When President Carter took the unprecedented step Jan. 4 of embargoing U.S. grain shipments to the Soviet Union in retaliation for the invasion of Afghanistan, corn was selling in the farm belt for $2.40 a bushel and wheat for $3.87.

Last week corn was averaging 19 cents a bushel below that level, wheat was down 34 cents and the commodity tickers had become one of the barometers of potential political and economic trouble for the Carter administration this election year.

Many farmers and agricultural industries which once loyally supported the president's anti-Soviet action -- and applauded his pledge to shore up prices -- have changed their minds. They say the president has not kept his promise to cushion them against the impact.

The American Farm Bureau Federation, the largest U.S. farm organization, has withdrawn support, saying that its members are "the hardest hit among the citizens making economic sacrifices."

And embargo critics in industry and grain point to the depressed prices as evidence that the government's use of grain as a diplomatic level is a counterproductive effort that hurts this country more than the Soviets.

For their part, officials of the Department of Agriculture acknowledges that there have been snags and miscalculations in connection with the Jan. 4 action. But they insist that U.S. agriculture's grievances this spring flow from causes that are deeper and broader than the loss of the Soviet market.

"A whole series of factors is involved," said the department's chief economist, Howard W. Hjort. "Tight farm credit, last fall's rail transportation problems, the normal seasonal slump and the concern over the commodity markets caused by silver are all contributing to low prices," he said.

This is true: agriculture is one of the first sectors of the economy to have felt the pinching effects of the president's anti-inflation policies, particularly scarce credit and high interest rates.

Cattlemen, hog farmers and chicken producers, none of whom is directly affected by the grain embargo, have experienced plunging prices and heavy losses in recent weeks.

These farmers, whose businesses are financed with borrowed money, have been slaughtering their animals rather than pay high interest rates.That means cheap beef, pork and chicken for consumers now, but high prices later, when livestock supplies have been thinned out.

In the grain-growing parts of the nation, high interst rates also have tended to push more commodities off farms and into markets, depressing prices. Credit is reported to be extremely tight in the Dakotas, Iowa, Minnesota and Nebraska, so farmers in those states have been selling their stocks of grain to local elevators at distress prices to raise cash for taxes, new seed, fertilizer and equipment.

Still, officials at the Department of Agriculture say they are puzzled by the depths to which prices have fallen. World demand is high relative to grain stocks, most of which are still located in the United States; it seems a seller's market.

Even after subtracting 17 million tons of lost Soviet sales, foreign countries are expected to buy more than 100 million tons of last summer's harvest, a record volume that is 10 percent more than in 1979. Exports of corn and animal feedgrains, of which the Soviets were to be heavy buyers, will be up to 69 million tons from 57.1 million last year.

But farmers care about local prices, not world export volumes, and for many of the frustrated the embargbo presents a big target.

"It's killng us here," said Mark W. Seetin, Minnesota's commissioner of agriculture. " wish I could blame interest rates, but I'm telling you that that the Russians are paying premium prices to Canada and Argentina that our farmers should have had."

For Seetin, the grain embargo is the final blow after a long strike by grain handlers at the port of Duluth last summer kept large amounts of Minnesota barley, corn and wheat blocked inland, with no access to markets abroad.

Corn sold recently at Minnesota elevators for less than $2 a bushel, a price close to what it was in the early 1970s, before double-digit inflation and rising farming costs.

But if the embargo is to blame for the current situation, officials ask, why did grain prices rebound in February, a month after the president's action?

After a sharp initial drop, wheat futures prices, which are higher than actual farm prices because they include storage costs and transportation to delivery points, exceeded their Jan. 4 level, and corn futures came back to nearly that level, before starting their current slide.

Industry and farm critics say the initial rebound came because the argricultural community expected the administration to keep its promise to move quickly to shore up prices. When the administration delayed, prices plunged.

Only now, the critics say, is the government finally beginning to move, a fact that could account for a minor rally in the commodity markets at the end of the week.

Not since the bygone days of giant grain surpluses in the 1950s and 1960s has the federal government had such sweeping authority to affect prices.

That is a consequence of steps taken at the time of the embargo.

The president's Jan. 4 action, taken for "urgent reasons of national security and foreign policy" denied the Soviets all but 8 million tons of the 25 million tons of U.S. grain they had planned to buy between last Oct. 1 and Sept. 30, 1980. Eight million tons was the minimum established under a five-year Soviet-American agreement that expires in 1981.

When the president acted, however, commercial grain companies already had shipped some 5 million tons from U.S. ports and had contracted to sell the Soviets 16.2 million tons more. To limit total Soviet purchases to 8 million tons, the government has to block delivery of 8.9 million tons of U.S. corn and 4.3 million tons of U.S. wheat for which the Soviets had signed contracts.

