Up to one-half of the total amount of phosphorus in the A horizon of soils from the Midwest and other temperate zones may be present as phosphorus bound to soil humic substances. All current evidence indicates that only inorganic forms of phosphate, orthophosphate in the form of HPO 2- and H PO – ions, are taken up by plants. The phosphate bound to soil humic substances must be mineralized to these inorganic forms before it is available for plant uptake. The process of mineralization is mediated by enzymes, especially phosphatase and phytase.