That sea of green changes aquatic plant communities, interferes with recreation and navigation, and clogs filters at water-treatment plants. When the plants die and decompose after their growth binge, they use up the water's oxygen, leading to fish kills and shifts in fish populations. Beyond that point, plants don't benefit from extra phosphorus, and the environment can suffer. For years, College soil scientists have recommended soil tests to determine how much phosphorus to apply for optimum yields. Ideally, the amount applied would equal the amount removed in harvested crops.
Farm animals complicate that formula, however.
Manure contains valuable nutrients, but not always in the right proportions. As far as crops are concerned, manure usually has too much phosphorus and not enough nitrogen. Where we make repeated applications of manure to meet the nitrogen needs of crops, we end up accumulating large quantities of phosphorus in the soil. This poses environmental risks if that soil phosphorus is lost to lakes or streams. New federal rules, part of the Clean Water Act, promise to tighten restrictions on phosphorus runoff.