The knowledge on the incidence of the soil functional factors on the productivity and particularly on the qualitative response of the olive tree is still very limited. During two years research (2002-2003), a study was conducted on the evaluation of soil suitability for the cultivation of the olive tree in the Province of Siena. In particular, the trial concerned the relationship between soil nature and olive tree behavior, through the determination of the olive ripening index and the chemical organoleptic analysis of the oil produced. Two soils with same climate, similar chemical characters but with different physical and hydrological properties were compared. They belonged to the Typic Ustorthents, loamy skeletal, somewhat excessively drained, and to the Udic Calciustepts, coarse-loamy, moderately well drained. Two cultivars, "Frantoio" and "Moraiolo", were monitored during both productive seasons, and two harvesting periods (October and November). The experiment underlined that the fruits on the Udic Calciustept showed a lower olives ripening index, in the same harvesting period, for both cultivars, than the Typic Ustorthent. The different characteristics of the two soils did not seem to have any significant influence on the fatty acid composition, which did not show substantial variations in any of the years and harvesting period considered. In fact, the quantity of every single fatty acid remained constant in both soils and for both cultivar. The different soil characteristics affected the total polyphenol content as well as every single phenolic component of the oil, particularly the hydroxytyrosol and the oleuropein aglycon. A greater concentration of these components in the oils of the Typic Ustorthent was recorded in both seasons and cultivars. This would indicate that different soil water contents induce a higher variation on the phenolic and polyphenolic compounds and, consequently, on the oil stability and on its antioxidant features, than the influence demonstrated by the cultivar. Finally, soil nature was crucial in determining organoleptic characteristics: the Typic Ustorthent produced a qualitatively superior oil, characterized by a marked green and grass fruity taste.