Bioavailability of Citrate-Insoluble Phosphorus in Monoammonium Phosphate and Triple Superphosphate Fertilizers
1995
Sikora, F. J. | Mullins, G. L.
The Association of Official Analytical Chemists (AOAC) method for determining bioavailable P was rigorously tested in agronomic studies during the 1930s when the principle constituents of the citrate-insoluble (CI) fraction were Ca phosphates. The presence of Fe, Al, and Mg phosphate compounds in the CI fraction in today's fertilizers requires a reassessment of the AOAC method. A greenhouse study was conducted to determine if the CI-P fractions of five monoammonium phosphate (MAP) and five triple superphosphate (TSP) fertilizers were unavailable to sorghum-sudangrass [Sorghum bicolor (L.) Moench]. Plant shoot weight (SW) and P uptake from fertilizer applied at 15 mg total P kg⁻¹ soil was compared with the same indices obtained from reagent-grade MAP and monocalcium phosphate (MCP) applied at rates of 13.8, 14.2, 14.6, and 15 mg P kg⁻¹ soil (corresponding to 8, 5.3, 2.7, and 0% reduction in P supply). With a few exceptions, there was good agreement in SW and P uptake among fertilizer and reagent-grade materials in regard to the percentage of CI-P in the fertilizer, which lead to the conclusion that the AOAC method correctly determined unavailable P as CI-P for short-term plant growth. Residual availability is discussed with reference to the solubility of suspected P-impurity compounds in the CI fraction.
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