Officials say that the companies had already procured most of these commodities -- either from farmers or in the futures markets.

To prevent the commercial companies from dumping this unsold grain on the markets, a move that was sure to depress prices, the government announced Jan. 7 that it would assume the cannceled contracts.

With that, the Department of Agriculture took on the role of a huge grain company, agreeing either to accept delivery of the commodities destined for the Soviets, or, like a middle-man, to resell the contracats to other private companies as the delivery periods specified in the documents arrived.

The government's aim to extricate itself slowly from its embargo-related involvement in grain markets.

So far, though, the Agriculture Department has postponed its day of reconing with the commerical grain companies by extending the delivery dates on all the corn and wheat contracts, at least through May. By then, officials hope, prices will have risen to a level where the contracts can be resold to private traders without depressing the markets.

Unlike private grain companies, the Department of Agriculture can legally manipulate the grain markets by buying commodities, changing price-support levels and other steps.

But in the view of a number of farm economists, the department has been too slow to do this. In this view, the factor that is depressing the markets is lack of confidence in the Carter administration to take effective measures. i

Although the department has $2.5 billion allocated to buy corn and wheat as a means of shoring up prices, it has purchased only small amounts of these commodities since January.

"There is a general aura that the government hasn't made good on its promises and probably doesn't know how," said one leading economist.

In early February, the department suffered a major embarrassment when it offered to purchase corn. Bids were received frfom 5,000 bidders in 19 states offering 1.3 million tons. However, chagrined officals said later, the bidders set unrealistically high prices on their corn, so only 7,000 tons actually were purchased.

Soon after that the corn prices began to sag.

Now, officials insist, they are gearing up for major buying that will pull millions of tons of grain off the market and, possibly, push prices up to where they were earlier.

In addition, farmers since January have added 5 million tons of corn to their privately held reserve, bringing it up to 19.6 million tons, and Congress last week authorized more farmers to use the reserve than now do. This farmer-owned corn is kept off the market until the price reaches a minimum of $2.63 a bushel, while the government pays storage costs.

Though Carter has won easy primary victories in such key farm states as Kansas, Wisconsin and Iowa, the approach of the November election could signal new administrative farm initiatives.

For this reason, officials say, it is too early to pass judgment on the success or failure of the anti-Soviet grain embargo, not just this year but future years as well. Weather, politics, and the cooperation of other grain exporting countries all will have a bearing.

Since the president's action was announced, the always rumor-filled international grain trade has outdone itself with reports of diversions of U.S. grain to Soviet agents in Europe, and of other countries making huge sales to the Soviets at premium prices.

We have not uncovered a single case of an actual diversion, and I don't think diversions are taking place," said Thomas R. Saylor, associate administrator of the Foreign Agriculture Service.

In several cases in which U.S. grain or soybeans were reported to have been transshipped to the Soviet Union via the port of Rottendam, the commodities turned out to have been licensed before the embargo, he said.

The Department of Agriculture's Jan. 7 agreement with the commerical grain compaines required the firms and their foreign affilitates to comply with U.S. export restrictions, as well as restrictions imposed by other governments on exports to the Soviet Union.

"I would be very surprised if the multinationals will risk breaking the law," said Christian Zimmerman, who is handling negotiations with the companies over the canceled contracts.

USDA and CIA estimates currently predict that the Soviet Union will be 4.5 million tons short of its target of 35 million tons of imported grain for the year ending June 30. The Soviets wanted the grain mainly to feed farm animals and produce meat.

By Sept. 30, as the effects of the U.S. embargo deepen, the Soviets will be running 10 million tons behind their import goal for the year ending Sept. 30, these estimates suggest.

Much depends on getting other big grain-exporting countries to honor a Jan. 12 agreement not to replace the lost U.S. imports with their own sales to the Soviet Union.

The Department of Agriculture already has protested a Feb. 29 Canadian announcement of a 2-million-ton-sale of wheat, oats and barley.

Argentina, whose government is angry at the Carter administration for trade and aid sanctions imposed under the human rights policy, has indicated that it will not attempt to limit the grain exports.

U.S. officials say that this could result in a jump in Argentine sales to the Soviets from 1.6 million tons in 1979 to more than 5 million tons this year.

However, the Aregentine corn and sorghum crop is turning out to be poor because of drought, and U.S. officials still hope that Argentine exporters will continue to supply regular cutsomers in Japan and Europe rather than take windfalls profits in this year's Soviet trade.

Even without the Soviet market, many forecasters see a growth of U.S. grain exports that could put increasing strains on the farm economy throughout the 1980s.

"The economic factors that produced the phenomenal growth in U.S. grain exports [in the 1970s] continue at work," according to Robbin S. Johnson of Cargill, the world's leading grain trading company.