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Growing nitrogen in the forest garden

Crawford in his forest gardenMy favorite part of Creating a Forest Garden was Crawford's simple numerical explanation of how to create a closed loop garden with nitrogen-fixing plants interspersed among your fruit trees.  His rule of thumb is to fill about 40% of your canopy with nitrogen fixers if your forest garden is made up of high demand fruit trees (with lower percentages possible if you're willing to truck in fertility or use lower-yielding, less mainstream types of fruits).  The nitrogen will make its way from these trees, shrubs, and herbs to your fruit- and nut-producing plants when the former lose their leaves, discard feeder roots, or hook into the mycorrhizal network, and you can expedite nitrogen movement by cutting branches and laying them underneath your high value plants as a nutritive mulch.

Alder in a forest gardenLet's do some fun nitrogen math!  First of all, you have to consider whether your nitrogen-fixing plants are growing in full sun, partial shade, or full shade --- the plants need full sun for peak nitrogen-fixing ability, with the amount of nitrogen sucked out of the air declining linearly as the plants are subjected to more and more shade.  In full sun, an average nitrogen fixer will make available 0.033 ounces of nitrogen per square foot of canopy area, meaning that a hefty alder 16 feet in diameter would fix about 6.6 ounces of nitrogen per year.

Meanwhile, heavy feeders (including apples, peaches, and most of the mainstream fruit trees) need about 0.026 ounces of nitrogen per square foot of canopy.  So a semi-dwarf apple tree with a canopy spread of 15 feet would need about 4.6 ounces of nitrogen per year.  It's easy to see how planting one nitrogen-fixing shrub (like the alder detailed above) per fruit tree could do the trick.

Comfrey stole this tree's nitrogenCrawford included numbers on alternative source of nitrogen too, such as 0.18 ounces of nitrogen per cutting for one comfrey plant, 0.20 ounces of nitrogen from each peeing episode, and 0.096 ounces of nitrogen per pound of manure applied.  I'm a bit disappointed that he didn't make the distinction that comfrey is not a nitrogen-fixer, though, and that although you can cycle nitrogen with its leaves, the plant has a tendency to suck that nitrogen right back up in poor soil.  On the other hand, if you plant comfrey beyond the eventual canopy spread of your fruit trees, fertilize the comfrey with urine, and cut leaves regularly, the dynamic accumulator could provide quite a bit of nitrogen and organic matter for your fruit trees.

Mulch-producing plantsMy other complaint with the fertility section of the book is that it doesn't consider mulch, which seems to be a much more time-consuming trucked-in component during the establishment stages of a forest garden than nitrogen is.  (If you include animals in your forest gardening system, nitrogen is pretty easy to come by.)  I suspect that this lack is due to Crawford gardening on high quality soil where he can begin to use herbaceous plants to keep weeds at bay under trees within just a couple of years --- in the sad soil of my forest garden, I'm not so sure I'll ever be able to plant inside the dripline without negatively impacting my trees.  Instead, I'm considering planting chop 'n drop plants between the eventual canopies of the fruit trees for building organic matter.

Any handy rules of thumb out there on what percent of the growing area should be devoted to chop 'n drop mulch producing plants?  I'm also still taking suggestions for top perennial species, even though I'm focusing on annual cover crops at the moment.

The Weekend Homesteader starts at the beginning with choosing easy fruit trees and berries.


This post is part of our Creating a Forest Garden lunchtime series.  Read all of the entries:




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So, wait... If you pee on your tree, on average, just once every other week,[1] you'll give it all the nitrogen it needs for the WHOLE YEAR?!?? I mean, I'm assuming that the nitrogen needs fluctuate throughout the seasons, so you might need to "fertilize" more often in the spring and summer and less in the winter, but have I understood the maths right?

[1] (4.6 oz per year / 0.2 oz per pee) = 23 pees per year = about 1x every 2 weeks

Comment by Seth near Philly Thu Nov 29 13:10:32 2012
Seth --- You got it! Of course, trees (even fruit trees) need a lot less nitrogen than annual vegetables, but we overlook the power of pee way too often.
Comment by anna Thu Nov 29 14:17:11 2012

And of course, many people say that pee repels deer, which might keep them from nibbling little leaves.

Incidentally, in Charles Frazier's book "Thirteen Moons," he describes an old Indian's apple trees as having huge apples, "like the apples in dreams" because he planted them over his old outhouse sites. Maybe that's a good alternative for vegetarians who don't want to keep / eat chickens.)

(Frazier's book is worth a read, by the way, if you live in the Appalachians. The audio book was very good. Not much about farming, but it gives a strong sense of what the place might have been like in the frontier days.)

Comment by Faith T Thu Nov 29 15:38:27 2012

N content of various plants: http://www.helpguide.org/life/organic_foods_pesticides_gmo.htm

Only N fixing bacteria actually add N to the soil, using atm. N2 as the outside source, so only the legumes actually build the soil around themselves. Non-legume Chop & Drop plants would merely take up soil N, use it, then return it as they decay: no net change unless grown elsewhere & carried in....Using manure as a N source also works by transporting the N from the pasture, depleting it there, to your garden. The pasture needs to contain legumes to be sustainable.

Your figures for "one pee supplying the N for a year's tree growth" doesn't sound right. Plants are 2-5% N according to the site above, so every 1 lb of foilage would contain ~ 0.5-0.8oz of N. and we won't even mention the inefficiencies of converting the urea to nitrates. But still, why waste the pee?

