Identification of residual P compounds in fertilized soils using density fractionation, x-ray diffraction, scanning and transmission electron microscopy

Residual fertilizer grains were recovered using density fractionation from three Western Australian soils fertilized five years previously with North Carolina apatitic rock phosphate (RP), Queensland apatitic RP and calcined calcium-iron-aluminium RP (Calciphos). Electron micrographs, energy dispersive X-ray analyses (EDS) and unit cell parameters derived from X-ray diffraction patterns (XRD) of recovered apatitic grains do not differ from those for the original fertilizers. Dissolution of these apatitic fertilizers in soils proceeds by surface etching of apatite crystals and is congruent with no formation of intermediate P compounds. Calciphos grains also did not differ in mineralogy and composition but they had developed a porous fabric due to dissolution along subgrain boundaries. Poorly soluble superphosphate residues were mainly composed of dicalcium phosphate dihydrate, FeAl phosphate and apatite together with quartz and other soil minerals. The P concentrations in various particle size fractions of recovered fertilizer materials were related to the particle size distribution of fertilizers originally added to the soils. These results support the agronomic results that the low agronomic effectiveness of RP fertilizers in these soils is primarily due to the small extent of dissolution of RP.