Phytoavailable K in soil is a key to control the transfer factor of radiocesium from soil to brown rice. The transfer factors were determined for paddy fields cultivated in 2017 and 2018 under different K fertilization regimes in Fukushima Prefecture, Japan. Two phytoavailable forms of K, the exchangeable and nonexchangeable K contents were investigated for the surface soil sampled after the transplanting and fertilization as well as after harvest of rice in the same paddy fields. The exchangeable K content largely decreased from after transplanting and fertilization to after harvest, and the exchangeable K of the soil after harvest was negatively correlated with the transfer factor (rs = -0.70, p < .001). Most soil samples after harvest showed that the transfer factors exponentially increased as the exchangeable K decreased; however, some of the samples indicated considerably low transfer factors (<0.005) despite being exchangeable K deficient, i.e., exchangeable K < 25 mg K2O 100 g-1. Even though this value before usual fertilization has been effectively used as a threshold to determine whether supplemental K fertilization is required to reduce the radiocesium content in brown rice, additional screening was needed to estimate this radiocesium transfer more precisely. Thus, we found that not only the exchangeable K but also nonexchangeable K contents had a negative correlation with the transfer factor (rs = -0.60, p < .001) of the soil samples after harvest but were not correlated with each other (rp = -0.10). Furthermore, the results revealed that soil with nonexchangeable K > 50 mg K2O 100 g-1 indicated a considerably low transfer factor, even if exchangeable K deficient. Thus, via our field-scale experiments, we concluded that the criterion nonexchangeable K > 50 mg K2O 100 g-1 can be used as another threshold for use along with that of exchangeable K to differentiate soil with a low radiocesium transfer rate from exchangeable K deficient soil.