Field Days & Legumes
keitheg at tpgi.com.au
Tue Aug 26 12:44:13 EDT 1997
Hi to all!
A couple of weekends ago I attended the South Coast Field day at
Candelo, NSW. It was refreshing to go to an Ag Show and not have
to walk around $1 million combine harvesters or be bombarded with
the latest agro-chemical free samples and trees-worth of glossy
hype, although I did accept a sample bag of seaweed compost :-)
However, for me the highlights of the weekend were the lectures
given by David Holmgren who spoke on 'Farm Forestry and Regrowth'
on the Saturday, and 'A Permaculture Approach to Weeds' on the
Sunday. It was the first time I had heard David speak, and was
very impressed at both his professionalism, presentation skills,
and content. His talks were interesting and always tinged with
a hint of anarchism (particularly regarding his definition of
weeds, which made some Landcare adherents shift uneasily a couple
of times!) Pat Coleby also spoke on 'Natural Animal Care and Farming'
- although I did not catch Pat's lecture. Having read much of her
material, I'm sure it was another fascinating hour.
The third speaker for the weekend (which is really the reason for
this post) was Greg Bender, a research biologist who spoke on
'The Role of Legumes Working for Us'. Greg showed how the fixation
of nitrogen by legumes is not an automatic process simply because
the plant is growing. 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.
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