11

CHAPTER 1: SOIL COMPOSITION AND IMPORTANCE

OVERVIEW

We have a working knowledge of what is and what is not soil. Yet whatever this stuff is,it nearly defies formal definition. To some it is the natural medium for the growth of landplants. To others it is materials on which, or with which, we build structures. Soils arethe uppermost parts of the Earth’s crust. The sands, silts, and clays comprising soilsoriginated from the rocks and minerals of the Earth’s crust. Soils also have an organiccomponent with various carbon-rich materials that affect soil properties in many ways.

Soils have profoundly impacted human history. Civilizations prosper or fail toprosper, in part, because of their use and misuse of their land. Archaeologists often studysoil. Soil science is among the oldest of sciences, although it has only been called soilscience recently.

At least thirteen essential plant nutrients are provided by soil, as are water andother plant needs. Our need for crops and timber is enormous and ever growing. Most ofthe wood, textile and paper fibers, food, and other resources for 6 billion humans mustcome from the soil. Increasingly we look to the soil for the future supply of alcohols andother biofuels for the motors of industry and transportation.

We often misuse our soils by leaving them unprotected, by compacting them, andby mismanaging chemicals and irrigation water. Soil degradation contributes todisastrous flooding, localized food shortages, and ironically, the degradation of still moresoil as forests and rangelands are plowed. One of the great challenges facing thisgeneration is to find ways to sustain our soils.

QUESTIONS AND ANSWERS

1. What is soil?Answer: Various definitions or descriptions would be appropriate. Except for

soil that has been buried by sediments, soil is the surface land material (mineral and/ororganic) capable of supporting plants. The depth of soil is the depth of plant roots. Incontrast to “soil,” the word “land” is normally used to mean the non-water portion of theEarth’s surface, without regard to its depth or its ability to grow plants.

2. Name the ten most common elements in the soils of the Earth.Answer: From “Insight: The Importance of Silicon and Oxygen,” the ten most

common elements, in descending order, are: oxygen, silicon, aluminum, iron, carbon,calcium, potassium, sodium, magnesium, and titanium.

3. (a) What is a mineral? (b) How does a mineral differ from a rock?Answers: (a) A mineral is a uniform crystalline substance containing a definite

and repeating pattern of the atoms that comprise it. (b) A rock is a mixture of minerals ora mixture of atoms in a non-regular pattern (amorphous, such as obsidian).

4. Differentiate between igneous, sedimentary, and metamorphic rock.

22

Answer: An igneous rock is formed by exposing Earth’s hot magma to a coolerenvironment on or near the surface. A sedimentary rock is one formed from sedimentsand hardened by pressure and/or cementation. A metamorphic rock is an igneous orsedimentary rock that has been altered (and usually hardened) by high temperaturesand/or pressures.

5. Differentiate between primary and secondary minerals.Answer: Primary minerals are those crystallizing from the original molten

minerals before they were modified by water and weathering. Secondary minerals areproduced by resolidification (crystal growth or precipitation) of materials undergoingdissolution in water from primary or other secondary minerals.

6. How is an organic soil different from a mineral soil?Answer: A mineral soil forms as various chemical, physical, and biological

processes act upon unconsolidated mineral materials. In contrast, an organic soil ischaracterized by an accumulation of organic material, primarily plant residues andmicrobial biomass. This accumulation often occurs in stagnant waters such as swamps,marshes, and fens, where decomposition of organic material is slow because of theanaerobic conditions.

7. What roles do soils play in facilitating plant growth?Answer: Soils provide all thirteen of the mineral nutrients essential for plants, in

addition to providing those nutrients that might be beneficial but not essential. Soils alsoprovide the water and anchorage a plant needs. In addition, soils provide oxygen to plantroots, and provide habitat for other organisms, especially microbes, that are beneficial toplants.

8. Besides plant growth, why is soil important to man?Answer: Soil, water, air, and organisms comprise Earth’s ecosystems. The most

important and direct role of soil is in plant growth support. However, soil is importantfor other reasons, too. Soil conditions determine in large part the temperature of theEarth. Soil is our waste disposal system—everything from crop residues to sewagesludge is deposited on soil for eventual decomposition. For engineering uses, soil is thefoundation for roads and buildings. The properties of soil determine whether or not a siteis suitable for homes, parks, roads, or other uses.

9. Discuss the amount of arable land in the world in relation to food needs.Answer: This is open-ended and subject to each student’s worldview. A few

facts are presented here. (1) The amount of arable land per person is sharply declining.(2) The amount of arable land per person varies widely by region of the world. (3) Theworld does not contain enough readily farmable land to produce the food, fiber, and otherresources necessary to provide all 6 billion humans an affluent lifestyle.

10. What is the projected world population for the year 2050?Answer: The United Nations population estimate for the year 2050 is

9,322,000,000 people.

33

CHAPTER 2: SOIL PHYSICAL PROPERTIES

OVERVIEW

Physical properties of soil include texture, structure, bulk density, pore space, aeration,temperature, color, and consistence. Soil water-holding properties are also physicalproperties, but are discussed extensively in Chapter 3.

Texture refers to the proportion of clay, silt, and sand in a soil. Soil texturepartially regulates the rate of water and air movement into and through soil. It influencesthe ease of tillage and the stability against wind and water erosion. Compared to sandysoils, clays are less permeable, hold greater amounts of water when wetted, have slowerair and water flow through them, and are more difficult to till when wet (clays are sticky)or dry (clays are hard).

Soil structure describes the aggregation of soil particles into larger masses. Thesemasses are called aggregates, or peds (or clods if man-made). They can be rounded orangular, large or small, and can take various shapes including prismatic, blocky, platy ormassive. Small, poorly aggregated particles are easily transported by wind or water to anew location.

Bulk density is a measure of soil weight per volume. A typical soil is about 45 to55 percent pore space. The solids comprising most soils weigh about 2.65 g/cm3.Because about half of a soil volume is comprised of these solids, a typical bulk soilweighs about 1.3 or 1.4 g/cm3. Pore space or porosity is inversely related to bulk density.The lower the bulk weight of a soil, the more porous it is likely to be. A porous soil mayor may not be well aerated; it depends on the tendency for the pores to be filled withwater, thereby excluding air.

The soil surface absorbs solar radiation, then heats the air by convection.However, soil is a good insulator; therefore, deeper soil depths may experience littlechange in temperature compared to the dramatic changes on the soil surface. Soil colorand wetness affect soil temperature by affecting its tendency to reflect light, water alsosignificantly increases soil resistance to changes in temperature.

When using soil for engineering purposes, the load-bearing and stress-resistingproperties of soil are crucial. These properties are sometimes described by the termconsistence.

QUESTIONS AND ANSWERS

1. Using the textural triangle, determine the texture for the following three soils: (a) 10%sand, 15% clay, and 75% silt; (b) 41% clay, 40% silt, and 19% sand; and (c) 16% clay,28% silt, and 56 % sand.

Answers: (a) silt loam, (b) silty clay, (c) sandy loam.

2. State how the following are altered, in general, by increased clay content: (a) waterinfiltration, (b) pore size, (c) total pore space, and (d) bulk density.

Answers: (a) Water infiltration slows, (b) pore size decreases, (c) total porespace increases, and (d) bulk density decreases as clay content increases.

44

3. In detail, state what each of the following names tells about a soil: (a) gravelly loam,(b) very cobbly loamy sand, and (c) stony sandy loam.

Answers: (a) A gravelly loam has 15 to 35% by volume as gravel (20 to 75 mmdiameter) and the rest is a loamy texture. (b) A very cobbly loamy sand has 35 to 60% byvolume as cobbles (75 to 250 mm diameter) and the rest has the texture of loamy sand.(c) A stony sandy loam contains 15 to 35% by volume as stones (250 to 600 mmdiameter) and the rest is sandy loam.

4. (a) How does soil air differ from atmospheric air? (b) What is the critical gas in thesoil air for root growth?

Answers: (a) Soil air is normally higher in water vapor (relative humidity) andmuch higher in carbon dioxide than atmospheric air. It is normally lower in O2 thanatmospheric air. (b) The critical gas for root growth is O2, needed for respiration.Seldom are concentrations of other gases critical for plant growth.

5. What is measured to indicate soil strength?Answer: Various terms collectively called consistence relate to soil strength.

The Atterberg limits are used, as is resistance to rupture, and many other measurements.

6. Of the colors 5YR 2/2, 5YR 6/2, 5YR 3/6, and 5Y 6/2, state which is (a) the blackest,(b) the least red, and (c) the most brilliant colored.

Answer: (a) 5YR 2/2, (b) 5Y 6/2, (c) 5YR 3/6.

7. What are some of the practical uses of soil color information?Answer: Color changes over distance indicate changes in parent material. White

colors indicate lime or salts; mottling indicates periodic waterlogging; gleying indicatesfrequent waterlogging; dark colors indicate high levels of humus.

8. State how each of the following affects the warming or cooling of soils: (a)transparent plastic mulches, (b) organic mulches, (c) water content in the soil, and (d) soilcolor.

Answers: (a) Transparent mulches cause greenhouse warming because theyallow heat waves to reach the soil but they retain the heat rather than allowing convectiveheat transfer to the air. (b) Organic mulches may heat or cool the soil, depending on thecolor, moisture content, and season of the year. Organic mulches usually are goodinsulators and tend to increase soil resistance to temperature change. In the summer alight-colored organic mulch may be used to keep the soil cooler. In the winter, a darkmulch may be used to keep the soil warmer. Because organic mulches may holdconsiderable water, their water content must also be considered. The wetter the mulch,the greater will be the resistance to change in temperature. (c) Water resists temperaturechange because of its high specific heat. It also absorbs considerable heat, converting itto latent heat through evaporation. Most wet soils are cooler than adjacent dry soils. (d)Dark soil surfaces heat up much more readily than light-colored soil surfaces. The light-colored surfaces reflect much incoming radiation.

9. Explain how and why large bodies of water modify nearby land area temperatures.

55

Answer: Water requires 5 times more heat to raise a gram of material 1 degreethan does dry soil. On a sunny, summer day air over land will heat up much faster thanair over water because the land heats up much faster than the water. This cooler airabove the water absorbs heat from the surrounding land. As winter approaches and thebody of water is slow to cool, the air above the water is warmer than above land,especially if the land is frozen or snow-covered. This relatively warm air above waterwill warm the surrounding land areas.

10. By what mechanisms do soils lose heat?Answer: Most of the heat lost by soil is transferred as latent heat to evaporating

water. A lesser amount of heat is transferred to the air, thereby warming the air.

11. Define the terms: liquid limit, plastic limit, plasticity index, well graded, and fines.Answer: Liquid limit is the water content in the soil at which the soil will flow

under a standardized agitation procedure. Plastic limit is the minimum water content atwhich the mixture acts as a plastic solid. Plasticity index is the numerical differencebetween the liquid limit and the plastic limit. Well graded indicates a mixture of particlesizes. Fines refers to fine-textured soil particles (silts and clays).

66

CHAPTER 3: SOIL WATER PROPERTIES

OVERVIEW

Water management is among mankind’s most powerful tools. Adding water where it isdeficient and eliminating water where it is in excess can greatly alter the growth of plants.Water is held in soils by hydrogen bonding. The strength of the attraction between soiland water varies according to soil type and other conditions, and is quantified using thevarious units of water potential. Sands have little hold on water and cannot retain verymuch. Conversely, clays have strong hydrogen bonding and hold much more water.Loams are intermediate, but are often the best soils for providing water to plants. This isbecause they hold a substantial amount of water but readily relinquish it to plants.

Water availability to plants is of great concern. When a soil is wetted as after aheavy rain then excess water is allowed to drain away, the soil is in a condition calledfield capacity (FC). This is the maximum amount of water a non-wetlands soil can holdagainst the pull of gravity, and is about –0.33 bars of potential. As plants transpire waterand as water evaporates from the soil, the soil dries and its water potential becomes morenegative. Eventually the soil no longer provides enough water to prevent plants fromwilting. This condition occurs at the permanent wilting point (PWP). The PWP occurs atabout –15 bars of potential. The amount of water between these two extremes isavailable water capacity (AWC). Though the magnitude of AWC varies from soil to soil,it can be calculated knowing that AWC = FC – PWP.

Mass water content is the mass of water in a soil divided by the mass of the soilsolids. Volumetric water content can be easily calculated as the mass water contentmultiplied by the soil’s bulk density. This term tells the volume of water per volume ofsoil; but also tells the depth of water in a specified depth of soil. Various devicesincluding resistance blocks and electromagnetic waves can measure water content.Simple devices called tensiometers can measure water potential.

Water is lost from the soil by evaporation and from plants by transpiration. Oftenthe two are combined and called evapotranspiration. Irrigation is used to replace thewater lost. To avoid stressing crops, irrigation water is often added when about one-halfof the available water is depleted.

Water flows from high potential to lower potential. This flow can be undersaturated conditions, where gravity is the primary driving force; or, the flow can be underunsaturated conditions where matric forces pull water from a wetter region (higherpotential) to a drier region (lower potential) of the soil. Saturated flow is rapid,especially through sandy soils, but ceases when the soil is not saturated with water.Unsaturated flow need not be downward, and is constantly occurring, sometimes soslowly as to be imperceptible.

QUESTIONS AND ANSWERS

1. How does hydrogen bonding hold water to soil mineral surfaces?Answer: Oxygen exhibits great electronegativity, meaning that when it forms a

covalent bond, it keeps more than its “fair share” of the electrons and therefore has aweak negative charge. Hydrogen, conversely, relinquishes electrons and tends to have a

77

weak positive charge. Therefore the hydrogen in a covalently bonded molecule, such aswater, is attracted to the oxygen in other covalently bonded materials, including water oroxygen-rich soil minerals.

2. Define (a) water potential, and (b) matric potential.Answers: (a) Water potential is the ability of water to do work (move, react, etc.)

relative to the ability of pure free water to do work. (b) Matric potential is one of fourcomponents of water potential. Matric potential is caused by the attraction between soiland water due to hydrogen bonding. Matric potential is negative, because water attractedto soil has less ability to do work than does pure free water.

3. Using kilopascals, what is the equivalent of a water potential value of –1 bar?Answer: -1 bar = -100 kilopascals.

4. Which water in a film of water on a soil particle is held most strongly to the soil: thewater near the particle or the water at the outer edge of the film? Explain.

Answer: The water near the particle is held more strongly. The closer the wateris to the surface, the greater the adhesive force between the oxygens of the soil and thehydrogens of the water.

5. What is the difference between available water capacity and field capacity?Answer: Available water capacity is the amount of water a soil holds between its

field capacity and its permanent wilting point. Field capacity is the amount of water asoil holds after gravitational water has drained away. Field capacity is greater thanavailable water capacity because some of the water in the soil, i.e., the water present atthe permanent wilting point, is held too tightly to be available to plants.

