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.

As stressed in the Millennium Ecosystem Assessment, over the last 50 years, human beings have modified the ecosystems to an unpreceded point in humankind history, in order to meet the increasing world demand in food, drinking water, wood, fibers and energy (Tilman 1999). Such changes much contributed to improving humankind well-being, but this was achieved at the expense of a degradation of numerous ecosystem services and increasing poverty of the poorest populations. Prediction models forecast further degradation of ecosystem services in the coming 50 years,