Journal of Agriculture and Ecology Research International 5(1): 1-10, 2016; Article no.JAERI.20016 ISSN: 2394-1073

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Diagnosis of Soil Fertility and Nutritional Status and Characterization of Olive Orchard in Central Morocco

Karima Bouhafa 1,2*, Lhoussaine Moughli 2, Amal Hadiddou 1, Khadija Bouchoufi 1 and Abdelkader Sdouq 1

1Regional Agricultural Research Center, P.O.Box 578, Meknes, Morocco. 2Department of Soil Science, Hassan II Institute of Agronomy and Veterinary Medicine, P.O.Box 6202, Rabats-Instituts, Rabat, Morocco.

Authors’ contributions

This work was carried out in collaboration between all authors. Author KB designed the study, completed the surveys and collection of soil and leaf samples, performed the statistical analysis, and wrote the first draft of the manuscript. Author LM supervised the study and managed the literature searches. Authors AH and AS has contributed to the investigation and collection of soil and leaf samples. Author K. Bouchoufi managed the laboratory analyzes. All authors read and approved the final manuscript.

Article Information

DOI: 10.9734/JAERI/2016/20016 Editor(s): (1) Małgorzata Pietrowska-Borek, Department of Biochemistry and Biotechnology, Pozna ń University of Life Sciences, Poland. Reviewers: (1) Raul Leonel Grijalva Contreras, Instituto Nacional de Investigaciones Forestales Agrícolas y Pecuarias, Mexico. (2) Walid Fediala Abd El-Gleel Mosa, Alexandria University, Alexandria, Egypt. Complete Peer review History: http://sciencedomain.org/review-history/11641

Received 7th July 2015 Accepted 10 th September 2015 Original Research Article Published 30 th September 2015

ABSTRACT

This study was conducted in 2009/2010 with 58 olive orchards randomly selected at the Sais region. The main objectives of this work are: i) Characterization of olive orchard ii) Determining the soil fertility levels and the assessment of nutritional status of these olive orchards. Composite soil samples from two depths (0-30 cm and 30-60 cm) and composite leaf samples were collected during the December/January period. Surveys were conducted on these orchards to collect data for characterization and analysis of different fertilization modes adopted in relation to olive yield. More

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*Corresponding author: Email: [email protected];

Bouhafa et al.; JAERI, 5(1): 1-10, 2016; Article no.JAERI.20016

than half of the surveyed groves (57%) are conducted in rain fed conditions. The surveys have revealed shortcomings as regards fertilization practices of olive cultivation. Indeed, approximately 48.3% of the farmers surveyed provide no fertilizer for their olive trees. The laboratory analyzes of soil and leaf samples collected in these orchards have confirmed this. T he study showed no correlation between the olive yield and soil fertility parameters and between the olive yield and the olive leaf parameters. The relationship between the olive nutrition parameters and the soil fertility parameters, at the 0-30 cm horizon, showed that olive leaf parameters were not linked to those of soil fertility. However, much of the leaf nitrogen content (about half) has been explained by the soil nitrate content in irrigated orchards. The determination of the olive tree fertilization standards is a necessary step for a rational management of fertilizers for the olive trees. These standards should be developed through field trials.

Keywords: Olea europaea; nitrogen; phosphorus; potassium; soil; leaves; surveys.

1. INTRODUCTION This work done at Sais region in Central Morocco, has as main objectives: The general situation of the olive orchards productivity throughout the world seems to be • Characterization of olive orchard at Sais unsatisfactory. Indeed, about 70% of the olive region in central Morocco. orchards are traditional and marginal with a • Determining the soil fertility levels and the medium to very low productivity due, in a assessment of nutritional status of these significant degree, to the lack of appropriate olive orchards. orchard management [1]. Under-application of inputs may lead to unsatisfactory yields, under 2. MATERIALS AND METHODS the potentiality of the orchard [2]. The new intensive orchards (about 30% of the total) 2.1 Surveys present a suitable productivity but are often associated with higher environmental impacts This study was conducted at Sais region, through [3,4]. Olive growing sustainability in the surveys, soil testing and leaf analyzes.

