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Article

Soil and Leaf Nutrient Status of Selected Valencia Orange Orchards in the Gharb Plain of Morocco

by
Rania Brital
1,2,
Mohammed Ibriz
2,
Ahmed Mansour Benmrich
1,2,
Hamid Benyahia
1,
Rachid Aboutayeb
3 and
Zhor Abail
1,*
1
Regional Center of Kenitra, National Institute of Agricultural Research, Avenue Ennasr, BP 415 Rabat Principale, Rabat 10090, Morocco
2
Faculty of Sciences, University Ibn Tofail, Kenitra 14000, Morocco
3
Regional Center of Settat, National Institute of Agricultural Research, Avenue Ennasr, BP 415 Rabat Principale, Rabat 10090, Morocco
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(12), 3040; https://doi.org/10.3390/agronomy14123040
Submission received: 27 May 2024 / Revised: 12 August 2024 / Accepted: 21 September 2024 / Published: 20 December 2024
(This article belongs to the Section Soil and Plant Nutrition)
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Abstract

:
Monitoring the nutrient status of citrus orchards is fundamental to ensure optimum fruit yield and quality. In the present study, soil and leaf samples of 20 Valencia Late orange orchards were collected in the Gharb plain of Morocco, the second-largest citrus area in the country. The objective was to assess the status of essential macronutrients (N, P, K, Ca, and Mg) and micronutrients (Fe, Zn, Cu, and Mn) in Valencia orange orchards and investigate the relationships between soil properties and nutrient contents in soils and leaves. Soils of the studied orchards had a medium to heavy texture, with low to moderate levels of organic matter content (6–31 g kg−1). They were also non-saline and mostly alkaline and calcareous. These soils exhibited a wide range of macro- and micronutrients. Suboptimum levels of total N, available Fe, and Cu were observed in most soils. Most of soils had also sufficient levels of available P, Mn, and Zn. All soils were sufficiently supplied with available Ca, Mg, and K. Similarly to soil analysis, leaf analysis indicated the prevalence of adequate to very high levels of P, Ca, Mn, and K. Leaf N and Fe status were below optimum levels in most orchards, which is in line with the observed low levels in soils. Nevertheless, unlike soils, leaves did not show any deficiency of Cu; instead, most orchards had adequate to excessive levels of this micronutrient. Additionally, leaf Mg and Zn status were deficient in most orchards, conversely to that of soils. This discrepancy between soil and leaf analysis was also noted in the lack of correlation we observed between soil nutrients and their respective levels in leaves. Correlation analysis revealed also an antagonistic interaction between K–Mg and Ca–Mg, which explained the widespread suboptimum levels of Mg in leaves despite its sufficient status in soils. Such antagonism was also observed between Fe–Mn. In the case of Cu, we suspect the use of Cu-containing plant protection products to contribute to the high levels in leaves despite its low levels in soils. Overall, our results showed that nutrient imbalances leading to antagonistic interaction heavily impacted nutrient status in our study area. We expect unbalanced fertilization to contribute to this issue. Therefore, fertilization practices should be managed judiciously to maintain an adequate nutrient balance in the soil and trees of citrus orchards and ensure their sustainable production.

