A yearly spraying of olive mill wastewater on agricultural soil over six successive years: Impact of different application rates on olive production, phenolic compounds, phytotoxicity and microbial counts

Olive mill wastewater (OMW) spraying effects onto olive-tree fields were investigated. Three OMW levels (50, 100 and 200m3ha−1year−1) were applied over six successive years. Olive-crop yields, phenolic compounds progress, phytotoxicity and microbial counts were studied at different soil depths. Olive yield showed improvements with OMW level applied. Soil polyphenolic content increased progressively in relation to OMW levels in all the investigated layers. However, no significant difference was noted in lowest treatment rate compared to the control field. In the soil upper-layers (0–40cm), five phenolic compounds were identified over six consecutive years of OMW-spraying. In all the soil-layers, the radish germination index exceeded 85%. However, tomato germination test values decreased with the applied OMW amount. For all treatments, microbial counts increased with OMW quantities and spraying frequency. Matrix correlation showed a strong relationship between soil polyphenol content and microorganisms, and a negative one to tomato germination index. ► Spreading olive mill wastewater (OMW) on olive-tree field was investigated. ► Three levels of OMW were applied to an olive tree field over six successive years. ► Soil phenolic content showed progressive increase with OMW levels in all the layers. ► Five polyphenolic compounds were identified by HPLC in the soil layer composition. ► Correlation matrix showed correlation between soil phenol content and microorganisms.