Soil serves as the major source of phosphorus or plant nutrition. There are nearly 200 mineral forms of phosphorus and the most commonly occurring form of P in the soil is apatite, which is an insoluble form of phosphorus. Insoluble phosphates that are not available to the plant or microorganisms comprise nearly 95 to 99 percent of the total phosphates in the soil. Therefore phosphorus is considered as one of the essential plant nutrients making up 0.2% of the plant dry weight. It is involved in photosynthesis, seed formation and numerous other plant functions, hence the application of phosphatic fertilizers becomes inevitable in modern agriculture.

Various methods has been proposed to solubilize the insoluble phosphates and increase its availability by biologically mediated processes such as mineralization and immobilization by phosphate solubilizing organisms have been extensively studied. An increase in P availability to plants through the inoculation of phosphate solubilizing microorganisms has been reviewed under pot culture conditions and field conditions. There are several studies indicating that soil inoculation with Phosphate Solubilizing Bacteria (PSB) has been shown to improve solubilization of fixed soil P and applied phosphates resulting in higher crop yields. A considerable number of bacterial species are able to exert a beneficial effect upon plant growth and mostly they are associated with the plant rhizosphere, and thus known as rhizobacteria. This group of bacteria has been termed plant growth promoting rhizobacteria, and among them are strains from genera such as Alcaligenes, Acinetobacter, Arthrobacter, Azospirillum, Bacillus, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Paenibacillus, Pseudomonas, Rhizobium, and Serratia. They are used as biofertilizers or control agents for agriculture improvement and there are numerous researchers for the area with the agricultural environment conservation.

The term P solubilization by microbes seems to have undergone a paradigm shift, since recent literature on P solubilization also includes the release of organic sources of P by the action of phosphatase and phytase enzymes. The most efficient Phosphorus solubilizing microbes belong to genera Bacillus and Pseudomonas among bacteria and Aspergillus and Penicillium among fungi. The recent reported on PSB were from several bacilli include B.brevis, B.cereus, B.circulans, B.firmus, B.licheniformis, B. megaterium, B. mesentricus, B.mycoides, B.polymyxa, B.pumilis, B.pulvifaciens and B.subtilis from the rhizosphere of the legumes, cereals (rice, maize), arecanut palm, oat, jute and chilli. while the reported Pseudomonas includes P.putida, P.aeruginosa, P.corrugata, P.lutea , P.fluorescens , P.rhizosphaerae, P. stutzeri , P.trivialis, P.poae and P.lurida are already reported for high phosphorous-solubilizing ability and relevant information of plant growth promotion activities are also available.

About Author / Additional Info:
Myself Piyush Joshi, perusing my Phd. in Cold tolerant Phosphates solubilization from NW Himalayas and its plant growth promotion abilities.