For half a century, Tim Hemphill grew corn and soybeans on his 720-acre farm in northern Iowa. Then five years ago, as he readied his son to take over the business so he could retire, catastrophe struck: Local corn prices plummeted. “It was about the worst thing that ever happened to farmers,” he says. And it’s happening all over the country: Slumps in commodity prices, paired with rising costs of pesticides and seeds, have driven