In developed countries, as well as in some developing nations such as China, total cropland actually has declined. The International Irrigation Management Institute of Colombo, Sri Lanka, predicts that the area of irrigated land, which now produces about one-third of the world's harvest, could shrink by some 12 percent in the next 15 years. Much of that loss is due to the sprawl of urban centers, which often are located on the most fertile land. Additional losses are due to the accumulation of salt on irrigated land. There is also a decline in the amount of water available for irrigation, due to increased demands for domestic and industrial uses from the growing population, according to the Irrigation Institute.
Soil erosion accounts for further losses.
In some areas, destructive farming practices have exhausted the soil and the pressure for more crops has pushed marginal land into production. Meanwhile, heavy applications of fertilizers, pesticides, and insecticides have polluted ground water. Many experts warn that the widespread use of the same type of seed leaves vast fields of cropland vulnerable to a single disease or infestation. The Worldwatch Institute's Brown points with alarm to the example of China. After four decades of progress, in which it became the world's leading grain producer, the country in the spring of 1994 faced a grain shortage.