Compared with the control plots, application of fertilizer K significantly (P<0.05) increased maize grain yields, and treatments with K application rates of 113 and 225 kg K2Oha1 increased the average yields by 15.1 and 13.8% over the course of the twenty years of the field experiment, but the variation from year to year was large (Table 3). The optimum K rate was approximately 150 kg K2Oha1 according to the K balance calculation (Fig. 3). If the K application rate was exceeded or notsynchronized to crop K demand, not only did grain yield cease to increase, but the K use efficiency also decreased and leaching of exchangeable K occurred (Fig. 2a). The K application rate can be
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