Over the last three decades, both the crop yields and population in China have steadily increased due to continued agricultural investments. K fertilizer and the return of straw have played vital roles in the yield and quality of crops. However, the effects of long-term fertilization on soil K to a soil depth of 1 m have received little focus, and unbalanced K fertilization frequently occurs because local farmers lack sufficient information about soil K. In this study, the spatial distribution of available potassium (AK) in East China was predicted using kriging with external drift (KED) and random forest (RF) techniques. A comparison of the trends in the 1980s and 2010s showed that the mean AK values in East China generally increased. The mean AK values at soil depths of 0–20 cm, 20–50 cm and 50–100 cm increased by 17%, 12% and 23%, respectively, and the AK stocks increased from 0.14 Pg to 0.17 Pg. However, the spatial patterns of AK at the regional scale suggested that approximately 68.12% of the study area lacked AK. AK deficiency was found in Northeast and South China. The ratios of AK concentrations in the subsoil to those in the topsoil suggested that less K was stored in the subsoil than in the topsoil. Relatively more AK was found in the subsoil (20–50 cm) of paddy fields in the middle of East China than in the other areas. The maps produced in this study could be used to support the decisions of regional agriculture management, and our study demonstrates that effective K fertilizer application in East China should be taken into consideration.