III. SOILS, AGRICULTURE, AND ENVIRONMENT
Potentials for Soil and Water Degradation Next Section>>
This section considers the potential of the soil to become degraded (damaged) by soil erosion and compaction, and the potential of drinking water supplies to become contaminated. Soils are ranked according to how readily the soil itself becomes degraded and how readily the water that percolates through the soil or flows over it may become contaminated. In the following sections, although low, medium, and high potentials are discussed, “YES” is marked only if the potential is high for degradation.
Subsections:Soil erosion by water | Soil erosion by wind | Soil compaction | Water pollution
Soil erosion is the most serious soil-degrading process, not only in Indiana, but also in the rest of the world. Each year, over 100 million tons of soil erodes from Indiana’s cropland, pastures, forests, and other places where people live and work, such as residential areas, industrial areas, and parks. Another 17 million tons of soil erodes from stream banks, gullies, roadsides, and construction sites.
Soil erosion processes are discussed briefly here and are described more fully in Chapter VI, in which the Universal Soil Loss Equation (USLE) is presented. You can learn to evaluate soils without reading Chapter VI, but the information in it will help you understand erosion processes better. Chapter VI also explains how cropping and tillage practices control erosion within tolerable limits. Soil erosion by water depends on:
• The energy of the rainstorms. In Indiana, this energy increases from northeast to southwest (R factor of USLE).
• The erodibility of the soil (K factor). In general, medium-textured soils, especially silty ones, are more erodible than sandy or clayey soils. Also, soils low in organic matter (gray and brownish surface horizons) are more erodible than those high in organic matter (dark colored).
• Length of slope (L factor). Long slopes erode much more than short ones because the amount of runoff water increases with distance down the slope.
• Gradient (%) of slope (S factor). Water runs faster on steep slopes than on nearly level ones, and faster moving water can remove and carry more soil material.
• Cropping and management practices (C factor). These include the kind of tillage used and the kind and amount of cover on the soil. Living or dead plant material breaks the impact of raindrops on the soil and anchors soil particles in place.
• Mechanical practices (P factor). This includes farming on the contour (level rows across the slope) and terraces. These practices reduce the speed of down-slope water flow and thus decrease erosion.
These factors can be divided into two classes. The factors, R, K, L, and S are natural features of the soil and climate that man cannot change. Of these four factors, the amount of erosion depends mainly on slope gradient, S.
Farmers and others, however, can set the C and P factors by the kind of tillage and cropping systems they decide to use. The cropping practices most used in Indiana, which mainly affect the C factor, are described later in this chapter.
The influence of the C (cropping and management practices) and S (slope) factors on soil erosion is shown in Fig. 21. Whether the soil is cleanly plowed or forested has a tremendous effect on soil erosion. Also, percent slope greatly affects erosion under spring plowing and most other practices, but in the forest there is very little erosion regardless of slope.
Fig 21. Effect of percent slope on annual erosion for six tillage-cropping systems. The flat "T" line is the tolerable soil loss of four tons per acre. Conditions are typical for Indiana: slop length = 200 ft., K = 0.37, R = 180.
Three properties, slope gradient (S factor), surface texture, and surface color mainly affect water erosion potentials identified in soil evaluation. Potential classes are:
LOW: All soils on 0% to 2% slopes and soils on 3% to 6% slopes that do not qualify for medium potential.
MEDIUM: All soils on 7% to 12% slopes, and soils on 3% to 6% slopes in which the surface horizon is gray or brownish, and the surface texture is medium or moderately clayey.
| Mark High Potential for soil erosion by water YES for soils on slopes steeper than 12%. |
Soils with medium and high potential for erosion by water are called highly erodible land. To be eligible for certain kinds of financial assistance from the government, a farmer might need to follow a conservation plan for highly erodible land. Check with your local Soil and Water Conservation District office for details about the current program.
back to topSoil Erosion by Wind
Soil erosion by wind depends on wind speed, soil cloddiness (which depends largely on soil texture), surface roughness, soil moisture (related to natural drainage class), field size, and cover. Several of these factors are the same as those that affect water erosion. Soils with sandy or moderately sandy surface horizons are especially susceptible to wind erosion. For soil evaluation, the potentials for soil erosion by wind are:
LOW: Soils in which drainage is poor or somewhat poorly, and surface texture is clayey or moderately clayey.