6. Give an approximate volumetric water percentage for the wilting point of a typical (a)loamy sand, (b) loam, and (c) clay soil.

Answers: Soils vary considerably, but typical values from Table 3-2 are (a) 3 %to 10 %, (b) 7 % to 17 %, and (c) 20 % to 24%.

7. Calculate the mass water content of a soil if a sample of the soil at field capacity plusthe can containing the sample weigh 248 g, the dry soil plus the can weight 233 g, and thecan weight is 141 g.

Answer: (248 g - 233 g)/(233 g - 141 g) = 0.163

8. If a soil at field capacity has a mass water content of 0.24 and a bulk density of 1.3g/cm3, calculate its volumetric water percentage.

Answer: (0.24) (1.3) (100%) = 31.2%

9. (a) How much water (in cm of water per 30 cm depth of soil) does the soil described inQuestion 8 contain at field capacity? (b) About how many centimeters of plant-availablewater will the soil hold in the top 90 cm? (Assume that the soil is uniformly wet to adepth of 90 cm.)

88

Answers: (a) (0.312) (30 cm) = 9.4 cm. (b) Because permanent wiltingpercentage is not given you need to assume a PWP value. PWP is often about 50% offield capacity. With this assumption the estimated available water capacity is: 31.2% ÷ 2= 15.6 %. The estimated amount of available water in 90 cm is (0.156) (90 cm) = 14 cm.

10. Describe how tensiometers are used to determine the soil water status for a crop.Answer: Tensiometers measure matric potential, which is normally about equal

to the total water potential in a soil. When the tensiometer indicates considerable tensionon the water, one might irrigate to avoid crop stress.

11. In terms of forces and flow rates, describe the difference between saturated flow andunsaturated flow.

Answer: Saturated flow is rapid and is caused by gravity. Unsaturated flow isslow, and is mainly caused by differing matric potentials in the soil.

12. List factors that influence rates of water infiltration into soils.Answer: soil texture, soil structure, degree of compaction, presence of swelling

clays, presence of an impermeable layer, presence of overhead water, type of vegetation,quantity of water already infiltrated.

13. Describe the phenomenon of passive water uptake by plants.Answer: Passive uptake is the absorption of water without the plant expending

metabolic energy. The water is pulled into the root as the water column in the root-stem-leaf system is moved upwards as in a wick, by evaporation from the leaves.

14. (a) Define evapotranspiration. (b) Define consumptive use. Estimate a dailyconsumptive use rate for soybeans in Nebraska.

Answers: (a) Evapotranspiration is the water lost by both evaporation andtranspiration. (b) Consumptive use is evapotranspiration, plus the small amount of waterretained in a plant. Table 3-6 indicates that peak consumptive use for soybeans inNebraska is 11.7 mm/day.

15. (a) How do the losses by ET compare to water losses from a pan of water at the samelocation and time? (b) Why is knowledge of this relationship useful?

Answers: (a) Table 3-5 gives the relation. Notice that ET values range from12% to 107% of pan evaporation. (b) This relationship is useful because pan evaporationis easily measured and sometimes reported by the weather service.

16. If an average transpiration ratio is about 450:1, why is this ratio for some crops higherand other crops lower than this?

Answer: Crops require different periods of time to reach maturity, they grow indifferent climates, and they produce different kinds of yields (grain, fruit, forage, root).

17. What can be done to conserve water on a farm or other parcel of land?Answer: Many answers will be correct. Examples of conserving practices

include: using mulches and crop residues, using water-conserving tillage systems, using

99

efficient irrigation practices, using water-efficient plants, controlling run-off of waterwith terraces or other devices, and controlling blow-off of snow with windbreaks.

1010

CHAPTER 4: SOIL CHEMICAL PROPERTIES

OVERVIEW

The interesting chemistry in soils is found mostly in the soil water and at the surfaces ofsoil colloids—clay and humus. Humus is the partly decomposed organic matter residues.Most clays are minerals formed from the dissolution of other minerals, followed byrecrystallization. Crystalline clays are stacked layers of oxygen bonded to such elementsas aluminum and silicon. One kind of clay, kaolinite, has one layer of silicon and onelayer of aluminum bonded through oxygen atoms. This clay is non-sticky, non-expanding, and not very fertile. Montmorillonite is a clay known for stickiness, swellingand high fertility. It has one layer of aluminum between two layers of silicon, eachbonded through oxygen atoms. Illite and vermiculite are similar to montmorillonite butwith little swelling when wet. Some clay, such as the volcanic allophane, is amorphousrather than crystalline. The tropical sesquioxide clays are products of extremeweathering, and are relatively inert and infertile.

Most colloids have a negative charge. The charge on clay results fromisomorphous substitution, which can be considered as impurities in the clay crystals.Often a Mg2+ substitutes for an Al3+, or an Al3+ substitutes for a Si4+. Additional chargeon clays arises from deprotonation on the edges of crystals. Humus has a negative chargebecause humic substances are weak acids, deprotonating in neutral or high pHenvironments. Cations are attracted to the negative sites on clay and humus. The abilityto hold plant-nutrient cations is an important part of soil fertility. Humus has a largenegative charge. Clays have moderate (montmorillonite) to low (kaolinite) charge. Theamount of charge on colloids is measured as cation exchange capacity.

Other important chemical properties of soil include redox potential, acidity,salinity, fertility, and factors affecting the fate of pollutants. These properties arediscussed separately in subsequent chapters of the text.

QUESTIONS AND ANSWERS

1. (a) What are colloids? (b) How large are they? (c) What substances are colloids insoils?

Answers: (a) Colloids are particles so small that their surface area and surfaceproperties assume enormous importance compared to the importance of their mass.Particles this small may not obey the laws of classical physics, but rather behave likelarge molecules. (b) Clay colloids are smaller than 0.002 mm in diameter (and smallerthan bacteria). Other solids of similar size are also called colloids. (c) The principal soilcolloids are clays and humus.

2. (a) What are the clay minerals? (b) Are they inherited in some soils? (c) What arethree different meanings of the word clay?

Answers: (a) Clay minerals are secondary minerals formed from aluminum andsilicon (as with kaolinite, montmorillonite, illite, vermiculite) or from other positive ionsbonded to oxygens in crystalline or amorphous configurations. (b) Yes, some soil parentmaterial contains clay from the day it arrives on location. In general, however, soils are

1111

clay factories in which clay is produced from chemical weathering processes. (c) Clayrefers to a soil separate smaller than 0.002 mm in diameter, to a textural class on thetextural triangle, and to specific types of minerals regardless of their sizes.

3. From what materials do clays form?Answer: In neoformation (formation of entirely new minerals), clays form from

soluble silica and alumina and a few metals such as magnesium and iron. Clays can alsoform from degradation of small non-clay minerals such as mica, which transforms intoillite upon weathering.

4. (a) What charge [+ or -] do most clays carry? (b) What causes the charge?Answers: (a) Most clays have a negative charge. (b) The negative charge is

caused by isomorphous substitution, and by deprotonation on the crystal edges. Thissecond type of charge is referred to as pH-dependent charge.

5. How does (a) montmorillonite differ from kaolinite? (b) allophane differ fromsesquioxides?

Answers: (a) Montmorillonite has a large negative charge, is sticky, expandswhen wet, and has two silica tetrahedral layers to one alumina octahedral layer. Kaolinitehas little negative charge, does not swell, becomes only slightly sticky, and has one silicatetrahedral layer to one alumina octahedral layer. (b) Allophane is amorphous but is analumino-silicate. Sesquioxides may or may not be amorphous, but are primarily iron andaluminum sesquioxides formed in the wet, warm tropics.

6. In what climatic conditions are each of the various clays expected to form? Explain forsesquioxides, kaolinite, and montmorillonite.

Answer: Leaching removes silicates much more readily than aluminum or iron.Therefore, the clay with the most silicates (montmorillonite) will form in the environmentwhere the least leaching occurs. This may be a dry environment that leaches littlebecause little rain falls, or it might be in a landscape where the soil is so impermeable thatleaching is negligible. Kaolinite forms in the subtropics where substantial leachingoccurs. Sesquioxides form in the environments most conducive to leaching. Usually thatenvironment is the warm and wet tropics.

7. What is the nature of organic colloids?Answer: Organic colloids are humic acids, fulvic acids, and humin. These

organic molecules differ considerably, but all are composed of carbon chains and carbonstructures, often with carboxyl and phenol groups.

8. (a) Define cation exchange. (b) Illustrate cation exchange by an equation.Answers: (a) Cation exchange is the changing of places of two positively

charged ions. One such ion may migrate from the soil solution to the clay surface,trading places with another ion that was adsorbed to the clay. (b) See Section 4:4 for anexample equation.

1212

9. (a) Which ions are involved in cation exchange? (b) Which ions are dominant instrongly acidic soils?

Answers: (a) Any cation can participate in cation exchange. The most commoncations include Ca2+, Mg2+, K+, Na+, H+, and Al3+. The heavy metals and some molecularcations such as ammonium also participate. (b) In strongly acidic soils the dominantcations are Al3+ and H+.

10. List some typical CEC ranges for (a) pure clays, (b) humus, and (c) typical soils.Answers: (a) See Table 4-3. (b) 150-300 cmolc/kg. (c) See Table 4-6.

11. How does soil pH alter CEC?Answer: As pH increases, CEC increases. This is because OH groups on crystal

edges act like weak acids and deprotonate as the pH increases.

12. Under what conditions might soils have appreciable AEC?Answer: In highly acidic environments, especially when the dominate clays are

sesquioxides, the AEC may exceed the CEC.

13. (a) What is soil pH? (b) What are the extreme pH values found in soils?Answers: (a) Soil pH is a measure of the acidity or basicity of the soil solution.

The pH is the negative logarithm of the hydrogen ion activity. (b) Soil pH values rangefrom about 3.5 to about 9.5.

14. In what ways might soil pH adversely affect plant growth?Answer: Strongly acidic soils often have toxic amounts of elements, particularly

aluminum and manganese. Strongly basic soils often have deficiencies of nutrients suchas iron. Many other effects can exist; for example, the bacteria necessary for a healthycrop of alfalfa cannot thrive in acidic soils even though the alfalfa itself could.

15. (a) Define BCSP. (b) Is the BCSP high or low in the high pH soils of arid regions?Answers: (a) BCSP (basic cation saturation percentage) is the percentage of

cations on the cation exchange surfaces that are not acid, i.e., that are not Al3+ or H+. (b)BCSP is usually at or near 100% in arid soils.

16. Explain why two soils might have different buffering capacities.Answer: Soils with large percentages of clay and/or humus are highly buffered

because of the many sites on these colloids where deprotonation occurs. Each clay orhumus colloid acts as a weak acid when protonated and a weak conjugate base whendeprotonated. Sandy soils, having little clay or humus, lack these buffering sites. Othersoils might be buffered at a high pH value because of the presence of large quantities ofbasic minerals, especially lime (CaCO3).

1313

CHAPTER 5: ORGANISMS AND THEIR RESIDUES

OVERVIEW

Soils contain arrays of organisms, ranging from the highly beneficial decomposing andnitrogen-fixing bacteria, to the pathogenic fungi, to the landscape-altering prairie dogs, tothe roots of higher plants. These organisms alter the soil−especially the organicmatter−and eventually become non-living organic matter themselves. The term organicmatter refers collectively to humus and to larger, recognizable organic tissues, all ofwhich are comprised largely of organic (i.e., reduced) carbon. A few animals greatlyimpact the soil. One, of course, is the human. He is not discussed in this chapter.Burrowing mammals may aerate the soil, but also consume organic products, and may domore harm than good. Earthworms aerate the soil, usually in ways that leave the soil inbetter condition. Unwise use of farm chemicals can harm populations of earthworms.Invertebrates such as insects, slugs, and nematodes often prey upon beneficial plantsgrowing in the soil.

Green plants have a tremendous impact on the soil. They are the ultimate sourceof nearly all soil organic carbon. Smaller amounts of soil organic carbon originate withalgae, which often fix both atmospheric carbon and atmospheric nitrogen. Differentspecies of fungi are of great importance to soil, for one of three reasons: (1) many aredecomposers, converting organic residues back to atmospheric CO2, (2) many arepathogens, inhibiting the growth of higher plants, and (3) many form mycorrhizalrelations with higher plants, effectively increasing the ability of roots to acquire plantnutrients.

Bacteria may be the most essential of all organisms in the ecosystem. Like thefungi some are decomposers and others pathogens. Unlike the fungi, however, somespecies of bacteria have the ability to acquire atmospheric nitrogen, thus contributing tothe fertility of the soil. Also, autotrophic bacteria can obtain energy from sources otherthan reduced carbon. The cyanobacteria, formerly called blue-green algae, are primitiveorganisms with the ability to fix atmospheric nitrogen and to perform photosynthesis in ahuge range of aquatic and moist soil environments. Filamentous bacteria known asactinomycetes are important decomposers and can occasionally be pathogenic.

Encouraging healthy soil microorganisms is a vital component for improving soilquality. Wise management of tillage, water, and farm chemicals is crucial to wisemanagement of organisms. Levels of soil organic matter are generally thought to belower than optimum. Greater amounts of soil organic matter would protect soil againsterosion, improve soil water properties, and generally enhance plant growth. Muchattention is presently focused on improving farming systems to encourage healthier andmore sustainable soil environments. Organic farming seeks to minimize or eliminate theuse of pesticides and manufactured fertilizers on soils and crops.

QUESTIONS AND ANSWERS

1. (a) In which ways can earthworms be beneficial to the growth of plants and yet benonessential? (b) What are the practical solutions to a nematode infestation?

1414

Answers: (a) Earthworms dissolve some nutrients, and aggregate and aerate thesoil as it passes through their digestive systems. However, plants grow almost as wellwithout them as with them. (b) Nematodes can be controlled with pesticides, resistantplant varieties, and following principles of sanitation whereby soils are not contaminatedwith infested materials.

2. Does the rhizosphere have a different pH and/or microbial activity than the bulk soil?Explain.

Answer: The rhizosphere, the zone within about 1 mm of the plant root, containsroot exudates, dead root cells, and numerous microorganisms. The rhizosphere pH isusually 1 or 2 pH units more acidic than the bulk soil. This is due primarily to theexcretion of H+ ions as the root takes in cation nutrients. Microbial activity is muchgreater in the rhizosphere than the bulk soil because the supply of carbon food is muchricher there.

3. How important are fungi as (a) organic matter decomposers? (b) plant pathogens? (c)toxins to people or animals?

Answers: (a) Fungi are very effective decomposers of organic matter, especiallywoody material or material in acidic environments. (b) Most plant diseases are caused byfungi. (c) Fungal molds growing on grains can be highly toxic to humans. The dreadedaflatoxin is the best-known example of fungus toxicity to humans. A fungal endophytethat lives as a parasite of fescue can be toxic to livestock.

