Soil samples were collected form the 7 treatments (NPK, NP, NK, PK, CK, CK + BM, and CK + SR) of a long-term upland fertilization field experiment, which started in 1988, for determination of phosphate sorption index (PSI) with soil maximum P sorption capacity (X_m), and hence discussions on effects of the soil matrix component, like soil pH, organic matter (OM), clay, iron-aluminum oxides and inorganic phosphates on PSI. Results show that long-term application of phosphorus fertilizer or combined application of organic and chemical fertilizer could significantly reduce PSI. With rising soil pH and organic matter and iron phosphate (Fe-P) content, PSI decreased markedly. But the higher the contents of free iron-aluminum oxides and clay in the red soil, the higher the soil PSI. Soil phosphate sorption index (PSI) showed a significant linear correlation with soil maximum P sorption capacity (X_m), (X_m = 0.5PSI + 412.8, r = 0.967^**, p < 0.01). Therefore, the former could used to replace soil maximum P sorption capacity (X_m) to characterize soil phosphorus sorption capacity and soil phosphorus supply capacity as well.