PHOSPHORUS CHEMISTRY AND MINERALOGY IN EXCESSIVELY FERTILIZED SOILS - QUANTITATIVE-ANALYSIS OF PHOSPHORUS-RICH PARTICLES

There has been little success with direct methodologies in isolating or characterizing phosphate minerals or P-bearing solids in soils, due primarily to the fact that P solids represent a relatively minor component of the soil matrix. This study was conducted to evaluate a method for concentrating P in soils and to characterize P-solids phases by a variety of direct mineralogical methods. Eleven soil samples representing six soil series from Illinois, Michigan, Wisconsin, and New York were fractionated according to particle size, with the clay-sized fractions being further separated into the density ranges of < 2.2, 2.2 to 2.5, and > 2.5 Mg/m3. The density separates were examined by scanning transmission electron microscopy in conjunction with energy dispersive x-ray analysis, x-ray diffraction (XRD), and Fourier-transformed infrared (FTIR) spectroscopy. Total P concentrations were generally highest in the clay-sized fractions, compared with the sand- and silt-sized fractions, and always highest in the lowest-density separates, compared with the remaining two density fractions. Discrete P-rich particles were found in most density separates, with the highest abundances occurring in the < 2.2 and 2.2 to 2.5 Mg/m3 fractions. Particles generally contained detectable quantities of Al and Si and various combinations of K, Ca, and Fe. Phosphorus concentrations within individual P-rich particles were highly variable. Analysis of the density separates with XRD and FTIR spectroscopy was not successful in identifying discrete P minerals.