The Yangtze River basin is distributed across subtropical monsoon climate regions, and has four seasons, including a hot rainy season. These climatic conditions provide favorable conditions for paddy-upland rotation. This paper summarizes the spatiotemporal changes in soil potassium (K) and K cycles in soil-plant systems, as well as environmental impacts on K changes, and provides information for optimal K management. During the past 30 years, soil available K increased by -7.1% to 103.4%. The increase was lower in Hunan, Guizhou, Zhejiang, and Jiangsu provinces (<10%) and higher in Anhui, Jiangxi, Henan, and Chongqing provinces (>30%), demonstrating that soil K pools were enhanced. Farm manure was gradually replaced by synthetic K sources, such as straw and mineral fertilizers, which contributed to an increase in crop yields and soil available K. The meta-analysis results showed that comprehensive K management strategies increased crop yield and soil available K by 11.0% and 44.3%, respectively, on average. Other factors such as balanced fertilization, recycling of straw, increase in atmospheric deposition, decrease in leaching, runoff, and soil K fixation also greatly influenced soil K changes, leading to improvements in crop yields, soil structure, soil fertility, and nutrient availability. Positive K cycles and appropriate K fertilizer use will facilitate proper K management, including cycling of straw, improving machinery and equipment, and estimating the optimal K fertilizer dose after straw. Future studies should focus on tradeoffs between different forms of K under various environmental conditions and accurate estimates of reductions in mineral-K fertilizer requirements following straw return.