A Plate Method for estimating the Numbers of Phosphate-Dissolving and Acid-Producing Bacteria in Soil

Abstract

IN the past, the preparation of plates for detecting phosphate-dissolving bacteria in soil has involved precipitation of the insoluble phosphate in the medium^1–3. This method is not very suitable for use with the dilution-plate counting technique. For this purpose we have adapted and extended the suggestion of Johnson⁴ in which finely divided dicalcium phosphate is incorporated in agar medium. Other forms of insoluble phosphates can also be used as well as insoluble carbonates. We have used calcium carbonate in this connexion for detecting and estimating acid-producing organisms in soil. Stock suspensions of the insoluble substances are prepared by grinding them in a ball-mill in the presence of gum arabic. The latter is necessary to prevent clumping of the finely divided substances during autoclaving.