The loss of arable land through various forms of soil degradation, particularly soil erosion, is the result of a complex set of often interacting economic, social and demographic forces and conflicting policy objectives, as well as of topographical and climatic factors. Relentless desertification has caused a continuing shrinkage of arable land in areas such as the Sudano-Sahel. Deterioration or loss of the productive capacity of the soil is often connected with salinization, including that caused by irrigation. Salt deposits, which are left in the soil after water runs off, can significantly reduce yields; and, in the long run, can make the land unsuitable for cultivation. At the same time, measures normally undertaken to prevent salinization are not always effective. In some cases they may also jeopardize the crop itself, especially when the effort to leach the excess salt from the surface of the soil leads to waterlogging which is itself damaging to crops. In many areas of the world, especially where irrigation is critical for crop production, the twin problems of waterlogging and salinity have become acute and their solution difficult and controversial.
A number of human activities so reduce the fertility of land, or degrade the soil, that large tracts of land must eventually be abandoned as being totally unproductive. This is the case in several agricultural methods. The failure to use the various forms of agricultural rotation - grass to crop to fallow, for instance - which were previously employed to remedy the 'soil fatigue' which uninterrupted growth of one crop often brings, has resulted in a decline in the organic humus content of the soil. Loss of fertility is usually quite irreversible, though there are examples where the process of intensive land-use, abandonment, and regeneration, have been incorporated into agricultural practices, as in the cyclical movements of 'slash-and-burn' forest tribes and of nomadic herdsmen on brush-land.
In many developing countries a heavy pressure of population, leading to increased demand for food, has been a major factor behind the cultivation of marginal lands, such as hill-sides and river banks, which directly leads to soil erosion. The increased irrigation needed for food production may also lead to problems such as salinization. Agricultural and animal husbandry practices, such as the shortening of the fallow period under shifting cultivation due to demographic pressure, and overgrazing, have often contributed to soil degradation. Rising costs of alternative energy as well as poverty and increase in population have led to increased demand for fuelwood and hence to accelerated deforestation. Crops that have high economic yield or commercial profitability (such as, in recent years, maize and soyabeans) have also been found on occasion to aid soil degradation.
By the year 2000, with population growth pushing ahead of the increase in potential production, the situation will have deteriorated for most developing countries: 64 countries (more than half the total) might then be in a critical position. Indeed, 38 of these would be able to support less than half their projected populations. It is estimated that if no long-term conservation measures are taken, land degradation in developing countries may in the long run depress food production by an average of 19%. The area of rain-fed cropland could shrink by as much as 544 million hectares - an area greater than the entire potential cropland of Southeast Asia. 30% of Central America's rain-fed cropland could be lost - and 36% of Southeast Asia's. The entire potentially cultivable lands of the 117 developing countries would be sufficient to support only 1.6 times the expected population of 2000 even if such land were used only for food crops or as grassland supporting livestock. This potential area is at least three times greater than the present cultivated area.