President Reagan promised American grain farmers today that they will be able to sell a "record volume" of corn and wheat to the Soviet Union this year as he sought to placate them and ease their concern over the recession.

Reagan's normally harsh anti-Soviet rhetoric was considerably toned down in a speech to the National Corn Growers Association, as he sounded almost apologetic that he had to suspend negotiations on a new long-term grain agreement with the Soviets to pressure them to ease martial law in Poland.

"We had no choice but to impose a number of sanctions" against Poland and the Soviet Union after the military crackdown last winter, he said.

"There is still no cause to celebrate in Poland," he added.

"I am, however, somewhat encouraged by indications martial law may be relaxing."

This was an obvious reference to the announcement last month that the military government in Poland would release some of those imprisoned for political reasons in the crackdown.

But, in a speech that was a paean of praise to the importance and industry of the American farmer, Reagan said that under the one-year extension of the current grain agreement that his administration will seek with the Soviet Union, farmers will "be able to sell large quantities during the next year."

"In other words, the granary door is open and the exchange will be cash on the barrelhead," he said, drawing applause from the more than 8,000 farmers at the corn growers' convention.

Reagan promised the grain farmers, who are facing major economic problems because of overproduction, that his administration will vigorously seek to find new markets abroad for their harvest. He said the administration already has dispatched trade teams to 23 nations in Europe, Africa, Latin America, the Middle East and the Far East to push for more export sales.

At the same time, he said the administration had formed a "united front" to challenge the "unfair competition" from nations that subsidize agricultural exports. He said he also intended to push European Common Market nations to liberalize agricultural trade and will take steps to have farm products included in the General Agreement on Tariffs and Trade (GATT) talks this fall.

The trip here and a stop Tuesday in Connecticut where he will address the Supreme Council of the Knights of Columbus, are political fence-mending missions to shore up constituencies that supported Reagan in 1980 but are now tending to wander off the reservation.

Sharply criticized for ignoring the plight of farmers when he stumped for his "New Federalism" program on his last visit to Iowa in February, Reagan went to considerable lengths in his speech to do so today.

"The farmers of America are very much in my mind," he said. He read from letters he had received describing farmers' problems and he praised them for their effiency and pluck, saying at one point that the American farmer is a "bigger risk taker than Evel Knievel."

The farmers who gathered in the cavernous Veteran Memorial Auditorium here to listen to his speech frequently interrupted his remarks with applause but afterwards expressed reservations.

"Very good speech. Nothing new. No solutions," was the way Lloyd Ritland who grows corn and soybeans on a 600-acre spread near here summed up his reaction.

The Des Moines Register published a statewide poll this morning that indicated that 8 of 10 Iowans see no sign of the recession ending. Only 46 percent approved of the way Reagan was handling the job, with 43 percent disapproving.

Sounding the GOP rallying cry in the upcoming election campaigns, Reagan blamed former President Jimmy Carter and past generations of Democrats for pursuing a "reckless course of fiscal insanity that had us careening toward catastrophe". He argued that over the long haul his budget and tax cuts and a constitutional amendment making it difficult to unbalance budgets will sort the problems out.

In the prepared text of his speech, Reagan said, "This administration does not have, nor will we have, a grain embargo on the Soviet Union."

But when he spoke, that promise was deleted because he always adds and subtracts things as he gives a speech, Reagan said at first. Later, however, he said "I had that covered later in the speech" and the pledge was redundant.

Earlier, however, White House chief of staff James A. Baker III said it had no business being in the speech because it was a "categorical statement" that conflicted with administration policy.

That policy, which Reagan repeated in his speech, is that grain farmers would not be singled out to bear the burden of future embargoes. The Reagan administration, if it sought to use trade as a weapon against the Soviets, would have a more broadly based embargo and would act jointly with allies, he said.