The nitrogen is an essential constituent of protein - the building block of all living cells. It is also a major constituent of the atmosphere (79 per cent). Although organisms live in an atmosphere rich in gaseous nitrogen yet the organisms cannot use this nitrogen. It can be utilised only after gaseous nitrogen has been 'fixed' into some economically usable form. The transformation whereby molecular nitrogen is converted into a variety of nitrogenous compound and its release again into the atmosphere, is what constitutes the nitrogen cycle.