Soil is a store house for organic and inorganic plant nutrients and water.
Some soils are rich in organic and humus contents and are more productive while others are less productive and have very poor percentage of organic contents.
The soil is subjected to a continuous and simultaneous depletion or loss and addition of soil resources.
The term soil erosion covers a wide range of physical and chemical actions, such as removal of soluble matters, chemical changes, disintegration by frost or by rapid changes of temperature, attrition by dust charged wind, scouring by silt laden currents, alternate impact and succession by storm waves, landslides and so on – Fox (1950).
The various soil components are being removed by living organisms and are returning to the soil by way of death and decay of organisms on it. If the rate of removal or loss of components is greater than the rate of addition, the soil will naturally become less fertile.
The various factors which make the soils less fertile are:
(2) Biological agencies, and
(3) Soil erosion.
Leaching is an important factor which makes the soil poor in its resources. In this process, minerals and organic substances are removed from the top layer of soil by rain water. Biological agencies are also active in causing loss of resources from the soil. Cultivation of crops regularly year after year makes the soil less productive.
In cultivation there are little chances for the compensation of lost nutrients of the soil by way of death and decay of the vegetation which grows on it. Leguminous plants, however, compensate the loss of nitrogenous compounds because bacteria inhabiting in their root nodules fix considerable amount of free atmospheric nitrogen into its compounds, such as nitrites, nitrates, ammonia and so on.
The word ‘erosion’ literally means to wearing away. In soil erosion, fertile soil surfaces are detached and removed from their original places and are deposited at some other places. According to Fox (1950), “the term soil erosion covers a wide range of physical and chemical actions, such as removal of soluble matters, chemical changes, disintegration by frost or by rapid changes of temperature, attrition by dust charged wind, scouring by silt laden currents, alternate impact and succession by storm waves, landslides and so on”. Thus, soil erosion is the removal of soil from its upper part.
There are two main types of erosions, namely:
(1) Normal erosion and
(2) Accelerated erosion.
1. Normal Erosion:
When the top soils are gradually removed under normal conditions of physical, biotic and hydrological equilibrium it is called normal erosion. Sometimes, it is also called geologic erosion. It is very slow process in which complete equilibrium is maintained between soil removing and soil forming processes. The normal erosion tends to produce wavy or undulating land surface with alternating ridges and depressions. This is accomplished chiefly by means of slow migration of soil particles from soil surface in successive rains. In arid region, wind during the long dry season is an important factor for normal erosion.
2. Accelerated Soil Erosion:
When the removal of soil does not keep harmony with the soil formation and it is much faster than the latter, it is called accelerated soil erosion.
Agencies Causing Soil Erosion:
Soil erosion is caused by the following two agencies:
A. Climatic Agencies Causing Erosion of Soil:
These are water and wind.
Water is an important factor in soil erosion. Snow and melting ice also remove the top soil to considerable extents. Soil is directly affected by heavy rainfall, rapidly running water and by wave action.
Erosion caused by water may be of the following types:
(i) Sheet erosion:
Uniform removal of a thin layer of soil from large area is called sheet erosion. It is affected by run-off effect of rain water.
(ii) Rill erosion:
In this type of soil erosion, heavy rainfall and rapidly running water produce finger-shaped grooves or rills over the entire field (Fig. 25.1).
(iii) Gully erosion:
It is more prominent type of erosion in which heavy rainfall, rapidly running water and transporting water may result in deeper cavities or grooves called gullies. Gullies may be ‘V shaped or ‘U’ shaped. Gullies cut the fields into small fragments and make them uncultivable (Fig. 25.2). Continuous flow of water through gullies further deepens the grooves and may ultimately result in ravines. Ravines are 15 to 30indeep and with steep vertical sides.
(iv) Landslides or slip erosion:
This type of soil erosion is caused by heavy rainfall and it occurs in sloppy lands, such as mountains and hills. In this type of erosion when the running water percolates through the crevices of rocks great masses of soils and loose rocks lying on the steep slopes slip downward.
(v) Stream bank erosion:
On the banks of swollen rivers it is most active. During the rainy season when fast running water streams take turn in some other directions, they cut the soil and make caves in the banks. As a result of this, quite often large masses of soils become detached and washed away from the banks and are deposited at places in course of streams.
