Soil-to-Crop Transfer Factor: Consideration on Excess Uranium from Phosphate Fertilizer

Abstract

In this chapter, we used the initial U/Th concentration ratio in nonagricultural soil samples to estimate the concentrations of excess U (U-ess) in agricultural fields in Japan due to long-term application of phosphate fertilizers as a means to understand the bioavailability of added U to soil. We carried out a data survey and obtained initial U/Th ratios from 210 nonagricultural soil samples collected throughout Japan; the values are distributed log-normally, and they ranged from 0.006 to 0.9 with the geometric mean value of 0.259. Because concentrations of Th are lower in phosphate fertilizers than those of U, we assumed that Th concentrations in agricultural fields kept their initial values. Thus, the geometric mean of initial U/Th and Th concentration in each agricultural soil sample was used first to calculate the initial U concentration in the soil sample, and then, by subtracting the value from total U in the soil sample, the U-ess concentration in the soil was estimated. Uranium in phosphate fertilizer was thought to be soluble so we hypothesized that U-ess would be easily taken up by crops; however, there were no correlations between brown rice and U-ess nor for potato and U-ess. These results suggested that the added U was fixed to the soil particles so that U-ess would not be easily taken up by crops. We also found that there was no significant difference between soil-to-crop transfer factors obtained in low and high U-ess soil conditions for brown rice and potato; this means that the U-ess and the initial U in soil-to-crop systems had the same fate. Thus, soil-to-crop transfer factors obtained using U in agricultural fields should be valid in long-term radiation dose assessment models.