The Soviet leadership, frustrated by repeated setbacks in agricultural production, today launched another ambitious plan to put vast areas of the nation's virgin lands to use, particularly in the arid south.

The meeting here of the Soviet Communist Party Central Committee coincided with reports that this year's grain harvest, hit hard by droughts in critical regions, has fallen well below target for the fifth consecutive year. A recent forecast by the U.S. Department of Agriculture estimated the Soviet grain harvest this year at 170 million tons, the worst crop since 1981.

The one-day meeting produced no announcement of changes in the party's upper echelons, as had been widely predicted. The meeting of the Central Committee's 300 members was described as a "regular" session, even though it did not immediately precede a biannual meeting of the Supreme Soviet, as is the custom.

The aims of the agricultural program were spelled out in a speech by Soviet party leader Konstantin Chernenko and in a report by Premier Nicolai Tikhonov. The fact that Mikhail Gorbachev, the party's second-ranking official who has been responsible for agriculture in the Politburo since 1979, did not speak on the issue suggests that he may be shedding responsibility for what continues to be the Soviet Union's most intractable problem.

The program adopted by the Central Committee today is primarily a continuation of long-standing agricultural policies that put heavy emphasis on land reclamation.

It seems to be in contradiction to more recent suggestions that better management, not expensive land reclamation programs, is the most cost-efficient way to increase agricultural yields.

Chernenko today stressed the need to make good on the nation's growing financial commitment to agriculture, which has been estimated to consume 27 percent of national capital investment.

Chernenko's speech seemed to cast doubt on the future of a controversial scheme to redirect the flow of Siberian rivers away from the Arctic Ocean toward Soviet Central Asia. That plan, aired in the Soviet press as recently as June, has been critized as enormously costly and possibly dangerous ecologically.

Chernenko did not refer specifically to reversing the flow of Siberian rivers, but he did warn against "hasty and ill-considered decisions" in carrying out large-scale land improvements. "This should be done very carefully so that in transforming land, we should not do harm to it. . . ," he said.

The plan adopted by the Central Committee today set a new goal to increase reclaimed lands 50 percent by the year 2000, bringing the total of both irrigated and drained lands up to between 121 and 130 million acres. Since 1966, the area of reclaimed land in the Soviet Union has increased from 42 million to 81 million acres at a cost of "about 115 million rubles", Tikhonov said.

In announcing the new goal, Chernenko also said there is a need for a "drastic rise" in yields from lands already in production.

"It is unforgivable that projected crop yields are so far obtained in the country only from one-third of irrigated areas," he said.