PEDOGEOCHEMICAL EVOLUTION AND TRACE-ELEMENTS AVAILABILITY TO PLANTS IN OPHIOLITIC SOILS

The results of a study on the distribution of major and trace elements in the bulk soils and in the EDTA-extractable fraction in nine soil profiles and in some related plant species from the Mt. Ferrato area (Florence, Italy) are reported. The topsoils display a moderate enrichment in those elements generally related to the organic matter content (e.g. Cd, Co, Cu, Pb, Zn) although many of the parent material characteristics, especially with regard to the major elements (high Mg content, low Ca/Mg ratios, low K concentrations) are still preserved. The data obtained from the EDTA-extractable element concentrations show a general decreasing trend with depth within the profiles. Aluminium, Cr and Ti are the least EDTA-extractable elements, while Mg presents a high tendency to form organic complexes with EDTA. Heavy metals are generally enriched in the topsoil in relation to the organic matter content, confirming they enter the biogeochemical cycle. No 'toxic levels', especially as regards heavy metals, have been found. Data on trace element content in plants are below the concentrations considered excessive or toxic. However, high Mn abundances (420 mg/kg), Cr (5.5 mg/kg) and Ni (> 10 mg/kg) have been sporadically found. Therefore, no evident signs of pollution occur in the area investigated. The well known infertility of the ophiolitic soils could be a consequence of a 'synergism' resulting from the combined effects of major and trace elements.