Estimation of soil phosphate adsorption capacity by means of a pedotransfer function

The capacity of soil to protect the aquatic environment against pollution is an important soil function. Thus, the phosphate adsorption capacity (PAC) is important in predicting the risk of phosphate pollution of the aquatic environment because of overfertilisation and because of reestablishment of former drained and cultivated wetlands. Moreover, PAC is an essential property in assessing the risk of drain and ground water contamination with the widely used glyphosate herbicide, which may compete with phosphate for adsorption sites on soil solids. As aluminium and iron oxides are the main phosphate adsorbents in many soils, PAC can be predicted by pedotransfer functions based on various aluminium and iron oxide fractions such as oxalate-extractable aluminium and iron (Al-o, Fe-o) and dithionite-extractable iron (Fe-d). Accordingly, experimentally determined PAC taken as the sum of oxalate-extractable phosphate and the Langmuir maximum of the phosphate adsorption isotherm was found to be well predicted by the pedotransfer function P-calc = 0.22 x Al-o+0.12 x Fe-o +0.02 x (Fe-d -Fe-o) for a wide range of noncalcareous soils including Alfisols, Entisols, Inceptisols, Oxisols, Spodosols, and Ultisols from various parts of the world. A set of noncalcareous soil samples from Denmark, Ghana, and Tanzania was used in developing this pedotransfer function, while another set of soils from Canada and Tanzania was used in testing the function. While close relationships between experimentally determined and predicted PACs were obtained with these soils, the function failed, however, to predict PAC of two Tanzanian Andisols. (C) 2003 Elsevier Science B.V All rights reserved.