Washington Post: Erwin Johnson picks up a clump of the dark, rich soil that he has farmed for 35 years, like his father and grandfather before him. In a few months, this flat expanse of northern Iowa will be crowded with corn ready to be trucked to market. A year ago, that market got a little closer — and a lot better. Instead of sending his corn to a barge company to be shipped down the Mississippi River for export, Johnson now loads it into an open truck and sends it just two miles up the gravel …