Availability of Phosphorus in the Water-Insoluble Fraction of Monoammonium Phosphate Fertilizers

Abstract

Declining reserves of premium-grade phosphate rock (PR) have resulted in a reliance on lower grade ore to met the needs for phosphate fertilizer production. Elemental impurities, primarily Fe, Al, Mg, and K, are released from closely associated accessory minerals during acidulation of PR. These impurities result in the formation of sludge compounds in the wet-process acid, and in impurity compounds in commercial monoammonium phosphate (MAP, NH4H2PO4) fertilizers. Plus, these impurity compounds are generally water insoluble, thus reducing the level of water-soluble P in commercial MAP fertilizers. A greenhouse study was initiated to determine the availability of P in the water-insoluble fractions of MAP fertilizers. Seven commercial MAP fertilizers were selected to represent the major U.S. sources of PR (Florida, North Carolina, and Idaho). The water-soluble P was removed by washing with deionized water, and the remaining material having low water-soluble P was compared with reagent-grade MAP (100% water soluble). The water-insoluble-P-containing materials produced lower sorghum-sudangrass (Sorghum bicolor [L.] Moench) yield and less P uptake than water soluble reagent-grade MAP. Forage yields and P uptake were dependent on the source of the water-insoluble fraction. Phosphorus soluble in neutral ammonium citrate was not a reliable measure of short-term P availability in the water-washed fertilizer fractions.