MAPLE PARK, Ill. — Snow and sleet were falling on Eldon Gould’s 500 acres this week, but he was already looking ahead to planting season; depending on ground conditions and the temperature, that could be just several weeks away. But now the prospect of steel and aluminum tariffs was adding to the list of worries and uncertainties that come with every corn and soybean season. “It’s the retaliation risk,” Mr. Gould said from his kitchen table in Maple Park, in a region of northern Illinois where farmland runs on for miles. “The world’s already awash in grain,” Mr. Gould said,...