Effect of soil pH on availability to crops of metals in sewage sludge-treated soils. II. Cadmium uptake by crops and implications for human dietary intake.
Appropriate pH-related permissible soil-limit concentrations for cadmium in sewage sludge-treated agricultural soils were estimated from the proportional changes in concentrations of cadmium in potatoes, oats and ryegrass grown on two sludge-amended soils and at different pH values. Implications for potential human dietary intake of cadmium were also assessed. Yields of crops increased with increasing soil pH, probably in response to decreasing uptake of zinc as soil pH value was raised. In general, cadmium concentrations in peeled potato tubers, potato peelings, oat straw and ryegrass decreased as simple linear functions of increasing soil pH over the range of pH values measured (pH 3.9-7.6). Cadmium concentrations in potato peel were particularly sensitive to changing pH conditions, whereas cadmium levels in oat grain were independent of soil pH. On the basis that a highly precautionary approach is adopted in setting soil standards for heavy metals, appropriate permissible concentrations of cadmium in sludge-treated agricultural soil which protect the human food chain were determined as 2.0 and 2.5 mg Cd Kg(-1) for banded pH ranges of 5.0-5.5 and 5.5-6.0, respectively.