Soil-111 : Principles of Soil Science

Dr. Pabitra Kumar Mani pabitramani@gmail.com www.bckv.edu.in www.slideshare.net.in/pabitramani

Professor, Agril. Chemistry and Soil Science,

F/Ag, BCKV

J N MukherjeeJustus von Liebig, 1840

What is Soil?What is Soil?

Soil: Unconsolidated mineral or material on the surface of the earth resulting from and influenced by time, parent material, climate, organisms, and topography.

Not all soil is created equal, Not all soil is created equal, ‘‘the soil’ vs. a soil. the soil’ vs. a soil.

The soil is the link between the rock core of the earth and the living things on its surface. It is the foothold for the plants we grow. (Simonson,1957)Soil refers to the weathered and fragmented outer layer of the earth’s terrestrial surface. (Hillel, 1982)

Joffe (1949), modified by Birkeland(1999)

Soils Defined• Natural Body that Occurs on the Land Surface that

are Characterized by One or More of the Following:

– Consists of Distinct Horizons or Layers– The Ability to Support Rooted Plants in a Natural

Environment– Upper Limit is Air or Shallow Water– Lower Limit is Bedrock or Limit of Biological Activity– Classification based on a typical depth of 2 m or

approximately 6.0 feet

The pedosphere: abiotic and biotic entities interacting in the soil matrix

Better knowledge of the soils on which this road was built may have allowed its engineers to develop a more stable design, thus avoiding this costly and dangerous situation. (R. Weil)

Transcosna Grain Elevator, Canada (Oct. 18, 1913)

Shear Failure under Foundation load

Branches of soil science

It deals with soil formation, soil genesis, soil classification and description of soil properties.

A pedologist studies, examines, and classifies soils as they occur in their natural environment.

(from Gr. Pedos, “ground,” and logos, “science”;original formed as Russian, pedologiya)

A pedon is the smallest volume of soil that should be recognized as a soil individual

The range of properties observed in several similar pedons, a polypedon are aggregated to define a soil individual.

A soil profile is a vertical cross section of a soil. It is divided into a number of distinct layers, referred to as horizons.

The horizons are normallydesignated by symbols and letters.

The presence or absence ofparticular horizons allows pedologists to classify the soil.

In addition, the organic or O horizon can form above the mineral soil commonly in forested areas,resulting from the dead plant andanimal remains.

The basic unit of study: Soil Profiles

TOPSOIL,upper or Ahorizon

SUBSOIL,middle or Bhorizon

PARENTMATERIAL,lower or Chorizon

Information important to different soil functions and applications is most likely to be obtained by studying different layers of the soil profile.

The Regolith – What ?• The Regolith is the Unconsolidated

material overlying Rock.The Overburden !

• This material may be rather thin to hundreds of feet thick and can include material dislodged or weathered from local rock, transported to the area by wind, water, ice, or gravity.

Image Source: http:///soils.ag.uidaho.edu

The Soil or Solum• The Soil or Solum is the portion of

the Regolith that has been influenced by the 5 Soil Formation Factors.

• The processes are controlled by Time, Climate, Topography (Landform and Position), Organisms, and Parent Material.

• Formation Process are in 4 Broad Categories (additions, losses, translocation, and transformations)- Genetic Soil Forming Processes. Rock – R Horizon

SOILRegolith

Young Soil

Old Soil

Well Defined Horizons

High Degree of Weathering

Highly Leached

Thick Solum

http://www.mo15.nrcs.usda.gov/features/gallery/

Poorly Defined Horizons

Low Degree of Weathering

Slightly Leached

Thin Solum

Sediment

Precipitationplus solutes, aerosols and particulates

Organic residues humus Primary Secondary minerals minerals

Leaching ofdissolved ions & OM

E(i)lluviationof

clayCaCO3

dissolved ionsdissolved OM

Capillary rise ofsalts

Capillary rise of

dissolvedions

Loss of gases

N 2, N2O

, CO 2

Sediment

Soil forming processes

TRANSFORMATIONS

Adapted from Stewart (1990)

AdditionsLosses

Losses

Add

ition

s

Additions

TRAN

SLOCA

TIONS

Loss

es

TRANS

LOCA

TIONS

Climate

Organisms

ReliefParent material

Time

H. Jenny refined the ideas of Russian scientist V.V. Dokuchaev into the following equation:

5 factors of soil formation

Soil = f (Cl, O, R, P, T…)

Edaphology:Study of soil in relation to growth, nutrition and

yield crops or plants.Edaphologists consider the various properties of soils in relation to plant production

Average Soil Composition

                                                                                                   { }Pore space 50%

Solids 50%

25% Water

25% Air 5% Organic Matter

45% Inorganic (mineral materials)

Soil pH - a “master variable”Soil pH - a “master variable”

Acid (pH=1.0)

Neutral (pH=7.0)

Alkaline (pH=14.0)

• A measure of the hydrogen (H+) ion activity

• One pH unit change = a ten fold change in acidity or alkalinity

Why called “master variable”?Why called “master variable”?

