Dairy farmer William C. Crossman Jr. is plowing away from Washington and all the way to the bank.

The Crossman farm has moved three times during the last 30 years, from Arlington to Fairfax to Loudoun to Westmoreland counties. Each time, Crossman says, he was pushed farther out by the spread of urbanization as spiraling property taxes drove his farm costs through the roof.

Each time, Crossman sold his property to land developers, who subdivided to bring in the very urban growth that was driving farmers out. He made a bundle. On his most recent sale, of his 777-acre Brooksmeade dairy farm in the shadow of the Blue Ridge, near Middleburg, Crossman netted more than $2 million.

"I asked myself: why should I stay here and pay taxes on all this damn land to send somebody else's kids to school?" Crossman says. "You've got to look after yourself first. I'll look after Billy, and leave the rest of them to look after themselves."

Crossman, whose grandfather began farming in Arlington after the Civil War, personifies the contradictions facing farmers in the rapidly growing metropolitan area as the 1980s approach.

Although Northern Virginia's soil is ideal for farming, the last decade's dizzying influx of people and industry into the area makes the land look even more enticing to builders of subdivisions, shopping centers and parking lots. Virginia farmers, pinched by high taxes and tempted by rising real estate values, are selling their property and moving out at a rate that area officials find alarming.

In Loudoun County alone, an estimated 1,500 to 2,000 acres of farmland are swallowed up each year by growing suburbanization, and the prospect are for a rapid acceleration of that process in the 1980s.

If nothing is done to reverse the trend, county leaders say, an area that had been populated almost exclusively by farmers as recently as 40 years ago will have no family farmers at all by the year 1990.

"We're trying to prevent Loudoun County from being paved over with wall-to-wall townhouses and shopping centers, but I think we're losing the battle," says Virginia Sen. Charles Waddell (D-Loudoun), who had been in the forefront of the fight in the state legislature for agricultural preservation. "I must say I'm very pessimistic about the future of farming in Northern Virginia."

Elsewhere in the state, the situation is much the same. Legislative moves to grant tax breaks for farmers and create agricultural districts that discourage urban development have so far been sufficient to halt the loss of an estimated 100,000 acres of farmland annually.

Crossman, who served for the past eight years on the Loudoun County Board of Supervisors and spent six of those years as its chairman, says he doubts the power of local governments to do anything to keep farmers on their land. Unless the state takes strong action like buying up farmers' development rights, he says, farmers will keep selling out for top dollar and joining the wagon train west.

"Our hands are tied by the state legislature," he says. "Sometimes I think we don't even need the durn local government, for all the good we can do."

Crossman's personal odyssey across Northern Virginia began in the late 1940s, when he moved from his father's farm in Arlington to set up a dairy farm of his own on 400 acres in what is now the outskirts of wealthy suburban Reston.

Although the area was a "wilderness" when Crossman first set up shop with 100 dairy cows, the coming of Dulles Airport, the growth of the federal government and the installation of new sewer lines changed the character of Fairfax County radically during the 1950s. Feeling the squeeze of quadrupling taxes and rising labor costs, Crossman "read the handwriting on the wall" and sold to a developer in 1961 for $400,000 -- far more than the $70,000 he had paid 13 years earlier.

Hundreds of brick colonial homes now occupy quarter-acre lots where Crossman's cows used to graze, in a subdivision called Fox Mill Estates. A solitary mature tree, the only one in the subdivision that stands taller than 20 feet, is all that is left to mark the place where his farmhouse stood.

Crossman is now moving out of his Loudoun County dairy farm, which he sold to a Chicago developer for $2 million after he saw his original property tax bill of $2,500 a year rise to almost $10,000. Neighbors around that homestead, located near Middleburg in the heart of Virginia's rolling hunt country were outraged when Collins Development Corp. announced plans to build 150 homes on the 777-acre site. But Crossman is not apologetic.

"All I had was a pretty farm up there on the hill for people to ride up and down and look at on weekends," he says. "But I never had any money in my pocket. Why should I stay and work myself to death for taxes? I couldn't take a shovelfull of dirt into the Safeway and buy groceries with it."

Now, he's starting out on yet another farm, this one on the banks of the Potomac in Virginia's Northern Neck -- about 40 miles south of Washington -- a spot some see as the next frontier for resort development.

Loudoun officials, nervous as the web of development spreads outward from the District's core, say they have only to look at the parking lots of neighboring Fairfax County to see what they want to avoid in the coming 10 years. But they are not united on a method to preserve the county's rural open spaces -- a goal they have set as one of their top priorities for the coming decade.

June Bachtell, Loudoun's director of economic development, says the way to fight residential development is to attract light industry and build up the county tax base.

"If we are successful, what we will do, hopefully, is bridge the gap between the cost of the services demanded of county government while keeping our tax rate low so that farmers can continue to operate," she says.

A recent move by the Loudoun supervisors to create two agricultural districts also is seen as a step in the right direction, but supervisor-elect Tom Dodson, the mayor of Middleburg, says the move may turn out to be largely symbolic.

In exchange for a four-to-eight-year commitment to continue farming, agricultural districts, under Virginia law, guarantee farmers that their land will be taxed by use rather than fair market value. They also restrict local governments from passing so-called nuisance ordinances that interfere with farming and require that local governments take the agricultural purposes of the district into account in area land-use decisions.

A total of 21,000 acres of Loudoun land has been placed in two such districts, one near Purcellville and another near Middleburg.

"We're hopeful that the agricultural districts may slow things down somewhat, but we're not really certain," Dodson says of the method that was passed by the state in 1977. "They haven't been tested in the courts yet."

According to Waddell and Del. Raymond E. Vickery (D-Fairfax), who have worked for farmland preservation in Virginia's legislature over the past several years, only a state law mandating the purchase of development rights by state and local governments can halt the endless spread of the suburbs. Such legislation is just not in the cards, they say.

"The agricultural district idea is about as far as they're going to go at the present time," says Vickery, "and that is basically just an act that exhorts people to do something rather than giving them any real tools."

Maryland's Howard County is one of only three counties in the nation that are actively pursuing the development rights of farmers. Under a measure approved by the voters in 1978, county officials there are empowered to buy easements on farmland at a price equal to the difference between the land's fair market value and its agricultural value.

A prohibition against development is then placed on the deed to the property, but the land remains in the possession of the farmer.