In a move that may help it catch up with other area jurisdictions, Loudoun County officials are starting to crack down on landowners who receive major tax breaks designed to preserve active farms but who never lift a pitchfork.

Early indications are that between 5 and 10 percent of landowners in the Loudoun program do not qualify for the real estate tax breaks, which can reduce the owner's tax bill by as much as 95 percent. Those who cannot comply with the rules will be removed from the program and forced to pay five years' back taxes, officials say.

Under narrowly drawn state legislation, Loudoun, Prince William and other urbanizing counties can grant varying degrees of local tax relief to owners of active farms, forests and vacant land that will be excluded from development.

Qualifying land is assessed at values significantly lower than those assigned to parcels that are developed or can be developed. The program is designed to encourage agriculture by basing assessments on the ability of the land to support crops or animals rather than its potential sales value for house sites.

In Prince William and many of the other Virginia counties that offer the agricultural tax break, assessors conduct regular inspections of affected land parcels. Supervising assessor Bill Hewett said fewer than 2 percent of the parcels in the Prince William program have been removed each year since a crackdown weeded out longtime violators in 1988.

A similar program in Maryland counties also features regular inspections and a low violation rate, assessment officials said. Two additional Maryland programs provide tax breaks to owners of farmland who pledge to keep the properties permanently undeveloped.

Loudoun officials are targeting their enforcement efforts at "minifarms" of five to 50 acres, which have been popular among urban residents fleeing to the countryside in recent years.

In past summers, Catherine Ashby, Loudoun's commissioner of revenue, and Peter Holden, manager of the Loudoun Soil and Water Conservation District, navigated rural Loudoun's gravel roads in search of suspected violators. In the future, site inspections will increase, and all program participants will be required to file a report outlining the uses of their land.

Starting this spring, "I'll have no choice" but to end the tax break for landowners who don't comply with the rules or who fail to fill out the required report, Ashby said.

"We don't want to see {the program} abused," said Board of Supervisors member James F. Brownell (R-Blue Ridge), who owns a farm in western Loudoun. "If you don't qualify, we're going to see you get out of it."

Ashby said she could not estimate how much revenue the government is losing. Loudoun is facing a budget crisis as severe as any in the Washington area, but officials say the crackdown was not spurred by the county's financial problems.

In 1990, Loudoun property owners received about $17 million in tax breaks under the program, which includes land having a total potential sales value of more than $2 billion.

Some rural Loudoun land has been assessed at more than $10,000 an acre. Under state guidelines for the tax break program, farm owners can have their land reassessed at about $500 an acre.

At the current Loudoun tax rate of 85 cents per $100 of assessed value, the annual tax bill on a 20-acre farm with a market value of $10,000 an acre could be cut from about $1,700 to less than $100.

Farmhouses and the yards around them are not eligible for the tax reductions. A minimum of five acres can qualify a parcel for the agricultural or open-space tax break in Loudoun. Prince William authorities recently raised the minimum for open space to 20 acres.

Loudoun officials say recipients of the agricultural tax break will have to show that they produce a marketable crop or product. Many violations of the program are described as inadvertent, such as residents or out-of-state landowners who sign up, expecting to grow hay, but never bale it.

Authorities are encouraging citizens to report any suspicions that their neighbors may be claiming the tax break improperly.

Although most Loudoun participants are in the rural western half of the county, some officials say the program benefits all residents by restraining development and its related cost to all taxpayers. Other officials note that the tax break for farmers must be made up by the rest of the taxpayers.