Soil productivity is reduced by loss of soil nutrients and organic matter and the water-holding capacity of soil. Over the last 100 years, with the growth in consumption, world population and man's technical capacity, the cultivation of soils has increased, both extensively and intensively, but without a parallel increase in soil conservation measures. The side-effects of present agricultural practices are loss of fertility and structure of the soil and deep erosion. To increase the yields developing countries need to invest in irrigation, mechanization, pesticides, fertilizers and storage that require expensive energy and resources. Soil-destructive practices continue because the prevailing system of grants and subsidies reward intensive, mechanized agro-industrial farming. The result is accelerated soil degradation.
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.
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).