Amendment addition did not significantly alter initial soil pH values. Final pH values were: 5.7–5.8 for V1, 5.9–6.0 for V2, 6.1–6.4 for NV1 and 6.4–6.8 for NV2. Two and six months after soil treatment, a decrease in Pb and Zn bioavailability was in general (differences were not always statistically significant) observed as compared to control plots (Fig. 4). This was more pronounced in the V2 and NV1 soils (p b 0.05) and may be due to compounds in the applied amendments (Ross, 1994) than to a vegetation effect, since this response was observed in both vegetated and non-vegetated sites. In general, COWand POULTRY treatments were more effective reducing Pb bioavailability in non-vegetated sites, while, in vegetated sites, COW-treated plots showed the highest decrease in metal bioavailability at 6 months (Fig. 4). The input of organic matter under COW treatment might have contributed to metal immobilization in soil through sorption processes.