The influence of sewage sludge on the content of heavy metals in potatoes and on tuber yield

Summary

A small plot field experiment with two types of sewage sludge, one poor and one rich in heavy metals, applied in moderate and heavy quantities, and compared with NPK-fertilizer, was carried out 1973 and 1974, in potatoes. The chemical composition of the NPK-fertilizer and the sludges, and the amounts applied are found in Tables 1 and 2.

The sludge increased the content of total Cd, Ni and Pb and the content of readily soluble Cu and Zn in the soil. The increase was greatest for Cu and Zn, and was more pronounced the second than the first year (Table 3). The small quantities of heavy metals in NPK-fertilizer did not influence the soil analytical values.

Digested sludge increased the yield of tubers significantly, but based on the amounts of nutrients applied, NPK-fertilizer was much more efficient than sludge (Table 4). It is suggested that low utilization of N, or lack of K, is mainly responsible for the lower efficiency of sludge. Toxicity in the plants due to sludge was not observed.

Application of 40–80 tons/ha of sludge dry matter, rich in heavy metals, increased considerably the concentration of Cu and Zn in the tubers, whereas 10–20 tons/ha did not influence the concentration (Table 6). The content of Hg, Ni and Pb in the tubers was very little influenced by sludge application. The Cd-concentration was mostly below 0.05 mg/kg of fresh tubers, and the analytical technique was not accurate enough to detect possible influence of increasing amounts of Cd in sludge. Generally, less than 0.5% of the heavy metals applied was accumulated in the tubers. The concentration of heavy metals in fresh tubers was in all cases below suggested maximum tolerable values for food. re]19760203