Leaf nutrient spatial variability and site-specific fertilization maps within olive (Olea europaea L.) orchards

Spatial variability of leaf nutrients (N, P, K, B and Fe) in a 30 ha olive orchard located in southern Spain was examined in two consecutive years for determining their spatial distribution for the implementation of a site-specific fertilization programme. A regular 75m x 75m sampling grid was established and intersection points (nodes) were georeferenced. Every sampling node was composed of an experimental unit of four nearest olive trees. Leaf samples were collected within every experimental unit. The leaf nutrients were analysed statistically and geostatistically. Based on the semivariograms, kriged estimates were used to draw contour maps for each leaf nutrient and to study the possibility of saving fertilizer recommendations. Positive correlations existed during both years between N and P (r = 0.8, P < 0.01), N and B (r = 0.48, P less than or equal to 0.01), and P and B (r = 0.37 and 0.51 for 1999 and 2000, respectively, P less than or equal to 0.01). Correlation between K with B was strong in 1999 (r = 0.54, P less than or equal to 0.01), and not so strong in 2000 (r = 0.22, P less than or equal to 0.05). Fe did not show any significant correlation during both years, except with K (-0.29, P less than or equal to 0.05). Geostatisitcal analysis of leaf nutrients revealed that N had both strong and moderate patchy distribution depending on the year, whereas P, K and B had strong patchy distributions both years. Fe showed a lack of spatial dependence both years. Contour maps of each leaf nutrient achieved by kriging were used to estimate the percentage of farm surface needing fertilization where concentration of the respective nutrients did not exceed the fertilization threshold. The study revealed the necessity of determining spatial variability in nutrient status of olive trees before planning a differential fertilizer programme. A consistent saving in N, K and B fertilizers could be achieved in the studied olive orchard both years, for example only 3 and 17% of the surface should be fertilized with N in 1999 and 2000, respectively. D 2003 Elsevier B.V. All rights reserved.