Factbox 3.
What Is Soil Erosion

By Professor Nils Nykvist

Soil erosion means that soil particles are detached and carried away by the action of water, wind, ice, and gravity. Blocks, stones, and gravel can be moved by the ice, but not by water or wind, except on steep slopes, where water with help of gravity can transport them down the slope. Coarse sand can be transported away with running water and with wind, by rolling on the ground.

All material finer than these factions can, depending on slope, be removed from the site by running water, and except for the clay soils, also by the wind. The steeper the slopes, the more the fine-grained soils can be eroded away by the running water. The detached soil particles are deposited when the wind speed decreases or the running speed of the water subsides.

The largest water erosion is often found in comparatively dry areas, because the sparse vegetation provides little protection against the often very heavy rains (Fig. 31).
Figure 31. Vegetation is a good protection against soil erosion. Tanzania.

Heavy exploitation of the vegetation by humans has in many areas led to soil erosion so severe that most of the fine soil particles have been eroded away from the bedrock. The barren mountains that are common in many tropical areas consist mainly of bare rock or coarse-grained soils with little ability to retain water.

Because of strong winds, sand blows away on the hills and settles down in lower-lying areas when the wind speed decreases. The moving sand can form sand dunes capable of burying villages and farmland when the wind is strong enough.

By planting trees, bushes, and grasses on the dunes, the wind erosion can be decreased considerably. The particles somewhat finer than sand, that is, silt, can, when they are dry, be lifted by the wind and carried far away from the place where they were picked up.

When the wind speed decreases the particles fall to the ground, where in more humid regions they take up water and lose their ability to be transported away by the wind. The well-known loess soil in China comes mainly from the large deserts in the north, where it was formed by physical weathering.

If the vegetation on such soils becomes impoverished by excessively hard cultivation or grazing, the soil can again fly away, if it dries out.

During the severe dry years in West Africa in the early 1970s, satellite images revealed how dust clouds were carried out over the Atlantic. Since it was the top, most nutritious parts of the soil that blew away, the wind erosion caused a severe loss of plant nutrients.

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