Abstract: Soil potassium balance (KB) is an indicator for potassium input and output in croplands. It is also a simple method to judge the situation of soil potassium deficiency, which has been extensively used in soil nutrient condition evaluation and fertilizer recommendations for crops. In this paper, KB in croplands in China was reviewed on four fronts—calculation method, spatio-temporal variability analysis, crop type difference analysis and soil potassium content analysis. The studies showed an overall deficient KB in croplands in China. Regional variations in potassium deficiency were significant, with surplus farmland potassium in some areas that resulted in KB polarization. The difference in nutrient uptake among crops, the fertilizer volatility and agricultural market prices together determine the difference in KB among different crop types or even same crop in different years. Due to differences in the calculation parameters or regional planning, vast differences were noted in different reports (literatures) in terms of the amounts of total deficient potassium, deficient potassium per hectare and spatial variation in potassium deficiency. Thus although direct comparison of these results for different studies was apparently inappropriate, such comparison should be done after comprehensive analysis of related index systems. Irrespectively, in temporal variations, the effects of different planting on KB in croplands and soil potassium accumulation were basically identical. In essence, the current state of soil nutrient pool was the result of long-term accumulation of regional cropping systems (including rotation) and fertilization behaviors, which in turn determined future trend in the development of soil nutrient pool. In order to control soil potassium polarization and to ensure a stable promotion of soil potassium pool in China, it was recommended to focus future research on areas such as the optimization of nitrogen and potassium application amounts, KB in annual crop rotation systems, seasonal differentiation of potassium fertilization in accordance with soil application and crop adsorption characteristics.