Phosphorus sorption by some coconut‐growing acid soils of Sri Lanka and its relationship to selected soil properties

Abstract Phosphorus (P) sorption in the presence of 10−2M calcium chloride by ten coconut‐growing acid soil samples of Sri Lanka belonging to the Ultisol, Alfisol and Entisol orders was evaluated using the Langmuir sorption isotherm. The data for all samples fitted the single‐site Langmuir sorption isotherm when equilibrium P was less than 1 μg ml−1, but showed some deviations at larger concentrations of P. A two‐site Langmuir sorption isotherm was successfully used to describe the sorption data of three samples at these high concentrations. The single‐site Langmuir sorption maxima were greater for Ultisols (349–825 μg g−1) than for the other soils (136–345 μg g−1). These sorption maxima were significantly correlated with %‐clay, %‐silt, dithionite‐extractable iron, and 1M ammonium acetate (pH 4.8)‐extractable aluminium. Regression analysis of P sorption on different forms of iron (Fe) and aluminium (Al) indicated that crystalline ‘free Fe oxides’ (dithionite‐extractable Fe minus 0.3M oxalate‐extractable Fe) and ‘active Al’ which consists of hydroxy Al monomers and polymers [1M ammonium acetates, (pH 4.8)‐extractable Al minus 1M potassium chloride extractable Al] were the important factors contributing towards P sorption.