The addition of organic matter in the form of compost improves overall physical, chemical and biological properties of soils but, to be really sustainable, the composting process should be carried out using the by-products available in situ. Two different soils of a Mediterranean olive orchard, one managed traditionally (NAS) and the other amended with compost (AS), were investigated in a two-year experiment. Increases in total organic matter, total nitrogen and pH, were detected in AS if compared to NAS. Significant increases in total and specific microbial counts were observed in AS, with a clear ame-lioration of microbiological soil quality. The results demonstrated that soil amendment using compost deriving from olive mill by-products can be an important agricultural practice for supporting and stimulat-ing soil microorganisms and, at the same time, for re-using these by-products, so avoiding their negative environmental impact.