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Establishing regional reference values for nutrients fills an important research and extension gap and can contribute to optimizing fertilizer use, increasing yield, and reducing production costs and environmental impacts. In the present study, with the objective of determining reference values and the optimal ranges of nutrients, 88 olive orchards (differing in yield performance) located in six provinces of Iran (Fars, Zanjan, Qazvin, Gilan, Isfahan, and South Khorasan), which were managed under an integrated soil fertility and plant nutrition management system, were investigated during 2020 and 2021. Since at least 30 pilot orchards are required for calculating nutrient reference values for each dataset entered into the model, and in most provinces the selected number of pilot orchards was lower than this threshold, provinces and similar regions were clustered into groups based on the national agro-climatic zoning map. Accordingly, olive orchards in Isfahan, Zanjan (Tarom), Qazvin (Lower Tarom), Gilan (Roudbar), and South Khorasan (Khusf), with a total of 48 pilot orchards, were categorized as belonging to the semi-arid temperate agro-climatic class. The province of Fars, however, was evaluated separately and independently during the two years of study, because it provided a sufficient number of pilot orchards (40 orchards). The dominant olive cultivars in these pilot orchards were ‘Zard’ and ‘Roghani’. In each pilot orchard, 30 trees of approximately 15 years old were marked. Leaf sampling was performed in mid-August from current-year non-fruiting shoots. A composite leaf sample was prepared from each of the 30 selected trees, collected from non-fruiting shoots across all directions, and transferred to the laboratory for analysis. The concentrations of nitrogen, phosphorus, potassium, iron, manganese, copper, zinc, and boron were determined. At harvest, fruit yield was estimated for each orchard based on the average yield of the 30 sampled trees. Finally, the methods of Compositional Nutrient Diagnosis (CND) and Deviation from Optimum Percentage (DOP) were applied to determine nutrient reference values and to assess the nutritional balance/imbalance in olive orchards of the studied provinces. The results for Fars province showed that, among 80 orchards evaluated across two years, 6 orchards were classified as high-yielding, while 74 orchards fell into the low-yield category. The CND-derived nutrient reference values for Fars olive orchards were: N = 2.31 ± 0.066%, P = 0.14 ± 0.088%, K = 1.58 ± 0.066%, Ca = 3.1 ± 0.074%, Mg = 0.60 ± 0.055%, Mn = 58.50 ± 0.175 mg kg⁻¹, Zn = 24.17 ± 0.179 mg kg⁻¹, Fe = 220 ± 0.109 mg kg⁻¹, Cu = 6.17 ± 0.163 mg kg⁻¹, and B = 65.50 ± 0.124 mg kg⁻¹. Based on nutrient indices, Zn, Cu, Mn, and Ca were balanced, whereas P, B, Fe, K, Mg, and N were found in excess.
For the semi-arid temperate agro-climatic class (48 orchards from multiple provinces), the CND reference values were: N = 1.31 ± 0.35%, P = 0.09 ± 0.39%, K = 1.63 ± 0.39%, Ca = 1.50 ± 0.25%, Mg = 0.11 ± 0.055%, Zn = 14.43 ± 0.205 mg kg⁻¹, Fe = 36.102 ± 2.45 mg kg⁻¹, Cu = 11.45 ± 0.49 mg kg⁻¹, and B = 27.35 ± 0.197 mg kg⁻¹. Nutrient indices indicated that P, Mg, Fe, Cu, and B were balanced, while N, K, Ca, and especially Zn were excessive.
Applying the DOP method in Fars province revealed that the highest management priority should be correcting N and P deficiencies across most orchards, followed by balanced supply of Ca and Mg, then addressing widespread deficiencies of Zn and B, and finally, managing K based on soil and leaf analysis rather than uniform application across all orchards. A key recommendation for this region is to shift nutrient management from generalized approaches to site-specific management, as a uniform prescription for all orchards proved ineffective.
In the semi-arid temperate class, DOP results showed that N and K were the most limiting nutrients, with widespread deficiencies across most orchards. Ca and Mg displayed large fluctuations, being deficient in some orchards and excessive in others. Among micronutrients, Fe often appeared excessive or imbalanced, Zn and Cu were frequently above optimum levels, while B showed deficiency in some orchards. Interestingly, orchards with medium yield (3,000–3,500 kg ha⁻¹) usually exhibited better nutrient balance (i.e., lower ΣDOP values), suggesting that nutrient balance is more important than absolute concentrations.
Overall, correcting N and K deficiencies, managing the balance of Ca and Mg, and controlling micronutrient applications, particularly Zn and Cu, should be prioritized for olive nutrition management in the studied regions. A key insight from this study is the apparent contradiction between the interpretation of some key nutrients when comparing CND and DOP methods. This discrepancy reflects fundamental philosophical and computational differences between the two diagnostic tools. Specifically, CND is based on compositional analysis and evaluates the balance and ratios of all nutrients within the closed system of the leaf, without considering absolute concentrations. By contrast, DOP helps identify the nature of imbalance (true deficiency vs. true excess). This integrated approach prevents superficial interpretations and leads to more precise and balanced fertilizer recommendations, which was the main aim of this study.
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