APPLICATION OF COMPOSTED SEWAGE SLUDGES CONTAMINATED WITH HEAVY-METALS TO AN AGRICULTURAL SOIL - EFFECT ON LETTUCE GROWTH

To elucidate the mechanism of transfer of heavy metals into the food c hain, an experiment was carried out with a calcareous soil, to which t wo different doses of a sewage sludge compost contaminated with either Cd or Zn, Cd, Cu, and Ni were applied, A crop of lettuce was then gro wn in the amended soils, The application of sewage sludge composts to a calcareous soil lowered the soil's pH, although the value was always around 8 at the end of the experiment, Electric conductivity rose wit h organic amendment. As anticipated, such an amendment improved the nu tritional level of the soils, particularly N and P, both total and ava ilable, Plant yields were negatively affected by organic amendments co ntaminated with heavy metals, the most dangerous in our experiment bei ng Cd and Zn since this metals easily taken up by plants, As Ni and Cu form insoluble complexes with the organic matter of the sewage sludge composts they are not readily absorbed, Of the metals studied, Cd and Zn showed the highest bioavailability index.