It depend only on atmospheric Nitrogen

N2 triple bond requires high energy in order to convert to NH3, 225Kcal + 8e-. The Bacteria that do it require 'Energy' from a plant host carbon breakdown and do not reduce N2 if not hosted. Then you can redraw the scheme into N2 fixation synbionts http://www.ncbi.nlm.nih.gov/books/NBK22522/

 Dear Prof. G., Bongi, thank you for your contribution. Pls can you help me with a link to a manual that could help me in the separation of the above mention organism

Not sure if I understand what are you looking for but if you mean microorganisms that fix nitrogen, there are some selective media you could use to isolate specific microbes. Biological nitrogen fixation occur in soil by both bacteria and archaea, free living or symbiotic so it depends on what microbes are you looking for?? for example, it would be easy to isolate Rhizobia from nodules in the root of  legume plant. this article might be useful http://link.springer.com/article/10.1007/s11104-014-2186-6 

Thank you Ibrahim S.A., I really appreciate

thank you Prof K.M., Towe

I am providing you two documents describing isolation, and characterization of a free living N2 fixing bacteria (Azotobacter spp) from soil.

 Thank you so much Prof. M.A., Kader, I am sincerely greatful

You can relatively easy isolate so-called diazotrophic bacteria (able to fix atmospheric N2) by using selective media (N-free medium with plentiful supply of sugars or other available C, Mg, P, Mo and other needed elements). However these isolates will instantly switch from atmospheric N2 to other less expensive N-sources (NH4, NO3, org-N, volatile NH3) as soon as they will become available. 

 Thank you Prof. N.S., Panikov I really appreciate your kind contribution