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Home » Biology Homework Help » Biotechnology » Biofertilizers
Biofertilizers
Micro-organisms employed to enhance availability of nutrients, viz., nitrogen (by fixing atmospheric N2) and phosphorus (by solubilizing soil phosphorus), to the crops are called biofertilizers. The various micro-organisms having realized/potential applications as biofertilizers are: bacteria (Rhizobium spp., Azospirillum, Azotobacter), fungi (mycorrhizae like Glomus), blue-green algae or cyanobacteria (Anabaena, Nostac etc.) and Azolla (a fern containing symbiotic Anabaena azollae).

Rhizobium spp.

These are gram-negative soil bacteria capable of forming root nodules in most leguminous plants and some nonleguminous plants. In some cases, stem nodules are also produced. Rhizobium is divided into several species chiefly on the basis of the legume species they are able nodulate, e.g. R. leguminosarum (nodulates pea), R. phaseoli (Phaseolus sp.), R. trifolii (Trifolium sp.), R. lupine (lupins), R. melilotii (Melilotus sp.) etc. Cowpea rhizobia are now classified as Bradyrhizobium. Rhizobium cells contain genes for nitrogen fixation (nig genes) on a megaplasmid. The bacteria enter the roots through root hairs, the interaction being highly specific and progressing through several steps; it ultimately results in nodule formation. Many genes of Rhizobium as well as the host legume are involved in the process. Inside the nodule many bacteria cells change into nondividing bacterioids which produce nitrogenase, the enzyme which reduces atmospheric nitrogen into ammonia. Nitrogenase is highly sensitive to O2; it is protected from O2 by the pink pigment leghaemoglobin, which binds to O2, produced by the legume and present in the nodules.

Azotobacter and azospirillum

Azotobacter uses the organic matter present in soil to fix nitrogen asymbiotically; it is capable of fixing upto 30 kg nitrogen/ha/yr. Azospirillum species occur in association with the roots of many plants of the grass family, e.g. jowar, wheat, bajra etc. These bacteria are capable of fixing over 30 kg N/ha/yr. Field inoculation of crops with Azotobacter or Azospirillum is estimated to save 15-25 kg N/ha.

Blue-green Algae and Azolla

Blue-green algae (cyanobacteria) are photosynthetic, prokaryotic organisms which fix N2 asymbiotically; some cyanobacteria are known to form symbiotic associations, e.g. Azolla (a fern)-Anabaena azollae (a blue-green alga). Examples of cyanobacteria are Anabaena, Nostoc, Plectonema etc. Usually composite cultures containing 2 or more genera are used for field inoculation since they are often superior to single strain inoculations. Cyanobacteria produce nitrogenase and N2-fixation occurs in specialized structures called heterocysts in which the nif region becomes reorganized (this is essential for N2-fixation). In addition, heterocysts act as O2-proof compartments which protect nitrogenase from O2 inactivation.

Phosphate solubilizing micro-organisms

Some bacteria, e.g. Thiobacillus etc., convert nonavailable inorganic phosphorus present in soil into an available from utilizable crop plants. These bacteria also produce iron chelating substances, e.g. pseudobactin, called siderophores which chelate the iron present in the root zone; this iron becomes nonavailable to harmful micro-organisms and, in this manner, crop plants are protected from them. In addition, certain fungi, e.g. Glomus, form associations with plants roots; these are called mycorrhizae. The fungus may be located at the root surface (ectomycorrhiza) or it may be present inside the roots (endomycorrhiza). These fungi convert nonavailable phosphorus into an available form, produce growth promoting substances and also protect against soil pathogens.

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