Microbiology of Soils
Nitrogen fixation

both (a) and (b)
especially changes nitrogen compounds, mostly amines such as NH2
fix the free nitrogen (N2) by which it should not be usable by plants
changes the free nitrogen (N2) to a form usable by plants

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Microbiology of Soils
The groups of symbiotic bacteria, which have the ability to fix nitrogen

All of these
grow together for a mutual benefit is called symbiosis and so these bacteria are called symbiotic nitrogen-fixing bacteria
these bacteria are from the genus, Rhizobium
derive their food and minerals from the legume, and in turn they supply the legume with some or all of its nitrogen

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