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Pee on my Ashes...Says the Tree

Human pee with ash is a natural fertilizer, study says

When it comes to cultivating a green thumb, gardeners perhaps need only look to their urinals and fireplaces.
That's because human urine mixed with wood ash can help produce bumper crops of tomatoes, new research shows. In many ways the substances are natural complements, explained study leader Surendra Pradhan, an environmental scientist at University of Kuopio in Finland. (Related: "Urine Battery Turns Pee Into Power.")

Urine is high in nitrogen, while wood ash is rich in nutrients not found in urine, such as calcium and magnesium. Human urine and wood ash have each separately been used as fertilizer for centuries. But until now, no one had explored applying them together.

Pee Productivity

The scientists fertilized several groups of greenhouse tomato plants: one with human urine and birch ash, another with commercial mineral fertilizer, and another with just urine.  Plants fertilized with urine and ash yielded nearly four times more tomatoes than nonfertilized plants. This compared favorably with commercial mineral fertilizers, which produced roughly five times as much fruit as nonfertilized plants.

To the team's surprise, urine alone produced a slightly greater yield than those of urine and ash together. (Read about sustainable-agriculture projects around the world.)

But the urine-and-ash plants became larger than the other groups, and they bore tomatoes with significantly higher levels of the nutrient magnesium, which is key for bone, muscle, and heart health, among other biochemical functions.

A group of 20 taste testers ranked tomatoes grown by all methods as equally tasty.

Simple Process

The best part of this type of fertilization is that "it is a very simple process," Pradhan said. Urine can be collected from eco-friendly, urine-diverting toilets. Or farmers could just collect their pee in cans. The researchers estimate a single person could supply enough urine to fertilize roughly 6,300 tomato plants a year—yielding some 2.4 tons of tomatoes. The farmer would just need to give plants ash three days or more after applying urine. Pradhan and his colleagues are now trying to implement this idea in Nepal, where Pradhan is originally from.

Hormone Concerns

One potential setback may be that pharmaceuticals and hormones excreted in human urine—such as remnants of birth control pills—could negatively impact crops, Pradhan said. For instance, such byproducts could promote antibiotic resistance in local bacteria or get absorbed by the plants.

(Related: "Sex-Changing Chemicals Can Wipe Out Fish, Study Shows."

"However, in small scales in a single family, the pharmaceutical residue present in urine is very low and it can be acceptable," he said.

He also argued that pharmaceutical and hormone residues have been in animal-manure fertilizer for years, and that past studies have not found them to pose a risk to agriculture.

Findings appear in the August issue of the Journal of Agricultural and Food Chemistry

National Geographic News