## 1 Introduction  
One of the most intractable environmental issues in Greece, and also in the Mediterranean basin countries, is the treatment of olive mill wastes (OMW). Olive oil producers have to deal with serious disposal problems, especially when massive amounts of liquid and solid wastes are generated over a few months a year [1], since olive oil production is seasonal, so the treatment process should be flexible enough to operate in a non-continuousmode; otherwise, olive mill wastes should be stored. The problem is enhanced by the fact that this period of the year coincides with frequent and intense rainfalls [2]. Currently, OMW is improperly treated and being left out in outdoor storage/evaporation lagoons during periods of high precipitation, so they may reach water bodies and cause serious modifications to the underground water properties [3]. Specifically, olive oil mill wastewaters (OMWW) cannot be directly disposed into any natural recipient or domestic wastewater treatment plant due to their high pollution load (i.e., high values of biological oxygen demand, chemical oxygen demand, organic matter content, salinity, total phenols and low pH values); thus, they may affect the soil and underground water quality [4]. Additionally, the situation is distressed by the fact that no treatment plants are available at the mills due to cost restrictions. Many researchers have investigated the critical handling of this so-called \"burden\" by various treatment techniques, including physicochemical, biological and advanced oxidation methods [5]. Numerous methods, individual or combined, such as filtration, ultrafiltration, oxidation, ozonation, coagulation, centrifugation, flocculation, incineration, reverse osmosis and photolysis have been carried out to treat OMWW. Most of these approaches are quite useful as pretreatment methods, since they are inefficient regarding pollutant removal, but they are costly and do not generate valuable sub-products [6]. The aim of these methods is water recovering from olive oil mill wastewaters, without taking into consideration the intriguing potential of this waste as a whole entity. In light of the above, several studies have been carried out working towards using OMWW as a renewable resource and aspiring to transform them from a pollutant to agricultural water resource or organic fertilizer [7]. Meanwhile, olive mill by-products and their compost with other plant materials have been widely used as soil amendments in agriculture, contributing significantly in circular and sustainable economy, as they lie in supporting agricultural recycling of the generated wastes [8; 9]. Besides the reduction in the annual amount of olive mill point source pollution to surface water and underground aquifers, many researchers have proven that they can improve soil chemical and physical characteristics [8; 10; 11; 12], increase the organic matter content in non-fertile soils [10; 13] and amplify nutrient absorption [14]. In addition, their application to landfills has the positive effect of reducing organic loads, and their use in horticulture may reduce and replace peat. This study aims to present the results of soil samples analysis after their mix with a biobed bio-mixture of straw, soil and compost, used for two consecutive years as the organic bio-filter of olive oil mill wastewater [15; 16]. So far, exhausted bio-mixtures used in bio-beds to minimize pesticide point-source contamination turned out to contain residues of pesticides and they are considered hazardous wastes; thus, they require special treatment before their disposal [17]. Contrariwise, saturated bio-mixtures from biobed systems utilized for OMWW treatment not only do not require any special treatment before their final disposal, they can also be exploited as a soil amendment. To this end, the effects of the used bio-mixture application in different proportions as a soil amendment on the physical and chemical properties of a medium-texture cultivated soil were investigated.