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Soybean Growth and Elemental Content as Influenced by Soil Amendments of Sewage Sludge and Heavy Metals: Seedling Studies¹

Many research studies have substituted soluble metal salts for sludge borne metal in attempts to define heavy metal uptake by plants. To assess this experimental approach and the effect of sewage sludge on the availability of soluble metals, soybean uptake of sludge and salt borne heavy metals was measured.

Soybeans (Glycine max L. var. ‘Corsoy’) were grown 30 days in a growth chamber on an Aquic Hapludoll loam soil receiving the following treatments: a) sewage sludge applications of 0, 34, 67, and 134 metric tons/ha; b) addition of a combination (L) of heavy metals, as inorganic salts, of 10 mg Cu, Cd, Cr, and 18 mg Zn/kg soil at three levels, L, 2L, and 4L; and c) sludge plus metal additions. Sludge applications increased dry matter production, tissue P and N content and reduced root nodulation. Heavy metals depressed plant growth and nodulation in the absence of sludge. A 34 metric tons/ha sludge application eliminated these adverse effects. The Zn concentration of seedling tissue increased with applied Zn, from both sludge borne and salt borne sources. However, per unit of added Zn, more Zn accumulated in tissue grown on sludge amended soil. The tissue concentration of Cu and Cd increased with added sludge. A 34 metric tons/ha sludge application depressed salt borne Cd uptake. Sludge borne Cr and Ni were not taken up by the plants. Plant uptake of salt borne Cd and Ni was enhanced by the presence of Zn, Cu, and Cr. Hence, the uptake of sludge borne metals cannot be predicted from data obtained in studies using inorganic salts as the metal source.

Key Words: Zn • Cu • Cd • Ni • Cr • Nodulation

² Soil scientist, USDA, ARS, and associate professor, Univ. of Minnesota; and associate professor, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication July 6, 1976.