Bio fertilizers are the living organisms capable of fixing atmospheric nitrogen or making native soil nutrients available to crops. Atmospheric nitrogen is fixed effectively by the microorganisms either in symbiotic association with plant system (Rhizobium, Azolla) or in associative symbiosis (Azospirillum) or in free living system (Azotobactor, phosphobacterium, blue green algae) or in micorhizal symbiosis (VAM fungi).
(a) Rhizobium – Rhizobium bacteria can fix atmospheric nitrogen symbiotically. They live in the nodules of host plants belonging to the family leguminoceae. The quantities of nitrogen fixed by Rhizobia differ with the rhizobial strain, the host plant and the environmental conditions under which the two develop. The species of the genus Rhizobium are numerous and require certain host plants. For example, the bacteria that live symbiotically with soybean will not do so with alfalfa.
Fixation of nitrogen by the leguminous plants will be at maximum only when the level of available soil nitrogen is at the minimum. It is sometimes advisable to include a small amount of nitrogen in the fertilizer of legume crops at sowing time (as a starter dose) to ensure that the young seeding will have an adequate supply until the rhizobia can become established. Larger quantity of nitrogen or continued applications of nitrogen, however reduce the activity of the rhizobia and therefore they are generally uneconomical. Rhizobial inoculation was found to fix 15-35 kg N per ha in a season on different pulse crops. Rhizobial inoculation can save up to 25% N fertilizer application to crops.
(b) Azolla – It is a small water fern of worldwide distribution under natural conditions. It contains the heterocystous blue green algae Anabaena azollae as a symbiont in an enclosed chamber in the dorsal leaf lobes. Azolla derives all its total nitrogen requirement by the symbiotic association with the algae.
The Azolla–Anabaena system is agronomically most signification plant algal association and this is being employed as a nitrogen source for rice culture. There are six species of Azolla. They are Azolla caroliniana, Azolla filiculoides, Azolla mexicana, Azolla nilotica, Azolla microphylla and Azolla pinnata. Azolla contains 3.1-4.2% N; 0.16% P2O5 and 0.18% K2O on dry weight basis.
(c) Azospirillum – This bacterium is associated with cereals like rice, sorghum, maize, cumbu, ragi, tenai and other minor millets and also for cotton, sugarcane, oilseeds and fodder grasses. These bacteria colonizing in the roots not only remain on the root surface, but also a sizable proportion of them penetrates into the root tissues and lives in harmony with the plants. They do not, however, produce any visible nodules or out growth on the root tissue.
In the absence of any plant, azospirillum live in the soil just like any other micro organism saprophytically, however, when a suitable crop is raised, they are attracted towards the root system, where they colonize and grow in almost a symbiotic manner.
(d) Azatobacter – The beneficial effects of Azatobacter on plants was associated (non-symbiotically) not only with the process of nitrogen fixation but also with the synthesis of complex of biologically active compounds such as nicotinic acid, pyridoxine, biotin, gibberellins and probably other compounds which stimulate the germination of seeds and accelerate plant growth. Azatobacter population in soil or near the root zone of crops (Rhizosphere) is very low when compared to other soil bacteria.
The nitrogen fixation potential of this bacterium is also not very high and appreciable (20 to 30 kg of N per ha per year). A fairly high population is required for substantial nitrogen fixation. Enormous energy is required by Azatobacter for nitrogen fixation. The possible source of energy for Azatobacter is the soil organic matter. The energy generated during the utilization of organic matter is used for nitrogen fixation.
(e) Blue green algae – The blue green algae occur under a wide range of environmental conditions. They are completely auto tropic and require light, water, free nitrogen (N2), carbon dioxide (CO2) and salts containing the essential mineral elements. They play a major role in the nitrogen economy of paddy soils in tropical countries.
Different algal species available are:
• Tolypothric tenuis,
• Aulosira fertilization,
• Anabaena, and
(f) Phosphobacterium – In most of the acid and clayey soils, the applied phosphorus either as super phosphate or mussoriphos will not be available to crops due to fixation. It is essential to use the phosphobacteria (a free living bacteria in soils like Bacillus megatherium) for proper solubilisation of fixed P and release them in the available form for the crop to take-up for its growth. Dual inoculation of the phosphobacteria with rhizobium or azospirillum can provide both N and P to the crop.
(g) Mycorrhizae (VAM) – Vesicular Arbiscular Mycorrhiza is a fungi used as bio-fertilizer. The mycorhizal symbiosis is an intimate association between plant root system and certain group of soil fungi. The plant provides carbon as energy source to the fungus which in turn helps the plant in better uptake of nutrients (especially P).
The VAM fungi form either a mantle of hyphae around the root or penetrate inside the roots spreading intra or intracellularly in the cortical region. The fungal mycelium also extends several centimeter, away from the root in the soil. The area that the plant can explore for nutrients thus greatly increase due to colonization of plant roots by the mycorrhizal fungi. The development of mycorrhiza network is much more in soils with low fertility.
In nutrient rich soils, there is very little extension of mycelial network. The mycelial growth is confined to the close proximity of roots. Mycorrhiza increases crop yield, protect against certain root pathogen, helps in uptake of P, Cu, Zn and B and increases tolerance to environmental stress.
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