At least the effect of the gypstacks in mining operations is fairly local; the damage caused by farm runoff is far broader. Studies show that only about 30 percent of the nitrogen fertilizers used in agriculture are actually taken up by the crops they are intended to feed. The rest remains in the soil or is carried off in rainwater. Phosphorus runoff creates algal blooms in freshwater lakes and streams. Nitrogen runoff gives rise to grander algal blooms at the mouths of rivers.

The photo shows agricultural runoff in Vermont's Lake Champlain following a heavy rainstorm. But this runoff is nothing compared to the flood of nitrogen that emerges from the mouth of the Mississippi River, where the fertilizer nourishes a colossal green mat of algae. As the algae die and decompose they become food for bacteria, which consume so much dissolved oxygen that resident fish and marine crustaceans cannot survive. A giant dead zone is the tragic result.