Main processes governing the plant-soil interactions in adult olive (Olea europaea L.) trees under fertigation were studied to better understand the response of the trees to this agricultural practice widely used in new olive orchards. Our final objective was to obtain soundly based scientific evidences for a rational choice of the fertilizer dose. Measurements were made in a ‘Manzanilla de Sevilla’ olive orchard in which 200 g N, 400 g N and 600 g N per tree and irrigation period (T200, T400 and T600, respectively) of a 4N-1P-3K fertilizer were applied by fertigation for 5 years (1999–2003); a control treatment (unfertilized) was also established. Four years after the start of the experiment mean values of soil P and K concentrations were greater in the fertigation treatments than in the control. For K, concentrations increased with fertilizer dose. The profile of NO3-N, P and K concentrations within the irrigation wetted zone was studied in 2003; in the top soil layer, the concentrations of the three elements increased with fertilizer dose, generally showing linear responses to the different doses; in deeper soil layers, concentrations also increased with fertigation, but to a lesser extent; the concentrations of NO3-N, P and K recorded at 0.8–0.9 m depth in the soil of T600, together with observations of root distribution, were enough to suggest leaching losses and possible groundwater contamination. As a consequence of the higher soil nutrient availability, leaf N, P and K increased generally with dose. Leaf N deficiencies and low, but not deficient, leaf K levels were found in control trees in 2002 and 2003, as well as in T200 trees in 2003. Differences between treatments in shoot length, trunk circumference and canopy volume were not significant, for any studied year. Nevertheless, between February 1999 and November 2003 there was a significant increase in canopy volume with fertilizer dose. In 2003, fruit yield increased with fertilizer dose, as a consequence of an increase both in fruit number and weight. Cumulative yield for the experimental period also increased with fertilizer dose. These results are further evidence to confirm previous research made with the same experimental set-up: T400 for oil quality and T600 for table olive quality seem to be the most appropriate treatments, although there is a risk for leaching losses and the possibility of groundwater contamination with T600.