Phosphate (PO4) and organic matter (OM) compete for adsorption to metal (hydr)oxides. Our objective was to quantify the effect of OM on PO4 solubility in forest and arable soil by desorption experiments and surface complexation (SC) modelling. We sampled different types of soil along an age gradient (˜50–2500 years) and from different depths (0–80 cm). The soil types are calcareous and cover a range of soil organic carbon (SOC) contents (5.6–43.5 g kg-1), PO4 contents (0.2–5.9 mmol kg-1) and water-soluble PO4 concentrations (0.03–13.4 µm). Assuming that PO4 concentrations are controlled by desorption, PO4 concentrations were expected to correlate with the PO4 loading on metal-(hydr)oxide surfaces. However, we show that the PO4 loading alone is a poor predictor of PO4 solubility because its solubility increases with increasing SOC content. These data were explained by SC modelling, which shows a decrease in the apparent adsorption affinity of PO4 with increasing OM loading on to the metal (hydr)oxides. As a consequence, if the competition with OM is disregarded in SC modelling, it results in underestimation of the PO4 concentration by several orders of magnitude. For forest soil, predicted OM loadings increase slightly with increasing soil age. For arable soil, however, OM loadings were much smaller, which we explain by the replacement of PO4 with OM. Overall, adsorption interactions strongly affect PO4 solubility and levels of OM and PO4 stabilization in soil.