1. Introduction

Citriculture is one of the most important activities in Morocco’s national economy, especially in terms of job creation and export revenue accounting for 3 to 4 billion MAD (279 to 372 million euros) annually [1]. The total production of citrus has increased steadily in the last two decades placing Morocco among the main citrus producers in the world and one of the top 10 in the Mediterranean region [2]. However, the increase in total production was largely associated with an increase in the planted area, rather than an improved productivity of citrus orchards. Despite the considerable efforts made through the renewal of plantations and varieties, the dissemination of localized irrigation, and other modern production techniques, most citrus orchards in Morocco are still suffering from low yields and poor fruit quality. According to FAOSTAT data [2], the average yield of the main citrus types (i.e., easy-peelers and oranges) was about 17 tons ha−1 for the period 2010–2020, which is low compared to other major producing countries. The scarcity of irrigation water is undoubtedly the first reason put forward for this insufficient productivity, particularly over the last five years where decreasing rainfall and increasing temperature have adversely affected orange production in Morocco.
Nutrient management of citrus orchards in Morocco also poses a daunting challenge to achieve their full potential production and reach the quality standards for the international fresh fruit market. Citrus crops are generally highly demanding in nutrients, which makes fertilization often necessary to meet their nutritional requirements. As emphasized by Embleton et al. [3], the key to achieving greater yields and improving the fruit quality of citrus crops lies in the provision of a sufficient amount of nutrients. It is estimated that, on average, fertilization accounts for 20–30% of the total cost of citrus production worldwide [4], and this cost can be substantially higher if not managed appropriately. In Morocco, fertilization of citrus orchards can represent over 40% of the total variable costs of production [5], which requires special attention from farmers to nutrient management practices to ensure optimum productivity and profitability. Through proper nutrient management, farmers not only cut down on production expenses but also safeguard the ecological equilibrium, promoting more productive and sustainable citrus farming practices. In this sense, monitoring the nutrient status of citrus orchards is a fundamental step to accurately determine the actual tree nutrient requirements and plan accordingly the appropriate fertilization program.
Soil and leaf analyses are widely used to assess the nutrient status of citrus orchards. In general, soil analysis provides an estimation of available nutrients for plant uptake indicating the soil nutrient-supplying capacity [6]. However, these estimates may not reflect the actual tree uptake. Tree nutrient uptake can be modified due to various factors related to the tree itself (size, rooting pattern, variety, rootstock–scion combination, etc.), soil properties, nutrient mobility in soil and plant, nutrient interrelation, climate, management practices, and others [7,8,9,10]. These factors, through a synergistic effect, may enhance tree nutrient uptake even though the soil test is low. Or, conversely, they may exert an antagonistic effect and consequently inhibit plant nutrient uptake despite a high supply from the soil. Additionally, trees can store large amounts of nutrients in their biomass and use these reserves during deficiency periods (internal remobilization) [10]. For these reasons, soil analysis was regarded as useful for diagnosing the conditions of nutrient availability for citrus trees, but it is of limited usefulness in predicting their actual nutrient needs. Alternatively, leaf analysis was considered as the most reliable tool to diagnose and predict the nutritional requirements of citrus trees because of the intimate relationship between leaf nutrient concentrations, tree growth, and fruit production. Yet, leaf and soil analysis should be used together for sound nutrient management of citrus orchards.
Tremendous efforts have been spent in Morocco to disseminate the use of soil and leaf analysis by citrus growers. Locally adapted standards were also developed for interpreting soil and leaf analysis of the widely grown varieties of sweet orange and mandarin-clementine [11,12]. However, the number of citrus growers who routinely conduct soil and leaf analysis of their orchards is still far below expectations. A recent survey conducted by Sedki et al. [13] in the Gharb region revealed that nearly 70% of the interviewees do not use soil and/or leaf analysis for managing the fertilization of their citrus orchards. This should result in situations of over- or under-fertilization leading to nutrient imbalances and consequently hampering orchard productivity. Unfortunately, little is known about the current nutritional status of citrus orchards at the national and regional levels. Most available information derives from old surveys whose observations and conclusions may not apply to the present situation.
Therefore, the objective of the present study was to provide a general appraisal of the macro- and micronutrient status of Valencia orange orchards at a regional scale and investigate the relationships between soil properties and nutrient contents in soils and leaves. For this purpose, the Gharb plain was selected as the target study area to conduct our soil and leaf analysis-based survey. The Gharb plain ranks as the second-largest citrus-producing area in the country after the Souss region, with orange farming covering 60% of the total citrus cultivation [1]. Nevertheless, the soil nature (e.g., calcareousness, high pH, and low organic matter) and mismanagement practices may pose multiple nutrient limitations that should be considered for an optimized use of the available water resources and sustainable orange production. We thus hypothesize the presence of multiple nutrient disorders in our study area. Furthermore, we expect most of these nutritional disorders to be likely associated with soil calcareousness and alkalinity or induced by nutrient imbalances rather than insufficient soil fertility.

2. Materials and Methods

2.1. Description of the Study Area

This study was carried out in the Gharb plain, covering the three provinces of Sidi Slimane, Sidi Kacem, and Kenitra, located in the northwestern part of Morocco. The Gharb plain is the first agricultural area in Morocco and the second-largest citrus-producing region in the country [1]. Valencia Late orange is the dominant variety of citrus grown in the region, occupying 37% of the total citrus planted area [14]. The climate is of Mediterranean subhumid type, with an average annual temperature of 19 °C and rainfall of 530 mm for the period 2010–2020 [15].
To address the objective of our study, 20 Valencia Late orange orchards were randomly selected across the major citrus-producing municipalities in the Gharb plain (Figure 1). These orchards were localized under the most dominant soil types (Vertisol and Cambisols, according to the WRB classification [16]) and represented the main features of Valencia orange orchards in the region such as the farm size (small to large), tree age (young to very old), irrigation system (drip and flood irrigation), and grafting rootstock (sour orange, Macrophylla, Carrizo citrange, or Volkameriana). In addition, these orchards were all conventionally managed, meaning that their soil was tilled (one to three passages of a harrower per year after pruning) and various agrochemicals (fertilizers, fungicides, herbicides, and insecticides) were applied at variable rates with or without organic fertilizers. General characteristics and management practices of the selected orchards were summarized in Supplementary Table S1.

