How legumes fix nitrogen in the soil

My wife and I were talking about legumes (pod producing plants such as peanuts, peas, beans as well as plants such as clover and buckwheat) and wondering about how they fix nitrogen in the soil. Do they store it up within themselves and then is it released into the soil upon there decay? Do they suck it out of the air, and put straight into the soil? If so how?

Well I did a little research and it appears as though the correct answer is, mostly the former.  Let me try and explain. As I understand it The legumes's roots form a symbiotic relationship with specialized bacteria called Rhizobia, that help convert the nitrogen from the air into something the plant can use. During the lifetime of this plant these bacteria continue to convert the nitrogen to the plant, but do not release it to the surrounding soil/plant life in the garden. Upon the death of the legume, the nitrogen is released from the plant, and becomes available for use by other plants in the garden.

These types of plants are great for crop rotations when you have previously planted high nitrogen demand plants.

A few things to keep in mind:

1. Nitrogen is only released to the soil upon the death of legume plants.
2. Calcium can intensify the ability to produce nitrogen by the bacteria.
3. If the soil already contains a fair amount of nitrogen, nitrogen fixing plants/bacteria will not be able to add much.

So if you ever wondered how that works now you know! Hope that helps and if you have any thing to add or corrections please feel free to comment below.

P.S My wife was right, as usual. ;)