Mankind has lost a considerable part of its cultivable soil during the course of recorded history. We can distinguish three zones on the globe which were successively the homes of dominating civilizations and where the soil has become progressively devastated in proportion to the time that has elapsed since they were first settled.
[First zone]In the Sahara hundreds of archaeological finds and cave paintings indicate that this was once a flourishing country with many lakes and rivers.
[Second zone]From west China, Turkey, Afghanistan, Iran, Iraq, Jordan, Sinai up to North Africa, there is presently a continuous range of stone, salt and sand deserts. In ancient times, these latitudes were inhabited by Sumerians, Babylonians, Persians, Macedonians, and Phoenicians - names which are all connected with world power and wealth.
[Third zone]Palestine, Syria, Asia Minor, Greece, Italy and Spain.
It is true that the Southern European States, (Greece, Italy and Spain), are neither steppes nor deserts, nevertheless their denuded mountains justify the statement once made by Henry C. Wallace, USA Minister of Agriculture: 'Nations live as long as their humus'. These countries, to which for centuries numerous foreigners came in search of fertile land, nowadays see their own inhabitants emigrating to all parts of the world to find better living conditions.
Statistics for 1970 to 1980 show that soil degradation – erosion, salinization and alkalinization and chemical degradation – occurred in all parts of the world to varying degrees and caused production losses. According to various estimates, between 30 and 80% of all lands under irrigation have been subject to salinization, alkalinization and waterlogging. Salinization and waterlogging are believed to be seriously affecting 200,000 - 300,000 hectares of the world's best land each year.
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).
The soils of our planet are known to have been formed over the lapse of thousands of years in conditions which have long since ceased to exist. The destruction of soils through man's mistakes, however, can take place in just a few years, and in most cases this process is irreversible or difficult to correct. The soil cover as the bearer of fertility and a component of the biosphere must be guarded from destruction and preserved for future generations.
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.