A 3-year-old effort to combat erosion wearing away three billion tons of topsoil a year in the Farm Belt is beginning to produce results, conservationists and farm lawmakers say.

"It is saving a half a billion tons of soil from erosion each year," says Senate Agriculture Committee Chairman Patrick Leahy, D-Vt. "It has helped the farmer by stabilizing farm income and land values. It has helped the public by reducing pollution and lowering the cost of farm programs."The Conservation Reserve Program, created under wide-ranging 1985 farm legislation, marked a sharp departure from soil erosion programs in the past.

Rather than taking the traditional, year-to-year approach, it envisions long-term idling of huge amounts of highly erodible land.

Some 25 million highly erodible farm acres have been placed in the conservation reserve thus far. The government is paying rent to keep the land out of cultivation except for trees and cover crops that halt erosion.

The aim is to place 45 million such acres into the reserve by 1990.

Farmers are banned under the program from using land placed in reserve for grazing and production of hay or row crops, such as corn and soybeans, that wear away highly erodible soil.

Preliminary indications cited by conservationists are that the program has reduced erosion by 450 million to 500 million tons of soil a year.

The government estimated annual erosion at three billion tons in 1985 when the program was approved.

"We're saving 10 times more soil than we were in the previous programs," says Bob Gray, a policy consultant to the American Farmland Trust, a Washington group that spearheaded the campaign for the reserve.

Rental payments are admittedly costly to the government but the program has thus far escaped the type of criticism that has been focused on some of the crop subsidy provisions of the 1985 law.

One reason may be that the program limits crop production and thus is a factor in holding down surpluses that can cause an explosion of federal payments under price and income support programs as well as storage costs.

Moreover, there are costs to the farmer in placing the land in reserve.

"This is not just money coming to the farmer to put in the bank or go on a holiday with it," says House Agriculture Committee Chairman E. "Kika" de la Garza, D-Texas. "There is a cost to preserve the land, or just to plant a vegetative cover."

Rents range from $45 to $50 an acre.

One problem that has cropped up, however, is that rents have been high enough to drive up the going rate for land in some parts of the country.

"This is a big problem and a big program," says conservationist Ken Cook, a major architect of the program. "There are going to be some mistakes and misfits . . . But this is a gigantic stride away from what we had before."

Not everyone views the program with equal enthusiasm. Rather than blasting it, however, potential critics tend to say it has done some good but could benefit from modifications.

Pierre Crosson, a senior fellow with Resources for the Future, a Washington-based, non-profit research group, says it is still to soon to make "really firm judgments as to precisely what the payoff of the program is."

"We don't know enough about what the various benefits are to be very conclusive about that sort of thing," he says.

Crosson is finishing a report that he says will show the program has caused little improvement of water quality because most of the land in the reserve contributes little to the sedimentation that causes water quality problems.

Agricultural runoff has been a major contributor to water problems. A 1985 report from the Conservation Foundation estimated the off-farm costs of soil erosion at perhaps $6.1 billion a year and possibly even as high as $13 billion.

One oft-cited cost is the Army Corps of Engineers dredging of reservoirs that have become filled with silt.

Agricultural runoff that finds its way down the Mississippi to the Gulf of Mexico is also blamed for much of the nagging problem of seafood contamination.

Still, water quality improvement is a secondary objective of the program. The main goal is cutting back on soil erosion.

"From a conservation standpoint, the Conservation Reserve Program is a distinct improvement over earlier USDA land conservation programs," Crosson says. "That doesn't mean that it couldn't be done still better."

Some of the most serious erosion problems are focused in the High Plains region reaching from eastern Montana to Colorado and Nebraska and to parts of Iowa, Missouri and Kansas.

The problem intensified in the 1970s when agricultural exports - and farming - were expanding dramatically.

"We're talking about thin and fragile soils that in many cases were not meant to be farmed," says Sierra Club lobbyist Daniel Weiss. "They were plowed out in the 1970s to make a quick buck."