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In a message dated 8/30/97 1:13:56 PM, you wrote:
>Two major factors which determine the quantity
>of atmospheric nitrogen fixed are the level of nutrients in the soil
>and the availability of the correct mycorrhizal bacteria.
>If the soil is already rich in nutrients (particularly nitrogen)
>then the legume will simply grow like a normal plant using the
>elements from the soil, and very little atmospheric nitrogen will
>be fixed. The nitrogen nodules may still grow on the roots, but
>they will not be functioning (Greg showed how to test a nodule
>to see if atmospheric fixing was taking place). In other words,
>it is the poorest soils which will fire legumes into fixing the
>most atmospheric nitrogen.
>However, no nitrogen will be fixed at all if the correct
>mycorrhizal bacteria is missing, as it is the symbiotic
>relationship between the plant and the bacteria which fixes
>the nitrogen - the plant cannot do it on its own. Most native
>legumes are able to find the appropriate mycorrhiza in our soils,
>but introduced species may have a hard time finding the one they
>need. This is why legume crops are now inoculated with a species-
>specific mycorrhiza (this is the area Greg now works in -
>identifying and manufacturing the specific mycorrhiza for farm
>And so, finally, to the questions :-)
>1. Are the non-native species of legumes that we use in
>permaculture actually fixing much nitrogen, seeing very few of
>us would be inoculating our seeds?
>2. At what point does the nitrogen fixed into nodules become
>available? Do we have to wait for the plant to die? or are nodules
>'sloughed off' during the life of the plant?
>3. Is there much point in planting legumes in rich soil
>specifically for nitrogen fixing purposes?
>Hope someone can give some light on this. I am just establishing
>a small permaculture demonstration orchard, and need to think about
>what to grow underneath the trees. Traditional permaculture lore has
>it that a mixture of perennial low-growing legumes is the way to go
>(pigeon pea, etc), but I now think that ground cover acacias may
>be a better bet. Any thoughts?
>Regards, and thanks for your patience.
We seem to be going round and round on this. Let me try to clarify that parts
of this post that I know about. Maybe others can fill in the blanks.
1) Rhizobia do indeed reduce N fixation if enough mineral nitrogen is
present. Nitrogen in organic form does not suppress fixation. This is from
research reported in Farm Journal circa 1967, where "conventional" chemical
nitrogen and nitrogen as organic chemicals, in the form of soybean meal, were
compared. it is a little like putting a steak under your cabbage, which
would indeed increase results, but of course science likes nice uniform stuff
to measure and compare. Therefore soil very high in N, but holding the vast
majority of N in organic form (micro-organisms, etc.), are great for the
legume crop and do not discourage it from playing its role. This is
corroborated by my personal experience. We have photographs showing
nodulation in rich soil in the Soils issue of THE INTERNATIONAL PERMACULTURE
SOLUTIONS JOURNAL. Email me for details.
2) Generally, legumes do not nodulate well in compacted soil. (Nodulation
does not guarantee fixation, but it is not likely that much fixation takes
place without nodules.) In a compacted alfalfa field (organic, by the way),
I could not find nodules to show to my students! There is little point in
trying to fix dirt into nitrogen. Rhizobia need air and compaction, by
definition, means that air pockets in soil have been driven away.
3) Rhizobia, and maybe other N fixers--someone might be able to shed light
on thihs--require specific trace minerals. They are more likely to be
present in highly organic soils, though not necessarily in optimum amounts.
As I said, I use wood ash, when available, as trees have a long time to
sequester scarce nutrients. I've also used glacial gravel dust and granite
dust to good effect, though not so much as with wood ash, possibly due to pH
factors and added P.
4) Crop legumes require adequate Phosphorous. Though they are often
efficient P accumulators, they just don't grow if the P isn't present in
adequate amounts. Stunted little legumes don't fix much N. In this case, it
is the plant, not the rhizobium, that is limited, but the rhizobium needs
those "surplus" sugars from the plant.
5) pH is important and this must be learned for each legume species.
6) Non leguminous species, such as alder or autumn olive, that use Frankia
instead of Rhizobia, are much more forgiving regarding soil conditions, as a
rule and are better pioneers in depleted conditions, some tolerating
waterlogged sites, such as alders, and some tolerating very scant moisture,
such a sweet fern.
The various bay laurels run the gamut--we have them here thriving where
temporarily flooded and/or subjected to drought, and at random seasons (A
"rainyi season" drought just broke last night with 3.4 inches of rain in an
7) Who isn't innoculating their legumes? Why not?
8) Mycorhizzae are often much species specific than rhizobia. Alien strains
of rhizobia (from the exotic plants viewpoint) often work just fine. And
then there are exceptions. It is very complicated. Because a mycorrhiza is
specific doesn't mean that the host plant and its rhizobia are. It is a bit
complicated. Get those soil organic levels up. There are likely to be more
fungal species in highly organic soil for the same reason that most of us
would prefer a table full of food to an empty one. These fungi are not
obligates (to my knowledge, probably there are exceptions), fortunately, and
can eat dung too. It's very complicated. However start by introducing the
desired plant species, always innoculating (even if growing it may not have
the rhizobia needed for fixation as nursery fertilizar can keep it going.)
Rhizobium innoculant is cheap--I innoculate even if the desired species is
"supposed" to be in my soil. You can't eat suppositions.
9) I am not sure that every species of rhizobium on one of its preferred
species of legume requires mycorrhizae. I'm pretty sure that well matched
fungi will always help, but not positive. We seem to be getting
oversimplifications here. Again, refer to the soils issue of our journal,
10) No legitimate permaculturist is going to broadscale plant an exotic
legume without trials. If they don't nodulate or if the nodules aren't
fixing, changes, including possibly legume species, need to be made. In warm
climates, there is a good chance that a single rhizobium will fix for most of
your legumes, though they may do better with a more slective species of
fixer. If in doubt, try the cowpea/peanut type rhizobium. It fixes for many
many species. Cool and cold temperate species tend to be a bit fussier. We
tried all conventional garden inoculatnts on caragana to no availa in one of
our Massachusetts places. Finally I bought some caragana seedlings (mine
were from seed), stripped off the nodules, made a slurry in a blender, and
washed it into the soil around the plants. This worked and they took off
without further ado. It is more conservative of inoculum to treat seeds, but
it is quite effective to treat established plants. Even drought-resistant
species like Caragana like mulch, by the way. This also made a large
difference for plants.
Well I feel as though I left out a few points. I guess I should have made
notes before starting. Well I'm sure someone else will fill in.
Oh yes, Keith. Now I remember two.
10) Pigeon pea is innoculated with cowpea type, though there are probably
more species specific rhizobia species in Africa as well. I have no trouble
with ours fixing here once we were able to get the inoculant (this can be a
hassel in backwaters like most of Florida). The ones planted before
rhizobium inoculation died.
Well I forgot the other one again.
Time to sign off.
For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since
1982), Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
courses via email. Next starts in Oct. 1997. Internships available.
Copyright, 1997, Dan & Cynthia Hemenway, P.O. Box 52, Sparr FL 32192 USA
The organizing principle of the Universe is silliness.