IN RECENT years, record surpluses have sent farm prices plummeting and forced the federal government into increasingly expensive subsidies to support farm income. Meanwhile, precious topsoil is eroding at a faster rate than in the Dust Bowl days of the 1930s. Should the government be encouraging farmers to bring new, fragile cropland into production?

Sen. Bill Armstrong (R-Colo.) has drawn the obviously correct conclusion that such encouragement makes no sense at all. Together with 28 cosponsors, he is pushing a bill that would stop all government subsidies to persons who cultivate previously unplowed land determined by the Agriculture Department to be highly susceptible to erosion.

After 45 years of government soil conservation programs, erosion is a more serious problem than ever before. The Agriculture Department estimates that every year more than 5 billion tons of topsoil are lost through erosion, a loss that can ultimately turn current farm surpluses into future deficits. The reasons for this loss are not technical--soil conservation techniques are highly developed--but economic. They arise primarily from the incentives that farm subsidies provide for ever more intensive and extensive cultivation of farmland.

Sodbusting, the plowing up of marginal grazing land for wheat farming, is one of the most pernicious practices encouraged by current policies. Enticed by the prospect of subsidies that can immediately double the value of grazing land, speculators have been plowing millions of acres. Farmers who buy the land can claim subsidies for their contribution to the nation's wheat surpluses or they can take their land out of production and get paid by the government for their forbearance. Once plowed, the fragile topsoil blows or washes away.

Farm groups have traditionally opposed linking subsidies to their methods of cultivation. But grasslands destruction has become such a threat that almost every major farm organization has joined with conservation groups in supporting Sen. Armstrong's measure and a companion bill introduced by Rep. Hank Brown in the House. The measure is also supported by the administration, which is trying to focus soil programs--long a favorite source of congressional pork barrel--more closely on areas that need help and on practices that foster conservation rather than increased production.

Last year, a similar measure was killed on technical grounds in an appropriations bill conference. This year, Sen. Armstrong hopes that prompt action by the Senate Agriculture Committee, which is scheduled to take up the matter today, and a strong supporting vote in the Senate will encourage the House to move quickly as well. Time is important. There are 250 million acres of unplowed land at risk, and farmers will soon be making their planting decisions for next year.