The nitrogen glut (and the uneven distribution that causes shortages in some places, particularly sub-Saharan Africa) is damaging the biosphere in many ways. Recent studies show that its harmful effects will be intensified by climate change.  It is painfully clear that any serious effort to prevent ecological catastrophes in this century must include reining in the overproduction of reactive nitrogen.   To determine how that can be done, we need to understand how and why the glut occurred. How did the metabolic rift in 19th century agriculture, characterized by the depletion and waste of essential nutrients, lead in the 20th century to planetary rifts caused by massive oversupply and overuse of the same elements? Part Three will address that question.