Core Ideas Understanding P sorption complexity is critical to managing alkaline soil‐P. Molecular and macro‐scale sorption are characterized in representative systems. Preferential P sorption on ferrihydrite is seen in ferrihydrite–calcite mixtures. Nonlinear P sorptive behavior in mineral–mixtures indicates mineral interaction. An increase in P bioavailability is expected with soil reduction and pH increase. A fundamental understanding of phosphorus (P) bioavailability in alkaline soils is critical to its better management in agricultural and environmental applications. The main objective of this study is to characterize phosphate speciation in the alkaline pH range (pH 8–9.5) in binary mixtures of ferrihydrite and calcite using a combination of wet‐chemical and P K‐edge X‐ray absorption near edge structure spectroscopy. Our results showed that ferrihydrite and calcite sorbed phosphate as an inner‐sphere surface complex with binuclear‐bidentate mode of phosphate bonding on ferrihydrite in both single‐ and mixed‐mineral systems of ferrihydrite and calcite. Phosphate sorbed preferentially on ferrihydrite in binary mixtures of ferrihydrite and calcite particularly at higher concentrations of sorbed phosphate, probably due to greater concentration of metal‐OH surface sites in ferrihydrite than in calcite. In addition, phosphate sorption in mineral mixtures did not behave as a linear combination of phosphate sorption in their single‐mineral components, indicating mineral interactive effects. Molecular‐scale insights from this study have important implications for macro‐scale behavior of P bioavailability in alkaline soils, such as an increase in P bioavailability and subsequent P release with soil reduction and pH increase.