In Situ Digestion of Rock Phosphates to Mobilize Plant-Available Phosphate for Organic Farming

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 ...