• soil pH controls:1) soil microbe activity2) nutrient exchanges3) nutrient availability4) gaseous exchanges5) chemical degradation6) CEC

Possible pH Ranges Under Natural Soil Conditions

black walnut: 6.0-8.0

Most desirable

carrot: 5.5-7.0cucumber: 5.5-7.0

spinach: 6.0-7.5tomato: 5.5-7.5

white pine: 4.5-6.0

Verystrong Strong Moderate Slight Slight Moderate Strong

Verystrong

NeutralAcid Basic

3 4 5 6 7 8 9 10 111 2 12 13 14

Most agricultural soilsExtreme pH range for most mineral soils

cranberry:4.2-5.0

apple: 5.0-6.5

Soil pH

0

100

4.0 5.0 6.0 7.0 8.0

Plant benefit

Plant Injury

Toxicity DeficiencyRel

ativ

e pl

ant y

ield

(%)

(Weil and Kroontje,1984)

General relationship of plant health and soil pH. Some nutrients can reach toxic levels at low pHs, while deficiencies can occur at high pHs.

Soren P.L. Sorensen invented the pH scale in 1909.

HAcaK

AcH

-AcH HAcKa

HAc

AclogapKpH

This form of ionisation constant equation is called the Henderson- Hasselbalch equation.

pH of an weak acid

AcH HAc

pH meter When one metal is brought in contact with another, a voltage difference occurs due to their differences in electron mobility. When a metal is brought in contact with a solution of salts or acids, a similar electric potential is caused, which has led to the invention of batteries. Similarly, an electric potential develops when one liquid is brought in contact with another one, but a membrane is needed to keep such liquids apart. 

A pH meter measures essentially the electro-chemical potential between a known liquid inside the glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions or the potential of hydrogen.

To complete the electrical circuit, also a reference electrode is needed. Note that the instrument does not measure a current but only an electrical voltage, yet a small leakage of ions from the reference electrode is needed, forming a conducting bridge to the glass electrode.

The majority of pH electrodes available commercially are combination electrodes that have both glass H+ ion sensitive electrode and additional reference electrode conveniently placed in one housing.

In principle it should be possible to determine the H+ ion activity or concn. of a soln by measuring the potential of a Hydrogen electrode inserted in the given soln. The EMF of a cell, free from liquid junction potential, consisting of a Hydrogen electrode and a reference electrode, should be given by,

E = E ref – RT/F ln aH+ F= Faraday constant ,96,485 E = E ref + 2.303 RT/F pH R= 8.314 J/mol/°K

pH = ( E- Eref )F/2.303 RT T= Kelvin scale

So, by measuring the EMF of the Cell E obtained by combining the H electrode with a reference electrode of known potential, Eref , the pH of the soln. may be evaluated.

The electric potential at any point is defined as the work done in bringing a unit charge from infinity to the particular point

Reduced state Oxidised state + n Electron⇋ M = Mn+ + nE E(+) = E0 – (RT/F) ln aM

n+ Nernst Equation

Principles of pH Meter

Soil pH & Nutrient Availability

Factors Influencing pH Organic Matter Soil Parent Material (carbonates, etc.) Temperature and Precipitation Agricultural Practices

Organic matter affects the buffering capacity of the soil (more SOM-resistance to change pH)Soil parent materials such as basalt have higher pH levels than those of parent

materials from granite.High temperature and moisture leach bases from soil .

Agricultural practices tend to lower pHAgricultural practices tend to lower pH•crops removing nutrients • leaching nutrients through soil; •decomposition of organic materials; • fertilization, particularly ammonium fertilizers; •Liming materials raise soil pH

Soil Fertility: it is the potential of the earth or inherent capacity of the soil to supply plant nutrients in quantity, forms and proportion required for the growth and development of the crop.

Fertility is measured by the amount of chemical elements or compounds required for plant growth

Productivity of a soil is defined as its capacity to produce plants under specified programme of management.

It is measured by the yield of the crop per unit area of the land

Fertility is one of the factors of soil productivity. Sometimes a soil may be fertile but may not be productive.

Types of soilsAlluvial SoilBlack SoilMountain SoilDesert soilRed soilLaterite Soil

Alluvial Soil This is the most wide spread

soil in india.This soil is formed by the deposition of materials by the rivers namely Indus,the Ganga, the Brahmputra. It is mainly found in Northern Plains and Eastern Coastal Plains. It is the most fertile soil. Due to high fertility, they are intensively cultivated and are densely populated.

