One of the earliest methods of conservation tillage came to be known as contour plowing, or "plowing on the contour." Tilling the soil along the gentle slopes of a piece of cropland, instead of up and down the gradient, prevents fertile topsoil from being carried downhill by flowing rainwater. This preventive measure is most important in areas which are prone to violent storms or heavy rains. Not only is the topsoil kept in place, minerals like salt or additives such as fertilizers, insecticides or weed control agents, as well as bacteria from animal waste are not swept away to pollute bodies of potable water.
In Thomas Jefferson's time, contour plowing was called more simply "horizontal plowing." Jefferson had won a coveted medal from the major agricultural society in France for his design of the moldboard plow, but he began to notice drawbacks to the heavy use of that instrument. One of his relatives, a politically active farmer named Thomas Mann Randolph, was inspired to develop a new plowing technique in order to salvage the hilly areas in Virginia. Instead of funneling water down, like shingles on the roof of a house, it caught the rain in little ridges of upturned earth. Jefferson commented on a noticeable improvement, specifying that the horizontal furrows retained surplus rainwater and allowed it to evaporate back into the soil.
Even after this successful experiment, later versions of the moldboard plow caused damage to the delicate top-soil of the great plains and prairies of the Midwest United States. The most dramatic evidence of soil erosion took the form of huge dust storms and crop failures during the Great Depression. Since then, contour plowing and other forms of conservation tillage have been reinstituted.
Drawbacks to contour plowing have caused it to be less widely used than conventional tillage methods. Some farmers may not have been fully aware of erosion damage and prevention. Lack of access to equipment, funding, or training sometimes take their toll. One of the main limitations of contour plowing results from its contribution of pockets of untilled land. These untended spots eventually develop weeds, which require extra herbicides. Killing off the weeds sometimes destroys surrounding grasses, which in turn leaves another opportunity for rainwater runoff to arise. To combat this possibility, contour plowing is often applied in combination with other soil conservation techniques, such as terracing.
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