhours (less than a day). Two yew plants that wereplanted along the side of a house at the same timeare shown in Figure 3.11. The plant on the rightside died from oxygen stress caused by water-saturated soil as a result of flooding from thedownspout water. The soil had a high clay con-tent. The use of pipes to carry water away frombuilding foundations is also important in prevent-ing building foundation damage, where soils withhigh clay content expand and contract because ofcycles of wetting and drying.

When changes in soil aeration occur slowly,plants are able to make some adjustment. Allplant roots must have oxygen for respiration, andplants growing on flooded soil or in swamps havespecial mechanisms for obtaining oxygen.

SOIL COLORColor is about the most obvious and easily deter-mined soil property. Soil color is important be-cause it is an indirect measure of other importantcharacteristics such as water drainage, aeration,

FIGURE 3.11 Both of theseyews were planted at thesame time along the side of anew house. The failure toi nstall a pipe on the right, tocarry away the downspoutwater, caused water-saturatedsoil and killed the plant.

and the organic matter content. Thus, color isused with other characteristics to make many im-portant inferences regarding soil formation andland use.

Determination of Soil ColorSoil colors are determined by matching the colorof a soil sample with color chips in a Munsellsoil-color book. The book consists of pages, eachhaving color chips arranged systematically ac-cording to their hue, value, and chroma, the threevariables that combine to give colors. Hue refersto the dominant wavelength, or color of the light.Value, sometimes color brillance, refers to thequantity of light. It increases from dark to lightcolors. Chroma is the relative purity of the domi-nant wavelength of the light. The three propertiesare always given in the order of hue, value, andchroma. In the notation, 10YR 6/4, 10YR is thehue, 6 is the value, and 4 is the chroma. This coloris a light-yellowish brown. This color system en-ables a person to communicate accurately thecolor of a soil to anyone in the world.

Factors Affecting Soil ColorOrganic matter is a major coloring agent that af-fects soil color, depending on its nature, amount,and distribution in the soil profile. Raw peat isusually brown; well-decomposed organic matter,such as humus, is black or nearly so. Many or-ganic soils have a black color.

In most mineral soils, the organic matter con-tent is usually greatest in the surface soil horizonsand the color becomes darker as the organic mat-ter content increases. Even so, many A horizonsdo not have a black color. Black-colored A hori-zons, however, are common in soils that devel-oped under tall grass on the pampa of Argentinaand the prairies of the United States. An interest-ing color phenomenon occurs in the clayey soilsof the Texas Blacklands. The A horizon has ablack color; however, the black color may extendto the depth of a meter even though there is a

SOIL COLOR 37

considerable decrease in organic matter contentwith increasing soil depth. In some soils, finelydivided manganese oxides contribute to blackcolor.

The major coloring agents of most subsoil hori-zons are iron compounds in various states of oxi-dation and hydration. The rusting of iron is anoxidation process that produces a rusty orreddish-colored iron oxide. The bright red colorof many tropical soils is due to dehydrated andoxidized iron oxide, hematite (Fe2O3). Organicmatter accumulation in the A horizons of thesesoils results in a brownish-red or mahogany color.Hydrated and oxidized iron, goethite (FeOOH),has a yellow or yellowish-brown color, and re-duced and hydrated iron oxide has a gray color.Various combinations of iron oxides result inbrown and yellow-brown colors. Thus, the colorof the iron oxides is related to aeration and hydra-tion conditions as controlled by the absence orpresence of water. Soils on slopes that never satu-rate with water have subsoils indicative of well-drained and aerated soil-subsoils with reddishand brownish colors. Soils in depressions thatcollect water, and poorly drained locations inwhich soils are water saturated much of the time,will tend to have gray-colored B horizons. Soilsin intermediate situations will tend to haveyellowish-colored B horizons. If a soil has a gray-colored B horizon, the soil is likely to be watersaturated at least part of the time unless it isartificially drained (see Figure 3.12).

The light and grayish colors of E horizons arerelated to the illuviation of iron oxides and loworganic matter content. Some soil horizons mayhave a white color because of soluble salt accu-mulation at the soil surface or calcium carbonateaccumulation in subsoils. Horizons in young soilsmay be strongly influenced by the color of the soilparent material.

Significance of Soil ColorMany people have a tendency to equate black-colored soils with fertile and productive soils.

38

FIGURE 3.12 Mineral soils that develop under thei nfluence of much soil wetness have a dark-coloredA horizon and a gray-colored subsoil. The A horizonis about 40 centimeters thick.

Broad generalizations between soil color and soilfertility are not always valid. Within a local region,i ncreases in organic matter content of surfacesoils may be related to increases in soil fertility,because organic matter is an important reservoirof nitrogen.

Subsoil colors are very useful in predicting thelikelihood of subsoil saturation with water andpoor aeration. Gray subsoil color indicates a fairlyconstant water-saturated condition. Such soils arepoor building sites because basements tend to bewet and septic tank filter fields do not operateproperly in water-saturated soil. Installation of adrainage system is necessary to use the soil as abuilding site successfully. Subsoils that havebright brown and red colors are indicative of goodaeration and drainage. These sites are good loca-tions for buildings and for the production of treefruits. Many landscape plants have specific needsand tolerances for water, and aeration and soil


color are a useful guide in selection of plantspecies.

Alternating water saturation and drying of thesubsoil, which may occur because of alternatingwet and dry seasons, may produce an interme-diate color situation. During the wet season, ironis hydrated and reduced, and during the dry sea-son the iron is dehydrated and oxidized. Thiscauses a mixed pattern of soil colors called mot-tling. Mottled-colored B horizons are indicative ofsoils that are intermediate between frequent watersaturation and soils with continuous well-drainedconditions.

SOIL TEMPERATUREBelow freezing there is extremely limited biologi-cal activity. Water does not move through the soilas a liquid and, unless there is frost heaving, timestands still for the soil. A soil horizon as cold as5° C acts as a deterrent to the elongation of roots.The chemical processes and activities of microor-ganisms are temperature dependent. The al-ternate freezing and thawing of soils results in thealternate expansion and contraction of soils. Thisaffects rock weathering, structure formation, andthe heaving of plant roots. Thus, temperature is animportant soil property.

Heat Balance of SoilsThe heat balance of a soil consists of the gainsand losses of heat energy. Solar radiation receivedat the soil surface is partly reflected back into theatmosphere and partly absorbed by the soil sur-face. A dark-colored soil and a light-coloredquartz sand may absorb about 80 and 30 percentof the incoming solar radiation, respectively. Ofthe total solar radiation available for the earth,about 34 percent is reflected back into space, 19percent is absorbed by the atmosphere, and 47percent is absorbed by the land.

Heat is lost from the soil by: (1) evaporation ofwater, (2) radiation back into the atmosphere,

(3) heating of the air above the soil, and (4) heat-i ng of the soil. For the most part the gains andl osses balance each other. But during the daytimeor in the summer, the gains exceed the losses,whereas the reverse is true for nights and winters.

The amount of heat needed to increase thetemperature of soil is strongly related to watercontent. It takes only 0.2 calories of heat energy toi ncrease the temperature of I gram of dry soil 1 ° C;compared with 1.0 calorie per gram per degree forwater. This is important in the temperate regionswhere soils become very cold in winter and plant-ing dates in the spring depend on a large rise insoil temperature. In general, sandy soils warmmore quickly and allow earlier planting than dofine-textured soils, because sands retain less wa-ter and heat up faster.

Location and TemperatureIn the northern hemisphere, soils located onsouthern slopes have a higher temperature thansoils on north-facing slopes. Soils on south-facingslopes are more perpendicular to the sun's raysand absorb more heat energy per unit area than dosoils on northern slopes. This is very obvious intundra regions where soils on north-facing slopesmay have permanently frozen subsoil layers

FIGURE 3.13 Dates for last killing frost in spring and first killing frost in thefall at left and right, respectively, for Erie County, Pennsylvania. The longergrowing season along Lake Erie favors land use for fruits and vegetables.(Data from Taylor, 1960.)

SOIL TEMPERATURE 39

within the normal rooting depth of trees, whereassoils on southern slopes do not. Trees tend togrow only on the south-facing slopes without per-manently frozen subsoils. Considerable variationi n the microenvironment for plants can existaround a building. Southern exposures arewarmer, drier, and have more light than northernexposures.

Large bodies of water act as heat sinks andbuffer temperature changes nearby. In the springand fall the temperature changes more slowly andgradually in the area adjacent to the Great Lakesthan at locations further inland. Near these lakesin the spring, the temperatures of both air and soili ncrease slowly, which delays the blossoms onfruit trees and thereby reduces the hazard of latespring frosts. Killing frosts in the fall are delayed,resulting in an extension of the growing season.As a consequence, production of vine and treefruits is concentrated in areas adjacent to theGreat Lakes. The effect of distance from Lake Erieon the dates of last killing frost in spring and firstkilling frost in the fall is shown in Figure 3.13.

Control of Soil TemperatureTwo practical steps can be taken to change soiltemperature. Wet soils can be drained to remove

40

water, and mulches can be applied on the surfaceof the soil to alter the energy relationships. Whenused for agriculture, wet soils are drained tocreate an aerated root zone. The removal of wateralso causes the soil to warm more quickly in thespring.

A light-colored organic matter mulch, such asstraw, will tend to lower soil temperature because(1) more solar radiation will be reflected and lessabsorbed, and (2) the water content of the soil willtend to be greater because more water will in-filtrate. The mulch will, however, tend to increasesoil temperature because the mulch tends to:(1) reduce loss of heat by radiation, and (2) re-duce water evaporation from the soil surface,which requires energy. The net effect, however, isto reduce soil temperature in the spring. In thenorthern corn belt, crop residues left on the soilsurface promote cooler soils in the spring andtend to result in slightly lower corn yields. A blackplastic mulch increases soil temperature becauseit increases absorption of solar radiation, reducesheat loss by radiation, and reduces evaporation ofwater from the soil surface. Black mulches havebeen used to increase soil temperatures in orderto produce vegetables and melons for an earliermarket.

PermafrostWhen the mean annual soil temperature is below0° C, the depth of freezing in winter may exceedthe depth of thawing in summer. As a conse-quence, a layer of permanently frozen soil, calledpermafrost, may develop. Permafrost ranges frommaterial that is essentially all ice to frozen soil,which appears ordinary except that it is frozenand hard. The surface layer that thaws in summerand freezes in winter is the active l ayer. The activelayer is used by plants. Permafrost is widespreadin tundra regions and where it occurs there is,generally, an absence of trees. Low-growingshrubs, herbs, and grasses grow on the soils withpermafrost if the temperature is above freezing inthe summer.


FIGURE 3.14 Soil with permafrost having a layer ofwater-saturated soil overlying the permafrost. Even insummer, with little precipitation, soils are wet. Frostheaving causes an irregular soil surface with roundedhummocks.

The base of the active layer is the upper surfaceof the permafrost, or the permafrost table. Themelting of frozen soil ice in summer results in wetsoil conditions, even if there is little or no rain-fall, because the permafrost inhibits the down-ward movement of water from the soil. Alternatefreezing and thawing of wet soil above the perma-frost produces cryoturbation (movement and mix-

FIGURE 3.15 The utilidor system for servicingbuildings in the Northwest Territories, Canada. Utilitylines (water, sewage) in above-ground utilidor areprotected from breakage resulting from soilmovement associated with freezing and thawing insoils with permafrost. The utilidor also heats thewater and sewage pipes to prevent freezing.

ing of soil due to freezing and thawing). Soil wet-ness from melting of ice and the irregular soilsurface (hummocks) due to cryoturbation areshown in Figure 3.14. Excessive soil movementnecessitates the distribution of utilities to build-ings above ground with utilidors, as shown inFigure 3.15.

When the vegetative cover is disturbed by trafficor building construction, more heat enters the soiland more ice melts. In some cases enough icemelts to cause soil subsidence. Buildings arebuilt on stilts to shade the soil in summer, topromote minimum heat transfer from buildings tosoil, and to allow maximum cooling and freezingof soil in winter.

SUMMARYSoil physical properties affect virtually every usemade of the soil.

Texture relates to the amount of sand, silt, andclay in the soil, and structure relates to the ar-rangement of the sand, silt, and clay into peds.Texture and structure greatly affect plant growthby influencing water and air relationships. Soilsthat expand and shrink with wetting and dryingaffect the stability of building foundations.

About one half of the volume of mineral soils ispore space. Such soils have a bulk density ofabout 1.3 g/cm 3 and 50 percent porosity. In soils

REFERENCES

with favorable conditions for water retention andaeration, about one half of the porosity is macro-pore space and one half is micropore space.

Soil color is used as an indicator of organicmatter content, drainage, and aeration.

Soil temperature affects plant growth. Soil tem-perature is greatly affected by soil color, watercontent, and the presence or absence of surfacematerials, such as mulches. Permafrost occurs insoils with average temperature below freezing.

REFERENCESChesters, G. 0., 0. J. Attoe, and 0. N. Allen. 1957. "Soil

Aggregation in Relation to Various Soil Constitu-ents." Soil Sci. Soc. Am. Proc. 21:272-277.

Day, P. R. 1953. "Experimental Confirmation of Hy-drometer Theory."Soil Sci. 75:181-186.

Foth, H. D. 1983. "Permafrost in Soils." J Agron. Educ.12:77-79.

Soil Survey Staff. 1951. Soil Survey Manual. U.S.D.A.Handbook 18. Washington, D.C.

Taylor, D. C. 1960. Soil Survey of Erie County, Pennsyl-vania. U.S.D.A. and Pennsylvania State University

Tedrow, J. C. F. 1977. Soils of Polar Landscapes. Rut-gers University Press, New Brunswick, N.J.

Wascher, H. L. 1960. "Characteristics of Soils Associ-ated with Glacial Tills in Northeastern Illinois." Univ.Illinois Agr. Epp. Sta. But. 665.

Wilde, S. A. 1935. "The Significance of Texture in For-estry and Its Determination by a Rapid Field Method."Jour. For. 33:503-508.

41

The beginning of agriculture marks the beginningof soil tillage. Crude sticks were probably the firsttillage tools used to establish crops by planting thevegetative parts of plants, including stem sec-tions, roots, and tubers. Paintings on the walls ofancient Egyptian tombs, dating back 5,000 years,depict oxen yoked together pulling a plow madefrom a forked tree. By Roman times, tillage toolsand techniques had advanced to the point thatthorough tillage was a recommended practice forcrop production. Childe (1951) considered thediscovery and development of the plow to be oneof the 19 most important discoveries or applica-tions of science in the development of civili-zation. Now, paradoxically, the use of the plow forprimary tillage is being challenged.

EFFECTS OF TILLAGE ON SOILS ANDPLANT GROWTHTillage is the mechanical manipulation of soil forany reason. In agriculture and forestry, tillage isusually restricted to the modification of soil con-ditions for plant growth. There are three com-

CHAPTER 4

TILLAGE AND TRAFFIC

42

monly accepted purposes of tillage: (1) to man-age crop residues, (2) to kill weeds, and (3) toalter soil structure, especially preparation of thesoil for planting seeds or seedlings.

Management of Crop ResiduesCrops are generally grown on land that containsthe plant residues of a previous crop. The mold-board plow is widely used for burying these cropresidues in humid regions. Fields free of trashpermit precise placement of seed and fertilizer atplanting and permit easy cultivation of the cropduring the growing season (see Figure 4.1).

In subhumid and semiarid regions, by contrast,the need for wind erosion control and conser-vation of moisture have led to the development oftillage practices that successfully establish cropswithout plowing. The plant residues remaining onthe soil surface provide some protection from wa-ter and wind erosion. Plant residues left on theland over winter may trap snow and increase thewater content of soils when the trapped snowmelts.

FIGURE 4.1 Tillage (disking) in the spring toprepare the land for corn planting where the landwas plowed the previous fall and virtually all of theprevious crop residues were buried. This type oftillage allows planting and cultivating withoutinterference from plant residues, but leaves the soilbare and susceptible to water runoff and soil erosion.Such tillage is representative of conventional tillagesystems.

Tillage and Weed ControlWeeds compete with crop plants for nutrients,water, and light. Weeds, however, can be con-trolled with herbicides. If weeds are eliminatedwithout tillage, can cultivation of row crops beeliminated? Data from many experiments supportthe conclusion that the major benefit of culti-vating corn is weed control. Cultivation late in thegrowing season may be detrimental, becauseroots may be pruned and yields reduced. Culti-vation to control weeds is important in sub-humid regions to increase and conserve soil wa-ter when crops are not growing during a fallowperiod.

Effects of Tillage on Structureand PorosityAll tillage operations change the structure of thesoil. The lifting, twisting, and turning action ofa moldboard plow leaves the soil in a more ag-gregated condition. Cultivation of a field to killweeds may also have the immediate effect ofloosening the soil and increasing water infil-tration and aeration. The resistance of peds to

EFFECTS OF TILLAGE ON SOILS AND PLANT GROWTH

43

disintegration or breakdown remains unchanged.Consequently, tillage with cultivators, disks, andpackers crushes some of the soil peds and tendsto reduce soil porosity. Exposed cultivated landsuffers from disruption of peds by raindrop im-pact in the absence of a vegetative cover. In addi-tion, tillage hastens organic matter decompo-sition. Therefore, the long-term effect of plow-ing and cultivation is a more compacted soil as aresult of the crushing of peds and subsequentsettling of soil.

When forest or grassland soils are converted touse for crop production, there is a decline in soilaggregation and soils become more compact be-cause of the long-term effects of tillage. Compac-tion results in a decrease in the total pore spaceand an increase in the bulk density. Pushing parti-cles together as a result of tillage results in adecrease in the average pore size. Some of themacropores are reduced to micropores, and thusthere is an increase in the volume of microporespace and a decrease in the amount of macroporespace. The overall decrease in total porosity re-sults from a greater decrease in macropore spacethan the increase in micropore space. An increasei n micropore space, or water-filled space, be-cause of the compaction of clayey soils, is detri-mental because of reduced aeration and watermovement. By contrast, the compaction of a sandsoil could be desirable because the soil couldthen retain more water and still be sufficientlyaerated.

A study was made of the effects of growingcotton for 90 years on Houston Black Clay soils inthe U.S. Texas Blacklands. Soil samples from cul-tivated fields were compared with samples ob-tained from an adjacent area that had remaineduncultivated in grass. Long-time cultivationcaused a significant decrease in soil aggregationand total porosity and an increase in bulk density.Changes in total porosity inversely paralleled thechanges in bulk density. These changes areshown in Figure 4.2. The most striking and impor-tant change was the decrease in macroporespace, which was reduced to about one half of

44

that of the uncultivated soil. Note that thesechanges occurred to a depth of at least 75 cen-timeters.

Surface Soil CrustsDuring crop production, the surface of the soil isexposed and left bare for varying periods of timewithout the protection of vegetation or crop resi-dues. Soil peds at the soil surface are broken apartby raindrops, and the primary particles (sand, silt,clay) are dispersed. The splashing raindrops andthe presence of water cause the smallest particlesto move into the spaces between the larger parti-cles. A thin, dense-surface crust may form andreduce the water infiltration rate by a thousand-fold or more. Thus, surface crust formation in-creases water runoff and erosion on sloping land.When the surface crust dries, the crust may be-come very hard and massive, inhibiting seedlingemergence. Crust hardness increases with in-creased clay content of the soil and the extent ofsoil drying.

Minimum and Zero Tillage ConceptsIt is obvious that plants in natural areas growwithout tillage of the soil. Even though plants

TILLAGE AND TRAFFIC

grow well in experiments without tillage, the pro-duction of most crops will generally require sometillage. Sugar beet yields were lowest when thel and was not worked or tilled at all and when thel and was worked four or more times, betweenplowing and planting. The highest yields occurredwhere the land was worked or tilled only once ortwice between plowing and planting, as shown inTable 4.1. The data in Table 4.1 show that exces-sive tillage is detrimental. Some tillage is neces-sary for the practical production of some crops;however, the total amount of tillage used is de-clining. The result has been considerable pro-

TABLE 4.1 Sugar Beet Yields as Affected by theNumber of Times a Field Was Worked Priorto Planting

Adapted from Cook, et al., 1959.

FIGURE 4.2 Effects of 90years of cultivation on porespace and bulk density (gm/CM

³)

of Houston Black Claysoils. Macropore and totalpore space decreased and themicropore space increased.Decreases in total porosityresulted in increases in bulkdensity. (Data from Laws andEvans, 1949.)

gress in the development of minimum tillage andno-till tillage systems.

Minimum tillage is the minimum soil manipu-lation necessary for crop production under theexisting soil and climatic conditions. Some of theminimum tillage equipment prepares the land forplanting, plants the seeds, and applies fertilizerand herbicide in one trip over the field. Underfavorable conditions, no further tillage will be re-quired during the growing season. Press wheelsbehind the planter shoes pack soil only where theseed is placed, leaving most of the soil surface ina state conducive to water infiltration. Experimen-tal evidence shows that minimum tillage reduceswater erosion on sloping land. It also makes weedcontrol easier because planting immediately fol-lows plowing. Weed seeds left in loose soil are ata disadvantage because the seeds have minimumcontact with soil for absorbing water. Crop yieldsare similar to those where more tillage is used, butthe use of minimum tillage can reduce energy useand overall production costs. Some of the sav-ings, conversely, may be used to pay for herbicidecosts.

No-till is a procedure whereby a crop is planteddirectly into the soil with no preparatory tillagesince the harvest of the previous crop. The plant-ing of wheat in the wheat stubble from the pre-vious crop without any prior tillage (such as plow-ing) is shown in Figure 4.3.

FIGURE 4.3 No-till planting of wheat in the plantresidues of the previous year's wheat crop.(Photograph courtesy Soil Conservation Service,USDA.)

EFFECTS OF TILLAGE ON SOILS AND PLANT GROWTH 45

In no-till systems for row crop production, anarrow slot is made in the untilled soil so thatseed can be planted where moisture is adequatefor germination. Weed control is by herbicides.The residues of the previous crop remain on thesoil surface, which results in greatly reduced wa-ter runoff and soil erosion. There was 4,000pounds per acre of wheat residue on the fieldwhen the soybeans were planted in the fieldshown in Figure 4.4. The coverage of the soilsurface with crop residues with various tillagesystems is shown in Table 4.2. Tillage systemswith reduced tillage and those where crop resi-dues are left on the soil surface minimize waterrunoff and soil erosion; this is called conservationtillage.

From this discussion of tillage, it should beobvious that tillage is not a requirement of plantgrowth. Generally, gardens do not need to beplowed. The only tillage necessary is that neededfor the establishment of the plants and the con-trol of weeds. Mulches in gardens, like crop resi-dues in fields, protect the soil from raindropimpact and promote desirable soil structure.A major benefit is a greater water- infitrationrate and a shorter amount of time needed toirrigate.

Tilth and TillageTilth is the physical condition of the soil as relatedto the ease of tillage, fitness of soil as a seedbed,and desirability for seedling emergence and rootpenetration. Tilth is related to structural condi-tions, the presence or absence of impermeablelayers, and the moisture and air content of thesoil. The effect of tillage on tilth is strongly relatedto soil water content at the time of tillage. This isespecially true when tillage of wet clay soilscreates large chunks or clods that are difficult tobreak down into a good seedbed when the chunksdry. As shown earlier, cropping usually causesaggregate deterioration and reduced porosity.Farm managers need to exercise considerablejudgment concerning both the kind of tillage andthe timing of tillage operations to minimize the

detrimental effects and to maximize the beneficialeffects of tillage on soil tilth.

TRAFFIC AND SOIL COMPACTIONThe changes in soil porosity, owing to compac-tion, are caused by the long-term effects ofcropping and tillage and by the pressure of tractortires, animal hooves, and shoes. As tractors andmachinery get larger, there is greater potential for

TABLE 4.2 Effect of Tillage System on Amount ofResidue Cover in Continuous Corn ImmediatelyAfter Planting

soil compaction. These changes in porosity areimportant enough to warrant their summarizationas an aid to a further discussion of the effects oftraffic on soils. Soil compaction results in: (1) adecrease in total pore space, (2) a decrease inmacropore space, and (3) an increase in micro-pore space.

Compaction LayersCompaction at the bottom of a plow furrow fre-quently occurs during plowing because of tractorwheels running in a furrow that was made by theprevious trip over the field. Subsequent tillage isusually too shallow to break up the compactedsoil, and compaction gradually increases. Thistype of compaction produces a layer with highbulk density and reduced porosity at the bottom ofthe plow layer, and it is appropriately termed aplow pan, or pressure pan. Pressure pans are aproblem on sandy soils that have insufficient claycontent to cause enough swelling and shrinking,via wetting and drying, to naturally break upthe compacted soil. Bulk densities as high as

FIGURE 4.4 Soybeansplanted no-till in the residuesof a previous wheat crop. Theprevious crop left about 4,000pounds of wheat residue peracre. (Photograph courtesySoil Conservation Service,USDA.)

1.9 g/cm ³ have been found in plow pans of sandycoastal plain soils in the southeastern UnitedStates. Cotton root extension was inhibited inthese soils when the bulk density exceeded1.6 g/cm3 . The effect of a compact soil zone onthe growth of bean roots is shown in Figure 4.5.

Effects of Wheel Traffic on Soilsand CropsAs much as 75 percent of the entire area of analfalfa field may be run over by machinery wheelsi n a single harvest operation. The potential forplant injury and soil compaction is great, becausethere are 10 to 12 harvests annually in areas wherealfalfa is grown year-round with irrigation. Wheeltraffic damages plant crowns, causing plants tobecome weakened and more susceptible to dis-ease infection. Root development is restricted.Alfalfa stands (plant density) and yields havebeen reduced by wheel traffic.

FIGURE 4.5 Bean root growth in compacted soil at4-inch depth on left and no soil compaction on theright.

TRAFFIC AND SOIL COMPACTION 47

Potatoes are traditionally grown on sandy soilsthat permit easy development of well-shapedtubers. Resistance to tuber enlargement in com-pacted soils not only reduces tuber yield but in-creases the amount of deformed tubers, whichhave greatly reduced market value. In studieswhere bulk density was used as a measure of soilcompaction, potato tuber yield was negativelycorrelated and the amount of deformed tuberswas positively correlated with bulk density.

Effects of Recreational TrafficMaintenance of a plant cover in recreational areasis necessary to preserve the natural beauty and toprevent the undesirable consequences of waterrunoff and soil erosion. Three campsites in theMontane zone of Rocky Mountain National Park inColorado were studied to determine the effects ofcamping activities on soil properties. Soils inareas where tents were pitched, and where fire-places and tables were located, had a bulk densityof 1.60 g/cm ³ compared with 1.03 g/cm³ in areaswith little use. The results show that campingactivities can compact soil as much as tractorsand other heavy machinery.

Snowmobile traffic on alfalfa fields significantlyreduced forage growth at one out of four locationsin a Wisconsin study. It appears that injury toalfalfa plants and reduced yields are related tosnow depth. There is less plant injury as snowdepth increases. The large surface area of thesnowmobile treads makes it likely that soil com-paction will be minimal or nonexistent. Moderatetraffic on fields with a good snow cover appears tohave little effect on soil properties or dormantplants buried in the snow.

Human traffic compacts soil on golf coursesand lawns. Aeration and water infiltration can beincreased by using coring aerators. Numeroussmall cores about 2 centimeters in diameter and10 centimeters long are removed by a revolvingdrum and left lying on the ground, as shown inFigure 4.6.

One of the most conspicuous recent changes in

48

some of the arid landscapes of the southwesternUnited States has been caused by use of all-terrainvehicles. In the Mojave Desert one pass of a mo-torcycle increases bulk density of loamy sandsoils from 1.52 to 1.60 g/cm 3 , and 10 passes in-crease the bulk density to 1.68 g/cm 3 , accordingto one study. Parallel changes in porosity oc-curred with the greatest reduction in the largestpores. Water infiltration was decreased and waterrunoff and soil erosion were greatly accelerated.Recovery of natural vegetation and restoration ofsoils occur very slowly in deserts. Estimates in-dicate that about 100 years will be required torestore bulk density, porosity, and infiltrationcapacity to their original values based on extrapo-lation of 51 years of data from an abandoned townin southern Nevada.

Effects of Logging Traffic on Soils andTree GrowthThe use of tractors to skid logs has increasedbecause of their maneuverability, speed, andeconomy. About 20 to 30 percent of the forest landmay be affected by logging. Tractor traffic can

TILLAGE AND TRAFFIC

disturb or break shallow tree roots that are grow-i ng just under the surface soil layer.

The ease with which water can flow through thesoil was found to be 65 and 8 percent as great oncutover areas and on logging roads, respectively,in comparison with undisturbed areas in south-western Washington. The reductions in perme-ability increase water runoff and soil erosion, re-sulting in less water being available for the growthof tree seedlings or any remaining trees. Reducedgrowth and survival of chlorotic Douglas-fir seed-lings on tractor roads in western Oregon was con-sidered to be caused by poor soil aeration and lownitrogen supply.

Changes in bulk density and pore space of for-est soils, because of logging, are similar tochanges produced by longtime cropping. Afterlogging, and in the absence of further traffic,however, forest soils are naturally restored to theirformer condition. Extrapolation of 5 years' datasuggests that restoration of logging trails takes 8years and wheel ruts require 12 years in northernMississippi. In Oregon, however, soil compactionin tractor skid trails was still readily observableafter 16 years.

FIGURE 4.6 Soil aeratorused to increase soil aerationon a college campus inCalifornia. The insert showsdetail of removal of smallcores that are left lying on thesurface of the lawn.

Controlled TrafficThe pervasive detrimental effects of soil compac-tion have caused great interest in tillage research(see Figure 4.7). The extensive coverage of thesoil surface by machinery wheels, producing soilcompaction, has resulted in the development ofcontrolled traffic tillage systems. Controlled trafficsystems have all tillage operations performed infixed paths so that recompaction of soil by traffic(wheels) does not occur outside the selectedpaths. In fields with controlled traffic, only thesmall amount of soil in the wheel tracks is com-pacted, and this compacted soil has almost noeffect on crop yields. One additional benefit ofcontrolled traffic is that the machines run on hardand firm paths that permit the planting of crops insoils with much greater water content than nor-mal. Tractors do not get stuck and crops can beplanted at an earlier date.

An experiment was conducted in Minnesota tostudy the effects of machinery wheels on soilcompaction in two tillage systems. The 9-year ex-periment showed there was no difference in soil

TRAFFIC AND SOIL COMPACTION

bulk density between a regular tillage systemusing a moldboard plow and a conservation (re-duced) tillage system with normal trafficking ofmachinery wheels. When the tillage was carriedout with the machinery wheels running on thesame path, no-wheel or controlled traffic, com-paction caused by wheel traffic was eliminatedand bulk densities were lower in both moldboardplow and conservation tillage than for wheel traf-fic. In the no-wheel traffic experiments, the con-servation tillage treatment eventually showedlower bulk densities (less compaction) thanthe moldboard plow treatment, as shown inFigure 4.8.

A controlled traffic experiment on sandy loamsoil in California showed that alfalfa yields withnormal traffic were 10 percent less as comparedto controlled traffic. The lower yields were alsoassociated with more compact soils havinggreater bulk density. An experimental controlledtraffic machine for planting small grain is shownin Figure 4.9.

In instances where repeated traffic is necessary,

FIGURE 4.7 Tillage research facilities at the USDA National Soil DynamicsLaboratory in Auburn, Alabama. (Photograph courtesy of James H. Taylor.)

49

50

FIGURE 4.8 Bulk density of the 0- to 15-centimetersoil layer as affected by tillage and wheel traffic overtime. (Reproduced from Soil Science Society AmericaJournal, Vol. 48, No. 1, January-February 1984, p.152-156 by permission of the Soil Science Society ofAmerica.)

it is generally best to use the same tracks becausemost of the compaction occurs with the first passover the soil.

FLOODING AND PUDDLING OF SOILRice or paddy (wetland rice) is one of the world'sthree most important food crops: more than 90

FIGURE 4.9 I ncorporation of wheat seed at a depthof 5 centimeters in a wet soil in a controlled trafficsystem where machinery wheels constantly run in thesame tracks. (Photograph courtesy National SoilDynamics Laboratory, USDA.)

TILLAGE AND TRAFFIC

percent of the rice is grown in Asia, mostly insmall fields or paddies that have been leveled andbunded (enclosed with a ridge to retain water).The paddy fields are flooded and tilled before ricetransplanting. A major reason for the tillage is thedestruction of soil structure and formation of soilthat is dense and impermeable to water. The prac-tice permits much land that is normally perme-able to water to become impermeable and suitedfor wetland rice production. In the United States,naturally water-impermeable soils are used forrice production and the soils are not puddled. Asmall amount of upland rice is grown, like wheat,on unflooded soils.

Effects of FloodingFlooding of dry soil causes water to enter pedsand to compress the air in the pores, resulting insmall explosions that break the peds apart. Theanaerobic conditions (low oxygen supply) fromprolonged flooding cause the reduction and dis-solution of iron and manganese compounds andthe decomposition of organic structural-bindingmaterials. Ped or aggregate stability is greatly re-duced and the aggregates are easily crushed. Thedisruption of soil peds and the clogging of poreswith microbial wastes reduce soil permeability.

Effects of PuddlingPuddling is the tillage of water-saturated soilwhen water is standing on the field, as shown inFigure 4.10. Peds that are already weakened byflooding are worked into a uniform mud, whichbecomes a two-phase system of solids and liq-uids. Human or animal foot traffic is effective informing a compact layer, or pressure pan, about 5to 10 centimeters thick at the base of the puddledl ayer. The development of a pressure pan allowsfor the conversion of soils with a wide range intexture and water permeability to become imper-meable enough for rice production. Puddling in-creases bulk density, eliminates large pores, andincreases capillary porosity in the soil above the

pressure pan. Soil stratification may occur in soilswhen sand settles before the silt and clay. Thisresults in the formation of a thin surface layerenriched with silt and clay, also having low per-meability. These changes are desirable for riceproduction because soil permeability is reducedby a factor of about 1,000 and the amount of waterneeded to keep the field flooded is greatly re-duced. Much of the rice is grown without a de-pendable source of irrigation water, and rainfallcan be erratic. Low soil permeability is, therefore,of great importance in maintaining standing wateron the paddy during the growing season. Standingwater promotes rice growth (because of no waterstress and an increased supply of nutrients), andthe nearly zero level of soil oxygen inhibits growthof many weeds.

Paddies are drained and allowed to dry beforeharvest. Where winters are dry and no irrigationwater is available, a dryland crop is frequentlyplanted. Drying of puddled soil promotes ped for-mation; however, large clods often form that aredifficult to work into a good seedbed. Pressurepans are retained from year to year and create ashallow root zone that severely limits the supplyof water and nutrients for dry season crops.

FLOODING AND PUDDLING OF SOIL

FIGURE 4.10 Puddling soilto prepare land for paddy(rice) transplanting.

Oxygen Relationships in Flooded SoilsThe water standing on a flooded soil has an oxy-gen content that tends toward equilibrium withthe oxygen in the atmosphere. This oxygenatedwater layer supplies oxygen via diffusion to a thinupper layer of soil about a centimeter thick, asshown in Figure 4.11. This thin upper layer retainsthe brownish or reddish color of oxidized soil.The soil below the thin surface layer and abovethe pressure pan is oxygen deficient, and reduc-ing conditions exist. Colors indicative of reducediron and manganese occur, that is, gray and blackcolors. The soil in the immediate vicinity of riceroots is oxidized because of the transport of oxy-gen into the root zone from the atmosphere. Thisproduces a thin layer of yellowish soil around theroots where reduced (ferrous) iron has been oxi-dized to ferric iron and precipitated at the rootsurfaces.

The soil below the pressure pan can be eitherreduced or oxidized. If the soil occurs in a depres-sion and is naturally poorly drained, a gray sub-soil that is indicative of reduced soil is likely.Many rice paddies are formed high on the land-scape, and they naturally have well-aerated sub-soils below pressure pans that may retain their

5 1

52

FIGURE 4.11 Oxidized and reduced zones inflooded paddy soil.

bright oxidized colors after paddies are estab-lished.

SUMMARYTillage is the mechanical manipulation of soil tomodify soil conditions for plant growth. Tillage isbeneficial for preparing land for planting, control-ling weeds, and managing crop residues. Tillageinvolves mechanical manipulation of soil and ve-hicular or animal traffic, which results in soilcompaction.

TILLAGE AND TRAFFIC

All soil compaction results in higher bulk den-sity and lower total porosity. The volume of mac-ropores is greatly reduced with an accompanyingincrease in micropores. Soil layers with a bulkdensity greater than about 1.6 to 1.8 g/cm ³ arebarriers for root penetration.

Pressure pans, or plow soles, tend to develop atthe bottom of plow furrows, especially in sandysoils with minimal capacity for expansion andcontraction due to wetting and drying.

Minimum and no-till tillage systems have beendeveloped to minimize the effects of tillage andtraffic on soil compaction. Only the minimumamount of tillage necessary for the crop and cli-matic conditions is used in the first method. Inno-till, crops are planted directly in the residues ofthe previous crop with no prior tillage. For rowcrops, a slit is made in the soil in which the seed isplanted. Minimum and no-till tillage leave moreresidues on the soil surface than conventionaltillage, resulting in reduced water runoff and soilerosion, thus they are considered conservationtillage.

In Asia, many soils are flooded and puddled tocreate a soil with minimal water permeability forflooded rice production. A pressure pan is createdbelow a puddled soil layer (root zone).

REFERENCESChilde, V. E. 1951. Man Makes Himself. Mentor, New

York.Cook, R. L., J. F. Davis, and M. G. Frakes. 1959. "An

Analysis of Production Practices of Sugar Beet Farm-ers in Michigan." Mi. Agr. Exp. Sta. Quart. Bul.42:401-420.

Dickerson, B. P. 1976. "Soil Changes Resulting fromTree-Length Skidding." Soil Sci. Soc. Am. Proc.

40:965-966.Dotzenko, A. D., N. T. Papamichos, and D. S. Romine.

1967. "Effect of Recreational Use on Soil and Mois-ture Conditions in Rocky Mountain National Park."J. Soil and Water Con. 22:196-197.

Froehlich, H. A. 1979. "Soil Compaction from Logging

Equipment: Effects on Growth of Young PonderosaPine." J. Soil and Water Con. 34:276-278.

Griffith, D. R., J. V. Mannering, and W. C. Moldenhauer.1977. "Conservation Tillage in the Eastern Corn Belt."

J. Soil and Water Con. 32:20-28.Grimes, D. W. and J. C. Bishop. 1971. "The Influence

of Some Soil Physical Properties on Potato Yieldsand Grade Distribution." Am. Potato 1 48:414-422.

Iverson, R. M., B. S. Hinckley, and R. M. Webb. 1981."Physical Effects of Vehicular Disturbances on AridLandscapes." Science. 212:915-916.

Laws, W. D. and D. D. Evans. 1949. "The Effects ofLong-Time Cutltivation on Some Physical and Chemi-cal Properties of Two Rendzina Soils." Soil Sci. Soc.

Am. Proc. 14:15-19.Meek, B. D., E. A. Rechel, L. M. Carter, and W. R. DeTar.

1988. "Soil Compaction and its Effect on Alfalfa in

Zone Production Systems." Soil Sci. Soc. Am. J.

52:232-236.Moormann, F. R. and N. van Breemen. 1978. Rice: Soil,

Water and Land. Int. Rice Res. Inst., Manila.

REFERENCES 53

Phillips, R. E., R. L. Blevins, G. W. Thomas, W. R. Frye,and S. H. Phillips. 1980. "No-Tillage Agriculture."

Science. 208:1108-1113.Steinbrenner, E. C. and S. P. Gessel. 1955. "The Effect of

Tractor Logging on Physical Properties of Some For-est Soils in Southwestern Washington." Soil Sci. Soc.A. Proc. 19:372-376.

Taylor, J. H. 1986. "Controlled Traffic: A Soil Compac-

tion Management Concept." SAE Tech. Paper Series860731. Soc. Auto. Eng. Inc., Warrendale, Pa.

Voorhees, W. B. and M. J. Lindstrom. 1984. "Long-TermEffects of Tillage Method on Soil Tilth Independent ofWheel Traffic Compaction." Soil Sci. Soc. Am. J.48:152-156.

Walejko, R. N., J. W. Pendleton, W. H. Paulson, R. E.Rand, G. H. Tenpas, and D. A. Schlough. 1973."Effect of Snowmobile Traffic on Alfalfa." J. Soil andWater Con. 28:272-273.

Youngberg, C. T. 1959. "The Influence of Soil Condi-tions Following Tractor Logging on the Growth ofPlanted Douglas-Fir Seedlings." Soil Sci. Soc. Am.

Proc. 23:76-78.

Water is the most common substance on theearth; it is necessary for all life. The supply offresh water on a sustained basis is equal tothe annual precipitation, which averages 66 cen-timeters for the world's land surface. The soil,located at the atmosphere-lithosphere interface,plays an important role in determining theamount of precipitation that runs off the land andthe amount that enters the soil for storage andfuture use. Approximately 70 percent of the pre-cipitation in the United States is evaporated fromplants and soils and returned to the atmosphereas vapor, with the soil playing a key rolein water retention and storage. The remaining30 percent of the precipitation represents thelongtime annual supply of fresh water foruse in homes, industry, and irrigated agriculture.This chapter contains important concepts andprinciples that are essential for gaining an under-standing of the soil's role in the hydrologic cycleand the intelligent management of water re-sources.

CHAPTER 5

SOIL WATER

54

SOIL WATER ENERGY CONTINUUMAs water cascades over a dam, the potentialenergy (ability of the water to do work) decreases.If water that has gone over a dam is returned to thereservoir, work will be required to lift the waterback up into the reservoir, and the energy contentof the water will be restored. Therefore, in a ca-scading waterfall, the water at the top has thegreatest energy and water at the bottom has thelowest energy. As water cascades down a falls, thecontinuous decrease in energy results in anenergy continuum from the top to the bottom ofthe falls. As an analogy, when wet soil dries, thereis a continuous decrease in the energy content ofthe remaining water. When dry soil gets wetter,there is a continuous increase in the energy of soilwater. The continuous nature of the changes inthe amount of soil water, and correspondingchanges in energy, produce the soil water energycontinuum.

Adhesion WaterWater molecules (H 20) are electrically neutral;however, the electrical charge within the mole-cule is asymmetrically distributed. As a result,water molecules are strongly polar and attracteach other through H bonding. Soil particles havesites that are both electrically negative and posi-tive. For example, many oxygen atoms are ex-posed on the faces of soil particles. These oxygenatoms are sites of negativity that attract the posi-tive poles of water molecules. The mutual attrac-tion between water molecules and the surfaces ofsoil particles results in strong adhesive forces. If adrop of water falls onto some oven dry soil, thewater molecules encountering soil particles willbe strongly adsorbed and the molecules willspread themselves over the surfaces of the soilparticles to form a thin film of water. This layer, orfilm of water, which will be several water mole-cules thick, is called adhesion water (see Figure5.1).

Adhesion water is so strongly adsorbed that itmoves little, if at all, and some scientists believethat the innermost layer of water molecules existsin a crystalline state similar to the structure of ice.Adhesion water is always present in field soils andon dust particles in the air, but the water can beremoved by drying the soil in an oven. Adhesionwater has the lowest energy level, is the mosti mmobile water in the soil, and is generally un-available for use by plant roots and microor-ganisms.

Cohesion WaterThe attractive forces of water molecules at thesurface of soil particles decrease inversely andl ogarithmically with distance from the particlesurface. Thus, the attraction for molecules in thesecond layer of water molecules is much less thanfor molecules in the innermost layer. Beyond thesphere of strong attraction of soil particles forwater molecules of the adhesive water layer, co-

SOIL WATER ENERGY CONTINUUM

FIGURE 5.1 Schematic drawing of relationshipbetween various forms of soil water. Adhesion wateris strongly absorbed and very immobile andunavailable to plants. The gravitational water, bycontrast, is beyond the sphere of cohesive forces,and gravity causes gravitational water to flow rapidlydown and out of the soil (unless downward flow isinhibited). The cohesion water is intermediate inproperties and is the most important for plantgrowth.

hesive forces operate between water moleculesbecause of hydrogen bonding. This causes watermolecules to attach themselves to the adhesionwater, resulting in an increase in the thickness ofthe water film. Water that is retained in soils be-cause of cohesive forces is called cohesion water,and is shown in Figure 5.1.

About 15 to 20 molecular layers is the maxi-mum amount of water that is normally adsorbedto soil particle surfaces. With increasing distancefrom the soil particle surface toward the outeredge of the water film, the strength of the forcesthat holds the water decreases and the energy andmobility of the water increase. Adhesion water isso strongly held that it is essentially immobile,and thus is unavailable to plants. The cohesionwater is slightly mobile and generally available toplants. There is a water-energy gradient across the

55

56

water films, and the outermost water moleculeshave the greatest energy, the greatest tendency tomove, and the greatest availability for plant roots.As roots absorb water over time, the soil watercontent decreases, and water films become thin-ner; roots encounter water with decreasing energycontent or mobility. At some point, water maymove so slowly to roots that plants wilt because ofa lack of water. If wilted indicator plants areplaced in a humid chamber and fail to recover, thesoil is at the permanent wilting point (see Figure5.2). This occurs when most of the cohesion wa-ter has been absorbed.

When soil particles are close to, or touch, adja-cent particles, water films overlap each other.Small interstices and pores become water filled.In general, the pores in field soils that are smallenough to remain water filled, after soil wettingand downward drainage of excess water has oc-curred, are the capillary or micropores. At thispoint, the soil retains all possible water againstgravity (adhesion plus cohesion water), and thesoil is considered to be at field capacity. Theaeration or macropores at field capacity are airfilled. A typical loamy surface soil at field capacityhas a volume composition of about 50 percentsolids, 25 percent water, and 25 percent air.

FIGURE 5.2 Wilted coleus plant on the right. If theplant fails to revive when placed in a humidchamber, the soil is at the permanent wilting point inregard to this plant.

SOIL WATER

Gravitational WaterIn soils with claypans, a very slowly permeablesubsoil layer permits little water, in excess of fieldcapacity, to move downward and out of the soil.After a long period of rainfall, water accumulatesabove the claypan in these soils. As the rainyperiod continues, the soil above the claypan satu-rates from the bottom upward. The entire A hori-zon or root zone may become water saturated.Soil water that exists in aeration pores, and that isnormally removed by drainage because of theforce of gravity, is gravitational water. When soilsare saturated with water, the soil volume compo-sition is about 50 percent solids and 50 percentwater. Gravitational water in soil is detrimentalwhen it creates oxygen deficiency. Gravitationalwater is not considered available to plants be-cause it normally drains out of soils within a dayor two after a soil becomes very wet (see Figure5.1).

The soil in the depression, of Figure 5.3 is watersaturated because underlying soil is too slowlypermeable. Note the very poor growth of corn inthe soil that remains water saturated after rainfall.Along the right side, the soil is not water satu-rated. The gravitational water has drained out, theaeration pores are air filled, and the soil is at fieldcapacity. Here, the growth of corn is good.

An energy gradient exists for the water within amacropore, including the gravitational water.When water filled macropores are allowed todrain, the water in the center of the macropore hasthe most energy and is the most mobile. Thiswater leaves the pore first and at the fastest speed.This is followed by other water molecules closerto the pore edges until all of the gravitationalwater has drained, leaving the cohesion and ad-hesion water as a film around the periphery of themacropore.

Summary Statement1. The forces that affect the energy level of soil

water, and the mobility and availability of wa-

ter to plants, are adhesion, cohesion, andgravity.

2. A water-saturated soil contains water withwidely varying energy content, resulting in anenergy continuum between the oven dry stateand water saturation.

3. The mobility and availability of water for plantuse parallel the energy continuum. As dry soilwets over time, the soil water increases inenergy, mobility, and availability for plants.As wet soil dries over time, the energy, mobil-i ty, and plant availability of the remaining wa-ter decreases.

4. Plants wilt when soil water becomes so im-mobile that water movement to plant roots istoo slow to meet the transpiration (evapora-tion from leaves) needs of plants.

ENERGY AND PRESSURE RELATIONSHIPSThere is a relationship between the energy of wa-ter and the pressure of water. Because it is mucheasier to determine the pressure of water, theenergy content of water is usually categorized onthe basis of pressure.

The hull of a submarine must be sturdily built towithstand the great pressure encountered in adive far below the ocean's surface. The column(head) of water above the submarine exerts pres-sure on the hull of the submarine. If the waterpressure exerted against the submarine was di-rected against the blades of a turbine, the energy

ENERGY AND PRESSURE RELATIONSHIPS 5 7

in the water could be used to generate electricity.The greater the water pressure, the greater thetendency of water to move and do work, and thegreater the energy of the water.

The next consideration will be that of waterpressure relationships in water-saturated soils,which is analogous to the water pressure relation-ships in a beaker of water or in any body of water.

Pressure Relationships in Saturated SoilThe beaker shown in Figure 5.4 has a bottom areaassumed to be 100 cm² . The water depth in thebeaker is assumed to be 20 centimeters. The vol-ume of water, then, is 2,000 cm ³ and it weighs2,000 grams. The pressure, P, of the water at thebottom of the beaker is equal to

P =force = 2,000 g

= 20 /cm ²area

100 cm²

g

The water pressure at the bottom of the beakercould also be expressed simply as equal to acolumn of water 20 centimeters high. This is anal-ogous to saying that the atmospheric pressure atsea level is equal to I atmosphere, is equivalent toa column of water of about 33 feet, or to a columnof mercury of about 76 centimeters. These arealso equal to a pressure of 14.7 lb/in. ²

At the 10-centimeter depth, the water pressurein the beaker is one half of that at the 20- cen-timeter depth and, therefore, is 10 g/cm ² . Thewater pressure decreases with distance towardthe surface and becomes 0 g/cm², at the free

FIGURE 5.3 Water-saturatedsoil occurs in the depressionwhere corn growth is poor.The surrounding soil at higherelevation is at field capacitybecause of recent rain, andcorn growth is good owing tohigh availability of both waterand oxygen.

58

FIGURE 5.4 A beaker with a cross-sectional area of100 square centimeters contains 2,000 grams of waterwhen the water is 20 centimeters deep. At the watersurface, water pressure is 0; the water pressureincreases with depth to 20 grams per squarecentimeter at the bottom.

water surface, as shown in Figure 5.4. In water-saturated soil, the water pressure is zero at thesurface of the water table and the water pressurei ncreases with increasing soil depth.

Pressure Relationships inUnsaturated SoilIf the tip of a small diameter glass or capillary tubeis inserted into the water in a beaker, adhesioncauses water molecules to migrate up the interiorwall of the capillary tube. The cohesive forcesbetween water molecules cause other water mol-ecules to be drawn up to a level above the water inthe beaker. Now, we need to ask: "What is thepressure of the water in the capillary tube?" and"How can this knowledge be applied to unsatu-rated soils?"

As shown previously, the water pressure de-creases from the bottom to the top of a beakerfilled with water and, at the top of the water sur-face, the water pressure is zero. Beginning at the

SOIL WATER

open water surface and moving upward into acapillary tube, water pressure continues to de-crease. Thus, the water pressure in the capillarytube is less than zero, or is negative. As shown inFigure 5.5, the water pressure decreases in thecapillary tube with increasing height above theopen water surface. At a height of 20 centimetersabove the water surface in the beaker, the waterpressure in the capillary is equal to -20 g/cm ² .Figure 5.5 also shows that at this same heightabove the water surface, the water pressure inunsaturated soil is the same at the 20-centimeterheight. The two pressures are the same at any oneheight, and there is no net flow of water from thesoil or capillary tube after an equilibrium condi-tion is established.

Applying the consideration of water in a capil-lary tube to an unsaturated soil, allows the follow-ing statements:

FIGURE 5.5 Water pressure in a capillary tubedecreases with increasing distance above the wateri n the beaker. It is -20 grams per square centimeterat a height 20 centimeters above the water surface.Since the water column of the capillary tube iscontinuous through the beaker and up into the soilcolumn, the water pressure in the soil 20 centimetersabove the water level in the beaker is also -20grams per square centimeter.

1. Water in unsaturated soil has a negative pres-sure or is under tension.

2. The water pressure in unsaturated soil de-creases with increasing distance above a freewater surface or water table.

THE SOIL WATER POTENTIALMost of the concerns about soil water involvemovement: water movement into the soil surface,water movement from the soil surface downwardthrough soil, and movement of water from the soilto, and into, roots, microorganisms, and seeds.The pressure of soil water, which is an indicationof tendency for soil water to move, is expressed bythe soil water potential. Technically, the soil wa-ter potential is defined as the amount of work thatmust be done per unit quantity of water to trans-port or move reversibly (without energy loss dueto friction) and isothermally (without energy lossdue to temperature change) an infinitesimalquantity of water from a pool of pure water atspecified elevation and at atmospheric pressureto the soil water at the point under consideration.It is important to note that the water potential isthe amount oph work needed to move water phrom arepherence pool to another point.

The symbol for the water potential is psi, 0. Thetotal water potential, q t , i s made up of severalsubpotentials, including the gravitational, matric,and osmotic.

The Gravitational PotentialThe gravitational potential, g , is due to theposition of water in a gravitational field. The gravi-tational potential is very important in water-saturated soils. It accounts primarily for the move-ment of water through saturated soils or themovement of water from high to low elevations.Customarily, the soil water being considered ishigher in elevation than the reference pool of purewater. Since the gravitational potential representsthe work that must be done to move water from a

THE SOIL WATER POTENTIAL 59

reference pool (which is at a lower elevation) tosoil that is at a higher elevation, the sign is posi-tive. The higher the water above the referencepoint, the greater the gravitational water potential.

Visualize a water-saturated soil that is under-lain at the 1-meter depth with plastic tubing con-taining small holes. Normally, the plastic tubingallows gravitational water to drain out of the soil.Suppose, however, that the tube outlet is closedand the soil remains water saturated. The gravita-tional water potential at the top of the soil that issaturated, relative to the plastic tubing, is equal toa 1-meter water column. This is the same as say-ing that the water pressure is equal to a 1-metercolumn of water in reference to the point of waterdrainage through the tubing.

If the drainage tube outlet is opened, gravita-tional water will leave the water-saturated soil anddrain away through the plastic tubing. The soilwill desaturate from the surface downward. Thewater pressure or gravitational potential at the topof water-saturated soil will decrease as the waterl evel drops. When the gravitational water hasbeen removed, and drainage stops, the top of thenew level of water-saturated soil will be at thesame elevation as the drainage tubing (the refer-ence point). At this elevation, the gravitationalpotential will be zero.

The Matric PotentialWhen a soil is unsaturated and contains no gravi-tational water, the major movement of water isl aterally from soil to plant roots. The importantforces affecting water movement are adhesionand cohesion. Adhesion and cohesion effects arei ntimately affected by the size and nature of pri-mary soil particles and peds. The resulting physi-cal arrangement of surfaces and spaces, owing totexture and structure, is the soil matrix. The inter-action of the soil matrix with the water producesthe matric water potential. The matric potentiali s 1m .

When rain falls on dry soil, it is analogous tomoving water molecules from a pool of pure water

60

to the water films on soil particles. No energyinput is required for the movement of water inraindrops to water films around dry soil particles.In fact, a release of energy occurs, resulting in adecrease in the potential of the water. Adsorbedwater has lower potential to do work than water inraindrops. In other words, negative work is re-quired to transfer water from raindrops to thefilms on soil particles. This causes the matricpotential to have a negative sign.

The matric potential varies with the content ofsoil water. The drier a soil is, the greater is thetendency of the soil to wet and the greater is therelease of energy when it becomes wetted. In fact,when oven dry clay is wetted, a measureable in-crease in temperature occurs due to the release ofenergy as heat. This release of heat is called theheat oph wetting. Generally, the drier a soil is, thelower is the matric potential. The numberpreceded by a negative sign becomes larger (be-comes more negative) when a soil dries from fieldcapacity to wilt point. In reference to the previoussituation where the gravitational water had justexited a soil with drainage tubing at a depth of 1meter, the matric water potential in unsaturatedsoil at the soil surface is equal to a -1-metercolumn of water.

The Osmotic PotentialThe osmotic potential, o, is caused by the forcesi nvolved in the adsorption of water molecules byions (hydration) from the dissolution of solublesalt. Since the adsorption of water molecules byi ons releases heat (energy of hydration), the signof the osmotic potential is also negative.

The osmotic potential is a measure of the workthat is required to pull water molecules away fromhydrated ions. Normally, the salt content of thesoil solution is low and the osmotic potential haslittle significance. In soils containing a largeamount of soluble salt (saline soils), however, theosmotic potential has the effect of reducing wateruptake by roots, seeds, and microorganisms. The

SOIL WATER

osmotic potential has little, if any, effect on watermovement within the soil, since diffusion of thehydrated ions creates a uniform distribution andthe hydrated ions do not contribute to a differen-tial in matric potential in one region of the soil ascompared to another.

Measurement and Expression ofWater PotentialsSince the gravitational potential is the distancebetween the reference point and the surface of thewater-saturated soil, or the water table, it can bemeasured with a ruler. Reference has been madeto a gravitational potential equal to a meter-longwater column. Water potentials are frequently ex-pressed as bars that are approximately equal toatmospheres. A bar is equivalent to a 1,020 cen-timeter column of water.

The matric potential can be measured in sev-eral ways. Vacuum gauge potentiometers consistof a rigid plastic tube having a porous fired claycup on one end and a vacuum gauge on the other.The potentiometer is filled with pure water, and atthis point the vacuum gauge reads zero. Thepotentiometer is then buried in the soil so that theporous cup has good contact with the surround-i ng soil (see Figure 5.6). Since the potential ofpure water in the potentiometer is greater thanwater in unsaturated soil, water will move fromthe potentiometer into the soil. At equilibrium, thevacuum gauge will record the matric potential.Vacuum gauge potentiometers work in the range0 to -0.8 bars, which is a biologically importantrange for plant growth. They are used in irrigatedagriculture to determine when to irrigate.

A pressure chamber is used to measure matricpotentials less than -0.8 bars. Wet soil is placedon a porous ceramic plate with very fine pores,which is confined in a pressure chamber. Air pres-sure is applied, and the air forces water throughthe fine pores in the ceramic plate. At equilibrium,when water is no longer leaving the soil, the airpressure applied is equated to the matric poten-

tial. At equilibrium, an air pressure of 15 bars isequal to a matric potential of -15 bars. This tech-nique is used to determine the matric potential inthe range of -0.3 to -15 bars, which is the rangebetween field capacity and the permanent wiltingpoint, respectively.

Many books and publications express the waterpotential in bar units. However, kilopascals andmegapascals are being used more frequently toexpress the water potential. A bar is equivalent to100 kilopascals (kPa). Using kilopascals, the wa-ter potential at field capacity is -30 kPa and at thepermanent wilting point is -1,500 kPa.

SOIL WATER MOVEMENTWater movement in soils, and from soils to plantroots, like water cascading over a dam, is fromregions of higher-energy water to regions of lower-energy water. Thus, water runs downhill.

The driving force for water movement is thedifference in water potentials between two points.The water potential gradient (t) is the waterpotential difference between two points dividedby the distance between the two points. The rate

SOIL WATER MOVEMENT 61

of water flow is directly related to the water poten-tial difference and inversely related to the flowdistance.

The velocity of water flow is also affected by thesoil's ability to transmit water or the hydraulicconductivity (k). Water flow through large pores isfaster than through small pores; flow is fasterwhen the conductivity is greater. Therefore, thevelocity of flow, V, is equal to the water potentialgradient times the hydraulic conductivity, k. Math-ematically,

V = kphThe hydraulic conductivity, or the ability of the

soil to transmit water, is determined by the natureand size of the spaces and pores through whichthe water moves. The conductivity of a soil isanalogous to the size of the door to a room. As thedoor to a room becomes larger, the velocity o1

speed at which people can go in or out of theroom increases. As long as the door size remain:constant, the speed with which people can movei nto or out of the room remains constant. As longas the physical properties of a soil (includingwater content) remain constant, k is constant.

Conductivity (k) is closely related to pore size

FIGURE 5.6 Soil waterpotentiometers for measuringthe matric potential. The oneon the left has been removedfrom the soil to show theporous clay cup.

62

Water flow in a pipe is directly related to the fourthpower of the radius. Water can flow through alarge pore, having a radius 10 times that of a smallpore, 10,000 times faster than through the smallpore. In water-saturated soils, water moves muchmore rapidly through sands than clays becausesands have larger pores. The larger pores give thesandy soils a greater ability to transmit water or agreater conductivity when saturated as comparedto clayey soils.

The size of pores through which water movesthrough soil, however, is related to the water con-tent of the soil. In saturated soil, all soil pores arewater filled and water moves rapidly through thelargest macropores. At the same time, watermovement in the smaller pores is slow or nonexis-tent. As the water content of soil decreases, watermoves through pores with decreasing size, be-cause the largest pores are emptied first, followedby the emptying of pores of decreasing size.Therefore, k (hydraulic conductivity of the soil) isclosely related to soil water content; k decreaseswith decreasing soil water content.

Water Movement in Saturated SoilAs shown previously, the potential gradient (ph) isthe difference in water potentials between the twopoints of flow divided by the distance between thepoints of flow. The difference in water potentialsbetween two points in water-saturated soil is thehead (h). The gradient, f, is inversely related to thedistance of flow through the soil, d. Therefore,

SOIL WATER

This equation is named after Darcy, who stud-ied the flow of water through a porous mediumwith a setup like that shown in Figure 5.7. Thesetup includes a column in which a 12-centimeter-thick layer of water rests on top of a15-centimeter-thick layer of water-saturated soil.I n this case, the potential difference between thetwo points is equal to 12 centimeters (point A topoint B) plus 15 centimeters (point B to point C)or 27 centimeters; h equals 27 centimeters. Thedistance of flow through soil, d, is 15 centimeters.If the amount of water that flowed through the soili n 1 hour was 360 cm ³ , and the cross-sectionalarea of the soil column is 100 cm² , k can becalculated as follows:

360 cm ³ = k x27 cm

x 100 cm² x I hr15 cm

Then, k = 2 cm/hrThe value fork of 2 cm/hr is in the range of soils

with moderate saturated hydraulic conductivity;i n the range of 0.5 cm/hr to 12.5 cm/hr. Soils withslow saturated hydraulic conductivity range fromless than 0.125 cm/hr to 0.5 cm/hr. Soils withrapid saturated hydraulic conductivity have a con-ductivity in the range 12.5 cm/hr to more than

FIGURE 5.7 A setup similar to that which Darcyused to study water movement in porous materials.

25 cm/hr. Thus, a sand soil could have a saturatedhydraulic conductivity that is several hundredtimes greater than that of a clay soil.

According to Darcy's equation, if the distanceof water movement through soil is doubled, thequantity of water flow is halved. This is an impor-tant consideration in soil drainage, because thefurther soil is from the drain lines, the longer itwill take for water to exit the soil. Therefore, thedistance between drainage lines is dependent onthe hydraulic conductivity. Another important de-cision based on the saturated hydraulic conduc-tivity includes determining the size of seepagebeds for septic tank effluent disposal.

Water Movement in Unsaturated SoilIf water is allowed to drain from a saturated soil,water leaves the soil first and most rapidly via thelargest pores. As drainage proceeds over time,water movement shifts to smaller and smallerpores. The result is a rapid decrease in both hy-draulic conductivity and in the rate of drainage, ormovement of water from the soil. Field soils withan average saturated hydraulic conductivity will,essentially, stop draining within a day or two aftera thorough wetting, meaning the gravitational wa-ter has exited the soil and the soil is retaining themaximum amount of water against the force ofgravity. Such soil in the field is at field capacityand will have a matric potential varying fromabout -10 to -30 kPa (-0.1 to -0.3 bars). Thedata in Table 5.1 show that the conductivity of theGeary silt loam decreased from 9.5 cm/day atwater-saturation (matric potential 0) to only1.1 x 10 - ' near field capacity (matric potentialequal to -33 kPa or -0.3 bars). Thus, the hydrau-lic conductivity at field capacity was about 1 per-cent as great as at saturation.

Gravity is always operating. After field capacityis attained, however, matric forces control watermovement. Additional movement is very slow andover short distances. The flow tends to be hori-zontally from soil to roots, except for some verti-cal movement at the soil surface, resulting from


loss of water by evaporation. As a consequence,gravity plays a minimum role in the movement ofwater in water-unsaturated soil.

Darcy's law also applies to the movement ofwater in unsaturated soil. The value for h, thehydraulic head, becomes the difference in thematric potentials between the points of flow. Forwater movement from soil to roots, h will com-monly be a few hundred kPa (a few bars);for example, the difference between -100 and-300 kPa (-1 to -3 bars).

A most important consideration for unsaturatedflow is the fact that unsaturated hydraulic conduc-tivity can vary by a factor of about 1 million withinthe range that is biologically important. Unsatu-rated hydraulic conductivity decreases rapidlywhen plants absorb water and soils become drierthan field capacity. For the Geary soil (see Table5.1) , k near field capacity at -33 kPa is 1.1 x 10 - 'compared with 6.4 x 10 -5 cm/day for a potentialof -768 kPa. This means that the conductivity inthe soil at field capacity is more than 1,700 timesgreater than in soil midway between field capacityand the permanent wilting point. The water inunsaturated soil is very immobile and significantmovement occurs only over very short distances.The distance between roots must be small to re-cover this water. It is not surprising that Dittmer(1937) found that a rye plant, growing in a cubicfoot of soil, increased root length about 3 milesper day when consideration was made for roothair growth. When plants wilt for a lack of water,the wilting is due more to the slow transmissionrate of water from soil to roots rather than a lowwater content of the soil per se.

Water Movement in Stratified SoilStratified soils have layers, or horizons, with dif-ferent physical properties, resulting in differencesin hydraulic conductivity. As a consequence, thedownward movement of water may be alteredwhen the downward moving water front encoun-ters a layer with a different texture.

When rain or irrigation water is added to the

64 SOIL WATER

TABLE 5.1 Volume Water Content, Hydraulic Conductivities, and MatricPotentials of a Loam and Silt Loam Soil

Data from Hanks, 1965.

surface of dry, unsaturated soils, the immediatesoil surface becomes water saturated. Infiltration,or water movement through the soil surface, isgenerally limited by the small size of the pores inthe soil surface. Water does not rush into theunderlying drier unsaturated soil, but movesslowly as unsaturated flow. The downward flow ofwater below the soil surface is in the micropores,while the macropores remain air-filled. The watermoves slowly through the soil as a wetting front,creating a rather sharp boundary between moistand dry soil. Similarly, water moves slowly froman irrigation furrow in all directions. Matric forces(cohesion and adhesion) control water move-ment; water tends to move upward almost as rap-idly as downward.

If the water is moving through loam soil and thewetting front encounters a dry sand layer, the wet-ting front stops its downward movement. The wet-ting front then continues to move laterally abovethe sand layer, as shown in Figure 5.8, becausethe unsaturated hydraulic conductivity in the drysand is less than that of the wet silt loam soilabove. Water in the wetting front is moving as aliquid and, in dry sand, there are essentially nocontinuous water films between sand grains. Thatis, the path of water movement is interrupted be-cause the water films in many adjacent sandgrains do not touch each other. The abrupt stop-page of the water front, shown in Figure 5.8, isanalagous to a vehicle moving at a high speed andabruptly stopping when it reaches a bridge that

has collapsed. There is a lack of road continuitywhere the bridge has collapsed. The distance be-tween particles in clay soils is small, and there isgreater likelihood of water films bridging betweenthe particles in dry clay soil as compared to drysand. This gives dry clay soils greater hydraulicconductivity than dry sands.

As the soil under the irrigation furrow, andabove the sand layer, becomes wetter over time,the water potential gradient between the moistsoil and dry soil increases. In time, the waterbreaks through the silt loam soil and enters thesand layer, as shown in the lower part of Figure5.8. The low unsaturated hydraulic conductivity of


any sand and gravel layers underlying loamy soils,enables the loam soil layer to retain more waterthan if the the soil had a loam texture throughout.A gravel layer under golf greens results in greaterwater storage in the overlying soil. Conversely,during periods of excessive rainfall, drainage willoccur and prevent complete water saturation ofthe soil on the green.

If a clay layer underlies a loamy soil, the wettingfront will immediately enter the clay. Now,however, the clay's capacity to transmit water isvery low by comparison, and water will continueto move downwward in the loam soil at a fasterrate than through the clay layer. Under these con-

FIGURE 5.8 Photographsillustrating water movement instratified soil where water is movingas unsaturated flow through a siltl oam into a sand layer. In the upperphotograph, water movement intothe sand is inhibited by the lowunsaturated hydraulic conductivityof dry sand. The lower photographshows that after an elapsed time of1.5 to 5 hours, the water content inthe lower part of the silt loam soilbecame high enough to cause waterdrainage downward into the sand.(Photographs courtesy Dr. W. H.Gardner, Washington StateUniversity.)

66

ditions, the soil saturates above the clay layer, asit does when a claypan exists in a soil. From thisdiscussion it should be apparent that well-drainedsoils tend never to saturate unless some underly-i ng layer interrupts the downward movement of awetting front and soil saturates from the bottomupward.

At the junction of soil layers or horizons ofdifferent texture, downward moving water tendsto build-up and accumulate. This causes a tempo-rary increase in the availability of water and ofnutrients, unless very wet and saturated soil isproduced, and results in increased root growthand uptake of nutrients and water near or at thesej unctions.

Water Vapor MovementWhen soils are unsaturated, some of the porespace is air filled and water moves through thepore space as vapor. The driving force is the dif-ference in water potentials as expressed by differ-ences in vapor pressure. Generally, vapor pres-sure is high in warm and moist soil and low incold and dry soil. Vapor flow, then, occurs fromwarm and moist soil to and into cold and drysoil. In the summer, vapor flow travels from thewarmer upper soil horizons to the deeper andcolder soil horizons. During the fall, with transi-tion to winter, vapor flow tends to be upward. Iti s encouraged by a low air temperature and up-ward water vapor movement, and condensationonto the surface of the soil commonly createsa slippery smear in the late night and early morn-ing hours.

Soil conductivity for vapor flow is little affectedby pore size, but it increases with an increase inboth total porosity and pore space continunity.Water vapor movement is minor in most soils, butit becomes important when soils crack at the sur-face in dry weather and water is lost from deepwithin the soil along the surfaces of the cracks.Tillage that closes the cracks, or mulches thatcover the cracks, will reduce water loss by evapo-ration.

SOIL WATER

PLANT AND SOIL WATER RELATIONSPlants use large quantities of water, and this sec-tion considers the ability of plants to satisfy theirwater needs by absorbing water from the soil andthe effect of the soil water potential on nutrientuptake and plant growth.

Available Water Supplying Powerof SoilsThe water-supplying power of soils is related tothe amount of available water a soil can hold. Theavailable water is the difference in the amount ofwater at field capacity (-30 kPa or -0.3 bar) andthe amount of water at the permanent wiltingpoint (-1,500 kPa or -15 bars).

The amount of available water is related to bothtexture and structure, because it is dependent onthe nature of the surfaces and pores, or soil ma-trix. Soils high in silt (silt loams) tend to have themost optimum combination of surfaces andpores. They have the largest available water-holding capacity, as shown in Figure 5.9. Thesesoils contain about 16 centimeters of plant-available water per 100 centimeters of depth, or 16percent by volume. This is about 2 inches of waterper foot of soil depth. When such soil is at thepermanent wilting point, a 2-inch rain or irri-gation, which infiltrates completely, will bring thesoil to field capacity to a depth of about 1 foot.

From Figure 5.9, it can be noted that sands havethe smallest available water-holding capacity.This tends to make sands droughty soils. Anotherreason why sandy soils tend to be droughty is thatmost of the available water is held at relativelyhigh water potential, compared with loams andclays. It should be noted in Figure 5.10 that fieldcapacity for sand is more likely nearer -10 kPa(-0.1 bar) than -30 kPa (-0.3 bar) as in loamyand clayey soils. This means that much of theplant available water in sandy soils can moverapidly to roots when water near roots is depleted,because of the relatively high hydraulic conduc-tivity at these relatively high potentials. Plantsgrowing on clay soils, by contrast, may not absorb

as much water as they could use during the daybecause of lower hydraulic conductivity andslower replacement of water near roots when wa-ter is depleted. This results in a tendency to con-serve the available water in fine-textured soils. Inessence, the water retained at field capacity insands is more plant available than the water infine-textured soils at field capacity. Therefore,

0

PLANT AND SOIL WATER RELATIONS

sands tend to be more droughty than clays be-cause they retain less water at field capacity, andthe water retained is consumed more rapidly.

Water Uptake from Soils by RootsPlants play a rather passive role in their use ofwater. Water loss from leaves by transpiration is

67

FIGURE 5.9 Relationship of soiltexture to available water-holdingcapacity of soils. The differencebetween the water content at fieldcapacity and the water content at thepermanent wilting point is theavailable water content.

FIGURE 5.10 Soil watercharacteristic curves for threesoils. Field capacity of thesandy loam is about -0.1kPa x 10 -2 (-10 kPa) and forthe finer-textured soils is about-0.3 kPa x 10 -2 (-30 kPa).

68

mainly dependent on the surrounding environ-ment. Water in the atmosphere has a much lowerwater potential than water in a leaf, causing theatmosphere to be a sink for the loss of water fromthe plant via transpiration. The water-conductingtissue of the leaves (xylem) is connected to thexylem of the stem, and a water potential gradientdevelops between leaves and stems due to waterloss by transpiration. The xylem of the stem isconnected to the xylem of the roots, and a waterpotential gradient is established between the stemand roots. The water potential gradient estab-lished between roots of transpiring plants and soilcauses water to move from the soil into roots. Thiswater potential gradient, or continuum, is calledthe soil-plant-atmosphere-continuum, or SPAC.Some realistic water potentials in the continuumduring midday when plants are actively absorb-i ng water are -50,000 kPa (-500 bars) in theatmosphere, - 2,500 kPa (-25 bars) in the leaf,- 800 kPa (-8 bars) in the root, and -700 kPa(-7 bars) in the soil. There is considerable resis-tance to upward water movement in the xylem, sothat a considerable water potential gradient canbe produced between the leaf and the root.

SOIL WATER

Diurnal Pattern of Water UptakeSuppose that it is a hot summer morning, andthere has been a soaking rain or irrigation duringthe night. The soil is about at field capacity andwater availability is high. The demand for water isnil, however, because the water-potential differ-ences in the leaves, roots, and soil are small.There is no significant water gradient and no wa-ter uptake. After the sun rises and the temperatureincreases, thus decreasing relative humidity, thewater potential gradient increases between theatmosphere and the leaves. The leaves begin totranspire. A water potential gradient is establishedbetween the leaves, roots, and soil. Roots beginwater uptake. The low water conductivity in (theplant causes the water potential difference be-tween leaves and roots, and between roots andsoil, to increase to a maximum at about 2 P.M., asshown in Figure 5.11. This is a period of greaterwater loss than water uptake, which creates awater deficit in the leaves. It is normal for theseleaves to be less than fully turgid and appearslightly wilted.

After 2 P.M., decreasing temperature and in-creasing relative humidity tend to reduce water

FIGURE 5.11 Diurnalchanges in water potential ofsoil, roots, and leaves duringa period of soil drying. (AfterSlatyer, 1967.)

l oss from leaves. With continued water uptake,the plant absorbs water faster it is lost, and thewater potential gradient between leaves and soildecreases. The water deficit in the plant is de-creased, and any symptoms of wilting disappear.By the next morning, the water potential gradientbetween the leaves and soil is eliminated (seeFigure 5.11).

As this pattern is repeated, the soil dries and thehydraulic conductivity decreases rapidly and thedistance water must move to roots increases. Inessence, the availability of soil water decreasesover time. As a consequence, each day the po-tential for water uptake decreases and theplant increasingly develops more internal waterdeficit, or water stress, during the afternoon.The accumulated effects of these diurnalchanges, in the absence of rain or irrigationor the elongation of roots into underlying moistsoil, are shown in Figure 5.11. Eventually,l eaves may wilt in midday and not recover theirturgidity at night. Permanent wilting occurs, thesoil is at the permanent wilting point, and thesoil has a water potential of about -1,500 kPa(-15 bars).

The sequence of events just described is typicalwhere roots are distributed throughout the soilfrom which water uptake occurs. For the se-quence of events early in the growing season of anannual plant, continual extension of roots intounderlying moist soil may occur to delay the timewhen the plant wilts in the absence of rain orirrigation.

The permanent wilting point is not actually apoint but a range. Experiments have shown thatsome common plants could recover from wiltingwhen the potential decreased below -1,500 kPa(-15 bars). High temperature and strong winds,on the other hand, can bring about permanentwilting at potentials greater than - 1,500 kPa. If aplant can survive and, perhaps, make slow growthwith a small water demand, the soil water po-tential may be much less than -1,500 kPa.For most soils and plant situations, the perma-nent wilting range appears to be about -1,000 to

PLANT AND SOIL WATER RELATIONS 69

-6,000 kPa (-10 bars to -60 bars), which repre-sents a rather narrow range in differences in soilwater content. Desert plants can thrive with verylow water potentials and, typically, have somespecial feature that enables them to withstand alimited supply of soil water. Some plants shedtheir leaves during periods of extreme waterstress.

Pattern of Water Removal from SoilWith each passing summer day, water movesmore slowly from soil to roots as soils dry, and theavailability of the remaining water decreases. Thisencourages root extension into soil devoid ofroots where the water potential is higher and wa-ter uptake is more rapid. For young plants with asmall root system, there is a continual increase inrooting depth to contact additional supplies ofavailable water in the absence of rain or irrigationduring the summer. In this way the root zone isprogressively depleted of water as shown in Fig-ure 5.12. When the upper soil layers are rewettedby rain or irrigation, water absorption shifts backtoward the surface soil layer near the base of theplant. This pattern of water removal results in:(1) more deeply penetrating roots in dry yearsthan in wet years, and (2) greater absorption ofwater from the uppermost soil layers, as com-pared to the subsoil layers, during the growingseason. Crops that have a long growing seasonand a deep root system, like alfalfa, absorb agreater proportion of their water below 30 cen-timeters (see Figure 5.13).

Soil Water Potential VersusPlant GrowthMost plants cannot tolerate the low oxygen levelsof water-saturated soils. In fact, water saturationof soil kills many kinds of plants. These plantsexperience an increase in plant growth from satu-ration to near field capacity. As soils dry beyondfield capacity, increased temporary wilting duringthe daytime causes a reduction in photosynthesis.

70

FIGURE 5.13 Percentage of water used from each30-centimeter layer of soil for crops produced withirrigation in Arizona. Onions are a shallow-rootedannual crop that is grown in winter and used a totalof 44 centimeters of water. Alflalfa, by contrast, grewthe entire year and used a total of 186 centimeters ofwater, much of it from deep in the soil. (Data fromArizona Agr. Exp. Sta. Tech. Bull. 169, 1965.)

SOIL WATER

Thus, plant growth tends to be at a maximumwhen growing in soil near field capacity, wherethe integrated supply of both oxygen and water

are the most favorable.During periods of water stress, the damage to

plants is related to the stage of development. Thedata in Table 5.2 show that four days of visiblewilting during the early vegetative stage caused a5 to 10 percent reduction in corn yield comparedwith a 40 to 50 percent reduction during the timeof silk emergence and pollination. Injury fromwater stress at the dough stage was 20 to 30

percent.

TABLE 5.2 Effect of Four Days of Visible Wilting onCorn Yield

FIGURE 5.12 Pattern ofwater used from soil by sugarbeets during a 4-week periodfollowing irrigation. Water inthe upper soil layers was usedfirst. Then, water was removedfrom increasing soil depthswith time since irrigation.(Data from Taylor, 1957.)

Role of Water Uptake forNutrient UptakeMass flow carries ions to the root surfaces, wherethey are in a position to be absorbed by roots. Ani ncreased flow of water increases the movementof nutrient ions to roots, where they are availablefor uptake. This results in a positive interactionbetween water uptake and nutrient uptake.Droughts reduce both water and nutrient uptake.Fertilizers increase drought resistance of plantswhen their use results in greater root growth androoting depth (see Figure 5.14).

FIGURE 5.14 Fertilizer greatly increased the growthof both tops and roots of wheat. Greater root densityand rooting depth resulted in greater use of storedsoil water. (Photograph courtesy Dr. J. B.Fehrenbacher, University of Illinois.)

SOIL WATER REGIME 71

FIGURE 5.15 Climatic data and average, soil waterstorage capacity were used to construct the waterregime for soils near Memphis, Tennessee. (Datafrom Thornthwaite and Mather, 1955.)

SOIL WATER REGIMEThe soil water regime expresses the gradualchanges in the water content of the soil. For exam-ple, a soil in a humid region may at some timeduring the year have surplus water for leaching,soil water depletion by plant uptake, and waterrecharge of dry soil. These conditions are illus-trated in Figure 5.15 for a soil with an averagewater-storage capacity in a humid region, such asnear Memphis, Tennessee.

The early months of the year (January to May)are characterized by surplus water (see Figure5.15). The soil has been recharged with water by

72

rains from the previous fall, when the soil wasbrought to field capacity. The precipitation in theearly part of the year is much greater than thepotential for water evaporation (potential evapo-transpiration), and this produces surplus water.This water (gravitational) cannot be held in a soilat field capacity, and it moves through the soil,producing leaching. During the summer monthsthe potential evapotranspiration is much greaterthan the precipitation. At this time, plants depletestored soil water. Since there is insufficient storedwater to meet the potential demand for water(potential evapotranspiration), a water deficit isproduced. This means that most of the plantscould have used more water than was available.Abundant rainfall in some years prevents a deficit;in very dry years, the water deficit may be quitelarge and a drought may occur. In the fall thepotential evapotranspiration drops below thelevel of the precipitation, and water recharge ofdry soil occurs. Recharge, in this case, is com-pleted in the fall, and additional pecipitation con-tributes mainly to surplus (gravitational) waterand leaching. Leaching occurs only during theperiod when surplus water is produced.

The water regime in desert climates is charac-terized by little water storage, a very small soilwater depletion, a very large deficit, and no sur-plus water for leaching.

SUMMARYThe water in soils has an energy level that varieswith the soil water content. The energy level ofsoil water increases as the water content in-creases and decreases as the water content of thesoil decreases. This produces a soil water-energycontinuum.

The energy level of soil water is expressed asthe soil water potential, commonly as bars orkilopascals.

Water moves within soil and from soil to rootsi n response to water potential differences. Watermovement is directly related to water potential

SOIL WATER

differences and inversely related to the distance offlow.

The rate of water flow is also directly related tosoil hydraulic conductivity. The hydraulic con-ductivity decreases rapidly as soils dry.

Plant uptake of water reduces the water contentof the soil and greatly decreases the hydraulicconductivity. Thus, as soil dries, water movementto roots becomes slower. Plants wilt when watermovement to roots is too slow to satisfy the plant'sdemand for water.

During the course of the year the major changesi n soil water content include: recharge of dry soil,surplus water and leaching, and depletion ofstored water. Water deficits occur when waterl oss from the soil by evapotranspiration exceedsthe precipitation plus stored water for a period oftime.

REFERENCESClassen, M. M. and R. H. Shaw. 1970. "Water Deficit

Effects on Corn:II Grain Components." Agron. J.

62:652-655.Dittmer, H. J. 1937. "A Quantative Study of the Roots

and Root Hairs of a Winter Rye Plant." Am. J. Bot.

24:417-420.Erie, L. J., 0. F. French, and K. Harris. 1965. "Con-

sumptive Use of Water by Crops in Arizona." ArizonaAgr. Exp. Sta. Tech. Bull. 169.

Gardner, W. H. 1968. "How Water Moves in the Soil."Crops and Soils. November, pp. 7-12.

Hanks, R. J. 1965. "Estimating Infiltration from SoilMoisture Properties." J Soil Water Conserv. 20:49-51.

Hanks, R. J. and G. L. Ashcroft. 1980. Applied SoilPhysics. Springer-Verlag, New York.

Slayter, R. 0. 1967. Plant-water Relationships. Aca-demic Press, New York.

Taylor, S. A. 1957. "Use of Moisture by Plants." Soil,USDA Yearbook. Washington, D.C.

Thornthwaite, C. W. and J. R. Mather. 1955. "Clima-tology and Irrigation Scheduling." Weekly Weatherand Crop Bulletin. National Summary of June 27,1955.

The story of water, in a very real sense, is the storyof humankind. Civilization and cities emergedalong the rivers of the Near East. The world'soldest known dam in Egypt is more than 5,000years old. It was used to store water for drinkingand irrigation, and perhaps, to control flood-waters. As the world's population and the need forfood and fiber increased, water management be-came more important. Now, in some areas ofrapidly increasing urban development and limitedwater supply, competition for the use of water hasarisen between agricultural and urban users.

There are three basic approaches to water man-agement on agricultural land: (1) conservationof natural precipitation in subhumid and aridregions, (2) removal of water from wetlands, and(3) addition of water to supplement the amount ofnatural precipitation (irrigation). Two importantneeds for water management in urban areas in-clude land disposal of sewage effluent and pre-scription athletic turf.

WATER CONSERVATIONWater conservation is important in areas in whichlarge water deficits occur in soils within arid and

CHAPTER 6

SOIL WATER MANAGEMENT

73

subhumid climates. Techniques for water conser-vation are designed to increase the amount ofwater that enters the soil and to make efficient useof this water.

Modifying the Infiltration RateInfiltration is the downward entry of water throughthe soil surface. The size and nature of the soilpores and the water potential gradient near thesoil surface are important in determining theamount of precipitation that infiltrates and theamount that runs off. High infiltration rates, there-fore, not only increase the amount of water storedfor plant use but also reduce flooding and soilerosion.