MEDIUM: Soil that is not rated HIGH or LOW.
| Mark High potential for soil erosion by wind YES for soils with sandy or moderately sandy surface texture. |
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Soil Compaction
Soil compaction is also a serious problem in Indiana. It occurs when a certain amount of soil is packed into a smaller volume. The main cause of compaction is excessive tillage and traffic, especially when the soil is wet. Therefore, soil compaction can be minimized by reducing the amount of tillage and by avoiding driving on the soil when it is too wet. Some of the practices that control erosion, such as conservation tillage, also minimize compaction. Compaction is discussed in more detail in Chapter VI.
Nearly all soils can be seriously compacted. Poorly and somewhat poorly drained soils are most subject to compaction because they are often wet when they are tilled or driven on. Also, soils low in organic matter (grayish or brown in color) are most at risk. Soil compaction potentials should be kept in mind in deciding when tillage, planting, or harvest operations should be done. Very few Indiana soils have low potential for compaction. For soil evaluation, the potentials for soil compaction are:
LOW: Soil meets all three requirements:
1. Soil drainage is well or moderately well,
2. Surface texture is medium,
3. Surface color is dark.
MEDIUM: Soil that is not rated HIGH or LOW.
Mark High Potential for soil compaction YES for soils that have both: 1. somewhat poor or poor natural soil drainage, and |
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Water Pollution
Essentially all the water we drink either percolates through the soil or flows over it, so soil properties and land use largely determine the quality of the water we drink. Drinking water can be contaminated by point and non-point sources. Point sources of pollution enter a stream at a specific place. Outflow from municipal sewage treatment facilities is one kind of point-source pollution. It contains nitrogen (N), phosphorus (P), and other plant nutrients, and it might contain pathogenic (disease-carrying) organisms, especially if the treatment plant is taxed beyond its capacity during heavy rains. Storm drainage water does not go through a treatment plant and often may carry oil, gasoline, and de-icing chemicals directly into a stream. Industries may contribute inorganic chemicals, including heavy metals, and organic chemicals, such as PCBs. Leaking underground storage tanks (e.g., gasoline tanks) are another source of pollution. Point sources of pollution are important but are beyond the scope of soil evaluation.
Non-point sources of pollution, considered for soil evaluation, come from broad areas, such as farmland and onsite waste disposal systems. These are some potential pollutants:
• Nitric and sulfuric acids in rainwater. They are released to the atmosphere largely by burning fossil fuels and are washed back to the earth by the rain.
• Nitrates derived from chemical fertilizer, manure or onsite waste disposal systems. The amount of N farmers apply to soils depends on the kind of crop to be grown and its yield potential (see the Crop Nutrient and Pest Management section). If farmers apply too much nitrogen, the excess is usually converted to nitrate which can be removed from the field via surface runoff or tile drainage flow and eventually get into streams and lakes. A high concentration of homes with septic waste disposal systems also adds much N to the soil. Since none of it is removed with a crop, nitrate can seep into surface water and leach to groundwater. See the Soil and Water Chemistry section of Chapter IV for more details about the chemistry of N in the soil.
• Phosphorus also comes from chemical fertilizer, manure, and onsite systems. Phosphorus, in the form of phosphates, is bound tightly to soil particles at ordinary levels of P in the soil. However, if the P level becomes very high, often from large manure applications, it can move in the soil profile and get into the water. See the Soil and Water Chemistry section of Chapter IV for more details about the chemistry of P in the soil.
• Pathogens Under some circumstances, pathogenic microorganisms such as E-coli, usually from manure or septic effluent, can leach through the soil to the groundwater. If a manure lagoon breaks, streams may become contaminated with pathogens, as well as with N and P.
• Pesticides Several pesticides have been identified in groundwater. Most of those added to farm fields at recommended rates are broken down to harmless products in the soil, but some are not. Pesticide pollution may come from spills or over-application.