4. (a) Define mycorrhizae. (b) How extensive are mycorrhizae in nature?Answers: (a) Mycorrhizae is a symbiotic association between fungus and the

roots of higher plants. (b) Most plants are mycorrhizal; however, few plants are obligatemycorrhizal (i.e., unable to grow without the fungal association).

5. (a) How are nitrogenase and Rhizobium related? (b) What are legume nodules? (c)How much nitrogen is fixed by some common legumes?

Answers: (a) Nitrogenase is the enzyme that Rhizobium possesses, allowing it tofix atmospheric nitrogen. (b) Nodules are the enlarged root areas in which colonies ofRhizobium reside. (c) See Table 5-3 in the text.

6. (a) What are actinomycetes? (b) What beneficial functions do they perform?Answers: (a) Actinomycetes are filamentous bacteria, outwardly resembling

small, simple fungi. (b) They decompose organic materials, and are especially effectivein high temperatures or when decomposing some highly resistant materials. Someactinomycetes have the ability to fix atmospheric nitrogen.

7. (a) Are viruses living organisms? (b) How do viruses cause damage?Answers: (a) Viruses do not metabolize energy and are not living organisms.

They typically are genetic material encased in protein. (b) The genetic material from avirus can disrupt the normal function of a host cell, often causing the cell to producemore viruses.

1515

8. For the majority of soil organisms, what might be considered optimal conditions of soilmoisture, temperature and pH?

Answer: Most organisms prefer a soil moisture content of 80 to 90% of fieldcapacity to allow easy access to water, but with adequate aeration. Organisms vary intheir preferred temperatures and pH values, but many thrive at temperatures between 30and 35 oC, and at moderate pH values, such as 6.

9. What management practices encourage growth of beneficial organisms?Answer: Many practices could be considered. In general maintaining high levels

of soil organic matter, providing adequate aeration, and avoiding excessive use ofpesticides and some fertilizers will help soil microorganisms.

10. What are the origins and general composition of soil humus?Answer: Humus is the remains of previously living organisms, mostly plant

roots. It is in various stages of decomposition, but is not the most actively decomposing,fresh organic materials. Soil organic matter is not called humus until it has stabilized intolarge molecules of humic acid, fulvic acid, and humin.

11. What is an enzyme?Answer: An enzyme is an organic catalyst that lowers the activation energy of a

reaction, normally facilitating a reaction that would not otherwise occur.

12. For what plant nutrient is soil organic matter the most important?Answer: The nutrient for which organic matter is most important is nitrogen.

Common soil minerals do not contain nitrogen, so the soil storehouse of nitrogen is theorganic matter.

13. How does organic matter affect soil aggregation?Answer: Many organic substances can cement soil particles together, thereby

creating aggregates. Also, soil is bound together by plant roots and fungal mycelia, eitherliving or dead.

14. Briefly discuss possible beneficial and detrimental effects of allelopathy.Answer: Some plants release organic compounds that are toxic to other plants.

This phenomenon, called allelopathy, can interfere with the establishment of newplantings. In addition to its natural purpose, allelopathy can be used beneficially forweed control without artificial chemicals.

15. Evaluate animal manures as soil amendments and fertilizers.Answer: Animal manures can be valuable amendments for improving soil

organic matter and providing the many benefits that organic matter imparts. However,manures can also be deleterious to the soil because they contain excessive levels of salts,heavy metals, or weed seeds. Manures contain plant nutrients, especially nitrogen, andcan be useful as low-grade fertilizers.

16. List the conditions and materials required for making compost.

1616

Answer: Composting requires a good source of easily decomposed carbonmaterials, adequate heat, moisture, aeration, and may require addition of fertilizer orlime.

17. Define LISA and tell why the concept of LISA is important.Answer: LISA is an acronym for low-input sustainable agriculture. Low-input

sustainable agriculture will become more important in the future as resources such asfuel, water, and chemicals become more scarce and demand for those resources increases.

18. Discuss the pros and cons of organic farming.Answer: Organic farming avoids chemical contamination of crops and soils.

Well-managed organic farms can produce more product per unit of fuel consumed thando conventional farms. Organic farming often produces crops with fewer and cheaperinputs, but does not produce greater total yields. Organic farming requires differentmanagement skills, including an understanding of pest life cycles and organic fertilizerreactions.

1717

CHAPTER 6: SOIL FORMATION AND MORPHOLOGY

OVERVIEW

Soils vary tremendously. Hundreds of different minerals make up soils and theseminerals change as they weather over centuries of time. Weathering refers to physicaland chemical changes, such as fracturing, dissolving, and oxidizing. Each soil isdifferent from another because of the effects of the five soil-forming factors: climate,relief, organisms, parent material, and time. Water moving through soil dissolves andtransports materials though the soil column. Many agents transport and sort soilmaterials. Glaciers grind and move rocks, depositing moraines. Wind sorts and depositsloess (silty material) and fine sands. Streams deposit alluvium along their flood plains.Because of the action of the various agents, each soil has a unique story and uniqueproperties. Some soils are fertile, some are acid, and some are alkaline or salty. Somesoils are deep while others are shallow. Some are young, while others are ancient. Theformation of soils and the formation of geologic landforms such as flood plains andmoraines are closely linked.

As soils undergo changes they develop horizons. Horizons form approximatelyparallel to the soil surface. Some horizons have accumulated organic material andbecome brown, black, or gray. Subsurface horizons may accumulate clays or solublesalts that dissolved in the upper layers of the soil and were washed downward byrainwater. Upper case letters are used to designate main (master) horizons. A designatestopsoil; B designates subsoil; and C designates parent material (the material in whichother horizons formed but relatively unaffected by weathering because of its depth in thesoil). In addition to these master horizons, three other master horizons, though lessfrequently encountered, are also used. O designates a horizon that is organic rather thanmineral; E designates an eluvial horizon from which material has been leached (washed);R designates bedrock. Each of the master horizons can be described by coded adjectivesthat are lower-case letters. For example a Bt horizon is subsoil with a clay accumulation.In addition, various diagnostic horizons are also used to classify and describe soils. Forinstance, the mollic epipedon is a diagnostic horizon. It is topsoil that is deep, dark, andfertile, indicating a particular type of soil (Mollisols).

New technology such as GIS and GPS are useful in soil science. GeographicalInformation Systems (GIS) allow practical use by municipalities and landowners ofdetailed soils information. The Global Positioning System (GPS) facilitates precisionfarming based on soil properties in specific regions of a farmer’s field.

QUESTIONS AND ANSWERS

1. (a) What is a soil profile? (b) What is changed in a soil profile as the soil develops? Answers: (a) A profile is a vertical cut, often about 2 meters deep, showing thesoil horizons and characteristics. (b) As a soil develops, soluble materials may be moveddownward with percolating water. Organic matter accumulates from old plant roots, cropresidues, leaf fall, etc. Clays form in the soil. Soils in humid areas may become moreacidic, while soils in arid regions may become more salty. Many other changes occur.

1818

2. What is (are) the likely soil texture(s) of soils moderately developed from (a) groundmoraine? (b) loess? (c) flat lowland river terraces? (d) glacial moraine outwash? (e)soil creep materials? and (f) dune landforms?

Answers: (a) Rocky but with anything from sandy to clayey in the finer portion.(b) Silt as a dominant particle size (e.g., silt, silt loam, silty clay loam). (c) Clay becauseof the gentle slope. (d) Gravelly and sandy textures. (e) Creep normally comes fromslippery clays, so the soil will likely be clayey. (f) Dunes are nearly always sandy;however, clay dunes exist in a few locations.

3. (a) Define landform. (b) Define a soil horizon.Answers: (a) A landform is a natural Earth surface left by the deposition and/or

mixing by action of wind, water, ice, or gravity. Examples include sand dunes, riverflood plains, and glacial moraines. (b) A soil horizon is a horizontal layer of soilapproximately parallel to the soil surface and that has properties different from otherlayers of that soil profile.

4. Describe in simple terms (a) an A horizon, (b) an R horizon, (c) a B horizon, and (d) anE horizon.

Answers: (a) The A horizon is a surface mineral horizon. (b) The R horizon isbedrock below the soil. (c) The B horizon is a subsoil horizon that accumulates colloidssuch as clay and humus from upper horizons. (d) The E horizon is a mineral horizonfrom which humus and other colloids have been leached.

5. Describe how to calculate the slope gradient of the land surface.Answer: Expressed as a percentage, the slope gradient is: ((difference in

elevation) ÷ (horizontal distance)) X 100%.

6. Much amorphous silica in soils is quite soluble in alkaline solution. How can this helpexplain the presence of duripans below about a meter (39 inches) in many arid-regionsoils?

Answer: Arid-region soils are usually alkaline, and thus will dissolve silica.Occasional heavy rains in some arid regions will move silica to depths of 2 to 4 feet,where drying allows the silica to precipitate and form silica-cemented hardpans calledduripans.

7. What materials cement mineral particles together in various horizons?Answer: From the most common to the least, the materials are clays and humus

(which forms soil aggregates), calcium carbonate (which forms calcic and petrocalcichorizons), silica (which forms duripans and fragipans), gypsum, and iron oxides.

8. Briefly define the following horizons: (a) mollic, (b) albic, (c) argillic, (d) calcic, and(e) ochric.

Answers: (a) Deep, dark, fertile topsoil. (b) Light-colored strongly leachedhorizon, usually near the soil surface. (c) Subsurface accumulation of clay. (d)Subsurface accumulation of calcium carbonate. (d) Thin, light-colored surface horizon.

1919

9. Describe, generally, the following landforms: (a) plateau, (b) alluvial fan, (c) delta, (d)river terrace, (e) floodplain, (f) eolian deposits, (g) loessial deposits, (h) ground moraine,and (i) colluvium.

Answers: (a) Plateaus are land surfaces, often expansive, of relatively flat areasusually eroded from layered rock such as sandstone. (b) Alluvial fans are slopingdeposits from water flowing out of canyons, as the water reaches less steep foot slopesthe velocity of the water decreases and some sediments are fall out producing sandy andgravelly deposits. (c) Deltas are stream sediments deposited into lakes or seas. Thesedeposits can range from clay to gravel. (d) River terraces are old river bottoms or floodplains, now higher than the river as the river cuts deeper into the Earth. (e) Flood plainsare low-lying areas where rivers commonly flood. (f) Eolian deposits are sandy or siltywind-blown deposits. (g) A loessial deposit is a particular type of eolian depositdominated by silt-sized particles. (h) Ground moraines are the land areas left from underthe glacial ice when it melted. (i) Colluvium is a general term for material moved downslope by the force of gravity. Colluvial deposits can be fine textured materials frommudslides, or can be coarse rock debris.

10. Write horizon symbols for (a) a B horizon with large sesquioxide accumulations, (b) aB horizon with high accumulation of “normal” clays, (c) a gleyed C horizon, (d) a Bhorizon with high carbonate accumulations, and (e) a cemented C horizon.

Answers: (a) Bs, (b) Bt, (c) Cg, (d) Bk, and (e) Cm.

11. What is meant by two capital letters together such as EB or BA when referring to soilhorizons?

Answer: Each combination indicates a soil layer that contains features of eachhorizon and the two horizons cannot be separated easily. The first letter indicates eitherthe dominant upper portion of the layer, or it indicates the horizon comprising more thanhalf of the mixed horizon.

12. What is a soil individual?Answer: A soil individual is the minimum volume that can be called a soil. It is

also called a pedon. A pedon must be uniform, and of at least one square meter in area.

13. Why would one soil form an ochric epipedon while another soil forms a mollic?Answer: The ochric epipedon is likely the result of low organic matter because

of low rainfall or very rapid organic matter decomposition. The mollic epipedontypically forms in a prairie where much organic matter is continually added to the soil.

14. (a) List the five soil-forming factors. (b) Is erosion one of these factors? (c) Howdoes each factor affect the final soil that is formed?

Answers: (a) The five soil-forming factors are: climate, relief, organisms, parentmaterial, and time. (b) Erosion is not a soil-forming factor, it is a soil-destroying factor atthe site of the erosion, but it may produce new parent material for soil formationelsewhere. (c) Climate affects the rates and types of weathering that occurs on a site andalso directly affects the vegetation on a site. Relief modifies climate by affecting waterdistribution, temperature, and wind. It also affects whether a site gains or loses soil as

2020

soil particles move due to other agents. Organisms, especially higher plants, profoundlyaffect soil conditions. They determine the amount of ground cover to protect againsterosion. They control nutrient cycling. They produce residues that will become humus.Parent material regulates the rate of weathering and which minerals will be released inweathering processes. Time is required for each process to occur. Longer times for soil-forming processes to act result in more mature soil with more extensively changedfeatures.

15. How can soils be degraded?Answer: Soils can be degraded in many ways, including: leaving them bare and

unprotected, compacting them thereby reducing porosity, adding pesticides or otherchemicals that adversely impact beneficial organisms, and failing to replace organicmatter and nutrients that are lost from the soil.

16. Define (a) GIS, and (b) database.Answers: (a) A GIS is a geographical information system that relates many sets

of information (databases) to geographical sites, thereby providing a multitude of factspertaining to a given site. (b) A database is a set of facts or data. A soils database mightinclude rainfall data for various sites, or depth to water table, for example.

17. Describe the GPS system and describe some of its uses.Answer: GPS is a global positioning system. This system allows one to locate

their longitude and latitude position. It uses radio waves from Department of Defensesatellites. It is used to indicate locations and routes for vehicles, to pinpoint locations forocean-going vessels, to locate sampling or work sites, and to track animals.

2121

CHAPTER 7: SOIL TAXONOMY

OVERVIEW

Soil Taxonomy, as described in the textbook, is the official USDA system for describingand classifying soils. The highest and most general category in the system is the order.Twelve soil orders are now recognized. The twelve orders are; Gelisols (soils withpermafrost), Entisols (soils with little or no profile development), Inceptisols (soils withweak profile development in non-arid climates), Andisols (soils formed in volcanicparent material), Histosols (soils dominated by organic matter accumulations in wetlandsor forests), Aridisols (soils developing in dry climates), Mollisols (grassland soils withdeep, dark surface horizons), Vertisols (soils dominated by shrink-swell clays), Alfisols(soils of deciduous forests with moderately fertile topsoil and with subsoil clayaccumulations), Spodosol (soils with leached, acidic, sandy layers), Ultisols (leached,weathered, and acidic soils of the humid subtropics), and Oxisols (excessively weatheredsoils of the tropics).

