EROSION, COMPACTION, AND CONTAMINATION

EROSION

Soil loss during agricultural production is mainly caused by water, wind, and tillage. Additionally, landslides (gravitational erosion) may occur on very steep slopes. While water erosion and landslides occur under extremely wet soil conditions, wind erosion is a concern with very dry soil. Tillage erosion occurs on fields that are either steep or have undulating topography and is not affected by soil moisture conditions, because the soil movement downslope is caused by the action of farm implements.

Erosion is the result of the combination of an erosive force (water, wind, or gravity), a susceptible soil, and several other management- or landscape-related factors. A soil’s inherent susceptibility to erosion (its erodible-it) is primarily a function of its texture (generally, silts more than sands and clays), its aggregation (the strength and size of aggregates, which are related to the amount of organic matter), and soil water conditions. Many management practices can reduce soil erosion, although different types of erosion have different solutions.

Water Erosion

Water erosion occurs on bare, sloping land when intense rainfall rates exceed a soil’s infiltration capacity and runoff begins. The water concentrates into tiny stream-lets, which detach the saturated soil and transport the particles downhill. Runoff water gains more energy as it moves down the slope, scouring away more soil and also carrying more agricultural chemicals and nutrients, which end up in streams, lakes, and estuaries u

Reduced soil health in many of our agricultural and urban watersheds has resulted in increased runoff dur-ing intense rainfall and increased problems with flood-ing. Also, the lower infiltration capacity of degraded soils reduces the amount of water that is available to plants, as well as the amount that percolates through the soil into underground aquifers. This reduction in under-ground water recharge results in streams drying up during drought periods. Watersheds with degraded soils thus experience lower stream flow during dry seasons and increased flooding during times of high rainfall.

Soil erosion is of greatest concern when the surface

is unprotected and directly exposed to the destruc-tive energy of raindrops and wind (figure 6.1). While degraded soils tend to promote erosion, the process of erosion in turn leads to a decrease in soil quality. Thus, a vicious cycle is begun in which erosion degrades soils, which then leads to further susceptibility to erosion, and so on. Soil is degraded because the best soil material— the surface layer enriched in organic matter—is removed by erosion. Erosion also selectively removes the more easily transported finer soil particles. Severely eroded soils, therefore, become low in organic matter and have less favorable physical, chemical, and biological

characteristics, leading to a reduced ability to sustain crops and increased potential for harmful environmen-tal impacts.

Wind Erosion

The picture of wind erosion from the Dust Bowl era provides a graphic illustration of land degradation. Wind erosion can occur when soil is dry and loose, the surface is bare and smooth, and the landscape has few physical barriers to wind. The wind tends to roll and sweep larger soil particles along the soil surface, which will dislodge other soil particles

and increase overall soil detachment. The smaller soil particles (very fine sand and silt) are lighter and will go into suspension. They can be transported over great distances, sometimes across continents and oceans. Wind erosion affects soil quality through the loss of topsoil rich in organic matter and can cause crop dam-age from abrasion . re In addition, wind erosion affects air quality, which is a serious concern for nearby communities.

The ability of wind to erode a soil depends on how that soil has been managed because strong aggregation makes it less susceptible to dispersion and transpor-tation. In addition, many soil-building practices like mulching and the use of cover crops protect the soil surface from both wind and water erosion.

Landslides

Landslides occur on steep slopes when the soils have become supersaturated from prolonged rains. They are especially of concern in places where high population pressure has resulted in the farming of steep hillsides The sustained rains saturate the soil (especially

in landscape positions that receive water from upslope areas). This has two effects: It increases the weight of

the soil mass (all pores are filled with water), and it decreases the cohesion of the soil (see the compaction of wet soil in figure 6.10, right, p. 64) and thereby its ability to resist the force of gravity. Agricultural areas are more susceptible than forests because they lack large, deep tree roots that can hold soil material together. Pastures on steep lands, common in many mountainous areas, typically have shallow-rooted grasses and may also experience slumping. With certain soil types, landslides may becomes liquefied and turn into mudslides.

Tillage Erosion

Tillage degrades land even beyond promoting water and wind erosion by breaking down aggregates and exposing soil to the elements. It can also cause erosion by directly moving soil down the slope to lower areas of the field. In complex topographies—such as seen in figure 6.4—till-age erosion ultimately removes surface soil from knolls and deposits it in depressions (swales) at the bottom of slopes. What causes tillage erosion? Gravity causes more soil to be moved by the plow or harrow downslope than upslope. Soil is thrown farther downslope when tilling in the downslope direction than is thrown uphill when tilling in the upslope direction ().

Downslope tillage typically occurs at greater speed than when traveling uphill, making the situation even worse. Tillage along the contour also results in downslope soil movement. Soil lifted by a tillage tool comes to rest at a slightly lower position on the slope . A more serious situation occurs when using a moldboard plow along the contour. Moldboard plowing is typically performed by throwing the soil down the slope, as better inversion is thus obtained than by trying to turn the fur-row up the slope . One unique feature of till-age erosion compared to wind, water, and gravitational

the potential for further soil losses from water or wind erosion. On the other hand, tillage erosion does not generally result in off-site damage, erosion is that it is unrelated to extreme weather events and occurs gradually with every tillage operation.

Soil loss from slopes due to tillage erosion enhances because the soil is merely moved from higher to lower positions within a field. However, it is another reason to reduce tillage on sloping fields.

SOIL TILTH AND COMPACTION

A soil becomes more compact, or dense, when aggregates or individual particles of soil are forced closer together. Soil compaction has various causes and different visible effects. Compaction can occur either at or near the surface (surface compaction, which includes surface crusting as well as plow layer compaction) or lower down in the soil (subsoil compaction). .

Surface Compaction

Plow layer compaction—compaction of the surface layer—has probably occurred to some extent in all intensively worked agricultural soils. It is the result of a loss of soil aggregation that typically has three primary causes—erosion, reduced organic matter levels, and force exerted by the weight of field equipment. The first two result in reduced supplies of sticky binding materi-als and a subsequent loss of aggregation.

Surface crusting has the same causes as plow layer compaction but specifically occurs when the soil surface is unprotected by crop residue or a plant canopy and the energy of raindrops disperses wet aggregates, pound-ing them apart so that particles settle into a thin, dense surface layer. The sealing of the soil reduces water infil-tration, and the surface forms a hard crust when dried. If the crusting occurs soon after planting, it may delay or prevent seedling emergence. Even when the crust is not severe enough to limit germination, it can reduce water infiltration. Soils with surface crusts are prone to