Mediterranean region is highly dependent on 58 olive orchards, distributed on the territory of sustainability of soil resource management the study area (Moulay Driss, Mhaya, Ain Jemaa, practices [1]. However, there is almost no Dkhissa, Agourai, Ras Jerry, Oued Jdida, Ain information on the overall effect of soil status on Kerma, Ain Orma, Ain Chkef, Sfassif, Laksir, olive yield [5]. Sebt Jehjouh, Ait Ouallal) were chosen at random. A plug survey was developed and In Morocco, the olive tree is the main fruit served as a support for data collection in the species cultivated, with about 840 000 ha until field. These relate to the culture system, planting 2011 [6], and a large proportion of its total area density, tree age, variety, soil type, and the located in rainfed areas. Olive growing is an technical route of culture including mineral and intensive agricultural activity which generates organic fertilization adopted by farmers. These almost 100 000 permanent jobs [6]. The olive data were used for the characterization of the sector contributes up to 5% of upstream sampled groves and analysis of different modes agricultural GDP and accounts for 15% of of fertilization adopted in relation to the olive agricultural foodstuff exports. The annual yield. production for 2014 was 100 000 tones of table olives and 120 000 tones of olive oil. Almost 75% 2.2 Soil Testing of the olives produced in Morocco go for olive oil whereas table olive represents 25% of olive At these selected orchards, composite soil production [6]. Olive yields are still below the samples were collected in the diagonal method, olive sector potential. Indeed, the weather, in the at two depths 0-30 cm and 30-60 cm, in the drought case, and weak technical management December/January period. They were the subject are the main factors generating the low of analyzes for their characterization (limestone, productivity registered for olives. Fertilization is organic matter, pH) and the determination of their one of the techniques non mastered by the fertility levels (nitrogen, phosphorus and majority of Moroccan growers and which potassium). The analytical methods used are: consequently leads to low yields of the olive tree. extraction by chromotropic acid [7] for nitrates,

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Olsen [8] for phosphorus, Walkley and Black [9] 3.1.2 Driving mode for organic matter, Drouineau [10] for active limestone extraction and ammonium acetate [11] More than half of the surveyed groves (57%) are for potassium. conducted in rainfed conditions (Fig. 2). At irrigated orchards, the gravity irrigation system 2.3 Leaves Analyzes dominates with a part of 24% of all surveyed orchards. As for drip irrigation, it is only present in 19% of studied farms. Composite leaf samples were also collected from the same orchards and in the same period of soil 3.1.3 Density sampling. The nitrogen and phosphorus mineralization was created by concentrated Most plantations (84.5%) have a planting density sulfuric acid and the potassium with perchloric between 100 and 350 trees.ha -1 (Fig. 3). acid. The leaf nitrogen contents were determined Extensive densities (<100 trees.ha -1) represent by Kjeldahl distillation, those of phosphorus by only 5% of respondents orchards. The rest (10%) colorimetry and those of potassium by is represented by orchards with the density photometrically [12]. exceeds 350 trees.ha -1.

2.4 Statistical Analysis 3.1.4 Intercropping

The percentage of surveyed orchards where the Correlation and regression analyzes between olive tree was associated with intercropping was some parameters were performed using the important (48.3%). The species used by farmers software Genstat. are mainly cereals (barley, wheat, and oats); legumes (Lentil, bean, peas, chickpeas, and 3. RESULTS AND DISCUSSION beans); the vegetable crops (tomato, potato, onion, turnip, zucchini, cauliflower, pepper); 3.1 Characterization of Olive Orchards fodder (alfalfa).

3.1.1 Age 3.1.5 Fertilization

48.3% of surveyed growers use no mineral The sample is characterized by different age fertilizer for their olive trees. The main fertilizers groups ranging from a minimum of 4 years to 70 used by farmers are ammonium nitrate, sulphate years. Young orchards whose age does not of ammonia, diammonium phosphate, phosphate exceed 10 years represent only 5% of all monoamonium, potassium nitrate, urea and surveyed orchards while older orchards some fertilizer formulas such as 14/28/14, (> 40 years) have a part of 35% of the studied the 10/25/20/S and the 0/16/30/S. orchards. The orchards belonging to the age 41.4% of farmers do not use manure. The type of group between 11 and 40 years account for more manure used in these orchards is dominated by than half (60%) of all surveyed groves (Fig. 1). sheep/beef mixture (Table 1).

