A Soviet trade mission has arrived to negotiate the purchase of Argentine grain less than a week after the military government promised not to take advantage of the partial U.S. grain embargo against the Soviet Union.

Well-informed observers said that the arrival of three Exportley officials yesterday does not necessarily mean the Soviets are planning to buy more grain from Argentina this year than last.

Nor, these observers said, does the fact that the Soviets are here mean Argentina is reneging on its pledge last week not to take commercial advantage of the U.S. embargo, announced three weeks ago by President Carter after the Soviet invasion of Afghanistan.

But the visit is nonetheless being watched closely by the United States and other Western countries to determine whether the Soviet Union will attempt to offset its lost American feed grain by buying here and whether the Argentine government will intervene if that turns out to be the case.

The Soviet delegation began meeting with private grain trading companies that control Argentina's grain exports. It was understood that the exportley officials are primarily interested in buying corn, of which Argentina is the second largest exporter after the United States.

Last year, the Soviet Union purchased about 1.8 million tons of corn here and informed sources said the Soviets had indicated, before the U.S. embargo, their interest in purchasing at least 1.2 million tons this year.

It had been expected that Argentina would have 5.5 million tons of corn available for export this year. But informed sources said the figure could be significantly less if a drought in this country's corn-growing areas does not end soon.

In addition, Argentina should have about 3.2 million metric tons of sorghum and 3.5 million metric tons of soybeans to export. Last year, the Soviet Union did not buy either of these crops, which also can be used for livestock feed.

President Carter sent Lt. Gen. Andrew J. Goodpaster, a former supreme commander of NATO forces in Europe, to Argentina and Brazil last week after the administration belatedly recognized that both countries could largely undermine the embargo by selling their crops to the Soviets.

Argentina's government, which initially said it would not interfere in the essentially free market for its crops, issued a statement after the Goodpaster visit saying it would try not to take advantage of the U.S. embargo -- as long as its traditional purchasers continued to buy here.

Brazil took a harder position, refusing to consider any limitation on its agricultural exports to the Soviet Union.