Soviet President Leonid Brezhnev hinted today that the Soviet Union's good shortages are reaching serious proportions in a speech in which he asserted that "improvement of the food supply comes first" in raising Soviet living standards.

He acknowledged "considerable difficulties" in the economy, which is now facing the second poor harvest in a row and which has been plagued by shortages of meat, milk and other consumer goods. "Certainly not all things went smoothly," he added in an apparent reference to intensified consumer frustration here.

Brezhnev's speech, at a Communist Party Central Committee plenum, appeared to reflect growing official concern about the current food situation in the wake of the Polish crisis that began last summer with workers' strikes over food price increases and eventually toppled Polish leader Edward Gierek.

In another move revealing Kremlin concern about its troubled agriculture, the Central Committee elevated a 49-year-old agriculture specialist, Mikhail Gorbachov, to full membership in the ruling Politburo, presumably to take full charge of that sector of the economy. He becomes its youngest member.

The 1970 Polish food riots, which brought down Wladyslaw Gomulka, led the Brezhnev Politburo to sharply expand livestock herds and general agricultural investment. But this year's expected poor results will be further complicated by President Carter's grain embargo imposed following the Soviet invasion of Afghanistan. The Russians already have had distress livestock slaughters because of grain shortages.

Moreover, earlier this year there were reported work stoppages at major Soviet automobile plants over poor food supplies.

Brezhnev devoted the core of his speech to consumer problems, according to a transcript distributed tonight by the news agency Tass. He hinted that this year's grain harvest may be even lower than recent Soviet estimates of 190 million tons, the figure which itself was about 45 million tons below target.

In a tacit admission of its power and sophistication compared to the civilian economy, Brezhnev called on the nation's defense industry to assist civilian managers "in the development of highly effective and high-quality types of machinery, in drawing up concrete programs and assignments." He implied the civilians, especially in the deeply troubled machine-tool industry, can no longer do without such help.

The 73-year-old party chief's address will be closely studied in Western capitals for clues to the 1981-85 five-year five-year-plan to be adopted next February by a national party congress.

Brezhnev did not specify the size of the 1980 harvest, but instead praised the fact that Soviet harvests since 1976 have averaged better than 200 million tons annually. This implies the present crop, cut by a long, cold spring and unusually wet summer, could dip as low as 165 million tons. Last year's harvest totaled 179 million tons, also well below target.

World grain harvests this year have been hurt by adverse weather and Moscow clearly faces major supply shortages ahead. Argentina, for example, which became a major grain supplier to Moscow following Carter's embargo, has announced recently that it was cancelling all export contracts because of its disastrous harvest this fall.

Brezhnev, however, made clear the problems are more basic than annual shortages, saying it is essential for the country at last to design and build a farm tractor and harvest combine to "meet the highest modern standards."

While noting that per capita food consumption is up, he asserted, "we still encounter difficulties supplying the cities and industrial centers with such foodstuffs as milk and meat."

Official figures published today show Soviet industrial output up 3.9 percent through September, below plan target of 4.5 percent. Labor productivity rose just 2.9 per cent compared with a 3.8 goal. Crude oil totaled 450 million tons in the first nine months. The 1980 goal is 606 million tons.