The Fairfax County Board of Supervisors gave preliminary approval yesterday to a last-ditch effort to save the county's remaining farms, scheduling public hearings on a plan to give tax breaks to owners of farm and forest land.

The plan would allow people who own 25 acres or more to reduce their property taxes by more than 90 percent, in some cases, in return for promising not to develop their property. Currently, only parcels of 500 acres or more qualify for such tax breaks.

Some supervisors said the proposal would help save some open space in the rapidly developing county, while others said that it would force subdivision homeowners to subsidize wealthier landowners in western Fairfax. The board decided by voice vote, with only one apparent dissent, to hold public hearings on the measure, but Supervisor Thomas M. Davis III predicted a close vote when it comes up for final passage.

Both sides acknowledged that the tax breaks would do little to keep Fairfax rural in the long run. Even now, the county has only about 250 parcels of land that might be eligible for the program, according to a staff report.

"It's not so much going to prevent the ultimate development of the land as it is initially going to put it into another location," said Janet O. Kilby, who heads the county's farm preservation program. "It does provide some delay in that development, and it gives the farmers who would like to remain in the county an opportunity to do so."

Kilby said that the delay also would give the supervisors time to decide whether to adopt more permanent solutions, such as purchasing development rights from farm owners.

Landowners seeking tax breaks under the proposal would have to promise not to develop their land for at least eight years. Anyone who withdrew from the program would have to pay six years of "rollback" taxes--the difference between what they paid and what they ordinarily would have paid--plus interest and a substantial penalty.

Supervisor Nancy K. Falck, whose Dranesville district includes one of the county's two existing 500-acre-plus agricultural districts, said the rollback penalties are so severe that the program would not be abused.

But Supervisor Martha V. Pennino, who voted against holding public hearings on the idea, said that the mass of the county's homeowners would have to make up the difference in lost taxes, with no ultimate benefit for the county.

Public hearings were scheduled before the Planning Commission on April 20, and the board on May 9. If it wins final approval, the law would take effect on June 30.