2.2. Soil, Leaf Sampling, and Analysis

Composite soil and leaf samples were collected simultaneously from each orchard in November/December 2020. Each composite soil sample consisted of 10 to 20 soil cores, depending on the field size which varied between 1 and 8 ha. A soil core was taken under the crown of a randomly selected tree at 0–25 cm depth. Soil cores were thoroughly mixed to form a composite soil sample for each orchard. The composite leaf sample consisted of about 100 randomly collected leaves from the same trees used for soil sampling. One to two leaves were taken from each side of the trees, at the fourth position from fruiting terminals. Composite soil and leaf samples were brought to the laboratory at INRA-Kenitra for further analysis.
Composite soil samples were air-dried and ground to pass through a 2 mm sieve. Leaves were washed using a detergent solution, rinsed three times with deionized water, oven-dried at 65 to 70 °C for 72 h, and ground to pass 0.5 mm sieve. Three subsamples were used in subsequent analysis. Soil particle size distribution (sand, silt, and clay contents) was determined by the method of Bouyoucos hydrometer [17]. Soil pH and electrical conductivity were determined in a 1:2 (soil–water) suspension. Total and active calcium carbonate were measured according to the method described by Estefan [18]. The method of Walkley and Blake [19] was used for organic carbon determination. Soil available P was measured by bicarbonate extraction [20]. Exchangeable cations (K, Ca, and Mg) were extracted with the ammonium acetate method [21] and available micronutrients (Fe, Mn, Zn, and Cu) with the method of Lindsay and Norvell [22]. Soil and leaf content of total N was determined by the Kjeldahl method [23]. For the other macronutrients (P, K, Ca, and Mg) and micronutrients (Fe, Zn, Mn, and Cu), 1 g of the ground leaf samples was ashed at 550 °C for 5 h and digested with 2 N hydrochloric acid [18]. Leaf P concentration was then determined by the yellow color method [24]. The concentrations of K and Ca in soil and leaf extracts were determined by a flame photometer (Elico. Cl 378, Sanathnagar, India). Concentrations of Mg, Fe, Mn, Zn, and Cu were measured by an atomic absorption spectrophotometer (Analyst 300, PerkinElmer, Shelton, CT, USA).

2.3. Statistical Analysis

Statistical analysis was carried out by the software SPSS (IBM SPSS Statistics 22.0). Data were tested for normality with the Shapiro–Wilk test and homogeneity of variances using Levene’s test. Since most variables did not follow the normal distribution, U Mann–Whitney and Kruskal–Wallis non-parametric tests were conducted to determine the significance of differences in soil attributes and soil and leaf nutrients among orchard-related factors, namely, the farm size, orchard age, tree density, irrigation system, and grafting rootstock. For this reason, the farm size was classified into two categories based on the total area, such as small farm (<=10 ha) and large farm size (>10 ha). Orchard age was also distinguished into two classes based on tree age, which was homogeneous within each surveyed orchard: young (<=10 years) and old (>10 years). Two categories of tree density were differentiated: low (below 500 tree ha−1) and high (500 tree ha−1).
Multiple correspondence analysis (MCA) was performed to evaluate the relationships between the categorical orchard-related factors and their influence on the variability of the entire dataset. Principal component analysis (PCA) was conducted separately to analyze the relationships between soil nutrients and leaf nutrients. The PCA of nutrient concentrations in soils showed that data could be summarized in three significant components with an eigenvalue above one. Most of the variance was loaded in the first two PC axes, which explained 53% of the total variance, meaning that the PC1–PC2 plan is good enough to assess the relationships between soil nutrients. The PCA of leaf nutrients indicated that four components had an eigenvalue above one. Leaf PC1 explained about 30% of the total variance, while PC2 and PC3 added an additional 16% and 14% to the total explained variance. The first three components, describing almost 60% of the total variance, were thus considered to examine the relationships among leaf nutrients. Spearman’s rank correlation analysis was also performed to identify the bivariate relationships between soil chemical attributes and soil and leaf nutrient contents, and the most significant principal components for soil and leaf nutrients.
The MCA and PCA were performed using “FactoMineR” and “Factoshiny” packages in R software version 4.3.2 (R Foundation for Statistical Computing). Spearman correlation matrix was established using the package Corrplot of R software. The symbol “rs” is used to refer to the Spearman correlation coefficient.

3. Results

3.1. Soil Properties and Nutrient Status

Table 1 summarizes the average soil physicochemical properties of the 20 surveyed orchards. Generally, the soils of these orchards were medium to heavy in texture, with silty clay loam and silty loam soils being dominant. Organic matter content (6 to 31 g kg−1) was low to moderate in most of these soils. Soil pH, CaCO3 content, and EC values indicate the prevalence of moderately to strongly alkaline, calcareous, and non-saline soil conditions in the studied orchards.
As shown in Table 2, these soils exhibited a variable range of macro- and micronutrient status. In general, soil Ca had the lowest variability among soil nutrients (CV = 18%), whereas soil Mn showed the highest variability (CV = 94%). According to the recommended standards for citrus soils in Morocco (Table 2), soil available Ca levels (5.3–9.7 g kg−1), K levels (144–540 mg kg−1), and Mg levels (0.3–2 g kg −1) were above the optimum range in all orchards. Most orchards (65%) had optimum to high levels of soil P (7.5–59.1 mg kg−1) and Mn (2.7–70.9 mg kg−1) and optimum levels of TN in 80% of the orchards. Soil content of available Zn (0.1–3.8 mg kg−1) was in the optimum range in 60% of the orchards and deficient in the rest. Additionally, suboptimum levels were observed for available Fe (1.7–27.3 mg kg−1) and Cu (0.1–3.8 mg kg−1) in soils of 90% and 60% of the orchards, respectively.

