[cuongt]

ammonia and its derivatives are important as sources of nitrogen for plants.

Describe one way in which atmospheric nitrogen is made available to plants by natural means.

i think it has something got to do with the soil assimilating nitrates but i cant get it out

[Albert]

Begin checking this out: http://www.physicalgeography.net/fundamentals/9s.html

Then, if you want to go deeper, I would look for a biochemistry book at a University/High School library.

[Yggdrasil]

Here are some pages from wikipedia which may serve as good starting points:

http://en.wikipedia.org/wiki/Nitrogen_fixation
http://en.wikipedia.org/wiki/Rhizobia
http://en.wikipedia.org/wiki/Nitrogenase