Soil is formed at a rate of only 1 cm every 100 to 400 years and it takes 3000 to 12000 years to build enough soil to form productive land. This means that soil is a nonrenewable resource and once destroyed it is gone forever. If we disregard this, a time will come when there would not be enough soil left to sustain life on earth, because the soil is a necessary growth medium for plants, a home for certain insects and animals, as well as a medium from which we get minerals, such as gold. It is important therefore to treat soil, especially topsoil, as a living entity.
Soil erosion is when the soil is blown away by the wind or washed away by the rain. Soil erosion is common in areas with steep slopes, where trees have been cut down, in droughts when crops and other vegetation grow poorly and in rural areas which are overpopulated. Nepal, in the Himalayan Mountains, has severe problems caused by increased population density and steep slopes. Soil erosion can be reduced by building terraces on hillsides, irrigation schemes to overcome droughts, planting more trees to bind the soil together and make wind breaks, and using fertilizers in overpopulated areas to make the soil more fertile. It is very important that the farming techniques used do not damage the structure of the soil, as this makes it easily eroded. Good farming techniques include contour ploughing, crop rotation and keeping the soil rich in humus. An example of poor techniques was the "Dust Bowl" in the mid-western states of the U.S.A. in the 1930's. Farmers exhausted the soil by monoculture and left the soil bare after harvesting. Soil erosion is a problem of the developed world as well as the developing TYPES OF SOIL EROSION
Water erosion causes two sets of problems:
* An on-site loss of agricultural potential
* An off-site effect of downstream movement of sediment, causing flooding and the silting up of reservoirs. Sheet erosion
* Soil erosion is characterized by the down slope removal of soil particles within a thin sheet of water. * Sheet erosion occurs when the entire surface of a field is gradually eroded in more or less uniform way. * It is a gradual process and it is not immediately obvious that soil is being lost. Gully erosion (dongas)
Dongas usually occur near the bottom of slopes and are caused by the removal of soil and soft rock as a result of concentrated runoff that forms a deep channel or gully. On steep land, there is often the danger of gullies forming. Water running downhill cuts a channel deep into the soil and where there is a sudden fall, a gully head forms at the lower end of the channel and gradually works its way back uphill. As it does so, it deepens and widens the scar that the gully makes in the hillside. Gully erosion is related to stream bank erosion, in which fast-flowing rivers and streams increasingly cut down their own banks. Rill erosion (channel erosion)
Channel erosion can occur on steep land or On land that slopes more gently. Because there are always irregularities in a field, water finds hollows in which to settle and low-lying channels through which to run. As the soil from these channels is washed away, channels or miniature dongas are formed in the field. Wind erosion
Wind erosion occurs when the land surface is left bare in regions that are arid enough, as a result of low rainfall, to allow the soil to dry out, and flat enough to allow the wind to carry the soil away over several consecutive days. Land may become susceptible to wind erosion through grazing animals, which remove the protective plant cover, and whose hooves break up the soil, especially round watering points. Arable land that has been left bare is also a major problem. FACTORS DETERMINING SOIL EROSION
There are various factors determining soil erodibility of which the following are the most important: Slope
The steeper the slope, the greater the erosion, as a result of the increased velocity...