Nitrogen is a vital nutrient for crops, but soils rarely produce enough of it to sustain the relentless farming needed to feed a world population that has more than doubled in the past 50 years. For almost a century synthetic fertilizer has made up the difference, driving up crop yields. Yet we have overindulged, creating vast quantities of waste. Globally, more than 90 percent of the nitrogen used to produce meat and dairy products, along with 80 percent used to grow plant-based foods, is lost to the environment. It contaminates streams, lakes and oceans as well as drinking water, and fuels the growth of algae and other organisms that can suffocate fish and kill plants. And much of the nitrogen that crops and livestock do absorb, and we thus consume, exits our bodies as sewage, a significant source of nitrogen pollution when it is not sufficiently treated.