The profitable use of the large amounts of olive oil mill wastes produced in Greece, as source of soil organic matter, might be probably beneficial to soil microorganism activity.
A high rate of organic matter biodegradation was observed in soil samples amended with the liquid form of olive mill wastewater, whereas the vice versa results were obtained with the solid form. When the soil was amended with a mixture of both olive mill waste forms, liquid (L) and solid (S), the organic matter of the solid waste (S) showed a well-improved biodegradation; the available forms of P, K, Zn, Mn, and Cu were increased, especially in treatments where the olive mill wastewater, liquid form, was threefold in comparison to the solid form. Moreover, the soil amended with the solid (S) form of olive mill waste reduced bacterial growth significantly, and both waste forms act negative impacts to soil-borne fungi belonging to the genus Rhizopus.
The results of this work demonstrated the high potential of olive mill waste, solid or liquid form, added to sandy loam soil in an incubation experiment in vitro. The better results for soil quality were obtained when a combining mixture of these materials was added in the ratio 1:3