Surface soils protected by vegetation in a natu-ral forest or grassland tend to have a higher in-filtration rate during rains than exposed soils incultivated fields or at construction sites. In anexperiment in which excessive water was applied,the protection of the soil surface by straw or bur-lap maintained high and constant infiltrationrates, as shown in Figure 6.1. When the straw andburlap were removed, the infiltration rates de-creased rapidly. The impact of raindrops on ex-

74 SOIL WATER MANAGEMENT

FIGURE 6.1 Effect of straw and burlap and theirremoval on soil infiltration rate. (Source: Miller andGiffore, 1974.)

posed soil breaks up soil peds and forms a crust.The average pore size in the surface soil de-creases, causing a reduction in the infitration rate.I nfiltration is also decreased by overgrazing anddeforestation, or by any practices that removeplant cover and/or cause soil compaction. Thebare field in Figure 6.2 shows the detrimentaleffects of raindrop impact on soil structure at thesoil surface, decreased water infiltration and,then, increased water runoff and soil erosion.

Since water movement into and through a soilpore increases exponentially as the radius in-creases, a single earthworm channel that is openat the soil surface may transmit water 100 times

faster than the same volume of smaller pores. Innatural forest and grassland areas, an abundantfood supply at the soil surface encourages earth-worms, resulting in large surface soil burrows thatare protected from raindrop impact by the vegeta-tive cover.

I n fields, it is common for infiltration rates todecrease over time during rainstorms. This isbrought about by a combination of factors. First, areduction in hydraulic conductivity occurs at thesoil surface, owing to physical changes. Second,as the soil becomes wet, and the distance of watermovement increases to wet the underlying drysoil, the velocity of water flow decreases. In-filtration rates from irrigation furrows, however, afew hours after the start of irrigation have beencaused by earthworms that moved toward themoist soil and entered the waterfilled furrows,leaving behind channels that increased infil-tration.

Many farmers modify the soil surface condi-tions with contour tillage and terraces. Thesepractices temporarily pond the water and slowdown the rate of runoff, which results in a longertime for the water to infiltrate the soil. In somesituations, where there is little precipitation, run-off water is collected for filling reservoirs. Then,surface soils with zero infiltration rates are desir-

FIGURE 6.2 Effects ofraindrop impact on soilstructure at the soil surfaceand on water infiltration andwater runoff. The soil isunprotected by vegetation orcrop residues, resulting in agreatly reduced infiltration rateand greatly increased runoffand soil erosion. Such effectsaccentuate the differences inthe amount of water stored insoils for plant growth on theridge tops, side slopes, and inthe low areas where runoffwater accumulates andsedimentation of eroded soiloccurs.

able. In these cases, the important product pro-duced by the soil is runoff water.

Summer FallowingMost of the world's wheat production occurs inregions having a subhumid or semiarid climate.I n the most humid parts of these regions, soilshave enough stored water (water from precipi-tation in excess of evaporation during the winterand early spring), plus that from precipitation dur-i ng the growing season, to produce a good crop ofwheat in almost every year. In the drier part ofthese regions, the water deficit is larger and inmost years insufficient water is available to pro-duce a profitable crop. Here, a practice calledsummer phallowing is used. Summer fallowingmeans that no crop is grown and all vegetativegrowth is prevented by shallow tillage or herbi-cides for a period of time, in order to store waterfor use by the next crop.

After wheat is harvested, the land is left un-planted. No crop is grown and weeds are con-trolled by shallow cultivation or herbicides tominimize the loss of water by transpiration. Dur-ing the year of the fallow period, there is greaterwater recharge than if plants were allowed to growand transpire. This additional stored water, to-gether with the next year's precipitation, is usedby the wheat. Fallowing produces a landscape ofalternating dark-colored fallow strips of soil and

WATER CONSERVATION

yellow-colored strips where wheat was produced,as shown in Figure 6.3. In order to explain howwater storage occurs in fallowed land, both evap-oration of water from the soil surface and watermovement within the soil must be considered.

After a wheat crop has been harvested, the wa-ter storage period begins. The surface soil layer isair dry and the underlying soil is at the approxi-mate permanent wilting point (see Figure 6.4).Rain on dry soil immediately saturates and in-filtrates the soil surface, and water moves down-ward as a wetting front, similar to water movingout of an irrigation furrow as shown in Figure 5.8.The depth of water penetration depends on theamount of infiltration and the soil texture. Eachcentimeter of water that infiltrates will moistenabout 6 to 8 centimeters of loamy soil, which isnear the permanent wilting point. A heavy rain-storm will produce an upper soil layer at fieldcapacity, whereas the underlying soil remains atthe wilting point (see Figure 6.4).

After the rain, some water will evaporate fromthe soil surface. This creates a water potentialgradient between the surface of the drying soiland underlying moist soil, and some water in theunderlying moist soil will migrate upward. Thisphase of soil drying is characterized by rapid wa-ter loss. As drying near the soil surface occurs, arapid decrease in hydraulic conductivity occurs atthe soil surface. Water migration upward is greatlyreduced, and the evaporative demand at the sur-

7 5

FIGURE 6.3 Characteristic patternof alternate fallow and wheat stripswhere land is summer fallowed forwheat production.

76

FIGURE 6.4 Changes in soil water during a summer fallow period for wheatproduction.

face greatly exceeds the water movement upwardfrom moist soil on a hot summer day. As a result,the immediate surface of the soil becomes air drywithin a few hours and the hydraulic conductivityfor liquid water approaches zero. This phase isassociated with a sharp decline in water loss.

Once a thin layer of surface soil is air dry, andfew water films bridge particles, water can movefrom the underlying moist soil to and through thesurface as vapor flow. Vapor movement underthese conditions is so slow as to be largely dis-counted, unless cracks exist. The result is a cap-ping effect that protects the stored water frombeing lost by evaporation. When another rain oc-curs, the surface soil is remoistened and addi-tional water moves through the previously moist-ened soil to increase the depth of the rechargedsoil. Repetition of this sequence of events duringa fallow period progressively increases the depthof recharged soil and the amount of stored soilwater. The sequence of events during a fallow-cropping cycle is shown in Figure 6.4.

Many studies have shown a good correlationbetween water stored at planting time and grainyield. The fallowing system, however, is not 100percent efficient. The capping feature does not


work perfectly, some evaporation occurs eachtime the soil surface is wetted and dried, andsome runoff occurs. A good estimate is that about25 percent of the rainfall during the fallow periodwill be stored in the soil for use in crop produc-tion. This extra quantity of water, however, has agreat effect on yields, as is illustrated in Table 6.1.The frequency of yields over 1,345 or 2,690 kg/hectare (20 or 40 bushels/acre) was significantlyincreased by summer fallowing. Fallowing wasalso more effective at Pendleton, Oregon, wherethe maximum rainfall occurs in winter, unlikeAkron, Colorado, and Hays, Kansas, where maxi-mum rainfall occurs in summer. Stored soil mois-ture has been shown to be as effective as precipi-tation during the growing season for wheatproduction.

Saline Seep Due to FallowingMost of the world's spring wheat is grown in sub-humid or semiarid regions where winters are se-vere, such as the northern plains areas of UnitedStates, Canada, and the Soviet Union. Low tem-perature results in low evapotranspiration, whichincreases the efficiency of water storage during

TABLE 6.1 Percentage Distribution of Wheat Yield Categories at Two Locations in the Great Plains and OneLocation in the Columbia River Basin

the fallow period. In fact, if a fallow period isfollowed by a wet year, surplus water may occurand percolate downward and out of the root zone.Where surplus water encounters an impermeablelayer, soil saturation occurs and water tends tomove laterally because of gravity above the imper-meable layer. This may create a spring (seepagespot) at a lower elevation. The evaporation ofwater from the area wetted by the spring results inthe accumulation of salts from the water, and anarea of salt accumulation called a saline seep isformed. The osmotic effect of soluble salt reducesthe soil water potential, water uptake, and wheatyields. In some instances the soils become toosaline for plant growth, because of very low waterpotential.

Control of a saline seep depends on reducingthe likelihood that surplus water will occur. Man-agement practices to control saline seeps in-clude: (1) reduced fallowing frequency, (2) use offertilizers to increase plant growth and water use,and (3) the production of deep-rooted perennialcrops, such as alfalfa, to remove soil water. Whenalfalfa is grown and has dried out the soil, thealfalfa grows very slowly, which indicates that it isonce again time to begin wheat production.

Effect of Fertilizers on WaterUse EfficiencyPlants growing in a soil that contains a relativelysmall quantity of nutrients grow slower and tran-

WATER CONSERVATION 77

spire more water per gram of plant tissue pro-duced than those growing where the plant nutri-ents are in abundance. Since it has been shownthat water loss (or use) is mainly dependent onthe environment, any practice that increases therate of plant growth will tend to result in produc-tion of more dry matter over a given period of timeand per unit of water used. The data in Table 6.2show that the yield of oats increased from 86 to145 kg/hectare (2.4 to 4.0 bushels/acre) for each2.5 centimeters of water used in conjunction withmore fertilizer. In humid regions, it has commonlybeen observed that fertilized crops are moredrought resistant. This may be explained on thebasis that increased top growth results in in-creased root growth and root penetration, so thatthe total amount of water consumed is greater.

Fertilizer use in the subhumid region may occa-sionally decrease yields. If fertilizer causes a cropto grow faster in the early part of the season, thegreater leaf area at an earlier date represents agreater potential for water loss by transpiration. Ifno rain occurs, or no irrigation water is applied,plants could run out of water before harvest, caus-ing a serious reduction in grain yield.

Because water use in crop production is mainlyenvironmentally determined, fertilizer use and thedoubling or tripling of yields has little effect onthe sufficiency of the water supply. In under-developed countries, large increases in cropyields can be produced without creating a needfor significantly more water. Conversely, if water


TABLE 6.2 Oats Production per 2.5 Centimeters of Water Used

From Hanks and Tanner, 1952.

supplies become very limited, it may be desirable gravitational water from drainage ditches intoto use less land and more fertilizer to obtain the higher canals for return to the sea. Now, 800 yearssame amount of total production with less water later, about one third of the Netherlands is pro-used.

tected by dikes and kept dry with pumps andcanals. Soil dewatering or drainage is used tolower the water table when it occurs at or near thesoil surface, or to remove excess water that has

SOIL DRAINAGE

accumulated on the soil surface. The use ofdrainage is commonplace to dewater the soil in

About A.D. 1200, farmers in the Netherlands be- the root zone of crops, to dewater the soil nearcame engaged in an interesting water control building foundations to maintain dry basementsproblem. Small patches of fertile soil, affected by and for the successful operation of septic sewagetides and floodwaters, were enclosed by dikes disposal systems, and to remove excess surfaceand drained. Windmills provided the energy to lift

water (see Figure 6.5).

FIGURE 6.5 Dewatering, surface and subsurface drainage, is required tomake this land suitable for both building construction and cropping.(Photograph courtesy Soil Conservation Service, USDA.)

Water Table Depth Versus Air andWater Content of SoilIf a well is dug and the lower part fills with water,the surface of the water in the well is the top of thewater table. Water does not rise in a well abovethe water table. In the adjacent soil, however,capillarity causes water to move upward in soilpores. A water-saturated zone is created in the soilabove the water table, unless the soil is so gravellyor stony that capillarity does not occur. The water-saturated zone above the water table is the capil-lary fringe and is illustrated in Figure 6.6.

Soils typically have a wide range in pore sizes.Some pores are large enough so that they areair-filled above the capillary fringe, depending onpore size and height above the capillary fringe.

SOIL DRAINAGE

This creates a zone above the capillary fringe thathas a decreasing water content and increasing aircontent toward the soil surface. This capillary-affected zone, plus the capillary fringe, generallyranges in thickness from several centimeters toseveral meters. For all practical purposes, soilmore than 2 meters above the water table isscarcely affected by it. This means that mostplants, even those in humid regions, do not usewater from the water table.

Generally, the soil above the capillary fringewill have a water content governed by the balancebetween infiltration and loss by evapotran-spiration and percolation. Soils in deserts andsemiarid regions typically have a permanently dryzone between the root zone and the water table.

FIGURE 6.6 Generalized relationshipof the water table to the soil waterzones, and the air and water contentof soil above the water table. Thecapillary fringe is water saturated.Upward from the capillary fringe thewater content of the soil decreasesand the air content increases withinthe capillary fringe affected zone.

7 9

80

Benefits of DrainageRoot tips are regions of rapid cell divison andelongation, having a high oxygen demand. Rootsof most economic crop plants do not penetratewater-saturated soil or soil that is oxygen defi-cient. The major purpose of drainage in agricul-ture and forestry is lowering the water table toincrease the plant rooting depth.

Some of the largest increases in plant growthresulting from drainage occur in forests. Drainageon the Atlantic Coastal Plain increased the growthof pine from 80 percent to 1,300 percent (seeTable 6.3). Few management practices are as ef-fective as drainage for increasing tree growth. Ad-ditional benefits of drainage in forests are easierlogging, with less soil disturbance, and easier sitepreparation for the next crop.

Drainage of wet soil also increases the length ofthe growing season in regions with low soil tem-perature and frost hazard. Soils with a lower watercontent warm more rapidly in the spring. Seedgermination is more rapid and roots grow faster.The net result is a greater potential for plantgrowth.

Surface DrainageSurface drainage is the collection and removal ofwater from the surface of the soil via open ditches.

TABLE 6.3 Effect of Drainage on Mean Annual Growth of Pine on PoorlyDrained Soils of the Atlantic Coastal Plain

Adapted from Terry and Hughes, 1975.a Cubic meters per hectare.


Two conditions favoring the use of surfacedrainage are: (1) low areas that receive a largeamount of water from the surrounding higherland, and (2) impermeable soils that have an in-sufficient hydraulic conductivity to dispose of theexcess water by movement downward andthrough the soil to drainage tubing. Surfacedrainage is favored on the Sharkey clay soils alongthe lower Mississippi River, because the soil isslowly permeable and receives water from sur-rounding higher land. Sugarcane, which is widelygrown and sensitive to poor aeration, is plantedon ridges to improve aeration in the root zone.

It is difficult to irrigate without applying excesswater in many instances. Surface drainage ditchesare used to dispose of excess water and to preventsaturation of the soil on the low end of irrigatedfields. Surface drainage is also widely used alonghighways and in urban areas for surface watercontrol. Rapid removal of surface water from lowareas of golf courses and parks is essential topermit their use soon after a rain.

Subsurface DrainageDitches for the removal of gravitational water canbe made quickly and inexpensively. Drainageditches, however, require periodic cleaning andare inconvenient for the use of machinery. There

are also many situations in which ditches are notsatisfactory, for example, water removal aroundthe basement walls of a building.

Drain lines are installed in fields with trenchingmachines. Perforated plastic tubing is very popu-l ar and is automatically laid at the bottom of atrench as the trenching machine moves across thefield (see Figure 6.7). Unless some unusual con-dition prevails, plastic tubing should be laid atl east 1 meter deep to have a sufficiently low watertable between the drains to permit crops to de-velop an adequate root system, as illustrated inFigure 6.8.

Drainage in Soil ofContainer-Grown PlantsMany plants are grown indoors in containers. Oneof the most common problems of growing plants

FIGURE 6.7 I nstallation of perforated plastic tubingto remove water from a field during the wet season.The plastic tubing carries water to an outlet wherethe water is disposed. (Photo courtesy SoilConservation Service, USDA.)

SOIL DRAINAGE 81

in containers is poor soil aeration caused by over-watering. How can this be true, since mostflowerpots have a large drainage hole in the bot-tom? Knowledge of changes in air and water con-tent of soil above the water table can help providethe answer.

Consider a flowerpot filled with soil that is wa-ter saturated. Water is allowed to drain out of thel arge hole in the bottom of the pot. Surplus waterwill exit rapidly from the largest soil pores at thetop of the pot and air will be pulled into these porespaces. Gradually the zone of water saturationwill move progressively downward. Shortlythereafter, the loss of water by gravity may essen-tially stop. At this time, some of the soil at thebottom may still be water saturated, if all the poresi n the soil are sufficiently small to attract watermore strongly than gravity. This creates a "capil-lary fringe" in the bottom of the pot. Then, the airand water content of the soil above the saturatedsoil at the bottom is analogous to soil above thesaturated capillary fringe. Compare the drawing inFigure 6.9 with Figure 6.6 and note, in particular,the decrease in water and increase in air withdistance upward in the soil above soil that is watersaturated.

If all soil pores are very small, all the soil in aflowerpot could remain saturated after watering. Agood soil for container-grown plants has manypores too large to retain water against gravity.After wetting and drainage, these large pores willbecome air filled, even at the bottom of the con-tainer. Special attention must be given to soil usedfor container-grown plants. Ordinary loamy fieldor garden soils are unsatisfactory. A mix of one-half fine sand and one-half sphagnum peat moss,by volume, is recommended by the University ofCalifornia to produce a mix with excellent physi-cal properties for plant growth. When sand andsoil are mixed together, the small soil particles fillthe spaces between the sand particles, producinga low-porosity mix with few large aeration pores.

Placing large gravel in the bottom of a flowerpotor in a hole dug outdoors will create a barrier tothe downward flow of water from an overlyingfiner-textured soil layer or soil mix. The gravel

acts in the same manner as a large hole in acontainer that is filled with a soil mix. A saturatedsoil zone tends to form above the gravel layer,whatever the depth of the gravel.

I RRIGATIONIrrigation is an ancient agricultural practice thatwas used 7,000 years ago in Mesopotamia. Otherancient, notable irrigation systems were locatedi n Egypt, China, Mexico, and Peru. Today, approx-

FIGURE 6.9 Illustration of conditions in a flowerpotafter a thorough watering and drainage. At thebottom of the pot, the soil is saturated. Soil-aircontent increases and water content decreases withdistance upward above the water-saturated layer. Thesoil pore spaces operate in a manner similar to aseries of capillary tubes of varying sizes.

FIGURE 6.8 The effect ofdrainage lines in lowering thewater table. The benefit ofdrainage is first evidentdirectly over the drainagelines, and it gradually spreadsto the soil area between them.

imately 11 percent of the world's cropland is irri-gated. Some of the densest populations are sup-ported by producing crops on irrigated land, as inthe United Arab Republic (Egypt), where 100 per-cent of the cropland is irrigated. This land is lo-cated along the Nile River. More than 12 percentof the cropland, or about 24,000,000 hectares(61,000,000 acres) are irrigated in the UnitedStates. Irrigation is extremely important in in-creasing soil productivity throughout the world.

Water SourcesMost irrigation water is surface water resultingfrom rain and melting snow. Many rivers in theworld have their headwaters in mountains andflow through arid and semiarid regions where thewater is diverted for irrigation. Examples includethe Indus River, which starts in the HimalayaMountains, and the numerous rivers on the west-ern slope of the Andes Mountains, which flowthrough the deserts of Chile and Peru to the Pa-cific Ocean. Much of the water in these riverscomes from the melting of snow in the highmountains. In fact, the extent of the snowpack ismeasured in order to predict the stream flow andamount of water that will be available for irri-gation the next season. Many farmers who usewell water have their own source of irrigationwater. About 20 percent of the water used forirrigation in the United States comes from wells.

Important Properties of Irrigated SoilsSoils well suited for irrigation have an interme-diate texture to a depth of I to 2 meters andwithout compact or water-impermeable layers.


Loamy soils that are underlain by coarse sandsand gravel have greater retention of water in theoverlying soil than soils that are loamy-texturedthroughout. The texture, together with the struc-ture, should result in an infiltration rate of about0.5 to 8 centimeters per hour and a moderatewater-storage capacity. The soil should not havean injurious soluble salt content.

Water Application MethodsChoice of various methods of applying irrigationwater is influenced by a consideration of: (1) in-filtration rate, (2) slope and general nature of thesoil surface, (3) supply of water and how it isdelivered, (4) crop rotation, and (5) seasonal rain-fall. The methods of distributing water can beclassified as flood, furrow, subsurface, sprinkler,and drip or trickle.

Flood Irrigation Flood irrigation floods theland by distributing water by gravity. Water is leti nto the upper end of the field, usually from a largeditch. Sometimes the water is distributed into ba-sins, which is a common practice for orchards,pastures, and some grain crops, including rice(see Figure 6.10). Flood irrigation can easily over-water some parts of; a field while leaving someparts underirrigated.

I RRIGATION 83

Furrow Irrigation Furrow irrigation is similarto flood irrigation, the difference being that thewater distribution is down furrows instead of be-ing allowed to flood over the land. In furrow irri-gation, which is used for row crops, water is dis-tributed down between the rows. Furrows thathave a slight grade or slope are made across thefield. The slope of the furrow bottoms depends onsoil infiltration rate, which should be sufficient todistribute water along the entire length of the fur-row (see Figure 6.11). Furrow irrigation is unsatis-factory on sand soils, because of the very highinfiltration rate and limited distance of water flowdown the furrows.

Many methods are used to distribute water intothe furrows. Gated pipe is shown in Figure 6.11,and the use of siphon tubes is shown in Figure6.12. Siphon tubes require extensive manuallabor. A major advantage of flood and furrow irri-gation is that the water is distributed by gravity, sowater distribution costs tend to be low.

Sprinkler Irrigation Sprinklers are widely usedto irrigate lawns. Sprinkler systems are versatileand have some special advantages. On sands,where high infiltration rates permit water to flowonly a short distance down a furrow, sprinklersare essential to spread water uniformly over thefield. On an uneven land surface, expensive land

FIGURE 6.10 Flood irrigationof grapes in the Central Valleyi n California.

84

FIGURE 6.11 Planned land with proper grade to give uniform distribution ofwater down the rows. Gated pipe is being used to distribute water into thefurrows.

leveling and grading can be avoided by the use ofsprinklers. The portable nature of many sprinklersystems makes them ideally suited for use whereirrigation is used to supplement natural rainfall.When connected to soil water-measuring appa-rati, sprinkler systems can be made automatic.

Large self-propelled sprinkler systems, whichpivot in a large circle around a well, have becomepopular. Such systems can irrigate most of thel and in a quarter section (160 acres, or 65 hec-tares). In the western part of the United States,l arge green circular irrigated areas produced by


pivot sprinklers are clearly visible from the air.Their use on many rolling and sandy soils hasgreatly increased the production of corn and othergrain crops on the Great Plains. In some areas,however, underground water reservoirs are beingdepleted, and some irrigated areas are again be-ing used as grazing land and for fallowed wheatproduction.

Sprinkler irrigation modifies the environmentby completely wetting plant leaves and the soilsurface and by affecting the surrounding air tem-perature. The high specific heat of water makes

FIGURE 6.12 Use of siphon tubes totransfer water from the head ditch intofurrows.

sprinkling an effective means of reducing frosthazard. Sprinkler irrigation has been used for frostprotection in such diverse situations as for wintervegetables in Florida, strawberries in Michigan,and citrus in California.

Subsurface Irrigation Subirrigation is irrigat-ing by water movement upward from a water tablelocated some distance below the soil surface.Some areas are naturally subirrigated where watertables are within a meter or two of the soil sur-face. This occurs in the poorly drained areas ofthe Sandhills in north-central Nebraska. Artificalsubirrigation is practiced in the Netherlands,where subsurface drainage lines are used to drainsoils in the wet seasons and then used duringdrought for subirrigation. The subirrigation de-pends on the use of drainage ditches and pumpsto create and control a water table. Subirrigationworks well on the nearly level Florida coastalplains. The sandy soils have high saturated hy-draulic conductivity, and the natural water table is1 meter or less from the soil surface much of thetime.

Subirrigation is more efficient in the use of wa-ter than other systems, and is adapted only forspecial situations. In arid regions, the upwardmovement of water and its evaporation at the soilsurface results in salt accumulation. Subirrigationworks best where natural rainfall leaches any saltsthat may have accumulated.

Drip Irrigation Drip irrigation is the frequent oralmost continuous application of a very smallamount of water to a localized soil area. Plastichoses about 1 or 2 centimeters in diameter, withemitters, are placed down the rows or aroundtrees. Only a small amount of the potential rootzone is wetted. Roots in the localized area canabsorb water rapidly because a high water poten-tial, or water availability, is maintained. A majoradvantage of drip irrigation is the large reductioni n the total amount of water used. Other advan-

I RRIGATION 85

tages include adaptability of the system to verysteep land, where other methods are unsuited,and the ability to maintain a more uniform soilwater potential during the growing season.Potatoes develop an irregular shape and be-come less desirable if tubers alternately growrapidly and slowly because of large changes inavailable water during the period of tuberdevelopment.

Rate and Timing of IrrigationAn ideal application of water would be a sufficientquantity to bring the soil in the root zone to fieldcapacity. More water may result in waterlogging ofa portion of the subsoil or in the loss of water bydrainage. If water percolates below the root zoneover a period of many years, the water may ac-cumulate above an impermeable layer and pro-duce a perched water table. Conversely, unlessenough water is applied to result in some drain-age, it is difficult to remove the salt in the irriga-tion water and prevent the development of soilsalinity.

An accepted generalization is that the time toirrigate is when 50 to 60 percent of the plant-available water in the root zone has been used.Potentiometers are used to measure the matricpotential in various parts of the root zone, and thetiming of irrigation is based on water potentialreadings. Computer programs are available forirrigation scheduling that are based on maintain-ing a record of gains and losses of soil water.Farmers commonly irrigate according to the ap-pearance of the crops. Such crops as beans, cot-ton, and peanuts develop a dark-green color un-der moisture stress. Some species of lawn grassesturn to an almost black-green color when wilting.In most instances, crops should be irrigated be-fore marked wilting occurs. When irrigation wateris in short supply, it may be better to curtail wateruse early in the season (delay irrigation), so thatsufficient water is available during the reproduc-tive stages of plant growth, when water stress isthe most injurious to crop yields.

86

Water QualityWater in the form of rain and snow is quite pure.By the time the water has reached farm fields,however, the water has picked up soluble materi-als (salts) from the soils and rocks over andthrough which the water moved. Four importantfactors in determining water quality are: (1) totalsalt concentration, (2) amount of sodium relativeto other cations, (3) concentration of boron andother toxic elements, and (4) the bicarbonateconcentration.

Total Salt Concentration An acre-foot of waterweighs about 2,720,000 pounds. If it contained

1 ppm of salt, it would contain 2.72 pounds of salt.Water containing 735 ppm of salt would contain 1ton of salt per acre-foot of water. An annual appli-cation of 4 acre-feet of irrigation water containing735 ppm of salt would involve adding salt equal to4 tons per acre each year. Since such water wouldbe of good quality, there is great potential fordeveloping sufficient soluble salt content in soilsso as to seriously reduce soil productivity. Thel ongtime use of soils for irrigation necessitatessome provision for the leaching and removal ofsalt in drainage water in order to maintain a favor-able salt balance in the soil, where the salts arenot naturally leached out of the soil.

The total salt concentration, or salinity, is ex-pressed in terms of the electrical conductivity ofthe water, and it is easily and precisely deter-mined. The electrical conductivity of irrigationwater is expressed as decisiemens per meter (dS/m);in earlier literature as millimhos per cen-timeter. Four water quality classes have been es-tablished with conductivity ranging from 0.1 tomore than 2.25 dS/m, as shown in Figure 6.13.The salinity hazard ranges from low to very high.

The four salinity classes of water have the fol-lowing general characteristics. Low salinity water(C1) can be used for irrigation of most crops, withlittle likelihood that soil salinity will develop.Some leaching is required, but this occurs undernormal irrigation practices, except in soils withextremely low hydraulic conductivity.


Medium salinity hazard water (C2) can be usedif a moderate amount of leaching occurs. Plantswith moderate salt tolerance usually can begrown without special practices for salinitycontrol.

High salinity hazard water (C3) cannot be usedon soils with restricted drainage. Even with ade-quate drainage, special management for salinitycontrol may be required. Plants with good salttolerance should be selected.

Very high salinity hazard water (C4) is not suit-able for irrigation under ordinary conditions, butmay be used occasionally under very special cir-cumstances. The soil must be permeable andhave adequate drainage. Excess irrigation watermust be applied to bring about considerableleaching of salt, and very salt-tolerant cropsshould be selected.

Sodium Adsorption Ratio The water qualityclassification in Figure 6.13 is also based on thesodium hazard, along the left vertical side of thefigure. The sodium hazard is based on the sodiumadsorption ratio (SAR), which is the amount ofsodium relative to other cations.

The negative electrical charge of clay and hu-mus particles is neutralized by cations that areadsorbed and balance the negative charge. Thischarge, which is discussed in Chapter 10, is differ-ent and much stronger than the electronegativitythat attracts water molecules. When 15 percent ormore of the charge is neutralized by sodium(Na'), clay and humus particles tend to disperseand decrease the soil's hydraulic conductivity.The SAR expresses the relative amount of sodiumin irrigation water relative to other cations and thelikelihood that the irrigation water will increasethe amount of adsorbed sodium in the soil.

The SAR of irrigation water is calculated asfollows:

SAR =(sodium)

(calcium + magnesium) 1 1²

The ion concentrations, denoted by parentheses,are expressed in moles per liter. The classifi-

cation of irrigation waters with respect to SAR isbased primarily on the effect of the exchangeablesodium on the physical condition of the soil. Irri-gation water with SAR values of 18 to 26 has a highsodium hazard and above 26 the sodium hazard isvery high. The four SAR classes combine with thefour classes of salinity hazard to give 16 classes ofwater as shown in Figure 6.13.

Boron Concentration Boron is essential toplant growth, but the amount needed is verysmall. A small amount of boron in soil may be

I RRIGATION 87

FIGURE 6.13 Diagram for theclassification of irrigation waters. (Datafrom USDA Handbook 60, 1954.)

toxic. Plants have a considerable range in toler-ance. The occurrence of boron in toxic concentra-tion in certain irrigation waters makes it necessaryto consider this element in assessing water qual-i ty. The permissible limit of boron in severalclasses of irrigation water is given in Table 6.4,and the relative tolerance of some plants is givenin Table 6.5.

Bicarbonate Concentration In waters contain-ing high concentrations of bicarbonate ion(HC0 3

-), calcium and magnesium tend to pre-

TABLE 6.4 Permissible Limits of Boron for Several Classes of IrrigationWaters in Parts per Million

From Agriculture Handbook 60, USDA, 1954.

cipitate as carbonates as the soil solution be-comes more concentrated. This reaction does notgo to completion under ordinary circumstances,but when it does proceed, the concentrations ofcalcium and magnesium are reduced and the rel-ative proportion of sodium in the soil solution isincreased. This results in an increase in the SARand a decrease in water quality.

TABLE 6.5 Tolerance of Plants to Boron (In each Group, the Plants FirstNamed are Considered as Being More Tolerant, and the Last Named MoreSensitive)


From Agriculture Handbook 60, USDA, 1954.

In making an estimate of water quality, the ef-fect of salts on both the soil and the plant must beconsidered. Various factors such as drainage andsoil texture influence the effects on soil. The finerthe size of clay particles and the greater the ten-dency for clay particles to expand and contractwith wetting and drying, the greater is the effect ofa given SAR on dispersion of clay particles and

Boron ClassSensitive

CropsSemitolerant

CropsTolerant

Crops

1 <0.33 <0.67 <1.002 0.33-0.67 0.67-1.33 1.00-2.003 0.67-1.00 1.33-2.00 2.00-3.004 1.00-1.25 2.00-2.50 3.00-3.755 >1.25 >2.50 >3.75

reduction in hydraulic conductivity. The ultimateeffect on the plant is the result of these and otherfactors that operate simultaneously. Conse-quently, no method of interpretation is absolutelyaccurate under all conditions, and interpretationsare based commonly on experience.

Salt Accumulation and Plant ResponseSoil salinity caused by the application of irrigationwater usually is not a problem in humid regionswhere rainfall leaches the soil some time eachyear. Most irrigation, however, is done in low rain-fall areas where leaching is limited and salt tendsto accumulate in irrigated soils. Today, crop pro-duction is reduced on 50 percent of the world'sirrigated land as a result of salinity or drainageproblems.

Saline soil contains sufficient salt to impairplant growth. The salts are mainly chlorides, bi-carbonates, and sulfates of sodium, potassium,calcium, and magnesium. The best method forassessing soil salinity is to measure the electricalconductivity of the saturated soil extract, the EC e .This procedure involves preparing a water-saturated soil paste by adding distilled waterwhile stirring the soil until a characteristic endpoint is reached. A suction filter is used to obtaina sample of the extract for making an electricalconductivity measurement. The saturated-extractelectrical conductivity is directly related to thefield moisture range from field capacity to thepermanent wilting point; this is a good parameterfor evaluating the effect of soil salinity on plantgrowth.

Salinity effects on plants, as measured by theelectrical conductivity of the saturated extract,ECe , in decisiemens per meter, dS/m, are asfollows:

1. 0-1: Salinity effects mostly negligible.2. 2-4: Yields of very salt-sensitive crops may be

restricted.3. 4-8: Yields of many crops restricted.4. 8-16: Only salt-tolerant crops yield satisfac-

torily.

I RRIGATION

5. Over 16: Only a few very salt-tolerant cropsyield satisfactorily.

The salt-tolerance figures of crops given in Ta-ble 6.6 are arranged according to major crop divi-sions. Within each group the crops are listed inorder of decreasing salt tolerance. The EC e valuesgiven represent the salinity level at which 10, 25,and 50 percent decreases in yields may be ex-pected, as compared with yields on nonsalinesoils under comparable growing conditions. Stud-ies of the effects of salt on plants have shownsignificant differences in varieties of cotton, bar-ley, and smooth brome, whereas variety differ-ences were of no consequence for green beans,lettuce, onions, and carrots.

The effect of salts on plants is mainly indirect,that is, the effect of salt on the osmotic waterpotential. Decreasing water potential due to saltreduces the rate of water uptake by roots andgerminating seeds. Thus, an increase in soil salin-ity produces the same net effect on water uptakeas that produced by soil drying. In fact, the effectsof decreases in osmotic and matric water poten-tials appear to be additive.

Salinity Control andLeaching RequirementFor permanent agriculture, there must be a favor-able salt balance. The salt added to soils in irri-gation water must be balanced by the removal ofsalt via leaching. The fraction of the irrigationwater that must be leached through the root zoneto control soil salinity is termed the leaching re-

quirement (LR). Assuming that the goal is to irri-gate a productive soil with no change in salinity,and where soil salinity will not be affected byleaching from rainfall or other factors, the leach-ing requirement is the ratio of the depth ofdrainage water, D dw , to the depth of irrigation

water, D 1 :

LR= Data

D;W

89


TABLE 6.6 Salt Tolerance of Crops Expressed as the EC, at 25° C for YieldReductions of 10, 25, and 50 Percent, as Compared to Growth onNormal Soils

TABLE 6.6 (continued)

Adapted from Agr. Information Bull. Nos. 283 and 292 and Western Fertilizer Handbook,1975.

LR, under the conditions specified (no change insoil salinity over time), is also equal to

LR =EC;,

EC dW

If the electrical conductivity of the drainage water(ECdW) that can be tolerated without adverselyaffecting the crop is 8 dS/m:

ECiwLR -

8 dS/m

For irrigation water with an electrical conductivity(ECi w) of 1, 2, and 3 dS/m, LR = 0.13, 0.25, and0.38, respectively. Of the irrigation water appliedto the soil, 13, 25, and 38 percent, respectively,represent the amounts of the water that must beleached through the soil. Under these conditions,the salt in the irrigation water is equal to the salt inthe drainage water and the salt content of the soilremains unchanged. When the irrigation need is10 centimeters, the total water application neededis calculated as follows for an LR of 25 percent:

LR = 10 cm + 0.25 (10 cm) = 12.5 cm

The need for leaching soils to control salt high-

I RRIGATION 91

lights a major disadvantage of drip irrigation: themethod is not suitable for application of largeamounts of water. Leaching of salts without pro-vision for drainage in upland soils, as on the highplains of Texas, and other irrigated areas, resultsin salt being deposited below the root zone. Mostof the irrigated land in arid regions is located inbasins or alluvial valleys containing stratified soilmaterials. Long and continued leaching for saltcontrol commonly results in the accumulation ofsurplus water above impermeable layers or theformation of perched water tables. When watertables rise to within 1 to 2 meters of the soilsurface, water moves upward by capillarity at arate sufficient to deposit salt on top of the soilfrom the evaporation of water, as shown in Figure6.14. It is essential in these cases that subsurfacedrainage systems be installed to remove drainagewater and maintain deep water tables. Conse-quently, salinity control in arid regions is depen-dent on drainage.

Archaeological studies in Iraq showed that theSumerians grew almost equal amounts of wheatand barley in 3500 B.C. One thousand years later,only one-sixth of the grain was the less salt-


tolerant wheat; by 1700 B.C., the production ofwheat was abandoned. This decline in wheat pro-duction and increase in salt-tolerant barley coin-cided with soil salinization. Records show wide-spread land abandonment, and that saltaccumulation in soils was a factor in the demiseof the Sumerian civilization.

Even though good irrigation practices are fol-

FIGURE 6.14 Water tables(water-saturated soil) a shortdistance below the soilsurface result in the upwardmovement and evaporation ofwater and the accumulation ofsalt at the soil surface. Theprocess results in thedevelopment of saline soil.

l owed, important differences can exist in the saltcontent of soil that is immediately under the irri-gation furrow and the tops of beds. Downwardwater movement under the furrow leaches the soiland produces low salinity. At the same time, watermoves to the top of the beds and salt is depositedas water evaporates. Important differences in saltconcentrations are produced (see Figure 6.15).

FIGURE 6.15 Saltaccumulation on the ridgesresulting from the upwardmovement and evaporation ofwater in a cotton field. Saltcontent under the furrows isl ess than on the ridges due tol eaching under the furrows.(Photo courtesy SoilConservation Service, USDA.)

Effect of Irrigation on RiverWater QualityAbout 60 percent of the water diverted for irri-gation is evaporated or consumed. The remainderappears as irrigation return flow (which includesdrainage water) and is returned to rivers for dis-posal. As a consequence, the salt concentrationof river water is increased. A 2- to 7-fold increasein salt concentration is common for many rivers.Along the Rio Grande in New Mexico, water diver-ted at Percha Dam for Rincon Valley irrigation hadan average of 8 milliequivalents per liter of saltfrom 1954 to 1963. Salt content increased to 9 atLeasburg, 13 at the American Diversion Dam, and30 milliequivalents per liter at the lower end of ElPaso Valley. Rivers serve as sinks for the deposi-tion of salt in drainage water from irrigated land.The Salton Sea, whose surface is below sea level,serves as a salt sink for the Imperial Valley insouthern California.

Nature and Management of Saline andSodic SoilsThe development of saline soils is a naturalprocess in arid regions where water tables arenear the soil surface. Large areas have becomesaline from irrigation and from saline seeps as aresult of summer fallowing. When sodium is animportant component of the salt, a significantamount of adsorbed sodium may occur and maydisperse soil colloids and develop undesirablephysical properties. Such soils are sodic. Thequantity, proportion, and nature of salts that arepresent may vary in saline and sodic soils. Thisgives rise to three kinds of soils: saline, sodic, andsaline-sodic.

Saline Soils Saline soils contain enough salt toimpair plant growth. White crusts of salt fre-quently accumulate at the soil surface. The elec-trical conductivity of the saturated-soil extract is 4or more dS/m. The pH is 8.5, or less, and less than15 percent of the adsorbed cations are sodium.

I RRIGATION 93

Soil permeability, or hydraulic conductivity, hasnot been adversely affected by adsorbed sodium.

Reclamation of saline soils requires the re-moval of salt by leaching and maintenance of thewater table far enough below the soil surface tominimize upward capillary water flow. Importantconsiderations are soil hydraulic conductivity andan adequate drainage system. It is not economicalto reclaim some saline soils that have high claycontent and a very low hydraulic conductivity.

Several practices are recommended for usingsaline soils. The soil should not be allowed to drycompletely, because the osmotic potential retardswater uptake. Sufficient water should be appliedwith each irrigation to result in leaching of somesalt in drainage water. Emphasis must be placedon the selection and growth of salt-tolerant crops.Considerable research effort is being directed to-ward the development of cultivars with greatersalt tolerance.

Sodic Soils Sodic soils have been called alkalisoils. Sodic soils are nonsaline and have an ad-sorbed or exchangeable sodium percentage(ESP) of 15 or more. The adsorbed cations (so-dium, calcium, magnesium, and potassium) areconsidered exchangeable, because they competefor adsorption on the negatively charged soil col-loids (clay and humus), are in motion, and ex-change sites with each other. The sodium ad-sorption ratio (SAR) of a soil-saturated pasteextract is about equal to the ESP. A SAR of 13 ormore is the same as ESP of 15 or more. The pH isusually in the range of 8.5 to 10.0. Sodium disso-ciates from the colloids and small amounts ofsodium carbonate form. Deflocculation or dis-persion of colloids occurs, together with abreakdown of the soil structure as shown in Fig-ure 6.16. This results in a massive or puddled soilwith low water infiltration and very low hydraulicconductivity within the soil. The soil is difficult totill, and soil crusting may inhibit seedling emer-gence. Organic matter in the soil disperses alongwith the clay, and the humus coats soil particles

94

FIGURE 6.16 Distintegrated soil structure in anirrigated field caused by high adsorbed sodium.(Photo courtesy Soil Conservation Service, USDA.)

to give a black color. Sodic soils may developwhen the leaching of a saline soil results in highexchangeable sodium and low exchangeable cal-cium and magnesium.

The basis for treatment of sodic soil is thereplacement of exchangeable sodium with cal-cium and the conversion of any sodium carbonateinto sodium sulfate. Finely ground gypsum (Ca-S04 -21-1²0) is broadcast and mixed with the soil.The calcium replaces or exchanges for the so-dium adsorbed on the negatively charged colloidsurfaces. The amount of gypsum needed toreplace the exchangeable sodium is the gypsumrequirement.

The calcium sulfate also reacts with any so-dium carbonate that is present to form sodiumsulfate and calcium carbonate. Then, leaching ofthe soil removes both the adsorbed sodium andthe sodium that was in the carbonate form, assoluble sodium sulfate. Gradually, the ex-changeable sodium percentage decreases to lessthan 15 percent, and the soil's structure and per-meability improve.


Saline-Sodic Soils Saline-sodic soils are char-acterized by salinity and 15 percent or more ofexchangeable sodium. As long as a large quantityof soluble salts remains in the soil, the exhange-able sodium does not cause dispersion of col-loids. The high concentration of ions in the soilsolution keeps the colloids flocculated. If the sol-uble salts are leached out, however, colloids maydisperse with an accompanying breakdown ofsoil structure and loss of water permeability. Themanagement of these soils is a problem until theexcess soluble salt has been removed and theexchangeable sodium level is reduced to below15 percent. If the soluble salts are removed byl eaching without lowering the exchangeable so-dium, saline-sodic soils may be converted in so-dic soils. Consequently, gypsum must be addedto the soil at the time leaching of salts is initiated.

WASTEWATER DISPOSALThe billions of gallons of wastewater produceddaily in the United States create a need for envi-ronmentally acceptable water disposal methods.The three major wastewater disposal techniquesare: (1) treatment and discharge into surface wa-ters, (2) treatment and reuse, and (3) land dis-posal via septic tanks and spray irrigation. Majorconsideration will be given to land disposal ofsewage effluent from septic systems and the man-agement of municipal wastewater that results inreturn of treated water to underlying aquifers forreuse.

Disposal of Septic Tank EffluentPeople who reside beyond the limits of municipalsewer lines have used septic sewage disposal sys-tems for many years. The expansion of rural resi-dential areas, and the development of summerhomes near lakes and rivers have greatly in-creased the need for waste disposal in rural areas.The sewage enters a septic tank, usually con-structed of concrete, where solid material is di-gested or decomposed. The liquid portion, or ef-

WASTEWATER DISPOSAL

fluent, flows out of the top of the septic tank andi nto drainage lines of a filter field. The drainagelines are laid in a bed of gravel into which theeffluent seeps and drains through the soil, as illus-trated in Figure 6.17.

The major factor influencing the suitability ofsoil for filter field use is the water-saturated hy-draulic conductivity. The hydraulic conductivity

2.of sands may be too high, because the sewageeffluent may move so rapidly that disease organ-isms are not destroyed before shallow-water sup-plies become contaminated. Soils with low hy-draulic conductivity are not suited for septic drainfields, because the sewage effluent may saturatesoil and contaminate the surface soil.

3.Most local regulations require that trained per-

sonnel make a percolation test before approvingthe installation of a septic system. The steps formaking a soil percolation test (perc test) are asfollows:

1. Dig six or more test holes about 15 cen-timeters in diameter and to the depth of theproposed drainage lines or seepage bed; the

4.

FIGURE 6.17 The layout of a septic tank and filter field for disposal ofwastewater (effluent) through the soil.

95

holes should be uniformly spaced over thearea selected for the filter field. Roughenthe sides of the holes to remove smeared soilthat might interfere with water entry into thesoil. Add 5 centimeters of sand or fine gravelto the bottom of the holes to reduce the soil'stendency to seal.Pour enough water into each hole so that thewater is at least 30 centimeters deep. Addwater as needed to keep the water depth to 30centimeters for at least 4 hours-preferablyovernight. This wets the soil and producesresults representative of the wettest season ofthe year.The next day, adjust the water level in the testholes so that the water is 30 centimeters deep.Measure the drop in water level over a 30-minute period and calculate the percolationrate.Soils with a percolation rate less than 2.5centimeters (1 in.) per hour are unsuited forfilter fields because of the danger of satura-tion of the soil above the filter bed with efflu-ent and the leakage of effluent from the soil

96

surface. Soils with percolation rates greaterthan 25 centimeters (10 in.) per hour are un-suited for filter fields because rapid move-ment of effluent might pollute groundwater.

Within the acceptable range of percolationvalues, increasing percolation rates result in de-creasing seepage-bed area. The more people liv-ing in a dwelling, the more effluent for disposal. Agraph is used to determine the area of seepagebed needed based on soil percolation rate andnumber of house occupants.

Several other factors are considered in selec-tion of filter field or seepage-bed sites. Theyshould be located at least 50 feet from any stream,open ditch, or other watercourse into which ef-fluent might escape, and the land slope should beless than 10 percent. Further, septic seepage bedsshould never be installed in poorly drained soilsor where soils become seasonally saturated withwater (see Figure 6.5). There is danger that shal-l ow groundwater may be contaminated with dis-ease organisms and that water may seep out of thesoil surface and contaminate work or play areas.


Land Disposal of Municipal WastewaterA relatively small community of 10,000 peoplemay produce about a million gallons of waste-water, or sewage effluent, per day from its sewagetreatment plant. The effluent looks much like ordi-nary tap water; when chlorinated, it is safe fordrinking. Discharging the effluent into a nearbystream does not create a health hazard. The ef-fluent, however, is enriched with nutrients, soplant growth may be greatly increased in streamsand lakes where the effluent is discharged. Weedgrowth in lakes and streams may reduce fish pop-ulations and interfere with boating andswimming. To combat these problems, research-ers at the Pennsylvania State University designedexperiments to use the soil as a living filter toremove the nutrients in sewage effluent. The wa-ter was applied with sprinklers, as shown in Fig-ure 6.18.

As the effluent water percolates through thesoil, nutrients are absorbed by plant roots andmicoorganisms. More water is applied than lostby evapotranspiration and the excess water per-colates from the bottom of the soil and is added to

FIGURE 6.18 Application ofwastewater (effluent) in aforest with a sprinkler systemin winter at Pennsylvania StateUniversity. (Photo courtesyUSDA.)

the underground water reservoir (aquifer). A com-munity of 10,000 people would need 50 hectares(about 125 acres) and a weekly application of 5centimeters of water to dispose of their effluent.Under these conditions about 80 percent of theeffluent water will contribute to aquifier rechargeand about 20 percent will be evapotranspired. Inthe Pennsylvania State University experiment, analmost complete removal of the nitrogen andphosphorus in the effluent occurred-the two nu-trients most closely tied to eutrophication. Fur-thermore, tree and crop growth on the land weregreatly stimulated.

Today, more and more cities are utilizing landdisposal of sewage effluent. In some areas, thetrees produced in the disposal area arechipped and used as fuel to dry and process thesolid sewage sludge. Marketing the sludge as afertilizer results in a considerable cost saving insewage disposal and conservation of nutrients.Additional benefits include the existence of a pro-tected forested or green zone near the communityand recharge of underground water aquifers. Suchwastewater management systems can be eco-

PRESCRIPTION ATHLETIC TURF

nomically and environmentally sound. An exam-ple of a wastewater land disposal system that usesnutrients for plant growth and return of water to anunderlying aquifer is shown in Figure 6.19.

PRESCRIPTION ATHLETIC TURFPrescription athletic turf (PAT) is a system of pro-ducing vigorous turf for use on athletic fields. Itcombines several aspects of water management,including irrigation and drainage, and was devel-oped at Purdue University, where football isplayed on natural grass turf.

The existing turf area is excavated to a depth ofabout 18 inches and the soil is removed. A plasticliner is laid on the newly exposed surface, 2-inchdiameter slitted plastic drain lines are laid on topof the plastic, and sensors for measuring waterpotential and salinity are installed. The mainfeatures of the PAT system are shown in Figure6.20. The sand layer forms the base for a 2-inch-thick surface layer of sand, peat, and soil. Thewater potential sensors maintain a thin layer of

FIGURE 6.19 Sewage-treatment system designed to apply wastewater to theland for crop and forest production. The water in excess of evaporationpercolates to the water table and contributes to the groundwater (aquifer).(Modified from Woodwell, 1977.)

97

98

water above the plastic liner. Grass roots are sup-plied by water that moves slowly upward by capil-larity. During periods of rain, pumps pull excesswater out of the soil so that the field is playablewith firm traction regardless of the weather. Thesalt sensors monitor the buildup of salts fromirrigation and fertilization. The sandy nature of thesoil resists compaction by traffic.

SUMMARYThe three approaches to water management onagricultural fields include the conservation of nat-ural precipitation in arid and subhumid regions,the removal of water (drainage) from wetlands,and irrigation to supplement natural precipi-tation.

Protection of the land surface by vegetation andplant residues shields the soil surface from rain-drop impact and encourages water infiltration.Summer fallowing results in storage of water forcrop production at a later time. Fertilizer use mayincrease the efficiency of water use by plants byincreasing the rate of plant growth.

Removal of water by drainage provides an aer-ated root zone for crops and dry soil for buildingfoundations. Upward from the surface of a water-saturated soil layer, there is an increase in the air


FIGURE 6.20 Diagram ofl ayout for prescription athleticturf, PAT. The drainage linesremove excess water in wetseason and are used forsubirrigation in dry seasons.Sensors monitor water andsalinity. (Data provided byPrescription Athletic Turf,Inc.)

content of soil, a decrease in the water content,and a decrease in the matric water potential.

Irrigation water contains soluble salts andsome provision (e.g., drainage) must be made tomaintain a favorable salt balance to keep irrigatedsoils permanently productive. Salt accumulationresults in formation of saline soils. The accumu-l ation of exchangeable sodium, greater thanabout 15 percent of the adsorbed cations, resultsi n the formation of sodic soils. High sodium satu-ration results in a deterioration of soil structureand a loss of soil water permeability. Sodic soilscan be improved by applying gypsum and thenleaching the soil.

Wastewater (sewage effluent) disposal on thel and results in removal of nutrient ions and returnof water to the underlying aquifers.

Prescription athletic turf is an artifical systemfor the production of vigorous grass on athleticfields by precise control of water, aeration,drainage, salinity, and nutrient supply.

REFERENCESBaker, K. F., ed. 1957. "The U. C. System for Producing

Healthy Container-Grown Plants." Ca. Agr. Exp. Sta.Manual 23. Berkeley.

Bender, W. H. 1961. "Soils Suitable for Septic TankFilter Fields." Agr. Information Bull. 243. USDA,

Washington, D.C.Bernstein, L. 1964. "Salt Tolerance of Plants." Agr Infor-

mation Bull. 283. USDA, Washington, D.C.Bernstein, L. 1965. "Salt Tolerance of Fruit Trees." Agr.

Information Bull. 292. USDA, Washington, D.C.Bower, C. A., 1974. "Salinity of Drainage Waters."

Drainage in Agriculture. Am. Soc. Agron. Madison.pp. 471-487.

Edminister, T. W. and R. C. Reeve. 1957. "DrainageProblems and Methods." Soil. USDA Yearbook of Ag-riculture. Washington, D.C., pp. 379-385.

Evans, C. E. and E. R. Lemon. 1957. "Conserving SoilMoisture." Soil. USDA Yearbook of Agriculture.Washington, D.C.

Hanks, R. J. and C. B. Tanner. 1952. "Water Consump-tion by Plants as Influenced by Soil Fertility. "Agron. J.44:99.

Jacobsen, T. and R. M. Adams. 1958. "Salt and Silt inAncient Mesopotamian Agriculture." Science.128:1251-1258.

Kardos, L. T. 1967. "Waste Water Renovation by theLand-A Living Filter." in Agriculture and the Qual-

REFERENCES 99

ity of Our Environment. AAAS Pub. 85, Washing-ton, D.C.

Kemper, W. D., T. J. Trout, A. Segeren, and M. Bullock.

1987. "Worms and Water." J. Soil and Water Conser-vation. 42:401-404.

Mathews, O. R. 1951. "Place of Summer Fallow in theAgriculture of the Western States." USDA Cir. 886.Washington, D.C.

Miller, D. E. and Richard O. Gifford. 1974. "Modification

of Soil Crusts for Plant Growth," in Arizona Agr. Exp.Sta. Tech. Bull. 214.

Soil Improvement Comm. Calif. Fert. Assoc. 1985.Western Fertilizer Handbook. 7th ed. Interstate, Dan-ville, Ill.

Terry, T. A. and J. H. Hughes. 1975. "The Effects ofIntensive Management on Planted Loblolly PineGrowth on the Poorly Drained Soils of the AtlanticCoastal Plain." Proc. Fourth North Am. Forest SoilsConf. Univ. Laval, Quebec.

United States Salinity Laboratory Staff, 1969. "Diagnosisand Improvement of Saline and Alkali Soils." USDAAgr. Handbook 60. Washington, D.C.

Woodwell, G. M. 1977. "Recycling Sewage throughPlant Communities." Am. Scientist 65:556-562.

Soil formation and soil erosion are two naturaland opposing processes. Many natural, undis-turbed soils have a rate of formation that is bal-anced by a rate of erosion. Under these condi-tions, the soil appears to remain in a constantstate as the landscape evolves. Generally, therates of soil erosion are low unless the soil surfaceis exposed directly to the wind and rainwater. Theerosion problem arises when the natural vegeta-tive cover is removed and rates of soil erosion aregreatly accelerated. Then, the rate of soil erosiongreatly exceeds the rate of soil formation andthere is a need for erosion control practices thatwill reduce the erosion rate and maintain soilproductivity.

Erosion is a three-step process: detachment fol-lowed by transportand deposition. The energy forerosion is derived from falling rain and the sub-sequent movement of runoff water or the wind.

WATER EROSIONSeveral types of water erosion have been iden-tified: raindrop (splash), sheet, rill, and gully orchannel. In addition, undercutting of stream

CHAPTER 7

SOIL EROSION

100

banks by water causes large masses of soil to fallinto the stream and be carried away. Very wet soilmasses on steep slopes are likely to slide slowlydownhill (solifluction).

Raindrops falling on bare soil detach particlesand splash them up into the air. When there islittle water at the soil surface, the water tends torun over the soil surface as a thin sheet, causingsheet erosion. Rill andgully erosion are due to theenergy of water that has concentrated and is mov-ing downslope. As water concentrates, it firstforms small channels-called rills-followed bygreater concentrations of water. Large concentra-tions of water lead to gully formation. Some of thehighest soil erosion rates occur at constructionsites where the vegetation has been removed andthe soil is exposed to the erosive effects of rain, asshown in Figure 7.1. The tendency of water tocollect into small rills that coalesce to form largechannels and produce gully erosion is also shownin Figure 7.1.

Predicting Water Erosion Rates onAgricultural LandThe rate of soil erosion on agricultural land isaffected by rainfall characteristics, soil erodibility,

FIGURE 7.1 Removal of the vegetative coveri ncreases water runoff and leaves soil unprotectedfrom the erosive effects of the rain, resulting inaccelerated soil erosion. As water concentrates inchannels, its erosive power increases. (Photographcourtesy Soil Conservation Service, USDA.)

slope characteristics, and vegetative cover and/ormanagement practices. The first plots in theUnited States for measuring runoff and erosion, asinfluenced by different crops, were established atthe University of Missouri in 1917 (see Figure 7.2).Since 1930, many controlled studies on field plotsand small watersheds have been conducted tostudy factors affecting erosion. Experiments wereconducted on various soils at many different loca-tions in the United States, using the same stan-dard conditions. For example, many erosion plotswere 72.6 feet long on 9 percent slopes and weresubjected to the same soil management practices.

WATER EROSION 101

These quantative data form the basis for pre-dicting erosion rates by using the Universal Soil-Loss Equation (USLE) developed by Wischmeierand Smith. The USLE equation is

where A is the computed soil loss per unit area astons per acre, R is the rainfall factor, K is thesoil-erodibility factor, L is the slope-length factor,S is the slope-gradient factor, C is the cropping-management factor, and P is the erosion-controlpractice factor. The equation is designed to pre-dict water erosion rates on agricultural land sur-faces, exclusive of erosion resulting from the for-mation of large gullies.

R = The Rainfall FactorThe rainfall factor (R) is a measure of the erosiveforce of specific rainfall. The erosive force, oravailable energy, is related to both the quantityand intensity of rainfall. A 5-centimeter rain fallingat 32 kilometers per hour (20 mph) would have 6million foot-pounds of kinetic energy. The tre-mendous erosive power of such a rain is sufficientto raise an 18-centimeter-thick furrow slice to aheight of 1 meter. The four most intense storms ina 10-year period at Clarinda, Iowa, accounted for40 percent of the erosion and only 3 percent of thewater runoff on plots planted with corn that weretilled up and down the slope. Anything thatprotects the soil from raindrop impact, such asplant cover or stones, protects the soil from ero-sion, as shown in Figure 7.3.

FIGURE 7.2 Site of the firstplots in the United States formeasuring runoff and erosionas influenced by differentcrops. The plots wereestablished in 1917 at theUniversity of Missouri.(Photograph courtesy Dr. C.M. Woodruff, University ofMissouri.)

102 SOIL EROSION

FIGURE 7.3 Columns of soil capped by stones thatabsorbed the energy of raindrop impact andprotected the underlying soil from erosion.

The rainfall factor (R) is the product of the totalkinetic energy of the storms times the maximum30-minute intensity of fall and modified by anyinfluence of snowmelt. Rainfall factors have beencomputed for many locations, with values rangingfrom less than 20 in western United States to 550along the Gulf Coast of the southeastern UnitedStates. An iso-erodent map of average annualvalues of R is shown in Figure 7.4.

K = The Soil Erodibility FactorSoil factors that influence erodibility by water are:(1) those factors that affect infiltration rate, per-meability, and total water retention capacity, and(2) those factors that allow soil to resist disper-

sion, splashing, abrasion, and the transportingforces of rainfall and runoff. The erodibility factor(K) has been determined experimentally for 23major soils on which soil erosion studies wereconducted since 1930. The soil loss from a plot72.6 feet long on a 9 percent slope maintained infallow (bare soil surface and with no cropplanted) and with all tillage up and down theslope, is determined and divided by the rainfallfactor for the storms producing the erosion. Thisis the erodibility factor. Values of K for 23 soils arelisted in Table 7.1.

LS = The Slope Length and SlopeGradient FactorsSlope length is defined as the distance from thepoint of origin of overland water flow either to a

TABLE 7.1

WATER EROSION

Computed K Values for Soils

point where the slope decreases to the extent thatdeposition occurs, or a point where water runoffenters a well-defined channel. Runoff from theupper part of a slope contributes to the totalamount of runoff that occurs on the lower part ofthe slope. This increases the quantity of waterrunning over the lower part of the slope, thusi ncreasing erosion more on the lower part of theslope than on the upper part. Studies have shownthat erosion via water increases as the 0.5 powerof the slope length (L ° 5). This is used to calculatethe slope length factor, L. This results in about 1.3times greater average soil loss per acre for everydoubling of slope length.

As the slope gradient (percent of slope) in-creases (S), the velocity of runoff water increases,which in turn increases the water's erosive power.A doubling of runoff water velocity increases ero-

From Wischmeier and Smith, 1965.a Evaluated from continuous fallow. All others were computed from row crop data.

103

104

sive power four times, and causes a 32-time in-crease in the amount of material of a given parti-cle size that can be carried in the runoff water.Splash erosion by raindrop impact causes a netdownslope movement of soil. Downward slopemovement also increases with an increase in theslope gradient. Combined slope length and gradi-ent factors (LS) for use in the soil-loss predictionequation are given in Figure 7.5.

C = The Cropping-Management FactorA vegetative cover can absorb the kinetic energyof falling raindrops and diffuse the rain's erosivepotential. Furthermore, vegetation by itself retainsa significant amount of the rain and slows the flowof runoff water. As a result, the presence or ab-sence of a vegetative cover determines whethererosion will be a serious problem. This is shownin Figure 7.6 where erosion on plots 72.6 feet longwith an 8 percent slope had about 1,000 timesmore erosion when in continuous corn as com-pared with continuous bluegrass.

The C factor measures the combined effect of

SOIL EROSION

all interrelated crop cover and management vari-ables including type of tillage, residue manage-ment, and time of soil protection by vegetation.Frequently, different crops are grown in sequenceor in rotation; for example, corn is grown one yearfollowed by wheat, which is then followed by ahay or meadow crop. This would be a three-yearrotation of corn-wheat-meadow (C-W-M). Thebenefit of a rotation is that a year of high erosion,during the corn production year, is averaged withyears of lower erosion to give a lower averageerosion rate, than if corn was planted every year.However, when corn is grown no-till, there maybe little soil erosion because of the protection ofthe soil by previous crop residues, as shown inFigure 7.7.

Complicated tables have been devised for cal-culating the C factor. As an illustration, the Cfactor for a 4-year rotation of wheat-alfalfa andbromegrass-corn-corn (W-AB-C-C) in central In-diana with conventional tillage, average residuemanagement, and average yields is 0.119. Somecomparison values are as low as 0.03 for cornmulched in no-till with 90 percent of the soil

FIGURE 7.5 Slope length andgradient graph for determining thetopographic factor, LS.

FIGURE 7.6 A continuous plant cover (e.g.,bluegrass) is many times more effective for erosioncontrol than cropping systems that leave the landbare most of the time. Continuous corn productionresulted in the longest amount of time with the soilunprotected by vegetation and the most erosion.

covered with crop residues and 0.48 for continu-ous corn with conventional tillage.

P = The Erosion Control Practice FactorI n general, whenever sloping land is cultivatedand exposed to erosive rains, the protection of-

WATER EROSION 105

fered by sod or closely growing crops in the sys-tem needs to be supported by practices that willslow runoff water, thus reducing the amount ofsoil carried. The most important of these prac-tices for croplands are contour tillage, stripcropping on the contour, and terrace systems.Contour tillage is tillage operations and plantingon the contour as contrasted to tillage up anddown the slope or straight across the slope. No-tillplanting of corn on the contour between terracesi n an Iowa field is shown in Figure 7.8.

Limited field studies have shown that contourfarming alone is effective in controlling erosionduring rainstorms of low or moderate intensity,but it provides little protection against the occa-sional severe storm that causes breakovers byrunoff waters of the contoured ridges or rows. Pvalues for contouring are given in Table 7.2, fromwhich it is apparent that contouring is the mosteffective on slopes of 2.1 to 7 percent. Stripcropping is the practice of growing two or morecrops in alternating strips along contours, or per-pendicular to surface water flow. Strip cropping,together with contour tillage, provides moreprotection. In cases where both strip croppingand contour tillage are used, the P values listed inTable 7.2 are divided by 2.

Terraces are ditches that run perpendicular to

FIGURE 7.7 No-till corn withthe residues of a previoussoybean crop providingprotection against soilerosion. (Photograph courtesySoil Conservation Service,USDA.)

106

the direction of surface water flow and collectrunoff water. The grade of the channel dependson the amount of runoff water. In areas of limitedrainfall and permeable soil, channels may have azero grade. In humid regions, the terrace channelusually is graded, and the runoff water that iscollected by the terrace is carried to an outlet fordisposal. Terraces are an effective way to reduceslope length. To account for terracing, the slopelength used to determine the LS factor shouldrepresent the distance between terraces. Shortslope length between terraces is shown in Fig-ure 7.8.

Application of the UniversalSoil-Loss EquationThe USLE was designed to predict the long-termerosion rates on agricultural land so that manage-ment systems could be devised that result in ac-

TABLE 7.2 Practice Factor Values for Contouring

From Wischmeier and Smith, 1965.

SOIL EROSION

FIGURE 7.8 Factorscontributing to soil erosioncontrol in this field includeno-till tillage on the contourwith terraces that greatlyreduce the effective length ofslope. (Photograph courtesySoil Conservation Service,USDA.)

ceptable levels of erosion. Having been devel-oped under the conditions found in the UnitedStates, one must be cautious in applying the equa-tion to drastically different conditions, such asexist in the tropics. The procedure for computingthe expected, average annual soil loss from aparticular field is illustrated by the following ex-ample.

Assume that there is a field in Fountain County,Indiana, on Russell silt loam. The slope has agradient of 8 percent and is about 200 feet long.The cropping system is a 4-year rotation of wheat-alfalfa and bromegrass-corn-corn (W-AB-C-C)with tillage and planting on the contour, and withcorn residues disked into the soil for wheat seed-i ng and turned under in the spring for second-yearcorn. Fertility and residue management on thisfarm are such that crop yields per acre are about100 bushels of corn, 40 bushels of wheat, and 4tons of alfalfa-bromegrass hay. The probability ofan alfalfa-bromegrass failure is slight, and there islittle risk that serious erosion will occur during thealfalfa-bromegrass year due to a lack of a goodvegetative cover.

The first step is to refer to the charts and tablesdiscussed in the preceding sections and to selectthe values of R, K, LS, C, and P that apply to thespecific conditions on this particular field.

The value of the rainfall factor (R) is taken fromFigure 7.4. Fountain County, in west-central In-diana, lies between isoerodents of 175 and 200.

By linear interpolation, R equals 185. Soil scien-tists consider that the soil erodibility factor (K) forRussell silt loam is similar to that for Fayette siltloam, being 0.38, according to Table 7.1.

The slope-effect chart, Figure 7.5, shows that,for an 8 percent slope, 200 feet long, the LS is 1.41.For the productivity level and management prac-tices, assumed in this example, the factor C for aW-AB-C-C rotation was shown earlier to be 0.119.Table 7.2 shows a practice factor of 0.6 for con-touring on an 8 percent slope.

The next step is to substitute the selected nu-merical values for the symbols in the soil erosionequation, and solve for A. In this example:

If planting had been up and downslope insteadof on the contour, the P factor would have been1.0, and the predicted soil loss would have been185 x 0.38 x 1.41 x 0.119 x 1.0 = 11.8 tons peracre (26.4 metric tons per hectare).

If farming had been on the contour and com-bined with minimum tillage for all corn in therotation, the factor C would have been 0.075. Thepredicted soil loss would then have been 4.5 tonsper acre (10.1 metric tons per hectare).

The Soil Loss Tolerance ValueThe soil loss tolerance value (T) has been definedi n two ways. This is an indication of the lack ofconcensus among soil scientists as to what ap-proach should be taken with T values or howmuch erosion should be tolerated. First, the Tvalue is the maximum soil erosion loss that isoffset by the theoretical maximum rate of soildevelopment, which will maintain an equilibriumbetween soil losses and gains. Shallow soils overhard bedrock have small T values. Erosion onsoils with claypans will result in shallower rootingdepths and in the need for smaller T values thanfor thick permeable soils, when both kinds ofsoils have developed from thick and water-permeable unconsolidated parent materials. The

WATER EROSION 107

emphasis is on the maintenance of a certain thick-ness of soil overlying any impermable layer thatmight exist.

Second, the T value is the maximum averageannual soil loss that will allow continuouscropping and maintain soil productivity withoutrequiring additional management inputs. Fertil-izers are used to overcome the decline in soilfertility caused by erosion.

On thick, water-permeable soils, such asMarshall silt loam in western Iowa, artificial re-moval of the surface soil reduced yields to lessthan one half of the crop yield obtained fromuneroded soil when no fertilizer was applied. Theapplication of fertilizer containing 200 pounds ofnitrogen per acre, however, resulted in similaryields of corn on both the normal soil and the soilwith the surface soil removed. The Marshall soil isa thick permeable soil that developed in a thicklayer of loess (parent material). Loess is a mate-rial transported and deposited by wind and con-sisting predominantly of silt-sized particles. Manysoils that have developed from thick sediments ofl oess are agriculturally productive.

There was some evidence that in years of mois-ture stress, the normal fertilized Marshall silt loamoutproduced the soil where the surface soil hadbeen removed and fertilized. As a result of thisand similar experiments, another approach wasdeveloped to establish Tvalues. This method con-siders the maximum soil loss consistent with themaintenance of productivity in terms of the main-tenance of a rooting depth with desirable physi-cal properties. Where subsoils have physicalproperties unsuitable for rooting, erosion resultsin reductions in soil productivity that cannot beovercome with only fertilizer application. Thedata in Figure 7.9 show that removing the surfacelayer from a fine-textured Austin soil in the Black-lands of Texas resulted in longtime or permanentreductions in soil productivity. Soil loss tolerancevalue guidelines from the Soil Conservation Ser-vice of the USDA are given in Table 7.3.

The data in Figure 7.6 show that average annualsoil loss with continuous bluegrass was 0.03 tons

108

FIGURE 7.9 Effect of removal of 38 centimeters oftopsoil on grain sorghum yields on Austin clay atTemple, Texas, 1961-1965. (Data from Smith et al.,1967.)

per acre and resulted in the removal of about 2.5centimeters (1 in.) of soil every 5,000 years. Obvi-ously, this soil loss is well below the T value forthe soil. On the other hand, continuous corn re-

SOIL EROSION

suited in a soil loss exceeding 50 tons per acre or2.5 centimeters (1 inch) every 2 to 3 years, a lossfar exceeding the Tvalue. In the Indiana case, thecalculated soil losses ranged from about 4 to 12tons per acre. The average annual rate of soilerosion on cropland land in the United States is 5tons per acre. It has been estimated that soil ero-sion is the most important conservation problemon 51 percent of the nation's cropland. In general,Tvalues in the United States range from 1 to 5 tonsper acre (2 to 11 metric tons per hectare). TheUSLE can be used to determine the managementrequired to maintain soil losses at or below acertain T value.

Water Erosion on Urban LandsHistoric soil erosion rates have changed as landuse has changed. Much of the land in the easternUnited States, which contributed much sedimentto streams and rivers a hundred or more yearsago, has been removed from cropping. Lower ero-sion rates resulted from abandonment of land forcropping and returning it to forest. In the past fewdecades, much of this land has experienced ahigh erosion rate because of urbanization, con-struction of shopping centers, roads, and housingareas, as shown in Figure 7.10.

TABLE 7.3 Guide for Assigning Soil Loss Tolerance Values to Soils withDifferent Rooting Depths

Data from USDA-SCS, 1973.° t Soils that have a favorable substratum and can be renewed by tillage, fertilizer, organicmatter, and other management practices.

$ Soils that have an unfavorable substratum, such as rock, and cannot be renewedeconomically.

WATER EROSION

Now, about 50 percent of the sediment instreams and waters throughout the country origi-nates on agricultural land; the other 50 percentoriginates on urban lands. It has been noted thatexposed land on steep slopes is very susceptibleto erosion. In fact, exposure of land during high-way and building construction commonly resultsin erosion rates many times those that typicallyoccur on agricultural land. Urban erosion rates asl arge as 350 tons per acre in a year have beenreported. It is important to plan construction sothat land will be exposed a minmum period oftime (see Figure 7.11).

Water Erosion CostsIn northern China, there is an extensive loess de-posit that is locally over 80 meters thick. Thisloess area is the major source of sediment for theHuang Ho (Yellow) River. This sediment is themajor parent material for the alluvial soils of thevast north-China plain, which supports millions ofpeople. The loessial landscape is very distinctive,with deeply entrenched streams and gullies. It hasbeen estimated that the loess will be removed byerosion in 40,000 years, which would be an an-nual soil loss of 15 tons per acre (33 metric tonsper hectare). This appears to be an excessive soilerosion rate.

The costs and benefits of the erosion and sedi-mentation are difficult to assess. In the short term,food production costs are increased in areas

109

where erosion occurs. Excessive water runoff andsoil erosion contribute to flooding and addition offresh sediments to the soils of the north-Chinaplain.

Soil erosion results in the preferential loss ofthe finer soil components that contribute the mostto soil fertility. Loss of organic matter, and thesubsequent reduction in nitrogen mineralization,are particularly detrimental to the production ofnonleguminous grain crops. As mentioned, thisloss can in some cases, be overcome by the addi-tion of nitrogen fertilizer on responsive soils.However, there is an increase in the cost of cropproduction.

Exposure of clay-enriched Bt horizons (be-cause of surface soil erosion) reduces water in-filtration and increases water runoff. This resultsin more soil erosion and retention of less water forplant growth. Seedbed preparation is more diffi-cult and the emergence of small plant seedlingsmay be reduced, resulting in a less than desirabledensity of plants. Summarization of data frommany studies showed that 2.5 centimeters (1 in.)of soil loss reduced wheat yield 5.3 percent, corn6.3 percent, and grain sorghum 5.7 percent. Someeffects of soil erosion on crop growth and produc-tion costs are given in Table 7.4. Increased pro-duction costs justify a long-term investment inerosion control measures to maintain erosion ator below acceptable T values.

Off-site costs result from sediment in water set-tling and filling reservoirs and navigation chan-

11 0

nels. The sediments adversely affect the ecologyof streams and lakes, as well as municipal watersupplies. In 1985 it was estimated that the cost ofsoil erosion resulting from all phases of pollution,both agricultural and urban, was $6 billion annu-ally in the United States. This is a loss greater thanthat estimated for the loss in soil productivity fromthe eroded land.

WIND EROSIONWind erosion is indirectly related to water conser-vation in that a lack of water leaves land barrenand exposed to the wind. Wind erosion is greatestin semiarid and arid regions. In North Dakota,wind-erosion damage on agricultural land wasreported as early as 1888. In Oklahoma, soil ero-sion was reported 4 years after breaking of thesod. Blowing dust was a serious hazard of earlysettlers on the Plains, and was one of the firstproblems studied by the Agricultural ExperimentStations of the Great Plains.

Types of Wind ErosionSoil particles move in three ways during winderosion: saltation, suspension, and surface creep.During saltation, fine soil particles (0.1 to 0.5 mm

SOIL EROSION

FIGURE 7.11 Slurry truckspraying fertilizer and seed onfreshly prepared seedbedalong road in Missouri. A thinl ayer of straw mulch will besecured with a thin spray oftar to protect the soil untilvegetation becomesestablished. (Photographcourtesy Soil ConservationService, USDA.)

in diameter) are rolled over the surface by directwind pressure, then suddenly jump up almostvertically over a short distance to a height of 20 to30 centimeters. Once in the air, particles gain invelocity and then descend in an almost straightline, not varying more than 5 to 12 degrees fromthe horizontal. The horizontal distance traveled bya particle is four to five times the height of itsjump. On striking the surface, the particles mayrebound into the air or knock other particles intothe air before coming to rest. The major part ofsoil carried by wind moves by this process. About93 percent of the total soil movement via windtakes place below a height of 30 centimeters.Probably 50 percent or more of movement occursbetween 0 and 5 centimeters above the soilsurface.

Very fine dust particles are protected from windaction, because they are too small to protrudeabove a minute viscous layer of air that clings tothe soil surface. As a result, a soil composed ofextremely fine particles is resistant to wind ero-sion. These dust-sized particles are thrown intothe air chiefly by the impact of particles moving insaltation. Once in the air, however, their move-ment is governed by wind action. They may becarried very high and over long distances via sus-pension.

Relatively large sand particles (between 0.5 and

TABLE 7.4 Effects of Erosion on Corn Growth and Production Costs

1.0 mm in diameter) are too heavy to be lifted bywind action, but they are rolled or pushed alongthe surface by the impact of particles during salta-tion. Their movement is by surface creep.

Between 50 and 75 percent of the soil is carriedby saltation, 3 to 40 percent by suspension, and 5to 25 percent by surface creep. From these facts, itis evident that wind erosion is due principally tothe effect of wind on particles of a size suitable tomove in saltation. Accordingly, wind erosion canbe controlled if: (1) soil particles can be built upi nto peds or granules too large in size to move bysaltation; (2) the wind velocity near the soil sur-face is reduced by ridging the land, by vegetativecover, or even by developing a cloddy surface;and (3) strips of stubble or other vegetative coverare sufficient to catch and hold particles movingi n saltation.

Wind Erosion EquationThe factors that affect wind erosion are containedin the equation to estimate soil loss by winderosion:

where: E equals the soil loss in tons per acre, Iequals the soil erodibility, K equals the soil rough-ness, C equals the climatic factor, L equals the

WIND EROSION 11 1

field length, and V equals the quantity of vegeta-tive cover. Wind erosion tolerance levels (Tvalues) have been established, just as for watererosion. The use of the wind erosion equationis more complex than that used for calculatingwater erosion losses. Details for use of thewind erosion equation are given in the NationalAgronomy Manual of the Soil Conservation Ser-vice, 1988.

Factors Affecting Wind ErosionSoil erodibility by wind is related primarily to soiltexture and structure. As the clay content of soilsincreases, increased aggregation creates clods orpeds too large to be transported. A study in west-ern Texas showed that soils with 10 percent clayeroded 30 to 40 times faster than soil with 25percent clay, as shown in Figure 7.12.

Rough soil surfaces reduce wind erosion byreducing the surface wind velocity and by trap-ping soil particles. Surface roughness can be in-creased with tillage operations, as shown in Fig-ure 7.13. Tillage and rows are positioned at rightangles to the dominant wind direction for maxi-mum effectiveness.

Climate is a factor in wind erosion via its effecton wind frequency and the velocity and wetnessof soil during high-wind periods. Wind erosion

11 2 SOIL EROSION

FIGURE 7.12 Amount of wind erosion in relation tothe amount of clay in the soil. (Data from Chepil etal., 1955.)

and blowing dust are persistent features of desertsi n which plants are widely spaced and dry soil isexposed most of time. When surface soils containa wide range in particle sizes, wind erosion

preferentially removes the finer particles, leavingthe larger particles behind. In this way a desert

pavement is created, consisting of gravel particlesthat are too large and heavy to be blown away.Consequently, deserts are an important source ofloess, which is composed mainly of silt-sized

particles. Furthermore, climate affects the amountof vegetative cover. During the Dust Bowl of the1930s, both wheat yields and wind erosion wererelated to rainfall. The lowest yields and highestsoil erosion rates occurred during years of leastrainfall.

Wind erosion increases from zero, at the edgeof an open field, to a maximum, with increasingdistance of soil exposed to the wind. As particlesbounce and skip, they create an avalanche effecton soil movement. Windbreaks of trees and al-ternate strips of crops can be used to reduce the

FIGURE 7.13 Contour ridges reduce wind velocity and collect much of theblowing soil. (Photograph courtesy soil Conservation Service, USDA.)

effective field length exposed to the wind. Finally,wind erosion is inversely related to the degree ofvegetative cover. Growing crops and the residuesof previous crops are effective in controlling winderosion. Stubble mulch tillage-tillage that leavesplant residues on the soil surface-is popular inthe wheat producing region of the Great Plains.

Deep Plowing for Wind Erosion ControlDeep plowing has been used to control wind ero-sion where sandy surface soils are underlain by Bthorizons that contain 20 to 40 percent clay. InKansas and Texas, plowing to control wind ero-sion must be deep enough to include at least Icentimeter of Bt horizon for every 2 centimeters ofthe surface soil (A horizon) thickness. In manyplaces, the soil must be plowed to a depth of 50 to60 centimeters. This kind of plowing increases theclay content of the surface soil by 5 to 12 percent,in some cases. Since 27 percent clay in the sur-face was required to halt soil blowing, deep plow-ing by itself resulted in only partial and temporarycontrol of wind erosion. When deep plowing wasnot accompanied by other soil erosion controlpractices, wind erosion removed clay from thesurface soil, and the effects of deep plowing weretemporary.

Wind Erosion Control on Organic SoilsOrganic soil areas of appreciable size are fre-quently protected from wind erosion by plantingtrees in rows-windbreaks. The use of trees aswindbreaks must be limited, however, because ofthe large amount of soil that they take out of cropproduction. A number of shrubs are also used; forexample, spirea is popular as a windbreak. Treesand shrubs also provide cover for wildlife.

The lightness of dry organic soils contributes tothe wind erosion hazard. Moist organic soil isheavier than dry organic soils, and particles tendto adhere more, reducing the wind erosion haz-ard. Accordingly, use of an overhead sprinklerirrigation system is helpful during windy periods,before the crop cover is sufficient to provideprotection from the wind. Rolling the muck soil

REFERENCES 11 3

with a heavy roller induces moisture to rise morerapidly by capillarity, thus dampening the soilsurface. The water table must be quite close to thesoil surface for this practice to be effective.

SUMMARYThe factors that affect soil erosion by water arerainfall, soil erodibility, slope length and gradient,crop management practices, and erosion controlpractices. The universal soil loss equation is usedto estimate soil erosion loss on agricultural land.

The permissible soil loss is the T value. Theuniversal soil loss equation can be used to deter-mine the soil management systems that will resultin acceptable erosion levels.

Soil erosion is greatly accelerated when thevegetative cover is removed. Thus, soil erosion isa serious problem in both rural and urban areas.

Cost of soil erosion includes reduced soil pro-ductivity and higher crop production costs. Offsitecosts are due to silting of watercourses and reser-voirs and reduced water quality.

Wind erosion is a major problem when sandysoils are used for crop production in regions withdry seasons, where the soil surface tends to beexposed to the wind because of limited vegetativecover. Wind erosion control is based on one ormore of the following:

1. Trapping soil particles with rough surfaceand/or the use of crop residues and stripcropping.

2. Deep plowing to increase clay content of sur-face soil and simultaneous use of other ero-sion control practices.

3. Protecting the soil surface with a vegetativecover.

REFERENCESAnonymous. 1982. Soil Erosion: Its Agricultural, Envi-

ronmental, and Socioeconomic Implications. Coun-cil for Agr. Sci. and Technology Report No. 92, Ames,Iowa.

11 4

Beasley, R. P. 1974. "How Much Does Erosion Cost?"Soil Survey Horizons. 15:8-9.

Browning, G. M., R. A. Norton. A. G. McCall, and F. G.Bell. 1948. "Investigation in Erosion Control and theReclamation of Eroded Land at the Missouri ValleyLoess Conservation Experiment Station." USDA Tech

Bull. 959, Washington, D.C.Burnett, E., B. A. Stewart, and A. L. Black. 1985. "Re-

gional Effects of Soil Erosion on Crop Productivity-Great Plains." Soil Erosion and Crop Productivity.American Society of Agronomy, Madison, Wis.

Chepil, W. S., N. P. Woodruff, and A. W. Zingg. 1955."Field Study of Wind Erosion in Western Texas."Kansas and Texas Agr. Exp. Sta. and USDA Washing-

ton, D.C.Colacicco, D., T. Osborn, and K. Alt. 1989. "Economic

Damage from Soil Erosion." J. Soil and Water Con.44:(1), 35-39.

Massey, H. F. and M. L. Jackson. 1952. "Selective Ero-sion of Soil Fertility Constitutents." Soil Sci. Soc. Am.Proc. 16:353-356.

Smith, R. M., R. C. Henderson, E. D. Cook, J. E. Adams,

SOIL EROSION

and D. 0. Thompson. 1967. "Renewals of DesurfacedAustin Clay." Soil Sci. 103:126-130.

Soil Conservation Service, 1988. National AgronomyManual. USDA, Washington, D.C.

Tuan, Yi-Fu. 1969. China. Aldine, Chicago.USDA-SCS. 1973. Advisory Notice, 6. Washington, D.C.Wischmeier, W. H. and D. D. Smith. "Predicting

Rainfall-Erosion Losses from Cropping East of theRocky Mountains." USDA Agr. Handbook 282. Wash-i ngton, D.C.

Wischmeier, W. H. and D. D. Smith. 1978. "PredictingRainfall Erosion Loss-A Guide to Conservation Plan-ning." USDA Agr. Handbook 537. Washington, D.C.

Woodruff, N. P. and F. H. Siddoway. 1965. "A WindErosion Equation." Soil Sci. Soc. Am. Proc. 29:602-608.

Wolman, M. G. 1967. "A Cycle of Sedimentation andErosion in the Urban River Channels." Geog. Ann.49A:385-395.

Zobeck, T. M., D. W. Fryrear, and R. D. Petit. 1989."Management Effects on Wind-eroded Sediment and

Plant Nutrients." J Soil Water Con. 44:160-163.

The soil is the home of innumerable forms ofplant, animal, and microbial life. Some of thefascination and mystery of this underworld hasbeen described by Peter Farb:

We live on the rooftops of a hidden world. Be-neath the soil surface lies a land of fascination,and also of mysteries, for much of man's wonderabout life itself has been connected with the soil.It is populated by strange creatures who havefound ways to survive in a world without sunlight,an empire whose boundaries are fixed by earthenwalls.

Life in the soil is amazingly diverse, rangingfrom microscopic single-celled organisms tolarge burrowing animals. As is true with organ-isms above the ground, there are well-definedfood chains and competition for survival. Thestudy of the relationships of these organisms inthe soil environment is called soil ecology.

* Peter Farb, Living Earth, Harper and Brothers Publishers,

New York, 1959.

Im

CHAPTER 8

SOIL ECOLOGY

115

THE ECOSYSTEMThe sum total of life on earth, together with theglobal environments, constitutes the ecosphere.

The ecosphere, in turn, is composed of numerousself-sustaining communities of organisms andtheir inorganic environments and resources,called ecosystems. Each ecosystem has its ownunique combination of living organisms andabiotic resources that function to maintain a con-tinuous flow of energy and nutrients.

