In France, the government guarantees wheat farmers more than $5 a bushel for their grain. Not surprisingly, French farmers produce a lot of wheat -- enough to feed their own countrymen, other Europeans and some Africans and Latin Americans as well.

The guarantee in the United States is more than $1.50 below that. Anything U.S. wheat farmers make above this minimum depends mainly on how much surplus grain the country can sell abroad.

But the more the Europeans, the Japanese and some wealthier developing countries subsidize and protect their farmers, the more difficult it is for the dwindling ranks of U.S. farmers to export.

These realities define the limits of the possible for U.S. negotiators who are about to wind up five years of talks in Geneva aimed at easing agricultural trade restrictions as part of a global trade package.

Once again, as has been the case in every major trade negotiation since World War II, an irresistible force -- America's surging exports -- has been pitted against an immovable object -- European and Japanese agricultural protectionism.

It is a contest in which powerful vested political interests have dug in on both sides, making movement extremely difficult.

Given this economic fact of life, American officials says they are gratified with their progress.

They cite a number of concessions to buttress their claim that, true to the promise of the president's special trade representative, Robert S. Strauss, the Carter administration has gained more for U.S. agriculture in this round of talks than any previous one.

Producers of tobacco, poultry, citrus fruit, peaches, rice and possibly almonds stand to benefit from trade concessions obtained so far.

And the talks have opened up the intriguing possibility that both Europe and Japan eventually will become major markets for choice, grainfed American beef. The Europeans, who are becoming increasingly concerned about their own high food prices, have agreed to lower tariffs on steaks, ribs and other U.S. beef delicacies hitherto little known there. Similarly, the Japanese have agreed to increase their quotas from 16,000 tons in 1978 to 31,000 tons in 1983.

U.S. officials say that if the Europeans get hooked on American-style cuts they could sharply increase their beef consumption from the present 57 pounds per person a year to something closer to America's 120 pounds.

Agricultural concessions obtained from the United States' trading partners so far apply to trade currently totaling about $3.5 billion a year. Officials estimate that the concessions could increase U.S. farm sales by as much as $700 million a year. (Trade officials say the impact on domestic food prices should be negligible. The more food exported, the less left behind. But most crops are now in surplus.)

Nevertheless, the trade concessions are a drop in the bucket compared with the nation's $27.2 billion in farm sales in 1978.

Of that amount, nearly $18 billion was from the sale of grain (wheat, corn, barley, sorghum, and rice) and oilseeds (primarily soybeans). In this sector, the negotiators have had limited success.

Japan has agreed to make permanent its present practice of not placing duties on U.S. soybeans -- a concession that could be more valuable than it looks since it rules out Japan's discriminating against U.S. soybeans in favor of Brazilian beans.

U.S. negotiators also are pressing the Franch to go along with a subsidy code that could curb its dumping of wheat in such favorite American markets as Morocco and Brazil.

But beyond that, the fundamental obstacles that prevent a really massive increase in U.S. grain exports to Europe (and to a lesser extent to Japan) have not been budged by the last five years of talks.

These obstacles have less to do with trade policy than with the internal agricultural systems of Europe and Japan.

In the United States, where labor has been costly and capital relatively cheap, government policies have encouraged the development of big, efficient farms. Capital and technology in the postwar era fueled dramatic increases in productivity, which in Turn, produced huge surpluses that had to be sold abroad.

It also encouraged a relentless concentration of farm holdings as farmers sought to take advantage of the benefits of equipment and technology. Today some 165,000 commercial farms produce about half the nation's food.

Families still operate most of these farms, but as the costs and risks of running them have increased, so have the demands for more government support.

U.S. farm policy has been underpinned by relatively low price supports, which were seen as encouraging the exportation of crops. The Kennedy administration dropped corn price supports to world market levels in 1961, and U.S. wheat prices have been the same as world prices since 1972.

Exports have been the key to U.S. agricultural prosperity. Today, the nation exports three bushels of its rice out of four, more than one bushel of wheat and soybeans out of two, and one bushel of corn out of four.

But as this system was emerging in America, the Europeans and Japanese were following diametrically opposed policies.

Their objective for most of the postwar period was self-sufficiency and massive support for small farmers regardless of the budgetary costs or the inefficiency or many of the less-productive farms.

The underpinning of the system was extremely high price supports and, in Europe, huge tariff barriers to imported wheat and corn.

Common Market officials justified their "common agriculture policy" as beging in the interest of preserving family farmers and preventing social disruption caused by the massive flight of people from rural to crowded urban areas.

America has been able to market its wheat and corn in Europe only when heavily subsidized European farmers have sold their grain, and imports are needed to make up the deficit. Because of the high support prices, Europe now has a wheat surplus. And a tariff of $80.71 a ton is now being tacked onto American corn valued here at $120 a ton. The effect of these two things is to make it increasingly difficult for U.S. grain to move into Europe.

The Carter administration has abandoned the unsuccessful pressure tactics used by previous administrations to get the Europeans to dismantle their agricultural system.

In effect, trade officials have acknowledged that the vested political interests in the European system are so powerful that outside pressure is futile.

Thus, the fortunes of U.S. grain farmers will be determined in Paris and Brussels as well as in Washington. This is a fact that adds to the frustration of the western grain farmers who have come to Washington to advertise their problems and demand price supports along European lines.

Fred Sanderson, a trade specialist for the Brookings Institution, describes resistance to more basic reforms of agricultural trade as "tremendous," but adds that change could come gradually.

Farm blocs in Europe are still politically powerful despite a steady decline in the number of farmers there, too. Farmers, for example, form a power base for the coalition government of French President Valery Giscard d'Estaing, and Bavarian farmers also hold a key to delicately balanced power alignments in West Germany.

However, financial pressures have begun to make more likely the policy changes that could open Europe to more U.S. grain. The subsidy systems keep European food prices high and cost the European Common Market $13.5 billion a years.

Significantly, the Common Market held the increase in farm price supports last year to 2.1 percent. And the European Commission's vice president, Finn Olav Gunderlach, has taken a firm position in favor of a price freeze in 1979 and 1980. This could bring European and American grain prices closer.