We know that urine has nutrients which plants use, but how to give it to them? The simplest guideline is to pee in a spot once and only once. Here are some details and research that support this guideline.
One approach is to use the following statement by Håkan Jönsson, a researcher at the Swedish University of Agricultural Sciences, in this article from Scienceline: The urine that one person produces can fertilize about ten square feet of soil a day, he says. Jönsson has been studying urine as fertilizer for many years and he has also been applying it himself in his own vegetable garden.
(I don’t know exactly what he means by “fertilize” since all plants have slightly different requirements and tolerances of various nutrients. I also don’t know for sure how long he means the urine will fertilize that ten square feet, but I have to assume he means for a short summer growing season since in Sweden that’s all there is — there’s no growing broccoli through the winter as we do here, for example.)
Nevertheless, most adults urinate about five times per day. So using Jönsson’s calculation, a person can fertilize a spot about the size of two square feet with a single urination. Or, if you’re not urinating directly on the ground, then once a day spread the collected urine over ten square feet of ground. Therefore the guideline: Pee in a spot (a spot of about two square feet), and only pee there once (once per growing season for vegetables, which would be twice a year here in Southern California — once in the summer and once in the winter).
Another way to come up with the guideline of applying urine as fertilizer by peeing in a spot once is to look at how much nitrogen you are adding to the soil with each single urination, and compare that to the need of the plant. Let’s look at a tree for this example. The suggested application amount of nitrogen for a mature citrus tree is about one pound per year, according to the University of California Master Gardener Handbook.
And how much nitrogen is in urine, such that we can apply one pound of it over the course of a year? We can find that by first determining how much urea is in urine because it is within that molecule that nitrogen is held. (Urea’s chemical makeup: CO(NH2)2.) An estimate on this Wikipedia page is that there are 1.318 ounces of urea in a gallon of urine. Because urea is about 45% nitrogen, that’s 0.593 ounces of nitrogen per gallon of urine.
A person produces between a quarter and a half gallon of urine per day, which means a person produces between 0.148 and 0.297 ounces of nitrogen per day. And as a person urinates about five times per day, then each single urination contains anywhere from 0.0296 to 0.0594 ounces of nitrogen.
From this we can estimate that in order to satisfy a mature citrus tree’s nitrogen requirement of one pound per year a person would have to pee beneath it between 269 and 541 times. This is something like once per day. It could be an evening routine.
To relate this approach with Jönsson’s, take a look at my own mature citrus tree, a Valencia orange that is over twenty feet tall. It has a canopy of about 450 square feet. But the roots of this tree extend beyond the canopy edge, as I know from digging near the tree, and many people say that a mature citrus tree can have roots extending up to twice as far as the canopy edge. The roots then might cover between 450 and 900 square feet. Urinating beneath it once per two square feet would mean urinating between 225 and 450 times. That’s pretty close to the 269 to 541 times above, which was estimated to add the one pound of nitrogen.
Again, the simple rule of thumb: Pee in a spot once.