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Olive mill by-products could be composted and applied to olive orchard soils. These practices solve the problem of these wastes disposal and reduce the need for chemical fertilizers. Therefore, the aims of this research were: (i) proposing ‘on-farm’ composting process of different olive mill waste mixtures; (ii) investigating the chemical, physical, and microbiological characteristics of produced composts; (iii) evaluating the agronomical performance of the composts. Two on-farm composting trials were carried out in Southern Italy by using “two-phase” and “three-phase” olive mill wastes. The obtained composts were analyzed for their main characteristics and were spread in two different olive orchards (Nocellara and Leccino). At the end of field trial, soil properties, olive tree yield, and oil production were determined. The results highlighted that both composts reached a chemical composition in line with the thresholds established by the Italian fertilizers legislation for “green wastes compost”. When the two compost piles became stable and mature, their microbial properties reached similar values. Also, the results suggested the efficiency of the composting process and good hygienic conditions of the matrices. Soil application of composted olive mill by-products increased olive yields on average by 9% compared to the untreated soils. Both olive orchards showed good results in productive parameters. In particular, oil ha^-1 increased by 166.4 and 179.9 kg in treated olive orchards, compared with untreated soils. However, more experimental data might be needed to confirm the effects of compost application in the long time and on different olive orchard soils.

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Seasonal accumulation and incorrect disposal of olive mill by-products may be detrimental to the environment. Conversely, their proper recycling as soil amendments may be a sustainable solution. Therefore, the objectives of this three-year field trial were: (i) Investigating the effects of Olive Mill Wastewater (OMW) and Pomace Compost (PC) on plant nutritional status and yield in an organic olive grove; (ii) The impact on main soil properties, and (iii) Verifying if these experimental fertilizers can replace the widespread fertilization practices. The OMW and PC treatments were compared to a commercial Organic-mineral Fertilizer (OF) and green manure of horse bean (MV). Plant nutritional status, soil properties and agronomical performance of treatments were assessed. The OMW and OF determined yield that was on average significantly higher than MV and PC by 191 and 55%, respectively. The best leaves P contents in PC indicated a more effective release of this nutrient as compared to the other treatments, which can be matched with more favorable soil conditions. The comparable yield and leaves composition for OMW and OF suggested the possibility to replace the OF with the OMW. The effects on olive oil production and soil fertility highlighted that OMW (and PC as a second choice) could be applied to sustain olive tree production, substituting traditional fertilizers.