Long-Term Changes in Soil Properties According to Different Strategies of Ferti-irrigation with Olive Oil Mill Wastewater in Olive Groves of a Mediterranean Region

The effects of application of olive mill wastewaters (OMW) has been mainly studied in the short-term, while the literature about its impacts after many years has been much lower. This study has monitored some soil physico-chemical parameters after 20 years from OMW application. Two OMW application strategies were tested: intermittent irrigation (i.e., every two years) and continuous irrigation (i.e., each year), both at a dose of 50 m³/ha per year; a non-irrigated soil was assumed as control, since never treated with OMW. Comparisons between long-term and short-term changes (from a previous investigation) were also carried out for key soil properties. In comparison to the control sites, all physico-chemical properties of soils treated with OMW significantly changed, regardless of the irrigation strategy. Noticeable increases were measured for soil salinity (up to + 70%) and content in polyphenols (+ 120%), which suggests paying attention to avoid degradation in soil quality. These effects were lower in the case of intermittent irrigation. The study also evidenced that some short-term undesired effects of OMW application decreased several after irrigation (e.g., increase in soil pH and salinity) down to tolerable values. Therefore, it can be concluded that the annual or inter-annual applications of OMW make the soil fertility stable or even increase it in the short term, but intermittent irrigation is advisable to avoid undesired impacts for crops and ecosystem.