**3. Olive tree fertilization**

### **3.1 Synthesis of research work carried out in the Mediterranean basin**

Mineral nutrition is one of the major factors in optimizing fruit yield and quality [7]. For the olive tree, nitrogen (N), phosphorus (P), and potassium (K) are essential nutrients. Marin and Fernández-Escobar [4] reported that annual intake is not necessary for good olive productivity. Hence, technical management can be inefficient following an underestimation or an overestimation of inputs at the orchard level. In fact, under-fertilized areas do not reach optimum yield levels, whereas, in over-fertilized areas, there could be a high risk of environmental pollution and an increase in costs [8]. Centeno and Gómez del Campo [9] reported an increase in olive yield after N application to the soil and P and K application by foliar spraying, although initial leaf analyses indicated adequate nutrition levels. After five trial years, Fernandez-Escobar et al. [10] reported that when olive tree fertilization is based on foliar diagnosis, it satisfies crop nutrient needs, minimizes environmental impact, improves crop quality, and avoids excessive and systematic use of fertilizer. In Spain, Garcia [11] proposed, for the olive tree, a balanced formula between the macro elements of 20-8-14 (N-P2O5-K2O) based mainly on the olive's nutrients exports.

#### *3.1.1 Nitrogen*

A survey carried out across the Mediterranean basin where about 98% of the 10 million hectares of existing olive groves in the world are located [12], showed that nitrogen is present in most fertilizer applications, even when potassium is the element that causes most severe nutritional disorders [13]. In a long-term experiment conducted with rain-fed olive orchards in several localities in Spain, Ferreira et al. [14] found that only trees with productivity below 35 kg.tree−1 showed a positive response to N intake. For a period of experimentation of 13 years, Fernández-Escobar et al. [15] found that nitrogen fertilization did not have significant effects on yield, fruit

characteristics, and tree growth in two typical orchards of the Mediterranean region so leaf nitrogen concentration increased with nitrogen dose. They also noted the absence of a yield decrease or olive tree growth decrease even when leaf N content was below the established deficiency threshold (1.4%), thus suggesting that this deficiency threshold should be inferior. A combination of soil N inputs and foliar N applications (50% to soil and 50% foliar) was more effective in increasing the olive leaf nitrogen, compared to the supply of the totality of N to the soil; this can reduce the amount of nitrogen fertilizer needed to correct a possible N deficiency [15]. Rodrigues et al. [16] reported a gradual and significant decrease in olive yield when nitrogen was removed from the fertilization plan for 4 years, compared to treatments where nitrogen was added annually. Jasrotia et al. [17] also found a significant increase in olive tree productivity with increasing nitrogen doses. After 5 years of study in olive orchards in southern Spain, Fernández-Escobar et al. [18] found no significant differences in terms of olive yield between trees subjected to a fertilization program based on foliar diagnosis, and those receiving, annually, the current fertilization in this region (500 kg.ha−1 of an NPK fertilizer (15-15-15) plus three foliar sprays of trace elements and amino acids). They also found that traditional fertilization practiced by farmers increased fertilization cost by more than ten times without increasing yield, vegetative growth, or oil content. In addition, the excess nitrogen affected negatively olive quality by inducing a decrease in polyphenol content with important antioxidant effects for olive oil. Nitrogen promotes an increase in the oleic and stearic acid contents of drupe and its deficiency is accompanied by an increase in palmitic and linoleic acid levels [19]. Too much nitrogen can cause environmental degradation [20] and affect negatively the groundwater quality [21]. This excess of N can also affect the olive oil quality [22] and the flower quality by reducing the egg's longevity [13]. These latter authors have also shown that a nitrogen deficiency caused an increase in the pistil abortion for the olive tree but only during the year when rainfall was low during the period preceding flowering. They suggested that a pre-flowering water deficit coupled with nitrogen deficiency induces an increase in pistil abortion for olive trees.

#### *3.1.2 Phosphorus*

Generally, phosphate fertilization is not recommended or practiced in rain-fed olive orchards [23]. Several authors have tried to determine the limiting and optimal values of soil available phosphorus concentration. Gargouri and Mhiri [6] found a critical value of 8 mg P.kg−1 obtained by the Olsen method. Previous work has shown that responses to phosphorus are rare in fruit trees [24] and it has not been clearly demonstrated for olive trees in the field [25, 26]. Rodrigues et al. [27] suggested that regular intakes of P might not be necessary, in agreement with other opinions [25, 26]. They also reported that the low level of olive phosphorus exports may explain the crop's lack of response to P fertilizer inputs observed in field trials. In contrast, Fontanazza [28] reported that phosphorus deficiency limits the absorption of nitrogen, magnesium, calcium, and boron and consequently reduces plant growth.

