How should government help farmers conserve soil?

In a world threatened by loss of needed farmland, experts are searching for a basic strategy for conserving soil and realizing its productive potential.

Pierre R. Crosson, senior fellow at Resources for the Future here, says increased investment in agricultural research can provide the answer. Give the world's farmers new generations of high-yielding crops, he explains, and then market forces will help supply the necessary incentives for farmers both to meet world food needs and to maintain the world's agricultural land. Public investment in basic research, he adds, is vitally needed because ''We can't assume that the market will provide the right amount of investment in new land-saving technologies.''

Across the street at the Worldwatch Institute, president Lester R. Brown puts much less faith in the ability of market forces to solve food production dilemmas. He insists that ''supply-side economics, with its near blind reliance on the marketplace, will not protect the basic resources on which humanity depends.''

Mr. Brown, a former US Department of Agriculture adviser to foreign governments, explains that ''the per capita production of many of our basic resources is now declining.'' He points out that the world's forests hit peak per capita wood production in 1964, fish catch peaked in 1970, and beef production has dropped 9 percent since peaking in 1976. He says that only collective action, government and private, can preserve these resources.

Citing a University of Iowa study, Brown notes that, for the average American farmer, ''the cost of reducing soil loss to a tolerable level is about three times the immediate economic benefit.'' Without government support, he insists, farmers are left in a no-win situation.

''If you are a farmer in Iowa, hanging on by your fingernails as most farmers are today, and losing soil at an excessive rate, you have two choices,'' he says. ''You can invest in soil conservation practices, such as crop rotation and terraces. But if you're already just barely hanging on, these additional expenditures without any immediate economic return could drive you into bankruptcy. The alternative is not to do anything and face the prospect of bankruptcy 20 or 30 years down the road when there is no topsoil left on your farm.''

Dr. Crosson acknowledges the challenge. But he says that Brown leaves one key ''safety valve'' out of the picture - the world's demonstrated ability to increase crop yields by applying high technology to farming skills.

Crosson agrees that the world's agricultural resources are being stressed by population growth and competing demands for land. ''Based on the expectation that the longer term will be characterized by scarcity rather than surplus,'' he says, ''clearly we need to do something to reduce emerging pressures on the land.''

Looking back, Crosson argues that past surpluses directed research efforts into finding ways to market surpluses. With the shift toward global supply shortages, he calls on the world's governments and research institutions to concentrate their efforts on developing new high-yielding technologies for agricultural production.

Crosson's answer is not government intervention to prop up prices or control land use. He says once governments develop high-yield technologies, farmers will implement these practices in their own economic interest.

Complicating the picture, agricultural exports play an increasingly important role in the economy. Last year they earned a record $43.3 billion, nearly double the 1975 total of $21.9 billion. Critics of export promotion policies charge rising exports encourage farmers to till fragile marginal land, increasing the nation's erosion problems.

The export problem, says Brown, ''comes down to whether we ought to cut back exports to save topsoil or continue mining this resource.'' He favors turning some 20 million acres of cropland back into pasture and using more conservation tillage practices.

Crosson says that ''over the last 50 years, (US government) soil conservation programs have spent in excess of $15 billion without knowing very much about soil erosion or its consequences.'' He maintains that ''soil conservation will be undertaken by farmers when it is in their economic interest, when high yields make their fields so valuable that they will want to keep their soil.''

Lester Brown and others warn that relying on ''a technological quick fix'' is too risky given today's resource problems. Many experts such as Crosson foresee dramatic breakthroughs as a result of genetic engineering and similar new techniques.

Crosson adds, ''Before these good things happen, such as transfering the nitrogen-fixing capacity to grains, major breakthroughs in areas of basic science have to be made, and this requires a large-scale and sustained effort in which the government will have to play a leading role.''