Bio-fertilizers

Biofertilizers are the living organism which bring about nutrient enrichment of soil. Biofertilizers are bacteria, cynobacteria and fungi. These organisms improve the soil fertility by adding nutrient due to their biological activity.
1) Bacteria as biofertilizers –
Some bacteria enrich the soil by fixing free atmospheric nitrogen into nitrogen compounds. These bacteria and their roles are as follows –
i. Rhizobium – rhizobium is a symbiotic bacterium which is found in soil and forms nodules in the roots of legume plants cannot use free nitrogen from atmospheric but can use in form of nitrates, therefore, bacteria fix free atmosphere nitrogen into nitrates. The leguminous plants utilize this nitrogen in form of nitrates.
ii. Azotobacter – Azotobater is a free living and non-shmbiotic bacteria found in soil. It can fix atmospheric nitrogen is utilized by crops like cereals (rice, maize), millets, fruits, vegetables, etc.
2) Cyanobacteria as biofertilizers
Certain cyanobateria that fix the free nitrogen of the air are –
I. Anabaena azolla – Anabaena azolla is a symbiotic cyanobacteria found in the cavities of leaves of fast growing and floating fern, Azolla. This cynobacterium fixes free atmospheric nitrogen and exudates nitrogenous compounds into the cavities of leaves. Such leaves can be an excellent biofertilizer.
II. Anabaena, nostoc, Spirulina – These are free living cynobacteria (green algal bacteria) which fix the nitrogen through photosynthesis. These organisms have been used as bio-fertilizers in India.
3) Fungi as bio-fertilizers
Certain fungi make a symbiotic association with the roots of certain seeds bearing plants. This association is called mycorrhyzal. Mycorrhyzal associated converts a marginal land into fertile land and reduces the dependency on irrigation and fertilizers. Mycorrhyzal are of two types –
I. Ectomycorrhyza – It is an association in which fungus lives outside of the roots of oak, pine, peach, Eucalyptus,etc. These absorb and store nitrogen, calcium, potassium and phosphorus in the fungal mantle. This result is greater plant vigour, growth and yield.
II. Endomycorrhyza – It is an association in which fungus lives between and within the cells of cortex. Ti is found in many herbaceous species like orchids and some woody plants. In some endomycorrhyzae fungal hypae penetrate into the cortical cells, these are called vesicular arbuscular mycorrhyza (VAM). VAM are important in the phosphate nutrition.

Nitrogen fixation
Nitrogen fixation may be defined as the phenomenon of the conversion of free nitrogen of the atmosphere into nitrogenous salts (in form of nitrate) which are readily absorbed by the plants.
Certain symbiotic bacteria like Rhizobium and Bacillus radiciola live in the nodule of the roots of the leguminous plants. These bacteria on entering the roots through the root hairs from the soil form the root nodule appears pinkish due to the presence of a pigment, leghaemoglobin. This pigment is related to haemoglobin, the red pigment of human blood. Like haemoglobin, leghaemoglobin also absorbs oxygen. Enzyme nitrogenase catalyses the fixation of nitrogen under anaerobic conditions.

Certain free-living microorganisms like cyanobacteria and photosynthetic bacteria also fix nitrogen. Certain cyanobateria live in symbiotic association with lichens Anhoceros (a liverwort), fronds of Azolla (a water fern) and roots of Cycas.