Comment by doc Fri Nov 30 07:20:26 2012

Faith --- Thanks for the book recommendation! I tried to get into Cold Mountain, but as I recall there was too much war for my tastes. Hopefully this one will be more up my alley!

Doc --- That's the exact point I was getting at when I wrote "I'm a bit disappointed that he didn't make the distinction that comfrey is not a nitrogen-fixer, though, and that although you can cycle nitrogen with its leaves, the plant has a tendency to suck that nitrogen right back up in poor soil." However, what you may not realize (I didn't until I started reading my first forest gardening book) is that legumes aren't the only nitrogen-fixing plants. Alders, autumn-olives, and other actinorhizal plants also fix nitrogen.

To be honest, I haven't checked his math with the trees, but I only use half to one five gallon bucket of composted manure on each fruit tree each year, so it sounds about right. Of course, it would be better for the earth if people used rotational grazing and kept all the manure on the pastures, but when you milk cows or stable horses inside in preparation for riding them, the manure builds up and ends up being a waste in today's society. So I'm glad to turn that waste into my garden fertility. :-)

Comment by anna Fri Nov 30 07:41:28 2012

RE: peeing on trees-- there's one good horticultural use for my son even before he learns how to selectively weed. He loves peeing outside. Maybe I can train him to look for the plants with yellowing leaves.

I don't know about the chop and drop question.I'm curious now, though.

Comment by Sara Fri Nov 30 08:25:32 2012
I just found your blog and LOVE IT. I am also sitting here feeling a little better that I am not the only one who misunderstood the use of comfrey. I planted it around the bottoms of my fruit trees, because it's a good source of nitrogen. It was a few years later before I also learned it takes a lot of nitrogen out of the ground and doesn't fix nitrogen. The leaves are nitrogen rich though. A neighbor showed me that, when building a compost pile, layers of comfrey leaves put down periodically help it to "cook" faster.
Comment by Kimberly @ Adventures in Mothering Fri Nov 30 10:15:46 2012
As doc says, comfrey accumlates nitrogen, the nitrogen in the soil, while leguminious plants fix the N2 nitrogen molecules in the air that plants cant take up in the sil in a frorm taht plants can take up, so they add nitrogen from the air to the soil instead of just turning over whats already there. that plants can absorbe an element like nitrogen when it is in one molecule and not when it is in another is easier to usnderstand when you think that the bubbles in champagn are carbon and there are a lot of carbon atoms in sugar and in coal and diamonds, then it is easy enough to understand why it can be hard to get at an atom of any particular type when it was in one molecule and easy enough in another. It would be hard to eat diamonds as a way of getting at carbon instead of sugar and flour so you can seesee why plants can take up some types of molecules with the nitrogen they want in them but not others). Accumulating plants like comfrey and say mulberry trees do however increase the nitrogen in your soil even though they dont create it, because they bring up the nitrogen the rain has washed down into the soil that could, if there were not deep rooted plants around to take it up, get washed right out of the garden and end up in some river. It is a sort of cycle, the rain washes nitrogen down into the soil and plant roots take it back up to the top again, by using the nitrogen their roots take up in their leaves, which, when they fall, leave the nitrogen on top again./ As to pee and how useful it is to plants, I dont know exactly all the ins and outs of how easily they use the nitrogen in pee, they do take up amonia NH3 which pee gets converted to befor bacteria convert it to nitrite, Plants take p amonia NH3, nitrite, NO2 and nitrate NO3, so nitrite is not the only form of molecule holding nitrogen atoms that plants can take up according to nitrogen cycle. Also it has been found by scientists, who are very useful, much as some imagine they only study silly things, that they can take up amino acids, so if well we once decided that they took up the chemicals they needed as straight chemicals rather than elaborated in organic compounds, meat say or plant matter say, it is now known that they can take it up in amino acids, which are àrt of meat as far ias i can remember, so maybe they can take up the nitrogen in pee pretty directly, I suppose their maybe amino acids in pee. I have even brought a bottle of fertiliser for house plants that says it contains amino acids. /Also, in the the hugglekulture tradition, soil is enriched with wood chips in order to make it suitable for heavy feeding plants, this is explained by Austrian permaculturist Sepp Holzer if you read his books, this is not well explained in his you tube videos. It seems that wood in soil feeds plants while mulch on top has to rot before it gives them a supply of nitrogen. Sepp Holzer does not talk of leaving the chips to break down for a while before putting his heavy feeders in the bed so maybe they feed off something like amino acids straight out of wood. Tree stumps can be a good place for young trees to start their lives and example of a similar principle. Sepp Holzer in his book says that two years after he makes the bed he can plant plants with more moderate nitrogen needs it is in the first two years that he uses such a bed for heavy feeders. In the end he got fed up with using wood chips and experimented with simply digging trenches and filled them with logs and covering these with the earth he had got out of the trench, which, less hard work system, he found to work and which did not suppose that there was a risk of the bed feeding plants too heavily in the first years, as the trunks only rotted from outside in, bit by bit, so fertilised the soil less. He says that if you build the bed wrong you can get the wood rotting like a swamp rots, which is not so cool. It looks like plants have an greater number and more complex feeding habits than we imagined. Maybe it is fungi breaking down wood held in damp soil that make the nutrients in wood availiable to plants planted in a hugglekulture beds. I have not read any scientific explaination of why this should work, it is just a system that is empirically right if it is a system regularly used by austrians. You can read about it by googling it, though the best explaination of this form of market gardening I have found is in the famous Sepps Holzers book, "Permaculture". A practical guide to smale scale intergrative farming and gardening. agri rose macaskie
Comment by rose macaskie Fri Apr 12 20:57:46 2013

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