M. van der Laan

Obtaining the parameters required to model labile phosphorus for South African soils

Abstract Modelling phosphorus (P) in the environment can increase our understanding of potential transfer pathways into receiving water bodies as well as the plant availability of this nutrient in soil. Many current models make use of algorithms originally developed for the EPIC model over two decades ago. These algorithms were developed primarily using continental USA soils. Obtaining the required input parameters can therefore be challenging when applying this approach to soils not classified according to the USA system, and for soils for which similar parameters are not available. In this paper, new equations for the estimation of labile P from Ambic P, Bray 2 P and the modified ISFEI method are proposed. Guidelines for the classification of South African soils as calcareous, slightly weathered and highly weathered are further suggested, and we propose that only topsoil properties be used for this purpose. Depending on the amount of soil information available, this classification can be achieved using the clay fraction SiO2:Al2O3 molecular ratio, the sum of exchangeable Ca, Mg, K and Na, or a newly proposed categorization system for South African soil forms. It is clear that the above approaches should be thoroughly tested and relevant local research carried out to improve our ability to model P in South African soils.