This study investigated the potential of exchangeable radiocaesium (Ex-¹³⁷Cs) and non-exchangeable potassium (Nex-K) as soil indicators for identifying paddy fields where radiocaesium (¹³⁷Cs) transfer to brown rice is likely to be high under exchangeable potassium (Ex-K) depletion. The characteristics of both indicators were compared. Using a dataset of 152 paddy fields in Fukushima Prefecture, Japan, we investigated the relationships between the ¹³⁷Cs transfer factor to brown rice and three parameters: Ex-K content, Ex-¹³⁷Cs fraction (i.e., the percentage of exchangeable ¹³⁷Cs in total soil ¹³⁷Cs), and Nex-K content (determined via hot nitric acid extraction). Both the Ex-¹³⁷Cs fraction and Nex-K content were significantly correlated with the ¹³⁷Cs transfer factor. Partial rank correlation analysis controlling for Ex-K (ρ_EK) revealed a stronger correlation for the Ex-¹³⁷Cs fraction (ρ_EK=0.65) than for the Nex-K content (ρ_EK=−0.50). A significant negative correlation (ρ=−0.47) was also found between the two soil indicators. To distinguish between their respective effects, a rice cultivation test was conducted using soils from 19 paddy fields in which the two values deviated from the correlation found in the analysis. The Ex-¹³⁷Cs fraction fluctuated significantly during cultivation (up to a 42-fold increase in some soils), whereas Nex-K content showed only minor variation, with an average decrease of 8％. Among soil indicators measured at different times, only the post-cultivation value of Ex-¹³⁷Cs fraction was significantly correlated with the ¹³⁷Cs transfer factor to aerial plant parts (ρ_EK=0.51). In conclusion, given the different properties of Ex-¹³⁷Cs and Nex-K, their use should be tailored to specific implementation contexts. Ex-¹³⁷Cs has a higher correlation with the transfer of ¹³⁷Cs to paddy rice, whereas Nex-K has the advantage of less fluctuation in measured values.