The upper layer of the soil consists of fine soil particles. It is rich in minerals and has humus. Hence, humus makes the soil fertile. Sometimes heavy rain, running water and wind remove the top layer of soil. This phenomenon is soil erosion.
Water and wind moving across the surface of the soil always carry away some of the soil. If the erosion processes remove top-soil only as rapidly as top-soils formed from the parent material beneath, no harm results. This is geological erosion.
Causes of Soil Erosion
Man has cut too many jungles, has grazed the grasses excessively, and ploughed the land too often. These activities have laid bare the formerly protected soil and exposed it to beating raindrops and angry winds.
The result was a removal of the top-soil classified into water erosion and wind erosion. As a result, this is man-made erosion and is harming harm ecological balance.
TYPES OF SOIL EROSION
Water erosion starts when beating raindrops strike bare soil and churn it into flowing mud. The muddy water tries to soak down through the natural cracks in the soil. But cannot do so because the mud seals the cracks.
There is no other place for the muddy water to go. It has to flow over the surface of the soil scouring rills and gullies. As it moves down the slope towards the small streams and rivers.
The scouring and corrosive action of the muddy water continues. As long as the amount of water is great and the slopes are steep.
As soon as the volume of water becomes less and the slopes become more gentle a large part of the mud (silt) settles out. the deposition of this part happens at the bottom of sewers. Depositions also take place in the quiet waters of lakes and reservoirs.
As a result, the fertile topsoil erodes the bare fields. Poorly managed grasslands and forests, sewers and river beds choke with sediment. Thus the useful life of lakes and reservoirs reduces greatly. As a result, the ecological balance is disturbed. Hence this is very bad.
The millions of the finest soil particles and organic matter carried annually by the rivers are a monument to careless soil management. This waste is visible everywhere.
Water erodes soil mainly in two ways:
1. By the violent splash of the falling raindrop on bare soil.
2. By the scouring action of soil-laden water moving down the slopes.
Falling raindrops splash soil in the same manner that a rock will splash in a pool of water. Raindrops fall at a speed of about twenty miles per hour.
A single raindrop may splash wet soil. As much as two feet high and five feet from the spot where the raindrop hits.
Continuous bombardment in a rainstorm by thousands of raindrops causes damage by beating the bare soil into flowing mud.
This flowing mud moves into and seals the wormholes, root channels cracks, and larger soil pores. Movement of water down into the soil thus retards.
This means that during rain, as lesser water enters the soil more of the rainfall moves over the surface, carrying soil particles with it. Movement of soil by raindrop splash erosion is the primary cause of sheet erosion.
Sheet Type Soil Erosion
Sheet erosion means the removal of soil in a more or less continuous, thin sheet over the surface of the soil. This type of erosion is usually so slow that the cultivator is seldom conscious of its existence
Rill Type Soil Erosion
Rill erosion takes place when run-off waters laden with splash erosion sediment concentrate in many channels to form streamlets.
Such streamlets cut incisions into the face of the earth, just as catclaws the back of your hand and leaves scratches. Greater concentrations of soil-laden water will move even larger amounts of soil, soon resulting in gully erosion.
Gully Type Soil Erosion
Gullies often start along bullock cart tracks, livestock trails, footpaths, or burrows of animals. Gully erosion is much more spectacular than either sheet or rill erosion.
Continuous exposure to beating raindrops and to the cutting action of muddy water producer deeper and deeper gullies, thus completely destroying the land for any productive use.
Wind erosion starts when bare sandy soil becomes dry and high winds roll the sand grains over each other, resulting in shifting sand dunes.
During dust storms, the finer particles of silt may be picked up by the wind and carried for miles. The results are depletion of the soil, a covering up of good farmland by worthless sand, and menacing dust storms.
Along sandy seashores, sand dunes are evidence of geological wind erosion. Further inland, wind erosion is evidence of poor land use.
Sandy soils from which the vegetative cover has been over-cut,over-grazed, or over-ploughed soon dry out and are subject to blowing by the wind.
When the original grass sod was ploughed in preparation for growing farm crops, many grassroots bound the soil particles together.
As the roots delayed and the soil surface became dry, sandy soils shift about by the wind, especially during droughts which were accompanied by high winds. As the wind increases in drying power and velocity, a deeper layer of soil becomes dry.
