Prospects for using soil microorganisms to improve the acquisition of phosphorus by plants

This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000 Microorganisms play an important role in the acquisition and transfer of nutrients in soil. For phosphorus (P), soil microorganisms are involved in a range of processes that affect P transformation and thus influence the subsequent availability of P (as phosphate) to plant roots. In particular, microorganisms can solubilize and mineralize P from inorganic and organic pools of total soil P. In addition, microorganisms may effectively increase the surface area of roots. Also, the microbial biomass itself contains a large pool of immobilized P that potentially is available to plants. Given that most soils are deficient in plant-available P and that P fertilizer represents a significant cost for agricultural production throughout the world, there is interest in using soil microorganisms as inoculants to mobilize P from poorly available sources in soil. Although potential clearly exists for developing such inoculants, their widespread application remains limited by a poor understanding of microbial ecology and population dynamics in soil, and by inconsistent performance over a range of environments. Furthermore, promotion of growth of plants in soil, as a consequence of microbial inoculation, may not necessarily be associated with characteristics such as P solubilization, which are manifest under laboratory conditions.