nitrogen fixation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The conversion of atmospheric nitrogen into compounds, such as ammonia, by natural agencies or various industrial processes.
• noun The conversion by certain soil microorganisms, such as rhizobia, of atmospheric nitrogen into compounds that plants and other organisms can assimilate.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry ,(biochemistry) the conversion of atmospheric nitrogen into ammonia and organic derivatives, by natural means, especially such conversion, by microorganisms in the soil, into a form that can be assimilated by plants

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the assimilation of atmospheric nitrogen by soil bacteria and its release for plant use on the death of the bacteria