Each year,about 75 billion tons of soil are eroded from the world's terrestrial ecosystems.Most agricultural land in the world is losing soil at rates 13~40 t/(hm~2·a).Because soil is formed very slowly,this means that soil is being lost(13~40) times faster than the rate of renewal and sustainability.Rain and wind energy are the two prime causes of erosion from tilled or bare land.Erosion occurs when the soil lacks protective vegetative cover.Soil erosion reduces the productivity of the land by loss of water,soil organic matter,nutrients,biota,and depth of soil.The greatest threat to providing food for a rapidly growing human population is soil erosion.Abandoned,eroded agricultural land is replaced by clearing forested ecosystems.