A confidential study prepared for the top Soviet leadership has outlined a nearly disastrous decline in the Soviet Union's ability to feed itself, demonstrating a compelling need for agricultural and other economic reforms.

The study, made available here, provided figures that showed a tenfold increase in Soviet food imports during the past decade, staggering levels of mishandling of agricultural equipment and "direct losses" of harvested crops due to negligence and lack of storage or drying facilities.

The document, prepared by a special government commission during the past year, said one-fifth of the grain harvest is lost because it is harvested late or left to rot.

The study said one-third of the country's potato crop is left to rot, and experts who took part in drafting the document put the losses even higher. They said about half of potato production--or the equivalent of the annual American production--is lost each year because of a chaotic distribution system and lack of storage facilities.

As a result, the study said, an average Soviet citizen is poorly fed, consuming 54 pounds of meat per year less than required by medical standards.

The commission, which prepared the study in cooperation with the state planning commission and 38 ministries and scientific institutes, concluded that "the existing economic mechanism does not provide necessary economic incentives for production increases and fuller use of the potentially available land."

The study provided the basis for an internal debate leading to a special Central Committee meeting on agriculture, to be held Monday.

The Soviets indicated Saturday, during talks with U.S. officials in Paris, that they will increase 1982 imports of grain to more than 45 million tons, United Press International reported. U.S. Undersecretary of Agriculture Seeley Lodwick ended two days of consultations with Soviet Deputy Foreign Trade Minister Boris Gordeev, but they failed to extend the current eight-year contract to sell U.S. grain to the Soviets beyond the Sept. 30 expiration date.

The gloomy assessments and devastating statistics in the Soviet report suggest that the crisis in agriculture may become a touchy political problem. The Kremlin's course of action is not clear, although experts involved in the preparation of the study talked about financial incentives and about the need to adopt parts of the "Hungarian model." In Hungary, most land is run by cooperatives that have become very profitable because of financial incentives for productivity. Hungary not only produces enough food for its own needs but also exports about $2 billion worth of food products.

A similar Central Committee meeting devoted exclusively to agriculture was held in March 1965, shortly after Leonid Brezhnev replaced former leader Nikita Khrushchev. At that time, most of the failures of previous years were blamed on Khrushchev.

As it stands, the study prepared for the Brezhnev Politburo appears to be an indictment of the present leadership.

Well-informed observers here said that recognition of the need for radical changes may suggest that the leadership is prepared for decisive action. In this view, the U.S. grain embargo two years ago forced Kremlin leaders to move toward reforms and reduce Soviet dependence on foreign food.

The study covered the period 1966 to 1980, and it appeared to reflect a broad consensus that a decisive change in domestic policies is required.

Monday's plenum is expected also to deal with organizational problems, including the filling of Politburo slots vacated by the recent death of the number two party secretary, Mikhail Suslov. There are widespread rumors of a broader shake-up of the ruling body.

Moscow's ever-increasing dependence on food imports is also cited as a major strategic concern.

For the period 1966 to 1970, the Soviet Union had to import 15 million tons of food, mainly grain and meat. From 1976 to 1980, food imports have risen to 80 million tons.

As an illustration of rising costs, the study said that Soviet food imports in 1970 cost $700 million while in 1980 Moscow had to pay $7.2 billion--10 times more.

Before 1970 the country resorted to food imports in extraordinary circumstances, the study said, but imports of grain, meat and other food products have now become "a permanent component" of Moscow's economic thinking.

The document did not mention the 1981 grain harvest, however. Western experts said it was about 165 million tons, considerably below the disastrous yields of the preceding two years. Besides the grain the Soviets are expected to buy in 1982, their food imports for this year are expected to be three times greater than the combined figures for the period from 1966 to 1970.

It is very difficult to say how bad the food situation is on a national basis. The Soviet Union is not hungry. But demand and expectations have risen dramatically to make the issue of food one of the most critical problems faced by the Kremlin.

According to the study, the crisis that developed during the past 15 years was caused by several factors. They include "a decline in the level of growth" of agricultural production, increasing production costs accompanied by a drop in labor productivity and a growing discrepancy between relatively declining supplies and rising demand.

While incomes have doubled during the past 15 years, the production of food has risen by only 23.1 percent.

The problem is compounded by an enormous exodus from rural areas. According to official figures, 17 million persons have left villages and settled in urban areas in the years 1976 to 1980.

According to the study, production costs in the same period have risen tenfold as compared to the level of production increases--the statistical ratio given is 60 to 5.2.

As a result, the state is subsidizing an extraordinary price system. A pound of meat, when available in state shops, costs $1.40. The state buys the meat from collective farms for $4 a pound. The real market price at Moscow's open markets is around $6 a pound.

The study also reveals staggering problems caused by shoddy agricultural equipment, poor maintenance and inefficient use of machinery.

Despite the fact that the Soviet Union produces 550,000 tractors each year, the number of tractors available to collective farmers remains fairly constant. The number of tractors functioning in 1976 was 2.4 million. Five years and more than 2.5 million new tractors later, the number available in 1980 was 2.4 million, the report said.

At the current rate of use and production, the Soviet Union will not have enough tractors before 1997, the study said. Experts also calculated that under current conditions the number of needed combines would not be reached for 34 years.

To illustrate the inefficient production, the study said that the Soviet Union in its potato cultivation uses land acreage that is 14 times greater than the combined total used by U.S. potato farmers. Yet, it said, the Soviet production is only twice that of the United States. The Soviets lose 33 percent of their potato crop each year because of inadequate storage facilities or negligence.

In the case of fruits and vegetables, such losses amount to about 20 percent, the study said. An estimated 25 percent of the sugar beet crop is left to rot each year, it added.

It also pointed out that 55 percent of U.S. potatoes are used for processed foods while in the Soviet Union it is less than 0.5 percent.

Although the overall figures for sugar beet production have increased, the production of sugar in 1980 was at the 1965 level.

A year and a half ago, Mikhail Gorbachov, an agricultural expert and former party secretary in the generally prosperous grain producing Stavropol region, in northern Caucasus, was promoted to full membership in the Politburo. At 50, he is the youngest member of the ruling body and clearly a man to watch in the future. He is also said to be behind the push to restructure Soviet agriculture.