Changes in organic matter and residual effect of amendment with two-phase olive-mill waste on degraded agricultural soils

Agricultural soils from many parts of the Mediterranean region are very poor in organic matter and are exposed to progressive degradation processes. Therefore, additions of the olive-mill waste from a continuous two-phase system as an organic amendment can improve soil quality and hence mitigate the negative environmental and agronomic limitations of these soils. A field study under semiarid Mediterranean conditions was conducted to evaluate the changes in organic matter after four consecutive annual additions of olive-mill watery husk (OMWH) waste and the de-oiled two-phase olive pomace (TPOP) waste on an olive grove soil: a cutanic Luvisol (CL). Treatments included a control, OMWH (30 and 60 Mg ha − 1 , DW equivalent), and TPOP (27 and 54 Mg ha − 1 , DW equivalent). Also, a two-year greenhouse study using wheat ( Triticum aestivum L.) was conducted to evaluate the residual effects of adding OMWH to CL soil and to a degraded Leptic Cambisol (LC). Treatments included five OMWH rates ranging from 0 to 40 Mg ha − 1 . Significant increases in total organic carbon (TOC), water soluble organic carbon (WSOC), humic and fulvic acids, and aggregate stability were observed in the treated plots, and the highest humification index was obtained when OMWH was applied at the lowest rate. The increase in aggregate stability correlated positively and highly significantly ( P < 0.01) with the humic and fulvic acid and WSOC contents. In the greenhouse, significant increases in TOC, carbohydrates, aggregate stability, total N, available K, and cation exchange capacity were observed in both soils. However, available P decreased significantly. There was an increased residual fertilization effect of OMWH for both soils. The best fits to grain yield and OMWH rates were with a linear regression for the CL soil ( R 2 = 0.957 and P < 0.01), and a quadratic regression for the LC soil ( R 2 = 0.960 and P < 0.01).