Other categories of the taxonomy system are suborder, great group, subgroup,family, and series. Soil moisture regimes, temperature regimes, mineralogy, and manyother factors are used in the lower categories to differentiate soils. Taxonomic names andformative elements were invented from Latin, Greek and other roots. A completetaxonomic name indicates each level of the taxonomy. The Elrose soil is a member ofthe Fine-loamy, siliceous, thermic Typic Paleudalfs. “Fine-loamy, siliceous, thermic”indicates the family. “Typic” indicates a “typical” subgroup. “Pale” indicates an “old”great group; “ud” following “pale” indicates a “udic” moisture regime, meaning thatwater is not limiting to plant growth. The final syllable “alf” indicates the Alfisol soilorder.

QUESTIONS AND ANSWERS

1. Define soil taxonomy.Answer: Soil taxonomy is a system to classify and describe soils.

2. Define (a) a root of an order, and (b) a formative element.Answers: The root of an order is a syllable to be used in a taxonomic name to

indicate the soil order. It is the two or three letters preceding the vowel that precedes“sol” in the order name. For example, the root of Spodosol is “od”; therefore, a soil nameending in “od” is a Spodosol. (b) Formative elements are words or syllables used toindicate suborders, great groups, etc. For example Paleudalf” uses the formativeelements “Pale” and “ud.”

3. Soils of which order are the most extensively weathered?Answer: Oxisols

4. Which soil order has the least developed soils?Answer: Entisols

2222

5. Tell which soil order probably fits soils in these locations: (a) recent deep ash deposits,such as that from Mt. St. Helens’ eruption in 1980, (b) old alluvium in arid Nevada, (c)tall grass prairies of the Great Plains, and (d) the well-weathered, old, red soils ofGeorgia.

Answers: (a) Andisols, (b) Aridisols, (c) Mollisols, (d) Ultisols

6. Why are some Alaskan soils and some wet, tropical-island rice-paddy soils bothconsidered to be Inceptisols?

Answer: Inceptisols are developing but are not yet mature, or fully developed.This situation can occur in many diverse locations.

7. (a) How are xeric and ustic moisture regimes different? (b) How do udic and aquicdiffer?

Answers: (a) Xeric and ustic moisture regimes are both somewhat waterdeficient for plants; however, the wetter season in the xeric regime is the off-seasonswhen plants are not growing, while the wetter season in the ustic regime is the growingseason. (b) Udic indicates moisture is nearly always adequate; aquic indicates that thesoil is saturated and anaerobic at times.

8. (a) How are temperature regimes categorized? (b) What does iso mean in thesenames?

Answers: (a) Temperature regimes are based on the mean annual soiltemperatures at a soil depth of 50 cm. The coldest regime is cryic, followed by frigid,mesic, thermic, and hyperthermic, the warmest. (b) The prefix iso means the summer andwinter temperatures differ by less than 6 oC.

9. What are some challenges in managing Histosols?Answer: If wetlands, draining them is legally restricted. If drained they

normally subside and can be flammable. They cannot support the weight of moststructures such as roads and buildings.

10. (a) Are Entisols productive? (b) Are Entisols unweathered materials?Answers: (a) Entisols vary widely from sand dunes to flood plains. Their

productivity also varies from highly productive to essentially non-productive. (b) Soilsare not made from unweathered materials, because nearly all parent materials have beenweathered at some point in geologic time. Soils made from newly formed volcanic ashare the exception to this rule. Entisols lack profile development and have not weatheredappreciable on their present site.

11. (a) What is unique about Vertisols? (b) What specific management problems existwith Vertisols?

Answers: (a) Vertisols are high in clays that shrink when dry and swell when wet.(b) Vertisol management is challenging. They are hard to wet by irrigation or rainbecause the surface swells and seals when exposed to water. They are sticky when wetand hard when dry. Aeration can be poor. They are often treated like shallow soils.

2323

They are unstable for engineering purposes and often cause cracked building foundations,or damaged roads.

12. To what extent are fertilizer and lime needed in Oxisols and Ultisols?Answer: Because these soils are highly leached, both fertilizer and lime are

usually needed for good crop production. Many Ultisols and Oxisols support forests orgrasslands, with little if any commercial inputs of lime and fertilizer.

13. (a) Where might you find extensive areas of Aridisols? (b) What are some typicalcharacteristics of Aridisols?

Answers: (a) Aridisols are found in areas with low effective precipitation.Extensive Aridisol acreage is found in the southwestern U.S., northern Africa, the MiddleEast, and Australia. (b) Aridisols are low in organic matter, have little profiledevelopment, and often have high salinity.

14. (a) How good are Mollisols for crops? (b) How should Mollisols be managed? (c)Are Mollisols important soils for the United States? Explain.

Answers: (a) Mollisols are naturally fertile but sometimes moderately dry. Theyare among the best soils for crops. (b) Management of Mollisols is not difficult exceptfor their need for protection against erosion. (c) Mollisols are extremely important to theUnited States because they are the most abundant soils in the United States and arenormally quite productive.

15. (a) Why are Alfisols naturally productive soils? (b) Why might some Alfisols bemore productive than many Mollisols?

Answers: (a) Alfisols are moderately high in fertility and usually occur in amoisture regime providing plenty of water. (b) Alfisols usually receive greater rainfallthan Mollisols. Water is often a limiting factor for plant growth in Mollisols, but is lesslikely to limit plant growth in Alfisols.

16. (a) Why are Spodosols poorly suited for growing a variety of crops? (b) Where areSpodosols found?

Answers: (a) Spodosols are infertile, being sandy, acidic, and lacking in plantnutrients. (b) Most Spodosols occur in cool forests such as in New England orScandinavia.

17. Locate on the U.S. soil map the dominant soil orders of (a) the Midwest, (b) theSoutheast, (c) the Great Plains, (d) the Southwest, and (e) the Northeast.

Answers: (a) Alfisols, (b) Ultisols, (c) Mollisols, (d) Aridisols, and (e)Spodosols.

18. Locate on the world map the large areas of (a) Mollisols, (b) Aridisols, (c) Alfisols,and (d) Ultisols.

Answers: (a) U.S. Great Plains, Argentina, Eastern Europe, (b) southwesternU.S., northern Africa, Middle East, Australia, (c) eastern Europe, sub-Saharan Africa, (d)southeastern U.S., southeastern Asia.

2424

CHAPTER 8: ACIDIC SOILS

OVERVIEW

Soil acidity refers to the relative abundance of H+ and Al3+. Soils in humid regionsbecome acidic as rainwater flows through them, replacing the basic cations with H+.Various aluminum species regulate and buffer the pH of acid soils. When soils are tooacidic for their intended use, such as crop production, they are treated with lime. Lime isa base (i.e., raises the pH) and Ca2+. Usually the base is carbonate but it can behydroxide, oxide or other materials. In the case of dolomitic lime, both Ca and Mg arepresent. When lime is added to soil the Ca2+ exchanges with the Al3+ and H+ onadsorption surfaces. The Al3+ then precipitates, and the H+ links with OH- to becomewater.

Some plants thrive in acidic soils. Azaleas, blueberries, cranberries, andpineapples are examples. However, Al3+ is abundant below pH of about 5.5, and thisaluminum species is toxic to most plants. Most crops prefer soils that are just slightlyacidic, with pH values between 6 and 7. Nutrient availability is closely linked to soil pH.In acidic soils phosphorus and molybdenum are not readily available, but manganese maybe excessively available, so that some plants experience both manganese and aluminumtoxicity. At high pH values iron, manganese, and zinc are often deficient for plants.

In the rare instance that one would wish to acidify soil, this could beaccomplished using elemental sulfur, iron, or aluminum compounds.

QUESTIONS AND ANSWERS

1. Explain why so many soils are acidic and why some soils remain alkaline.Answer: Soils become acidic as their “basic cations” are leached away. This

loss of cations allows the H+ in rain to attach to the soil solids. Where leaching isminimal in relation to the amount of basic cations in the soil, the soil may remain basic.

2. Explain how (a) humus decomposition, (b) root respiration, and (c) N fertilizersproduce acidity.

Answers: (a) As humus decomposes, carbon dioxide forms in the soil. Carbondioxide combines with water to form carbonic acid. (b) Root respiration also releasescarbon dioxide. (c) Most N fertilizers are based on ammonia products that are quicklyconverted by soil microbes to nitrate and H+.

3. What are the dominant ions on the cation exchange sites of low-pH soils?Answer: H+ and Al3+.

4. Why are strongly acidic soils poor growing media for most plants?Answer: Strongly acidic soils often have toxic levels of aluminum and

manganese, and reduce microbial action.

5. (a) Define lime. (b) What actual material is most used as agricultural lime?

2525

Answers: (a) Lime is a base, added to the soil to raise the pH. (b) The mostcommon lime is CaCO3.

6. What are the important considerations in selecting a lime?Answer: Quality of lime is determined by the chemical compound involved, the

fineness of the product, and the purity of the product.

7. (a) In general terms, what does lime do when added to acidic soils? (b) How doesliming improve the soil for growth of most plants?

Answers: (a) Lime neutralizes the acid, turning it into harmless products. (b)Liming improves microbial activity, controls aluminum and manganese toxicity, and addscalcium to the soil.

8. Compare the importance of lime versus fertilizer.Answer: Any soil used for cropping systems that break the nutrient cycle by

removing products from the field, either presently needs fertilizer or will eventually needfertilizer. Many soils need regular inputs of lime just as they need regular inputs offertilizer. However, unlike fertilizers, many soils do not and will not need lime.

9. Do all crops require liming of acidic soils? Discuss.Answer: No, not all crops require liming of an acidic soil. Some crops are quite

tolerant of acidic soils; however, they tend to be specialty crops like tea and blueberries,not the staple crops that feed the human race.

10. What is the preferred method of adding lime to soils? Why?Answer: Lime is normally broadcast in large doses, such as a few tons per acre.

The lime is then incorporated by tillage. Broadcast followed by incorporation isnecessary because of the large amounts usually needed.

11. Is lime effective on no-till lands? Explain.Answer: Lime performance on no-till land has been surprisingly effective. It

works slower without tillage, as one would expect. However, over time the acidity of thetreated soil can be neutralized under no-till systems.

12. In relative terms, evaluate clays, sands, low CEC, and high CEC acidic soils for theirneed for lime.

Answer: Assuming that all soils listed have the same pH, the clay would containmuch more residual acidity (i.e., more H+ ions on surfaces) than the sand and would needmuch more lime. Likewise, the high CEC clay would require more lime than the lowCEC clay.

13. (a) How can soils be deliberately acidified? (b) Which materials are used?Answers: (a) Soils can be acidified either by directly adding acid, or by adding a

material that produces acid. (b) One can apply acids such as waste product sulfuric acidto the soil. Many materials produce acidity in the soil, including: elemental sulfur,ammonium fertilizer, Fe3+ salts, and Al3+ salts.

2626

CHAPTER 9: SALT-AFFECTED SOILS

OVERVIEW

The word “salt” is not limited to table salt. All ionic compounds in soils are called salts.Those salts that are more soluble than gypsum are considered to be soluble salts. Theseare the ones that cause problem soils. Salts can rise to upper parts of the soil fromshallow groundwater, can be left over from salty parent material, or can be added withsalty sea spray or irrigation water. Regardless of the source, salts can interfere with plantgrowth and various intended uses of soil. Extracting water from soil becomesincreasingly difficult for plants as soil salinity increases. Certain elements common tosalty soils, most notably Na, Cl, and B, are toxic to many plant species. Soils containinghigh levels of soluble salts are called saline soils. Furthermore, sodium can profoundlyaffect soil properties by dispersing clay colloids. Once dispersed, these colloids form ahard surface or subsurface layer that tends to seal the soil against incoming water. Soilsthus affected are called sodic soils, and are very unproductive.

Plants differ greatly in their response to salty soil. Most fruit and vegetable cropsare highly susceptible to salt damage. Some field crops such as beans, rice, and soybeanare highly susceptible to salts; whereas others, such as barley and cotton are very salttolerant. Likewise, with the forage crops, some are salt-susceptible, others are salt-tolerant.

Soil salinity is evaluated by the electrical conductivity (EC) of a saturated extract,by the exchangeable sodium percentage (ESP) and sodium adsorption ratio (SAR) of thesoil. If a soil is judged to be unacceptably salty the problem may or may not be treatable.If the problem arises from a salty shallow groundwater, the problem can only be solved ifsubsurface drainage is possible. Managing irrigation water to leach salts below the rootzone of plants is a standard approach to soil salinity. If the soil is sodic, a form ofcalcium is needed to exchange with the adsorbed sodium. Gypsum is usually used tosupply this calcium. Choosing salt-tolerant plants, and keeping a salt-affected site wellvegetated will help remedy a salt problem.

QUESTIONS AND ANSWERS

1. Can soluble salts be seen? Explain.Answer: Dissolved salts cannot be seen in the soil. As salts precipitate in dry

soils they may or may not be visible. Low or moderate concentrations of soluble saltsnormally cannot be seen in the soil. However, where salts concentrate and precipitateinto large subsoil specks, or where salts rise and precipitate on the soil surface, they canbe seen.

2. Explain what each of these tells about salt in soil: ESP, SAR, and EC.Answer: ESP and SAR are both used to determine the extent to which sodium

dominates the soil salts. An ESP greater than 15 or an SAR greater than 13 indicates asodic soil. EC is a measure of total soluble salts in the soil or in water. Measuring ECdoes not identify specific ionic species. An EC value greater than 4 dS/m from asaturated extract indicates a saline soil.

2727

3. (a) What are the sources of soluble salts? (b) What are the most numerous ions ofsoluble salts?

Answers: (a) Soluble salts in soils come from shallow groundwater, from parentmaterial, from sea spray, and from irrigation water. (b) The most numerous ions ofsoluble salts are: Na+, K+, Ca2+, Mg2+, Cl-, SO4

2-, and HCO3-.

4. What is the effect of a high ESP or SAR value?Answer: A high ESP or SAR values indicates a high tendency for the soil to be

hard when dry and to have low permeability to water. Either SAR or ESP can be used toidentify a sodic soil. The two values tend to be numerically similar in the range commonto soils (ESP values from 5 to 20 equal SAR values from 5 to 18), but SAR is easier tomeasure.

5. If calcium precipitates as insoluble carbonates, how will this change the soil SAR?Answer: Soluble calcium is in the denominator of the SAR formula. If dissolved

calcium precipitates it is no longer soluble, so the denominator becomes smaller and theSAR increases.

6. Give definitions for sodic, saline, and saline-sodic soils.Answer: See Table 9-2.

7. Could a non-saline soil contain enough salt to damage crops?Answer: Highly salt-sensitive crops such as strawberries and beans are damaged

by salt levels much lower than the saline soil threshold of 4 dS/m.

