Desirable levels of exchangeable K and Ca and their concentration in the soil solution to reduce uptake of radioactive Cs by rice plants

K in the soil solution can control the uptake of radioactive Cs by rice plants, but this control is not accomplished only by K; it is affected by other ionic species. It is therefore important to investigate uptake of radioactive Cs from the perspective of the concentration of major cations such as Ca in the soil solution and the levels of exchangeable cations in the soil. To clarify the effects of K and Ca in the soil solution and of the levels of soil exchangeable cations to prevent uptake of radioactive Cs, we conducted a pot experiment and field experiments in a paddy soil in 2011 and 2012. To reduce the uptake of radioactive Cs, it was necessary to achieve a K concentration in the soil solution of 0.5 mmol L^-1, and a Ca concentration higher than 2 mmol L^-1 based on the results of the pot experiment. In addition, we obtained the desirable levels of exchangeable cations and the cation exchange capacity (CEC) in the soil from previous reports and the results of our field experiments. On this basis, we propose the following threshold levels for exchangeable cations and CEC in the soil as a standard: 0.53 K cmol_c kg^-1, 18.0 Ca cmol_c kg^-1, 2.0 Mg cmol_c kg^-1, i.e. 25 mg K₂O 100 g^-1, 505 mg CaO 100 g^-1, 40 Mg O mg 100 g^-1, and a CEC of 30 cmol_c kg^-1. Converting these values into the corresponding levels in the soil solution, we obtained concentrations of 0.71 mmol K L^-1, 4.22 mmol Ca L^-1, and 1.35 mmol Mg L^-1. These levels are within the improving standard for fertility of paddy soils in Japan. Consequently, it will be necessary to improve the fertility of paddy soils to control the uptake of radioactive Cs by rice plants. (author)