SOME farmland, such as the highlands of Nepal and the steep slopes of Washington State, is highly susceptible to wind and water erosion and would best be left untilled. But even suitable land can be subjected to erosion.
Soil is naturally generated at a rate of five tons per acre - about the thickness of a dime. But Iowa, in the heart of America's Grain Belt, has been losing eight tons per acre per year, and up to 40 tons in some areas. The world's farms annually lose an amount of soil equal to that of Australia's wheat-growing land, according to Lester Brown of the Worldwatch Institute. To avoid erosion, farmers increasingly let crops lie on the ground after harvest rather than plowing them under and use herbicides rath er than machinery to uproot weeds - though application of chemicals is another concern. Water depletion
Irrigation has made farming possible where inadequate rainfall once prevented it. Dr. Brown notes that the amount of irrigated land per person rose 30 percent from 1950 to 1978. However, in the US, 21 percent of all irrigated land receives water from underground aquifers that Brown says are being depleted. "At some point, some of that will have to stop." Other nations face similar challenges, he says. Salination
Salination occurs on irrigated farms in hot climates. Rapid evaporation builds up salt on top of the soil, harming the crops. "A lot of the irrigated lands in the world are salt-affected," says James Biggar, a professor of hydrologic science at the University of California, Davis. He expects the problem to increase. Production has stopped in some parts of California and Arizona because of salination. Waterlogging
Land in low-lying areas with high water tables and poor drainage is subject to waterlogging, which hurts deep-rooted crops. This is a significant problem for farmers in Sri Lanka, for example.