Groundwater pollution
The potential for groundwater pollution depends on soil properties and how the land is used. Most rural families depend on a well for their drinking water supply. Any pollution of groundwater on the farm will probably affect that farmer’s well before it affects others, so farmers are especially concerned about the quality of groundwater. In general, water from deep wells is less likely to be polluted than water from shallow wells. For this discussion, we assume that the wells are shallow. Soils can remove many potential contaminants from water. Some are filtered out by plants growing on the soil or by the soil itself. Other contaminants are absorbed on soil particles, or are chemically or biologically changed into harmless materials in the soil. Groundwater is especially subject to pollution in soils where rain water moves rapidly to the water table. These are soils that have a high infiltration rate (low runoff rate), have a high water table, and transmit water so fast to a shallow well that there is little time for purification. On the other hand, soils with the least potential for groundwater pollution:
• have a deep water table (are well drained),
• have high surface runoff and therefore less infiltration,
• have a high capacity to adsorb pollutants (fine texture),
• conduct water slowly, allowing time for degradation, for example, soils
with high clay content, fragipan, or dense till.
For soil evaluation, the potentials for groundwater pollution are:
LOW: Soils have all these properties:
1. well drained, and
2. surface texture of medium or finer, and
3. have one or more of the following above 40 inches:
a. moderately clayey or finer subsoil, or
b. fragipan limiting layer, or
c. dense till limiting layer.
MEDIUM: Soils that do not qualify for LOW
or HIGH.
Mark High potential for groundwater pollution YES for soils that have both: 1. poor or somewhat poor natural drainage, and |
Groundwater pollution is also greatly affected by how the land is used. Forest land has very little possibility of contaminating the water that percolates through the soil because farm chemicals and manure are seldom applied to forest land, and the soil effectively cleans up the rain that falls on the forest. Cropland, however, has more potential for water pollution because fertilizer, manure, and pesticides are often added to the soil. Pasture land is intermediate in groundwater pollution potential. Usually fewer chemical fertilizers and pesticides are applied to pasture than to cropland, but manure may be applied to pasture.
Surface water pollution
Much surface water pollution originates from point sources: pipes from municipal storm drains and sewage disposal systems, factories or other facilities that empty into a stream. For soil evaluation, however, we are more concerned about non-point sources of pollution, such as fields farmyards and concentrated onsite waste disposal systems.
The greatest non-point contaminant of surface water is sediment produced by soil erosion and the materials that accompany the sediment. Sheet erosion removes a thin layer of surface soil, where many of the surface-applied chemicals are held. Nutrients held on the soil surface accompany the soil material that is eroded and transported to a lake or reservoir. In many surface waters, especially lakes, phosphorus (P) is the nutrient that limits the growth of algae, the tiny plants that live near the surface of lakes, streams, and bays. When P levels increase, algae reproduce rapidly and block sunlight from the water plants at the bottom of the lake. Without sunlight this vegetation dies, and the decomposition processes uses up the dissolved oxygen in the water. The lack of oxygen kills fish and other animals in the water.
The soils that have the greatest potential for contributing contaminants to surface waters are the ones most subject to erosion by water. Therefore, the rules for potential for surface water pollution are the same as the rules for potential for erosion by water:
LOW: All soils on 0% to 2% slopes, and soils on 3% to 6% slopes that do not qualify for medium potential.
MEDIUM: All soils on 7% to 12% slopes, and soils on 3% to 6% slopes in which the surface horizon is gray or brownish, and the surface texture is medium or moderately clayey.
| Mark High potential for surface water pollution YES for soils on slopes steeper than 12%. |
Similar to groundwater, surface water pollution is also greatly affected by how the land is used. Forest land has little potential to pollute surface water. In the forest, most of the rain infiltrates into the soil rather than running off, but the part that runs off is cleaned up by the organic materials (leaves, dead plant material) on the forest floor. Cropland has the most potential for surface water pollution because fertilizer, manure, and pesticides added to the soil may be eroded off in a hard rain. Pasture is intermediate in surface water pollution. Usually chemical fertilizer and pesticide applications are less on pasture than on cropland, but manure may be applied to pastures.
Another potential source of pollution of surface water is tile drain outflow. Plant nutrients, especially N, move readily through the soil and the into the drain lines that flow to ditches, streams, and reservoirs. Phosphate was once thought to be immobile in the soil, but it has been found that if the P content becomes high enough, it can also move into and through the tile. This is important, but is not considered in the rules above.
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