Nitrogen Fixing Plants

I'm relatively new to the permaculture concepts of guild/companion planting and have encountered some conflicting information about the use of nitrogen fixing plants (namely legumes inoculated with bacteria) in guilds. Some people and resources have advocated planting things like goumi and black locust because they are nitrogen fixers and will make nitrogen available to plants around them but I recently heard that these plants only really fix nitrogen for themselves and it would only become available if you cut back the plants so the root matter died and released it into the soil. Is there enough ephemeral root matter to die off in the soil to release nitrogen or is it really all stuck inside said plant?

I have also read that nitrogen fixing plants such as alfalfa or beans must be tilled back into the soil to release the nitrogen and make it available to other plants because the nitrogenis stored in little nodules in the roots. Maybe there something I missing or maybe each year some of the roots die back and introduce the nitrogen to the surrounding soil. either way from my research I have concluded that nitrogen fixing plants are a much more complicated scenario than they are often presented as. I personally will stick to them as cover crops in a cultivated garden or utilize them if they have other redeeming qualities as companion plants but don't paln on wasting much time or space on them just for the sake of thier abilitity to fix nitrogen.

Killing any plant releases it's nitrogen back into the soil. 

The concern about feeding surrounding plants while living is an excellent concern.  This area of study is pretty massive. 

For example, I remember reading Toby's book to see that black locust and honey locust are both N-fixers.  So I used to be a big fan of thornless honey locust for a variety of reasons.  But!  Toby pointed out via a mailing list that Honey Locust makes just enough for itself and does not share.  But black locust makes enough to share. 

Further, I think it was Dave Boehnlein (of bullock brother's farm fame) was giving a tour and mentioned that if he grew an alder and then cut off branches on one side, that within weeks he would see greening of the area under where those branches used to be.  As if the roots tat would mirror the branches underground had died.  (my memory on this is pretty sketchy - perhaps Dave will clarify)

The important thing is:  if black locust shares and honey locust does not, that means that there is a massive spectrum of N-fixing plants and how much they share.  (and, of course, there is a spectrum of N-consuming plants and how much they consume)

How does a plant "share" the nitrogen it fixes?

As I understand it you don't necessarily have to cut or till in an n-fixing plant to have it benefit those around it.

If you did take one of those black locusts you mentioned and whacked off the top and 50% of the branches there would be a corresponding amount of root death (not necessarily a 1:1 ratio, but roots will certainly die in amounts relative to the amount you whack). This root death and decay is what releases nitrogen into the soil.

Does that mean you have to whack off the tops of all these n-fixers to gain their benefits? Nope. Just like your skin is constantly going through a process of replacing itself, so are a plant's hair roots. Roots are dying all the time and releasing nitrogen. New roots are growing as well and forming nodules to generate more nitrogen. Therefore, in many cases you don't need to whack everything to take advantage of it's nitrogen-fixing nature.

Now that said, Paul is correct that if you do whack a nitrogen-fixer back at the right time of year (generally a few weeks before the associated plants will be entering a growth period) you will create a large charge of nitrogen in the soil. My sample size is unscientifically small, but I've been whacking a set of alders each spring that had been planted in the same hole as black walnuts. I've certainly noticed improved growth on the walnuts, but I can't draw a distinct relationship between my pruning and the growth (since mulching, companion planting, and irrigation all went in in the last few years).

The "nitrogen-fixer whacking" strategy is applied extensively in the tropics with species such as Gliricidia, Erythrina, & Leucana. In fact, if you didn't whack these species in the tropics they would be very likely to completely smother the target species (avocado, jackfruit, mango, etc.) since they grow so fast. I don't know of any scientific research, but if it exists, I suspect the examples are in the tropics. Perhaps there is further info at http://agroforestry.net or the Nitrogen-Fixing Tree Association's (NFTA) website...

Paul, with regard to the honeylocust example, you may have stumbled upon a species that falls into the "exception" category. Last I heard there was widespread disagreement in the scientific world as to whether honeylocust fixes nitrogen or not. If not, it would be kind of weird as it is a legume. However, I believe that finding nodules on the roots is very uncommon (if anyone has found them at all). However, due to the fact that the tree can handle growing in harsh conditions it is possible that something unique is happening. For instance, the tree could be fixing nitrogen in a different way, it could have fungal associates instead of bacteria, or it could be a "sometimes" nitrogen fixer (it is possible that it will form a relationship with nitrogen-fixing bacteria when planted in tough conditions and not bother when the soil is rich). I believe another species of some confusion is Redbud (Cercis spp.).

