The Soviet Union has been given another go-ahead to buy more grain from the United States, the Agriculture Department said Tuesday. Undersecretary Richard T. Crowder said Moscow now has permission to buy up to 22 million metric tons of U.S. grain without further consultations, an increase of two million tons. A long-term grain agreement between the two countries specified initially that Moscow could buy up to 12 million tons of grain a year without further U.S. approval. That limit was raised to 16 million tons last October and to 20 million tons in December. The new limit of 22 million tons applies to the 12-month period that began Oct. 1. The agreement itself runs through Dec. 31. Officials said the increase resulted from talks with the Soviets in recent days and takes into consideration recent large purchases of corn by Moscow. In all, since Oct. 1, the Soviet Union has bought about 18.9 million tons of corn and wheat for delivery in 1989 and 1990. Those orders include more than 15.6 million tons of corn and 3.3 million tons of wheat. Purchases also include 342,300 tons of soybeans, 1.2 million tons of soybean meal and 7,300 tons of barley. In 1988-89, sales were a record of more than 21.7 million tons, including more than 16.3 million tons of corn and nearly 5.4 million tons of wheat. 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.