BOULDER — Boulder County will pay $6.7 million to buy 190 acres of irrigated farmland straddling the Boulder-Weld county line, under an open space purchase unanimously approved Tuesday by Boulder’s Board of County Commissioners.

Boulder County intends to keep title to the 80 acres that lie immediately west of East County Line Road between Oxford and Pike roads. That land probably will be leased out for continued agricultural use.

The county plans to sell the 110 acres that lie within Weld County. Boulder County would retain a conservation easement over that land to prevent any intensive development by future private owners.

The overall 190 acres, called the Ross Property, is “the last piece in a beautiful mosaic of protected open space that covers nearly 5,000 acres,” according to Mel Stonebraker, a land officer for the Boulder County Parks and Open Space Department.

Commissioner Will Toor called the purchase “an important opportunity to complete preservation of that corridor along the Boulder County line.”

The 80-acre Boulder County portion is bordered by county-owned open space on its north, west and south. Stonebraker said the property, together with much of the surrounding open space, “is arguably the best agricultural land in Boulder County. The preservation of the Ross property has always been a high priority, and the county has tried for years to purchase it.”

Commissioner Ben Pearlman said acquiring and keeping the Boulder County portion would fill “an 80-acre gap” of county open space and conservation easements in an area he said “is really a Boulder County breadbasket” of productive ag land.

Stonebraker said the county staff “feels that the purchase of the Ross Property is a wonderful opportunity to protect the last piece in an extraordinary expanse of open space while at the same time helping to create a buffer along the Boulder County-Weld County line.”

The $6.7 million purchase price would cover the cost of buying the land, associated water rights, a small house, two small barns and several sheds. The property’s current owners would keep ownership of mineral rights under the 110 acres and could continue to collect royalties on a number of oil and gas wells there.

The 80-acre portion of the property would be bought with $2.8 million from county open space sales tax revenues. Boulder County’s general fund or state Lottery Conservation Trust Fund grants would be tapped to cover the $3.9 million cost of buying the 110-acre Weld County portion of the property.