40 35 30 25 20 15 10

% oforchards % 5 0 [4-10] [11-25] [26-40] [>40] Age groups (years)

Fig. 1. Olive orchards distribution according to their ages

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Drip irrigation 19% Gravity irrigation Rainfed conditions

56.90% 24.14%

Fig. 2. Olive orchards distribution according to the driving mode

Fig. 3. Olive orchards distribution according to their planting densities

Table 1. Use of manure by olive growers 3.2 Assessment of Soil Fertility of Olive Orchards Sampled Manure % of orchards Sheep manure 12.1 Based on analyzes of samples taken from soil, Cattle manure 1.7 the majority of these are basic, calcareous, and largely poor to moderately filled in organic Sheep manure + Cattle 44.8 matter. manure No 41.4 3.2.1 Organic matter content

3.1.6 Variety The organic matter content was highly variable from one floor to another. It was between 0.1 and The dominant variety in olive orchards studied 4.5% for the soil layer 0-30 cm, and between was the Moroccan Picholine with a percentage of 0.04 and 2.7% for the soil layer 30 -60 cm. The 98.3% of the total surveyed orchards. surface horizon is usually provided by organic matter (mean = 2.1%) than the 30 -60 cm horizon 3.1.7 Yield (mean = 1.3%). The olive trees are reported to grow quite well on soils containing more than 1% The yields reported by growers surveyed vary of organic matter [13], even if a threshold of 1.5% widely from one farm to another. They vary is considered low in other conditions [14]. between 0 and 14.3 t.ha -1. Olive trees under drip -1 3.2.2 Active limestone content irrigation recorded an average yield of 5.1 t.ha , those led by gravity irrigation recorded an The average content of active limestone was average yield of 1.9 t.ha -1. For orchards in -1 12.5% and 10.9%, respectively, for the soil layers rainfed conditions, the average yield is 1.8 t.ha . 0-30 cm and 30-60 cm. It varied between 0 and

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18% for the 0-30 cm horizon and 0 to 19.8% for topsoil (0-20 cm), the P values ranging from 2.4 the 30-60 cm horizon. Excellent yield and to 23.2 mg.kg -1 and K values between 14 and vegetative growth can be observed, in both soils 386 mg.kg -1. [19] reported, in Andalusia, with small amount of limestone and where this soil potassium contents between 61 and 171 amount reaches 50%, with a limit of 76% [15]. mg.kg -1) in the 0-30 cm layer. Pa against, [20] found, in the Andalusian olive, soil potassium 3.2.3 pH contents between 105 and 565 mg.kg -1 and soil phosphorus contents between 60 and 162 mg.kg - The soil pH ranged from 6.3 to 8.8 for the 0-30 1 in the 0-30 cm layer. cm horizon and 6.4 to 8.9 for the 30-60 cm horizon. Alkaline soils predominate in the olive 3.3 Determination of the Nutritional orchards studied. Indeed, in the two soil layers 0- 30 and 30-60 cm, 96.6% and 93.1% of soils, Status of the Olive Orchards respectively, are basic soils according to standards set by [16] (Fig. 4). It is well known The olive leaf analysis revealed low leaf nitrogen, that the olive-tree tolerates a wide margin of soil phosphorus and potassium contents which have pH, but the neutral, slightly alkaline values to varied, respectively from 0.224 to 0.602%; from alkaline ones, between 7 and 8.5, assure its best 0.036 to 0.255 and from 0.338 to 1.078%. [18] development [17]. found values between 0.058% and 0.14% for P and between 0.43% and 0.93% for K. [5] 3.2.4 Soil fertility reported values ranging from 0.95 to 1.56% for nitrogen, from 0.07 to 0.14% for phosphorus and For a given site, soil fertility levels have declined from 0.12 to 1.07 for potassium. The leaf mineral in the lower layers of the soil in comparison to contents were in most cases lower than the the topsoil [5]. In our study, the soil nitrate levels deficiency levels reported in the literature. -1 ranged between 1.8 and 71.4 mg.kg and Indeed, all of the studied orchards require -1 between 1.5 and 40 mg.kg , respectively, for the nitrogen inputs, according to the fertilization soil layers 0-30 cm and 30-60 cm. For standards defined by [21] and [22] (Table 2). phosphorus, soils showed levels ranging from 91% of orchards need potassium fertilization. As -1 1.3 to 59.3 mg.kg for the 0-30 cm layer and for phosphorus, it was less of a problem -1 from 1.4 to 41.7 mg.kg for the 30-60 cm layer. compared with nitrogen and potassium as about The soil potassium ranged between 43.8 and a third of sampled soils need a supply of -1 1456.5 mg.kg for the 0-30 cm horizon, and phosphorus. -1 between 34.4 and 997.7 mg.kg for 30-60 cm horizon. The soil P and K levels are higher than Table 2. Percentage orchards should be the values found in other studies in Tunisia, Syria fertilized based on leaf analyzes and Spain. Indeed, in a soil fertility survey of olive trees in Sidi Bouzid (northwest Tunisia), -1 Mineral element % of orchards [18] reported values between 6 and 19.9 mg.kg N 100 % to P (average of 11.8 mg.kg -1) and from 90 to -1 -1 P 36.2 % 270 mg.kg for K (average of 133 mg.kg ) in the K 91.4 % 0-30 cm soil layer. In Syria, [5] reported in the

