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/m 3 . 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/m 3 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.
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Pierzynski et al. (1990) studied this question.