Applying much more phosphorus fertilizer and manure than crops can use builds up phosphorus in soil. After continuous over-fertilization, agricultural fields become a persistent reservoir for phosphorus that ends up polluting water bodies. Recent USGS study found that in the agricultural Midwest, many streams have their highest nitrogen concentrations in the spring, peaking in April-May. Nitrogen concentrations in many of these streams drop after crops emerge and begin taking up nitrogen, but phosphorus levels are often higher in the summer, creating ideal conditions for cyanobacteria blooms. In the Mississippi River basin, agricultural sources are the primary contributor of phosphorus, according.
