Over the ensuing six weeks the Russians bought up more than 10 million metric tons of corn and wheat. The biggest summer buying spree since 1972 has continued, with another large sale reported last Thursday. Analysts believe the sales will keep grain prices firm even if there is a bountiful crop. And if, as now seems likely, the United States harvest falls short of lavish early expectations, the Soviet purchases could help push corn prices up. Because of ample supplies, the price of hard red winter wheat probably won't rise, but the Soviet buying will help steady the price at about $3.90 a bushel.

While most grain purchases are tracked from July 1 to June 30, the five-year grain agreement signed with the Russians last year runs on an October-to September calendar. For the year ending Sept. 30, the Russians have so far snapped up 14.4 million metric tons of corn and wheat, and they have already contracted for 8.8 million tons in the next year, most of it corn.

Under the terms of the 1983 pact, the Soviet Union must buy at least 9 million metric tons of grain a year, and are permitted to buy as much as 12 million metric tons. (A metric ton is about 2,205 pounds.) The Administration gladly waived the ceiling for the current year and is expected to do the same for the next year after a consultation scheduled for Nov. 20.

MR. Malish said the consultation was postponed from October at Soviet request - presumably because the Russians did not want to hand Mr. Reagan the added satisfaction of announcing even more grain sales before the election.

Based on estimates of Soviet crops and Moscow's eager advance purchases, forecasts of total purchases in the grain-buying year that will begin next month range from 15 million to upwards of 20 million metric tons, with most analysts closer to the more optimistic figure.

Economists differ on how much of a financial windfall the Soviet purchases will bring to American farmers. That depends largely on how much farmers have to sell, the most basic price determinant. The last official harvest forecast, issued by the United States Department of Agriculture last month, foretold a bumper crop of 7.67 billion bushels of corn, and the third-largest wheat harvest in history. But bad August weather has made those forecasts look somewhat optimistic. (See box.)

''We were looking at our biggest corn crop we'd ever had back in the middle of July,'' said Morris Johnson, who farms 400 acres south of Moline, Ill. ''Then it stopped raining.''