Black Soil This soil is black in colour and is

best for growing cotton and sugarcane. This soil is formed by the weathering up of igneous rocks.It is rich in minerals like iron and magnesium. It is mainly confined to the north western part of peninsular plateau, Deccan lava plateau which includes the states of Maharashtra, Saurashtra, Malwa, Madhya Pradesh and Chhattisgarh.

Laterite Soil These soils are formed by the

intense leaching in tropical regions where the rainfall is high. This soil is devoid of nutrients and has low humus content and is not suitable for crop cultivation. It is mainly found in Karnatka, Kerala, TamilNadu,Madhya Pradesh and hilly areas of Orissa and Assam. Through soil conservation techniques they are useful for growing tea and coffee.

Desert Arid Soils These are sandy,porous

and saline in nature. They lack humus and moisture. Hence they are not suitable for cultivation except in areas where irrigation is available. This soil is mainly found in Rajasthan and some parts of Gujarat.

Red Soil Red soil develops on

crystalline igneous rocks in areas of low rainfall in the eastern and south eastern parts of the Deccan Plateau. This soil is rich in iron compounds and is reddish in colour while it is yellowish in colour when it occurs in hydrated form.

Pedon.—The pedon is presented in Soil Taxonomy (Soil Survey Staff, 1975) as a unit of sampling within a soil. The limits on the area of a pedon establish rules for deciding whether to consider one or two or more kinds of soil within a small-scale pattern of local lateral variability. A pedon is regarded as the smallest body of one kind of soil large enough to represent the nature and arrangement of horizons and variability in the other properties that are preserved in samples.A pedon extends down to the lower limit of a soil. It extends through all genetic horizons and, if the genetic horizons are thin, into the upper part of the underlying material. The pedon includes the rooting zone of most native perennial plants. For purposes of most soil surveys, a practical lower limit of the pedon is bedrock or a depth of about 2 m, whichever is shallower. A depth of 2 m provides a good sample of major soil horizons, even in thick soil. It includes much of the volume of soil penetrated by plant roots, and it permits reliable observations of soil properties.The surface of a pedon is roughly polygonal and ranges from 1 m2 to 10 m2 in area, depending on the nature of the variability in the soil. Where the cycle of variations is less than 2 m and all horizons are continuous and nearly uniform in thickness, the pedon has an area of approximately 1 m2. Where horizons or other properties are intermittent or cyclic over an interval of 2 to 7 m, the pedon includes one-half of the cycle (1 to 3 1/2 m). If horizons are cyclic over an interval greater than 7 m, each cycle is considered to contain more than one soil. The range in size, 1 to 10 m2, permits consistent classification by different observers where important horizons are cyclic or repeatedly interrupted over short distances.Polypedon.—The pedon is considered too small to exhibit more extensive features, such as slope and surface stoniness. The polypedon is presented in Soil Taxonomy as a unit of classification, a soil body, homogeneous at the series level, and big enough to exhibit all the soil characteristics considered in the description and classification of soils 

Bulk Density DeterminationBulk Density Determination

For our example, let’s assume we have 1 cubic centimeter of soil that weighs 1.33 grams

Soil is made of solids andpore spaces

1.33 grams

To calculate Bulk Density:Volume = 1 cm3

Weight = 1.33 grams

Bulk Density = Weight of SoilVolume of Soil

Bulk Density = 1.33

1

Bulk Density = 1.33 grams/cm3

{ }

Porosity value generally ranges from 0.3 to 0.6 (30–60%).

In clayey soils, the porosity is highly variable because the soil alternately swells, shrinks, aggregates, disperses, compacts, and cracks

it ranges between 0.3 and 2.

Comparison of Coarse Textured and Fine Textured Soils

Coarse Textured Soil

Less porespace but more macropores

Fine Textured Soil

More total porespace

Texture and Pore SpaceTexture and Pore Space

Clay

Silty Clay

Silty Clay Loam

Clay Loam

Sandy Clay

Sandy Clay Loam

LoamSilt Loam

SiltSandy Loam

Sand

Percent Clay

Percent Silt

Percent Sand

FineMediumCoarse

Loamy Sand

Soil StructureSoil Structure• Soil structure is the combination or

arrangement of primary soil particles into secondary units

• The way soil particles are arranged to form stable aggregates

• Compare this to clods, which are caused by disturbance (plowing or digging)

• Compaction – results from implement traffic, stable soil aggregates are broken down

Common Types of Soil Common Types of Soil StructureStructure

Common to Ohio soils just below Ap (> 8”)

Plow layer

Sand

Deeper in profile (>3-4’)

