Bigger Picture Web, who captured a picture of nitrogen nodules on a field bean I took along to show the group last week.
Over the winter months I've been growing field beans in one of my veggie beds as a green manure. Green Manures are used as a means of adding organic matter back into the soil, and are particularly handy for people who're growing veg and don't have a ready supply of organic matter (compost or manure).
As members of the legume (pea and bean) family, they're able to make their own nitrogen and are known as nitrogen fixers. Legumes store it in little nodules (as can be seen here) and once the nodules have separated from the plant or the plant decomposes, the nitrogen is released and is available to other plants. Plants from other vegetable families get their nitrogen from the soil, usually from plant debris (or from fertilisers).
Green manures from the legume family are therefore great to grow before anything from the brassica family (cabbages etc) as the big leafy green crops will relish the additional nitrogen and are unable to make it themselves.
*It might surprise many gardeners who are familiar with crop rotation that botanists now believe that the root nodules accumulate half of the total nitrogen and that it only becomes available to other plants when the nodules are removed from the plant. This only happens when the plant is severely stressed from shade or drought or when the root dies.
Also, when the plant is young about 40% of the nitrogen is in the roots with the rest in the foliage and stems. Once the plant has flowered the reserves of nitrogen in the roots drop to 3-6% with 8-10% in the leaves and stems. The remaining 70-90% is stored in the seeds and seed pods.
What this means for most of us hobby gardeners is that the roots of the pea and bean crops that we have allowed to flower and fruit for the cooking pot are unlikely to be of any nutritional benefit to the veg following them in our crop rotations as is currently believed.... green manures are the key.
* Source Chris Beardshaw - How Does Your Garden Grow