8. How does a high salt content reduce plant growth?Answer: Soil salinity interferes with the plant’s ability to extract water from the

soil. Also, some components of soil salt, such as Na, Cl, and B are toxic to many plants.

9. (a) What are the three key requirements needed to accomplish reclamation of salt-affected soils? (b) How does reclamation of sodic soils differ from reclamation of salinesoils?

Answers: (a) Reclamation requires adequate internal drainage, water to leachexcess salts, and an outlet for the leached salts. (b) Reclaiming saline soil requires onlythe removal of the salts. Reclaiming sodic soils requires that a harmless cation such ascalcium replace the sodium on the soil colloids, then that the sodium by removed byleaching.

10. (a) Why is gypsum sometimes used for reclamation? (b) What will happen if gypsum-treated soils are not adequately leached?

Answers: (a) Gypsum, unlike other common forms of calcium such as lime, issoluble in the high-pH environment of a sodic soil. Gypsum is the amendment of choicefor sodic soils because it is inexpensive and it dissolves readily. (b) If treated soils arenot leached they contain excess salts and are likely to be saline or saline-sodic soils untilthe excess salts are removed.

2828

11. Why is waste-salt disposal (reclamation waters, irrigation runoff water, etc.) a seriousproblem?

Answer: These wastewaters contaminate the body of water in which they aredisposed unless disposed in a salt lake or in the sea. The problem of salt-water disposalwill likely become more serious because fresh water is becoming more scarce, andincreased demand for crops from irrigated land will result in the production of more saltywastewater.

12. What are some management techniques for reducing salt buildup in irrigated soils?Answer: Many techniques can be used, singly or in combination. One can form

or level the land to minimize run-off of rain that can then be used to remove salts. Onecan perform periodic leaching of salts be adding an excess of irrigation water. One canselect the highest possible quality of irrigation water. One can install subsoil drains toremove shallow groundwater.

13. How could a farmer improve conditions in a saline seep?Answer: The farmer may be able to identify the source of the water in the seep.

If the source is a fallow field, as it often is, the fallow field could be converted to apermanently vegetated field. Also the farmer may be able to vegetate the region aroundthe seep, so the plants will extract water, lowering the water table.

14. How is salt measured in situ (in field soil at normal water content)?Answer: Various field devices based on the flow of electric current can be used

to measure soil salinity in the field.

2929

CHAPTER 10: PLANT NUTRIENTS: NITROGEN, PHOSPHORUS, ANDPOTASSIUM

OVERVIEW

Plants require thirteen mineral nutrients for growth. Roots encounter these nutrientsthrough the mechanisms of mass flow, diffusion, and interception. Some nutrients enterthe root passively, but others such as phosphorus and potassium are acquired actively asthe plant expends energy to secure the nutrient. The three major nutrients, nitrogen (N),phosphorus (P), and potassium (K), are discussed in this chapter. Much of themanagement of soils hinges on the management of N, P, and K. Ready availability ofthese nutrients is essential for healthy plants. In addition, N and P are among the mostcommon nonpoint-source pollutants.

The atmosphere is a great reservoir of nitrogen. A few types of microorganismscan convert atmospheric N2 to plant-available nitrogen. These organisms are the primarysuppliers of soil nitrogen in natural ecosystems. Industrial processes are used to convertatmospheric N2 to ammonia. Nitrogen undergoes many transformations in the soil, mostof which are mediated by microorganisms. Nitrogen can be lost from the soil as avolatile gas, as protein in a harvested crop, as a passenger on eroding soil, or as solublenitrate percolating toward groundwater, where it becomes a pollutant. Nitrogen fertilizeris used to overcome the various losses. It is by far the most common kind of fertilizerused worldwide.

Phosphorus is deficient for many crops, because it is present in soils in relativelysmall amounts, and because the P in the soil is relatively insoluble. Unlike nitrogen,phosphorus does not cycle back into the soil after removal. Phosphorus-deficient soilshave no natural source of supply, so fertilizer is applied to the soil. This fertilizer, thoughessential in one location, is deadly in another. Phosphates in streams and lakes contributeto algal blooms, after which the dissolved O2 needed by gilled organisms diminishes andfish die.

Potassium, though used by plants in large amounts, is often naturally adequate inarid and semi-arid soils. In soils of humid regions the potassium may be leached out. Asfarming continues decade after decade, however, the potassium is sure to be depletedfrom even the most fertile soils. It can easily be replaced by an inexpensive andenvironmentally safe potash fertilizer.

QUESTIONS AND ANSWERS

1. Draw a nitrogen cycle including these boxes: N2-fixation, mineralization, nitrification,denitrification, and leaching.

Answer: See Figure 10-3.

2. About how much nitrogen is released during decomposition of soil organic matter eachseason?

Answer: This varies enormously, probably from near 0 to about 100 kg/ha.Mineralization values in Table 10-3 range from 6 to 65 kg/ha.

3030

3. (a) How fast is nitrification: completed in seconds, days, or years? (b) Explain whynitrification rates differ in different environments.

Answers: (a) Assuming favorable conditions, most nitrification occurs withinseveral days. (b) Rates depend on temperature, moisture, pH, microbial population, andaeration.

4. (a) Is leaching loss of nitrogen typically great? (b) Under which conditions will thelosses be greatest?

Answers: (a) Leaching losses typically range from 0 to 80 kg/ha. (b) Losses aregreat when the soil is sandy or otherwise porous, when the clays have low CEC, whenwater flux through the soil is large, and when the dominant form of nitrogen is nitrate.

5. Tabulate the necessary conditions for (a) ammonia volatilization and (b) denitrificationlosses of nitrogen.

Answers: (a) Volatilization requires warm temperatures, high pH, and a supplyof ammonium on or near the soil surface. (b) Denitrification requires anaerobicconditions, decomposing organic matter, and a supply of nitrate.

6. Briefly tabulate some typical N additions and N losses to show how a soil’s nitrogenbalance changes.

Answer: Additions: fertilizer, biological fixation, organic residues and lightning.Losses: harvest, volatilization, denitrification, erosion, and leaching.

7. (a) Why are nitrogen fertilizers only 40% to 70% efficient? (b) What happens to thatpart not used?

Answers: (a) Nitrogen is inefficient because it is lost so easily and by so manyways. (b) The nitrogen not used by plants is immobilized into microbial biomass or islost by one or more of volatilization, denitrification, erosion, and leaching.

8. What are the purposes for using controlled-release N fertilizers?Answer: They are used primarily to keep nitrogen in a less vulnerable condition

until the plant needs the nutrient. This helps reduce loss of nitrogen.

9. Briefly discuss the problem of adequate phosphate uptake by plants as influenced by(a) solubility and (b) mobility in soil.

Answers: (a) Low solubility of phosphorus is the main reason that little of thephosphorus in soil in available to plants. Of course, it is also the main reason why soilshave any phosphorus in them at all. (b) Because phosphorus has poor mobility in soil, itis normally not delivered to roots to an appreciable extent by mass flow. It slowlymigrates toward the root by diffusion.

10. Explain why plant efficiency of phosphorus fertilizers is only about 10%-30%, muchlower than efficiency of nitrogen fertilizers.

Answer: Phosphorus is not lost from the soil (other than by erosion), but is lostfrom the realm of availability. Phosphorus dissolves, then precipitates as relativelyinsoluble compounds, or adsorbs very firmly to soil surfaces.

3131

11. (a) How much phosphorus is lost by leaching? (b) How much phosphorus is lost byerosion?

Answers: (a) Very little P is lost by leaching except on sandy soils in rainyclimates. (b) Eroding soil can carry much P with it. Erosion loss of P ranges from 0 toabout 60 kg/ha.

12. How important are mycorrhizae in facilitating phosphorus uptake by plants?Answer: Mycorrhizae is very important for most plants. Plants with

mycorrhizae are normally larger and healthier, due in large part to the greater supply ofphosphorus.

13. (a) What sources of potassium do plants use during a growing season? (b) Whichplants have high potassium requirements? (c) What is the ionic form of potassium insolution?

Answers: (a) Plants use mostly exchangeable potassium, but some potassium canbe supplied from weathering minerals. (b) Plants producing much sugar or starch havehigh potassium needs. Such plants include bananas, potatoes and sugarcane. (c) K+.

14. Explain why humid areas are likely to have inadequate available potassium, whereasarid regions are likely to have adequate supplies.

Answer: Potassium leaches out of humid-region soils, but remains in the soilprofile in arid regions.

15. (a) Is potassium volatile? (b) Is potassium released during humus decomposition?Explain.

Answers: (a) No. (b) Little potassium is released during humus decompositionbecause potassium is not a part of organic structures in plants.

3232

CHAPTER 11: CALCIUM, MAGNESIUM, SULFUR, ANDMICRONUTRIENTS

OVERVIEW

Calcium and magnesium behave similarly in soil. The chemistry of sulfur is much morecomplex than that of calcium or magnesium because sulfur undergoes many redoxreactions mediated by microbes. These three elements are the secondary plant nutrients.In addition, at least seven other elements, termed micronutrients, are essential to plants.Still other elements may not be essential to plants in general but are essential to at leastsome plants, or perhaps beneficial to a wide range of plants. These nutrients are termedbeneficial elements.

Calcium is normally the dominant cation in soil. Plants use it as Ca2+. It flowsreadily in soil. In those rare instances where calcium is deficient, it can be added as limeor gypsum. Magnesium like calcium moves readily in soil and is supplied to plants bymass flow. Plants use it as Mg2+. Various magnesium fertilizers are available, includingmagnesium sulfate and magnesium potassium sulfate. Some soil sulfur is replenished byatmospheric sulfur fallout from air pollution, volcanoes, etc. Microbes oxidize andreduce soil sulfur in performing metabolic functions. Consequently sulfur exists in manydifferent oxidation states ranging from the reduced S2- to the oxidized SO4

2-. Manysulfate fertilizers are available to correct deficiencies.

The seven generally recognized essential micronutrients include boron, chlorine,copper, iron, manganese, molybdenum, and zinc. Each has unique chemical behaviorand a unique function in the plant. All micronutrients are used by plants in smallamounts. Applying too much of any given micronutrient to the soil can often be moredamaging to a crop than not applying any. At least five additional elements are known tobe beneficial to at least some plants. These elements are cobalt, nickel, silicon, sodium,and vanadium.

QUESTIONS AND ANSWERS

1. How plentiful in soils are the nutrients calcium, magnesium, and sulfur?Answer: Calcium is plentiful and is rarely deficient. Magnesium is deficient in

heavily leached soils. Sulfur is an important component of organic matter; but in soilslow in organic matter or heavily leached, sulfur may be deficient.

2. Explain the unique calcium requirement of peanuts.Answer: Peanuts are typically grown in acidic soils where calcium is relatively

low. Because plants cannot translocate calcium in their phloem, a phloem-fed organ likea peanut needs special provisions to receive adequate calcium. Gypsum is often used tocorrect calcium deficiencies in peanuts.

3. How is grass tetany in cattle related to the nutrient magnesium?Answer: Grass tetany is caused by a low level of magnesium in the blood of

cattle. Cattle consuming forages low in magnesium may experience this disorder.

3333

4. What functions in the plant are performed by calcium and magnesium?Answer: Calcium is an important component of the cell wall. Magnesium is the

only metal in the chlorophyll molecule and is also needed for some metabolic cycles.

5. To what extent is sulfur deficiency likely to be more common in the future? Explain.Answer: Sulfur deficiency will likely be more common in the future for two

reasons. First, due to pollution control efforts, the air is cleaner than it was several yearsago so less sulfur is added to the soil from atmospheric fallout. Second, we continue topush crops for higher yields on soils that experience greater sulfur exhaustion.

6. (a) Which ion forms of sulfur are most common in the soil solution? (b) Which naturalsoil sources supply sulfur for plants?

Answers: (a) Sulfate (SO42-) is most common but sulfide (S2-) can be abundant in

wetlands. (b) Organic matter, gypsum, and various sulfur minerals are natural sources ofsoil sulfur.

7. Describe management practices that might allow cultivation of acid sulfate soils.Answer: Ponding, as done for paddy rice, prevents oxidation of these soils and

therefore prevents the formation of sulfuric acid.

8. (a) What is the form of boron in soil solution? (b) In which climatic region isdeficiency most likely? Explain.

Answers: (a) Soil boron is usually H3BO3. (b) Because boron is soluble, soils inhigh rainfall regions are often boron deficient.

9. (a) Although iron occurs in large amounts in soils, why is iron often deficient forplants? (b) If iron compounds have such low solubility, why are not more cropsexhibiting iron deficiency? (c) How is iron deficiency corrected?

Answers: (a) Soil forms of iron have very low solubility, especially in high-pHsoils. (b) Plants use very little iron. Also, some organic compounds form chelates withiron, greatly enhancing its availability to plants. (c) Iron deficiency is corrected byspraying iron on plant foliage, by applying chelated iron to soil, or by acidifying the soil.

10. Is cobalt an essential plant nutrient? Explain.Answer: Cobalt is not known to be essential for higher plants. However, it is

essential for healthy populations of Rhizobium bacteria, which have symbioticrelationships with alfalfa, clovers, beans, and other legumes.

11. (a) What is the ionic form of zinc in the soil solution? (b) Explain the mobility ofzinc in soils.

Answers: (a) Zn2+. (b) Zinc is adsorbed strongly to soil colloids and therefore hasvery low mobility in soils.

12. Which materials are used to correct deficiencies in (a) zinc, (b) iron, (c) sulfur, and(d) boron?

3434

Answers: Examples include: (a & b) sulfates or organic complexes such aschelates, (c) sulfate, elemental sulfur, and thiosulfates, and (d) frits, solubor, and borax.

13. Which crops are most likely to exhibit a molybdenum deficiency? Explain.Answer: Those plants that fix atmospheric nitrogen use the nitrogenase enzyme,

which requires molybdenum. Alfalfa and other legumes are most likely to exhibitmolybdenum deficiency.

14. If soluble boron amendments are easily applied to correct a boron deficiency, why isit hazardous to apply enough for several years in one addition?

Answer: The margin between sufficient boron and toxic boron is small. Addinglarge amounts of boron to soil runs the risk of creating toxic conditions for plants.

3535

CHAPTER 12: SOIL FERTILITY MANAGEMENT

OVERVIEW

Fertilizer is the land management input used most universally. Large amounts ofnutrients are harvested and removed from fields, forests, and grasslands. Fertilizersreplace those losses. The need for fertilizer can be diagnosed by soil testing or planttesting. In many instances, soil testing is the preferred approach. Soil samples can bequantitatively tested for the various plant nutrients, and for pH, salinity, texture, andorganic matter. The results of these tests are then interpreted and correlated to plant-response data, and finally converted to a fertilizer recommendation for the given site.With trees and other perennial crops, plant testing of often preferred. One can digestspecific plant tissues in strong acid and measure the amounts of various minerals in thetissue. Again, this data eventually leads to a fertilizer recommendation.

