The Fairfax County Supervisors decided yesterday to develop a program designed to preserve some open space in the rapidly developing county by paying farmers and other landowners not to develop their land for a set number of years.

The potentially costly program, under which Fairfax would essentially lease or purchase the development rights of farms and forests, got preliminary approval from the Board of Supervisors, although the county is continuing to explore alternative options to encourage farm preservation.

Only 22,000 of Fairfax's 262,000 acres are still being farmed, and the farmland is disappearing at a rate of about 1,000 acres a year. The program to be planned for Fairfax is similar to one approved but not yet implemented in Montgomery County, where 40 percent of the land is still being farmed.

Supervisor James Scott proposed the "open-space easement" program as a way to preserve woods and farmland without allowing developers to profit. The county could choose the program's beneficiaries, refusing to deal with owners who appear to be waiting for the optimal moment to subdivide.

The more common preservation technique in Virginia, land use or preferential tax assessment, permits all landowners to pay sharply reduced taxes as long as their land remains undeveloped. "It's been a disaster everywhere it's been tried," said Fairfax Board Chairman John F. Herrity, because developers have taken advantage of it.

Preservationists in the county welcomed the board's commitment to save farmland, but said they are concerned that Scott's proposal could cost millions of dollars that would have to be appropriated, rather than written off as lost revenue. Although the ultimate cost of a land-use tax might be as great, they say, the political risks of appropriating large sums of money to lease development rights might discourage future supervisors.

In addition, the board would be faced with the sensitive task of deciding which landowners best qualified if it could not afford to buy easements from everyone. And some farmers argue that only a permanent purchase of development rights, rather than an easement that would expire after several years, will reassure local farmers.

"There are few farmers here, but they want a permanent arrangement, not a continuous series of five-year agreements," said farmer Tony Newcomb. "To them the question is, 'Is Fairfax County willing to provide a secure system under which farmers will be taxed as farmers and not as land speculators?' "

Suffolk County in Long Island began purchasing development rights from farmers several years ago to preserve farmland from New York City's spreading suburbs. But buying the rights -- which is essentially buying much of the value of the land while leaving the physical property in others' hands -- was so expensive that Suffolk had to stop long before it had planned.

The Fairfax supervisors unanimously instructed their staff yesterday to develop a plan for seven-year or 10-year easements, with landowners paying a penalty if they eventually choose to develop.