This simply refers to the complex process by which nitrogen is added or removed from the soil.
The earth's atmosphere contains about 78% of nitrogen.
Plants utilize nitrogen when it is converted to nitrate for making chlorophyll and protein.

  Nitrate is the form in which atmospheric nitrogen is absorbed by plants.

            Ways through which atmospheric nitrogen can be converted to nitrate;

     *Nitrogen fixing bacteria(symbiotic bacteria).

     *Nitrogen fixation.

     *Blue-green gae(anabaena and nostoc)

     *Thunderstorm.

     *Decay.