Bioavailability of Citrate‐Insoluble Phosphorus in Monoammonium Phosphate and Triple Superphosphate Fertilizers

Abstract

AbstractThe 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.