The Soviet Union apparently has failed to fulfill an agreement to buy minimum amounts of U.S. grain, but a top Agriculture Department official said yesterday that the United States can do little about it.

Although Soviet purchases in the fiscal year that ended Monday were a record of at least 18.6 million metric tons, the total apparently fell about 1.1 million tons short of the 4 million tons of wheat Soviets are are obligated to buy under a long-term grain trading agreement.

The agreement, an extension of a series of pacts signed with the Soviets since 1975, sets minimum purchase levels and stipulates terms for sales beyond those levels but contains no penalty provisions.

Richard Goldberg, a deputy undersecretary of agriculture, said yesterday that the Soviets did not meet Monday's deadline on wheat purchases but said that "technically it still could happen."

Goldberg said, "We consider it a breach and we're disappointed." He noted that Agriculture Secretary John R. Block, in a meeting with Soviet officials in Moscow a month ago, received assurances that the agreement would be met.

Goldberg and other officials indicated that the pact could have been met but that the USDA has not yet learned about it because of a time lag in sales reporting by grain-trading companies. Goldberg said the department would know more "in a couple of days."

Block, responding to questions yesterday at a forum, said he could provide no details about the Soviets' reported failure to comply.

The agreement requires Moscow to buy at least 8.5 million metric tons of grain and soybeans each year, including 4 million tons each of wheat and corn. Corn purchases of 15.8 million tons in the 1984-1985 year far surpassed the requirement, but only 2.9 million tons of wheat sales have been reported.

Goldberg said he agreed with speculation the Soviets may have balked at wheat purchases because the U.S. price is higher than the world price. U.S. corn is more competitive on world markets.