Soil is one of the most important natural resources of man. Soils are essential for man for growing crops, fodder and limber. Once the fertile portion of the earth’s surface is lost, it is very difficult to replace it. In India, the destruction of the top-soil has already reached an alarming proportion.
Land degradation problems have resulted in increasing depletion of the productivity of the basic land stock through nutrient deficiencies. In addition to the direct loss of crop producing capacity, soil erosion increases the destructiveness of floods and decreases the storage capacity of water in reservoirs.
It is therefore essential that the soils should not be allowed to wash or blow-away more rapidly than they can be regenerated, their fertility should not be exhausted and their physical structure should remain suited to continued production of desired plant materials.
Protection of land from further degradation, adoption of various conservation measures, including reclamation and scientific management of available land stock is very important for a country like India to achieve higher productivity of food, fodder, fuel and industrial raw materials on a substantial basis.
Besides, demand for land for providing social priorities such as shelter, roads, industrial activities is increasing at a very fast rate with the rise in population and very often good agricultural and forest lands are being diverted to such use.
It is, therefore, necessary to keep soil in place and in a state favourable to its highest productive capacity.
The process of destruction of soil and the removal of the destroyed soil material constitute soil erosion.
According to Dr. Bennett “the vastly accelerated process of soil removal brought about by the human interference, with the normal disequilibrium between soil building and soil removal is designated as soil erosion”.
Erosion of soil by water is quite significant and takes place chiefly in two ways (a) Sheet erosion, (b) Gully erosion.
(a) Sheet movement of water causes sheet erosion and depends on the velocity and quantity of pronounced surface runoff and the erodability of the soil itself. In such cases, the soil is eroded as layers from the hill slopes, sometimes slowly and insidiously and sometimes more rapidly. Sheet erosion is more or less universal on:-
– all bare follow land,
– all uncultivated land whose plant cover has been thinned out by over grazing, fire or other misuse, and
-all sloping cultivated fields and on sloping forest, scrub jungles where natural porosity of soil has been removed by heavy grazing, felling of trees or burning etc.
The particles loosened and shifted by the rain drops are carried down slope by a very thin sheet of water which moves along the surface. The impacts of the raindrops increases the turbulance and transporting capacity of this unchannelized sheetwash which results in the uniform skimming of the top soil.
Sheet erosion is considered as dangerous as it may continue for years but may or may not leave any trace of the damage. Sheet erosion is common in the Himalayan foothills, in Assam, Western ghats and Eastern ghats.
When sheet erosion continues unchecked, the silt laden run-off forms well-defined minute finger shaped grooves over the entire field. Such thin channeling is known as ‘rill-erosion’, which is active over wide areas in Bihar, Uttar Pradesh, Madhva Pradesh and in semiarid areas of Maharashtra, Karnataka, Andhra Pradesh and Tamil Nadu.
(b) Gully erosion
On a gentle slope, adequately covered by vegetation, clay soil will resist erosion to a great extent and the water forms small rivulets which can then erode deeper. The rivulets in turn join together to form larger channels until gullies are formed gradually deep gullies cut into the soil and then spread and grow until all the soil is removal from the sloping ground.
This phenomenon once started and if not checked, goes on extending and ultimately the whole land is converted into a bad-land topography. Gully erosion is more common in areas where the river system has cut down into elevated plateaus so that feeders and branches carve out an intricate pattern of gullies.
Apart from this, it also takes place in relatively level country whenever large blocks of cultivation give rise to concentration of field run-off.
It occurs in dry climatic areas having a sparse and low vegetation cover on mechanically weathered, loosened surficial material. Dust storms are the principal agents of wind erosion.
The top soil is often blown off from the surface rendering it infertile. Besides, with the decrease in the wind velocity coarse sand particles get deposited in some areas covering the existing soil and rendering it unproductive.
The removal of soil by running water and wind is known as soil erosion. The soil-forming process and the erosional process of running water and wind are continuous. Generally, there is a balance between these two processes.
The rate of removal of fine particles from the surface is the same as the rate of addition of particles to the soil layer. Sometimes such a balance is disturbed by natural or human factors, leading to a greater rate of removal of soil. When this happens, the entire soil layer may be removed in a few years.
Wind and water are powerful agents of soil erosion because of their ability to remove soil and transport it.
(a) Erosion by Water: Erosion by water can be of several types, for example, sheet erosion, gully erosion, stream bank erosion, shore erosion and slip erosion.
1. Sheet Erosion: When a layer of the soil on the surface is removed over a large area by running water, it is called sheet erosion. Sheet erosion is harmful since it removes the finer and more fertile top soil.
2. Rill Erosion: This is the second stage of sheet erosion. Small finger-like rills begin to appear on the landscape. Over a period of time, the fine rills increase in number and also become deeper and wider. This reduces the actual area under cultivation and the yield of crops decline.
3. Gully Erosion: When soil is removed by water flowing along definite paths downs the slope or in channels, it is called gully erosion. Gullies cut up agricultural land and make it unfit for cultivation. Badland is a region with a large number of deep gullies or ravines, e.g., Chambal Valley in Madhya Pradesh.
4. Stream Bank Erosion: The continuously flowing water erodes the banks of streams and rivers. Gradually the bed of the river widens.
5. Shore Erosion: The tidal waves dash against coastal rocks, causing them to erode bit by bit.
6. Slip Erosion: During heavy rains, water percolates into the soil until it is unable to penetrate further by the underlying impervious rocks. On steep land, the heavy moisture-laden soil often comes down bodily, resulting in a landslide.
(b) Wind Erosion is significant in desert and semi-desert regions. In regions with heavy rainfall and steep slopes, erosion by running water is more significant.
In some regions, the top soil is blown away by wind in the dry season, and washed away by running water in the wet season.
Causes of Soil Erosion in India: Soil erosion takes place when soil is removed faster than it is formed. There are many different processes by which soil is removed but the cause is usually the removal of trees and other vegetation that hold the soil together and in place.
1. Topography and Slope of the Land: Hilly Region: Rainfall occurs in torrents and washes away the top layer of the soil. Also, the steep slopes stimulate the eroding power of the rainwater.
Plains: Here the erosion is comparatively less than on slopes. But in regions where the rivers overflow onto the plains, the erosion is severe. For example, the entire basin of the Kosi River is threatened by the over-flowing of river.
2. Nature of Rainfall: Floods and torrential rains cause more damage than light or moderate showers spread over many days.
i. The action of heavy rain is stronger when there are no trees and the plains are bare.
ii. When a prolonged dry spell is followed by sudden heavy rain, sheet erosion takes place. This is because the ground gets baked hard and the soil is unable to absorb the water easily.
3. Nature of Soil: Porous soils with good water-absorbing capacity are least subject to erosion, while the impervious soils are gradually eroded by the action of water.
4. The Human Factor: Man and his activities are responsible for soil erosion to a great extent. As the human population increases, the demand on the land also increases. Forest and other natural vegetation are removed for human settlement, for cultivation, for grazing animals and for various other needs. Improper use of land leads to soil erosion.