I've designed four food forests in the last two and a half years, and included nitrogen-fixing shrubs and understory plants in all the designs on principle. I knew in the back of my mind that pissing in the garden must help a lot with nitrogen, but only this past week have I sat down and penciled out the numbers based mostly on data from Martin Crawford in Agroforestry News, Volume 3 Number 3. He categorizes overstory canopy plants (trees and shrubs) by their nitrogen demand:
- Very demanding: require 10 grams nitrogen per square meter of canopy per year. Chestnuts, Citrus sp, Plums, Walnuts, & blackberries
- Demanding: require 6 grams nitrogen per square meter of canopy per year. Apples, Apricots, Bamboos, Filberts, Hazelnuts, Medlars, Mulberries, Peaches, Pears, Persimmons, Quinces, Blackcurrants, Gooseberries.
- Slightly demanding: require 2 grams nitrogen per square meter of canopy per year. Cherries, serviceberries, Cornus sp, Elderberries, Yellowhorn, Raspberries, and many others
- Undemanding: need no extra nitrogen. Figs, Spice bush, Pines for nuts, red currants, hazels for poles, and many others.
- Ground covers: 1 gram nitrogen per square meter of canopy per year. Doesn't specify whether ground covers actively harvested for leaves or fruit would need extra nitrogen or whether the figure already takes that into account.
Crawford also mentions that one pee, which he defines as half a liter, contains about 5.6 grams of nitrogen.
So I crunched the numbers to figure out how much forest garden I can fertilize each year with my very own piss! I used the following assumptions/number bases/conversions (I don't do British math):
- An average person pees about 1/3 gallon urine per day
- 1/3 gallon = 1.26 liters
- 1/3 gallon contains 14.112 grams nitrogen
- 1 square meter = 10.764 square feet
- 28 grams = one ounce. 16 ounces = 1 pound. 1 pound = 448 grams
- Our forest garden canopy at our house here demands an average of about 8-10 grams/square meter for canopy areas. (In the heaviest demanding spots we have walnut or chestnut overstory with hazelnut understory, with ground cover beneath the hazel, for a total of 17 grams per square meters. Most areas have a demanding overstory with ground cover, for 7 grams per square meters. Many areas have even lower demand than that.)
- I assume that nitrogen returned to the garden in the winter does not fertilize as effectively as during the more active growing season, and that some of that nitrogren leaches away or otherwise goes to waste. So I assume that 8 months of the year, 100% of the nitrogen gets used, but during the other 4 months of the year only 25% of the nitrogen gets used. So effectively you have 9 months worth of useful nitrogen.
- According to this random website, vegetables typically remove 30-100 lbs nitrogen er acre, which is 13,440 - 44,800 grams Nitrogren / 43,560 square feet = 1249 - 4162 grams nitrogen / 4047 square meters = .3 - 1 gram nitrogren/square meter. So even assuming the heaviest demand, and more intensive gardening than standard far-spaced monocultures, 2 grams / square meter seems a reasonable high end guess for annual bed / open area nitrogen demand
Crunching the numbers for an average nitrogen demand of 8 grams per square meter gives the following:
1 day of nitrogen (14.112 grams) feeds 1.764 square meters of canopy.
9 months (270 days) of nitrogen feeds 476.28 square meters of canopy.
476.28 square meters of canopy = 5127 square feet.
If you assume the forest garden uses an average of 10 grams nitrogen per square meter, you knock off 20% of the coverage you got with 8 grams, so you get: 4101 square feet.
If you were only fertilizing open area using 2 grams nitrogen per square meter, you'd multiply by 4 to get 20,508 square feet, almost half an acre!
So between Theressa and me, we should be able to fertilize 8200 - 10,250 square feet of forest garden, without even taking into account our large open sun vegetable area. And that doesn't even take into account the nitrogen I bring into the yard each week in coffee grounds, and dumpstered bread and bins of produce scraps for the chickens. Nor does it include the nitrogen in our poop in the humanure pile. Not only do we not really need nitrogen-fixers, we may have an excess of nitrogen in the yard! Anyone know if an excess could occur in this situation, and whether we might harm the water table with nitrogen leachings?
In general, it seems urban lots don't need nitrogen-fixers for the nitrogen if the house inhabitants recycle most of their urine. Interesting!
I'll post later with a few redemptive reasons to keep including nitrogen-fixers in your designs. And I'll probably post a summary of Martin Crawford's recommendations for how much nitrogen-fixing plant canopy to plant for a given design.
Note: I wrote a few follow-up posts related to this one: