Legumes

Jack Rowe jackrowe at compuserve.com
Tue Aug 26 12:45:38 EDT 1997


Keith queried:

>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?
Depends... some, as the medics, are great nitrogen-fixers and also great at
finding the needed mycorhyzal (sp?) cohorts, at least here. Look to how the
plants grow in depleted deep sandy soils for a hint as to whether they have
found the bacteria. 

>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?
Most of the nitrogen fixed by a legume -- from 40 to 90 percent or so --
ends up in the leaves (since leaf production is the major use of the
nitrogen by the plant), so the tops are most important. Tops which are cut
release from 40% (cut and let lie) to 60% (cut and dig in) of their
nitrogen the first year, on average, and one might expect something
analogous for the nitrogen in the roots. 

>3. Is there much point in planting legumes in rich soil 
>specifically for nitrogen fixing purposes?
In accord with what you've written (nitrogen-fixing is suspended when soil
nitrogen is adequate), no. Much more effective under conditions of high
nitrogen is either 1) cropping, or 2) catch-cropping. Cropping is good
sense, usually the soil is improved just for the purposes of cropping and
receiving a yield. If season or other conditions are inappropriate, "catch
crops" are used to capture nitrogen and any other available nutrients. For
these purposes a catch crop is wanted that has high nutritional
requirements for the mineral element in question. In the case of nitrogen,
it's hard to beat the grasses -- annual rye, oats or wheat (crops), corn,
etc. Dry matter production by grasses can easily double that of a good
legume. I like the idea of a mix, such as of a grass and a leafy green such
as mustard. 

On the other hand, the Land Institute in Kansas, USA has found in
establishing prairie plant mixes that the more legume species that are
included in an initial seeding, the greater will be the number of ALL
species (whether legume, grass, broadleaf, etc.) which become established
on a permanent basis. So nitrogen is not always the main or only issue. 

As for planting under trees, tap-rooted plants are best since fruit trees
tend to concentrate feeder roots on or near the surface. Mustard is good
here, too, comfrey, etc., but most grasses have phenomenal root systems and
will compete strongly with fruit trees. Pasture mixes of non-competitive
grass/legume mixes can be gotten from such supply houses as Peaceful Valley
in California. 

Jack Rowe
Cross Timbers Permaculture Institute
Seeds of Texas Seed Exchange
jackrowe at compuserve.com



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