The Fairfax County Board of Supervisors last night rejected a proposal to give farmers a tax break designed to save some of the county's disappearing agricultural land.

Most supervisors said they feared such a measure would cost the county millions of dollars in lost revenue and benefit investors -- who buy land and wait years for the right time to develop it -- rather than farmers.

The board voted instead to ask the General Assembly for a new law that would allow Fairfax to offer tax breaks that developers could not take advantage of. Although the supervisors said they wanted to support the county's remaining 400 farmers, some advocates of a land-use tax said they were discouraged.

"The General Assembly isn't going to do anything in time to save any farmlands in Fairfax County," Ray Vickery Sr., a longtime Fairfax resident and U. S. Agriculture Department official, said after the vote.

Vickery and a small committee of Fairfax farmers and environmentalists, along with the Federation of Citizens Associations, had urged the board to follow the leads of Montgomery, Prince William and most other Virginia and Maryland counties by adopting a land-use tax that would keep farming economically feasible amid the mushrooming subdivisions.

The land-use tax allows farmers to pay property taxes based on how much their land earns, not how much it would fetch on the open market; the Fairfax supervisors want permission to exclude developers from the tax break. But a number of studies have questioned its value in urban areas, where so many pressures besides rising property tax rates force farmers off their land.

A county study last spring found 22,000 acres of farmland left in Fairfax, all of which would be gone by the year 2005 at the current rate of development.

"The land-use tax will not contribute to the preservation of open space and farmland, but will provide windfalls to developers and increase the tax burden" for homeowners, said Supervisor Thomas M. Davis III.

Only Supervisor Audrey Moore argued strongly for the land-use tax, and no official vote was taken on the measure. "It does work. It is a valid concept even if it is a little late," Moore said, adding that development would also cost the county because of the increased services that would be necessary.

Vegetable farmer Tony Newcomb, who owns 26 acres not far from Tysons Corner, said he was encouraged by the board action because the supervisors unanimously committed themselves to finding some way to keep farmers in the county.

"That's an amazing testimonial," Newcomb said. "I wouldn't mind if it takes two years, as long as there's a real program and the board supports it."

The board voted to ask the General Assembly to allow it to tailor agricultural districts to the kinds of small truck farms that remain in Fairfax. Unlike under a standard land-use tax, the agricultural districts would allow county officials to exclude developers.