It used to be a simple trade: To qualify for federal subsidies, a farmer who grew wheat, corn, rice or cotton had to agree to leave part of his land fallow.

But Congress changed the rules in last year's farm bill. Now a farmer can keep his subsidy, yet plant his "idled" land with another crop that is not subsidized: vegetables, for example, or dry beans or alfalfa.

Some members of Congress are nervous about what they may have wrought -- gluts and price wars, perhaps.

And growers of beans, potatoes, fresh vegetables and some other crops are up in arms. They fear a great, price-depressing influx into their unsubsidized markets as subsidized farmers rush to put idled land back to work growing items usually beneath their notice.

The prospect of oversupply and lower prices may appeal to consumers, but the specialty farmers who get no federal supports charge that they face unfair competition from subsidized farmers and are urging Congress and the administration to change the rules.

The potato and dry edible bean markets, depressed because of 1985 surpluses, are reacting adversely to new surpluses that could result as the subsidized farmers turn their idled land to those crops.

"No one contemplated creating surpluses in other commodities when the farm bill was written," said one Senate Agriculture Committee aide, "but it seems that every time you cure one problem, you create another."

Legislators and growers from dry edible bean, potato and sugar beet areas are calling on the Agriculture Department to bar subsidized farmers from turning their land over to these crops, lest they add to surpluses of these items.

USDA officials contend that the new farm law does not empower the secretary of Agriculture to change the situation. "We'll probably stay where we are," said one. "We are locked in because we've already announced that corn and wheat farmers can grow these other commodities . . . . It was clear to us that Congress wanted it this way when it enacted the bill."

But members of the Michigan congressional delegation, worried about the dry bean industry, have introduced an amendment that would direct the secretary to limit the specialty crop plantings if adverse market impact was anticipated.

Heavy political fallout from this dispute is predicted.

"It is a sleeping giant, and by fall it will become a political nightmare, with a tremendous oversupply of these specialty commodities and everyone pointing fingers of blame," said William G. Lesher, a former USDA official who is involved in a rescue effort for growers of dry edible beans.

Lesher and others expect that, as provisions of the new law become more widely known, the subsidized farmers -- particularly in irrigated sections of the West -- will plant specialty items and move into lucrative markets usually dominated by growers not in the federal support programs.

"Alfalfa, potatoes and fresh vegetables all are concerned about this, and so are the banks that finance the traditional growers," said Dick Klein of Klein Brothers Ltd., in Stockton, Calif., one of the nation's major processors and producers of dry edible beans. "We have very thinly traded markets . . . . I see the federal program farmers trying for a 'perfect' crop, so to speak, such as beans, and they will destroy our market system."

A Washington representative of the National Potato Council said, "My people are up in arms . . . . It has the potential for creating a sort of range war that pits nonprogram farmers against program farmers. It is going to create a problem -- and I don't think Congress or the secretary of agriculture want to get caught in the middle of it."

Rep. Bill Schuette (R-Mich.), a House Agriculture Committee member who is from a bean-growing district, said that the unintended impact of the farm bill provision "came as a surprise to many of us." But, he said, "I view this as one of those kinks that we need to unkink. Let's not make matters worse for farmers who are not involved in the federal programs."

The first alarms over potential impacts on specialty crops were rung by growers of dry edible beans, who were aroused by reports of a demand for bean seeds by farmers who traditionally have not grown the crop and by a falling market that anticipates an upsurge in bean plantings this year.

"They'll be giving beans away next fall," said Tim Courneya of Northarvest Beans, a farmers' association in Frazee, Minn. "Our dry edible bean market can only absorb so much, and we already are in heavy surplus . . . . Now, we will have farmers getting federal subsidies competing against bean growers who get no federal assistance."

Added Klein: "Everybody has listened to us; everybody understands the problem. But my only frustration is whether something will be done before a disaster hits our industry."