The Department of Agriculture announced yesterday that 6 million acres, about 3 percent of the national forest system, have been identified for possible sale under the Reagan administration's surplus property disposal plan.

The U.S. Forest Service property that could go on the block is in 44 states, Puerto Rico and the Virgin Islands, with parcels representing as much as 36 percent of a state's federal forest land (Ohio) down to less than 1 percent (Florida).

Before the land can be sold it must clear several hurdles, including further study by the USDA, congressional approval of legislation authorizing the sales and criticism from environmentalists.

The first criticism came yesterday from Gaylord Nelson, chairman of the Wilderness Society, who said the announcement "represents the opening salvo in an unprecedented assault on the integrity of our national forests . . . . It is a short-sighted and irresponsible approach to the management of our irreplaceable natural resources."

But the USDA stressed that not all of the 6 million acres is certain to be put up for sale. The department said that after, and if, Congress passes authorizing legislation, "Intensive studies of these areas will be made to determine their suitability for disposal before any are actually proposed for sale."

The Forest Service lands were chosen after President Reagan last year created a Property Review Board to oversee the identification and disposal of unneeded federal holdings. The USDA and the Department of the Interior, the largest land-holding agencies, set up asset management programs to pinpoint excess land.

The USDA review of forest lands held that 134 million acres of the system were unsuitable for further study and would be added to 51 million acres labeled to be kept as wilderness, wild and scenic rivers, recreation areas and monuments.

The remaining 6 million acres, the USDA said, are isolated parcels, lands in checkerboard ownership patterns, lands needed for community expansion and some under special-use permits.

The study areas include 48,577 acres in Virginia, 42,011 in West Virginia, 15,154 in Pennsylvania and none in Maryland. The USDA said maps showing targeted lands will be available in each national forest, in regional forestry offices and at the USDA in Washington.