WASHINGTON (AP) _ Millions of tons of farm soil are being swept away by water and wind, leaving the Earth increasingly unable to feed the more than 5 billion humans now living on the planet.

David Pimentel of Cornell University reports today in the journal Science that soil erosion is washing way the world's ability to feed itself, with more than 1 billion people already malnourished and with the future looking even more grim.

Pimentel said Thursday that it takes about 1.2 acres of land per year to provide a varied diet to one person, but there's only about 0.6 acre per person worldwide available for farming now. In 40 years, erosion may reduce the farm land to only 0.34 acres per person, he said.

``We know what to do to control erosion, but we aren't doing it,'' he said. ``Few people really appreciate the seriousness of the problem.''

In a worldwide study in Science, Pimentel and his associates at Cornell found that erosion is destroying about 29 million acres of farm land a year. About 3.7 billion acres worldwide can still grow crops.

``In many regions, limited land is a major cause of food shortages and undernutrition,'' said Pimentel. He estimates that about 20 percent of the world's population is poorly nourished.

Erosion, he said, is hardly recognized by most people because its effects are subtle. A single rainstorm on bare ground can strip away a millimeter (0.03 inch) of topsoil, he said.

``If you look at the land, you wouldn't even notice the loss,'' said Pimentel. ``But that amounts to tons of topsoil lost per acre. It's insidious. Eventually, it catches up with you.''

The effect is cumulative. In 1776, for instance, the average U.S. topsoil was nine inches deep. The average soil now is 5.9 inches deep.

``That's not going to be replaced,'' said Pimentel. ``It takes nature more than 200 years to form just one inch of topsoil and we're losing it about 17 times faster than its being replaced.''

More than 247 million acres of American agricultural land has been abandoned over the last 200 years, much of it because of erosion, the study shows.

Pimentel said that erosion, by taking away soil quality and nutrients, also reduces crop yields. In Kentucky, Illinois, Indiana and Georgia, corn yields have dropped by between 12 percent and 65 percent on eroded soils, the study shows.

Soil erosion could be slowed by changing farming methods, said Pimentel. Such measures as crop rotation and contour planting have controlled erosion and increased yields in experiments on damaged lands in Texas, Missouri and Illinois, the study shows.

The United States is now the world's top food exporter, but Pimentel said that if the present rate of soil erosion and population growth continues, America will only be able to feed itself for a few more decades and then will have to import food for a population that will double in 60 years.

Science is the journal of the American Association for the Advancement of Science.