soil erosion

Signs that heavy machinery has caused ground compaction (also called ponding). Natural vegetation has been removed, reducing the amount of water that is intercepted or captured as it passes through the soil. The soil is compacted, and becomes impermeable (does not let water in). The result is an increase in overland runoff (water flowing over the surface), causing an increase in soil erosion and flooding.

Spectacular earth pillars are found near Bolzano in the Dolomites, northern Italy. The large rocks protect the soft boulder clay beneath from erosion. As a result, there are rock-caps supported by very steep slopes. However, in recent decades earthquake activity has shaken many of the pillars, causing the rocks to fall.

Wearing away and redistribution of the Earth's soil layer. It is caused by the action of water, wind, and ice, and also by improper methods of agriculture. If unchecked, soil erosion results in the formation of deserts (desertification). It has been estimated that 20% of the world's cultivated topsoil was lost between 1950 and 1990.

If the rate of erosion exceeds the rate of soil formation (from rock and decomposing organic matter), then the land will become infertile. The removal of forests (deforestation) or other vegetation often leads to serious soil erosion, because plant roots bind soil, and without them the soil is free to wash or blow away, as in the American dust bowl. The effect is worse on hillsides, and there has been devastating loss of soil where forests have been cleared from mountainsides, as in Madagascar.

Improved agricultural practices such as contour ploughing are needed to combat soil erosion. Windbreaks, such as hedges or strips planted with coarse grass, are valuable, and organic farming can reduce soil erosion by as much as 75%.

Soil degradation and erosion are becoming as serious as the loss of the rainforest. It is estimated that more than 10% of the world's soil lost a large amount of its natural fertility during the latter half of the 20th century. Some of the worst losses are in Europe, where 17% of the soil is damaged by human activity such as mechanized farming and fallout from acid rain. In Mexico and Central America, 24% of soil is highly degraded, mostly as a result of deforestation.