The beets Surendra Pradhan and Helvi Heinonen-Tanski grew were perfectly lovely: round and hefty; with their skin a rich burgundy; their flavor sweet and faintly earthy, like the dirt from which they came. Unless someone told you, you'd never know the beets were fertilized with human urine.
Pradhan and Heinonen-Tanski, environmental scientists at the University of Kuopio in Finland, grew the beets as an experiment in sustainable fertilization. They nourished the root vegetables with a combination of urine and wood ash, which they found worked as well as traditional mineral fertilizer.
"It is totally possible to use human urine as a fertilizer instead of industrial fertilizer," says Heinonen-Tanski, whose research group has also used urine to cultivate cucumbers, cabbage and tomatoes. Recycling urine as fertilizer could not only make agriculture and wastewater treatment more sustainable in industrialized countries, the researchers say, but also bolster food production and improve sanitation in developing countries.
Urine is chock full of nitrogen, potassium and phosphorus, which are the nutrients plants need to thrive—and the main ingredients in common mineral fertilizers. There is, of course, a steady supply of this man-made plant food: an adult on a typical Western diet urinates about 500 liters a year, enough to fill three standard bathtubs. And despite the gross-out potential, urine is practically sterile when it leaves the body, Heinonen-Tanski pointed out. Unlike feces, which can carry bacteria like salmonella and E. coli, urine poses no health risks—astronauts on the International Space Station even drink the stuff—after it's purified.
The nutrients in urine are also in just the right form for plants to drink them up, says Håkan Jönsson, a researcher at the Swedish University of Agricultural Sciences in Uppsala who was not involved in the beet study but has researched urine recycling for over 15 years. Food gives us nutrients like nitrogen as parts of complex organic molecules, but our digestive system strips them down into the basic mineral form that plants need—so "we have done half of the job," Jönsson says.
A small but dedicated contingent of organic gardeners in the U.S. and Europe already fertilize with urine at home, and researchers in Scandinavia have run pilot projects to recycle locally collected urine on small farms. But urine recycling may never become a part of large-scale farming in industrialized countries, because implementing it would mean drastically remodeling sewage systems in order to collect and transport liquid waste.
It would also mean swapping regular flush toilets for separating toilets, where a divided bowl and independent set of pipes separate urine from everything else. This detail is a roadblock, Jönsson says, because many people don't want a toilet that looks strange. "Acceptance is a big problem for this kind of system," he adds.
For the recent experiment with beets, the urine was obtained from specialized toilets in private homes. Heinonen-Tanski's group planted four plots of beets and treated one with mineral fertilizer, one with urine and wood ash, one solely with urine, and one with no fertilizer, as a control.
After 84 days, about 280 beets were harvested. The beetroots from the urine- and urine/ash–fertilized plants were found to be 10 percent and 27 percent larger by mass, respectively, than those grown in mineral fertilizer. By subjecting some of the beets to chemical analysis, the researchers determined that all of them had comparable nutrient contents—and according to a blind taste-testing panel, their beety taste was indistinguishable. The results were published in the February 10 issue of the Journal of Agricultural and Food Chemistry.
Effective fertilization is not the only benefit of recycling urine, Heinonen-Tanski suggested in a review paper in the January 2010 issue of Sustainability. The separating toilets that collect urine use less water than flush toilets, she wrote, and the simplified waste stream requires less energy in sewage treatment.
"Agricultural and health organizations should encourage people to use human urine as a fertilizer," Heinonen-Tanski concluded in the paper, especially in areas where wastewater treatment is unavailable or ineffective.
Though Jönsson is skeptical that micturition farming will ever happen on a large scale, his own family does practice urine fertilization: He and his wife use what they collect from their separating toilet to nourish their garden at home in Sweden. The urine that one person produces can fertilize about one square meter of soil a day, Jönsson said—but there's been less to go around since his three children left home.
"It's enough for the vegetables and the flowers," he said, "but I can only fertilize very lightly on the lawn. Otherwise I run out of urine."
This article is provided by Scienceline, a project of New York University's Science, Health and Environmental Reporting Program.