Christine wrote:The other thing about the nitrogen nodules is that they are depleted by the plant producing flowers and fruit 
This got me wondering (which is a dangerous thing  

 )
According to New Mexico State University ....<<quote>> Almost all of the nitrogen fixed goes directly into the plant. 
Little leaks into the soil for a neighboring nonlegume plant. However, nitrogen eventually returns to the soil for a neighboring plant when vegetation (roots, leaves, fruits) of the legume dies and decomposes.
Then I thought I would test it out.
I dug up a broad bean plant that had finished producing beans and carefully harvested the nodules. I then ground them up and soaked them in 5ml distilled water, left them for 10 minutes while I washed up the mortar and pestle before OH came home and found out what I was doing, and then tested the water for NH3/NH4.
The results were a huge 100 ppm, bearing in mind that artificially fertilised soil aims at about 20 ppm and if all the nodules in my bean row all broke down at the same time it would cause pollution, but of course they don't and anyway they spread into un-noduled (new word) soil nearby.
So I still think it's definitely worth taking advantage of a free resource.