The United States informed the Soviet Union yesterday that it could buy nearly twice as much corn and wheat in the next 12 months as authorized in the two countries' 1975 grain agreement because of large surpluses on American farms.

THe lifting of the 8-million-ton ceiling, which had been in effect since last October, was announced by Assistant Secretary of Agriculture Dale E. Hathaway at a meeting with senior Soviet trade officials.

Hathaway said afterward that the decision to let the Soviets buy up to 15 million tons without further consulation with the U.S. government had been approved by other parts of the administration, including the White House. This tonnage is the equivalant of 10 per cent of U.S. grain and soy-bean exports.

Huge, unexpected Soviet grain purchases here strained relations between the two superpowers from 1972 until 1975. In October, 1975, the United States negotiatied an agreement at least 6 million tons annually.The same agreement sent the ceiling on Russian buying at 8 million tons, unless Washington expressly increased that maximum volume.

This year, there are strong pressures on both the United States and the Soviet Union to liberalize those trading rules.

The Carter administration is facing a bill of as much as $8 billion for price-support programs and payments to farmers suffering from the lowest grain prices since 1972. The new administration farm bill committs the government to substantial payment to help these grain farmers. But if foreign customers begin buying more grain prices are likely to rise and the amount of government aid to farmers would go down.

For its part, the Soviet Union may want to increase its buying while prices are at their present low levels.

Russian traders contacted private grain companies this week, reliable sources said. Hathaway said yesterday that the Soviets did not revel wheter they planned to buy more than the 6-million minimum requirements, but said, "It is quite possible."

The Department of Agriculture now estimates that the 1977 Russian havest will be 215 million tons of grain. Officials say that is a good corp, although it is less than the 1976 record of 224 million tons.

The Soviet Union has seldom harvested three good crops in a row since 1945. Therefore, some officials speculated privately that the Russians may want to buy more grain than they need this year as a precaution against a 1978 harvest failure.

Polish, a close Soviet ally which imports between 1 and 2 million tons a year from Russia, has suffered severe harvest setbacks in recent months.

Polish requirements could be a factor in a Russian decision to step up their purchases. The country is expected to buy 6 to 7 million tons of grain this year, making it one of the world's largest grain importers.

Poland spends an estimated 100 billion zlotys a year - about $3 billion at the tourist exchange rate - to subsidize retail food prices. Although the Warsaw government increased prices paid to grain farmers last year, the output of the farmers did not increase.

At the same time, the government withdrew food price increases after riots in 1976. Thus, the margin between what the government pays farmers for food and what it sells the food to Polish consumers for widened.

The U.S. government gave Poland $150 million in food-buying credits earlier this year, but the regime in Warsaw is reported to want another $150 million to help it pay its food import bill. That matter may be settled before President Carter visits Warsaw next month.