Abstract
Heavy metal ions in the soil solution are considered to be in equilibrium with the complexes of soluble ligands, exchangeable ions, what adsorbed on hydroxides of Al and Fe, and aluminol group of allophane, and the complex of humic substances and insoluble metal conpounds. Equilibrium equations derived from the law of mass action and donnan* s equilibrium theory fitted with Ca—Zn and Ca—Cd exchange on montmorillonite, while Gapon’s equation did not. The solubility decrease, due to the fall of the redox potential of soils which causes sulfides to form, was most prominent in Cd. Fractionation of heavy metals in the soil showed that the amount of Cu was relatively high in the humus complexes and that of Cd was so in the exchangeable form, while that of Zn was relatively low in the fractions which react directly with soil solution. Soil contamination by heavy metals raised relative amounts of these fractions. The concentrations of metals in the percolated water of uncontaminated paddy soil in lysimeter experiments were about the same as those in clean streams. The income and outgo of Cd in the soils used in the experiments were most severely influenced by the redox state of the soils through the uptake by rice plants, while the incomes of Cu and Pb balanced their outgoes approximately.
