All organisms require nitrogen to make amino acids, which are used to build nucleic acids and proteins. About 78% of the Earth's atmosphere is made up of nitrogen gas (N₂). This form of nitrogen gas is not usable by most organisms. Nitrogen containing substances such as ammonium (NH₄), nitrate ions (NO₃-),
and nitrite ions (NO₂-) are found in the soil, in the waste produced by organisms, and in dead and decaying organic matter. Dissolved nitrogen is found in the ocean and other large bodies of water. The nitrogen cycle is how nitrogen moves between these forms. There are several processes involved in the nitrogen cycle including nitrogen fixation, ammonification, nitrification, assimilation and denitrification.

Watch this video to learn more about the nitrogen cycle.