Phytotoxicity and fertilising potential of olive mill wastewaters for maize cultivation

The use of organic wastes as agricultural amendment is an effective strategy to regenerate degraded soils and represents an economical alternative that provides a local source of fertiliser. Olive mill wastewaters are widely known for their fertilising value. However, they can still not be recommended as a safe practice because of their potential ecological risks. Here, the effect of olive mill wastewaters on maize is investigated. Olive mill wastewater samples were collected in semi-modern and traditional mills of the Marrakech area, Morocco. Samples were analysed for pH, electrical conductivity and total phenolic content. Major phenolic compounds were identified and quantified. Phytotoxicity was assayed in the laboratory using maize seedlings, and under field conditions. Field fertilisation was conducted through land application accompanied or not by foliar spray according to a fractionated application mode. Our results show a high variability in physicochemical characteristics of wastewaters, notably for phenolic profiles. Variable toxicity levels were also attributed to each sample according to its salinity and phenolic load. Under field conditions, a 9% reduction of seedling emergence was observed in olive mill wastewater-pre-treated soil as compared with the control. However, olive mill wastewater-fertilised plots showed a net improvement in plant height of 10–11% compared with the control. A yield improvement reached 28% over that of the control when the plants were amended both by land application and foliar spray. These results show that the large variability in olive mill wastewaters composition constitutes a major limiting factor for their re-use in agricultural systems. Olive mill wastewaters can be used as a safe agronomic amendment provided the application is moderate and progressive and critical development stages are avoided, notably the preliminary development stages. Foliar fertilisation using olive mill wastewaters is also revealed to be a promising way to manage these effluents