WASHINGTON (AP) _ The Soviet Union has bought an additional 750,000 metric tons of U.S. corn for delivery under a newly extended supply agreement between the two countries, the Agriculture Department said Monday.

The latest sales brought total corn sold to the Soviets to more than 6.28 million tons in the agreement's sixth year, which began Oct. 1. Last year, U.S. corn sales to the Soviet Union totaled slightly less than 5.2 million tons.

A metric ton is about 2,205 pounds and is equal to 39.4 bushels of corn or 36.7 bushels of wheat or soybeans. The 750,000 tons of corn would be about 29.5 million bushels.

Corn prices have been averaging about $2.58 per bushel at the farm level, according to USDA estimates, meaning that the latest sale could have a farm value of around $76 million.

Although no wheat has yet been sold for 1988-89 delivery under the pact, the Soviets have bought 500,000 tons of soybeans and 835,000 tons of soybean meal, plus 500,000 tons of grain sorghum.

The United States and the Soviet Union on Nov. 28 signed an extension of the previous five-year grain agreement, which had expired on Sept. 30. The extension of the previous pact will run through Dec. 31, 1990.

Under terms of the agreement, the Soviet Union is committed to buy at least 9 million tons of U.S. grain annually, including a minimum of 4 million tons each of wheat and corn.

The Soviets have the option of buying the remaining 1 million tons as wheat, corn or soybeans, with every ton of beans counting as two tons of grain - up to a maximum of 1 million tons of grain equivalent.