Fairfax County, talking a cautious step patterned on its neighbors' actions, has set up its first agricultural district in an effort to preserve some of its dwindling farmland.

The new district totals 639 acres, including two dairy farms in northwest Fairfax and a parcel owned by the Nature Conservancy, a national land-bank organzation.

"I'm just so delighted," said Supervisor Nancy K. Falck (R-Dranesville) after her motion sailed through the board on an 8-to-0 vote.

"There was a time when only two of us on the board supported this," added Supervisor Marie B. Travesky (R-Springfield). "It was a long time coming."

"Creation of the Patowmack agricultural and Forestal District will mean an immediate and substantial tax benefit for the families who own the dairy farms. For example, real estate taxes for the 471-acre Patowmack Farm, owned by the Sidney P. Spalding family, will be reduced by almost $21,500, to less than $2,000.

The other farm, 132 acres owned by the E.J. Nalls family, will have its taxes reduced by almost $8,500, to less than $600.

The Nature Conservancy does not pay any taxes on its 36 acres because it is a tax-exempt organization.

Among those who came to the board meeting rooting for the district was Collas G. Harris, a member of the Great Falls Civic Association, which has been a strong backer of such legislation.

"I've been birddogging this since the beginning of 1977, when the General Assembly made it possible," Harri said. Like some other residents in the Dranesville district, Harris wants to see the supervisors take another, more controversial step, and adopt a countywide land-use tax that would give similar benefits to hundreds of landowners who farm part of their land or maintain forestland.

The main difference between an agricultural district and the land-use tax is that the agricultural district is confined to one area and has stricter guidelines on eligibility.

However, supervisors gave little hope for a land-use tax. "If it went to a vote now," said supporter Falck, "it would lose 7 to 2."

Three neighboring counties -- Loudoun, Prince William and Faquier -- have adopted the land-use tax and agricultural districts.