Most land that is best suited for crop production is already being farmed. If the current rates of conversion of agricultural land to non-agricultural uses continue in industrialized countries, and if the current rates of land degradation continue in developing countries, within 20 years more than one-third of the world's arable land could be lost or destroyed. The quality of arable land is being impaired by a combination of urbanization, desertification, erosion and salinization; and in most countries the rate of soil loss from croplands is far in excess of the rate of soil formation. Expensive efforts to bring new lands under agriculture are partially offset by the loss of croplands to dehydration. As population pressure continues to increase, more people are being forced onto land that is becoming less and less productive, including important watersheds which become degraded and in turn reduce the productivity of agricultural land in their catchment areas.
The world's cropland currently occupies 14 million square km and, even using the most optimistic assumptions, it would appear that croplands world-wide could be no more than doubled. In addition the loss of arable land through desertification each year is 6 million hectares each year, with another 400,000 hectares lost to waterlogging and salinity. Per capita arable land declined at a rate of about 25% over the period 1950-75, and will likely drop another 15% by the turn of the century. In the early 1970s, one hectare of arable land supported an average of 2.6 people, but by the year 2000, with present population projections, one hectare will have to support four people. Because of ecological and economic constraints, only moderate additions to global arable land may be feasible. In developing countries, the expense involved in bringing new land into production generally results in preference for intensified use of existing farmland. Reform of the land tenure system would normally be necessary to make adequate land available for small farmers and landless labourers.
Arable land is almost fully utilized in most parts of the world: 83% is already cultivated in Asia, 88% in Europe, 64% in the USSR, 51% in North America. In 1975 only 21% of the potential arable land in the developing countries of Asia was unused, in the Near East the proportion was 37%; but in Africa, 70% of the potential arable land was unused, and in Latin America the figure was 75%.
As people move onto marginal lands to ensure their livelihood, the global losses to desertification will accelerate. In Mexico one half of the country's territory was suffering soil erosion by 1950 due to felling of trees for crops, pasture, fuelwood or intensive cattle raising. In some areas of Ethiopia, people trying to eke out a living on eroded land are eroding it even more, cutting down the remaining trees for fuel, and denuding the countryside. Approximately half of India is already experiencing some form of soil degradation. Pressures of population are so severe in parts of the country that it is difficult to maintain a balance between food production and land degradation. In Java, soil erosion is an ecological emergency, a result of overpopulation which has led to deforestation and misuse of hillside areas by land-hungry farmers. Loss of arable land to erosion and urbanization is not confined to the developing world. The problem is acknowledged in the USA and Japan. The United Nations Environment Programme forecasts that over one-third of the world's arable land may be lost or destroyed by the turn of the century.