Soil erosion is the wearing away of land surface by the action of such natural agencies as water and wind .

The following types of erosion are observed in India :

Normal or geologic erosion.   This is a normal feature of any landscape. Geologic erosion takes place steadily but so slowly that ages are required for it to make any marked alteration in the major features of the earth's surface. There is always an equilibrium between the removal and formation of soil, so that unless the equilibrium is disturbed by some outside agency, the mature soil preserves more or less, a constant depth and character indefinitely.

Accelerated soil erosion.   The removal of the surface soil from areas denuded of their natural protective cover as a result of human and animal interference takes place at a much faster rate than that at which it is built up by the soil-forming process. This accelerated detachment rapidly ravages the land and it is with this type of soil erosion that we are so seriously concerned. Nature requires, on an average, about 1000 years to build up 2.5 cm of top soil, but wrong farming methods may take only a few years to erode it from lands of average slope.

Wind erosion   Wind erosion takes place normally in arid and semi-arid areas devoid of vegetation, where the wind velocity is high. The soil particles on the land surface are lifted and blown off as dust storms. When the velocity of the dust-bearing winds is retarded, coarser soil particles are deposited in the form of dunes and thus fertile lands are rendered unfit for cultivation. In other places, fertile soil is blown away by winds and the subsoil is exposed, as a result the productive capacity of the soil is considerably reduced.

Water erosion.   Soil erosion caused by water can be distinguished in three forms, viz. (1) sheet erosion, (2) rill erosion, and (3) gully erosion.

Sheet erosion.   Sheet erosion removes a thin covering of soil from large areas, often from entire fields, more or less, uniformly during every rain which produces a run-off. This type of erosion is very insidious, since it keeps the cultivator almost ignorant of its ill-effects. It is generally neglected, although the soil deteriorates slowly and imperceptibly. Its existence, however, can be detected by the muddy colour of the run-off from the fields.

Gully erosion.   When rill erosion is neglected, the tiny grooves develop into wider and deeper channels, which may assume a huge size. This is called 'gully' erosion. Gullies are the most spectacular evidence of the destruction of soil. The gullies tend to deepen and widen with every heavy rainfall. They cut up large fields into small fragments and, in course of time, make them unfit for cultivation.

Landslides or slip erosion.   A landslide is defined as an outward and downward movement of the slope-forming material, composed of natural rocks, soil, artificial fills, etc. The fundamental causes of landslides are topography of the region and geological structure, the kinds of rocks and their physical characteristics. The immediate cause of a slide may be an earthquake, or a heavy rainfall, which unduly saturates the ground or a part of a road. However, these are accidents rather than fundamental causes.

Stream-bank erosion.   Torrents are defined as hill streams characterized by wide-spreading beds on emergence from the hills with ill-defined banks, flashy flows and swift currents. Usually, they are dry water-courses, except during the rainy season when with every downpour in their catchment, they get very much swollen with flood and subside almost to its normal tiny size immediately after the storm is over.

These sudden and violent flows are responsible for moving immense quantities of detritus, comprising boulders, shingles, sand and silt, depending upon the geology of the terrain. This debris gets deposited in the torrent bed in the form of scattered islands owing to the sudden widening of the torrent channel after it emerges from the hills, or owing to the flattening of the gradient in the lower reaches, or because of obstructions caused by wild vegetation and uprooted trees. The bed level of the torrent is raised by these deposits. These deposits, in turn, reduce the transporting capacity of the torrent, resulting in overflows and the meandering of the course and in the erosion of the banks.