Laboratory tests can predict beneficial effects of phosphate-solubilising bacteria on plants

Phosphorus is important in plant growth but in the soil it reacts readily to form insoluble compounds, which are not readily available for plant utilisation. Although the presence of soil microorganisms capable of solubilising phosphate has been known for many years, their isolation and use as crop inoculants have met with only limited success. This project aimed to develop a more robust method of selecting strains, which would give a more reliable indication of their usefulness for Australian crops, with a particular focus on wheat. Bacteria were isolated from a typical black wheat growing soil from the North-Western wheat belt of New South Wales using a differential medium containing insoluble phosphate. In all, 48 isolates were tested in the laboratory for their ability to colonise the roots of wheat seedlings in sterile perlite and to solubilise phosphate in the rhizosphere of plants grown on agar plates. To establish whether strains selected for their laboratory performance could benefit wheat grown in unsterilised field soil, a pot trial was carried out in a high-illumination growth chamber. Seven strains of varying capabilities were used to determine the effects of inoculation on releasing unavailable phosphorus and improving growth and yield, in comparison with conventional phosphorus fertiliser. Strains that were good phosphorus solubilisers in laboratory tests resulted in increased grain yield and grain phosphorus content in the presence or absence of applied dicalcium phosphate. It was also seen that the ability to colonise plant roots in high numbers was not always necessary for a positive effect. Strains that were predicted in the laboratory to be either poor or highly variable phosphorus solubilisers performed worst in the pot trial. The laboratory tests were found to be useful tools for quickly ranking isolates in terms of their ability to supply the phosphorus requirements of wheat grown in pots of non-sterile soil.