Discarded food atop a compost pile at an organic farm. Photograph by Hannele Lahti, National Geographic

People have been using manure as fertilizer for millennia. But scientists now believe they can turn human urine into liquid gold—as composting material.

The premise is simple: Pee is rich in nitrogen, which plants desperately need. Commercial fertilizers boost plant growth and yield by providing abundant nitrogen to the plant’s roots.

Of course, commercial fertilizers can harm the environment if they get into lakes and streams. As well, not all farmers in the developing world can afford to buy fertilizer for their crops. Enter pee. (Related: “Human Pee With Ash Is a Natural Fertilizer.“)

Debendra Shrestha, a researcher at Tribhuvan University in Kathmandu, Nepal, noted that Nepalese farmers have been applying human urine to their crops for centuries.

Unlike commercial fertilizers, urine is free and abundantly available. Plus, it doesn’t seem to have any harmful environmental effects. The main question that remained was whether it actually worked: Would plants grow better when their soil was treated with human urine?

Pee Power

To answer this question, Shrestha and colleagues grew sweet peppers (Capsicum annum) in soil that had various combinations of human urine, compost, and urea—the main nitrogen-containing chemical in human urine. The urine was collected from communal toilets in Kathmandu, whereas the compost was sourced from cow manure. (Also see “Urine Battery Turns Pee Into Power.”)

The plants grown in soil that had a combination of human urine and compost grew the tallest, yielded the most peppers, and had the most total fruit weight per plant, according to the study, published recently in the journal Scientia Horticulturae.

The scientists say the pee was so effective because of several factors working together. For instance, the mix of compost and urine decreased the amount of nitrogen lost in the soil while making more carbon available to the plants.

“We need to start moving toward the application of urine in combination with compost,” Shrestha said in an interview with <a ” href=”http://www.scidev.net/en/south-asia/news/crop-thrives-on-urine-compost-mix.html” target=”_blank”>SciDev.Net.

To Pee or Not to Pee?

Still, not everyone is convinced. Other studies in Africa that used a combination of human urine, human manure, and poultry manure found that these substances did not yield more crops than commercial fertilizers did. (See “Human Waste to Revive Haitian Farmland?”)

The use of human urine, noted Surendra Pradhan, a researcher at the International Water Management Institute who is based in Ghana, has major problems, according to SciDev.Net.

For one, although urine is freely available, not all cultures might take to the idea of using it on their crops. What’s more, it needs to be used along with compost for it to be effective, since urine alone doesn’t have enough nutrients to sustain plant growth over several years. (Read more about sustainable agriculture.)

Last, although commercially available fertilizers aren’t free, many governments do subsidize their availability, which may decrease the overall appeal of urine-based fertilizers.