Demand for phosphorus and nitrogen fertilizer is rising steadily as the world grows more crowded. "Over the next 50 years, the human population is predicted to grow from nearly 7 billion to 9 billion," says Abdullah Jaradat, research leader at the soil lab run by the U.S. Department of Agriculture in Morris, Minnesota. "To feed those people, we must double our agricultural output, and that means more nitrogen and phosphorus."

This photograph shows an outlet pipe from a phosphate waste impoundment in Lakeland, Florida; the waste forms a stream that joins the water table. The phosphate mines in central Florida pose an environmental threat to the region.