Phytotoxicity Evaluation of Olive Mill Solid Wastes and Their Feasibility as Biofertiliser

ABSTRACT Olive oil‐extracting industry generates huge quantities of wastes commonly known as olive mill wastewater (OMWW) and olive mill solid waste (OMSW) that create a major environmental problem in olive‐producing countries. Agricultural reuse of olive mill residues through soil application has been proposed as a valorisation strategy of these wastes. The aim of this study was to assess the efficiency of olive mill solid waste (OMSW) application in improving soil properties and promoting pomegranate growth. The experiment included three treatments: fresh‐OMSW (F‐OMSW), exhausted‐OMSW (E‐OMSW), and an unfertilised control. OMSW was applied at a rate of 25% of the total substrate volume, and the experiment was conducted over a 90‐day period in a greenhouse pot trial using 2‐year‐old pomegranate plants. Soil chemical properties, microbial populations and plant growth parameters were evaluated to determine the impact of OMSW application. In both treated soils, the pH was slightly affected by OMSW treatments. However, an increase in electrical conductivity (EC) (51% and 30%), humidity (103% and 96%) and total phenol content (169% and 287%) following F‐OMSW and E‐OMSW application was observed. Application of OMSWs has significantly increased soil nutrient content. Furthermore, treated soils with F‐OMSW showed an improvement in organic matter content. Fungal and bacterial communities were not negatively affected by OMSW amendments. Nonetheless, enzyme activities in the soil were enhanced. No phytotoxic effects were observed, and F‐OMSW‐treated plants showed higher vegetative growth and chlorophyll index. The overall results indicate that incorporating F‐OMSW or E‐OMSW as biofertilisers into the soil can significantly enhance the vegetative growth of trees while preserving the soil's chemical and microbiological characteristics without causing any detrimental effects. To enhance agricultural sustainability, future research will explore novel applications of OMSW in agriculture, such as developing bio‐based fertilisers and soil amendments. Additionally, optimising application rates is essential to maximise agronomic benefits while mitigating environmental impacts.