Ask a Question forum: nitrogen fixing in legumes

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Connecticut
NutmegCT
Dec 1, 2017 6:05 PM CST
"Nitrogen nodules" in legumes - how long does the nitrogen stay in the nodules?

I've had a vague understanding of legumes and nitrogen fixation for years. But I've never understood how the legume actually uses the nitrogen stored in those root nodules.

I seem to recall that rhizobia bacteria work with the plant itself to accumulate nitrogen in the early stages of plant growth, but the plant then uses the accumulated nitrogen to produce the growth to maturity.

Could you point me to an explanation of this "accumulation - distribution" process?

For years I've advised gardeners to clip off the roots of beans and peas at harvest time, to use as a manure, due to the roots' stored nitrogen content. I'd show people the nodules in the root system.

But if the plant uses that stored nitrogen as it's growing, then I'd think the root nodules are actually pretty much empty of their accumulated nitrogen. In other words, the plant works with the rhizobia to fix nitrogen for its own growth - not to store permanently in the nodules even after growth has ceased.

So when the plant dies back at frost, are the nodules still full of nitrogen?

If you can point me to further information, I'd sure appreciate it!
Thanks.
Tom M.
Name: Philip Becker
Fresno California (Zone 8a)
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Philipwonel
Dec 1, 2017 9:07 PM CST
The bean roots store the energy ! Not to be used by themselves.
It will be used by the next generation of plants, whatever, that be. Usually , it's a high nitrogen user, as say, corn.
Your welcome to use my name, as reference.
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Anything i say, could be misrepresented, or wrong.
Name: Daisy I
Reno, Nv (Zone 6b)
Not all who wander are lost
Garden Sages Plant Identifier
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DaisyI
Dec 1, 2017 9:16 PM CST
Yes and no. Or maybe no and yes. It depends upon how much nitrogen is in the soil to begin with. If there's not enough, the nodules will not form. They store extra nitrogen. When farmers grow alfalfa to fix nitrogen, they don't let the alfalfa die back naturally, they plow it under at (in their minds) the optimum time. Of coarse, the tops have been cut and added to some bales of hay (alfalfa hay) which is very high in nitrogen.

If you are trying to fix nitrogen, don't throw away the tops of your legumes, use them too.

Hope this helps.
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Name: Sue
Ontario, Canada (Zone 4a)
Daylilies Birds Enjoys or suffers cold winters Native Plants and Wildflowers Butterflies Annuals
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sooby
Dec 2, 2017 6:17 AM CST
Welcome!

Good question! According to this article from New Mexico State University, if the nodules are on a perennial plant they will continue to fix atmospheric nitrogen (convert N2 gas to NH3) throughout the growing season, but for annuals the nodules are short-lived and are replaced. The nodules on annuals become less able to fix nitrogen when the pods are filling.

The article tells how to determine the status of a nodule by its colour.

http://aces.nmsu.edu/pubs/_a/A...

Lower down on the page the article also discusses the eventual return of the nitrogen to the soil.
[Last edited by sooby - Dec 2, 2017 6:18 AM (+)]
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Connecticut
NutmegCT
Dec 2, 2017 7:00 AM CST
This is excellent. Thanks for all the advice.

Note particularly the section Nitrogen Return to the Soil in that NMSU article:

"Almost all of the fixed nitrogen goes directly into the plant. However, some nitrogen can be "leaked" or "transferred" into the soil (30–50 lb N/acre) for neighboring non-legume plants (Walley et al., 1996). Most of the nitrogen eventually returns to the soil for neighboring plants when vegetation (roots, leaves, fruits) of the legume dies and decomposes."

It seems to my non-expert brain that if the plant has matured, the nitrogen is no longer in the nodules. If the plant is harvested, then the only "plant harvested nitrogen" would be what leaked from the nodules into the soil during plant growth. If the plant is plowed under as green manure, then the maximum amount of nitrogen is returned to the soil.

Anyway, this certainly opens my eyes to another aspect of the traditional "leave the roots in the soil" advice.

I've attached a photo of part of the garden last year, showing young cucurbits, pole beans, and biennials for seed.

Tom M.

