The mechanism of phosphate bacteria in increasing the solubility of phosphorus in Indonesian Andisols

The purpose of this research was to determine: 1) phosphate bacteria (PB) mechanisms responsible for enhancing the soil’s soluble phosphorus (P), using the processes of inorganic P solubilisation, organic P mineralization, and blocking of soil colloidal adsorption site, and 2) to investigate the factors contributing to this increase. Phosphate bacteria (PB) was inoculated into sterile Andisols in three separate compositions, termed 1 kg P∙kg–1 (2.82 g phosphate rock, 0.5 g Ca3(PO4)2, 0.4 g Al3(PO4)2, or 0.4 g Fe3(PO4)2), organic P (0.5 cm3 para-Nitrophenylphosphate (pNPP) or 0.5 g Na-phytate), and 1 kg P (KH2PO4)∙kg–1, in order to analyse inorganic P solubilisation, organic P mineralization, and evaluate blocking soil colloidal site for adsorption P, respectively. Furthermore, spectrophotometry technique was applied to determine the amount of dissolved P. The PB showed an improvement in inorganic P solubilisation from 147.66 to 194.61 mg P∙kg–1, and also in organic P from 63.6 to 91.7 mg P∙kg–1, compared to control, (31.06 mg P∙kg–1) and (23.7 mg P∙kg–1), respectively. Meanwhile, the micro-organisms were known to decrease P adsorption by 13.43%, beyond the restraint set at 85.34%. Therefore, increased soluble P in Andisols is possibly expressed, using the equation as follows: soluble P (mg P∙kg–1) = 1201.96 + 1.18 inorganic P solubilisation (mg P∙kg–1) + 1.09 organic P mineralization (mg P∙kg–1) – 0.92 adsorption P (mg P∙kg–1) (R2 = 0.99).