The potential influence of sludge-borne heavy metals on crop growth and the food chain is a major concern with the utilization of municipal sludges in agricultural crop production. The objectives of this field research were to compare soybean plant growth and metal uptake of salt sludge-borne metals and to determine the partitioning of these heavy metals into the various plant parts.
Soybeans (Glycine max L. Merrill) were grown on soil to which air-dried, anaerobically digested sludge was applied at rates of 25, 50, and 100 and 200 metric tons/ha. In addition, Zn, Cu, Ni, Pb, Cd, and Cr salts were added individually at levels equivalent to the quantity added in 50 metric tons sludge/ha. Sludge and metal salts were mixed into the top 20 cm of soil by roto-tilling.
Adding sewage sludge did not significantly influence soybean seed yield. The seed concentration of N, P, K, Zn, and Cd increased, whereas the Fe concentration decreased. Total uptake of elements, other than N, P, K, Ca, Mg, and Fe, was less than 1 kg/ha although the concentration increased significantly in some cases.
Adding Cu salt increased seed yield and Cr salt decreased seed yield; otherwise metal salts did not influence yields. The metal salts of Ni, Zn, and Cd increased their concentration in mature seed.
When mature plants were separated into leaves, stems, husks, or pods and seed, their analyses indicated that Zn was concentrated in the leaves, increasing up to 331 μg Zn/g leaf at the 200 metric ton/ha sludge rate (the Zn concentration in the stems and husks also increased with increasing sludge rates). In contrast, the Zn concentration of the seed leveled off at 100 metric tons sludge/ha. Cadmium and Cu were concentrated in the mature leaf tissue.
Sewage sludge of the type applied in this study could be used as a source of P without yield reductions. These data did not show large increases of sludge-borne metals in the seed.