peas beans and nitrogen fixing

[dave45]

Is it really true that peas and beans fix nitrogen out of the air? Does that mean that my old pea plants are better for the compost heap than other veggie waste?

[ojay54]

Well I always thought that nitrogen was fixed from the atmosphere via lightening,but,encouraged by your question, I've had a word with a guy who passed me through the science bit of RHS and a few other exams and it would seem the answer is yes.

I've always left leguminous roots in the soil,and sort of dug them in.For sure,the best matierial you can put on your compost heap is a VARIETY of stuff,the more the merrier.

[Green Aura]

I'm dragging this up from school biology so it's probably been long altered but yes, legumes have nitrogen fixing nodes in their roots.

The easiest way to utilise it is to cut the plant down at ground level and leave the roots in the soil. They'll rot fairly quickly. Then you can grow your nitrogen greedy plants e.g. brassicas the following season.

You can compost the rest of the plant but to the best of my knowledge it has no more N2 than other plants.

[ojay54]

I was under the misconception that the legume roots fixated nitrogen from the soil,but apparently the plant is also doing it via the atmosphere...I think...

[Odsox]

All legumes have root nodules that contain symbiotic bacteria that convert nitrogen to nitrates that the plant can use, in return the bacteria get a source of sugar from the plant's sap.
Someone on the forum a few years ago was convinced that when the bean plants die the nitrogen is NOT released into the soil for the use of the next crop. I tested the soil where I had pulled up a row of broad beans a few days earlier and found that there was a very high concentration of nitrates compared to soil a few inches away.
If you pull old legume plants up, a high proportion of the root nodules are stripped off and left behind.

I'll see if I can find that old post later.

[diggernotdreamer]

I went to a talk by the Elm Farm Research Station in Newbury. They had been doing research into the nitrogen fixing ability of legumes including field beans and lupins, They concluded that once a crop had been harvested, most of the available nitrogen had gone into producing the seeds, the only way to utilise all the nitrogen produced on the nodules was to incorporate it into the ground before they were too old. If you have ever looked at the nitrogen nodules on young legume plants, they are very fat compared with the nodules that are left on a spent plant, which is why winter tares are a good green manure for the gardener to grow, I plant them, use some of the tops for animal fodder through the winter, come the spring, I just shear off the growth, leave it on the surface and then mulch over the top of it, you need to use green manures before they flower as the energy and nitrogen goes into producing flowers and pods.

[Odsox]

OK, I found that old post (it probably didn't still exist until John sorted the forum)
http://selfsufficientish.com/forum/view ... n&start=60
About half way down the page.
Bear in mind that these broad bean plants were picked and finished old ones, not young healthy plants.

Also as someone pointed out earlier in the post, if there is no nitrogen left after a legume crop, why do all the old gardening books recommend planting nitrogen hungry crops after them. They must have come to that conclusion by observation, long before they knew what nitrates were, let alone how to test for it.

[doofaloofa]

AFAIK the nitrogen fixed by legumes goes into thier seeds in the form of protien

but what DnD said