Heavy metal-sensitive `Express Orchid' petunias (Petunia ×hybrida Hort Vilm.-Andr. `Express Orchid') were grown in substrates of 2 green yard waste compost: 3 peat (v/v) with target Cu contents of 100 and 200 mg·kg-1 at varying pH. Iron supply was also varied. Copper contents of the substrate were determined by H2O, NH4NO3, NH4OAc, CaCl2, CaCl2-DTPA, and aqua regia extraction. Plant Cu concentration increased with increasing Cu supply and decreasing pH, indicating that Cu phytoavailability depended on substrate pH. Extraction of fresh substrates with CaCl2-DTPA provided a good prediction of plant Cu concentration and reflected well the influence of pH on Cu phytoavailability. The percentage of CaCl2-DTPA extractable Cu increased with decreasing pH. Extractions of Cu with NH4NO3, H2O, NH4OAc, and CaCl2 resulted in very low extractable amounts and hence were not suitable. Plants showed Cu toxicity induced iron-like deficiency chlorosis, which was alleviated by additional Fe supply. This Fe supply did not seem to affect total Fe concentration 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 toxicity level of 12.3 mg.kg-1 plant dry weight. From this, a threshold toxicity level for CaCl2-DTPA extractable Cu in compost-peat substrates of 3 mg.L-1 substrate was determined. Chemical name used: diethylenetriamine-pentaacetic acid (DTPA).
Diptera of sanitary importance associated with composting of biosolids in Argentina
