Phosphate Solubilization and Mobilization in Soil Through Microorganisms Under Arid Ecosystems

Phosphorous is going to be plant nutrient that will limit the agricultural production in the next millennium. It is a major growth-limiting nutrient, and unlike the case of nitrogen, there is no large atmospheric source that can be made biologically available (Ezawa et al., 2002). As regards the role of P, it stimulate root development and growth, gives plant rapid and vigorous start leading to better tillering, essential for many metabolic processes in plant life and for seed formation and organization of cells, encourages earlier maturity, In most soils, its content is about 0.05% of which only 0.1% is plant available (Achal et al., 2007). The total phosphorous content in arid soils in India ranges from 560 to 900 kg ha-1, the available phosphorous is quite low i.e. 15-25 kg ha-1 (Dhir, 1977). The soils being poor in organic matter (1.54.2 mg kg-1), most of these phosphorous is present in inorganic form as calcium and aluminium phosphatase. About 20-25% of total phosphorous in arid soils of India is organic in nature and 68% organic phosphorous in the soil is present as phytin (Yadav & Tarafdar, 2007), which are not directly available to plants. Therefore application of phosphatic fertilizers to the soil is essential to maintain adequate amount of soluble P in the soil solution for optimum plant growth as well as to maintain soils sustainability. Efficiency of P fertilizer throughout the world is around 10-25% (Lindsay, 1979), and concentration of bioavailable P in the soil is very low reaching the level of 1.0 mg kg-1 soil (Goldstein, 1994). Phosphorous is taken up from the soil in the form of soluble orthophosphate ions; H2PO4-1, HPO4-2 and PO4-3 and generally the availability of these ions to the plants is in the order of H2PO4-1> HPO4-3>PO4-3. The type of the orthophosphate ion present in the soil is depending on soil reaction. At the relatively low pH of 4 to 5, orthophosphate usually exist as H2PO4-1 ions. On increasing pH, first HPO4-2 ions are formed which convert to PO4-3 as the soil reaction become alkaline. Large amount of P applied as fertilizer enters in to the immobile pools through precipitation reaction with highly reactive Al3+ and Fe3+ in acidic soil and Ca2+ in calcareous or normal soils (Gyaneshwar et al., 2002; Hao et al., 2002). Although total P pool is high, only a part is available to plants. So, the release and mobilization of insoluble and fixed forms of P is an important aspect of increasing soil P availability. Soil