Soil erosion (Types and Causes)

Go to page 180 of Edexcel Geography book, I've adapted the page here, and have added some explanations where I thought they were needed. :)

Soil erosion is the washing away or blowing away of top soil. Basically the wearing away and loss of exposed top soil, mainly by the action of wind and rain (as surface run-off). 

Soil erosion can result in reduce soil fertility. It is a natural process, but it is made worse by people.

There are three main types of soil erosion:

• sheet erosion: occurs in parts of the world where there is moderate rainfall. When this falls on bare soil, the top of the soil will be removed down slope. 
• gully erosion: where there is intense rainfall, as during tropical storms, the force of the water can cut gullies in slopes. This is most likely to happen where there is little vegetation cover. 
• wind erosion: in dry parts of the world, loose dry soil is readily blown away by the wind

Thus, areas most at risk of soil erosion:
-mountainous areas with steep slopes (e.g. Himalaya slopes in Nepal), so the topsoil is removed as rain flows downslope. One quarter of a million tonnes of topsoil are washed off the deforested mountain slopes of Nepal and northern India each year only to be deposited into the Bay of Bengal.
-areas with unreliable rainfall, as the soil dries and becomes really light so the wind can carry it away quickly

Also keep in mind that exposed soil doesn't just get washed downhill by water, it can move downwards slowly under gravity too. Soil erosion can be particularly bad during wet seasons as it is washed away by the storms and it is blown away during dry season..

Soil erosion is made more rapid and severe where there is misuse of the land.
Activities that cause problems include:

• removing vegetation by cutting down trees and bushes for fuel or to make way for more farmland. This exposes the soil to the wind and rain.
• overgrazing by animals-rearing too many animals in relation to amount of grass available. Result: ditto above.
• overcultivating the soil by failing to 'feed' it with fertilisers or by growing the same crop  in the same field year after year. This monoculture weakens the soil structure and removes vital minerals from the soil. (The same crop uses up the same nutrients until it is all depleted.) The net result is that crops will fail and the soil will be left exposed to the forces of erosion. 
• compacting the soil by the use of heavy machinery. (e.g. if a tractor went over a patch of soil, the weight of it would increase pressure on the soil, squashing it together and making it more compact.) This reduces the rate at which rainwater is able to infiltrate the soil. So much of the rainwater flows across the soil surface and erodes the soil as it does so. 
• ploughing fields in the same direction as the slope. This readily encourages gullying. (Ploughing up and down hill creates channels down which rain water can flow. Increases amount and speed of surface run-off.)

In general, you can say that the threat of soil erosion increases with the sparseness (lack of) of vegetation. It also does so where population numbers are so great that they put pressure on the land. (E.g. increase in population means more food is needed, so farmers try to grow more crops but on the same piece of land..growing crops so intensively to feed a growing population can only result in decreasing fertility of the soil, especially when fertilisers are not added to replenish nutrients taken up by the crop!)