How Legumes 'Fix' Nitrogen in Your SoilLegumes (peas, vetches, clovers, beans and others) grow in a symbiotic relationship with soil-dwelling bacteria. The bacteria take gaseous nitrogen from the air in the soil* and feed this nitrogen to the legumes; in exchange the plant provides carbohydrates to the bacteria. This is why legume cover crops are said to "fix" or provide a certain amount of nitrogen when they are turned under for the next crop or used for compost.
Rhizobacteria are naturally present in the soil, but their populations are often too low to maximize nitrogen fixation. For the best nitrogen fixation, inoculate or coat the seed with purchased rhizobium. Specific strains of rhizobacteria work with different legumes. Read the packages carefully to ensure that you purchase the correct rhizobacteria for the legume cover crop you have chosen to sow.
To coat the seed, put it in a container and moisten it slightly with water or milk. The liquid will help the inoculant coat the seed. Sprinkle approximately 1 heaping tablespoon of inoculant per ½ pound of seed.
The rhizobia are living organisms, so you should sow the seed as soon as possible after coating it. Do not leave inoculated seed in the sun because the soil-dwelling creatures can’t live in UV light.
All legumes need adequate supplies of phosphorus, calcium and sulfur. Fall is a good time to test the soil for these major nutrients and adjust the soil content by adding lime, rock phosphate or gypsum as needed.
*this is one reason it is important to have well aerated soil in your gardens and lots of organic matter in your soil as well.
**This explanation found on http://www.seattletilth.org/learn/resources-1/almanac/october/octobermngg
Image source: https://en.wikipedia.org/wiki/Nitrogen_fixation
Rain brings nitrogen into the soil, as well, in a form that plants can use. Lightening facilitates this process which is why plants do so well after rain and especially after a thunderstorm.***