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, in cluding mineralogy, of weathered Hawaiian soils in terms of their util ity as predictors of P sorption. Chemical and physical properties asso ciated with P sorption were measured and correlated. In addition, a th eoretical quantity, potential P-sorption sites, was estimated from the mineralogical characteristics of the clay fraction. Rietveld refineme nt was used to determine the quantity of soil minerals. Crystallite si ze was determined by curve-fitting of selected x-ray diffraction peaks . These parameters were then used to predict the soil P-sorption poten tial by estimating the density of A-hydroxyl sites per gram of soil. T his estimate of potential P-sorption sites provided the best correlati on (R-2 = 0.94) with measured P-sorption capacity in 0.001 M CaCl2, wh ich 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 t he moisture content at 1.5 MPa, specific surface area (glycerol retent ion), and the amount of amorphous (i.e., oxalate extractable) soil mat erial, also correlated well with P sorption but yielded lower R-2 valu es. Clay content and soil chemical properties atone were not good pred ictors of P-sorption capacity across the full range of soils. Better e stimates of the quantity of sorption sites, especially those contribut ed by amorphous oxides, would yield further improvements in this appro ach.