When the House takes up the farm bill this week, much more will be at stake than the matter of how many billions of dollars of subsidy to spend on agriculture over the next four years. It's also an important environmental bill. At issue is whether our multibillion-dollar farm programs will continue to subsidize soil erosion and water pollution or will be used to help solve these problems.

According to a pioneering study released in May by the Conservation Foundation, soil sediment washed from farm fields causes $3 billion to $13 billion in damage each year by clogging navigation channels, water conveyances and treatment facilities, prematurely filling reservoirs and ruining recreational lakes and streams. Erosion hurts farmers too. If soil loss continues at the current pace, national crop yields could be reduced by an average of 10 percent over the next 50 years, according to some scientists.

Improper land use is at the heart of the problem. According to Agriculture Department surveys, farmers are planting corn, wheat, soybeans, cotton and other crops on about 53 million acres of land that is so susceptible to soil erosion that it should never have been cropped in the first place. A farmer who plows a steep pasture or a tract of thin-soiled prairie may lose more soil to erosion in one year than nature can produce in a thousand. He may send soil, pesticides and fertilizers flowing into the Chesapeake Bay. Yet the federal government will subsidize those abuses with crop price supports, direct cash payments and other benefits.

The House will vote on a tough "sodbuster" policy designed to keep highly erodible land blanketed with grass and trees. Any farmer who plowed up highly erodible land to plant wheat, corn or other crops without an approved conservation plan would be ineligible for most Agriculture Department programs -- including vital price supports -- on any land he farmed. A companion measure, the so-called "swampbuster" policy, applies the same penalties to farmers who drain ecologically rich wetlands for crop production.

The principle behind both measures is simple: taxpayers will no longer subsidize such abuses of the land -- especially at a time of mounting crop surpluses.

To help farmers deal with severe erosion problems on land already in production, both committees and the administration have approved a new "soil conservation reserve program." The bill before the House would offer farmers annual rental payments to take up to 25 million acres of the country's most erodible cropland out of product it with grass or trees.

The conservation reserve has broad political appeal because it would achieve two objectives at the same time. Since excessive soil erosion is highly concentrated on a fraction of the nation's 421 million acres of cropland, a 25-million-acre reserve could cut excessive cropland erosion by 30 percent within a few years. In addition, the estimated $1.2 billion in annual reserve payments would provide much-needed financial assistance to farmers and help reduce crop surpluses.

Environmental groups oppose any weakening of these reforms, and strongly support a crucial amendment by Reps. Dan Glickman (D-Kan.) and Howard Wolpe (D- Mich.). It sets a deadline of 1990 for farmers to enroll their erosion-prone cropland in the conservation reserve, retire it voluntarily or adopt a farming system that will bring soil loss into line. If they don't comply, they'll lose eligibility for price and income support payments and a host of other program benefits.

This amendment, in combination with the reserve program, is a firm but fair way to redress a classic inequity in farm programs. Because program benefits are directly related to the amount of land farmers plant to program crops, conservation-minded farmers have been penalized for keeping their erodible acres in grass or trees. After 1990, conservation compliance would handicap rather than reward farmers who abuse fragile soils in order to produce surplus crops.

Environmentalists aren't the only ones supporting such a policy. In a 1984 survey some 8,000 farmers from 17 states were asked whether farmers should be required to adopt recommended soil conservation practices in order to qualify for USDA price support programs. A clear majority said they should.

Unfortunately, experience shows that farmers -- including family farmers -- probably won't invest heavily in conservation when good times return. To the contrary, when crop prices and farm incomes soared in the 1970s, farmers plowed up millions of acres of fragile pasture and range, bulldozed and burned windbreaks and hedgerows, drained wetlands, abandoned terraces and soil- saving crop rotations and farmed right up to the streambank. Today farmers -- and taxpayers -- are suffering not only from the turnabout in the export market but also in part from those excesses.

Major environmental groups favor generous aid for agriculture. In return, though, it is perfectly reasonable to expect that farmers and farm policy makers will deal in earnest with soil erosion and other critical environmental problems, even if that means placing conditions that some farmers don't like on the programs to aid them.