Fertilizer labeling includes a grade designation for the percent of nitrogenphosphorus and potassium in the product. The grade is three numbers (x-y-z) indicatingpercent N, percent P2O5, and percent K2O. Nitrogen fertilizers containing ammonium ormaterials that convert to ammonium (ammonia and urea) are acid forming in soils. Thisis because the process of nitrification converts one ammonium to one nitrate and 2 H+.Fertilizers can be applied in a variety of ways including: starter (with seed), broadcast(spread over surface), banding (alongside seed), top dressing (on established crop),fertigation (with irrigation water), and foliar (on leaves). Micronutrients are often appliedas foliar sprays. Nitrogen and potassium can be added in many ways, but phosphorus ismost efficient when applied in a band.

Fertility management is the primary focus of precision agriculture. The fertilizerneeds in a typical field vary over distance in the field. Fertilizer can be used lessexpensively and with less environmental danger if only those parts of the field needinghigh doses receive them.

Livestock manure is a valuable but troublesome source of fertilizer nutrients. Itoften contains a higher P:N ratio than is needed by crops. Therefore applying enoughmanure to satisfy crop nitrogen needs may result in excessive soil phosphorus levels, andsubsequently to pollution of surface waters as phosphorus-rich water runs off the land.

QUESTIONS AND ANSWERS

1. When taking standard soil samples, how do you determine (a) depth to sample, (b)number of subsamples per composite sample, (c) areas to avoid, and (d) when to sample?

Answers: (a) One should sample to the rooting depth, normally breaking thesample up into two or more separate depth increments. (b) The number of subsamplesvaries depending upon the variability in the field—more variability requires moresubsamples. Three samples per field is minimal, and would often be insufficient. (c)Avoid anomalous areas such as small rock outcrops, roadways, ditches, or weed patches.(d) Sample as close to the time of planting as operating schedules allow; but leave timefor sample analysis, fertilizer application, and pre-plant tillage.

2. Explain why sampling and testing for soil nitrogen are problematic.

3636

Answer: Sample handing is critical. If a sample from a cold field is stored in awarm room, the microbes in the sample are more active than those in the field—they willperform nitrogen transformations in the sample that are not performed in the field. Also,nitrogen in the soil is quite fleeting. It can quickly leach or undergo denitrificationfollowing heavy rains, it can transform to biomass, it can transform from unmeasuredorganic nitrogen forms in biomass and crop residues to readily available forms.

3. Explain what a soil test for phosphorus actually measures.Answer: Each type of phosphorus soil tests measures some fraction of the total

soil phosphorus. The amount measured is called extractable or labile P. Tests do notattempt to measure total soil phosphorus, because plants cannot access all the phosphorusin the soil.

4. Tell for which cases or which crops plant tissue testing would be preferred over soiltesting.

Answer: Tissue testing is preferred over soil testing for perennial crops such astree fruits, grapes, and pastures.

5. Describe visual symptoms for deficiencies of nitrogen, phosphorus, and iron.Answer: Nitrogen deficiency appears as stunted plants and chlorotic leaves on the

older parts of the plant. Phosphorus also appears as stunted plants, but leaves on theolder parts of the plant appear to be too dark green, or to be red or purple. Irondeficiency appears as a distinct interveinal chlorosis on the younger leaves of plants.

6. Explain how environmental concerns have altered our views and practices of fertilizeruse.

Answer: Both nitrogen and phosphorus are potential pollutants. Therefore we nolonger think that if a little is good, a lot is better. The goal now is to obtain high fertilizeruse efficiency by applying no more than is needed.

7. Discuss the mobility and acid-forming tendencies of typical nitrogen, phosphorus, andpotassium fertilizers.

Answer: Of the three fertilizers N, P & K, only nitrogen is acid forming.Nitrogen is the most mobile of the three, especially when in the extremely mobile nitrateform. Phosphorus is very immobile because of its tendency to precipitate in soil and toadsorb to soil surfaces. Potassium is moderately immobile because it participates incation exchange on clay and humus sites.

8. What is the grade of diammonium phosphate, and what do the grade numbers mean?Answer: The grade is 18-46-0, meaning 18% nitrogen, 46% P2O5, and 0% K2O.

9. Calculate the amount of 82-0-0 and 0-46-0 fertilizers needed to meet the need for 150kg/ha of N and 60 kg/ha of P2O5 on a 200-ha field.

Answer: For N: (150 kg/ha)(200 ha)(100 kg 46-0-0/82 kg N) = 36585 kg 82-0-0.For P2O5: (60 kg P2O5/ha)(200 ha)(100 kg 0-46-0/46 kg P2O5) = 26087 kg 0-46-0

3737

10. Compare the benefits of broadcast application of fertilizer to the benefits of deepbanding.

Answer: Broadcasting is more feasible than deep banding on annual non-rowcrops such as small grains, and on permanent crops whose roots would be damaged bydeep banding. Also, broadcasting may fit better into a grower’s pesticide or limeapplication scheme. Where its use is suitable, deep banding normally provides betterfertilizer efficiency with greater crop response per unit of fertilizer applied.

11. Explain the difference between fertigation and foliar application.Answer: With fertigation, the fertilizer is added in small amounts to large

amounts of irrigation water. With foliar application, the water is only a carrier, used insmall amounts to place the fertilizer on the leaves of plants.

12. Discuss how various factors affect fertilizer efficiency.Answer: Soil water affects the leaching and denitrification of nitrogen. Depth of

placement affects root access to fertilizers. Form of fertilizer affects its solubility andreactivity. Soil texture, mineralogy, and organic matter content affect the tendency forfertilizer to be retained by the soil. Soil pH affects the tendency for phosphorus to befixed and the tendency for ammonium to be volatilized.

13. Explain the extent to which crop nutrient needs can be met by applying manure.Answer: Manure is often cheap and available near large livestock confinement

enterprises such as dairies, feedlots, and chicken houses. Transporting manure greatdistances from the source is not economically feasible. Manure is a low-grade fertilizerand is applied in much heavier doses than is used for commercial fertilizer. The balanceof nutrients in manure may be quite different than the needs of the crop to be fertilized, sothe grower may find it wise to use both manure and supplemental applications ofcommercial fertilizers.

14. Explain why spatial variation in a field is important to a producer, and what theproducer can do to optimize production in a field with considerable spatial variation.

Answer: In a field with considerable spatial variation no single fertilizer dose willbe right for the whole field. If the field is “averaged” by taking one composite sample,some areas get too little fertilizer and will yield poorly. Other areas get too much,wasting the farmers money and threatening to pollute ground or surface waters. Thefarmer could either divide the field into smaller fields, each being relatively uniform, orcould practice precision farming.

3838

CHAPTER 13: TILLAGE SYSTEMS AND ALTERNATIVES

OVERVIEW

For thousands of years people have tilled the soil to grow crops. Tillage implementsadvanced from forked sticks to large, elaborate machines. Tillage is performed forvarious reasons, among them are: seedbed preparation, weed control, soil loosening, soilshaping, lime incorporation, fertilizer incorporation, pest control, water management, soilaeration, and soil warming. Although tillage works for all the above purposes, it alsotends to leave the soil more vulnerable to erosion by wind and water. Tillage that movessoil down-slope is itself an agent of soil erosion. Current thinking favors minimizingtillage to protect the soil from erosion. In addition to erosion control, minimizing tillagereduces fuel costs, reduces topsoil drying, reduces soil compaction, and reduces labor forthe farmer. On a larger scale, minimizing tillage aids in global carbon sequestration.

Primary tillage is the tillage used to break up soil and bury residues for the firsttime after harvest. Secondary tillage refers to the subsequent procedures after theprimary tillage operation. Primary tillage usually involves large, heavy equipment suchas chisel plows, moldboard plows, or large disks. Secondary tillage usually involveslighter equipment and less soil manipulation than with primary tillage. Sometimes theseolder terms (primary and secondary tillage) do not fit our tillage practices. Conservationtillage is any of a number of farming systems that leave at least 30% of the surfacecovered by plant residues for control of erosion by water. This is normally accomplishedby strip-till, mulch-till, ridge-till or no-till. Conservation tillage requires unconventionalapproaches to seeding, fertilizing, and controlling pests. However, it improves soilcarbon levels and greatly reduces erosion compared to conventionally tilled, bare fields.Tillage systems should be matched to the needs of the soil and crop. The development oftransgenic crops, though controversial, reduces the threat of weeds and insects. Thesecrops require fewer tillage operations and pesticide applications.

QUESTIONS AND ANSWERS

1. Describe the following tillage implements (a) bedder, (b) chisel, (c) coulter, (d)cultivator, (e) disk, (f) harrow, and (g) rotary plow.

Answers: See Section 13:3.

2. Define (a) conservation tillage, (b) reduced tillage, (c) conventional tillage, and (d)minimum tillage.

Answers: (a, b, c) See Section 13:3. (d) a nonspecific term used to mean the leastamount of tillage necessary for a cropping system.

3. Discuss the purposes of tillage.Answer: Tillage is done to prepare seedbeds, control pests, loosen the soil, warm

the soil, improve water relations, aerate the soil, incorporate chemicals, or shape the soil.

4. Discuss the advantages and disadvantages of reduced tillage.

3939

Answer: Reduced tillage protects the soil against erosion and saves time and fuel.However, it also requires specific management skills, and may not improve yields.

5. What advantages does conservation tillage have over no-till systems?Answer: Unlike no-till, conservation tillage allows some direct weed control and

incorporation of lime, fertilizers, and pesticides.

6. How does reduced tillage affect physical properties of soils?Answer: Compared to conventional tillage, reduced tillage usually improves soil

water content, reduces subsoil compaction, and normally results in cooler soils due to theincreased residue on the surface.

7. List some recommendations and precautions for weed control in reduced tillage.Answer: Weed populations are normally greater with reduced tillage. Herbicides

may become inactive by adsorbing to the organic residues on the soil. Post-emergenceherbicides may be more effective than pre-emergence formulations. Farming withreduced tillage relies much more heavily on herbicides than with conventional tillage.Also, crop rotations and variety selection is more critical with reduced tillage.Transgenic crops are more appealing in reduced tillage than in conventional tillage fields.

8. How effective are fertilizer and lime in no-till systems? Explain.Answer: They can still be effective, but farmers should expect a time lag between

application and availability, especially with lime. Sometimes lime or fertilizers areplaced in a slot at planting time using a system that, with a liberal interpretation of thedefinition, might qualify as no-till.

9. When considering reduced tillage, which particular problems occur if the land is to beplanted with small-seeded crops?

Answer: Small-seeded crops do not respond well to no-till because they need afine seedbed with lots of soil contact to protect against desiccation in shallow soilplacement. A method such as strip-till may be best, in which a narrow strip of soil istilled for each crop row but still leaving most of the field untilled.

10. How does tillage or the lack of it influence quality of the environment?Answer: Most of the effects of reducing tillage are favorable for the environment.

With less erosion: the soil is more productive, less sediment clogs waterways, and lesssoil blows in the air. However, large healthy crops are good for the environment becausethey stabilize soil and sequester carbon from the atmosphere. If a tillage systemsacrifices yield, some of the benefits are lost. Also, in some circumstances reducingtillage may result in greater pesticide and nutrient pollution of water.

11. Write a paragraph explaining why conservation tillage should be used on mostfarmland.

Answer: An example might read as follows. Conserving our valuable farmlandrequires that we control soil erosion. The most practical way to control erosion oncropland is to reduce the number and severity of tillage operations, leaving more crop

4040

residues on the soil surface. Besides protecting the land, this will keep many nutrient-and pesticide-laden sediments out of our water. Additional benefits to reduced tillageinclude saving fuel and labor costs, and reducing soil compaction. In some instances thesoil conditions, crop needs, or pest situation may not allow reduced tillage.

4141

CHAPTER 14: SOIL EROSION

OVERVIEW

Soil erosion has been America’s most devastating environmental disaster. It is similarlydisastrous in many diverse regions of the Earth. Although natural or geologic erosion isinevitable, man has greatly accelerated the loss of soil by clearing land for farming andfor roads and structures. Recently, much progress has been made in developing landmanagement methods to protect the soil. Still, the United States loses about 5 tons of soilper acre annually from farmland. Both the loss of soil, and the subsequent sedimentationof the eroded particles are economic and environmental problems. The sediments plugand elevate rivers, fill reservoirs, and suffocate fish. The nutrients and pesticidesattached to the sediments pollute surface waters. Dust from wind erosion fouls machinesand human lungs.

Raindrops detach small soil particles that are then transported from their locationin flowing water. If non-turbulent sheets of water exit the field, damage is slight tomoderate. If the water flows in turbulent channels, rills and gullies form, altering thelandscape and often destroying the land for its intended use. The extent of erosion due torain is affected by rainfall intensity, the soil erodibility, the length and slope of the field,and the farming or land-shaping practices used. The Universal Soil Loss Equation andother models use these factors to predict annual soil loss. The seemingly non-ambitious,yet unachieved goal for the United States is to limit annual soil loss to less than 5 tons peracre.

Wind erosion is more severe than water erosion in dry regions, where lightweight,dry, poorly aggregated soil particles move easily. In the presence of wind, soil canbecome suspended, can creep along the ground, or can move in short hops in a processcalled saltation. Wind erosion can be predicted similar to water erosion.

Both wind and water erosion can be controlled somewhat by improving soilstructure, by crop selection, by reorienting or shaping the field, by using cover crops, andby construction of buffer strips. The most powerful and practical tool for soil erosioncontrol is vegetative cover. Cropping and tillage systems that produce and retainsubstantial amounts of soil cover can minimize soil erosion.

QUESTIONS AND ANSWERS

1. What is the typical tolerable erosion rate (T value) in tons per acre per year?Answer: 5

2. Explain the damages caused by allowing rainfall to hit bare soil.Answer: Raindrops carry enough momentum to dislodge soil particles, leaving

them vulnerable to transport off the field in flowing water. Also the splashaccompanying the striking raindrop will move small drops of water that containsuspended soil particles. This splash movement is mostly down slope.

3. List practices that decrease detachment of soil by raindrops.

4242

Answer: Improving soil structure with soil-building farming systems, increasingground cover with crop residues, and increasing living vegetative cover by crop selectionor by use of cover crops will greatly reduce soil detachment by raindrops.

4. Discuss the damages of erosion from two points of view: (a) quality of the soil lost and(b) rates of loss versus the rates of soil formation.

Answers: (a) The best soil is lost to erosion. The topsoil that blows away orwashes away in erosion had probably been leached of soluble salts, and probablycontained beneficial humus and plant nutrients. (b) The USDA optimistically estimatesthat soils form at a rate of about 5 tons per acre per year. Coincidentally, soils erode inthe U.S. at an average rate of about 5 tons per acre per year.