Given all this confusion about honeylocust, it is possible that Toby views it as a marginal tree for nitrogen-fixation (thus not assuming it makes enough to share). However, there are a couple selected honeylocusts that have been bred for the nutritional value of their sweet pods (both for animal and human food). I believe these are 'Milwood' & 'Calhoun'. You can learn more about these in J. Russell Smith's Tree Crops: A Permanent Agriculture. And lest one start poo-pooing the honeylocust I'd also like to mention that they are one of the best shade trees for the northern climates and provide good timber to boot.

Anyway, that's the latest knowledge I've got. If anyone knows of any new discoveries regarding honeylocust I'd love to hear about it.

Dave

Some time back, I was reading about feeder roots (those out past the dripline of the plant), and the article indicated that these roots only live for a few days, they die and more are formed.  When they die, their nodules break down into nitrogen that becomes available to other plants.

Sue

Would it be wise to plant a few red alders in a food forest?  I know the cambium is edible, and they fix nitrogen, and provide mulch with their leaves. 

I have limited space and want to put something there that will benefit the forest the most.  The two trees I were thinking of were red alder and black locust.  These would be part of an apple-based food forest.

Alders will mine nutrients (esp nitrogen) and shed them in their leaves, a benefit.  But using alder as a food plant... I don't know.  Just because something is said to be edible doesn't mean you would ordinarily eat it unless you were literally starving.  'Edible' tends to mean that native peoples may have used it as a food source, and that it isn't poisonous in normal amounts.  It doesn't mean that it tastes good, that you would want to eat it unless you were starving, that the tree would even survive if you harvested much of it.

I my opinion, it would be too large and cast too much shade to be valuable for your uses.  There are enough alder trees growing wild around here that it might be more effective to just collect the leaves in baskets off your property, and use them a beneficial, nitrogen-rich mulch in your growing area.

Sue

That is true.  We have alders on the property too.  But I am thinking of putting them on the northmost end of the forest.  Maybe letting them mulch the forest themselves.  Then I don't have to gather leaves for mulch. 

I tried the cambium of alder once.  It was cool, eating wild and all, and I felt like a bear, but it was a little woody... you are right about it not being choice. 

Would it be a violation of permaculture principles to have species in a FOOD forest that aren't food?

Permaculture is not a religion (no matter what some people think).  It is a way of using Nature's methods to work for us.

There is an old story of a young Native American that was sent out into the wilderness by the medicine man (probably a trainee) and told to bring back samples of all the plants he could find that had no benefit of any kind.  He returned in a month empty-handed, explaining to the elder that he couldn't find any plant that was useless.

You need to tailor your planting to YOUR wants and needs. If your neighbor is doing permaculture and only wants to grow rutabagas, sunflowers and roses, that's fine.  You can grow kumquats, sweet corn, beans, squash and Iceberg lettuce.  Suddenly you decide that you also MUST have nasturtiums and alyssum.  That's fine, too.  And if your other neighbor doesn't want to grow anything but strawberries, more power to them.

After all, there's no reason for you to grow turnips and peas if you despise turnips and peas.

Grow what you'll eat, grow what you like, grow what you think is pretty.

Vegetables and fruit are foods for the body, flowers are food for the soul.  And there are osier willows for basketry, rhubarb for making ornamental concrete leaves, mulberries to make paper, and sweet sorghum and millet to feed the birds.

Sue

I thought we were trying to grow what will be best for all life.  If I just want apple trees and plant an apple orchard, that is not permaculture to me.

Ornamental concrete leaves?  How do they function?

"I thought we were trying to grow what will be best for all life."

YOU may grow whatever you want.  *I* am not going to waste time, energy and assets to grow something that is not useful to me.  I will eat ants raw before I eat cauliflower.

Concrete leaves are just interesting and pretty! 

Sue

I don't think permaculture is growing what is best for all life. but often things that are good for the ecosystem are good for us too. part of it is doing the best we can to support ourselves without harming anything else. ultimately because that helps us. am I selfish? yes. I love my life at least as much as any animal loves his. I find it disturbing that the human struggle is so often dismissed. do we need to do some things different as a society? of course.

why is planting dill to help distract insects from my vegies not permaculture? because I don't have enough stuff? I don't have any illusion that Ican design a system better for the world than already exists. I think our 'mission' is to find a way to live within it not alter it. find a  way to fulfill our needs just as every other animal on the planet does. if we all just do our part.....

if you want to "help" all life. the best thing for us to do is leave it alone and not put our bumbling hands in the matter at all. I think it is very arrogant of people to think they can design a system better for "all life" then nature can.

I'm with Leah.  Trying to fix all life is trying to play God. How can we provide all the answers when we don't even know the questions?

Sue

80% of the earths atmosphere is Nitrogen (N2) gas

Bacteria and fungi make atmospheric nitrogen available to the plants they associate with.

