Potential Use of Olive Mill Wastewater as Amendment: Crops Yield and Soil Properties Assessment

Abstract

The traditional olive oil extraction process generates a large amount of liquid by-product that could be recycled as an organic fertilizer. The aim of this 3-year field experiment was to assess the effects of olive mill wastewater (OMWW) application on yields of ryegrass, proteic pea, and clover crops and on soil properties. The following treatments were compared: OMWW at 80 m3 ha−1 (80_OMWW), OMWW at 120 m3 ha−1 (120_OMWW), and an unfertilized control (Contr). The yields of ryegrass increased 40% and 41.6% compared to Contr, for 80_OMWW and 120_OMWW, respectively, while a significant enhancement was found at the lowest level of OMWW in proteic pea (27.0% increase). Clover showed a species-specific sensitiveness, but the OMWW applications increased the protein content 26.3% and 28.7% for 80_OMWW and 120_OMWW, respectively, in comparison with Contr. The OMWW rates also enhanced the total organic content in the soil compared to the unfertilized control. At the end of the 3-year experiment, total extracted carbon and humified organic carbon were greater than the initial ones. The values of available phosphorus (P) and potassium (K) determined at the end of experiment in Contr were almost the same as those recorded at the beginning of the research, confirming that the increases found in 80_OMWW and 120_OMWW plots were due to the OMWW applications. The findings suggest that repeated applications of OMWW could both sustain fodder crop performance and support soil fertility, with the possibility to recycle the organic amendment and reduce the risks of soil degradation.