Assessment of soil fertility variation in an olive orchard and its influence on olive tree nutrition.

Soil fertility variation at olive orchard scale was studied in a rainfed olive orchard located on a topographical sequence in the centre of Tunisia (34.37 N 10.16 E). Soil parameters such as pH, organic matter, gypsum, lime, N, P and K contents and soil electrical conductivity were determined in samples obtained at 1m depth and used to characterize soil fertility. Leaf samples from olive trees (cv Chemlali) 80 yrs old and at squared spacing distance (24x24 m apart) were collected in mid-July 2005 to verify plant nutritional status. A regular 200m x 200m sampling grid was established and the intersection points were georeferenced. Each soil fertility component was analyzed statistically and geostatistically. Interpolations were realized according to thresholds and standard deviation of every parameter. Estimates were used to draw variation maps of each soil fertility component (Kriging method). High geodistribution variation was detected. The results showed that an important area is menaced by K deficiency. Indeed, in this area soil K2O content revealed to be under the threshold of 80 ppm. Another area, located in the higher part of the topographical sequence resulted to be affected by a high concentration of sulfates, carbonates and sodium. The study of the relationship between soil and plant concentrations for several nutrients did not allow to single out any correlation. Nevertheless, the carbohydrates allocation in the tree was influenced by soil fertility status. The hypothesis that to overlap stress caused by soil limiting conditions the tree concentrate assimilates in the active parts to increase osmotic pressure is discussed. Under these conditions, olive tree may develop a protection system leading to low reserve carbohydrates and yield.