All ecosystems have two types of organismsbased on carbon source: (1) producers, and (2)the consumers and decomposers. The producersuse (fix) inorganic carbon from carbon dioxide,and are autotrophs. The consumers and the de-composers use the carbon fixed by the producers,such as glucose, and are heterotrophs.

ProducersThe major primary producers are vascular plantsthat use solar energy to fix carbon from carbondioxide during photosynthesis. The tops of plantsprovide food for animals above the soil-

11 6

atmosphere interface. Plants produce roots, tu-bers, and other underground organs within thesoil that serve as food for soil-dwelling organisms.A very small amount of carbon is fixed from car-bon dioxide by algae during photosynthesis thatoccurs at or near the soil surface. Some bacteriaobtain their energy from chemical reactions, che-

moautotrophs, and fix a tiny amount of carbonfrom carbon dioxide. The material produced bythe producers serves as food for the consumersand decomposers.

Consumers and DecomposersConsumers are, typically, animals that feed onplant material or on other animals. For example,very small worms invade and eat living roots. Theworms might be eaten or consumed by miteswhich, in turn might be consumed by centipedes.Eventually, all organisms die and are added to thesoil, together with the waste material of animals.

All forms of dead organic materials are attackedby the decomposers, mainly by bacteria andfungi. Through enzymatic digestion (decom-position), the carbon is returned to the atmo-sphere as C0 2 and energy is released as heat. Thenutrients in organic matter are mineralized andappear as the original ions that were previouslyabsorbed by plant roots. The result is that themajor function of the consumers and decom-posers is the cycling of nutrients and energy. Inthis context it is appropriate to consider soil as the

stomach of the earth. Without decomposers torelease fixed carbon, the atmosphere would be-come depleted of C0 2 , life would cease, and thecycle would stop.

MICROBIAL DECOMPOSERSAll living heterotrophs, animals and microor-ganisms, are both consumers and decomposers.They are consumers in the sense that they con-sume materials for growth and, at the same time,they are decomposers in the sense that carbon isreleased as C02 . Mineralization is the conversion

SOIL ECOLOGY

of an element from an organic form to an inor-ganic state as a result of microbial activity. Anexample is the conversion of nitrogen in proteininto nitrogen in ammonia (NH3). The microor-ganisms are considered to be the major or ulti-mate decomposers. They are the most closelyassociated with the ultimate release of the energyand nutrients in decomposing materials and inmaking the nutrients available for another cycle ofplant growth. Only about 5 percent of the primaryproduction of green plants is consumed byanimals. By contrast, about 95 percent of the pri-mary production is ultimately decomposed by mi-croorganisms.

General Features of DecomposersThe decomposers secrete enzymes that digest or-ganic matter outside the cell, and they absorb thesoluble end products of digestion. Different en-zymes are excreted, depending on the decom-posing material. For example, the enzyme cellu-lase is excreted to decompose cellulose and theenzyme protease is excreted to decomposeprotein. These enzymes and processes are similarto those that occur in the digestive system ofanimals.

The microorganisms near plant roots share thesame environment with roots, and they competewith each other for the available growth factors.When temperature, nutrients and water supply,pH, and other factors are favorable for plantgrowth, the environment is generally favorable forthe growth of microorganisms. Both absorb nutri-ent ions and water from the same soil solution.Both are affected by the same water potentials,osmotic effects of salts, pH, and gases in the soilatmosphere. Both may be attacked by the samepredators. On the other hand, plants and microor-ganisms also play roles that benefit each other.Roots slough off cells and leak organic com-pounds that serve as food for microorganisms,whereas the microbes decompose the organicmaterials, which results in the mineralization ofnutrient ions for root absorption.

In the case of symbiotic nitrogen fixation, the

nitrogen-fixing organisms obtain energy and foodfrom the plant while the plant benefits from thenitrogen fixed. Nitrogen fixation is the conversionof molecular nitrogen (N 2) to ammonia and sub-sequently to organic combinations or to formsutilizable in biological processes. The net effectof the fixation is the transfer of nitrogen in theatmosphere, which is unavailable to plants, toplants and soils where the nitrogen is available.

The major distinguishing feature of microor-ganisms is their relatively simple biological orga-nization. Many are unicellular, and even multi-cellular organisms lack differentiation into celltypes and tissues characteristic of plants andanimals. They form spores, or a resting stage,under unfavorable conditions and germinate andgrow again when conditions become favorable.

BacteriaBacteria are single celled, among the smallestliving organisms, and exceed all other soil organ-isms in kinds and numbers. A gram of fertile soilcommonly contains 10' to 10 10 bacteria. The mostcommon soil bacteria are rod-shaped, a micron(1 /25,000 of an inch) or less in diameter, and upto a few microns long (see Figure 8.1). Research-ers have estimated that the live weight of bacteriain soils may exceed 2,000 kilograms per hectare(2,000 pounds per acre). The estimated weightsand numbers of some soil organisms are given inTable 8.1.

Most of the soil bacteria are heterotrophs andrequire preformed carbon (such as carbon in or-ganic compounds). Most of the bacteria areaerobes and require a supply of oxygen (0 2) in thesoil atmosphere. Some bacteria are anaerobicand can thrive only with an absence of oxygen inthe soil atmosphere. Some bacteria, however, arefacultative aerobes. They thrive well in an aerobicenvironment but they can adapt to an anaerobicenvironment.

Bacteria normally reproduce by binary fission(the splitting of a body into two parts). Somedivide as often as every 20 minutes and may multi-

MICROBIAL DECOMPOSERS 11 7

FIGURE 8.1 Rod-shaped bacteria magnified 20,000times. (Courtesy Dr. S. Flegler of Michigan StateUniversity).

ply very rapidly under favorable conditions. If asingle bacterium divided every hour, and everysubsequent bacterium did the same, 17 millioncells would be produced in a day. Such a rapidgrowth rate cannot be maintained for a long pe-riod of time because nutrients and other growthfactors are exhausted and waste products accu-mulate and become toxic. In general, the growthof the decomposers is chronically limited by thesmall amount of readily decomposable substratethat is available as a food source. Most of theorganic matter in soils is humus; organic materialthat is very resistant to further enzymatic decom-position.

FungiFungi are heterotrophs that vary greatly in size andstructure. Fungi typically grow or germinate fromspores and form a threadlike structure, called the

11 8

mycelium. Whereas the activity of bacteria is lim-ited to surface erosion in place, fungi readily ex-tend their tissue and penetrate into the surround-ing environment. The mycelium is the workingstructure that absorbs nutrients, continues togrow, and eventually produces reproductivestructures that contain spores, as shown in Fig-ure 8.2.

It is difficult to make an accurate determinationof the number of fungi per gram of soil, becausemycelia are easily fragmented. Mycelial threads orfragments have an average diameter of about 5microns, which is about 5 to 10 times that ofbacteria. A gram of soil commonly contains 10 to100 meters of mycelial fragments per gram. Onthis basis, the live weight of fungal tissue in soilsis about equal to that of bacteria (see Table 8.1).

The most common fungi are molds and mush-rooms. Mold mycelia are commonly seen growingon bread, clothing, or leather goods. Rhizopus isa common mold that grows on bread and in soil. Ithas rootlike structures that penetrate the breadand absorb nutrients, much like the roots ofplants. The structure on the surface of the breadforms the working structure, or mycelium, fromwhich fruiting bodies that contain spores develop.

FIGURE 8.2 A soil fungus showing mycelium andreproductive structures that contain spores.(Photograph courtesy of Michigan State UniversityPesticide Research Electron Microscope Laboratory.)

SOIL ECOLOGY

Some molds grow on plant leaves and producewhite cotton-colored colonies that produce a dis-ease called downy mildew. Mushroom fungi havean underground mycelium that absorbs nutrientsand water, and an above-ground mushroom thatcontains reproductive spores. Many mushroomsare collected for food, such as the shaggy-manemushroom shown in Figure 8.3.

Fungi are important in all soils. Their tolerancefor acidity makes them particularly important inacidic forest soils. The woody tissues of the forestfloor provide an abundance of food for certainfungi that are effective decomposers of lignin. Inthe Sierra Nevada Mountains, there is a largemushroom fungus, called the train wrecker fun-gus, because it grows abundantly on railroad tiesunless the ties are treated with creosote.

ActinomycetesActinomycetes refers to a group of bacteria with asuperficial resemblance to fungi. The actino-mycetes resemble bacteria in that they have a verysimple cell structure and are about the same sizei n cross section. They resemble filamentous fungii n that they produce a branched filamentous net-work. The network compared to fungi, however, isusually less extensive. Many of these organismsreproduce by spores, which appear to be verymuch like bacterial cells.

Actinomycetes are in great abundance in soils,as shown in Table 8.1. They make up as much as50 percent of the colonies that develop on platescontaining artificial media and inoculated with asoil extract. The numbers of actinomycetes mayvary from 1 to 36 million per gram of soil. Al-though there is evidence that actinomycetes areabundant in soils, it is generally concluded theythat are not as important as bacteria and fungi asdecomposers. It appears that actinomycetes aremuch less competitive than the bacteria and fungiwhen fresh additions of organic matter are addedto soils. Only when very resistant materials re-main do actinomycetes have good competitiveability.

SOIL ANIMALS

TABLE 8.1 Estimates of Amount of Organic Matter and Proportions, DryWeight, and Number of Living Organisms in a Hectare of Soil to a Depth of 15Centimeters in a Humid Temperate Region

Vertical Distribution of Decomposers inthe SoilThe surface of the soil is the interface between thelithosphere and the atmosphere. At or near thisinterface, the quantity of living matter is greaterthan at any region above or below. As a conse-quence, the A horizon contains more organicdebris or food sources than do the B and C hori-zons. Although other factors besides food supplyi nfluence activity and numbers of microor-ganisms, the greatest abundance of decomposerstypically occurs in the A horizon (see Figure 8.4).

Adapted and reprinted by permission from Soil Genesis and Classification by S. W. Buol,F. D. Hole, and R. J. McCracken, copyright© 1972 by Iowa State University Press, Ames,I owa 50010.

SOIL ANIMALS

Soil animals are numerous in soils (see Table7.1). Soil animals can be considered both con-sumers and decomposers because they feed on orconsume organic matter and some decom-position occurs in the digestive tract. Animals,however, play a minor consumer-decomposerrole in organic matter decomposition. Someanimals are parasitic vegetarians that feed onroots, whereas others are carnivores that prey oneach other.

11 9

120

FIGURE 8.3 Mushroom fungal caps that containspores-an edible type.

WormsThere are two important kinds of worms in soils.Microscopic roundworms, nematodes, are veryabundant soil animals. They are of economic im-portance because they are parasites that invadeliving roots. The other important worm is the ordi-nary earthworm.

Earthworms Earthworms are perhaps the bestknown of the larger soil animals. The commonearthworm, Lumbricus terrestris, was imported

SOIL ECOLOGY

into United States from Europe. This worm makesa shallow burrow and forages on plant material atnight. Some of the plant material is dragged intothe burrow. In the soil, the plant materials aremoistened and more readily eaten. Other kinds ofearthworms exist by ingesting organic matterfound in the soil. Earthworms eat their waythrough the soil by ingesting the soil en masse.Excreted materials (earthworm castings) are de-posited both on and within the soil. Althoughcastings left on the soil surface of lawns are objec-tionable, earthworms feed on thatch, thus' helpingto prevent thatch buildup in turf grass. The inti-mate mixing of soil materials in the digestive trackof earthworms, the creation of channels, and pro-duction of castings, alter soil structure and leavethe soil more porous. Channels left open at thesoil surface greatly increase water infiltration.

Earthworms prefer a moist environment with anabundance of organic matter and a plentiful sup-ply of available calcium. As a result, they tend tobe least abundant in dry and acidic sandy soils.They normally avoid water-saturated soil. If theyemerge during the day, when it is raining, they arekilled by ultraviolet radiation unless they quicklyfind protection. Obviously, the number and activ-ity of earthworms vary greatly from one location toanother. Suggestive numbers of earthworms in anAp horizon vary from a few hundred to more thana million per acre. Estimated weights of earth-worms are 100 to 1,000 pounds per acre (kilo-grams per hectare). Attempts to increase earth-

FIGURE 8.4 Distribution ofmicroorganisms in the A, B,and C horizons of a cultivatedgrassland soil. All values referto the number of organismsper gram of air-dry soil. (Datafrom Iowa Res. Bul. 132.)

worm activities in soils have been disappointing,because the numbers present probably representan equilibrium population for the existing condi-tions.

Nematodes Nematodes (roundworms) are mi-croscopic worms and are the most abundantanimals in soils. They are round shaped with apointed posterior. Under a 10-power hand lensthey appear as tiny transparent threadlike worms,as illustrated in Figure 8.5. Most nematodes arefree-living and inhabit the water films that sur-round soil particles. They lose water readilythrough their skin, and when soils dry or otherunfavorable conditions occur, they encyst, orform a resting stage. Later, they reactivate whenconditions become favorable. Based on food pref-erences, nematodes can be grouped into thosethat feed on: (1) dead and decaying organic mat-ter, (2) living roots, and (3) other living organismsas predators.

Parasitic nematodes deserve attention owing totheir economic importance in agriculture. Manyplants are attacked, including tomatoes, peas,carrots, alfalfa, turf grass, ornamentals, corn,soybeans, and fruit trees. Some parasitic nema-todes have a needlelike anterior end (stylet) usedto pierce plant cells and suck out the contents.Host plants respond in numerous ways: plants,for example, develop galls or deformed roots. In

SOIL ANIMALS 121

the case of a root vegetable such as carrots, themarket quality is seriously affected.

I nvestigations indicate that nematode damageis much more extensive than previously thought.Nematodes are a very serious problem for pineap-ple production in Hawaii, where the soil for a newplanting is routinely fumigated to reduce nema-tode populations. Parasititic nematodes attack awide variety of plants throughout the world, re-ducing the efficiency of plant growth, analogousto worms living in the digestive track of animals.Agriculturally, the most important role of nema-todes in soils appears to be economic, as para-sitic consumers.

ArthropodsA high proportion of soil animals is arthropods;

they have an exoskeleton and jointed legs. Mosthave a kind of heart and blood system, and usu-ally a developed nervous system. The most abun-dant arthropods are mites and springtails. Otheri mportant soil arthropods include spiders, insects(including larvae), millipedes, centipedes, woodlice, snails, and slugs.

Springtails Springtails are primitive insectsless than 1 millimeter (2s in.) long. They are dis-tributed worldwide and are very abundant (seeTable 8.1). Springtails have a springlike ap-

FIGURE 8.5 A nematode anda piece of ordinary cottonthread photographed at thesame magnification (270). Thenematode is ,'55 as thick as thethread and is not visible to thenaked eye. (Courtesy H. H.Lyon, Plant PathologyDepartment, CornellUniversity.)

122

pendage under their posterior end that permitsthem to flit or spring in every direction. If the soilin which they are abundant is disturbed and bro-ken apart, they appear as small white dots dartingto and fro. They live in the macropores of the litterlayers and feed largely on dead plant and animaltissue, feces (dung), humus, and fungal mycelia.Water is lost rapidly through the skin, so they arerestricted to moist layers. Springtails in the lowerlitter layers are the most primitive and are withouteyes and pigment.

Springtails are in the order Collembola, andthey are commonly called Collembola. Their ene-mies include mites, small beetles, centipedes,and small spiders. Collembola that were grown inthe laboratory, and are in various stages of devel-opment, are shown in Figure 8.6.

Mites Mites are the most abundant air-breathing soil animals. They commonly have asaclike body with protruding appendages and arerelated to spiders. They are small, like Collem-bola; a mite photographed on the top of a pinheadis shown in Figure 8.7.

Some mites are vegetarians and some are carni-vores. Most feed on dead organic debris of allkinds. Some are predaceous and feed on nema-

FIGURE 8.6 Springtails and eggs photographedthrough a light microscope.

SOIL ECOLOGY

FIGURE 8.7 Electron microscope photograph of amite on the top of a pinhead; magnified about 50 to100 times. (Photograph courtesy Michigan StateUniversity Pesticide Research Laboratory.)

todes, insect eggs, and other small animals, in-cluding springtails. Activities of mites include thebreakup and decomposition of organic material,movement of organic matter to deeper soil layers,and maintenance of pore spaces (runways).

Millipedes and Centipedes Millipedes andcentipedes are elongate, fairly large soil animals,with many pairs of legs. They are common inforests, and overturning almost any log or stonewill send them running for cover. Millipedes havemany pairs of legs and are mainly vegetarians.They feed mostly on dead organic matter, butsome browse on fungal mycelia. Centipedes typi-cally have fewer pairs of legs than millipedes, andare mainly carnivorous consumers. Centipedeswill attack and consume almost any-sized animalthat they can master (see Figure 8.8). Worms are afavorite food of some centipedes. The data inTable 8.1 show that the numbers of millipedesand centipedes are small compared withspringtails and mites, but that their biomass maybe larger.

White Grubs Some kinds of insect larvae playamore important role in soils than do the adults as

FIGURE 8.8 A centipede, a common carnivoroussoil animal.

does, for example, the white grub. White grubsare larvae of the familar May beetle or june bug.The grubs are round, white, about 2 to 3 cen-timeters long, and curl into a C shape when dis-turbed. The head is black and there are three pairsof legs just behind the head. White grubs feedmainly on grass roots and cause dead spots inl awns. A wide variety of other plants are alsoattacked, making white grubs an important agri-cultural pest. Moles feed on insect larvae andearthworms, resulting in a greater likelihood ofmole damage in lawns when white grubs arepresent.

Ants and termites are important soil insects andwill be considered later in relation to the earth-moving activities of soil animals.

I nterdependence of Microbes andAnimals in DecompositionMicroorganisms and the animals work together asa decomposing team. When a leaf falls onto theforest floor, both micoorganisms and animals at-tack the leaf. Holes made in the leaf by mites andspringtails facilitate the entrance of microor-ganisms inside the leaf. Soil animals also breakup organic debris and increase the surface areafor subsequent attack by bacteria and fungi. Soilanimals ingest bacteria and fungi, which continueto function within the digestive tracks of the

NUTRIENT CYCLING

animals. The excrement of animals is attacked bymicrobes and ingested by animals. Thus, materialis subjected to decomposition in several stagesand in widely different environments. Organicmatter, and the soil en masse, may be ingested byearthworms, thereby producing an intimate mix-ing of organic and mineral matter. Again, diges-tion results in some decomposition and, ulti-mately, the material is excreted as castings. Thenet result of these activities is the mineralizationof elements in organic matter, conversion of car-bon to C0 2 , and release of energy as heat. Themajor credit for nutrient mineralization and recyl-ing of nutrients and energy, however, goes to themicrobes (bacteria and fungi).

NUTRIENT CYCLINGNutrient cycling is the exchange of nutrient ele-ments between the living and nonliving parts ofthe ecosystem. Plants and microbes absorb nutri-ents and incorporate them into organic matter,and the microbes (with aid of animals) digest theorganic matter and release the nutrients in min-eral form. Nutrient cycling conserves the nutrientsupply and results in repeated use of the nutrientsin an ecosystem.

Nutrient Cycling ProcessesTwo simultaneous processes, mineralization andimmobilization, are involved in nutrient cycling.Immobilization is the uptake of inorganic ele-ments (nutrients) from the soil by organisms andconversion of the elements into microbial andplant tissues. These nutrients are used for growthand are incorporated into organic matter. Mineral-ization is the conversion of the elements in or-ganic matter into mineral or ionic forms such asNH 3 ,

Cat-,H2P0 4

- , S0 42- , and K+. These ions

then exist in the soil solution and are available foranother cycle of immobilization and mineral-ization.

Mineralization is a relatively inefficient process

123

124

in that much of the carbon is lost as CO 2 andmuch of the energy escapes as heat. This typicallyproduces a supply of nutrients that exceeds theneeds of the decomposers; the excess of nutrientsreleased can be absorbed by plant roots. Only inunusual circumstances is there serious compe-tition between the plants and microbes for nu-trients.

A Case Study of Nutrient CyclingOne approach to the study of nutrient cycling is toselect a small watershed and to measure changesin the amounts and locations of the nutrients overtime. An example is a 38-acre (15-hectare) water-shed in the White Mountains of New Hampshire.In this mature forest, the amount of nutrientstaken up from the soil approximated the amountof nutrients returned to the soil. For calcium, thiswas 49 kilograms per hectare (44 lb per acre)taken up from the soil and returned to the soil, asillustrated in Figure 8.9. About 9 kilograms of cal-cium per hectare were added to the system bymineral weathering and 3 kilograms per hectarewere added in the precipitation. These additionswere balanced by losses due to leaching and run-off. This means, in effect, that 80 percent of thecalcium in the cycle was recycled or reused by theforest each year. Plants were able to take up 49kilograms of calcium per hectare, whereas only12 kilograms were added to the system annually.Researchers concluded that northern hardwoodforests have a remarkable ability to hold and torecirculate nutrients.

The data in Table 8.2 show that 70 to 86 percentof four major nutrients absorbed from the soilwere recycled by return of the nutrients in leavesand wood to the litter layer each year. Without thisextensive recycling, the productivity of the forestswould be much lower. Nutrient cycling accountsfor the existence of enormous forests on somevery infertile soils. In some very infertile soils,most of the nutrients exist in the organic matterand are efficiently recycled.

SOIL ECOLOGY

FIGURE 8.9 Calcium cycling in a hardwood forestin New Hampshire in kilograms per hectare per year.(Data from Bormann and Likens, 1970.)

Effect of Crop Harvesting onNutrient CyclingThe grain and stalks (stover) of a corn (Zeamaize) crop may contain nitrogen equal to morethan 200 kilograms per hectare (200 lb per acre). Itis not uncommon for farmers in the Corn Beltregion of the United States to add this amount ofnitrogen to the soil annually to maintain highyields. According to the data in Table 8.2, by con-trast, only 10 kilograms of nitrogen per hectarewere stored annually in wood or removed fromthe system by tree growth in a beech forest. Natu-rally occurring nitrogen fixation and the additionvia precipitation can readily supply this amount ofnitrogen. Basically, food production represents

SOIL MICROBE AND ORGANISM INTERACTIONS

TABLE 8.2 Annual Uptake, Retention, and Return of Nutrients to the Soil ina Beech Forest

nutrient harvesting. The natural nutrient cyclingprocesses cannot provide enough nutrients togrow high-yielding crops. Yields of grain in manyof the world's agricultural systems stabilize atonly 500 to 600 kilograms per hectare (8 to 10bushels per acre) where there is little input ofnutrients as manure or fertilizer.

When the agricultural system is based on feed-i ng most of the crop production to animals, as indairying, most of the nutrients in the feed areexcreted by the animals as manure. On the aver-age, 75 to 80 percent of the nitrogen, 80 percent ofthe phosphorus, and 85 to 90 percent of the po-tassium in the feed that is eaten by farm animals isexcreted as manure. Crops can be harvested withonly a modest drain on the nutrients in the cycle ifthe nutrients in the manure are returned to thel and. Careful management of manure and the useof legumes to fix nitrogen, together with lime toreduce soil acidity, were the most important prac-tices used to develop a prosperous agriculture indairy states like New York and Wisconsin.

Specialization in agriculture has resulted invery large crop and animal farms. Today, manylarge livestock enterprises have more manure,and nutrients, than they can dispose of by applica-

Stoeckler, J. H., and H. F. Arneman, "Fertilizers in Forestry," Adv. in Agron. 12:127-195,1960. By permission of author and Academic Press.

12 5

tion of manure to the land. Waste disposal is animportant problem. Conversely, large crop farmswithout animals have no manure to apply to theland, so they balance the nutrient cycle deficitwith the application of fertilizer.

SOIL MICROBE ANDORGANISM INTERACTIONSThe role of soil organisms in nutrient cycling hasbeen stressed, and it has been shown that someorganisms are parasites that feed on plant roots.In this section the specialized roles of microor-ganisms living near and adjacent to plant rootswill be considered.

The RhizospherePlant roots leak or exude a large number of or-ganic substances into the soil. Sloughing of rootcaps, and other root cells, provide much neworganic matter. More than 25 percent of the photo-synthate produced by plants may be lost from theroots to the soil. These substances are food formicroorganisms, and they create a zone of in-

126

tense biological activity near the roots in an areacalled the rhizosphere. The rhizosphere is thezone of soil immediately adjacent to plant roots inwhich the kinds, numbers, or activities of micro-organisms differ greatly from that of the bulk soil.Although many kinds of organisms inhabit therhizosphere, it appears that bacteria are benefitedthe most. The bacteria appear mainly in coloniesand may cover 4 to 10 percent of the root surface.The intimate association of bacteria and rootsmay result in mutually beneficial or detrimentaleffects. Nutrient availability may be affected andplant-growth-stimulating or toxic substances maybe produced. Since roots tend to grow better insterile soil inoculated with organisms than in ster-ile soil that is not inoculated, it appears that thenet effect of bacteria growing in the rhizosphere isbeneficial.

DiseaseSoils may contain organisms that cause bothplant and animal disease. Bacteria are responsi-ble for wilt of tomatoes and potatoes, soft rots of anumber of vegetables, leaf spots, and galls. Somefungi cause damping-off of seedlings, cabbageyellows, mildews, blight, and certain rusts. Thecatastrophic potato famine in Ireland from 1845 to1846 was caused by a fungus that produced po-tato blight. Certain species of actinomycetescause scab in potatoes and sugar beets. The soilused for bedding and greenhouse plants is rou-tinely treated with heat or chemicals to kill plantpathogens.

The organisms that produce disease in humansmust be able to tolerate the soil environment, atleast for a period long enough to cause infection.Many disease organisms occur in fecal matter andhuman sewage, such as the virus that producesinfectious hepatitis and the protozoan (amoeba)that produces amebic dysentery. When materialscontaining animal disease organisms are addedto soil, the organisms encounter a very differentenvironment than the one they occupied in ananimal. Consequently, many animal disease or-

SOIL ECOLOGY

ganisms quickly die when added to soils. Somehuman disease organisms, however, are very per-sistent. The disposal of human feces by back-packers in wilderness areas poses a health threat,because some organisms are not readily killed byshallow burial. There is also danger of causingdisease in humans when human feces are usedfor fertilizing gardens or fields, because of resis-tant spore forms of some disease organisms. InChina, much of the human excrement is used forfertilizer, but this material is composted for a suf-ficiently long period of time to kill the diseaseorganisms before the compost is applied to thefields.

Historically, human excrement (night soil) hasplayed an important role in the maintenanceof soil fertility. In 1974 the value of the nitro-gen, phosphorus, and potassium in the humanexcreta in India was estimated to be worth morethan $700 million as compared with the cost of thenutrients as fertilizer. In countries where most ofthe food is consumed by humans, careful use ofhuman excrement can be very helpful in main-taining soil fertility. In highly industrialized soci-eties, the nutrients are disposed of through sew-age systems and lost from the food productionenterprise. The low cost of energy means that thecost of nutrients in the form of fertilizer is lessexpensive than processing waste to recover andreuse the nutrients.

MycorrhizaSome fungi infect the roots of most plants. Fortu-nately, most of the fungi form a symbiotic rela-tionship-one that benefits both fungi and higherplants. After the appropriate fungal spores germi-nate, hyphae, or small segments of the mycelium,invade young roots and grow a mycelium both onthe exterior and interior of roots. Fungal hyphaeon the exterior of roots serve as an extension ofroots for water and nutrient absorption. Theseexternal hyphae function as root hairs and arecalled mycorrhiza. Mycorrhiza literally means"fungus roots."

There are two types of hypha or mycorrhiza:ectotrophic and endotrophic. The ectotrophic hy-phae exist between the epidermal cells, usingpectin and other carbohydrates for food. Contin-ued growth of the hyphae outside the root resultsin the formation of a sheath (mantle) that com-pletely surrounds the root, as shown in Figure8.10. Literally thousands of species belong in thegroup of fungi that produce ectotrophic mycor-rhiza and produce mushroom or puffball fruitingbodies. They are associated typically with treesand, thus are beneficial to trees, as illustrated inFigure 8.11.

Endomycorrhiza are more abundant than ecto-mycorrhiza. They benefit most of the field andvegetable crop plants. Hyphae invade roots andramify both between and within cells, usuallyavoiding the very central core of the roots. Thehost plants provide the fungi with food. The bene-fits to the host plant include: (1) increased effec-

SOIL MICROBE AND ORGANISM INTERACTIONS

FIGURE 8.10 Ectomycorrhiza. (a) Uninfected pine root. (b) I nfected rootwith fungal mantle. (c) Diagram of the anatomy. The Hartig net is the hyphaebetween root cells. (Courtesy D. H. Marx, University of Georgia.)

tive surface area of roots for the absorptionof water and nutrients, particularly phosphorus;(2) increased drought and heat resistance; and(3) reduced infection by disease organisms.

Nitrogen FixationWe live in a "sea" of nitrogen, because the atmo-sphere is 79 percent nitrogen. In spite of this,nitrogen is generally considered the most limitingnutrient for plant growth. Nitrogen exists in the airas N 2 and, as such, is unavailable to higher plantsand most soil microbes. There are some speciesof bacteria (nitrogen fixers) that absorb N 2 gasfrom the air and convert the nitrogen into ammo-nia that they and the host plant can use. Thisprocess of nitrogen fixation is symbiotic. The bac-teria obtain food from the host plant and the hostplant benefits from the nitrogen fixed. The bac-teria responds to and invades the roots of the host

127

128

FIGURE 8.11 Effect of mycorrhizal fungi on thegrowth of 6-month-old Monterey pine seedlings.(a) Fertile Prairie soil. (b) Fertile Prairie soil plus 0.2percent by weight of Plainfield sand from a forest.(c) Plainfield sand alone. Only plants of b and c werei nfected with mycorrhiza. (Photograph courtesy of thelate Dr. S. A. Wilde, University of Wisconsin.)

plant, and the host responds by forming a nodulethat surrounds the bacteria, and in which nitrogenis fixed, as shown in Figure 8.12.

In aquatic ecosystems, blue-green algae fix ni-trogen. They are the land counterpart by being themajor means by which nitrogen from the atmo-sphere is added to aquatic ecosystems. A detailedconsideration of nitrogen fixation occurs in Chap-ter 12, where the nitrogen cycle is discussed.

SOIL ORGANISMS ANDENVIRONMENTAL QUALITYDuring the billions of years of organic evolution,organisms evolved that could decompose thecompounds naturally synthesized in the environ-ment. Today, humans have become importantcontributors of both natural and synthetic com-pounds to the environment. The problems of pes-ticide degradation and contamination of soilsfrom oil spills will be considered as they relate toenvironmental quality. The application of sewagesludge to the land is covered in Chapter 15.

SOIL ECOLOGY

Pesticide DegradationPesticides include those substances used to con-trol or eradicate insects, disease organisms, andweeds. One of the first successful synthetic pesti-cides, DDT, was used to kill mosquitoes for ma-laria control. After decades of use it was found inthe cells of many animals throughout the world.This dramatized the resistance of DDT to biode-gradation and its persistence in the environment.The structure of DDT appears to be different fromnaturally occurring compounds and, therefore,few if any organisms have enzymes that candegrade DDT. The difference in structure and de-gradability of 2,4-D and 2,4,5-T is illustrated inFigure 8.13.

Generally, the pesticides used have almost nosignificant effect on soil microbes. The soil mi-crobes attack the organic pesticides similar totheir attack of other organic substrates. In someinstances, the microbes detoxify the pesticide, forexample, by the shifting of chlorine atoms. Inother cases, the pesticide is metabolized or de-graded. In general, those pesticides that resistdetoxification and/or degradation and persist inthe environment long after they are applied, arecalled hard pesticides. The others that degradeeasily are soft pesticides. The challenge for scien-tists is to develop pesticides that can perform theiruseful function and disappear from the ecoystemwith minimal undesirable side effects.

Oil and Natural Gas DecontaminationMany species of microbes can decompose petro-l eum compounds. Crude oil and natural gas are agood energy source, or substrate, for many mi-crobes, and their growth is greatly stimulated.

The first result of an oil spill or natural gas leakis the displacement of soil air and creation of ananaerobic soil. Any vegetation is likely to be killedby a lack of soil oxygen. The reducing conditionsresult in large increases in available iron andmanganese. It is suspected that toxic levels ofmanganese for higher plants are produced insome cases. In time, the oil or natural gas is

FIGURE 8.12 Symbiotic nitrogen-fixing nodules on the roots of soybeanroots (left), and on sweet clover roots (right). (Photographs courtesy NitraginCompany.)

FIGURE 8.13 Structure representing 2,4-D on thel eft and 2,4,5-T on the right. The structures are verysimilar except for the additional C1 at the metaposition of 2,4,5-T, which is metabolized with greatdifficulty (if at all) by soil microorganisms.

SOIL ORGANISMS AND ENVIRONMENTAL QUALITY 129

decomposed and the soil returns to near normalconditions. A study of 12 soils exposed to con-tamination near Oklahoma City showed that or-ganic matter and nitrogen contents had been in-creased about 2.5 times. The increased nitrogensupply in soils that had been contaminated proba-bly explains the increased crop yields obtainedon old oil spills.

Natural gas leaks are common in cities. Thedestruction of grass and trees is due to a lack ofnormal soil air, which is displaced by the naturalgas (methane). Sometimes the problem can bediagnosed by detecting the odor of a pungentcompound in the natural gas that escapes from acrack in a nearby sidewalk or driveway. Because

130

methane (CH4) is odorless, an odor has beenadded to public gas supplies.

EARTH MOVING BY SOIL ANIMALSAll soil animals participate as consumers andplay a minor role in the cycling of nutrients andenergy. Many of the larger animals move soil tosuch an extent that they affect soil formation.

Earthworm ActivityEarthworms are probably the best known earthmovers. Darwin made extensive studies andfound that earthworms may deposit 4 to 6 metrictons of castings per hectare (10 to 15 tons peracre) annually on the soil surface. This couldresult in the buildup of a 2.5-centimeter-thicklayer of soil every 12 years. This activity producesthicker than normal, dark-colored surface layersin soils. Stones and artifacts are buried over time,which is a subject of great interest to archae-ologists.

As a result of their earth-moving activities,earthworms leave channels. Where these chan-nels are open at the soil surface, they can tranportwater very rapidly into and through the soil. Ear-lier it was reported that infiltration of water fromirrigation furrows increased after earthworms hadburrowed to and entered the irrigation furrows.Renewed interest in the role of earthworms hasbeen created in regard to no-till systems, whichleave plant residues on the soil surface. The resi-dues provide a source of food for the earthworms,which forage on the soil surface, and the residuesand growing plants protect the soil surface fromraindrop impact so that the channels remain openand functional for water infiltration.

Ants and TermitesThe activities of ants and termites are, perhaps,more important than the activities of earthworms.Ants transport large quantities of material from

SOIL ECOLOGY

within the soil, depositing it on the surface. Someof the largest ant mounds are about 1 meter talland more than 1 meter in diameter. The effect ofthis transport is comparable to that of earthwormsin creating thick A horizons and in burying objectslying on the soil surface.

A study of ant activity on a prairie in southwest-ern Wisconsin showed that ants brought materialto the surface from depths of about 2 meters andbuilt mounds about 15 centimeters high and morethan 30 centimeters in diameter (see Fig. 8.14).Furthermore, it was estimated that 1.7 percent ofthe land was covered with ant mounds. Assumingthat the average life of a mound is 12 years, theentire land surface would be reoccupied every600 years. The researchers believe that the earthmovement activity of ants resulted in a greaterthan normal content of clay in the A horizon,because of the movement to the surface of mate-rial from a Bt horizon, a clay-enriched B horizon.

Ants also harvest large amounts of plant mate-rial and in some ecosystems are important con-sumers. Their gathering of seeds retards naturalreseeding on range lands. Leaf harvester antsmarch long distances to cut fragments of leavesand stems and bring them to their nests to feedfungi. The fungi are used as food. Organic matteris incorporated into the soil depths and nutrientsconcentrate in the nest sites.

Termites inhabit tropical and subtropical soils.They exhibit great diversity in food and nestinghabits. Some feed on wood, some feed on organicrefuse, and others actively cultivate fungi. Pro-tozoa (single-celled microscopic animals) in thedigestive tract of many termites aid in the diges-tion of woody materials. Some species build hugenests up to 3 meters high and several meters indiameter. Most mounds are on a smaller scale.Some termites have nests in the soil and tunnnelson the surface to permit foraging for food. Soilmaterial is brought to the surface from depths asgreat as 3 meters. Termites have had an enormouseffect on many tropical soils, where their earth-moving activity has been occurring for hundredsof thousands of years.

FIGURE 8.14 Ant (Formica cinera) in a Prairie soili n southwestern Wisconsin. Upper photo shows antmounds more than 15 centimeters high and morethan 30 centimeters in diameter. The lower sketchshows soil horizons and location of ant channels;numbers refer to the number of channels observed atthe depth indicated. (Photographs courtesy Dr. F. D.Hole, Wisconsin Geological and Natural HistorySurvey, University of Wisconsin.)

In summary, ants and termites create channelsi n soils and transport soil materials to such anextent that soil horizons are greatly affected andsometimes obliterated. A concentration of nutri-

SUMMARY

ents occurs at the nest sites, which is why somefarmers in Southeast Asia make use of the highersoil fertility in areas occupied by mounds.

RodentsMany rodents, including mice, ground squirrels,marmots, gophers, and prairie dogs inhabit thesoil. A characteristic microrelief called mimamounds, which consists of small mounds ofearth, is the work of gophers in Washington andCalifornia. Mima mounds occur on shallow soils.They appear to be the result of gophers buildingnests in the dry soil near the tops of the mounds.Successive generations of gophers at the samesite create mounds that range from 0.5 to 1 meterhigh and more than 5 to 30 meters in diameter.

Extensive earth moving by paririe dogs hasbeen documented. An average of 42 mounds perhectare (17 per acre) were observed near Akron,Colorado. Each mound consisted of an average of39 tons of soil material. The upper 2 to 3 meters ofthe soil formed from loess, a silty wind-depositedmaterial, and was underlain by sand and gravel.All of the mounds observed contained sandand gravel that had been brought up fromdepths of more than 2 meters. Prairie-dogactivity had changed the surface soil texture fromsilt loam to loam on one third of the area.Abandoned burrows filled with dark-coloredsurface soil, called crotovinas, are common ingrasslands.

SUMMARYHigher plants are the major producers contribut-ing to the supply of soil organic matter. The mi-croorganisms (bacteria and fungi) are the majordecomposers and are mainly responsible for thecycling of nutrients and energy in soil eco-systems.

Soil animals play a minor role in the cycling ofnutrients and energy, but play an important role inearth-moving activities.

131

132

Nutrient cyling results in reuse of the nutrientsin an ecosystem. Nutrients are efficiently recycledin natural ecosystems. Interference of the cycle,such as cropping and removal of nutrients infood, results in reduced soil fertility. Manures andfertilizers are used to maintain soil fertility in agri-culture.

Soil organisms and higher plants engage inmany interactions related to disease, mycorrhiza,and nitrogen fixation, and soil organisms andhigher plants compete for the same growthfactors.

A zone adjacent to plant roots with a high popu-l ation of microorganisms is the rhizosphere.

Microorganisms play important environmentalquality roles, such as detoxification of chemicalsand decomposition of oil from spills.

Earthworms, ants, termites, and rodents movel arge quantities of soil and may greatly alter thenature of soil horizons.

REFERENCESAlexander, M. 1977. Soil Microbiology, 2nd ed. John

Wiley, New York.Arkley, R. J. and H. C. Brown. 1954. "The Origin of Mima

Mound Microrelief in the Far Western States." SoilSci. Soc. Am. Proc. 18:195-199.

Barley, K. P. 1961. "The Abundance of Earthworms inAgricultural Land and Their Possible Significance in

SOIL ECOLOGY

Agriculture." Adv. in Agronomy. 13:249-268. Aca-demic Press, New York.

Baxter, F. P. and F. D. Hole. 1967. "Ant Pedoturbation ina Prairie Soil." Soil Sci. Soc. Am. Proc. 31:425-428.

Bormann, F. F. and G. E. Likens. 1970. "The NutrientCycles of an Ecosystem." Sci. Am. 223:92-101.

Bowen, E. 1972. "The High Sierra." Time, Inc., NewYork

Buol, S. W., F. D. Hole, and R. J. McCracken. 1973. SoilGenesis and Morphology. I owa State UniversityPress, Ames.

Ellis, R., Jr. and R. S. Adams, Jr. 1961. "Contaminationof Soils by Petroleum Hydrocarbons." Adv. inAgronomy. 13:197-216. Academic Press, New York.

Flaig, W., B. Nagar, H. Sochtig, and C. Tietjen. 1977.Organic Materials and Soil Productivity. FAO SoilsBull. 35. United Nations, Rome.

Richards, B. N. 1974. Introduction to the Soil Ecosys-tem. Longmans, New York.

Thorp, J. 1949. "Effect of Certain Animals that Live inSoils." Sci. Monthly. 42:180-191.

Schaller, F. 1968. Soil Animals. University of MichiganPress, Ann Arbor.

Smucker, A. J. M. 1984. "Carbon Utilization and Lossesby Plant Root Systems." Roots, Nutrient and WaterFlux and Plant Root Systems. ASA Spec. Pub. 49, SoilSci. Soc. Am., Madison.

Stoeckeler, J. H. and H. F. Arneman. 1960. "Fertilizers inForestry." Adv. in Agronomy. 12:127-195.

Temple. K. L. A. K. Camper, and R. C. Lucas. 1982."Potential Health Hazard from Human Wastes in Wil-derness." J. Soil and Water Con. 37:357-359.

Walters, E. M. P. 1960. Animal Life in the Tropics.George Allen and Unwin, London.

CHAPTER 9

SOIL ORGANIC MATTER

Almost all life in the soil is dependent on organicmatter for nutrients and energy. People have longrecognized the importance of organic matter forplant growth. Although organic matter in soils isvery beneficial, Justus von Liebig, a famous Ger-man chemist, pointed out about 150 years agothat soils composed entirely of organic matter arenaturally infertile. This chapter considers the ori-gin, nature, and importance of the organic matteri n soils.

THE ORGANIC MATTER IN ECOSYSTEMSThe organic matter in an ecosystem consists ofthe organic matter above and below the soil sur-face. The distribution of organic matter in a pon-derosa pine forest ecosystem is shown in Table9.1. The distribution of organic matter, expressedas organic carbon, was 38 percent in the trees andground cover and 9 percent in the forest floor. Theremaining 53 percent of the organic matter was inthe soil and included the roots plus the organicmatter associated with soil particles. In the forest

133

there is a continual growth of plants and additionsto these three pools of organic matter: standingcrop, forest floor, and soil. These three pools oforganic matter represent a supply of nutrients forfuture growth.

In grassland ecosystems, much more of theorganic matter is in the soil and much less occursin the standing plants and grassland floor. Al-though approximately 50 percent of the total or-ganic matter in the forest ecosystem may be in thesoil, over 95 percent may be in the soil, wheregrasses are the dominant vegetation.

DECOMPOSITION AND ACCUMULATIONThe primary or original source of soil organicmatter (SOM) is the production of the primaryproducers-the higher plants. This organic mate-rial is subsequently consumed and decomposedby soil organisms. The result is the decom-position and the accumulation of organic matterin soils that has great diversity and a highly vari-able composition.

134

TABLE 9.1 Quantity and Distribution of Organic Carbon (organic matter) ina Ponderosa Ecosystem in Arizona

Adapted from Klemmedson, 1975.

Decomposition of Plant ResiduesThe plant residues, including tops and roots, con-tain a wide variety of compounds: sugar, cellu-lose, hemicellulose, protein, lignin, fat, wax, andothers. Observe from Table 9.2 that the composi-tion of SOM is distinctly different than that of theoriginal plant material added to the soil.

In an experiment, wheat straw was added tosoil and the changes in the major components inthe straw were followed over time. It was foundthat proteins, the soluble fraction, and the cellu-lose and hemicellulose disappeared or decom-posed very rapidly, whereas lignin decomposedvery slowly, as shown in Figure 9.1a. As a result ofthe rapid decomposition of proteins and the solu-

TABLE 9.2 Partial Composition of Mature PlantTissue and Soil Organic Matter

SOIL ORGANIC MATTER

Organic Carbon

ble fraction, cellulose and hemicellulose, andother readily degradable compounds, there was acorresponding and rapid increase in microbialproducts (see Figure 9.1b).

Microbial products include living and dead mi-crobial cells and their waste or excretion prod-ucts. Some of the organic compounds that aresynthesized in the soil during decomposition re-act with each other and with mineral soil com-ponents. Consequently, the decomposition ofplant residues results in: (1) the production ofa considerable mass of microbial products, and(2) the production of a wide variety of materials ofvarying resistance to decomposition. As a resultof the addition and decomposition of plant andanimal residues in soils, labile and stable frac-tions of organic matter are produced. Labile or-ganic matter is organic matter that is liable tochange or is unstable. The liable and stable or-ganic matter fractions correspond, in general, tothe organic residues and humus fractions, respec-tively.

Labile Soil Organic MatterThe labile fraction of SOM consists of any readilydegradable materials from the plant and animalresidues, and readily degradable microbial prod-

ucts. The labile organic matter composes about10 to 20 percent of the total SOM. Labile organicmatter is an important reservoir of nutrients be-cause the nutrients, especially nitrogen, are rap-i dly recycled in the soil ecosystem. The additionto the soil of a relatively small amount of easilydegraded organic matter, such as manure, or-ganic waste materials, and plant residues, maycontribute more to the available nutrient supplythan the slow release of nutrients from the muchlarger amount of stable organic matter. The labileorganic matter is rapidly degraded when condi-tions are favorable. As a result, any labile organicmatter incorporated within soils tends to disap-pear quickly if conditions are favorable for de-composition. Therefore, the decomposers are fre-quently quite inactive, owing to the rapiddisappearance of readily decomposable sub-strates or labile organic matter.

Stable Soil Organic MatterThe stable SOM fraction consists of resistant com-pounds: (1) in the decomposing residues, (2) inmicrobial products, and (3) that formed as a re-sult of interaction of organic compounds witheach other and with the mineral components ofthe soil, especially the clay. The stable organicmatter is equivalent to humus. The nutrientswithin it are recycled very slowly. Radioactive

DECOMPOSITION AND ACCUMULATION 135

FIGURE 9.1 Decompositionof wheat straw in soil in thelaboratory. (a) Decreases overtime: proteins and solubles(C1), cellulose andhemicellulose (C2), and lignin(C3). (b) Changes over time inamount of carbon in originalplant material and inmicrobial products. (Datafrom Van Veen and Paul,1981.)

studies have shown that certain stable SOM frac-tions are commonly more than a few hundred orthousands of years old. The stable organic matterhas a long residence time in the soil and playsi mportant roles in structure formation and sta-bility, water adsorption, and adsorption of nutri-ent cations.

Stable soil organic matter, or humus, originatesfrom several processes. Lignin, a 6-carbon ringstructure, is a plant compound and is very resis-tant per se. Thus, lignin accumulates in soils be-cause of its resistant or stable nature. Organiccompounds of varying resistance interact with lig-nin and form new compounds that are resistant.This accounts for the significant quantity ofprotein or proteinlike material in humus (see Ta-ble 9.2).

Much of the stable organic matter is intimatelyassociated with the mineral particles. As such,stable organic matter plays an important role inthe formation of soil structure and the ability ofsoil peds to resist crushing and breakdown fromtillage and raindrop impact. Studies have shownthat some readily decomposable materials(sugars and proteins) are decomposed by mi-crobes in less than a day under laboratory condi-tions. When the same materials are added tosoils, the decomposition time is greatly in-creased. The adsorption of organic compoundsonto soil, particles makes these compounds less

136

decomposable. Scientists believe that the organicmatter is protected from decomposition; thisprotection helps to increase the stability of bothreadily decomposable and resistant organic mat-ter. Much of the humus is associated intimatelywith clay and silt particles, and the humus playsan important role in formation of soil structure, asshown in Figure 9.2.

Polysaccharides include microbial gum andtwo other abundant compounds found in micro-bial cell walls: hemicellulose and cellulose.These polysaccharides are organic polymers (gi-ant organic moleclules formed by the combina-tion of smaller molecules) having high molecularweight and are present as ropes and nets. Thesepolymers are widely and intimately associatedwith the soil fabric. On drying, soil particle sur-faces are brought into very close contact with thenetlike arrangement of the organic compounds.The hydroxyls of the polymers and the exposedoxygen atoms of the mineral soil particles bindstrongly to each other. The result is an intimate

FIGURE 9.2 Some arrangements of organic matterand mineral soil particles. Organic matter intimatelyassociated with mineral particles is protected fromdecomposition and contributes to the formation andstability of soil structure. (a) Quartz-organicmatter-quartz. (b) Quartz-organic matter-clay.(c) Organic matter-clay-organic matter.

SOIL ORGANIC MATTER

association of mineral and organic matter thatprotects the organic matter from decompositionand forms stable soil peds. One half or more ofthe SOM is likely to be protected.

Decomposition RatesAlthough SOM has been characterized as havingtwo forms-labile and stable-there is great vari-ety in the nature and decomposition rates of vari-ous organic fractions of the SOM. In a nativegrassland soil, about 9 percent of the SOM.wereshown to consist of roots and plant residues and 4percent consists of unprotected and decomposa-ble liable organic matter with a decompositionrate of 8 percent per day. This rate, which as-sumes optimal conditions, is shown in Table 9.3.Soil organisms were 1.3 percent of the total or-ganic matter with a decomposition rate of 3 per-cent per day.

The other fractions of the SOM, 84 percent, hadmuch slower decomposition rates. The bulk ofthe SOM (54 percent) consisted of decomposableorganic matter that was protected by attachmentto clays and other mineral components, and had adecomposition rate of 0.08 percent per day. Twofractions, composed mainly of ligneous material,decomposed at 0.0008 and 0.000,008 percent perday, for unprotected and protected ligneous mate-rials, respectively. Thus, the decomposition rateof the most readily decomposable material isabout 1 million times that of the most stable (seeTable 9.3). The most resistant organic matter frac-tions have been called recalcitrant, meaningstubborn.

Assuming that plant residues, roots, decom-posable unprotected organic matter, and soil or-ganisms are liable organic matter, the data inTable 9.3 show that the soil contained about 15percent liable organic matter and 85 percent sta-ble organic matter.

Properties of Stable Soil Organic MatterStable soil organic matter (humus) is a heteroge-neous mixture of amorphous compounds that areresistant to microbial decomposition and possess

TABLE 9.3 Soil Organic Matter and Decomposition Rates under OptimumConditions for a Native Grassland Soil

Adapted from Van Veen and Paul, 1981.

a large surface area per gram. The large surfacearea enables humus to absorb water equal tomany times its weight.

During decomposition of plant and animalsresidues, there is a loss of carbon as carbon diox-ide and the conservation and reincorporation ofnitrogen into microbial products, which eventu-ally are incorporated into humus. As a conse-quence, humus contains about 50 to 60 percentcarbon and about 5 percent nitrogen, producing aC : N ratio of 10 or 12 to 1. The more decomposedthe humus is the lower or narrower will be theC : N ratio. Compared with plant materials, hu-mus is relatively rich in nitrogen and, when miner-alized, humus is a good source of biologicallyavailable nitrogen. Humus is also a significantsource of sulfur and phosphorus. The ratio ofC:N:P:S is about 100:10:1:1.

Humus contains phenolic groups, 6-C ringswith OH, that dissociate H + at pH above 7.0.During humus formation there is an increase incarboxyl groups, R-COOH, and these groups tendto dissociate H+ at pH less than 7:

R-COOH = R-C00 - + H+

As a result, humus has a considerable amount of

DECOMPOSITION AND ACCUMULATION

Organic carbon, upper 16 inches

137

negative charge. Cations in the soil solution tendto be weakly adsorbed onto the negativelycharged phenolic and carboxyl groups. However,adsorbed cations can be readily exchanged byother cations remaining in solution. The total neg-ative charge of the SOM, or ability to adsorb ex-changeable cations, is called the cation exchangecapacity (CEC). The CEC of organic matter actssimilarly to that of clay particles, in that the chargeadsorbs and holds nutrient cations in position tobe available to plants, while effectively holdingthese cations against loss by leaching. The mostabundant nutrient ions adsorbed are Ca.²1 , M g ²+ ,

and K+ . Adsorbed H + affects the pH of the soilsolution, depending on the amount adsorbed andthe degree of dissociation into the soil solution.

Clay and SOM are the source of most of thenegative charge in soils. The sum of the CEC of theSOM plus the CEC of the clay is generally con-sidered equivalent to the CEC of the total soil. Thesource and amount of CEC of soil clays are dis-cussed in Chapters 10 and 11.

Protection of Organic Matter by ClayThe amount of organic matter present in a soil atany given time is the net effect of the amount that

138

has been added minus the amount that has beendecomposed. The major factors affecting the SOMcontent are soil texture, vegetation type, watercontent or aeration of the soil, and soil temper-ature.

Locally, within a given landscape, a correlationhas been found between soil texture and the or-ganic matter content of soils as a result of the roleof the soil matrix, especially clay, in protectingorganic matter from decomposition. Organic mat-ter is positively correlated with the clay contentand negatively correlated with the sand content insome grassland soils in Wyoming (see Figure9.3). These soils had a mean average soil temper-ature of 8 to 15° C (47 to 59° F).

In the same study, soils with lower averagetemperature showed a weaker relationship be-tween organic matter content and texture. Thissuggests that low soil temperature, by decreasingthe rate of decomposition, appeared to have hadan important effect on the organic matter contentof the soils. The data in Figure 9.4 are generallyrepresentative of the plains of the United Statesbetween Canada and Mexico, and show that highprecipitation at low temperature results in muchmore organic matter in soils as compared to highprecipitation and high temperature. In most local(farm) situations, where temperature is a con-stant, clay content is positively associated withorganic matter content. Thus, soils with apprecia-

SOIL ORGANIC MATTER

FIGURE 9.4 Model of soil nitrogen variation of theGreat Plains from Canada to Mexico (back to frontcurves) and from the desert to the humid regions(left to right curves). The NSQ is the precipitationdivided by the absolute saturation deficit of air; it is ameasure of the effectiveness of the precipitation.Decreases in temperature and increases in humidityare associated with increases in total soil-nitrogencontent. (From Jenny and used by permission ofSpringer-Verlag, New York, copyright (D 1980.)

ble clay content tend to have a higher content oforganic matter and, therefore tend to have a goodcombination of soil fertility and physicalproperties.

FIGURE 9.3 Relationshipsbetween the content oforganic matter to a depth of40 centimeters and the clayand sand content of someWyoming grassland soils.(Adapted from McDaniel andMunn, 1985.)

ORGANIC SOILS

The emphasis thus far has been on mineral soils,soils with a volume composition consisting ofabout 50 percent solids, and these solids com-posed of 45 percent mineral matter and 5 percentorganic matter. In such soils the mineral fractionhas a dominating influence on soil properties.Organic soils, by contrast, have properties that arecontrolled mainly by the organic matter.

Organic Soil Materials DefinedOrganic soil materials are saturated with water forlong periods or are artificially drained and, ex-cluding roots, (1) contain 18 percent (dry-weightbasis) organic carbon if the mineral fraction is 60percent or more clay; (2) contain 12 percent ormore organic carbon if the mineral fraction has noclay; and (3) contain a proportional content oforganic carbon between 12 and 18 percent, de-pending on clay content, as shown in Figure 9.5. Ifthe soils are never water saturated for more than afew days each year, the organic carbon contentmust be 20 percent or more.

Organic soils are those soils composed of or-

ORGANIC SOILS 139

ganic soil materials that are of a significant thick-ness, depending on the depth to rock and otherlithic materials.

Formation of Organic SoilsHigh organic matter contents in soils are the re-sult of slow decomposition rates rather than highrates of organic matter addition. Organic soils arevery common in tundra regions where the rate ofplant growth is very slow. In the shallow water oflakes and ponds, plant residues accumulate in-stead of decomposing because of the anaerobicconditions. Consequently, organic soils may con-sist almost entirely of organic matter. In somecases, an infusion of mineral sediment from soilerosion may result in the accumulation of somemineral matter, together with the organic matter.Some organic soils, peat, for example, are minedand used for fuel.

Formation of many organic soils consists of theaccumulation of organic matter in a body of wa-ter. After glaciation, many depressions receivedclay and other materials that sealed the bottomsand created ponds and lakes. As the plant mate-rial accumulated on the bottom of the bodies of

FIGURE 9.5 Organic carboncontent (dry weight basis) oforganic and mineral soilmaterials. Mineral soilmaterials below the heavysolid line and organic soilmaterials above the heavysolid line. Organic soilmaterials above the dashedline are water saturated for nomore than a few days peryear.

140

water, the water depth decreased and the kinds ofplants contributing organic matter changed. An-other important organic component is the re-mains and fecal products of small aquaticanimals.

Minor climatic changes have occurred sincethe last glaciation, and this has had an effect onthe water levels and types of vegetation and theanimal populations. Thus, a series of layers or soilhorizons develop from the bottom upward thatreflect the conditions at the time each layer wasformed. Pollen grains are preserved in the peat,which can be readily identified. The vegetationtype can be verified by pollen analysis. A total of 5meters of peat soil accumulated in Sweden in9,000 years or at a rate equal to 1 centimeter every18 years. This is illustrated in Figure 9.6.

Properties and UseOrganic soils are composed mainly of plant andanimal residues and some microbial decom-position products. The soils are very light andhave low bulk density, high CEC, and a high nitro-gen, phosphorus, and sulfur content.The absenceor low content of sand and silt particles, com-posed of minerals containing essential nutrients,may result in low amounts of available potassium.

Organic soils must be drained before they canbe used for crop production. The upper soil layersthen become aerobic and the organic matter be-gins to decompose. This converts undecomposedpeat into well-decomposed organic soil calledmuck. In time, the entire soil may disappear. Thisis a serious problem in a warm climate with arapid decomposition rate, as in the Florida Ever-glades, where sugarcane is a major crop, togetherwith a wide variety of vegetable crops (see Figure9.7). Organic soils typically occupy the lowestelevation in the landscape, and crops grown insuch soils are the most likely to be killed by frost.Their low bulk density makes them susceptible towind erosion and desirable for sod production(see Figure 9.8).

SOIL ORGANIC MATTER

FIGURE 9.6 Stratification found in a peat soil inSweden, where changes in climate produced changesi n the kind of vegetation that formed the peat. (Datafrom Lucas, 1982.)

Archaeological InterestThe excellent preservation ability in a peat bog isillustrated by examination of the famous Tollundman, who was found in 1950 in a peat bog inDenmark. The facial expression at death andbristles of the beard were well perserved. Excel-lent fingerprints were made, and an autopsyrevealed the contents of the last meal, which con-sisted mainly of seeds, many of them weed seeds.When found, the Tollund man was buried under 2meters of peat that was estimated to have formedin the 2,000 years since his burial.

THE EQUILIBRIUUM CONCEPTDuring the early phase of soil formation, soil or-ganic matter content increases. After a few centu-ries, the soil attains a dynamic equilibrium con-tent of organic matter; additions balance losses.Even though the organic matter content remainsconstant from year to year, there is a significantturnover of organic matter and recycling of nu-trients.

FIGURE 9.8 Organic soil landscape. Crop on left isgrass sod used for landscaping and in the rear is awindbreak of willow trees. The lightness of the soilmakes the soil desirable for sod production;however, it also contributes to susceptibilty to winderosion.

THE EQUILIBRIUUM CONCEPT 141

FIGURE 9.7 The subsidencerates for organic soils dependson the depth to the water table;the lower the water table thegreater is the soil aeration anddecomposition of organic matter.Subsidence is greater in Floridathan in Indiana because of highertemperatures in Florida.

A Case ExampleThe pool, or amount of organic matter in a soil,can be compared to a lake. Changes in the waterl evel in a lake depend on the difference betweenthe amount of water entering and leaving it. Thisidea, applied to soil organic matter, is illustratedat the top of Figure 9.9, showing that the change insoil organic matter is the difference between theamount of organic matter added and the amountthat is decomposed.

Soil organic matter, labile and stable con-sidered together, decomposes in mineral soils ata rate equal to about I to 3 percent per year.Assuming a 2 percent decomposition rate, and40,000 kilograms in the plow layer of a hectare,800 kilograms of soil organic matter would bedecomposed or lost each year. Conversely, if 800kilograms of humus or stable organic matter wereformed from decomposition of residues added tothe soil, the organic matter content of the soilwould remain the same from one year to the next.The soil would be at the equilibrium level illus-trated in Figure 9.9.

Decomposition of 800 kilograms of organicmatter containing 5 percent nitrogen would resultin mineralization of 40 kilograms per hectare ofnitrogen. On an acre basis, this would amount toabout 40 pounds of available nitrogen per acre.

142

Assuming that the ratio of N:P:S in the soil organicmatter is 10:1:1, there would be 4 kilograms perhectare (4 lb/acre) of phosphorus and sulfur min-eralized. Other nutrients are also mineralized, buttheir availability is more closely related to the soilmineral fraction and mineral weathering.

Effects of CultivationEven on nonerosive land that is converted fromgrassland or forest to cultivated land, a rapid lossof organic matter resulting from cropping occurs.

SOIL ORGANIC MATTER

FIGURE 9.9 Schematic illustration of the equilibrium concept of soil organicmatter as applied to a hectare plow layer weighing 2 million kilograms andcontaining 2 percent organic matter. (Values are similar on a pounds per acrebasis for a 2-million pound acre-furrow-slice.)

At the Missouri Agricultural Experiment Station,as a result of 60 years of cultivation, the soil lostone third of its organic matter (see Figure 9.10).The organic matter content decreased 25 percentduring the first 20 years, 7 percent during the next20 years, and only 3 percent during the third 20-year period. The organic matter content of the soilwas moving to a new equilibrium level. In general,cultivated soils today contain about one half asmuch organic matter as they did before culti-vation. Theoretically, if a soil is fallowed, so thatno plants grow and decomposition continues, the

FIGURE 9.10 Decline in soil nitrogen (or organicmatter) with length of time of cultivation underaverage farming practices in Missouri. (Data fromJenny, Missouri Agr. Exp. Sta. Bull. 324, 1933.)

organic matter content would eventually ap-proach zero. On the other hand, if cultivated fieldsare allowed to revert to their original native vege-tation, the organic matter content will graduallyi ncrease toward the equilibrium level that existedprior to cultivation.

Soils in arid regions naturally have a low or-ganic matter content. Irrigating such soils andproducing crops that return a large amount ofplant residues to the soil results in an increase in

THE EQUILIBRIUUM CONCEPT

FIGURE 9.11 Relationshipbetween annual applicationrates of crop residues andcontent of organic carbon insoils after 11 years. (Data ofFrye, Bennett, and Buntley,1985).

143

the SOM content. In this case, converting land toagricultural use results in an equilibrium levelthat is higher than that of the virgin land.

Maintenance of Organic Matter inCultivated FieldsThe organic matter content of a soil is more diffi-cult to determine than the carbon content. As aresult, changes in the soil carbon content areused as a measure of changes in organic mattercontent. Multiplying the percent of soil carbon by1.72 is generally considered to equal the percentof organic matter.

Soil organic matter is constantly subject to de-composition and loss. Therefore, each soil andcropping situation, requires the addition of a cer-tain amount of organic matter each year to main-tain the status quo. In a 16-year experiment inMinnesota, 5 tons of crop residues per acre annu-ally were required to maintain the original carboncontent of the soil at 1.8 percent (see Figure 9.11).Addition of more than 5 tons per acre increasedSOM content and lesser amounts resulted in adecline in the organic matter content. While landi s being farmed (excluding the desert land), it is

144

virtually impossible, to maintain the organic mat-ter content of the virgin soil. It is, therefore,prudent to accept the organic matter content thatresults from economical or profitable farming.

In Figure 9.12, the effect of different yields ofcorn grain on soil carbon is given for a typicalwell-drained agricultural soil in southern Michi-gan. The grain was harvested and all of the re-maining crop residues were returned to the soil,including the roots that equal about 15 to 20 per-cent of the total crop residues. Note that a corngrain yield of 3,150 kilogram per hectare (kg/ha)(50 bushels per acre) is predicted to maintain anequilibrium level equal to 1.0 percent organic car-bon. By contrast, yields of 6,300 and 9,450 kilo-grams per hectare (100 and 150 bushels per acre)are predicted to maintain equilibrium carbon lev-els of 1.7 and 2.3 percent, respectively. Higheryields return more residues and maintain a higherorganic matter content, which in turn makes stillhigher yields more likely.

Another observation can be made from Figure9.12. A yield of 6,300 kilograms per hectare wouldresult in about a 400 kilograms per hectare loss ofcarbon if the soil contained 2.5 percent organiccarbon. Conversely, if the soil contained only 0.5percent organic carbon, the same yield of 6,300

FIGURE 9.12 Estimated annual soil-humus carbonchanges as related to corn (maize) yields andorganic carbon content of loamy soils in southernMichigan. (Adapted from Lucas and Vitosh, 1978.)

SOIL ORGANIC MATTER

kilograms per hectare would increase the soil car-bon content by about 800 kilograms per hectare.Since World War II there have been large in-creases in crop yields in many countries in spiteof claims of declining soil organic matter and soildepletion. It appears that soil management sys-tems that result in high crop yields maintain SOMat acceptable contents. Many soil managementprograms focus on the production of high cropyields and proper crop residue managementrather than the organic matter content of soils, perse. One of the most serious causes of organicmatter loss is soil erosion that tends to prefer-entially remove the organic matter fraction. Highcrop yields also mean more vegetative cover andreduced soil erosion.

Effects of Green ManureGreen manuring is the production of a crop for thepurpose of soil improvement. These crops areeasily and inexpensively established and, fre-quently, are grown during the fall, winter, and/orspring when the land would normally be un-protected by vegetation. Benefits include erosioncontrol and the accumulation of nutrients (intoorganic matter) that might otherwise be lost byleaching. There is also less likelihood of pollutionof water tables with nitrate nitrogen. In addition,organic matter from the green manure crop iseventually incorporated into the soil, and the fol-lowing crop benefits from the rapid release ofnutrients from a large amount of readily decom-posable (labile) material. The humus that isformed contributes to the maintenance of the con-tent of soil organic matter. However, this is usu-ally a secondary reason why green manure cropsare grown.

HORTICULTURAL USES OFORGANIC MATTEROrganic materials used for horticultural purposesconsist mainly of naturally occurring peat andmoss and the products of composting. High-

content organic matter materials are used as acomponent of soil mixes and as a mulch.

Horticultural PeatsPeats that are used for soil amendments are gen-erally classified as moss peat, reed-sedge peat,and peat humus. Moss peat forms from moss veg-etation and reed-sedge peat from reeds, sedges,cattails, and other associated plants. Someproperties of common horticultural peats aregiven in Table 9.4. The low nitrogen content ofsphagnum peat means that it may not haveenough nitrogen to satisfy the needs of decom-posers and, thus, its use creates competition be-tween decomposers and plants for any availablenitrogen that might be present in the soil mix. Thelow pH of sphagnum peat, however, makes itdesirable as a mulch for plants, such as azaleasand rhododendrons, that grow well in acid soil.Peat moss makes a neat-looking surface that setsoff plants and protects the soil from the disruptivei mpact of raindrops. The soil remains more po-rous and receptive to rain or irrigation water (seeFigure 9.13).

CompostsMany gardners have organic residues with wideC : N ratios such as tree leaves, weeds, grass clip-

TABLE 9.4 Characteristics of Common Horticultural Peats

HORTICULTURAL USES OF ORGANIC MATTER

From Lucus, et al., Ext. Bull., 516, Micoigan State University, East Lansing, Micoigan.a Oven dry basis.

FIGURE 9.13 Mulching a rose garden with peatmoss. The mulch protects the soil surface fromraindrop impact and maintains a highwater-infiltration rate.

pings, or other plant and garbage wastes. Thesematerials can be composted with a resulting de-crease in the C:N ratio and production of an or-ganic material used for mulching and as a com-ponent of soil mixes. Composting consists ofstoring or maintaining the organic materials in apile while providing favorable water content, aer-ation, and temperature. As the organic matter de-composes, much of the carbon, hydrogen, andoxygen is released as carbon dioxide and water.Nutrients, like nitrogen, are continuously reusedand recycled by the microbes and conserved

145

146

TABLE 9.5 Materials Recommended forMaking Compost

From Kellogg, 1957.

within the composting pile. Thus, while there is aloss of carbon, oxygen, and hydrogen, other nutri-ent elements are being concentrated. The occa-sional turning of the compost pile increases aer-ation and generally hastens rotting. The rottedmaterial has good physical properties and is agood source of nutrients. Small amounts of mate-rials, from the weeding of a garden, for example,can be composted in a black plastic bag.

The low nitrogen content of many compostingmaterials may retard their decomposition. For thisreason, most composters add some nitrogen fer-tilizer to the compost pile. The nutritive value ofthe compost can also be increased by the additionof other materials. Some recommendations ofthe U.S. Department of Agriculture are given in Ta-ble 9.5.

SUMMARY

The total soil organic matter is composed of alabile fraction and a stable fraction. The labilefraction consists of readily decomposable plant,

SOIL ORGANIC MATTER

microbial, and animal products. Even though it isa minor amount of the total soil organic matter,labile organic matter is very important in nutrientcycling. The stable organic matter is humus,which contributes to cation exchange capacity,water adsorption, and soil structure stability. Sta-ble organic matter decomposes very slowly, andas a result, releases (mineralizes) nutrients veryslowly.

Organic soils consist mainly of organic matter.They have light weight and are susceptible towind erosion. Most organic soils require drainagefor cropping, which aerates the soil and greatlyincreases the rate of decomposition. The absenceor very low content of minerals is associated withvery low potassum supplying power.

The organic matter content of a soil tends toapproach an equilibrium. When the losses of or-ganic matter exceed additions, the soil organicmatter content declines. Cultivated soils containabout half as much organic matter as they didbefore the soils were converted to cropping. Theexception is desert soils where cropping may re-sult in an increase in organic matter content.

Horticultural peats and composts are organicmaterials used for mulching and for soil mixes.

REFERENCES

Alexander, M. 1977. Soil Microbiology. 2nd ed. JohnWiley, New York.

Allison, F. E. 1973. Soil Organic Matter and Its Role inCrop Production. Elsevier, New York.

Frye, W. W., 0. L. Bennett, and G. J. Buntley. 1985."Restoration of Crop Productivity on Eroded or De-graded Soils." Soil Erosion and Crop Productivity. pp.335-356. Am. Soc. Agronomy. Madison, Wis.

Glob, P. V. 1954. "Lifelike Man Preserved 2,000 Years inPeat." Nat. Geog. Mag. 105:419-430.

Jenny, Hans. 1933. "Soil Fertility Losses Under MissouriConditions." Missouri Agr. Exp. Sta. Bull. 324. Colum-bia, Mo.

Jenny, Hans. 1980. The Soil Resource. Springer-Verlag,New York.

Kellogg, C. E. 1957. "Home Gardens and Lawns". USDA

Agricultural Yearbook. pp. 665-688. Washington,D.C.

Klemmedson, J. O. 1975. "Nitrogen and CarbonRegimes in an Ecosystem of Young Dense Pon-derosa Pine in Arizona." Forest Science. 21:163-168.

Lucas, R. E. 1982. "Organic Soil (Histosols)." Mi. Agr.Exp. Sta. Res. Report 435. East Lansing, Mi.

Lucas, R. E. and P. E. Rieke, and R. S. Farnham. 1971."Peats for Soil Improvement and Soil Mixes." Mi. Agr.Ext. Bull. 516. East Lansing, Mi.

Lucas, R. E. and M. L. Vitosh. 1978. "Soil Organic MatterDynamics." Mi. Agr. Exp. Sta. Res. Report 358. EastLansing, Mi.

REFERENCES 147

McDaniel, P. A. and L. C. Munn. 1985. "Effect of Tem-perature on Organic Carbon-texture Relationships in

Mollisols and Aridisols." Soil Sci. Soc. Am. J. 49:1486-1489.

Paul, E. A. 1984. "Dynamics of Organic Matter in Soils."Plant and Soil. 76:275-285.

Schreiner, O. and B. E. Brown. 1938. "Soil Nitrogen."

Soils and Men. USDA Yearbook of Agriculture, Wash-

ington, D.C.Van Veen, J. A. and E. A. Paul. "1981. Organic Carbon

Dynamics in Grassland Soils." Can. J. Soil Sci.

61:185-210.

CHAPTER 10

SOIL MINERALOGY

Minerals are natural inorganic compounds, withdefinite physical and chemical properties, that areso conspicuous in many granitic rocks. They arebroadly grouped into primary and secondary min-erals. Primary minerals have not been alteredchemically since their crystallization from moltenlava. Disintegration of rocks composed of primaryminerals (by physical and chemical weathering)releases the individual mineral particles. Many ofthese primary mineral particles become sand andsilt particles in parent materials and soils. Primaryminerals weather chemically (decompose) andrelease their elements to the soil solution. Someof the elements released in weathering react toform secondary minerals. A secondary mineralresults from the decomposition of a primary min-eral or from the precipitation of the decom-position products of minerals. Secondary miner-als originate when a few atoms react andprecipitate from solution to form a very small crys-tal that increases in size over time. Because of thegenerally small particle size of secondary miner-als, they dominate the clay fraction of soils.

Consideration of the characteristics of mineralsfound in soils, and their transformation from one

148

form to another, is essential to understandingboth the nature of the soil's chemical propertiesand the origin of its fertility. Since soils developfrom parent material composed of rocks and min-erals from the earth's crust, attention is directedfirst to the chemical and mineralogical composi-tion of the earth's crust.

CHEMICAL AND MINERALOGICALCOMPOSITION OF THE EARTH'S CRUSTAbout 100 chemical elements are known to existin the earth's crust. Considering the possible com-binations of such a large number of elements, it isnot surprising that some 2,000 minerals havebeen recognized. Relatively few elements andminerals, however, are of great importance insoils.

Chemical Composition of theEarth's CrustApproximately 98 percent of the mass of theearth's crust is composed of eight chemical ele-ments (see Figure 10.1). In fact, two elements,

FIGURE 10.1 The eight elements in the earth's crustcomprising over 1 percent by weight. The remainderof elements make up 1.5 percent. (Data from Clark,1924.)

oxygen and silicon, compose 75 percent of it.Many of the elements important in the growth ofplants and animals occur in very small quantities.Obviously, these elements and their compoundsare not evenly distributed throughout the earth'ssurface. In some places, for example, phosphorusminerals (apatite) are so concentrated that they

WEATHERING AND SOIL MINERALOGICAL COMPOSITION

TABLE 10.1 Average Mineralogical Composition of Igneous and SedimentaryRocks

° Present in small amounts.

149

are mined; in other areas, there is a deficiency ofphosphorus for plant growth.

Mineralogical Composition of RocksMost elements of the earth's crust have combinedwith one or more other elements to form the min-erals. The minerals generally exist in mixtures toform rocks, such as the igneous rocks, granite andbasalt. The mineralogical composition of igneousrocks, and the sedimentary rocks, shale and sand-stone, are given in Table 10.1. Limestone is alsoan important sedimentary rock, which is com-posed largely of calcium and magnesium carbon-ates, with varying amounts of other minerals asimpurities. The dominant minerals in these rocksare feldspars, amphiboles, pyroxenes, quartz,mica, apatite, clay, iron oxides (goethite), andcarbonate minerals.

WEATHERING AND SOILMINERALOGICAL COMPOSITIONThe soil inherits from the parent material a min-eral suite (a set of minerals) that typically con-tains both primary and secondary minerals.

150

Weathering in soils results in the destruction ofexisting minerals and the synthesis of new miner-als. The minerals with the least resistance toweathering disappear first. The minerals most re-sistant to weathering such as quartz, make up agreater proportion of the remaining soil and ap-pear to accumulate. As a consequence, the miner-alogy of a soil changes over time, when viewed ona time scale of hundreds of thousands of years.

Weathering ProcessesNumerous examples of weathering abound andcan be observed every day. These include therusting of metal and the wearing down and disin-tegration of brick walls. Mineral weathering isstimulated by acid conditions. Respiration ofroots and microorganisms produces carbon diox-ide that reacts with water to form carbonic acid(H ²C0 3). An acid environment stimulates the re-action of water with minerals and is one of themost important weathering reactions. For exam-ple, the reaction of a feldspar (albite) with water(hydrolysis) and H + is as follows:

SOIL MINERALOGY

leached from the soil, particularly in humid re-gions.

The kaolinite, even though very resistant to fur-ther weathering, may be decomposed and disap-pear from the soil. The reaction:

The decomposition of kaolinite resulted in theformation of gibbsite, a mineral more resistant toweathering than the kaolinite. Both kaolinite andgibbsite are clay minerals that have distinctly dif-ferent properties. Thus, as weathering proceedsand the mineralogical composition of the soilchanges, the chemical and physical propertiesalso change.

The loss of silicic acid by leaching results in theprogressive loss of silicon. There is a progressiveincrease in the accumulation of aluminum, be-cause the aluminum tends to be incorporated intoresistant secondary minerals that accumulate inthe soil. For this reason, an indicator of weather-ing intensity is the silica:alumina ratio (Si0 ² /A1 ²0 3). As silicon is lost from the soil by leachingand aluminum accumulates, the silica:aluminaratio decreases. The Si0 ² /A1 ²03 ratio decreasesfrom 3:1 for albite to 1:1 for kaolinite.

Other weathering reactions include oxidation,dissolution, hydration, reduction, and carbon-ation. An example of mineral weathering and theformation of clay minerals on the surface of apiece of basalt is shown in Figure 10.2.

Summary Statement1. Mineral weathering is stimulated by an acid

soil environment.2. Plant-essential nutrient ions are released to

the soil solution and clay minerals (second-ary) are formed.

3. Primary minerals disappear and secondaryminerals, especially the clay minerals, accu-mulate.

4. Much of the silicon released in weathering isremoved from the soil by leaching, unless

In the reaction a primary mineral, albite, was con-verted to kaolinite. The first-formed kaoliniteparticles are very small. They increase in size asadditional ions attach or join the crystal, resultingi n an gradual increase in particle size. The clayfraction of soils tends to be dominated by parti-cles formed in this manner and particles like kao-linite are called clay minerals. Clay minerals tendto be resistant to further weathering. Therefore, asprimary minerals, like albite, weather and disap-pear, clay minerals, like kaolinite, increase inabundance.

In the reaction, sodium was released as an ion,Na'. In addition, some of the silicon in the albitewas incorporated into kaolinite and some intoH4SiO4 (silicic acid). The silicic acid tends to be

there is limited precipitation and leaching isi neffective. The silica:alumina ratio de-creases as soils become more weathered.

Weathering Rate and Crystal StructureParticle size, through its effect on specific surface,is an important factor affecting the weatheringrate of minerals. That is, a given mineral in the siltfraction weathers faster than if the mineral is inthe sand fraction. Chemical bonding within themineral crystal, however, is the major factor af-fecting weathering rate.

Oxygen, silicon, and aluminum are the threemost abundant elements in the earth's crust, com-

WEATHERING AND SOIL MINERALOGICAL COMPOSITION

TABLE 10.2 Size, Percent Volume in Earth's Crust, Coordination Numberand Valence of the Most Abundant Elements in Soil Minerals

151

prising about 47 percent, 28 percent, and 8 per-cent by weight, respectively (see Figure 10.1).Many soil minerals are silicate or aluminosilicateminerals. Oxygen is abundant on a weight basis,and because of the large size of the oxygen atom,oxygen occupies more than 90 percent of the vol-ume of the earth's crust (and soil), as shown inTable 10.2.

Note the generally small atomic size of theabundant cations listed in Table 10.2 as com-pared with the oxygen. In the minerals, the nega-tive charge of oxygen is balanced by the positivecharge of cations. It is helpful to think of soilminerals as being composed of Ping-Pong ballswith much smaller balls (like marbles), in thei nterstices. The Ping-Pong balls represent largenegatively charged oxygen atoms, and the smallballs represent the positively charged cations inthe interstices.

Silicon is a very abundant cation, which per-forms a role in the mineral world similar to therole carbon plays in the organic world. The siliconion fits in an interstice formed by four oxygenions. The covalent bonding (electron sharing) be-tween oxygen and silicon forms a tetrahedron asshown in Figure 10.3. The silicon-oxygen tetrahe-dron is the basic unit in silicate minerals. Thevalence of the silicon is +4 and that of the oxygen

152

The coordination number is the number of ionsthat can be packed around a central ion. Fouroxygen ions can be packed around a silicon ion,resulting in a coordination number of 4 (as shownin Figure 10.3). Note that larger cations have alarger coordination number. Aluminum, iron, andmagnesium ions are larger than silicon ions, andaluminum, iron, and magnesium typically have acoordination number of 6. Potassium has a coor-dination number of 8 and 12 (see Table 10.2).

Different silicate minerals are formed, depend-i ng on the way the silicon-oxygen tetrahedra arelinked together and how the net charge of thetetrahedra is neutralized. In olivine, the silicon-oxygen (Si-0) tetrahedra are bonded together bymagnesium and/or iron. The crystal is neutral andhas the formula of MgFeSi0 4 . The ratio of magne-sium and iron, however, is highly variable, result-ing in several olivine species. The model of oliv-ine in Figure 10.4 shows that each magnesiumion, or iron ion, is surrounded by six oxygen ions;three oxygen ions form each of the faces of twoadjacent Si-0 tetrahedrons. Magnesium and ironions are larger than silicon and more oxygen ionsare required to form an interstice large enough forthem to occupy. The coordination number ofmagnesium and iron is 6.

The oxygen-silicon bonds are much strongerthan the magnesium and iron bonds with oxygen.Consequently, reaction of olivine with water re-

SOIL MINERALOGY

suits in the H+ of the water replacing magnesiumand iron ions from the crystal face, rather than thesilicon. The weathering of olivine can be viewedas the separation of the Si-O tetrahedra with therelease of the

Mg2+and Fe

²+,as illustrated in

Figure 10.4. The Si-O tetrahedra and the magne-sium and iron ions encounter other ions in thesoil solution, where the ions can react and formnew minerals. The magnesium and iron ions canbe absorbed by roots. Olivine weathers rapidlyand soils with a high content of olivine may haveso much

Mg²+in solution as to be detrimental to

some plants. Conversely, olivine is absent inmany soils because of its low resistance to weath-ering.

The net charge of individual Si-O tetrahedramay be partly or entirely neutralized by the com-mon sharing of oxygen between adjacent tetrahe-dra. If adjacent Si-O tetrahedra share one oxygen,single chains are formed as in pyroxene and au-gite (see Figure 10.5). As shown in Figure 10.5, asmore sharing of oxygen by silicon occurs, miner-als with double chains, sheets, and three-dimensional structures are formed. Mica, and ka-olinite, have a sheet structure, and quartz has athree-dimensional structure. In the case of quartz(Si0 ²) all of the charge is neutralized by the shar-ing of all of the oxygen with silicon. The result is amineral with strong chemical bonding within thecrystal and great weathering resistance. As more

FIGURE 10.3 Models showing the tetrahedral (four-sided) arrangement ofsilicon and oxygen ions in silicate minerals. The silicon-oxygen tetrahedronon the left has the apical oxygen set off to one side to show the position ofthe silicon ion.

oxygen sharing occurs, the oxygen-silicon ratio(ratio of oxygen ions to silicon ions) of the min-eral decreases. This is associated with increasedweathering resistance.

Mineralogical Composition VersusSoil AgeThe difference in the weathering resistance ofminerals has been used to develop a series ofweathering stages that parallel soil age (amount

WEATHERING AND SOIL MINERALOGICAL COMPOSITION 153

FIGURE 10.4 Modelsrepresenting the weathering ofolivine. Olivine is composed ofsilicon-oxygen tetrahedra heldtogether by iron and/ormagnesium. Every othertetrahedron is "inverted," as shownby the light-colored tetrahedra inthe olivine model on the left.During weathering, the silicon-oxygen tetrahedra separate withthe release of iron andmagnesium.

of weathering and soil formation). The weatheringstages are based on the representative minerals inthe fine silt and clay fractions. Table 10.3 contains13 weathering stages grouped into minimal, mod-erate, and intensive weathering stages.

Weathering stage I represents soils that havebeen subjected to little, if any, weathering andleaching, because the soils contain weatherableminerals such as gypsum, (CaSO 4 2H²0), and ha-lite (NaCI). Each higher weathering stage is domi-nated by minerals with greater weathering resis-

FIGURE 10.5 Modelsshowing the commonarrangements of silicon-oxygen tetrahedra in silicateminerals and their relation toweathering resistance.

154

TABLE 10.3 Representative Minerals and Soils Associated withWeathering Stages

tance, and in effect, more developed or oldersoils. Soils in which the fine silt and clay fractionsare dominated by minerals of the first five weath-ering stages are considered to be minimallyweathered. These soils have sand and silt frac-tions that are rich in primary minerals. The soilstend to occur where there has been a lack of timeor water for weathering or where temperatures aretoo low for effective weathering. Minimally weath-ered soils are generally fertile.

Soils dominated by minerals represented bymoderate weathering are in stages 6 thru 9. Thesesoils tend to have a significant amount of bothprimary minerals (quartz and muscovite mica)and secondary minerals (vermiculite andsmectite). Many temperate region soils are mod-

SOIL MINERALOGY

erately weathered and they are quite fertile soils.Most of the world's corn and wheat are producedon these soils.

Soils of weathering stages 10 through 13 areconsidered to be intensively weathered. Mineralsrepresentative of the intensive weathering stagesare secondary minerals with great weathering re-sistance (see Table 10.3). These soils tend to oc-cur in warm and humid tropical regions wherethere has been sufficient time to weather almostall of the primary minerals . Many intensivelyweathered soils have clay fractions dominated bykaolinite, gibbsite (Al(OH) 3), and iron oxides, in-cluding hematite. The world's oldest and mostweathered soils are high in content of oxides ofaluminum (gibbsite), iron (hematite), and/or tita-

environment of somewhat limited leaching. Crys-tallization of volcanic ash products in seawateroften produces montmorillonite. The expansionof layers permits easy diffusion of hydratedcations into the interlayer space and adsorptionon the planar surfaces. The dominant features ofmontmorillonite are shown in Figure 10.12 andsome properties are given in Table 10.4.

