Confronted by the serious illness of Soviet Premier Alexei Kosygin and deepening problems with the national economic plan lie oversees, the Kremlin leaderhip tonight named an aging but apparently vigorous longtime associate of President Leonid Brezhnev's to the ruling Politburo to bolster the government's executive strength.

The elevation of Nikolai Tikhonov, 74, came as Brezhnev announced that the Soviet grain harvest this year totaled just 179 million tons, or 48 million tons below the target. This year's figure is 58 million tons short of last year's crop.

Responding to what is one of the worst Soviet grain harvests since World War II, the Central Committee named Mikhail Gorbachov, 48, an alternate member of the Politburo to take charge of agriculture.

The promotion of Tikhonov, a metallurgical engineer who has specialized in economic planning for 22 years, raises to 14 the number of full Politburo members and brings the ruling body's average age close to 70.

Tikhonov has been a deputy to the 75-year-old Kosygin since 1976 and an alternate Politburo member since last year. His selection raises the eventual possibility that he may succeed the ailing premier, and effectively consolidate Brezhnev's sway over the complex Soviet economic mechanism, which has faltered below most projected outputs all through the 1970s.

Meanwhile, a senior Soviet source confirmed reports that Kosygin is seriously ill and will not attend Wednesday's public session of the Supreme Soviet, the figurehead parliament, at which the 1979 economic figures will be announced and the 1980 plan described.

The source indicated that Kosygin has suffered a heart attack and although recovering, will be incapacitated for considerable time. Other official sources reported that Brehnev, in an address delivered before a closed-door plenum of the Central Committee, sharply criticized the country's economic performance and called for stern measures to improve productivity.

It was not known precisely what the party would do to improve lagging sectors of the cumebersome economy, but one well-informed source said the plenum approved "serious" and "thorough" measures.

One step was the naming of Gorbachov, an agricultural specialist, to take full charge of problem-plagued agriculture.

At enormous cost in precious hard currency, the Soviets have had to arrange to import up to 25 million tons of grain from the United States to make up for this year's shortfall.

Gorbachov fills a vacancy at the top of the agricultural sector that has existed since the death in 1978 of Fyodor Kulakov, 60, a Politburo member who held the agriculture portfolio.

Tikhonov's promotion to the Politburo reestablishes a general tradition that the premier's principal deputy should also be a voting member of the ruling body. The post had been vacant since last November, when Kirill Mazurov, at that time Kosygin's chief assistant, was dropped from the Politburo, ostensibly for health reasons.

Mazurov, 64, had been seen by some Western Kremlinologists as differing with the general view of the Brezhnev circle opposing fundamental reform of the economy that could lead to reduced party control.

Kosygin is seen by analysts as having struggled for such reform in the 1960s and lost to Brezhnev, whose conservative leadership has always taken great care to preserve and strengthen the party over all other sectors in the Kremlin. Tikhonov got his start with Brezhnev in the Dnepropetrovsk region of the Ukraine in the 1930s and has moved steadily upward with his mentor.

The Soviet economy had expanded throughout the 1970s, but at decreasing rates each year. This year, official figures show that production in such key areas as steel, fertilizer, synthetic fibres, cement, and major durables such as autos, refrigerators, and washing machines are lagging below even 1978 productin levels. Energy production is above 1978, but the Soviets have had trouble meeting their 1979 production goals in oil.

In general, the persistent failure to hit annual targets has made it virtually impossible for the nation to achieve the overall goals of the five-year plan by the time it expires at the end of 1980.

The news agency Tass, in reporting Brezhnev's speech complained about low worker productivity and pointed to unsatisfactory shortages of food, machinery, adequate transport. Brezhnev said 1980 must be a year of "shock work" in order to achieve the five-year-plan goals and that energy conservation must be a big factor in Soviet economic future.