Changes in Soil Macronutrients after a Long-Term Application of Olive Mill Wastewater

The land spreading of olive mill wastewater (OMW) derived from olive oil production can represent a suitable option to enrich and maintain agriculture soils under south Mediterranean climates. Therefore, OMW spreading field may represent a low cost contribution to crop fertilization and soil amendment. The main objective of this study was to investigate the long-term effects of raw OMW application on soil macronutrients and phenolic compounds dynamics. The results showed that regular application of three doses: 50, 100 and 200 m3·ha-1 of OMW for nine successive years increased the soil electrical conductivity significantly (p ≤ 0.05%) with the increase of OMW rates at the depth 0 - 20 cm. The pH variations were not detected after ten months of the spreading date. Furthermore, soil sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) values were substantially affected by OMW salinity. The soil organic matter (SOM) increased from 0.068% observed for the control sample to 0.2%, 0.34% and 0.48%, respectively, with the increase of OMW rate in the top layer (0 - 20 cm). The potassium, phosphorus and nitrogen increased gradually with the OMW application dose. The Ca2+ contents on soil decreased with the spreading of OMW rate, as referred to control. In addition, the phenolic compounds variations were not proportional to doses applied and its levels remained high as compared with the control essentially on top layers (0 - 40 cm). This practice should be beneficial to organic farming and is an alternative solution to direct spreading of raw OMW on soil.