… Across some of the most productive farmland in the United States, a nutrient called phosphorus has been accumulating in the soil for decades, at levels far beyond what crops actually require. … Unlike nitrogen, which easily escapes from soil into the air or groundwater, phosphorus sticks to soil particles. Once it’s added, it tends to remain in place. That trait made phosphorus seem environmentally benign. However, phosphorus can still be carried off fields when rain or irrigation water erodes phosphorus-rich soil, or some of the built-up phosphorus dissolves into runoff. Years of application have led to something no one initially planned for: accumulation.