All organisms require nitrogen to make amino acids, which are used to build nucleic acids and proteins. About 78 percent of the Earth's atmosphere is made up of nitrogen gas (N₂).

Source: Nodules on roots of legumes, Indiana Public Media

This form of nitrogen gas is not useable by most organisms. The nitrogen gas must be “fixed” or changed into a useable form. Nitrates, NO₃, are the usable form of nitrogen for plants. Special bacteria called nitrifying or nitrogen-fixing bacteria are found living in nodules on the roots of certain plants. These bacteria can change the nitrogen gas into nitrates.

The nitrogen cycle is how nitrogen moves between different forms of nitrogen. Watch this video to learn more about the nitrogen cycle.

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