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DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

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

^a Dep. of Soil and Plant Nutrition, Univ. of São Paulo/ESALQ, C.P. 9, 13418-900, Piracicaba, Brazil
^b IFDC, P.O. Box 2040, Muscle Shoals, AL 35662
^c Dep. of Plant and Soil Sciences, Alabama A&M Univ., Normal, AL 35762

^* Corresponding author (nchien{at}ifdc.org).

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.

Abbreviations: AOAC, Association of Official Analytical Chemists • EDS, energy-dispersive spectroscopy • H8, Fe₃KH₈(PO₄)₆·6H₂O • H8-syn, synthesized H8 • H14, Fe₃KH₁₄(PO₄)₈·4H₂O • H14-syn, synthesized H14 • ICDD, International Center for Diffraction Data • MCP, monocalcium phosphate • NAC, neutral ammonium citrate • PDF, powder diffraction file • PR, phosphate rock • RMSE, root mean square of error • SEM, scanning electron microscope • SSP, single superphosphate • TSP, triple superphosphate • WSP, water-soluble P

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