The resulting map can then either be viewed on screen or printed out. There are four main types of land degradation. It occurs where steep land is being unwisely farmed and where gently sloping land is left exposed to the effects of heavy rain. In the United States the annual loss of topsoil still averages 12 tonnes per hectare, and an estimated 50 million tonnes a year of plant nutrients are washed away with it. Wind erosion caused the famous dust bowls in the Great Plains of the United States in the 1 930s.
Biological damage occurs when soils are deprived of their nutrients, organic matter or humus.
Deep-rooting crops are needed to cure physical damage, and crop rotation and good farming practices to cure biological damage. After several years, the soil becomes highly saline and affects plant growth. If drainage is very poor, the soil may also become waterlogged. It is estimated that about 40 million out of 200 million irrigated hectares are waterlogged, affected by salt, or both. The area of land being abandoned every year for these reasons is roughly equal to the amount of land being reclaimed and irrigated.