Across the Bootheel of Missouri, cotton picking machinery is rusting in barns this summer and managers of the few remaining cotton gins are wondering if this will be their last year in business.

Thousands of acres that once grew cotton have been planted in neat, dark green rows of soybeans. Here and there new metal gain bins gleam in the sun. And down on the Mississippi River, barges pull up to huge terminals at such places as Cottomwood Point and Huffman's Landing to take on cargos of soybeans destined for Eupore and Japan.

American farmers will harvest more acres of soybeans in 1979 than of either corn or wheat, the Department of Agriculture extimates. Soybeans are now the nation's leading cash crop, producing more revenue than corn for farmers in Iowa and more than cotton for growers in Mississippi.

Soybeans exports bring in more revenue than any other U.S. crop -- $6.9 billion for feed grains and $4.6 billion for wheat.

"soybeans are key to the economic health of the United States," says the American Soybean Association, a non-profit organization that receives funds from the federal government and farmers to promote the use of soy products.

As production has increased from 15 million tons in 1960 to a predicted 50 million tons this year, a new industry has grown up around soybeans.

Soybeans are processed at multimillion-dollar plants to produce two primary products: one, a dry, high protein meal, has become a principal ingredient for livestock feed; the other, soy oil, is used in margarine or as a cooking and salad oil.

The industry is experimenting with numerous exotic uses for soy products, from meat extenders to industrial solvents. And in Japan and some other Asian countries, soya is a mainstay of human diets.

The rise of soybeans to the status of glamor crop came about through changes in diets at home and abroad since World War II. In the 1920s, soybeans were a minor crop with little commercial value. Farmers often grew them only to plow them under to restore nutrients to cornfields.

As the nation's hog and poultry industry grew, however, soybeans became a handy source of protein. And as other nations developed their own meat industries, soybeans began to move off the fertile lands along the Ohio, Illinois and Mississippi rivers into export channels.

These developments have altered the agricultural landscape in Pemiscot Country, an area of rich farmland created from swampy timberland after the turn of the century.

Farmers in these parts still recall with a touch of nostalgia the days when cotton was the main crop -- when handpickers fanned out in fields to gather the cotton, and mules hauled the bales to local gins.

"everything was cotton when I was young," says Robert O. Pierce, who now grows registered and certified soybean seed on 3,000 acres that he leases and owns.

Until recently, Pierce grew 1,000 acres of cotton or more a year, and he owns an interest in a loca gin. But three years ago, he says, he put his three cotton pickers in the barn and went over to soybeans.

Economics, in one form or another, dictated his decission.

Pierce and other farmers around here say there hasn't been a good cotton crop locally since 1972. Yields that run 700 pounds an acre in the 1960s have been averaging as little as 350 pounds.

Pierce attributed this to a series of wet, cool springs -- adverse wheather for planting cotton -- and possibly to an accumulation of chemical weedkiller that some say has damaged the soil.

Pierce's father started raising soybeans in the 1940s. Today Pierce is one of the largest and most prosperous darmers in the country. He sells soybean seeds to local farmers and bags additional quantities for Pfizer, one of several pharmaceutical firms that have entered seed business.

"cotton is a good product, but we just couldn't make the profit we needed," says Pierce whose son bob says he has no intention of going back to cotton.

Horace Dunagan Jr., president of the First State Bank of Caruthersville, believes that the transistion to soybeans might have been even more rapid were it not for old patterns of land ownership by local cotton gins. To ensure that there was an adequate sypply of cotton for these gins, gin owners leased out the land on the condition that the tanant farmers continue to plant cotton. These arrangements appear to be disappearing, Dunagan said.

Robert Winston, manager of a local gin, says business has dropped from 6,765 bales in 1972 to 1,323 bales last year. Plants that produced vegetable oil from crushed cottonseed also are closing.

David Guethle, an extension service agronomist of the University of Missouri, confirms that farmers have had troubled growing cotton, but he says that the principal reson for the changeover is economic - - the fact that the soybean "is a money-making crop."

Farmers here can produce more than 40 bushels of soybeans an acre. With prices last year running around $7 a bushel, earnings per acre have averaged $300, compared with $280 an acre for a typical field of cotton.

And raising sybeans tends to be cheaper and easier than growing cotton. In its early stages, cotton requires constant attention to guard against diseases and boll insects. A mechanical cotton picker costs about $80,000 and covers only two rows at a time, while a soybean combine can harvest seven rows each trip -- meaning lower fuel cost for soybean growers.

Although soybean promoters note that "a hungary world needs protein," only a small percentage of U.S. soybeans are used in poor countries where malnutrition is prevalent. Half are used in the Unites States, and the rest are exported to Europe, Japan or to advanced developing countries in the form of meal, oil or umprocessed beans.

Because the Mississippi River -- the "Main Street" of the world grain trade -- flows right by Pemiscot County, most of the soybeans grown here will end up abroad.

To accommodate the growth, the Missouri Farmers Association, a cooperative, has expanded its depot in Coruthersville from a 300,000 bushel capacity in 1970 to 2.2 million bushels today.

Elevator manager Melvin Dowling says he sells most of the beans he buys to Farmers Export, another cooperative located downstream, north of New Orleans.

But for the most part soybean trade up and down the river to dominated by multinational grain compainies.

In a 180-mile section of river between Osceola to Cargil, Continental and Bunge, three of the largest of these companies. Most of the beans moving into these terminals are shipped abroad -- sometimes to processing plants owned by overseas subsidiaries of these same companies.

Not surprisingly, the grain companies and several cooperatives have moved aggressively into the processing of the beans -- much as oil companies are in volved in petroleum refining.

Provate security analysts list Minneapolis-based cargill, the world's largest grain company, as the leading U.S. soybean processor, with nearly 20 percent of the country's total crushing capacity.

Large agricultural firms such as Ralson Purina and Archer Daniels Midland also have invested in processing.

As the need for high-protein anilal feeds has increased, new crushing plants have been build overseas. The problem has been how to market the vegetable oil byproduct, which competes with other oils made from palm, coconut, peanuts and cottonseed.

The Japanese government has limited soybean oil imports by domestic processors to amounts for the Japanese may have to import meal, which is turn would curb purchases of improcessed U.S. soybeans, unless the Japanese can be persuaded to use more soybean oil.

In West Germany soybean oil has gained wide acceptance and is produced by such major companies as Unilever.

But the oil has encountered strong resistance in France as a cooking oil because "the French prefer a richer, peanut smell and like butter," says the American Soybean Association's Michael A. Phillips.

Efforts to promote soybean oil in France also have encountered strong resistance from French agricultural interests.

Earlier predictions that Brazilian soybean products would supplant those of the United States in markets abroad have proved to exaggerated.

Since the early part of the decade, foreign and local interest have invested massively in processing plants in Brazil. The brazilian government backed this development with generous concessions to foreign investors and with enormous subsidies to exporters of meal. These subsidies have amounted to as much as $1 a bushel and have enable exporters in Brazil to offer European and Japanese feed buyers discounts of $20 to $30 a ton below the U.S. price.

But this year U.S. trade negotiators -- spurred by the powerful Midwest farm bloc -- obtained a promise from Brazil to phase out the subsidies. CAPTION: Picture, The Caruthersville elevator Dowling manages held 300,000 bushels of soybeans in 1970. Today it has a capacity of 2.2 million bushels. For Lee A. Meyer for The Washington Post; Graph, The Soybean Giants, The Washington Post; Map, Principal U.S. Soybean Areas, The Washington Post