Thumb of 2017-12-02/NutmegCT/12d5d2

Name: Philip Becker
Fresno California (Zone 8a)
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Philipwonel
Dec 2, 2017 8:13 AM CST
Sue : Hi there 😀 .
I'm not a scientist, only diploma I have , is a high school diploma.
Your answer/explaination made no since to me. N2 and NH3 ? Is everybody suppose to know what they are ?

I come from a family of mostly farmers . Self-made, old time farmers, no higher education. But they knew how to farm and how things work.
My Family and organic garden magazine, plus, anything else I could find and read were my education. Everything in plain ol laymen's terms. I guess, highly educated people of today, would call it , ( poor man's terms. )

Nitrogen is taken out of air by legumes, stored in roots, and available for successive years crops, that require lots of nitrogen. So the rotation starts.

IE : Yr 1 Beans, yr 2 corn, yr 3 tomatoes. Then they start all over again.
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Anything i say, could be misrepresented, or wrong.
Name: Sue
Ontario, Canada (Zone 4a)
Daylilies Birds Enjoys or suffers cold winters Native Plants and Wildflowers Butterflies Annuals
Region: Canadian Keeps Horses Dog Lover Plant Identifier Garden Sages
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sooby
Dec 2, 2017 8:33 AM CST
Philipwonel said:Sue : Hi there 😀 .
I'm not a scientist, only diploma I have , is a high school diploma.
Your answer/explaination made no since to me. N2 and NH3 ? Is everybody suppose to know what they are ?


I'm sorry Philip, I was anwering the OP's question, who appeared to understand the process. You could, however, have read the very first paragraph in the link I gave which said:

"Approximately 80% of Earth's atmosphere is nitrogen gas (N2). Unfortunately, N2 is unusable by most living organisms. Plants, animals, and microorganisms can die of nitrogen deficiency, surrounded by N2 they cannot use. All organisms use the ammonia (NH3) form of nitrogen to manufacture amino acids, proteins, nucleic acids, and other nitrogen-containing components necessary for life."

Actually, your first post above is incorrect. You said "The bean roots store the energy ! Not to be used by themselves." We are talking about nitrogen, not energy, and the nitrogen is used by the plant that has the nodules as well as the crops that follow.

Name: Philip Becker
Fresno California (Zone 8a)
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Philipwonel
Dec 2, 2017 9:11 AM CST
Thank You Sue I tip my hat to you.
I'll have to check out article you posted, I'm still not to old to learn more than I know. 👍
Never know ! One day, I may be able to talk like a wizard, as good as you !

Oh !!! By the way, that's meant as a compliment. 👍
Bye bye birdie 😎😎😎
Anything i say, could be misrepresented, or wrong.
Name: Prabhi Setty
Trinidad West-Indies
prabhisetty
Dec 3, 2017 10:19 AM CST
sooby said: Welcome!

Good question! According to this article from New Mexico State University, if the nodules are on a perennial plant they will continue to fix atmospheric nitrogen (convert N2 gas to NH3) throughout the growing season, but for annuals the nodules are short-lived and are replaced. The nodules on annuals become less able to fix nitrogen when the pods are filling.

The article tells how to determine the status of a nodule by its colour.

http://aces.nmsu.edu/pubs/_a/A...

Lower down on the page the article also discusses the eventual return of the nitrogen to the soil.


Name: Prabhi Setty
Trinidad West-Indies
prabhisetty
Jan 24, 2018 5:59 PM CST
sooby said: Welcome!

Good question! According to this article from New Mexico State University, if the nodules are on a perennial plant they will continue to fix atmospheric nitrogen (convert N2 gas to NH3) throughout the growing season, but for annuals the nodules are short-lived and are replaced. The nodules on annuals become less able to fix nitrogen when the pods are filling.

The article tells how to determine the status of a nodule by its colour.

http://aces.nmsu.edu/pubs/_a/A...

Lower down on the page the article also discusses the eventual return of the nitrogen to the soil.


Name: Kat
Magnolia, Tx (Zone 8b)
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kittriana
Jan 24, 2018 6:38 PM CST
Ummm, for a farmers world, to acquire the benefit of the legume fixing nitrogen, the plants are grown together( 3 sisters planting). Since after blooming the plant is using that nitrogen, you would till it under to use as green manure and plant seed immediately the tilling under. The roots are left in ground for several reasons, the mycorrhiza in the old roots assists new growth, as well as the nitrogen property of legumes, or the sulfur attributes of say radishes.
kitt

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