The individual clay layers in soils tend to occuras clusters that are much like a stack of playingcards. The more expansive the clay, the morelikely layers will separate, causing the clay to existas very small-sized particles. As a consequence,smectites occur as very small particles, whichgive this type of clay great capacity for swellingand shrinking with wetting and drying. The small

SOIL CLAY MINERALS 1RQ

spaces between smectite particles contribute tohydraulic impermeability. Houston Black Claysoils in Texas have a high smectite content andare characterized by contraction and develop-ment of wide cracks during the dry season and bygreat expansion and closure of the cracks duringthe wet season.

KaoliniteReference to the formation of kaolinite was madein regard to the weathering of albite feldspar. Inthat case, kaolinite formation was by crystalliza-tion from solution. Another mechanism for kao-linite formation is the partial disintegration of 2:1clays. The stripping away of one Si-0 sheet, from

FIGURE 10.13 Model ofkaolinite showing two offsetlayers. The upper layersurfaces are composed ofhydroxyls (light-colored).These hydroxyl are bondedby the hydrogen of thehydroxyl to the oxygen ofadjacent layers, producing anonexpanding clay. Cationexchange capacity is low andoriginates at the edges due todeprotonation of exposedhydroxyls. A hydrated cationis adsorbed to such a cationexchange site.

160

a 2:1 layer, converts the 2:1 layer into a 1:1 layer,typical of kaolinite. This mode of formation isfavored by an acid weathering environment that isfound in older soils or intensively weathered soilswhere silicon is being removed by leaching.

The 1:1 structure has layers that have a plane ofoxygens on one side and a plane of hydroxyls onthe other. Stacking of the layers results in a strongattraction between oxygen of one layer and thehydrogen of hydroxyl (hydrogen bonding) of anadjacent layer, resulting in a nonexpanding clay.Kaolinite particles tend to be large in size (manyare silt sized), and to have great resistance tofurther weathering. The tetrahedral and octahe-dral sheets may retain some isomorphous substi-tution that can contribute to cation exchange ca-pacity. This appears to be very minor and thecation exchange capacity is low, relative to theother clay minerals discussed thus far (see Table10.4). The cation exchange capacity occursmainly because of deprotonation of exposed hy-droxyl at the edges. A model showing someproperties of kaolinite is given in Figure 10.13.

The large size of kaolinite particles often givessoils high in kaolinite clay content good physicalproperties. Many of the kaolinite particles occurin the coarse clay (0.0002 to 0.002 mm diameter)and silt fractions. Smectite particles typically oc-cur in the fine clay (less than 0.0002 mm diame-ter). Two Bt horizons may contain the sameamount of clay but, if the clay is kaolinite, thehorizon could be quite water permeable com-pared with a Bt horizon containing smectite.Smectite is a common clay of U.S. midwesternand Great Plains soils, in United States, and asignificant number of the soils have very slowly orimpermeable Bt horizons. By contrast, kaolinite iscommon in soils on the coastal plain of south-eastern United States where soils are much older.Even so, impermeable claypan Bt horizons are notcommon on the southeastern coastal plain.

Allophane and HalloysiteNoncrystalline (amorphous) minerals exist whereglassy ash and cinders are thrown into the air by

SOIL MINERALOGY

volcanoes and are deposited on land masses be-fore the atoms become organized into crystals. Inthe humid tropics, amorphous minerals weatherrapidly into an amorphous clay mineral calledallophane. Soils high in allophane have a gel-likeconsistency when wet and lack the grittiness as-sociated with sand particles. There is an enor-mous surface area and great reaction with, oradsorption of, organic matter, giving the organicmatter protection against decomposition. Charac-teristic features of soils high in allophane includea high content of organic matter, high water-holding capacity, high and variable cation ex-change capacity, and low load-bearing strength.

Slowly, allophane becomes more organized(more crystalline) and becomes halloysite-amineral with structure and composition similar tokaolinite. A weathering sequence from volcanicash weathering is allophane-halloysite-kaolinite.Soils high in allophane are important in the moun-tain chain that extends from southern SouthAmerica to Alaska and, also, in Hawaii.

Oxidic ClaysKaolinite formation from 2:1 minerals is indicativeof a leaching regime that is very effective in re-moving silicon. Under such conditions, kaolinitemay slowly weather, resulting in the decom-position of the layer sheets into their componentparts. The Si-O tetrahedra are transformed intoSi-OH tetrahedra, H4SiO4 , (silicic acid), that issoluble and removed by leaching. Aluminum-OHoctahedra are released and slowly polymerize toform an aluminum hydrous oxide (Al(OH) 3),called gibbsite. Gibbsite particles tend to be platyand hexagonal in shape like the sheet shown inFigure 10.7. Many layers stack on top of each otherand exist as clay particles in soils. Gibbsite is veryi nsoluble and resistant to further weathering andis representative of weathering stage 11 (see Ta-ble 10.3). The mineral is common in intensivelyweathered soils that also contain abundant ironoxides, such as hematite (Fe ²03) and goethite(FeOOH). Only rare and unusual soils have a sig-nificant amount of titanium oxide (anatase),

which is representative of weathering stage 13.Few parent materials contain enough titanium fora significant amount of anatase to accumulate viaweathering and soil formation.

The oxide clays are abundant in many red-colored soils in the humid tropics. These soils aretypically acid and, in an acidic environment,some of the exposed hydroxyls on particle sur-faces of oxidic clays adsorb a H + (protonate)and form sites of positive charge (Al-OH + H + =Al-OH² +). The kaolinite clay has a net negativecharge, which promotes attraction of the kaoliniteand oxidic clays and formation of extremely sta-ble aggregates or peds. The peds, which aredense and frequently the size of sand grains, arevery resistant to deformation and give the soilphysical characteristics similar to soils domi-nated by quartz sand; that is, soils with an abun-dance of oxidic clays have high water-infiltrationrates and resist erosion. The soil can be plowedshortly after a rain. A small amount of availablewater is retained at field capacity. The water isheld at low tension or high potential, and thewater is easily and rapidly absorbed by roots.Short periods of water stress for crops may occureven though the soils are in the tropical rain forestzone.

Summary Statement1. There is a kinship amoung the crystalline alu-

minum silicate clay minerals because all con-sist of Si-0 tetrahedra sheets and Al-OH octa-hedra sheets.

2. The major differences in the aluminum sili-cate clays result from the ratio of tetrahedraand octahedra sheets and the nature andamount of isomorphous substitution. A sum-mary of the major features of these clays isgiven in Table 10.4.

3. The 2:1 clays are transformed into 1:1 clay bythe loss of a Si-O tetrahedral sheet. This is oneformation mode of kaolinite.

4. Near the end of the weathering sequence,gibbsite formation becomes important. In theoldest and most weathered soils, the mineral-

ION EXCHANGE SYSTEMS OF SOIL CLAYS

ogy of the clay fraction is dominated by oxidicclays, together with kaolinite.

5. Volcanic activity produces amorphous ashand cinders that weather to form amorphousallophane. Allophane slowly crystallizes toform kaolinite.

6. Over time, weathering produces changes inthe mineralogical composition of soils,which is reflected in the soil's physical andchemical properties. The cation exchange ca-pacity, for example, increases to a peak withweathering in soils dominated by 2:1 clays(moderately weathered), which is followedby a very low cation exchange capacity insoils dominated by oxidic clays (intensivelyweathered).

I ON EXCHANGE SYSTEMS OFSOIL CLAYSSome soils are dominated by layer silicate clays,some by oxidic clays, and some soils have bothkinds of clay in abundance. This results in threeunique ion-exchange systems in mineral soils:(1) layer silicate, (2) oxide, and (3) oxide-coated,layer silicate. The type and kind of ion exchangehave important consequences for plant growthand soil management.

Layer Silicate SystemLayer silicate system soils are overwhelminglynegatively charged with high cation exchange ca-pacity (CEC). The cation exchange capacity origi-nates mainly from isomorphous substitution.These negatively charged sites constitute perma-nent charge that is not affected by pH. The chargemay be modified, however, to a small extent byminor or thin coatings of oxidic clays on the parti-cle surfaces. Some cation exchange capacity orig-inates from the dissociation of hydrogen (depro-tonation) from edge AI-OH groups that is favoredby high pH (high OH concentration):

[ clay edge]AI-OH + OH-

= [clay edgelAl-0 - + H ²O

16 1

diate between the other two systems. Many of thered-colored tropical soils have an ion exchangesystem that is greatly affected by both silicate andoxidic clay.

SUMMARYIn an acid soil environment, weathering of miner-als is stimulated. Minerals weather at differentrates, depending on their resistance to weath-ering.

In general, primary minerals weather and sec-ondary minerals are formed. Over time, the miner-alogical composition of the soil changes and,consequently, soil properties change.

Weathering releases ions to the soil solution.Many of these ions are plant nutrients. Some ionsprecipitate to form secondary minerals and someions are leached from the soil.

Minimally and moderately weathered soils tendto be rich in primary minerals and to be fertilewith high cation exchange capacity. Intensivelyweathered soils are depleted of primary minerals.Few nutrient ions are released and soils are infer-tile. The clay minerals in intensively weatheredsoils have low cation exchange capacity.

Nutrient ions adsorbed to the surfaces of clayparticles represent a reservoir of nutrients that isexchangeable and available to plants.

REFERENCES 163

Ultimately, soils become "weathered out" andhave almost no capacity to support plants. Suchsoils may be mined for ore. Clay edge Al-OHprotonate in acid soils and form sites of an-ion exchange capacity and deprotonate in alka-line soils and form sites of cation exchange ca-pacity.

REFERENCESBirkeland, P. W. 1974. Pedology, Weathering, and Geo-

morphological Research. Oxford, New York.Bohn. H. L., B. L. McNeal, and G. A. O'Connor. 1985.

Soil Chemistry, 2nd ed. John Wiley, New York.Clarke, F. W. 1924. "Data of Geochemistry." U.S. Geo.

Survey Bull. 770. Washington, D.C.Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. John

Wiley, New York.Jackson, M. L. and G. D. Sherman. 1953. "Chemical

Weathering of Minerals in Soils." Advances inAgronomy. 5:219-318.

Jenny, H. 1980. The Soil Resource. Springer-Verlag,New York.

Soil Sci. Soc. Am. 1977. Minerals in the Soil Environ-ments. Soil Sci. Soc. Am. Madison, Wis.

Uehara, G. and G. Gillman. 1981. Mineralogy, Chem-istry, and Physics of Tropical Soils with VariableCharge. Westview, Boulder.

White, R. E. 1987. Introduction to the Principles andPractice of Soil Science. 2nd ed. Blackwell Sci. Pub.London.

A total chemical analysis of a soil may reveal littlethat is useful in predicting the ability of the soil tosupply plant nutrients. Instead, it is usually a rela-tively small part of the total amount of an elementthat is available for plant growth. For these rea-sons, the discussion of soil chemistry focuses onion exchange reactions, solubilities, and mineraland biochemical transformations. These reac-tions are illustrated in Figure 11.1.

CHEMICAL COMPOSITION OF SOILSThe average chemical composition of igneousrocks is given in Table 11.1. On an oxide basis,silicon and aluminum are first and second inabundance. This reflects the dominance of sili-cate and aluminosilicate minerals in igneousrocks. Iron is next in abundance in igneous rocks.Next most abundant, are four cations that arei mportant in balancing the charge of silicon-oxygen tetrahedra in minerals-calcium, magne-sium, sodium, and potassium. The remaining 1 or2 percent includes the other elements. The aver-

CHAPTER 11

SOIL CHEMISTRY

164

age chemical composition of igneous rocks issimilar to the mineralogical composition of manyminimally and moderately weathered soils. Suchsoils contain much quartz, feldspar, and mica inthe sand and silt fractions, 2:1 layer silicate claysin the clay fraction, and much of the negativecharge of the clays neutralized by the adsorptionof calcium, magnesium, sodium, and potassiumions.

When soils weather and the mineralogicalcomposition changes over time, there is a corre-sponding change in chemical composition. Dur-ing soil formation, there is a preferential loss ofsilicon relative to aluminum and iron. This is re-flected in the chemical analysis of the intensivelyweathered Columbiana clay soil in Costa Rica(see Table 11.1). The soil contains only 26 per-cent silica, and the sum of iron and alumumium is69 percent (on an oxide basis). The silica:alumina ratio decreased from 3.75 to 0.53 as aresult of weathering and soil formation. Thechemical composition of the Columbiana soil re-flects a high oxidic clay content. Release and lossby leaching of calcium, magnesium, sodium, andpotassium were more rapid than silica, and this is

reflected in the low content of these four cationsi n the intensively weathered soil relative to igne-ous rocks.

ION EXCHANGEIon exchange is of great importance in soils (seeFigure 11.1). Ion exchange involves cations andanions that are adsorbed from solution onto nega-

TABLE 11.1 Chemical Composition of AverageIgneous Rocks and an Intensively Weathered Soil

Adapted from Bohn, McNeal, and O'Connor, 1985.

I ON EXCHANGE

FIGURE 11.1 Major chemicalprocesses and reactions insoils. Weathering of mineralsand decomposition of organicmatter supply ions to the ionpool of the soil solution. Ionsare lost from the pool by plantuptake and leaching. Somei ons precipitate and someions form secondary minerals,especially clay minerals, andare subject to dissolution.Ions also sorb onto anddesorb from cation and anionexchange sites.

165

tively and postively charged surfaces, respec-tively. Such ions are readily replaced or ex-changed by other ions in the soil solution ofsimilar charge, and thus, are described by theterm, ion exchange. Of the two, cation exchange

is of greater abundance in soils than anion ex-change.

Nature of Cation ExchangeCation exchange is the interchange between acation in solution and another cation on the sur-face of any negatively charged material, such asclay colloid or organic colloid. The negativecharge or cation exchange capacity of most soilsis dominated by the secondary clay minerals andorganic matter (especially the stable or humusportion of SOM). Therefore, cation exchange re-actions in soils occur mainly near the surface ofclay and humus particles, called micelles. Eachmicelle may have thousands of negative chargesthat are neutralized by the adsorbed or exchange-able cations. Assume for purposes of illustrationthat X- represents a negatively charged ex-changer that has adsorbed a sodium ion (Na),producing NaX. When placed in a solution con-taining KCI, the following cation exchange reac-tion occurs:

166

I n the reaction, K+ in solution replaced or ex-changed for adsorbed (exchangeable) Na', re-sulting in putting the adsorbed Na' in solutionand leaving K + adsorbed as KX. Cations are ad-sorbed and exchanged on a chemically equiva-lent basis, that is, one K+ replaces one Na', andtwo K+ are required to replace or exchange forone Ca

t+ .

The negatively charged micellar surfaces form aboundary along which the negative charge is lo-calized. The cations concentrate near this bound-ary and neutralize the negative charge of themicelle. The exchangeable cations are hydratedand drag along the hydration water molecules asthey constantly move and oscillate around nega-tively charged sites. The concentration of cationsis greatest near the micellar surfaces, where thenegative charge is the strongest. The chargestrength decreases rapidly with increasing dis-tance away from the micelle, and this is associ-ated with a reduction in cation concentrationaway from the micelle. Conversely, the negativelycharged micellar surface repels anions. This re-sults in a decreasing concentration of cations andan increasing concentration of anions with dis-tance away from the micellar surface. At somedistance from the micellar surface, the concentra-tion of cations and anions is equal. These featuresare illustrated in Figure 11.2.

An equilibrium tends to be established betweenthe number of cations adsorbed and the numberof cations in solution. The number of cations insolution is much smaller than the number ad-sorbed (generally 1 percent or less) unless thecontent of soluble salts is high. Roots absorbcations from the soil solution and upset the equi-librium. The uptake of a cation is accompanied bythe excretion of H+ from the root and this restoresthe charge equilibrium in both the plant and soil.Excretion of hydrogen ions tends to increase theacidity in the rhizosphere and the H + may beadsorbed on micelles. The H + is weakly adsorbedonto clay (ionic bonding); however, it is stronglyadsorbed to carboxyl groups of humus (covalentbonding).

SOIL CHEMISTRY

Distancefrom clay surface

FIGURE 11.2 Distribution of cations and anions inthe vicinity of a negatively charged surface.

Cation Exchange Capacity of SoilsThe cation exchange capacity of soils (CEC) isdefined as the sum of positive (+) charges of theadsorbed cations that a soil can adsorb at a spe-cific pH. Each adsorbed K+ contributes one +charge, and each adsorbed Ca

2+contributes

two + charges to the CEC. The CEC is the sum ofthe + charges of all of the adsorbed cations.(Conversely, the CEC is equivalent to the sum ofthe - charges of the cation exchange sites.)

The CEC is commonly expressed as centimolesof positive charge per kilogram [cmol(+)/kg],also written as cmol/kg, of oven dry soil. A moleof positive charge (+) is equal to 6.02 x 10

23

charges. For each cmol of CEC per kg there are6.02 x 1021 + charges on the adsorbed cations.

For many years the CEC of soils was expressedas milliequivalents of cations adsorbed per 100grams of oven dry soil (meq/ 100 g). Many agricul-tural publications still use meq/100 g, whichmeans that many persons need to be familiar withboth systems. The CEC values are the same inboth systems; that is, a CEC of 10 cmol/kg is equalto a CEC of 10 meq/100 g

The CEC of a soil is equal to the CEC of both themineral and organic fractions. The major sourceof CEC in the mineral fraction comes from theclay. The negative charge of organic matter is due

mainly to dissociation of H+ (deprotonation)from the -OH of carboxyl and phenolic groups.The CEC of soil organic matter (SOM) ranges from100 to 400 cmol/kg, depending on degree of de-composition. There is an increase in the groupscontributing to the CEC as organic matter be-comes more decomposed or more humified. Asmentioned in the previous chapter, the clays havegreat variation in CEC, ranging from less than10 cmol/kg for oxidic clay, to a 100 cmol/kg ormore for some 2:1 clay. The sand and silt fractionscontribute little to the CEC of soils. As a conse-quence, the CEC of soils is affected mainly by theamount and type of clay and the amount anddegree of decomposition of the organic matter.

In the A horizons of mineral soils, the organicmatter and clay frequently make similar contribu-tions to the CEC. In subsoils, such as Bt horizonswhere clay has accumulated, the clay commonlycontributes more to the CEC than the organic mat-ter. The accumulation of humus in Bhs or Bhhorizons contributes much CEC to these subsoilhorizons.

Cation Exchange Capacity versusSoil pHThe definition of CEC specifies that the CEC ap-plies to a specific pH because the CEC is pHdependent. To make valid comparisons of CECbetween soils and various materials, it is neces-sary to make the determination of CEC at a com-mon pH. The CEC is positively correlated with pH;therefore, acid soils have a CEC less than themaximum potential CEC.

Most of the CEC of 2:1 clays is permanent ornon-pH dependent. By contrast, the CEC of SOM isentirely pH-dependent. Soils high in organic mat-ter content, kaolinite (1:1 clay), and oxidic clayshave relatively large changes in CEC with changesin pH owing to the predominance of pH-dependent charge. The changes in the CEC withchanges in pH of the horizons in a very sandy soilthat has a Bh horizon is shown in Figure 11.3.

Changes in CEC with changes in soil pH are

I ON EXCHANGE 167

FIGURE 11.3 The cation exchange capacities in asand-textured soil at the soils natural pH anddetermined at pH 7.0 and 8.2. The increases incation exchange capacity with increasing pH areclosely related to the content of organic carbon(organic matter) in this soil which has a Bh horizonand contains very little clay. (Adapted from Holzhey,Daniels, and Gamble, 1975).

important in the management of intensivelyweathered and acid soils in tropical regions be-cause of the generally low CEC and the highlypH-dependent nature of the CEC. For minimallyand moderately weathered mineral soils, wheremuch of the CEC originates from the permanentcharge of 2:1 clays, changes in CEC resulting fromchanges in soil pH have much less importance.For plants, the current soil pH is the relevant pH.Values given for the CEC are for soils at theirnatural, or current pH, unless otherwise noted.The CEC at the soils' current pH is called theeffective cation exchange capacity (ECEC).

Kinds and Amounts ofExchangeable CationsThe major sources of cations are mineral weather-i ng, mineralization of organic matter, and soilamendments, particularly lime and fertilizers. Theamounts and kinds of cations adsorbed are theresult of the interaction of the concentration ofcations in solution and the energy of adsorption ofthe cations for the exchange surface. The cationscompete for adsorption. As the concentration of acation in the soil solution increases, there is ani ncreased chance for adsorption. The morestrongly a cation is attracted to the exchange sur-face (energy of adsorption); the greater is thechance for adsorption.

The energy of adsorption of specific ions isrelated to valence and degree of hydration. Theenergy of adsorption of divalent cations is abouttwice that of monovalent cations. For cations ofequal valence, the cation with the smallest hy-drated radius is most strongly adsorbed becauseit can move closer to the site of charge. The dehy-drated and hydrated radi of four monovalentcations are given in Table 11.2. Rubidium (Rb) isthe most strongly adsorbed, whereas lithium (Li)is the most weakly adsorbed. Note that the hy-drated radius is inversely related to the nonhy-drated radius. This is because the water mole-cules can move closer to the charge that is locatedat the center of a small ion and, therefore, thewater molecules are more strongly attracted tosmaller ions. The more strongly that water mole-

TABLE 11.2 Ionic Radii and Exchange Efficiency ofSeveral Monovalent Ions

cules are attracted to ions, the greater is the num-ber of hydrated molecules and the hydrated ra-dius of the ions.

The differences in the hydrated radius and va-l ence of Ca t+ and Na' are illustrated in Figure11.4. Calcium is adsorbed more strongly than so-dium because: (1) it is divalent, and (2) it has thesmallest hydrated radius. As a result of the smallenergy of adsorption of sodium, it is more likely toexist in the soil solution and be removed from thesoil by leaching. As a result of the strong energy ofadsorption, calcium is typically more abundant asan exchangeable cation than are magnesium, po-tassium, or sodium. The energy of adsorption se-quence is: Ca >Mg > K > Na. The cmol/kg(meq/ 100 g) of these four cations for a moderatelyweathered soil is given in Table 11.3. Note thattheir abundance follows the energy of adsorptionsequence and the dominance of calcium.

The symbol, X, has already been used in acation exchange reaction, as in NaX. In NaX, aNa' is adsorbed onto a cation exchange site. Bytransposing the X to form XNa, exchangeable so-dium is formed. XNa is an abbreviation forexchangeable Na. Pronounce XNa, as "X Na,"or as exchangeable Na, and write it as XNa. Ex-changeable calcium is XCa. This nomenclaturewill be used in reference to the exchangeablecations.

168 SOIL CHEMISTRY

TABLE 11.3 Exchangeable Calcium, Magnesium,Potassium and Sodium in Tama Silt Loam

Adapted from Soil Survey Investigations Report, No. 3, I owa,SCS, USDA, 1966.

Exchangeable Cations as a Source ofPlant NutrientsOf the four cations referred to in Table 11.3, threeare essential for plants. The sodium may be ab-sorbed by plants, may substitute for some po-tassium, but sodium is not considered an essen-tial element. It is, however, needed by animals,and its uptake by plants and its presence in food isi mportant. It is paradoxical that, in regard to XCaand XK, the XCa is much more abundant in soilsthan is XK. On the other hand, plants generallyabsorb much more potassium than calcium. Theresult is that calcium is rarely deficient in soils forplant growth; by contrast, potassium frequentlylimits crop yields in the humid regions and is acommon element in fertilizers. This is reflected inthe data of Table 11.4, which are based on the

I ON EXCHANGE 169

data in Table 11.3. A plow layer would containenough XCa to last a high-yielding corn crop 112years, the XMg would last for 27 years, whereasthe XK would provide only a 2-year supply.

When the exchangeable cations in the underly-ing horizons are considered, it can be seen thatthe exchangeable cations are an important supplyof available nutrients. Small, but important, quan-tities of other cations, including iron, manganese,copper, and zinc also exist in the soil as ex-changeable cations.

Anion ExchangeAnion exchange sites arise from the protonationof hydroxyls on the edges of silicate clays and onthe surfaces of oxidic clays. The anion exchangecapacity is inversely related to soil pH and is,perhaps, of greatest importance in acid soils dom-inated by oxidic clays. Nitrate (N03

-) is veryweakly adsorbed in soils and, as a consequence,remains in the soil solution where it is very sus-ceptible to leaching and removal from soils.Anion exchange reactions with phosphate mayresult in such strong adsorption as to render thephosphate unavailable to plants. Uptake of anionsby roots is accompanied by the excretion of OH -

or HC03- .

In general, plants absorb as many anions ascations. The availability to plants of the anionsnitrate, phosphate, and sulfate is related to miner-alization from organic matter, as well as anionexchange.

TABLE 11.4 Amounts of Exchangeable Calcium, Magnesium and Potassiumin 2 Million Pounds in the Ap horizon of Tama Silt Loam and Adequacy forPlant Growth

17 0

SOIL pH

The pH is the negative logarithm of the hydrogen

ion activity. The pH of a soil is one of the mosti mportant properties involved in plant growth.There are many soil pH relationships, includingthose of ion exchange capacity and nutrient avail-ability. For example, iron compounds decrease insolubility with increasing pH, resulting in manyi nstances where a high soil pH (soil alkalinity)causes iron deficiency for plant growth.

Determination of Soil pHSoil pH is commonly determined by: (1) mixingone part of soil with two parts of distilled water orneutral salt solution, (2) occasional mixing over aperiod of 30 minutes to allow soil and water. toapproach an equilibrium condition, and, (3) mea-suring the pH of the soil-water suspension using apH meter. A rapid method using pH indicator dyeis illustrated in Figure 11.5. The common range ofsoil pH is 4 to 10.

Sources of AlkalinityParent materials have a wide ranging mineral-ogical composition and pH, and young soils in-

SOIL CHEMISTRY

herit these properties from the parent material.Throughout the world there are parent materialsand soils that contain several percent or more ofcalcium carbonate; they are calcareous. Whencalcareous soil is treated with dilute HCI, carbondioxide gas is produced, and the soil is said toeffervesce.

Carbonate Hydrolysis The hydrolysis of cal-cium carbonate produces OH - , which contrib-utes to alkalinity in soils:

Calcium carbonate is only slightly soluble, andthis reaction can produce a soil pH as high as 8.3,assuming equilibrium with atmospheric carbondioxide. In calcareous soil, carbonate hydrolysiscontrols soil pH. When a soil contains Na 2 CO 3 ,the pH may be as high as 10 or more, which iscaused by the greater solubility of Na 2 CO 3and greater production of OH- by hydrolysis ina similar manner. Sodium-affected soils with15 percent or more of the CEC saturated withNa' are called sodic and are also highly alka-line.

FIGURE 11.5 To determinesoil pH with color indicatordye solution: (1) Place a smallamount of soil in the foldedcrease of a strip of wax paper;(2) add 1 or 2 drops moreindicator than the soil canabsorb, tilt up and down toallow indicator to equilibratewith soil, and (3) separateexcess dye from soil with tipof a pencil and match dyecolor with the color chart.

http://water.to

Mineral Weathering Some rocks and mineralsweather to produce an acidic effect. The weather-i ng of many primary minerals, however, contrib-utes to alkalinity. This is the result of the con-sumption of H + and the production of OH - . Forexample, the hydrolysis of anorthite, (calciumfeldspar), produces a moderately strong base:

In the reaction, an aluminosilicate clay mineralwas formed together with the moderately strongbase, calcium hydroxide. The net effect is basic.The generalized weathering reaction, in which Mrepresents metal ions such as calcium, magne-sium, potassium, or sodium, is

As long as a soil system remains calcareous,carbonate hydrolysis dominates the system andmaintains a pH that ranges from 7.5 to 8.3 ormore. Mineral weathering is minor under theseconditions, and the mineralogy and pH of thesoils change little, if at all, with time. The develop-ment of soil acidity requires the removal of thecarbonates by leaching. When acidity develops,the weathering of primary minerals is greatly in-creased, causing an increase in the release ofcations-calcium, magnesium, potassium andsodium. Leaching of these cations in humid re-gions, however, eventually results in the develop-ment of soil acidity.

Calcium and magnesium are alkaline earthmetals or cations, and potassium and sodium arealkali metals. These cations are called basic

cations because soils tend to be alkaline (basic)or neutral when the CEC is entirely saturated withthem. Leaching of the basic cations (Ca, Mg, K,and Na) results in their replacement by Al" andH+, and soils then become acid. The cations A13 +and H+ are called the acidic cations.

SOIL pH

171

Sources of AcidityThree important processes, or reactions, contrib-ute to a more or less continuous addition of H + tosoils, and these processes tend to make soilsacid. In all soils respiration by roots and other soilorganisms produces carbon dioxide that reactswith water to form carbonic acid (H2CO3). This isa weak acid, which contributes H+ to the soilsolution. Second, acidity is produced when or-ganic matter is mineralized, because organicacids are formed and the mineralized nitrogenand sulfur are oxidized to nitric and sulfuric acid,respectively. Third, natural or normal precipi-tation reacts with carbon dioxide of the atmo-sphere and the carbonic acid formed gives naturalprecipitation a pH of about 5.6. The results ofthese reactions add acidity continuously to soils.

In desert regions there is little precipitationand, thus, these reactions contribute little acidicinput to soils. As precipitation increases there isan increase in all three of the previously men-tioned processes. A larger amount of precipitationcontains more acidity. Greater precipitation re-sults in more plant growth, causing more respira-tion and organic matter mineralization. Manystudies have shown a relationship between an-nual precipitation, the leaching of carbonates,and soil pH, as given in Figure 11.6.

In soils with a pH of 7, there is a balance in theprocesses or reactions that produce alkalinity andacidity. On the central U.S. Great Plains this oc-curs with about 26 inches (65 cm) of annual pre-cipitation (near the Iowa-Nebraska border). Thedominant upland soils have developed under agrass vegetation and are quite fertile for agricul-tural crops.

Development and Properties ofAcid SoilsI n a calcareous soil, OH - from carbonate hydroly-sis reacts with any H + that is produced to formwater; thus, calcareous soils remain alkaline inspite of the near constant production or additionof H+ . As long as soils remain calcareous, they

172

remain alkaline. Development of neutrality, andeventually acidity, is dependent on the removal ofthe carbonates. This requires sufficient precipi-tation to cause effective leaching. This amount ofprecipitation also results in considerable acidici nput. The weathering of primary minerals and therelease of basic cations tend to repress the acidi-fying effects of respiration, precipitation, and min-eralization of organic matter. Any H + in the soilsolution neutralizes the OH - produced by min-eral weathering and shifts the weathering reac-tions to the right, or encourages further weather-ing of primary minerals and formation of OH - .Despite the neutralizing effect of mineral weather-ing, soils in humid regions become increasinglyacid with time as leaching continues.

The effects of the acidifying processes are pro-gressive; the effects producing acid A horizonsfirst, then acid B horizons, and, lastly, acid Chorizons. Many minimally weathered and leachedsoils have a neutral or alkaline reaction in allhorizons and in intensively weathered and

SOIL CHEMISTRY

FIGURE 11.6 Generalrelationship of upland soilsbetween annual precipitation,depth of leaching of carbonates,and pH of the surface soil in thecentral United States. (Adaptedfrom Jenny, 1941.)

leached soils, all of the horizons are acid. Bycontrast, many moderately weathered andleached soils have acid A and B horizons andnuetral or alkaline (many being calcareous) Chorizons.

Role of Aluminum After leaching has removedthe carbonates, a progressive loss of XCa, XMg,XK, and XNa occurs as leaching continues. Forexample, the removal of an exchangeable po-tassium ion, K+, is accompanied by the removalof an acid anion, like nitrate. The H + associatedwith the nitrate remains behind and competes foradsorption onto the exchange position vacated bythe potassium. Adsorption of H+ at the edge ofsilicate clay minerals makes aluminum unstable,and it exits from the edge of the clay lattice.Subsequent hydrolysis of the released A1 3 + pro-duces H + :

Hydroxy-aluminum from the preceding reactionhydrolyzes to produce additional H + :

Consequently, acidity in soil stimulates the devel-opment of additional acidity through aluminumhydrolysis, and aluminum hydrolysis becomes avery important source of H+ when soils becomeacid.

The hydroxy-aluminum (hydroxy-Al) formed inthe preceding reactions consists of AI-0H octahe-dra that polymerize and produce large cationswith a high valence. These large cations are verystrongly adsorbed onto cation exchange sites be-cause of their high valence. In fact, the adsorptionis so strong that they are not exchangeable in thesense that XCa and XNa are exchangeable. Sitesthat adsorb hydroxy-aluminum cease to func-tion for cation exchange, and the adsorption ofhydroxy-aluminum reduces the CEC. This is onemore reason for the soil's pH-dependent cationexchange capacity. The adsorption of hydroxy-aluminum in the interlayer space of expandingclays renders them nonexpandable. The ad-sorption of large hydroxy-aluminum cations in theinterlayer space of 2:1 clay is illustrated in Figure11.7.

When soil pH declines below 5.5, A1 3+ beginsto occupy cation exchange sites and exist as an

FIGURE 11.7 Drawing of two layers of a 2:1expanding clay with hydroxy-Al adsorbed in thei nterlayer space. This adsorption is very strong andreduces the cation exchange capacity. The mutalattraction of the two clay layers to the positivelycharged hydroxy-Al causes the clay to become anonexpanding, hydroxy-interlayered clay.

SOIL pH

173

exchangeable cation (XAI). The aluminum ionhas a valence of 3 + , and it is adsorbed much morestrongly than divalent and monovalent cations.The amount of XAI increases over time as the soilbecomes more leached and more acid. The in-crease in XAL is accompanied by a decrease inXCa, XMg, XK, and XNa.

Moderately versus Intensively WeatheredSoils As long as soils remain only moderatelyweathered, the weathering of primary mineralsand the release of calcium, magnesium, po-tassium, and sodium contribute to the mainte-nance of a good supply of these elements. Theseelements, or basic cations, occupy most of thecation exchange sites, and there is little ex-changeable aluminum. As moderately weatheredsoils evolve into intensively weathered soil,however, a considerable change in mineralogicalcomposition occurs. The high CEC 2:1 clays dis-appear, and low CEC clays, such as kaolinite andoxidic clay, accumulate. There is a large decreasein the CEC, and soils have a small capacity foradsorption of nutrient cations. In addition, withthe disappearance of most of the primary miner-als, there is a very limited release of calcium,magnesium, potassium, and sodium by weather-ing. The aluminum saturation of the CEC rangesfrom about 0 percent aluminum saturation at pH5.5 to nearly 100 percent aluminum (XAl) satura-tion at pH 4. The saturation of the CEC with basiccations (calcium, magnesium, potassium, andpotassium) decreases from nearly 100 percent atpH about 5.5 or 6 to about 0 percent at pH 4.0,causing a great decline in the amount of ex-changeable nutrient cations. In essence, intensiveweathering results in acid soils that are naturallyinfertile.

The data in Table 11.5 are for three intensivelyweathered soils on the coastal plain of the south-eastern United States. The pH is below 5, andthe CEC averages only 2.68 cmol/kg (2.68 meq/100 g). These soils are characterized by very lowfertility as suggested by the small amount of XCa

174

TABLE 11.5 The pH, Exchangeable Cations (KCI Extractable), Cation Exchange Capacity, and AluminumSaturation in Three Intensively Weathered Soils

and XMg (XK and XNa are also low). The averagealuminum saturation of the cation exchange ca-pacity (cmol of XAI divided by the CEC and times100) is 88 percent. When the aluminum saturationof soils exceeds 60 percent, there is frequentlyenough aluminum (A1 3+) in solution to be toxicto plant roots. The relationship between theamounts of exchangeable aluminum and calciumand the growth of corn roots is shown in Figure11.8.

Whereas calcium and magnesium deficiencesfor plants are rather uncommon on acid soils thatare moderately weathered, low XMg and XCacommonly result in plant deficiences on acid andintensively weathered soils. A comparison of thedata in Table 11.5 with the data in Table 11.3shows these large differences in exchangeablecations.

Role of Strong Acids A soil pH below about 3.5and down to 2 is indicative of strong acid in thesoil. Pyrite (FeS 2) occurs in some soils and thesulfur is oxidized to sulfuric acid. Soils containingstrong acid may produce a toxicity of both alumi-num and iron; the soil may be too acid for plantgrowth. These soils are called acid sulfate soils orcat clays, and they tend to form in mangroveswamps along saltwater beaches (see Figure11.9). Sometimes the tailings from mining opera-tions contain pyrite, and the formation of sulfuric

SOIL CHEMISTRY

acid and low pH make reclamation and revegeta-tion of such materials difficult.

Acid Rain Effects The water in precipitationreacts with carbon dioxide in the atmosphere toform carbonic acid. This causes natural, or ordi-nary, precipitation to be acidic and have a pH ofabout 5.6. Industrial gases contain nitrogen andsulfur that result in the formation of acids andcause rainfall to have a pH less than 5.6. Such rainis called acid rain. Studies have shown thatthe amount of acidity contributed to soils by acidrain is usually much less than that normallycontributed by respiration, organic mattermineralization, and the acidity in normal rain-fall.

In soils that are underlain by calcareous layers,any acid water that percolates through the soil willbe neutralized and will not contribute to theacidity of groundwater or nearby lakes. On theLaurentian Shield in northeastern Canada, soilsgenerally are underlain by acid parent materialsand rocks. Here, additions of acidity from acidrain to acid soils contribute to the acidification ofgroundwater and lakes.

Soil Buffer CapacityThe buffer capacity is the ability of ions associ-ated with the solid phase to buffer changes in ion

SOIL pH

FIGURE 11.8 On left, poor corn root growth in a Hawaiian soil associatedwith low exchangeable calcium and high exchangeable aluminum. On right,good corn root growth associated with low exchangeable aluminum and highexchangeable calcium. (Drawn from Green, R. E., Fox, R. L., and D. D. F.Williams, 1965.)

FIGURE 11.9 Mangrove swamps are one of thetypical environments where acid sulfate soilsdevelop.

175

concentration in the solution phase. In acid soils,buffering refers to the ability of the XAl, XH, andhydroxy-aluminum to maintain a certain concen-tration of H+ in solution. The amount of H+ in thesoil solution of a soil with a pH of 6.0, for exam-ple, is extremely small compared with the non-dissociated H+ adsorbed and the amount ofaluminum that can hydrolyze to produce H + . Neu-tralization of the active or solution H + results inrapid replacement of H+ from the relatively largeamount of H+ associated with the solid phase.Thus, the soil exhibits great resistance to undergoa pH change.

176

Summary StatementWith a long period of time, soil evolution in humidregions causes changes in both the mineralogicaland chemical properties and changes in soil pH.Minimally weathered soils tend to be alkaline orslightly acid and fertile. By contrast, intensivelyweathered soils are very acid and infertile. A sum-mary of the major processes and changes thatproduce various ranges in soil pH and accom-panying properties is as follows.

1. In the pH range 7.5 to 8.3 or above, soil pH iscontrolled mainly by carbonate hydrolysis.

2. If a soil is calcareous, removal of carbonatesby leaching is required before a neutral oracid soil can develop.

3. Soils naturally at or near neutral tend to occurin regions of limited precipitation where thereis a balance between the production of H +

and OH - . The acidic inputs from biologicalrespiration, organic matter mineralization,and precipitation are balanced by the basici nputs of mineral weathering.

4. In humid regions, soils tend to become acidover time as basic cations decrease andacidic cations increase. In the range of about7 to 5.5, H+ for the soil solution is supplied

SOIL CHEMISTRY

mainly by hydroxy-aluminum hydrolysis, andin the range 5.5 to about 4 the H+ for the soilsolution comes mainly from exchangeablealuminum hydrolysis. Soils high in organicmatter may receive a significant amount ofH+ from exchangeable hydrogen.

5. Intensively weathered soils in the advancedstages of weathering have: (1) low CEC,(2) high saturation of the cation exchangecapacity with aluminum, and (3) very smallamounts of exchangeable calcium, magne-sium, potassium, and sodium. Intensivelyweathered soils tend to produce aluminumtoxicity and deficiencies of magnesium andcalcium for plant growth.

6. In the most acid soils, pH < 4, free strongacid exists.

SIGNIFICANCE OF SOIL pHStudies have shown that the actual concentrationsof H+ or OH - are not very important, except un-der the most extreme circumstance. The associ-ated chemical or biological environment of a cer-tain pH is the most important factor. Some soilorganisms have a rather limited tolerance to varia-

FIGURE 11.10. Red pine wasplanted along the edges ofthis field to serve as awindbreak. The red pine inthe foreground andbackground failed to survivebecause these areas were partof an old limestone road andthe soil was alkaline. Whitecedar was later planted and isgrowing very well on the oldroad.

tions in pH, but other organisms can tolerate awide pH range. The effects of limestone gravelfrom an old roadway on the growth of red pineand white cedar trees is shown in Figure 11.10.

Nutrient Availability and pHPerhaps the greatest general influence of pH onplant growth is its effect on the availability ofnutrients for plants, as shown in Figure 11.11.Nitrogen availability is maximum between pH 6and 8, because this is the most favorable range forthe soil microbes that mineralize the nitrogen inorganic matter and those organisms that fix nitro-gen symbiotically. High phosphorus availability athigh pH-above 8.5-is due to sodium phos-phates that have high solubility. In calcareoussoil, pH 7.5 to 8.3, phosphorus availability is re-

SIGNIFICANCE OF SOIL pH 177

duced by the presence of calcium carbonate thatrepresses the dissolution of calcium phosphates.Maximum phosphorus availablity is in the range7.5 to 6.5. Below pH 6.5, increasing acidity isassociated with increasing iron and aluminum insolution and the formation of relatively insolubleiron and aluminum phosphates.

Note that potassium, calcium, and magnesiumare widely available in alkaline soils. As soilacidity increases, these nutrients show less avail-ability as a result of the decreasing CEC anddecreased amounts of exchangeable nutrientcations: decreased amounts of XCa, XMg, and XK.

Iron and manganese availability increase withincreasing acidity because of their increased sol-ubility. These two nutrients are frequently defi-cient in plants growing in alkaline soils becauseof the insolubility of their compounds (see Figure

FIGURE 11.11 Generalrelationship between soil pH andavailability of plant nutrients inminimally and moderatelyweathered soils; the wider thebar, the more availability.

178

11.11). Boron, copper, and zinc are leachable andcan be deficient in leached, acid soils. Con-versely, they can become insoluble (fixed) andunavailable in alkaline soils. In acid soils, molyb-denum is commonly deficient owing to its reac-tion with iron to form an insoluble compound.

The relationships shown in Figure 11.11 are forminimally and moderately weathered soils. Forplant nutrients as a whole, good overall nutrientavailability occurs near pH 6.5. In intensivelyweathered soils, some nutrients such as boronand zinc may be in very short supply and becomedeficient at lower pH than for minimally and mod-erately weathered soils. As a consequence, a de-sirable pH is more typically 5.5 in intensivelyweathered soils.

Effect of pH on Soil OrganismsThe greater capacity of fungi, compared with bac-teria, to thrive in highly acid soils was cited inChapter 8. The pH requirement of some diseaseorganisms is used as a management practice tocontrol disease. One of the best known examplesis that of the maintenance of acid soil to controlpotato scab. Potato varieties have now been de-veloped that resist scab organisms in neutral andalkaline soils. Damping-off disease in nurseries iscontrolled by maintaining soil pH at 5.5 or less.

Earthworms are inhibited by high soil acidity.Peter Farb (1959) relates an interesting case inwhich soil pH influenced both earthworm andmole populations. On a certain tennis court inEngland, each spring after rain and snow hadwashed away the court markings, the markinglines were still visible because moles had uner-ringly followed them. This intriguing mystery wasinvestigated and produced the following solution.The tennis court had been built on acid soil andchalk (lime) was used to mark the lines. As aresult, the pH in the soil below the lines wasincreased. Earthworms then inhabited the soil un-der the markings, and the soil remaining on thetennis court remained acid and uninhabitated by

SOIL CHEMISTRY

earthworms. Since earthworms are the primaryfood of moles, the moles restricted their activityonly to the soil under the marking lines.

Toxicities in Acid SoilsFigure 11.11 shows that iron and manganese aremost available in highly acid soils. Manganesetoxicity may occur when soil pH is about 4.5 orless. High levels of exchangeable aluminum inmany acid subsoils of the southeastern UnitedStates cause high levels of solution aluminum andrestrict root growth. Plants, and even varieties ofthe same species, exhibit differences in toleranceto high levels of aluminum, iron, and manganesein solution and to other factors associated withsoil pH. This gives rise to the pH preferences ofplants.

pH Preferences of PlantsBlueberries are well known for their acid soil re-quirement. In Table 11.6, the optimum pH rangefor blueberries is 4 to 5. Other plants that prefer apH of 5 or less include: azaleas, orchids, sphag-num moss, jack pine, black spruce, and cran-berry. In Table 11.6 the names of plants that prefera pH of 6 or less are italicized. Most field andgarden crops prefer a pH of about 6 or higher.Because plants have specific soil pH require-ments, there is a need to alter or manage soil pHfor their successful growth.

MANAGEMENT OF SOIL pHThere are two approaches to assure that plantswill grow without serious inhibition from unfa-vorable soil pH: (1) plants can be selected thatgrow well at the existing soil pH, or (2) the pH ofthe soil can be altered to suit the needs of theplants. Most soil pH changes are directed towardreduced soil acidity and increased pH by liming.

MANAGEMENT OF SOIL pH

The Lime Requirement acidity and to furnish calcium and magnesium forAgricultural lime is a soil amendment containing plant growth. Most frequently, agricultural lime iscalcium carbonate, magnesium carbonate, and ground limestone that is mainly CaC0 3 and isother materials, which are used to neutralize soil

calcitic lime. If the lime also contains a significant

179

180

amount of MgC03 , the lime is dolomitic. Theamount of lime needed or the lime requirement isthe amount of liming material required to producea specific pH or to reduce exchangeable and soilsolution aluminum.

Lime Requirement of Intensively WeatheredSoils Intensively weathered acid soils are char-acterized by low CEC that is highly aluminumsaturated and with low calcium and magnesiumsaturation. The main benefits from liming are:(1) reduced exchangeable aluminum and re-duced aluminum in solution, (2) an increase inthe amount of XCa and/or XMg, and (3) increasedavailability of molybdenum.

The lime requirement of intensively weatheredsoils is the amount of liming material needed tochange the soil to a specific soluble aluminumcontent. This is commonly considered to be equalto 1.5 times the amount of exchangeable alumi-num. Thus, the lime requirement of the Rains soil(see Table 11.5) is equal to

The lime requirement for the Rains example isthe amount of lime equal to 2.43 cmol of + chargeper kilogram of soil. A mole of CaC03 weighs 100grams. Because of the divalent nature of the cal-cium, a half mole of CaC03, 50 g, will neutralize amole of + charge. The amount of CaC03 equal toI cmol of + charge is 0.5 g (50 g/100). Therefore,1.21 g/kg (2.43 cmol/kg x 0.5 g/kg) of CaC0 3 isthe lime requirement. Assuming a 2 million kgfurrow slice of soil per hectare, the lime needed is2,420 kg/ha.

The milliequivalent weight of CaC0 3 is 0.05 g. IfCaC0 3 is used, the lime requirement is 0.12 g(2.43 meq x 0.05 g per meq) of CaC03 per 100 gof soil. Assuming an acre furrow slice weighs 2million pounds, 2,400 pounds of CaC0 3 are re-quired per acre. The rate must be adjusted toaccount for different thicknesses or weights ofplow layers.

In many soils, roots are susceptible to alumi-

SOIL CHEMISTRY

num toxicity in the subsoil that restricts rootingdepth and water uptake. Soils are droughty, whichis a difficult problem to solve, because the limehas low solubility and moves downward slowly.Overliming the plow layer, to promote the down-ard movement of lime, may increase pH enoughto cause micronutrient deficiences or toxicities.

Lime Requirement of Minimally and Moder-ately Weathered Soils Moderately and mini-mally weathered acid soils typically have a largeCEC that has low aluminum saturation. Aluminumtoxicity and calcium deficiency are rare and, onlyoccasionally, is there a magnesium deficiency.The lime requirement is the amount needed toincrease or adjust pH to a more favorable value, asshown in Figure 11.12 for production of soybeans.

Consider a moderately weathered soil with apH of 5.0 for the growth of soybeans, which have apH preference of 6 to 7. The lime requirement isthe amount of lime required to adjust the pH to amore favorable value for the crop being grown; forsoybeans it would be about 6.5. The same is truefor many other legumes, including alfalfa andclover. Farmers who continuously grow corn canmaintain a lower soil pH than those producingsoybeans and alfalfa, because corn is less respon-

FIGURE 11.12 Growth of soybeans versus soil pH.A pH of 6.5 is good for soybean production.

sive to pH adjustment than are soybeans and al-falfa.

The amount of lime required to adjust the pH ofsoils depends on the amount of pH adjustmentneeded and on the amount of buffer acidity. Theamount of buffer acidity is related to the CEC. Asshown in Figure 11.13, much less base (or lime),is required to increase the pH of Plainfield sandthan of Granby loam.

It is a common practice in soil testing laborato-ries to make a direct measurement of the limerequirement by using a buffer solution. A buffersolution with a pH of 7.5, for example, is mixedwith a known quantity of soil. After a standardperiod of stirring, the pH of the buffer-soil suspen-sion is made. The amount of depression in pHfrom that of the buffer solution is directly relatedto the amount of lime needed to adjust soil pH to aparticular value.

A less accurate but useful method, especiallyfor the gardener or homeowner, is to determinesoil pH with inexpensive color indicator dyes. Thelime requirement is obtained from a chart thatrelates lime need to soil pH and texture, essen-

MANAGEMENT OF SOIL pH 181

tially CEC, as shown in Table 11.7. The data inTable 11.7 show lower lime requirements in thesouthern coastal states, because the soils containclays (such as kaolinite) with lower CEC.

The Liming Equation and Soil BufferingIn acid soils, H+ in the soil solution comes mainlyfrom aluminum hydrolysis. Generally, a minoramount comes from XH unless there is a highorganic matter content. Below pH 5.5, the hydro-lysis is of exchangeable aluminum and, between5.5 and 7, mainly from hydroxy-aluminum hydro-lysis. The dominant aluminum form at pH 7 isAI(OH) 3 , which has very low solubility and, for allpractical purposes, is inert. Lime hydrolyzes inthe soil to form OH- :

The hydroxyls neutralize the H + that are producedfrom aluminum hydrolysis, gradually the ex-changeable aluminum and hydroxy-aluminumare converted to AI(OH) 3 , and the pH increases.

FIGURE 11.13 Titration curvesshowing the different amounts ofbase needed to increase the pHof soils having increasing cationexchange capacities from left toright.

182

The overall reaction representing the neutraliza-tion of Al-derived soil acidity is

Exchangeable and hydroxy aluminum are precipi-tated as insoluble aluminum hydroxide, and theamount of XCa is increased. The reduction inacidic cations and increase in basic cations re-sults in an increase in soil pH. Liming can beviewed as reversing the processes that producedthe soil acidity.

Some Considerations in Lime UseIn agriculture, liming is usually done by applyingground limestone with spreaders, as shown inFigure 11.14. Major considerations are the purity,or neutralizing value, and fineness of the lime.Calcium carbonate is quite insoluble, and parti-cles that will not pass a 20-mesh screen are quiteineffective. It takes at least 6 months for lime toappreciably increase soil pH, and lime may beapplied whenever it is convenient to do so. Cal-cium hydroxide (Ca(OH) 2) is also used as a lim-ing material. It has greater neutralizing value perpound and is more soluble than the carbonateform. Wood ashes can also be used to increasesoil pH.

SOIL CHEMISTRY

TABLE 11.7 Suggested Applications of Finely Ground Limestone to Raisethe pH of a 7-inch Layer of Several Textural Classes of Acidic Soils, in Poundsper 1,000 ft2

Acid soils that have been limed are still sub-j ected to the natural acidifying effects of biologi-cal respiration, organic matter mineralization,and precipitation. Thus, soils again become acidand must be relimed about every 2 to 5 years,depending on variables such as lime applicationrate, CEC, rainfall, crop removal of calcium andmagnesium, and fertilization practices.

Management of Calcareous SoilsMillions of acres of soil are calcareous in aridregions. In humid regions, many calcareous soilsoccur on floodplains and on recently drained lakeplains. The pH ranges from about 7.5 to 8.3, andmany of these calcareous soils are excellent for

FIGURE 11.14 Broadcast application of lime onacid soil.

agricultural use. Plants growing on calcareoussoils are sometimes deficient in iron, manganese,zinc, and/or boron. To lower soil pH would re-quire the leaching of the carbonates, which isi mpractical. As a result, crops are fertilized withappropriate nutrients to overcome the deficien-cies, or crops that grow well on calcareous soilare selected.

Soil AcidulationThe addition of acid sphagnum peat to soil mayhave some acidifying effect. However, significantand dependable increases in soil acidity are prob-ably best achieved through the use of sulfur. Sul-fur is slowly converted to sulfuric acid by soilmicrobes, and the soil slowly becomes more acidover a period of several months or a year. As withli me, the amount of sulfur required varies with thepH change desired and the soil CEC. Sulfur isused to change soil pH in large agricultural fieldsi n arid regions, but it is less common a practicethan the use of lime in humid regions. Sulfur iscommonly used in nurseries and gardens. Rec-ommendations for small areas are given in Table11.8.

EFFECTS OF FLOODING ONCHEMICAL PROPERTIESI n well-aerated soils, the soil atmosphere con-tains an abundant supply of oxygen for microbialrespiration and organic matter decomposition.

From Kellogg, 1957.

EFFECTS OF FLOODING ON CHEMICAL PROPERTIES

TABLE 11.8 Suggested Applications of Ordinary Powdered Sulfur to Reduce the pH of 100 ft 2 of a 20Centimeter Layer of Sand or Loam Soil

183

Oxygen serves as the dominant electron acceptori n the transformation of organic substrates to car-bon dioxide and water. When soils are floodednaturally, or artificially as in rice production, aunique oxidation-reduction profile is created. Athin, oxidized surface soil layer results from thediffusion of oxygen from the water into the soilsurface. This layer is underlain by a much thickerlayer that is reduced (see Figure 4.11). Oxygen isdeficient in the reduced zone, which initiates aseries of reactions that drastically alter the soil'schemical environment.

Dominant Oxidation andReduction ReactionsFlooded soils with a good supply of readily de-composable organic matter may become de-pleted of oxygen within a day. Then, anaerobicand facultative microbes multiply and continuethe decomposition process. In the absence of ox-ygen, other electron acceptors begin to function,depending on their tendency to accept electrons.Nitrate is reduced first, followed by manganiccompounds, ferric compounds, sulfate and, fi-nally, sulfite. The reactions are as follows (reduc-tion to the right and oxidation to the left):

Weeks submergedFIGURE 11.15 Flooding soil increases pH of acid soils and decreases thepH of alkaline soils. The pH changes are related to the soil's content of ironand organic matter. (Data from F. N. Ponnamperuma, 1976.)

184

Soils have various levels of reducing condi-tions, depending on the quantity of decomposa-ble organic matter, the amount of oxygen broughtin by water, and the temperature. The formation ofS2- (sulfide) occurs in a strongly reduced envi-ronment, resulting in the formation of FeS 2 andH 2S. In some instances, toxic amounts of H 2S areproduced for rice production. About two weeksare needed after flooding to create rather stableconditions.

Changes in Soil pHFrom the reactions just cited, there is a con-

sumption of H + and an increase in hydroxyls. Inmost flooded rice fields, iron is the most abun-

SOIL CHEMISTRY

dant electron acceptor (equation 3), and iron re-duction tends to increase soil pH. The productionof carbon dioxide, and the accompanying forma-tion of carbonic acid, have an acidifying effect.Alkaline soils tend to be low in available iron,minimally weathered, and the production of car-bonic acid lowers soil pH. In acid soils the abun-dance of soluble or available iron results in a netincrease in pH due to hydroxyl production. As aconsequence, flooding of most rice fields causesthe pH to move toward neutrality, as shown inFigure 11.15. There is an increase in nutrient avail-ability, and soils generally do not need lime if theyare flooded and puddled for rice production. Theincrease in pH of highly acid soils is sufficient toreduce the threat of aluminum toxicity.

SUMMARYThe kinds and amount of clay minerals and or-ganic matter determine the cation exchange ca-pacity.

The abundance of adsorbed cations in neutraland slightly acid soils is Ca > Mg > K > Na.These cations are held against leaching and areavailable for plant uptake.

The anion exchange capacity is typically muchless than the cation exchange capacity, and itresults from protonation of hydroxyls.

Calcareous soils are alkaline as a result of hy-drolysis of calcium carbonate. Leaching of thecarbonates in calcareous soils produces a pH ofabout 7. Continued leaching results in decreasedamounts of exchangeable calcium, magnesium,potassium, and sodium. There is an increase inexchangeable H and Al with increasing acidity.

In acid mineral soils, H+ for the soil solutioncomes mainly from hydrolysis of aluminum, bothhydroxy and exchangeable forms. ExchangeableH+ increases as organic matter content increasesin acid soils.

The lime requirement of intensively acid soils isequal to about 1.5 times the amount of exchange-able aluminum. In minimally and moderatelyweathered soils, the lime requirement is theamount of lime needed to increase soil pH to adesired value.

Sulfur is used to increase soil acidity.Flooding soils results in a decrease in the pH of

alkaline soils and an increase in the pH of acidsoils.

REFERENCESBohn, H. L., B. L. McNeal, and G. A. O'Connor. 1985.

Soil Chemistry, 2nd ed. Wiley, New York.Binkley, D. 1987. "Use of the Terms 'Base Cation' and

REFERENCES 185

' Base Saturation' Should Be Discouraged." Soil Sci.Soc. Am. J. 51:1089-1090. Madison, Wis.

Clarke, F. W. 1924. "Data of Geochemistry." U.S. Geo.Sur. Bul. 770. Washington, D.C.

Evans, C. E. and E. J. Kamprath. 1970. "Lime Responseas Related to Percent Aluminum Saturation, SolutionAluminum, and Organic Matter Content." Soil Sci.Soc. Am. Proc. 34:893-896.

Farb, P. 1959. Living Earth. Harper & Row, New York.Foth, H. D. 1987. "Proposed Designation Change for

Exchangeable Bases." Agronomy Abstracts, Madi-son, Wis., pp. 1.

Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. Wiley,New York.

Foy, C. D. and G. B. Burns. 1964. "Toxic Factors in AcidSoils." Plant Food Review. Vol. 10.

Green, R. E., R. L., Fox, and D.D.F. Williams. 1965. "SoilProperties Determine Water Availability to Crops."Hawaii Farm Science. 14, (3):6-9. Hawaii Agr. Exp.Sta., Honolulu.

Holzhey, C. S., R. B. Daniels, and E. E. Gamble. 1975."Thick Bh Horizons in the North Carolina CoastalPlain: 11 Physical and Chemical Properties and Ratesof Organic Additions from Surface Sources." Soil Sci.Soc. Am. Proc. 39:1182-1187.

Krug, E. C. and C. R. Frink. 1983. "Acid Rain on AcidSoils." Science. 221:520-525.

Jenny, Hans. 1941. Factors of Soil Formation. McGraw-Hill, New York.

Kellogg, C. E. 1957. "Home Gardens and Lawns." Soil.USDA Yearbook. Washington, D.C.

Kamprath, E. J. and C. D. Foy. 1985. "Lime and FertilizerInteractions in Acid Soils. Fertilizer Technology andUse. Am. Soc. Agron. Madison, Wis.

Ponnamperuma, F. N. 1976. "Specific Soil ChemicalCharacteristics for Rice Production in Asia." IRRIRes. Papers Series, No. 2. Manila.

Spurway. C. H. 1941. "Soil Reaction Preferences ofPlants." Mich. Agr. Exp. Sta. Spec. Bull. 306. EastLansing.

Thomas, G. W. and W. L. Hargrove. 1984. "The Chem-istry of Soil Acidity." In Soil Acidity and Liming.Agronomy 12:1-58. Am. Soc. Agron. Madison, Wis.

http://Mich.Agr.Exp.Sta.Spec.Bull.306.East

http://Mich.Agr.Exp.Sta.Spec.Bull.306.East

PLANT-SOIL MACRONUTRIENTRELATIONS

For many centuries, people knew that manure,ashes, blood, and other substances had a stimu-lating effect on plant growth. The major effect wasfound to result from the essential elements con-tained in these materials. Six of the essential nutri-ents are used in relatively large amounts and arethe macronutrients-usually over 500 parts permillion (ppm) in the plant. Macronutrients in-clude nitrogen, phosphorus, potassium, calcium,magnesium, and sulfur. A list of the macronu-trients and their major roles in plant growthare given in Table 12.1. The quantities of themacronutrients contained in harvested crops aregiven in Table 12.2.

DEFICIENCY SYMPTOMSTranslocation of nutrients within a plant is anongoing process. Early in the gowing season, acorn plant consists mainly of leaves, and the nitro-gen exists mostly in the leaves, as shown in Figure12.1. As the growing season continues, consider-able nitrogen accumulates in the stalks and cobs.The grain (fruit) develops last and has a highpriority for the nutrients within the plant. In thelatter part of the growing season, mobile nutrients

CHAPTER 12

186

are translocated from leaves, stalks, and cobs andare used for the development of the grain (seeFigure 12.1 for nitrogen). When the nutrient con-tent of leaves decreases below critical values dur-i ng the growing season, owing to translocation toother plant parts, specific symptoms develop.

There is a considerable difference in the mobil-ity of various nutrients within plants. A shortage ofa mobile nutrient, such as nitrogen, results in thetranslocation of the nutrient out of the older andfirst formed tissue into the younger tissues. Thiscauses the deficiency symptoms to appear first onthe older or lower leaves. This is true for nitrogen,phosphorus, potassium, and magnesium. Immo-bile plant nutrients cannot be remobilized whena shortage of the nutrient occurs, resulting in adeficiency in the most recent or newly formedtissues. Macronutrients with limited mobility inplants are calcium and sulfur.

NITROGENThe atmosphere is made up of 79 percent nitro-gen, by volume, as inert N2 gas. Although a largequantity of nitrogen exists in the atmosphere, thenutrient that is absorbed from the soil in the great-

TABLE 12.1 Essential Macronutrient Elements andRole in Plants

Element

Role in Plants

Nitrogen (N)

Constituent of all proteins,chlorophyll, and in coenzymes,and nucleic acids.

Phosphorus (P)

I mportant in energy transfer aspart of adenosine triphosphate.Constituent of many proteins,coenzymes, nucleic acids, andmetabolic substrates.

Potassium (K)

Little if any role as constituent ofplant compounds. Functions inregulatory mechanisms asphotosynthesis, carbohydratetranslocation, proteinsynthesis, etc.

Calcium (Ca)

Cell wall component. Plays rolein the structure andpermeability of membranes.

Magnesium (Mg) Constituent of chlorophyll andenzyme activator.

Sulfur (S)

I mportant constituent of plantproteins.

Complied from many sources.

est quantity and is the most limiting nutrient forfood production is nitrogen.

The Soil Nitrogen CycleThe nitrogen cycle consists of a sequence of bio-chemical changes wherein nitrogen is used byliving organisms, transformed upon death and de-composition of the organisms, and converted ulti-mately to its original state of oxidation. The majorsegments of the soil nitrogen cycle are shown asfive steps or processes in Figure 12.2.

The reservoir of nitrogen for plant use is essen-tially that in the atmosphere, N2 . There is virtuallyan inexhaustible supply of nitrogen in the atmo-sphere, since (at sea level) there are about 77,350metric tons of N 2 in the air over I hectare (34,500,tons per acre). It takes about a million years forthe nitrogen in the atmosphere to move throughone cycle.

NITROGEN 187

The N2 is characterized by both an extremelystrong triple bonding between the two nitrogenatoms and a great resistance to react with otherelements. The process of converting N2 into formsthat vascular plants can use is termed nitrogenfixation, (step 1 of Figure 12.2). The other majorsteps in the soil nitrogen cycle are: (2) minerali-zation, (3) nitrification, (4) immobilization, and(5) denitrification.

Dinitrogen FixationDinitrogen fixation is the conversion of molecularnitrogen (N2) to ammonia and subsequently intoorganic forms utilizable in biological processes.Some N2 is fixed by lightning and other ionizingphenomena of the upper atmosphere. This fixednitrogen is added to soils in precipitation.

Most nitrogen fixation is biological, being ei-ther symbiotic or nonsymbiotic. Nitrogen-fixingorganisms contain an enzyme, nitrogenase,which combines with a dinitrogen molecule (N 2 )and fixation occurs in a series of steps that re-duces N 2 to NH3 , as shown in Figure 12.3. Molyb-denum is a part of nitrogenase and is essential forbiological nitrogen fixation. The nitrogen-fixingorganisms also require cobalt, which is an exam-ple of plants need for cobalt.

Symbiotic Legume Fixation Legume plantsare dicots that form a symbiotic nitrogen fixationrelationship with heterotrophic bacteria of the ge-nus Rhizobiurn. When the root of the host planti nvades the site of a dormant bacterial cell, aso-called "recognition event" occurs. A root hairi n contact with the bacterial cell curls around thecell. The dormant bacterial cell then germinatesand forms an infection thread that penetrates theroot and migrates to the center of the root (seeFigure 12.4). Once inside the root, bacteria multi-ply and are transformed into swollen, irregular-shaped bodies called bacteroids. An enlargementof the root occurs and, eventually, a gall or noduleis formed (see Figure 8.12). The bacteroids re-

188 PLANT-SOIL MACRONUTRIENT RELATIONS

TABLE 12.2 Approximate Pounds per Acre of Macronutrients Contained in Crops

PhosphorusPotassium

CropAcreYield Nitrogen as P as K Calcium Magnesium Sulfur

GrainsBarley (grain) 40 bu. 35 7 8 1 2 3

Barley (straw) I ton 15 3 25 8 2 4

Corn (grain) 150 bu. 135 23 33 16 20 14

Corn (stover) 4.5 tons 100 16 120 28 17 10

Oats (grain) 80 bu. 50 9 13 2 3 5

Oats (straw) 2 tons 25 7 66 8 8 9

Rice (grain) 80 bu. 50 9 8 3 4 3Rice (straw) 2.5 tons 30 5 58 9 5

Rye (grain) 30 bu. 35 5 8 2 3 7Rye (straw) 1.5 tons 15 4 21 8 2 3

Sorghum (grain) 60 bu. 50 11 13 4 5 5Sorghum (stover) 3 tons 65 9 79 29 18

Wheat (grain) 40 bu. 50 11 13 1 6 3Wheat (straw) 1.5 tons 20 3 29 6 3 5

HayAlfalfa 4 tons 180 18 150 112 21 19Bluegrass 2 tons 60 9 50 16 7 5Coastal Bermuda 8 tons 185 31 224 59 24 -Cowpea 2 tons 120 11 66 55 15 13Peanut 2.25 tons 105 11 79 45 17 16Red clover 2.5 tons 100 11 83 69 17 7Soybean 2 tons 90 9 42 40 18 10Timothy 2.5 tons 60 11 79 18 6 5

Fruits and vegetablesApples 500 bu. 30 5 37 8 5 10Beans, dry 30 bu. 75 11 21 2 2 5

Cabbage 20 tons 130 16 108 20 8 44Onions 7.5 tons 45 9 33 11 2 18Oranges (70 pound

boxes) 800 boxes 85 13 116 33 12 9

Peaches 600 bu. 35 9 54 4 8 2Potatoes (tubers) 400 bu. 80 13 125 3 6 6

Spinach 5 tons 50 7 25 12 5 4Sweet potatoes

(roots) 300 bu. 45 7 62 4 9 6Tomatoes (fruit) 20 tons 120 18 133 7 11 14Turnips (roots) 10 tons 45 9 75 12 6

Other cropsCotton (seed and

lint) 1,500 lb 40 9 13 2 4 2Cotton (stalks,

leaves, and burs) 2,000 lb 35 5 29 28 8

TABLE 12.2 (continued)

Data from Plant Food Review, 1962, pp. 22-25, publication of The Fertilizer Institute.a These values are about the same as kilograms per hectare (multiply pounds per acre by 1.121).

ceive food, nutrients, and probably certain growthcompounds from the host plant. The fixed nitro-gen in the nodule is transported from the noduleto other parts of the host plant, which is beneficialfor the host.

One method used to study the amount of nitro-gen that is fixed is to compare the amount ofnitrogen in nodulated and nonnodulated legume

NITROGEN

plants. Weber (1966) used this method and foundthat as much as about 160 kilograms of nitrogenper hectare (140 lb per acre) was fixed whensoybeans were grown. This is shown in Figure12.5 for the zero amount of nitrogen applied perhectare. In addition, about 75 percent of the nitro-gen in the tops of the soybean plants was derivedfrom nitrogen that was fixed, and 25 percent was

189

FIGURE 12.1 Growth andnitrogen in corn plants. Part ofthe nitrogen taken up andpresent in the leaves, stalks, andcobs early in the growing seasonis later moved to other organs.The grain has high priority for thenitrogen as shown by the largeamount in the grain at harvesttime. (From J. Hanway, 1960.)

PhosphorusPotassium

CropAcreYield Nitrogen as P as K Calcium Magnesium Sulfur

Peanuts (nuts) 1.25 tons 90 5 13 1 3 6

Soybeans (grain) 40 bu. 150 16 46 7 7 4

Sugar beets (roots) 15 tons 60 9 42 33 24 10

Sugarcane 30 tons 96 24 224 28 24 24

Tobacco (leaves) 2,000 lb 75 7 100 75 18 14

Tobacco (stalks) 35 7 42

190

FIGURE 12.3 A simplified series of steps in nitrogen, fixation. (Adaptedfrom Delwiche, in The Science Teacher, Vol. 36, p. 19, March 1969.)

PLANT-SOIL MACRONUTRIENT RELATIONS

FIGURE 12.2 The majorprocesses of the soil nitrogencycle are fixation, mineralization,nitrification, immobilization, anddenitrification. Nitrogen can alsobe lost from the soil by leachingand volatilization.

FIGURE 12.4 Early stages in nodule formation. (a) Recognition eventbetween the root hair and bacteria. (b) Curling of root hair. (c) Earlypenetration of the infection thread. (From P. W. Wilson, The Biochemistry ofSymbiotic Nitrogen Fixation, The University of Wisconsin Press, 1940, p. 74.Used by permission of P. W. Wilson and the University of Wisconsin Press.)

derived from the soil. Also note in Figure 12.5 thatthe amount of fixed nitrogen decreased as theamount of nitrogen applied to the soil as fertilizerwas increased. At the highest nitrogen fertilizerrate, little nitrogen fixation occurred, and nearlyall of the nitrogen in the plant was absorbed from

FIGURE 12.5 Amount of total nitrogen fixed andpercent of nitrogen in soybean plants from symbioticfixation in relation to the application of nitrogenfertilizer. (Data from C. R. Weber 1966.)

NITROGEN 191

the soil. From these and other data, it is reason-able to conclude that 50 to 200 kilograms perhectare (or pounds per acre) are commonly fixedannually when legumes such as soybeans, clover,or alfalfa are grown. Peas and navy beans fix lessnitrogen, so it is a standard practice to use nitro-gen fertilizers to increase pea and bean yields.

Many trees are legumes and fix nitrogen. Blacklocust is an example of a nitrogen-fixing legumetree. In the tropics, Leucaena trees have beenused to supply nitrogen for food crops in mixedcropping systems. The trees are also effective forerosion control. Other additional benefits of theLeucaena are that the leaves can be used as for-age for animals and the wood can be used for fuel.The same benefits are obtained from the growth ofcowpeas, which forms a small shrub.

In actual farming practice, the amount of nitro-gen added to soils by the growth of legume cropsdepends on the amount of the crop that is har-vested and removed from the soil. If an entire cropof legumes is plowed under, all the fixed nitrogenis added to the soil. If all of the legume crop isharvested and removed from the field, there may

192

actually be a net loss of soil nitrogen by the pro-duction of the legume crop. This occurs whensoybeans are grown and only the beans are har-vested. Legume residues-roots, stems andleaves-are relatively rich in nitrogen and are de-composable (mostly labile). Even though the pro-duction of a legume crop may not increase totalsoil nitrogen, the decomposition of the plant resi-dues releases considerable available nitrogenduring the following months.

Nonlegume Symbiotic Fixation Many nonle-gume plant species have root nodules and also fixnitrogen symbiotically. This means that symbioticfixation of nitrogen is important in natural ecosys-tems by both legume and nonleguminous plants.Red alder is an example of a nonlegume capableof symbiotic nitrogen fixation. This feature makesred alder (Alnus) a good pioneer species for in-vading freshly exposed parent materials and landsthat are burned over, where soils have low ni-trogen-supplying capacity because of low organicmatter content. Ceanothus species (a floweringshrub), in the forests of the U.S. Pacific Northwest,fixes a large amount of nitrogen. The nitrogen-fixing organisms in this nonlegume are actino-mycetes. The nonlegume symbiotic nitrogen-fixing systems result in more fixation of nitrogenin natural ecoystems than in the symbiotic herba-ceous legumes found in agricultural fields.

I n water, the fern, Azolla, forms an associationwith blue-green algae. The blue-green algae livein the fronds of the fern and fix nitrogen. The algalpartner can fix all the nitrogen that the associationcan use. Prolific growth of Azolla forms a floatingmat of biomass on the surface of the water in ricepaddies and other bodies of water. In Asia, Azolla

is cultivated, and the biomass is harvested andused as a manure for crop production and asanimal feed.

Nonsymbiotic Nitrogen Fixation Certaingroups of bacteria living independently in the soilcan fix nitrogen. These bacteria fix nitrogen non-


symbiotically, and a dozen or more have beenfound. The two organisms that have been studiedmost belong to the genus Azotobacter and thegenus Clostridium.

Azotobacter are widely distributed in nature.They have been found in soils with pH 6 or abovein almost every instance where examinationshave been made. The greatest limiting factor af-fecting their distribution in soil appears to be pH.These organisms are favored by pH 6 or more,good aeration, abundant organic matter, and suit-able moisture. Clostridium is also a heterotrophicbacteria, but is an anaerobe that can thrive in soilstoo acid for Azotobacter. It exists mostly as aspore and becomes vegetative for brief periodsafter rains when anaerobic conditions exist. Bothof these heterotrophs depend on decomposableorganic matter for their energy supply and thus,have limited activity in soils and contribute littlenitrogen for plant growth in cultivated fields.

Summary Statement In every ecological nichethere are nitrogen fixing organisms and systemsthat transfer N2 from the atmosphere to the soil.Plants that fix nitrogen are good pioneers for thecolonization of parent materials and soils that arelow in organic matter and nitrogen. In cultivatedfields, legume-Rhizobia symbiotic fixation of ni-trogen has traditionally been a primary source ofnitrogen in agriculture. The amount of nitrogenfixed symbiotically in most cultivated soils is re-lated to the legume species, the nitrogen-fixingRhizobia species, and the amount of nitrogenavailable in the soil. Whether or not the produc-tion of a legume crop will increase the total con-tent of soil nitrogen is dependent on the amountof the crop that is harvested and removed from thefield. Azolla fixes nitrogen symbiotically and iscultivated in Asia. The biomass is harvested and isused as manure in crop production and is used asanimal feed.

MineralizationAt any given time, about 99 percent of soil nitro-gen is organic nitrogen in organic matter. About 1

to 2 percent of the total organic nitrogen is de-composed and the nitrogen mineralized eachyear. Many different kinds of heterotrophic organ-i sms are involved. The first inorganic or mineralform of nitrogen produced by decomposition(mineralization) is ammonia (NH 3 ). For this rea-son, the nitrogen mineralization process has alsobeen called ammonification; (see step 2 in Figure12.2).

Some NH 3 volatilizes from animal droppings ormanure, and some NH 3 produced at or near thesoil surface escapes into the atmosphere by vola-tilization, especially when soil pH is 8 or more.Ammonia is a polar gas and, in the soil, NH 3 re-acts with water and then combines with a H+ toform ammonium, NH4'. Ammonmium is ad-sorbed onto the cation exchange sites and isretained in soils as an exchangeable cation.The ammonium ion is about the same size asthe potassium ion, and some NH4' becomes en-trapped in the interlayer space of micaeous min-erals, somewhat like the entrappment of K + . Thisentrappment of ammonium nitrogen is called am-monium fixation. Fixed NH 4 ' has low plant avail-ability.

It appears that all plants obtain some of theirnitrogen from the soil. Plants that participate insymbiotic nitrogen fixation may obtain the greatbulk of their nitrogen through fixation. Plants thatdo not have a symbiotic nitrogen-fixing relation-ship must meet all of their nitrogen needs from thesoil. How much nitrogen the soil can supply

NITROGEN

° Time for one-half of the material to decompose.

193

plants depends on the amount of nitrogen miner-alized, which is dependent on the kinds andamounts of soil organic matter and the rate ofnitrogen mineralization.

The total nitrogen in a grassland soil in Canadawas divided into four pools, depending on de-composability, as shown in Table 12.3. The livingbiomass accounted for only 4 to 6 percent of thesoil nitrogen, but contributed 30 percent to theamount of nitrogen mineralized in a 12-week pe-riod. By contrast, the oldest and most stable nitro-gen compounds accounted for 50 percent of thetotal soil nitrogen and contributed a mere 1 per-cent to the mineralized nitrogen (see Table 12.3).

Each percent of soil organic matter representsabout 1,000 pounds of nitrogen per acre-furrow-slice, assuming soil organic matter is 5 percentnitrogen. If 1 percent of the total organic nitrogenis mineralized each year, the nitrogen mineraliz-ing potential of the plow layer is 10 pounds peracre annually for each percent of organic matter.A plow layer containing 3 percent organic matterwould annually mineralize about 30 pounds ofnitrogen per acre.

NitrificationNitrification is the biological oxidation of ammo-nium (NH4') to nitrite (N02 ) and to nitrate(N03

-). This process is shown as step 3 in Figure12.2. Nitrification is a two-step process, with ni-trite (NO 2

-) as the intermediate product. Specific

TABLE 12.3 Nitrogen Pools and Their Contribution to Mineralized Nitrogenin a Chernozemic (grassland) Soil

194

autotrophic bacteria are involved. The reactionsand organisms are as follows.

The nitrifying bacteria obtain energy from theseoxidizing reactions. Note also that the first step innitrification produces H+, which contributes tosoil acidity. Nitrification changes the form ofavailable nitrogen from a cation to an anion. Thisis of no great concern in plant nutrition, sincemost plants readily use both nitrate and am-monium.

Nitrification is affected by oxygen supply andsoil pH. Nitrification is inhibited in water-saturated soils. Plants that are native to water-saturated environments, where the lack of 0 2

does not permit nitrification to occur, tend toprefer ammonium. Paddy rice is more productivewhen an ammonium fertilizer rather than a nitratefertilizer is applied.

When soils have a pH of 6 or more and are wellaerated, there is an abundance of nitrifyingbacteria-bacteria that quickly oxidize NH4' toN03- . Well-aerated soils with pH of 6 or morealso provide an environment that is favorable forproduction of most economic crops. Thus, condi-tions favorable for crop production are favorablefor nitrification, resulting in N03 being a com-mon form of nitrogen absorbed by plants andmicroorganisms.

In most well-aerated soils the dominant form ofavailable nitrogen is nitrate. It remains soluble inthe soil solution and moves readily to roots inwater by mass flow. In essence, plants can rapidlyabsorb both the water and nitrate from a soil ifboth are in good supply (available). This has twoimportant consequences. First, mass flow is themajor means by which nitrogen is moved to rootsurfaces for absorption. It accounts for about 80percent of the nitrogen absorbed by corn. Second;the nitrogen from fertilizer is quickly used, andmore than one application of nitrogen fertilizer


per year is usually necessary for lawns and otherlong-season crops to maintain vigorous growth.

Nitrate is subject to leaching, whereas,NH4' is

adsorbed onto the cation exchange sites and re-sists leaching. This has important implications forthe economy of use of nitrogen fertilizer and ni-trate pollution of groundwater. During the winterand spring, plants may be dormant and not beactive in nitrogen uptake. This is the time of yearwhen surplus water and groundwater recharge arelikely to occur and nitrate leaching is most likely.Nitrogen fertilizer applied in the fall is usually asammonium, commonly with a nitrification inhibi-tor. The nitrification inhibitor is used to maintainthe nitrogen in the ammonium form and reducepossible leaching loss of nitrogen as nitrate, andthis reduces the danger of groundwater pollution.

ImmobilizationBoth ammonium and nitrate are available formsof nitrogen for roots and microorganisms. Theiruptake and use result in the conversion of inor-ganic (mineral) forms of nitrogen into organicform. The process is immobilization (shown asstep 4 in Figure 12.2). Immobilized nitrogen isstable in the soil, in the sense that is is not readilylost from the soil by leaching, volitalization, ordenitrification, and is not taken up by roots andmicroorganisms. Soil nitrogen is subject to re-peated cycling within the soil through mineraliza-tion, nitrification, and immobilization.

Mineralization and immobilization are two op-posing processes that generally control the levelof available nitrogen that exists in the soil. Asnoted earlier, a soil with 3 percent organic mattermight mineralize 30 pounds of nitrogen per acreper year. Part of this nitrogen is used by the miner-alizing organisms. Perhaps, only one half of theamount of nitrogen mineralized might be in ex-cess of that needed by the microorganisms. Thiswould result in only 15 pounds of nitrogen avail-able for uptake by roots. Large crop yields maycontain several hundred pounds of nitrogen peracre, which means that large amounts of nitrogen

fertilizer are commonly applied to produce non-leguminous crops.

Carbon-Nitrogen Relationships Some plant-residue materials such as wheat straw, cornstover, and sawdust have a very small nitrogencontent relative to their carbon content. As a con-sequence, the ratio of carbon to nitrogen (C : Nratio) is relatively large compared with soil humusand the leguminous plants, which tend to be richi n nitrogen (see Table 12.4). Bacteria requireabout 5 grams of carbon for for each gram ofnitrogen assimilated or use carbon and nitrogenin a ratio of 5:1. Consequently, when straw isadded to the soil, there is insufficient nitrogen tomeet the needs of the decomposing organisms.Under these conditions, decomposition of thestraw is limited by an insufficient supply of nitro-gen; unless, nitrogen is available from some othersource. This other source could be nitrogen min-eralized within the soil from SOM. The decom-posing microorganisms have first priority for anymineralized nitrogen (nitrogen released in de-composition). When there is insufficient nitrogeni n the straw to supply the needs of the decom-posers, they will use any nitrogen that is available.This results in a deprivation of nitrogen for grow-

Data are taken from several sources. The values are approxi-mate's only, and the ratio in any particular material may varyconsiderably from the values given.

NITROGEN 195

ing plants, and means that plants may be starvedfor nitrogen for a period of several weeks. Eventu-ally, the straw material will be consumed and thecarbon and nitrogen assimilated end up as liveand/or dead microbial tissue. This tissue, alongwith other SOM (with relatively small C : N ratios)become the major substrate for the decomposers,and the temporary period of nitrogen starvationceases. These changes are illustrated in Figure12.6.

In general, materials with C : N ratios between20 and 30 have just about enough nitrogen tosatisfy the needs of the decomposers. Materialswith higher ratios produce a temporary period ofnitrogen deficiency for plant growth. Humus,animal manure, and legume residues are rela-tively rich in nitrogen, have a relatively small C:Nratio, and contain more nitrogen than the decom-posers need. These materials with small C:N ra-tios are excellent materials to add to soils to in-crease the available nitrogen for growing plants.

An addition of about 20 pounds of nitrogen tothe soil as fertilizer per ton of straw is required toprevent a temporary nitrogen deficiency, as a re-sult of the microbial decomposition of the straw.When a large amount of straw or other similarmaterial is added to soils without supplementalnitrogen fertilizer, a month or so is required for thetemporary nitrogen-deficient period to pass underconditions favorable for decomposition.

DenitrificationDenitrification is the reduction of nitrate or nitriteto molecular nitrogen or nitrogen oxides by mi-crobial activity. Many of the denitrification prod-ucts are gaseous and escape from the soil. Just asit is natural for nitrogen to be added to soils byfixation, it is natural for nitrogen to be lost fromthe soil by denitrification. In ecosystems wherethe total amount of nitrogen remains constantfrom year to year, the annual additions of nitrogenvia fixation tend to balance the losses of nitrogenby denitrification. Thus, denitrification is one ofthe most significant processes in the nitrogen cy-


cle, because denitrification accounts for largelosses of nitrogen from soils.

Denitrification is carried out by certain faculta-tive and anaerobic organisms. Nitrate-nitrogen inpoorly drained soil is especially subject to denitri-fication. Anaerobic conditions may exist in well-drained soils after rains, when the interiors of soilaggregates are wet. Denitrification is shown asstep 5 in Figure 12.2, and the reaction is asfollows.

C6H 1 20 6 + 4NO3 = 6CO2 + 6H 20 + 2N 2

Nitrite (NO2 -), nitric oxide (NO), and nitrous ox-ide (N 20) are also produced.

Normally, denitrification is detrimental to agri-culture, because nitrogen is lost from soils. Deni-trification, however, helps to prevent an excess ofnitrate in the groundwater of irrigated valleyswhere high rates of nitrogen fertilizer have beenused. In a large irrigated valley, denitrificationoccurs as nitrate in shallow groundwater movesslowly down the length of the valley. This denitri-fication contributes to a reduced nitrate content ofthe drainage water that exits the valley, and thusless threat of nitrate pollution. Where nitrate iscarried downward below the biologically activezone where denitrifers and an energy source exist,denitrification does not occur. Thus, nitrate indeep ground water is not denitrified.