N-fixing fungi are called Mycorrhizal fungi
N-fixing bacteria also are found in soil.

Mycorrhizal fungi associate with roots. The fungi form nodules on the roots providing nitrogen to the plant to make proteins. The plant in turn feeds the Mycorrhizal fungi with sugars produced through photosynthesis.

This is cool!!!
Plants up to 80% of the energy is used to feed soil organisms. These organisms work as extensions of the roots. Fungi create networks underground between all the plants. If one plant is in need the fungi will feed it from the energy from the nearby plants.

Not all plants have n-fixing associations... So where do they get the nitrogen the need to make proteins???

All plants contain nitrogen in the form of proteins, but N-fixing plants have much higher concentrations. When they die or leaves fall to the ground. Soil organisms break them down (bacteria and fungi). Nematodes, protozoa, bugs, worms, etc. in turn eat the Bacteria and fungi. They then turn it in to plant available nitrogen.

Mycorrhizal fungi do not form nodules and do not fix nitrogen. They may facilitate nitrogen uptake from the soil, and may associate with nitrogen fixing bacteria, but I don't believe there is any documented case of Mycorrhizal fungi fixing nitrogen.

Well Mycorrhizal fungi host n-fixing bacteria. So it is the bacteria that fixes nitrogen, but you still need the fungi to host the bacteria. I apologize for not being clear.

Hey,

During their active growth, plants are constantly exuding soluble organic matter (so including nitrogen) and more or less giving it away to the great hungry microbial community. In addition plants are constantly growing new roots and the finer roots tend to have short lifetimes (think of how grasses produce so much underground biomass in a single season - this is mainly due to fine root turnover), so living plants are constantly contributing readily-decomposable organic matter to the soil all the time. Although some legumes like beans and chickpeas may divert a large fraction of their fixed nitrogen into the grains - the reason they're so rich in nitrogen - most legumes contain a higher concentration of N in their biomass than most other plants, so they will always be providing some nitrogen to the soil in their rhizosphere zone (the extent of their root system).

There are a number of scientific studies which have shown that legumes intercropped with grains benefited the grain crop during their active growing period i.e. without the legumes being mowed or tilled in. Similarly a Master's student of mine from Zambia studied the effect of Faidherba trees (an african savannah tree similar to acacias) on soil nitrogen levels and found the beneficial effect on crop yields (400% increase in maize yield under the canopies of these trees!) was due to the trees themselves, and not just a result of N-rich leaves falling down to from a thick litter just before the maize growing season (we determined this by applying Faidherba mulch to soil outside the tree canopy - still much less nitrogen enrichment than soil from under the canopy from which litter had been removed..). There's not a great deal of studies about these processes out there, as it's kind of technically difficult to study how a leguminous tree influences the belowground nitrogen economy without adversely affecting the tree itself. However, based on the little evidence I have encountered, I would say you can definitely count on legume trees providing extra nitrogen in the area that their roots have explored (to not say exploited), but it won't just be a result of leaf litter fall - that's just a bonus

But it is perhaps important to remember that how much nitrogen a legume fixes varies enormously between and even with species - I've seen studies where a same species of clover had a tenfold difference (50-500 kg N/ha to be specific) in how much nitrogen was fixed! Again estimates are sketchy if existent at all for many species of interest to permies, as it's not easy to measure how much nitrogen a large tree with an enormous root system has fixed in a given year without completely excavating the whole thing. So it could well be that many legume trees may in fact provide very little nitrogen on a yearly basis compared to clovers used as green manures. But no doubt the additive effect over the years of a perennial crop will make up for the lower N fixing ability.

I'm experimenting with different green manures this year, and have also just discovered some black locust growing wild (it's considered an invasive plant here in Belgium) in our young food forest on an alluvial, low-lying wet clay soil, and i'm keen to make the most of it! I have pruned the largest stems down to waist-chest height so i can harvest many young shoots of green growth to use as mulch, as the young trees are located under a large ash tree (it was there first) in a more shady position where we haven't planted much else yet. any other thoughts how to make best use of these young black locust trees?

Greetings,

Dave

Thanks Dave, such an informative post! I always wondered about that.

Black locust sticks -- great firewood and you won't have to split the sticks. Or poles and posts for garden use? Presumably they'll be excessively slow to break down so they won't serve certain purposes of mulch, though they'd shade the soil and all.

Looking to do a "Permaculture Orchard" here in central coast range Oregon. 20 miles SW of Corvallis.

N-Fixers planned, Siberian Pea shrubs, Goumi, Black locust, Sitka Alder.

Sitka Alder already on the property, easy to propagate, have seeds of Siberian Pea, Comfrey growing wild on a nearby river bank, Goumi & Black Locust, I will have to buy.

