ABSTRACT
Nitrogen is a key element in the nutrition of living things because of its importance in nucleic acids, which are concerned with heredity, and in proteins, which inter alia provide the bases for enzymes. Gaseous nitrogen is present in abundance in the Earth’s atmosphere forming about 80% of atmospheric gases. Indeed it has been estimated that each acre of land has about 3,500 tons of N2 above it. Unfortunately, most living organisms cannot utilize gaseous nitrogen but require it in a fixed form; that is, when it forms a compound with other elements. Nitrogen can be fixed both chemically and biologically. Chemical fixation is employed in the production of nitrogenous chemical fertilizers, which are used to replace nitrogen removed from the soil by plants. The ability to carry out biological fixation is found only in the bacteria and blue-green algae. Some of these organisms fix nitrogen in the free-living state and thereby contribute to the improvement of the nitrogen status of the soil. Others do so closely associated (in symbiosis) with higher plants. In some of these associations such as with some tropical cereals, the organisms live on the surface of the plant roots and fix the nitrogen there. In some others the microorganism penetrates the roots and forms outgrowths known as nodules within which the nitrogen is fixed. Of the nodule-forming nitrogen fixing associations between plant and micro-organisms, the most important are the legumebacteria associations. There are about 1,200 legumes species and their nodulation is important because about 100 of them are used for food in various parts of the world.
