Legumes and nitrogen

General tips / questions on seeding & planting

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Monika
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I notice that in the current KG magazine (July), it states that it is a myth that leaving the roots of legumes in the ground adds nitrogen to the soil. To quote 'to have a noticeable effect, plants need to be dug into the soil before they flower ...'.
What then are the many little beige nodules attached to the roots, especially of broad beans when they have been harvested? I always assumed they are/contain the nitrogen and leave them in the ground until they disappear or are dug in.
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retropants
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I thought the same thing Monika. ???
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Geoff
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I've read that the effect is minimal but not all references say the same, there is an awful lot of reading to do to get a definitive answer. For example Wikipedia https://en.wikipedia.org/wiki/Nitrogen_cycle has:

The conversion of nitrogen gas (N2) into nitrates and nitrites through atmospheric, industrial and biological processes is called nitrogen fixation. Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants. Between 5 and 10 billion kg per year are fixed by lightning strikes, but most fixation is done by free-living or symbiotic bacteria known as diazotrophs. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds. Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two-component enzyme that has multiple metal-containing prosthetic groups.[22] An example of free-living bacteria is Azotobacter. Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. A few non-legumes can also form such symbioses. Today, about 30% of the total fixed nitrogen is produced industrially using the Haber-Bosch process,[23] which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.[24]


As I said there are lots of detailed papers to read such as https://www.csun.edu/~hcbio027/biotechn ... emann.html. I haven't read KG yet but I think the consensus is some benefit but perhaps not as much as we might have thought.
Last edited by Geoff on Thu Jun 06, 2019 10:23 am, edited 1 time in total.
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Tony Hague
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I suppose the question is if the nitrogen fixed in the root nodules is quickly used by the plant, or if a significant amount remains in the nodules. And how long the bacteria can continue to do the business after you've cut the plant down.
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I shall continue as usual, I don't leave the roots in tough, just strip and nodules off as I pull them up, every little helps.
On a bit of a tangent did you see the bit on Springwatch about the damage to soil done by digging last night?
Been gardening for over 65 years and still learning.
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Primrose
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No Tiger I didn,t see that. Must catch up on iPlayer. I I don,t deep dig, just fork over lightly because when i look at the compacted soil after winter and think of the minute little fragile roots of seedlings trying to push downwards I always wonder how they miraculously find the strength to do so if the soil around them seems to be as solid and compacted as a concrete block.

We are also given to believe that frost will help to break up the solid earth clumps and make the soil more friable. Are gardeners making all this up as they go along .....just like politicians? I,m beginning to wonder what to believe about anything any more !
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Geoff
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Reading round the subject it seems the plants feed the nodules which then give off ammonia that the plant is able to use in place of soil based nitrogen. Ammonia being a gas will not be there after the plant and nodules have gone, I've not seen any mention of useful nitrates being produced. When you read the long article about the stages of growth at which they are most active it perhaps explains why green manuring with legumes works as you are returning the whole plant to the soil when it probably has its highest nitrogen content.

I don't watch Seasonwatch as I can't stand the childishness, I hate the naming of animals (though we have named the dog!) and I don't like the lack of recognition of pests or vermin. I think you can take the all life is sacred mantra a bit too far. Sometime I'll have to read about cultivation damage and no dig. I am confused, as I have said before, why did cultivation arise if it wasn't effective and fed the world? Surely early man had more to occupy himself with if cultivation wasn't time effective.
Monika
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Thank you for the nitrogen explanation, Geoff. I do use green manure as well, but it's usually rye grass and phacelia, so the nitrogen bit doesn't arise.

And I do love my digging - it might not do my soil any good but it does me good, so I am going to carry on digging ....
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Interesting, thanks for posting Monika! (& repliers)!

I can't say I have ever thought about the dynamics behind it but it pops up in most gardening books & sites so you make it part of your routine as they must be clever people as that advice has been around for decades. As for dig v's no dig, personally I don't care how people choose to grow but I find it a bit uncomfortable on some sites when it is rammed down your throat as the only way we should grow. It is almost like a cult with the zealots following but pretty cost prohibitive, not saying wrong just not for me. How would I ever get my fix of bind weed chasing for a start? ;)
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Stephen
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So what did Springwatch say about digging?

I can certainly see a case for disturbing the soil structure, bringing layers from below to the top. However, does anyone really do double depth digging now? And, when it was practiced and commended, were the results so poor? No one could possibly have double dug my allotment, you hit sub-soil (think of a hoggin-like mix of clay and chalk) at one spit of depth. But surely there is value in de-compacting and aerating the soil?
For Charles Dowding, down in Somerset, his "no-dig" system certainly gives good results but he is in rural Somerset, surrounded by cows where the manure is plentiful and he has the space to let it stand allowing the first year's weeds germinate and be hoed off.

No one is going to grow spuds without digging anyway.
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Colin2016
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There is a guy that double digs his plot and says his soil is not very good/fertile, but hey stuff grows same as my beds which are no dig,

There is no right or wrong way just what suites you.

Note the compost has to be turned over so there is digging of a sorts.
Colin2016
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"No one is going to grow spuds without digging anyway"

How about these, planted 4 in deep in composted beds, covered with straw a tip from the guy that double digs, rather than making a ridge.

Hopefully there will be spuds on the plants.
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Stephen
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I'm sure there will be plenty of tubers Colin.
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robo
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I would be proud of them colin
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Primrose
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Looks a very healthy crop. Hope it works.
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