Soil erosion
Soil erosion, the term used to describe the washing away or blowing away of the upper part of the soil cover, is causing major concern worldwide. There are few countries in the world in which soil erosion is not a problem. It not a new development for it has been occurring on and off for many millions of years, during geological times and more recently. So why the concern? The reason is that there is increasing evidence that the human activities are accelerating erosion in many parts of the world with catastrophic results. Just think about the consequences of having the material on which we rely for our food, forestry, wildlife and several other important uses, gradually disappearing and the remaining land being unable to produce these necessities of life. Soil erosion has been linked with the downfall of some of the great civilisations of the world such as the Aztec, the Mayan and the some of the civilisations of the Near East and the Mediterranean Basin. It is now well established that the current continuing misuse of land in some parts of the world is continuing to cause severe erosion.
There are two main causes of erosion, wind and water. It was wind erosion that first grabbed the headlines when soil particles from the cotton belt and the Great Plains of the United States began to settle over Washington, and the 'Dust Bowl' came to be recognised in the United States in the 1930s. Wind erosion occurs when wind scours the soil surface, lifts particles into the air and transports them over distances ranging from a few centimetres to hundreds of kilometres. Wind erosion not only moves particles around arid and semi-arid landscapes but deposits in regions where the input of such particles causes nuisance and expense. Wind erosion is particularly a problem on sandy and organic soils. It can, however, become a problem wherever soils that have been trampled excessively by animals, loosened by frequent ploughing and tillage and denuded of vegetation, become exposed to periodic winds. Although attention to wind erosion has been dwarfed somewhat by the now more dominant and spectacular water erosion it continues nevertheless to be a major problem in some areas.
Water erosion of soil can occur in virtually all parts of the world. With increasingly intensive farming there has been a tendency for topsoil structure to weaken as organic matter is used up and not returned to the soil. Weakly aggregated soils disintegrate under the influence of heavy rainfall and soil particles become mobilised. In recent years cultivation has been extended more and more to sloping fields. The combination of weakly structured soils and sloping fields provide ideal conditions for soil runoff. What begins as a small rill becomes extended into larger gullies, and fields becomes broken up into rills and gullies, the erosion accelerates and a situation of soil loss and gully formation that is difficult to retrieve soon develops. This is happening in many countries of the world where overgrazing, decreasing organic matter in topsoils, excessive cultivation, and deforestation all contribute to increasing soil erosion.