Any opinions on what will be the best, My plan is to polyculture all of these varieties and see what works best. But any info from someone with experience ion this would be appreciated.

Chhers!

Leah Sattler wrote:I have also read that nitrogen fixing plants such as alfalfa or beans must be tilled back into the soil to release the nitrogen and make it available to other plants because the nitrogenis stored in little nodules in the roots. Maybe there something I missing or maybe each year some of the roots die back and introduce the nitrogen to the surrounding soil. either way from my research I have concluded that nitrogen fixing plants are a much more complicated scenario than they are often presented as. I personally will stick to them as cover crops in a cultivated garden or utilize them if they have other redeeming qualities as companion plants but don't paln on wasting much time or space on them just for the sake of thier abilitity to fix nitrogen.

Following this debate with interest as this is precisely what I researched.

My research showed that they do release some nitrogen into the rhizosphere, but there is a time lag. I studied grass and clover, in the absence of grazing animals, significant amounts of fixed nitrogen did start to appear in the grass around September/october. I suspect that a big part of that was leaves dying back because of frost etc. Over winter probably a lot more fixed nitrogen was released because over the course of 3-4 years, nitrogen was building up in the system.

IMO, growing legumes purely for their rhizodeposition is not a good way to use them. Their are many ways you could use them without tillage though. You could use a cut and drop method or feed the cuttings to animals. I would suggest cutting and dropping the mulch somewhere else, rather than back onto the legumes (legumes prefer to grow in low nitrogen). MOST OF THE NITROGEN IS IN THE LEAVES, not in the root nodules.

The way I think of it is that legumes put a lot of nitrogen into THE SYSTEM. "The system" to organic growers, means soil, plants, animals, manure and people. If you are organic, these things should all be connected. If you are only thinking "how much nitrogen will this legume provide for my plants RIGHT NOW RIGHT HERE", you are not thinking organically/holistically. If someone grows a monoculture of beans, harvests them and sells them, and at the end of the season rips up the plants and bins them before cultivating the soil and leaving it bare over winter, they cannot expect to have an extra 400kg of nitrogen ready for their next crop. Likewise with intercropping. Some nitrogen will find its way to neighbouring plants, but because of the time lag, if you are growing summer annuals, think hard about what will happen after the crops are harvested. A winter cover crop (best a non legume) would be a very good idea, as there will be a lot of nitrogen in the form of organic compounds and decaying bits of root and leaf and exudate which will not sit around in the winter rain.

If, on the other hand, you are growing a perennial polyculture including legumes, the nitrogen from the legumes will eventually find its way into the other plants, it might take a few years but basically, with legumes, the system will be self sustaining for nitrogen at least.

I once worked on a farm where the farmer was attempting to improve his soil with an alfalfa monoculture. He planned to grow it for a few years and then plough it in. It was an OK plan, but I suggested as an improvement, I cut some of the alfalfa and mulched gardens with it. He was very surprised by the results of this which were extremely good vegetable yields. He thought that it would rob his poorest soil, but the alfalfa grew back fine. Ploughing up legumes does produce good results but a lot of nitrogen gets wasted, especially if there is a long delay between cultivation and planting. Actually I suspected that the real problem he had wasn't lack of nitrogen, but compaction from too much heavy machinery and cultivation.

If you intercrop right you get two crops where you would otherwise get one, and the yield of each crop will be roughly similar to what you would get in a monoculture. Legumes and non legumes can share soil. Caveat: Patrick Whitefield has noted many cases where intercropping doesn't work so good, and emphasises that companion plants should share space as well as soil, so tall plants and an understorey for instance. He also noted that the three sisters planting never seems to work outside Mexico.

Legumes do have to be used right. But basically sustainable agriculture could not work without them. Not using them is not an option, however, for some of us, with small gardens there may be more practical ways to quickly build soil fertility than planting legumes and waiting for rhizodeposition.

Here is a rather excellent page:  http://www.perennialsolutions.org/all-nitrogen-fixers-are-not-created-equal

Eric has broken down our favorite n-fixing plants as

high (160+ lbs/acre)

red alder
russian olive
scotch broom
kudzu
prairie clover
chickpea milkvetch
alfalfa
white clover

medium (85-160lbs/acre)

black locust
false indigo
ohio buckeye
silverberry
northern bayberry
senna
buffaloberry
siberian peashrub
autumn olive
groundnut
wisteria
northern sweetvetch
trefoil
yellow sweetclover
crownvetch
red clover

low (1-85lbs/acre)

catalpa
water locust
Kentucky coffeetree
green alder
purple vetch
common vetch
Canadian milkvetch

(note that these are just a few plant samples.  You gotta go to eric's page to see the real list)

Here is a good explanation of how nitrogen goes from 1 plant to another: