Nitrogen is an essential macroelement which regulates plant growth. It is the main constituent of proteins, enzymes, hormones, alkaloids etc. Which are used for the formation and functions of cellular components? The main source of nitrogen is air which contains 78 percent; by volume.

It reaches into the soil in the form of nitrogenous compounds by different methods. Most of the plants can not utilise atmospheric nitrogen in its elementary form. Only certain bacteria, cyanobacteria and fungi can utilise the atmospheric nitrogen.

These organisms are termed as nitrogen fixing organisms. Plants utilize nitrogen only when the nitrogen is fixed i.e., it is converted into nitrogenous salts or nitrate (XO3) and ammonium ions (XH4+). “The fixation of atmospheric nitrogen (X2) into nitrogenous salts with the help of microorganisms like bacteria, algae .and fungi etc. to make it available for absorption by plants are called Biological nitrogen fixation. Biological nitrogen fixation is of two types; such as asymbiotic nitrogen fixation and symbiotic nitrogen fixation.

Asymbiotic Nitrogen Fixation:

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The fixation of free nitrogen by the microorganisms living freely i.e. outside the plant cell is called Asymbiotic or Xon-symbiotic biological nitrogen fixation. Free living bacteria and cyanobacteria can fix nitrogen in the soil; the nitrogen fixing cyanobacteria are Anabaena, Nostoc, Spirulina, Stigonema, Scytonema etc.

These cyanobacteria have some thick walled cells called heterocysts which fix atmospheric nitrogen. There also occurs some free living aerobic bacteria such as Azotobacter, Derxia and anaerobic bacteria e.g. Clostridium, Desulphovibrio, Methanobacterium etc. capable of nitrogen fixation. Besides, Rhodotorula a yeast can fix atmospheric nitrogen in the soil.