Pros and Cons of Olive Fertigation: Influence on Fruit and Oil Quality

Agronomic practices can modify olive fruit and oil quality. However, there is little information on the influence of fertigation, a common practice in most intensive orchards. We studied nutrient distribution in the soil profile following fertigation with different doses of N-P-K fertilizer, and its effect on nutrient concentrations, yield and both table olive and oil quality. Measurements were performed in an adult ‘Manzanilla de Sevilla’ olive orchard in which 100, 200 and 400 g N per tree and irrigation period of a 4N-1P-3K fertilizer were applied by fertigation from 1999 to 2001 (three growing seasons) and 200, 400 and 600 g N of the same fertilizer were applied in the two following growing seasons (2002-2003). A control treatment, irrigation without fertilizer, was also established. Irrigation amounts were similar in all treatments. In 2003, NO3-N, P and K concentrations in the root zone wetted by irrigation were studied: they increased with respect to those in the drying zone, showing a general linear relationship with fertilizer dose, particularly in the top soil layer where most of the olive roots were active. In the 600 g N treatment, leaching losses were observed at 0.8-0.9 m depth, possibly leading to groundwater contamination. We found an increase in fruit yield with increasing fertilizer dose, likely due to the observed greater concentrations of NO3-N, P and K in the soil. In fact, our data show a positive relationship between increased soil NO3-N, P and K availability and higher leaf N, P, K concentrations. This could have accounted for the observed increase in canopy volume, fruit number per tree and fruit weight with the amount of fertilizer. Despite the fact that fruit weight, pulp/stone ratio and volume increased with fertilizer dose, reducing sugars, necessary for olive fermentation, and pulp texture decreased. Differences in texture remained after ‘Spanish-style’ green olive processing. In addition, no differences were found in oil content but its quality was negatively affected with increasing fertilizer: in particular, polyphenol total content, bitterness, oxidative stability and the relation of monounsaturated/polyunsaturated fatty acids decreased with fertilizer dose. INTRODUCTION Many of the new olive orchards have localized irrigation, which enables highfrequency fertigation. It is assumed that the benefits of fertigation derive from both being able to adjust the water and fertilizer supplied and their application in wetted soil zones where most active roots are located. Fertigation increases both the root growing period and nutrient assimilation (Fernández et al., 1991). Nevertheless, fertigation also has some drawbacks, such as soil nutrient depletion and acidification (Peryea and Burrows, 1999; Mmolawa and Or; 2000; Neilsen et al., 2004). In addition, fertigation may favor N-NO3 leaching, resulting in nutrient losses and groundwater contamination. Olive oil and table olive consumption has increased in recent years because of the recognized nutritional value of the Mediterranean diet (Patumi et al., 2002). The table olive industry demands fruits with good size, shape, high pulp/stone ratio, good texture and color. The nutritional and biological value of the fruit depends on some chemical components of its pulp such as water, oil, reducing sugars, polysaccharides, polyphenols Proc. IS on Olive Irrigation and Oil Quality Eds.: U. Yermiyahu et al. Acta Hort. 888, ISHS 2011 270 and minerals. Fruit texture is also important for table olive production (Morales-Sillero et al., 2008). Factors such as cultivar, weather and soil conditions, fruit ripeness, agronomic practices and oil-extraction process influence the fruit and oil characteristics. Among the agronomic practices, fertigation holds great importance (Morales-Sillero et al., 2007, 2008). MATERIALS AND METHODS The experiment was carried out from 1999 to 2003 in a ‘Manzanilla’ olive orchard close to Seville in Spain. The trees, at 7 m x 7 m spacing, were 10 years old in 1999. A randomized complete block design with six blocks per treatment (repetitions) and four trees per block was established. Each block was surrounded by guard trees. The trees received 100, 200 and 400 g of N per tree and irrigation period with 4N1P-3K fertilizer that was applied by fertigation from 1999 to 2001 (three summer growing seasons) and 200, 400 and 600 g of N per tree of the same fertilizer in the two following summer growing seasons (2002-2003); these treatments were named T200, T400 and T600, respectively. A control treatment, irrigated but without fertilizer, was also established. Irrigation amounts were calculated according to crop evapotranspiration (ETc) estimated by the crop coefficient approach (Allen et al., 1998), using the Kr and Kc coefficient values estimated for this orchard by Fernández et al. (2006) (Kr = 0.7; Kc values were 0.76 in May, 0.70 in June, 0.63 in July and August, 0.72 in September and 0.77 in October). The methods used for root-distribution, soil and plant analyses were as described by Morales-Sillero et al. (2009). Fruit quality was evaluated according to Morales-Sillero et al. (2008), and oil quality was characterized as described by Morales-Sillero et al. (2007). Analysis of variance (ANOVA) was used to determine statistical differences between treatments of all studied variables, and analysis of covariance was used for fruit yield. Polynomial contrasts were obtained when a significant F test was observed. RESULTS AND DISCUSSION Fertigation and Soil-Plant Interactions The average estimated evapotranspiration was 250 mm per irrigation season. Hence, each tree received the same quantity of irrigation water, enough to maintain the soil close to field capacity. The soil outside the wetted zone was under drying conditions, since no rain fell during the summer seasons. We estimated that about 81% of the young active roots were located in the wet zones. Similar results were reported by Fernández et al. (1991). The soil concentrations of N-NO3 and available P and K showed a linear increase with fertilizer dose (Table 1), which was less evident at increasing soil depth. The high contents of N-NO3 recorded in the T600 trees at 80-90 cm depth, together with the absence of active roots, indicated a significant risk of groundwater contamination by nitrate leakage. The contents of N, P and K in the leaves showed a linear increase with treatment dose, likely because of the increasing availability of these elements in the soil. It should be noted that N and K, but not P levels were low in the control trees prior to irrigation (Table 2). The increasing fertilizer dose increased total fruit production, but the average fruit weight was not significantly affected. Nevertheless, there was a tendency toward increasing fruit weight with fertilizer dose, parallel to the increase in fruit yield (Table 3).