Abstract
This study was conducted to estimate the concentration of cadmium (Cd) in soybean seeds in different upland fields converted from paddy fields using Cd concentration in soil extracted with 0.1M HCl, soil pH, phosphate absorption coefficient and history of field conversion. A summary was follows. (1) The concentration of Cd in soybean seeds grown in the converted upland field from paddy field at the second year or longer period was significantly lower than that of seeds in the first year after conversion. This reason was the increase of soil pH due to the application of liming material. (2) The Cd concentration in soil extracted with 0.01M HCl was correlated with the Cd concentration of soybean seeds. The extraction efficiency of Cd with this solution was decreased as rising of the phosphate absorption coefficient, which is a major indicator of Cd absorption in soil. (3) The exchangeable Cd concentration extracted with 0.05M Ca(NO_3)_2 had a high correlation to the Cd concentration of soybean seeds in low phosphate absorption coefficient soil, because the Cd was adsorbed with the different surface functional group in soil and main adsorbents of Cd was the negative charge of siloxane surface in 2:1 type clay mineral. (4) In upland fields which were in the first year converted from paddy field and less than 1,100 of the phosphate absorption coefficient, the estimation of Cd concentration of soybean seeds using the soil Cd concentration extracted with 0.1M HCl and soil pH was practical used. The Cd concentration of soybean seeds in other soils was lower than that of above descript soils significantly. (5) Thus, it had been possible to estimate concentration of Cd in soybean seeds at the less than 5% level of significance using above data, to set the target of improvement of soil pH suitable for different soil Cd level and to make the Cd risk map for soybean.
