Water‐Insoluble Impurity Effects on Phosphorus Availability in Monoammonium Phosphate Fertilizers

Abstract

AbstractWater‐insoluble, P‐containing impurity compounds can form during the production of commercial monoammonium phosphate (MAP, NH4H2PO4) fertilizers. Increased reliance on lower grade phosphate rock (PR) could result in an increase in these water‐insoluble fertilizer impurity compounds, thus lowering the percentage of water‐soluble P in commercial P fertilizers. There is some concern regarding the effect that impurity compounds may be having on P‐fertilizer effectiveness. Therefore, a greenhouse study was initiated to determine the concentration at which water‐insoluble impurity compounds will reduce MAP‐fertilizer effectiveness. Five commercial MAP fertilizers, representing the major U.S. sources of PR (Florida, North Carolina, and Idaho), were selected for study. Fertilizer impurities were collected as water‐insoluble residues by washing the MAP fertilizers with deionized water. Mixtures of water‐soluble, reagent‐grade MAP and the water‐insoluble fertilizer residues were prepared to give a wide range in water‐soluble P. The fraction of water‐soluble P was controlled by supplying 0, 20, 40, 60, 80, and 100% of the total P as reagent‐grade MAP. Fertilizer mixtures were applied at a rate of 80 mg P kg−1 soil, and sorghum‐sudangrass [Sorghum bicolor (L.) Moench] was grown in the treated soil. Four harvests were taken at 21‐d intervals. Forage yields and P uptake were affected by the content of water‐soluble P and the source of fertilizer P. Statistical analysis of the data for the five sources produced two separate regression yield models. The two respective fertilizer groups required 57 and 68% water‐soluble P to obtain 90% of the maximum yield. Results from this study indicate that current impurity levels in MAP fertilizers are not a major agronomic problem.