Many studies have been conducted on the interactions of olive mill wastewater (OMW) with soils at the laboratory scale, but less attention has been paid to the effect of OMW on soils at a field scale, particularly over a long period. Additionally, little is known about the effect of OMW on soil microorganisms in terms of the soil enzymatic activities. The storage and evaporation in open tanks is a strategy followed by many producers to reduce the volume of OMW and to valorize OMW through the settlement of suspended sludge. Olive mill wastewater sludge (OMWS) was applied to a fig orchard with 15-year-old trees (cv. 'Sarilop') during the winter for 6 consecutive years at a maximum annual rate of 100 kg tree(-1). In particular, nine treatments were considered: 100, 75, 50, 25 and 0 kg of OMWS tree(-1) both every year (EY) and once every two years (ETY). The long-term trial aimed to assess the cumulative (EY) and residual (ETY) effects of the incorporation of increasing amounts of OMWS on soil biological properties, which were evaluated by measuring the soil microbial biomass carbon (MBC), basal soil respiration (BSR), N-mineralization (N-min), several soil enzyme activities (hydrolases and oxidoreductases) involved in the cycles of the main biological nutrients (C, N, and P), and plant yield as yield efficiency per unit of trunk cross-sectional area (TCSA). The results of this study demonstrated that OMWS significantly stimulated soil microbial activity and fig yield at the levels applied and was not harmful to soil microflora in general, except for the highest dose with yearly treatment (100 EY). OMWS applications provided 41-1046% of the increases in the level of microbial activity with respect to the control at the end of the experiment. For this reason, no toxic effect of OMWS was observed. The application of OMWS to the soil caused the highest increase in DHG activity, while the lowest increase was observed for MBC content. Microbial activities were most stimulated by 75 kg OMWS tree(-1) with yearly application, while they were least stimulated by 25 kg OMWS tree(-1) with application every two years. This agro-industrial waste could represent a strategic resource in the integrated management of agricultural systems, as its use enhances soil biological activities and improves soil fertility, and consequently, the use of chemical fertilizers can be reduced. Additionally, the results suggest that 75 kg tree(-1) applied every year and 100 kg tree(-1) applied every two years applications are the best amendment rates for degraded and poor Mediterranean soils and fig orchards in this region.