Hopefully the title of this article has got you thinking ...... legumes fix nitrogen from the atmosphere, so what is he talking about.
All of that is true. All legumes have small nodules on their roots that contain many specialised bacteria that convert atmospheric nitrogen (not used by plants) into nitrites and ultimately nitrates, which are used by plants. This is an adaptation that legumes have developed that enable them to thrive in soils that are deficient in nitrates - soils that most heavy nitrate feeders such as corn and leafy vegetables have difficulty growing in.
For hundreds of years, farmers and gardeners have taken advantage of this by growing a crop of legumes to replenish soil nitrates prior to growing a crop of heavy nitrate feeders.
This is a good practice, but it needs to be noted that they ploughed or dug the legume crop into the soil before it reached maturity and started flower and produce seeds.
So what does this mean for the backyard organic vegetable gardener? Well, it is a myth that by growing a legume crop such as beans or peas, enjoying the harvest and then digging the remaining plants back into the ground will give you significant nitrate replenishment to the soil; some benefit will be gained, but no where near as much as you may have thought.
The reason for this is because of why the legume has the adaptation of the nitrogen fixing bacteria in nodules on their roots - this is quite simple .... it is for their benefit, they use the nitrates produced for their own growth and maturity through to flowering and seed development. When you harvest the beans or peas, you are removing from that little part of the ecosystem the nitrogen that was fixed by the bacteria.
Therefore, if your purpose for growing legumes is nitrate replenishment in the soil, consider growing a legume that you then dig into the soil before it start to flower and produce seeds. This way you will maximise the nitrogen replenishment into the soil rather than removing it to your dinner plate.