Imagine if aliens in a spacecraft were studying the United States from above. Their conclusion, I expect, would be that the single most important decision these inscrutable humans make each year is whether to plant corn, soybeans or wheat. No decision, year after year, transforms more square miles of American land than the choice farmers make about which crops to grow. In some years, those aliens would see huge swaths of tight green rows of soybean; in others, more tall corn with yellow tufts; in others, an unusual abundance of wheat in, well, amber waves. Earlier this month, Rick Stern, who farms 1,500 acres in central New Jersey with his father, told me that he had just spent weeks making this very decision. He carefully studied the global market reports to learn about recent yields in Argentina and Brazil, America’s leading rivals for soybean exports. He grew up watching his father and grandfather face the same dilemma each fall, using as much information as they could scrape together.

But this year, there’s another variable that everyone is talking about — a factor that Stern doesn’t quite understand but knows will have a huge impact on the commercial grain business: whether, or when, the U.S. Federal Reserve will raise interest rates. Stern knows that the Federal Reserve is a powerful group of people who make decisions in Washington, but he doesn’t know exactly how it works. ‘‘That’s something I’ll never be privy to,’’ he told me, standing in a field of soybeans, in front of his enormous combine (‘‘It’s like a rocket ship with wheels,’’ said my son, amazed). ‘‘I just know their decision affects everybody.’’

Here’s what Stern does know: If the Fed raises rates, it would be bad for farmers like him. It would strengthen the dollar against the Argentine peso and Brazilian real, which in turn would mean that importers in China, Vietnam and the Middle East would buy more from South America and less from him. He also knows a rate increase would make it more expensive to borrow money to upgrade his equipment. He points out that his combine is 10 years old: If it broke down and he had to replace it, higher interest rates would make that enormous purchase significantly more costly to finance.

Over all, Stern has learned, a Fed rate increase — and there will possibly be one before the end of the year — will mean more acres of corn and fewer of soybeans, because American farmers won’t be able to rely as much on export markets to sell corn. It also means Stern will pay more attention to the few acres he has set aside for specialty crops, like watermelon, cantaloupe, tomatoes and hay. These he grows for local sale, so a stronger dollar won’t affect them. He just wishes the Fed would leave rates alone, and he can’t understand why they would even consider raising them. ‘‘It’s politics,’’ he says. ‘‘Most politicians forget who they are and why they went to Washington.’’