Optimization the fertilization of olive orchards in Tunisia

This project was initiated with the objective to increase the revenues of Tunisian olive farmers through the adoption of best fertilizer management practices, and consisted of three research activities. The objective of the first activity was the assessment of the nutrient status of olive orchards in rain-fed and irrigated areas. The study continued in 2017 (‘on’ year) in the rain-fed area of northwest Tunisia. The olive orchards were fertilized by an application of 2 to 4 kg ammonium nitrate (AN)/tree in February. Foliar analysis for nitrogen (N), phosphorus (P), and potassium (K) showed that nutrient contents decreased significantly, and were lower than those observed during the ‘off’ year (2016). In consequence, application of N, P, and K remain indispensable to overcome constraints of the conventional fertilization.

The objective of the second activity was the development of the Deviation from Optimum Percentage (DOP) approach as alternative to the critical concentration diagnosis. A survey of 20 farmers was carried out to assess farmers’ fertilization practices. Furthermore, a foliar diagnosis [N, P, K, calcium (Ca), and magnesium (Mg)] was carried out in the same farms to make a diagnosis on the fertilization efficiency practiced by the farmers using the critical concentration and DOP index. Results of the survey showed that most of the farmers (61%) apply only N in the fertilization program, and in spite of the presence of the drip irrigation system in all surveyed orchards, only 39% of farmers apply fertilizers through the drip irrigation system. According to the results of foliar analysis, we found 45, 50, and 80% of N, P, and K deficiency cases, respectively. The DOP analysis showed that most of the orchards (60%) have a severe nutritional imbalance while only 16% have an equilibrated nutritional balance.

The objective of the third activity was the assessment of the effect of foliar fertilization on olive trees under saline conditions in an irrigated area of northern Tunisia. The electrical conductivity of the irrigation water was 4.6 mS/cm. Foliar fertilization consisted of different sources of N, P, K, Ca, and boron (B) using two types of water: tap and saline water. Results of three experimentation years showed that foliar fertilization increased cumulated olive yield by 22% and 8% with tap water and saline mixture, respectively. An improvement of the nutritional status was also observed during all of the years, except for sodium, which increased above the threshold level when saline water was used for foliar fertilization.