FIGURE 12.6 Addition of organicmaterial with high C : N ratioresults in depletion of soil nitrate.The activity of the decomposers isgreatly stimulated and they usewhatever nitrogen is present. Intime, most of the added material isdecomposed and the soil nitratelevel increases.

Near Atlanta, Georgia, there is a large sewagedisposal project that uses sprinklers to dispose ofthe effluent in a forest. The rate of effluent applica-tion is controlled to produce an anaerobic soilenvironment and denitrification of the nitrates inthe effluent before the water leaves the disposalarea. This use of denitrification is likely to in-crease as more and more communities becomeinvolved in disposing of sewage effluent withoutcontaminating groundwater, and at the sametime, contributing water to groundwater recharge.

Human Intrusion in the Nitrogen CycleThe amount of nitrogen in a corn crop is large(see Table 12.2) compared to the nitrogen-supplying power of soils. The United States pro-duces more than 50 percent of the world's corn,and this requires a large amount of nitrogen fertil-izer. The balance sheet for the earth's nitrogencycle in Figure 12.7 shows that the amount ofi ndustrially fixed nitrogen was equivalent to theterrestrial (natural) fixation and about twice asgreat as the amount of nitrogen fixed by legumecrops. The biosphere now receives about 11 per-cent more nitrogen than is lost. It is obvious thathumans have become an important factor in thenitrogen cycle through industrial nitrogen fixa-tion. The long-term effects of a nitrogen buildup in

FIGURE 12.7 Nitrogen balance sheet for the world'sbiosphere. Nine million metric tons (92-83) morenitrogen is being added than is being removed bydenitrification (and loss to sediments). (Data fromC. C. Delwiche, "The Nitrogen Cycle, " 1970, ScientificAmerican, Inc. All rights reserved.)

the biosphere are unknown. There is, however,i ncreased potential for groundwater pollution andeutrophication of lakes. In agriculture, as long asthe use of nitrogen fertilizer does not result inadding more nitrogen than crops and microor-ganisms can immobilize, there is little danger ofgroundwater pollution.

Summary Statement on Nitrogen CycleFor the earth there is a quasi-equilibrium betweenfixation and denitrification that maintains a ratherconstant soil nitrogen content. It is difficult to

PHOSPHORUS 197

increase the nitrogen content of soils because ofthe natural, large losses of nitrogen by denitri-fication (and also leaching and volatilization).

Within the soil there is an internal nitrogencycle that shuttles nitrogen back and forththrough mineralization, nitrification, and immobi-lization. The amount of nitrogen available forcrops is the nitrogen in excess of the needs of themineralizing microorganisms. The low nitrogen-supplying power of soils results in large additionsof nitrogen to soils as fertilizers to meet the nitro-gen needs of high-yielding nonleguminous crops.The large amount of industrially fixed nitrogenhas resulted in a significant intrusion in theearth's nitrogen cycle by humans.

Plant Nitrogen RelationsNitrogen is a part of chlorophyll, and nitrogen-deficient plants have light-green or yellow leaves.Nitrogen-deficient plants are stunted, producinglow crop yields. The yellowish color of lawns inthe growing season is usually an indication ofnitrogen deficiency. Many plants develop distinctnitrogen-deficiency symptoms, including yellowmidribs of the lower leaves of corn (maize) plants,as shown in Color Plate 1. Yellow and nitrogen-deficient corn leaves have at low nitrate content,according to tissue analysis (see Color plate 1).

An excess of nitrogen causes rapid vegetativegrowth and dark-green leaves. The vegetation issucculent and has reduced resistance to injuryfrom insects, disease, and frost. For some crops,succulent tissue means greater damage from me-chanical harvesting and reduced storability. Ce-real grains develop tall, weak stalks that are easilyblown over or broken during rainstorms, when theheads are heavy and filled with grain near harvesttime.

PHOSPHORUSThe earth's crust contains about 0.1 percent phos-phorus (P). On this basis, the phosphorus in aplow layer is equivalent to the phosphorus in

198

20,000 bushels of corn for an acre. This does noti nclude phosphorus that could be absorbed byroots at depths below the plow layer. Phosphorus,however, commonly limits plant growth. The ma-jor problem in phosphorus uptake from soils byroots is the very low solubility of most phosphoruscompounds, resulting in a low concentration ofphosphate ions in the soil solution at any onetime.

Soil Phosphorus CycleMost phosphorus occurs in the mineral apatite i nigneous rocks and soil parent materials. Fluora-patite (Ca 1o(P04 ) 6 F2) is the most common apatitemineral. Fluorapatite contains fluorine (F), whichcontributes to a very stable crystalline structurethat has great resistance to dissolution or weath-ering. The structure is similar to that of teeth den-tine. Fluoridation of water and tooth paste is de-signed to incorporate fluorine into dentine toincrease resistance to tooth decay.

Apatite weathers slowly, producing the phos-


FIGURE 12.8 Major processes in the soil phosphorus cycle. The availabilityof phosphorus to plants is determined by the amount of phosphorus in thesoil solution.

phate ion, H 2P04- , which exists in the soil solu-

tion, as shown as step I of Figure 12.8. TheH 2 P0 4

- is immobilized when roots and microor-ganisms absorb it and convert the phosphorusinto organic compounds. This results in a signifi-cant amount of phosphorus in soils as organicphosphorus. Commonly, 20 to 30 percent of thephosphorus in plow layers of mineral soils isorganic phosphorus. As with nitrogen, organicphosphorus is mineralized by microorganismsand is again released to the soil solution asH 2P04

- . The organic phosphorus cycle, a sub-cycle in the overall soil phosphorus cycle, is simi-lar to the soil nitrogen cycle, where phosphorus isshuttled back and forth by mineralization and im-mobilization (steps 2 and 3 in Figure 12.8).

A major difference between the nitrogen andphosphorus cycles in soils is that the availableform of nitrogen is mainly nitrate and is stable inthe soil solution. Nitrate remains in solution andmoves rapidly to roots by mass flow, unless deni-trified or leached. The phosphate ion, by contrast,quickly reacts with other ions in the soil solution,

resulting in precipitation and adsorption to min-eral colloids that convert the phosphorus to anunavailable or fixed form (step 4 in Figure 12.8).As a consequence, most of the phosphate ionsfrom mineralization of organic phosphorus, ormineral weathering, may be converted to an un-available form before plants have an opportunityto absorb the phosphorus and before loss byleaching can occur. The kinds of ions in the soilsolution that render phosphate insoluble are re-l ated to soil pH.

Effect of pH on Phosphorus AvailabilityThe ions in the soil solution are a function of pH.In calcareous parent materials and soils, the pH iscommonly in the 7.5 to 8.3 range. In these soils,apatite is the dominant phosphorus mineral.Apatite has very low solubility, and even plants invirgin soils being cropped for the first time, havedifficulty satisfying their phosphorus needs. Theions in the soil solution include Ca

21from the

hydrolysis of calcium carbonate. Any phosphateions released, predominantly HP0 4

2- above pH7.2, tend to precipitate as insoluble calcium phos-phate compounds that are slowly converted toapatite. Therefore, in calcareous soils most of thephosphorus tends to exist as apatite and tendsto remain as apatite. Phosphorus availability toplants is low.

Over the course of soil evolution in humid re-gions, the calcium carbonate dissolves, the cal-cium is leached, and the soils become acid. Inacid soils, there is much less Ca t+ and muchmore Al 3+ and Fe

31in solution than in calcareous

soils. This results in precipitation of phosphatei ons as insoluble aluminum and iron phosphates.Again, these phosphorus compounds have veryl ow solubility, resulting in a low concentration ofphosphate ions in solution.

The formation of these insoluble calcium, alu-minum, and iron compounds is called phos-phorus fixation (step 4 in Figure 12.8). In the caseof phosphorus, the fixation, is usually the result ofprecipitation or adsorption. Phosphate ions are

PHOSPHORUS 199

fixed by adsorption onto clays and other soil con-stituents. Fixed phosphorus dissolves or is re-leased slowly into the soil solution (step 5 ofFigure 12.8). The very low concentration of phos-phorus in the soil solution results in (1) very littlephosphorus being moved to roots by mass flow,and (2) minimal loss of phosphorus from soils byleaching. The optimum pH for phosphorus avail-ability is near 6.5, where there is the least poten-tial for phosphorus fixation. Even at this pH only avery small amount of phosphorus is found in thesoil solution, the amount in solution being of theorder of 0.04 pounds per acre-furrow-slice.

Newly formed aluminum and iron phosphatesare amorphous (not crystalline) and have a largesurface area per gram. With time, the amorphousforms become more crystalline, as shown in Fig-ure 12.9. There is a large decrease in surface areawith increasing crystallinity, and this is associatedwith decreased solubility or availability of the alu-minum and iron phosphate minerals. In an exper-iment with Sudan grass grown in quartz sand,plants growing in soil that received crystallineforms of iron and aluminum phosphate grewsomewhat like plants that had not received phos-phate. By contrast, the plants growing in soil thatreceived colloidal iron or aluminum phosphategrew much better and absorbed much more phos-phorus, as shown in Table 12.5.

Changes in Soil Phosphorus Over TimeThe forms of phosphorus in soils change duringthe long period of soil evolution. In young cal-careous soil, the phosphorus is mainly apatitephosphorus. Plant growth and organic matter ac-cumulation in the soil result in the conversion of asignificant amount of apatite phosphorus into or-ganic phosphorus. As soils become acid, phos-phorus is fixed as iron and aluminum phosphate.I n acid soils, phosphorus released by dissolutionof apatite is gradually converted into iron andaluminum phosphates. When first formed, theiron and aluminum phosphates are amorphousand, over time, they gradually become crystalline.

200

FIGURE 12.9 Transformation of soluble phosphorus into less availableforms. Availability decreases with increases in crystallinity. (From A. S. R.Juo, and B. G. Ellis, 1968. "Chemical and Physical Properties of Iron andAluminum Phosphates and Their Relation to Phosphorus Availability." SoilSci. Soc. Am. Proc. 32:216-221. Used by permission of the Soil ScienceSociety of America.)

Eventually, in intensively weathered soils rich iniron and aluminum oxides, the crystalline formsof iron and aluminum phosphate become encap-sulated (occluded) by iron and aluminum oxides.Occluded phosphorus is the least available formof soil phosphorus. Note in Figure 12.10 that mostof the phosphorus is calcium phosphate in a mini-mally weathered North Dakota soil, little phos-phorus is aluminum and iron phosphate, and

TABLE 12.5 Yield and Phosphorus Uptake by Sudangrass after Growing 30 Days in Quartz Sand Culture

Adapted from Juo and Ellis, 1968.a Relative to colloidal aluminum phosphate as 100.


there is no occluded phosphorus. By contrast, theintensively weathered soil from Hawaii, has nocalcium phosphate, and a small amount of ironand aluminum phosphorus, but the phosphorusis mainly occluded. These soils have the leastability to supply plants with phosphorus. Themoderately weathered Wisconsin soil is interme-diate and is the best suited for supplying phos-phorus to plants.

Plant Uptake of Soil PhosphorusThe dominant form of phosphorus available toplants exists in the soil solution mainly as H 2PO 4

-

below pH 7.2 and mainly as HPO 42- above pH

7.2. The potential for P fixation tends to maintainphosphorus in insoluble forms and, therefore, thephosphorus concentration in the soil solution isvery low. Thus, mass flow cannot supply plantswith sufficient phosphorus except in unusualcases. As a result, most of the H 2PO4

- at rootsurfaces has been moved there by diffusion. It hasbeen reported that about 90 percent of the phos-phorus in corn is supplied by diffusion and about

FIGURE 12.10 Percentage distribution of four formsof phosphorus as related to weathering or soil age.(Adapted from S. C. Chang and M. L Jackson. 1958."Soil Phosphorus Fractions in Some RepresentativeSoils." Jour. Soil Sci. 9:109-119. Used bypermission.)

10 percent is supplied by mass flow and rootinterception. Diffusion of phosphorus to roots isthe major limiting step in phosphorus uptake fromsoils by roots.

Obviously, the greater the phosphate concen-tration of the soil solution, the greater is the avail-ability of soil phosphorus. The problem in agricul-ture is to maintain an adequate phosphorusconcentration in the soil solution, even thoughthere is rapid fixation of phosphorus. An adequatesupply of solution phosphorus can result fromthe use of phosphorus fertilizers. The first fixationcompounds, whether in calcareous or acid soils,tend to be amorphous, and they are more avail-able than the crystalline apatite and aluminumand iron phosphates, which form slowly overtime. The insoluble crystalline forms require yearsto form, but in the meantime, the amorphous and

PHOSPHORUS 201

adsorbed phosphorus formed from the use ofphosphorus fertilizers can provide adequate solu-tion phosphorus for high crop yields on mostsoils.

Fixation of fertilizer phosphorus results in lowuptake of the fertilizer phosphorus during the yearof application. Commonly, only 10 to 20 percentof phosphorus in fertilizer is used by plants duringthe year of fertilizer application. Therefore, re-peated use of phosphorus fertilizers results in anincrease in soil phosphorus content, and in manyinstances, soils have become sufficiently high inphosphorus that further additions of phosphorusfertilizers do not increase plant growth.

Plant Phosphorus RelationsPhosphorus plays an indispensable role as a uni-versal fuel for all biochemical activity in livingcells. High-energy adenosine triphosphate (ATP)bonds release energy for work, when converted toadenosine diphosphate (ADP). Phosphorus isalso an important element in bones and teeth. Therelation of phosphorus in soils to plants andanimal health, and the extensive occurrence ofphosphorus deficiency in grazing animals, arewell known.

Some of the phosphorus deficiency symptomsof plants are not specific. The growth of bothshoots and roots is greatly reduced. One of thecommon diagnostic aids is foliage color. Phos-phorus is mobile in plants and, when a deficiencyof phosphorus occurs, phosphorus is moved fromthe older leaves to the younger leaves at the top ofthe plant: Deficiency symptoms first appear on theolder, or bottommost, leaves. The deficiencysymptom is commonly a purplish color as a re-sult of increased anthocyanin development inphosphorus-deficient tissue, as shown for corn inColor Plate 2.

Low phosphorus in animal diets is a commonproblem. Some relationships between soil phos-phorus levels, phosphorus levels in plant tissue,and phosphorus deficiency in dairy cattle areshown in Figure 12.11. As the amount of fertilizer

202

FIGURE 12.11 Effect of phosphrus fertilizerapplication on concentration of phosphorus in oatsand alfalfa. About 0.3 percent of phosphorus (dashedline) is required by dairy cattle for normal growth.(From W. H. Allaway, 1975.)

phosphorus increased (or available soil phos-phorus increased), the phosphorus content of oatgrain and alfalfa hay was markedly increased. Be-cause cattle require about 0.3 percent phos-phorus in their diet for optimum growth, oat graincontained sufficient phosphorus without fertili-zation. Oat straw remained phosphorus deficientfor cattle, even with phosphorus fertilization ofthe oat crop. Cattle diets consisting mainly ofgrasses usually require a phosphorus supplementto insure adequate dietary phosphorus. The use ofphosphorus fertilizer changed alfalfa hay from aninadequate phosphorus source to an adequatesource. It is interesting to note that the U.S. race-horse industry is centered on the high phos-phorus soils of Kentucky and Florida.

Phosphorus deficiency is not a serious problemin human nutrition, because of the large amountof cereal grains and meat consumed, which aregood phosphorus sources. For humans, the use ofphosphorus fertilizer is more important for in-creasing the quantity of food than for increasingthe phosphorus content of the food.

POTASSIUM

There is a wide range in the potassium content ofsoils and availability of potassium for plant


growth. Some soils are very deficient in availablepotassium, whereas others are very sufficient.This is in stark contrast to the general needs ofnitrogen and phosphorus for high-yielding crops.Basically, potassium in soils is found in mineralsthat weather and release potassium ions (K+).These ions are adsorbed onto the cation ex-change sites. The exchangeable potassium tendsto maintain an equilibrium concentration with thesoil solution from which roots absorb K + . Organicsoils tend to be deficient in available potassiumbecause they contain few minerals with po-tassium.

Soil Potassium CycleThe earth's crust has an average potassium con-tent of 2.6 percent. Parent materials and youthfulsoils could easily contain in a plow layer 40,000 to50,000 pounds per acre, or kilograms per hectare.The potassium content below the plow layercould be similar. About 95 to 99 percent of thispotassium is in the lattice of the following min-erals:

Micas, especially biotite, weather faster and re-l ease their potassium much more readily than dofeldspars. The feldspars tend to exist as largerparticles than micas, with the feldspars largely inthe sand and silt and the micas in silt and clayfractions.

Weathering of feldspar results in the dissolu-tion of the feldspar crystal and release of K+ to thesoil solution. The potassium in micas is interlayerpotassium. Weathering of micas results in themigration of K+ out of the interlayer space alongthe edge of weathering mica particles; the weath-ered edge of mica is shown in Figure 12.12. Theweathered edge of the mica particle in Figure12.12 is vermiculite.

The K+ appears in the soil solution or is ad-sorbed onto a cation exchange site (step 1 inFigure 12.13). An equilibrium tends to be estab-lished between the solution potassium and ex-changeable potassium (steps 2 and 3). Theamount of solution potassium is about 1 percentas large as the exchangeable potassium.

As weathering proceeds, in the absence of theremoval of K+ from the soil by plant uptake orleaching, there is a buildup of exchangeable po-tassium as a result of weathering. The exchange-able potassium maintains a quasi-equilibriumwith the potassium entrapped or fixed, as shownin step 4 in Figure 12.13. Fixation is the reverse ofthe weathering of mica and occurs as the amountof exchangeable potassium increases and K+move back into voids where interlayer potassiumweathered out at an earlier time. Release, theopposite of fixation and shown as step 5, is en-couraged when plant uptake and leaching (steps6 and 7 in Figure 12.13), result in reduced solutionpotassium. A reduced concentration of solutionpotassium is followed by reduced exchangeablepotassium, which is then followed by the releaseof fixed potassium.

FIGURE 12.12 Weathered mica silt particle showingloss of potassium from the darkened area around theedges. The loss of potassium results in expansion ofthe 2:1 mica layers. The weathered zone isvermiculite. (Photograph courtesy M. M. Mortland.)

POTASSIUM 203

Plant removal and leaching tend to occur dur-ing the spring and summer, when the supply ofavailable or exchangeable potassium is reduced.In winter, by contrast, plant uptake and leachingof potassium may be minimal and the release ofpotassium by weathering results in an increase inexchangeable potassium. In other words, in win-ter the direction of the net effect of the reactions isof the order 1, 2, and 4. In spring and summer, thedirection is 6 (and 7), 3, and 5.

Potassium fixation is affected by the equilib-rium conditions in the soil and soil drying andwetting. The application of potassium fertilizergreatly increases the amount of solution po-tassium. Drying the soil at this time results inincreased concentration of K+ in the soil solutionat the edges of clay particles and the movement ofpotassium into the interlayer spaces, where it be-comes entrapped or fixed. By contrast, soon afterthe harvest of a crop, the concentration of K + i nthe soil solution is low. Drying the soil at this timeresults in the migration of potassium from inter-l ayer spaces and increases the amount of avail-able or exchangeable potassium.

Summary Statement When plants are dor-mant and leaching is minimal, weathering of po-tassium minerals results in an increase in solu-tion potassium, followed by an increase ofexchangeable potassium, followed by an increasei n the amount of fixed potassium. Normally, thisoccurs in fall and winter in minimally and moder-ately weathered soils. When plant uptake andleaching occur, normally in spring and summer,there is a decrease in solution potassium, fol-lowed by a decrease of exchangeable potassium,and then a decrease in the amount of fixed po-tassium.

Weathering and subsequent loss of potassiumby leaching with time can deplete the soil ofpotassium minerals and create soils with fewpotassium minerals and very low available po-tassium. This is generally true for intensivelyweathered soils.

Potassium does not complex with organic com-

204

FIGURE 12.13 Major processes in the soil potassium cycle.

pounds; that is, it is not an integral part of anyorganic molecules in plants. Therefore, po-tassium availability is only minimally related tothe soil's organic matter content in mineral soils.In fact, the most potassium-deficient soils arethose composed mostly of organic matter, suchas organic soils.

Plant Uptake of Soil PotassiumThe soil's ability to maintain a desirable concen-tration of solution potassium is determined by theit's potassium buffer capacity. The potassium buf-fer capacity is the ability of the solid phase of thesoil to buffer changes in the K+ concentration ofthe soil solution. Soils with high potassium buffercapacity tend to be fine-textured and contain al arge amount of fixed potassium or interlayer po-tassium (contain abundant mica and vermiculite)


that can be released. In these soils, the solution

potassium is quickly replenished when removedby roots. It would seem that potassium fixation iscompletely beneficial because potassium is pro-tected from leaching, yet has good potential for

plant availability in the future. However, somesoils have an unusually large potassium-fixing ca-

pacity (vermiculitic soils), which competes withplants for potassium in fertilizer that is applied tosoils, and much of the applied potassium is "lost"to fixation.

The clays in intensively weathered soils aremainly kaolinite and oxidic clay. These soils con-tain little if any fixed potassium and have low

potassium buffering capacity.In many minimally and moderately weathered

soils, the exchangeable potassium is about equalto the needs of the crop, and the amount in solu-tion at any one time is equal to I to 3 percent of

the exchangeable. As with the case of phos-phorus, the amount of potassium in the soil wateris small compared with plant needs. The result isthat diffusion is very important for moving po-tassium to roots for uptake, because mass flowcannot provide an adequate amount. For corn, ithas been reported that diffusion accounts forabout 80 percent of the potassium uptake. Diffu-sion is the rate-limiting step for plant uptake ofpotassium from most soils.

Very high potassium fertilization may producean unusually high concentration of solution po-tassium. Then, plants may absorb much morepotassium than is needed for growth-producingluxury uptake, which is the absorption of nutri-ents by plants in excess of their need for growth.The luxury uptake of potassium reduces the up-take of other cations, especially magnesium, and,to a lesser extent, calcium (see Figure 12.14).

Plant Potassium RelationsPotassium enhances the synthesis and translo-cation of carbohydrates, thereby encouraging cellwall thickness and stalk strength. A deficiency issometimes expressed by stalk breakage, or lodg-ing. Potassium can be substituted for by sodiumi n some of its physiological roles in some plants.

The deficiency symptoms are the most severeon old growth, because potassium is mobile inplants. Potassium deficiency symptoms are quitecharacteristic on many plants as a yellowing, andeventually death, of leaf margins. For alfalfa,white spots first appear along leaf margins fol-lowed by a coalescing of the spots to give a mar-ginal edge effect. The dying of the edges of olderleaves is commonly called necrosis. Potassiumdeficiency symptoms of corn and alfalfa areshown in Color Plate 1.

CALCIUM AND MAGNESIUMThere are many similarities between the behaviorof calcium and magnesium with those of po-tassium in soils. They are all released from miner-

CALCIUM AND MAGNESIUM 205

FIGURE 12.14 The calcium, magnesium, andpotassium content of alfalfa plants are a function ofthe potassium concentration in the nutrient solution.(Adapted from Wallace, 1948.)

als by weathering and occur as exchangeablecations. They are the most abundant in young andminimally weathered soils and least abundant ini ntensively weathered and leached soils. All threeelements are absorbed by roots as cations fromthe soil solution.

Some important calcium minerals include cal-cite, dolomite, gypsum, feldspar, apatite, and am-phibole. Important magnesium minerals include:dolomite, biotite, serpentine, hornblende, andolivine. In contrast to potassium, there is no sig-nificant fixation of calcium or magnesium intounavailable forms. The cations set free by weath-ering are adsorbed onto the cation ex-change sites. An equilibrium tends to be estab-lished between the exchangeable and solutionforms. Compared to potassium, the amounts ofexchangeable and solution calcium and magne-sium are usually many times larger than theamounts of exchangeable and solution po-

206

tassium. By contrast, plant needs for calcium andmagnesium are much less than for potassium.Mass flow commonly moves more calcium andmagnesium to root surfaces than plants need,whereas at the same time, plants need more po-tassium than is moved to surfaces surfaces. Thus,calcium and magnesium limit plant growth onlyoccasionally. In fact, calcium deficiencies inplants are rare, and magnesium deficiencies oc-cur only occasionally. When deficiencies of cal-cium do occur, it is usually on intensively weath-ered soils.

Magnesium is less strongly adsorbed to cationexchange sites than is calcium. The cationexchange capacity of neutral soils may be 75percent or more calcium saturated and only 15per cent magnesium saturated. Magnesiumdeficiency is usually associated with acidic sandysoils. In Michigan, acidic sandy soils with lessthan 75 pounds of exchangeable magnesium peracre-furrow-slice are likely to be magnesium de-ficient for corn. By contrast, soils formed fromserpentinite (magnesium rich) parent materialcontain abundant magnesium and very low ex-changeable calcium. The extreme imbalance be-tween calcium and magnesium causes severe cal-cium deficiency. Plant growth on such soils ismeager and the soils are called serpentine

barrens.

Plant Calcium and Magnesium RelationsMagnesium is a constituent of chlorophyll. Aswith several other nutrients, a magnesium defi-ciency results in a characteristic discoloration ofleaves. Sometimes, a premature defoliation of theplant results. The chlorosis of tobacco, known assand drown, is due to magnesium deficiency. Cot-ton plants suffering from a lack of magnesiumproduce purplish-red-colored leaves with greenveins. Leaves of sorghum and corn becomestripped; veins remain green with yellow interveinstrips (as shown for corn in Figure 12.15). Themagnesium is mobile in plants and is moved fromthe older, lower leaves to the younger, upper


l eaves. This causes the lower leaves to exhibit thedeficiency symptoms. The magnesium-deficiencysymptom on coffee leaves is shown in ColorPlate 4.

Calcium is immobile in plants. When a de-ficiency occurs, there is malformation and dis-integration of the terminal portion of the plant.The deficiency symptom has been establishedfor many plants by the use of greenhouse meth-ods. As indicated earlier, calcium-deficiencysymptoms are seldom seen in fields. This isbecause other serious nutrition problems usu-ally occur before calcium becomes sufficientlydeficient.

Soil Magnesium and Grass TetanyGrass tetany (grass staggers) is a major disease ofcattle, which is characterized by muscle spasms,especially in the extremities. The disease is due toa magnesium deficiency. The incidence of grasstetany is more common when cattle consume adiet consisting mainly of grass rather than le-gumes. This is because the magnesium content ofgrasses is lower than that of legumes. The inci-dence of grass tetany is generally low whengrasses have 0.2 percent or more magnesium. Thei ncidence of grass tetany is related to levels ofavailable soil magnesium, plant species, and tem-perature. Conditions that favor the incidence ofgrass tetany include: (1) very low exchangeablemagnesium (2) reduced uptake of magnesium byplants caused by high levels of available po-tassium (see Figure 12.14), sometimes causedby the application of potassium fertilizers; and(3) cool, wet spring weather that encourages thegrowth of grasses, relative to legumes, in pastureswhere the animals graze.

SULFURSulfur exists in some soil minerals, including gyp-sum (CaSO4

- 2H 20). Mineral weathering releasesthe sulfur as sulfate (SO4

2 -) which is absorbed byroots and microorganisms. Sulfur is released as

FIGURE 12.15 Magnesium deficiency symptoms oncorn. The veins are green and the intervein areas ofthe lower leaves are yellow.

sulfur dioxide (SO2) into the atmosphere by theburning of fossil fuels and becomes an importantconstitutent of the precipitation. In many loca-tions, more sulfur is added to soils (via precipi-tation) than plants need. Sulfur accumulates insoils as organic sulfur in plant residues and isthen mineralized to 504

2 -. The net effect is theaccumulation of sulfur in soils as organic sulfur.Plants, depending on the SO 2 content of the air,may absorb sulfur through leaf stomates. In thepast, sulfur has been an important element in

SULFUR 207

fertilizers used in the United States. For these rea-sons, sulfur is seldom limiting for plant growtheven though plants require about as much sulfuras phosphorus. Sulfur deficiencies tend to occurwhere there is little input of sulfur by precipitationdue to low precipitation and little input of sulfurinto the atmosphere by burning of fossil fuels. Theoutput of sulfur is so great near some mining andsmeltering operations that nearby vegetation iskilled.

The addition of sulfur to soils via precipitationcontributes to the development of soil acidity.Sulfate is adsorbed onto the surfaces of iron andaluminum oxides and onto the edges of silicateclays, where aluminum is located. As soil pHdecreases, the capacity for sulfur adsorption in-creases. This is one reason why acid rain has hadonly a minimal effect on many soils; the more acidthe soil becomes, the more sulfur is adsorbed andfixed or inactivated. Sulfur is lost from soils assulfate by leaching and as H2S gas, which is pro-duced by microbial reduction of sulfates inanaerobic soils.

Areas in the United States in which crops haveresponded to sulfur are shown in Figure 12.16.Most of the locations occur where low amounts ofsulfur are added to soils by precipitation and/orsoil parent materials have low sulfur content. Theannual addition of sulfur to soils from precipi-tation was more than 200 pounds per acre nearChicago, Illinois, and only 6 pounds per acre ineastern Nebraska. Note in Figure 12.16 that noresponse to sulfur is shown for Illinois, whereassome response to sulfur has been obtained ineastern Nebraska. Generally, the industrial North-east receives sufficient sulfur from precipitation toensure sufficient amounts for crops. In many irri-gated fields in semiarid and arid regions, enoughsulfur is added by the irrigation water to ensureadequate sulfur for crops.

Plants that have the greatest need for sulfurinclude: cabbage, turnips, cauliflower, onions,radishes, and asparagus. Intermediate sulfur us-ers are legumes, such as alfalfa and cotton andtobacco. Grasses have the lowest sulfur require-


FIGURE 12.16 Locations where crops have responded to sulfur fertilizers.(Data from Jordan and Reisenauer, 1957.)

ment. Sulfur deficiency symptoms are similar tonitrogen in that sulfur-deficient plants are stuntedand light-green to yellow in color.

SUMMARYMacronutrients are used by plants in relativelylarge amounts and include nitrogen, phosphorus,potassium, calcium, magnesium, and sulfur.

Plant-deficiency symptoms for specific nutri-ents are related to the mobility of the nutrientswithin the plants.

Nitrogen availability in soils is controlledmainly by mineralization and immobilization. Ni-trogen is added to soils by fixation. and an ap-proximate amount is lost annually by denitri-fication. The dominant form of available nitrogen

is nitrate, which is readily moved to roots by massflow and lost from soils by leaching and denitri-fication.

Phosphorus originates from apatite weatheringand exists in soils in many different insoluble

minerals and in soil organic matter. Soil pH, by itscontrol of the ions in solution, greatly affectsphosphorus fixation and, therefore, phosphorusavailability in soils. The low concentration of so-lution phosphorus limits phosphorus uptake byplants and results in very limited leaching of phos-phorus.

Potassium is released by mineral weatheringand the available supply is the exchangeable

potassium. During periods of limited plant uptakeand leaching, release of potassium by weatheringresults in increases in solution, exchangeable,fixed potassium in most minimally and moder-ately weathered soils. During periods of activeplant uptake and leaching, there is a decrease insolution, exchangeable, and fixed potassium. Therate of potassium diffusion to roots is the limitingstep in potassium uptake from soils.

Calcium and magnesium have many parallelswith potassium in terms of the forms in soils. Theamounts of exchangeable calcium and magne-sium usually exceed the amount of exchangeablepotassium, yet, plants use much more potassium

than calcium or magnesium. Calcium is rarelydeficient for plant growth, and such a deficiencyoccurs usually only in intensively weathered soils.Magnesium is sometimes deficient in plantsgrowing on acid and sandy soils with low cationexchange capacity.

Sulfur is released from sulfur minerals. Much ofthe sulfur in soils has been added by precipi-tation. Near industrial areas, more sulfur is addedto soils by precipitation than plants need. In someplaces, excessive additions of sulfur kill the vege-tation.

Allaway, W. H. 1975. "The Effect of Soils and Fertilizerson Human and Animal Health." USDA Agr. Informa-tion Bul. 378.

Barber, S. A. 1984. Soil Nutrient Bioavailability. Wiley,New York.

Black, C. A. 1968. Soil-Plant Relations. Wiley, NewYork.

Chang, S. C. and M. L. Jackson. 1958. "Soil PhosphorusFractions in Some Representative Soils." Jour. SoilSci. 9:109-119.

Cook, R. L. and C. E. Millar. 1953. "Plant Nutrient Defi-

REFERENCES 209

ciencies." Mich. Agr. Exp. Sta. Spec. Bull. 353, EastLansing.

Delwiche, C. C. 1970. "The Nitrogen Cycle." ScientificAmerican, September, pp. 137-146.


Hanway, J. 1960. Growth and Nutrient Uptake by Corn.Iowa State University Extension Pamphlet 277.

Jordan, H. V. and H. M. Reisenauer. 1957. "Sulfur andSoil Fertilty." USDA Yearbook Soil, Washington, D.C.

Juo, A.S,R. and B. G. Ellis. 1968. "Chemical and Physi-cal Properties of Iron and Aluminum Phosphates andTheir Relation to Phosphorus Availability." Soil Sci.Soc. Am. Proc. 32: 216-221.

Mengel, K. and E. A. Kirby. 1982. Principles of PlantNutrition. 3rd. ed. International Potash Institute,Bern, Switzerland.

Mortland, M. M. 1958 "Kinetics of Potassium Releasefrom Biotite." Soil Sci. Soc. Am. Proc. 22:503-508.

Paul, E. A. 1984. "Dynamics of Organic Matter in Soils."Plant and Soil. 76:275-285.

Stevenson, F. J. 1986. Cycles of Soil. Wiley, New York.Wallace, A. et al. 1948. "Further Evidence Supporting

Cation-Equivalent Constancy in Alfalfa." J. A. Soc.Agron. 40:80-87.

Weber, C. R. 1966. "Nodulating and Nonnodulating Iso-lines." Agron. Jour. 58:43-46.

CHAPTER 13

MICRONUTRIENTS ANDTOXIC ELEMENTS

Micronutrients are required in very small amountsand function largely in plant-enzyme systems (seeTable 13.1). The factors that determine theamounts of micronutrients available to plants areclosely related to soil conditions and plant spe-cies. For example, a change in soil pH can changea deficient situation for plants into a toxic situa-tion. Four of the seven micronutrients are used byplants as cations, and they will be consideredfirst.

IRON AND MANGANESEIron and manganese are weathered from mineralsand appear as divalent cations in solution; assuch, they are available to plants. Generally, inacid soils, sufficient Fe

2+and Mn

2+exist in the

soil solution to meet plant needs. In some veryacid soils, manganese, and to a lesser extent iron,are toxic because of the high amounts in solution.Deficiencies are common in alkaline soils whereoxidized forms of iron and manganese exist asinsoluble oxides and hydroxides. As a conse-quence, deficiencies are common in arid regionswhere many soils are calcareous and alkaline. In

210

humid regions, many calcareous soils exist onrecent lake plains and on exposed subsoil or par-ent materials. At building sites, such as home-sites, acid soil may be contaminated with enoughmortar from brick wall construction, or the sur-face soils may be mixed with highly calcareousparent material during excavation and becomealkaline. Shrubs and flowers grown on these loca-tions commonly have deficiency symptoms foriron and/or manganese (see Figure 13.1).

Cereals and grasses, including sugarcane, tendto have a manganese deficiency when grown onalkaline soils. Plants particularly susceptible toiron deficiency include: roses, pin oaks, azaleas,rhododendrons, and many fruits and orna-mentals.

Deficiency symptoms of iron and manganeseare striking and, for some plants, are very similar.In some cases, it is impossible to know whetherthe symptom is due to an iron deficiency or amanganese deficiency unless trials are conductedto determine which element alleviates thesymptoms. In other cases, it is known from experi-ence which element is deficient when the defi-ciency symptoms occur. Typical iron-deficiencysymptoms or iron-chlorosis appears on leaves asyellow interveinal tissue and dark-green-colored

I RON AND MANGANESE

TABLE 13.1 Essential Micronutrient Elements and Role in Plants

veins, as shown for pin oak and roses in ColorPlate 2. The younger leaves tend to be most af-fected because iron is immobile in plants.

The absence of sufficient manganese stunts to-matoes, beans, oats, tobacco, and various otherplants. Associated with this stunting is a chlorosisof the upper leaves. The interveinal tissue turnsyellow, whereas the veins remain dark-green-colored, as shown for kidney beans in Color Plate3. The gray-speck disease of oats is attributedto manganese deficiency and appears as gray-colored areas on stems. For many nutrients,varietal or cultivar differences result in differingplant responses to the same level of available nu-trients. This is illustrated by varietal response tomanganese by soybeans as shown in ColorPlate 4.

Many chlorotic plants growing on alkaline soilshave an iron deficiency and many have a manga-nese deficiency. Zinc may also be a contributingfactor. Healthy green pin oak trees were found toneed 55 centimeters of acid soil when growing inan area where many soils were alkaline and thetrees were chlorotic (see Figure 13.2).

211

Plant "Strategies" for Iron UptakeIron is fourth in abundance in the earth's crust,following oxygen, silicon, and aluminum. Yet,iron deficiencies in plants are common becausethe amount of soluble iron in the soil solution isfrequently too low to meet plant needs. Plantshave developed special "strategies" for copingwith this situation in alkaline soils where the oxi-dized or ferric form of iron has very low solubility.

Plants increase their ability to absorb iron fromcalcareous and highly aklaline soils in two ways.First, plant roots decrease the pH in the rhizo-sphere by the excretion of H+ that solubilizesferric iron. Second, for monocots (grasses),siderophores are excreted by the roots. Sidero-phores (iron bodies) are metabolites secreted byorganisms that form a highly stable coordinationcompound (organic chelate) with iron. The side-rophores solublize ferric iron, which is subse-quently absorbed by the roots and the iron usedby the plant. Microorganisms also excrete sidero-phores. These mechanisms enable certain plantspecies to satisfy their iron needs when growingon alkaline soils. Many plants, however, are not

212

FIGURE 13.1 Iron-deficient symptoms frequentlyoccur on isolated pin oak trees growing in areaswhere soils are normally acid. Because of localcontamination of the soil with alkaline materials fromroad and building construction, the soil is alkaline.Note die-back at the top of the tree. Chlorotic leavesare yellow with green-colored veins.

efficient in utilizing ferric iron and develop defi-ciency symptoms or fail to grow.

COPPER AND ZINCCopper and zinc are released from mineral weath-ering to the soil solution as Cue' and Zn

21.These

micronutrient cations can be adsorbed ontocation exchange sites. Little Cu

2+exists as ex-

changeable copper, however, but tends to bestrongly adsorbed to the inorganic fraction or

MICRONUTRIENTS AND TOXIC ELEMENTS

FIGURE 13.2 Soil reaction (pH) profiles associatedwith untreated green and chlorotic pin oak trees. Pinoak trees were healthy if the upper 55 centimeters ofsoil was acid. (Data from Messenger, 1983.)

complexed with organic matter. As a result, cop-per is quite immobile in soils and the copperconcentration of soil solutions tends to be verylow. Strongly adsorbed and nonexchangeablecopper equilibrate with Cu e+ in solution, andmuch of the copper in solution is complexed withsoluble organic matter of low molecular weight.Even though there is a very low concentration of

copper in solution, plants require very little cop-per, which is generally obtained in sufficientquantity by root interception and mass flow. Cop-per deficiencies in plants tend to occur on newlydeveloped organic soils and leached sandy soilsthat are characterized by low amounts of totalcopper. Copper is not easily leached and, afterabout 20 to 40 pounds of copper per acre havebeen applied to deficient soils, little additionalcopper is needed.

Zinc, as Zn 2 +, occurs as an exchangeablecation, is strongly absorbed onto several soil con-stitutents, and is complexed by organic matter.There is considerable similarity in the forms ofzinc and copper in soils and plant uptake relation-ships. Zinc, however, appears to be complexedwith organic matter to a lesser degree and hassharply reduced availability with increasingsoil pH.

Plant Copper and Zinc RelationsCopper is quite immobile in plants and deficiencysymptoms are highly variable among plants. Incereals the deficiency shows first in the leaf tips at

COPPER AND ZINC 21 3

tillering time. The tips become white and theleaves appear narrow and twisted. Growth of thei nternodes is depressed. In fruit trees, gum pock-ets under the bark and twig dieback occur.

Zinc deficiency was first recognized in plantsgrowing on organic soils in Florida. Zinc defi-ciencies occur similarly to copper on organicsoils and leached acid sandy soils from whichzinc has been leached. Zinc is commonly defi-cient on alkaline and/or calcareous soils. An ex-treme case of zinc deficiency of navy beans oncalcareous soil is shown in Color Plate 3. Applica-tion of phosphorus fertilizers at high rates hasalso been found to reduce zinc availability insoils, especially when levels of soil zinc are onlymarginally sufficient (see Figure 13.3).

Zinc deficiencies are widespread in the UnitedStates on corn, sorghum, citrus and other fruits,beans, vegetable crops, and ornamentals. Pecanrosette, the yellows of walnut trees, the mottle-leafof citrus, the little-leaf of the stone fruits andgrapes, the white-bud of corn, and the bronzing oftung tree leaves are all due to zinc deficiency. Intobacco plants, a zinc deficiency is characterizedby a spotting of the lower leaves.

FIGURE 13.3 Navy beanplants growing on soils low inavailable zinc grew less wellwith increasing amount ofapplied phosphorus (zero to720 pounds per acre) fertilizerowing to reduced availabilityof zinc to the plants.

21 4

BORON

The most abundant boron mineral in soils is tour-maline, a borosilicate. The boron is released byweathering and occurs in the soil solution mostlyas undissociated boric acid, H 3B0 3 . The boronin solution tends to equilibrate with adsorbedboron.

Boron is leached from acid sandy soils, and thisresults in low availability. Boron availability isreduced by increasing pH due to strong boronadsorption to mineral surfaces (fixation). Thus,boron availability is a maximum in soils with in-termediate pH. Boron deficiency tends to occur indry weather; which is likely because of reducedmovement of boron to roots by mass flow inwater.

Plants absorb boron as uncharged boric acid,and it appears to be a simple diffusion of theuncharged molecules into roots. Once the boronis stablized in the plant, it is quite immobile.Apical growing points and new plant leaves be-come chlorotic on boron-deficient plants, andgrowing points frequently die. Many physiologicaldiseases of plants, such as the internal cork ofapples, yellows of alfalfa, top rot of tobacco, andcracked stem of celery are caused by a borondeficiency. Boron deficiency of sugar beetscauses heart rot of the beet, as shown in ColorPlate 3.

CHLORINE

The chlorine requirement of plants is very small,even though chlorine may be one of the mostabundant anions in plants. Chlorine is unique inthat there is little probability that it will ever bedeficient in plants growing on soils. Chlorine en-ters the atmosphere from ocean spray, is distrib-uted around the world, and is added to soils viaprecipitation. The presence of chlorine in fertil-izer, as potassium chloride, is an additionalsource of chlorine for many agricultural soils.


MOLYBDENUM

Weathering of minerals releases molybdenumthat is adsorbed to various soil constituents. Theadsorbed molybdenum maintains an equilibirumwith molybdenum in solution. The solution formis mainly as the molybdate ion, M004

2- . Molyb-denum also accumulates in soil organic matter.

I n acid soils, molybdenum is strongly adsorbedor fixed by iron oxides. In many instances, molyb-denum deficiencies are corrected by liming, ow-i ng to increased molybdenum availability. Thedata in Figure 13.4 show that increasing soil pH byliming increased the molybdenum content of cau-liflower leaves. Some calcareous soils in westernUnited States developed from high molybdenumcontent parent materials and have very high levelsof available molybdenum. The plants, however,are not injured by the high molybdenum level.

FIGURE 13.4 Increased soil pH due to limingi ncreased the availability of soil molybdenum andresulted in an increased concentration ofmolybdenum in cauliflower leaves. (Data from Turnerand McCall, 1957.)

Plant and AnimalMolybdenum RelationsMolybdenum is needed for nitrogen fixation inlegumes. When molybdenum is deficient, le-gumes show symptoms similar to those of nitro-gen deficiency. In cauliflower, molybdenum de-ficiency results in cupping of leaf edges. It iscaused by a reduced rate of cell expansion nearthe leaf margin, compared with that in the centerof the leaf. Leaves also tend to be long and slen-der, giving rise to the symptom called whiptail.Interveinal chlorosis, stunting of plants, and gen-eral paleness are also exhibited, depending onthe kind of plant.

Plants and animals need very small amounts ofmolybdenum. Plants deficient in the molybde-

MOLYBDENUM 21 5

num needed for animal consumption have beenfound growing on certain acid soils. The majornutritional problem is molybdenum toxicity, mo-lybdenosis, which develops in grazing animalswhen forage has over 10 to 20 ppm molybdenum.

The molybdenosis problem arises because for-age plants have a wide range of tolerance formolybdenum, whereas animals do not. Legumesaccumulate more molybdenum than do commongrasses, and they take up more molybdenum inwet than in dry soils. The data in Figure 13.5 showthat large increases in molybdenum (and manga-nese) can occur in plants with large increases inmolybdenum (and manganese) in the growingmedium. By contrast, note the virtual lack of anincrease in iron content of the plant with an in-

FIGURE 13.5 The iron (andcopper) content of tomatoleaflets was little affected by alarge increase in the supply ofnutrient in the growing medium.The molybdenum andmanganese content of leafletswas greatly increased. (FromBeeson, 1955.)

21 6

creasing amount of available iron. For soils withvery high available molybdenum, there is no ef-fective method for reducing molybdenum uptakefrom soils by plants.

Most problem areas of molybdenosis are re-lated to the geologic origin and wetness of soils.Common sources of molybdenum in the westernUnited States are granite, shale, and fine-grainedsandstone. Soils producing forages with high mo-l ybdenum level are generally confined to valleysof small mountains streams (western UnitedStates). Only a very small part of any valley actu-ally produces high molybdenum forages. Thesesoils are wet, or poorly drained, alkaline, and highi n organic matter. Molybdenosis in Nevada andCalifornia is associated with soils formed from thehigh-molybdenum-content granite of the SierraNevada Mountains. A typical location where highmolybdenum content forage is produced isshown in Figure 13.6.

COBALT

Cobalt is required by microorganisms that sym-biotically fix nitrogen. This is the only knownneed for cobalt by plants. Microorganisms in the


stomachs of ruminants incorporate cobalt intovitamin B 12 , and this vitamin provides theseanimals with cobalt. Single-stomached animals,i ncluding humans, must obtain their cobalt fromfoods containing vitamin B12, such as milk ormeat products. Only persons who eat strictly veg-etarian foods are likely to have a cobalt-deficientdiet. Cattle and sheep that are not fed legumesusually need supplementary cobalt.

Cobalt deficiency of animals was observed bythe earliest settlers in parts of New England. InNew Hampshire, it was called Chocorua's Curseand Albany Ail, from local place names. In south-ern Massachusetts, it was known as neck's ailbecause the disease was most common on necksof land that projected into bays. It is now knownthat the cobalt deficiency in New England had itsorigin in the geologic history of the soils. Thelow-cobalt soils formed in glacial deposits de-rived from the granite of the White Mountains,which contained very little cobalt. The soils arealso sandy and tend to lose cobalt by leaching.The location of low-cobalt soils in New England isshown in Figure 13.7.

Cobalt deficiency among grazing animals is aproblem on the wet, sandy soils of the lowercoastal plain in the southeastern United States.

FIGURE 13.6 Plants containingtoxic levels of molydbenum arefound only on wet soils fromedfrom high-molybdenum parentmaterials, area C, in this mountainvalley in Nevada. Areas A and Bhave well-drained soils in whichthe molybdenum is not readilyavailable to plants. Area D is wet,but has soils formed low-molybdenum parent materials.(Data from Allaway, 1975.)

FIGURE 13.7 Low cobalt areas (shaded) in NewEngland. The arrows indicate the direction of glacialice movement from the White Mountains containinglow-cobalt rocks. (Data from Kubota, 1965.)

The low cobalt soils are very sandy, poorlydrained, and have humic or organic pans (Bhhorizons). The original sandy parent materialshad little cobalt, and the cobalt was leached fromthe upper soil horizons during soil development.These soils have the lowest cobalt content of anysoils in United States. Consequently, little cobaltis available to forage plants growing on thesesoils.

SELENIUMSelenium (Se) is not required by plants but isneeded in small amounts by warm-bloodedanimals and, probably, humans. Large areas of

POTENTIALLY TOXIC ELEMENTS FROM POLLUTION 217

the United States have soils so low in seleniumthat forages for livestock are deficient in sele-nium. Acid soils formed from low-selenium par-ent materials produce forages that are seleniumdeficient for good animal nutrition. In the westernstates, a selenium toxicity is a problem associatedwith many soils.

In 1934, mysterious livestock maladies on cer-tain farms and ranches of the Plains and RockyMountain states were discovered to be caused byplants with high selenium content, which weretoxic to grazing animals. Affected animals hadsore feet, excessive shedding, and some died.During the next 20 years, scientists found that thehigh levels of selenium occurred only in soilsderived from certain geologic formations with ahigh selenium content. Another important discov-ery was that a group of plants, selenium accumu-

lators, had an extraordinary ability to extract sele-nium from the soil. Selenium accumulatorscontained about 50 ppm of selenium comparedwith less than 5 ppm in nonaccumulator plants.One selenium accumulator is a palatable legume,Astragalus bisulcatus. This plant was found tohave a selenium content that was about 140 timesmore than that of adjacent plants. The adequacyof selenium in forages for farm animals in the

United States is shown in Figure 13.8.

POTENTIALLY TOXIC ELEMENTSFROM POLLUTIONPotential poisoning of humans by arsenic, ca-dimum, lead, and mercury are real concerns to-day. Arsenic has accumulated in soils from spraysused to control insects and weeds and to defoliatecrops before harvesting. Although arsenic accu-mulates in soils and has injured crops, it has notcreated a hazard for humans or animals.

Cadmium poisoning has occurred in Japanfrom the dumping of mine waste into rivers, wherefish ingested the cadmium from the mine waste.Cadmium also appears in some sewage sludges.Using soils for the disposal of sewage represents a

21 8 MICRONUTRIENTS AND TOXIC ELEMENTS

potential danger. Mine spoils may have toxic lev-els of cadmium, but natural agricultural soils donot contain harmful levels of cadmium.

Lead is discharged into the air from automobileexhausts and other sources, and it eventuallyreaches the soil. In soil, lead is converted to formsunavailable to plants. Any lead that is absorbedtends to remain in plant roots and is not trans-ported to the shoots. Soils must become very pol-l uted with lead before significant amounts movei nto the tops of plants.

Mercury is discharged into the air and waterfrom use in pesticides and industrial activities.Under conditions of poor aeration, inorganic mer-cury is converted to methyl mercury, which is verytoxic. Plants do not take up mercury readily fromsoils; however, soils should not be used to dis-pose of mercury because of the highly toxic na-ture of methyl mercury.

The soil is being used increasingly for sewagedisposal. The application of high levels of heavymetals, and their potential uptake by plants, rep-

FIGURE 13.8 Areas where foragesand feed crops contain variouslevels of selenium. (Data fromAllaway, 1975.)

resents an important limitation in the use of soilsfor sewage and industrial waste disposal. Sewageknown to be lacking in heavy metals can be addedto soils without any danger of heavy metal poi-soning of humans or other animals.

RADIOACTIVE ELEMENTSRocks and soils naturally contain radioactive ele-ments. Currently, there is concern about radon.Many radioactive elements have very short half-lives, and contamination of the soil with theseelements is of little concern. Radioactive cesium(Cs) is produced by atomic bombs and has a longhalf-life. Radioactive cesium appears to be fixedi n vermiculitic minerals much like potassium.This limits its availability to plants. The soil tendsto slow down the movement of radioactive ce-sium from the soil into plants and, later, intoanimals.

The contamination of soils is via the atmo-

sphere, and radioactive elements that fall on vege-tation are absorbed by the leaves. Testing of nu-clear weapons prior to 1962 resulted in significantcontamination of tundra vegetation in Alaska. In-creased radioactivity was found in the caribouthat grazed on the tundra. People whose majorfood was caribou were found to have about 100times more radioactivity in their bodies than peo-ple in the continental United States. More re-cently, a similar contamination occured in north-ern Europe as a result of the Chernobyl nuclearreactor explosion in the Soviet Union.

Nuclear bomb tests were conducted on BikiniAtoll of the Marshall Islands between 1946 and1958. The soils were heavily contaminated withradioactive cesium-137. Several approaches havebeen suggested to decontaminate the soil. First,mix a large quantity of potassium with soil andirrigate it with seawater to increase the likelihoodof potassium uptake by plants relative to cesium.A second approach is to remove the top 16 inchesof topsoil and replace it with uncontaminatedsoil. The Bikinians favor the second approach.

SUMMARYMicronutrients are needed by plants in very smallamounts and function largely in plant-enzymesystems.

The reduced (divalent) forms of iron and man-ganese are available to plants. Plant deficienciesare associated with the oxidized forms, especiallyi n alkaline soils. Toxic concentrations may occuri n very acid soils. Plants have developed specialways to obtain iron from alkaline soils, includingthe excretion of hydrogen ions and siderophores.

Copper and zinc are released to the soil solu-tion by mineral weathering. Uptake is mainly asdivalent cations. Copper deficiencies tend to oc-cur on acid sandy soils and organic soils. Plantdeficiencies of zinc are common on alkaline min-eral soils and organic soils.

Boron is released by the weathering of tourma-line, and plant uptake is mostly as undissociated

REFERENCES 219

boric acid. Boron is leached from acid mineralsoils, resulting in plant deficiency. In alkalinesoils, boron is fixed.

Chlorine is picked up by winds from seawaterand distributed via precipitation throughout theworld. Chlorine deficiency in plants growing onsoils is highly unlikely.

Molybdenum availability is greatly affected bysoil pH. In acid soils molybdenum is fixed by iron,and plant deficiencies occur. If some of thesesoils are limed and the pH is increased, molybde-num may become toxic for plants. Molybdenumtoxicity for grazing animals is related to soil pH,soil organic matter content, and soil wettness.

Cobalt is needed by microrganisms that fix ni-trogen.

Selenium is not needed by plants but is neededfor animals. The selenium content of forages ishighly variable, depending on soil, producing for-ages for farm animals that range from deficient totoxic.

Potentially toxic elements sometimes added tosoils include arsenic, cadimum, lead, andmercury.

Plants and soils receive readioactive elementsvia the precipitation or fall-out from the atmo-sphere.

REFERENCESAllaway, W. H. 1975. "The Effects of Soils and Fertilizers

onHuman and Animal Health." USDA Agr. Inform.Bull. 378, Washington, D.C.

Anonymous. Fallout in the Food Chain. Time, Septem-ber 13, 1963.

Beeson, K. C. 1955. "The Effects of Fertilizers on theNutritional Quality of Crops," in Nutrition of Plants,Animals, and Man Symposium Proc. Michigan StateUniversity, East Lansing.

Bienfait, H. F. 1988. "Mechanisms in Fe-Deficient Reac-tions of Higher Plants." J. Plant Nut. 11(6-11):605-629.

Chaney, R. L. 1988. "Recent Progress and Needed Re-search in Plant Iron Nutrition." J. Plant Nut. 11(6-11):1589-1603.

220

Elliott, S. 1988. "Bikini Still Atomic No-man's Land."Lansing State Journal, May 22, 1988.


Kubota, J. 1965. "Soils and Animal Nutrition." Soil Con.

November, pp. 77-78.Kubota, J. 1969. "Trace Element Studies Link Animal

Ailments with Soils." Soil Conservation. Vol. 35, No-vember, pp. 86-88.


Lewin, R. 1984. "How Microorganisms Transport Iron."Science. 225:401-402.

Messenger, A. S. 1983. "Soil pH and the Foliar Macro-nutrient/Micronutrient Balance of Green and Inter-veinally Chlorotic Pin Oaks." J. Environ. Hort. 1(4):

99-104.Turner, F. and W. W. McCall. 1957. "Studies on Crop

Response to Molybdenum and Lime in Michigan."Mich. Agr. Exp. Sta. Quart. Bul. 40:268-281.

Manures and other materials have been applied tosoils for thousands of years to increase plantgrowth. About 150 years ago, some of the essen-tial plant nutrients were discovered and, subse-quently, fertilizers containing essential plant nu-trients were manufactured and applied to soils toincrease fertility. By 1855, researchers at the Rot-hamsted Agricultural Experiment Station, nearLondon, England, declared that soil fertility couldbe maintained permanently with mineral fertil-izers. Today, fertilizer use has been estimated toaccount for 20 to 25 percent, and 30 to 40 percent,of the total crop production in the world andUnited States, respectively. The close parallel be-tween the yields of corn (maize) and the applica-tion of nitrogen fertilizer in Illinois is shown inFigure 14.1.

FERTILIZER TERMINOLOGYA fertilizer is any organic or inorganic material ofnatural or synthetic origin (other than liming ma-terials) that is added to soils to supply one ormore essential nutrient elements. Most of the fer-tilizers considered in this chapter are inorganicmaterials that are the product of sophisticated

CHAPTER 14

FERTILIZERS

221

manufacturing methods and have a uniform com-position.

Grade and RatioThe fertilizer grade expresses the nutrient contentof fertilizers. The grade is the percentage compo-sition, expressed in the order: N-P 20 5-K20. Thegrade, 27-3-9, contains 27 percent nitrogen, phos-phorus equivalent to 3 percent P205 , and po-tassium equivalent to 9 percent K20. Most of thenitrogen and potassium in fertilizers is water solu-ble. In the United States the phosphorus in thefertilizer must be sufficiently soluble to dissolve inneutral ammonium citrate. Other countries havedifferent laws regarding the solubility or availabil-i ty of the nutrients in fertilizers. Comparisons be-tween fertilizers can be made only on the basis oftheir nutrient content, not according to the kindsof compounds in the fertilizer.

The Soil Science Society of America favors afertilizer grade based on the percentages of nitro-gen, phosphorus, and potassium. In some cases,the fertilizer composition is expressed on thisbasis.

The fertilizer ratio is the relative proportions ofprimary nutrients in a fertilizer grade divided by

222

FIGURE 14.1 Corn yields and nitrogen fertilizer usei n Illinois; 5-year averages. (Data from Welch, 1986.)

the highest denominator for that grade. A 27-3-9fertilizer has a ratio of 9-1-3. This fertilizer is highi n nitrogen, low in phosphorus, and moderate inpotassium, relatively speaking. Such a fertilizerprovides good maintenance for lawns in whichthe soil phosphorus level from past fertilizer use ishigh and the major need is for nitrogen.

General Nature of Fertilizer LawsIn general, the laws controlling the sales of fertil-izers in various states are similar. They all requireperiodic registration of the brands or grades andaccurate labeling. Most states require that the fol-lowing information be made available to pur-chasers: (1) name or brand; (2) grade, which be-

FERTILIZERS

comes the guaranteed analysis; and (3) name andaddress of manufacturer.

Fertilizers that are offered for sale may besampled by inspectors at any time, and analyzedto determine whether the fertilizers meet the guar-anteed analysis. The inspection and analysis maybe under the control of the Department of Agricul-ture, the Agricultural Experiment Station, or astate chemist. In this way purchasers are assuredthat manufacturers comply with the fertilizer laws.

Types of FertilizersFertilizers that contain only one compound andsupply one or two plant nutrients are called fertil-

izer materials or carriers. Fertilizer carriers or ma-terials that are mixed together and processed toproduce fertilizers containing at least two or morenutrient elements are mixed fertilizers. Fertilizermay be in the form of a solid, liquid, or gas.

FERTILIZER MATERIALSThe first fertilizer material was manufacturedwhen ground bones were treated with sulfuricacid to increase the solubility (availability) of thephosphorus. This occurred in England in about1830, and was the beginning of the modern fertil-izer industry. This was followed by the develop-ment of potassium materials from dried sea de-posits late in the nineteenth century, and themanufacture of nitrogen materials from the fixa-tion of atmospheric N2 in the early twentiethcentury.

Nitrogen MaterialsThe source of essentially all industrial nitrogen,including fertilizer nitrogen, results from the fixa-tion of atmospheric N 2 , according to the followinggeneralized reaction:

The hydrogen is usually obtained from natural gas(CH4) and the overall reaction, using 8N 2 and 20 2

to represent air, is

The NH 3 is ammonia, commonly called an-hydrous ammonia. Much of the ammonia is pro-duced in the southern states near petroleum andnatural gas fields and is transported by pipeline tovarious parts of the United States. Being 82 per-cent nitrogen, ammonia is the most concentratednitrogen material and the least expensive on a perpound of nitrogen basis. Low cost has contributedto its widespread use.

At normal temperature and pressure, ammoniais a gas, but it is stored and transported undersufficient pressure to keep it a liquid. Locally,ammonia is handled by tank trucks and trailers fordirect application to fields, as shown in Figure14.2. When the ammonia is released from injec-tors at a depth of about 15 centimeters and reactswith the water of moist soil, NH4' is formed,which is adsorbed onto cation exchange sites.However, application of ammonia at shallowdepths in dry soil can result in considerable lossof ammonia by direct escape from the soil asa gas.

About 98 percent of the nitrogen fertilizer pro-duced in the world is ammonia or one of its deriv-atives. The manufacture of ammonia and five ofits derivatives is shown diagrammatically in Fig-

FIGURE 14.2 I njection of anhydrous ammonia intosoil. The ammonia is handled as a liquid underpressure.

FERTILIZER MATERIALS 223

ure 14.3. Oxidation of NH 3 to nitric acid and neu-tralization of the HN03 with NH3 produces ammo-nium nitrate (NH4NO3 ). The CO2 from NH 3

manufacture is used to produce urea [CO(NH2)2]:

Urea [CO(NH2)2] hydrolyzes in soil in thepresence of urease to form ammonia. The appli-cation of urea, in effect, is the same as applyingammonia. The placement of urea on the soil sur-face (as for lawn fertilization) results in the signif-icant loss of NH 3 to the atmosphere by volatiliza-tion unless rain or irrigation leach the urea intothe soil. Urea and ammonium nitrate should notbe mixed or stored together, because the mixadsorbs much water from the air and forms anunfavorable physical condition.

Reaction of NH3 with (1) sulfuric acid producesammonium sulfate, (2) phosphoric acid producesammonium phosphate, and (3) water and ammo-nium nitrate produce nitrogen solutions (see Fig-ure 14.3). All of these nitrogen materials are watersoluble.

In general, water-soluble nitrogen materials areabout equally effective when they are used on thebasis of an equivalent amount of nitrogen and areplaced within the soil. For ammonium nitrate(NH4NO 3), dissolution into the soil solution pro-duces both N0 3

- and NH4'for plant uptake. Un-

der conditions of well-aerated soil and a pH 6 orabove, NH4' is quickly nitrified to N03. Theresult is that it often makes little difference,whether the nitrate or ammonium form of nitro-gen is applied. A few exceptions include the useof ammonium for flooded rice production toavoid denitrification and the strong acid-formingeffect of ammonium sulfate. For this reason,(NH 4) 2SO4 is preferred if an increase in soilacidity is beneficial.

The application of a soluble nitrogen fertilizertypically results in large, rapid increases in thenitrate concentration of the soil solution. Underthese conditions, plant uptake of nitrogen is rapidand the fertilizer nitrogen is quickly depleted. Anapplication of nitrogen fertilizer on a lawn in May

224

may result in little, if any, fertilizer nitrogen left inJune. Consequently, there is a need for a lesssoluble nitrogen fertilizer. Urea-formaldehydeand sulfur-coated urea are insoluble or slow-release fertilizers designed to supply nitrogenslowly and reduce the frequency of nitrogen appli-cation. These fertilizers are more expensive andtheir use is restricted to lawns, golf courses, andhorticulture.

A brief summary of some properties of nitrogenmaterials is given in Table 14.1.

TABLE 14.1 The Principal Fertilizers Supplying Nitrogen

FERTILIZERS

FIGURE 14.3 A diagrammaticrepresentation of the manufactureof ammonia and some of itsfertilizer derivatives.

Phosphorus MaterialsThe starting point for the manufacture of phos-phorus fertilizers is phosphate rock, which is acalcium fluorophosphate of sedimentary or igne-ous origin. Huge deposits of phosphate rock, alsocalled rock phosphate, are located east of Tampa,Florida, where about 70 percent of the phosphateore in the United States is mined. The sedimentarydeposits were formed 10 to 15 million years agoand are buried under 5 to 10 meters of sand.Mining consists of removing the sandy overbur-

den and using a hydraulic gun to break up the oreto form a slurry, as shown in Figure 14.4. Theslurry is pumped to a recovery plant, where rockphosphate pebbles are separated from sand andclay by a washing and screening process. Therock phosphate pebbles are ground to a powder,which is sometimes used directly as a fertilizer.

The primary phosphorus mineral in phosphaterock is fluoroapatite, Ca 1O F2 (PO4)6. Apatite isalso the source of phosphorus in most soil parentmaterials. Apatite is quite insoluble, and whenphosphate rock is applied to soil, very little phos-phorus dissolves. Its use is restricted to areasclose to the mines, where it is inexpensive, or toits application in very acid soil, where soil acidspromote dissolution. Rock phosphate is a pre-ferred source of phosphorus by some organicfarmers.

As with ground bones, acidulation of rockphosphate increases solubility and plant avail-ability of the phosphorus. The acidulation of rockphosphate with sulfuric acid produces ordinary


superphosphate, as is shown by the followingequation:

The ordinary superphosphate produced in thepreceding reaction consists of about one halfmonocalcium phosphate and one half calciumsulfate. Hydrogen fluoride is one of the most haz-ardous pollutants if released into the atmosphere.The gases from manufacture of the phosphorusfertilizer are passed through a scrubber and thefluorine is recovered. Some of the fluorine is usedto fluorinate water. Ordinary superphosphate isabout 9 percent phosphorus, which equals 20percent P205 equivalent and has a grade of 0-20-0.The phosphorus in monocalcium phosphate iswater soluble. Very little 0-20-0 is used today,because other phosphorus fertilizers are less ex-pensive on an equivalent phosphorus basis.

FIGURE 14.4 Mining rockphosphate . The overburden is,removed and a hydraulic gunis used to break up the orelayer and form a slurry that ispumped to a recovery plant.(Photograph courtesyI nternational Minerals andChemical Corporation.)

226

Under proper conditions, the reaction of rockphosphate with excess sulfuric acid will producephosphoric acid (H3 PO 4). Treatment of phos-phate rock with phosphoric acid produces super-phosphate with a higher content of phosphorus.The reaction is:

FERTILIZERS

In the washing process to remove rock phos-phate pebbles, the very small rock phosphateparticles (many colloidal-sized) are put into wasteponds. Here, the particles settle out and the waterevaporates. These low-soluble and low-value ma-terials are processed and marketed under variousnames, including colloidal phosphate. Bonemealis also a phosphorus source, but it is too expen-sive to use on farm fields. Both of these materialsare natural products and are preferred by somegardeners. A summary of major materials contain-ing phosphorus, including their phosphorus con-tent and some remarks, is given in Table 14.2.

Potassium MaterialsWood ashes contain several percent potassiumand are an important source of this element. Po-tassium mines were opened in Germany in thelate nineteenth century and at Carlsbad, NewMexico, in the early twentieth century. Mining ofthe hugh potassium deposits in Saskatchewan,Canada, began in 1959 (see Figure 14.6.). Thesepotassium deposits, which were formed when an-cient seas evaporated, are called evaporate de-posits. The salts in the ocean water precipitatedas the water evaporated and the salts in the waterbecame more concentrated. Later, these salt de-

FIGURE 14.5 Routes for themanufacture of phosphorusmaterials.

The same water-soluble monocalcium phosphateis produced, but without the CaSO 4 . The grade is0-45-0 and has been called concentrated, or triplesuperphosphate. Early production of superphos-phate was mainly with sulfuric acid, and its useadded a significant amount of sulfur to soils. Now,the extensive use of 0-45-0 has reduced sulfuradditions to soils.

Ammonium phosphates are produced by neu-tralizing phosphoric acid with ammonia. Twopopular kinds are produced, depending on theextent of neutralization-monoammonium phos-phate (MAP), 11-48-0, and diammonium phos-phate (DAP), 18-46-0. The actual grade is variable,depending on the proportions of the reactants.Ammonium polyphosphate (APP), 10-34-0, ismanufactured by reacting NH3 and H3 PO4 in across-pipe reactor. These phosphates containboth nitrogen and phosphorus in water-solubleform. Routes for the manufacture of the majorphosphorus materials are shown in Figure 14.5.

TABLE 14.2 The Principal Phosphatic Materials

° Total phosphoric acid instead of amount that is available.

posits were buried under various kinds of over-burdens and rocks.

The minerals in the deposits are representativeof the salts in seawater, mainly salts of sodiumand potassium. Sylvite is KCI and sylvinite is amixture of KCI and NaCI. Langbeinite is a mixtureof potassium and magnesium sulfates. Process-i ng of most of the ores consists of separating KCIfrom the other compounds in the ore; more than95 percent of the potassium in fertilizers is KCI.


Some of the Canadian deposits are relatively pureKCI and require little, if any, processing.

The most popular method of processing po-tassium ores is flotation. The ore is ground, sus-pended in water, and treated with a flotation agentthat adheres to the KCI crystals. As air is passedthrough the suspension, KCl crystals float to thetop and are skimmed off. The material is dried andscreened to obtain the proper particle size. Thematerial, KCI, is marketed as 0-0-60.

FIGURE 14.6 A continuousmining machine at work about1 kilometer underground inpotash mine at Esterhazy,Saskatchewan, Canada.(Photograph courtesyAmerican Potash Institute andInternational Minerals andChemical Corporation.)

228

Some KCI is obtained from brine lakes atSearles Lake in California, and at the Great SaltLake in Utah. Minor potassium materials includeK2S04 and KN0 3 . All of the potassium materialscontain water-soluble potassium.

Micronutrient MaterialsAlthough the bulk of fertilizer consists of the mate-rials containing nitrogen, phosphorus, and po-tassium, micronutrients are sometimes added tofertilizer to supply one or more of the micronu-trients. In these instances, the percentage of theelement in the fertilizer becomes a part of theguaranteed analysis. Some common micronu-trient fertilizers are listed in Table 14.3.

MIXED FERTILIZERSLarge quantities of materials containing only oneor two nutrients are applied directly to the land.This is particularly true for NH3, which is handledas a liquid under pressure and applied with spe-cial applicators. Because of the unique timing ofnitrogen fertilizer applications, much of the nitro-gen is applied alone. The mixing together of dif-ferent nutrient-containing materials producesmixed fertilizers that contain two or more plantnutrients. Mixed fertilizer is commonly applied inrelatively small amounts with or near seeds for thepurpose of accelerating early growth of cropplants.

FERTILIZERS

TABLE 14.3 Some Common Micronutrient Carriers

The formula is a recipe for making the fertilizer.The mixing together of 364 pounds of NH 4 NO3

(33-0-0), 1,200 pounds of ordinary superphos-phate (0-20-0), and 400 pounds of KCI (0-0-60),plus 36 pounds of conditioner, would produce2,000 pounds of fertilizer with a grade of 6-12-12.The major systems for mixing fertilizers are granu-lation, bulk blending, and fluid.

Granular FertilizersI n the manufacture of granular fertilizers, dry ma-terials are mixed together with a little water, or drymaterials are mixed with acids and other fertilizersolutions to form a thick slurry. The mixing occursi n a heated, revolving drum where particles ofdifferent size are formed as the product dries,producing granules. Materials like diatomaceousearth (a powdery siliceous material formed fromthe remains of diatoms) or kaolinitic clay areadded to produce a coating on the granules thati nhibits adsorption of water from the air andprevents caking (agglomeration or lumping). Cak-i ng is due to soft granules that coalesce at contactpoints, causing a lumpy or massive condition. Oilis added to reduce dust. A desirable granular fer-tilizer contains hard granules of uniform size andcomposition and has good storage and handlingqualities. Such materials are required by today'smodern fertilizer applicators. Granular fertilizersare sold in bags or as bulk materials for directspreading onto fields, and are also used to makebulk blended fertilizers.

Bulk Blended FertilizersGranular fertilizers can be mixed together withoutadditional processing to produce bulk blends.Such fertilizers, when handled in bulk form, areconsiderably cheaper than regular granular mixedfertilizers. Other advantages include the ease withwhich a wide variety of grades can be producedquickly and with negligible need for storage offinished products. In many bulk-mixing plants,the formula is calculated by using a computerprogram, and the bulk blend is loaded into a truckand immediately applied to fields. Currently, 54percent of the mixed fertilizers in the United Statesare bulk blends.

Each of the materials in a bulk blend can sepa-rate from each other, or segregate, because nochemical reaction between the materials has oc-curred. Thus, a major precaution in bulk bendingis that materials be of similar particle size; other-wise, segregation will occur during handling andapplication. The result is nonuniform distributionof the fertilizer nutrients in the field. In addition,urea should never be added to a blend containingammonium nitrate, because the blend adsorbswater from the atmosphere and cakes.

Fluid FertilizersFluid fertilizers are easily made and handled. Mi-cronutrients and pesticides are readily incorpo-rated. Fluid fertilizers can be applied to irrigationwater and can be used for direct application to

MIXED FERTILIZERS 229

plant foliage. They are either solutions or suspen-sions.

The principal materials used for making solu-tions include urea, urea-ammonium nitrate solu-tion, ammonium polyphosphate solution (10-34-0), and finely ground KCI. Liquid fertilizer gradesare usually lower than dry fertilizers because ofthe limited solubilities of the materials in water. Ingeneral, they are more expensive than dry fertil-izers. Additionally, low temperature can causecrystallization, or salting-out of the salts. Mostpopular liquid fertilizers have a salting-out tem-perature of freezing or below. A salting-out prob-l em occurs only if liquids are stored in cold cli-mates during the winter months. The applicationof liquid fertilizer is shown in Figure 14.7.

Suspension fertilizers are manufactured fromliquid fertilizers similar to those that are used tomake solution fertilizers. Suspensions, however,have more material added to the water than candissolve, so it becomes a thin slurry. One to 2percent clay (attapulgite, an alumino-Mg-silicateclay) is added to help hold particles in suspen-sion. This allows for preparation of higher grades,as compared with liquid fertilizers. Conversely,salt crystals in suspension may grow over timeand particles may settle out. This limits the lengthof the storage period.

The same amount of nutrients in fluid or dryform generally produces similar results when timeof application and soil placement are similar. Al-though fluids are very easy to handle, they need

FIGURE 14.7 Applying liquidfertilizer broadcast.(Photograph courtesy Ag-Chem Equipment Company.)

230

specialized application equipment, especially forsuspensions. The solution fertilizers are quitepopular and often are surface applied with herbi-cides mixed into the solution. Suspension fertil-izers have attained only modest acceptance.

NATURAL FERTILIZER MATERIALSThe emphasis in this section is on those generallyavailable materials that have had minimalprocessing and that are sources of nutrients. Theyare of both organic and inorganic origin.

Animal manures have been used almost fromthe beginning of agriculture and are a goodsource of plant nutrients. A ton of wet (fresh)animal manure contains approximately 10pounds of nitrogen, phosphorus equivalent to 5pounds of P205 , and potassium equivalent to 10pounds of K20. Animal manures are an excellentsource of nutrients, because of the availability ofnutrients in the liquid portion and the decom-posability of the solid fraction. Other organic ma-

TABLE 14.4 Nutrient Composition of Some NaturalFertilizer Materials

Complied from several sources, including Taber, Davison, andTelford, 1976.

FERTILIZERS

terials with a C:N ratio of 20 or less are also valu-able as nitrogen sources. If material such assawdust is used, it can be valuable as a mulch,but additional nitrogen is needed to prevent com-petition for nitrogen between plants and mi-crobes. Materials high in protein are high in nitro-gen. Some nitrogen materials are listed in Table14.4.

Ground rock phosphate and waste pond or col-l oidal phosphate are sources of phosphorus. Thephosphorus is in calcium phosphate compounds,which are very slowly soluble in alkaline soils; thesame is true for bonemeal. These compounds aremore satisfactory when used on acid soils.Animal tankage (refuse from animal meat pro-cessing) is also a source of phosphorus. The ap-proximate amount of phosphorus in some materi-als is given in Table 14.4.

Potassium is not an integral part of plant tis-sues, and sources of potassium are inorganic orfrom the ashes resulting from the burning of or-ganic materials. The potassium in ashes is in theform of K2 CO3 . Repeated use of wood ashes maytend to cause dispersion of soil structure; thealkalinity effect may produce an undesirable in-crease in soil pH, because many plants grown onalkaline soil are prone to iron deficiency.

Finely-ground granite is available, however; itcontains highly insoluble potassium mineralssuch as feldspar and mica. The potassium ingreensand (glauconite) is interlayer potassium ina micaeous mineral and similar to the potassiumin the mica of granite fines. These sources ofpotassium have low availability. The compositionof some potassium materials is given in Table14.4.

SUMMARYFertilizers contain mainly soluble forms of plantnutrients. The grade indicates the nutrient com-position. A grade of 27-3-9 contains on a weightbasis 27 percent nitrogen, phosphorus equivalentto 3 percent P 20 5 , and potassium equivalent to 9percent K20.

Most of the nitrogen in fertilizers comes fromfixation of atmospheric nitrogen. Reacting nitro-gen with hydrogen produces ammonia, NH 3 . Thereaction of ammonia with other chemicals pro-duces a wide array of nitrogen fertilizers.

The phosphorus in fertilizers comes from phos-phate rock. The rock is acidulated with sulfuric orphosphoric acids to produce superphosphatesthat contain citrate soluble phosphorus. Phos-phoric acid is also used in the manufacture ofammonium polyphosphates.

The potassium in fertilizers comes mainly fromevaporite deposits. The potassium chloride in thedeposits is recovered and marketed as 0-0-60.

Many micronutrient carriers are available tosupply micronutrients.

Mixed fertilizers are marketed mainly as granu-lar, bulk blended, or fluid (liquid and suspension)fertilizers.

Various naturally occurring organic and inor-

REFERENCES 23 1

ganic materials are used as sources of nitrogen,phosphorus, and potassium. The materials havelower nutrient content and generally fair to lowavailability.

REFERENCESEngelstad, O. P., ed. 1985. Fertilizer Technology and

Use, 3rd ed. Soil Sci. Soc. Am., Madison, Wis.Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. Wiley,

New York.I nternational Fertilizer Development Center. 1979. Fer-

tilizer Manual. TVA. Muscle Shoals, Ala.Taber, H. G., A. D. Davison, and H. S. Telford. 1976.

"Organic Gardening." Washington State UniversityExt. EB 648. Pullman, Wash.

Welch. L. F. 1986. "Nitrogen: Ag's Old Standby." Solu-tions: J. Fluid Fert. and Ag. Chem. July/August, pp.18-21.

CHAPTER 15

SOIL FERTILITY EVALUATIONAND FERTILIZER USE

Soil fertility is the status of a soil with respect to its

ability to supply elements essential for plant

growth without a toxic concentration of any ele-

ment. Soil fertility is determined or evaluated withvarious techniques, and this information is thebasis for making fertilizer recommendations. Theemphasis in this chapter is the evaluation of soilfertility, making fertilizer recommendations, andthe use of fertilizers.

SOIL FERTILITY EVALUATIONEvaluations of soil fertility are based on visualobservations and tests of both plants and soils.The major techniques used to evaluate soil fertil-i ty are plant deficiency symptoms, analysis of thenutrient content of plant tissue, and soil tests forthe amount of available nutrients.

Plant Deficiency SymptomsColor Plates I to 4 in Chapter 12 contain defi-ciency symptoms for several nutrients on severaldifferent plants. By the time plant nutrient defi-ciency symptoms become obvious, it may be toolate in the season to apply fertilizer. Such informa-

232

tion, however, is useful in determining which nu-trients are deficient. Lawns commonly turn light-green or yellow when nitrogen is deficient, andthis indication of nitrogen deficiency can be veryuseful for home owners. Many trees, shrubs, andflowers develop iron deficiency symptoms whengrowing on alkaline soils and these symptoms arevery important to many gardeners. Deficiencysymptoms can be useful to researchers in deter-mining the location of soil fertility experiments ona deficient nutrient. Care must be exercised inusing deficiency symptoms to evaluate soil fertil-ity, because several different deficiencies can pro-duce similar symptoms.

Plant Tissue TestsTissue or plant analyses are frequently used as thebasis of fertilizer recommendations for crops thatrequire one or more years to mature. This in-cludes many tree crops, sugarcane, and pineap-ple. The nutritional status of the plant can beassessed and the appropriate fertilizer can be ap-plied long before harvest, which allows plenty oftime for the plants to benefit from the fertilization.As a result, plant analyses are popular with horti-culturists and foresters who grow tree crops. In

addition, it is difficult to evaluate soil fertility fortree crops by using soil samples, because of thewidespread and deeply penetrating root systemsof trees and the difficulty of getting a representa-tive soil sample.

The use of diphenylamine as an indicator of theconcentration of nitrate in the sap of corn isshown in Color Plate 1. Although the test is quali-tative, it is very useful. Other similar easy-to-usetests are available to verify plant nutrient defi-ciency symptoms. Simplicity of use and immedi-ate results are beneficial to consultants and oth-ers who diagnose plant growth problems in thefield.

Exacting quantitative chemical tests of planttissue are used to determine the degree of defi-ciency or sufficiency of nutrients in plants. Anenormous amount of research data has been sum-marized into tables and figures that relate nutrientcomposition of particular plants to growth oryield. For example, corn plants have sufficientnitrogen for maximum yield when the nitrogencontent of the leaf opposite and below the upper-most ear is 3 percent.

The relationship between the percent of po-tassium in first-year needles of Red Pine andgrowth, or plant height, is shown in Figure 15.1.Note that needles with less than 0.35 percent po-tassium had deficiency symptoms. Growth wasmaximum, with about 0.5 percent or more po-tassium in needles. To obtain the informationneeded to make a specific potassium fertilizerrecommendation based on tissue analysis, thepercentage of potassium in needles must be re-lated to the plant's response to potassium fertil-izer. For example, suppose in an experiment in aRed Pine plantation where the potassium contentof the needles was 0.25 percent potassium, that 50pounds of K20 per acre resulted in growth repre-sented by 60 percent of the relative maximumyield, and the application of 100 pounds of K 20produced growth equal to 100 percent of relativeyield (maximum yield). A forester having a partic-ular goal for growth, could then fertilize accord-i ngly, based on the experimental data.

SOIL FERTILITY EVALUATION 233

FIGURE 15.1 Relationship between the potassiumcontent of first-year needles of Red Pine, Pinusresinosa, and site index or tree growth. (Data ofStone et al., 1958.)

Plant analyses have been used to determinelocations where the soil has sufficient availablenutrients to produce maximum yields in orchardswithout the use of additional fertilizer. For long-maturing crops, such as sugarcane and pineap-ples, periodic plant analyses are used to producea log of the nutritional status of the crop, and thisserves as a guide for proper timing of fertilizerapplications.

Soil TestsOne of the weakest steps in making good fertilizerrecommendations by using soil tests is the obtain-ing of a representative soil sample. A soil sampleof 1 pint (about 1 pound or 500 grams) when usedto represent 10 to 15 acres, means that 1 pint ofsoil is used to characterize 10,000 to 30,000 tonsof soil.

Fairly uniform areas of fields should besampled separately on the basis of uniformity insoil characteristics and past soil management.Sampling of unusual areas, including the locationof manure or lime piles, should be avoided. Col-l ect about 20 individual samples of the plow layer

234

and mix them together in a clean plastic pail (seeFigure 15.2). From this large soil sample, about 1pint is sent to the testing lab. Because differentlabs use different kinds of tests and procedures,persons interested in soil tests should consult asoil testing lab for proper sampling proceduresand care of samples after they have been col-lected. Most states have soil testing laboratoriesassociated with both the Agricultural ExperimentStation and agribusiness firms. Many fertilizer dis-tributors provide soil testing services for their cus-tomers.

Shallower sampling is advised in no-till systemswhere nutrients accumulate in the upper fewinches of soil as a result of the surface applicationof fertilizer (see Figure 15.3). In some instances,samples of the subsoil are taken.

Soil tests have been developed that measurethe fraction of nutrients (available fraction) in theplow layer, which is closely related to plantgrowth. Soil tests for potassium, calcium, andmagnesium measure the exchangeable amounts.As with the procedures for developing fertilizerrecommendations based on plant tissue tests, soiltests must be related to plant growth or crop yield.For example, the relationship between phos-phorus soil tests and the relative yield of corn innorth-central Iowa is shown in Figure 15.4. Notethat a soil test of 61 or more pp2m (pounds ofphosphorus per acre-furrow-slice) resulted in a

FIGURE 15.2 Guides for taking soil samples. Avoidunsual areas and take about 20 random samples perarea.

SOIL FERTILITY EVALUATION AND FERTILIZER USE

FIGURE 15.3 Distribution of soil phosphorus afterthree years of tillage and fertilization. (From Potashand Phosphate Institute, 1988.)

relative yield of nearly 100 percent. This meansthat a soil test of 61 or more pp2m is high andindicates a situation where the application ofphosphorus fertilizer is not expected to increaseyield. The various soil test levels are categorizedas high, medium, and low. For example, lowphosphorus tests range from 16 to 30 poundsof phosphorus pp2m, and medium from 30 to61 pp2m. These results are based on the use of asoil extractant composed of dilute HCI and NH 4 F,called Bray's P1. This test is used in almost alllabs in the midwestern region in the United States.

FIGURE 15.4 Relationship between phosphorus soiltest and the growth of corn in north-central Iowa.(Data courtesy of R. Voss, Iowa State University.)

Some regions use other extractants because thesoils contain different forms of phosphorus, andthe results from Bray's P1 test do not relate well tocrop yields.

