Effects of spreading olive mill wastewater on soil properties and crops, a review

The disposal of olive mill wastewaters (OMWs) is a major problem that affects soil and groundwater. Different papers have discussed the effects of OMW discharge on soil characteristics, plant production and phytotoxicity. In this work, a holistic approach is proposed and the main research studies regarding the effects of OMWs on crops and soils are reviewed. Generally, direct application of OMWs exerts a temporary positive effect on soil physical properties. However, in clay soils, the accumulation of salts from these wastewaters could lead to the disintegration of the soil structure. Deterioration of soil structure would decrease the soil hydraulic conductivity, which is a key parameter for determining the volume of OMW that can be spread on a soil. Organic matter (OM) makes up approximately 65% of the OMW dry weight. Among the soil chemical characteristics, the soil pH remains almost constant following OMW applications. Polyphenols are the most limiting factor for spreading OMWs on soils because of their antimicrobial and phytotoxic effects. Nevertheless, these polyphenols are rapidly degraded depending on environmental conditions. Due to the highly variable chemical composition of the OMWs, these effluents provide variable macronutrient inputs (especially potassium and phosphorus), which potentially reduce necessary fertilizer inputs. Regarding soil microflora, the OMWs exert the following two contrasting actions: stimulating microflora development by temporarily enriching soil carbon and inhibiting certain microorganisms and phytopathogenic agents by adding antimicrobial substances to the soil. For many crops, spreading OMWs benefits crop yield. However, the application of OMWs may also damage germination. Thus, the application of OMWs should adequately precede sowing. The beneficial effects of OMWs on crop yields indicate that these wastewaters may become very important sources of OM and nutrients in agriculture soils and positively impact soil fertility. However, to improve their effectiveness, some use restrictions based on soil characteristics were outlined.