Argentina and Brazil, South America's largest grain exporting nations, indicated today they will not participate in the U.S. partial embargo on grain exports to the Soviet Union.

The immediate effect of the separate decisions to turn aside U.S. pleas to join the partial embargo appeared to be political, since it is unclear if Argentina and Brazil will in fact ship large new amounts of grain to the Soviet Union.

The two countries could theoretically make up much of the 17 million metric tons of grain that the United States is withholding from the Soviets. mBut Moscow would have to pay premium prices to get the South American suppliers to divert supplies from traditional purchasers.

Argentine Agriculture Minister Jorge Zorreguieta, declaring that his nation's position has been "erroneously interpreted" at a meeting in Washington on Saturday of grain exporters, declared: "Argentina will not participate in an embargo of grain to the Soviet Union.

"The final destination, either direct or indirect, of our grain, in a free economy like Argentina's, will be determined by the market."

Brazilian representatives came away from a meeting in Washington today of soybean exporters refusing to make any commitment to hold down exports to the Soviet Union. Two days ago, Brazilian Treasury Minister Karlos Rischbieter said in Sao Paulo that the U.S. decision not to sell grain to Moscow offered his country the opportunity to sell soybeans there.

On the Argentine side, today's strong declarations appeared to be a reaction to what was taken as misrepresentations by the Department of Agriculture on Saturday.

A U.S. diplomat there declared that the accounts of that meeting had indeed misstated the Argentine position.

"It was an attempt to put the best face on things," the U.S. diplomat here said, "A typical Madison Avenue PR job."

After the meeting of the world's major grain exporting countries, Under Secretary of Agriculture Dale Hathaway announced that "general agreement" had been reached not to take advantage of a unilateral decision by President Carter to embargo 17 millin metric tons of grain that the Soviet Union had sought in the United States.

Hathaway's statement gave the impression that the administration had achieved success in convincing Argentina and Brazil, the two most recalcatrant exporters, to go along with the embargo.

[A spokesman for the Agriculture Department acknowledged that the material presented at Saturday's press conference did not state the entire Argentine position. He said this was inadvertent.]

David Lacroze, the president of the Argentine Grain Board and this country's representative at Saturday's meeting in Washington, on arrival here gave this an Argentina's position: That is would not deliberately take advantage of the U.S. embargo by going out of its way to sell the Soviet Union.But the second part, Lacroze said, had not been contained in the communique -- the part which said that Argentina would make no effort to stop the Soviet Union from buying additional grain on Argentina's open grain exchange.

The question of whether Argentina should or should not go along with the U.S. embargo had become as important a domestic political issue here -- enmeshed with U.S. criticism of human rights violations.

U.S. Embassy officials say that in three important crops -- corn, soybeans, and sorghum -- Argentina is expected to have an exportable yield this growing season of 12 million tons.

The Soviets do not need large quantities of wheat, according to experts here, because the Carter administration agreed to ship the 8 million tons of wheat already contracted. Most of the embargoed 17 million tons or cereals consisted of corn, soybeans and sorghum, these experts say.

Staff writer Jerry Knight reported from Washington:

Monday's meeting here of representatives of the three largest producers of soybeans, the United States, Brazil and Argentina, produced only a noncommital communique -- in contrast to the upbeat declarations Saturday.

The Agriculture Department said only that Brazil and Argentina were asked to hold soybean sales to the Soviets to "normal trade levels."

A spokesman for the Brazilian Embassy said his country had attended only to gain information about U.S. actions. It was understood that Brazil will make a delcaration later. Treasury Minister Rischbieter said on Saturday that the situation was one of the few in which Brazil could benefit from an international crisis.

The Agriculture Department estimates that Brazil will grow 13.5 million metric tons of soybeans this year and Aargentina will produce about 4.5 million tons. Brazil's exports of soybeans and soy products to the Soviets have been as much as a million tons a year in the past, but Argentina has made no significant sales to Moscow.

Before Carter's embargo, the U.S. expected to sell the Soviets 1.4 million tons of soybeans 400,000 tons of soybean meal and 79,000 tons of soy oil this year. The suspension will cancel all the meal sales, 30,000 tons of oil and 740,000 tons of beans.

While the South American exporters run a calender of crop harvests contrasting with the north -- where commitments on upcoming crops largely have been made by now -- Argentina and Brazil both have informal commitments to traditional customers that would be broken only in the face of much higher bids from the Soviets.

Tom Saylor, deputy assistant secretary of Agriculture, said that there had been a discussion with the Brazilians of "trade flows," including Brazil's large wheat imports from the United States. Brazil imports nearly four million tons of wheat a year from the United States, while exporting its own soybeans.