3.2. Leaf Nutrient Status

Leaf concentrations of macro- and micronutrients were summarized in Table 3. The concentrations of all macronutrients (N, P, K, Ca, and Mg) and Fe showed low variation across orchards, while the concentrations of the three micronutrients (Zn, Mn, and Cu) exhibited high variation with the maximum recorded for Zn (CV = 75%). According to the optimum range recommended by Lekchiri [11] for Valencia orange in Morocco, almost 90% of the orchards had suboptimum levels of nitrogen. Nitrogen concentration varied from 6.8 to 21.1 g kg−1. Leaf concentration of P (0.9–2.2 g kg−1) was mostly above the optimum range, showing moderately high levels in 35% of the orchards and very high levels in 60% of the orchards. Potassium concentration ranged from 2.8 to 9 g kg−1, with 65% of the orchards having optimum to high levels. Interestingly, 35% of the orchards had low to moderately low levels of K. Leaf Ca status (46–96 g kg−1) was above the optimum range in 65% of the orchards. Magnesium (0.5–1.4 g kg−1) was deficient in leaves of all orchards (Table 3).
With respect to micronutrients, Fe concentration in leaves varied within a narrow range (92–124 mg kg−1) across the studied orchards. Meanwhile, higher variability was observed for Zn, Mn, and Cu. Copper varied mainly in the range of 7.3–20.6 mg kg−1, with only one orchard exhibiting a largely high concentration of 36.9 mg kg−1. Zinc and Mn concentrations were in the range of 2.9–38.9 mg kg−1 and 2.5–63.3 mg kg−1, respectively. Based on leaf micronutrient standards recommended for Valencia orange in Morocco [11], Fe content was below the optimum range in all orchards, and most orchards (80%) had low to moderately low content of Zn. Manganese was found within the optimum range in 30% of the orchards and moderately high to very high in 40% of the orchards. Most of the orchards (80%) showed moderately high to very high leaf concentrations of Cu, with the rest of the orchards (20%) in the optimum range.

3.3. Effects of Orchard-Related Factors on Soil and Leaf Nutrients

Apart from soil Fe and leaf K, the concentration of other soil and leaf nutrients did not show any significant differences with respect to irrigation, rootstock, planting age, tree density, and farm size types. The concentration of soil Fe was significantly higher in large (10.9 ± 2.3) than in small (6.1 ± 1.8) farms (Mann–Whitney, p = 0.035). Similarly, leaf K was significantly higher in the large (5.9 ± 1.5) than the small (4.3 ± 0.4) farm size group (Mann–Whitney, p = 0.015). These results may indicate differences in Fe and K fertilization management practices or other cultivation practices that influenced the nutrient status of these elements between small and large farm sizes. Soil P and K did not differ significantly between old and young orchards but showed a significant positive correlation with the planting age (P: rs = 0.45, p < 0.05; K: rs = 0.49, p < 0.05), suggesting a possible accumulation of these nutrients with supplemental fertilization along years.
It is worth noting that soil and leaf nutrient concentrations in the studied orchards may also have been affected by the interaction between orchard factors rather than a single factor. We were unable to examine these interactive effects considering the smallness of our sample size and the observational nature of this study. Nevertheless, the results of MCA analysis indicated that planting age, rootstock, and farm size were the most influential categorical variables in the overall variation of the dataset. Planting age was closely related to the first axis (R2 = 0.754, p < 0.0001), while the farm size was related to the second axis (R2 = 0.406, p < 0.01). Rootstock correlated with both axes (MC1: R2 = 0.613, p < 0.01; MC2: R2 = 0.532, p < 0.01). The MCA first and second axes explained 52% of the total variance, and the projection of orchards in the MC1–MC2 plan discriminated two main opposite categories: young orchards using Macrophylla rootstock under drip irrigation and high planting density and belonging to large farm size vs. orchards with older planting age using sour orange rootstock under flood irrigation and low density and belonging small farm size (Supplementary Figure S1).

