Fairfax County has perhaps 4,000 or 5,000 acres of farmland still under cultivation amid its sprawling 250,000 acres of subdivisions and shopping centers, but no one knows for sure since the county's modern computerized records have no category for farms.

That is one reason the Board of Supervisors last week asked county planners to go back to the drawing board and out into the countryside to get more information for a proposed land-use tax, which supporters believe could help save the few working farms left in the county.

The tax is one of the few methods permitted by the state legislature to perserve open space, farmland and forests, and it has some drawbacks, according to even its most ardent supporters.

The tax, now in effect in Prince Williams, Loudoun and more than 60 other Virginia countries, assesses rural land on its use and value as farmland instead of its fair market value as land that can be turned into housing subdivisions.

The Fairfax supervisors have twice rejected the tax for fear it would reduce county tax revenues and help developers more than farmers.

"But it's already almost too late for Fairfax . . . there's not much farmland left to save," says Wallace Covington, whose family has farmed Fairfax land for more than a century.

Covington's 76-year-old mother still grows vegetables and lives on what's left of the family farm -- which once included the site of the new Pan Am Shopping Center near Fairfax City -- and he and his family keep sheep on about five acres near Centreville. He farms 200 acres in Prince William, land included in that county's land-use tax program, and expects he'll soon have to move out of Fairfax County altogether because he believes the present county zoning system discourages farming.

"You've got to be a good neighbor, a compatible neighbor and we've got 150 townhouses next door already. They won't put up with me. My life here is just about ended," Covington said. "We used to have a poultry farm next door. He had 15,000 hens. The courts gave him a year to go."

Most of the farmers already have left, says Robert McNair, who owns a 250-acre dairy farm near Herndon and Dulles International Airport.

"I've been hearing about this kind of tax for 25 years," said McNair, who has one of the last four dairy herds in the county. "I don't think it ever will be enacted. I'd use it for at least part of my land, and I think it would benefit the county. But the Board of Supervisors apparently doesn't think so or they would have enacted it.

"The board's scared to death some tax revenue would be lost, but land is not a burden on the county. It's development that's the tax burden."

As for county fears that speculators, not farmers, would benefit from a land-use tax, McNair said agreed that some might.

"Everybody who owns his own home is a speculator," he said. "I'm a speculator. But I'm not an overnight speculator. My family's had this farm for over 75 years."

He and Covington both said that the county's pro-development policies and taxes have forced out farmers but that this might slow the exodus.

One of the land-use tax's chief supporters, Eleanor White of McLean, who heads the Citizens Committee for Preservation of Farmland, contends that the tax would aid county taxpayers.

"The board's own figures show that it is 18 times as expensive to develop land as not to develop it," she said.

She said county figures show it costs Fairfax taxpayers $185,000 a year for schools, roads and services for every 100 acres of developed land (at 2.5 units per acre) in the western part of the county and $11,800 a year for 100 acres of undeveloped land, used for farming or open space.

The Fairfax County Federation of Citizen Associations also supported the land-use tax at last week's board hearing, but called it, at best, only "an interim method to slow development" of farmland and key open space areas threatened by development.

The federation pointed out that other urban counties like Henrico, outside Richmond, and Virginia Beach have adopted the land-use tax.

It also argued that a land-use tax is not unique and would be little different than the present county method of taxing commercial and rental properties, which "is basically determined by return on investment," not the value of the land.

While there is no estimate of the amount of taxes Fairfax could lose if it lowered tax rates for farmland, the lost taxes would be recouped by the county up to a point, with 8 percent interest, whenever land-use tax property is sold for development, according to county officials.

However, the "rollback tax" only extends for five years under present state law, and county officials say the 8 percent interest offers a bargain rather than a penality.

Prince William County, which has put 70,000 acres under the land-use tax program in the past six years, "this year will lose forever about $900,000 in taxes from 1974," said Prince William County tax assesor Ben Kelsey.

Prince William now is foregoing about $1.7 million in taxes a year from farmlands and forests under the landuse tax.

Kelsey added that "development of Prince William is continuing at a very rapid pace and there's been no visible-slowing because of the land-use tax."

But Kelsey said 60 percent of the county is still classed as agricultural land and the tax "has helped some bonafide farmers out in the middle of nowhere."

The state program allows counties to choose the kind of land it wants to tax for land-use. Kelsey and said he thought some forest land was perhaps inappropriately in the program, "like two corners of the Dale City interchange on I-95 which soon are to become shopping centers," and both he and Loudoun County officials support changes in the state law to tighten controls on the program and extend the rollback period in which counties can recover taxes when land is sold for development.

Milton Herd, Loudoun's agricultural planner, a recently created post in a county that prides itself on preserving its history and having Virginia's most fertile farm belt, said Loudoun now has about 90 percent of its eligible farmland, about 200,000 acres, under land-use taxes.

The program costs the county about $3 million a year in foregone taxes, Herd said, "but that's not the true cost since it costs the county money if the land is developed. It is in effect a low-cost loan, forgiven after five years, but we see this as a small step, no panacea, for preserving agriculture."

Loudoun and other counties have asked the legislature to consider "new ideas, like transferring development rights (in use in Maryland), zoning alternatives exclusive of agricultural zones, cluster developments and other things," Herd said.