Nitrogen fixation how nitrogen in the air (N₂) is changed (converted) into ammonia (NH₃) or other nitrogenous compounds in the soil.

Nitrogen fixation is essential to life because nitrogen compounds are needed for making all nitrogen-containing organic compounds, such as amino acids, proteins, and nucleic acids.

Atmospheric nitrogen is a relatively unreactive molecule. It is useless to all except a few bacteria and archaea. Biological nitrogen fixation converts N₂ into ammonia, which is used (metabolized) by most organisms.