Suitability of Untreated and Composted Olive Mill By-Products as Amendments in Organic Olive Orchards

ABSTRACT. In the Mediterranean area, the proper soil application of olive pomace compost (OPC) and olive mill wastewater (OWW) could play a crucial role in replacing the annual losses of organic matter, thus sustaining agriculture. Therefore, the objectives of this five-year field trial were: (i) to investigate the effects of OWW and OPC, applied at different doses, on soil fertility in an organic Mediterranean olive grove; (ii) to assess the influence of treatments on production and quality responses of olive trees. In a randomized block experimental design, the following experimental treatments were compared: (i) OWW; (ii) OPC; (iii) the same olive pomace compost used in treatment (ii) distributed to supply a double rate of N per plant (OPCx2). These treatments were compared to: (a) commercial NPK organic-mineral fertilizer (Org-min); and (b) green manure of broad bean (GM). Soil parameters were analyzed to assess soil fertility at the beginning of the field trial and in the medium and longer term. At harvest, agronomical performance of treatments was also evaluated. At the end of the field trial, olive pomace compost (mean of OPC and OPCx2) significantly increased total organic carbon by 47.5% compared to Org-min, and the humified organic carbon values were significantly higher for OWW, OPC, and OPCx2, as compared to GM. The values for olive yield components in GM were lower than in OPC and OWW. Therefore, results showed that OWW and OPC application could be a suitable substitute of traditional methods (GM) to improve overall soil fertility and sustain yield in organic olive grove.