Sulfur, an essential nutrient for plants, was as free as air back in the 1980s, drifting down onto farmers' fields from the polluted sky. The nutrient also caused acid rain, however, and it triggered chemistry that meant more mercury in fish. Regulations led to less sulfur in the air, but in the Midwest, where sulfur-hungry corn and soybean fields were proliferating, crops still needed the nutrient.