Modelling phosphate adsorption to the soil: application of the non-ideal competitive adsorption model.

Phosphorus (P) transport in subsurface runoff has increased despite the limited mobility of P in soils. This study investigated the ability of the non-ideal competitive adsorption (NICA) model to describe phosphate (PO(4)) adsorption for soils in southern Quebec (Canada). We measured the surface charge and PO(4) adsorption capacity for 11 agricultural soils. Using the experimental data and a nonlinear fitting function, we derived the NICA model parameters. We found that the NICA model described accurately the surface charge of these soils with a mean R(2)>0.99, and described the adsorption data with a mean R(2)=0.96. We also found that the variable surface charge was distributed over the two binding sites with the low pH sites demonstrating a stronger binding energy for hydroxyl and PO(4) ions. We established that the NICA model is able to describe P adsorption for the soils considered in this study.