| Abstract: | Sustainable phosphorous (P) management is a key problem in organic farming. In situ digestion of naturally occurring rock phosphates (RPs) may be a solution. This would require the application of fertilizers consisting primarily of RP mixed with elemental sulfur (S). Through microbial action, the S is oxidized into sulfuric acid, which then transforms the RP into soluble, plant-available forms. By means of an incubation experiment, this study characterized the in situ digestion of RP and revealed how it is influenced by temperature and microbial action. When either S alone or S together with RP (SP) was added to soil that had been inoculated with S-oxidizing microorganisms, the soil pH decreased rapidly from about 7.3 to 3.2 over 12 weeks of incubation. In soil that had not been inoculated with of S-oxidizing microorganisms, the pH of the soil treated either with S or with SP decreased only slightly. The pH of the inoculated soil to which either S or SP had been added decreased more rapidly at 30 °C than at 23.8 °C during the first 4 weeks. The oxidation rate in inoculated soil was much greater than in noninoculated soil and greater at 30 °C than at 23.8 °C. The S oxidation rate in inoculated soil was significantly greater in the SP treatment than in the S treatment both at 23.8 °C and at 30 °C. After incubation, the amounts of water-soluble P and of CAL P (calcium-acetate-lactate–extracted P) were large only in the SP treatment in inoculated soil. |
