Soil erosion is defined as the wearing-away of topsoil. Topsoil is the most fertile part of the soil because it contains the most organic, nutrient-rich materials. Therefore, this is the layer that farmers want to protect for growing their crops and ranchers want to protect for growing grasses for their cattle to graze on. There are various ways by which this fertile topsoil is lost and wasted.
Types of soil erosion:
1. Water Erosion: It is caused by the action of water, which removes the soil by falling on as rain drops as well as by its surface flow action. Depending upon the form of the lost soil it may be:
(a) Sheet erosion: The removed soil is like a thin covering from large area. This sheet is lost more or less uniformly.
(b) Rill erosion: If sheet erosion occurs with full force, the runoff water moves rapidly over the soil surface cutting well defined finger-shaped groove like structures, appearing as thin channels or streams.
(c) Gully erosion: This results due to the convergence of several rills (thin channels formed during rill erosion) towards the steep slope, which form together wider channels (grooves) of water, known as gullies.
2. Wind erosion: It is common in dry (arid) region where soil is chiefly sandy and the vegetation is very poor or even absent. Once the top soil is laid bare to the fury of strong winds, it gets blown off in the form of dust storm and sand storm. Wind erosion may be of the following three types:
(a) Saltation: In the arid regions of low rainfall, drainage is poor and high temperatures prevail. Water evaporates quickly leaving behind the salts. Salt accumulation occurs mainly in lowlands around the oceans. The salts are mainly chlorides, sulphates; carbonates, and nitrates of potassium, magnesium and sodium, and chlorides and nitrates of calcium.
(b) Suspension: The wind throws away smallest soil particles into air, which moves as fine clust with the wind. By this way soils are transported to fairly long distances.
(c) Surface creep: the heavier particles of soil that are not easily thrown up by wind, are simply pushed or spread along the surface by wind.
3. Landslides or slip erosion: The hydraulic pressure caused by heavy rains increases the weight of rocks at cliffs which come under the gravitational force and finally slip or fall off.
4. Stream bank erosion: The rivers during floods splash their water against the banks and thus cut through them. Particularly at meanders. This type of erosion is known as riparian erosion.
5. Deforestation and over-grazing: Deforestation makes soil cover vulnerable for wind and water erosion. Over grazing is a major hazard affecting pastures, forests, and mountains. Grazing destroys the little cover and enhances wind and water erosion.
Methods of controlling Soil Erosion
1. Biological methods: It includes the use of plant of vegetation cover.
(i) Agronomic practices: It includes natural protection by growing vegetation in a manner that reduces soil loss. These are:
(a) Contour farming: In which preparation of fields with alternate furrows and ridges to reduce water flow. Ridges at the same level are known as contour. On slopes, however, this type of farming is coupled with terracing.
(b) Mulching: It is effective against wind as well as waster erosion. Some such plants as maize stalks, cotton stalks etc.., are used as a 'mulch' (a protective layer formed by the stubble). Mulches reduce soil moisture evaporation and increase amount of soil moisture by addition of organic matter to soil.
(c) Crop rotation: It decreases soil loss and preserves the productivity of land.
(d) Strip cropping: It involves the planting of crop in rows or strips to check flow of water
(ii) Agrostological methods: Grasses such as Cynodon dactylon are utilized as erosion-resisting stabilizer plants. They are grown in strips between the crops. Such methods include:
(a) Hay farming: This aims to grow grasses in rotation with the fields crops, which helps in building up the structure of soil, preventing soil erosion and improving its fertility
(b) Retiring lands to grass: It involves to grow grasses on such lands where major proportion of the top soil has been eroded. Generally grasses are allowed to grazing under suitable climate conditions.
2. Mechanical methods: These methods are used as supplements to biological methods. These are:
(i) Basin listing: i.e. to construct small basin along the slope to intercept and divert the runoff water.
(ii) Contour terracing: To construct a channel along the slope to intercept and divert the runoff water. This may be:
(a) Channel terrace: To dig channels at suitable intervals and the excavated soil deposited as a wide, low, ridge along, the lower edge of the channel.
(b) Broad based ridge terrace: i.e. to construct ridge along both the sides of the channel:
(c) Bench terrace: To construct a number of platforms along contours or suitable graded lines across the slope.
3. Other methods: These include:
(i) Stream bank protection: To grow vegetation alongside the river bank, to construct drains, concrete or stone Pitching etc. for checking & cutting and carving 'of riverbanks.
(ii) Afforestation: Trees as windbreaks are planted at 90° to the prevailing wind in deserts which check the velocity of wind. They check the spread of sand dunes or desert conditions or blowing away of the fertile top soil. Windbreaks may be planted in several rows.
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