The next question involving the use of soil teststo make fertilizer recommendations is: If the soiltest for phosphorus is low, how much phos-phorus fertilizer will be required to obtain a partic-ular crop-yield goal? In general, the most profit-able rate of fertilizer application is the amountneeded to obtain about 90 to 95 percent of therelative maximum yield. Data from experimentsrelating crop response to varying amounts of fer-tilizer for various soil test levels are placed intables, which are used to make fertilizer recom-mendations. The phosphorus fertilizer recom-mendations for corn in north-central Iowa aregiven in Table 15.1. The data in this table also takeinto consideration the level of subsoil phos-phorus. A soil phosphorus test of 35 pp2m ismedium, and the phosphorus recommendationfor corn would be 70 pounds of P 20 5 per acre, ifsubsoil phosphorus is very low or low, and 60pounds of P205 per acre if the subsoil test ismedium or high in phosphorus. A series of tablesare constructed to make recommendations forvarious crops in different climate and soil regions.The procedures for developing fertilizer recom-

TABLE 15.1 Phosphorus FertilizerRecommendations for Corn or Sorghum Graini n Iowa

Basic Recommended Amounts for Soil Test Class andSubsoil P Levels (P20 5 , lb/acre)

SOIL FERTILITY EVALUATION 235

mendations for potassium and some of the mi-cronutrients are the same as those for phos-phorus. Nitrogen fertilizer recommendations areusually based on consideration of the nitrogenmineralizing power of the soil, the kind of pre-vious and current crop, the application of ma-nure, and the return of crop residues to the soil.

An example of soil test results and fertilizerrecommendations is given in Figure 15.5. The soiltesting laboratory report in this figure containsthree sections: First, there is an identification sec-tion, giving sample numbers and record of theprevious crop. The soil analysis section gives thesoil test results for soil pH (and lime index), phos-phorus, potassium, calcium, magnesium, cationexchange capacity, and percentage of organicmatter. The fertilizer recommendations sectioncontains the yield goals and recommendations fortwo or three years for various crops.

Fertilizer recommendations for home gardensand lawns are much like those made for commer-cial producers. For home gardens and lawns,however, there is usually little, if any, profit motiveinvolved, so a less rigorous procedure will satisfythe needs of many persons. For example, in theabsence of a soil test, a reasonable fertilizer appli-cation for a lawn or garden is approximately 15 to20 pounds of 10-10-10 per 1,000 square feet. Thisis equivalent to 1.5 to 2 pounds of nitrogen, P205,

and K20 per 1,000 square feet. Many lawns andgardens have been heavily fertilized for manyyears. Frequently, soil tests are high for certainnutrients, which indicates that further fertilizationis not needed. In these instances, the soil testi ndicates which nutrients are sufficient.

Computerized FertilizerRecommendationsThe data in fertilizer recommendation tables areput into equations and fertilizer recommenda-tions are generated by using a computer. Informa-tion regarding crop, soil, yield goals, soil pH, andother critical factors are entered, and the com-puter generates the recommendations and pro-

236 SOIL FERTILITY EVALUATION AND FERTILIZER USE

FIGURE 15.5 Sample of acomputer-generated laboratoryreport showing soil test resultsand fertilizerrecommendations. (CourtesyDr. J. Alhrichs, PurdueUniversity.)

vides a printout of soil test results and the recom-mendations (see Figure 15.5). Some specificillustrations of the equations that are used forcomputing fertilizer recommendations follow.

The K20 recommendation for potatoes growni n Michigan is based on the following equation:

Ks = 175 + (0.5 x YG) - (1.0 x ST)

where Ks is pounds of K20 recommended peracre for loamy sand and sandy loam soils, YG isyield goal in hundred weights per acre, and ST isthe soil test value in pounds of K pp2m(poundsper acre-furrow-slice). This recommendationconsiders: (1) differences in the soil's ability tosupply potassium by relating the recommen-dation to soil texture, (2) yield goal sought bythe grower, and (3) the soil test level of the field.For a yield goal of 500 cwt/acre and a soil testof 200 pounds of potassium pp2m, the K 20recommended per acre is 225 pounds(175 + 250 - 200).

For micronutrients, availability is closely re-l ated to soil pH so that soil tests for nutrients, plussoil pH, are used as the basis for recommenda-tions. An example of a manganese recommenda-tion in pounds of manganese per acre for respon-sive crops is:

pound of manganese per acre136 + (6.2 x pH) - (0.35 x ST)

Here, the amount of manganese recommended isdirectly related to soil pH and inversely related tothe manganese soil test (ST). In addition, differen-tial crop response to manganese is considered;responsive crops include oats, beans, soybeans,and sugar beets.

No commonly accepted soil test exists for mak-ing nitrogen fertilizer recommendations. Rather, anitrogen balance system is commonly used, inwhich estimates are made of the amount of nitro-gen needed by the crop and the amount of nitro-gen likely to be available. The amount of nitrogenneeded minus the amount of nitrogen availableequals the nitrogen fertilizer recommendation.The recommendations vary greatly between crops

APPLICATION AND USE OF FERTILIZERS 237

and yield expectations. For example, the nitrogenrecommendation for corn grain in Michigan iscalculated with the following equation:

pounds of fertilizer nitrogen per acre =[ -27+(1.36xYG)]-[40+(0.6xPS)]-(4xTM)

In this equation, YG is the yield goal in bushelsper acre and PS refers to percent stand for le-gumes, as alfalfa and clover, that precede the corncrop. Five or six alfalfa plants per square footis a 100 percent stand. A nitrogen credit of100 pounds is made when an alfalfa crop with100 percent stand precedes the corn crop[ 40 + (0.6 x 100) = 100 pound credit]. The TMrefers to tons of manure, crediting each ton with 4pounds of nitrogen fertilizer equivalent. Thus,corn with an expected yield of 150 bushels peracre and following an 80 percent stand of alfalfawith 10 tons of manure applied per acre, will havea nitrogen recommendation of 49 pounds per acre(-27 + 204 - 88 - 40).

Nitrogen does not accumulate and buildup insoils where leaching occurs as do phosphorusand potassium. For this reason, the nitrogen needfor a lawn remains nearly the same year after year.Many home owners can maintain a good lawnfertilization program by applying only nitrogenfertilizer. A moderate recommendation is 1 poundof nitrogen per 1,000 square feet in the spring andagain in late summer and/or fall. A serious gar-dener will be able to assess the nitrogen need of alawn by careful observation of the color and therate of growth when water does not seriously limitgrowth.

APPLICATION AND USE OF FERTILIZERSAfter a fertilizer recommendation has been made,the major factors to consider in the use of fertilizerare time of application and placement. High andvery high soil tests mean that fertilizer is not ex-pected to increase plant growth or yields signifi-cantly. Obviously, under such conditions, a con-sideration of time of application and placement of

238

fertilizer is meaningless. In other words, thehigher the soil fertility level, the less important aretime of application and placement. In many farmfields, however, time of application and place-ment are important considerations.

Time of ApplicationThe closer the time of fertilizer application is tothe time of maximum plant need, the more effi-cient plants will be in using the fertilizer nutrients.This is especially true for nitrogen, which is lostby denitrification whenever the soil is anaerobic.Nitrate is mobile and lost by leaching during pe-riods of surplus water, and NH3 is subject to vola-tilization when it exists on the soil surface. This isshown in Figure 15.6, where nitrogen is applied asa side dress, placement of the nitrogen fertilizer in

FIGURE 15.6 Effect of fall, spring, and sidedressnitrogen rates on corn yields at DeKalb, Illinois.Average for 1966-1969. (Reproduced from AmericanSociety of Agronomy, Vol. 63, No. 1, p. 119-123,copyright © 1971.)


the soil along the rows when the corn is a foot ortwo high, and as a preplanting broadcast applica-tion onto the soil surface in early spring weremuch more effective than application the pre-vious fall. Side dress application resulted in pro-viding nitrogen near the time of maximum plantneed and with the least amount of time availablefor nitrogen loss.

The time of application of phosphorus and po-tassium fertilizer is less critical than for nitrogen.However, fixation of phosphorus, and to a lesserextent potassium fixation, reduce their availabilityand reduce the plant's ability to recover the ap-plied fertilizer phosphorus and potassium. Thel onger the period of time between application andplant use, the greater is the fixation and ineffi-ciency of the nutrient use. In general, the levels ofphosphorus and potassium in many soils inUnited States have increased to the point wheretime of application has little effect, and much ofthe phosphorus and potassium fertilizer is bulk-blended and applied broadcast (spread uniformlyon the soil surface) at a time when it is mostconvenient.

Methods of Fertilizer PlacementSide dressing of nitrogen fertilizer for corn hasalready been mentioned as being an applicationof fertilizer along the rows after the corn is a footor two high. A starter fertilizer application is onethat contains a relatively small amount of nutrientelements and is applied with or near the seed in aband for the purpose of accelerating early growthof crop plants. Starter fertilizer is commonly ap-plied an inch or two below and to the side ofseeds, so that the nutrients will be near youngroots and in moist soil. The result can be a fasterstart for the plants and a greater ability to competewith weeds. The earlier start, however, does notnecessarily mean higher yields at harvest time.Another benefit of band placement is less contactof the soil with the phosphorus, thus reducingphosphorus fixation.

The amount of fertilizer that can be safely

placed in a band near seeds is related to the salteffect of the fertilizer. As shown in Figure 15.7,fertilizer in contact with seed may reduce orprevent germination. If, however, the seed andfertilizer are separated by a distance of only 2 to 3centimeters (an inch), germination is barely af-fected.

Placement of nitrogen fertilizers is of little con-cern because nitrate has great solubility and mo-bility. Essentially, all of the nitrate is in solutionand moves readily to roots by mass flow. Theuptake of fertilizer phosphorus and potassium,compared with nitrogen, is more affected by lowconcentrations of phosphorus and potassium inthe soil solution, and their uptake is more depen-dent on rate of diffusion. The diffusion rates ofphosphorus and potassium are so low that theymove only 0.02 and 0.5 centimeters or less, re-spectively, to roots in loam soils during the entiregrowing season.

The major effect of the phosphorus and po-tassium fertilizer placement results from the neteffect of the amount of roots in the fertilized soil

FIGURE 15.7 Effects of fertilizer placement (shownby arrows) on the germination of wheat. On left it iswith seed and, on the right, the fertilizer and seedsare separated.

APPLICATION AND USE OF FERTILIZERS 239

volume versus the concentration of phosphorusand potassium in the soil solution. Consider that agiven amount of phosphorus fertilizer is placed ina small volume of soil, and it produces a very highconcentration of phosphate ions in solution.Phosphorus uptake could be rapid, even with asmall amount of root length or surface area-den-sity for absorption. This situation is representativeof band placement. As the soil volume increases,with a constant amount of fertilizer, more soil andphosphorus contact results in greater phosphorusadsorption (fixation), reducing the phosphorusconcentration in the soil solution. Conversely, agreater soil volume means more roots and moreroot surface for phosphorus absorption. Theoreti-cally, there is a most desirable volume for eachsoil into which the fertilizer should be mixed orplaced to achieve maximum uptake of phos-phorus, and potassium, and crop yield. Experi.mental data in Figure 15.8 show that when 50pounds of P205 per 2 million pounds of soil wereplaced with varying amounts of the soil volume,the maximum predicted phosphorus uptake oc-curred with soil volumes ranging from 2 to 20percent of the soil volume, depending on soil. Thegreater the amount of available phosphorus al-ready in the soil, the larger the volume of fertilizedsoil for maximum phosphorus uptake. Localizedplacement of phosphorus in a band along thecrop rows is especially effective in intensivelyweathered soils. These soils are rich in iron andaluminum oxides, have high phosphorus fixationcapacity and, generally, have a very low phos-phate concentration in the soil solution.

In summary, localized placement of fertilizer ina band puts fertilizer in contact with a small soilvolume in which there are few roots; however,there is a high concentration of fertilizer nutrientsin solution. By comparison, when fertilizer isbroadcast uniformly over the entire soil surface,the fertilizer is eventually mixed with a very largesoil volume. This results in many roots in the soilvolume but a much lower concentration of fertil-izer nutrients in solution.

240

FIGURE 15.8 The effect of fraction of soil fertilizedon predicted uptake of phosphorus by corn plantsfrom the application of 50 pounds of P205 per acre.Uptake values are relative. The Raub soil has thehighest soil test for phosphorus, and the Pope soilhas the lowest. (Data from Kovar and Barber, 1987.)

Broadcast fertilizer application is the surfaceapplication in a more or less uniform rate (seeFigure 15.9). If the broadcast application is madeafter the crop has been planted, it is called top

dressing. Broadcasting or top dressing is the typi-


cal method used to fertilize a lawn, forage field, orforest. The fertilizer is placed on the soil surface. Itmay be ineffectively used if the soil surface re-mains dry. Rain and irrigation water move nitro-gen rapidly into the soil, but they move potassiumto a lesser extent, and phosphorus very little. Ingeneral, the mobility of nitrogen as nitrate in soilsresults in little if any placement effect for nitratefertilizers. Urea, and other NH3-producing fertil-izers, including animal manures, are subject tosignificant nitrogen loss by volatilization whenbroadcast onto the soil surface, and the lossescannot be easily avoided.

Fertilizer application by airplane or helicopteris common for flooded rice fields, forests, andrugged pasturelands. Fertilizer is added to irri-gation water and is applied to leaves as a foliarspray. Foliar sprays are important for supplyingplants with certain micronutrients, such as iron,that are rapidly precipitated when a soluble formis added to the soil as fertilizer.

Capsules can be inserted into tree trunks tosupply certain micronutrients. Fertilizer "spikes"can be inserted in the soil of container grownplants to increase the supply of nutrients in thesoil solution.

Salinity and Acidity EffectsFertilizers are soluble salts and their effect is tol ower the water potential of the soil. The rate of

FIGURE 15.9 Broadcastapplication of dry fertilizer.(Photograph courtesy Ag-Chem Company.)

water uptake by seeds and roots is reduced. Fertil-izers placed directly with seeds can delay and/orreduce germination, as shown in Figure 15.7. Bio-logical oxidation of ammonia or ammonium, NH 3

or NH4', produces H+ and contributes to soilacidity. Continuous corn grown on acid soils thatis fertilized mainly with NH3 can eventually resultin very acid soils and accompanying acid soilinfertility. Toxicities of manganese and/or alumi-num and a deficiency of molybdenum are likely.

Each pound of nitrogen from ammonium sul-fate produces acidity equal to that neutralized byabout 5.5 pounds of CaC0 3 . Ammonium nitrate,anhydrous ammonia (NH3), and urea produceacidity equivalent to about 1.8 pounds of CaC0 3

for each pound of nitrogen. Where increased soilacidity is desirable, (NH4 ) 2 SO 4 is recommendedas the nitrogen fertilizer; it is frequently used forblueberry production. The acidity effects of phos-phorus and potassium fertilizers are generallyunimportant, except for that of the ammoniumnitrogen in ammonium phosphates. The wheatshown in Figure 15.10 failed to grow because ofacidity produced by heavy and continued applica-tion of ammonmiurn chloride over several years.

FIGURE 15.10 Very little wheat was produced onthe plot in the foreground with a pH of 3.7, becauseof long and continued application of ammoniumchloride. Note excellent wheat growth on the plot inthe rear with a favorable soil pH.

ANIMAL MANURES 241

Ammonium chloride is not a common fertilizer;however, in an experiment it produced similarresults on soil pH and crop yields as did ammo-nium sulfate.

ANIMAL MANURESAfter centuries during which animal manure washighly prized for its fertilizing value, manure onmany American farms has become a liability. Spe-cialized animal farms with many animals producemore manure than can be used effectively on thecropland. The emphasis has changed from theapplication of manure onto soils to increase soilfertility to the use of the soil for animal wastedisposal, as shown in Figure 15.11. The increas-ing shortage of fuel wood has increased the use ofmanure as a source of energy in developingcountries.

Manure Composition andNutrient ValueMany factors affect the quantity of manure pro-duced and its composition, including: (1) thekind and age of the animal, (2) the kind andamount of feed consumed, (3) the condition ofthe animal, and (4) the milk produced or workperformed by the animal.

Farm manures consist of solids and liquids.The phosphorus and dry matter are concentratedin the solid portion (feces). On average, the liquidportion of cattle manure contains about two thirdsof the potassium and one half of the nitrogen asshown in Figure 15.12. Because the compositionof manure is so variable, data on manure compo-sition presented in Table 15.2 can only be approx-imate. Earlier, it was shown that a ton of averagefarm manure was considered the equivalent of 4pounds of nitrogen in fertilizer for corn grain pro-duction. The phosphorus and potassium in a tonof manure are equated to about 2 pounds of P 20 5

and 8 pounds of K20 from fertilizer. The nutrientvalue of the manure from 500 beef cattle is

242

$19,600 per year. This is based on the assump-tions that the cattle weigh 750 pounds, and thevalue of each pound of nutrients is worth 18 centsfor nitrogen, 21 cents for P 20 5 , and 9 centsfor K20.

Nitrogen Volatilization Lossfrom ManureLarge amounts of NH 3 are produced in manure,especially by the decomposition of urea and uricacids. The pungent odor in a closed barn is usu-


FIGURE 15.11 Aerial view ofl arge feedlot at Coalinga,California. The waste-disposalproblem for each 10,000 cattleis equal to that of a city of164,000. (Photograph courtesyEnvironmental ProtectionAgency.)

ally due to NH3 . Manure contains many microbialdecomposers, and there is a continuous forma-tion and loss of NH 3 as long as favorable condi-tions exist. After manure is spread in the field,drying and/or freezing can accentuate NH 3 l oss.As a result, only a few practical things can bedone to conserve the nitrogen in manure duringstorage and application. During storage, one or afew large piles is better than many small piles ofmanure. The sooner the manure is incorporatedi nto the soil after spreading, the more nitrogen isconserved. An ideal handling of manure occurs

FIGURE 15.12 Distribution ofnitrogen, phosphorus, potassium,and dry matter in cattle manure.(From Azevedo and Stout, 1974.)

Compiled from Van Slyke, 1932.° Multiply by 0.5 to convert to kilograms per metric ton.

Clear manure without bedding; tons excreted by 1,000 lb of live weight of various animals.

when manure in pen or stall barns is madeanaerobic by cattle tramping, the manure is occa-sionally removed from these barns and thenplowed under immediately after application.

Many farmers are moving to a liquid manure-handling system. The manure is accumulated inpits and the liquid manure is injected into the soil.

Manure as a Source of EnergyBiogas production using manure and other or-ganic wastes is based on anaerobic decom-position of the wastes. During anaerobic decom-position, the major gas produced is methane(CH4). The remaining manure sludge and liquideffluent contain the original plant-essential ele-ments of the digested materials. Because muchcarbon is lost as C0 2 , the sludge and effluent areenriched with nitrogen and phosphorus, relativeto the original materials.

Basically, a slurry of manure and other organicwastes is fed into a hopper that is connected to amicrobial digestion chamber. The gas producedis piped off to light lamps, cook food, or run small

ANIMAL MANURES 243

engines. The sludge and effluent are excellent foruse as fertilizer. A biogas generator, with a metalfloating digestion chamber, is shown in Figure15.13. The dairy farm with the largest herd ofregistered Holstein cattle in the United States hasa biogas generation plant that produces all theelectricity needed to operate the farm.

Several million biogas generators are used inChina, where the primary interest is in the nutrientvalue of the digested sludge and the effluent. Be-cause most harmful disease organisms are killedi n digestion, biogas generation is also a goodmeans for disposing of human wastes and forkeeping the environment more sanitary. The gasis combustible and so safety precautions areneeded to prevent an explosion resulting fromexposure of the gas to an open flame.

During the settlement of the Great Plains in theUnited states, bison manure (chips) was usedfor fuel. Today, many people live in areas thatare not forested and fuel wood is scarce. InIndia, about 80 percent of the cattle manure isformed in cakes, dried, and used as fuel (seeFigure 15.14).

244

LAND APPLICATION OFSEWAGE SLUDGELand application of wastes to soils has been prac-ticed since ancient times and still represents animportant part of many soil management pro-grams. The federal mandate on water pollutioncontrol and waste disposal, however, marks the

FIGURE 15.14 Cow dung shaped into cakes,plastered on a wall, and allowed to dry. The driedchips are used for cooking food where fuel wood isi n short supply.


FIGURE 15.13 Biogasgenerator with intake hopper(at left), floating metal gasholder and anaerobicdigestion chamber in thecenter, and sludge dischargeat the right.

beginning of an era when land disposal of sewagewastes is being intensively studied and acceptedby the public. Sewage sludge is the solid residueremaining after sewage treatment. The increasingcost of disposal by sanitary landfills has increasedinterest in land disposal. Seventy to 80 percent ofthe municipal sewage treatment plants in Michi-gan dispose of sludge by land application. About2 percent of the land in the United States would berequired to dispose of all of the sludge produced.

Sludge as a Nutrient SourceSewage sludge is used as a source of nutrients forcrop production. The median contents, on a dry-weight basis, of sewage sludge produced in theUnited States is about 3 percent nitrogen, 2 per-cent phosphorus, and 0.3 percent potassium. A10-ton application per acre would contain 600pounds of nitrogen, which is mainly organic nitro-gen, and about 20 to 25 percent is estimated to bemineralized the first year. A 10-ton applicationwould supply about 120 to 150 pounds of nitrogenfor plant uptake. The regular nitrogen recommen-dation for crops could then be reduced by this

amount. About 3 percent of the nitrogen remain-ing in the sludge will be mineralized in the sec-ond, third, and fourth years after application.Where sludge is applied repeatedly, allowancemust be made for the release of nitrogen fromresidual-sludge. Excessive applications of sludgeresult in reduced yields, so there is a limit on howmuch sludge can be applied to land and stillproduce acceptable yields. Excessive applicationof sludge also poses a hazard for pollution ofgroundwater with nitrate.

The phosphorus in sludge is equated to that infertilizers. The phosphorus content of sludge istypically high enough when using sludge to satisfyall or meet most of the nitrogen needs of crops,that more phosphorus will be applied in thesludge than is needed. This is not a serious prob-lem, because soils have great capacity to fix phos-phorus. On coarse-textured soils very high levelsof soil phosphorus could eventually pose a haz-ard for leaching of some phosphorus to thegroundwater.

The potassium content of the 10 tons of sludgewould be equated to about 60 pounds of fertilizerpotassium.

Solid dry-sludge application to the land as abroadcast application may encounter an odorproblem, which is why sludge is sometimes han-dled as a slurry and injected directly into the soil.This has the added advantage of greatly reducingnitrogen loss by volatilization.

Heavy Metal ContaminationCertain industrial processes, such as metal plat-ing, contribute heavy metals to sludges. Amongthese are cadimum (Cd), lead (Pb), zinc (Zn),nickel (Ni), and copper (Cu). The greatest detri-mental impact of applying sludge to agriculturalland is likely to be associated with the cadmiumcontent of the sludge. Many crops may containelevated concentrations of cadmium in their vege-tative tissues without showing symptoms of toxic-ity. However, cadmium increases in the grainhave been much lower than in the vegetative tis-

LAND APPLICATION OF SEWAGE SLUDGE

sues. Potential hazards of heavy metals in theenvironment are discussed in Chapter 13.

The heavy metal content of the sludge limits thetotal amount of sludge that may be applied toland. Metal loading values have been developed,which are the total amounts of the heavy metalsthat can be applied before further sludge applica-tion is disallowed. The reaction and fixation ofheavy metals into insoluble forms are related tosoil texture. Therefore, loading values for heavymetal application on agricultural land have beentied to the cation exchange capacity, which is anindication of the organic matter and clay contentin soils. Loading values are given in Table 15.3.

An average loading value for cadmium is 10pounds per acre. For a sludge containing 14 ppmof cadmium, there is 0.028 pounds of cadmiumper ton. The amount of sludge that would need tobe applied to attain the cadmium loading value of10 pounds per acre is 357 tons.

Heavy metals are less soluble in alkaline soils,and maintaining high soil pH is one way to reducethe likelihood of heavy metal contamination offood crops. Soil pH should be maintained at 6.5 orhigher. The lime requirement of soils receivingsludge is that amount of lime needed to increasesoil pH to 6.5. The great importance of soil pH indeactivating heavy metals has overshadowed the

TABLE 15.3 Total Amount of Sludge Metals Allowedon Agricultural Land

245

246

importance of cation exchange capacity as thebasis for determining heavy metal loading values.

FERTILIZER USE ANDENVIRONMENTAL QUALITYEnvironmental concerns regarding fertilizer, ma-nure, and sludge use center around the pollutionof surface waters with phosphate and pollution ofgroundwater with nitrate-N.

Phosphate PollutionPhosphate from fertilizer and organic wastes re-acts with soil and becomes adsorbed (fixed) ontosoil particles. The adsorbed H2P04 - is carriedalong and deposited in surface waters as soil sedi-ment because of erosion. This adsorbed phos-phate in the sediment establishes an equilibriumconcentration of phosphate with the surroundingwater, just as it does in the soil with the soilsolution. The result is an increased level of avail-able phosphorus for plants growing in the waterbecause of the desorption of the H2P0 4

- . I n-


creased growth of algae and other aquatic plants,and their subsequent microbial decomposition,depletes the water of oxygen and kills fish living inthe water. Sometimes, excess weed growth ham-pers swimming and boating. The real problem iscaused by erosion of soil rather than the use ofphosphorus fertilizer. In fact, much of the pol-lution occurs at construction sites where unpro-tected soil is exposed to serious erosion. Wherephosphorus fertilizer increases the vigor of a vege-tative cover, erosion and pollution of surface wa-ters are reduced.

In some states, one half or more of the soilsamples test high or very high for phosphorus.More and more farmers are receiving fertilizer rec-ommendations for only small maintenance levelsof phosphorus, which should help reduce thedanger of phosphorus pollution of waters.

Nitrate PollutionGroundwater naturally contains some nitrate(N03

- ). Since ancient times, the disposal of hu-man and animal wastes from villages createdvarying degrees of nitrate pollution. Today,

FIGURE 15.15 Relationshipbetween the amount of appliednitrogen fertilizer, corn yield, andthe amount of nitrogen recoveredin grain and remaining in soil asleachable nitrogen. (Data ofBroadbent and Rauschkolb, 1977.)

however, the problem is more serious because ofthe extensive and high rates of nitrogen fertilizerused for crop production, and the concentrationof animals on large farms and ranches. Some ofthe initial concerns of nitrate pollution in theUnited States were the result of the pollution ofwater wells on farms. These wells became pol-luted with nitrate produced in barnyards frommanure.

The extent to which nitrate moves to the watertable as water passes through soil is a function ofthe amount of nitrate in the soil and the amount ofnitrate immobilized by the crop and microor-ganisms. Thus, the use of nitrogen fertilizer couldhave no effect or it could increase or decrease theamount of nitrate leaching through the soil to thewater table. Nitrate pollution (an increase in thenitrate level of groundwater above the normallevel) is a problem when more nitrogen fertilizeris added than crops and microorganisms can im-mobilize or absorb during growth. The data inFigure 15.15 show that when more than 220pounds of nitrogen per acre were applied, thecrop yield and uptake of nitrogen did not continueto increase significantly, and additional fertilizernitrogen appeared as nitrate that could beleached from the soil.

The nitrate pollution problem can be solved bymaking a realistic assessment of yield goals andthe application of nitrogen fertilizer and organicwastes accordingly. Efforts are being made bymany Agricultural Experiment Station and Agri-cultural Extension personnel in various states toencourage formers to avoid excessive applicationrates of both nitrogen and phosphorus fertilizers.Recommendations of nitrogen for soybeans arebeing reduced or eliminated.

Nitrate ToxicityNitrate is absorbed by roots, translocated to theshoots, and converted into proteins. Usually, thenitrate content of food and feed crops is not toxicto animals. Whenever nitrate moves into the plantmore rapidly than it is converted into protein,

SUSTAINABLE AGRICULTURE 247

nitrate accumulates in the plant. Sometimes asmuch as 5 percent or more of a plant's dry weightmay be nitrate-nitrogen. Under these conditions, ahigh concentration of nitrite may be produced inthe digestive tract of ruminants (cud-chewinganimals). Nitrites are very toxic and interfere withthe transport of oxygen by the bloodstream.

High nitrate levels in plants usually result fromhigh soil nitrate plus some environmental factorthat results in nitrate accumulation in plants. Dur-ing a drought, plants absorb nitrates, but the lackof water interferes with the normal utilization ofthe nitrate for growth. Then, nitrate accumulatesin plants. Other factors that may contribute tonitrate accumulation are cloudy weather and re-duced photosynthesis.

Various plants show different tendencies to ac-cumulate nitrate. Annual grasses, cereals cut atthe hay stage, some of the leafy vegetables, andsome annual weeds are most likely to containhigh nitrate levels. Pigweed is a nitrate accumula-tor. High concentrations of nitrate are less likelyto accumulate in legumes and perennial grasses,but even these species are not completely freefrom the problem. Within any one plant, the ni-trate level in the seeds or grain is almost alwayslower than in the leaves. High levels of nitratehave not been found in the grains used as foodand feed crops.

Monogastric (single-stomached) animals arenot likely to develop a nitrite problem from theingestion of nitrates. No authenticated cases ofhuman poisoning have been traced to high levelsof nitrate in U.S. food crops. Cases of nitrite poi-soning have been reported in European infants asa result of consumption of high nitrate-contentvegetable baby foods that were improperly stored.,

SUSTAINABLE AGRICULTUREWestern agriculture, characterized by high tech-nology, has been considered to be efficient. Thequote "one farmer producing enough food to feed70 people" fails to convey the fact that four

248

tractors, thousands of dollars of capital invest-ment, large trucks, miles of concete highway, andpeople in processing and marketing activitiesare required. Now, there is growing concernabout future energy supplies and costs. Addi-tionally, today's agriculture experts are findingthat the solution to one problem frequentlycreates other, more difficult problems. Manyfarmers are seeking alternative farming methods,because costs seem to be overtaking the benefitsfrom the adoption of newer technology.

Another aspect of the interest in sustainableagriculture is the concern over food contamina-tion from harmful chemicals. For many years, or-ganic gardeners have been devising methods togrow crops and animals without chemical fertil-izers, pesticides, and animal growth stimulators.For some, farming and food production have a"holistic" component that is reflected in a certainpersonal philosophy and way of life.

Slash-and-burn farmers in the humid tropicshave a self-sustaining system that is ecologicallysound and permanent, provided the populationdoes not overwhelm the system and make fallowperiods too short to regenerate the soil. Manysubsistence farmers throughout the world haveminimal reliance on outside production inputs.Agricultural development in Europe and theUnited States was based largely on self-sustainingsystems. During the nineteenth century, crop rota-tions, including legumes to fix nitrogen, andanimals to provide both power and cycling ofnutrients in manure, resulted in fairly satisfactorycontrol of pests and fairly high yields. Farm ma-nures were highly valued for their nutrientcontent.

Crop production research in this area has pro-duced promising results. In some cases, the re-sults of reduced crop yields have been canceledout by lower input costs. This problem, however,is not only a technological one and it is not anisolated one within society. Rather, it is an expres-sion of societal concerns that invades all areas oflife. Technology has brought apparent abundanceto millions, yet millions are as improvished as


ever. Significant movement toward and accep-tance of a restructing of agriculture to a sustain-able mode will require a change in society's choicei ndicators. That is, total costs of production musti nclude offsite costs and environmental damage,and a desire for economic profit in the short termwill need to be tempered by a greater concern forthe quality of life over the long term.

SUMMARYSoil fertility is the soil's ability to provide essentialelements for plant growth without a toxic concen-tration of any element. Soil fertility is assessed byuse of deficiency symptoms, plant tissue analysis,and soil testing.

Plant tissue analysis and soil tests are the basisfor making fertilizer recommendations.

Fertilizer nutrients are absorbed most effi-ciently when they are applied near the time ofmaximum plant uptake, owing to the potentialloss of nitrogen by leaching and volatilization andthe fixation of phosphorus and potassium. Thesefactors are also important considerations forplacement of fertilizers.

I n some cases, the acidity and salinity effects offertilizers are important considerations in the se-lection and application-of fertilizers.

Manure contains nutrients and is a substitutefor chemical fertilizers. Manure is also an energysource for biogas generation and as a substitutefor fuel wood.

Much of the sewage sludge produced in theUnited States is applied to land. This sludge con-tains nutrients that can substitute for fertilizers.Some sludges contain heavy metals, and theamount of sludge that can be applied is limited tothe heavy metal content of the sludge. Soil pHshould be maintained at 6.5 or higher to inactivateheavy metals.

Modern agriculture is energy intensive and hasbeen becoming increasingly so. Sustainable agri-culture concerns stem from the uncertainity of thefuture supply and cost of energy.

REFERENCES

Anonymous. 1986. Utilization, Treatment, and Disposalof Waste on Land. Soil Science Soc. Am., Madison,Wis.

Anonymous. 1988. Fertilizer Management for Today'sTillage Systems. Potash and Phosphate Institute, At-l anta, Ga.

Azevedo, J. and P. R. Stout. 1974. "Farm Animal Ma-nures: An Overview of Their Role in the AgriculturalEnvironment." Ca. Agr. Exp. Sta. Manual 44, Davis,Cal.

Bezdicek, D. F. and J. F. Power (eds.) 1984. "OrganicFarming: Current Technology and its Role in a Sus-tainable Agriculture." ASA Spec. Pub. No 46, Amer.Soc. Agronomy, Madison, Wis.

Broadbent, F. E. and R. S. Rauschkolb. 1977. "NitrogenFertilization and Water Pollution." CaliforniaAgricul-ture, 31 (5):24-25.

Cressman, D. R. 1989. "The Promise of Low-input Agri-culture." J. Soil and Water Con. 44:98.

Edens, T. C., C. Fridgen, and S. L. Battenfield. 1985.Sustainable Agriculture and Integrated FarmingSystems. Michigan State University Press, EastLansing.

Engelstad, O. P. (ed.) 1985. Fertilizer Technology andUse. 3rd ed. Soil Sci. Soc. Am., Madison, Wis.

FAO. 1978. China: Azolla Propagation and Small-scaleBiogas Technology. FAO Soils Bull, 41, Rome.


Illinois Agronomy Department. 1984. Illinois Agron-omy Handbook 1985-1986. University of Illinois,Urbana.

Jacobs, L. W. 1977. "Utilizing Municipal Sewage Waste-

REFERENCES 249

waters and Sludges." North. Cent. Reg. Ext. Pub. 52.Michigan State University, East Lansing.

Kovar, J. L. and S. A. Barber. 1987. "Placing Phosphorusand Potassium for Greatest Recovery." Jour. Fert. Is-sues. 4(1):1-6.

Neely, D. 1973. "Pin Oak Chlorosis-Trunk ImplantationsCorrect Iron Deficiency." Jour. For. 71:340-342.

Page, A. L., T. J. Logan, and J. A. Ryan. 1989. LandApplication of Sludge. Lewis Publishers, Chelsea,Mich.

Stoeckeler, J. H. and H. F. Arneman. 1960. Fertilizers in

Forestry. Adv. Agron. 12:127-165.Stone, E. L., G. Taylor, and J. DeMent. 1958. "Soil and

Species Adaption: Red Pine Plantations in New

York." First North Am. Forest Soils Conf. Proc. Michi-

gan State University, East Lansing.Van Slyke, L. L. 1932. Fertilizers and Crop Production.

Orange Judd, New York.Voss, R. 1982. "General Guide for Fertilizer Recommen-

dations in Iowa. "Agronomy 65. Iowa State University,Ames.

Warncke, D. D., D. R. Christensen, and M. L. Vitosh.1985. "Fertilizer Recommendations: Vegetable andField Crops in Michigan." Mich. State Univ. Ext. Bull.E-550. East Lansing, Mich.

Welch, L. F., D. L. Mulvaney, M. G. Oldham, L. V.Boone, and J. W. Pendleton. 1971. "Corn Yields withFall, Spring, and Sidedress Nitrogen." Agron. Jour.

63:119-123.Welch. L. F. 1986. "Nitrogen: Ag's Old Standby." Solu-

tions: J. Fluid Fert. and Ag. Chem. July/August, pp.18-21.

Wolcott, A. R., H. D. Foth, J. F. Davis, and J. C.Shickluna. 1965. "Nitrogen Carriers: Soil Effects."Soil Sci. Soc. Am. Proc. 29:405-410.

http://North.Cent.Reg.Ext.Pub.52.Michigan



Soil genesis deals with the factors and processesof soil formation. The formation of soil is theresult of the interaction of five soil-forming fac-tors: parent material, climate, organisms, topo-

graphic position or slope, and time. The domi-nant processes in soil genesis are: (1) mineralweathering, (2) humification of organic matter,(3) leaching and removal of soluble materials,and (4) translocation of colloids (mainly silicateclays, humus, and iron and aluminum oxides).The net effect of these processes is the develop-ment of soil horizons, that is, the genesis of soil.

ROLE OF TIME IN SOIL GENESIS

Soils are products of evolution, and soil proper-ties are a function of time or soil age. The age of asoil is expressed by its degree of development andnot the absolute number of years. In a sense, soilshave a life cycle that is represented by variousstages of development. Reference has been madeto minimally, moderately, and intensively weath-ered soils. To study time as a soil-forming factor,

CHAPTER 16

SOIL GENESIS

250

the other soil-forming factors must remain con-stant, or nearly so.

A Case Study of Soil GenesisAt the end of the Wisconsin glaciation, about10,000 years ago in the upper Midwest of theUnited States, the Great Lakes were larger thanthey are today. The lowering of lake levels oc-curred in stages to produce a series of exposedbeach surfaces of different ages. In the case of thestudy area, there are three different soils on lakebeaches aged 2,250 years, 3,000 years, and 8,000

years. All slopes were nearly level. Similar cal-careous, quartzitic sand parent material occurredat each location, as shown in Figure 16.1.

For a short period of time after the glacial icemelted, a tundra climate existed. Soon after, theclimate became favorable for trees. The climatewent through a warming trend between 4,000 and6,000 years ago; today, the climate is humid-temperate. In general, the vegetation was a mixedconiferous-deciduous forest.

Two factors that greatly affected soil genesis in

this study were (1) the quartzitic sand-parent ma-terial, and (2) a humid climate that producesmuch leaching. Very little clay existed in the par-ent material and very little clay was formed viaweathering. Clay translocation was insignificant,and Bt horizons could not develop. Under theconditions of a high-leaching regime, colloidalhumus and oxides of iron and aluminum (sesqui-oxides) were translocated downward and accu-mulated in the subsoil.

The changes that occurred during soil genesisare summarized in Table 16.1. During the first2,250 years, humification and accumulation oforganic matter near the soil surface resulted in theformation of an A horizon. The lime present in theupper part of the parent material was leached outand the A horizon became acid. The acid A hori-zon overlies a slightly discolored parent materiallayer. This young soil, which has only A and Chorizons, is the Deer Park soil.

During the next 750 years, leaching continuedto remove lime, and acidity developed in the soilto a depth of 1 meter or more. Sufficient sesquiox-ides accumulated in the subsoil to form a Bshorizon (s from sesquioxide). Simultaneously, anE horizon evolved. This 3,000-year-old soil on theNipissing beach has A, E, Bs, and C horizons andis called the Rubicon soil.

During the next 5,000 years, there was consider-able movement and accumulation of humus inthe subsoil, and the Bs horizon became a Bhs

ROLE OF TIME IN SOIL GENESIS 251

FIGURE 16.1 Relationshipbetween lake beach locations andages for soil genesis in northernMichigan. Ages of deposits givenas years before the present (BP).(Data from Franzmeier andWhiteside, 1963.)

horizon (h from humus). This 8,000-year-old soilhas A, E, Bhs, and C horizons and is called theKalkaska soil. A soil similar to the Kalkaska isshown in Figure 16.2 (also see book cover).

Note from Table 16.1, that as colloidal andamorphous sesquioxides and humus (noncrys-talline materials with high specific surface) accu-mulated in the subsoil, the tree species changed.The change was toward species more demandingof water and nutrients, and this parallels the soil'sability to retain more water and nutrients asamorphous colloidal materials accumulated inthe B horizon. Therefore, as the soils evolved, soilproperties changed and this caused a change inthe composition of the forest species on eachterrace. The most productive soil in the sequencefor forestry is the Kalkaska soil.

Time and Soil Development SequencesIt should be recognized that the rate of soil gene-sis in a particular situation is highly dependent onthe nature of the soil-forming factors. In somesituations, soil genesis is rapid, whereas in oth-ers, soil genesis is very slow.

Some soil features or properties developquickly, but the development of other propertiesrequires much more time.

A horizons can develop in a few decades. Con-sequently, soils with only A and C horizons can

252

develop in 100 years or less. The formation of aBw horizon involves changes in color and/orstructure and requires about 100 to 1,000 years.Soils with an A-Bw-C horizon sequence, therefore,require commonly several hundred years for de-velopment. The development of the Bt horizon isdependent on some clay formation within the soiland significant translocation of clay. Theseprocesses usually require a minimum of severalthousand years. Soils with an A-E-Bt-C horizonsequence need about 5,000 to 10,000 years toform under favorable conditions. The time to formi ntensively weathered soils containing a highcontent of kaolinite and oxides of iron and alumi-

SOIL GENESIS

TABLE 16.1 Major Processes and Changes During Soil Evolution inNorthern Michigan From Sand Parent Material

Adapted from Franzmeier and Whiteside, 1963.

num is approximately 100,000 years. Some ofthese soils in the tropics are more than a millionyears old.

ROLE OF PARENT MATERIAL INSOIL GENESISParent material has a great influence on theproperties of young soils, including color, texture,structure, mineralogy, and pH. Over time the ef-fects of the parent material decrease, but someeffects of the parent material still persist in oldsoils.

ROLE OF PARENT MATERIAL IN SOIL GENESIS

FIGURE 16.2 Soil with A, E, Bhs, and C horizons.(The metal frame is used to collect a vertical slab ofsoil to mount on a board and treat with plastic, to beused for display purposes.)

Consolidated Rock as a Source ofParent MaterialConsolidated rocks are not parent material butserve as a source of soil parent material. Soilformation may begin immediately after the depo-sition of volcanic ash but must await the physicaldisintegration of hard rocks when granite, basalt,or sandstone are exposed to weathering. During

253

the early stages of rock weathering and soil for-mation, the formation of parent material and soilmay occur simultaneously as two overlappingprocesses. Soils formed in parent materials de-rived from the weathering of underlying rocks arecommon in the U.S. Appalachian Highlands andInterior Highlands in the eastern states and in theCordilleran Region in the western states (see Fig-ure 16.3). Within these regions, however, manysoils have formed in the sediments that occur atthe base of steep slopes or in small depressions,basins, or areas beside streams and rivers.

Soil Formation fromLimestone WeatheringParent material formation from most hard rocks isby the physical disintegration and chemical de-composition of the mineral particles in the rock. Aparent material derived from sandstone is, essen-tially, composed of the sand particles that werecemented together in the sandstone. In the case oflimestone, the soil develops in the insoluble im-purities that remain after the calcium and magne-sium carbonate have been dissolved and leachedfrom the weathering environment. Clay is a com-mon impurity in limestone, giving rise to the finetexture of soils derived from limestone weather-i ng. Chert is a compact, siliceous material thatoccurs in some limestones. The chert resists dis-solution, and soils formed from the weathering ofcherty limestone are stony.

Many centimeters of limestone are required toform a centimeter of soil, because the impurities(residues) typically comprise only a small per-centage of the limestone and some of the residuesare carried away. It has been estimated that a footof soil accumulation in the Blue Grass region ofKentucky required limestone of the order of 100feet and more than half a million years. Most soilsof the Kentucky Blue Grass region of Kentuckyand the Nashville Basin of Tennessee have devel-oped from limestone and were prized by settlersfor their suitability for agriculture. A soil devel-

254 SOIL GENESIS

oped from limestone weathering is shown in Fig-ure 16-4.

Sediments as a Source ofParent MaterialMost soils have developed from sediments thatwere transported by water, wind, ice, or gravity.Colluvial sediments occur at the base of steepslopes where gravity is the dominant force, caus-ing movement and sedimentation. Colluvial sedi-ments are common and are an important parentmaterial in mountainous areas.

Alluvial sediments are ubiquitious because ofthe widepread existence of streams and rivers(see Figure 16.5). The most extensive area of allu-vial sediments in the United States occurs alongthe Mississippi River. Most of the agricultural soils

FIGURE 16.4 Parent material derived fromlimestone is the residual accumulation of impuritiesi n the limestone after water has leached away thecarbonate materials. Because clay is a majori mpurity, parent materials and soils derived fromlimestone weathering tend to be fine textured.

FIGURE 16.3 Physiographic regions of North America.

in California occur in valleys where alluviumis the dominant parent material. Colluvial andalluvial sediments occur as inclusions in areasdominated by hard rocks or other kinds of sedi-ments. Regions of the United States in whichsediments are extensive and important sourcesof parent material are shown in Figure 16.3.The regions include the Gulf and AtlanticCoastal Plains, the Central Lowlands, theI nterior Plains, and the Basin and Range.Sediments were the most common source of

parent material for the soils of the UnitedStates.

Gulf and Atlantic Coastal Plains The sedi-ments of the Gulf and Atlantic Coastal Plains areof marine origin. They were derived from theweathered products of the adjacent highlands thatwere deposited when the area was submergedbelow the sea. Where calcareous clays rich insmectite were deposited, soils developed withhigh clay content and great expansion and con-traction during alternating wet and dry seasons.Soils have developed in this kind of parent mate-rial in Texas, Mississippi, and Alabama. Proper-ties of the soils are the result of the dominanteffect of the parent materials.


The majority of the other soils on the coastal

plains developed in sediments consisting mainlyof quartz sand and kaolinite clay. The quartz andkaolinite represent the "end products" of highlyweathered sediments and soils. The sedimentsaccumulated over a long period of time and someof the material in the sediments participated inseveral weathering cycles before coming to rest inthe sea. The coastal plains slope downward to-ward the sea, resulting in increasing age of sedi-ments and a period of soil formation from theseacoast inward. In the southeastern UnitedStates this has produced three major surfaces-the lower, middle, and upper coastal plains. Thesoils generally have sandy A horizons and lownatural fertility. Clay translocation has producedBt horizons in most of the soils. Many of the sub-soils are deficient in calcium for good root

growth.

Central Lowlands Most of the Central Low-lands (see Figure 16.3) was glaciated, with theOhio and Missouri rivers forming the approximatesouthern boundary of the glaciation. Four majoradvances of the ice occurred during a period ofabout 1.5 million years. Each advance of the ice

255

FIGURE 16.5 Alluvialsediments, deposited alongstreams and rivers, arecommon parent materials forsoil formation throughout theworld.

256

was followed by a long interglacial period. The

glacial and interglacial periods and approximatetimes of their occurrence are listed in Table 16.2.

During deglaciation, rivers and streams of waterflowed from the melting ice and carried and de-

posited material called outwash near the icefront. Outwash consists of coarse-textured sedi-ments. Clay, and to a lesser extent silt, werecarried by water into glacial lakes, where theclay settled to form calcareous, fine-texturedlacustrine (lake) sediments. The melting of ice

produced moraine sediments by the direct depo-sition of material in the ice onto the ground sur-face. The moraine sediments are unsorted and arecalled till. Sometimes the ice front readvancedover previously deposited sediments. As a resultof these varied activities, glacial parent materialsare very heterogenous, variable in composition,and commonly stratified, as shown in Figure 16.6.

The nature of the sediments was affected by thekinds of rocks the glacier moved over and thetypes of materials that were incorporated into theice. As a result, some glacial materials containsome boulders and stones, but most are com-

posed of highly variable amounts of sand, silt, andclay. Another important component was calciumcarbonate whose content was related to the extentto which the glacier moved over limestone andother calcareous rocks. In the northern part ofWisconsin, Michigan, and the New England

TABLE 16.2 Glacial and Interglacial Ages inNorth America

SOIL GENESIS

states, most of the glacial sediments have a highsand content.

During deglaciation, there was rapid ice melt-i ng in summer and large rivers carried away themelt water. As a result, stream valleys with verywide floodplains were produced on which sedi-ments accumulated. During the winter, there waslittle meltwater, and the sediments on the flood-

plains were exposed to the winds. The floodplainmaterials dried, and in the absence of a vegetativecover, the silt-sized particles were preferentially

picked up by westerly winds and deposited alongthe eastern sides of the major rivers in the degla-ciated area. These sediments contain mainly silt,with smaller amounts of clay, and are calledloess. The loess is thickest near the river anddecreases in thickness with distance away fromthe river. In some cases, the loess was more than100 feet thick. Loess about 50 feet thick along theMississippi River in western Tennessee is shownin Figure 16.7.

Loess is the dominant parent material in thesoils of Iowa and Illinois. The high silt contentof the loess parent material is reflected in theabundance of silt loam A horizons and the highwater-holding capacity and fertility of manyof the Corn Belt soils. Many soils beyond the so-called loess area have been influenced by a thinl ayer of loess. In some of these soils, the up-per soil horizons have developed in loess andthe lower horizons have developed in till orlimestone.

Much loess exists outside of the glaciated partof the Central Lowlands and is found in the Inte-rior Plains, especially in Nebraska and Kansas.This is an area of limited precipitation and steppe(short grass) vegetation. The loess is believed tohave resulted from transport of silt from a sourcearea located to the west and under conditions oflimited protection of land by a vegetative cover.Extensive areas of nonglacial loess also occur inChina and Argentina. Thick loess overlies hills ofbasalt in the Palouse area of eastern Washingtonand western Idaho. The loess originated from thedeserts of central Washington. About 10 percent

ROLE OF PARENT MATERIAL IN SOIL GENESIS 257

FIGURE 16.6 Sand depositedby water and later overriddenby glacial ice,which meltedand left a layer of till; anexample of stratified parentmaterial.

of the world's land has been influenced by loess

Plains. After the Rocky Mountains were formed,deposition.

erosion and subsequent sedimentation producedsediments along the eastern side of the moun-

I nterior Plains The Interior Plains region is tains in a manner analogous to the formation ofcomparable to the western part of the Great

the Coastal Plains sediments east and south ofFIGURE 16.7 A loess depositabout 50 feet (15 ms) thick onthe uplands along the easternside of the Mississippi Riverfloodplain in westernTennessee.

258

the Appalachian Mountains. These sediments,however, contain more weatherable minerals,and the resulting soils are much more fertile thanthose formed from the coastal plains sands. Thereare considerable areas of sand and, in someplaces, erosion has exposed older sediments un-derneath. Glacial-derived parent materials coverthe Interior Plains region north of the MissouriRiver.

Basin and Range Region The Basin and Rangeregion consists of a series of low mountains sepa-rated by broad basins. The area was subjected toblock-faulting with the ranges representing theuplifted areas and the down-faulted blocks repre-senting the basins. Weathering and erosion haveproduced sediments of coalescing alluvial fans,which fill most of the basins. Lacustrine sedi-ments are of minor extent.

Volcanic Ash Sediments Volcanic ash distri-bution parallels that of volcanic mountains,which were the source of the ash. The 1980 erup-tion of Mount St. Helens brought a new awarenessof the existence and importance of volcanic ash inthe northwestern part of United States. Thoughmuch of the material ejected in 1980 was crystal-line in nature, ash is commonly amorphous andweathers to form amorphous allophane and ironand aluminum oxides. Allophane has a high pH-dependent cation exchange capacity. Soils withamorphous minerals lack the distinct sand andsilt particles found in most mineral soils. Themajor agricultural area in the United States wheresoils have developed from ash occurs on Hawaii,the large island of the Hawaiian chain, wheresugarcane is the dominant crop. Although ash is aminor parent material near Honolulu, Hawaii, onthe island of Oahu, the famous landmark, Dia-mond Head, is an old volcanic cone composed ofconsolidated ash, as shown in Figure 16.8.

SOIL GENESIS

Ash is widely distributed by winds and is aminor but frequent addition to many soils a greatdistance from the ash source. Ash is a significantcomponent of the loess in the Palouse in thenorthwestern United States and on the Pampa inArgentina. In some instances, an occasional addi-tion of ash significantly increases the fertility ofhighly weathered soils as occurs on the SerengetiPlain in Kenya, Africa.

Effect of Parent Material Properties onSoil GenesisHorizon development, especially many B hori-zons, depends on the translocation of fine-sizedparticles in water. A parent material composed of100 percent quartz sand would not weather toproduce mineral colloidal particles. In such par-ent material, little else can happen than someorganic matter accumulation due to plant growth.Early in this chapter, it was noted that a sand-textured parent material in a humid environmentled to the development of Bhs horizons. Withinthe parent material there was a small amount ofprimary minerals that weathered to release someiron and aluminum. There was insufficient clay toform Bt horizons, however. When soils develop ina parent material that is essentially all clay, littlewater will migrate downward to transport parti-cles. Parent materials of intermediate texture leadto the development of a wide variety of soils.

The texture and structure of a soil will affectsoil genesis by affecting water infiltration and ero-sion, permeability and translocation of materialsin water, protection and accumulation of organicmatter, and solum (A plus B horizon) thickness.The thicker solum of the Spinks soil, as comparedwith the St. Clair soil, is shown in Figure 16.9. Thethicker solum is due to the lower clay content,greater water permeability, and greater downwardmigration of clay before lodgement during theformation of the Bt horizon in the Spinks soil.Greater permeability of sandy material also con-tributes to the more rapid removal of lime (cal-


cium carbonate), resulting in more rapid develop-

Stratified Parent Materialsment of acidity and a greater leaching depth of the

A stratified parent material is shown in Figurelime. Note that the texture of the A and B horizons

16.6. A layer of loess over till was shown to be ai n Figure 16.9 is related to the clay content of the C

condition found in the glaciated part of the Cen-horizon (parent material).

tral Lowlands. Frequently, the A or the A and B

259

FIGURE 16.8 Diamond Headis a famous landmark nearHonolulu, Hawaii, and is anold volcanic cone composedof consolidated layers of . ash(tuff). Note that the cone ishighest on the right becausethe prevailing winds werefrom the left during ash fall.

FIGURE 16.9 Relationshipbetween texture of the parentmaterial and the thicknessand texture of horizons ofthree forest soils in the north-central part of the UnitedStates.

260

horizons develop in the loess and the lower part ofthe soil develops from till. Soil permeability towater (hydraulic conductivity) changes at theboundary of materials of different texture, whichaffects the downward movement of water. Alluvialparent materials typically are stratified. Wherestreams dissect the landscape, various materialsmay be exposed for soil formation, giving rise to aseries of soils whose differences are due mainly toparent material differences. Such a sequence ofsoils is a lithosequence, as shown in Figure 16.10.

In soils with horizons developed from differ-ent parent materials, numbers are used to indi-cate this feature. For example, a soil with A and Ehorizons developed in loess, and Bt and C hori-zons developed in till has a horizon sequence ofA-E-2Bt-2C. The 2 indicates that the Bt and C hori-zons developed in a different or second parentmaterial.

Parent Material of Organic SoilsIn locations where considerable quantities ofplant material grow and where decay is limited

SOIL GENESIS

because of water saturation and anoxia, low tem-perature, or acidity, an abundance of organic mat-ter may accumulate. Sometimes, this results in amat of organic matter on the surface of mineralsoil. In permanently ponded areas throughout theworld, thick deposits of peat and muck haveformed. The Florida Everglades is one of the best-known areas. Organic deposits are the parent ma-terial for organic soils.

ROLE OF CLIMATE IN SOIL GENESISClimate greatly affects the rate of soil genesis. Inareas that are permanently dry and/or frozen, soildoes not form. The two components of climate tobe considered are precipitation and temperature.

Precipitation EffectsWater is necessary for mineral weathering andplant growth. Water in excess of field capacity(surplus water) participates in the downwardtranslocation of colloidal particles and solublesalts. The limited supply of water in deserts re-

FIGURE 16.10 A roadcutshowing outwash overlyingsandstone. Strata of differentmaterials can produce alithosequence of soils on ahillside where the materialsare exposed, as shown in theleft side of the photograph.

suits in soils that tend to be alkaline, relativelyunweathered, low in clay and organic matter con-tent, and cation exchange capacity. Generally,soils of the arid and subhumid regions tend to befertile, except for the limited ability of the soilmicrobes to mineralize soil organic matter and tosupply available nitrogen. Where there is suffi-cient water for only limited leaching, the carbo-nates tend to be moved downward only a shortdistance, where they accumulate and form a zoneor horizon of calcium carbonate accumulation.

FIGURE 16.11 Grassland soil developed undersubhumid climate. Limited leaching is expressed bythe presence of a k layer (accumulation ofcarbonates) in the upper part of the C horizonbetween depths of 70 and 110 centimeters.(Photograph courtesy USDA.)

ROLE OF CLIMATE IN SOIL GENESIS 26 1

The symbol k is used to indicate this calciumcarbonate accumulation feature. This feature isshown in Figure 16.11 for a soil that has a Ckhorizon. Soils in deserts commonly have k hor-izons.

Increases in precipitation have been posi-tively related with greater: (1) leaching of lime andgreater depth to a k layer, (2) development of soilacidity, (3) weathering and clay content, and (4)plant growth and organic matter content. A studyof surface soils developed in loess along a tra-verse from Colorado to Ohio showed that as pre-cipitation increased, the depth to the k layer in-creased until the annual precipitation was about26 inches (65 cm). This occurred near theNebraska-Iowa border. At this location, pH of Ahorizons was about 7. Eastward along the tra-verse, sufficient leaching occurred to move cal-cium carbonate to the water table, and k layerswere generally absent. In addition, the surfacesoils became more acid and those in Ohio had apH of about 5.

Increasing precipitation from easternColorado to Indiana is associated with a changein native vegetation. The annual precipitation in-creases from about 16 to 36 inches (40 to 90 cm).The vegetation changes from widely spaced,bunch and short grasses in Colorado, to short andtall grasses in Nebraska, and to tall grasses inIllinois and Indiana. These changes are associ-ated with an increase in organic matter from 180to 360 metric tons per hectare (80 to 160 tons peracre) to a depth of 100 centimeters (see Figure16.12). Similar changes in organic matter contentoccur on the grasslands in the Pampa of Ar-gentina. Along these traverses with increasingprecipitation, increasing soil organic matter con-tents of well-drained upland soils are associatedwith greater annual production of the grasses.East of the grassland area the vegetation changesto forest, and the soil organic matter content isconsiderably less. The organic matter content oftall grass soils in Illinois decreases from 360 met-ric tons per hectare to 150 metric tons per hectarefor forest soils in Ohio (see Figure 16.12).

262

Increasing precipitation in the tropics alsoresults in increasing amounts of organic matter intropical soils. The relationship between grasslandand forest soils in the temperate region, however,is reversed in the tropics. The forested, humid,tropical soils have a greater organic matter con-tent than the soils of the adjacent drier grassland-shrub savannas. It has been suggested that highacidity and infertility of savanna soils are partiallyresponsible for the lower production of plant ma-terial on the tropical savannas than that in temper-ate grasslands.

Temperature EffectsEvery 10°C increase in temperature increases therate of chemical reactions approximately twotimes. Increased weathering and clay formationoccur with an average increase in soil temper-ature.

The relationship between average tempera-ture and plant growth and the accumulation oforganic matter is complex. The organic mattercontent of soil is the net result of plant growth orthe addition of organic matter, the rate of organicmatter mineralization, and the soil's capacity toprotect organic matter from mineralization. Many

SOIL GENESIS

FIGURE 16.12 Generalized mapshowing organic matter content ofsoils, in metric tons per hectare100 centimeters, as related toclimate and vegetation. (Adaptedfrom Schreiner and Brown, 1938.)

soils of tundra regions are rich in organic mattereven though plant growth rates are very slow.Even slower are the rates of microbial decom-position of organic matter due to both low temper-ature and soil wetness. The grassland soils of theGreat Plains have a gradual decrease in organicmatter with increasing annual temperature. (seeFigure 9.4).

The forested soils of the southeastern UnitedStates are shown to contain only 60 percent asmuch organic matter as do the forested soils fur-ther north (see Figure 16.12). This difference ispartially due to the generally more sandy nature ofthe A and E horizons of the southern soils on thecoastal plains as compared with the finer-textureof the forest soils further north.

Based on the observations of the relation be-tween temperature and total soil organic mattercontent in the eastern United States, one wouldexpect tropical soils to be low in organic matter.Tropical rain forests produce more organic matterand have greater organic matter decompositionthan temperate region forests, because of year-round favorable temperature and precipitation.Temperate region forests have a long dormantwinter period, resulting in less plant growth anddecomposition of organic matter than that in trop-

i cal rain forests. Therefore, the content of soilorganic matter tends to be similar in the temper-ate forests and tropical rain forests. Although re-searchers have found slightly more organic matterin the tropical counterparts, the differences are oflittle or no importance. The greater dominance ofamorphous clays in tropical soils may also be afactor in the protection of organic matter.

Climate Change and Soil PropertiesSome soils have been influenced by more thanone climate because of climate shifts. A study ofburied soils, paleosols, reveals the nature of theclimate that existed many years ago when ancientsoils formed. In the southwestern United Statesthe uplift of the Rocky Mountains cut off moisture-laden winds and created a drier climate east of themountains. Today, some of the soils on the south-western deserts have a strongly developed Bt hori-zon, which indicates that a humid climate existedthere when the Bt horizon was formed.

ROLE OF ORGANISMS IN SOIL GENESISPlants affect soil genesis by the production oforganic matter, nutrient cycling, and the move-ment of water through the hydrologic cycle. Mi-croorganisms play an important role in organicmatter mineralization and the formation of hu-mus. Soil animals are consumers and decom-posers of organic matter; however, the most obvi-ous role of animals appears to be that of earthmovers.

A most obvious effect of organisms on soilgenesis is that caused by whether the natural veg-etation is trees or grass. The following discussionwill emphasize the differential effects of trees andgrass on soil genesis.

Trees Versus Grass and OrganicMatter ContentFigure 16.12 shows that the forested soils of Ohioand Wisconsin contain significantly less organicmatter than grassland soils located to the west. A

ROLE OF ORGANISMS IN SOIL GENESIS 263

study of forest and grassland soils developed fromsimilar parent material in southern Wisconsin re-vealed that grassland soils contain about twice asmuch total organic matter as forest soils. In addi-tion there is a marked difference in the distribu-tion of organic matter within the soil profile (seeFigure 16.13).

The explanation for the differences in theamount and distribution of organic matter is re-lated to the differences in the growth habits of thetwo kinds of plants. The roots of grasses are short-lived and each year contribute to the soil largeamounts of organic matter that become humified.Furthermore, there is a gradual decrease in rootdensity with increasing soil depth, which parallelsthe gradually decreasing organic matter content.In the forest, by contrast, roots are long-lived andthe annual addition of plant residues is largely asleaves and dead wood that fall directly onto thesoil surface. Some of the plant materials decom-pose on the soil surface, and small animals trans-port and mix some of the organic matter with arelatively thin layer of topsoil. In the hardwoodforest of southern Wisconsin earthworms wereactive and 81 metric tons of organic matter was inthe top 6-inch layer per hectare and only 25 metrictons in the next deeper 6-inch layer. Thus, in theforest soil 47 percent of the soil organic matterwas in the top 6 inches as compared to 35 percentfor the grassland soil. The dark-colored surfacelayer enriched with organic matter, the A horizon,is thicker in the grassland soil than in the forestsoil.

Another interesting feature shown in Figure16.13 is the similar amount of total organic matter

i n each ecosystem. In the forest, however, most ofthe organic matter is found in the standing trees.When settlers cleared and burned the trees, morethan one half of the organic matter in the forestecosystem was destroyed. By contrast the conver-sion of the Prairie to use for cropping resulted inlittle loss of organic matter. In addition, erosion ofthe topsoil was more detrimental to forest soilsthan to grassland soils, because of the relativelygreater concentration of organic matter near thesurface of the soil.

264 SOIL GENESIS

FIGURE 16.13 Distribution of organic matter in forest and grasslandecosystems in south-central Wisconsin in metric tons per hectare. (Adaptedfrom Nielson and Hole, 1963, and used by courtesy of F. D. Hole, Soil SurveyDivision of the Wisconsin Geological and Natural History Survey, University ofWisconsin.)

Vegetation Effects on Leachingand EluviationWide differences in the uptake of ions and, conse-quently, in chemical composition of plants havebeen well documented. These differences play arole in soil development. Species that normallyabsorb large amounts of the cations calcium,magnesium, potassium, and sodium will delaythe development of soil acidity because they recy-cle more of these cations to the soil surface

through the addition of organic matter. The effectwill be to delay their removal from the cationexchange sites and to delay the development ofacidity and the accumulation of exchangeablealuminum and hydrogen. The data in Table 16.3support the fact that hardwoods maintain a higherpH than do spruce trees when grown on parentmaterial with the same mineralogical compo-sition.

Under the same climatic conditions, where for-

Adapted from R. Muckenhirn, 1949.

ests and grasslands soils exist side by side andhave comparable parent material and slope, for-est soils show a greater leaching of exchangeablecalcium, magnesium, potassium, and sodiumand greater eluviation of clay from the A to Bhorizon. Forest soils are more acid than grasslandsoils, due to greater leaching of basic cations;they also have less clay in the A horizon and moreclay in the B horizon. A comparative study of soilson 2 to 8 percent slopes in Iowa found that the B/Aclay ratio (percent clay in A horizon divided bypercent clay in B horizon) of the forest soil was 2.0compared with 1.04 for the grassland soil. Clay-pans develop in forest soils in less time than isrequired in grassland soils. The discussion of theeffect of trees versus grass on soil genesis showsthat the same fundamental processes occur inboth kinds of soils but differ in degree of expres-sion. Soils developed under trees and grass be-come strikingly similar when both have devel-oped impermeable clay-pan subsoils.

Role of Animals in Soil GenesisThe role of animals in incorporating organic mat-ter into the soil surface has been mentioned. Go-phers and prairie dogs are well known for theirburrowing and nest-building activities. Theseanimals transport and deposit material on the sur-

ROLE OF TOPOGRAPHY IN SOIL GENESIS 265

face of the soil. In some tropical soils that aremore than a million years old, and that have hadsignificant termite activity, the soil, in a sense, hasbeen turned over many times. This is one proba-ble cause for the uniformity in properties in manyintensively weathered soils in the tropics. For ad-ditional consideration of soil animals, see Chap-ter 8.

ROLE OF TOPOGRAPHY IN SOIL GENESISTopography is used here to refer to the configura-tion of the surface of a local or relatively smallarea of land. Differences in topography can causewide variations in soils within the confines of asingle field. Topography determines the local dis-tribution or disposal of the precipitation and de-termines the extent to which water tables influ-ence soil genesis. Water-permeable soils onbroad, level areas receive and infiltrate almost allof the precipitation. Soils on sloping areas infil-trate less than the normal precipitation; hence,there is runoff. Depressions and low areas receiveadditional water, making more water available forsoil genesis than the normal precipitation. Theeffects of water relations on soil genesis are morepronounced in humid than in arid regions.

Effect of SlopeBoth the length and steepness of slope affect soilgenesis. As the steepness of slope increases,there is greater water runoff and soil erosion. Thenet effect is a retardation of soil genesis. Gener-ally, an increase in slope gradient is associatedwith less plant growth and organic matter content,less weathering and clay formation, and lessleaching and eluviation. Consequently, soils havethinner sola and are less well developed onsteeper slopes. Even on the native prairies of thecentral United States, some grassland soils onsteep slopes have thin A horizons. The majorcause of different soil colors in many fields is soilerosion. On eroding slopes the removal of soil

266

produces light-colored soils and the sedimenta-tion and deposition of the eroded soil; wherechanges in slope occur, dark-colored soils areproduced (see Figure 16.14).

Many soils on sloping land are in equilibrium interms of erosion rate and rate of horizon forma-tion. As erosion reduces the thickness of the Ahorizon, the upper part of the B horizon is incor-porated into the lower part of the A horizon. Theupper part of the C horizon is slowly incorporatedi nto the lower part of the B horizon. Under theseconditions, the soil essentially maintains its char-acter over a long period of time as the landscapeevolves.

Effects of Water Tables and DrainageFor this discussion, it is assumed that soils arepermeable to water, so that landscape positionplays an important role in affecting the depth tothe water table and soil drainage. Drainage is ameasure of the tendency of water to leave the soil.Well-drained soils occur on slopes where the wa-ter table is far below the soil surface. The well-drained soils shown in Figure 16.15 are repre-

SOIL GENESIS

sented by the Gritney and Norfolk soils. Poorlydrained soils tend to occur in the low parts of thelandscape where water tables exist close enoughto the soil surface to cause various degrees ofanoxia and reduction. This landscape position isrepresented by the Bibb soils in Figure 16.15. TheRains soils are poorly drained because of a nearlylevel or slightly depressional position on a broadupland area. The Goldsboro soils (Figure 16.15)are intermediate, being moderately-well drainedor somewhat poorly drained.

Poorly drained soils have an A horizon that isusually dark-colored because of the high organicmatter content; the subsoil tends to be gray-colored. Well-drained soils have lighter colored Ahorizons and have uniformly bright-colored sub-soils. Intermediate drainage conditions producesoils with intermediate properties. Whenever thesoil is water saturated, downward translocation ofclay and removal of soluble materials are in-hibited.

As a result of differences in topography, soilsdeveloped from similar parent materials may varygreatly within a small area. A sequence of soilsalong a transect that differs because of topogra-

FIGURE 16.14 Cultivatedland showing light-coloredsoil on actively eroding slopesand dark-colored soils whereeroded material has beendeposited and isaccumulating.

phy is a soil catena, or toposequence. Locally,variations in soils are due primarily to variationsof topography and parent material.

Topography, Parent Material, andTime InteractionsOn slopes, water runoff and soil erosion occurwhile simultaneously run-on water and soil depo-sition occur on areas at a lower elevation. Thus, ina landscape, young soils tend to occur where bothsedimentation and erosion are active. The oldestsoils tend to occur on broad upland areas whereneither erosion nor sedimentation occurs.

About 500,000 or more years ago in the north-central United States, the Kansan glacier melted,l eaving a relatively flat land surface composed oftill and other glacial materials. During a long inter-glacial period, an old, well-developed soil withhigh clay content formed in the surface of the till,as shown in Figure 16.16a. Loess was later depos-ited on the soil developed from till, which wasburied and became a paleosol (see Figure16.16b). Erosion then dissected the till plain andremoved the loess from the sloping areas, whichreexposed the paleosol on the upper slopes. Ero-sion and dissection also exposed unweathered tillon the lower slopes in the valleys (see Figure16.16c). Today, the result is: (1) soils of moderateage formed in loess on broad, nearly level up-

ROLE OF TOPOGRAPHY IN SOIL GENESIS 267

FIGURE 16.15 Representa-tive landscape showing therelative location of someimportant soils and depth tothe seasonal high water tablei n Wilson County, NorthCarolina. (From WilsonCounty, North Carolina, soilsurvey report, USDA.)

lands; (2) young soils on upper slopes that devel-oped in the paleosol; and (3) young (less devel-oped) soils that developed from unweathered tillon the lower valley slopes. In stream valleys, soilsare very young and have developed from recentalluvium and colluvium. Within the landscapethere are four major kinds of parent material (eachof different age) and about six different topo-graphic positions, as well as some microclimaticdifferences. In such a landscape in southern lo-wa,these soil-forming factors result in formationof 11 major kinds of soils, as shown in Fig. 16.17.

Uniqueness of Soils Developed inAlluvial Parent MaterialWeathering typically is more intense near the soilsurface than in underlying layers, causing themineral material to be less weathered with in-creasing soil depth. For alluvial soils, the surfacehorizon is the least weathered because it formsfrom the most recently deposited alluvium.

Alluvium accumulates along the Nile River atthe rate of about 1 millimeter annually, resultingin a meter of alluvium every 1,000 years. It iscommon for alluvial sediments to be more thanseveral hundred meters thick. After subsoil mate-rial is buried by more recent alluvium, the subsoilreceives very little additional organic matter fromroot growth and the organic matter present con-

268

(c) Modern soil LandscapeFIGURE 16.16 Steps in the formation of a soillandscape in southern Iowa. (Adapted fromOschwald et al., 1965.)

SOIL GENESIS

FIGURE 16.17 Relationship of slope, vegetation, and parent material to soilsof the Adair-Grundy-Haig soil association in south-central Iowa. (FromOschwald et al., 1965.)

FIGURE 16.18 Earth-moving activities of humans toconstruct terraces on slopes for paddy riceproduction have resulted in great modification of thenatural soils.

tinues to mineralize. This results in a gradual de-crease in soil organic matter with increasingdepth. Alluvial soils tend to be young soils, andthe horizons are caused mainly by differences inthe properties of the layers of alluvium.

On floodplains, sand and the coarser particlesare deposited near the river and may form a levee.Further from the river, the silt and clay tend to bedeposited from slower-moving and, in somecases, very stagnant water. The result is a generalincrease in the clay content of soils with increas-ing distance from the river.

When rivers cause rapid down cutting, alluviumat the outer edges of the floodplain may becomeelevated relative to the river, so that they no longerflood. This results in the formation of terraces,and in the absence of flooding and deposition, thesoils develop normally with time.

HUMAN BEINGS AS ASOIL-FORMING FACTOROne of the most obvious and early effects of hu-man activity on soils has occurred at campsitesand villages. Organic residues have been addedto the soils from food preparation and preser-vation, together with other materials such asbones and shells. Human excrement and othermaterials have similarly contributed to the forma-tion of thick, dark-colored layers greatly enrichedwith phosphorus.

The use of land for agriculture, forestry, graz-ing, and urbanization has produced extensivechanges in soils. Many of these changes havebeen discussed, including soil erosion, drainage,salinity development, depletion and addition oforganic matter and nutrients, compaction, andflooding. Sometimes, solid waste disposal pro-duces new soils that are little more than an accu-mulation of trash. Millions of acres of land havesoils with properties that are due more to humanactivities than to soil-forming factors. In China,land has essentially been created to grow foodcrops. The enormous movement of soil and theshaping of the landscape for production of paddyrice is illustrated in Figure 16.18.

SUMMARYSoils are the products of evolution. Over time, thesoil properties are less affected by inheritance

REFERENCES 269

from the parent material, and soil properties be-come more closely related to the changes thatoccurred during soil genesis. The age of a soil isexpressed by the degree of development and notthe number of years. In one sense, soils have a lifecycle that is represented by soils expressing dif-ferent degrees of development.

Some soils form in parent material derived fromthe direct weathering of rocks. Most soils haveformed in sediments.

Soil genesis, in general, is accelerated by in-creases in precipitation and temperature.

Soil genesis is accelerated by tree rather thangrassland vegetation. Similar processes occur un-der both types of vegetation, and old soils aresimilar when developed under trees or grass.Grassland soils have much greater content of or-ganic matter within the soil than forest soils, andthe organic matter content decreases less rapidlywith increasing soil depth.

Topography affects soil genesis mainly by dis-persal of precipitation, erosion, and accompany-ing deposition of eroded material, and soildrainage. Youthful soils tend to occur where soilerosion and soil deposition are active.

Human beings have affected many soils at habi-tation sites and through agriculture, waste dis-posal, and earth moving.

REFERENCESAandahl, A. R. 1948. "The Characterization of Slope

Positions and Their Influence on the Total NitrogenContent of a Few Virgin Soils of Western Iowa." SoilSci. Soc. Am. Proc. 13:449-454.

Bidwell, 0. W. and F. D. Hole. 1965. "Man as a Factor inSoil Formation." Soil Sci. 99:65-72.

Blizi, A. F. and E. J. Ciolkosz. 1977. "Time as a Factor inthe Genesis of Four Soils Developed in Recent Allu-vium in Pennsylvania." Soil Sci. Soc. Am. J. 41:122-127.

Boul, S. W., F. D. Hole, and R. J. McCracken. 1980. SoilGenesis and Classification, 2nd ed., Iowa State Press,Ames.

Ericson, D. B., M. Ewing, and G. Wollin. 1964. "The

270

Pleistocence Epoch in Deep-Sea Sediments." Sci-

ence. 146:723-732.Fanning, D. S. and M.C.B. Fanning. 1989. Soil Morphol-

ogy, Genesis, and Classification. Wiley, New York.

Franzmeier, D. P. and E. P. Whiteside. 1963. A Chro-nosequence of Podzols in Northern Michigan: I, II,and III. Mich. Agr. Exp. Sta. Quart. Bull. 46:2-57. EastLansing.

Jenny, Hans. 1980. The Soil Resource. Springer-Verlag,New York.

Muckenhirn, R. J., E. P. Whiteside, E. H. Templin, R. F.Chandler, and L. T. Alexander. 1949. "Soil Classifi-cation and the Genetic Factors of Soil Formation."Soil Sci. 67:93-105.

Nielsen, G. A. and F. D. Hole. 1963. "A Study of theNatural Processes of Incorporation of Organic Matterinto Soil in the University of Wisconsin Arboretum."Wis. Acad. Sci. 52:213-227.

Oschwald, W. R. et al. 1965. "Principal Soils of Iowa."

SOIL GENESIS

Agronomy Special Report 42. Iowa State University,Ames.

Ruhe, R. V., R. B. Daniels, and J. G. Cady. 1967. "Land-scape Evolution and Soil Formation in SouthwesternIowa." USDA Tech. Bull. 1349, Washington, D.C.

Sanchez, P. A., M. P. Gichuru, and L. B. Katz. 1982."Organic Matter in Major Soils of the Tropicaland Temperate Regions." 12th Int. Congress SoilSci. Symposia Papers Vol 1. pp. 101-104, NewDelhi.

Schreiner, O. and B. E. Brown. 1938. "Soil Nitrogen."Soils and Men, USDA Yearbook of Agriculture, Wash-ington, D.C.

Twenhofel, W. H. 1939. "The Cost of Soil in Rock andTime." Am. J. Sci. 237:771-780.

Vance, G. F., D. L. Mokma, and S. A. Boyd. 1986. "Phe-nolic Compounds in Soils of Hydrosequences andDevelopmental Sequences of Spodosols." Soil Sci.Soc. Am. J. 50:992-996.

SOIL TAXONOMY

Classification schemes of natural objects seek toorganize knowledge, so that the properties andrelationships of these objects may be remem-bered and understood for some specific purpose.Many soil classification schemes have been de-veloped, and most have been on a national basis.In 1975, the United States Department of Agricul-ture, with the help of soil scientists from manynations, published Soil Taxonomy. Many of itsterms and ideas also appear in the legend of theworld soil map published by the Food and Agri-culture Organization (FAO) of the United Nations.Knowledge of either system makes use of theother system possible with minimal adjustments.

DIAGNOSTIC SURFACE HORIZONSDiagnostic horizons have unique properties andare used with other properties to classify soils.Diagnostic surface horizons (epipedons) form atthe soil surface and diagnostic subsurface hori-zons form below the soil surface. There are seven

CHAPTER 17

271

diagnostic epipedons that differ in thickness,color, structure, content of organic matter andphosphorus, mineralogy, and the degree ofleaching.

Mollic HorizonThe word mollic is derived from mollify, whichmeans to soften. The mollic horizon is a surfacehorizon that is soft rather than hard and massivewhen dry. Mollic epipedon formation is favoredwhere numerous grass roots permeate the soiland a moderate to strong grade of structure iscreated. Except for special cases, mollic horizonsare dark-colored, contain at least 1 percent or-ganic matter, and are at least 18 centimeters thick.They are only minimally or moderately weatheredand leached. The cation exchange capacity is 50percent or more saturated with calcium, magne-sium, potassium, and sodium when the cationexchange capacity is determined at pH 7. Themajor grassland soils of the world have mollicepipedons (see Figure 17.1).

272

ruuunw i i. i aon wun a mouic epipeaon.

Umbric and Ochric HorizonsOther diagnostic epipedons can be related to themollic horizon, as shown in Figure 17.2. The um-bric epipedon is similar to the mollic in overallappearance of thickness and color; however, it ismore leached and has a saturation with basiccations calcium, magnesium, potassium, and so-dium that is less than 50 percent when the cationexchange capacity is determined at pH 7. Umbrichorizons form in some very humid forests, as inthe Coastal Range of the northwestern UnitedStates. In most forests, the epipedon is thinnerthan is that of the umbric, is lower in organicmatter, and is lighter in color. This epipedon iscalled ochric.

Histic HorizonWhere soil development occurs under conditionsof extreme wetness, as in swamps or lakes, theepipedon is organic in nature and is, typically, a

SOIL TAXONOMY

histic epipedon. Histic epipedons are 0 horizons.A histic epipedon must be water saturated for atleast 30 consecutive days during the year, unlessthe soil has been drained. The degree of decom-position of the organic matter in histic horizons isindicated with the following symbols: i for fibric, efor hemic, and a for sapric. Fibric (i) is the leastdecomposed and contains a large amount of rec-ognizable plant fibers; sapric (a) is the most de-composed; and hemic (e) is of intermediate de-composition. The relationship of the histic,horizon to the mollic horizon is shown in Figure17.2.

Melanic HorizonMelanic epipedons are thick, black colored, andcontain high concentrations of organic matter.The moist Munsell value and chroma are 2 or less,and the organic carbon content is 6 percent ormore but less than 25 percent. The organic matter

is thought to result from the supply of largeamount of root residues from a graminaceousvegetation in contrast to organic matter that re-sults from a forest vegetation.

Melanic horizons also have andic soil proper-ties. Andic soil properties result mainly from theweathering of volcanic materials, which contain asignificant amount of volcanic glass (noncrystall-i ne ash, pumice, lava, etc). The dominant clayminerals include allophane and some oxides ofiron and aluminum. Recent work with electronmicroscopy, and other techniques, has shownthat these minerals are characterized by an or-derly arrangement of atoms. But the orderly ar-rangement is short range and does not in a regularmanner form a 3-dimensional pattern like thatfound in crystalline minerals such as vermiculiteor kaolinite. The particles are very small with anenormous surface area, which results in highanion exchange capacity and a large capacity tocomplex organic matter. These minerals are con-sidered to be amorphous or short-range minerals.

Melanic horizons are black and have high or-ganic matter content similar to some histic hori-zons. Even though the organic matter content ishigh, the organic matter in melanic horizons ismostly associated or complexed with minerals,which results in properties dominated by the min-eral fraction rather than the organic fraction. Thehorizon is also characterized by low bulk densityand high anion exchange capacity.

Anthropic and Plaggen HorizonsThe anthropic and plaggen horizons are formedby human activity. The anthropic epipedon re-sembles the mollic horizon in color, structure,and organic matter content; however, the phos-phorus content is 250 or more ppm of P 20 5 equiv-alent soluble in 1 percent citric acid solution. Theanthropic horizon occurs where human activityresulted in the disposal of bones and other refusenear places of residence, or in agriculture at sitesof long-continued use of soil for irrigated.cropping. Measurement of the phosphorus en-

DIAGNOSTIC SUBSURFACE HORIZONS 273

richment of soil is a standard technique used byanthropologists to determine where ancientcampsites, villages, and roads were located. Manyanthropic horizons have been found in Europe,and some have been found at old presettlementcampsites in the United States. Some anthropichorizons in the Tombigee River valley in northernMississippi are more than a meter thick, contain0.6 to 3 percent organic carbon, and 1 to 5 percentcharcoal fragments. In the deeper parts of theanthropic horizons the carbon had been dated asbeing 7,000 years old. The citrate soluble P 20 5

content of the anthropic horizons exceeded 250ppm compared with 80 and 20 ppm in the surfaceand subsoils, respectively, of adjacent soils.

The plaggen epipedon is found in Europewhere long-continued manuring has produced asurface layer 50 centimeters or more thick. Duringthe Middle Ages, farmers in northwestern Europecollected sods from forests and heath landswhere soils were very sandy. These clumps of sodcontained sand and were used for bedding inbarns. The manure from the barn was applied tothe land, which resulted in the slow accumulationof a thick, sandy epipedon enriched with organicmatter. In some places, a 1-meter thick plaggenepipedon was created in 1,000 years. Plaggen hor-izons typically contain artifacts, such as bits ofbrick or pottery, and today these soils are valuedfor agricultural use.

The derivation of the the epipedon names andtheir major features are given in Table 17.1.

DIAGNOSTIC SUBSURFACE HORIZONSGenerally, diagnostic subsurface horizons are Bhorizons. They may form immediately below alayer of leaf litter, as in the case of the E horizon.More than 15 subsurface diagnostic horizons havebeen recognized.

Cambic HorizonThe cambic horizon can form quickly, relativelyspeaking, because changes in the color and struc-

274

ture, and some leaching will convert the subsoilparent material into a cambic horizon. Cambichorizons are not illuvial horizons and, generally,they are not extremely weathered. A cambic hori-zon is a Bw horizon. A parent material with atexture of fine sand (or even finer) is required forthe development of cambic horizons.

Agrillic and Natric HorizonsThe gradual illuvial accumulation of clay in acambic horizon converts a cambic horizon (Bwhorizon) into a Bt horizon. Typically, clay skinsoccur on the ped surfaces of Bt horizons. Whenthe ratio of clay in the Bt horizon, compared withthat of the overlying eluvial horizon, is 1.2 ormore, the Bt horizon becomes an argillic horizon,in which the overlying eluvial horizon contains 15to 40 percent clay. If the eluvial horizon has lessthan 15 percent clay, there must be at least 3percent more clay in the illuvial horizon to qualifyas an argillic horizon. If the eluvial horizon has

SOIL TAXONOMY

more than 40 percent clay, the illuvial horizonmust have at least 8 percent more clay to qualifyas an argillic horizon. Argillic horizons usuallyrequire several thousands of years to form, asshown in Figure 2.7.

A natric horizon is a special kind of argillichorizon. These horizons have typically been af-fected by soluble salts. In addition to the proper-ties of the argillic horizon, natric horizons com-monly have a columnar structure and a sodiumadsorption ratio (SAR) of 13 or more (or 15 per-cent or more exchangeable sodium saturation).The natric horizon is a Btn horizon; the n indicatesthe accumulation of exchangeable sodium. Anexample of a natric horizon is shown in Figure17.3.

