Effect of pH on chemical forms and plant availability of cadmium,zinc, and lead in polluted soils

The effect of pH on chemical forms and plant availability of heavy metals in three polluted soils was investigated. The soils were adjusted to pH values of 7.0, 6.0, and 4.5, then sequentially extracted so that Cd, Zn, and Pb could be partitioned into five operationally defined chemical fractions: exchangeable, carbonate, Fe-Mn oxide, organic, and residual. Kidney beans were grown in the soils to investigate plant availability of the metals in relation to changes of their levels in chemical forms resulting from alteration of soil pH. Alteration of pH resulted in changes of chemical forms of the metals in the soils, and at lower pH the changes were more significant. When soil pH values were decreased from 7.0 to 4.55, levels of Cd, Zn, and Pb in exchangeable form increased, decreased in carbonates and decreased slightly in Fe-Mn oxide forms. Their levels in organic and residual forms were unchanged. Although concentration of metals in plants increased with reduction in soil pH values, dry matter yields were also restricted, so that the amount of metal uptake were almost similar. The uptake rate of the metals in the exchangeable + carbonate forms was the same for the three elements in all the cases.