Soil erosion refers to all physical processes that loosen or tear-off soil particles and displace the detached particles from the parent sites. Much of the erosion is due to instability of land mass, gravity and the balance is directly lost due to excessive moisture or water in the earth mass. Many times this is referred to as gravity erosion, such as, fall, slide or flow. Land slides, torrents or river erosion entail mass movement of fractured rock, other unconsolidated materials and of coarse soil and therefore, is also referred to as mass wasting. The process, as such, is a physical one, and can be understood by analyzing the resistance of soil in the form of cohesion and friction against the causative forces of rain drop impact and flow of the eroding agents.
Soil erosion is caused by natural processes as well as by human activities. The natural processes can be divided into: I) Geological Erosion, and II) Accelerated Erosion.
I) Geological Erosion :
Geological soil erosion is a natural process that tends to bring the earth's surface to a uniform level. The first phase of this process is weathering which is essentially physico-chemical in nature. This leads to simplification of substances through disintegration and is aided by certain biological influences causing further disintegration. The process leads to the development of complex soil bodies with definite physical, chemical and biological properties. Therefore, as a part of geological process, erosion takes place through various weathering processes.
II) Accelerated Erosion :
Under cultivation, the land experiences a lot of pressure from outside and consequently the balance between vegetation cover and climate is disturbed. Thus, removal of surface soil by natural agencies takes place at a faster rate than it can be built up by soil forming process. Erosion occurring under these conditions is referred to as accelerated erosion. Its rate and magnitude are higher than those in normal geological erosion.
Depending upon the agency causing it, erosion is classified as follows:
Water Erosion :
Water is an important force causing transport of soil. Erosion of soil by water can take place by means of rain drops, waves or ice. The erosion of soil by rain water including melted snow is raindrop erosion. Raindrops falling on the land surface cause detachment of soil particles which are subsequently transported -away with the flowing water. A thin film of soil layer is detached and transported by the rain water flowing on the land surface. This type of erosion is known as wash-off or sheet erosion. Sheet erosion may not be evident significantly, but lands subjected to sheet erosion loose a thin layer of top fertile soil every year. The second stage of sheet erosion in which finger like rills appear on the landscape is known as rill erosion. These rills are usually smoothened out every year by normal farm operations. But each year rills slowly increase in their number as well as in their shape and size. They become wider and deeper and affect crop production adversely.
Gully erosion is an advanced stage of rill erosion. Rills when neglected develop in size and become gullies. Gullies could also be caused by runoff concentrating at a point on the agricultural land. Gullies when not controlled expand year after year. Ravines are a form of gullies. The erosion of the stream banks by flowing water is known as stream bank erosion. In certain areas where the rivers or streams change course stream banks are eroded at an accelerated pace. Stream bank erosion damages adjoining agricultural lands, highways, railroads and bridges.
Coastal Erosion is caused on the sea-shore due to wave action and the advance of the sea at some places. Coastal erosion is more difficult to control than other forms of water erosion.
Wind Erosion :
Soil erosion by winds is very common in areas where vegetation is not enough to cover and protect the soil. Such conditions are found in arid lands and along the sand shores of oceans, lakes and rivers. The fine loose soil particles are blown from the surface of the land and transported by the winds. Soil transported in a series of short bounces is termed as siltation. The tiny soil particles are carried to long distances in the form of suspension. Such a movement of soil is termed as suspension movement. Heavier soil particles are transported at the ground level, as a result of high velocity of wind, this is defined as surface creep.
The sand is constantly blown away from one place to another. Agricultural fields, orchards, etc., whatever comes in the way, is covered with sand and becomes unproductive in the course of time. Plants get buried in the sand and are destroyed forever. As sand moves forward deserts expand in size.