Water erosion.   Soil erosion caused by rainfall is the result of the application of energy from two distinct sources, namely (i) the falling rain drops, and (ii) the surface flow. The energy of a falling rain drop is applied slantingly or vertically from above, whereas that of a surface flow is applied more or less horizontally along the surface of the ground. The chief role of the falling rain drop is to detach soil particles, whereas that of the surface flow (outside the rills and gullies) is to transport the soil. The falling rain drop also makes a major contribution to the movement of the soil on unprotected sloping lands during the period of heavy-impact storms, by splashing large quantities downslope and by imparting transporting capacity to the surface water by keeping it turbid. More than 100 tonnes of soil per hectare can sometimes be lost yearly in this fashion from a bare and highly detachable soil on slopping land.

Wind erosion.   Wind is responsible for three types of soil movement in the process of wind erosion. They are known as : (i) saltation, (ii) suspension, and (iii) surface creep.

(i) Saltation.   The major portion of the soil carried by the wind is moved in a series of short bounces called "saltations". The soil carried in a saltation consists of fine particles ranging from 0.1 to 0.5 mm in diameter. saltation is caused by the direct pressure of wind on soil particles and their collision with other particles. After being pushed along the ground surface by the wind, the particles leap almost vertically in the first stages of saltation. Some grains rise only a short distance; others leap 30 cm or higher, depending on the velocity of the rise from the ground.

(ii) Suspension   Very fine soil particles, less than 0.1 mm in diameter, are carried into suspension, being kicked up into the air by the action of particles in saltation. The movement of fine dust in suspension is completely governed by the characteristic movement of the wind. Suspended material is carried long distances from its original location and is thus a complete loss to the eroded area, especially when erosive winds are from different directions.

(iii) Surface creep   Soil particles, larger than about 0.5 mm in diameter but smaller than 0.1 mm, are too heavy to be moved in saltation but are pushed or spread along the surface by the impact of particles in saltation to form a surface creep.

About 90 per cent of the total soil movement in wind erosion is below the height of 30 cm, and about 50 per cent of it is within 5 cm of the ground level. The control of wind erosion is mainly based on the reduction or elimination of movement in saltation.

Factors influencing erosion.   Soil erosion by water is influenced greatly by (i) precipitation (its intensity and amount), (ii) the slope of the land (its degree and length), (iii) the type of soil, and (iv) the nature of the ground cover and land use.

(i)   PRECIPITATION.   Precipitation is the most important factor influencing soil erosion. The intensity of rainfall, its duration and frequency influence the rate and the volume of run-off. A light rain which can be easily absorbed in the soil causes no run-off and soil loss. As the intensity of rain increases and more rain falls than can enter the soil (infiltration), there is of run-off and soil loss. Rainfall of long duration and greater frequency increases both the total run-off and soil loss. Apart from the intensity and duration of rainfall, the soil moisture is also important in determining the run-off and soil loss by erosion. If the soil is already saturated with water, the same amount and intensity of rainfall will cause more run-off and soil loss from it than from a dry soil.

(ii)   DEGREE AND LENGTH OF SLOPE.   The speed and the extent of run-off depend on the slope of the land. The greater the slope, the greater is the velocity of the flow of the run-off. According to the law of falling bodies, velocity varies as the square-root of the vertical drop. Hence, if the land slope is increased four times, the velocity of the water flowing on the slope is approximately doubled. If the velocity of the run-off water is doubled, its energy, i.e. erosive power, is increased four times, as the latter varies as the square of the velocity. Similarly, the quantity of the material of a given size that can be carried is increased about thirty-two times (varies as the fifth power of the velocity), and the size of the particles that can be transported by pushing or rolling is increased about sixty-four times (varies as the sixth power of the velocity).

There is thus a rapidly increasing rate of soil loss as the slope of the field becomes steeper. The erosion hazard is not simply added but is multiplied as the field extends back on the steeper part of the farm.

(iii)   TYPE OF SOIL.   The type of soil, i.e. structure, texture, organic matter content, its infiltration capacity and permeability, greatly affects the soil loss and run-off. Fine soils are more susceptible to erosion than coarse soils, since rain-water enters in and passes through a dense clay much more slowly than through a porous sand or gravelly soil. In India, it has been observed that deep lateritic soils at Ootacamund and red soils at Deochanda have the lowest rate of run-off; the alluvial soils at Vasad and Dehra Dun have a very high rate of run-off; the black soils have an intermediate rate of run-off, but still the rate of run-off is high. Lateritic clays are less erodiable. The soil left in loose and pulverized condition is particularly liable to erosion through sheet-wash and gullying.

(iv)   NATURE OF GROUND COVER AND LAND USE.   When rain falls on a surface covered by a thick mantle of plants, its velocity and erosive power are reduced and most of the water either quickly percolates through the soil or moves over the surface with non-erosive velocity. Areas not protected with thick cover of plants are unable to absorb water effectively, because the dashing rains shatter the soil surface, the fine soil particles go into suspension and the thick mixture of water and soil quickly fills and closes the tiny interstices in the soil, reducing infiltration and consequently increasing run-off and soil loss.

The above data on run-off and soil loss under different soil, climatic and slope conditions clearly indicate that if the land is left undisturbed under a natural cover, the run-off and soil loss are the least; the soil loss and run-off increase steeply when the vegetation is removed and the land is cultivated.

Alongwith the loss of run-off of water and soil, considerable amount of plant nutrients are also lost.

The loss of plant nutrients in alluvial soil at Kanpur increased with the increase in the degree of the slope and the increase was very steep when the degree of slope increased from 1.5 per cent to 3.0 per cent.

SOIL AND WATER-CONSERVATION MEASURES

The key to soil and water conservation is the utilization and treatment of land according to its capability.

Land-capability classification.   Any soil and water-conservation project includes two distinct sets of operations, viz. (1) the mapping of land for classification according to its capability, and (2) planning and executing measures to check erosion, improve land productivity and reclaim wasteland. The farm plans for effective soil and water conservation are based largely on the capability of the land. The land-capability classification map is normally prepared by interpreting a standard soil-survey map.

Land-capability classification is a systematic arrangement of different kinds of lands according to those properties that determine the ability of the land to produce crops on a virtually permanent basis.

The factors determining land-capability.   These are the major soil characteristics of the land, e.g. the texture of the top soil, its effective depth, permeability of the top soil and subsoil, and associated land features, e.g. the slope of the the land, the extent of erosion, the degree of wetness and susceptibility to overflowing and flooding.

The grouping of soils into capability classes is done primarily on the basis of their capability to produce common cultivated crops and pasture plants without deterioration over a long period.

Land-capability classes.   The land-capability classes are based on the intensity of hazards and the limitations of use. The land-capability classes range from the best and most easily farmed land to that which has no value for cultivation, grazing or forestry, but which may be suited to wild-life, recreation or for watershed protection. They all fall into 2 broad groups : one suitable for cultivation and other land uses, and the other not suitable for cultivation, but suitable for other land uses.