The Fairfax County Planning Commission last unanimously approved the first agricultural district in the county, 639 acres north of Reston that includes two farms and a nature preserve.

The plan must be approved by the Board of Supervisors, which will consider the proposal at a meeting Jan. 26. If the plan is approved, property taxes on the two dairies would be reduced by about $30,000 a year through state tax benefits for agricultural districts.

The land would be assessed on its agricultural value, not on its potential market value for development. Currently the land is valued at $3,000 to $45,000 an acre, but would be valued at $300 an acre or less under the agricultural district, according to county planners. If land in the district is sold for development, taxes for the previous five years would have to be paid at the higher assessment rate, plus 8 percent interest, a rate the General Assembly may soon increase.

The new district has been endorsed by the county planning staff and a citizen advisory board appointed by the supervisors. But "it may be the first and also the last" ag district created in Fairfax County, planning commissioner Edward M Lightfoot said at last week's commission hearing, because the county has few large working farms left that could qualify as districts.

Just over a year ago the Board of Supervisors discouraged the county's largest dairy, Frankland Farms, from applying for agricultural district status. The farm, which includes 828 acres near Dulles Airport, has since been sold for a 1,600 home development.

The supervisors also were initially cool to proposal from the two dairies in the new district, the Spalding and Nalls farms, because of concern over loss of tax dollars and fear that developers might take advantage of agricultural or forest status as a tax dodge.

But sentiment apparently has, swung to Spalding and Nalls, and the county now seems ready to experiment with the agricultural district as one means of preserving farms and open space.

"Everybody loves cows. People in the suburbs have a soft spot in their hearts for farms, and this is going to pass -- absolutely," Supervisor, Martha Pennini (D-Centerville), said after last week's planning commission decision.

The two farms are off Seneca Road near Loudon County and the Potomac River. Part of the Spalding property has been sold to the Northern Virginia Regional Park Authority, and part to Nature Conservancy, a nonprofit conservation group that is including its 36 acres as part of the Nalls-Spalding ag district. Its land, and much of Spalding's, will remain in forest.