About three miles off the main highway between Managua and Leon, Domingo Vega Oviedo began this weekend to till a fertile but long-neglected field that until last month belonged to Anastasio Somoza.

The fallow field, in the potentially rich Nejapa Valley, forms a part of the 400-acre Rancho Grande. It has not been cultivated for at least three years because, government officials say, Somoza owned so many farms, cattle ranches and coffee plantations throughout Nicaragua that he had little need to use them all productively.

All of this land, hundreds of thousands of acres by the most conservative estimate, has been expropriated by the revolutionary junta. The land is being divided into workers' cooperatives, as is the case of Rancho Grande, or being given directly to landless peasants, depending on the circumstances.

Land redistribution was one of the basic tenets of the Sandinista National Liberation Front during its years of fighting the Somoza regime and it was one of the first things the new junta began to implement after the Sandinistas overthrew Somoza a little over two weeks ago.

In a normal year, Nicaragua earns more than $600 million from its principal agricultural exports - cotton, coffee and beef - and farming is the most important sector of the country's economy.

The Sandinistas have an opportunity to make fundamental changes in agriculture - without serious opposition - because the Somoza family and its allies owned an estimated 60 percent of the country's arable land.

Jaime Wheelock, the new minister of agrarian reform, has spoken often about the Sandinistas' commitment to Nicaragua's peasants, most of whom live in near-feudal conditions in rural areas like the Nejapa Valley, where there is no hospital, only one school, no electricity and no running water for a population of 10,000 peasant families.

"You must see the conditions in which these people live," Wheelock said in a recent interview, "to understand how much we have to do and why we have to do it. There cannot be social justice in Nicaragua until there is justice for the peasants."

Despite the importance the new government has attached to improving the economic and social conditions of the more than 1.5 million Nicaraguans who live in rural areas and the Sandinistas' own leftist talk before they came to power, Wheelock said that Nicaragua's land reform program will be carried out on "a totally realistic, totally pragmatic basis."

The only agricultural properties to be seized without compensation are those that belonged to the Somoza family, their top civilian supporters or National Guard officers who fled the country, Wheelock said.

Other farms are to remain in private hands, although the minister said the new government will take a dim view of large holdings that are not farmed productively and efficiently.

Wheelock said, "There are families in Nicaragua who have not visited their land for 20 or 30 years."

Nonetheless, he said, the new government envisions both privately owned agribusinesses and farms playing an important role.

"What we need in Nicaragua is to increase agricultural production," he said. "We will not imitate the Cuban revolution, where the land was organized into collectives, nor the Bolivian experience, where each peasant was given a plot of land so small that he was worse off than before. We will carry out a Nicaraguan revolution based on our own needs and our own possibilities."

Wheelock said some of the confiscated Somoza land will be given to individual peasants and some will be given to a peasants' cooperative. In still other cases, the government will keep title to the land and run the properties as state-owned farms.

Some of the confiscated ranches and plantations are so rich and so extensive, he said, that if they were divided among the peasants "we would be creating little Somozas."

The Sandinistas' idea is that profits from the cooperatives and the state-owned farms will be used to improve social conditions in rural areas and then to benefit the rest of the population, Wheelock said. He expects schools, hospitals and decent houses to be built where now there are few or none.

Wheelock, unknown outside Nicaragua before the Sandinista victory last month, has emerged as one of the most popular leaders of the new revolutionary government since it assumed power here July 19.

He is 31, handsome, well-spoken and has a gentle manner. He was a professor of sociology and hilosophy at Managua's National University before he went underground to fight with the Sandinistas several years ago.

He comes from a well-to-do family of English descent with large coffee plantations in the north of the country. He has written several books and said he became an expert in agricultural economics during his years in the underground.

In addition to being named minister of agrarian reform, he is on the nine-member Sandinista National Directorate. Many here believe the directorate holds the real power in Nicaragua, rather than the five-member junta named by the Sandinistas to govern the country.

Last week, Wheelock visited the Nejapa Valley, about 12 miles from Managua, and explained how the new cooperative at Rancho Grande would be run to the 150 peasant families living nearby who are now members of the Valle de Nejapa "community."

He explained that the land, largely uncultivated and in some areas badly overgrown with thick scrub and vines, will belong to all the families in the cooperatives. Those families, with the help of technicians sent by the government, will decide what they want to grow, he said.

Vega Oviedo, who helped organize the Nejapa Valley for the Sandinistas during the war, was named director of the new cooperative and began work Saturday tilling some of the fields at Rancho Grande in preparation for planting.

The problem the new cooperative here will face are the same that will be faced by other cooperatives throughout the country. There is no seed to plant at Rancho Grande - although Wheelock promised that it would arrive soon - and there is an urgent need to get something in the ground because the growing season is half over.

Because of the war, some areas of Nicaragua were not planted when they should have been. This coule force the country to live on international food aid for at least another year.

The peasants themselves, at least at Rancho Grande and at Granja Imporcesa, a pig farm in the Nejapa Valley that also was owned by the Somozas and was turned into a cooperative, are not convinced that cooperatives are the best way to run the seized properties.

Both Vego Oviedo and Juan Berrios, a worker at the Imporcesa pig farm, said they would prefer that the properties be divided up among the peasants rather than run as cooperatives, a concept they do not fully understand.

It is still too early to determine how Nicaragua's agraian reform program will work in practice. Or whether it will be accepted by peasants.

In the first flush of Somoza's overthrow, peasants in the Nejapa Valley seem very supportive of the new junta. The Somozas were as unpopular here, where the family owned five large properties comprising 4,800 acres of the best land, as they were elsewhere in Nicaragua, especially among the poor.

"There's much enthusiam now in Nejapa for the Sandinistas," Vega Oviedo said as he returned to plowing the fields at Rancho Grande Saturday morning. But he added: "We are hoping we won't have another government that only gives us promises." CAPTION: Map, no caption, The Washington Post