This review considers the direct spreading of olive mill wastewater (OMW) on soil properties and crops. In general this has a positive effect on soil structure, with polyphenol content the main limiting factor. OMW could be an important source of water and organic matter in the Mediterranean basin, since it has beneficial effects on soil fertility and the yield of important crops.
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.
This Italian study reviews the main research studies regarding the effects of OMWs on crops and soils, and proposes a holistic approach.
In general, the direct application of OMWs exerts a temporary positive effect on soil physical properties. Among the soil chemical characteristics, the soil pH remains almost constant following OMW applications.
However, in clay soils, the accumulation of salts from these wastewaters can lead to disintegration of the soil structure. This deterioration can reduce the soil's hydraulic conductivity, which is a key parameter for determining the volume of OMW that can be spread on a soil.
Polyphenols are the most limiting factor for spreading OMWs on soils, because of their antimicrobial and phytotoxic effects. Nevertheless, these polyphenols rapidly degrade, depending on environmental conditions.
Because of the highly variable chemical composition of the OMWs, these effluents provide variable macronutrient inputs (especially potassium and phosphorus), which potentially reduce the necessary fertiliser inputs.
For the soil microflora, OMWs exert 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 precede sowing by an adequate margin.
The beneficial effects of OMWs on crop yields indicate that these wastewaters may become very important sources of organic matter and nutrients in agricultural soils, and positively impact soil fertility. However, to improve their effectiveness, some use restrictions based on soil characteristics are outlined.
