ABSTRACT
Phosphorus is an essential macronutrient needed by plants and plays a vital role in maintaining soil fertility and plant nutrition. Phosphorus in the soil is present in the organic as well as inorganic combinations. The plant available forms of P in marginal soil can be increased with plant growth promoting rhizobacteria (PGPR) or application of vesicular arbuscular mycorrhiza (VAM) fungi. Globally, phosphorus is mined from geological sediments and most of the mined P is used to meet the critical need of crop plants for agronomic productivity. However, recovery of P by plants is comparatively low and major amount of applied P is fixed in the soil creating a need for addition of P fertilizer. Microorganisms play a crucial role in mineral mobilization 238in the rhizosphere soil. Insoluble phosphates are converted into available forms by phosphate-solubilizing microorganisms (PSMs) via the process of acidification, chelation, exchange reactions and production of organic acid. Several bacterial strains (Pseudomonas, Bacillus, Rhizobium, and Enterobacter) and fungal strains (Aspergillus and Penicillium) have so far been recognized as powerful phosphate solubilizers. However, plant inoculations with PSMs during field studies had inconsistent effect on plant growth and crop yields due to variations in soil, crop, and environmental factors affecting the survival and colonization of the rhizosphere. Increasing availability of soil P through microbial inoculation will require identification of the most appropriate strains, preparation of effective formulations, and introduction of efficient agronomic managements to ensure delivery and survival of inoculants and associated improvement of P efficiency.
