The Reagan administration's suspension of direct agricultural aid to Poland and speculation about possible trade sanctions against the Soviet Union are drawing an uneasy reaction from some farm groups concerned with U.S. export policy.

Nervousness over the Polish crisis and Washington's reaction to it have pushed already-low grain prices lower during recent days and drawn calls from farm groups for a restatement of Reagan's trade and export credit positions.

The National Corn Growers Association has called for "immediate clarification" of the administration's grain export and credit policies toward Eastern Europe and the Soviet Union.

The U.S. Feed Grains Council, which promotes overseas sales, has expressed similar concern. The National Grange called for an end to all U.S. trade with the Warsaw Pact nations, but urged the president this week not to single out agriculture in any foreign policy sanctions.

Their touchiness goes back to the Carter administration's partial embargo of farm sales to the Soviet Union in January, 1980, following the invasion of Afghanistan. Farmers were furious that they could not trade with the Russians, while most other industries could.

The new four-year farm legislation signed into law last week by President Reagan contains embargo-protection provisions for American farmers, extending them massive federal compensation for losses. But there appears to be little chance that the law would help them in the current situation.

Poland's farm trade with the United States apparently is so minimal that farmers will get none of the special financial aid allowed by the law. They would be compensated for an agriculture-only ban against the Soviets, but few farm observers believe sanctions would be limited to farm products.

The Polish crisis comes at the worst possible time for U.S. farmers, who have just produced record wheat and corn crops. A major bright spot in an otherwise generally depressed U.S. farm economy has been the prospect of big new grain sales to the Soviets, fresh from their third straight poor harvest.

Many members of Congress and farm leaders contend that Carter's 1980 embargo touched off an agricultural recession from which U.S. farmers have not yet recovered, despite Reagan's lifting of the ban and despite big new grain purchases by the Soviets since last summer.

This year's paltry harvest brought Moscow eagerly back into the U.S. market, contributed to a one-year extension of the U.S.-Soviet grain sales agreement and led to a start on talks aimed at a longer-term trade pact--an event widely cheered by American farmers.

But the latest events in Poland have cast a shadow over that shiny new prospect for disposal of the burgeoning American grain crops.

In his address to the nation Wednesday, Reagan held out the possibility of sanctions against Moscow for its role in the Polish crisis. Before adjourning this month, the Senate passed a resolution urging a full trade embargo if the Russians intervened militarily in Poland.

Given repeated assurances by Agriculture Secretary John R. Block and others in the administration that agriculture alone would never be singled out as an embargo tool, farm groups assume that any future trade sanctions would cover more than farm goods.

The embargo-protection issue consumed hours of debate as Congress wrangled this year over a new farm bill. To make it tougher on an executive branch to single out farmers, the law requires costly direct payments to farmers or crop loans at high rates in the event of a "selective" embargo.