For the first time since the early 1970s, the government is preparing to pay large numbers of farmers not to grow grain.

A new farm strategy, now on President Carter's desk, would reverse the policy the Department of Agriculture announced only two months ago of telling farmers to "plant fence row to fence row" and grow all the grain they could.

The president's decision to partially embargo grain sales to the Soviet Union and an unexpectedly abundant harvest have forced the sudden turnaround in farm policy.

The switch in signals would add a little to the federal budget deficit and would cost consumers money by artificially propping up food prices, which had been expected to decline this year.

The key tactic would be to pay farmers not to plant at least 3 million to 4 million acres of corn this spring. Agricultural economists figure the government would have to pay farmers at least $60 million to take that much land out of production.

A modest program to cut this spring's wheat harvest and other efforts to curb production of some minor crops are also under consideration. They would cost about $30 million.

No decision has been made on the new policy, but it was recommended to President Carter this week. He is expected to act on it before Monday's Democratic precinct caucuses in Iowa, the leading corn-growing state.

Paying farmers not to grow grain was the cornerstone of federal farm policy for many years. Since 1974, however, when the Soviets unexpectedly stepped in and bought up America's surplus grain, the government has moved away from production curbs.

Instead it has been able to support farm incomes by encouraging exports of grain. The 17 million metric tons of grain that the president withheld from the Soviets two weeks ago were to have been sold under a five-year program that has meant billions of dollars to American farmers.

To protect farmers from the loss of Soviet business, the administration has been forced to promise to buy the Russians' grain and hold it off the market.

That takes care of last year's record crop, but planting time is coming soon in the Corn Belt and farmers have indicated they want to plant even more land to corn this year.

Returning to a paid cropland diversion program would protect farmers from a precipitous drop in their incomes in this election year.

While no decision on the diversion program has been made, Agriculture Department officials have hinted at it repeatedly in the daily briefings they give on the grain situation.

Details of the administration's thinking have been shared with members of the congressional Agriculture committees. There is strong support for the plan among the farm state senators and representatives who help shape agricultural policy.

But there is equally strong opposition from the White House Office of Management and Budget, which objects to the cost of the program.

Budget watchdogs believe the administration already has done enough to protect farmers from the impact of losing the Soviet sales this year.

But future sales to the Soviet Union also could be lost, either because the United States cancels them or because the Russians say, "no thanks," and turn to suppliers who will not cut them off in the future.

Also going into the equation is the administration's push to use grain to make gasohol. That could use up some of the surplus Soviet grain, but not much.

Department of Agriculture officials have promised a decision on the diversion program by March 1, but the longer the administration waits, the more such a program will cost.

Many farmers have already bought their seeds and fertilizer. To get them not to plant, the government will have to pay for what the farmers have spent.

The way the program would probably work, farmers would be paid not to plant a specific percentage of their land. Farmers who signed up for the program would also be guaranteed a certain price for their crops.

Price guarantees were offered to farmers this year, but only 21 percent of them signed up. The rest figured they could make more money planting the land and selling the grain.

To make the new diversion program more attractive -- and therefore more effective -- the administration would probably have to increase the amount of those payments.