Long-term impacts of de-oiled two-phase olive mill waste on soil chemical properties, enzyme activities and productivity in an olive grove

Soils in semi-arid Mediterranean areas are generally characterized by low organic matter content and are subjected to progressive degradation and deterioration of workability. Because de-oiled two-phase olive mill waste (DW) contains an important level of organic matter content, its recycling as organic amendment or fertilizer may be an alternative for its disposal that also improves soil quality and productivity. A ten-year field study was conducted to evaluate the long-term sustainability of raw de-oiled two-phase olive mill waste (DW) disposal as a soil amendment on an olive grove in Elvas, Portugal. The soil was amended with DW at rates of 0, 27, and 54Mgha⁻¹, dry weight equivalent, for eight years, with cumulative and residual effects being assessed in the last year and two years after the last application. The DW amendments significantly increased the olive yield only in the residual year at the 27Mgha⁻¹ application rate. Long-term applications of DW to soil led to positive cumulative and residual effects on the soil's chemical (total organic carbon and its humified fractions, total N, available P, and K), properties. Simultaneously, dehydrogenase, urease, β-glucosidase, alkaline phosphatase, and arylsulfatase activities increased even at the higher DW application rates. Electrical conductivity rose significantly with DW application, especially in the residual year, ranging from 0.513dSm⁻¹ for the unamended soil to 1.89dSm⁻¹ at the 54Mgha⁻¹ application rate. The addition of raw DW to an olive grove may be considered to be a good strategy for recycling this waste, converting it into a resource that could be used for a long time as organic amendment without negatively affecting yield, while improving many soil properties. However, the greatest concern regarding the long-term use of DW is the risk of soil salinity, especially if application rates are greater than 27Mgha⁻¹.