120.0 0-30 30 -60 96.6 100.0 93.1 80.0 60.0 40.0

20.0 3.4 6.9 0.0 neutral soils basic soils

Fig. 4. pH of soil sampled groves

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3.4 Relations between the Parameters observed by [23] between the olive yield and leaf Studied phosphorus and potassium content.

The data statistical analysis showed no We also studied the relationship between the correlation between the olive yield and soil nitrate olive leaf nitrogen, phosphorus and potassium or phosphorus or exchangeable potassium contents, at the 58 sampled orchards, and soil contents (Table 3). [5] in their study in Syria, nitrate, available phosphorus and exchangeable found that the most important factor was the potassium contents, at the 0-30 cm horizon. amount of potassium available in the root zone Figs. 5, 6 and 7 present regressions curves explaining the two-thirds of the yield variability produced between these parameters for each (R 2 = 68%) followed by total nitrogen in the root driving mode orchards adopted by farmers zone that was ranked second with 58% of the (rainfed conditions, gravity irrigation and drip explained variability and mineral nitrogen in the irrigation). These have shown that nutrition root zone, which explained 44% of variability parameters of olive leaves are not linked to those (significant correlation). of soil fertility. With the exception of nitrogen where much (about half) of the leaf content has The olive yield is not correlated with the leaf been explained by the soil nitrate content but nitrogen or phosphorus or potassium contents only in irrigated orchards (Fig. 5). Note that all (Table 4). [5] found the same results for nitrogen these regressions are not statistically significant. and phosphorus when they noted a significant [18] also showed no relationship between the correlation between the olive yield and the leaf olive leaf phosphorus and potassium content and potassium contents with 26% of the explained the soil phosphorus and potassium content, variability. A strong negative correlation was respectively.

Table 3. Regressions between the olive yield and the soil nitrates, phosphorus and potassium contents, according to the three driving modes of olive groves

Yield Rainfed conditions Gravity irrigation Drip irrigation Model R2 Model R2 Model R2 Soil nitrates y = -0,0005x 2 R² = y = 2,96e- R² = 0,3033 y = -0,0007x 2 R² = 0,076 + 0,0257x + 0,0067 0,036x - 0,0638x + 1,5725 6,6668

Soil y = 0,0002x 2 R² = y = -0,0032x 2 R² = 0,1802 y = -0,017x 2 R² = 0,4261 phosphorus - 0,0365x + 0,0255 + 0,1696x + + 0,9779x - 2,3283 0,3605 3,967 Soil y = 1E-06x 2 - R² = y = -1E-05x 2 R² = 0,1166 y = 0,0002x 2 R² = 0,1734 potassium 0,0013x + 0,0062 + 0,0087x + - 0,094x + 2,0524 0,4994 17,129

Table 4. Regression between the olive yield and the leaf mineral contents, according to the three driving modes of olive groves

Yield Rainfed conditions Gravity irrigation Drip irrigation Model R2 Model R2 Model R2 % Leaf nitrogen y = -8,8455x 2 R² = y = 184,75x 2 R² = y = -166,6x 2 R² = + 8,6368x - 0,0045 - 118,7x + 0,3515 + 125,2x - 0,0299 0,1061 20,193 17,805 % Leaf y = 193,49x 2 R² = y = 300,44x 2 R² = y = -720,24x 2 R² = phosphorus - 38,882x + 0,0327 - 68,193x + 0,6275 + 253,39x - 0,1631 3,4603 4,9764 13,875 % Leaf y = -12,073x 2 R² = y = 4,2404e- R² = y = -185,75x 2 R² = potassium + 16,425x - 0,0299 1,676x 0,3184 + 201,21x - 0,2352 3,3019 46,614

