Some surprising data obtained in a 1977 survey, combined with 10 years of shrinking budgets, are causing the future of the nation's soil conservation programs to be, quite literally, remapped by the Department of Agriculture. On the new map will appear "targets": areas where soil erosion is considered a severe threat to agricultural production, and which, under Agriculture Secretary John Block's recent conservation proposal, will receive a transfusion of federal technicians and financial aid.

The exercise sheds a harsh light on what is known about the erosion problem in this country, and what is not. Though serious, erosion is by no means the pervasive threat it is made out to be by journalists and even by government officials. The 1977 survey revealed, for example, that about 6 percent of America's cropland (25 million acres) accounts for an astonishing 42 percent of the total erosion caused by rainfall (the major form of soil loss). Yet, the fertility of land with lower erosion rates may also be seriously jeopardized where thin topsoils or other factors come into play -- research on the matter is notoriously inadequate.

At a time when researchers, farmers and policy makers are scrutinizing every aspect of land use and policy, there can be no last word on the adequacy of America's agricultural resources. It is relatively clear, however, that a host of problems is sapping the strength of U.S. agriculture, as well as posing a serious threat to the environment. As our agricultural exports increase, the situation can only worsen, and we will not have the fanfare of another Dust Bowl to rally round.

The decay under way is insidious, but it amounts to one of the more pressing resource issues of the '80s, and Neil Sampson has done an admirable job of describing it. A well-known spokesman for strong federal conservation programs, Sampson has synthesized in this book many of the resource studies that have poured forth in recent years. "Farmland or Wasteland" is therefore essential reading for anyone interested in the present course of American agriculture.

Sampson provides an especially lucid discussion of the powerful economic forces working against soil and water conservation down on the farm. He explains how a booming grain export market (it has grown from $7 to $40 billion since 1970), coupled with an ebbing domestic demand for beef, have driven many farmers to drop cattle production, and grow only cash crops, instead of soil-saving pasture and hay. One of these, soybeans, leaves land particularly vulnerable to erosion, because very little plant material remains to hold the soil once the valuable beans have been removed. What is more, the land being added to grow these crops generally has a much higher potential for erosion than land now in cultivation.

Among experts, soil loss is considered the most serious agricultural resource problem, but there are others, including conversion of high-quality farmland to nonfarm development; damage to topsoil from surface mining; and water shortages and salt accumulation on irrigated land out West. Sampson reviews each trend in detail.

Taken singly, none of these threatens national agriculture. Together, in Sampson's view, they portend resource scarcity in the years ahead. "To argue about whether such trends will ultimately reach the stage where they result in problems in 1990 -- or 2000 or 2030 -- is to miss the whole point," he feels. "Wasting good farmland is senseless . . . It cannot be defended on any grounds -- legal, moral or economic."

Sampson has his biases, both in diagnosing the problems and suggesting remedies. He has his favored government agencies and his whipping boys, and it would be easy to assemble a chorus of experts to dispute him on any number of points. Here are two: He understates the extent to which erosion is concentrated on a smallish portion of cropland; and he cursorily sketches the important trend of conservation tillage, which requires increased use of pesticides to save soil, a trade-off meriting more than mention in a photo caption.

Still, "Farmland or Wasteland" mounts a stimulating assault on what Sampson terms the "surplus syndrome": the view that chronic overproduction remains the central problem of farm policy, making concerted efforts on conservation and research unnecessary. And what if we do not overcome the syndrome? One of our all-purpose national columnists ludicrously predicted not long ago that America "may not have many more harvests" if erosion goes unchecked. By comparison, Sampson's grim prognosis seems tame: thousands of ruined farmers, food prices pushed beyond the reach of more of the world's poor, continued pollution of America's water -- in short, the waste of the finest garden on earth.