Nitrogen is essential for plants and animal life as it is a constituent of major bimolecules like proteins and nucleic acids. It is available in plenty in the atmosphere. However, for plants to be able to use it efficiently, it has to be converted into some nitrogenous compounds. This conversion is called nitrogen fixation. Nitrogen in the combined form is also necessary for the manufacture of explosives and chemical fertilizers.
In nature, nitrogen is fixed by some microorganisms and by lightning. The nitrogen fixing bacteria living in the root of leguminous plants and some blue-green algae convert nitrogen gas into ammonium compounds which are then absorbed by plants. The plants supply the bacteria with food, which in turn secrete ammonium compounds that are absorbed and used by the plants.
Lightning causes atmospheric nitrogen to combine with oxygen of the air to form nitrogen oxides. These oxides then combine with moisture in the air and are carried down by rain as nitrates, which are taken up by plants.
