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"P" is for plants: Human urine plus ash equals tomato fertilizer, study says

tomato fertilizerGardeners, take note: the secret to growing hearty tomatoes is remarkably close at hand. Look no further than your fireplace and, er, your bladder.

According to a study from a group of environmental scientists at the University of Kuopio in Finland, human urine and wood ash make a reasonably potent tomato fertilizer, boosting plant growth and fruit yield dramatically over untreated plants and nearly keeping pace with conventional fertilizer. The research appears in the August 26 Journal of Agricultural and Food Chemistry.

The idea did not come completely out of left field—urine and ash have individually found use in helping plants grow, and their beneficial aspects appear complementary on paper. A commonly used nitrogenous fertilizer called urea is prevalent in urine, and wood ash (the Finnish group used birch) is rich in nutrients, such as potassium and calcium, that urine lacks.

In the greenhouse test, urine alone actually produced more tomatoes than urine with ash did—and neither treatment produced quite as much as did the researchers' mineral fertilizer. But both urine-based fertilizers roughly quadrupled fruit production when compared to unfertilized control plants. The researchers estimate that the product of a single individual's micturition could fertilize 6,300 tomato plants a year, yielding more than two tons of fruit.

The addition of ash did confer some benefits—those plants were larger and grew fruit with significantly higher magnesium and potassium content. A panel of 20 taste testers rated all growing methods as equally tasty.

Some caveats, remain, of course. The urine in the Finnish study was stored in cool conditions for six months before use, and it is unclear what effects this had on its fertilizing properties. What is more, plants are often highly salt-averse, and it seems reasonable to think that the salinity of urine could be harmful at high enough doses.

Then there's the inevitable gross-out factor: The researchers caution that even though urine is usually free of the harmful microbes found in fecal matter, care should be taken to avoid direct contact between urine-based fertilizer and the plants themselves to prevent contamination.

Photograph of tomatoes (used for illustration only and likely not grown with urine-based fertilizer): ©iStockphoto/Funwithfood

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