THE SWIFTLY growing demand for U.S. agricultural exports, while contributing more and more to the trade balance, has set American farm policy on a dangerous course. Larger and larger harvests, that both meet and require larger and larger exports, require farming practices that are destroying the resource on which all farming depends--the topsoil. For every bushel of corn harvested in Iowa these days, two irreplaceable bushels of soil are lost.

What's happening is no secret. Since the spring of 1977 when dust storms reminiscent of the 1930s swept across the Great Plains, a stream of studies and reports has documented the dimensions of the trend. Secretary of Agriculture John R. Block understands the threat, but economic demands--to keep revenues up, and the budget down--are forcing him to perpetuate the contradictory policies.

Last week Mr. Block told Congress that erosion, caused in large part by foreign demand for farm products, is already sharply reducing agricultural productivity and, without greatly expanded conservation programs, will grow much worse. When he testified, Mr. Block was just back from a foreign trade promotion trip that included a visit to woo the newest American agricultural customer. China, Mr. Block said, "could be our biggest market someday."

Soil erosion is difficult to take seriously when year after year, weather permitting, U.S. farmers produce record harvests. Yet experts agree that a dangerous loss in soil productivity is under way: it is only temporarily masked by heavier use of fertilizers and pesticides. The numbers are bleak. It takes nature 100 to 1,000 years to produce an inch of topsoil, and on much of American crop land soil is being lost at the rate of one inch every 15 years. About six inches of topsoil are necessary for commercial farming. Iowa, the heart of the nation's-- and now the world's--breadbasket once had 14 inches of topsoil. Many parts of the state are down to nearly 6 inches.

Soil erosion can be controlled, but conservation measures are very expensive. While acknowledging the need for a much greater effort, Secretary Block did no more than propose a new program combining existing funds into block grants. Most farmers do not have the necessary cash either. Those who do tend to be large landowners who do not live on their land and who are not interested in what the land will be good for years from now. Meanwhile, the search for export markets to keep surpluses down and prices up pushes farmers to plant on land that is more vulnerable to erosion. Federal policies are encouraging the practice by providing disaster aid that removes the risk for farmers who plow up these marginal acres.

The specter of soil erosion poses one more aching problem for a Congress struggling to control spending. With badly needed social programs being cut, there seems little room for an expensive new program, one whose effects will only be felt in the future. But if room cannot be found, the question must be asked whether some day in the not too distant future the country will discover to its horror that one of its greatest sources of strength has been irreparably damaged.