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Fig. 5. Evolution of the leaf nitrogen content according to the soil nitrate content (0-30 cm) for the three driving modes of the studied orchards

0.300 y = 0.0889x 0.0707 Rainfed conditions R² = 0.0233 0.200

Leaf P (%) Leaf 0.100

0.000 0 10 20 30 40 50 60

-1 Soil phosphorus (mg.kg )

0.300 y = -9E-05x 2 + 0.004x + 0.0813 Gravity irrigation R² = 0.0452 0.200

0.100 Leaf (%) P

0.000 0 10 20 30 40 50 60

-1 Soil phosphorus (mg.kg )

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2 0.300 y = 6E-05x - 0.0044x + 0.1924 R² = 0.0684 Drip irrigation 0.200 Leaf (%) P 0.100

0.000 0 10 20 30 40 50 60 -1 Soil phosphorus (mg.kg )

Fig. 6. Evolution of the leaf phosphorus content depending on the soil available phosphorus content (0-30 cm) for the three driving modes of the studied orchards

y = 2E-07x 2 - 0.0004x + 0.6413 Rainfed irrigation 1.200 R² = 0.0526 1.000 0.800 0.600

Leaf K Leaf (%) 0.400 0.200 0.000 0 200 400 600 800 1000 1200 1400 1600

-1 Soil potassium (mg.kg )

Gravity irrigation y = 3E-07x 2 - 0.0006x + 0.7126 1.200 R² = 0.0641 1.000 0.800 0.600 Leaf K Leaf(%) K 0.400 0.200 0.000 0 200 400 600 800 1000 1200 1400 1600

-1 Soil potassium (mg.kg )

y = 0.0003x + 0.4047 Drip irrigation 1.200 R² = 0.0474 1.000 0.800 0.600 0.400 Leaf K (%) K Leaf 0.200 0.000 0 200 400 600 800 1000 1200 1400 1600

-1 Soil potassium (mg.kg )

Fig. 7. Evolution of the leaf potassium content depending on the soil exchangeable potassium content (0-30 cm) for the three driving modes of the studied orchards

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4. CONCLUSION 2. Bouma J. Precision agriculture: introduction to the spatial and temporal • The surveys have revealed shortcomings variability of environmental quality. In: Lake as regards fertilization practices of olive JV, Bock GR, Goode JA. (Eds.), Precision cultivation at Sais region. Indeed, agriculture: Spatial and temporal variability approximately 48.3% of the farmers of environmental quality. Ciba Foundation surveyed provide no fertilizer for their olive Symposium, 210. Wiley, Wageningen, the trees. Netherlands. 1997;5-17. • The laboratory analyzes of soil and leaves 3. Michelakis N. Olive orchard management: th samples collected in these orchards have advances and problems. Proc. 4 confirmed these failures: Much of olive International Symposium on Olive orchards require additions in fertilizers. Growing. Eds. C. Vitagliano and G.P. • The study of the correlation between yield Martelli. Acta Hort. 2002;586:239-245. and some soil parameters showed no 4. Touzani A. Tour d’horizon de l’oléiculture correlation between the olive yield and soil mondiale et programme d’intervention du fertility parameters. COI. Olivea. 1998; French. • Also, the olive yield was not correlated with 5. Tubeileh A, Turkelboom F, Al-Ibrahem A, the leaf nitrogen or phosphorus or Thomas R and Tubeileh K.S. Modelling the potassium contents. Effects of Soil Conditions on Olive • The relationship between the olive nutrition Productivity in Mediterranean Hilly Areas. parameters and the soil fertility International Journal of Agronomy. parameters, at the 0-30 cm horizon in the 2014;(2014)12. Article ID 672123. olive groves, showed that olive leaf 6. General description of olive growing in parameters were not linked to those of soil Morocco. fertility. However, much of the leaf nitrogen Available:http://www.internationaloliveoil.or content (about half) has been explained by g the soil nitrate content but only in irrigated 7. Sims, James R, Jackson, Grant D. Rapid orchards. analysis of soil nitrate with chromotropic acid. Soil Science Society of America; This diagnostic work done at Sais region has 1971. identified problems concerning, especially, 8. Olsen SR, Cole CV, Watanabe FS, Dean fertilization of olive trees. The determination of LA. Estimation of available phosphorus in the olive tree fertilization standards is a soils by extraction with bicarbonate. U.S. necessary step for a rational management of Dep. Agric. Circ. 939, USA; 1954. fertilizers for the olive trees. These standards will 9. Allison LE. Organic Carbon. Dans CA, be developed through field trials. Black ed. Methods of soil analysis. Part II, chap. 90. American Society of Agronomy. ACKNOWLEDGEMENT Inc., Madison, Wis. 1965;1372-1376.