2. Wind erosion:
Removal of soil by wind is called wind erosion. Stormy winds carry the soil particles to distant places and sometimes form sand-dunes. Wind currents usually remove the top soil which is fertile and frill of humus and minerals. Wind causes the following three types of soil movements, viz.,
(ii) Suspension; and
(ii) Surface creep.
Under the influence of direct pressure of stormy wind small soil particles of 1 to 1.5 mm diameters move up from the soil surface, generally in vertical direction. Major part of wind carried soil is moved in a series of bounces, called saltation.
In this, fine soil particles (diameter less than 1 mm) are suspended in air. These suspended particles are kicked up when particles of saltation strike on the soil. The soil particles are deposited at distant places.
(iii) Surface creep:
In this, there are larger particles ranging from 5 to 10 mm in diameter. Because they are too heavy to move in saltation, they creep on surface of soil.
B. Biotic Agencies Causing Soil Erosion:
Excessive grazing, deforestation, undesirable forest biota, and mechanical practices by man are important factors which cause soil erosion. Deforestation is the commonest factor which is responsible for soil erosion. Ruthless felling has exposed soils to direct effects of rain, snow and drought and has set in soil deterioration of the gully and sheet types over extensive areas in Kashmir (J.S. Singh and M.K. Wali, 1961).
Grazing is yet another destructive biological factor for the soil erosion. Cattle and sheep during the summer graze the forest vegetation and make the soil bare (Fig. 25.3). Whyte (1957) while commenting on the problem of soil erosion of Kashmir Himalayas, mentioned, “control of fluctuating grazing was made the responsibility of forest department in 1939 but this appears to have little effect and the problem is as acute as ever”.
Some Other Causes of Soil Erosion:
1. Uneven distribution of rainfall.
2. Shifting cultivations. Shifting cultivations are usually noted in the mountains which are geographically young and degraded into soil easily and the whole of the land is covered with a thick mantle of tropical forest vegetation. The removal of the forest or bush cover by felling and burning for shifting cultivation and the resulting exposure of the bare soil to rams and sun, cause enormous soil losses especially on hill slopes. Both surface layer of the soil and large quantity of plant nutrients are washed away under the influence of intense rainfall. Shifting cultivation is a major problem in the hilly areas of Assam, Manipur, Tripura, Arunachal Pradesh, Nagaland and Orissa.
It is reported that about 207,267 hectares in Assam, 46,963 hectares in Tripura, 21 862 hectares in Manipur and 308,502 hectares in Orissa are under shifting cultivation. Shifting cultivation is practiced sporadically in Andhra Pradesh, Madhya Pradesh, Maharashtra, Kerala, Karnataka and Tamil Nadu also. The process of shifting the area of cultivation is called by various names Jhum or Jum in Assam, podu, dahi or kamana in Orissa, penda in Madhya Pradesh and so on. In English it is described as slash, burn or swidden or simply as shifting cultivation.
3. Fields on steep slopes are cultivated and top soil is washed away by rains. The loss of soil is too much and the fields become uncultivable.
4. Forest fires are responsible for burning down forest trees on huge scale.
5. Faulty agricultural methods—Sometimes farmers do not care towards leveling and terracing of their upland fields. Rainfall washes away the top soil and results in erosion.
6. Over-grazing by cattle causes removal of vegetational cover of the soil.
Factors Affecting Soil Erosion:
There are several factors which affect the soil erosion in a particular area.
These factors are given below:
1. Nature of soil.
2. Distribution, nature and amount of precipitation.
3. General slope of the soil.
4. Vegetational cover.
5. Soil management.
6. Land use practices.
Consequences of Soil Erosion:
There are several serious effects of soil erosion which are as follows:
1. Due to uprooting of trees the shortage of timber and fuel wood results.
2. Loss of soil stability and fertility.
3. Shortage of fodder.
4. Destruction of land in plains.
5. Formation of sand dunes.
6. Greater frequency of floods and threat to communication channels.
7. Silting of river bed, lakes and dams.
8. Higher temperature and scanty rainfall.