Effect of solid olive-mill waste amendment on soil fertility and olive (Olea europaea L) tree activity

A field trial was carried out for four years to evaluate long-term effects of soil amendment with solid olive-mill waste (SOMW). Fifty tonnes per hectare of untreated SOMW were spread annually in an olive (Olea europaea L.) grove. Soil chemical characteristics, microbial populations, olive tree growth and oil quality were determined. During the experimentation there was no increase in organic C following SOMW spreading, likely due to the rapid utilisation of the organic matter by microorganisms. However, exchangeable K in the treated soil increased as a consequence of its high concentration in the SOMW. As expected, SOMW did not have long-term inhibitory effects on the soil microflora, but the number of viable bacteria, bacterial spores and mycetes increased slightly. No phytotoxic effects were observed; on the contrary, SOMW-treated trees showed higher vegetative activity, evaluated as shoot length, canopy volume and pruning weight, and greater olive fruit productive efficiency. The fruit dry weight was also higher. The chemical and sensorial features of the olive oil extracted were not affected by SOMW treatment. The overall results indicate that SOMW can be used as a soil amendment in olive orchards increasing the vegetative and productive activities of the trees, without long-term negative consequences on the chemical and microbiological characteristics of the soil and on the olive oil quality.