Nitrogen fixation is a chemical process which fixes molecular nitrogen into a compound that plants can use. The air contains almost 80% of nitrogen, which however is not available for plants.
Some microorganisms can fix the nitrogen from the air and make it available for plants. Some of them live in symbiosis with plants. The bacteria of the genus Rhizobium live in the roots of leguminous plants like those of the bean family (and so the plants that are grown as pulses) which therefore are well provided with nitrogen and so can build up much protein. For this reason these plants when cultivated, are goodcompanions for nitrophilous cultures.
French: fixation de l'azote