Characterization of phytoavailable copper in compost-peat substrates and determination of a toxicity level

Heavy metal-sensitive 'Express Orchid' petunias (Petunia xhybrida Hort Vilm .-Andr. 'Express Orchid') were grown in substrates of 2 green yard waste co mpost : 3 peat (v/v) with target Cu contents of 100 and 200 mg.kg(-1) at va rying pH. Iron supply was also varied. Copper contents of the substrate wer e determined by H2O, NH4NO3 NH4OAc, CaCl2, CaCl2-DTPA, and aqua regia extra ction. Plant Cu concentration increased with increasing Cn supply and decre asing pH, indicating that Cu phytoavailability depended on substrate pH. Ex traction of fresh substrates with CaCl2-DTPA provided a good prediction of plant Cu concentration and reflected well the influence of pH on Cu phytoav ailability. The percentage of CaCl2-DTPA extractable Cn increased with decr easing pH. Extractions of Cu with NH4NO3, H2O, NH4OAc, and CaCl2 resulted i n very low extractable amounts and hence were not suitable. Plants showed C u toxcity induced iron-like deficiency chlorosis, which was alleviated by a dditional Fe supply, This Fe supply did not seem to affect total Fe concent ration of petunias, but reduced Cu concentration of the shoots. Since yield reduction was not observed, the occurrence of chlorosis during the culture period was chosen as the toxicity parameter, resulting in a Cu threshold t oxcity level of 12.3 mg(.)kg(-1) plant dry weight. From this, a threshold t oxicity level for CaCl2-DTPA extractable Cu in compost-peat substrates of 3 mg(.)L(-1) substrate was determined. Chemical name used: diethylenetriamin e-pentaacetic acid (DTPA).