Kandic HorizonIn intensively weathered soils, the genesis of sub-soil layers that have more clay than the overlying Ahorizon is obscure because of the very long time

FIGURE 17.3 Soil with a natric horizon. The upperpart of the natric horizon is at 20 centimeters and ischaracterized by whitish-topped columnar peds.

of soil formation. Subsoils with much higher claycontent than overlying horizons may show no evi-dence of clay skins. Therefore, many tropical soilshave a subsoil horizon similar to the argillic hori-zon, except that the clay minerals have very lowcation exchange capacity, which is indicative oflow activity clay. The clay minerals are predomi-nately kaolinite and oxidic clays. The cation ex-change capacity of the clay, determined at pH 7, is16 cmol/kg (16 meq/100 g) or less or the ECEC is12 cmol/kg or less. These clay-rich horizons arecalled kandic.

Spodic HorizonI n sand-textured parent material there is little clayfor illuviation and rapid water permeability. Underconditions of high precipitation, iron and alumi-num oxides (sesquioxides) are released in weath-ering. These oxides complex or chelate with or-ganic matter and are transported to the subsoil,resulting in the formation of an illuvial horizon.An illuvial subsoil horizon that is greatly enrichedwith these amorphous compounds is a spodichorizon. Spodic horizons can be Bs, Bh, or Bhshorizons, depending on the degree of accumu-

DIAGNOSTIC SUBSURFACE HORIZONS 275

l ation of sesquioxides, humus, or humus plussesquioxides, respectively. Evolution of the spo-dic horizon is summarized in Table 16.1.

Albic HorizonThe loss by eluviation of sesquioxides and clay,during the formation of spodic and argillic hori-zons, tends to leave behind a light-colored overly-ing eluvial horizon called the albic horizon. Albicis derived from the word white. The horizon iseluviated and is labeled an E horizon. The color ofthe albic horizon is due to the color of the primarysand and silt particles rather than to the particlecoatings. Albic horizons are commonly underlainby spodic, argillic, or kandic horizons.

Oxic HorizonThe oxic horizon (Bo horizon) is a subsurfacehorizon at least 30 centimeters thick that is in anadvanced stage of weathering. It is not dependenton a difference in the clay content of subsoil ver-sus the topsoil horizons. Oxic is derived from theword oxide. Oxic horizons consist of a mixture ofiron and/or aluminum oxides with variableamounts of kaolinite and highly insoluble ac-cessory minerals such as quartz sand. There arefew if any weatherable minerals to weather andsupply plant nutrients, and the clay fraction has asmall cation exchange capacity-16 cmol/kg(meq/ 100 g) or less as determined at pH 7 or 12 orless cmol/kg at the soil's natural pH. Soils withoxic horizons have essentially reached the endpoint of weathering.

Calcic, Gypsic, and Salic HorizonsThe calcic horizon is a horizon of calcium carbo-nate or calcium and magnesium carbonate accu-mulation. Calcic horizons develop in soils inwhich there is limited leaching and the carbo-nates are translocated downward. However, thecarbonates are deposited within the soil profilebecause there is insufficient water to leach the

276

carbonates to the water table. The symbol k indi-cates an accumulation of carbonates, as in Bwk orCk. Gypsic horizons have an accumulation of sec-ondary sulfates, and that is indicated with thesymbol y. Cemented calcic and gypsic horizonsare petrocalcic and petrogypsic horizons, respec-tively. Salic horizons contain a secondary enrich-ment of salts more soluble than gypsic and areindicated with the symbol z.

Subordinate DistinctionsWithin HorizonsOther symbols and meanings used for subordi-nate distinctions within soil horizons or layersi nclude the following:

b

Buried genetic horizonc

Concretions or hard nonconcretionarynodules

f

Permafrostg

Strong gleying (indicating iron reduction)m Cementation or indurationq

Accumulation of silicar

Weathered or soft rockv

Plinthite (iron-rich reddish material thatdries irreversibly)

x

Fragipan character (high density andbrittleness)

SOIL MOISTURE REGIMESThe soil moisture regime is an expression of thechanges in soil water over time. It refers to thepresence or absence of either groundwater or ofwater held at a potential of less or more than-1,500 kPa (-15 bars) by periods of the year. Threesoil water conditions are recognized: soil that iswater saturated (wet soil), soil between saturationand the permanent wilting point (moist soil), andsoil drier than the wilting point (dry soil).

The water content of the upper soil horizonchanges frequently throughout the year becauseof repeated rainfall and subsequent drying. As a

SOIL TAXONOMY

result, the soil moisture regime is based on amoisture control section that is represented by thedifference between the depth of soil at the perma-nent wilting point that is wetted by 2.5 centimetersof water as compared with the depth wetted by 7.5centimeters of water. The moisture control sec-tion is generally within the 10-centimeters and90-centimeters depths, being thicker in sandysoils and thinner in clay soils. The amount ofwater in the moisture control section provides ani ndication of the sufficiency or deficiency of waterfor plant growth and the water status of the soil forother uses.

Aquic Moisture RegimeSoils with aquic moisture regime are wet. Atsometime during the year, the lower soil horizonsare saturated. In many instances, the entire soil issaturated part of the year. Water saturation occursbecause of the location of the soil in a landscapeposition that is naturally poorly drained or wherean impermeable layer causes saturation. Typi-cally, the top of the saturated soil fluctuates dur-ing the year, being deeper during the driest part ofthe year. Artificial drainage is needed to growplants that require an aerated root zone. Soils withan aquic moisture regime are unsuitable for build-i ng sites unless these soils are dewatered with adrainage system.

Subsoil colors in mineral soils with aquic re-gime are frequently gray, which indicates reduc-i ng conditions and low oxygen supply. Alternatingreducing and oxidizing conditions are associatedwith a fluctuating water table that produces mot-tled colors. The high water content tends to beassociated with a high content of organic matterand, when properly drained, many aquic soilsbecome very productive agricultural soils.

Udic and Perudic Moisture RegimesUdic means humid. Soils with a udic moistureregime are not dry in any part of the moisturecontrol section as long as 90 cumulative days in

most years. Udic moisture regimes are commonin soils of humid climates that have well distrib-uted precipitation. The amount of summer rain-fall, plus stored water, is approximately equal toor exceeds the potential evapotranspiration (PE).In other words, if droughts occur, they are shortand infrequent. Forests and tall grass prairies aretypical native vegetation.

In most years there is some surplus water thatpercolates downward and produces leaching. Nu-trient ions in the water are leached, removing asignificant amount of plant nutrients from the soil.Leaching losses of calcium and magnesium maybe greater than the amount required to producean average crop. Udic soils are common from theEast Coast of the United States to the westernboundaries of Minnesota, Iowa, Missouri, Ar-kansas, and Louisiana. Old soils with udic mois-ture regime tend to be naturally acid and infertileas a result of leaching. An example of soil waterchanges over time in a soil with a udic soil mois-ture regime is shown in Figure 17.4 and the udicmoisture regime diagram for Memphis, Tennes-see is given in Figure 5.15.

If the regime is characterized by very wet condi-tions, the regime is perudic. In the perudic re-gime, precipitation exceeds evapotranspirationevery month in most years, water moves throughthe soil every month unless the soil is frozen, andthere are only occasional periods when somestored soil water is used.

Ustic Moisture RegimeThe concept of the ustic moisture regime relatesto soils with limited available water for plants, butwater is present at a time when conditions aresuitable for plant growth. Soils with ustic moistureregime are dry in some part of the moisture con-trol section for as long as 90 cumulative days inmost years. There is moisture available, storedsoil water plus precipitation, for significant plantgrowth at some time during the year and a deficitpart of the year, as shown in Figure 17.4. Mostsoils on the central and northern Great Plains

SOIL MOISTURE REGIMES 277

have an ustic moisture regime. Native vegetationwas mainly mid, short, and bunch grasses. Sur-plus water is rare, leaching is limited, and soilstend to be fertile.

In areas where soils have an ustic moistureregime, there is enough rainfall in winter, earlyspring, and summer to produce a significantamount of soil water recharge or storage. Thiswater, plus the precipitation during the spring andearly summer, is sufficient for the production of awheat crop that ripens before the soil becomesdry in mid- and late summer. Corn does not ripenuntil fall and generally cannot be produced unlessirrigated. Droughts are common. Sorghum is apopular crop because it can interrupt growthwhen water is lacking and grow again if morerainfall occurs. Cattle grazing is an importantl and use.

Soils in monsoon climates have ustic moistureregimes because there is enough water duringthe rainy season for significant plant growth;however, there is a long dry season with a deficit.

Aridic Moisture RegimeThe driest soils have an aridic moisture regime.The subsoil, or moisture control section, is dry(permanent wilting point or drier) in all parts formore than half the growing season, and is notmoist (wetter than the permanent wilting point) insome parts for as long as 90 consecutive daysduring the growing season in most years. Mostsoils with an aridic moisture regime are found inarid or desert regions with widely spaced shrubsand cacti as the native vegetation. A crop cannotbe matured without irrigation. Some climatic dataand the soilwater balance typical of an aridicmoisture regime are given in Figure 17.4. Theprecipitation is lower than the potential evapo-transpiration (PE) during most months of the year.There is very little soil water storage or recharge,which means there is little water retained for plantgrowth. The result is a frequent lack of water forplants and a large water deficit. Many of the nativeplants have an unusual capacity to endure very

278

low water potentials in the soil and within theirtissues. Grazing is the dominant land use exceptwhere irrigation permits crop production.

Xeric Moisture RegimeSoils in areas where winters are cool and moistand summers are hot and dry have a xeric soilmoisture regime. This kind of climate is known asa Mediterranean climate. The precipitation occursin the cool months when the evapotranspiration islow and surplus water may readily accumulate.This frequently results in considerable soil wet-

SOIL TAXONOMY

ness during the winter. Leaching and weatheringmay occur during the winter even though sum-mers are long, very dry, and hot. With limitedwater storage, and little if any summer rainfall, alarge water deficit occurs in summer. Xeric soilsare typical of the valleys of California where alarge variety of crops, including vines, fruits,nuts, vegetables, and seeds are produced withirrigation. Xeric soils of the Palouse in Wash-ington and Oregon are used for winter wheat.Here, there is a good match between the timewhen the wheat needs water and when soil wateris available.

TABLE 17.2 Definitions and Features of Soil Temperature Regimes

° Frigid soils have warmer summers than cryic soils; both have same MAST. Frigid soilshave more than 5° C temperature difference between mean winter and mean summertemperatures at a depth of 50 centimeters (or lithic or paralithic contact, if shallower).

SOIL TEMPERATURE REGIMESSoil temperature regimes are defined according

to the mean annual soil temperature (MAST) inthe root zone (arbitrarily set at 5 to 100 cm). Re-gime definitions and some characteristics aregiven in Table 17.2. Both the soil moisture re-gimes and the temperature regimes are used toclassify soils at various categories in Soil Tax-onomy.

CATEGORIES OF SOIL TAXONOMY

CATEGORIES OF SOIL TAXONOMYThe categories of Soil Taxonomy are order, su-border, great group, subgroup, family, and series.The order is the highest category and the series isthe lowest category.

Soil OrderThere are 11 orders, each ending in -sol; L. solum,meaning soil. The orders, together with their deri-

279

280 SOIL TAXONOMY

TABLE 17.3 Formative Syllables, Derivations, and Meanings of Soil Orders

The Entisol order includes all soils that do notfit into one of the other 10 orders and is composedof soils with great heterogeneity. Extensive treat-ment of the soil orders can be found in the nextchapter.

CATEGORIES OF SOIL TAXONOMY 28 1

282

(Table 17.4, continued)

Suborder and Great GroupOrders are divided into suborders. The subordername has two syllables. The first syllable con-notes something of the diagnostic properties ofthe soil, and the second syllable is the formativeelement or syllable for the order. Suborder names

SOIL TAXONOMY

and their formative elements, meaning, and con-notation are given in Table 17.4.

For the suborder Aqualfs, aqu is derived fromL. aqua, meaning water, and alf from Alfisol.Thus, Aqualfs are Alfisols that are wet; they havean aquic soil moisture regime. Boralfs are Alfisols

of the cool regions. Udalfs have an udic moistureregime, Ustalfs have an ustic moisture regime,and Xeralfs have a xeric moisture regime. Notethat many of the formative elements of the su-border names connote soil moisture or tempera-ture regimes. The suborder is the most commoncategory used in the discussion of soil geographyand in the legends of the soil maps in the nextchapter.

The legends for the soil maps also use somegreat group names. In the legend of Figure 18.1(soil map of the United States), Ala refers to areaswhere soils are predominatly Aqualfs with signif-icant amounts of other soils in decreasing order ofimportance. Udalfs are the second most extensivesoils, followed by Haplaquepts, which is a greatgroup name. Hapl means minimal horizon andrefers to Aquepts that are weakly or minimallydeveloped. Areas of A2a are dominated by Bor-alfs, with Udipsamments second in importance.Udipsamments are Psamments (sand-texturedEntisols) with a udic moisture regime. Thus,some of the formative elements given in Table17.4 for suborders are also used to form greatgroup names. Prefixes and their connotations forgreat group names are given in Appendix 3.

Subgroup, Family, and SeriesThe taxonomic categories that follow great groupare subgroup, family, and series. The subgroupdenotes whether or not the soil is typical (typic)for the great group. Soils that are not typical of thegreat group are indicated as being intergrades.The family category provides groupings of soilswith ranges in texture, mineralogy, temperature,and thickness. The primary use of the series nameis to relate soils delineated on soil maps to theintrepretations for land use.

Series names are taken from the name of a townor landscape feature near the place where the soilseries was first recognized. Many of the names arefamilar, such as Walla Walla in the state of Wash-i ngton, Molokai in Hawaii, and Santa Fe in NewMexico. The Antigo soil is the state soil in Wis-

AN EXAMPLE OF CLASSIFICATION: THE ABAC SOILS 283

consin. More than 16, 000 series have been recog-nized in the United States.

AN EXAMPLE OF CLASSIFICATION:The ABAC SOILSSoils of the Abac series are Typic Ustorthents,meaning the soils are Entisols (ent), that occur onrecent slopes, subject to erosion (orth), have anustic moisture regime (ust), and are typical for theUstorthent great group. The family is loamy,mixed, calcareous, frigid, and shallow. Thismeans, for example, that the texture is finer thanloamy sand (loamy), no one mineral dominatesthe mineralogy and the soil is calcareous (mixed,calcareous), the mean annual soil temperature isbetween 0 and 8° C (frigid), and there is rockwithin 50 centimeter of the soil surface (shallow).The Abac soil is a loamy, mixed, calcareous,frigid, shallow typic Ustorthent.

From this information it can be concluded thatthe soil is not likely to be cultivated and used forcropping. Such soils can produce a moderateamount of forage for grazing in the spring andearly summer. The soils occur on steep slopeswhere it is shallow to rock in Montana. Such soilscan be desirable locations for vacations and rec-reational activities, as shown in Figure 17.5.

FIGURE 17.5 Landscape in the mountains wheremany of the soils are-Ustorthents. The soils providesome pasture for grazing animals; however, the idealsummer climate makes such locations desirable forcamping, horseback riding, and hiking.

284

THE PEDONSoil bodies are large, and hence there is a need fora smaller soil unit that can be the object of scien-tific study. The pedon is this unit. A soil pedon isthe smallest volume that can be called a soil, andit is roughly polygonal in shape. The lower bound-ary is the somewhat vague boundary between soiland nonsoil at the approximate depth of root pen-etration. Lateral dimensions are large enough torepresent any horizon. The area of a pedon rangesfrom I to 10 square meters. The pedon is to thesoil body what an oak tree is to an oak forest. Asoil body is composed of many pedons, which iswhy a soil body is called a polypedon (see Figure17.6).

FIGURE 17.6 Illustration of the relationshipbetween the pedon and polypedon.

SOIL TAXONOMY

SUMMARYDiagnostic surface and subsurface horizons, to-gether with other diagnostic features, are used toclassify soils in Soil Taxonomy.

Soil moisture regimes express the changes insoil water with time. Aquic soil moisture regimesare characterized by water-saturation. Aridic,ustic, and udic are three regimes with an increas-ing amount of water available for plant growthduring the summer. Soils with perudic regime arecontinuously wet. Xeric soil moisture regime ischaracterized by cool, rainy winters and hot, drysummers.

Soil temperature regimes reflect the mean aver-age soil temperature (MAST) in the root zone.

There are more than 16,000 soils (series) in theUnited States and they are classified into orders,suborders, great groups, subgroups, families, andseries based on the presence of various diagnos-tic horizons and other criteria.

The pedon is the smallest volume that can becalled a soil.

REFERENCESBoul, S. W., F. D. Hole, and R. J. McCracken. 1980. Soil

Genesis and Classification. 2nd ed. Iowa State Uni-versity Press, Ames.

Fanning, D. S., and M. C. B. Fanning. 1989. Soil Mor-

phology, Genesis, and Classification. Wiley, NewYork.

Foth, H. D. and J. W. Schafer. 1980. Soil Geography andLand Use. Wiley, New York.

Soil Survey Staff. 1975. Soil Taxonomy. USDA Agr.Handbook 436, Washington, D.C.

Soil Survey Staff. 1987. "Keys to Soil Taxonomy." SoilManagement Support Services Tech. Monograph 6.USDA Washington, D.C.

Soil Survey Staff, 1989. Amendments to Soil Taxonomy.USDA, Washington, D.C.

Wilding, L. P., N. E. Smeck, and G. F. Hall. 1983. Pedo-

genesis and Soil Taxonomy: I Concepts and Interac-

tions. Elsevier, New York

The extent and abundance of the orders for theUnited States and the world are shown in Table18.1. Aridisols are the most abundant on a world-wide basis, occupying 19.2 percent of the land,whereas Mollisols are the most abundant soils inUnited States with 24.6 percent. Inceptisols andAlfisols are ranked 2 and 3, respectively, in boththe United States and the world. Histosols andOxisols are the least abundant soils in UnitedStates, and Histosols and Andisols are the leastabundant soils worldwide.

The geographic distribution of soils in theUnited States is given in Figure 18.1 and for theworld in Figure 18.2. The consideration of thesoils is primarily at the suborder level.

ALFISOLSThe central concept of Alfisols is soils with anochric epipedon and an argillic subsurface hori-zon, only moderately weathered and leached, andenough available water for the production ofcrops at least three months during the year. Thereis sufficient exchangeable calcium, magnesium,potassium, and sodium to saturate 35 percent ormore of the CEC (at pH 8.3) at a depth of 1.25

CHAPTER 18

SOIL GEOGRAPHY ANDLAND USE

285

meters below the upper boundary of the argillichorizon or 1.8 meters below the soil surface,whichever is greater.

In humid forested regions, many of the Alfisolshave an albic (E) horizon between the ochric andargillic horizons. Alfisols that develop in regionsor areas adjacent to grasslands lack E horizonsand have properties very similar to Mollisols. Thelimited accumulation of organic matter and ab-sence of a mollic horizon may result in a grass-land soil being an Alfisol rather than a Mollisol.This is the situation in many areas in which thedominant soils have an ustic moisture regime. Forthese reasons, many Alfisols are quite similar toMollisols in native fertility and ability to producecrops. Large areas of each of the suborders ofAlfisols occur in United States.

AqualfsAqualfs have an aquic soil moisture regime andare shown in Figure 18.1 as Ala. In the area westof Lake Erie in Ohio and Michigan, the Aqualfshave a naturally high water table. The Aqualfsformed mainly on glacial lake beds from fine-textured parent material. They occur on nearly 0percent slopes and have high natural fertility.

286

When artificially drained, the soils are very pro-ductive for corn, soybeans, wheat, and sugarbeets. The soils are poorly suited to septic filter-field sewage disposal systems and other useswhere natural soil wetness is undesirable.

Another area of Aqualfs (Ala in Figure 18.1)occurs in southern Illinois and west into Missouri.These soils are wet because of thick argillic hori-zons (claypans). The soils developed on nearlylevel upland areas on land surfaces that wereformed by some of the earlier glaciations. Anotherarea of claypan Aqualfs occurs in southwesternLouisiana, where wetland rice is a major crop.

BoralfsBoralfs are the northern or cool Alfisols with cryicor frigid temperature regime. The mean annualsoil temperature is warmer than 0° C (32° F) andcolder than 8° C (47° F). Forestry rather than agri-culture tends to be the major land use. The soilsare too cold for winter wheat, corn, and soybeans.

Two kinds of Boralf areas are shown in Figure18.1 -A2a and A2s. The A2s areas occur on steepslopes in the Rocky Mountains. The soils of theBlack Hills in western South Dakota are alsomainly Boralfs. These areas are important for rec-reation and forestry uses, they provide runoff wa-

SOIL GEOGRAPHY AND LAND USE

ter for collection in reservoirs, and they are usedfor some grazing of cattle. The Boralfs, area Ala inWisconsin and Minnesota, developed from recentglacial parent materials. Much of the land is foundin forests. Areas in agriculture are devoted todairying and specialty crops, such as potatoes. Avery large area of Boralfs, similar to that in Minne-sota and Wisconsin, occurs in the Soviet Union(area Ala of Figure 18.2). Moscow is located inthis area; important crops are rye, oats, flax, sugarbeets, and forage crops. Dairy cattle and poultry,together with pigs, are important livestock prod-ucts. Boralfs represent some of the most northerlysoils used for agriculture.

UdalfsUdalfs are warmer than Boralfs and have a udicsoil moisture regime. The major area of Udalfs inUnited States occurs east of the Prairie region inthe north-central states. These areas (A3a in Fig-ure 18.1) have trees for native vegetation becausethey are more humid than the major grasslandslocated to the west. The Udalfs have developedmainly from mid- to late-Pleistocene loess and till.In Indiana, Ohio, and southern Michigan, the soilsare nearly as productive as are the grassland soils(Mollisols) in Illinois and Iowa. These soils are

Legend for general soil map of the United States.

ALFISOLSAqualfsAla-Aqualfs with Udalfs, Haplaquepts, Udolls; gently sloping.BoralfsAla-Boralfs with Udipsamments and Histosols; gently and

moderately sloping.A2S-Cryoboralfs with Borolls, Cryochrepts, Cryorthods, and

rock outcrops; steep.UdalfsA3a-Udalfs with Aqualfs, Aquolls, Rendolls, Udolls, and

Udults; gently or moderately sloping.UstalfsA4a-Ustalfs with Ustochrepts, Ustolls, Usterts,

Ustipsamments, and Ustorthents; gently or moderatelysloping.

XeralfsA5S1-Xeralfs with Xerolls, Xerothents, and Xererts;

moderately sloping to steep.A5S2-Ultic and lithic subgroups of Haploxeralfs with

Andisols, Xerults, Xerolls, and Xerochrepts; steep.

ANDISOLSCryands11 a-Cryands with Cryaquepts, Histosols, and rock land; gently

and moderately sloping.

1 1 S1-Cryands with Cryochrepts, Cryumbrepts, andCryorthods; steep.

Udands1 1 52-Udands and Ustands, Ustolls, and Tropofolists;

moderately sloping to steep.

ARIDISOLSArgidsD1a-Argids with Orthids, Orthents, Psamments, and Ustolls;

gently and moderately slopingD1S-Argids with Orthids, gently sloping; and Torriorthents,

gently sloping to steep.OrthidsD2a-Orthids with Argids, Orthents, and Xerolls; gently or

moderately sloping.D2S-Orthids, gently sloping to steep, with Argids, gently

sloping; lithic subgroups of Torriorthents and Xerorthents,both steep.

ENTISOLSAquentsEla-Aquents with Quartzipsamments, Aquepts, Aquolls, and

Aquods; gently sloping.OrthentsE2a-Torriorthents, steep, with borollic subgroups of

Aridisols; Usterts and aridic and vertic subgroups of Borolls;gently or moderately sloping.

288

E2b-Torriorthents with Torrerts; gently or moderatelysloping.

E2c-Xerothents with Xeralfs, Orthids, and Argids; gentlysloping.

E2S1-Torriorthents; steep, and Argids, Torrifluvents, Ustolls,and Borolls; gently sloping.

E2S2-Xerorthents with Xeralfs and Xerolls; sleep.E2S3-Cryorthents with Cryopsamments and Cryands; gently

sloping to steep.PasmmentsE3a-Quartzipsamments with Aquults and Udults; gently or

moderately sloping.E3b-Udipsamments with Aquolls and Udalfs; gently or

moderately sloping.E3c-Ustipsamments with Ustalfs and Aquolls; gently or

moderately sloping.HISTOSOLS

HistosolsH1a-Hemists with Psammaquents and Udipsamments; gently

sloping.H2a-Hemists and Saprists with Fluvaquents and

Haplaquepts; gently sloping.H3a-Fibrists, Hemists, and Saprists with Psammaquents;

gently sloping.I NCEPTISOLS

Aquepts12a-Haplaquepts with Aqualfs, Aquolls, Udalfs, and

Fluvaquents; gently sloping.1 2P-Cryaquepts with cryic great groups of Orthents,

Histosols, and Ochrepts; gently sloping to steep.Ochrepts1 3a-Cryochrepts with cryic great groups of Aquepts,

Histosols, and Orthods; gently or moderately sloping.1 3b-Eutrochrepts with Uderts; gently sloping.1 3c-Fragiochrepts with Fragiaquepts, gently or moderately

sloping; and Dystrochrepts, steep.13d-Dystrochrepts with Udipsamments and Haplorthods;

gently sloping.13S-Dystrochrepts, steep, with Udalfs and Udults; gently or

moderately sloping.

Umbrepts14a-Haplumbrepts with Aquepts and Orthods; gently or

moderately sloping.1 4S-Haplumbrepts and Orthods; steep, with Xerolls and

Andisols; gently sloping.MOLLISOLS

AquollsMia-Aquolls with Udalfs, Fluvents, Udipsamments,

Ustipsamments, Aquepts, Eutrochrepts, and Borolls; gentlysloping.

BorollsM2a-Udic subgroups of Borolls with Aquolls and

Ustorthents; gently sloping.M2b-Typic subgroups of Borolls with Ustipsamments,

Ustorthents, and Boralfs; gently sloping.M2c-Aridic subgroups of Borolls with Borollic subgroups of

Argids and Orthids, and Torriorthents; gently sloping.M2S-Borolls with Boralfs, Argids, Torriorthents, and Ustolls;

moderately sloping or steep.UdollsM3a-Udolls, with Aquolls, Udalfs, Aqualfs, Fluvents,


Psamments, Ustorthents, Aquepts, and Albolls; gently ormoderately sloping.

UstollsM4a-Udic subgroups of Ustolls with Orthents, Ustochrepts,

Usterts, Aquents, Fluvents, and Udolls; gently or moderatelysloping.

M4b-Typic subgroups of Ustolls with Ustalfs, Ustipsamments,Ustorthents, Ustochrepts, Aquolls, and Usterts; gently ormoderately sloping.

M4c-Aridic subgroups of Ustolls with Ustalfs, Orthids,Ustipsamments, Ustorthents, Ustochrepts, Torriorthents,Borolls, Ustolls, and Usterts, gently or moderately sloping.

M4S-Ustolls with Argids and Torriorthents; moderatelysloping or steep.

XerollsM5a-Xerolls with Argids, Orthids, Fluvents, Cryoboralfs,

Cryoborolls, and Xerorthents; gently or moderately sloping.M5S-Xerolls with Cryoboralfs, Xeralfs, Xerothents, and

Xererts; moderately sloping or steep.SPODOSOLS

AquodsS1a-Aquods with Psammaquents, Aquolls, Humods, and

Aquults; gently slopingOrthodsS2a-Orthods with Boralfs, Aquents, Orthents, Psamments,

Histosols, Aquepts, Fragiochrepts, and Dystrochrepts; gentlyor moderately sloping.

S2S1-Orthods with Histosols, Aquents, and Aquepts;moderately sloping or steep.

S2S2-Cryorthods with Histosols; moderately sloping or steep.S2S3-Cryorthods with Histosols, Udands and Aquepts; gently

sloping to steep.ULTISOLS

AquultsUla-Aquults with Aquents, Histosols, Quartzipsamments, and

Udults; gently sloping.HumultsU2S-Humults with Andisols, Tropepts, Xerolls, Ustolls, Udox,

Torrox, and rock land; gently sloping to steep.UdultsU3a-Udults with Udalfs, Fluvents, Aquents,

Quartzipsamments, Aquepts, Dystrochrepts, and Aquults;gently or moderately sloping.

U3S-Udults with Dystrochrepts; moderately sloping or steep.

VERTISOLSUdertsVia-Uderts with Aqualfs, Eutrochrepts, Aquolls, and Ustolls;

gently sloping.UstertsV2a-Usterts with Aqualfs, Orthids, Udifluvents, Aquolls,

Ustolls, and Torrerts; gently sloping.

Areas With Little SoilX1-Salt flats.X2-Rock land (plus permanent snow fields and glaciers).

Slope ClassesGently sloping-Slopes mainly less than 10 percent, including

nearly level.

FIGURE 18.2 Broad schematic map of the soil orders and suborders of theworld.

Moderately sloping-Slopes mainly between 10 and 25percent.

Steep-Slopes mainly steeper than 25 percent.

SOILS OF THE WORLDDistribution of Orders and PrincipalSuborders and Great GroupsA ALFISOLS-Soils with subsurface horizons of

clay accumulation and medium to high basesupply; either usually moist or moist for 90

ALFISOLS

FIGURE 18.2 Broad schematic map of the soil orders and suborders of theworld.U. S. DEPARTMENT OF AGRICULTURE

consecutive days during a period whentemperature is suitable for plant growth.A1

Boralfs-cold.Ala with Histosols, cryic temperature

regimes commonAlb with Spodosols, cryic temperature

regimesA2

Udalfs-temperate to hot, usually moist.A2a with AqualfsAlb with AquollsA2c with Hapludults

289

FIGURE 18.2 (Continued)

290

A2d with OchreptsA2e with TroporthentsA2f with Udorthents

A3

Ustalfs-temperate to hot, dry more than 90cumulative days during periods whentemperature is suitable for plant growth.

A4Ma with TropeptsA3b with TroporthentsA3c with TropustultsA3d with UstertsMe with Ustochrepts


A3f

with UstollsA3g with UstorthentsA3h with UstoxA3j

Plinthustalfs with Ustorthents

Xeralfs-temperate or warm, moist inwinter and dry more than 45 consecutivedays in summer.Ma with XerochreptsA4b with XerorthentsA4c with Xerults

• ARIDISOLS-Soils with pedogenic horizons,usually dry in all horizons and never moist asl ong as 90 consecutive days during a periodwhen temperature is suitable for plant growth.Dl

Aridisols-undifferentiated.D1 a with OrthentsD1b with Psamments

D1c with UstalfsD2

Argids-with horizons of clayaccumulation.D2a with FluventsD2b with Torriorthents

•

ENTISOLS-Soils without pedogenic horizons;either usually wet, usually moist, or usually dry.El

Aquents-seasonally or perennially wet.E1a Haplaquents with UdifluventsE1b Psammaquents with Haplaquents

Etc Tropaquents with HydraquentsE2

Orthents-loamy or clayey textures, manyshallow to rock.E2a CryothentsE2b Cryorthents with OrthodsE2c Torriorthents with AridisolsE2d Torriorthents with UstalfsE2e Xerorthents with Xeralfs

E3

Psamments-sand or loamy sand textures.E3a with AridisolsE3b with UdoxE3c with TorriorthentsE3d with UstalfsE3e with UstoxE3f shifting sandsE3g Ustipsamments with Ustolls

•

HISTOSOLS-Organic soils.Hl

Histosols-undifferentiated.Hl a with Aquods

H1b with BoralfsH1c with Cryaquepts

I

I NCEPTISOLS-Soils with pedogenic horizons ofalteration or concentration but withoutaccumulations of translocated materials otherthan carbonates or silica; usually moist or moistfor 90 consecutive days during a period whentemperature is suitable for plant growth.11

Andepts-amorphous clay or vitricvolcanic ash or pumice. The suborderAndepts was converted to the Andisolorder in 1989.

I1a Dystrandepts with Ochrepts

ALFISOLS 291

1 2

Aquepts-seasonally wet.1 2a Cryaquepts with Orthentsl 2b

Haplaquepts with Salorthids1 2c

Haplaquepts with Humaquepts1 2d

Haplaquepts with Ochraqualfs1 2e Humaquepts with Psamments1 2f

Tropaquepts with Hydraquents1 2g

Tropaquepts with Plinthaquults1 2h

Tropaquepts with Tropaquents1 2j

Tropaquepts with Tropudults1 3

Ochrepts-thin, light-colored surfacehorizons and little organic matter.1 3a

Dystrochrepts with Fragiochrepts1 3b

Dystrochrepts with Udox1 3c

Xerochrepts with Xerolls1 4

Tropepts-continuously warm or hot.1 4a

with Ustalfs1 4b

with Tropudults1 4c

with Ustox1 5

Umbrepts-dark-colored surface horizonswith medium to low base supply.1 5a

with Aqualfs

MOLLISOLS-Soils with nearly black, organic-rich surface horizons and high base supply;either usually moist or usually dry.Ml Albolls-light gray subsurface horizon over

slowly permeable horizon; seasonally wet.M1a with Aquepts

M2 Borolls-cold.M2a with AquollsM2b with OrthidsM2c with Torriorthents

M3 Rendolls-subsurface horizons have muchcalcium carbonate but no accumulation ofclay.M3a with Usterts

M4

Udolls-temperate or warm, usually moist.M4a with AquollsM4b with EutrochreptsM4c with Humaquepts

M5 Ustolls-temperate to hot, dry more than90 cumulative days in year.M5a with ArgialbollsM5b with UstalfsM5c with UstertsM5d with Ustochrepts

M6 Xerolls-cool to warm, moist in winter anddry more than 45 consecutive days insummer.M6a with Xerorthents

292

•

OXISOLS-Soils with pedogenic horizons thatare mixtures principally of kaolin, hydratedoxides, and quartz, and are low in weatherableminerals.01

Udox-hot, nearly always moist.O1a with Plinthaquults

01 b with Tropudults02 Ustox-warm or hot, dry for long periods

but moist more than 90 consecutive daysi n the year.02a with Plinthaquults02b with Tropustults02c with Ustalfs

•

SPODOSOLS-Soils with accumulation ofamorphous materials in subsurface horizons;usually moist or wet.S1

Spodosols-undifferentiated.Sta cryic temperature regimes; with

BoralfsSib cryic temperature regimes; with

HistosolsS2

Aquods-seasonally wet.S2a Haplaquods with Quartzipsamments

S3

Humods-with accumulations of organicmatter in subsurface horizons.S3a with Hapludalfs

S4 Orthods-with accumulations of organicmatter, iron, and aluminum in subsurfacehorizons.S4a Haplorthods with Boralfs

• ULTISOLS-Soils with subsurface horizons ofclay accumulation and low base supply; usuallymoist or moist for 90 consecutive days during aperiod when temperature is suitable for plantgrowth.U1

Aquults-seasonally wet.U1a Ochraquults with Udults

U1b Plinthaquults with UdoxU1c Plinthaquults with Plinthaquox

U1d Plinthaquults with TropaqueptsU2

Humults-temperate or warm and moist allof year; high content of organic matter.U2a with Umbrepts

U3

Udults-temperate to hot; never dry morethan 90 cumulative days in the year.U3a with Andepts (Udands)U3b with DystrochreptsU3c with UdalfsU3d Hapludults with DystrochreptsWe Rhodudults with UdalfsU3f Tropudults with AquultsU3g Tropudults with HydraquentsU3h Tropudults with Udox


U3j Tropudults with TropeptsU3k Tropudults with Tropudalfs

U4 Ustults-warm or hot; dry more than 90cumulative days in the year.U4a with UstochreptsU4b Plinthustults with UstorthentsU4c Rhodustults with UstalfsU4d Tropustults with TropaqueptsU4e Tropustults with Ustalfs

V VERTISOLS-Soils with high content of swellingclays; deep, wide cracks develop during dryperiods.V1

Uderts-usually moist in some part in mostyears; cracks open less than 90 cumulativedays in the year.Via with Usterts

V2

Usterts-cracks open more than 90cumulative days in the year.V2a with TropaqueptsV2b with TropofluventsV2c with Ustalfs

X Soils in areas with mountains-Soils with variousmoisture and temperature regimes; many steepslopes; relief and total elevation vary greatly fromplace to place. Soils vary greatly within shortdistances and with changes in altitude; verticalzonation common.X1

Cryic great groups of Entisols, Inceptisols,and Spodosols.

X2

Boralfs and cryic great groups of Entisolsand Inceptisols.

X3

Udic great groups of Alfisols, Entisols, andUltisols; Inceptisols.

X4

Ustic great groups of Alfisols, Inceptisols,Mollisols, and Ultisols.

X5

Xeric great groups of Alfisols, Entisols,I nceptisols, Mollisols, and Ultisols.

X6

Torric great groups of Entisols; Aridisols.X7

Ustic and cryic great groups of Alfisols,Entisols, Inceptisols, and Mollisols; usticgreat groups of Ultisols; cryic great groupsof Spodosols.

X8

Aridisols, torric and cryic great groups ofEntisols, and cryic great groups ofSpodosols and Inceptisols.

Z MISCELLANEOUSZi

I cefields.Z2

Rugged mountains-mostly devoid of soil(includes glaciers, permanent snow fields,and, in some places, areas of soil).

. . . Southern limit of continuous permafrost.- -Southern limit of discontinuous permafrost.

FIGURE 18.3 Many Udalfs occur on gentle slopes ofglaciated areas in the north-central part of the UnitedStates and New York, where a wide variety of crops isgrown and dairying is a major industry.

well suited to the production of a wide variety ofcrops, especially corn, soybeans, wheat, and for-ages (see Figure 18.3). Udalfs have developedalong the lower Mississippi River in loess and aresimilar to Udalfs that developed from loess in theMidwest. A Udalf is shown in Color Plate 5.

Many of the best agricultural soils in northwes-tern Europe are Udalfs comparable to those of thenorth-central United States in that they developedin glacial parent materials of similar age. In bothareas, dairying is a major industry.

Soils of the Blue Grass region in central Ken-tucky are predominantly Udalfs that developedfrom a high phosphorus limestone capped by athin layer of loess. Early settlers prized the soilsfor growing food and, today, they are used forraising racehorses in addition to being used forgeneral agriculture.

UstalfsUstalfs have an ustic moisture regime and tend tooccur where temperatures are warm or hot andthe vegetation is grassland or savanna. The largeareas dominated by Ustalfs in Texas and NewMexico (A4a in Figure 18.1) occur south of areaswhere the soils are mainly Mollisols. In theseUstalf areas, higher temperature and sandy parentmaterials have contributed to the development ofsoils with ochric epipedons under grass or sa-

ALFISOLS 293

vanna vegetation. Much of the land is devoted tocattle grazing, and warm temperatures allow cot-ton to be grown.

The largest area in the world in which Alfisolsare the dominant soils occurs south of the SaharaDesert in Africa. The soils are Ustalfs (see areaA3a in Figure 18.2). The soils have undergone al ong period of weathering; many of them havekandic horizons, and would normally be Ultisols.However, calcareous dust from the Sahara Desertis carried south by seasonal winds and is depos-ited on the soils: This has kept the base saturationat 35 percent or more. Considerable cattle raising,subsistence farming, and shifting cultivation oc-cur in this region. The large size of raindrops ofthe tropical thunderstorms contributes to highrates of water runoff and soil erosion on exposedland.

XeralfsXeralfs have a zeric moisture regime and are thedominant soils in the coastal ranges and inlandmountains in California and southern Oregon(see areas A5s1 and A5s2 in Figure 18.1). Natural

FIGURE 18.4 Grazing of cattle is important onXeralfs on the lower slopes of the Sierra NevadaMountains in California. On the cooler and morehumid upper slopes are Xerults, Ultisols with a xericmoisture regime.

294

vegetation is a mixture of annual grasses, forbs,and shrubs. Many of these sloping lands are usedfor animal grazing, and some areas are irrigatedfor the production of horticultural crops. In themountains at highest elevations with greater pre-cipitation, Ultisols occur as shown in Figure 18.4.

ANDISOLS

Andisols are weakly developed soils produced bythe weathering of volcanic ejecta. Although An-disols occupy less than 1 percent of the world'sland surface, they are important soils of the Pa-cific Ring of Fire-that ring of active volcanoesalong the western coast of the American con-tinents, across the Aleutian Islands, down Japan,the Philippine Islands, other Pacific Islands, anddown to New Zealand. The most important areasof Andisols in the United States occur in the Pa-cific northwest and Hawaiian Islands.

Genesis and PropertiesAn important feature of Andisols is the presenceof volcanic glass. The dominant process in mostAndisols is one of weathering and mineral trans-formation. Volcanic glass weathers rapidly pro-ducing allophane and oxides of aluminum andiron. Translocation and accumulation within thesoil are minims). Repeated ash falls maintain theweakly developed nature of the soils and buriedhorizons are common.

Andisols have andic soil properties. The weath-ered minerals, including allophane and oxides ofaluminum and iron, are amorphous or short-range minerals with enormous surface area andgreat capacity to complex with organic matter.Andisols are characterized by high organic mattercontent, ion exchange capacity, and low bulkdensity. The capacity to hold available water forplant growth is high. Soils are generally con-sidered fertile, although high fixation and lowavailability of phosphorus are sometimes a prob-lem. Many Andisols have melanic epipedons.Large areas of intensively cropped black soils,


Andisols with melanic epipedons, are visible fromthe air in the Andes of South America.

SubordersMost Andisols suborders are based on moistureand temperature regimes. Aquands have an aquicmoisture regime, Cryands are cold (cryic temper-ature regime or colder), Torrands have an aridicmoisture regime, and Xerands have a xeric mois-ture regime. Vitrands have the greatest content ofglassy or vitreous materials. Sufficient weatheringof Vitrands and disappearance of glass results inUdands with udic moisture regime or Ustandswith ustic moisture regime.

The wide range in temperature and moistureregimes is associated with great variation in na-tive vegetation from bamboo, grass, sedges, andforest. Xerands are the dominant Andisols of thePacific northwest in the United States. Udandsand Ustands are the dominant Andisols of theIsland of Hawaii where they are important soils forsugarcane production and cattle grazing.

Locations where soils are likely to be Andisolsare shown on the world soil map as mountainousareas, X areas. One area of volcanic ash soils islocated in northern New Zealand. The soils areAndepts in the legend of Figure 18.2. In 1989 theAndept suborder was converted to the Andisolorder.

ARIDISOLSAridisols have an aridic moisture regime and arethe dominant soils of deserts. Large areas of des-ert occur in northern Africa (Sahara Desert), cen-tral Eurasia, central and western Australia, insouthern South America east of the Andes Moun-tains, in northern Mexico, and in the southwest-ern United States. Small deserts exist on manyislands in the rain shadow of mountains, as in theHawaiian and Caribbean islands. Aridisols are themost abundant soils worldwide and the fifth mostabundant soils in the United States (see Table18.1).

FIGURE 18.5 Vegetation on the Sonoran Desert inArizona. Note gravel pavement in the foregoundbecause of removal of fine particles by the wind.

Shrubs, the dominant vegetation, give way tobunch grasses with increasing precipitation.Plants are widely spaced and use the limited wa-ter supply quite effectively. Many desert plantsgrow and function during the wetter seasons andgo dormant during the driest seasons. Suprisingly,there can be great plant diversity and a consider-able amount of vegetation, as seen in Figure 18.5.On the more humid or eastern edge of the aridregion in southwestern United States, the desertbunch grasses give way to taller and more vigor-ous grasses; Aridisols merge with Alfisols andMollisols.

Genesis and PropertiesIn arid regions, there is a slow rate of plantgrowth, resulting in low organic matter accumu-

ARIDISOLS 295

lation and limited weathering, leaching, and elu-viation. A striking feature of most Aridisols is azone below the surface in which calcium carbo-nate has accumulated to form a calcic (k) hori-zone as a result of limited leaching by water.Winds play a role by removing the finer particlesfrom exposed areas and, where gravel occurs inthe soil, a surface layer of gravel (desert pave-ment) is formed (see Figure 18.5).

Some Aridisols, however, have a well-developed argillic horizon (Bt), which indicatesconsiderable weathering and clay movement. Theoccurrence of argillic horizons suggests that amore humid climate existed many years ago. TheMohave is a common Aridisol in the Sonoran Des-ert of Arizona; and some data are given in Table18.2 to illustrate properties commmonly found inAridisols. Note the Bt horizon (argillic) at the 10-to 69- centimeter depth. This is underlain by Bkand Ck horizons that contain 10 and 22 percentcalcium carbonate, respectively. The horizonshave low organic matter contents and low carbon-nitrogen ratios. The argillic horizon has high CECbecause of clay accumulation. Significant ex-changeable sodium and high pH values reflectlimited leaching, at least, in recent years. A photo-graph of an Aridisol with A, Bt, and Ck horizons isshown in Color Plate 6.

Aridisol SubordersAridisols are placed in suborders on the basis ofthe presence or absence of an argillic horizon.Orthids do not have argillic horizons, whereasArgids do. As shown in Table 18.2, the Mohavesoil has an argillic horizon and is an Argid. Thegeneral distribution of Orthids and Argids in theUnited States is shown in Figure 18.1 and for theworld in Figure 18.2.

It is believed that Orthids are young Aridisolsand that Argids, by comparison, are much olderAridisols. There is evidence that the Orthids in theUnited States developed largely within the past25,000 years in an arid climate. Orthids are usu-ally located where recent alluvium has been de-posited. Argids are common on old land surfaces

296 SOIL GEOGRAPHY AND LAND USE

in any landscape where there has been enoughtime for genesis of an argillic horizon during ahumid period that existed more than 25,000 yearsago. About 75 percent of the Aridisols in UnitedStates are Argids. Entisols occur on the most re-cent land surfaces. The desert landscape showni n Figure 18.6 has land surfaces with greatly differ-ent ages.

Land Use on AridisolsAridisols of the western United States occur al-most entirely within a region called the "westernrange and irrigated region." As the name implies,

grazing of cattle and sheep and production ofcrops by irrigation are the two major agriculturalland uses. The use of land for grazing is closelyrelated to precipitation, which largely determinesthe amount of forage produced. Some areas aretoo dry for grazing, whereas other areas receivemore favorable precipitation and thus may beused for summer grazing on mountain meadows.As many as 35 hectares (75 acres) or more in thedrier areas are required per head of cattle, thusmaking large farms or ranches a necessity. Mostranchers supplement range forage by producingsome crops on a small irrigated acreage as shownin the left foreground of Figure 18.6. Overgrazing,

FIGURE 18.6 Grazing landsdominated by Aridisols. Soilsi n the area are closely relatedto age of land surface withAridisols on the older surfacesand Entisols on the youngestsurfaces. Note the smallirrigation reservoir andirrigated cropland near ranchheadquarters. (USDAphotograph by B. W. Muir.)

which results in the invasion of less desirableplant species and increased soil erosion, is themajor hazard in the use of these grazing lands.

I n Arizona, for example, 1 or 2 percent of thel and is irrigated. Most of the irrigated land is lo-cated on soils developed from alluvial parent ma-terials along streams and rivers. Here, the land isnearly level, irrigation water is obtained from theriver, and water is distributed over the field bygravity. The alkaline nature of Aridisols may causedeficiencies of various micronutrients on certaincrops. Major crops include alfalfa, cotton, citrusfruits, vegetables, and grain crops. In Arizona,crop production on only I or 2 percent of the landthat is irrigated accounts for 60 percent of the totalfarm income. Grazing, by contrast, uses 80 per-cent of the land and accounts for only 40 percentof the total farm income.

ENTISOLSEntisols are soils of recent origin. The centralconcept is soils developed in unconsolidatedearth, or parent material, with no genetic horizonsexcept an A horizon. Some Entisols have rockclose to the surface and have A and R horizons asshown in Figure 18.7. All soils that do not fit intoone of the other 10 orders are Entisols. The resultis that the Entisol order is characterized by greatdiversity.

AquentsSmall areas of Aquents are widely distributed. Inthe United States, however, the only major areashown in Figure 18.1 is in southern Florida (areaE1a). Parent materials are mainly marine sandsand marl. The soils are wet as a result of a natu-rally high water table. Although they are drainedfor cropping, water for irrigation is pumped from ashallow water table when needed. The frost-freeweather in southern Florida makes these soilsvaluable for production of winter vegetables andfruits.

ENTISOLS

FIGURE 18.7 An Entisol developing in parentmaterial formed by the weathering of granite in theU.S. Rocky Mountains.

The largest area in which soils are mainlyAquents worldwide occurs on the North ChinaPlain near the mouths of Yellow, Yangtze, andseveral other rivers (area E1a in Figure 18.2). It is avast floodplain where inhabitants must cope withoccasional catastrophic floods. This large flood-plain is one of the most densely populated andintensively cropped areas of the world.

FluventsFluvents develop in alluvium of recent origin.They are, in large part, soils having properties thathave been inherited from the parent material. Fora soil to be a Fluvent it must have, with increasing

297

298

depth, an irregular distribution of organic matterthat reflects the organic matter content of differentlayers of recently deposited alluvium. The organicmatter in soils formed from alluvium decomposesover time. Therefore, in the absence of continuedflooding and deposition of alluvium, a normalgradual decrease in organic matter content withincreasing soil depth is produced with time. Inthis way, a Fluvent may develop in the absence ofcontinued flooding and deposition of alluviumi nto an Inceptisol, an Aquept. For this reasonmany soils on large river valley floodplains, suchas the Mississippi River floodplains, are notFluvents.

Alluvial sediments were deposited in the Impe-rial Valley in southeastern California by severalyears of uncontrolled flooding of the ColoradoRiver when the valley underwent widespread de-velopment of irrigation. Some Fluvents have de-veloped in these recent sediments, however, mostof the soils are Orthents. The uncontrolled flood-i ng created the Salton Sea in a depression belowsea level north of the valley. Now, the Salton Seaserves as a sink for the salt in the drainage waterfrom the irrigated fields.

PsammentsPsamments develop in sand parent materials anddo not have an aquic moisture regime.Psamments develop when unstablized sanddunes become stablized by vegetation. The tex-ture must be loamy fine-sand or coarser to a depthof 1 meter or more, or to a hard rock layer. TheSandhills of north-central Nebraska (see area E3cof Figure 18.1) are dominated by Psamments withan ustic moisture regime. These soils are Ustip-samments. This area consists of sand dunes sta-bilized by mid- and tall grasses and used for cattlegrazing. The major product is feeder stock that issold for fattening in feedlots. A typical Sandhillslandscape is shown in Figure 18.8. Numeroussmall lakes in the area provide watering places.The major soil management challenge is to con-trol grazing so that productive forage species


FIGURE 18.8 Cow-calf production is the major landuse on the Psamments of the Nebraska Sandhills.Maintenance of a grass cover is essential to preventserious wind erosion.

maintain a vegetative cover to control winderosion.

Psamments occupy most of the central ridge ofthe Florida peninsula. The hyperthermic tempera-ture regime provides frost-free winters and makesthe soils prized for citrus production. Outside theUnited States many large areas in north and southAfrica, Australia, and Saudi Arabia are dominatedby Psamments.

OrthentsThe suborder Orthents includes all Entisols thatdo not fit into one of the other suborders. As aresult, Orthents are very diverse and form in par-ent materials finer in texture than sands and with-out an aquic moisture regime. Delayed genesis iscaused by active erosion on steep slopes, shortperiod of time, and/or hardness of rock close tothe soil surface. Where the parent materials arederived from unconsolidated rock, A-C soils oc-cur. A-R soils occur where rock is near the soilsurface (see Figure 18.7). Active erosion on steepslopes accounts for many Orthents in the Bad-lands of the western United States, and activeerosion, together with rock close to the soil sur-face, account for many of the Orthents in theRocky Mountains. Orthents are common in thevalleys of California, including the large Central

Valley (area Etc of Figure 18.1). The soils have axeric moisture regime, are mainly Xerorthents,and were formed in sediments carried to the val-l ey floor as a result of erosion of the surroundinghills and mountains. The soils are intensively irri-gated, and many high value crops are produced.As a result, the value of agricultural crops pro-duced in California is the largest of any state (seeFigure 18.9).

HISTOSOLSHistosols are organic soils, and they have beencalled bogs, peats, mucks, and moors. Most His-tosols are water saturated for most of the yearunless they have been drained. Histosols containorganic soil materials to a significant depth. Or-ganic soil material contains a variable amount oforganic carbon, depending on the clay content.Soil material containing 60 or more percent clayhas an organic carbon content of 18 or more per-cent. Material containing no clay has an organiccarbon content of 12 or more percent. Interme-

FIGURE 18.9 Orthents are the most importantagricultural soils in California in large, broad, andnearly level valleys. A wide variety of crops is grown,including fruits, nuts, vegetables, and farm crops.Cotton is shown in the foreground and large stacks ofbaled alfalfa in the rear.

HISTOSOLS 299

diate clay contents have intermediate amounts oforganic carbon (see Figure 9.5).

Many Histosols have developed in organic ma-terials of great depth (many meters deep). Othershave developed in parent materials containingorganic layers of varying thickness. Where theorganic material overlies rock at a shallow depth,Histosols can be thin.

Histosol SubordersDuring water saturation, organic matter resists

decomposition but decomposes when the soildrains and desaturates. The degree of decom-position of the organic fibers or plant materialsi ncreases with increasing time of drainage or des-aturation. Three degrees of decomposition arerecognized and reflected in the suborders Fibrists,Hemists, and Saprists. Fibrists consist largely ofplant remains so little decomposed that their bo-tanical origin can be determined readily. Hemistsare decomposed sufficiently so that the botanicalorigin of as much as two-thirds of the materialscannot be determined. Saprists consist of almostcompletely decomposed plant remains and theyusually have a black color.

Histosol horizons are 0 horizons: Oi, Oe, andOa refer to 0 horizons with fabric, hemic, andsapric organic materials, respectively. Folists arethe other suborder and consist of organic soilmaterials resulting from the deposition of leaf orfoliar material. Folists may be thin over hard rock,and it is likely that they may never be water satu-rated.

One eighth of the soils in Michigan are Histo-sols, which occur in all the counties, with only afew areas being quite extensive. In Figure 18.1,two large areas of Hemists (H1a) are shown innorthern Minnesota, and one area is shown in thelower Mississippi River delta in Louisiana. TheEverglades in Florida contain Fibrists, Hemists,and Saprists (see area H3a in Figure 18.1). AnEverglades landscape is shown in Figure 18.10.Large acreages of Histosols occur in northernCanada (see H I b and H1c in Figure 18.2).

300

FIGURE 18.10 Histosol landscape of the FloridaEverglades.

Land Use on HistosolsA major limitation of northern Histosols for agri-cultural use is frost hazard. The Histosols tend tooccupy the lowest position in the landscape andare the first areas to freeze. Use for crop produc-tion requires water drainage, which enhances de-composition of the organic matter. The sub-sidence and loss of soil by oxidation anddisappearance of the soil is a major concern, asshown in Figure 18.11. A large acreage in theFlorida Everglades is used primarly for the pro-duction of sugarcane. During 50 years of agricul-ture the soils subsided an amount equal to anamount of soil that required 1,200 years to form.Since the Histosols of the Everglades overlie lime-stone, many areas now cultivated will be too thinfor continued cultivation past the year 2000. Thelikely use for such soils will then be pasture,which requires no tillage.

Other problems in the use of Histosols are firehazards and wind erosion during dry seasons.The soils contain few minerals for supplyingmany of the plant nutrients; however, the organicmatter contributes to high nitrogen-supplyingpower. Despite the problems in their use, manyHistosols are intensively managed for the produc-tion of a wide range of field and vegetable crops,including sod.

I NCEPTISOLSInceptisols typically have an ochric or umbric epi-pedon that overlies a cambic (Bw) horizon. They


FIGURE 18.11 I n 1924 the bottom of this 9-footconcrete post was placed on the underlyinglimestone at Belle Glade, Florida, and the top of thepost was at ground level. This photograph, whichwas taken in 1972, shows that subsidence of theorganic soil (Histosol) has been about 1 inch peryear.

typically show little evidence of eluviation, illuvia-tion, or extreme weathering. They are too old to beEntisols and lack sufficient diagnostic features tobe placed in one of the other orders. Inceptisolsoccur in all climatic zones where there is someleaching in most years, and are the second mostabundant soils both in the United States andworldwide. In the United States about 70 percentof the Inceptisols are Aquepts, 26 percent areOchrepts, and 4 percent are Umbrepts.

AqueptsAquepts are wet Inceptisols that have an aquicmoisture regime, unless they are artificiallydrained. Mottled rusty and gray colors are com-mon at depths of 50 centimeters or less. Aqueptsoccur in many depressions in geologically recentlandscapes where soils are water saturated atsome time during the year, and are surrounded bybetter drained soils that are Alfisols or Mollisols.Water saturation inhibits eluviation and illuvia-tion. With better drainage, Aquepts may developinto soils that are older or more mature. Aquepts

can have any temperature regime and almost anyvegetation.

The largest areas of Aquepts occur in the Tun-dra regions of the Northern Hemisphere in Alaska,Canada, Europe, and Asia (areas 12a in Figure18.2). Most of these cold Aquepts (Cryaquepts)have permafrost, which inhibits the downwardmovement of water and contributes to soil wet-ness when frozen soil melts during the summer.Trees grow poorly on soils with permafrost, soforestry is not important. Locally, above timber-line in many mountains, there are Cryaqueptswith permafrost (see Figure 18.12).

Few people inhabit tundra regions; some ofthose who do work in the mining and oil indus-tries, and a few people in northern Europe andAsia raise reindeer. The reindeer browse on thetundra vegetation and may be readily contami-nated with radioactive elements that fall out of theatmosphere. Reindeer farmers in Finland foundthat their animals were unfit for human consump-tion because of the radioactive contamination fol-lowing the Chernobyl (USSR) reactor explosion.

FIGURE 18.12 Dominant soils on the tundra abovetimberline in the mountains are Inceptisols, Aquepts.Soils have permafrost, vegetation is sparse, and theenvironment is fragile.

I NCEPTISOLS 301

Other large and important areas of Aquepts occurin the world's major river valleys. Note the large12a area in Figure 18.1 that includes the lowerMississippi River valley. The soils have formed inalluvial materials, but most of the materials arenot recent and the soils are not Entisols, becausethe river has wandered back and forth over a widefloodplain over a long period of time. TheseAquepts tend to be fine textured and very fertile.Good drainage and management have made theAquepts of the Mississippi River valley among themost productive soils in the United States.Sugarcane is an important crop in southern Lou-isiana where temperatures are warm. A landscapein which most of the soils are Aquepts in the lowerMississippi River valley is shown in Figure 18.13.

The Ganges-Brahmaputra River valley in north-ern India and Bangladesh is the largest area in theworld in which Aquepts are the dominant soils(see Area 12c of Figure 18.2). The presence ofAquepts in the river valleys of Asia, and the largerice consumption of the inhabitants makesAquepts the world's most important soil for riceproduction. Aquepts are also the dominant soilsi n the Amazon River valley, (see Figure 18.2).

OchreptsOchrepts are the more or less light-colored andfreely drained Inceptisols of the mid- to high lati-tudes. They tend to occur on geologically youngland surfaces and have ochric and cambic hori-zons. Most of them had, or now have, a forestvegetation. Ochrepts are the dominant soils of theAppalachian Mountains (area 13s of Figure 18.1).Erosion on steep slopes has contributed to theformation and maintenance of these Inceptisols.

UmbreptsUmbrepts are Inceptisols with umbric epipedons.Most of them occur in hilly or mountainous re-gions of high precipitation and under coniferousforest. Umbrepts in the United States are the mostextensive near the Pacific Ocean in the states of

302

Oregon and Washington (see areas 14S and l4a ofFigure 18.1). These Umbrepts have a mesic tem-perature regime and udic or xeric moisture re-gimes. Many of the soils are on steep sloopes thatare densely covered with coniferous forest andl umbering is the principal industry.

MOLLISOLSMollisols are the dominant soils of the temperategrasslands or steppes. These regions are fre-quently bordered on the drier side by deserts withAridisols and on the wetter side by forests inwhich the dominant soils are Alfisols. There issufficient precipitation to support perennialgrasses that contribute a large amount of organicmatter to the soil annually by deeply pentratingroots. Precipitation, however, is somewhat lim-ited so the soils are only minimally or moderatelyweathered and leached. Mollisols typically have amollic epipedon. Subsurface horizons are typi-cally cambic or argillic, and a few have natrichorizons. Mollisols are characterized by a moder-ate or high organic matter content and a highcontent of weatherable minerals. Many of thesoils are calcareous at the surface and the acidMollisols tend to be only mildly acid and withoutsignificant exchangeable aluminum. Two Molli-sols, both with k (calcic) horizons, are shown inColor Plate 6.


FIGURE 18.13 I ntensiveagriculture on Aquepts in thelower Mississippi River valley.

Mollisols are considered to be some of the mostnaturally fertile soils for agriculture. Where tem-perature and water are favorable, high grain yieldsare easily obtained. The most abundant soils inthe United States are Mollisols, being 25 percentof the total (see Table 18.1). Few Mollisols occuri n the tropics because of the generally long periodof weathering and leaching.

The world's major grasslands lacked trees forl umber, readily available water supplies, and nat-ural sites for protection against invaders. As aresult, these areas dominated with Mollisols werei nhabited mainly by nomadic people until about150 years ago. The farmers who settled in thecentral part of United States tended to avoid theMollisols because they were used to forest soils,and they found the sod of the Udolls difficult toplow and convert into cropland. The Virgin Landsof the Soviet Union were opened to settled agricul-ture in 1954. Today, these areas are characterizedby their excellent agricultural soil and a low popu-l ation density. Land use is related mainly to watersupply and temperature.

AquollsAquolls are the wet Mollisols that have an aquicmoisture regime or are artificially drained. Theyoccur as inclusions in many grassland land-scapes owing to water saturation of the soil inlocal depressions. Drummer silty clay loam is an

Aquoll and is the most abundant soil in Illinois.Much of the Corn Belt's reputation for corn pro-duction is due to large acreages of Aquolls thatdeveloped on nearly level and depressional areasunder poor drainage.

Aquolls are the dominant soils of the Red RiverValley bordering Minnesota and North Dakota(area M1a in Figure 18.1). These Aquolls haveformed from lacustrine sediments deposited inglacial Lake Agassiz about 9,000 to 12,000 yearsago. The soils tend to be fine-textured, to have arelatively high organic matter content, and to bewell suited for grain crops. The Red River Valley isan area of large-scale cash crop farming. Springwheat, barley, sugar beets, corn, and sunflowersare the major crops.

BorollsBorolls are Mollisols that do not have an aquicmoisture regime but are northerly or cool withcryic temperature regime (have mean annual soiltemperature less than 8° C or 47° F). Borolls aredominant on the northern Great Plains in theUnited States and Canada and in the Soviet Unionand northern China (areas M2a, b, and c in Figure18.2). Spring wheat is the major crop and much ofthe land is used for cattle grazing. Low tempera-ture is frequently coupled with low precipitationso that much of the wheat is produced in a wheat-fallow system.

Ustolls and UdollsUstolls and Udolls have warmer temperaturesthan do Borolls, and have ustic and udic moistureregimes, respectively. Ustolls typically have k hor-izons and a neutral or alkaline reaction. Udollsare without k layers. Udolls are typically acid, andlime is frequently applied. Cambic and argillichorizons are common in both.

In both the Great Plains and the north-centralstates, Ustolls are dominant west of the approxi-mate boundary between Iowa and Nebraska andUdolls are dominant east of the boundary. (Con-trast areas M4a, b, and c with the M3a areas in

MOLLISOLS 303

Figure 18.1). Winter wheat, produced in a wheat-fallow system, is the major crop grown on Ustolls.Kansas ranks first in wheat production in theUnited States. The winter wheat is planted in thefall. The milder winters on Ustolls (as comparedwith those on Borolls) allow the wheat to becomeestablished in the fall and to begin growing inearly spring. Consequently, winter wheat yieldsare greater than those for spring wheat. Springwheat is grown primarily on Borolls of the north-ern Great Plains in the United States and Canada,and in the Soviet Union. Cattle grazing is also amajor enterprise on Ustolls, especially on landnot suitable for wheat (see Figure 18.14). Irri-gation water pumped from the Ogallala formationis used for intensive cropping on the High Plainsof Texas. Common crops include sorghum, winterwheat, and peanuts.

Corn and soybeans are the major crops grownon Udolls, as shown in Figure 18.15. The two mostfamous areas of Udolls are the Corn Belt in theUnited States and the humid part of the Pampa inArgentina, which are areas of intensive corn pro-duction. There is limited production of corn in theSoviet Union, which is a reflection of the coolerand drier climate and the dominance of Ustollsand Borolls rather than Udolls on the grasslands.Note in Figure 18.2 that there is a west to eastdistribution of Aridisols, Ustolls, and Udolls in

FIGURE 18.14 Beef cattle grazing on Ustolls wherethe slopes are unfavorable for wheat production.(Courtesy USDA.)

304

Argentina, similar to that which occurs in theUnited States. Land uses in the two areas are alsosimilar, with wheat and cattle grazing on theUstolls and corn production concentrated on theUdolls. Most of the Mollisols of Argentina formedin loess that was contaminated with volcanic ash,and therefore the soils have a greater inherentfertility than do comparable Mollisols in theUnited States and the Soviet Union.

XerollsXerolls are Mollisols with warmer temperaturesthan are associated with Borolls and a xeric mois-ture regime. Most of the Xerolls in the UnitedStates occur in Washington, Oregon, and Idaho,where summers are dry and winters are cool andrainy. Water availability is suited for winter wheatproduction, and Whitford County in the state ofWashington produces more wheat than any othercounty in the United States. Whitford County islocated in the Palouse of eastern Washington andwestern Idaho. The Xerolls developed in loess


FIGURE 18.15 TypicalAquoll-Udoll landscape incentral Illinois with corn andsoybeans as the major crops.

that overlies basalt in a rolling landscape (seeFigure 18.16). The soils are very similar to Molli-sols developed from loess in the central UnitedStates. The xeric moisture regime precludes cornproduction because the summers are very dry.Much of the land with Xeroll soils is used for cattlegrazing.

OXISOLS

Oxisols are the most intensively weathered soils,consisting largely of mixtures of quartz, kaolinite,oxides of iron and aluminum, and some organicmatter. Many Oxisols contain little silt becausethe silt sized mineral particles have essentiallyweathered leaving a clay fraction composedmainly of oxidic clays plus kaolinte and varyingamounts of sand because of the great weatheringresistance of large quartz grains. Most Oxisolshave oxic horizons. Some have plinthite, whichforms a continuous layer within 30 centimeters ofthe soil surface and is water saturated at this

depth sometime during the year. Plinthite is aniron-rich material that forms some distance belowthe soil surface. When plinthite is exposed todrying, it dries irreversibly, forming a rocklike ma-terial.

Differences in properties with depth in Oxisolsare so gradual that horizon boundaries are diffi-cult to determine. A photograph of an Oxisol isshown in Color Plate 6.

Oxisols are characterized by a very low amountof exchangeable nutrient elements, very low con-tent of weatherable minerals, high aluminum sat-uration of the cation exchange capacity, and mod-erate acidity. Although aluminum saturation maybe high, the crops do not generally develop alumi-num toxicity because of the low CEC. The miner-alogy contributes to very stable peds or structurethat results in qualities associated with sandy,quartzitic soils in the temperate regions. Gener-ally, Oxisols represent the most naturally infertilesoils for agriculture. Some are, however, theworld's most productive soils because of inten-

OXISOLS 305

FIGURE 18.16 Landscape ofXerolls in the Palouse inWashington composed mainlyof loessial dunes. Winterwheat is the major crop.(Photograph courtesy Dr. H.W. Smith.)

sive management, as in the case of pineappleproduction in Hawaii, shown in Figure 18.17.

There are no Oxisols within the continentalUnited States; however, they are found in Hawaiiand Puerto Rico. As a result, Oxisols are the least

FIGURE 18.17 Pineapple production on Oxisols inHawaii.

306

abundant soils in United States. Worldwide, Oxi-sols are the sixth most abundant soils; however,Oxisols are the most abundant soils of the tropics,where they occur on about 22 percent of the land.

Oxisol SubordersOxisols tend to occur along or parallel to theequator. Near the equator, where there is signifi-cant rainfall each month, many Udox and Peroxsoils have developed. Udox have udic moistureregime and Perox have perudic moisture regime.North and south of the equator are regions inwhich there is abundant rain in summer but nonein winter. In these areas, the Oxisols that develophave an ustic moisture regime and are Ustox.About two thirds of the Oxisols are Udox (andPerox) and about one third are Ustox. Oxisols aredominant soils in two large areas. One is in theAmazon basin in South America and the other iscentered on the Congo River or Zaire basin incentral Africa. Note the distribution of Udox andthe Ustox soils by comparing the location of theUdox areas, O1b , with the Ustox areas, 02b, inFigure 18.2. Oxisols of the Aquox and Torroxsuborders are of minor extent.

Land Use on Udox SoilsThe Oxisols in the year-round rainy climates,Udox, are mainly in the tropical rain forests wherea small human population is supported by shift-ing cultivation or slash-and-burn agriculture.Trees are killed by girdling and/or burning, andthe ashes and decomposing plant remains pro-vide nutrients for crops for a year or two. Then,soil fertility exhaustion and weed invasion resultin abandonment of the land, which is graduallyreforested by invasion of trees and shrubs. After aperiod of about 20 years, sufficient nutrients willhave accumulated within living and dead plantmaterials and in soil organic matter to allow foranother short period of cropping. Land preparedfor growing crops is shown in Figure 18.18. Notethe large amount of organic residues on the soil


FIGURE 18.18 Preparation of land in the foregroundby slash and burn. Note stumps left in field and theconsiderable amount of organic matter on the soilsurface that will mineralize and provide plantnutrients.

surface and the dense rain forest growth in thebackground.

Frequently, a village or tribal chief allocatesland to the families. Vegetables grown in smallgardens near houses, and fish and game animalssupplement the diet. There are no farm animals toprovide manure for the maintenance of soil fertil-i ty, and only a sparse population can be sup-ported. Where population growth occurs, shorterfallow periods (periods when the forest is regrow-ing and accumulating nutrients within plant andsoil organic matter) are used and it may becomeimpossible to regenerate soil fertility. Then, thesystem becomes ineffective. The accumulation ofnutrients in the vegetation during forest fallow inZaire (Congo) is shown in Table 18.3. The 18- to

TABLE 18.3 Nutrient Accumulation in Forest Fallow

Adapted from C.E., Kellogg, "Shifting Cultivation," Soil Sci.,95:221-230, 1963. Used by permission of Williams and Wat-kins Co.

19-year-old forest vegetation contained five timesmore nutrients than were contained in the 2-year-old forest fallow. Forests grow vigorously on Oxi-sols in the humid tropics because water is gener-ally available and (1) plant nutrients are efficientlycycled and (2) deeply penetrating roots absorbnutrients from soil layers that are much lessweathered and more fertile than the A and B hor-izons.

Land Use on Ustox SoilsThe native vegetation of the savanna areas, whereUstox occur, is not suited for the type of slash-and-burn agriculture that occurs in the rain forestson Udox soils (see Figure 18.19). There are vari-ous kinds of fallow systems used. However, theopportunity to accumulate nutrients in plant ma-terial is much less in the savanna than in the rainforest. Farmers in these areas tend to be smallsubsistence growers who cultivate a smallacreage permanently and give greater attention tothe use of manures, plant residues, and fertilizersto maintain soil fertility. Periods of 6 months withlittle if any rainfall are common, and each familytends to farm a small acreage of wetland, (if it isavailable), where vegetables are harvested duringthe dry season. Many fruit trees are scattered

FIGURE 18.19 Natural vegetation on savannas inBrazil, consisting of small trees, shrubs, and grass inan area dominated by Ustox soils.

OXISOLS 307

FIGURE 18.20 Land preparation of Ustox soils onthe African savanna. Organic refuse, including anyanimal manure, is placed in the furrows between theridges. Then, each ridge is split and the soil is pulledi nto the furrow to form a new ridge where the furrowpreviously existed. Ridged land increases waterinfiltration and reduces erosion from torrentialsummer storms.

throughout the savanna and provide some foodwith minimal care. Corn, beans, cowpeas, andcassava are important crops. Some farmers havelarge herds of cattle. A farmer is shown preparingland for cropping on Ustox in Africa in Figure18.20.

Extremely Weathered OxisolsThe maximum effective cation exchange capacity(ECEC) of the clay fraction of oxic horizons is 12or less cmol/kg. By contrast, the ECEC of the mostweathered horizons in Oxisols is 1.5 or less cmol/kg. On this basis, soils with a horizon having anECEC of 12.5 to 1.5 cmol/kg are considered to beintensively weathered and soils with a horizonhaving an ECEC of 1.5 or less cmol/kg are con-sidered to be extremely weathered. The prefix acris used to indicate this low ECEC as in the case ofAcrudox, extremely weathered Udox. The horizonwith acricity must be 18 or more centimetersthick.

308

The pH of soils is closely related to the kinds ofexchangeable cations. Extremely weathered soilswith very low ECEC have a small capacity to de-velop low pH values even when nearly 100 per-cent aluminum saturated and tend to have a pHthat is greater than that for intensively weatheredsoils. Even though extremely weathered soils maybe essentially 100 percent aluminum saturated,there may be insufficient A1 3+ in solution to pro-duce aluminum toxicity. Such soils have verysmall amounts of exchangeable calcium andmagnesium. Lime is used in very small quantitiesto supply calcium and magnesium with minimaleffect on soil pH.

Horizons in extremely weathered soils that con-tain very little organic matter sometimes have anet charge of zero or a net positive charge. Soilswith a horizon 18 or more centimeters thick thathas a net charge of zero or a net positive chargeare considered anionic. Thus, an Acrudox with ananionic horizon is an Anionic Acrudox.

Plinthite or LateriteA soil horizon associated with the tropics andmany Oxisols, and also with Ultisols and Alfisols,is plinthite. It is an iron-rich mixture containingquartz and other dilutents that remains soft aslong as it is not dried. On exposure at the soilsurface, due to erosion, the material dries andhardens irreversibly with repeated wetting anddrying. The hard, rocklike material is hardenedplinthite and is commonly called ironstone orlaterite.

Plinthite can, perhaps, form from the concen-tration of iron in situ, but it more likely forms viathe influx of iron in moving groundwater. Plinthiteformation is characteristically associated with afluctuating water table in areas having a short dryseason. Reduced iron (ferrous), produced inwater-saturated soil, is much more mobile thanoxidized iron (ferric) and moves in water andprecipitates on contact with a good supply of oxy-gen. Plinthite normally forms as a subsurfacelayer that is saturated at some season, but it can


also form at the base of slopes where water seepsout at the soil surface. Plinthite is soft to the extentthat it can be cut with a spade and readily mined.On exposure to the sun, the material hardens to abricklike substance, which is used as a buildingmaterial (see Figure 18.21).

There is great diversity in plinthite, rangingfrom continuous thick layers to thin, discontinu-ous nodular forms. Small amounts of nodularplinthite or deeply buried, continuous layers be-low the root zone have little influence on plantgrowth. Where continuous plinthite is exposed byerosion, drying and hardening can seriously limitplant growth. As long as the layer remains moistand soft, and is below the root zone, soils can beeffectively used for a wide variety of crops. Insome places holes are dug in hardened plinthiteand then filled with soil to grow high-value crops.It is estimated that hardened plinthite, ironstoneor laterite, occurs on 2 percent of the land intropical America, 5 percent in Brazil, 7 percent inthe tropical part of the Indian subcontinent, and15 percent for Sub-Saharan West Africa.

SPODOSOLSSpodosols contain spodic horizons in whichamorphous illuvial materials, including organicmatter and oxides of aluminum and iron, haveaccumulated. Quartzitic sand-textured parent ma-terial and a humid climate with intense leachingpromote development of spodic horizons. As aresult, Spodosols are widely distributed from thetropics to the tundra. Trees are the common vege-tation.

Spodosol SubordersIn the United States, about 86 percent of the Spo-dosols are Orthods and the remainder areAquods. Aquods have aquic moisture regime andare the dominant soils in the state of Florida,which has abundant sand-textured parent mate-rial, abundant rainfall, and shallow water tables

that are close to the soil surface at least part of theyear (see the S1a areas for Florida in Figure 18.1).The two major kinds of soils in Florida are bothvery sandy and naturally infertile-Aquods andPsamments. The warm climate, together with in-tensive soil management, makes these soils pro-ductive for winter vegetables, citrus crops, andyear-round pastures.

Orthods are the most common Spodosols; anOrthod is shown in Color Plate 5. They are abun-dant in the northeastern United States where theyhave developed in late Pleistocene sandy parentmaterials (see areas S2a in Figure 18.1). Theyoccur in association with Alfisols in the GreatLake states where the Spodosols have developedon sandy parent materials and the Alfisols havedeveloped on the loamy parent materials. The soilboundary line across lower Michigan and north-ern Wisconsin is, in essence, a parent materialboundary. The boundary separates an area withloamy Alfisols to the south from an area to thenorth dominated by sandy Spodosols. This pat-tern is repeated across northern Europe and Asia,as shown in Figure 18.2.

SPODOSOLS

309

FIGURE 18.21 Miningplinthite in Orissa state, India.On drying, the bricks hardenand are used for construction.

Suborders of minor extent include Humods andFerrods with a relatively high organic matter andiron content in the spodic horizons, respectively.

Spodosol Properties and Land UseSpodosols typically have a very low clay contentin all horizons; less than 5 percent for the soilhorizons cited in Table 18.4. Sola are quite acid,with a pH less than 5 being very common. There isa small cation exchange capacity, ranging from1.3 cmol/kg (meq/100 g) for the C horizon to 5.2cmol/kg (meq/100 g) for the Bhs horizon. Thecation exchange capacity of each horizon isclosely related to the amount of amorphous ma-terial-greater in the spodic horizon (Bhs) than inthe E or lower B and C horizons. There is a modestaluminum saturation of the cation exchange ca-pacity which, coupled with a low cation exchangecapacity, results in a very low quantity of ex-changeable calcium, magnesium, and potassium(see Table 18.4). These properties contribute tosoils that are too infertile for general agricultureand have a low water-holding capacity.

31 0

TABLE 18.4 Selected Properties of a Spodosol (Orthod)

° Trace amount. Selected data for pedon 25 in Soil Taxonomy, 1975.

The natural infertility and droughtiness of Spo-dosols has been cited as an example of a casewhere virgin soils are not necessarily fertile andsuitable for agriculture. The following quote isfrom an article that appeared in Time magazine in1954:

The virgin soil under a long-established forest is

not always good. When the settlers cleared New

England forests 300 years ago, the topsoil they

found was only 2 to 3 inches thick. Below this was

sterile subsoil and when the plow mixed the two

together, the blend was low in nearly everything a

good soil should have. It was not the lavish virgin

soil of popular fancy. Such a soil could not sup-

FIGURE 18.22 Percent of improved farmland inNew Hampshire from 1790 to 1970. (Courtesy StevenHamburg, Yale University.)


port extractive agriculture which takes nutrientsout of the soil and does not replace them. ManyNew England lands that were treated in this waysoon went back to forest.

I n 1870, the widespread conversion of forestland in New Hampshire peaked to about 75 per-cent improved farmland and then rapidly de-creased to only about 10 percent improved farm-land by 1930, as shown in Figure 18.22. Assettlement moved westward, the plow followedthe cutting of the forest and a similar land-usepattern also occurred on the Spodosols in theGreat Lake states. The first bulletin published bythe Michigan Agricultural Experiment Station in1885 was devoted to solving problems on thesand plains of north-central Michigan. The aban-donment of land for agricultural use where soilsare Spodosols is shown in Figure 18.23.

The cool summers of the northern Spodosolregion attract many tourists. Many cities in thisarea owe their existence to mining and lumber-i ng. Although agriculture is of minor importance,there are localized areas of intense fruit, vegeta-

ble, and dairy farming. This farming occurs ontwo kinds of soils. In one kind the agricultureoccurs on inclusions of Alfisols within the Spo-dosol region. These are situations where the par-ent materials are loam-textured and the Alfisolsare Boralfs. In the other kind, the soils have asandy upper Spodosol solum that overlies a hori-zon of increased clay content-a Bt (argillic) hor-izon. These are bisequum soils that contain the E

and Bhs horizons of a Spodosol that typicallyoverlie some E and the argillic horizon of an Al-fisol. These are the dominant soils in the well-known potato-growing area in Aroostook County,Maine and in adjoining New Brunswick, Canada.

ULTISOLS

The central concept of Ultisols is ultimately

leached soils of the mid- to low latitudes that havea kandic or an argillic horizon but few weathera-ble minerals. In parent materials with almost noweatherable minerals, a Ultisol may develop in afew thousand years. Perhaps, 100,000 years ormore are required if the Ultisol develops from theweathering and leaching of a former Alfisol. Ulti-sols are the dominant soils of the Piedmont andcoastal plain of the southeastern United Statesbeyond the area of glaciation and young parentmaterials. The large Ultisol region extends fromWashington, D.C., south and westward into east-ern Texas, as shown by the U3a areas in Figure18.1. Ultisols are the fourth most abundant soils inthe United States (see Table 18.1) and the fourthmost abundant soils in the tropics, following Oxi-sols, Aridisols, and Alfisols.

Ultisol SubordersAquults have an aquic moisture regime and occurextensively along the lower coastal plain of the

ULTISOLS

FIGURE 18.23 Thesedisintegrating buildings tellthe story of forest removal,farming, and thenabandonment of landdominated by Spodosols inthe Great Lake States.

southeastern United States. The water tables arenaturally close to the soil surface at some timeduring the year. Further inland, at higher eleva-tions on the middle and upper coastal plains, theUltisols are mainly Udults. Contrast areas U1a(Aquults) with U3a (Udults) in Figure 18.1. Hum-ults are found in the coastal ranges of northernCalifornia, Oregon, and Washington (area U2s inFigure 18.1). Humults have more than the usualamount of organic matter and develop under con-ditions of high precipitation and cool tempera-tures, associated with mountain slopes.

A large region of mostly Udult soils is found insoutheastern China at a latitude and continentalposition similar to that of the southeastern UnitedStates (see U3d in Figure 18.2). Many smallerareas of Ultisols occur in Africa and SouthAmerica, usually in association with Oxisols.

Properties of UltisolsA common horizon sequence for many Ultisols isA, E, Bt and C. Ultisols that develop in iron-richparent materials may have intense red colors andlack an obvious E horizon. A photograph of aUdult is shown in Color Plate 5. The symbol, v, asin Btv, indicates plinthite, which in this instance isnot continuous and has mixed red and yellowcolor (see the Ultisol in Color Plate 5).

In the United States, 76 percent of the Ultisolsare Udults. The horizons of Udults are quite acidto a great depth and they contain a very small

31 1

31 2

amount of exchangeable calcium, magnesium,potassium, and sodium. The very low amounts ofthese exchangeable cations reflect the low con-tent of weatherable minerals-less than 10 per-cent in the coarse silt and fine sand fractions.Leaching removes cations from the soil more rap-idly than they are released by weathering. Ultisolsare required to have less than 35 percent of thecation exchange capacity, which is determined atpH 8.2, saturated with calcium, magnesium, po-tassium, and sodium in the lower part of the rootzone (at a depth of about 125 centimeters belowthe upper boundary of the argillic horizon or 180centimeters below the soil surface). By contrast,the exchangeable aluminum is high and rootsmay grow poorly in subsoils because of an alumi-num toxicity. Many subsoils contain too little cal-cium for good root growth. Most of the plant nutri-ent cations are bound within the plants and/orsoil organic matter. Ultisols are naturally too infer-tile for the production of many agricultural cropsbecause of their acidity, low content of exchange-able basic cations, and the potential for alumi-num toxicity.

Land Use on UltisolsDuring the early settlement of United States, agri-culture on Ultisols in Virginia and the surroundingstates consisted mainly of tobacco and cottonproduction for the European market. Afterclearing the forest and cropping for only a fewyears, the naturally infertile soils became ex-hausted. Abandonment of land and a shifting typeof cultivation became widespread as the frontiermoved westward. This was the plight of early Vir-ginians. One farmer in particular, Edmund Ruffin,described the situation as follows:

Virginians by the thousands, seeing only a dismalfuture at home, emigrated to the newer states ofthe South and Middle West. The populationgrowth dropped from 38 percent in 1820 to lessthan 14 percent in 1830, and then to a mere twopercent in 1840. All wished to sell, none to buy.

So poor and exhausted were Ruffin's lands and


those of his neighbors that they averaged only 10bushels of corn per acre and even the better landsa mere 6 bushels of wheat. Ruffin happened toread Elements of Agricultural Chemistry by SirHumphrey Davy, from which he learned thatacidity was making the soils sterile, and limingwas needed to neutralize soil acidity. The applica-tion of marl to the land was very successful. Now,it is known that the marl (or liming) added cal-cium and magnesium and reduced aluminumtoxicity. Liming and the increase in soil pH madethe application of manure and use of legumes inrotations more successful. For this discovery, Ruf-fin was credited with bringing about an agricul-tural revival in the South. With the long, year-round growing season and abundant rainfall, thisregion that once proved so troublesome is nowone of the most productive agricultural and for-estry regions in the United States (see Figure18.24).

Land use on Ultisols in the tropics is quite simi-l ar to land use on nearby Oxisols. S1ash-and-burnand subsistence agriculture are common. Manyplantations or estates produce tea, rubber, bana-nas, tobacco, and other valuable crops.

VERTISOLSVertisols are mineral soils that are more than 50centimeters thick, contain 30 percent or more clayin all horizons, and have cracks at least 1 cen-timeter wide to a depth of 50 centimeters, unlessirrigated at some time, in most years. The typi-cal vegetation in natural areas is grass or herba-ceous annuals, although some Vertisols sup-port drought-tolerant woody plants.

