Tricalcium phosphate is inappropriate as a universal selection factor for isolating and testing phosphate-solubilizing bacteria that enhance plant growth: a proposal for an alternative procedure

Literature analysis and chemical considerations of biological phosphate solubilization have shown that the commonly used selection factor for this trait, tricalcium phosphate (TCP), is relatively weak and unreliable as a universal selection factor for isolating and testing phosphate-solubilizing bacteria (PSB) for enhancing plant growth. Most publications describing isolation of PSB employed TCP. The use of TCP usually yields many (up to several thousands per study) isolates supposedly PSB. When these isolates are further tested for direct contribution of phosphorus to the plants, only a very few are true PSB. Other compounds are also tested, but on a very small scale. These phosphates (P), mainly Fe-P, Al-P, and several Ca-P, are even less soluble than TCP in water. Because soils greatly vary by pH and several chemical considerations, it appears that there is no metal-P compound that can serve as the universal selection factor for PSB. A practical approach is to use a combination of two or three metal-P compounds together or in tandem, according to the end use of these bacteria-Ca-P compounds (including rock phosphates) for alkaline soils, Fe-P and Al-P compounds for acidic soils, and phytates for soils rich in organic P. Isolates with abundant production of acids will be isolated. This approach will reduce the number of potential PSB from numerous isolates to just a few. Once a potential isolate is identified, it must be further tested for direct contribution to P plant nutrition and not necessarily to general growth promotion, as commonly done because promotion of growth, even by PSB, can be the outcome of other mechanisms. Isolates that do not comply with this general sequence of testing should not be declared as PSB.