Soil deterioration has many causes, the main ones being inappropriate land-use, erosion, salinization and waterlogging, and chemical degradation. In general, the removal or depletion of vegetation or vegetable residues which protect the land is the basic cause of soil degradation. Excessive or untimely tillage, over-fertilization, improper implements, burning crop residues and excessive livestock grazing have all contributed to this process. Lack of earthworms, humus and the roots of grasses make soil incapable of forming and maintaining a crumb-like structure. In wet areas, inability to maintain a "crumb-like" structure causes the soil to become waterlogged and turn into an intractable sticky mass with the consistency of butter. Repeated passage by tractors and other farm machinery consolidates such a mass to considerable depths below the surface, making it difficult for roots of plants to penetrate and for oxygen, moisture and nutrients to diffuse freely. During dry weather this clay shrinks and sets to a hardness like concrete. In dry areas, the crumb structure of the soil becomes less pervious or even impervious by the formation of a crust, which reduces water infiltration and the availability of water to plants. Because of the resulting increased run-off water, erosion may occur. Reduced protection of the soil by vegetation also makes the area more liable to wind erosion.
In advanced stages of erosion, all soil, and therefore all capacity for production, may be removed. More frequently, lack of soil conservation results in the loss of the most productive layers of the soil - those having the highest capacity for retention of moisture, the highest soil nutrient content, and the most ready response to artificial fertilization. Moderate to slight soil deterioration cannot be regarded as having serious social consequences, except over many decades. As an income depressant, however, it does prevent a community from reaching full productive potentiality. More severe erosion has led to very damaging social dislocation, reducing dependent communities to subsistence level. This has been illustrated in the hill and mountain lands of south-eastern USA and in Italy, Greece, Palestine, China, and elsewhere for many millions of peasant people. Illiteracy, short life spans, nutritional disease, poor communications and isolation from the rest of the world have been the marks of such communities. It has been estimated that about half the area of originally potentially productive land in the world has become useless because of the absence of soil conservation.
There is a lack of reliable data on land degradation but it is likely that soil degradation has affected some 1 900 million hectares of land worldwide (UNEP/ISRIC 1991). The largest area affected, about 550 million hectares, is in Asia and the Pacific. In China alone, between 1957 and 1990, the area of arable land was reduced by an area equal to all the cropland in Denmark, France, Germany and the Netherlands combined, mainly because of land degradation (ESCAP 1993).
In Africa, an estimated 500 million hectares of land have been affected by soil degradation since about 1950 (UNEP/ISRIC 1991) - including 65 per cent of the region's agricultural land (Oldeman 1994). Crop yields in Africa could be halved within 40 years if degradation of cultivated land continues at present rates (Scotney and Dijkhuis 1989). Land degradation affects about 300 million hectares of land in Latin America, as a result of soil erosion, loss of nutrients, deforestation, overgrazing and poor management of agricultural land (UNEP/ISRIC 1991). In Europe, some 12 per cent of the land area (115 million hectares) is affected by water erosion and some 4 per cent (42 million hectares) by wind erosion; in North America about 95 million hectares are affected by degradation, mainly erosion (UNEP/ISRIC 1991).
According to GLASOD, of the world's 1,900 million ha of land affected by soil degradation during the past 45 years, the largest area (around 550 million ha) is in the Asia-Pacific region (UNEP/ISRIC 1991). For Asia this constitutes about 20 per cent of total vegetated land. Dry parts of the region are particularly vulnerable, and it is estimated that 1,320 million people (39 per cent of the region's population) live in areas prone to drought and desertification (UNEP 1997). The more recent Assessment of Soil Degradation in South and South-East Asia (ASSOD 1997, see map) found that agricultural production is substantially reduced by degradation in dry areas. Nearly 180 million ha in China, including 90 per cent of China's extensive grasslands (SEPA 1998), 110 million ha in India and 62 million ha in Pakistan are degraded, representing 56, 57 and 86 per cent respectively of susceptible drylands (UNEP 1997).
World demand for food has nearly tripled since 1950. As it climbed, soil deterioration increased, accelerating sharply during the 1970s when world grain prices doubled after the massive Soviet wheat purchase in 1972. Idled, highly-erodible cropland was returned to use and steep and marginal lands were cultivated. By the early 1980s the inherent productivity of roughly a third of the world's cropland was falling, although increased use of chemical fertilizers often masked this deterioration.
More than three billion hectares, almost a quarter of the world's land surface, is now at risk from desertification, salinization from bad irrigation or other degradation. Worldwide, an estimated 26 billion tonnes of topsoil are washed or blown off cropland each year. Every year 6 million hectares of productive dryland becomes desert. US$6 billion damage a year is done off-site by eroded soil deposited on coral reefs, in dams and fisheries, in the USA each year. 8 million metric tonnes of hazardous chemical wastes are leaking into Dutch soil. Cleaning up the 21,000 abandoned chemical hazard sites in West Germany will cost US$4 billion.
Soil degradation erodes the overall resource base for agriculture. Loss of croplands encourages farmers to overuse the remaining agricultural land and to move into forests and onto rangelands. Such approaches to agriculture deplete the soil and are not sustainable.