Thirty years ago, the Fosters’ biggest business was exporting potatoes to Puerto Rico, and they sent out five 50,000-pound trailers a day, five days a week. This year, the Fosters have shipped only three trailer loads since the start of the harvest in mid-September. “The earth is gold to farm,” said Dean Foster, who learned before he was in kindergarten how to drive a tractor on his family’s potato farm in Sagaponack, N.Y. As a child, he never questioned going into the family business, which has been growing potatoes on the East End of Long Island for generations. In 2015, Sagaponack, a village in the Hamptons, was listed by Forbes as the second most expensive ZIP code in the country. Not great news for the humble potato farmer. The answer to their problems, unexpectedly, has turned out to be vodka. More