3.4. Relationships Between Soil Nutrients, Leaf Nutrients, and Other Soils Properties

The PCA of nutrient concentrations in soils showed that soil PC1 was strongly and negatively correlated with soil Mg and positively correlated with TN and Fe (Figure 2a). The PC1 had also a moderate positive correlation with soil Mn. These results align with the highly significant negative correlation (p < 0.01) observed between soil Mg and TN and Mn (Figure 3). The PC2 was positively related to increases in soil available K, P, Cu, and Zn (sorted from the strongest correlation) and decreases in Mn (Figure 2a). The concentrations of K and P and Cu and Zn were also significantly correlated (p < 0.05) among themselves (Figure 2a and Figure 3). Overall, the PC1–PC2 plan showed an opposite direction between soil concentrations of P, K, and Mg and the concentration of Mn. On the other hand, soil PC1 did not show any significant correlation with soil properties, whereas soil PC2 was significantly positively associated with soil OM (Figure 3 and Figure 4A). This is in accordance with the strong positive correlation observed between soil OM, soil K (rs = 0.78, p < 0.001), and soil P (rs = 0.74, p < 0.001). These two nutrients constitute in fact the main variables contributing to the PC2. Considering soil PC3, which is mainly related to soil P, we found this axis to be negatively linked to soil pH.
The PC analysis of leaf nutrients indicated that leaf PC1 was positively related to the concentration of leaf Mn, Fe, and Zn on the positive side and to leaf Mg, Ca, and TN on the negative side (Figure 2b). The correlation between PC1, leaf Mg, and Mn was very strong, while it was moderate with the other variables. The PC2 was strongly negatively linked to the concentration of leaf K, which was the only variable to have a loading value higher than 0.5 on the PC2 axis. The PC3 was also only significantly and strongly positively correlated with leaf Cu. According to the PC1–PC2 plan, it appears that orchards with high concentrations of leaf Mn and K tend to have low concentrations of Mg, and an increase in the concentration of Ca in leaves would be associated with a decrease in leaf Fe and Zn concentrations. This was also confirmed by the results of the bivariate correlation summarized in Figure 3. The projection of variables in the PC1–PC3 plan indicated a peculiar pattern of leaf Cu, which had no correlation with other leaf nutrients (Figure 2 and Figure 3). No soil properties correlated with either leaf PC axis, indicating a low direct influence of soil properties on leaf nutrient status (Figure 3).
When analyzing the relationship between soil nutrients and leaf nutrients, we found that soil main PC axes were not significantly linked to leaf main PC axes (Figure 3). Additionally, no significant correlation was detected between soil nutrients and their respective concentrations in leaves, and only a few weak to moderate significant correlations were observed between some soil and leaf nutrients, namely, soil K and leaf Zn (rs = 0.45, p < 0.05), soil P and leaf K (rs = −0.52, p < 0.05), soil Zn and leaf Mg (rs = −0.57, p < 0.01), and soil Mn and leaf Fe (rs = −0.67, p < 0.01). This suggests that the concentrations of nutrients in the soil had a marginal influence on their respective concentrations in leaves, while the interaction between some soil and leaf nutrients may significantly influence their status in orange orchards.

