The fate of yellow dent corn is traced from soil to supermarket in the new documentary “King Corn.” Abundant, cheap, and rich in calories, the crop is found in almost everything we eat. And, as the U.S. government continues to subsidize commercial farms to drive prices down and yield up, Americans struggle with the consequences of a high-calorie, low-nutrition diet. Living on Earth’s Bruce Gellerman gets an earful from co-producer Ian Cheney about the making of the movie.