Chemical characterization and bioavailability of phosphorus in water-insoluble fractions of three mono-ammonium phosphate fertilizers.

: Elemental impurities in mono-ammonium phosphate (MAP) fertilizers, such as Fe, Al, Ca, Mg, and F, result in compounds other than NH4H2PO4. The phosphorus availability of the impurity compounds was determined in 3 commercial MAP fertilizers produced from North Carolina, Florida, and Idaho phosphate rocks. Soluble compounds, including NH4H2PO4, were washed out of the fertilizers, and the water-insoluble fraction was collected. The North Carolina, Florida, and Idaho MAP fertilizers contained 13, 16, and 17% water-insoluble fractions, respectively. Availability of phosphorus in each MAP fertilizer and water-insoluble fraction was determined in a greenhouse pot study using sorghum-sudangrass (Sorghum bicolor) as the test crop. There were no differences in dry matter weights or phosphorus uptake between reagent grade MAP and each MAP fertilizer. Lower dry matter weights and phosphorus uptake were produced from the water-insoluble North Carolina, Florida, and Idaho fractions (NC greater than FL greater than ID). Chemical analysis and characterization with optical microscopy, infrared spectroscopy, and X-ray diffractometry indicated 77.3, 73.8, and 73.6% NH4H2PO4 in the North Carolina, Florida, and Idaho MAP fertilizers, respectively. MgAl(NH4)2H(PO4)2F2, AlNH4HPO4F2, and FeNH4(HPO4)2 were identified as water-insoluble phosphorus compounds. There was a significant negative correlation between the percent phosphorus present as MgAl(NH4)2H(PO4)2F2 and the phosphorus availability. Increased crystallinity of the various water-insoluble compounds also contributed to reduced phosphorus availability.