Sonoma County supervisors Tuesday are set to approve a $4 million contribution toward the purchase of Preservation Ranch, the 19,652-acre property that sprawls across the county's northwest corner.

The money would come from the county's taxpayer-supported Agricultural Preservation and Open Space District. It would be the final pot of public funds going to the $24.5 million purchase, set to close at the end of this month. About $10 million would come from the state and the remainder from private sources.

The county's participation was critical to getting the deal done, said Bill Keene, general manager of the Open Space District. "It's one of the best deals we've ever had," he said.

The conservation purchase — the largest by acreage in Sonoma County history — would shield the property from a controversial forest-to-vineyards project covering nearly 1,800 acres.

The property and the project is controlled by CalPERS, the giant state workers pension fund. A Napa vineyard development firm that managed the pension fund's vineyard portfolio bought the property in 2004 for $28.5 million.

The Conservation Fund, the private Virginia-based group that would own and maintain the property — managing it for timber and carbon credits and keeping it on the tax rolls — is contributing $6 million and has secured $3.5 million in financing. The group owns an adjoining 38,000 acres in Mendocino County, part of a former commercial timber holding that once included Preservation Ranch.

The Open Space District's contribution would buy a conservation easement over 18,200 acres of the ranch, which takes in a rugged 30-square-miles of second- and third-growth redwood and fir forest in the Gualala River watershed.

The county's easement would restrict use of the property to sustainable forestry, grazing and public recreation. It would ban forest conversions and limit any building to two residential structures.

The property takes in 160 parcels, about 60 of which would have been open to future development under the CalPERS proposal.

The state Coastal Conservancy, a public agency, last month approved $10 million toward the deal.

Sonoma Land Trust, the Santa Rosa-based nonprofit group, is set to contribute $1 million through a grant from the Gordon and Betty Moore Foundation.

The Conservation Fund is set to sell a 17-acre vineyard on the property to help recoup most of its financing costs.

Two other parcels that are non-contiguous with the main property on the north and west sides, totaling about 1,040 acres, could also be sold, with protections banning subdivision and forest conversions.

The Conservation Fund is set to take two years to draw up a plan for public access.

You can reach Staff Writer Brett Wilkison at 521-5295 or