IN EARLY FALL, when the potato greens have withered upon their hills, I take a spade and pierce black earth, cautious not to push too hard too quickly. I lift and place each load of soil on a screen stretched across a frame of two-by-fours, which in turn sits atop my wheelbarrow. Every six or seven spadefuls, I stop, put down the spade, and break apart the wedges of soil, letting the grains and smaller stones fall between the apertures. What accumulates below the screen is fine as flour, while upon the screen, plump and pockmarked and smeared with mud, sit the year’s potatoes. I brush them off, place them carefully in boxes, and carry them down to the cellar, where I consider them the golden currency they are, a…