Soil erosion occurs when soil is removed through the action of wind and water at a greater rate than it is formed
The soil covering the surface of the earth has taken millions of years to form and we must learn to respect it. Soil is formed at a rate of only 1 cm every 100 to 400 years and it takes 3 000 to 12 000 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.
WHAT IS SOIL EROSION?
When a raindrop hits soil that is not protected by a cover of vegetation and where there are no roots to bind the soil, it has the impact of a bullet. Soil particles are loosened, washed down the slope of the land and either end up in the valley or are washed away out to sea by streams and rivers. Erosionremovesthetopsoilfirst.Oncethisnutrient-richlayerisgone, few plants will grow in the soil again. Without soil and plants the land becomes desertlike and unable to support life.
CAUSES OF SOIL EROSION
Erosion occurs when farming practices are not compatible with the fact that soil can be washed away or blown away. These practices are: Overstockingandovergrazing Inappropriate farming techniques such as deep ploughing land 2 or 3 times a year to produce annual crops Lack of crop rotation Planting crops down the contour instead of along it.
Water erosion causes two sets of problems: Anon-sitelossofagriculturalpotential Anoff-siteeffectofdownstreammovementofsediment,causing floodingandthesiltingupofreservoirs.
Soil erosion is characterised by the downslope removal of soil particles within a thin sheet of water. Sheet erosion occurs when the entire surfaceofafieldisgraduallyeroded in more or less uniform way. —2—
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. Onsteepland,thereisoftenthedanger 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 relatedtostreambankerosion,inwhichfast-flowingriversandstreams 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 therearealwaysirregularitiesinafield, waterfindshollowsinwhichtosettleand low-lyingchannelsthroughwhichtorun. As the soil from these channels is washed away, channels or miniature dongas are formedinthefield.
Wind erosion occurs when the land surface is left bare in regions that are aridenough,asaresultoflowrainfall,toallowthesoiltodryout,andflat enough to allow the wind to carry the soil away over several consecutive days.Landmaybecomesusceptibletowinderosionthroughgrazing 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. —3—
FACTORS dETERmINING SOIL EROSION
There are various factors determining soil erodibility of which the following are the most important:
The steeper the slope, the greater the erosion, as a result of...