An investigation into the phytotoxicity of zinc, copper and nickel using sewage sludge of controlled metal content

A replicated factorial pot trial with perennial ryegrass and a sandy loam soil (pH 7·0) investigated the relative phytotoxicities of Zn, Cu and Ni, and the nature of their interaction. Sewage sludge of controlled metal content made from enriched sewage was used for the soil treatment so that each pot received the same amount of sludge dry solids whilst quantities of metals added were adjustable as required. Upper critical concentrations (total) of the three elements in soil were found to be 319 mg Znkg⁻¹, 105 mg Cukg⁻¹ and 221 mg Nikg⁻¹. The relative toxicities of Zn/Cu/Ni were 1·0/2·6/1·0 on a loading rate basis. The comparatively low toxicity of Ni was attributed to the pH value (7·0) of the soil used. At subcritical levels in the soil phytotoxic effects of mixtures of the three elements were independent but some additivity was seen when one or more element was already at a hypercritical level. At subcritical levels the element present in highest concentration relative to its critical concentration determined the phytotoxic effect on ryegrass yield of mixtures of Zn, Cu and Ni in soil. But where two or more elements exceeded their critical soil concentrations their combined phytotoxic effect could be taken as being additive. It was concluded that the three elements could be dealth with separately in guidelines for sludge utilisation on agricultural land.