A University of Illinois long-term study finds that air pollution legislation is controlling emissions so well that some farmers may need to begin applying sulfur fertilizers to their fields. Data shows that sulfur levels have declined in Midwest watersheds and rivers, and sulfur balances in many Midwest fields are negative. The research says more sulfur is removed each year during harvest and leaching than is put back in fertilizers and deposition. With the move from burning coal to natural gas and low sulfur coal, along with the increase in scrubbers, only about 25 percent of atmospheric sulfur is available, compared to 40 years ago. Sulfur comes from two main sources, the air and from groundwater. Fields with tile drainage quickly move sulfur to surface waters, contributing to the low levels in the soil. Rivers in agricultural watersheds have declining sulfur concentrations, a direct response to atmospheric deposition. As a result, farmers may need to start applying sulfur fertilizer, especially in areas with less soil organic matter.