Natural and Human-Accelerated Soil Erosion :
Soil does not stay in one place indefinitely. Soil erosion is the movement of soil components, especially topsoil, from one place to another. The two main forces causing soil erosion are wind and flowing water.
Some soil erosion always takes place because of natural water flow and winds. But the roots of plants generally protect soil from excessive erosion. Agriculture, logging, construction, and other human activities that remove plant cover greatly accelerate the rate at which -soil erodes.
Soil scientists distinguish between three types of erosion by water: sheet, rill and gully. Sheet erosion occurs when surface water moves down a slope or across a field in wide flow. Because, it erodes topsoil evenly, sheet erosion may not be noticeable unit much damage has been done. In rill erosion the surface water forms rivulets that flow at high velocities through miniature valleys. In gully erosion the rivulets join together, forming large, high velocity flows.
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Excessive erosion of topsoil reduces both the fertility and the water-holding capacity of soil. The resulting sediment, the largest single source of water pollution, clogs irrigation ditches, navigable waterways and reservoirs.
Soil, especially topsoil, is classified as renewable resources because it is continually regenerated by natural processes. In tropical and temperate areas, the renewal of 2.54 centimetres of soil takes from 200 to 1,000 years, depending on climate and type. However, if the average rate of topsoil erosion exceeds the rate of topsoil formation on a piece of land, the topsoil on that land becomes a non-renewable resource being depleted.
Annual erosion rates for agricultural land throughout the world are 18 to 100 times the natural renewal rate. Soil erosion on forestland and rangeland is not as severe as erosion on cropland, but forest soil takes two to three time longer to restore itself. Construction sites usually have the highest erosion rates erosion rates by far. After bulldozers remove all or most of the vegetation from a site. Most builders do little to prevent erosion of the bare soil.
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Today topsoil is eroding faster than it forms on about one-third of the world’s cropland. The amount of topsoil washing and blowing into the world’s rivers, lakes, and oceans each year would fill a train of freight cars long enough to encircle the planet 150 time. At this rate the world is losing about 7% of its topsoil from potential cropland each decade.
The situation is worsening as farmers cultivate areas unsuited for agriculture to feed the world’s growing population. Such mining of the soil converts potentially renewable resources into a nonrenewable resource.