Comparison of Qualitative and Quantitative Methods for Isolation of Phosphate Solubilizing Microorganisms

Phosphate solubilising bacteria possess the ability to solubilise insoluble phosphate to soluble forms enhancing the nutrient status of the soil. This process not only compensates increasing cost of phosphatic fertilisers but also minimises the negative environmental impacts associated with the application of inorganic fertilisers. Phosphate solubilising bacteria (PSB) were screened based on the size of a halo/ clear zone around the colony (NBRIP agar plate assay) and by measuring solubilise phosphorous content (colorimetric method). The aim of this work was to assess the comparative reliability of quantitative and qualitative methods of isolation of phosphate solubilising bacteria. Bacterial strains which showed very poor performance in qualitative method were proven to be good phosphates solubilisers in quantitative method and vice versa. Therefore no positive relationship among the values obtained from qualitative and quantitative methods could be observed. Furthermore qualitative method did not reflect the real ability of the phosphate solubilising bacteria to solubilise insoluble phosphates. From the results of the present study, it can be concluded that isolation of efficient phosphate solubilising bacteria through quantitative method could give better results than that of qualitative method.Keywords: clear zone, insoluble phosphates, phosphate solubilising bacteri