ANALYSIS OF MACRO AND TRACE-ELEMENTS IN H ORTICULTURAL SUBSTRATES BY THE CACL2 DTPA METHOD .7. ZINC/

Petunia plants were grown in different substrates with increasing amou nts of Zn to test the suitability of the CaCl2/DTPA method for estimat ing Zn in horticultural substrates. 1. No Zn deficiency was observed. Plants which contained >300 mg Zn/kg dry matter showed Zn toxicity sym ptoms. 2. Zn contents of substrates by different extraction methods we re linearly correlated with Zn contents of plants. 3. Regarding the wh ole concentration range CaCl2 extractable Zn showed the best correlati on to Zn content of plants. 4. With compost as substrate the CaCl2/DTP A method overestimated Zn in the substrate. 5. Regarding only Zn conte nts within the transition zone from normal to toxic concentrations CaC l2/DTPA extractable Zn was closer correlated to Zn content of plants t han CaCl2 extractable Zn. 6. Critical values for Zn toxicity were 87 m g Zn/l substrate for the CaCl2/DTPA method and 55 mg Zn/l substrate fo r the CaCl2 method. 7. One disadvantage of the CaCl2 method is a risk of underestimation Zn in substrates with high pH. 8. In some cases by the CaCl2 method no distinction between deficiency and normal supply i s possible. 9. From a practical point of view overestimation by the Ca Cl2/DTPA method is less unfavourable than underestimation by the CaCl2 method.