Fertilizers of organic origin are called biofertilizers. There are organisms which make the soil rich in nutrients. The main sources of biofertilizers are-
(i) Bacteria, (ii) Cyanobacteria, (iii) Fungi
(i) Rhizobium. Rhizobium and Legume symbiosis is very well known these days. It contributes substantially to total Biological Nitrogen Fixation (BNF). It produces nodules in the roots of leguminous plants and fixes nitrogen from the atmosphere. These days. Rhizobium cultures are being used to inoculate the legume's seeds to ensure better nitrogen fixation and better yield of the crop. Phosphorus is also added for better nitrogen fixation. According to Jordon (1984) the slow growing rhizobic are grouped under the genus Brady-Rhizobium and the fast growing under the genus Rhizobium. The ability to fix high amount of Nitrogen (efficiency) is governed by the symbiotic capability between Rhizobium and the host plant.
(ii) Azolla-anabaena Symbiosis. Azolla, a small, fast growing fern, occurs floating on water. The Azolla leaves contain a cyanobacterium-the Anabaena azollae. It fixes atmospheric nitrogen and excretes the nitrogenous compounds into the cavity of the leaf. It is believed that Azolla can increase crop yield upto 50%. A pinnate is a very good biofertilizer for rice crop. The agronomic potential of Azolla as biofertilizer for rice has been recognised in many countries including India, Philippines, USA, Sri Lanka and Thailand. Azolla can be produced by farmers by Rice-fish culture technique standardized by CRRI.
(iii) Loose association of Nitrogen Fixing Bacteria. According to Joahannah Dobreiner (1975) Azospirillum lipoferum, a nitrogen fixer, lives in loose association with the roots of certain Brazilian grasses and maize. As a result of this association Dobreiner reported sufficient increase in the yield of the plants. Several members of Azotobacter and Azospirillum genera have been widely tested to increase yields of cereals under field conditions.
(iv) Free Living Bacteria. Azotobacter and Bacillus polymyxa are free living soil bacteria. These fix up atmospheric nitrogen and make it available to crops (cereals, millets, fruits and vegetables). Azotobacter, if grown along with cotton, maize, jowar and rice, increases yield and saves nitrogen upto 10-20 kg/ha.
(v) Cyonobacterium. Cyanobacteria e.g., Anabaena, Nostoc Aulosira nodularia, Scytonema, Calothrix etc., are good nitrogen fixers. These derive the energy through photosynthesis for nitrogen fixation. This combination of photosynthesis and nitrogen fixation is excellent for agriculture. The scientist at the Indian Agricultural Research Institute (IARI), New Delhi have used these organisms as biofertilizers and it is observed by them that these organisms fix nitrogen upto 20-30 kg/ha.
(vi) Mycorrhiza. The root fungus symbiosis is caused by mycorrhizae. The association of mycorhizae is wide spread in bryophytes, pteridophytes, gymnosperms and about 90% or more of angiosperms. Frank (1885) for the first time reported association of fungus and roots of higher plants and he coined the term mycorrhiza. These mycorrhizae differ in their structure and in the systematic position of the fungi involved. The poor development of mycorrhizal root, results in the stunted growth and chlorosis of leaves. The mycorrhizal plants are able to withstand better water stress condition than the mycorrhizal seedlings. The mycorrhizae are described as "Biofertilizer and Biocide". The mycorrhizae can be broadly classified into two types; the ectomycorrhizae and endomycorrhizae.
(a) Ectomycorrhizae commonly found to occur on the roots of trees such as pine, oak, peach, eucalyptus etc. It is an association in which there is a well developed mycelium forming a mantle on the surface of the roots. These increase the water and nutrient uptake, resulting in greater plant vigour and growth, and crop yield.
The ectomycorrhizae fungi grow best at pH 5-6. Excess of chemical fertilizers suppress ectomycorrhizal development. Shading also suppresses ectomycorrhizal development.
(b) Endomycorrhizae. This is an association in which the fungus lives between and within the cells of the cortex, and growth on the outside of the root is limited or nil. This type of association is found in herbaceous plants such as orchids and a few wood plants. Endomycorrhizae are produced by both the septate and non septate fungi.
Benefits of soil from Biofertilizers : Biofertilizers are the substance that contains microbes, which helps in promoting the growth of plants and trees by increasing the supply of essential nutrients to the plants. It comprisesliving organisms which include mycorrhizal fungi, blue-green algae, and bacteria. Mycorrhizal fungi preferentially withdraw minerals from organic matter for the plant whereas cyanobacteria are characterized by the property of nitrogen fixation.