4. Discussion

Soils of the studied Valencia orange orchards were mostly alkaline and calcareous with low to moderate levels of organic matter. These are common features of soils in major citrus growing areas of Morocco and the Mediterranean region, which present a high risk of reduced availability of essential nutrients for citrus crops [28]. Referring to the recommended standards in Morocco, suboptimum levels of available Fe and Cu were observed in most soils of the studied orchards. Most soils were in the recommended range for N and had sufficient levels of available P, Mn, and Zn. All soils were sufficiently supplied with available Ca, Mg, and K, which is consistent with its calcareous nature and mineralogy very rich in K-bearing minerals like illite [29]. In accordance with soils, leaf samples indicated the prevalence of adequate to very high levels of P, Ca, Mn, and K in the studied orchards. Leaf Fe status was below optimum levels in most orchards, which is in line with the observed low levels in soils. Nevertheless, unlike soils, leaves did not show any deficiency of Cu; instead, most orchards had adequate to excessive levels of this micronutrient. Additionally, leaf Mg, N, and Zn status were deficient in most orchards, conversely to that of soils.
Nitrogen limitation revealed through leaf analysis is unsurprising and was reported in previous surveys [25,26,30,31]. It also indicated that total N levels in soils did not reflect the actual N status in trees, as suggested by the absence of relationships between soil and leaf N concentration. Limited availability of soil N to support tree needs is expected in our study area given the limited reserves of potentially mineralizable N due to the inherently low content of organic matter [31]. For this reason, N application through organic and mineral fertilizers was always highly recommended to support the N needs of citrus trees in Morocco [30]. Apart from organic fertilizers applied periodically (once every year to once every four years), NPK-mineral fertilizers were applied routinely in all selected orchards. Quantitative data on the applied rates of fertilizers were not collected during our survey. However, a recent study [13] conducted in the Gharb plain showed that the application rate of N is generally high in citrus orchards reaching over 300 kg of N/ha/year. Hence, rather than an insufficient N supply from fertilizers, the prevailing suboptimal levels of N are likely attributed to unbalanced fertilization, lowering the responsiveness to added N and, consequently, increasing its loss through volatilization and leaching.
Conversely to N, the absence of suboptimal levels of P in leaves suggests an overall satisfying P fertilization that has covered the potentially low supply from soils in some orchards. However, P fertilization seems also to be excessive in more than half of the orchards, which would increase the likelihood of nutrient imbalances and, in turn, negatively affect their productivity. Based on soil and leaf samples collected on a large number of citrus orchards several years ago in the Gharb plain, El Ayadi et al. [26] recommended the reduction in P and K fertilizer rates and even the non-application of these nutrients where they could be well supplied by soils. This still seems to apply to P status in more than half of the studied orchards, underlining the need for further attention to avoid overfertilization by phosphorus. However, in the case of K, our study showed that 35% of the orchards may suffer from K deficiency despite sufficient levels in soils. There is widespread agreement that high levels of Ca and Mg can reduce the K content in leaves to the point of deficiency [32,33,34,35]. This competitive interaction between K, Ca, and Mg results from an imbalance in the concentration of these nutrients, which is the likely explanation for what has occurred in the orchards with suboptimal K leaf content.
Accordingly, the observed Mg deficient levels in leaves despite the sufficient status in soils may have resulted from the K–Mg and Ca–Mg antagonism. It was suggested that Mg deficiency can be anticipated when Mg:K and Mg:Ca ratios in soil are respectively lower than 3.5 and 0.14 [36,37]. In the present study, the calculated Mg:K ratio was below the critical value in half of the orchards, whereas the Mg:Ca ratio exceeded the critical value in most of the orchards (70%). This result indicates that Mg–K antagonism is more likely to contribute to the observed Mg deficiency than Mg–Ca antagonism in our study area. This is further supported by the negative correlation found between Mg and K in leaves (Figure 2 and Figure 3), while no correlation was found between Mg and Ca. Such negative correlation also provides evidence that the antagonism between K and Mg occurred in our conditions leading to a reduced Mg uptake and translocation within the plant. The low leaf Mg in our study could also be the consequence of competition with other cations than K and Ca. In fact, the negative correlation we observed between leaf Mg and leaf Mn soil Mg and soil Mn (Figure 2 and Figure 3) seems to lend support to this assumption. Similar antagonistic interaction was also reported in previous studies [38,39,40]. Magnesium deficiency induced by competing cations is thus a prevalent phenomenon in our study area rather than absolute deficiency. During our survey, we often observed the presence of foliar symptoms of Mg deficiency in leaves (Supplementary Figure S2). Generally, visual symptoms of deficiency appear when the lack of nutrients is severe. Therefore, the application of Mg-containing fertilizers is necessary while ensuring a balanced supply of all required nutrients, particularly Mn, K, and Ca.
With regard to micronutrients, Fe and Zn were the most limited as indicated by soil and leaf analyses. The limiting status of Zn and Fe aligns with the previously reported incidence of Zn and Fe deficiency in citrus across the different growing areas of Morocco [26,31,41,42,43,44,45]. However, unlike these studies where Fe deficiency was considered of minor importance, our results showed that it could be a serious concern for citrus production along with Zn deficiency if not managed appropriately. Visual symptoms of Fe and Zn deficiency were also frequently observed on leaves from the surveyed orchards (Supplementary Figure S2). This is in agreement with the fact that Fe and Zn are the micronutrients mostly involved in nutrient deficiencies for citrus in Mediterranean countries and worldwide [10,46]. Despite the observed accordance between soil and leaf Fe and Zn status, we did not find any significant correlations between their concentrations in soils and leaves, suggesting the involvement of other factors in inducing their suboptimum status in leaves. In the case of Fe, we observed a negative significant correlation between leaf Fe and soil Mn (Figure 3), which may indicate an antagonistic interaction between these two cations. According to Rietra et al. [35], such antagonism might be associated with the inhibition of the ferric-chelate reductase activity due to a high content of Mn and other metals. With regard to Zn, no clear evidence was provided by our results to suspect an induced suboptimal Zn nutrition due to nutrient imbalances.
The status of the two remaining micronutrients (Mn and Cu) should be considered with caution given the risk of potential toxicity. Oppositely to Fe and Zn, Mn and Cu were mostly sufficient to high, and even excessive in the case of Cu, according to leaf analysis. Manganese sulfate is generally applied to citrus in our study area and was commonly recommended to correct the widely reported incidence of Mn in different growing areas of Morocco [25,31]. Copper sulfate is also widely applied to citrus [30,47], in addition to regular sprays with Cu-containing fungicides, which may explain the high to excessive levels in leaves recorded in our study [48]. This points out the need for a more integrated approach to managing citrus orchards which considers the indigenous supply provided by soils and the various exogenous sources of nutrients applied in the field (e.g., plant protection products containing mineral nutrients, and irrigation water).
Such an approach should also account for the complexity of interaction occurring in the soil–tree system of citrus orchards and the involvement of several factors in influencing the dynamic of nutrients and consequently their status. This was obvious through the discrepancy we observed for some nutrients between their status in soils and leaves and the lack of correlation between nutrient concentrations in soils and their respective concentrations in leaves (Figure 3). This lack of correlation was often reported for tree crops, including citrus [7,8,9,10]. As suggested by these authors, it could be attributed to several factors related to soil (e.g., soil properties, nutrient mobility and interrelation in soil, soil sampling), plant (e.g., nutrient mobility and interrelation in the plant, variety, scion–rootstock interaction), management practices, and others. In this study, we found that among the seven soil properties evaluated, only soil OM and pH were significantly linked to the concentration of soil nutrients. The increase in soil OM was associated with an increase in soil P and K, while the pH increase was associated with a decrease in soil P. On the other hand, no meaningful relationship was detected between any of the soil properties and leaf nutrients. These results are supported by other studies conducted in different countries and different tree crops [49,50].
Through this study, we were also able to examine the influence of orchard-related factors on the concentration of nutrients in soils and leaves. It seems that differences do exist in fertilization management between small and large farm sizes, which is expected. The planting age and grafting rootstock were found to have an influence on the concentration of nutrients. It is worth noting that soil and leaf nutrient concentrations in the studied orchards may also have been affected by the interaction of various factors rather than a single factor. We were unable to examine these interactive effects considering the smallness of our sample size and the observational nature of this study.

