Characterization and Agronomic Evaluation of Single Superphosphates Varying in Iron Phosphate Impurities

An increase in the concentrations of water-insoluble iron phosphate (Fe-P) compounds in acidulated P fertilizers has raised the concern that the agronomic effectiveness of these P fertilizers will decrease. This study was conducted to evaluate the agronomic effectiveness of three sources of single superphosphate (SSP) varying in Fe-P impurities for upland and flooded rice (Oryza saliva L.) grown for 55 d. A modal chemical analysis and X-ray diffraction were used to characterize the SSP samples and their water-leached forms. A greenhouse study was conducted where rates of 0, 5, 15, 30, 50, and 100 mg P kg -1 as total available P (water plus citrate-soluble P) were applied from each SSP source and monocalcium phosphate (MCP) to a Hiwassee clay loam (thermic Rhodic Kanhapludult). The watersoluble P contents of the SSP sources were 46, 80, and 86% of the total available P (water + citrate) corresponding to an increase of Fe content (2, 4, and 7%) in the phosphate rock sources used for SSP production. The main Fe-P impurities in the SSP samples were identified as Fe 3 NaH 8 (PO 4 ).6H 2 O and Fe 3 H 9 (PO 4 ) 6 .6H 2 O. Single superphosphate with only 46% of water solubility was 91% as effective as MCP in increasing dry matter yield and 76% as effective for P uptake by upland rice. The other two SSP sources were as good as MCP in effectiveness for upland rice. All of the SSP sources were equally as effective as MCP in producing dry matter yield and P uptake by flooded rice.