X‐Ray Diffraction Line Profile Analysis Vs. Phosphorus Sorption by 11 Puerto Rican Soils

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Abstract
The increase in the areas under goethite (110), gibbsite (002), and hematite (104) x‐ray diffraction lines, due solely to small crystallite effects, was plotted against phosphorus sorption for 11 Puerto Rican soils. This increase is shown to be proportional to the total surface area of each phase measured and is therefore referred to as the surface area index. The data showed a very high correlation (R2 = 0.95 for a straight line and R2 = 0.96 for a second‐order polynomial) between the surface area index of goethite and phosphorus sorption. In comparison, the correlation between the surface area index of gibbsite and phosphorus sorption was poor (perhaps because gibbsite was present in low concentrations relative to goethite) but judged to be significant (R2 = 0.56). The correlation between the surface area index of hematite and phosphorus sorption was not significant (R2 = 0.26) as compared to goethite and gibbsite. The sum of the surface area indices of goethite and gibbsite showed a near‐perfect correlation with phosphorus sorption (R2 = 0.98 for a second‐order polynomial). The study demonstrated that, in the case of goethite and to a lesser extent gibbsite, the mean crystallite size is more important than the abundances of the phases for explaining observed phosphorus sorption characteristics of soils.

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