5. Conclusions

The present study confirms again the complexity of factors affecting the nutrient status of citrus orchards, which renders its monitoring necessary through a combination of both soil and leaf analyses. These latter revealed that N, Mg, Fe, and, to some extent, Zn were the most limiting nutrients in the surveyed Valencia orange orchards. Additionally, high levels of Cu, P, Ca, K, and Mn were also observed through leaf analysis. Hence, expected, multi-nutritional disorders were detected in our study area. The concern is, indeed, not limited only to the risk of nutrient deficiency but also excess, particularly for Cu. In addition, our results showed that soil properties, namely, texture (clay, sand, and silt), active CaCO3, organic matter, and pH were closely related to the concentrations of some nutrients in soils and leaves. Yet, their influence on soil and nutrient status was not supported by our results, contrary to what was stated in our second hypothesis. On the other hand, the nutrient imbalance was found to heavily impact the nutrient status of citrus orchards studied herein. High levels of some nutrients led to an antagonistic effect, which likely reduced tree uptake of other nutrients despite their sufficient status in soil (e.g., the case of Mg). We suspect unbalanced fertilization to be one of the main contributing factors to such nutrient disorders. Unfortunately, we did not collect quantitative data to thoroughly examine this assumption. This was beyond the scope of this study, and it remains a topic for future investigations. We should also acknowledge that given the observational nature of this study and the small number of surveyed orchards, it was difficult to fully describe and evaluate the effect of management practices and orchard characteristics on soil and leaf nutrient status.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy14123040/s1, Table S1: General characteristics and management practices in the 20 surveyed orchards of Valencia Late orange in the Gharb plain of Morocco; Figure S1: Multiple correspondence analysis (MCA) of surveyed orchards according to irrigation type (drip and flood); Figure S2: Frequent deficiency symptoms observed during leaf sampling in the studied orchards of the Gharb plain in Morocco.

Author Contributions

Conceptualization, R.B., M.I., H.B. and Z.A.; data curation, R.B.; formal analysis, R.B.; investigation, R.B. and A.M.B.; methodology, R.B., A.M.B. and Z.A.; resources, R.B. and R.A.; software, R.B. and Z.A.; supervision, M.I. and Z.A.; validation, R.B., M.I. and Z.A.; visualization, M.I., H.B. and Z.A.; writing—original draft, R.B.; writing—review and editing, R.B. and Z.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Citrus-Megaproject Program at INRA Morocco.

Data Availability Statement

Data is contained within the article and supplementary material.

