Fig. 1. The distribution of Cd between the solid and solution phases (represented as K_d) for the soils as affected by changes of pH. Values and error bars represent the mean and standard deviation of triplicate measurements. The five-parameter nonlinear regression fitted for the acidic soil was K_d = 48.4 + 2499/(1 + exp[-(pH-5.77)/0.18]^0.86: R² = 0.99, P < 0.0001 and for the calcareous soil was K_d = 112.4 + 3421/(1 + exp[-(pH-7.49)/0.0034]^0.0112: R² = 1.00, P < 0.0001. See text for explanation of equations.