Periodically, there is talk of making Voronezh the capital of the Russian Republic, the largest of the Soviet Union's 15 republics, stretching from the Pacific to the Ukraine.

But so far it is just talk, since such a move would require displacing Moscow, now capital of both the republic and the nation. And as people here know, Moscow is not easily displaced.

But Voronezh, a city of 900,000 spreading ever closer to the Don River six miles away, knows nonetheless that it lies at the heart of Russia -- not the Soviet Union, but old Russia. And that gives it a certain place, culturally and historically.

Voronezh -- which has not changed its prerevolutionary name -- was where Peter the Great built his fleet for a surprise attack on the Turks in 1695.

During World War II, Voronezh was caught in the middle as the German and Red armies pushed the front line back and forth across the Voronezh River 30 times, at a cost of 15,000 lives.

The city was wiped out -- not a single downtown building was left standing.

By 1943, only a handful of people were left; the rest were killed or fled east.

Now the main street, Revolution Street, has been rebuilt. The old Bristol Hotel, renamed the Central and given a recreated Art Deco facade, faces a gleaming new children's theater, the city's latest pride.

The city's principal architect, Peter Danilenko, calls the new Voronezh unique because of the plan under which it was rebuilt, radiating out from the center, divided into circular bands, with its industrial base separated from its residential areas by a green zone.

The result is a low-lying, attractive provincial city with wide streets and a residential district of small houses hugging the bank of the Voronezh River.

With the characteristic Soviet penchant for quantification, city officials tell you proudly that Voronezh is the second greenest city in the Soviet Union.

FOR HISTORICAL reasons, Voronezh reckons itself the capital of Russia's "black earth" zone -- chernozemlya in Russian, chernozem for American agronomists brought up on the Russian system of soil classification.

Chernozem is earth derived from loess, a wind-blown deposit that appeared after glaciation. It is rich in organic material, and therefore highly productive. The United States and the Soviet Union share most of the world's supply of chernozem: Voronezh has a rival in Granville, Iowa, which calls itself the "black earth capital of the world."

In fact, the black earth zone here stretches far beyond Voronezh's region -- from the Altai range to the Ukraine, a huge swath of the continent that makes up about 10 percent of the Soviet land mass and produces 75 percent of the agricultural yield.

Over the centuries, the black earth zone was the route followed by invaders from the east who coveted its thick, rich grasses for their herds. Voronezh stood near the end of their path.

Around Voronezh the land is indeed black and when it gets wet, it clings to cars, boots and pant legs like chocolate. People here say that an institute in Paris with samples of soil from around the world has ranked Voronezh dirt the best.

There is a local saying that if you plant a matchstick in Voronezh earth, it will grow into a telephone pole.

Perhaps equally apocryphal is the story of how the occupying Germans loaded railroad cars with Voronezh earth to take west. Nobody here knows why the Nazis did it, but they know it was an unforgivable crime.

IN THE OFFICE of the mayor of Voronezh is a desk with a computer on top. Three years ago Voronezh became one of the first Soviet cities to computerize, not surprisingly, because this is one of the electronic capitals of the Soviet Union, a producer of television sets, stereos and, most recently, video cassette recorders.

The mayor, Igor Larin, 48, now depends on the computer for all aspects of city planning. He says the computer can give him updates on which streets need repair, which stores need new supplies and where the construction industry has fallen behind -- all the minutiae that are decided centrally in this country.

"It is very helpful in planning our five-year plan," said Larin, who came to Voronezh in 1964 as engineer, and worked his way through the local party committees to become mayor two years ago.

There are some problems that not even the computer can solve. As in many Soviet provincial cities, there is never any meat in the main state stores -- only at the market and in stores connected to factories or other workplaces.

In the clothing stores as well, the selection is smaller than in Moscow, although cursed by the same random pattern of distribution. As winter was setting in, the dress racks here were filled mostly with cotton shifts.

Privately, people in Voronezh say that problems of shortages and distribution are not of their making. The Voronezh region exports most of its products; what comes back is decided in Moscow.