5. Define (a) sheet erosion, (b) rill erosion, and (c) gully erosion.Answers: See Erosion in the Glossary.

6. Define each term in the Universal Soil Loss Equation (USLE).Answer: See Section 14:4.

7. List the condition for each factor in the USLE that will allow (a) very large amounts oferosion and (b) very low amounts of erosion.

Answers: (a) very large amounts of erosion come from frequent heavy rains (Rfactor), poorly aggregated soil (K factor), long and steep slopes (LS factor), fallow orintermittent row crops (C factor) and no conservation practices (P factor). (b) Very lowamounts of erosion come from gentle rains, well-aggregated soil, short and level fields,lush perennial vegetation, and the use of conservation practices such as contour tillage.

8. In general terms, how does the RUSLE differ from the USLE?Answer: The USLE is a rather crude tool for estimating soil erosion, particularly

in the way it deals with vegetative cover and the effects of slopes. The RUSLE is a moresophisticated model that refines the predicted effects of the factors affecting soil erosion.

9. Define and briefly discuss these terms (a) highly erodible land, and (b) erosivity index.Answers: (a) Highly erodible land has an erosivity index greater than or equal to

8. (b) The erosivity index equals (R)(K)(LS)/T.

10. Which causes the greatest increase in soil erosion by water: doubling the slopepercentage or the doubling the slope length?

Answer: Doubling the slope percentage has a much greater effect.

11. Describe characteristics of vegetative cover that would generally allow the least soilerosion.

Answer: Dense forest and permanent pasture allow the least erosion.

12. Define each term in the Wind Erosion Equation (WEQ).Answer: See Section 14:8.

4343

13. (a) Which soil factors influence the soil’s wind erodibility index (I)? (b) Describe asoil with a high I value.

Answers: (a) Soil texture and structure determine the I value. (b) A silty or finesandy soil would have a high I value.

14. Discuss the importance of soil cover for controlling wind erosion.Answer: Soil cover slows the velocity of wind reaching the soil surface. Where

this cover is present in sufficient amounts and for sufficient periods of time, wind erosioncan be negligible.

15. Briefly discuss methods to control wind erosion.Answer: Wind erosion can be reduced by increasing the amount and duration of

vegetative cover, by constructing soil ridges perpendicular to the prevailing wind, byplanting windbreaks, by keeping the soil moist, and by reducing the size of fields.

4444

CHAPTER 15: WATER RESOURCES AND IRRIGATION

OVERVIEW

Water strongly influences the biological and economic productivity of land. Some areasare too wet for our intended purposes, but much more vast are the areas too dry for ourpurposes. Water is woefully scarce in many parts of the world. About 16% of theworld’s cropland is irrigated. However, this cropland produces about one-third of theworld’s food. India, China, and the United States are the nations with the greatestirrigated acreage. In the U.S., California, Nebraska, and Texas all have more than 5million irrigated acres. Water for irrigation comes from surface sources and fromgroundwater. Thousands of dams have been built in the U.S. alone, to capture water formunicipal and agricultural use. However, because dams have fallen from public favor,irrigation expansion will depend upon groundwater. Some groundwater reserves arerechargeable, others like the enormous Ogallala Aquifer under the high plains of the U.S.,are fossil waters and become depleted with use. Ways to increase the water supply aresought, including methods of reclaiming water from the sea.

Water quality for irrigation depends on its salinity, sodium content, pH, sedimentload, and presence of toxic substances. Determining the optimal water quantity to beused for irrigation is a complex matter. Crops have different needs for water, differentrooting depths, and different reactions to drought. Much thought and research has goneinto developing techniques to determine when to irrigation. For nearly all crops, oneirrigates before the crop experiences water stress.

Furrow irrigation is an ancient practice, still used for many row crops. Sprinklerirrigation made great advances in the second half of the 1900s. Surface flooding is usedfor rice and many other crops. Presently much attention is given to drip or tricklemethods because of their high efficiency. Using low-pressure sprinklers with nozzlespositioned near the ground is another new technology aimed at improving irrigationefficiency.

QUESTIONS AND ANSWERS

1. What is an aquifer?Answer: An aquifer is a water-holding medium under the Earth’s surface. It is

usually gravel, sand, or fractured rock.

2. What causes a spring or an artesian well to flow?Answer: A spring or an artesian well flows because a low-lying surface outlet of

the aquifer is below the water level in other parts of the conterminous aquifer.

3. (a) Why would some aquifers produce lower flow from wells and recharge moreslowly than other aquifers? (b) Define overuse of groundwater.

Answers: (a) The conductivity of aquifers varies because of pore size and otherfactors. (b) Overuse means water from the aquifer is withdrawn at a rate that exceedsrecharge.

4545

4. (a) What causes saltwater intrusion? (b) What can be done to minimize it?Answers: (a) When fresh water is withdrawn from a coastal aquifer, pressure

from the sea forces seawater into the aquifer. (b) Some communities pump wastewaterinto the aquifers to keep seawater out. The other way to keep seawater out is to minimizewithdrawals so the pressure in the aquifer remains high.

5. What are the advantages and disadvantages of groundwater compared to surface watersas sources of irrigation water?

Answer: Advantages of groundwater include low sediment, consistent quality,consistent availability, and omnipresence. The main disadvantage of groundwater is theneed to expend energy pumping the water to the surface. Also, the properties of theaquifer may preclude pumping at the desired rate.

6. Which areas of the United States are overusing groundwaters?Answer: The greatest overuse occurs in the middle and central plains, the

intermountain area, and the Gulf Coast area.

7. How adequate are world water supplies? Give examples.Answer: Many countries lack the 500 m3 per person, considered to be a minimum

annual water requirement. See Table 15-1.

8. In water budget irrigation scheduling, how are the rate and amount of water lossmeasured?

Answer: One common way is to measure loss from a Class A Evaporation Pan,then to convert that amount to estimated crop use. Another method is to use one of manymathematical models.

9. Discuss briefly how dry a soil should be allowed to get before irrigating it. Consider(a) rooting depth and (b) the nature of the crop grown.

Answers: Irrigation is usually recommended when about 50 percent of availablewater has been used. (a) Most roots and nutrients are in the surface soil where thegreatest drying occurs. The moisture conditions in the upper 1 foot or so of soil is themost critical. (b) Because of different root morphology and different physiologicalfactors, crops respond differently to water stress. Rooting depth and allowable waterdepletion for various crops are tabulated in Chapter 3.

10. Define and describe each of the following irrigation techniques: (a) furrow, (b)border, (c) drip, and (d) sprinkler.

Answers: See Section 15:5.

11. What are the disadvantages of furrow irrigation, and how can the extent of thesedisadvantages be reduced?

Answer: The disadvantages include the need for much labor, the need for a levelfield, loss of tail waters at the end of the run, and the loss of water at the beginning of therun due to deep percolation. Capturing and recirculating the water back onto the field can

4646

correct the loss of tail water. The loss of percolating water can be minimized by surgeirrigation and by reducing furrow lengths.

12. List the disadvantages and advantages of using (a) drip systems and (b) sprinklersystems.

Answers: (a) Drip systems are expensive, require clean water, and plug easily;however, they are efficient, can be used in any terrain, and are good for fertigation. (b)Sprinklers are expensive, require high-pressure pumping, and perform poorly in windyclimates; however, they can achieve high efficiency, they work well on sand and unevenland, and they can easily cover large acreage.

13. Discuss the filtering requirement of drip irrigation systems.Answer: Drip emitters have very small holes that plug easily. Sands, organic

debris, and algae must be filtered out or they will plug the holes.

14. List a few ways that water supplies for irrigation could be increased.Answer: Water supplies could be increased by resolving political disputes,

terminating irrigation on non-responsive land, keeping pollutants out of existing supplies,and using water from the sea.

4747

CHAPTER 16: WETLANDS AND LAND DRAINAGE

OVERVIEW

Wetlands were once considered nuisances to be drained and made farmable. Now thatwe recognize their value to ecosystems, laws have been enacted to protect them. Manywetlands were drained decades ago, and are presently prime farmland. Some non-wetlands are fitted with drainage systems to facilitate irrigation or to otherwise enhancetheir productivity.

Natural wetlands (swamps, bogs, fens, marshes) preserve biodiversity, rechargegroundwater, provide flood protection, catch sediment, and produce economic products.Defining and delineating wetlands is not easy because wetlands are not necessarily wetall year long. They must have hydric soils, hydrophytic plants, and saturated conditionsfor at least some period during the year. Hydric soils have indicators of anaerobicconditions, such as mottles, gleying, or hydrogen sulfide.

Some land can legitimately be drained to improve its use. Surface draining willalleviate saturated conditions in depressions caused by poor soil permeability. If the soilis wet because of an elevated water table, subsurface drains are needed. Surface drainagecan be accomplished by shaping, leveling, or installing shallow open ditches. Subsurfacedrainage is accomplished by installing buried horizontal tile or tube lines a few feetbelow the soil surface at intervals of several feet throughout the field. The tubes or tilesare porous, allowing free water to enter the tube and run off the field to an outlet in adrainage ditch. Drained land is more productive, more manageable, and better suited forengineering purposes than land with wet spots or a shallow water table.

QUESTIONS AND ANSWERS

1. Define a wetland and discuss the problem of delineating its boundaries.Answer: See Section 16:2. Wetlands have hydric soil, hydrophytic plants, and

evidence of saturation, whether presently saturated or not. Delineating boundaries isdifficult. On any given day, a jurisdiction (legal) wetland may be dry, and land that iswet may not be a jurisdictional wetland.

2. What is hydric soil?Answer: Some soils are obviously hydric because they are saturated Histosols, or

have other obvious features. Often hydric soils are not obvious and are identified byindicators such as those in Table 16-2.

3. List six or seven reasons to retain wetlands.Answer: Wetlands provide wildlife habitat, preserve biodiversity, protect against

waves and hurricanes, collect sediments, recycle nutrients, recharge aquifers, and produceuseful products.

4. What are the points you would argue in a debate to save wetlands if given pressures forfood production, and draining more wetlands is proposed to produce food?

Answer: Same as above.

4848

5. What is the law regarding draining wetlands on your property?Answer: In general, no net loss of wetlands is allowed; therefore, if one parcel of

land is drained, another parcel of wetlands must be created. Otherwise, the landowner issubject to legal sanctions.

6. What evidence is visible that might indicate that a soil has very poor drainage?Answer: See wetland indicators in Table 16-2.

7. State how drainage affects these items: (a) rate of warming, (b) the variety of adaptablecrops, (c) toxic substances produced, (d) depth of root zone, and (e) salt accumulation.

Answers: (a) Wet soils stay cool longer; drained soils warm more quickly. (b)Draining soils greatly increases the number of adaptable crops. (c) Fewer toxicsubstances are produced in drained soils. (d) For most plants, roots only grow whereoxygen is available; therefore, drainage deepens the root zone. (e) Subsurface drainageallows removal of salts.

8. Name some kinds of toxins produced in poorly drained soils.Answer: Manganese and some organic toxins are produced in wet soils.

9. Define paddy rice and describe the changes in aeration and soil pH as paddies areflooded.

Answer: Paddy rice is a method of rice production in which the rice is grown in aponded field. Within a few days after ponding free oxygen decreases to almost zero.Soil becomes less acidic or alkaline as pH values tend toward neutral in a paddy.

10. List advantages and disadvantages of (a) open-ditch drains, (b) tile drains, and (c)mole drains.

Answers: (a) Open-ditch drains are cheap and easy to install and are effective;however, they require maintenance, may not be crossable with equipment, and are weedy.(b) Tile drains are relatively permanent (20-50 years), are buried out of the way, and areeffective; however, they are expensive to install. (c) Mole drains are quick and cheap toinstall; however, they are temporary, require a heavy tractor, and can only reach shallowdepths.

11. (a) What is the composition of the tile in tile drains? (b) How does water seep into thetile lines? (c) What problems can occur to tile lines?

Answers: (a) Tile is fired clay. Most so-called tile lines are now plastic tubes. (b)Tiles are not cemented together, allowing water to enter at the joints. When plastic tubesare used, the tubes are perforated. (c) Drain outlets can get plugged by vegetation or byrodents. Drain lines can shift out of alignment, so water no longer exits the field.

4949

CHAPTER 17: POLLUTION OF SOIL, WATER, AND AIR

OVERVIEW

Pollution happens as we use resources. Nature can handle moderate amounts of naturalsubstances, but we produce greater-than-moderate amounts of many substances, and weproduce many unnatural substances. We threaten our environment in various ways,including overexploiting species, introducing alien species, modifying the landscape,modifying the climate, adding chemicals to fresh water, and degrading the soil throughmismanagement.

Plant nutrients can improve the environment by producing more plant growth;however, fertilizer elements in bodies of water are pollutants. Eutrophication, thenutrient enrichment of water leading to algal blooms and the eventual loss of dissolvedoxygen, is usually caused by phosphorus. Much fertilizer phosphorus reaches bodies offresh water in run-off and on eroded sediments. Nitrates travel so easily through soilsthat they often end up in groundwater where they can cause methemoglobinemia (bluebaby syndrome) in young mammals.

Organic wastes can threaten the environment in various ways. In water thesewastes require oxygen as they decompose. As animal manure they can be useful toagriculture, but often threaten the groundwater because of the high nitrate levelsproduced in small areas such as feedlots and dairies. Sewage sludge also contains usefulnutrients but may contain dangerous levels of heavy metals. Present agriculturaltechniques rely heavily on pesticides, the danger of which is exacerbated when thepesticide is transported off the intended field as in runoff water.

Soil use affects the atmosphere and vice-versa. Some of our soil-managementpractices produce greenhouse gases such as nitrous oxide, methane, or elevated levels ofcarbon dioxide. Acid rain, formed as rain falls through a polluted atmosphere,accelerates the acidification of soils.

Salts added to soils or bodies of fresh water lower the quality of the soil or water.Salty water has few uses unless one expends the energy to clean it; salty soil can producelimited species of plants.

QUESTIONS AND ANSWERS

1. (a) What is meant by pollution? (b) Are pollutants always toxic or carcinogenic?Answers: (a) Pollution is the altering of some component in the environment

leaving it less useful to man or less suitable to ecosystems. (b) No, as in the case of salt, apollutant can simply lower the overall quality of the environment.

2. (a) What is meant by eutrophication? (b) Explain why phosphates have greaterinfluence than nitrates on eutrophication.

Answers: (a) Eutrophication is excessive nutrient enrichment, usually of water.(b) Phosphates are often limiting to growth of algae and cyanobacteria in aquatic systemsbecause the algae and cyanobacteria in the system can fix their own N.

