Synthesis, Characterization, and Agronomic Evaluation of Iron Phosphate Impurities in Superphosphates

Abstract

Two of the most common impurities found in superphosphates (single superphosphate [SSP] and triple superphosphate [TSP]) in the forms of Fe3KH8(PO4)6·6H2O and Fe3KH14(PO4)8·4H2O 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.