Granular

Platy

Prismatic

Columnar

Blocky

Single Grain

Massive

GRANULAR

BLOCKY

C

B

E

A

Granular

Platy

Blocky

Massive

. . . more on Soil Horizons. . . more on Soil Horizons

Mollisol Alfisol

B

C

Ap

A

Soil HorizonsA black Leaching zone: roots here (topsoil)

B “red” Accumulating zone: nutrients (subsoil)

C rocky Weathered bedrock

D solid Bedrock

Horizons subject to erosion

Black color is caused by organic particles Prevented by:

• Mulching

• Proper cultivation

• Perennial planting

Rotting material makes humus particles

• Improve drainage (air capacity)

• Increase water holding capacity

• Increase nutrient holding capacity leaves, grass clippings, compost, green manure, horse manure

Typical electrode system for measuring pH. (a) Glass electrode (indicator) and saturated calomel electrode (reference) immersed in a solution of unknown pH. (b) Combination probe consisting of both an indicator glass electrode and a silver/silver chloride reference. A second silver/silver chloride electrode serves as the internal reference for the glass electrode.

The two electrodes are arranged concentrically with the internal reference in the center and the external reference outside. The reference makes contact with the analyte solution through the glass frit or other suitable porous medium.

Combination probes are the most common configuration of glass electrode and reference for measuring pH.

Soil is a natural body consisting of layers (horizons) of mineral and/or organic constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical properties and their biological characteristics.

Built entirely from natural, locally occurring materials, this typical rural Tanzanian house has soil walls made by daubing mud onto a framework of sticks. Both its soil walls and grass thatch roof are shown in the process of being refurbished. For scale, note man standing left of the house. (Photo courtesy of R. Weil)

68

Shear Strength of Soils

Dr. Attaullah ShahShear strength of a soil is the resistance to deformation by continuous shear displacement of soil particles due to tangential (shear) stress.

Climate and Soil Development

Image Source: University of Wisconsin, 2002

Soil Development

There are 5 main factors that influenceThere are 5 main factors that influencesoil formation processes.soil formation processes.

1. Type of Parent Material1. Type of Parent Material

2. Climate2. Climate

3. Topography3. Topography

4. Biotic Components 4. Biotic Components

5. Time5. Time

Soil ProfileSoil ProfileWhat do we see?

• organic matter - surface soil is organic matter - surface soil is darker due to organic matterdarker due to organic matter

• iron oxides - subsoil has iron oxides - subsoil has brighter browns and tans due brighter browns and tans due to iron oxidesto iron oxides

• drainagedrainage

• horizons - layers of different horizons - layers of different color or texture; formed from color or texture; formed from the top downthe top down

Soil Quality Indicators

Physical Chemical Biological Organic Matter

ChemicalPhysical

Biological

OM

Physical Soil Quality Indicators

Soil Structure Aggregate Stability Bulk Density/Compaction Soil Pores Permeability Infiltration Surface Crusting Available Water Holding Capacity

Chemical Soil Quality Indicators

Soil pH Nutrient Supply Potentially Mineralizable Nitrogen Cation Exchange Capacity Calcium Carbonate Salinity and Sodicity

– Electrical Conductivity– Exchangeable Sodium Percentage

Soil Contaminants

Biological Soil Quality Indicators

The “Big Picture:” the Soil Food Web

Indicators – Microbial Biomass– Soil Enzymes– Soil Respiration– Plant Roots– Other Indicators

Soil Organic Matter Considered the single most important Considered the single most important

indicator of soil qualityindicator of soil qualityOrganic matter - Critical in soils since it contributes to infiltration, AWC, CEC, soil structure, and provides energy source for microorganisms, which are important in decomposition of OM and release of nutrients needed for crops and overall plant growth and aggregation of soils. Because OM is important to CEC as is clay; OM becomes very important in sandy soils since they are inherently low in clay

Pedon: : Ped Ped - Ground, - Ground, Logos Logos - Discourse/ Science- Discourse/ Science

Pedology- - The scienceThe science that deals with formation, that deals with formation, morphology and classification of soil morphology and classification of soil bodies as land scape componentbodies as land scape component

Pedogenesis--The natural process of soil FormationThe natural process of soil Formation Weathering Weathering Rock --------------Parent Material 1. Physical 2. Chemical 3. Biological

Pedogenic Processes:

Pedogenic processes areParent Material Soil extremely complex involving many physical, chemical & biological reactions

Different processes or their combinations Parent Material Soils (Same type) depends on variation in natural environment of different nature Parent Material Effect of same natural Soils Different types Environment of different nature

This is the material from which the soil has developed and can vary from solid rock to deposits like alluvium and boulder clay. It has been defined as ‘the initial state of the soil system’.