Microbiology of Soils
Nitrifying bacteria can not be isolated directly by the usual techniques employed to isolate hetrotrophic bacteria. The reasons may be due to

fast growth
medium growth
slow growth
none of these

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Microbiology of Soils
Nitrogen fixation

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

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