Synthesis, characterization, and agronomic evaluation of iron phosphate impurities in superphosphates

Two of the most common impurities found in superphosphates (single superphosphate [SSP] and triple superphosphate [TSP]) in the forms of Fe3KH8(PO4)(6).6H(2)O and Fe3KH14(PO4)(8).4H(2)O were synthesized (H8-syn and H14-syn, respectively), characterized, and agronomically evaluated to investigate cost-effective means to optimize the utilization of phosphate rocks (PRs) containing Fe impurities. A solubility study showed that more P was released from both compounds as pH increased in the 0.01 M KCl solutions (pH 3.0-7.5) and more P was released from H14-syn than H8-syn. The two Fe-K-P compounds were mixed and compacted with monocalcium phosphate (MCP) at 0, 25, 50, 75, and 100% of total P as MCP. In a greenhouse study, rates of P were applied at 0, 10, 20, 40 and 80 mg P kg(-1) from H8-syn, H14-syn, and MCP, while the compacted mixtures were applied only at 40 mg P kg(-1) to an Ultisol (thermic Rhodic Kanhapludults, pH 5.3) cropped with upland and flooded rice (Oryza sativa L.) for 65 d. The results showed that P uptake and dry-matter yield were greater with H14-syn than H8-syn for both crops and both compounds were more effective for flooded rice than upland rice. The calculated values of relative agronomic effectiveness (RAE) of H8-syn and H14-syn with respect to MCP were 32 and 72% in dry-matter yield for upland rice and 55 and 102% for flooded rice, respectively. To reach 90% of maximum dry-matter yield obtained with MCP it required approximately 43 and 35% of total P as water-soluble P (WSP) in the mixtures of H8-syn and H14-syn with MCP for upland rice and only 17 and 11% for flooded rice, respectively.