After secret talks in London, the United States has agreed to sell the Soviet Union an additional 10 million metric tons of wheat during the next 14 months and has told the Russians that they might be able to purchase at least that much corn.

At the same time, Agriculture Secretary Bob Bergland announced that the government would remove acreage restrictions on wheat growers for the first time in three years.

Agriculture Department officials estimate that, because of poor crops and increasing consumption, the Soviets may need to import a record 30.5 million tons of grain in the next year, with about 20 million metric tons coming from U.S Farmers. A metric ton consists of 2,200 pounds.

Sales that large would exceed Soviet purchases during the 1972 Russian grain deal and would rival Soviet imports in 1975-76, when production there fell to a disastrous 140 million tons.

Under a 1975 agreement, the Soviet are required to purchase at least 6 million tons of U.S. grain and may buy up to 8 million tons. Anything over 8 million tons must be approved by the government.

This year the Soviets were given permission to buy up to 15 million tons. But with two months left in the marketing year, the Soviets have purchased roughly 11.5 million tons of corn and 3.5 million tons of wheat.

Under the agreement announced yesterday, the Soviets can buy another 2 million tons of wheat in the next two months and up to 8 million more in the next year of the agreement.

U.S. officials said they refused in London to give the Soviets a firm commitment on future purchases of corn because of the uncertainty of this year's crop. But Howard Hjort, the department's chief economist, said it is likely that the corn crop will be sufficient to let the Soviets purchase about as much as they have this year.

The department has estimated a near-record wheat crop of about 2.2 billion bushels and a corn crop of 6.6 billion bushels. Last year, U.S. farmers produced a record 7.1 billion bushels of corn.

The announcement is likely to raise wheat prices on the futures market, but the companion decision to lift acreage restrictions could offset that somewhat.

Hjort said yesterday the sales agreement announced will add about $500 million, or 0.2 percentage points, to consumers food costs, boost farm income by $1.8 billion, add $1.4 billion to U.S. export earnings and save $660 million in federal subsidies to farmers.

The Soviet grain crop has been damaged by poor weather this year, and the Agriculture Department estimates that the Soviets will produce about 185 million tons of grain, compared with 237 million tons last year.

"They are using more grain, so their shortage will be as great as ever," Hjort said yesterday.

Dale Hathaway, assistant agriculture-secretary for international programs, flew to London earlier in the week for discussions with the Soviets, who asked if they could purchase more than the agreed-upon 15 million tons this year. Hathaway said that because the U.S. winter wheat crop, which is being harvested, was so good, that would be no problem, and said the Soviets could take up to 10 million tons over the next 14 months.

Hathaway reportedly told the Soviets that the United States would consider requests for additional sales of corn in the fall, after the corn harvest is under way.

Bergland said the London talks weren't announced because they were "market sensitive."

Wheat and corn prices rose dramatically in June after the department predicted a sharp decline in Soviet production this year. Those prices have fallen somewhat in recent weeks because of the large winter wheat crop and the prospect of a good corn crop.

Despite the forecasts of good crops, however, department officials decided it would be prudent to remove acreage restrictions on wheat growers for next year. A poor crop here or abroad, coupled with huge Soviet imports, could sharply reduce worldwide stocks and raise prices, the same thing that happened after the 1972 sale.

The acreage restrictions were designed to keep prices from falling too sharply.

Bergland also raised the loan rate on wheat for 1980 to $2.50 a bushel, up from $2.35 this year. He said the target price, which determines federal payments to farmers, would be about $3.07 a bushel this year, down from $3.40 last year.