10. Drouineau G. Rapid determination of the Our thanks go to Ms Sahar Bennani for her active limestone soil. Reportation new data assistance in statistical analysis. on the nature of the limestone fractions.

Ann. Agron. French. 1942;12:441-450. COMPETING INTERESTS 11. Chapman HD. Cation-exchange capacity. Dans C. A. Black, ed. Methods of soil Authors have declared that no competing interests exist. analysis. Part II, Chap. 57 and 58. American Society of Agronomy. Inc. REFERENCES Madison, Wis. 1965;891-903. 12. El Gharous M, El Amrani M, El Mjahed K. 1. Gargouri K, Sarbeji M, Barone E. Manuel des Méthodes d’Analyses de Sol Assessement of soil fertility variation in an et de la Plante. Aridoculture, Institut olive orchard and its influence on olive tree National de la Recherche Agronomique. nutrition. Second international seminar. Morocco. 1995; French. Biotechnology and quality of olive tree 13. Soyergin S, Moltay I, Genç C, Fidan AE, products around the mediterranean basin. Sutçu AR. Nutrient status of olives grown Marsala-Mazara del Vallo, Italy. 5-10 in the Marmara region. Acta Hort. 2002; November; 2006. 586:375-379.

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Bouhafa et al.; JAERI, 5(1): 1-10, 2016; Article no.JAERI.20016

14. Freeman M, Carlson RM. Mineral nutrient fertilization of rainfed olive orchards. availability. In: Olive production manual, Scientia Horticulturae. 2008;16(4):399-403. ferguson et al. eds. Publication 3353. 20. Ferreira Llamas J. Tests and standardized University of California. 1994;69-75. trials used for olive tree fertilization. In 15. Llamas JF. Basis of fertilization in olive international course on the fertilization and cultivation and the olive tree’s vegetative intensive cultivation of the olive. cycle and nutritional needs. Int. Course on UNDP/FAO/Spanish national institute for the fertilization and intensive cultivation of agricultural research, Cordoba, Spain, the olive. Cordoba. 1983;1-25. 1984;30-52. 21. Marin L, Fernández-Escobar R. 16. Chafai Elalaoui A. Guide engrais Optimization of nitrogen fertilization in olive fertilisation. Edition: Ecole Nationale orchards. In: Val J, Montanés L, Monge E. d’Agriculture. Meknès, Maroc; 2003. (Eds), Proceeding of the third International French. symposium on mineral nutrition of 17. Martinez Raya A. Suitable land for olive deciduous fruit trees, Zaragoza, Spain. cultivation. Inter. Course on the Ferti. and 1997;411-414. Intens. Cult. Of the Olive, Spain. 1983;26- 22. Fernandez-Escobar R, Moreno R, Garcia- 29. Creus M. Seasonal changes of mineral 18. Gargouri K, Mhiri A. Relationship between nutrients in olive leaves during the soil fertility and phosphorus and alternate-bearing cycle. Scientia potassium olive plant nutrition. Options Horticulturae. 1999;82:25-45. Mediterranean’s: Serie A. 2002;50:199- 23. Jasrotia A, Sing RP, Bhutani VP, Sing IM. 204. Influence of tree characters and nutrients 19. Restrepo-Diaz H, Benlloch M, Navarro C status of olive trees on their productivity. and Fernández-Escobar R. Potassium Acta Horticulturae. 1999;474:313-315. ______© 2016 Bouhafa et al.; This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/4.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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