Acknowledgments

We are deeply grateful to Hamid Bentaleb, may God rest his soul in peace, for his invaluable assistance in the selection of citrus growers. We also thank all the growers who allowed us to work in their orchards. We extend our heartfelt gratitude to Hassan Benaouda and the entire administrative team, as well as the URACRP staff at the Regional Center of Agricultural Research of Kenitra, for their invaluable support. We are also thankful to Ahmed Douaik for his assistance with statistical analysis of our data.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the study area at Gharb plain in the northwestern of Morocco.
Figure 1. Location of the study area at Gharb plain in the northwestern of Morocco.
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Figure 2. Correlation circle of the principal component analysis of soil (a) and leaf (b) nutrient concentrations. PC1 and PC2 represent the first and second main axes.
Figure 2. Correlation circle of the principal component analysis of soil (a) and leaf (b) nutrient concentrations. PC1 and PC2 represent the first and second main axes.
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Figure 3. Matrix illustrating the correlation coefficient (Spearman’s ρ) for soil properties, soil and leaf nutrient concentrations, and soil (S1, S2, S3) and leaf (L1, L2, L3) PCA three main axes. Blue and red colors indicate positive and negative significant correlations, respectively. High correlation coefficients are indicated with dark colors.
Figure 3. Matrix illustrating the correlation coefficient (Spearman’s ρ) for soil properties, soil and leaf nutrient concentrations, and soil (S1, S2, S3) and leaf (L1, L2, L3) PCA three main axes. Blue and red colors indicate positive and negative significant correlations, respectively. High correlation coefficients are indicated with dark colors.
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Figure 4. Bivariate correlations between soil organic matter and soil PC2 axis (A) and soil pH and soil PC3 axis (B). * p < 0.05 and ** p < 0.01.
Figure 4. Bivariate correlations between soil organic matter and soil PC2 axis (A) and soil pH and soil PC3 axis (B). * p < 0.05 and ** p < 0.01.
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Table 1. Mean values (±SE) of physicochemical properties of soils collected at 25 cm depth from the 20 surveyed orchards.
Table 1. Mean values (±SE) of physicochemical properties of soils collected at 25 cm depth from the 20 surveyed orchards.
Soil Physical PropertiesMean ± SE
pH8.28 ± 0.06
Electrical conductivity (EC) (mS/cm)0.29 ± 0.03
Organic matter (OM) (%)1.93 ± 0.15
Total CaCO3 (%)11.32 ± 1.51
Active CaCO3 (%)5.69 ± 0.78
Sand (%)11.86 ± 2.07
Clay (%)24.83 ± 2.86
Silt (%)63.30 ± 2.29
Table 2. Concentration of macro- and micronutrients (mean ± SE) in soils collected at 25 cm depth from the 20 surveyed orchards in the Gharb plain of Morocco.
Table 2. Concentration of macro- and micronutrients (mean ± SE) in soils collected at 25 cm depth from the 20 surveyed orchards in the Gharb plain of Morocco.
Soil ParametersMean ± SE (CV%)Optimum *Interpretation **
Total N (g kg−1)1.01 ± 0.07 (33%)0.6–1.6Low to optimum
Available P (mg kg−1)21.5 ± 3.1 (65%)13–22Low to high
Available K (mg kg−1)316.4 ± 27.7 (39%)75–375Optimum to high
Available Ca (g kg−1)4.5 ± 0.18 (18%)1.8–3.6High
Available Mg (g kg−1)0.96 ± 0.10 (53%)0.1–1.3Optimum to high
Available Fe (mg kg−1)8.48 ± 1.51 (80%)20–150Low to optimum
Available Zn (mg kg−1)1.82 ± 0.25 (60%)1.8–10Low to optimum
Available Cu (mg kg−1)1.34 ± 0.18 (60%)1.4–5.8Low to optimum
Available Mn (mg kg−1)19.0 ± 3.99 (94%)10–20Low to high
* Optimum range in soil nutrients recommended for Valencia Late orange in Morocco according to [12,25,26,27]. ** Interpretation based on the range (minimum to maximum value) of nutrients observed in all orchards. SE and CV denote standard error and coefficient of variation, respectively.
Table 3. Concentration of macro- and micronutrients (mean ± SE and range) in leaves collected from the 20 surveyed orchards in the Gharb plain of Morocco.
Table 3. Concentration of macro- and micronutrients (mean ± SE and range) in leaves collected from the 20 surveyed orchards in the Gharb plain of Morocco.
Mean ± SE (CV%)Optimum *Interpretation **
Macronutrients (g kg−1)
Total N13.8 ± 0.11 (4%)20–23.5Very low to optimum
Total P1.6 ± 0.01 (2%)0.9–1.1Optimum to very high
Total K5.1 ± 0.03 (3%)4.5–6.5Low to high
Total Ca70.7 ± 0.26 (2%)53–67Moderately low to very high
Total Mg1.0 ± 0.00 (2%)3–4Very low to low
Micronutrients (mg kg−1)
Total Fe109.5 ± 1.92 (8%)125–175Moderately low
Total Zn11.9 ± 1.99 (75%)16–28Low to moderately high
Total Cu16.1 ± 1.53 (50%)6.6–8.8Optimum to very high
Total Mn24.4 ± 3.33 (61%)14–27Moderately low to very high
* Optimum range recommended by [11] in Morocco for leaves of Valencia Late collected from non-fruiting terminals. ** Interpretation based on the range (minimum to maximum value) of nutrients observed in all orchards. SE and CV denote standard error and coefficient of variation, respectively.
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Brital, R.; Ibriz, M.; Benmrich, A.M.; Benyahia, H.; Aboutayeb, R.; Abail, Z. Soil and Leaf Nutrient Status of Selected Valencia Orange Orchards in the Gharb Plain of Morocco. Agronomy 2024, 14, 3040. https://doi.org/10.3390/agronomy14123040

AMA Style

Brital R, Ibriz M, Benmrich AM, Benyahia H, Aboutayeb R, Abail Z. Soil and Leaf Nutrient Status of Selected Valencia Orange Orchards in the Gharb Plain of Morocco. Agronomy. 2024; 14(12):3040. https://doi.org/10.3390/agronomy14123040

Chicago/Turabian Style

Brital, Rania, Mohammed Ibriz, Ahmed Mansour Benmrich, Hamid Benyahia, Rachid Aboutayeb, and Zhor Abail. 2024. "Soil and Leaf Nutrient Status of Selected Valencia Orange Orchards in the Gharb Plain of Morocco" Agronomy 14, no. 12: 3040. https://doi.org/10.3390/agronomy14123040

APA Style

Brital, R., Ibriz, M., Benmrich, A. M., Benyahia, H., Aboutayeb, R., & Abail, Z. (2024). Soil and Leaf Nutrient Status of Selected Valencia Orange Orchards in the Gharb Plain of Morocco. Agronomy, 14(12), 3040. https://doi.org/10.3390/agronomy14123040

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