The Calvert Farmland Trust closed in on its effort to preserve the 155-acre Mackall Farm on Tuesday when the county Board of Commissioners approved the nonprofit group's application for a $660,000 loan to help purchase the archaeologically important site.

The purchase -- if completed -- not only would protect the property from extensive development, but also would preserve land that is said to include the burial site of the family of Thomas Johnson, Maryland's first governor.

"I think it's a great victory for the heritage of the county," said Kirsti Uunila, a state historical specialist.

The Calvert Farmland Trust, which was formed in the mid-1990s, hopes to purchase the land, enroll it in a preservation program, and then sell three subdivided tracts, said Susan Hance-Wells, president of the nonprofit organization.

The trust has been working to complete the deal for about two years, Hance-Wells said. Neighbors of the farm, including the Jefferson Patterson Park & Museum, had voiced concerns about the fate of the property, Hance-Wells said.

Jefferson Patterson Park & Museum is home to the Maryland Archaeological Conservation Laboratory and is situated on a 512-acre tract that has been described as one of the richest archaeological sites in Maryland.

"Patterson Park was very concerned," Hance-Wells said.

The acquisition of the property also would represent a coup for the county's land preservation efforts. The Mackall Farm loan money would come from a special fund the county set up to support the acquisition of open land in Calvert. More than 20,000 acres have been preserved from development since the late 1970s, when Calvert became the first Maryland county to initiate a program to preserve land by providing incentives to owners.

The trust has been part of that effort. Because it is a nonprofit, tax considerations make possible deals that otherwise might not work. For example, landowners can sell property to the trust for less than market value but then be eligible for a tax deduction for a charitable contribution in the amount of the difference between the sale price and the actual value, according to the trust.

To help finance the purchase of the Mackall Farm, the trust may enroll the land in the county program that grants transferable development rights to property owners agreeing to preserve land, Hance-Wells said. Those owners can then sell the development rights to developers who trade them for county permission to build more densely in designated areas.

The trust could use money generated by the sale of development rights to help purchase the property, which has an estimated value of up to $4.9 million, officials said. In the deal being finalized, it is possible that the trust will be able to cover the purchase price through the profits realized from the sale of the development rights and the subdivided tracts. If so, the trust would not need the county loan, Hance-Wells said.

Nonetheless, the board's unanimous endorsement of the loan means that the county money would serve as a backup source of funds when the trust attempts to complete its purchase of the farm, Hance-Wells said.