Phosphorus nutrition of crops through arbuscular mycorrhizal fungi

Phosphorus (P) is one of the diffusion limited major nutrients, which is essential for plant growth. In soil, phosphorus occurs in three forms namely, soluble inorganic P, insoluble inorganic P and organic P. Uptake of P from soil solution is mediated by arbuscular mycorrhizal fungi (AMF) in addition to plant roots. AMF are ubiquitous occurring in most of the soils. They are commonly found in association with agricultural crops. It is now proved beyond doubt that AMF greatly enhance plant growth. The improved growth is mainly attributed to uptake of diffusion limited nutrients such as P, Zn, Cu, etc. from soil. The other beneficial effects are their role in the biological control of root pathogens, hormone production, greater ability to withstand water stress and synergistic interaction with beneficial microorganisms. It is believed that mycorrhizal plants absorb P only from the soluble P pools in the soil. Synergistic interactions between AMF and P-solubilizing microorganisms (PSM) are present which in turn helps plant growth. This is because PSM solubilize and release H2PO4 ions from unavailable forms of P and AMF help in the uptake of H2PO4 ions from soil. Field studies have shown that inoculation with efficient AMF not only increases growth and yield of crop plants but also reduces the application of phosphatic fertilizer by nearly 50%, especially in marginal soils deficient in nutrients. Though the rock phosphates available in India are of low grade and not fit for the manufacture of phosphatic fertilizer, they can be used with PSM plus AMF as a potential source of P for crop plants, thus bringing down the import of P fertilizers/rock phosphate in our country. Advantages of AMF have been attained through application of suitable AM fungal inoculum and augmenting native AM fungal activities in soil through manipulating agricultural practices in favour of these fungi.