Phosphate minerals and solubility in native and agricultural calcareous soils

Generally the direct identification of naturally occurring phosphate minerals, especially the reaction products of applied phosphate fertilizer with soil constituents, is very difficult. Nevertheless the characterization of phosphate (P) minerals and the understanding of their behavior in soils are very important with respect to fertilizer management and to safeguarding water quality. The objectives of this study were to determine the P minerals detectable in different particle size fractions of native and cultivated calcareous soils that are probably products of reactions between the soil and the fertilizer, and to examine the relationships between P mineral composition, phosphate solubility, chemical fractionation, and P activity. Fifteen calcareous soils representing four soil series were collected from native pineland and wetland, cultivated vegetable fields and fruit groves in south Florida. Soil samples were separated into sand, silt and clay with and without the removal of CaCO3 and organic matter. Mineralogy of selected fractions was determined by X-ray diffraction (XRD). Relatively distinctive crystalline apatite was identified by XRD of high-density separates from carbonate-free silt fractions of selected cultivated soils, but no phosphate minerals were detected in any fraction of the corresponding native soils. Soils were also incubated under field capacity moisture content for 30 days, either without P application or with 66.8 mg P kg− 1 soil applied as KH2PO4. Soil solutions were extracted and the concentrations of both anions and cations were determined. The activities of Ca2 + and H2PO4− calculated using the MINTEQA2 speciation program were adequate to support the stability of hydroxyapatite (HA), fluorapatite (FA) and tricalcium phosphate (TCP) minerals in farmed soils, while TCP was unstable in unfarmed soils, and the solution points of TCP were close to the equilibrium of HA. The results indicated that phosphate activities in native and heavily fertilized calcareous soils were controlled by the solubility of different phosphate minerals. Sequential fractionation data suggested that Ca-Mg-associated P accounted for a dominant proportion of total P in farmed soils, but this fraction was below the detectable limit in unfarmed soils. These findings demonstrate that the repeated heavy application of phosphate fertilizer during many years has resulted in the formation of stable, crystalline calcium phosphate minerals in these farmed calcareous soils.