A major problem for 21st century agriculture is the prospect of P scarcity. Adsorption of PO4 on the soil's solid phase is the primary mechanism regulating P availability. Release of citrate by roots is generally thought to increase the availability of P, which in turn improves P acquisition by plants. However, the interaction between citrate and PO4 remains poorly understood in soils and conflicting results are found in the literature. Here modeling is used to investigate the effects of citrate adsorption on P availability in a chromic cambisol, a luvisol and two ferralsols over a range of soil pH values. The effect of different levels of exchangeable Ca, soil organic C (SOC), dissolved organic C (DOC) and PO4 fertilization were also tested in order to cover a wider range of environmental conditions. Results showed that the competition of citrate for PO4 binding sites was not the only mechanism regulating P availability. Citrate adsorption can also increase PO4 adsorption through electrostatic interactions with adsorbed Ca2+ ions and actively reduce P availability. More precisely, it was found that the addition of 10 lmol citrate kg (1) soil decreased P availability in both chromic cambisol and luvisol. The same trend was predicted by the model with 100 mu mol of citrate kg (1) soil in the chromic cambisol, whereas available P was found to increase in the luvisol. In contrast, the addition of citrate at these two concentrations always increased P availability in the two ferralsols. Increasing exchangeable Ca further decreased P availability in the chromic cambisol and luvisol, while it further increased available P in ferralsols. Additional sensitivity tests showed that DOC concentration had little influence on these results. In contrast, increasing SOC concentration massively counteracted the deleterious influence of citrate in chromic cambisol, while the effect was amplified in luvisol. In ferralsols, it was mainly observed that the increase of SOC further promoted P availability. To conclude, it was found that citrate can either increase or decrease P availability in soil, depending mainly on the occurrence of 2:1 clay minerals and on the concentrations of citrate, adsorbed Ca, and soil organic C. Special attention should, therefore, be given to possible adverse effects of rhizosphere management for improved P nutrition.