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The main on-site impact is the reduction in soil quality which results from the loss of the nutrient-rich upper layers of the soil, and the reduced water-holding capacity of many eroded soils. In affluent areas of the world, accelerated water erosion’s on-site effects upon agricultural soils can be mitigated by increased use of artificial fertilizers; however this is not an option for much of the earth’s population.

Erosion’s removal of the upper horizons of the soil results in a reduction in soil quality i.e. a diminution of the soil’s suitability for agriculture or other vegetation. This is because the eroded upper horizons tend to be the most nutrient-rich. Also, because the finest constituents of eroded soil tends to be transported furthest, eroded soils become preferentially depleted of their finer fraction over time; this often reduces their water-holding capacity. In other words, “Erosion removes the cream of the soil”. Increased use of artificial fertilizers may to an extent, and for a time, compensate for erosion-induced loss of soil quality where economic circumstances are favorable. This is not usually feasible in developing countries however. Loss of soil quality is a long-term problem; globally, soil erosion's most serious impact may well be its threat to the long-term sustainability of agricultural productivity, which results from the the 'on-site' damage which it causes.

The on-site impact of erosion: severe rilling on a hillslope at Rottingdean on the UK South Downs in 1987. Photo: John Boardman

Crops are particularly reliant on the upper horizons of the soil, which are the most vulnerable to erosion by water and wind. In this sense, erosion removes 'the cream of the soil'. Agricultural tillage also redistributes soil, resulting in thinner soils on topographically convex areas within a field.

The damaging on-site effects of erosion, in terms of decreased agricultural yields, are well known in the developing countries of Africa and Asia. But even in the developed world there is cause for concern. Water erosion is locally severe in Australia, New Zealand, parts of the US, Southern Europe, and Eastern Europe (often as a result of the former large state-controlled farms).

In erosion-prone areas of the more affluent countries, productivity may be maintained in the short to medium term by increased fertiliser input. The effects of erosion are thus rarely acknowledged by farmers in richer countries. This strategy is however infeasible with regard to erosion in developing countries.

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.

منبع:http://soilerosion.net