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).
Compost of separately collected green yard and organic household wastes may contain high amounts of Zn. Hence, substrates basing on compost can result in Zn phytotoxicity. The aim of the investigations was to identify a suitable reagent to characterize phytoavailable Zn in compost‐peat substrates. Petunia hybrids were cultivated in compost‐peat substrates with increasing Zn amounts (basic load, 400, 800, 1600 mg kg−1 d.m. aqua regia soluble). Extractable Zn was determined by extraction of fresh and dried substrates with H2O, 1 M NH4NO3, 0.1 M CaCl2, 1 M NH4OAc, and CaCl2‐DTPA (0.01 M + 0.002 M). The Zn content of plants increased significantly with increasing Zn application and decreasing pH. The result of CaCl2 and NH4OAc extraction reflected the influence of pH on Zn phytoavailability quite well and the correlation with the plant Zn content was very good. Extraction with H2O and NH4NO3 did not give acceptable results. Extraction with CaCl2‐DTPA was inconsistent for all trials and thus not suitable. The percentage of CaCl2‐extractable Zn in relation to the aqua regia soluble content at the same pH varied over the trials. Thus, the phytoavailable content of the substrates is influenced by additional factors, besides Zn supply and pH. The result of CaCl2 extraction of dried substrates resulted in the best reflection of Zn phytoavailability and was used for all further investigations, including determination of critical values of phytotoxic Zn. Plant yield was not influenced by the Zn treatments. However, Zn induced chlorosis of petunias occurred at a plant Zn content > 160 mg kg−1 d.m. and a phytoavailable Zn content in the substrate > 6 mg l−1 in CaCl2 extract and > 2.6 mg l−1 in NH4OAc extract, respectively.
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