Question about legumes and nitrogen fixing

[StephenH]

I'd appreciate answers to the following from any grapes with more scientific or gardening knowledge or both than me.

According to the gardening section of last Saturday's 'Guardian Weekend' magazine, the idea that legumes increase the nitrogen content of the soil because they fix nitrogen on their roots, with the aid of bacteria, is false, because although they do fix nitrogen, they use it all themselves, so there's none left over. Is this correct?
Assuming it is, I'd've thought that even if legumes don't increase the amount of nitrogen in the soil, at least they don't deplete it, because they make their own, so it's still a good idea to rotate them ahead of nitrogen-hungry plants. Is that correct?

[chris]

I believe so. I asked a question about what to do with the roots and nodules left behind - and people advised me to leave them in the soil, dig them in/over so when they rot down they release the nitrogen.

If they're saying that, what's in the nodules then?

[Capsid]

Well the clue is in the name. They fix nitrogen ie they extract it from the atmosphere and then convert that to ammonia which then can be converted to nitrites and nitrates which can then be used by the plants. The bacteria use the nitrogen, yes but when they decompose the ammonia is broken down by other bacteria to generate the nitrates.

[Nicos]

This is what Wiki says...

Quote....
"Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clover, soybeans, alfalfa, lupines, peanuts, and rooibos.
They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants.
When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil "

so - according to that, you need to leave the plants to die back before removing the roots from the soil.

We usually leave the plants to die , then chop off the tops, and leave the roots in place until Spring when we turn over the soil.

[Capsid]

Or chuck the roots on the compost bin.

[zazen999]

It's a Which report about gardening old wives tales that has been reported by the Gaurdina.

In particular it says:
The theory Nodules on the roots of peas and beans (legumes) harness nitrogen from the air and make it available to plants.
The evidence Research has shown that almost all the nitrogen gathered by bacteria in the root nodules of legumes is passed straight into the plant. By the time legumes are in flower, most of the nitrogen is in the leaves and developing pods. So the only way nitrogen will benefit future crops is if the whole plant – including seeds, pods, leaves, stems and roots – is allowed to rot back into the soil.
The verdict False: picking peas and beans removes most of the nitrogen that was gathered by bacteria in root nodules. Put spent plants on your compost heap to harness any nitrogen left in the leaves.

Full report here

[Nicos]

Hmm...that's interesting zaz...esp the bit about sun not scorching recently watered leaves...cos I've most certainly done that in the past!!
....scientists????

[solway cropper]

I've always composted the entire pea/bean plant including the roots as I reckon the amount of nitrogen that might be in the root nodules is very small and certainly no substitute for a good dose of fertilzer.

[hailtryfan]

The evidence Research has shown that almost all the nitrogen gathered by bacteria in the root nodules of legumes is passed straight into the plant. By the time legumes are in flower, most of the nitrogen is in the leaves and developing pods. So the only way nitrogen will benefit future crops is if the whole plant – including seeds, pods, leaves, stems and roots – is allowed to rot back into the soil.
The verdict False: picking peas and beans removes most of the nitrogen that was gathered by bacteria in root nodules. Put spent plants on your compost heap to harness any nitrogen left in the leaves.

Dead right. That's why farmers plough the entire plant (seeds and all) into the ground when using legumes to improve soil as part of their crop rotation. This also improves soil structure as well as making nitrogen compounds available for their next crop.

[StephenH]

Some interesting answers there. Thanks, all!

[Two_Sheds]

Just my tuppenceworth too, from: http://aces.nmsu.edu/pubs/_a/a-129.pdf

"nodules on annual legumes generally lose their ability to fix nitrogen, because the plant feeds the developing seed rather than the nodule"

"Some legumes are better at fixing nitrogen than others. Common beans are poor fixers (less than 50
lbs per acre) and fix less than their nitrogen needs."

"The amount of nitrogen returned to the soil during or after a legume crop can be misleading. Almost all of the nitrogen fixed goes directly into the plant. Little leaks into the soil for a neighboring non-legume plant."

[marchogaeth]

I have always understood though, that if you have legumes you shouldn't give them nitrogen fertilizer or the bacteria don't come into play. You put much less nitrogen on a mixed grass legume pasture. (I don't fertilize the fields at all - totally organic hay meadows or muck from garzing stock so I may be out of date).

[VirginVegGrower]

I tend to chop mine back into and under the top six inches of soil. But composting them would do the same.

[binley100]

I'm trying field beans as a green manure and you chop down and dig the lot in with a sprinkling of poultry manure to aid decomposition (well so I've read )

[VirginVegGrower]

I've sown those this year. Didn't know about the chicken poo. We have that composted. Do you have access to any?