Optimization the fertilization of olive orchards in Tunisia

This project started in 2014 with the objective to increase the revenues of olive farmers of Tunisia by increasing production through the adoption of improved fertilizer management practices. The first activity of the project was the assessment of the initial nutrient status of olive orchards in rain-fed and irrigated areas. The study continued in 2016 (off year) in the rain-fed area of north-west of Tunisia. The olive orchards were fertilized by an application of 3 to 4 kg ammonium nitrate (AN)/tree in February. Foliar analysis for nitrogen (N), phosphorus (P), and potassium (K) showed that N was above the threshold values due to the application of AN, however all olive orchards were deficient in P and K with severe symptoms of deficiency in the leaves. In consequence, application of P and K remain indispensable to overcome constraints of conventional fertilization.

The second activity of the project was the assessment of the effect of foliar fertilization on olive trees under saline conditions in irrigated areas of northern Tunisia. The electrical conductivity of the irrigation water was 4.59 mS/cm. Foliar fertilization consisted of different sources of N, P, K, calcium (Ca), and boron (B) combined with two irrigation treatments: magnet-treated saline water and saline water. Two types of water were used for foliar fertilization: tap water and saline water. Results based on the average of 'on' (2015) and 'off' (2016) years showed that foliar fertilization, even when using a saline mixture, significantly improved olive yield per tree by 1.2 to 1.6 times greater than farmer fertilizer practices (FFP). In 2016, the use of magnetic water for irrigation did not affect nutritional status, growth or olive yields. However, foliar fertilization improved the olive oil content by 2 to 6%, compared to FFP.

The third activity under the project was to study the nutritional balance of super high-density olive orchards (SHD) by using the critical concentration method and the DOP (deviation from the optimum percentage) method. Foliar diagnosis was undertaken in four SHD at Mornag, Sminja, Sfax, and Gafsa. Fertilization consisted of the application of N as AN and urea, K as potassium nitrate and potassium sulfate, P as phosphorous acid, and Ca as calcium nitrate. Three to four foliar applications (B, amino acid, K) were done in these orchards. Values of the DOP indexes indicated that N and P were in the optimal ranges, however, Ca was deficient and K was slightly in excess.