Understanding Nitrogen Fixers

As I discussed in another thread, i will soon start my first guild using a dwarf fruit tree (haven't decided which yet). I'm wanting to add nitrogen fixers for obvious reason but have several questions.

1. How do they actually work? Does their mere existence in the guild spread nitrogen to the other plants underground through their roots or must they be chopped and dropped?

2. One thing I plan to have is a perennial clover to serve as a living mulch as well as a add nitrogen. If I do some form of chop and drop using another plant (comfrey for example), will dropping over the clover smother and kill out the clover? In other words, does chopping and dropping in an area with living mulch work?

3. I'm still struggling to choose an appropriate N-fixer understory herb or shrub for my guild. Some others were suggested in another thread but all seem too big to be under a dwarf tree. Can someone offer some additional recommendations? I would prefer it to offer something edible to me as well but that is secondary.

Brandon Greer wrote:I'm wanting to add nitrogen fixers for obvious reason but have several questions.

1. How do they actually work? Does their mere existence in the guild spread nitrogen to the other plants underground through their roots or must they be chopped and dropped?

Brandon, you have put your finger squarely on the knot of a controversy that is -- at least as far as I can tell -- unresolved. The religion of nitrogen fixing is divided into at least three sects. One sect holds that the nitrogen is shared with other plants while the fixer is alive, perhaps by means of fungal networks between plants. Another sect holds that the nitrogen is shared only when the fixer plant is killed (chopped and dropped, or chopped and tilled in). Yet a third sect maintains that if you chop part of a living nitrogen fixer, there will be some proportional amount of root death (because plants support roots in proportion to their above-ground greenery) and that root death will release nitrogen to share with other plants. I confess I have no idea which of these three sects has the truth of the matter.

Brandon Greer wrote:2. One thing I plan to have is a perennial clover to serve as a living mulch as well as a add nitrogen. If I do some form of chop and drop using another plant (comfrey for example), will dropping over the clover smother and kill out the clover? In other words, does chopping and dropping in an area with living mulch work?

In my limited experience, yes. Clover is pretty tough stuff. I am using it as a green mulch in some of the larger pots in my container garden, and I routinely add chopped-and-dropped weeds in thick mats as a mulch right over the living bed of clover. A lot of the clover is killed (presumably releasing a slug of nitrogen into the soil) but plenty of it comes up through a couple of inches of mulch and continues growing happily. I'm careful not to use many inches of mulch, just one or two.

Their mere existence continually spread 1 unit of nitrogen. Chopping releases 2 units from the decomposing leaves and 3 units from the self-pruned roots (5unit). Completely killing and tilling it will release 8 units. If the

Please add one sect, when bacteria are consumed by nematodes they excrete waste that are in plant available form and are rich in nitrogen. from my understanding and simplified teachings in my pdc's, i say that the nitrogen becomes available when the bacteria dies. not sure if that is entirely true but my hunch. so when you say chop a cover crop down the photosynthesis sugar pump is no longer there and the bacteria dies from a lake of symbiosis. Indeed when you till in more nitrogen is released than simply chopping and dropping, why because more bacteria are killed through this mechanized soil management. it does indeed set soil succession back through killing fungi as well amongst others.
https://treeyopermacultureedu.wordpress.com/chapter-2-3-or-the-11-design-principles-from-the-intro-book/accelerating-succession-and-evolution/

First science: nitrogen fixing bacteria that live in symbiosis with plants turn atmospheric nitrogen into ammonia witch than comes available to plants in one form ore other.
Is al the ammonia consummed by the host plant? Who knows.( no science )
I assume part of the ammonia comes available for neighboring plants .
Chopping and dropping creates a mulch witch later becomes humus and in due process will give some nitrogen.
Clover is an aggressive grower that can withstand mulching, though don't exaggerate with it.
Myself, I use caragena as a nitrogen fixer to dwarf apples. The understory is mostly alliums for disease protection . I am planning to sow some Lucerne as an nitrogen fixator with a return in edible seed.

Try. Observe .React . Enjoy! EAT!

dirk maes wrote:First science: nitrogen fixing bacteria that live in symbiosis with plants turn atmospheric nitrogen into ammonia witch than comes available to plants in one form ore other.
Is al the ammonia consummed by the host plant? Who knows.( no science )
I assume part of the ammonia comes available for neighboring plants .

I can give a tentative answer to the sharing question from my own personal observations. Was with my son out in the yard when I spotted a patch of clover. The grass in the immediate area was greener than the rest of the lawn. Got down on my knees to explain nitrogen fixation to my son. His reaction, was "yeah Dad, uh-ha sure". What I could see though was that only those blades of grass immediately adjacent to the clover was greener. One cm away and the effect dropped to zero.

So, my answer if you have a lot of clover, covering a very wide area, you'll get a substantial benefit, but don't expect that benefit to extend even one inch past where the clover is positioned.

I feel like I heard about using legumes as nitrogen fixers from my PDC with Geoff Lawton. Don't hold me to this, as it has been a couple of years now, but I feel like it was expressed to us that legumes hold Nitrogen on the ends of tiny root fibers until the plant fruits. Then the plant takes up that Nitrogen to produce fruit.

Therefore, if you cut the plant at half flower, the nitrogen it is holding underground remains there to be moved and delivered by the microorganisms in the soil to adjacent plants.

I've heard a lot recently about Nitrogen release from fresh hugel mounds because of the green wood used to build them. I'm not sure if there is much science in that though.