The efficiency of conservation tillage for reducing soil erosion and improving water storage is universally recognized. This is particularly important in arid and semi-arid zones, where water is the limiting factor for crop development under rainfed conditions. In these areas, management of crop residues is also of prime importance for obtaining sustainable crop production. The present study shows the main advantages derived from the long term application of conservation tillage on soil quality and crop performance. The experiment was established in 1991 on a sandy clay loam soil (Xerofluvent) in the province of Seville (southwestern Spain), using a wheat-sunflower crop rotation. Two tillage treatments were applied: Traditional Tillage (TT) used in the area for rainfed agriculture (burning the straw of the preceding crop, and mouldboard ploughing, 25-30 cm depth), and a Conservation Tillage (CT), characterized by leaving the straw of the preceding crop on the surface as mulch, chiseling (25-30 cm depth) after wheat, and disc harrowing (5-7 cm depth) after sunflower. The results show that differences in bulk density (Db) between TT and CT were observed during drought periods. In contrast, Db was very similar in both treatments during the years with much higher precipitation than the average. The presence of residues of the preceding crop in the soil surface in the CT treatment seems to be highly effective in enhancing water infiltration and water conservation under dry weather conditions. The hydraulic conductivity was sometimes significantly higher in CT than in TT, that could be also related with the existence of preferential paths created by an increase of the earthworm population in the CT treatment. During the dry years soil water recharge was better in the CT treatment than in the TT treatment. Thus, crop development and yield was better in the conservation tillage than in the traditional tillage. It can be argued that semi-arid conditions may suppose a limiting factor for the accumulation of Organic Carbon (OC) in the top soil layers. However, soils with low inherent levels of organic matter could be the most functionally improved with conservation tillage, despite modest or no change in total standing stock of soil organic carbon within the rooting zone. In this study we show that despite a slight increase of Soil Organic Carbon (SOC) and the Stratification Ratio (SR) of SOC under CT, with respect to TT, noticeable and significant increases of the SR of soluble OC, C of the microbial biomass and some enzymatic activities were recorded in CT, with respect to TT. These increases reveal that the stratification of SOC under CT have consequences on soil functions beyond that of potentially sequestering more C in soil. CT also improved soil quality by softening the loss of CaCO3, with respect to TT.