The head of a U.S. delegation here to discuss Soviet purchases of American grain said today the Russians have bought only 6 million metric tons this year, the minimum under a bilateral agreement, and blamed the Carter administration's grain embargo for "long-term damage to our trade" with Moscow.

The announcement, made by Acting Undersecretary of Agriculture Alan Tracy at the end of the latest round of Soviet-American grain talks, provided little good news for American farmers, who currently are suffering from a grain surplus.

Under the terms of the 1976 agreement, the Soviets agreed to purchase a minimum of 6 million metric tons a year, with an option for 2 million more. In October, three weeks before congressional elections, President Reagan said he would allow the Soviets to buy as much as 23 million metric tons in the current crop year that ends in September.

While American farmers held nearly 75 percent of the Soviet grain import market in the year prior to the embargo, that share is now expected to drop to about 25 percent.

The embargo was imposed by former president Jimmy Carter in retaliation for the Soviet invasion of Afghanistan. Carter's cancellation of contracts for 17 million metric tons of grain--above the exempted 8 million metric tons provided by the 1976 U.S.-Soviet grain pact--was the first major use by the United States of food sales as a weapon in foreign policy.

While President Reagan lifted Carter's embargo in 1981, he also has used export controls as a foreign policy tool, most notably in the case of the abortive sanctions against the Siberian natural gas pipeline.

The Soviets have become more wary in their dealings with the Americans and have raised the question of embargo use in virtually all bilateral forums, including the latest talks.

While continuing to buy the contracted amounts of U.S. grain, the Russians are believed to view grain trade as one place where they have some leverage for retribution, including the possibility to stir political trouble for the administration in the farm belt.

An American statement issued after today's talks said Soviet "difficulties with dependability of supply had contributed to lower levels of total imports."

The statement said that the American negotiators had pointed out to the Russians that the decline in Soviet purchases of U.S. grain had led indirectly to reduced production and that this could bring about an increase in world grain prices.

The Soviet Union is the world's largest importer of grain, while the United States is its largest producer. The Russians have diversified their purchases during the past several years, and their principal suppliers of grain are now Argentina and Canada.

Moscow also has complained about the quality of U.S. grain. Presumably due to inadequate inspection, some U.S. wheat shipments have been affected by a fungus-type disease known as "scab." Its discovery had led to fears here that the wheat shipments may also be affected by a poisonous mycotoxin associated with "scab."

Tracy said he was able to ease Soviet concerns about the quality of U.S. wheat shipments. "We have some new measures in place as of a few days ago," apparently to improve quality controls, he added.

Tracy's meeting with U.S. journalists stood in sharp contrast to a similar session last fall when his predecessor, former undersecretary of agriculture Seeley Lodwick, held out the hope that the Soviets would purchase at least 18 million metric tons in the current crop year.

"There is no question that we have lost our predominant share" of the Soviet import market, Tracy said. He added that additional U.S. sales may depend on the Soviet winter crop. Its prospects are not known at this time.

In the 1981-82 crop year, the Russians purchased 14 million metric tons of U.S. grain. Even that large amount comprised about 32 percent of Soviet grain imports for the period.

In the meantime, Moscow has sharply increased its grain purchases in Argentina, Canada and Western Europe. Additional food imports come from Brazil, Thailand, Australia and Eastern Europe.