APPARENT SOLUBILITY PRODUCTS OF PHOSPHORUS IMPURITY COMPOUNDS IN COMMERCIAL MONOAMMONIUM PHOSPHATE FERTILIZERS

Monoammonium phosphate (MAP) fertilizers contain P impurity compounds that have low solubility and bioavailability. Solubility products of fertilizer impurity compounds not currently documented would be useful data to predict the solubility and subsequent P bioavailability of the compounds in soil. Water-insoluble MAP fractions from seven commercial MAP fertilizers were equilibrated with water and the filtrate was analyzed for total concentration of chemical components. Activities of individual elements were calculated using a chemical speciation program, MINEQL, and were used to determine apparent solubility products (K(sp)o) of P impurity compounds. The apparent log K(sp)o values were -16.33, -9.33, and -14.59 for AlNH4HPO4F2, FeNH4(HPO4)2, and MgAl(NH4)2H(PO4)2F2, respectively. The values were only considered estimates because they were determined from a heterogeneous mixture of compounds in the water-in-soluble MAP fractions rather than pure solid phases. The approximate log K(sp)o values were instructive for comparing solubilities of the impurity compounds with solubilities of variscite, strengite, monetite, brushite, struvite, and FePO4(c) in water and soil. At pH 6 in water, the general order of decreasing P solubility was struvite > brushite > monetite > MgAl(NH4)2(PO4)2F2 = AlNH4HPO4F2 > FeNH4(HPO4)2 > > FePO4(c) > variscite almost-equal-to strengite. In soil, the P impurity compounds have a greater solubility than variscite, strengite, monetite, and brushite between pH 5.5 and 7. Therefore, the impurity compounds are not expected to remain intact in soil for long periods of time. Of the three impurity compounds, MgAl(NH4)2H(PO4)2F2 appears to be the least desirable because of lower calculated solubility in soil.