By the Numbers: Soil Erosion of Cropland in the U.S., 1982 to 1992; October 1996; Scientific American Magazine; by Doyle; 1 Page(s)

America¿s position as the world¿s leading exporter of grains depends largely on a layer of topsoil typically less than a foot thick. This layer usually erodes but can be replenished through the accumulation of organic matter, the process of weathering, the activity of earthworms and microorganisms, and other means. As a rule of thumb, it takes 30 years to form an inch of topsoil--and much longer in areas of little rainfall. An inch of topsoil, however, can be lost in less than a decade of such improvident farming practices as excessive grazing, monocropping and destruction of ground cover. Heavy and frequent rain can wash away topsoil, particularly where vegetation is sparse and where the ground slopes. Wind erosion is especially destructive during prolonged droughts, such as that of the 1930s, which produced the infamous dust bowl in parts of the Great Plains.

The devastation of the dust bowl led to the creation of the Soil Conservation Service, recently renamed the Natural Resources Conservation Service. Since 1982 the NRCS has systematically measured erosion and other soil characteristics for the entire country in its National Resources Inventory. The map, created from this database, shows that in most areas with extensive cropland, there has been an improvement or at least no increase in average erosion rates. In 1992 wind and water caused tolerable levels of erosion on 68 percent of cropland, an improvement of 21 percent over 1982. Some of the improvement was the result of crop rotation and better tilling methods but more important have been the efforts of the Conservation Reserve Program, in which the government pays farmers to remove environmentally sensitive cropland from use.