Effects of olive mill wastewater and olive mill pomace on soil physicochemical properties and soil polyphenols

Every year, worldwide olive oil extraction processes produce huge amounts of by-products such as (olive mill wastewater and olive mill pomace) in a short period of time. These products are a major problem that affects the soil. Therefore, the aim of this study was to investigate the single and simultaneous effects of different olive mill wastes and to determine phenolic compounds in soil using (FTIR) spectroscopy. Under laboratory conditions, increasing doses (12.5%, 25%, 50%, 75%, and 100% w/w) of olive mill wastewater (OMWW), olive mill pomace (OMP), and combinations thereof were applied to the soil. A non-significant decrease in soil pH was found, after treatment with olive mill wastewater (OMWW), while a significant decrease was measured after treatment with OMP and the combination of both. Moreover, treatment with OMWW, OMP, and their combination at relatively high doses significantly increased the values of electrical conductivity (EC), organic matter (OM), organic carbon (OC), and phenolic compounds (PP). At low doses, including the legally permissible doses (50 m 3 ha −1 y −1 for OMWW and 50–80 t −1 ha −1 y −1 for OMP), no significant effects were observed on total nitrogen (TN) and assimilable phosphorus (P) in the soil. The FTIR results show that OMWW and OMP have a high content of phenolic compounds. In addition, FTIR analysis provided valuable information on soil components after treatment with OMWW, OMP, and their combination. The overall results show that OMWW and OMP can be considered as helpful amendments and cost-effective fertilizers to improve soil quality.