Factors affecting phosphorus sorption in palexerults of western Spain

Plant availability of soil phosphorus (P) is a function of extractable P and properties affecting P sorption. Such properties have not been studied extensively in highly weathered soils with a xeric moisture regime. The relationships among P sorption and certain soil components were determined in 36 surface and subsoil samples from seven Palexerults of western Spain. Measures of P sorption were the maximum P sorption capacity obtained from the Langmuir equation and the Bache and Williams P sorption index, which were very closely correlated. Soil properties measured were pH, exchangeable Al, base saturation, organic matter, clay, and free iron oxides. The best correlation between either maximum P sorption capacity or P sorption index and another soil property was that found with logarithmically transformed exchangeable Al (r = 0.85). Exchangeable Al may be estimated by routine soil tests, so the relationship could be utilized to improve the P soil test interpretation. There were also strong, positive correlations between the two measures of P sorption and both clay and free iron oxides. In these soils, P sorption was negatively correlated with pH, base saturation, and organic matter. Maximum P sorption capacity could also be predicted well from logarithmically transformed values for clay and pH (R = 0.88), properties commonly determined in soil classification.