According to the Agriculture Department, the Soviets purchased an additional 500,000 metric tons of corn--about 19.7 million bushels--for delivery through Sept. 30. USDA officials said the latest sale was reported to the department by private exporters, as required by law. No prices or other details were disclosed in the announcement. However, the department said that the latest estimated farm price of corn is about $2.70 per bushel. A metric ton is about 2,205 pounds and is equal to 39.4 bushels of corn or 36.7 bushels of wheat. Thus, the latest sales would have an estimated U.S. farm value of about $53 million.