Initially, the organic amendments led to an increase in the organic-C concentration in the soil with respect to the control, especially in olive husk plots (Table 2). However, throughout the experiment, this parameter was only higher in olive husk-treated soils in comparison with the control. This fact could be due to the greater organic-C content of olive husk compared to cow manure (562 and 412 g kg−1 dw, respectively) and lower microbial degradation of olive husk. Riffaldi et al. (1997) found that the potentially mineralisable organic C of olive mill wastewater sludge was 8.9–12.0% of organic C in low organic matter calcareous soils, while an average of 47% of organic C from cow manure can be mineralised in soil after 30 days (Ajwa and Tabatabai, 1994). In all treatments, no significant differences were found in organic C content of soils during the experiment. The high metal concentrations of the soil could have negative effects on microbial activities (Dai et al., 2004), provoking a low organic matter mineralisation during the plant growth.