As foreign assistance pours into Tanzania in ever-increasing amounts, the debate over the merits of President Julius Nyerere's controversial rural-based socialist approach toward economic development goes on unresolved.

No African government has tried harder to lift its rural population out of poverty en masse, close the enormous gap between rich and poor and direct the main thrust of its development efforts to the destitute country-side.

In fact, it is the near-universal assessment of foreign aid experts that Tanzania has done just about everything the World Bank or the U.S. Congress could expect of one of the world's 25 least-developed nations to help the poorest of the rural poor.

Western donors heap praise upon President Nyerere for his commitment, honesty and vision even if they do not support his idea of ujamaa villages, the Tanzania version of China's rural communes that have become the country's basic social and productive units.

Yet, many of his admirers remain troubled by the uneven results of the billion-dollar plus international investment in Tanzania over the past decade.

"I don't understand it," said one puzzled African economist from the Africa Development Bank. "They have done everything by the books just as they should. Yet, we are all still waiting for something to happen. Maybe it's a question of time. I don't know."

The head of a Western aid mission expressed the general assessment of the strengths and weaknesses of ujamaa this way:

"There are two issues here in Tanzania, economic growth and how you close the gap between the rich and poor in terms of basic needs.

"So far as closing the gap in terms of per capita income, ujamaa is not there yet and neither is most of the Third World. On the basis of economic indexes, ujamaa is probably a failure," he said.

"But," he quickly added, "how can you say that when people are drinking clean water, kids are going to school and villages have clinics for the first time in their lives? In these terms, ujamaa is a success."

The aid director never got back to the issue of economic growth, which appears increasingly to be the crux of the problem in the Tanzanian approach to development.

Among American theorists of economic development it is fashionable to make a distinction between "development" and "growth" and to criticize many African countries for promoting "growth without development." This means that while economic indexes such as gross national product may be rising, real living conditions for the vast majority of people remain unchanged or deteriorate.

But Tanzania also raises the thought-provoking issue of whether too much emphasis on development without sufficient concern for growth is not as bad as the opposite. Nyerere raised the question two years ago in a frank analysis of the country's poor economic performance.

"Over the last 10 years, we have done quite well in spreading basic social services to more and more people in the rural areas," he wrote. "More remains to be done, but we shall only be able to do it if we produce more wealth. And we have not been doing very well on that front."

Ironically, while Nyerere has made self-reliance a basic tenet of his concept of socialist development, Tanzania has become increasingly dependent on foreign assistance to the point where Western and Tanzanian economists are wondering whether the nation can "afford" all the aid being offered.

Last year, Tanzania received between $450 million and $500 million in foreign aid -- $28 to $31 for every Tanzanian -- a rate that puts Tanzania among the highest aid recipients in Africa. The World Bank loaned the country $140 million in the last fiscal year, more than any other African nation received.

Sweden has put Tanzania at the top of its aid list in Africa, providing $70 million to $80 million a year. Other Western countries, such as West Germany, the United States and Canada, are now averaging $20 million to $40 million annually.

As a result, Tanzania is depending on foreign assistance to cover between 60 and 70 percent of its current five-year development plan, according to one Canadian economist.

Meanwhile, the government projects a deficit in the current fiscal year of more than $200 million in its budget resulting from an increase of nearly 20 percent in fixed costs and an 80 percent jump in development spending.

Western and Tanzanian economists agree that the underlying problem is Tanzania's inability to produce enough to support its expensive welfare goals, such as putting every child into school by next year, providing clean water to all 8,000 villages by 1981 and setting up a health system to reach every peasant in the back country.

Other than a small amount of diamonds, Tanzania has no mineral resources to generate capital and depends mainly on six export crops for the bulk of its foreign exchange -- coffee, cotton, cashew nuts, sisal, tea and tobacco.

The volume of these six cash crops, however, dropped 21 percent between 1965 and 1973, according to World Bank figures, and the downward trend apparently is not being reversed.

The Tanzania Sunday News of Dec. 31, announcing in banner headlines an overall 6 percent growth in the economy last year, also disclosed that sisal production fell from 88,373 tons to 76,319 tons; pyrethrum, a flower used to make insecticide, from 4,500 tons to 2,500; cashew nuts from 96,000 tons to 68,000 tons and coffee, the country's chief export crop, amounted to 845,000 bags instead of the projected 880,000.

This drop, compounded by falling world commodity prices, meant that last year Tanzania used up most of its foreign exchange reserves trying to finance increased output in its light industries, which depend heavily on the import of spare parts and raw materials.

Tanzanian and Western economists offer a wide variety of reason for this chronic problem of declining production in export crops.

One is the nationalization years ago of many European-owned estates producing these crops and poor state management of them since, including low investment and labor productivity. Another is the fact that Tanzania suffered a serious drought in the mid-1970s.

A third factor often cited, particularly by Western economists, is the campaign in 1974-75 in which five million peasants were uprooted and moved into villages, often disrupting production.

Apparently, however, this has been largely overcome. Taking advantage of good rains and rising state-set prices, Tanzanian farmers have produced excellent grain crops in the last two years, more than covering the country's needs.