Effect of 15-Year-Long Fertilization on Potassium Quantity/Intensity Relationships in Black Soil in Northeastern China

The effect of long-term fertilization and cropping on soil potassium (K)–supplying capacity has not been extensively studied. Five treatments control, nitrogen (N), N–potassium (P), NPK, and NPK + manure (NPKM)] were used to evaluate the quantity and intensity relationship (Q/I) of K in black soil (Luvic Phaeozems soil) after a 15-year (1990–2005) long-term fertilization with a continuous corn cropping located in Changchun, Jilin Province, northeast China. Results showed that the long-term K fertilization of NPK or NPKM markedly increased the values of equilibrium active ratio (AR₀ ^K) and nonspecifically available K (?ΔK₀) but decreased the potential buffering capacity (PBC^K) value. Values of the AR₀ ^K and??ΔK₀ in the NPK and NPKM treatment were about four and two times greater than those in the control, N, and NP treatments, respectively. Compared with the non-K fertilization, PBC^K values were decreased by 40–49% under the K fertilization. The black soil suffered from K deficiency even with NPK fertilization as evidenced from the greater free energies of K⁺ exchanging for calcium (Ca²⁺) and magnesium (Mg²⁺) (?ΔG) (14.7–18.8 kJ mol^?1) than the threshold value of 14.6 kJ mol^?1, from the lower soil K⁺ saturation (1.34%–1.44%) than the critical value of 2.00%, and from the response of corn yield to K fertilization. Our results demonstrated that the increment of the current K application rate through fertilizer and/or manure is needed to improve corn yield in the long run in northeastern China.