Water which formerly helped to bind the soil particles together is lost by evaporation. The result is sorting action of the wind which :
1. Picks up the finer and more fertile silt particles and move them many miles as clouds of dust.
2. Rolls and hops coarser sand particles and certain clay granules along near the surface.
These particles lodge against clumps of grass, crop residues or fence rows, and momentarily come to rest.
Effects of Soil Erosion
Erosion causes a loss of productive top-soil, a deposition of infertile sand on productive fields, a silting of reservoirs, and a lowering of the ground-water table.
1. Washing away of productive soil.
The top-soil is the most important soil layer for crop growth because most of the plant roots are in this layer.
The loss of soil fertility by erosion is more serious than the loss by plant removal. In this way, there is not only a loss of infant food but a loss of a portion of the soil body itself.
When the top-soil has been washed away, the sub-soil is less fertile and more cloddy – making it both more difficult and more expensive to raise satisfactory farm crops.
2. Deposition of sand on productive fields.
In the downstream areas, there is always a danger of the fertile fields being made productive by being covered with coarse material brought down the hill by running water.
3. Silting of lakes and reservoirs.
The removal of the most fertile part of the soil is not only a loss to the soil, but is often a menace to navigable streams which fill up with this material.
Reservoirs construction is done to generate electricity or for irrigation and for later use for drinking. If soil erosion goes unchecked, the reservoirs and lakes become filled with soil. Thus reducing their storage capacity and their useful life.
4. Lowering of the underground water table.
As the surface run-off increases, the quantity of water available for entering the soil decreases. Less water moving downward by percolation means a smaller supply of water to replenish the ground-water in wells. With less water in wells, irrigation reduces, and crop yields decline.
J. C. Ghosh, a Member of the Planning Commission, Government of India, has given a good definition of soil conservation.
Soil conservation is not merely fencing, contouring, strip cropping, filling gullies. Or planting steep or erodible land to grassland tree.
Soil conservation means draining the land if it is wet. Also irrigating it if it is dry. It means the addition of fertilizers and organic matter to the soil which is lacking these constituents. It is practicing all these measures and whatever else to keep the soil permanently productive.
How To Prevent Soil Erosion
Soil erosion caused by running water can be effectively controlled by increasing soil fertility, using sod-forming crops, contour soil conservation, dry farming of gullies cultivation, strip cropping, contour bunding, and by the reclamation of barren land.
The soil is most susceptible to erosion when the fields are bare of vegetation. Soils low in organic matter, low in fertility, and poor in soil structure are more easily eroded than those high in organic matter, fertile, and having a desirable soil structure.
1) Increasing Green Cover and forestation
When grasses and legumes secure a foothold, the soil is held in place by the network of roots. Old roots die and new ones take their place.
A part of the roots and a part of the tops from grasses and legumes remain to form organic matter in the soil. This improves the organic matter status, the physical condition of the soil and ultimately increases soil fertility.
Thus the soil is protected from beating raindrops and erosive action of surface water by being covered with grasses and legumes, both living and dead. Because of these characteristics, grasses and legumes are some of the best agents for controlling soil erosion.
2) Increasing Soil Fertility
Good root growth and protective cover reduce the harmful dispersive action of beating raindrops. The more fertile the soil, the more luxuriant the plant growth and the better the soil is protected against soil erosion.
One of the best ways of increasing soil fertility is to first have the soil tested. Based on these tests, fertilizer recommendations will be made, which, if followed, will result in larger crop yields with minimum expenditure on fertilizer.
The final result will be higher crop yields as well as more effective erosion control.
3) Using Sod-Forming Crops
Sod-like crops, such as grasses, sunn hemp, lucerne, berseem clover or groundnut offer excellent soil protection against erosion by water.
Such crops are effective primarily because they provide a cushion against which the raindrops break their fall and thus gently soak into the soil.
Such crops also effectively protect the soil against erosive winds because of their binding action. The denser the vegetation the more effective is the cushioning effect.
Close-growing crops also help to control erosion by providing root channels through which water moves downwards more freely. In addition, plant roots aid in making a desirable soil structure that does not melt during rain.
A final reason why crops aid in soil erosion control is that crops encourage the growth of earthworms and other desirable forms of soil life.
Because of the holes, they make these animals help to keep the soil open and receptive to raindrops to be effective in controlling erosion, the vegetation must be very dense at the time of the greatest erosion hazard.
4) Counter Cultivation
Sowing of crops and all tillage operations need to be carried out at right-angles to the slope of the land. Contour tillage helps in creating obstructions to the flow of water above every furrow and in a more uniform distribution of water.
