On a farm in coastal Maine, a barn is going up. Right now it’s little more than a concrete slab and some wooden beams, but when it’s finished, the barn will provide winter shelter for up to six cows and a few head of sheep. None of this would be remarkable if it weren’t for the fact that the people building the barn are two of the most highly regarded organic-vegetable farmers in the country: Eliot Coleman wrote the bible of organic farming, The New Organic Grower, and Barbara Damrosch is the Washington Post’s gardening columnist. At a time when a growing number of environmental activists are calling for an end to eating meat, this veggie-centric power couple is beginning to raise it. “Why?” asks Coleman, tromping through the mud on his way toward a greenhouse bursting with December turnips. “Because I care about the fate of the planet.”