When soil is exposed, it is removed by wind and rain. For example, rain pounding on the surface of soil breaks it apart, making it easy for running water to carry it away. At first, erosion may be sheets of water running down a slope. However, as time goes on, flowing water may become concentrated in tiny channels, or rills. With time these may enlarge to gullies, making the fields unusable because equipment can't get around. As the gullies enlarge and widen by lateral erosion, the stream banks are undercut, eating into adjacent land.
The question of how much soil is lost is uppermost in the minds of farmers, soil scientists, and other scientists interested in rates of erosion. Although the answer is difficult to determine, a clue is the amount of sediment that is being transported by streams. The material in streams includes not only dissolved minerals but also rock material from nonsoil sediments as well as soil, so that it becomes a task to assess how much is actually soil. In the United States alone, the Department of Agriculture has estimated that more than 4 billion tons of soil are lost each year. .
After soil dries it is often loose, allowing wind to carry it off, the amount being determined by wind speed. The rate of wind erosion of soils is difficult to determine, but it is thought to be usually less than that of water erosion except during times of drought.
A good example of wind erosion occurred in the Dust Bowl area of the United States during the 1930s, where 100 million acres of land centered on parts of Oklahoma, Texas, New Mexico, Kansas, and Colorado were affected by severe loss of soil. It resulted from a combination of factors, including the clearing of land of natural vegetation, drought, constant wind, and poor farming practices. Dust storms began in 1932 and carried dust eastward on the prevailing winds, dimming the sun and producing rain loaded with dust in distant places such as New York.
By the end of the decade increased rainfall and improved farming techniques had substantially reduced the problem. However, drought returned several times, each event producing substantial wind erosion of the remaining soil. One of the techniques developed to counter the problem of wind erosion is keeping the soil wet with irrigated water; that method can continue as long as the local aquifer contains adequate water. In May 2002 newspaper articles related that in Montana more than a thousand wheat farmers abandoned their farms because drought had once again resulted in failed crops and blinding dust storms—reminding many people of the Dust Bowl days of the 1930s. Worldwide, dust storms have substantially increased as a result of deforestation and the conversion of naturally vegetated land to farms.
Another type of soil destruction does not necessarily involve its loss, but rather heavy nutrient depletion by crops such as corn, cotton, and tobacco. In these cases the soil has to be substantially fertilized, which can indirectly cause streams and groundwater to become polluted.
Irrigation in desert areas where evaporation is high has deleterious effects on the usefulness of soil. For example, in Egypt irrigation water from the Nile is spread over fields. As the water evaporates it leaves behind dissolved salts in the open spaces of the soil. With time the soils become so salinized that they can no longer supports crops. In ancient Egypt, when the Nile flooded it removed the salt and deposited a new layer of silt. With the building of dams across the Nile, however, this process no longer occurs, because sediments are trapped in the reservoir.
Cultivation of soils for agriculture, deforestation, overgrazing, and other intrusions such as grading for highways, urban land use, and bulldozing areas for large-scale engineering works have caused the loss of natural vegetative cover and increased soil losses. This is a direct consequence of the growth of world population and improvements in technology upsetting the balance of nature.
Although it is hard to estimate how much soil is being formed today, it seems safe to say that erosion far exceeds soil production. For example, in the New York City area, glaciated bedrock still retains its smooth surface with no sign of soil formation even after 18,000 years. Researchers estimate that in California it takes as much as 2,000 years for one inch of soil to form, and that topsoil in various parts of the world is being lost at rates up to eighty times faster than it is being created.
On farms, as elsewhere, the roots of plants hold soil together. Farming practices that leave the remains of plants in fields after the crops have been harvested, and planting crops between the cash crops, help reduce erosion. These plants also protect the soil from erosion by wind and rain. Windbreaks using rows of trees, fences, and contour plowing are all techniques that have been introduced to reduce soil loss.
See also: Erosion; Geology, Geomorphology, and Geography; Soil; Topsoil Formation
Akin, Wallace. 1990. Global Patterns: Climate, Vegetation, and Soils. Norman: University of Oklahoma Press; Montgomery, Carla. 1999. Environmental Geology, 5th ed. Dubuque, IA: Wm. C. Brown Publishers; Morgan, Royston P. C. 1995. Soil Erosion and Conservation. New York: John Wiley and Sons.
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