Erosion and erosion control

DEFINITION: The loss of topsoil through the actions of wind and water, and the efforts undertaken to mitigate such loss

The control of erosion is vital because soil loss from agricultural land is a major contributor to nonpoint source pollution and desertification and represents one of the most serious threats to world food security.

In the United States alone, some two billion tons of soil erode from cropland on an annual basis. About 60 percent, or 1.2 billion tons, is lost through water erosion, and the remainder is lost through wind erosion. This is equivalent to losing 0.3 meter (1 foot) of topsoil from 810,000 hectares (2 million acres) of cropland each year. Although soil is a renewable resource, soil formation occurs at rates of just a few inches per hundred years, which is much too slow to keep up with erosive forces. The loss of soil fertility through is incalculable, as are the secondary effects of of surrounding waters and increase of in rivers and streams.

Erosion removes the topsoil, the most productive soil zone for crop production and the plant nutrients it contains. Erosion thins the soil profile, which decreases a plant’s rooting zone in shallow soils, and can disturb the topography of cropland sufficiently to impede the operation of farm equipment. Through erosion, nitrates, phosphates, herbicides, pesticides, and other agricultural chemicals are carried into surrounding waters, where they contribute to cultural eutrophication. Erosion also causes the deposit of increased sedimentation in lakes, reservoirs, and streams, which eventually require dredging.

There are several types of wind and water erosion. The common steps in water erosion are detachment, transport, and deposition. Soil particles become detached from soil aggregates, and the particles are carried, or transported, away; in the process, the particles scour new soil particles from aggregates. Finally, the soil particles are deposited when the water flow slows. In splash erosion, raindrops impacting the soil can detach soil particles and hurl them considerable distances. In sheet erosion, a thin layer of soil is removed by tiny streams of water moving down gentle slopes. This is one of the most insidious forms of erosion because the effects of soil loss are imperceptible in the short term. Rill erosion is much more obvious because small channels form on a slope. These small channels can be filled in by tillage. In contrast, ephemeral gullies are larger rills that cannot be filled by tillage. Gully erosion is the most dramatic type of water erosion. It leaves channels so deep that even equipment operation is prevented. Gully erosion typically begins at the bottoms of slopes, where the water flow is fastest, and works its way with time to the top of a slope as more erosion occurs.

Wind erosion generally accounts for less soil loss than does water erosion, but in some dry areas, such as much of the American Southwest, it is actually the dominant type of erosion. Wind speeds 0.3 meter (1 foot) above the soil that exceed 16 to 21 kilometers (10 to 13 miles) per hour can detach soil particles. These particles, typically fine to medium-size sand grains fewer than 0.5 millimeters (0.02 inches) in diameter, begin rolling and then bouncing along the soil, progressively detaching more and more soil particles by impact. The process, called saltation, is responsible for 50 to 70 percent of all wind erosion. Larger soil particles are too big to become suspended and continue to roll along the soil. Their movement is called surface creep.

The most obvious display of wind erosion is called suspension, when very fine and clay particles detached by saltation are knocked into the air and carried for enormous distances. The Dust Bowl of the 1930s was caused by suspended silt and clay in the Great Plains of the United States. It is also possible to see the effects of wind erosion on the downward sides of fences and similar obstacles. Wind passing over these obstacles deposits the soil particles it carries. Other effects of wind erosion are tattering of leaves, filling of road and drainage ditches, wearing of paint, and increasing incidence of respiratory ailments.

Control Measures

The four most important factors affecting erosion are soil texture and structure, roughness of the soil surface, slope steepness and length, and soil cover. Several passive and active methods of erosion control involve these four factors. Wind erosion, for example, is controlled through the creation of windbreaks, rows of trees or shrubs that shorten a field and reduce the wind velocity by about 50 percent. Tilling the land perpendicular to the wind direction is also a beneficial practice, as is keeping the soil covered by plant as much as possible.

Water erosion is controlled through a number of practices. In the United States, farmers can get help from the federal government-sponsored Conservation Reserve Program to protect highly erosive, steeply sloped land. Tilling land along the contours of slopes aids in preventing erosion, as does the shortening of long slopes by terracing, which also reduces the slope steepness. Permanent grass waterways can be planted in areas of cropland that are prone to water flow. Likewise, grass filter strips can be planted between cropland and adjacent waterways to impede the velocity of surface and cause suspended soil particles to sediment and infiltrate before they can become contaminants.

Conservation tillage practices such as minimal tillage and no-till farming, or zero tillage, have been widely adopted by farmers as a simple means of erosion control. As the names imply, these are tillage practices in which as little disruption of the soil as possible occurs and in which any crop residue remaining after harvest is left on the soil surface to protect the soil from the impacts of rain and wind. The surface residue also effectively impedes water flow, which results in less suspension of soil particles. Because the soil is not disturbed, practices such as no-till farming also promote rapid water infiltration, which reduces surface runoff. Zero tillage is rapidly becoming the predominant practice in southeastern states such as Kentucky and Tennessee, where rainfall levels are high and erodible soils occur.

Bibliography

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Faulkner, Edward. Plowman’s Folly. Norman: University of Oklahoma Press, 1943.

Field, Harry L., and John B. Solie. “Erosion and Erosion Control.” In Introduction to Agricultural Engineering Technology: A Problem Solving Approach. 3d ed. New York: Springer, 2007.

Mulvihill, Keith. "Soil Erosion 101." NDRC, 1 June 2021, www.nrdc.org/stories/soil-erosion-101#what-is. Accessed 20 Dec. 2024.

Plaster, Edward. Soil Science and Management. 5th ed. Clifton Park, N.Y.: Delmar Cengage Learning, 2008.

Schwab, Glen, et al. Soil and Water Conservation Engineering. 5th ed. Clifton Park, N.Y.: Delmar Cengage Learning, 2005.