Soil erosion is a natural phenomenon: it has occurred over the millenia as part of geological processes and climate change. However, erosion is more severe now: globally, moderate to severe soil degradation affects almost 2 000 million hectares of arable and grazing land - an area larger than that of the United States and Mexico combined. More than 55 percent of this damage is caused by water erosion and nearly 33 percent by wind erosion.

Every year soil erosion and other forms of land degradation rob the world of 5-7 million hectares of farming land. Every year 25 000 million tonnes of topsoil are washed away: China's Huang River alone dumps 1 600 million tonnes a year into the sea. The United States has lost about one-third of its topsoil since settled agriculture began. Worldwide, soil erosion puts the livelihoods of nearly 1000 million people at risk.

Impact of soil erosion

Degraded soils

Global status of human-induced soil degradation
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Soil degradation by area and type plus major causes

What are the causes of soil degradation?

Soil erosion mostly occurs when there is no vegetation to protect the soil from being washed or blown away. Clearing forests, growing crops on steep slopes or on large fields without protection, can all lead to erosion. So can ploughing too deeply, failing to rotate crops, planting crops up and down hills rather than along their contours or grazing too many animals on one piece of land. Soil degradation in developing countries is closely linked to poverty: both personal and national. Poor farmers, with no resources to fall back on, may be forced to put immediate needs before the long-term health of the land. Governments, under pressure from foreign debt, weak commodity prices and the needs of their urban populations, coupled with domestic policies that are biased against agriculture, often fail to give adequate support to rural people.

Fighting erosion

The effects of erosion are legion. Soil washed off bare hillsides ruins aquatic habitats and clogs waterways. Reservoirs silt up, cutting the lifespan of hydroelectric schemes. Riverbeds rise, increasing the risk of floods.

Erosion can, however, be reduced. And eroded land can be restored.

The weapons in the fight against erosion fall into two categories - biological and physical. The biological approach involves matching crops to soils, and farming methods to terrain. Physical techniques include building terraces and dams, controlling gullies (by, for instance, planting trees) and overall watershed management.

In the 1930s, wind erosion devastated millions of hectares of farmland in the United States. As a result the Government set up an agricultural extension service to train farmers in soil conservation. They were taught to farm along the contours, to plough less deeply and to plant trees, hedges and grass around the edges of fields. Crop rotation was introduced, giving soils a chance to recover nutrients, and irrigation was made available. Productivity was restored within a few years.

In 1979, the Chinese authorities, supported by the United Nations Development Programme (UNDP) and FAO, set up a project in Mizhi County on the Loess Plateau, one of the world's most eroded regions. Nearly two-thirds of the land in Mizhi slopes at an angle of over 20 degrees. Soil improvement methods included turning steep slopes over to permanent vegetation, terracing and gully control. Farmers were encouraged to replace annual crops with perennials, such as alfalfa, which would hold the soil in place from year to year, and to diversify into small animal husbandry and fruit growing. Total food production has risen by about 70 percent, in spite of the fact that the cultivated area has been halved.

In southern Morocco in the mid-1970s, palm plantations, villages and roads were being buried under wind-blown sand released by overgrazing and wood cutting. In the 1980s, three methods of stabilizing the sand dunes proved successful: using chequerboard patterns of palm branches to protect vegetation from the wind, erecting fibro-cement windbreaks and sculpting the sandbanks on road verges so as to encourage the wind to carry sand away rather than allow it to settle. These techniques saved villages, palm plantations and many irrigation canals, roads and railways from the desert.

In Niger, FAO's enormous Keita project has transformed a barren landscape into a flourishing environment for crops and livestock - so much so that the area can now be seen from space as a green patch in the desert.

Combating soil erosion
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Check dams, built by filling wire mesh cages with stones, reduce water erosion in seasonal floods.	New farming techniques allow crops to be grown on land never cultivated before.	The Keita project in Niger. Fences built of millet stalks hinder dune migration by wind action in the long dry season.