In this study we identified the nature of the root-induced chemical processes controlling changes in phosphate (P) availability in a soil with two P loadings resulting from long-term fertilization treatments. We used a set of mechanistic adsorption models (surface complexation and ion exchange) within the framework of the component additive approach to simulate the effect of durum wheat roots on P availability. We had to consider the influence of adsorption of other ions to ensure the goodness-of-fit of the simulations. We found that Ca(2+) uptake, in addition to P uptake and root-induced alkalization, controlled P availability in the rhizosphere regardless of the fertilization level. The relative influence of these three processes depends primarily on the extractant used to estimate P availability. Calcium uptake was the most significant process in water extracts, whereas P uptake was the dominant root-induced chemical process in CaCl(2) extracts. Under low Ca concentrations, Ca(2+) uptake decreased the promoting influence of Ca(2+) adsorption on P adsorption. In addition to confirming the validity of our approach to model P availability, the present investigation indicated that root-induced processes markedly affect P availability irrespective of the fertilization level.