Several years ago, a Moscow economist touring Uzbekistan's Andizhan Province in the fertile Fergana Valley east of here was astounded at the impact of Central Asia's remarkable population explosion on life in the region.

The close-knit Uzbeki families had accommodated new children by simply adding new rooms to the rambling mud-and-wattle houses so that, the economist recalled, the kishlaks (villages) seemed to merge into a shigle community snaking unbroken for miles across the rich farmland.

The Andizhan Province now had 285 persons a square kilometer, the highest population density of any rural region in the Soviet Union. Like Moslems elsewhere in Soviet Central Asia, the people of the Fergana Valley show little inclination to move away despite better money and better jobs elsewhere in the Soviet state.

But if the Soviet Union is to continue its ambitious planned economic expansion in the years ahead, the leaders of the authoritarian government must find ways to tap the growing manpower pool of the Moslem-oriented republics of Kirghizia, Tadzhikistan, Turkmenia and Uzbekistan.

Birthrates throughout European Russia have plummeted in the face of the demands made on life by modern Soviet industrialization. Young Slavic households have put children aside so that both parents can work. Despite state incentives to have children, the population of Great Russia, dominant for centuries in this multiracial population of 261 million, is static.

Meanwhile, central Asian birthrates have soared, the result of traditional Central Asian pride in large families, combined with Soviet-backed improvements in health and welfare and an economic boom caused by exploitation of gas and mineral reserves and vast agricultural irrigation projects. Birthrates here are three times as high as the one percent population growth nationally.

The slackened European birthrates mean that in the 1980s, with just 9 percent ofthe Soviet Union's entire population, the families of the four core Central Asian republics will account for about one-quarter of all Soviet citizens annually reaching jobtaking age in an economy that has had increasing difficulty raising efficiency and production without new infusions of manpower.

Some Western demographers have predicted that if the present birthrate trends continue, the Slavic Great Russians will become a minority in their own country by the turn of the century.

The impact of these remarkable demographic trends poses major questions for the aging Kremlin progeny of Lenin as they chart their country's future. The concern at the highest levels of the government is reflected in the increasingly candid debate in the official press over how the country should proceed.

Western capitals are closely following the Soviet economic and demographic trends and decisions over the integration into modern Soviet industrial society of millions of Moslem-oriented Uzbeks, Tartars, Tadzhiks, Bashkiris, Turkmenis and dozens of other Central Asian ethnic groups who together may total about 50 million in all 15 Soviet republics.

Zbigniew Brzezinski, President Carter's national security adviser, wrote some years ago that nationality questions in the Soviet Union "are becoming increasingly important and in many respects are, I believe, crucial to the future evolution of the [country]."

Western demographers and economists have issued reports on these questions and, congressional committees have heard expert testimony on the subject.

But as the economy's growth rate slows and the need to act becomes more pressing for Moscow, no one, including the Soviets, has a clear idea of how to bring the rural Moslems out of the countryside to the jobs that need them.

In Stalinist times, the answer would have been brutally simple: forced resettlement of masses of Central Asians at the vast new industrial and energy complexes being thrown out across Siberia and other remote places where, despite important money benefits, there is a persistent labor shortage.

Soviet President Leonid Brezhnev in an interview last year with the French newspaper Le Monde, gave a rosy picture of the Central Asian growth rate.

"As fas as population growth in one republic or another is concerned, this phenomenon doesn't disturb us," he said. "On the contrary, it gladdens us...."

But others find the population trends disquieting, among them the economist V. Perevedentsev, who was surprised at the burgeoning population of the Andizhan province.

He wrote that conventional Soviet economic theory holds that increased mechanization frees manpower to work elsewhere, but the traditional Moslem lifestyles of larger and larger families living comfortably together turn that theory upside down.

While commisars compete with each other for the European manpower pool, he wrote in Komsomolskaya Pravda, Andizhan collective farm managers are so overstaffed with extra hands that they "have to hunt around here, there, and everywhere in orde to find anything for their collective farmers to do."

