The conversion of atmospheric nitrogen into nitrogenous compounds so as to be made available for the plants to absorb is called as nitrogen fixation. If this fixation into nitrogenous form occurs with the help of living microorganisms, it is said to be the biological nitrogen fixation. Some of these organisms are bacteria, blue green algae, fungi and lichens, they are commonly known as nitrogen fixers. These microorganisms maybe either free living or symbiotic. Bacteria Azotobacter, Clostridium, Rhodospirillum and the cyanobacterium Nostoc are the free living nitrogen fixers. The symbiotic nitrogen fixers are the lichen Collema, the bacterium Rhizobium which is found in the root nodules of leguminous plants.
The nitrogen fixation i.e. conversion of nitrogen to ammonium takes place in two steps. In the first step splitting of nitrogen molecule and releases the nitrogen atom occurs in the presence of the enzyme nitrogenase. This phase is known as activation of nitrogen. This free nitrogen atom will then combines with the hydrogen atom and forms the ammonium.
