Physico-chemical, microbiological, agronomical, and phytopathological aspects in the recycling of olive waste composted residues

Olive mill wastes might impact plants, soil, microbial population, aquatic ecosystems and air media unfavourably. In the present work, olive waste composting confirmed to be a suitable microbial biotechnology for transformation of these by-products into organic fertilizers (cured composted residues) with no phytotoxicity, free of pathogens and able to improve soil fertility and plant production. In our experimental conditions, physico-chemical, biochemical, phytotoxicity and microbiological analysis confirmed the negligible agronomical qualitative characteristics of the cured composted residues. Moreover, olive mill residues after the composting process showed positive properties which suppressed soil-borne plant pathogens. Composted olive waste evidenced good suppressive activity against microsclerotia of V. dahliae and other fungal plant pathogens. Agronomical trials on tomato and sunflower crops showed that seed and fruit production was not decreased by the exclusive addition of compost. Compost amendment caused an increase in soil organic matter, although crop yield and plant growth increased only after mineral fertilization. Furthermore, olive yield and olive oil quality were not negatively affected by compost distribution. These results are encouraging for the agronomic use of composted residues in addition to mineral fertilizers. This may represent an initial phase to increase organic matter in the soil and possibly to reduce mineral fertilization. This process may be considered a new environmental opportunity for greater agriculture sustainability. All these aspects take into consideration that the application of good quality cured composts with high agronomic, microbiological and suppressive characteristics seem to be a very promising strategy for organic and integrated agriculture systems and for organic matter re-integration of soil.