Persistence of Metal Residues in Sewage Sludge Treated Soils Over Seventeen Years

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Abstract
Results from two long-term sewage sludge experiments conducted on different soil types are presented. Sewage sludges highly contaminated with Cr, Cu, Ni or Zn and a relatively uncontaminated sludge were applied at both sites at the same rates and metal contents in 1968. The Cr-rich sludge also had a high Cd content. Plot soils were sampled in 1972, 1976 or 1977, 1981 and 1985 and total and extractable metal contents determined. Metals added in sewage sludge may change their form but persist in soils in an extractable and plant available form for many years. There is little difference in soil extractable contents or plant uptake of Cu and Zn whether sludge is applied as one single application or as its equivalent in four separate annual applications of one quarter the amount. The percentages of the total chromium contents extractable by both EDTA and acetic acid were small and this was reflected in a low uptake of this element by pasture herbage species (<0.3 mg Cr/kg DM).