Nitrogen fixing and possible protonodules in Gleditsia:
Gleditsia triacanthos does accumulate substantial nitrogen, but does not nodulate - foliage, seeds and fruit are considered a good source of nitrogen for foraging animals. The species is also known for shedding limbs (like many Leguminous trees), has large seed pods and is deciduous - so probably a nitrogen rich litter is also produced.
----- Original Message -----
Sent: 4/20/09 6:04 PM
Subject: [pfaf] Re: Black locust and honey locust
Thank you for all of your responses. I am aware of the relationship between nitrogen fixing bacteria and certain trees and plants. I am also aware that honey locust and black locusts are legume trees.
However, there are sources that say (like the Plants for a Future website), that honey locust does not fix nitrogen, and that black locust may not either, despite them being in the legume family. Therefore, it is not necessarily a matter of whether the bacteria is present, it is a matter of whether they indeed provide habitat and starch for this bacteria and whether or not the bacteria have a nitrogen fixing relationship with the trees.
Although some sources say they fix nitrogen, others say they do not. Perhaps the sources that say they do assume it is so because they are legumes, and not based on evidence. I guess I'll have to dig up some roots and look for the nodules, and perhaps check them out under a microscope!
I am also aware that Italian Alders (Alnus cordata), Japanese Alders (Alnus japonica) and Grey Alder (Alnus incana) can tolerate dry sites and that they do fix nitrogen (using a different bacteria than the legumes).
Again, permanent, perennial nitrogen fixing crops are very important for sustainable agricultural systems, and for home scale gardens, since the nutrient cycle can not be nearly as efficient without them. Even better are trees and shrubs that can be coppiced or pollarded (they grow back when they are cut near ground level, or higher up, and you can then use the resultant material as mulch, which builds soil).
Another of my favourite plants that fixes nitrogen is sea buckthorn (using the same bacteria as alders). They are amazingly hardy, drought tolerant, incredibly nutritious (edible berries) small trees (20 feet or so). I highly, highly recommend looking into them. They are a very valuable crop. One berry has about 10 times more vitamin C than an entire orange, apparently, among other nutrients that they also have in high quantities (vitamin A, vitamin E and essential fatty acids).
--- In firstname.lastname@example.org, "trentrhode" wrote:
> Hi everyone,
> I am creating an edible forest garden in Ontario, Canada, and I am rather confused about whether black locusts and honey locusts fix nitrogen.
> Most reports say black locust does, and some reports say honey locust does, but many people seem to think it doesn't (the pfaf website does not list honey locust as fixing nitrogen).
> On the pfaf website, it says black locust fixes nitrogen at the top of the plant information, but then further down, Ken writes that he thinks it doesn't fix nitrogen, and that it is greedy for nutrients.
> Other websites mostly say black locust does fix nitrogen, and most people I talk to say it does, although everyone seems pretty divided about honey locust.
> The reason this is important to me is that there are barely any large nitrogen fixing trees that grow in my climate (zone 5). Italian Alders might, and Japanese Alders do.
> It is, of course, important to have large nitrogen fixing trees in an edible forest system (at least to me, as I really want to fill that niche).
> Any thoughts?
> ~Trent Rhode
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