The dislocations caused by labor shortages where they are not wanted and labor surpluses where they are not needed have become endemic to the Soviet economy, which functions inefficiently to begin with.

To help solve the imbalance between labor demand and supply, a number of economists are calling for a comprehensive population program that would spur the birthrate in Slavic Russia. Questions of national demographic policy pose a host of delicate issues centering on race, education, and lifestyles.The Moslem population quotient only serves to underscore the potential exposiveness of the problem.

In general, Central Asian education levels, while much higher than in czarist times, lag far behind those in European Russia.

For example, A. V. Bachuring, vice chariman of the State Planning Commission (Gosplan) reported last summer that "in the Central Asian Republics, a very small percentage of eighthgrade school graduates (roughly equivalent to second-year students in U.S. high schools) enter vocationaltechnical schools and even lower percentages enroll in secondary vocational-technical schools." He placed the highest portion in Kirghizia at 14 and 8 percent respectively, as contrasted with the Latvian republic's 27 and 16 percent.

Young Moslems with rudimentary education, lacking fluency in Russian, require enormous amounts of extra assistance to acquire the skills needed to make a meaningul difference in the higher end of Soviet industry. The press has raised this issue frequently in recent years.

This year demographer G. Litvinova obliquely raised the race question. "The state has an interest not only in the quantity of its citizens but also in their quality. It matters to the state what sort of population or what sort of manpower is increased -- whether these people have a high or low degree of training, whether they have a high degree of mobility or by virtue of a number of circumstances (including a tendency to have large families or a language barrier) are tied down to a specific region." She added, somewhat ominously, "The large family is becoming an outmoded demographic type, the support of which cannot be successful and can hardly be desirable."

She proposed adopting a differentiated demographic policy that would try to stimulate the birthrate where it is low and encourage its reduction where it is high.

Economist Manevich, a professor and department head at the economics institute of the prestigious Academy of Sciences, rejected the idea as "fundamentally mistaken," on the ground that it would "not help to increase the brithrate for the Soviet Union as a whole." Among the alternate sloutions he recommended were better pay incentives and living benefits in the areas of manpower shortage.

Left unmentioned by Manevich was the essentially Slavophile underpinning of the Litvinova suggestion. But a tradition of paternalism toward the Central Asians has existed here since czarist times, when the Russians came as conquerors and stayed living apart in their own compounds for protection and succor amidst the Moslem population. Historic remnants of this separateness can be seen today in the old Russian quarters preserved in Samarkand and other cities of the spice route.

Although the Soviets sharply rebuke suggestions that the old pattern lives on, Western analysts point out that the Central Asian economies originally shaped by czarist Russia continue along inescapably paternalistic lines today, producing raw materials such as cotton and gas that are mostly shipped out for sophisticated processing and consumption elsewhere.

As the years pass, a direct confrontation between the age-old lifestyles of the Moslem millions and the imperatives of the modern industrial state authored and directed by Soviet power becomes inevitable.

Moscow's economists, for example, frequently note that whereas women in Russia have been so completely drawn into industry and trades that there are no longer any available females in reserve, Moslem women have largely shunned such work, preferring instead the cares of home, raising an average of six children a family.

Bachurin characteristically observed that "in the Central Asian republics, relatively few young women, especially those living in rural areas, are enrolled in vocational-technical schools."

Meanwhile, the manpower problems stiffen. Bachurin pointed out that the annual increase in able-bodied cadres falls steadily: from 2.8 million in 1976, to 2 million this year, to 1.5 million in 1980.

Western analysts have suggested that the reduced manpower pool eventually will be reflected in a drop in the Soviet armed forces, which now stand at 3.6 million in all uniformed branches. The impact of this on Soviet strategic thinking is incalculable for a country that worries about attack from both the Europenan West and Chinese East, where it has stationed more than 500,000 men along the world's longest closed border.

Eventually, the rulers in Moscow will turn their stern attention to Central Asia and when they do, profound changes seem certain for the Moslem millions who have for so long kept to the traditions of their ancient culture.