Abstract:【Objective】This study was to explore effect of long-term potassium application on soil potassium supply capacity, so as to provide certain scientific basis for rationally potassium application and continuing research on development of agricultural soil.【Method】From a long term potassium fertilization experiment on a tobacco-rape rotation system, which was designed to have 0,187.5 or 375 kg·hm-2 K2O applied to flue-cured tobacco and 36 kg·hm-2 to rape, soil samples were collected for analysis of apparent balance and Quantity-Intensity (Q/I) of soil potassium, and relevant parameters, i.e. K equilibrium activity ratio (ARe), soil labile K (KL), non-specific labile K (-ΔK0) and specific labile K (KX), potential buffering capacity (PBC), K and Ca+Mg exchange free energy (-ΔG), and soil properties, like readily available K (Kav), slowly available K (Knex), K saturation degree (KS), clay minerals components and so on, and moreover, relationships between Q/I parameters and soil properties in the flue-cured tobacco-rape cropping system.【Result】 Results show that after 20 years of the stationary experiment, ARe varied in the range of 0.38×10-3~8.91×10-3(mol·L-1)0.5, -ΔK0 of 0.02~0.55 cmol·kg-1, KL of 1.61~2.16 cmol·kg-1, KX of 1.06-2.14 cmol·kg-1, PBC of 6.20~7.76 cmol·kg-1(mol·L-1)-0.5, and -ΔG of 11.70~19.72 kJ·mol-1. Obviously K application enhanced both ARE and -ΔK0, but reduced KL, KX, PBC, and -ΔG. Kav, Knex, KS and Kab were extre-significantly and positively correlated with ARe and -ΔK0, but negatively with KL, KX, PBC and -ΔG. Significant relationships were observed between the parameters of the Q/I curve of soil K, and these parameters could be integrated into a principal component index, which was significantly lower in K application treatments than in no K application treatments. Soil potassium was retained mainly interlaminarly in clay mineral crystals and lower in bio-availability in the soils with no K applied, but on joint position interface of the clay mineral crystals and higher in bio-availability in the soils applied with K. Hence, the soils with no K applied were higher in vermiculite content, but lower in illite content than the soils with K applied.【Conclusion】All the Q/I curve parameters, such as ARe, -ΔK0, KL, KX, PBC,-ΔG, can be used as indicators for evaluation of soil potassium supply capacity. Apparent balance of soil potassium is a major factor driving changes in K Q/I curve shape and differentiation of its parameters, variation of potassium supply capacity and evolution of soil minerals. The higher the K application rate, the higher the soil potassium surplus and the higher the soil supply capacity. In the soils under long term potassium deficiency, mineral potassium transforms into specific labile K at a higher rate than specific labile K does into non-specific labile K. Long-term potassium deficiency causes transformation of illite into vermiculite.