Plant nutrition models do not properly account for the effects of root-induced chemical processes in the rhizosphere, e.g. pH changes, on the availability of nutrients such as phosphorus (P). As a result, they underestimate the actual P uptake, i. e. P bioavailability for plants, in low P soils. We used a mechanistic description for the adsorption of cations and anions by soil constituents to simulate P availability in rhizosphere and bulk soil over a range of pH values. We also measured and simulated the availability of calcium (Ca) and the bioavailability of Ca and P for wheat. Results of modelling were found in excellent agreement with experimental data. In the rhizosphere, the achievement of the goodness-of-fit required to account for the uptake of Ca by plants, in addition to P uptake and root-induced alkalisation. Calcium uptake significantly increased P availability, when assessed by water extraction, by decreasing the promoting effect of the adsorption of Ca onto that of P. The calculated amounts of Ca and P taken up by plants corresponded to the measured amounts, i. e. Ca and P bioavailabilities. Our modelling investigation showed that P was primarily adsorbed onto Fe-oxides and clay minerals depending on soil pH. The major source of bioavailable P for wheat was P desorbed from goethite and kaolinite. In addition to confirming the validity of our approach to model P availability and bioavailability, the present investigation suggested that in the studied soil, a novel rootinduced chemical process was controlling P nutrition under low P conditions, namely the uptake of Ca.