5050

3. (a) To what extent was DDT more poisonous to animals than other pesticides? (b)What are the criteria a pesticide must meet to be acceptable for use today?

Answers: (a) DDT was less poisonous than many pesticides used today; however,DDT was very persistent in the food chain. (b) Modern pesticides must not becarcinogenic or mutagenic, must be effective, and must have a short life in theenvironment.

4. To what extent is agriculture responsible for pollution with pesticides?Answer: Agriculture accounts for at least two-thirds of pesticide use, and

undoubtedly accounts for much of the pesticide pollution.

5. (a) Name some of the common heavy metals. (b) Why are they of concern?Answers: (a) Cd, Ni, Pb, Zn, Cr and Cu. (b) Heavy metals are toxic, they do not

degrade, they can bioaccumulate in the food chain, and they are difficult to remove fromsoil.

6. How serious is the salt problem?Answer: The problem of excess salt on land and in water is very serious. Salts

continue to cause the abandonment of substantial acreage of farmland. Salts in rivers andlakes lowers their quality for drinking and other uses.

7. How is BOD (or COD) related to undesirable water conditions and waste disposal?Answer: Organic materials and some inorganic chemicals consume O2 as they

degrade, lowering levels of dissolved O2 in the water. Without dissolved O2 in the water,aquatic animals die.

8. What are the kinds and sources of radioactive pollutants most likely to occur on cropsand in soils?

Answer: Strontium-90, cesium-137, uranium-238, and iodine-131 havecontaminated soils in localized areas.

9. (a) How do soil sediments fit the definition of a pollutant? (b) What damage is done bysoil sediments?

Answers: (a) Sediments make water less useable to man and less functional inecosystems. (b) Sediments carry pesticides and fertilizer nutrients, thus chemicallyaltering the water. They raise the beds of rivers and fill reservoirs.

10. Although air pollutants have been given less attention than water pollutants,agriculture produces some air pollutants. List and briefly discuss these.

Answer: Nitrous oxide from denitrification and methane gas from rice paddiesand other wetlands are greenhouse gases. Airborne dust from eroding soil is a pollutantwhile it remains airborne and becomes unwelcome sediment when it falls.

11. Why is manure disposal an enormous problem?Answer: The problem is enormous because the quantity of manure to dispose is

enormous. The manure has some economic value, but not enough to justify shipping it

5151

great distances from the source to the user. Land near the source tends to receive toomuch manure, resulting in water pollution from the nutrients in the manure, andsometimes unwanted seeds and salts in the soil.

5252

CHAPTER 18: TOWARD ENVIRONMENTAL INTEGRITY

OVERVIEW

The soil is nature’s waste treatment facility and filter, and at times is a pollutant itself.Many laws have been enacted to protect the soil and to protect the air and water from thesoil. These laws include the Clean Water Act, the Clean Air Act, the PollutionPrevention Act, and others. Best Management Practices (BMPs) have been developed tooutline how to use agricultural inputs such as manure, fertilizer, and pesticides in amanner that poses the least threat to the environment. Users are expected to follow theseBMPs. Pesticides can experience many possible fates. Some pesticides are morepersistent than others. Some adsorb readily to the soil, some do not.

Soil can be managed to improve air and water quality. Land management systemsthat reduce greenhouse gases and improve carbon sequestration could help restore theatmosphere to a more natural condition, and greatly improve the quality of theenvironment. Also, land management systems that control nitrate leaching, controlerosion, decrease salt loading, and decrease runoff of pesticides and fertilizers could helprestore the waters to a more natural condition. Improved plant selection and farmingsystems, along with the use of buffer strips, have proved successful and can facilitatemuch more progress still.

When damaged or contaminated, soils require remediation. This can beaccomplished by washing a contaminant from the soil (leaching), by using plants toscavenge for an unwanted substance in the soil (phytoremediation), by extracting volatilematerials from the contamination site (vapor extraction), by using microbes to degradethe material (bioremediation), or, when all else fails, by soil disposal.

Soils that are degraded but not contaminated can be restored to greaterproductivity. Measures to improve soil quality include controlling acidity and erosion,adding plant nutrients and organic residues, and minimizing compaction.

QUESTIONS AND ANSWERS

1. In what ways does soil degrade pollutants or remove them from the environment?Answer: Soil filters some materials so they cannot leach to groundwater. Soil

provides a hospitable environment for the microbes that might degrade an unwantedchemical. Soil contains enzymes that degrade some pollutants.

2. What authority does the EPA have over land use?Answer: The EPA is the federal agency that oversees pollution control. They

have specific authority granted by the Clean Water Act, the Pollution Prevention Act, etc.

3. Describe the difference between point source and nonpoint source pollution.Answer: Point source pollution comes from an identifiable source or a number of

identifiable major contributors. Nonpoint source pollution arises from the smallcontributions from many individuals, such as with nutrient enriched waterways in afarming community and vehicle exhaust polluting the air in a city.

5353

4. For what kind of practices might one find an official description of best managementpractices (BMPs)?

Answer: In agriculture, BMPs cover such enterprises as manure use, fertilizeruse, pesticide use, and irrigation.

5. List five important soil characteristics contributing to soil quality.Answer: The list might include such characteristics as biological activity,

infiltration rate, compaction, salt concentration, acidity or alkalinity, nutrient levels, andaggregation.

6. What are the possible fates of a soil-applied pesticide?Answer: A pesticide may end up in a crop, in groundwater, on an eroding soil

particle, or in the atmosphere. The pesticide may degrade biologically, by nonbiologicalreactions such as hydrolysis, or by the action of light.

7. How safe is the application of biosolids to farmland? Explain.Answer: The degree of risk depends on the biosolids. The most safe materials,

Class A biosolids, are believed to pose little threat to the environment. Less safematerial, the Class B biosolids, pose greater risk and their use is more restricted. Thepotential for some biosolids to release pathogens, carcinogens, or heavy metals into theenvironment continues to worry citizens and scientists.

8. Explain various ways that soil management affects the quality of nearby waters.Answer: Soil management affects the quantity of runoff into surface waters and

the amount of sediment entering surface waters. It also affects the amount of salt,pesticide, and fertilizer entering bodies of water.

9. How would improved carbon sequestration benefit the average person?Answer: Improved carbon sequestration would reduce the level of carbon dioxide

in the atmosphere. The result would likely be twofold: less global warming and less acidrain.

10. What types of pollutants can be treated with bioremediation? How effective isbioremediation?

Answer: Organic solvents and fuels are the typical candidates for bioremediation.Bioremediation is reasonably effective, but can be slow and incomplete.

11. Give an example of successful phytoremediation.Answer: Sunflower extracts lead and uranium from water. Alpine pennycress

accumulates zinc. Brake fern extracts arsenic from soil. A planting of St. Augustinegrass, rye, and milo stimulated microbes to decompose hydrocarbons. Crabgrass andmulberry trees also can help decompose hydrocarbons.

5454

CHAPTER 19: SOIL SURVEYS AND LAND-USE PLANNING

OVERVIEW

Soil surveys are conducted to inventory, describe, and evaluate soils, usually on a countybasis. Both the act of studying the soil and report issued following the study are calledsoil surveys. In the United States soil surveys are conducted by USDA personnel incollaboration with local Experiment Stations and sometimes other Federal entities. Thesoil survey reports contain a wealth of information ranging from climatic data to detailedsoil descriptions, to evaluations of each soil’s suitability for numerous uses. Soils data isnow available in various electronic formats.

Soils that are extensive and of great importance to a state are designatedbenchmark soils, and are studied more intensively than others. Also, some land receivesspecial designation as prime farmland, unique farmland, or farmland critical to the state.Agricultural land is classified according to its capability to produce agricultural productsusing a Roman-numeral rating system in which the best land is designated Class I and thepoorest land is designated Class VIII.

The soil survey report describes the suitability of each soil for many diverse usesincluding campgrounds, golf courses, various crops, dwelling with or without basements,septic tank drain fields, and topsoil. New surveys will identify soils posing a risk tohumans because of such hazards as radon, contamination, and sinkholes.

Property rights are dear to property owners. Various laws are used to control landuse under the philosophy that one person’s land-use activities should not harm another’sability to use his or her land. Zoning laws are land-use controls. Differential taxation isalso widely used, whereby land is taxed at higher or lower rates depending on whichenterprises take place on the land, or how developed the land is. All states originally hada right-to-farm law protecting farmers from lawsuits of neighbors claiming the noise orodor from the farm is a nuisance. Such laws are now in question.

QUESTIONS AND ANSWERS

1. (a) What equipment is used by soil surveyors? (b) What profile features will a surveyordescribe and measure in the survey?

Answers: (a) The surveyor needs a GPS receiver, digging tools, pH tester,Munsell color book, HCl to test for lime, and a hand level. (b) The surveyors willidentify all horizons in a profile and describe their boundaries, acidity, color, texture,structure, and other visible features.

2. What information is in a soil survey report? List at least seven items.Answer: temperature, precipitation, land capability, physical properties, chemical

properties, water management, classification, etc.

3. Which information in a soil survey report might be useful to a building contractor or toarea planners?

5555

Answer: Contractors and planners would be interested in the survey rating of asoil’s suitability for: dwellings, landscaping, topsoil, septic tank drain fields, buriedpipelines, and other purposes.

4. What are some of the soil properties that may result in slight, moderate, or severeratings for lawns, landscaping, and golf fairways?

Answer: See Table 19-4.

5. List a few of the major factors limiting agricultural land capability worldwide.Answer: See Table 19-1.

6. In general terms define prime farmland, land critical to the state, and benchmark soils.Answer: See Section 19-5.

7. Describe (a) capability class III land, (b) class V land, (c) subclass IIs land, and (d)subclass IVw land.

Answers: (a) can be cultivated but requires intensive conservation practices. (b)cannot be cultivated, but otherwise not restricted. (c) can be cultivated but has alimitation affecting rooting, such as stoniness, shallowness, or salinity. (d) land with awetness problem but suitable for pastures.

8. Controlling land use is of increasing interest and concern. Discuss the mechanisms ofeminent domain and preferential tax assessments.

Answer: The law of eminent domain allows a government to buy land from anunwilling seller, provided that the property so acquired will be used for the public good.Preferential tax assessments are usually used to keep farmland in farms. Often farmlandis taxed according to its profitability, not its market value. If the land is sold fordevelopment the tax may then be based on market value.

9. Describe the economic threat to farmland.Answer: Farmland is threatened by the possibility of lawsuits because of the

nuisances created by farming, and by the farmers’ enormous need for capital, and hencefor credit. Farmland is also threatened by lucrative land deals in which a farmer canmake much more money by selling the land to developers than by farming it.

5656

CHAPTER 20: GREENHOUSE SOILS AND SOILLESS CULTURE

OVERVIEW

An important segment of the world’s plants are grown in containers—in homes andoffices, in greenhouses, in plant nurseries awaiting sale, in permanent landscape settings,and on rooftops. The desirable characteristics for container soils differ somewhat fromthose of field soils. A small volume of container soil is expected to provide plants thewater and aeration that field-grown plants would obtain from a much larger volume ofsoil. With suitable root media (potting soil) and lots of attention, container-grown plantsproduce higher yields than field plants.

Potting soil or soilless rooting media must be porous, must have sufficient weightto support plants without unnecessary weight adding to the cost of shipping, must have asuitable pH and CEC, and must be free of pests. Typical components of rooting mediaare bark, sawdust, straw, vermiculite, sand, perlite, polystyrene, rock wool, peat moss,and lime. Usually three or four components are mixed to give the media the desiredproperties. The media is then pasteurized by heat, steam, or chemicals.

Watering is critical for container-grown plants. The media must drain well toavoid disease, and must be porous to allow aeration. Therefore, the media will hold littlewater. Watering must be frequent. In some greenhouse operations watering iscontinuous. Also, fertility management is critical. The media or the plant tissue can betested by the same procedures used for field-grown plants. Fertilizer is often added inirrigation water or in slow-release forms.

Hydroponics is the practice of growing plants in solution culture. This practice isused commercially for high-value crops such as greenhouse tomatoes, and will be usedon long space flights to produce both food and O2. The plants’ need for physical supportmay be met with a material such as perlite or peat. A carefully regulated nutrient solutionis constantly metered to the plants. With intensive management, hydroponics operationscan be highly productive.

QUESTIONS AND ANSWERS

1. How do small pots affect root media aeration?Answer: Aeration is especially critical in a small pot because the bottom of a pot

is often saturated with water, allowing no room for air. The rooting media in a small potmust be very porous.

2. What is necessary to ensure that root media is adequately drained and aerated?Answer: Coarse-textured porous material is best for root media. A typical field

soil is a poor container soil, because it will not drain adequately.

3. Describe the composition and characteristics of peat moss, bark, perlite, andvermiculite.

Answer: See Section 20:2.

5757

4. List several root media and indicate why each material is used. Which properties doesit provide to the soil?

Answer: Peat provides water-holding ability. Sand provides aeration and weight.Perlite is a light-weight substitute for sand. Vermiculite provides both water-holdingcapability and aeration, and has some cation exchange capacity.

5. What are the properties that a root medium should have?Answer: A good root medium should have adequate water-holding ability while

providing adequate aeration, it should have the desired pH and a reasonable CEC, itshould be heavy enough to support the plant but not too heavy, it should be free of pests.

6. Describe a preferred watering program.Answer: Apply water when half of the available water in the pot is depleted. Wet

the entire media in each watering, allowing some water to drain from the bottom of thepot.

7. Which nutrients are added to the root media, or later to the plant, and how are theyadded?

Answer: All essential nutrients must be added unless the medium contains rottingorganic materials such as leaves or manure. Usually the less soluble materials such asphosphate, calcium, magnesium and micronutrients are added to the root medium as solidpowders. The more soluble nitrogen, sulfur, and potassium may be added in irrigationwater. Sometimes all nutrients are added in irrigation water, especially if the medium isextremely porous and non-retentive as with sand or perlite.

8. Why do most root media need lime? How much do they need?Answer: Peat moss and other commonly used organic components are very

acidic. Root media using these products need lime to neutralize the acids. About 6 to 8kg of lime per cubic meter of medium is common where peat moss is a major componentof the medium.

9. Why and how are root media pasteurized?Answer: The warm, moist environment of a greenhouse is ideal for the growth of

many pathogenic microbes. Root media must be pasteurized to kill these organisms.Pasteurization is usually done by steam or dry heat, or by using chemicals such aschloropicrin.

10. Describe (a) hydroponics, and (b) the nutrient film technique (NFT).Answers: (a) Hydroponics is the practice of growing plants in nutrient solution

without soil. (b) The NFT involves growing plants in a thin layer of nutrient solution thatcontinually flows across the roots.