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Potash resources in China are very scarce. It is especially important to correctly evaluate the potential potassium-supplying power of soils in northern China in order to use soil potassium sufficiently and potash fertilizer properly. Regional differences in crop yield responses to long-term potassium application in northern China were determined in this study. Twenty-five representative soil samples from 13 provinces, municipalities, and autonomous regions of major agricultural regions in northern China were collected from the surface layer (0-20 cm) before crop seeding in 1993. A soil potassium-depletion study was carried out in a pot experiment with successive planting of corn seedlings for 10 harvests in the 25 soils. Since 1993, field trials on wheat and corn response to long-term potassium application were conducted at the fixed sites of HLJ-SC (Shuangcheng of Heilongjiang), JL-LFZ (Liufangzi of Jilin), HB-XJ (Xinji of Hebei), SX-LF (Linfen of Shanxi), QH-NKY (Nongkeyuan of Qinghai), and XJ-CJ (Changji of Xinjiang). Soil-available potassium, slowly available potassium, total potassium, main clay minerals, cation exchange capacity, particle size, other available nutrients, and potassium concentration in plants were measured. The results showed that potential potassium-supplying powers of the 25 tested soils, which were respectively evaluated by the contents of slowly available potassium in soils and the amount of total net potassium uptake in the pot experiment, generally tended to increase from eastern to western regions in northern China. Significant wheat yield responses to long-term potassium application in the field trials were found since 2000 in the north-central region but not observed until 2004 in the northwestern region. Significant corn yield responses to long-term potassium application in the field trials were found starting in soils of the northeastern region, following in soils of the north-central region, and then in soils of the northwestern region. These were consistent with potential potassium-supplying power of the soils, which tended to increase from east to west regions. [ABSTRACT FROM AUTHOR]