This spring, for the first time in more than 20 years, Sidney P. Spalding filled the silos on his 470-acre farm in Fairfax County. At age 90, Spalding is getting back into the dairy business.

The retired Army general and his family are cutting 100 acres of hay and restoring the milk barn, long abandoned to the swallows. Heifers have been bred and are now grazing in fields near the Potomac.

But for all the family's work, Spalding says the key to the success of the new dairy will be a tax break from Fairfax County -- a tax break that is far from certain.

Spalding, who began dairy farming in 1935 but quit in 1960 because of a shortage of help, is bucking the trend in a county whose 400 square miles are nearly solid suburbs. With few farms and only three dairies left in Fairfax, Spalding admits, "It may be foolish to start a new dairy, but we're doing it."

A spry man who built most of the barns himself, Spalding lives in a tile-floored log cabin he erected in the 1930s when commuting daily to Army offices in Washington. And he still works daily on the farm, driving tractors, cutting grass with a scythe and helping rejuvenate the dairy, which his wife Edith and two of their nine children will jointly operate.

Crucial to the success of the farm and the new dairy, says Spalding, is a reduction in the farm's property taxes, which are close to $25,000 a year.

Spalding wants his farm to become the county's first agricultural district. If approved by the Board of Supervisors, it could reduce the farm's taxes "perhaps to one-fifth what they are now," says Spalding.

Spalding's farm currently is valued at nearly $1.5 million, or about $3,300 an acre. Because Fairfax does not have agricultural districts or a land-use tax, Spalding's farm is taxed at the same rate as that of residential owners, $1.54 for each $100 of assessed value.

"The taxes now are preposterous," Spalding says, "and if they don't go down, we'll have to sell the place and start a dairy somehwere else."

That is exactly what happened to the county's biggest dairy, Franklin Farms, little more than a year ago. The owners, Jean and James Franklin, sold the 828-acre farm off West Ox Road near Dulles International Airport to developers after county officials indicated that the Franklins' proposal for an agricultural district had little chance of approval. When the Franklins sold the farm, taxes were nearly $40,000 a year.

"Fairfax made it clear to us that we were in for a battle and my parents just threw up their hands," said the Franklins' son Stanley.

Jean Franklin said recently she and her husband had operated their dairy since 1936 and wanted to continue. "We would still be farming," she said, "but we couldn't do it with the taxes. It was very traumatic. We lost 40 years of our lives."

The Franklins now have bought a large farm in an agricultural district in neighboring Loudoun County and are starting another dairy.

In Loudoun, agricultural districts are becoming common, with 15,000 acres already incorporated into "ag" districts and another 45,000 acres under consideration for the state-sponsored program.

But Loudoun and most of Virginia are still largely rural, while Fairfax is almost yard-to-yard subdivisions.

Fairfax officials say they do not know how much farm land remains in the county. "But I do know there's not a single farm that doesn't have subdivisions nearby," said Supervisors Chairman John F. Herrity, who has opposed any change in the county tax structure.

The county gives tax breaks "to only one group of citizens, the elderly poor," said Herrity. Before the county starts giving breaks to rural landowners, he added, "we better know just how much farm land there is and how much in taxes it's going to cost the county. What is a benefit for some taxpayers is going to be an economic burden for others."

The Spalding and Franklin farms are both in the Centreville District of Supervisor Martha V. Pennino.

Pennino, who opposed an agricultural district for the Franklins, said this week she still is "not overly enthused with the concept" of preferential taxes for farm land.

"The Franklin family proved to me that I was correct," Pennino said. "They were sitting on prime land, near Rte. 50 and Dulles Airport. They sold out not because they were being squeezed by taxes but because the price was right. I'm not criticizing them . . . but as far as I know we only have three working farms left in the county and while it might be nice to preserve them, they are just like other businesses. We don't give preferential tax treatment to shoe outlets. But as for the Spalding family, I'll wait till the public hearing to see what they have to offer."

Another supervisor, Marie B. Travesky (R-Springfield), says she knows of only two supervisors who favor agricultural districts -- herself and her Republican colleague from the Dranesville District, Nancy Falck.

"But I think we've got to do something before we lose every last bit of open space we have," Travesky said. "The county has been shortsighted about this."

Springfield is the largest of the county's eight magisterial districts, both in area and population and probably also has the most rural land. Travesky says she would like to see agricultural districts around the Occoquan watershed: "That would also help protect the county's water supply."

Under state law, designed to promote the creation of ag districts and help preserve Virginia's open space and farm land, each landowner must apply separately to have land designated as an agricultural district. Last month, the supervisors approved the application forms for a district, which enables the Spaldings to begin the process. The county must act on the application, which has not yet been filed, within 90 days. However, the county need not approve it. At this point its chances are dubious.

Herrity calls ag districts the same thing as "that goddamned land-use tax" the supervisors recently postponed action on.

In fact, the farm district proposal is similar to land-use tax.

The land-use tax, now used in Loudoun, Prince William and about 60 of Virginia's 95 counties, allows certain rural land to be taxed according to its current use, such as farming or forestry, rather than its fair market value. The same tax method is used in ag districts, but would not be permitted countywide or for rural areas of less than 500 acres.

In Fairfax, it would mean that farm land now assessed at at least $2,000 an acre -- as are most potential development sites in the county -- could be assessed for as little as $300 an acre, says Phoebe Kilby, an associate county planner who helped draft the ag district proposal.

Under ag districts or a land-use tax, if farm land is sold for development the original owners must pay back taxes at the higher tax rate rate, plus 10 percent interest, but only for a five-year period. Herrity contends that the five-year rule effectively gives developers low-interest loans for five years and huge tax breaks if they hold land for more than five years.

The board has asked county planners to survey the county and report back on exactly how much farm land is left. The study is expected to take about two months and is being done by aerial photographs and door-to-door inquiries, says Kilby. That will give the board the background information it needs just before the Spalding ag district application comes before them, she added.

Herrity said recently he would be more inclined to permit tax breaks for rural landowners if the supervisors could be assured that only bonafide farms would be getting the benefits and that land specuators and developers would not be reaping profits at the expense of county homeowners.

"If we could isolate dairy farms, then we might have something," Herrity said.

One of the three remaining dairies in Fairfax is the Nalls' farm, on Seneca Road opposite the Spaldings. Like their neighbors, the Nalls are searching for relief from the high taxes. The Nalls own 131 acres, not enough to qualify for a 500-acre ag district, although they rent and farm considerably more.

"But we've got to find some answer," said Elma Nalls. "We've been here since 1916. We like it here."

In fact, the tax problem almost killed the Spaldings' plan for a dairy before the first cow was set out to pasture.

At one point, Spalding said, the family even had an architect draw up a development proposal, "(It) was a fine plan . . .," Spalding said, "with some 70 house lots, a beautiful plan . . . but we didn't want to do it."

Instead, they sold some riverfront land to the Nature Conservancy and the Northern Virginia Regional Park Authority to aid in the two groups' plan to preserve the Potomac shoreline as park land.

"That helped us," said Spalding, "and the Nature Conservancy is helping us now."

In return for the right of first refusal if the farm ever is sold, the Nature Conservancy is providing the 30 acres Spalding needs so his 470 acres will meet the 500-acre minimum for an ag district.

If the Spalding farm wins approval as the county's first ag district, the land would be used for forestry, farming and the dairy.

"We've got more than 200 acres in woodland, hardwood and some pine," said Spalding. "But we won't be cutting them down. We like animals and birds and wild flowers.

"The pileated woodpeckers like our woods, and we like them."