Tax breaks alone are not enough to save Fairfax County's steadily shrinking bank of farmland, a long-awaited county study has concluded.

The study, prepared by the environment and policy division of the county planning office, contends that preferential tax treatment "has not been effective" in preserving farmland elsewhere in Virginia, and sometimes has given speculators a tax shelter while they wait to develop vacant land.

Instead of establishing a land-use tax, the study recommends that Fairfax districts, which provide tax breaks but generally limit those breaks to bona fide farmers. Over the long run, however, the study suggests that the best way to preserve farmland would be through zoning or public purchase of such land.

On June 15, the county Board of Supervisors is scheduled to act on the recommendations, and already there are signs that several groups will be competing for an upper hand in the decision.

One group, led by the Citizens Committee for the Preservation of Farmland, is strongly behind enactment of a land-use tax to provide immediate relief for county farmers. Another group, consisting mainly of citizens in the older, heavily developed sections of the country, opposes the tax, viewing it as a subsidy for landowners in largely undeveloped areas farther out.

Some critics contend that a land-use tax simply wouldn't be competitive with financial inducements offered by developers.

"There is no way a lower tax rate will be able to match the prices developers have been paying farmers," said Martha V. Pennino (D-Centreville), whose once largely open district is being rapidly transformed into an urban area.

Other supervisors say a land-use tax could force the county to raise the overall real estate tax rate to make up for revenue losses. And those supervisors are reluctant to support any preservation plan that would involve raising the tax rate.

These supervisors found ample ammunition in the study, which estimated that a land-use tax could cost the county from $1.1 million to $7.2 million annually in real estate taxes, depending on its application.

But the preservation committee, which has been lobbying for swift county action to save the remaining 20,000 acres of county farmland, disputes those contentions, arguing that a land-use tax is the quickest and surest way of encouraging preservation.

Regarding the proposal for agricultural districts, for instance, committee spokeswoman Eleanor White said many small farmers would get no help from such districts, which generally require a minimum of 500 acres. Furthermore, she said, "There is no way the Virginia General Assembly is going to change the law so smaller districts can be created," as one proposal in the study suggests.

White also disputed the report's contention that a land-use tax could cost the county up to $7.2 million a year in tax revenues. If the preferential tax treatment were limited to agricultural and horticultural land (excluding vacant land being held by developers), the cost would be under $1 million, she said.

By comparison, the committee contends, a land-use tax not only would preserve farmland, it would encourage the establishment of more small truck farms, which would save tillable land and thus keep more agricultural dollars in Virginia.

But the county study disputes that argument, contending that preferential treatment "is not likely to be effective in preserving agricultural . . . land in Fairfax County since other factors, in addition to high property taxes . . . can cause these lands to convert to developed uses." The other factors, the study says, include intense development pressure, low profits from farming, the age of the farmers and encroachment by suburban development.