The international trade talks about have been going on in Geneva for four years have come down to a question of apples and oranges.

And chickens and beef and tobacco and wine.

Since the talks began, farm exports have been one of the major stumbling blocks to a successful treaty, with the United States seeking greater access to world markets for its agricultural products.

Now, with the major industrialized nations trying to reach agreement in principle on a new treaty as the economic summit in Bonn approaches, farm exports again stand on the way.

The farm export issue reappeared this week when Robert S. Strauss, the Carter administration's chief trade negotiator, called reporters to a breakfast news conference at the White House and warned that, unless Japan and the European community show more willingness to open their doors to American farm goods, there can be no trade agreement. He said he doubted that a compromise could be reached in time for the Bonn summit, which begins July 16.

Trade officials in the administration said it was a deliberate and calculated effort by Strauss to prod the Europeans and Japanese into conditions on various farm products - from citrus fruits to vegetables to tobacco to wine - that could be used to sell the treaty to Congress next year.

At the same time, it highlighted the importance of agricultural trade to the U.S. economy.

The United States exports about $24 billion worth of farm products and accounts for more than 16 percent of total farm exports in the world. It exports almost three-fifths of its wheat, more than half of its soybeans and rice, more than a third of its cotton and tobacco and more than a quarter of its corn.

In contrast, the United States imported about $13 billion in farm goods, including coffee and sugar, last year, leaving a net agricultural trade balance of about $11 billion.

The value of U.S. farm exports rose dramatically during the early 1970s as worldwide crop shortages pushed up prices and created additional demand for American grain. But this has stabilized. At the same time, the value of imported farm products has continued to increase. As a result, the relative trade balance in agriculture has shrunk, meaning there is less to offset the growing cost of other imported goods, from oil to machinery to manufactured goods.

In addition, several potentially significant changes have occurred in the pattern of U.S. imports. A comparison of import figures for 1977 and early 1978 shows that oil is no longer the leading imported product. It has been overtaken by machinery and transport equipment and by manufactured goods.

Oil, which had been the biggest drain on the U.S. trade balance, is now third on the list.

The changes, which may only be temporary, may affect the Carter administration's desire to gain additional access for various products, both agricultural and manufactured, through a new trade agreement.

But various trade barriers, which U.S. negotiators hope to break down, stand in the way of increased agricultural trade with industrialized countries.

Some progress has been made on such big-ticket items as wheat, corn, dairy products and some beef products. Negotiations on a new international wheat agreement are being carried on alongside of the trade talks, and a new pact could become part of the new trade agreement.

The Europeans want greater stability in world markets and prices, and the United States seeks greater access and agreement on a grain reserve.

Other talks, under the auspices of the Geneva trade negotiations, affect beef, dairy products and grains such as corn, rice and barley.

But the United States also wants access for a variety of other products, which are less important to the overall trade balance but may be even more crucial to the treaty's eventual success in Congress.

"They're not big-ticket items, but they are important to the districts of the members of Congress who are on committees, that must approve the agreement," one U.S. trade official said yesterday.

"What Strauss is trying to get is something for the people who count."

Strauss, at a trade conference in June sponsored by National Journal, said that unless the United States received "considerable improvement in market access . . . particularly for our agricultural products," he would not be willing to bring a trade agreement back to try to sell to Congress.

During that conference, Strauss met privately with the principal negotiators from Japan and the European Community, and at the time they jointly announced their belief that the outlines of an agreement could be reached in time for the Bonn summit.

But two events occurred since then that caused Strauss to issue his warning that the timetable might not be met. The European Community's Council of Ministers, meeting last week, agreed on a mandate for its negotiators that appeared to be extremely restrictive on the question of agricultural trade.

Then, last weekend, the Japanese issued a new trade proposal that, in the U.S. view, "went only about one-tenth as far" as expected.

Strauss, according to one U.S. official, concluded that, on the basis of those actions, it would be difficult to conclude an agreement in principle by late next year.

The Europeans and Japanese say they believe they are not overly restrictive toward agricultural goods. Japan is the single leading importer of U.S. farm products and the European community combined is the largest. Both the European community and Japan say they cannot eliminate their trade barriers without seriously disrupting their agricultural economies and causing poliical problems domestically.