Startling and rapid deterioration of the Soviet Union's agricultural system, which has forced the importation of about one-fourth of the country's grain supplies, is detailed in a new study by Worldwatch Institute, a Washington think tank.

The study by Worldwatch President Lester R. Brown, a former Department of Agriculture economist, goes far beyond other recent studies in documenting the breakdown of a farming system that as recently as a decade ago was exporting grain.

And, Brown said, the Soviet Union's new reliance on outside sources of basic foodstuffs adds major dimensions to its political and commercial ties with other countries, particularly the United States.

By measure of its ability to feed itself, Brown wrote, "the Soviet Union appears to be in deep trouble," with few immediate prospects for weaning itself from imports from the United States.

In contrast, the report noted, U.S. agriculture has continued to gain steadily in productivity, with dramatically more efficient use of soil, seed, fertilizers and pesticides than in the Soviet Union.

Brown said the Soviets' growing "food connection" with the United States -- the bulk of its 46 million tons of imported grain this year will come from U.S. farms -- bears importantly on future relations between the superpowers.

Moscow's need for grain from abroad and U.S. needs for selling its enormous overstocks may be "the most important change between the two countries since the Cold War began," he wrote. "It demonstrates in clear economic terms that the United States and the Soviet Union need each other.

"The long line of ships that now connects American farms with the dining tables of the Soviet Union constitutes a new economic tie between the two countries, one that could eventually transform their political relations as well," Brown wrote.

The study found that, after peaking about 1978, production in virtually every sector of Soviet agriculture has declined steadily, with no sign that reforms recently proposed by the Kremlin will stem the trend.

The study concluded that Soviet food shortfalls are more a result of a farming-system breakdown years in the making than of bad weather, which is most often blamed by specialists in Moscow and Washington.

"The fourth consecutive massive crop shotfall in 1982 signals a broad-based deterioration of Soviet agriculture that will create food shortages well into the future," Brown wrote.

Soviet dependence on foreign suppliers of grain and meat -- they lead the world in imports of both -- also has serious economic implications. Grain imports alone will cost more than $6.6 billion this year, Brown noted, at a time of declining world prices for oil and gold, major sources of foreign exchange for Moscow.

"In practical terms the Soviet government has little choice" but to continue high import levels to meet consumer demands and control political unrest over food shortages, the paper said.

U.S. and Soviet grain and livestock production trends were about equal between 1950 and 1978, with both increasing on parallel lines, although U.S. farmers were producing more on less land with far less labor, the analysis showed.

But then, for various reasons, Soviet lines on the production graph turned down precipitously, with output declining in virtually all major products--grains, potatoes, sugar beets, oil seeds, fruits, vegetables, livestock, poultry, milk and butter.

"The deterioration of agricultural production in the Soviet Union is too pervasive to blame on weather, as Soviet officials are inclined to do," Brown said. "Just as production declines cut across almost every segment of Soviet agriculture, the shortcomings involve virtually every agricultural input and every phase of agricultural management.

"Evidence now points to a basic conflict between a centrally planned and controlled agriculture and the evolution of a modern, highly productive agriculture. Centralized agriculture and Soviet [five-year] plan goals are incompatible," he wrote.

Brown catalogued a range of difficulties contributing to the marked drop in Soviet farm output. These include inefficient use of short supplies of fertilizer, pest infestations, a rustic transportation network, antiquated stock-breeding techniques, an inflexible marketing system, defective farm equipment and poor maintenance, massive soil erosion and poorly designed irrigation.

He said that the shift in the agricultural balance of power toward the United States, when the two countries are thought to be militarily equivalent, could give U.S. policy planners a chance to reshape relations with the Soviet Union.