Rietveld Estimates of Mineral Percentages to Predict Phosphate Sorption by Selected Hawaiian Soils

Predicting P sorption by weathered soils in order to manage soil, crop, and natural resources has been an important but elusive goal of soil science for decades. This study evaluated several soil properties, including mineralogy, of weathered Hawaiian soils in terms of their utility as predictors of P sorption. Chemical and physical properties associated with P sorption were measured and correlated. In addition, a theoretical quantity, potential P-sorption sites, was estimated from the mineralogical characteristics of the clay fraction. Rietveld refinement was used to determine the quantity of soil minerals. Crystallite size was determined by curve-fitting of selected x-ray diffraction peaks. These parameters were then used to predict the soil P-sorption potential by estimating the density of A-hydroxyl sites per gram of soil. This estimate of potential P-sorption sites provided the best correlation (R² = 0.94) with measured P-sorption capacity in 0.001 M CaCl₂, which varied from 79 to 3280 mg P kg⁻¹ soil at an equilibrium solution-P concentration of 0.2 mg P L⁻¹. Other bulk soil properties such as the moisture content at 1.5 MPa, specific surface area (glycerol retention), and the amount of amorphous (i.e., oxalate extractable) soil material, also correlated well with P sorption but yielded lower R² values. Clay content and soil chemical properties alone were not good predictors of P-sorption capacity across the full range of soils. Better estimates of the quantity of sorption sites, especially those contributed by amorphous oxides, would yield further improvements in this approach. Journal Series no. 4202, Hawaii Institute for Tropical Agriculture and Human Resources. This research was, in part, supported by the USDA under CSRS Special Grant Agreement no. 92-34135-7346, managed by the Pacific Basin Administrative Group (PBAG).