soil erosion

soil erosion Soil erosion, the loss of soil from the surface of the Earth, is principally due to the action of water, wind, and mass movement, alone or sometimes in combination. Other agents that contribute to erosion are animals and humans, for example, in footpath erosion.

Erosion is a natural process generally occurring at low rates. Under woodland or grassland, rates are less than one tonne per hectare per year. Human-induced or accelerated erosion may be a serious environmental problem which threatens the ability of communities to grow crops or graze animals. It may also damage property or pollute water some distance from the site of the erosion. The most famous example is the Dust Bowl of the American Great Plains in the 1930s, particularly between 1933 and 1938. These were drought years; but ploughing of grazing land for wheat farming led to severe wind erosion across a huge area and the migration westwards of hungry and disposessed farmers (as depicted in Steinbeck's The grapes of wrath). In Amarillo, Texas, during one month at the height of the period there were 23 days within each of which there were at least 10 hours when windblown soil was in the air, and one in five storms gave rise to zero visibility.

Blaikie has argued that the explanation for soil erosion is usually to be found in social and political relations in society, especially factors concerning land ownership. Such factors explain why unsuitable land is farmed and management decisions are taken which increase the risk of erosion occurring. Physical factors such as slope, soil type, vegetation cover, and the amount, timing, and intensity of rainfall and wind influence how much erosion occurs.

Soil erosion is a potentially serious problem because rates of soil formation are low, depending as they do on the weathering of rocks, the deposition of sediment, and the accumulation organic material from animal and vegetable life. Rates of formation are estimated to be less than one tonne per hectare per year (t ha⁻¹ year⁻¹). Rates of soil erosion can locally exceed 100 t ha ⁻¹ year ⁻¹; average rates are much lower, yet still frequently exceed rates of soil formation. Although rates of erosion are commonly quoted for various countries, there are problems with such figures. Many are obtained from small-plot experiments which poorly replicate field conditions. Others are based on the sampling of sediment carried away by rivers; this tells us little about amounts of erosion on fields within the river basin.

Soil erosion by water appears to be more extensive than that by wind. The characteristic forms of water erosion are rills and gullies. Rills are small, temporary channels that can be removed by cultivation, for example, by a plough. Gullies are more substantial and, unless filled in, will become permanent features of the landscape. In semi-arid landscapes gullies may be large: in the American south-west they are known as arroyos. Their development has been attributed to landscape destabilization resulting from the introduction of cattle in large numbers in the late nineteenth century. Climatic change or wetter periods have also been implicated. Landscapes which have suffered extreme dissection by gullies are known as badlands. Examples occur in Cappadocia (Turkey), Alberta (Canada), Swaziland, and southern Spain. These forms have probably developed over thousands of years.

The impact of raindrops on bare soil causes splash erosion. This is important since it disaggregates soil particles and allows them to be carried away by sheet flow (interrill erosion). Splash itself moves soil particles only short distances.

Wind and water erosion remove the most valuable part of the soil, the organic-rich upper horizon. This leads to reduction in its water-holding capacity and in the stability of soil aggregates. Fine-grained soil particles (clay and silt) and organic matter are preferentially removed by run-off. Soil nutrients are commonly attached to the finest particles, and their removal thus reduces soil fertility. Over long periods of time soils then become coarser and in some cases more stony. Erosion consequently results in thinning of the soil, and many experiments have shown that loss of soil leads to lower crop yields. This can be compensated by addition of fertilizers; these, however, may be costly, and excessive use of them can damage freshwater habitats and aquifers. Crops may be damaged by the abrasion of moving particles carried by the wind or buried by deposits of soil.

Overgrazing by animals is a common cause of erosion. Semi-arid areas are especially at risk because of seasonal or periodic drought. The loss of vegetation cover exposes the land to erosional processes. Overgrazing by goats and sheep has led to erosion in the Mediterranean basin both in the past and at the present day. Erosion is therefore an important component of desertification, which is defined as ‘land degradation in arid, semi-arid, and dry sub-humid areas resulting mainly from adverse human impact’. Adverse human impact on vegetation is also included in this concept.

The impact of mass movements (landslides, debris flows, etc.) is difficult to estimate because they tend to occur irregularly. They are of particular importance in upland or montane areas, such as parts of China, the Himalayas, and Tanzania. The extent to which such occurrences are due to human action as opposed to natural forces is in dispute. In the Himalayas there is strong evidence that rates of erosion are high because of steep slopes and high rainfall rather than farming and deforestation.

Soil erosion is not exclusively a modern phenomenon. In many societies population pressure has led to the cultivation of unsuitable land and erosion. In Greece, forest clearance and population increases led to severe soil erosion in the period 4000 to 3000 bc. On the southern chalklands of Britain, Neolithic and Bronze Age clearance of wooded landscapes and the introduction of arable farming gave rise to substantial erosion.

Soil erosion affects the quality of the soil and therefore the success of farming: these are known as on-farm impacts. From the point of view of short-term economics, the off-farm impacts appear to be much more serious. These include the pollution of freshwater habitats by soil and agricultural chemicals; flooding and damage to property by soil-laden water; and the silting up of dams, reservoirs, and river courses.

The distribution and the rate of soil erosion are predicted by compiling erosion hazard maps and by mathematical approaches, most commonly the Universal Soil Loss Equation (USLE): Soil loss = R × K × LS × C × P,

where R is an index of rainfall erosivity, K is an index of soil erodibility, LS is an index of slope length and steepness, C is an index of crop cover, and P is an index of conservation measures.

The terms in the USLE may be quantified and an estimate of soil loss obtained for a specific situation. Although the USLE is useful for identifying the factors that control erosion, it is poorly tested and there is little evidence that it gives reliable estimates of erosion on field-size areas. New models for estimating soil loss are being developed: these are based on detailed simulation of the processes of erosion.

Although understanding and predicting erosion is often imperfect, many societies have developed methods of preventing soil loss, such as the terracing of slopes. If well maintained, terraces are effective but they are costly to construct and constrain mechanized agricultural operations. Soil conservation techniques are difficult to introduce to societies where the main issue is producing the next harvest.

John Boardman

Bibliography

Bell, M. and and Boardman, J. (1992) Past and present soil erosion. Oxbow Books, Oxford.
Blaikie, P. (1985) The political economy of soil erosion in developing countries. Longman, Harlow.
Morgan, R. P. C. (1986) Soil erosion and conservation. Longman, Harlow.
Pimental, D. (ed.) (1993) World soil erosion and conservation. Cambridge University Press.