Worldwide, and in United States, Vertisols arethe ninth most abundant soils. The three largestareas are in Australia (70 million acres), India (60million acres), and the Sudan (40 million acres).These soils are mainly Usterts, and are shown asareas of V2a and V2c in Figure 18.2.

Vertisols in the United States have formedmainly in marine clay sediments on the coastalplains of Texas, Alabama, and Mississippi. Very

sharp boundaries between soils of different or-ders may occur in the area because Vertisols de-veloped on clay sediments, whereas Ultisols de-veloped on adjacent sandy sediments.

Vertisol SubordersApproximately 40 percent of the Vertisols inUnited States are Uderts and 60 percent areUsterts. Usterts are sufficiently dry so that cracksremain open more than 90 cumulative days in ayear. There are two areas in Texas where Ustertsare the dominant soils (areas V2a in Figure 18.1).Uderts have a wetter climate than do Usterts andhave cracks that are open 90 or fewer cumulativedays in a year. A large area of Uderts occurs alongthe Gulf Coast of Texas (V1a in Figure 18.1). Asmaller area occurs along the border of Alabamaand Mississippi. Torrerts and Xererts occur to alesser extent.

Vertisol GenesisConditions that give rise to the formation of Verti-sols are: (1) parent materials high in, or that

VERTISOLS 31 3

FIGURE 18.24 Udults on thecoastal plain in thesoutheastern United Stateshave nearly level slopes andsandy surface soils that areeasily tilled.

weather to form, a large amount of expandingclay, usually smectites; and (2) a climate withalternating wet and dry seasons. After the cracksdevelop in the dry season, surface soil materialand material along the crack faces, fall to thebottom of the cracks. The soil rewets at the begin-ning of the rainy season from water that quicklyruns into and to the bottom of the cracks, whichcauses the soil to wet from the bottom up duringthe early part of the wet season. After the cracks atthe surface close, the soil wets normally from thesurface downward. This results in a dry soil layersandwiched between two layers that are beingwetted and undergoing expansion. Thus, there isconsiderable moving and sliding of adjacent soilmasses and the formation of shiny ped surfaces-slickensides. A large slickenside is shown nearthe AC label of the Vertisol in Color Plate 5.

Expansion or swelling of soil in the vicinity ofcracks results in great lateral and upward pres-sures, which cause a slow gradual movement ofsoil upward between areas that crack. This soilmovement produces a microrelief called gilgai(see Figure 18.25). The major features of Vertisolsare illustrated in Figure 18.26.

31 4

Vertisol PropertiesThe high content of swelling clay and movementof soil by expansion and contraction retard thedevelopment of B horizons. Typically, the soilremains quite undeveloped and often has an A-Cprofile. Some properties of the Houston Blackclay from the Blackland Prairie region of Texas arepresented in Table 18.5. There is a high and con-sistent clay content in each horizon. Organic mat-ter content is typical for soils with high clay con-

Slickensides


FIGURE 18.25 Gilgaimicrorelief on Vertisols inTexas. (Photograph SoilConservation Service, USDA.)

tent, but the organic matter decreases graduallywith increasing soil depth. This pattern of organicmatter content is similar to that which occurs inmany soils; in Vertisols, the change in organicmatter content may be associated with no changein color (see Vertisol in Color Plate 5). This Verti-sol contains some CaCO 3 and has a pH of about8.3 in all horizons. Some Vertisols, Uderts, aresufficiently leached to be acid in some horizons.

The soil has high fertility for many agricultural

!j FIGURE 18.26 Schematicdrawing showing a soil profile ofVertisols. Major features includedeep cracks, parallelepipedstructure (rhombohedralaggregates), slickensides, andgilgai microrelief basin. (Adaptedfrom Dudal and Eswaran, 1988.)

VERTISOLS

crops and the soils can retain a large amount ofwater. Conversely, when the soils are wet the in-filtration of water is essentially zero, which resultsin excessive water runoff and soil erosion on slop-ing cultivated land.

Land Use on VertisolsIt is difficult for rainwater to move through Verti-sols and contribute to a groundwater aquifer.Those who settled on Vertisols had difficultydrilling wells and obtaining water. For this reason,and because many Vertisols occur in regions oflimited rainfall where moisture regimes are ustic,many Vertisols have been minimally or only mod-erately leached. Thus, Vertisols are consideredfertile for many agricultural crops (see Figure18.27).

Most soil management problems are the resultsof the high content of expanding clay and accom-panying physical properties. In Texas, the Verti-sols were referred to as "dinner-pail-land," beingtoo wet and sticky to till before the noontime meal(dinner) and too dry and hard to till after thenoontime meal.

Farmers in India found the Usterts too hard totill with their small plows and oxen at the end of along dry season. They were unable to plant cropsbefore the monsoon rains started, which meantthat crops were not produced during the seasonwhen water is most available. Then, crops wereplanted after the moonsoon rains stopped, and

Table is based on data of Kunze and Templin, 1956. Average of 5 profiles.

315

the crops depended primarily on stored soil wa-ter. Some creative research done at the Interna-tional Crops Research Institute for the SemiaridTropics (ICRISAT) in Hyderabad, India, resultedi n recommendations about some different tillagemethods. Tillage was to be done immediatelyafter harvest of the winter crops before the soilsbecame too dry to till. Some occasional tillageduring the dry season was also recommended.Then, farmers could establish the crop in loose,dry soil before the monsoon rains began. The

FIGURE 18.27 Post-monsoon crop of cotton onUsterts in India. Cotton is a major crop, which is whythe Usterts are called Black-Cotton soils. Note themany slickensides surfaces and black color to adepth of more than a meter.

31 6

result was high-yielding crops during the mon-soon season (in addition to the post-monsooncrops), and more than 100 percent more totalproduction per year as compared with the tradi-tional system.

Open cracks are a hazard for grazing animalsduring dry seasons. The expansion and contrac-tion of soil cause a misalignment of fence andtelephone posts, breaks pipelines, and destroysroad and building foundations as shown in Figure18.28.

SUMMARYHistosols are soils formed from organic soil mate-rials. Typically, they are water saturated for part ofthe year. Drainage is required for cropping, butsubsequently, decomposition of the organic mat-ter results in a gradual disappearence of the soiland subsidence of the soil surface.

Andisols, Spodosols and Vertisols owe theirexistence, largely to the parent material factor ofsoil formation. Andisols develop in volcanic


FIGURE 18.28 Structuralfailure caused by landslide onVertisols produced by wet soilsliding downhill. Note thatpipes are on top of the groundto prevent breaking caused byexpansion and contraction ofsoil.

ejecta. Spodosols develop from sand parent mate-rial in humid regions and are droughty, acid, andi nfertile. Vertisols develop in clay materials thatexpand greatly with wetting and shrink with dry-ing. They are frequently minimally weathered andleached, and are fertile soils for cropping. Verti-sols have unique use problems associated withtheir clayey nature.

Aridisols, Entisols, and Inceptisols typically areminimally weathered soils that tend to be rich inprimary minerals and to be fertile for cropping.Some Inceptisols in tropical regions, however,have developed in intensively weathered parentmaterial and are acid and infertile.

Moderately weathered and generally fertilesoils include Alfisols and Mollisols. These soilsare common in the temperate regions and aregenerally good for agricultural use.

Ultisols and Oxisols are intensively weatheredsoils. They are generally acid and infertile, andaluminum is toxic for many plants. Many are usedfor slash-and-burn agriculture and subsistencefarming. Intensive management can result in pro-ductive soils with high crop yields.

REFERENCES

Buol, S. W. 1966. "Soils of Arizona."Ariz. Agr. Exp. Sta.Tech. Bull. 117, Tuscon.

Dudal, R. and H. Eswaran. 1988. "Distribution, Proper-ties, and Classification of Vertisols," in Vertisols:Their Distribution, Properties, Classification, andManagement. L. P. Wilding and R. Puentes (eds.)Texas A & M University Printing Center, CollegeStation.

Foth, H. D. and J. W. Schafer. 1980. Soil Geography andLand Use. Wiley, New York.

Kanwar, J. S., J. Kampen, and S. M. Virmani. 1982."Management of Vertisols for Maximizing Crop Pro-duction-ICRISAT Experience." Vertisols and RiceSoils of The Tropics, Symposia Papers f/. pp. 94-118.12th Int. Congress Soil Sci., New Delhi.

Kedzie, R. C. 1885. "Early Amber Cane as a Forage

Crop." Agr. College of Michigan Bull. 1. East Lan-

sing.

REFERENCES 31 7

Kellogg. C. E. 1963. "Shifting Cultivation." Soil Sci.95:221-230.

Kunze, G. W. and E. H. Templin. 1956. "Houston BlackClay, the Type Grumusol: II Mineralogical and Chem-ical Characterization." Soil Sci. Soc. Am. Proc. 20:91-96.

Ruffin, Edmund. 1961. An Essay on Calcareous Ma-nures. Cambridge, Mass. (Reprint of original bookthat was published in 1832).

Sanchez, P. A. 1976. Properties and Management ofSoils in the Tropics. Wiley, New York.

Soil Survey Staff. 1975. "Soil Taxonomy." USDA Agr.Handbook 436. Washington, D.C.

Soil Survey Staff. 1987. "Keys to Soil Taxonomy." SoilManagement Support Services Tech. Monograph 6.Cornell University, Ithaca, New York.

Swanson, C. L. W. 1954. "The Road to Fertility." Time,January 18.

Tuan, Yi-Fu. 1969. China. Aldine, Chicago.Young, A. 1976. Tropical Soils and Soil Survey. Cam-

bridge, London.

A soil survey is the systematic examination, de-scription, classification, and mapping of soils inan area. The kinds of soil in the survey area areidentified and their extent is shown on a map. Thesoil map, together with an accompanying reportthat describes, defines, classifies, and makes in-terpretations for various uses of the soils, is pub-lished as a Soil Survey Report.

MAKING A SOIL SURVEYA soil survey requires making soil maps of thearea and writing an informational and interpreta-tive report. In many states, a soil survey is a coop-erative activity between the U. S. Department ofAgriculture and state Agricultural ExperimentStations. The size of the area surveyed and thei ntensity of mapping depend on the amount ofdetail required to make the necessary land-useinterpretations.

Making a Soil MapSoil maps are made by scientists who are familiarwith the local soil-forming factors, an understand-ing of soil genesis, and the ability to identify the

CHAPTER 19

SOIL SURVEYS AND LANDUSE INTERPRETATIONS

318

soils. An aerial photograph is used as the mapbase. The soil surveyor walks over the land andsimultaneously studies the aerial photograph andthe landscape. This includes observations ofground cover or vegetation, slope, and landforms.In a glacial landscape, outwash plains will have anearly level surface and typically contain consid-erable sand and gravel. Lake beds are nearly levelwith fine-textured soils. Ground moraines have anirregular surface with much variation in slope andtexture. These three different landforms will likelyhave different plant species in the native vegeta-tion or, if in a cultivated field, have growth differ-ences of crop plants. These three different situa-tions (outwash plain, lacustrine plain, groundmoraine)in a field will result in at least three dif-ferent kinds of soil. At selected locations, the soilsurveyor uses a soil auger to bore a hole andobserve the sequential horizons of the soil profileas the bore hole samples of horizons are extracted(see Figure 19.1).

Observations of each horizon include: thick-ness, color, texture, structure, pH, presence orabsence of carbonates, and so on. In addition, theslope and degree of erosion are noted. This infor-mation is then recorded directly on the aerial pho-tograph, and lines are drawn to separate different

FIGURE 19.1 A soil surveyor indentifies the soil byi nspecting the soil horizons. Lines are then drawnaround areas of similar soil on an aerial photographto produce a soil map. (Photograph courtesy USDA.)

soil areas (mapping units) to create the soil map.Other information is also recorded, such as loca-tion of gravel pits, farmsteads, cemeteries, dams,and lakes.

Areas of different kinds of soil typically appearon the aerial photograph as areas with differentintensity of black and white, as shown in Figure19.2. A large black area of Histosols is sand-wiched between two light-colored areas of Al-fisols in a recently cultivated field. A soil surveyorstanding at point X in Figure 19.2 would see thel andscape in late summer after grain harvest, asshown in Figure 19.3. It is obvious in Figure 19.3that at point X (Figures 19.2 and 19.3) the soil ison a slope. The soils near X are well-drainedmineral soils that have been subjected to erosion.These soils have a grayish-brown surface colorand appear as a light-colored area on the aerialphotograph. These soils are Alfisols. The slopeends in a depression in which the soils are verypoorly drained. These black-colored soils are or-

MAKING A SOIL SURVEY 31 9

FIGURE 19.2 Aerial photograph of a field in InghamCounty, Michigan, where soils are Alfisols andHistosols. Standing at point X and looking north aperson would observe a black area of Histosolssandwiched between areas of light-colored soils thatare Alfisols.

ganic soils (Histosols) and appear as a dark areaon the aerial photograph. Beyond the depression,the soils are again well-drained Alfisols on a slopesimilar to that at point X. From observations of theaerial photograph and landscape, the soil sur-veyor constructs a mental image of what kind ofsoil is located in each part of the landscape. Then,at selected locations, soil borings are made toverify and identify the soil. The result is that thesoil surveyor does not randomly bore holes andmake observations, but instead uses the auger toconfirm the hypotheses. This results in the boringof holes at strategic locations and the construc-tion of the soil map with minimum physical input.

The soil map made for the recently cultivatedfield (without a vegetative cover in Figure 19.2), isshown in Figure 19.4. The symbols on the maprefer to the mapping units. Typically, the mappingunit contains three symbols such as 3B1-3 forsoil, B for slope, and I for erosion. In this case itmeans: 3 for Marlette sandy loam, B for a 2- to6-percent slope, and 1 for slightly eroded. Somesoils are always located on level or nearly levelareas that are not eroded, and the slope and ero-

320

1A Capac loam, 0-3 percent slopes2

Colwood-Brookston loams3B1 Marlette sandy loam, 2-6 percent slopes4

Palms muck5B Spinks lomy sand, 0-6 percent slopes6A Urban land, Capac-Colwood complex, 0-4 percent slopes

SOIL SURVEYS AND LAND-USE INTERPRETATIONS

FIGURE 19.3 On-the-groundview of the landscape from X ofFigure 19.2 and looking north-northwest. The soil differencesshown on the aerial photographare also apparent after wheatharvest. The dark-colored areaof Histosols in the depressionarea can be seen to besandwiched between the Alfisolareas that are sloping.

sion notations are omitted. The mapping symbolsfor the soil map in Figure 19.4 are as follows:

IA

Capac loam, 0 to 3 percent slopes2

Colwood-Brookston loam3B1

Marlette sandy loam, 2- to 6-percentslope, slight erosion

4

Palms muck5B

Spinks loamy sand, 0- to 6-percent slope6A

Urban land, Capac-Colwood complex, 0-to 4-percent slope

The mapping symbol at point X in Figures 19.2and 19.3-3B1-is for the Marlette soil, which isa Udalf. In the center of the depressional area, thesoil is Palms muck (4)-a Saprist. These two soilshave very different properties, including slope,erosion, and drainage characteristics and woulddemand very different management regardless ofwhether they are used for agriculture, forestry, orfor a home-building site.

Writing the Soil Survey ReportTypically, soil survey reports are for a county,parish, or another governmental unit. The soil

maps usually form the latter part of the report.Anyone desiring information on a specific parcelof land can refer to the appropriate soil map sheetand determine the kinds of soils. The first part ofthe report is written and consists of informationabout the genesis and classification of the soils,descriptions and properties of each soil, and in-terpretative tables for various soil uses.

Using the Soil Survey ReportI magine that you are driving down the road alongthe eastern side of the cultivated field shown inFigure 14.2, because you are looking for a site onwhich to build a house in the country. Would yoube able to find a suitable parcel of land in thatfield to use as a home site? How could the soilsurvey report help you in making your decision?

To find the answers to these questions youwould need the Soil Survey Report for InghamCounty, Michigan. First, locate in the soil surveyreport the appropriate soil map in the back sec-tion and identify the soils within the parcel of landthat interests you. Then, refer to the descriptions

TABLE 19.1 L and-Use Intepretations

MAKING A SOIL SURVEY 321

of the soils for the mapping units in the frontportion of the report to learn about the generalfeatures of these soils. This includes the kinds ofhorizons and their properties and the conditionsunder which the soils developed. For example,the 4 mapping unit includes Palms muck, whichconsists of very poorly drained organic soil about36 inches thick that overlies moderately coarse tofine-textured deposits.

Factors that are important for building sites in-clude natural drainage, depth to water table,septic-field suitability (where municipal sewagedisposal is not available), supporting ability,shrink-swell potential, slope, and lawn and land-scaping suitability. Some hazards and suitabilitiesof the various mapping units, relative to construct-ing a building with a basement, are given in Table19.1. The Palms soils (4) have severe problemsrelated to flooding and wetness. The water table isat or within a foot of the soil surface from Novem-ber to May. These conditions make the use of aseptic filter field for sewage effluent disposal im-possible because of the high water table. A dry

322

basement would be a virtual impossibility.Driveways and sidewalks, without footings or sup-port on the mineral soil under the muck, wouldsubside as the muck dries out and decomposes.Palms soil is obviously poorly suited for a build-ing site.

From Table 19.1, the soil with the best overallsuitability is 5B (Spinks loamy sand). It is a well-drained and moderately to rapidly permeable soil.The water table is below 6 feet throughout theyear, and the soil has sufficient water permeabilityfor the septic-filter field and only moderate limita-tions for lawn and landscaping. Spinks soilswould also have a low shrink-swell potential.

A precaution is in order. Soils do not occur asuniform bodies. Mapping units typically contain15 percent or more inclusions. That is, an arealabeled 5B on a soil map could contain a signifi-cant amount of similar but different soils. Thus, toestablish whether or not a septic-sewage disposalsystem will operate successfully in a particularsoil mapping area, a percolation test is needed forthat small area within the mapping unit where thefilter field will be located.

SOIL SURVEY INTERPRETATIONS ANDLAND USE PLANNINGWhen a soil survey is made, there are specificneeds to be fullfilled by the survey. The writers ofthe soil survey report include many tables of soilproperties and characteristics, together with ta-bles that contain predictions of soil behavior formany uses. We have used this type of soil surveyinformation in regard to selection of a buildingsite for a house in the country. Estimates of cropyields and rates of forest growth are useful forprospective land buyers and land tax assessors.Highway planners need information about flood-i ng and wetness conditions and the ability of soilsto support roads. Landfills for waste disposal re-quire underlying layers that are impermeable towater (see Figure 19.5). Thus, land-use interpreta-tion maps are needed to satisfy the requirementsof various land users.

SOIL SURVEYS AND LAND-USE INTERPRETATIONS

FIGURE 19.5 Sites selected for solid waste disposal(landfills) need water-impermeable underlying stratato minimize groundwater pollution.

Examples of InterpretativeLand-Use MapsAll soils that are predicted to perform similarly fora particular use can be grouped together, and aninterpretative land-use map can be constructed.

FIGURE 19.6 Soil suitablility map for buildings withbasements of the recently cultivated field shown inFigure 19.2 (also in Figure 19.4).

SOIL SURVEY INTERPRETATIONS AND LAND-USE PLANNING

The suitability map for construction of buildingswith basements in Figure 19.6 is based on the soilmap of Figure 19.4 and the interpretations given inTable 19.1.

Soils directly affect the kind and amount ofvegetation available to wildlife as food and cover,and the soils affect the construction of water im-poundments. The kind and abundance of wildlifethat populate an area depend largely on theamount and distribution of food, cover, and wa-ter. Figure 19.7 shows a soil map and an accom-panying suitability map for the production of grainand seed crops for wildlife use. The requirementsfor grain and seed crops are much like those ofmost agricultural crops. Similar maps could beconstructed for suitability of land for wetland hab-itat for wildfowl, open-land habitat for pheasantand bobwhite quail, or woodland habitat forruffed grouse, squirrel, or deer.

Land Capability Class MapsThe Soil Conservation Service of the U.S. Depart-ment of Agriculture developed eight land-use ca-pability classes based on limitations due to ero-sion, wetness, rooting zone, and climate. Thelimitations and restrictions for these land use ca-pability classes are given in Table 19.2. Classes Ithrough IV apply to land suited for agriculture andclasses V through VIII to land generally not suitedto cultivation. Class I land has few use restrictions

323

FIGURE 19.7 Soil survey map onthe left was used to construct awildlife suitability map on the rightfor a 40-acre tract of land. (Basedon information from Allan, et al.,1963.)

in terms of erosion, wetness, rooting depth, andclimate. Class VIII land, by contrast, precludesnormal agricultural production and restricts useto recreation, wildlife, water supply, or estheticpurposes. A land-use capability class map for a200-acre dairy farm is Wisconsin in shown in Fig-ure 19.8.

Computers and SoilSurvey InterpretationsOne of the major problems with the use of SoilSurvey Reports is the large amount of availablei nformation and the difficulty of locating the spe-cific material needed by a particular user. In Min-nesota the most frequent nonfarm users wantedsoil survey information to determine land valuesfor purchase, sale, rent, or as collateral for loansby financial institutions. Other nonfarm uses weremade by local government officials, zoning ad-ministrators, sanitarians, and tax assessors. Manypersons make frequent use of the Soil Survey Re-port. To increase the usefulness and the speedwith which information can be extracted from SoilSurvey Reports, computers are being used to de-velop digitized records (files) of soil maps andsoil data bases. Computers help to solve the prob-lem of locating information by locating only theparticular part of the record that the user needs.Another big advantage of computerized soil sur-vey informtion is that the soils' data bases can be

324 SOIL SURVEYS AND LAND-USE I NTERPRETATIONS

TABLE 19.2 Land Capabilty Classes

Land Suited to Cultivation and Other UsesClass 1.

Soils have few limitations that restricttheir uses.

Class Il.

Soils have some limitations that reducethe choice of plants or requiremoderate conservation practices.

Class IIl.

Soils have severe limitations thatreduce the choice of plants, requirespecial conservation practices, orboth.

Class IV. Soils have very severe limitations thatrestrict the choice of plants, requirevery careful management, or both.

Land Limited in Use-Generally NotSuited for Cultivation

Class V.

Soils have little or no erosion hazard,but have other limitations impracticalto remove that limit their use largelyto pasture, range, woodland, orwildlife food and cover.

Class VI. Soils have severe limitations that makethem generally unsuited to cultivationand limit their use largely to pastureor range, woodland, or wildlife foodand cover.

Class VII. Soils have very severe limitations thatmake them unsuited to cultivationand that restrict their use largely tograzing, woodland, or wildlife.

Class VIII.

Soils and landforms have limitationsthat preclude their use forcommercial plant production andrestrict their use to recreation,wildlife, or water supply, or toesthetic purposes.

combined with systems to produce a GeographicInformation System (GIS). Within these computerfiles information is stored, such as land own-ership, elevation-topographic information, vege-tation, and land use. Systems are being developedcooperatively with university and government per-sonnel so that natural resources (soil, water, wet-land, etc.) can be better protected through im-proved land-use planning.

FIGURE 19.8 Land capability map for a 200-acredairy farm in Wisconsin. Roman numerals designatel and capability classes; other symbols indicate landcharacteristics. For example, 30E37, 30 designatestype of soil, E indicates slope, and 37 means thatmore than 75 percent of the topsoil has been lostand that there are occasional gullies. (CourtesyUSDA, Soil conservation Service.)

SOIL SURVEYS ANDAGROTECHNOLGY TRANSFER

Field experiments are conducted to test hypothe-ses for increasing the amount and efficiency ofagricultural production. Suppose an experimentindicated that a 3-ton application of lime was themost profitable rate of application for productionof alfalfa in a particular crop rotation system. The

question that arises is: Where can this informa-tion be used or applied? If the experiment wasconducted in the state of Arkansas, where in Ar-kansas are the results valid? Can the experimentalresults be used outside of Arkansas, and if so,where?

The transfer of this kind of information is validonly for locations with very similar soils, climate,and soil management practices. Thus, it is veryi mportant to (1) establish field experiments onthe kinds of soils or mapping units for whichi nterpretations or recommendations are needed,and (2) restrict the recommendations or pre-dictions to those soils or very similar soils andexperimental conditions. Because experimentscannot be conducted for every mapping unit orsoil, it becomes necessary to extrapolate and in-terpret the information for situations other thanthose represented by the experiments.

SUMMARYA soil survey is a systematic examination andmapping of the soils of an area. The soil map,together with information about the soils andl and-use interpretations, is published as a SoilSurvey Report.

Soil Survey Reports are used by a wide variety of

REFERENCES

persons, including farmers, governmental offi-cals, land appraisers and tax assessors, land-useand zoning personnel, and ordinary citizens.

Soil survey information and interpretations arebeing computerized to facilitate access by usersand to allow for better planning by various govern-mental agencies.

Because research information can only be ob-tained on a limited number of soils, soils that areexpected to perform similarly must be groupedtogether for making interpretations.

REFERENCESAllan, P. F., L. E. Garland, and R. F. Dugan. 1963. "Rat-

ing Northeastern Soils for Their Suitability for WildlifeHabitat." Trans. 28th Nor. Am. Wildlife and Nat. Res.Con.

Anderson, J. 1981. "A News Letter for Minnesota Coop-erative Soil Survey." Soil Survey Scene 3, (4):1-3.

University of Minnesota, St. Paul.Bartelli, L. J., A. A. Klingbiel, J. V. Baird, and M.K.R.

Heddleson. 1966. Soil Surveys and Land Use Plan-

ning. Soil Sci. Soc. Am., Madison, Wis.Soil Conservation Service, 1961. Land-Capability Classi-

fication. USDA, Washington, D.C.Soil Conservation Service. 1981. Soil Survey Manual.

430 V-SSM, USDA, Washington, D.C.

325

Since the beginning of time, increased food pro-duction was mainly the result of using more land.Now, new cropland supplies are diminishing asthe human population continues to increase. Thischapter considers the adequacy of the world'sland, or soil, resources for the future.

POPULATION AND FOOD TRENDSHuman population growth and food supply areancient problems. For several million years, hu-mans migrated onto new lands and slowly im-proved their hunting and gathering techniques.The population increased slowly; about 10,000years ago there were an estimated 5 million peo-ple in the world. At this time, people had justbegun to colonize the last remaining continents,the Americas, by crossing over the land bridgebetween Asia and Alaska during the last ice age.

Development of AgricultureSettled agriculture was just getting started in theFertile Crescent of the Middle East when the mi-gration to the Americas began. Agriculture mayhave developed at the same time in other isolated

CHAPTER 20

LAND AND THE WORLDFOOD SUPPLY

326

places in the world. It is uncertain why early peo-ple gave up the more leisurely hunting and gather-ing activities and took up a more rigorous way ofliving to produce crops and animals. Develop-ment of agriculture resulted in both an increasedfood supply and a population explosion. Afterreaching a population of 5 million over severalmillion years, the human population increased 16times to 86 million within the next 4,000 years(10,000 to 6,000 years before present). Populationgrowth tended to level off, and the world popula-tion increased only six to seven times to 545 mil-lion over the next 6,000 years. This was followedby the Industrial Revolution that began about1650.

The Industrial RevolutionThe Industrial Revolution brought energy and ma-chines into the food production process, resultingin an increased food supply. However, a new di-mension was added to population growth-a re-duced death rate because of the development ofmedicine and public sanitation. Now, the worldpopulation increases annually at the rate of 1.7percent and doubles every 40 years. The mostrapid rate of increase is in Africa, where the an-

nual increase is 2.9 percent, with a doubling every24 years. Western Europe has a growth rate of 0.2percent and a population doubling about every400 years. The slowest population growth is inEurope, where some countries have negative pop-ulation growth rates.

Estimates of world population exceeding 6 bil-lion by the year 2000 will require about 40 percentmore food just to maintain the status quo. Con-sidering that more than 500 million people arecurrently suffering from malnutrition, and in-creased food consumption rates are occurring inthe richest parts of the world, it appears that foodneeds, shortly beyond the year 2000, will be dou-ble today's needs. It is predicted that there will bea world population of 12 billion or more by thetwenty-second century.

POPULATION AND FOOD TRENDS

Recent Trends in Food ProductionAs recently as the period from 1950 to 1975, therewas a 98 percent increase in world cereal produc-tion and a 22 percent increase in cropland. Inrecent decades, during a period of rapid popula-tion growth, world food production was able tokeep pace with population growth. In fact, the rateof food production has exceeded the populationgrowth rate. For the period from 1971 to 1980,both the developing and developed countries in-creased food production faster than population,as shown in Figure 20.1. For Africa, however, pop-ulation increased faster than food production,which caused a 14 percent decrease in per capitafood production during the 1970's.

There has been a declining per capita grainproduction trend for the past 20 years in Africa

327

328

and in the Andean countries of Bolivia, Chile,Ecuador, and Peru. Although total world food pro-duction is increasing, the rate of increase is slow-ing. Since the late 1970s, there appears to be adeclining trend in world per capita food produc-tion and, now, 50 to 60 nations have declining percapita food production, as shown in Figure 20.2.

Recent Trends in Per Capita CroplandExpansion onto new lands has been the tradi-tional means to increase the food supply. Onereason for the present declining world per capitafood production is the slower rate of bringing newland into production. The rate of cropland in-crease has declined from an annual increase of1.0 percent in the 1950s to 0.3 percent in the1970s, and is expected to be 0.15 percent in the1990s. New settlement projects are underway in

LAND AND THE WORLD FOOD SUPPLY

the Amazon Basin and in the central uplands ofBrazil and in the outer islands of Indonesia. Con-versely, the area of cropland peaked in the 1950sand 1960s in many Western nations, with declinesfrom a maximum in 1980 of 29.4 percent for Ire-land, 21 percent for Sweden, and 19.6 percent forJapan.

About 2 billion people will be added to theworld in another two or three decades, whichmeans that a great deal of land will be needed fornonagricultural use. When new families areformed in many developing nations, houses arebuilt on cropland with fields immediately adja-cent to them. The use of local building materials(such as bamboo) make multistoried housing im-practical. Much land will also be used for roads,strip mining, and transportation facilities, asshown in Figure 20.3.

At the 1978 International Soil Science Congress

held in Edmonton, Canada, concern was ex-pressed about Canada's ability to remain self-sufficient in food supply by the year 2000 andbeyond. A population growth of 20 to 45 percent isexpected, and cities continue to expand ontoprime farmland. The amount of land for cerealgrains per capita was predicted to decline from0.184 hectares in 1978 to 0.128 hectares by 2000.Only three of Canada's Prairie provinces are sur-plus food producers. Only 10 percent of the foodproduced is now exported.

Land requires considerable inputs to remainproductive, and it is subject to degradation byerosion, salinity, water-logging, and deserti-fication. A recent United Nations report estimatedthat 10 percent of the world's irrigated land hashad a significant decline in productivity due towater-logging, and another 10 percent due to ani ncrease in salinity. In some irrigated regions,geologic groundwater will be exhausted. In othercases, the high cost of pumping water and compe-tition with urban water users will result in conver-sion of irrigated land to range land. Deserti-fication is a problem, especially in subSaharanAfrica. As the need for land grows, farmers are

POTENTIALLY AVAILABLE LAND AND SOIL RESOURCES 329

FIGURE 20.3 Farmland inthe United States is convertedto urban use at the annualrate of about 400,000 hectares(1 million acres).

forced onto lands with greater production hazardsand limitations as shown in Figure 20.4.

Summary StatementThe past few decades have brought an end to thetraditional expansion onto new lands as a majormeans to increase food production. For the fore-seeable future, it appears that an expanding worldpopulation will have to produce its food on adeclining amount of land per person.

POTENTIALLY AVAILABLE LAND ANDSOIL RESOURCESAbout 65 percent of the ice-free land has a climatesuitable for some cropping. Only 10 or 11 percentof the world's land is cultivated, which suggests aconsiderable possibility for increasing the world'scropland.

World's Potential Arable LandThe distribution of the world's cropland is shownin Figure 20.5. The five countries with the most

FIGURE 20.4 Farmers arepushed higher and higher onslopes in the Himalayanfoothills due to populationgrowth. Many such lands areused only a few years before itis no longer profitable to cropthe land due to severe soilerosion and landslides.

cropland, in decreasing order, are: USSR, UnitedStates, India, China, and Canada. The croplandtends to exist either north or south of a largecentral band where there is only a small percent-age of cropland, in northern South America andAfrica and through central Asia. Much of thecropland in the USSR, United States, and Canadais in areas where the dominant soils are Mollisols.Few new cropland areas are available in westernEurope and India, as can be seen by examiningFigure 20.5.

Many estimates of potential cropland, or arablel and, have been made. Several well-documentedstudies concluded that the present cultivated landarea could at least be doubled. The data from oneof the studies, (see Table 20.1) show the poten-tially arable land and currently cultivated land bycontinents. The data support the following con-clusions:

1. The world's arable land can be increased 100percent or more.

2. Of the potentially arable land that can be de-veloped, 66 percent is located in Africa andSouth America.

The 66 percent potentially arable land that canbe developed is located mainly in tropical and

POTENTIALLY AVAILABLE LAND AND SOIL RESOURCES 331

subtropical areas of Africa and South America.China and India are the two largest and mostpopulated countries in Asia. At present in Asia,there is only 0.7 acre of cultivated land per person,and little additional unused and potentially arableland exists. India is unique as a large country,since it has one of the the greatest agriculturalpotentials of any country, because of its soils andclimate. This is reflected in the fact that about 50percent of India's land is currently cultivated, ascompared to only 10 percent for the world and 20

percent for the United States.The developing countries, with 77 percent of

the world's population, have 54 percent of thecurrent cultivated land area and 78 percent of theundeveloped, potentially arable land. By contrast,the developed countries have 23 percent of theworld's population, 46 percent of the cultivatedl and, and 28 percent of the undeveloped andpotentially arable land. Currently, the developedcountries have about twice as much cultivatedl and on a per capita basis than do the developingnations. Note that Europe has about as much cul-tivated land per capita as does Asia, which hasessentially no unused, potentially arable land. Infact, the actual cropland area in many Europeancountries is declining.

332

Limitations of World Soil ResourcesOn a global basis, the main limitations for usingthe world's soil resources for agricultural produc-tion are drought (28%), mineral stress or infertility(23%), shallow depth (22%), excess water (10%),and permafrost (6%). Only 11 percent of theworld's soils are without serious limitations, asshown in Table 20.2. Because the currently usedagricultural land represents the world's best land,development of the unused, potentially arablel and would be expected to have more seriouslimitations than those shown in Table 20.2.

Future improvements in the world food situa-tion, in terms of new land for cultivation, willrequire developing land in Africa and SouthAmerica. Almost 50 percent of the South Ameri-can continent is centered on the Amazon Basinand central uplands of Brazil, where soils aremainly Oxisols and Ultisols. Not surprisingly, 47percent of soil-use limitations are related to min-eral stress and only 17 percent are due to drought.In Africa, 44 percent of the area has an arid orsemiarid climate. Many Oxisols and Ultisols oc-cupy the Zaire (Congo) basin, but an enormousarea of Ustalfs occurs along the subSaharan re-gion. This reflects the 44 percent drought and only18 percent mineral stress limitations in Africa.


TABLE 20.2 World Soil Resources and their Major Limitations for Agricultural Use

From The State of Food and Agriculture 1977, FAO, 1978.1 Nutritional deficiencies or toxicities related to chemical composition or mode of origin.

Summary StatementThe world's arable (or cultivated) land can bedoubled with reasonable inputs. Increasing foodproduction by bringing more land into cultivationin South America is limited mainly by soil inferti-lity, but also by available water or drought. InAfrica, the major limitations to bringing new landinto cultivation are first, lack of water or droughtand, second, soil infertility.

FUTURE OUTLOOKIn New York state, wheat yields increased only 3.1bushels per acre (209 kg per hectare) over the70-year period from 1865 to 1935. This also ty-pifies the nature of crop yield increases in Europeduring the same period. Beginning in 1935, im-proved varieties and better management practicesincreased wheat yields 20.1 bushels per acre dur-ing the next 40 years, as shown in Figure 20.6.Fifty-one percent of the increase was due to newtechnology and 49 percent of the increase wasdue to improved varieties of wheat. Low energycosts coupled with the adoption of improved vari-eties and other technology have resulted in a con-tinuing overproduction of food in the UnitedStates.

FUTURE OUTLOOK

Large yield increases are common when thebase yield is very low, which accounts for much ofthe relatively large increase in food production inmany developing nations in recent years (see Fig-ure 20.1). In the future, yield increases will be-come more difficult as the yield base increases.Thus, with very little new land coming into useand the need to increase yields on currentcropland, increases in food production are be-coming more difficult.

Beyond TechnologyFrom a technological point of view, a large poten-tial for increasing food production could be real-ized by greater photosynthetic efficiency, greateruse of biological nitrogen fixation, more efficientwater and nutrient use by plants, and greater plantresistance to disease and environmental stress. Inmany developing countries, technology hasbought about large and rapid increases in foodproduction, only to be overwhelmed by popula-tion growth. Without a suitable social, economic,and political environment, the benefits of technol-ogy have, in some cases, been temporal.

333

FIGURE 20.6 Wheat yields inNew York State from 1866-1975. Between 1935 and 1975,yield increase was the resultA 51 percent to technologyand 49 percent to geneticgain. (Adapted from Jensen,1978, and used by permissionof Science.)

The World Grain TradeThe extent of food production sufficiency in vari-ous parts of the world can be deduced in part froman observation of the world grain trade. WesternEurope, after the Industrial Revolution, becamedependent on other countries for food grains andremained a consistent importer for a very longperiod of time. We have noted that Europe haslittle more cropland per capita than does Asia,and Europe has little unused potentially arableland to bring into production. The large and con-sistent import of grain by Europe since the 1930sis shown in Table 20.3.

Many nations that are now importers of foodgrains were exporters during the 1930s (see Table20.3). India was a wheat exporter until populationgrowth overtook food production growth. LatinAmerica and even Africa were food grain export-ers. The USSR was an exporter before the 1917Russian revolution. Its inability to provide ade-quate diets, and the resulting riots, caused theUSSR to enter the world market and buy a largeamount of grain in 1973. This produced the so-called great grain robbery as the grain was se-cretly purchased without allowing the increased

334

demand to affect prices. Note from Table 20.3 thatthe USSR is a major grain importer in spite of anear stagnant rate of population growth. TheUSSR has more cultivated land per person thandoes North America (see Table 20.1). Productionproblems stem from their unique economic-political situation and less than ideal climaticconditions. The large imports of grain into theUSSR from the United States, which began in theearly 1970s, are continuing. Much of the importedgrain is fed to farm animals (see feed grains inFigure 20.7).

FIGURE 20.7 Grain-exports to the Soviet Union fromthe United States. The increase in exports hascontinued thorough the 1980s. (Data USDA.)


North America-the United States andCanada-together with Argentina, Australia, andNew Zealand, have become the world's exportersof food grains. The data in Table 20.3 show thatdespite great efforts to increase food productionin the developing countries, the food balance situ-ation has deteriorated, a trend that is expected tocontinue.

Population Control and PoliticsA population growth rate of zero is a mathemati-cal certainty. Even a low growth rate, over a longenough period of time, results in more peoplethan there is standing room available on earth. Atthe extreme of estimates for the earth's carryingcapacity, deWit (1967) calculated that if all of thel and surface produced calories equivalent to themaximum photosynthetic potential, there wouldbe enough calories for 1,000 billion people. Sucha world would have no place for people to live andwork and no room for animals.

Population control as government policy isdifficult because of long-time family traditions,religious beliefs, and a diminished sense of im-portance as the country's population declines. Aone-child family policy was adopted by China tocurb population growth. Today, the slowest ratesof population growth are in the areas with thegreatest amount of resources to invest in food

production. Population control programs warrantmore support.

In Africa, south of the Sahara, some countriesare contributing to a food reserve while othercountries are food deficient. Drought has beenused to explain the food shortage in some of theseAfrican countries. It is becoming more and moreapparent, however, that a lack of appropriate in-frastructures is a problem. In Zambia, there arefew people within a large area. In spite of lowpopulation, there has been considerable erosionand degradation of the most fertile soils. Over-grazing also contributes to soil erosion and landdegradation. Yet, in Zambia there is no soil con-servation service and little effort has been made atthe national level to control land use.

I n Nepal, population growth has forced farmersto plant crops on higher and steeper land in thehills. The result is extensive soil erosion and even-tual abandonment of the land as a result of land-slides. Erosion is further promoted by the removalof the forest cover for fuel wood, which has re-sulted in severe flooding downstream. Depletionof the forest (fuel wood) and subsequent conver-sion to kerosene to cook food may absorb 15 to 20percent of a poor family's income. Erosion is anever present and pressing problem for small farm-ers and results in higher fuel costs for the urbanpoor. The political power, however, rests withthose who are not directly affected. Some of theseinclude wealthy people who own rental propertiesin the capital city, the large landowners with es-tates on the fertile strip of land along the Indianborder (the Terai), and merchants who importgoods for resale. These groups wield the mostpolitical power in Nepal, so the needs of the urbanand rural poor are ignored.

SUMMARYIn 1948, the late Dr. Charles E. Kellogg pointed outthat sufficient land and soil resources (and otherphysical resources) are available to feed theworld, provided, that economic, social, and polit-

REFERENCES

ical problems are solved. Today, the statement isstill true. Looking at the food-population problemfrom a technological point of view, one can beoptimistic. Looking at the problem in its totality, itis hard to be optimistic. The difficult task ahead isto provide the economic, social, and political en-vironment in which hundreds of millions of smallpeasant farmers, and with simple tools, can pro-duce enough food for their families and somesurplus to sell to the urban sector. The problem isan ancient one, and the challenge for its solutioncontinues.

REFERENCESBentley, C. F. 1978. "Canada's Agricultural Land Re-

sources and the World Food Problem," in Trans. 11th

Int. Cong. Soil Sci. 2:1-26, Edmonton, 1978.Blaikie, Piers. 1985. The Political Economy of Soil Ero-

sion in Developing Countries. Longman, London.Brown, H. 1954. The Challenge ofMan's Future. Viking,

New York.Brown, L. R. 1978. "Worldwide Loss of Cropland."

Worldwatch Paper 24, Worldwatch Institute, Wash-ington, D.C.

Brown, L. R. 1985. "Reducing Hunger." State of the

World, pp. 23-41, Norton, New York.Brown, L. R. 1989. State of the World. Norton, New York.deWit, C. T. 1967. "Photosynthesis: Its Relationship to

Overpopulation." Harvesting the Sun, A. J. Pietro,F. A Greer, and T. J. Army, Eds, pp. 315-332, Aca-demic, New York.

FAO. 1978. The State of Food and Agriculture 1977.Rome.

FAO. 1981. The State of Food and Agriculture 1980.Rome.

Foth, H. D. 1982. Soil Resources and Food: A GlobalView, in Principles and Applications of Soil Geogra-

phy. E. M. Bridges and D. A. Davidson, eds., Long-man, London.

Jensen, N. F. 1978. "Limits to Growth in World FoodProduction." Science. 210:317-320.

Kellogg, C. E. 1948. "Modern Soil Science." Am. Scien-

tist. 38:517-536.Kellogg, C. E. and A. C. Orvedal. 1969. "Potentially

335

336

Arable Soils of the World and Critical Measures forTheir Use," in Advances in Agronomy. Vol. 21. Aca-demic Press, New York.

Morgan, Dan. 1979. Merchants of Grain. Viking, NewYork.


The White House. 1967. The World Food Problem. U.S.Government Printing Office, Washington, D.C.

USDA. 1964. "A Graphic Summary of World Agricul-ture." Misc. Pub. No. 705. Washington, D.C.

APPENDIX I

SOIL TEXTURE BY THEFIELD METHOD

337

338 SOIL TEXTURE BY THE FIELD METHOD

APPENDIX II

TYPES AND CLASSES OFSOIL STRUCTURE

APPENDIX III

PREFIXES AND THEIRCONNOTATIONS FOR NAMES

OF GREAT GROUPS IN THE U.S.SOIL CLASSIFICATION SYSTEM

(SOIL TAXONOMY)

341

A horizons. Mineral horizons that formed at the sur-face, or below an 0 horizon, and are characterized byan accumulation of humified organic matter intimatelymixed with the mineral fraction.

Acid soil. Soil with a pH value < 7.0.

Acidic cations. Hydrogen ions or cations that, onbeing added to water, undergo hydrolysis, resulting inan acidic solution. Examples in soils are

H+,A1 3+ , and

Fe 3+

Acidity, residual. Soil acidity that is neutralized bylime or other alkaline materials, but that cannot bereplaced by an unbuffered salt solution.

Acidity, salt-replaceable. The aluminum and hy-drogen that can be replaced from an acid soil by anunbuffered salt solution such as KCI. Essentially, thesum of the exchangeable Al and H.

Acidity, total. Sum of salt-replaceable and resid-ual acidity.

Actinomycetes. A nontaxonomic term applied to agroup of gram-positive bacteria that have a superficialresemblance to fungi.

Activity. I nformally, may be taken as the effectiveconcentration of a substance in a solution.

Aerate. To allow or promote exchange of soil gaseswith atmospheric gases.

Aeration porosity. The fraction of the bulk soil vol-ume that is filled with air at any given time, or under agiven condition, such as a specified soil-water matricpotential.

Aerobic. (1) Having molecular oxygen as apart of theenvironment. (2) Growing only in the presence of mo-lecular oxygen, as aerobic organisms. (3) Occurringonly in the presence of molecular oxygen, such asaerobic decomposition.

Aggregate. A unit of soil structure, usually formedby natural processes, and generally below 10 milli-meters in diameter.

* Adapted from Glossary of Soil Science Terms, published bythe Soil Science Society of America, July 1987, and reprintedby permission of the Soil Science Society of America.

GLOSSARY*

342

Air dry. The state of dryness at equilibrium with thewater content in the surrounding atmosphere.

Albic horizon. A mineral soil horizon from whichclay and free iron oxides have been removed or segre-gated. The color of the horizon is determined primarilyby the color of primary sand and silt particles ratherthan coatings on these particles. An E horizon.

Alfisols. Mineral soils that have umbric or ochric epi-pedons, agrillic or kandic horizons, and that have plant-available water during at least 90 days when the soil iswarm enough for plants to grow. Alfisols have a meanannual soil temperature below 8° C or a base saturationi n the lower part of the argillic horizon of 35 percent ormore when measured at pH 8.2. A soil order.

Alkaline soil. Any soil having a pH above 7.0.

Ammonification. The biological process leading tothe formation of ammoniacal nitrogen from nitrogen-containing organic compounds.

Ammonium fixation. The process of converting ex-changeable or soluble ammonium ions to those occu-pying interlayer positions similar to K + i n micas.

Amorphous material. Noncrystalline soil consti-tutents.Anaerobic. (1) The absence of molecular oxygen.(2) Growing in the absence of molecular oxygen, suchas anaerobic bacteria. (3) Occurring in the absence ofoxygen, such as a biochemical process.

Andisols. Mineral soils developed in volcanic ejectathat have andic soil properties. A soil order.

Anion exchange capacity. The sum total of ex-changeable anions that a soil can adsorb.

Aquic. A mostly reducing soil moisture regime nearlyfree of dissolved oxygen due to saturation by ground-water or its capillary fringe and occurring at periodswhen the soil temperature at 50 centimeters below thesurface is above 5° C.

Argillic horizon. A mineral soil horizon that is char-acterized by the illuvial accumulation of layer-silicateclays.

Aridic. A soil moisture regime that has no plant-available water for more than half the cumulative time

that the soil temperature at 50 centimeters below thesurface is above 5° C and has no period as long as 90consecutive days when there is water for plants, whilethe soil temperature at 50 centimeters is continuouslyabove 8° C.

Aridisols. Mineral soils that have an aridic moistureregime but no oxic horizon. A soil order.

Autotroph. An organism capable of utilizing C0 2 orcarbonates as a sole source of carbon and obtainingenergy for carbon reduction and biosynthetic processesfrom radiant energy (photoautotroph) or oxidation ofinorganic substances (chemoautotroph).

Available nutrients. Nutrient ions or compounds informs that plants can absorb and utilize in growth.

Available water. The portion of water in a soil thatcan be absorbed by plant roots; the amount of waterbetween in situ field capacity and the wilting point.

B horizons. Horizons that formed below an A, E, or 0horizon that have properties different from the overlyingand underlying horizons owing to the soil formingprocesses.

Bacteroid. An altered form of cells of certain bac-teria. Refers particularly to the swollen, irregular vacu-olated cells of Rhizobium i n nodules of legumes.

Bar. A unit of pressure equal to 1 million dynes persquare centimeter.

Basic cation saturation percent e. The extent towhich the cation exchange capacity is saturated withalkali (sodium and potassium) and alkaline earth (cal-cium and magnesium) cations expressed as a percent-age of the cation exchange capacity, as measured at aparticular pH, such as 7 or 8.2.

Biomass. The total mass of living microorganisms ina given volume or mass of soil.

Biosequence. A sequence of related soils, differingone from the other, primarily because of differences inkinds and numbers of plants and soil organisms as asoil-forming factor.

Biuret. A toxic product formed at high temperatureduring the manufacturing of urea.

Buffer power. The ability of ions associated with thesolid phase to buffer changes in ion concentration inthe solution phase.

Bulk density, soil. The mass of dry soil per unit bulkvolume, expressed as grams per cubic centimeter,

GLOSSARY 343

g/cm3 . The bulk volume is determined before drying toconstant weight at 105° C.

C horizons. Horizons or layers, excluding hard rock,that are little affected by the soil-forming processes. Chorizons typically underly A, B, E, or 0 horizons.

Calcareous soil. Soil containing sufficient free Ca-C0 3 , and/or MgC0 3 , to effervesce visibly when treatedwith cold 0.1 molar HCI. These soils usually containfrom as little as I to 20 percent CaC0 3 equivalent.

Calcic horizon. A mineral soil horizon of secondarycarbonate enrichment that is more than 15 centimetersthick, has a CaC0 3 equivalent above 150 g/kg, and hasat least 50 g/kg more CaC0 3 equivalent than the under-lying C horizon.

Caliche. A zone near the surface, more or less ce-mented by secondary carbonates of calcium or magne-sium precipitated from the soil solution. Caliche mayoccur as a soft thin soil horizon, as a hard thick bed, oras a surface layer exposed by erosion.

Cambic horizon. A mineral soil horizon that has atexture of loamy very fine sand or finer, has soil struc-ture rather than rock structure, contains some weather-able minerals, and is characterized by alteration orremoval of mineral material, or the removal of carbo-nates.

Capillary fringe. A zone in the soil just above thewater table that remains saturated or almost saturatedwith water.

Carbon-organic nitrogen ratio. The ratio of themass of organic carbon to the mass of organic nitrogenin soil, organic material, plants, or the cells of microor-ganisms.

Cat clay. Wet clay soils containing ferrous sulfide,which become highly acidic when drained.

Catena. A sequence of soils of about the same age,derived from similar parent material, and occurringunder similar climatic conditions, but having differentcharacteristics because of variations in relief and indrainage.

Cation exchange. The interchange between a cationi n solution and another cation on the surface of anynegatively charged material such as clay colloid ororganic colloid.

Cation exchange capacity (CEC). The sum of ex-changeable cations that a soil, soil constitutent, orother material can adsorb at a specific pH; commonly

344

expressed as milliequivalents per 100 grams or cen-timoles per kilogram.

Chemical potential. I nformally, it is the capacity of asolution or other substance to do work by virtue of itschemical composition.

Chlorosis. Failure of plants to develop chlorophyllcaused by a deficiency of an essential element. Chloro-tic leaves range in color from light green through yellowto almost white.

Chroma. The relative purity, strength, or saturation ofa color; directly related to the dominance of the deter-mining wavelength of the light and inversely related tograyness; one of the three variables of color.

Chronosequence. A sequence of related soils thatdiffer one from the other, in certain properties primarilyas a result of time as a soil-forming factor.

Citrate soluble phosphorus. That part of the totalphosphorus in fertilizer that is insoluble in water butsoluble in neutral 0.33 M ammonium citrate and which,together with water-soluble phosphorus represents thereadily available phosphorus content of the fertilizer.

Clay. (1) A soil separate consisting of particles<0.002 millimeters in equivalent diameter. (2) A tex-tural class.

Clay films. Coatings of clay on the surfaces of soilpeds, mineral grains, and in soil pores. (Also calledclay skins, clay flows, or agrillans.)

Clay mineral. Any crystalline inorganic substance ofclay size.

Claypan. A dense, compact layer in the subsoil hav-ing a much higher clay content than the overlying mate-rial, from which it is separated by a sharply definedboundary. Claypans usually impede the movement ofwater and air and the growth of plant roots.

Colluvium. A general term applied to deposits on aslope or at the foot of a slope or cliff that were movedthere chiefly by gravity.

Concretion. A local concentration of a chemicalcompound, such as calcium carbonate or iron oxide, inthe form of a grain or nodule of varying size, shape,hardness, and color.

Consistency. The manifestations of the forces of co-hesion and adhesion acting within the soil at variouswater contents, as expressed by the relative ease withwhich a soil can be deformed or ruptured.

Consumptive water use. The water used by plantsin transpiration and growth, plus water vapor loss from

GLOSSARY

adjacent soil or snow, or from intercepted precipitationi n any specified time. Usually expressed as equivalentdepth of free water per unit of time.

Creep. Slow mass movement of soil and soil materialdown relatively steep slopes primarily under the influ-ence of gravity.

Crust. A soil-surface layer, ranging in thickness froma few millimeters to a few tens of millimeters, which,when dry, is much more compact, hard, and brittle thanthe material immediately beneath it.

Cryic. A soil temperature regime that has mean an-nual soil temperatures of above 0° C but below 8° C at 50centimeters. There is more than a 5° C difference be-tween mean summer and mean winter soil tempera-tures.

Darcy's law. A law describing the rate of flow ofwater through porous media. Named for Henry Darcy ofParis, who formulated it in 1856 from extensive work onthe flow of water through sand filter beds.

Denitrification. Reduction of nitrate or nitrite to mo-l ecular nitrogen or nitrogen oxides by microbial activityor by chemical reactions involving nitrite.

Desert pavement. The layer of gravel or stones re-maining on the land surface in desert regions after theremoval of the fine material by wind erosion.

Diffuse double layer. A heterogeneous system thatconsists of a solid surface layer having a net electricalcharge, together with an ionic swarm under the influ-ence of the solid and a solution phase that is in directcontact with the surface.

Duripan. A mineral soil horizon that is cemented bysilica to the point that air-dry fragments will not slake inwater or HCI.

E horizons. Mineral horizons in which the mainfeature is loss of silicate clay, iron, aluminum, or somecombination of these, leaving a concentration of sandand silt particles of quartz or other resistant minerals.

ECe . The electrolytic conductivity of an extract fromsaturated soil, normally expressed in units of deci-siemens per meter at 25° C.

Ecology. The science that deals with the interre-lationships between organisms and between organ-isms and their environment.

Ecosystem. A community of organisms and the envi-ronment in which they live.

Ectomycorrhiza. A mycorrhizal association in whichthe fungal mycelia extend inward, between root corticalcells, to form a network ("Hartig net") and outward intothe surrounding soil.

Edaphology. The science that deals with the influ-ence of soils on living things, particularly plants.

Eluviation. The removal of soil material in suspen-sion (or in solution) from a layer or layers of soil.

Endomycorrhiza. A mycorrhizal association with in-tracellular penetration of the host root cortical cells bythe fungus as well as outward extension into the sur-rounding soil.

Entisols. Mineral soils that have no distinct sub-surface diagnostic horizons within 1 meter of the soilsurface. A soil order.Erosion. (1) The wearing away of the land surface byrunning water, wind, ice, and other geological agents,i ncluding such processes as gravitational creep. (2)Detachment and movement of soil or rock by water,wind, ice, or gravity.

Erosion potential. A numerical value expressing thei nherent erodibility of a soil.

Eutrophic. Having concentrations of nutrients opti-mal, or nearly so, for plant or animal growth.

Evapotranspiration. The combined loss of waterfrom a given area, and during a specified period of time,by evaporation from the soil surface and by transpira-tion from plants.

Exchangeable cation percentage. The extent towhich the adsorption complex of a soil is occupied tparticular cation.

Exchangeable ion. A cation or anion held on or nearthe surface of a solid particle, which may be replacedby other ions of similar charge that are in solution.

Fallowing. The practice of leaving land uncroppedfor periods of time to accumulate and retain water andmineralized nutrient elements.

Family, soil. I n soil classification one of the catego-ries intermediate between the great soil group and thesoil series. Families provide groupings of soils withranges in texture, mineralogy, temperature, andthickness.

Fertigation. Application of plant nutrients in irri-gation water.

Fertility, soil. The ability of a soil to supply elements

GLOSSARY

essential for plant growth without a toxic concentrationof any element.

Fertilizer. Any organic or inorganic material of natu-ral or synthetic origin (other than liming materials) thatis added to a soil to supply one or more elementsessential to the growth of plants.

Fibric material. Mostly undecomposed plant re-mains that contain large amounts of well-preserved andrecognizable fibers.

Field capacity, in situ (field water capacity). Thecontent of water, on a mass or volume basis, remainingi n a soil 2 or 3 days after having been wetted with waterand after free drainage is negligible.

Film water. A thin layer of water, in close proximityto soil-particle surfaces, that varies in thickness from Ior 2 to perhaps 100 or more molecular layers.

Fixation. The process by which available plant nutri-ents are rendered less available or unavailable in thesoil.

Floodplain. The land bordering a stream, built up ofsediments from overflow of the stream, and subject toinundation when the stream is at flood stage.

Flux. The time rate of transport of a quantity across agiven area.

Free iron oxides. A general term for those iron ox-ides that can be reduced and dissolved by a dithionitetreatment. Often includes goethite and hematite.

Friable. A consistency term pertaining to the ease ofcrumbling of soils.

Fluvic acid. The colored material that remains in so-lution after the removal of humic acid by acidification.

Gibbiste. A mineral with a platy habit that occurs inhighly weathered soils. Al(OH) 3 . .

Gilagi. The microrelief of soils produced by expan-sion and contraction with changes in water content, acommon feature of Vertisols.

Glacial drift. Rock debris that has been transportedby glaciers and deposited, either directly from the ice orfrom the meltwater.

Goethite. A yellow-brown iron oxide mineral that isvery common and is responsible for the brown color inmany soils. FeOOH.

Great soil group. One of the categories in the systemof soil classification that has been used in the UnitedStates for many years. Great groups place soils accord-

345

346

i ng to soil moisture and temperature, basic cation satu-ration status, and expression of soil horizons.

Green manure. Plant material incorporated into soilwhile green or at maturity; for soil improvement.

Groundwater. That portion of the water below thesurface of the ground at a pressure equal to or greaterthan atmospheric.

Guano. The decomposed dried excrement of birdsand bats, used for fertilizer purposes.

Gypsic horizon. A mineral soil horizon of secondarycalcium sulfate enrichment that is more than 15 cen-timeters thick.

Gypsum. The common name for calcium sulfate (Ca-S0 4

. 2H 20), used to supply calcium and sulfur toameliorate sodic soils.

Hardpan. A hardened soil layer, in the lower A or inthe B horizon, caused by cementation of soil particleswith organic matter or with materials such as silica,sesquioxides, or calcium carbonate.

Heavy metals. Those metals that have high density;i n agronomic usuage includes Cu, Fe, Mn, Mo, Co, Zn,Cd, Hg, Ni, and Pb.

Hematite. A red iron oxide mineral that contributesred color to many soils. Fe203.

Hemic material. An intermediate degree of decom-position, such as two-thirds of the organic materialcannot be recognized.

Heterotroph. An organism capable of deriving car-bon and energy for growth and cell synthesis by theutilization of organic compounds.

Histosols. Organic soils that have organic soil mate-rial in more than half of the upper 80 centimeters, orthat are of any thickness overlying rock or fragmentalmaterials, which have interstices filled with organic soilmaterials. A soil order.

Hue. One of the three variables of color.

Humic acid. The dark-colored organic material thatcan be extracted from soil by various agents and that isprecipitated by acidification to pH 1 or 2.

Humification. The process whereby the carbon oforganic residues is transformed and converted to hu-mic substances through biochemical and/or chemicalprocesses.

Humus. All of the organic compounds in soil exclu-sive of undecayed plant and animal tissues, their partial

GLOSSARY

decomposition products, and the soil biomass. Resis-tant to further alteration.

Hydraulic conductivity. The proportionality factori n Darcy's Law as applied to the viscous flow of water insoil.

Hydroxy-aluminum. Aluminum hyroxide com-pounds of varying composition.

Hyperthermic. A soil temperature regime that hasmean annual soil temperatures of 22° C or more and ahigher than 5° C difference between mean summer andmean winter soil temperatures at 50 centimeters belowthe surface.

Illuvial horizon. A soil layer or horizon in whichmaterial carried from the overlying layer has been pre-cipitated from solution or deposited from suspension.The layer of accumulation.

Illuviation. The process of deposition of soil materialremoved from one horizon to another in the soil, usu-ally from an upper to a lower horizon in the soil profile.

Immobilization. The conversion of an element fromthe inorganic to the organic form in microbial or planttissues.

I nceptisols. Mineral soils having one or more pedo-genic horizons in which mineral materials, other thancarbonates or amorphorus silica, have been altered orremoved but not accumulated to a significant degree.Water is available to plants more than half of the year ormore than 90 consecutive days during a warm season.A soil order.

I ndicator plants. Plants characteristic of specificsoil or site conditions.

I nfiltration. The downward entry of water throughthe soil surface.

I on activity. I nformally, the effective concentrationof an ion in solution.

Iron oxides. Group name for the oxides and hydrox-i des of iron. Includes the minerals goethite, hematite,lepidocrocite, ferrihydrite, maghemite, and magnetite.Sometimes referred to as sesquioxides or hydrousoxides.

Iron pan. An indurated soil horizon in which ironoxide is the principal cementing agent, as in plinthite orl aterite.

Ironstone. Hardened plinthite materials often occur-ring as nodules and concretions.

Isomorphous substitution. The replacement of oneatom by another of similar size in a crystal structurewithout disrupting or seriously changing the structure.

Jarosite. A yellow potassium iron sulfate mineral.

Kandic horizon. Subsoil diagnostic horizon having aclay increase relative to overlying horizons and havinglow-activity clays, below 16 meq/100 g or below 16cmol(+)/kg of clay.

Kaolinite. A clay mineral of the kaolin subgroup. Ithas a 1 : 1 layer structure and is a nonexpanding claymineral.

Labile. A substance that is readily transformed bymicroorganisms or is readily available to plants.

Landscape. All the natural features such as fields,hills, forests, water, and such, which distinguish onepart of the earth's surface from another part. Usuallythat portion of the land that the eye can comprehend ina single view.

Lattice. A regular geometric arrangement of points ina plane or in space. Lattice is used to represent thedistribution of repeating atoms or groups of atoms in acrystalline substance.

Leaching. The removal of materials in solution fromthe soil.

Lepidocrocite. An orange iron oxide mineral that isfound in mottles and concretions of wet soils. FeOOH.

Lime, agricultural. A soil amendment containingcalcium carbonate, magnesium carbonate and othermaterials; used to neutralize soil acidity and furnishcalcium and magnesium for plant growth.

Lithosequence. A group of related soils that differ,one from the other, in certain properties primarily , ; aresult of differences in the parent rock as a soil-formingfactor.

Loam. A soil textural class.

Loess. Material transported and deposited by windand consisting of predominantly silt-sized particles.

Luxury uptake. The absorption by plants of nutrientsin excess of their need for growth.

Macronutrient. A plant nutrient usually attaining aconcentration of more than 500 mg/kg in mature plants.

Maghemite. A dark, reddish-brown, magnetic ironoxide mineral chemically similar to hematite, but struc-

GLOSSARY 347

turally similar to magnetite. Fe 203 . Often found in well-drained, highly weathered soils of the tropical regions.

Magnetite. A black, magnetic iron oxide mineralusually inherited from igneous rocks. Often found insoils as black magnetic sand grains.

Manganese oxides. A group term for oxides of man-ganese. They are typically black and frequently occur asnodules and coatings on ped faces, usually in associa-tion with iron oxides.

Manure. The excreta of animals, with or without anadmixture of bedding or litter, fresh or at various stagesof further decomposition or composting.

Mass flow (nutrient). The movement of solutes asso-ciated with the net movement of water.Melanic horizon. A thick, dark colored surface hori-zon having andic soil properties.Mesic. A soil temperature regime that has mean an-nual soil temperatures of 8° C or more but less than 15°C, and more than 5° C difference between mean sum-mer and mean winter temperatures at 50 centimetersbelow the surface.

Mica. A layer-structured aluminosilicate mineralgroup of the 2: 1 type that is characterized by high layercharge, which is usually satisfied by potassium.

Microclimate. The sequence of atmosphericchanges within a very small region.

Microfauna. Protozoa, nematodes, and arthropodsof microscopic size.

Microflora. Bacteria (including actinomycetes),fungi, algae, and viruses.

Micronutrient. A chemical element necessary forplant growth found in small amounts, usually less than100 mg/kg in the plant. These elements consist of B, Cl,Cu, Fe, Mn, Mo, and Zn.

Mineralization. The conversion of an element froman organic form to an inorganic state as a result ofmicrobial activity.

Mineral soil. A soil consisting predominantly of, andhaving properties determined predominantly by, min-eral matter. Usually contains less than 200 g/kg of or-ganic carbon.

Mollisols. Mineral soils that have a mollic epipedonoverlying mineral material with a basic cation satura-tion of 50 percent or more when measured at pH 7.0. Asoil order.

Montmorillonite. An aluminum silicate (smectite)

348

with a layer structure composed of two silica tetrahe-dral sheets and a shared aluminum and magnesiumoctahedral sheet.

Mor. A type of forest humus in which the Oa horizonis present and in which there is almost no mixing ofsurface organic matter with mineral soil.Mottled zone. A layer that is marked with spots orblotches of different color or shades of color (mottles).Muck soil. An organic soil in which the plant resi-dues have been altered beyond recognition.

Mulch. Any material such as straw, sawdust, leaves,plastic film, and loose soil, that is spread upon the soilsurface to protect soil and plant roots from the effects ofraindrops, soil crusting, freezing, evaporation, andsuch.

Mulch farming. A system of tillage and planting op-erations resulting in minimum incorporation of plantresidues or other mulch into the soil surface.

Mull. A type of forest humus in which the Oe horizonmay or may not be present and in which there is no Oahorizon. The A horizon consists of an intimate mixtureof organic matter and mineral soil with gradual transi-tion between the A horizon and the horizon under-neath.

Munsell color system. A color designation systemthat specifies the relative degrees of the three simplevariables of color: hue, value, and chroma. For exam-ple: 10YR 6/4 is a color with a hue = 10YR (yellow -red), value = 6, and chroma = 4.

Mycorrhiza. Literally "fungus root." The association,usually symbiotic, of specific fungi with the roots ofhigher plants.

N value. The relationship between the percentage ofwater under field conditions and the percentages ofi norganic clay and of humus.

Natric horizon. A mineral soil horizon that satisfiesthe requirements of an argillic horizon, but that also hasprismatic, columnar, or blocky structure and a subhori-zon having 15 percent or more saturation with ex-changeable sodium.

Nitrification. Biological oxidation of ammonium tonitrite and nitrate, or a biologically induced increase inthe oxidation state of nitrogen.

Nitrogenase. The specific enzyme required for bio-logical dinitrogen fixation.

No-tillage system. A procedure whereby a crop isplanted directly into the soil with no preparatory tillage

GLOSSARY

since harvest of the previous crop; usually a specialplanter is necessary to prepare a narrow, shallow seed-bed immediately surrounding the seed being planted.

Nutrient antagonism. The depressing effect causedby one or more plant nutrients on the uptake and avail-ability of another.

Nutrient interaction. A statistical term used whentwo or more nutrients are applied together to denote adeparture from additive responses occurring when theyare applied separately.

O horizon. Layers dominated by organic material,except limnic layers that are organic.

Ochric epipedon. A thin, light colored surface hori-zon of mineral soil.

Organic soil. A soil that contains a high percentageof organic carbon (>200 g/kg or >120 - 180 g/kg ifsaturated with water) throughout the solum.

Outwash. Stratified glacial drift deposited by melt-water streams beyond active glacier ice.

Ovendry soil. Soil that has been dried at 105° C untilit reaches constant mass.

Oxisols. Mineral soils that have an oxic horizonwithin 2 meters of the surface or plinthite as a continu-ous phase within 30 centimeters of the surface, and thatdo not have a spodic or argillic horizon above the oxichorizon. A soil order.

Paleosol, buried. A soil formed on a landscape dur-i ng the geological past and subsequently buried bysedimentation.Pans. Horizons or layers in soils that are stronglycompacted, indurated, or having very high clay content.

Paralithic contact. Similar to lithic contact exceptthat it is softer, and is difficult to dig with a spade.

Parent material. The unconsolidated and more orless chemically weathered mineral or organic matterfrom which the solum of soils is developed by pedoge-nic processes.

Particle density. The density of soil particles, the drymass of the particles being divided by the solid volumeof the particles.

Pascal. A unit of pressure equal to 1 Newton persquare meter.

Peat. Unconsolidated soil material consisting largelyof undecomposed, or only slightly decomposed, or-ganic matter accumulated under conditions of exces-sive moisture.

Ped. A unit of soil structure such as an aggregate,crumb, prism, block, or granule, formed by naturalprocesses.

Pedon. A three-dimensional body of soil with lateraldimensions large enough to permit the study of horizonshapes and relations. Its area ranges from 1 to 10square meters.

Penetrability. The ease with which a probe can bepushed into the soil.

Percolation. The downward movement of waterthrough the soil.

Pergelic. A soil temperature regime that has meanannual soil temperatures of less than 0° C. Permafrost ispresent.

Permafrost. A perennially frozen soil horizon.

Permafrost table. The upper boundary of the perma-frost, coincident with the lower limit of seasonal thaw.

Permanent wilting point. The largest water contentof a soil at which indicator plants, growing in that soil,wilt and fail to recover when placed in a humid cham-ber. Often estimated by the water content at -15 bars,-1,500 kilopascals, or -1.5 megapascals soil matricpotential.

Petrocalcic horizon. A continuous, indurated calcichorizon that is cemented by calcium carbonate and, insome places, with magnesium carbonate.

Petrogypsic horizon. A continuous, strongly ce-mented, massive gypsic horizon that is cemented withcalcium sulfate.

pH-dependent charge. The portion of the cation oranion exchange capacity which varies with pH.pH, soil. The negative logarithm of the hydrogen ionactivity of a soil.

Phase. A utilitarian grouping of soils defined by soilor environmental features that are not class differentiaused in the U.S. system of soil taxonomy, for example,surface texture, surficial rock fragments, salinity, ero-sion, thickness, and such.

Phosphate. In fertilizer trade terminology, phos-phate is used to express the sum of the water-soluble and citrate-soluble phosphoric acid (P205);also referred to as the available phosphoric acid(P205)Phosphoric acid. I n commerical fertilizer manufac-turing, it is used to designate orthophosphoric acid,H3P04. In fertilizer labeling, it is the common term usedto represent the phosphate content in terms of availablephosphorus, expressed as percent P 2 0 5 .

GLOSSARY 349

P20 5 . Phosphorus pentoxide; fertilizer label designa-tion that denotes the percentage of available phos-phate.

Placic horizon. A black to dark-reddish mineral soilhorizon that is usually thin, is commonly cementedwith iron, and is slowly permeable or impenetrable towater and roots.

Plastic soil. A soil capable of being molded or de-formed continuously and permanently, by relativelymoderate pressure, into various shapes. See Consis-tency.

Plinthite. A nonindurated mixture of iron and alumi-num oxides, clay, quartz, and other diluents that com-monly occurs as red soil mottles usually arrangedin platy, polygonal, or reticulate patterns. Plinthitechanges irreversibly to ironstone hardpans, or irregularaggregates on exposure to repeated wetting and drying.

Plow pan. An induced subsurface soil horizon orlayer having a higher bulk density and lower total po-rosity than the soil material directly above and below,but similar in particle size analysis and chemicalproperties. The pan is usually found just below themaximum depth of primary tillage and frequently re-stricts root development and water movement. Alsocalled a pressure pan or plow sole.Polypedon. A group of contiguous similar pedons.

Potash. Term used to refer to potassium or po-tassium fertilizers and usually designated as K20.Potassium fixation. The process of converting ex-changeable or water-soluble potassium to that occupy-i ng the position of K+ i n the micas.Primary mineral. A mineral that has not been alteredchemically since deposition and crystallization frommolten lava.

Profile, soil. A vertical section of soil through all itshorizons and extending into the * C horizon.

R layer. Hard bedrock including granite, basalt,quartzite, and indurated limestone or sandstone that issufficiently coherent to make hand digging impractical.

Rainfall erosion index. A measure of the erosivepotential of a specific rainfall event.

Reaction, soil. The degree of acidity or alkalinity of asoil, usually expressed as a pH value.

Regolith. The unconsolidated mantle of weatheredrock and soil material on the earth's surface; looseearth materials above solid rock.

350

Residual fertility. The available nutrient content of asoil carried to subsequent crops.

Reticulate mottling. A network of streaks of differ-ent color, most commonly found in the deeper profilesof soil containing plinthite.

Rhizobia. Bacteria capable of living symbiotically inroots of leguminous plants, from which they receiveenergy and often utilize molecular nitrogen.

Rhizosphere. The zone immediately adjacent toplant roots in which the kinds, numbers, or activities ofmicroorganisms differ from that of the bulk soil.

Rill. A small, intermittent watercourse with steepsides; usually only several centimeters deep and, thus,no obstacle to tillage operations.

Runoff. That portion of the precipitation on an areawhich is discharged from the area through streamchannels.

Saline soil. A nonsodic soil containing sufficient sol-uble salt to adversely affect the growth of most cropplants.

Salic horizon. A mineral soil horizon of enrichmentwith secondary salts more soluble in cold water thangypsum.

Saline seep. I ntermittent or continuous saline waterdischarge at or near the soil surface under dry-landconditions, which reduces or eliminates crop growth.

Saline-sodic soil. A soil containing both sufficientsoluble salt and exchangeable sodium to adversely af-fect crop production under most soil and crop condi-tions.

Salt balance. The quantity of soluble salt removedfrom an irrigated area in the drainage water minus thatdelivered in the irrigation water.

Sand. (1) A soil particle between 0.05 and 2.0 milli-meters in diameter. (2) A soil textural class.

Sapric material. One of the components of organicsoils with highly decomposed plant remains. Materialis not recognizable and bulk density is low.

Saprolite. Weathered rock materials that maybe soilparent material.

Saturation extract. The solution extracted from asoil at its saturation water content.

Secondary mineral. A mineral resulting from the de-composition of a primary mineral or from the reprecipi-tation of the products of decomposition of a primarymineral.

GLOSSARY

Self-mulching soil. A soil in which the surface layerbecomes so well aggregated that it does not crust andseal under the impact of rain, but instead serves as asurface mulch upon drying.

Series, soil. See Soil series.

Sesquioxides. A general term for oxides and hydrox-ides of iron and aluminum.

Siderophore. A nonporphyrin metabolite secretedby certain microorganisms and plant roots that forms ahighly stable coordination compound with iron.

Silica-alumina ratio. The molecules of silicon diox-ide per molecule of aluminum oxide in clay minerals orin soils.

Silt. (1) A soil separate consisting of particles be-tween 0.05 and 0.002 millimeters in equivalent diame-ter. (2) A soil textural class.

Site index. A quantitative evaluation of the produc-tivity of a soil for forest growth under the existing orspecified environment. Commonly, the height in metersof the dominant forest vegetation taken or calculated toan age index such as 25, 50, or 100 years.Slickensides. Polished and grooved surfaces pro-duced by one mass sliding past another; common inVertisols.

Smectite. A group of 2:1 layer structured silicateswith high cation exchange capacity and variable inter-layer spacing.

Sodic soil. A nonsaline soil containing sufficient ex-changeable sodium to adversely affect crop productionand soil structure under most conditions of soil andplant type.

Sodium adsorption ratio (SAR). A relation betweensoluble sodium and soluble divalent cations thatcan be used to predict the exchangeable sodium per-centage of soil equilibrated with a given solution. De-fined as:

where concentrations, denoted by parentheses, are ex-pressed in moles per liter.

Soil. (1) The unconsolidated mineral or material onthe immediate surface of the earth that serves as anatural medium for the growth of land plants. (2) Theunconsolidated mineral or organic matter on the sur-face of the earth, which has been subjected to andi nfluenced by genetic and environmental factors of par-

ent material, climate, macro- and microorganisms, andtopography, all acting over a period of time and pro-ducing a product-soil-that differs from the materialfrom which it is derived in many physical, chemical,biological, and morphological properties and charac-teristics.

Soil association. A kind of map unit used in soilsurveys comprised of delineations, each of whichshows the size, shape, and location of a landscape unitcomposed of two or more kinds of component soils, orcomponent soils and miscellaneous areas, plus allow-able inclusions in either case.

Soil conservation. A combination of all manage-ment and land-use methods that safeguard the soilagainst depletion or deterioration by natural or human-i nduced factors.

Soil genesis. The mode of origin of the soil withspecial reference to the processes or soil-forming fac-tors responsible for development of the solum, or truesoil, from unconsolidated parent material.Soil horizon. A layer of soil or soil material approxi-mately parallel to the land surface and differing fromadjacent genetically related layers in physical, chemi-cal, and biological properties or characteristics such ascolor, structure, texture, consistency, kinds and num-ber of organisms present, degree of acidity or alkalinity,and so on.

Soil loss tolerance. (1) The maximum average an-nual soil loss that will allow continuous cropping andmaintain soil productivity without requiring additionalmanagement imputs. (2) The maximum soil erosionl oss that is offset by the theoretical maximum rate ofsoil development, which will maintain an equilibriumbetween soil losses and gains.

Soil management groups. Groups of taxonomicsoil units with similar adaptations or management re-quirements for one or more specific purposes, such asadapted crops or crop rotations, drainage practices,fertilization, forestry, and highway engi neering.Soil monolith. A vertical section of a soil profile re-moved from the soil and mounted for display or study.

Soil productivity. The capacity of a soil to produce acertain yield of crops, or other plants, with optimummanagement.

Soil science. That science dealing with soils as anatural resource on the surface of the earth, includingsoil formation, classification and mapping, geographyand use, and physical, chemical, biological, and fertil-

GLOSSARY 351

ity properties of soils per se: and those properties inrelation to their use and management.

Soil separates. Mineral particles, less than 2.0 milli-meters in equivalent diameter, ranging between speci-fied size limits.

Soil series. The lowest category in the U.S. system ofsoil taxonomy: a conceptualized class of soil bodies(polypedons) that have limits and ranges more re-strictive than all higher taxa. The soil series serves as amajor vehicle to transfer soil information and researchknowledge from one soil area to another.

Soil solution. The aqueous liquid phase of the soiland its solutes.

Soil structure. The combination or arrangement ofprimary soil particles into secondary particles, units, orpeds.

Soil survey. The systematic examination, descrip-tion, classification, and mapping of soils in an area.

Soil texture. The relative proportions of the varioussoil separates in a soil.

Soil water potential (total). The amount of workthat must be done per unit quantity of pure water inorder to transport reversibly and isothermally an infini-tesimal quantity of water from a pool of pure water, at aspecified elevation and at atmospheric pressure, to thesoil water (at the point under consideration).

Solum. The upper and most weathered part of thesoil profile; the A, E, and B horizons.

Spodic horizon. A mineral soil horizon that is char-acterized by the illuvial accumulation of amorphousmaterials composed of aluminum and organic carbonwith or without iron.

Spodosols. Mineral soils that have a spodic or aplacic horizon that overlies a fragipan. A soil order.

Strip cropping. The practice of growing crops thatrequire different types of tillage, such as row and sod, inalternate strips along contours or across the prevailingdirection of the wind.

Structural charge. The charge (usually negative) ona mineral caused by isomorphous substitution withinthe mineral layer.

Subsoiling. Any treatment to loosen soil below thetillage zone without inversion and with a minimum ofmixing with the tilled zone.

Surface charge density. The excess of negative orpositive charge per unit area of surface area of soil orsoil mineral.

352

Thermic. A soil temperature regime that has meanannual soil temperatures of 15° C or more, but less than22° C and more than 5° C difference between meansummer and mean winter soil temperatures at a 50-centimeter depth below the surface.Thermophile. An organism that grows readily at tem-peratures above 45° C.

Till. (1) Unstratified glacial drift deposited by ice andconsisting of clay, silt, sand, gravel, and boulders, inter-mingled in any proportion. (2) To prepare the soil forseeding; to seed or cultivate the soil.Tilth. The physical condition of soil as related to itsease of tillage, fitness as a seedbed, and its impedanceto seedling emergence and root penetration.

Top dressing. An application of fertilizer to a soilsurface, without incorporation, after the crop stand hasbeen established.

Toposequence. A sequence of related soils. Thesoils differ, one from the other, primarily because oftopography as a soil-formation factor.

Torric. A soil-moisture regime defined like aridicmoisture regime but used in a different category of theU.S. soil taxonomy.

Truncated. Having lost all or part of the upper soilhorizon or horizons.

Tuff. Volcanic ash usually more or less stratified andin various states of consolidation.

Udic. A soil moisture regime that is neither dry for aslong as 90 cumulative days nor for as long as 60 consec-utive days in the 90 days following the summer solsticeat periods when soil temperature at 50 centimetersbelow the surface is above 5° C.

Ultimate particles. Individual soil particles after astandard dispersing treatment.

Ultisols. Mineral soils that have an argillic or kan-dic horizon with a basic cation saturation of less than35 percent when measured at pH 8.2. Ultisols have amean annual soil temperature of 8° C or higher. A soilorder.

Umbric epipedon. A surface layer of mineral soilthat has the same requirements as the mollic epipedonwith respect to color, thickness, organic carbon con-tent, consistence, structure, and phosphorus content,but that has a basic cation saturation less than 50 per-cent when measured at pH 7.

Ustic. A soil moisture regime that is intermediate be-tween the aridic and udic regimes and common in

GLOSSARY

temperate subhumid or semiarid regions, or in tropicaland subtropical regions with a monsoon climate. Alimited amount of water is available for plants but oc-curs at times when the soil temperature is optimum forplant growth.

Value, color. The relative lightness or intensity ofcolor and approximately a function of the square root ofthe total amount of light. One of the three variables ofcolor.

Vermiculite. A highly charged layer-structured sili-cate of the 2:1 type that is formed from mica.

Vertisols. Mineral soils that have 30 percent or moreclay, deep wide cracks when dry, and either gilgai mi-crorelief, intersecting slickensides, or wedge-shapedstructural aggregates tilted at an angle from the horizon.A soil order.

Vesicular arbuscular. A common endomycorrhizalassociation produced by phycomycetous fungi of thefamily Endogonaceae. Host range includes most agri-cultural and horticultural crops.

Waterlogged. Saturated or nearly saturated withwater.

Water potential. See Soil water potential.

Water table. The upper surface of groundwater orthat level in the ground where the water is at atmo-spheric pressure.

Water table, perched. The water table of a saturatedlayer of soil that is separated from an underlying satu-rated layer by an unsaturated layer (vadose water).Wilting point. See Permanent wilting point.

Xeric. A soil moisture regime common to Mediterra-nean climates having moist, cool winters and warm, drysummers. A limited amount of water is present but doesnot occur at optimum periods for plant growth. Irri-gation or summer fallow is commonly necessary forcrop production.

Xerophytes. Plants that grow in or on extremely drysoils.

Yield. The amount of a specified substance produced(e.g., grain, straw, total dry matter) per unit area.

Zero point of charge. The pH value of a solution inequilibrium with a particle whose net charge from allsources is zero.

Zero tillage. See No-tillage system.

I NDEX

353

354 INDEX

I NDEX 355

356 I NDEX

I NDEX 357

358 I NDEX

I NDEX 359

360 I NDEX

COLOR PLAT[;

Nitrogen deficiency on corn .Lower leaves turn yellow alongmidrib, starting at the leaf tip .

Potassium deficiency on alfalfa(and clovers) shows as a seriesof white dots near leafmargins; in advanced stagesentire leaf margin turns white .(Courtesy American PotashInstitute .)

Left is nitrogen-deficientsection of corn leaf and cutopen section of stem thatremains uncolored when

treated with diaphenylamine,indicating low nitrate level inplant sap . On right the leaf is

dark green and diaphenylamineproduces a dark blue color

indicative of high nitratelevel in plant sap .

Potassium deficiency on corn .Lower leaves have yellow

margins .

COLOR PLATE 2

Iron deficiency on pin oak . Theleaf veins remain green as theintervein areas lose theirgreen color .

Phosphorus deficiency on cornis indicated by purplishdiscoloration .

Iron deficiency on roses . Greenveins with yellow intervein

areas, showing most on newestleaves .

--C^

Manganese deficiency onkidney beans . Leaf veins

remain green as the interveinareas lose their green color

and turn yellow .

COLOR PLATE 3

Zinc deficiency of navy beansgrown on calcareous soil . The

small unfertilized zinc-deficientplants stand in marked

contrast to the taller plantsthat were fertilized with zinc .

A case where zincfertilization is necessary

to produce a crop .

Boron deficiency on sugarbeets causes heart rot . Mostadvanced symptom is on left .

COLOR PLATE 4

Manganese-treated plants inrear showing no deficiency .

Plants in foreground areunfertilized and difference in

degree of manganesedeficiency symptoms is

indicative of varietal responseto limited soil manganese .

Magnesium deficiency oncoffee . Veins remain green andintervein areas turn yellow .

Excess soluble salt symptomson geranium . The leaf marginsturn yellow and becomenecrotic .

towf-

JCLI0J0

fundamentals of soil science 8th edition - henry foth

Documents

concepts of soil

soil physicalproperties

soil fertili

original chapter

root growth

united states copyrightact

mediumfor plant growth

importance of soils