Erosion: Soil Quality

Introduction

Soil erosion involves the detachment and removal of soil material from one site and its transport to another location. Soil erosion usually degrades soil quality and a soil of poorer quality is less able to withstand further erosion, thus creating a downward spiral of soil degradation. When the surface soil is removed through erosion, organic matter and clay particles may be lost, with consequent reductions in fertility, biological activity, aggregation and rooting depth. Other potential effects of erosion on soil quality include reduced porosity and infiltration, formation of crusts on the soil surface, changes in soil texture, and compaction. These changes in turn reduce the capacity of the soil to supply and cycle nutrients, filter and degrade toxic materials, store and supply moisture and sustain plant and biological productivity. They may also result in increased runoff, less biomass production and plant cover, and greater susceptibility to further erosion. Erosion increases the variability in soil quality across a field and, on a broad scale, is associated with widespread loss of agricultural productivity and declining water quality.

Soil erosion is a natural process by which soil particles are detached and moved by water, wind, gravity, or ice. All soils have an inherent erodibility, or natural susceptibility to erosion, based on soil features, topography, and climate. However, human activities, such as logging, livestock grazing, tillage, removal of vegetation, and urban development, can greatly accelerate natural rates of erosion. Cleared and managed as they are for crop and livestock production, agricultural soils are particularly susceptible to wind and water erosion (Figures 1 and 2), and also to a recently recognized process known as tillage erosion—the loosening of soil by tillage equipment and its downslope movement under gravity.¹¹¹

Eroded soil may move only a few meters in a field and come to rest in lower positions, resulting in increased variability of surface soil properties across the field as subsoil becomes exposed in some places and surface layers are buried and over-thickened in others. It may also move great distances, being deposited in neighboring fields, roadside ditches, and water bodies. In some cases of wind erosion, fine soil particles may travel many kilometers before being deposited.

Effects of Soil Erosion on Soil Quality

The loss of surface soil in a landscape may impair soil function, and thus soil quality, through adverse effects on many physical, chemical, and biological properties of the soil.¹²¹ Table 1 summarizes some of the effects of erosion on the soil quality functions listed by Gregorich.¹⁴¹

FIGURE 1 Deposition of wind-blown soil material. (From USDA, Natural Resources Conservation Service—Soil Quality Institute.)

FIGURE 2 Severe erosion by water has removed all of the surface layer and much of the subsoil. (From USDA, Natural Resources Conservation Service—Soil Quality Institute.)

TABLE 1 Some General Effects of Soil Erosion on Soil-Quality Functions

Physical Effects

Erosion selectively removes the finer, lighter particles from the soil surface, leaving coarser particles behind. Depending on the severity of erosion, an eroded soil may become very coarse in texture, sometimes with a gravelly surface. The deposition of the eroded material in lower topographic areas may result in a thickening of the topsoil and an increase in rooting volume.

Tillage of eroded soils may result in a mixing of subsoil with the surface soil, altering its composition. For example, cultivation of eroded soils having clay-enriched subsoils may increase the clay content of the surface soil. As erosion progresses, plant roots must enter progressively deeper into the subsoil layer to obtain nutrients and water. Where subsoil layers restrict root growth because of their physical and/or chemical properties, the depth of rooting is reduced, along with the capacity of the soil to supply water and nutrients to plants.

Individual soil particles are held together in aggregates, which form the structural fabric of the soil. Soils with good structural arrangement of aggregates and the pore spaces between them provide good aeration for soil roots and microbes; allow ready movement and storage of water and plant nutrients in the pore spaces; and retain their structure when exposed to stresses such as cultivation and the impact of raindrops.^13,41 Abrasion by wind, rain, and tillage can disintegrate aggregates at the soil surface, and the resulting fine particles can plug larger soil pores and form a hard, thin crust on the surface. This crust seals the surface and limits the infiltration of air and water, as well as impedes the emergence of seedlings. Finer soil particles are also more easily compacted as the pore spaces between them are reduced under the pressure of farm machinery. Deteriorating soil structure further increases the risk of soil erosion—fine soil particles created by the breakdown of aggregates at the soil surface are especially vulnerable to wind and water erosion. Furthermore, compacted, crusted soils resist the infiltration of water, increasing the volume of surface runoff and compounding the effects of erosion. Thus, erosion reduces soil quality, making the soil prone to further erosion and further degradation.

Chemical Effects

Organic matter and clay are important sites of cation exchange in the soil. Cation-exchange capacity is a measure of negatively charged sites on the soil particles that are capable of holding positively charged ions, including many plant nutrients. As organic matter and clay particles are lost from the soil surface through erosion, attached nutrients are relocated in the landscape, often to adjacent water bodies where they contribute to declining water quality. Loss of these fine soil particles also impairs the ability tostore nutrients, reducing soil fertility. This effect is more pronounced in sandy soils containing small amounts of clay, though it may be offset to some degree if erosion causes the surface layer to become more clayey. Where subsoils are more enriched in clay than the surface soil, the clay particles may form chemical bonds with phosphorus, fixing it into forms not easily available to plants and thus reducing fertility.

For some acid soils with subsoil pH <5.0, concentrations of available aluminum may be at levels toxic to plant roots and removal of surface soil by erosion can effectively reduce the rooting depth of the soil.

Subsoil layers having pH >8.5 often contain high levels of sodium. Surface soil removal and subsequent exposure of these subsoils to rainwater or irrigation waters with low ionic concentrations can lead to dispersion of clay particles, loss of soil structure, surface sealing, and greatly reduced water infiltration.

Biological Effects

Removal of organic matter and nutrients from the soil surface by erosion reduces the food and energy supply needed to sustain healthy populations of soil organisms and support plant growth. Soil organisms are the agents of organic matter decomposition and nutrient cycling in soil. In addition, they play an important role in the stabilization of soil aggregates through the production of binding agents such as roots, fungal hyphae, polysaccharides, gums, and complex molecules consisting of humic substances combined with iron, aluminum, or aluminosilicates.¹⁵'⁶! So, _as erosion proceeds, the food source for organisms is reduced, leading to declines in populations. Soil structure and stability then deteriorate, the soil becomes more susceptible to further erosion, and the cycle continues.

Conclusions

The interaction of accelerated soil erosion and soil quality is complex. Soil erosion usually reduces soil quality, and a soil of poorer quality is less able to withstand erosion, thus creating a downward spiral of soil degradation. However, many soil-conservation practices have been useful in mitigating the effects of erosion. Reduced tillage systems limit soil disturbance and build soil structure. Crop residue management, under seeding, cover cropping, and permanent cover (e.g., pasture) protect the soil from the action of wind and water. Contour cultivation, grassed waterways, and terracing alter the flow of surface water, curbing water erosion. Herbaceous wind barriers and woody windbreaks and shelterbelts help to control wind erosion. Nevertheless, large tracts of agricultural land throughout the world are still subject to the unsustainable loss of soil as a result of erosion, and continued adoption of preventive methods are needed to protect and restore soil quality.

References

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