Pros and cons of olive fertigation: Influence on fruit and oil quality

Abstract

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.