Sales missions and scientific teams have been crossing the Pacific with little fanfare for months, but behind that lies the biggest agricultural trade story in years--the dramatic inroads American farmers have made in China.

Five years ago, the Peking government bought $44,000 worth of onion seeds from the United States and that was it. This year, Chinese purchases of American farm products are expected to pass $2.5 billion. Prospects are for even more in the future.

China has become the largest foreign buyer of U.S. wheat ($1.1 billion last year) and cotton ($701 million) and this year it will rank with the Soviet Union as the third or fourth biggest customer, behind Japan and Mexico.

"China could be our biggest market someday," Agriculture Secretary John R. Block said last month after returning from a Far East trade promotion trip that included three days in China.

Block pinned down no specific new trade deals, but he reported other developments that could lead to broader sales and exchanges of research and technology. The "most significant," he said, was the Chinese agreement to allow private U.S. farm marketing groups to open permanent offices in Peking.

Although farm trade has waxed and waned since the first U.S. contacts with China were made in 1972, the principal assurances of continued American access did not come until last year, when an agreement was signed allowing the Chinese to buy up to 9 million metric tons of grain annually through 1984.

A natural extension of that was the Chinese decision to let the marketing groups establish resident offices. These organizations conduct educational and promotional programs designed to broaden the use of and demand for their products.

For instance, U.S. Wheat Associates, financed by American farmers, has been training Chinese bakers in the different uses of wheat and is responsible for construction of a modern bakery that will be operating in Peking by year's end. Block tried a made-in-China doughnut during his visit and declared it a winner.

Wheat Associates, the American Soybean Association and the U.S. Feed Grains Council, representing corn, sorghum and other grains and promoting swine breeding and dairy management, will set up operations there next year.

Block said the Cotton Council International is planning exchanges but has not decided on a permanent office. The Holstein Association, with its dairy breed, and the National Renderers Association, seeking to sell lard and tallow, also are active in China.

Block said his talks with officials in Peking also raised the possibility of American tobacco sales. China's crop was poor this year.

There also was an agreement to increase the level of scientific and technical exchanges, the secretary reported. Two years ago, five teams were exchanged; this year, it's up to 24 and projections now call for as many as 80 exchanges in the near future. They will deal with such topics as soil management, medicinal plants, animal health, biological pest control, plant genetics and germ plasm.

Block said that he and the minister of agriculture agreed to hold talks to work out difficulties in animal health regulations that have closed doors to the trading of animals and purebred stock. "We are at an impasse right now," Block said, "but we each have things that can benefit the other."

The Chinese are interested in obtaining top-grade American dairy and swine stock, while U.S. breeders have a special interest in Chinese hogs, which are prolific breeders.

"They are very interested in the technology exchanges, and we, of course, are interested in trade," Block said. "Their big concern is adopting our technology to their needs. They wanted reassurances from us that our technology will be available and we asked the same kind of right to theirs, such as the plant germ plasms."

American researchers, for example, are intrigued with the prospect of obtaining plasm from Chinese soybeans, which are extremely high in protein, to cross with American soybeans, which are high in oil content. With the Chinese plasm, an even more nutritive soybean could emerge.

USDA officials want to know more about Chinese biological pest control techniques, which are conceded to be considerably ahead of U.S. methods, and the Chinese told Block they want to learn American processes for producing corn sweeteners. China imports about 1 million tons of sugar annually.

Trade economists at USDA predict that population growth and transportation restraints may hinder China's long-range ability to continue steadily to increase agricultural imports. Some of the country's ambitious goals, such as its livestock development program, already are being trimmed.

But, as Block sees it, China, with 1 billion mouths to feed, will continue to need food from abroad and that alone will provide strong markets for U.S. farmers into the foreseeable future.