Total and extractable copper and zinc as assessors of phytotoxicity in soilless potting media

Abstract

Agrostis capillaris ’Parys’, Beta vulgaris cicla ’Fordhook Giant’ (Swiss chard), Brassica oleracea ’Lion Heart’ (cabbage), Cineraria ’Miranda White’, Festiica rubra litoralis ‘Merlin’, Matthiola incana ‘Austral Apricot’ (stock), Phaseolus vulgaris ’Hawksbury Wonder’ (bean), and Polycarpaea spirosrylis (Copper Weed) were grown in pine bark‐based soilless media of pH 4.5, 5.5, and 6.5 amended with copper or zinc sulfate or composts containing high concentrations of either copper (Cu) or zinc (Zn). Growth responses ranged from none, through iron (Fe) deficiency, to symptoms of severe Cu or Zn toxicity. Correlations between growth and shoot Cu or Zn concentration on the one hand and total metal content of the medium and several measures of extractable metal content on the other were obtained. Medium pH had to be included in regressions to obtain high correlation coefficients. Total metal content and DTPA‐ and NH₄OAc‐extractable metals were about equally well correlated with growth response in these experiments, but when the results were combined with those from an experiment in which sewage sludge was the source of Cu and Zn, DTPA‐and NH4OAc‐extractable metals gave better correlations than did total content.