The result is more infiltration of water, less runoff and erosion, more water in the soil available for plant growth, and higher crop yields.
5) Strip Cropping
Strip cropping includes growing erosion-resisting and erosion-permitting crops alternately on the approximate contour and across the slopes.
In this system, the crops such as groundnut and soya bean are included as erosion-resisting conservation as erosion-permitting crops.
6) Contour Bunding
Contour bunding is very effective in checking erosion in fields. Breaching is to be avoided. To do this bunds should be built with a gentle fall along their water channels. Waste weirs (outlets) should always be provided to avoid breaching.
A broken bund is worse than no bund at all because of the erosive action of the water that has been concentrated behind the bund. Bunds may breach for any of the following
1. The rainfall may come with a greater intensity than that for which the bunds were designed.
2. The soil in a low place in the bund may become saturated and the bund, therefore, weakened.
3. The intentional breaching of the bund by ryots (cultivators)to drain stagnant water.
4. Rat and other rodent holes in the bund.
5. Cattle, sheep, or goat trails across the bund.
6. The crossing of the bund with bullock carts, automobiles or lorries.
7. Ploughing too close to the bund and thus weakening it.
8. Lack of periodic maintenance of the bunds.
7) Gully Control
Gully control is necessary in order to prevent the complete destruction of cultivated lands and grasslands. In the first place, water now flowing into the gully should be diverted by means of a graded band Built above the head of the gully.
The second step is to build several obstructions in the gully such as sod, brush or rock dams. Next, the steep sides and head of the gully should be sloped and the loose soil put above the dams.
Some types of vegetation such as fast-growing trees, grasses or vines, should be planted in the loose soil. Above the dams and around the head of the gully. The vegetation should be protected from livestock.
If the soil is not fertile, manure, compost, or commercial fertilizers must be added to increase the growth of the vegetation.
When the vegetation has become well-established, the diversion bund at the head of the gully can be broken to allow the water to flow down the protected gully.
8) Increasing Soil Penetration
Water escaping from the soil by means of under drainage never carries with it any of the soil material other than the slight portions that are soluble.
It is, therefore, essential to establish systems of farming that will enable a large proportion of the rainfall to penetrate the soil.
9) Deep Ploughing
Erosion may be largely prevented on most farms by deep ploughing and by keeping the soil covered as much as possible with growing crops or their remains.
Deep ploughing encourages an increased penetration of the rainfall and, therefore, reduces the amount passing over the surface of the soil.
The presence of growing plants retards the movement of surface water and holds back the soil particles. An abundance of roots in the soil helps to hold it together and prevent erosion. The application of barnyard and green manure also retards erosion.
Controlling Wind Erosion
1. Controlled grazing by livestock.
2 Planting trees.
3. Contour trenching on the gentle slopes.
4. Check dams in the gullies, sewers, and streams.
Soil Erosion in India
Causes of Soil Erosion
There is barely enough productive land in India to provide adequate food, feed, and fodder for the people and the livestock. Because of this pressure on the land, most of the soils have become over-cropped and over-grazed, and the trees and shrubs over-cut.
Too little of the vegetation has been allowed to remain as a protective blanket to cushion the blow of the falling raindrops and the scouring action of the surface runoff water. The result is severe soil erosion, less available water for the growth of crops, and a continuously decreasing productivity of India’s soils.
Government Data on Soil Erosion
The Central Soil Conservation Board estimates that two of every three acres of India’s croplands are suffering from soil erosion. And in immediate need of soil and water conservation practices.
This is especially serious because these three acres must produce enough food for three people. And also of the feed fodder for three domestic animals.
With excellent management, India’s most productive soil may yield enough food and feed to achieve the target. But when two-thirds of the cropland becomes less productive by erosion, the goal of self-sufficiency will be very difficult to achieve.
Soil Erosion Control
One partial solution to the problem is to improve soil and water conservation practices. National and State schemes alone, no matter how well-financed cannot adequately solve the serious problem of soil and water conservation. Nor can the cultivators alone work effectively.
The responsibility is a triangular one with three segments. The Central Government to lead in conservation education and to work on major watersheds (catchments) that involve more than one state.
The States Governments to develop conservation works that lie wholly within their own States. And the cultivators and landowners to cooperate with the Central and State Governments.
In the establishment and maintenance of soil and water conservation practices such as bunding, irrigation, drainage, strip cropping, contour tillage, proper grazing management, effective cropping systems, and recommended management of the forests.