SOIL EROSION

Soil erosion is the wearing away and redistribution of the earth's soil layer. It is caused by the action of water, wind and ice, and also by improper methods of agriculture. If unchecked, soil erosion results in the formation of deserts. It has been estimated that 20 per cent of the world's cultivated topsoil was lost between 1950 and 1990.

If the rate of erosion exceeds the rate of soil formation (from rock), then the land will decline and eventually become infertile.

The removal of forests or other vegetation often leads to serious soil erosion because plant roots bind soil, and without them the soil is free to wash or blow away, as in the American dust bowl. The effect is worse on hillsides, and there has been devastating loss of soil where forests have been cleared from mountain sides, as in Madagascar. Improved agricultural practices such as contour ploughing are needed to combat soil erosion. Windbreaks, such as hedges or strips planted with coarse grass, are valuable, and organic farming can reduce soil erosion by as much as 75 per cent.

In India, severe soil erosion is a characteristic of areas with heavy rainfall and improper land management. Vari­ous types of erosion take place. Normal or geologic erosion is a slow, long process which ensures an equilibrium between soil removal and formation. Accelerated soil erosion is erosion of the soil surface due to animal or human interference at a rate faster than that involved in the soil's building up.

In arid and semi-arid parts with high wind velocity, wind is a major cause of erosion. Finer particles are generally carried farther than coarse particles are. Erosion by water is of three types. Sheet erosion is when rainfall pounds soil grains loose, carrying them downwards. As the huge sheet of water running downslopes gains acceleration, erosion of the soil underneath also takes place. When sheet erosion continues for long, the silt-laden runoff forms many finger-shaped grooves over a large area.

This is rill erosion. Again due to sheet erosion, concentration of water along depressed areas results in heavy erosion in these areas which is termed gully erosion. In hilly parts, heavy rainfall and earthquakes result in limdslides, which bring about slip or landslide erosion. When cobbles, silt and boulders get deposited on the torrent bed, the bed level of the torrent is raised. This reduces the transporting capacity of the torrent, causing erosion (stream-bank ero­sion).