By some estimates, we could hit "peak fertilizer" within 20 years, after which supplies of mass-produced nitrogen, phosphorus, and potassium may begin running low. That's scary for two reasons. One, we need these minerals for growing food and sustaining life. And two, resource shortages tend to lead to high prices, and market domination by a select few. If you don't like Saudi Arabia controlling the oil supply, you may not like Morocco—which has 85% of the world's phosphorus—with a stranglehold on what you eat.
What can we do? Nitrogen can be synthesized from air, but the other two elements are finite. Once we've dug up the easy stuff, we either have to find new sources (like at the bottom of the ocean), or recycle what's already in the environment. That means looking to the waste stream, even the human waste stream.
Urine could be particularly useful because it's pretty harmless once it leaves the body, we produce a lot of it, and it contains plenty of fertilizer. The practical issues are around separation (feces are much harder to treat), collection, and re-distribution.
A small project in Vermont is showing how we can use urine in agriculture. Last year, the Rich Earth Institute, based in Brattleboro, gathered 3,000 gallons of urine from 350 people, sanitizing it either by leaving it in a tank for 30 days or by heating it with a solar pasteurizer. It then spread the urine on hayfields, boosting the productivity of the land.
Rich Earth has received support from the U.S. Department of Agriculture and the Environmental Protection Agency. In National Geographic, co-founder Kim Nace describes urine as a "local, accessible, free, sanitary source of nitrogen and phosphorus," and a help to farmers who can't afford prices for mainstream products.
Annually, Americans each produce sufficient urine for 5,000 grams of fertilizer, including 1 pound of phosphorus, Rich Earth says. That's enough to grow one person's vegetables for a whole year. Used across the economy, urine might help cut agricultural inputs and reduce environmental damage caused by agricultural run-off. It would also cut the need for so much energy-intensive processing at wastewater plants.
Aside from the gross factor, the argument against human waste for fertilizer is the potential infrastructure investment needed. Ideally, you'd want the urine separated from feces, so you would need to get people using separation toilets and put in extensive new plumbing. That, however, has to be stacked against the costs of future phosphorus and potassium production. If the mining's no longer easy, urine collection may eventually pass the smell test.