Friday, December 07, 2007

Nitrogen-fixers vs liquid gold

Every book and article on forest garden design stresses the importance of including nitogren-fixing plants in your plantings. Very few give any pointers as to how much nitrogen-fixing capacity you need. And even fewer talk about, let alone quantify, the nitrogen recycled by returning your own urine to your garden.

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:


Bpaul said...

Awesome post. People can get so squeamish about this stuff, but it's just closing a loop: remove nutrients from soil via plant food, return it with your waste.


FarmerScrub said...

I just edited the post to correct the figure for fertilizing "open area" requiring 2 grams nitrogen per square meter. I originally multiplied the figure for an 8 grams nitrogen canopy by a factor of 5, when of course I should have multiplied by 4.

Paul, glad you enjoyed the post! It does seem obvious, doesn't it? :)


Bpaul said...

You compost in the style of The Humanure Handbook do ya?

I'd love to hear how that's working out for you. I thought the book brilliant, and was very jazzed by the simplicity of his ideas.

FarmerScrub said...

Hey Paul,

Yeah, we do the humanure thing. Since you asked, I'll plan a write-up about it soon!


Bpaul said...

I'd appreciate that.

Anonymous said...

You know, I did take a year worth of soil science and still have all my notes... from what I can remember off the top of my head, most env. issues with nitrogen are due to surface runoff that ends up in surface water, but it can get into the groundwater. It takes a lot to poison people but a little bit goes a long way towards eutrophication. I forget in what form it ends up in the water. I think you've got NH3, NH4, NO3, NO4, N2 all floating around out there... I'll dig up my stuff and you can see it next time we visit.

FarmerScrub said...

Hey Joanna,

Thanks for the info...I would really appreciate tapping into your knowledge & notes the next time we see each other!


Anonymous said...

Interesting post.

If you're concerned about having too much nitrogen in the soil nitrogen fixing plants probably won't contribute to the problem because in high nitrogen conditions legumes do not fix much nitrogen.

This is because the legume "trades" carbohydrates with rhizobium bacteria in exchange for nitrogen. The plant will not give up those carbohydrates if it can acquire the required nitrogen from the soil.

Anonymous said...

I hope that you and your wife maintain a very low sodium diet, or are fortunate enough to have a high average annual rainfall. Build up in the soil of sodium will eventually kill all but the salt tolerant plants. I would recommend doing calculations including how many inches of rain you need annually to wash your soil clean of your salt poisoning. BTW Other animals do not present this problem as they do not eat the salt we do.

rogerhall said...

Thanks for the post! My urine goes on my leaf bin (although I don't get to capture it at work, which I regret whenever I go to the bathroom there :).

United Diversity said...

Great post. I'm in the very early stages of designing a 1 hectare forest garden/ camping site. I think it could prove very productive what with all the urine and humanure the happy campers will provide!

In his excellent book Creating a Forest Garden, Martin Crawford says "as a general rule a total of about 25-30 per cent area of nitrogen fixers is enough, though if you have mostly heavily cropping plants with a high nitrogen demand then this may rise to 40 per cent" (p58).

About urine he says "you can pee in situ - this is ideal" and "urine does not need diluting before applying" (p55-6)

However, Patrick Whitefield in his "How to make a Forest Garden" book says of liquid manures:

"A healthy soil will support healthy plants, whereas plants which are fed directly with highly soluble nutrients are sappy and vulnerable. What is more, a high level of soluble nutrients in the soil is actually harmful to the soil microbes which are t he power-house of natural fertility. Liquid manures contain highly soluble nutrients, so their place in forest gardening is rather limited"

"The plus side of being highly soluble is that they are quick-acting, so they can be useful as a tonic for sick plants. If the weather is very wet, vegetables can get seriously short of nitrogen and other soluble nutrients, especially if they are growing on a sandy soil. Urine, diluted two to one, can bring back their colour and vigour and get them growing again. If used when the soil is dry it should be diluted more, down to ten to one in soil so dry that the plants need watering anyway. If should be used sparingly on clay soils, as its salt content can harm the soil structure." (p66)

I think I'm going to email Martin and Patrick to get more clarification on this issue... will try to remember to report back here...

United Diversity said...

I got a pretty swift response from both Martin and Patrick...

Martin says:

"It all depends of the scale of use. A little used here and there will be fine used neat – although of course leaves can be burnt by neat urine – this is what happens in nature. If you are intending to use a lot then it is probably best used diluted and/or composted with straw etc."

And Patrick adds:

"I think the distinction here is between a mature forest garden and a typical vegetable garden consisting of annual plants, which for most of the growing season are immature. The former has a very robust, diverse and mature ecology, well equipped to deal with the neat piss of mammals, which, as Martin says, is just what happens in nature. Very different from a veg garden, both from the point of view of plant damage and that of working within natural cycles. When I wrote my book Martin's garden didn't even exist and I was writing about something I still hadn't experienced first hand (Robert Hart's garden was very much a prototype). I was transposing from my experience of annual vegetable gardening.

"Even so, if I had a garden like Martin's I'm not sure whether I would piss in it willy nilly - no pun intended."