The three main causes are removal of vegetation (through removal of forest and other natural growth); water (this is mainly the effects of heavy rainfall removing the top layer of soil) and wind, which plays a major role in creating deserts. You can find out more about these causes, and ideas on how to prevent erosion,here.
Soil erosion is a term for soil either gradually being taken from the land, or very suddenly being washed away or blown away. The gradual erosion of soil from the land is something which is a natural process. Water and wind naturally remove soil from the land, but usually at a rate which is the same as soil is formed, so some soil is taken away but this is replaced. Soil erosion which is sudden has a negative impact on the land. Sudden erosion is caused by extreme rainfall literally washing away soil, rivers bursting their banks and taking soil with them to the sea or by tsunamis. tornadoes etc. When this happens then the soil is not replenished and the land is less productive so crops do not grow as well. Soil contains many nutrients which are beneficial to crops or to stock feeding off the land, so the land cannot immediately recover after widescale erosion has occurred. There are fears that soil erosion will become much more widespread as a result of extremes of weather caused by global warming.
Soil is naturally removed by the action of water or wind: Such 'background' (or 'geological') soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil. Even before this, natural processes moved loose rock, or regolith, off the Earth's surface, just as has happened on the planet Mars.
In general, background erosion removes soil at roughly the same rate as soil is formed. But 'accelerated' soil erosion — loss of soil at a much faster rate than it is formed — is a far more recent problem. It is always a result of mankind's unwise actions, such as overgrazing or unsuitable cultivation practices. These leave the land unprotected and vulnerable. Then, during times of erosive rainfall or windstorms, soil may be detached, transported, and (possibly travelling a long distance) deposited.
Accelerated soil erosion by water or wind may affect both agricultural areas and the natural environment, and is one of the most widespread of today's environmental problems. It has impacts which are both on-site (at the place where the soil is detached) and off-site (wherever the eroded soil ends up).
More recently still, the use of powerful agricultural implements has, in some parts of the world, led to damaging amounts of soil moving downslope merely under the action of gravity: This is so-called tillage erosion.
Soil erosion is just one form of soil degradation. Other kinds of soil degradation include salinisation, nutrient loss, and compaction.