#### *3.1.3 Potassium*

Potassium fertilization is considered essential for the olive tree, especially because the fruit is highly concentrated in K [29]. Ben Mimoun et al. [30] reported a positive effect of potassium fertilization on olive yield and oil content under rainy conditions. Potassium is known not only for its significant effect on yield and fruit quality but also

#### *Management of Olive Tree Fertilization in Morocco DOI: http://dx.doi.org/10.5772/intechopen.104644*

for its effect on water use efficiency [31]. Adequate potassium fertilization allows better tolerance to a drought season [32], which is very common under our Mediterranean conditions [21]. In their study, Ben Mimoun et al. [30] found that fractional foliar potassium inputs had a greater effect than soil potassium inputs on the olive tree. This implies that this technique is preferable especially under rainy conditions because the lack of moisture in the soil during the plant's growth period could limit potassium absorption by the roots. Nutrient uptake depends on the supply of nutrients to the root system, namely their availability, the nutrient requirement level, and the absorption period [33]. Fine-textured soils are characterized by potassium uptake, so the addition of potassium to the soil surface is almost ineffective [34]. Foliar nutrient inputs are, in general, useful for meeting plant requirements and have high efficiency [35]. Potassium is particularly well suited to this fertilization form because just after foliar spraying, its translocation takes place quickly through the leaves [34]. The minimum threshold of the soil's available potassium content correlated with its clay content. This threshold is 80 mg K.Kg−1 when the clay percentage is less than 15% and 150 mg K. Kg−1 beyond this limit [6]. These potassium thresholds were obtained by the K extraction method with ammonium acetate. Sarrwy et al. [36] reported a remarkable improvement in leaf nutritional status, yield, and fruit quality after the application of potassium nitrate or mono-potassium phosphate, compared to control trees. The best result has been obtained with potassium nitrate, which is probably due to the high nitrogen requirement in the olive tree nutrition compared to phosphorus. Garcia [11] recommended 1 to 2 Kg K2O.tree−1, based mainly on the exports of olives in potassium. Although potassium is often a nutritional problem in olive orchards [25, 26], high doses of fertilizer may not be necessary [27]. The need for a regular supply of potassium, and the dose that must be provided for each application, depends on the availability of K in the soil and on the latter's capacity to retain it adsorbed by colloids or fixed by clay minerals. In sandy soils, for example, the strategy for supplying K should be similar to that for N, based on a regular supply of a limited amount of fertilizer. In clay soils, it is possible to provide higher doses with less frequent applications [27].

From this literature review, we can say that nitrogen and potassium are the most elements required by the olive tree, compared to phosphorus, which poses fewer problems.

#### **3.2 Some results on olive tree fertilization in Morocco**

In Morocco, studies on olive tree fertilization are almost non-existent. Few studies have looked at this aspect. Generally, the olive tree is considered, especially by small farmers, as a hardy species that does not require maintenance. As a result, determining the fertilization standards for the olive tree is essential for the rationalization of fertilizer inputs, in particular nitrogen, phosphates, and potassium. These macro elements are generally the most required by olive trees and will help improve crop yield levels. In this part, we will report some results related to three olive fertilization trials.

Three trials were carried out in rainy conditions in 3 different sites (S1 = Taza, S2 = Taounate, and S3 = Fez) belonging to the Fez-Meknes region which encompasses 33% of the total national olive tree area. Two orchards among the three chosen are planted by the Moroccan variety Picholine that dominates in Morocco. For the same variety, we considered two different age categories: a young orchard (S1: 9 years old) and an old orchard (S2: 35 years old) but with, nearly, similar planting densities (10 \* 10 for S1 and 9 \* 9 for S2). The third site was represented by a young orchard (S3) of the Spanish variety Arbequina with a higher planting density (3 \* 5).

Before the installation of these experiments, a soil physicochemical characterization in the three sites was carried out through laboratory analyzes of the soil samples taken from two soil layers 0–30 and 30–60 cm. The analysis results showed that the studied soils are basic, poor in organic matter, non-saline for olive trees, and moderately to strongly calcareous for S1 and S3 and non-calcareous for S2 (**Table 7**). We noted low soil available phosphorus content at S1 and S2 and lower soil nitrate content at S2. For exchangeable potassium, these soils were well provided with this element [3].

The study design adopted for these trials is factorial in incomplete random blocks with two blocks. Each elementary plot consists of four trees. Four doses of each of the elements N, P, and K were tested. The nitrogen was fractionated into two inputs—1/2 in March and 1/2 in May. Phosphorus and potassium were brought in March.

At S1 and S3, the olive yields were equal, while the planting densities, as well as the olive tree varieties, were different. Generally, an olive tree in a dense orchard (S3) would produce less compared to another in an orchard where planting density is low (S1). In the latter case, competition between trees for nutrients, water, and light is weak. The yield recorded at the S3 level can be explained on the one hand, by the significant amount of rain (797.4 mm) that it received during this year compared to S1 (580.1 mm) and S2 (499.6 mm), and on the other hand, by the significant production potential of the Arbequina variety planted in this orchard. This potential was proved by a study in Tunisia where the behavior of different introduced varieties and Tunisian varieties was studied, the evaluation of the production potential of these varieties showed that Arbequine comes in the first position next to the variety Chemlali about cumulative production [37].

### *3.2.1 Nitrogen*

The result showed that at S1 (9 years) and S3 (7 years), nitrogen input was not necessary since it did not improve the productivity parameters of these olive orchards (**Table 8**) and negatively affected the olive oil quality, especially peroxide index (**Table 9**). This could be due to the availability of soil mineral nitrogen, needed by olive trees in these orchards (**Table 7**). On the other hand, at S2 (35 years old), the addition of nitrogen fertilizer was beneficial since it improved both yields, yield efficiency as well as olive oil content. In the latter site, the nitrogen requirement of the olive tree was relatively high given


#### **Table 7.**

*Physicochemical characteristics of the experimentation soils sites.*
