Subject: Re: Nitrogen fixing beans
From: kjohnsto@mesastate.edu (karen johnstone)
Organization: Mesa State College
Date: 12 Oct 1997 18:37:43 -0600
Message-ID: <MOD$971012.735@rec.gardens.ecosystems>
References: <MOD$970927.29878@rec.gardens.ecosystems> <MOD$970928.20324@rec.gardens.ecosystems> <MOD$971003.14144@rec.gardens.ecosystems> <MOD$971006.15925@rec.gardens.ecosystems> <MOD$971011.5846@rec.gardens.ecosystems>


I'm not a botanist, and don't have expert information, but somehow the
thread still seems to lack information. I recently did a modeling
project on nitrogen cycling with cowpeas/corn. Had to find a lot of
specific numbers. Conclusions I came up with from my research:

The nitrogen fixed by the rhyzobial bacteria is only a part of the total
nitrogen returned to the soil by a legume crop. In the case of large
nitrogen contributors, like cowpeas and fava beans, the decay of the top
growth is the major contributor. During the growth stages, legumes will
deplete nitrogen in competition with other crops in an intercropped system.
In soils where nitrogen is deficient, the contribution is most significant.
Given the figures that I have seen, they must pull some nitrogen from the
air, since the organic nitrogen present in the above ground part of the
plants far exeeds the available nitrogen from the soil at the time of
planting. If surplus nitrogen is present, the legumes will take all of
their nitrogen needs from the soil. I can only assume, until someone with
expert information tells me otherwise, that in the poor-soil situation, the
legumes have the ability to pull nitrogen from the air. (Which should be
easy for a plant, which is so very good at pulling a much less plentiful
gas, CO2, from the air). The nitrogen must come from somewhere, since a
rotation of legumes will leave a field with a higher nitrogen level than it
started with.

Todd M. Bolton (tmbolton@erols.com) wrote:

* Allyn Weaks wrote:
* >
* > In article <MOD$971003.14144@rec.gardens.ecosystems>, "Nick LaRocca"
* > wrote:
* >
* > > Jack Dingler wrote in article
* > > <MOD$970928.20324@rec.gardens.ecosystems>...
* > > >
* > > > Chuck, beans and other legumes fix nitrogen by using nitrogen
fixating
* > > > bacteria that are harbored in root nodules under the soil. So to
answer
* > > > your question the notrogen is already in the bed.
* > > >
* > >
* > > Actually, this isn't quite the whole story. Most of the nitrogen
produced
* > > by the nodule bacteria is immediately absorbed by the legume,
* >
* > Yes, but there is some leakage from the nodules, on the order of 10%?
* > (From one or another of my first-year bio books; I can't remember which,
* > maybe Campbell.) Also, root systems are much less static than the upper
* > parts, so as the roots keep growing and breaking off, nodules get
detached
* > and returned to the detritus system, releasing even more.
* >
* > > and that's
* > > where it stays until the plant is dug in. If you remove the plant and
take
* > > it somewhere else, most of the nitrogen goes with it.
* >
* > Yes, though don't discount the roots, which can be pretty extensive in
some
* > species. If you carry off the tops and leave the roots behind, you're
* > still ahead of the game, though not by as much. For me, it's worth not
* > tilling in the tops, because tilling does me a lot of no-good in turning
up
* > huge amounts of weed seeds, as well as disturbing my little helpers (ants,
* > worms, beetles, etc.). Mostly I can leave the tops as mulch though, and
it
* > vanishes faster every year.

* Accordong to NRCS "Nutrient Management" booklet dealing with
* conventional farming and the amount of reduction in N fert if the field
* is put on a legume rotation. The first year alfalfa is planted it
* allows reducing N fertilizer app by 100 to 140 lbs. per acre. This is
* out of a range of 90 to 160 lbs acre normal application for high yield
* corn.

--
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