Olive tree (Olea europaea subsp. Europaea var. Europaea L.) is likely the oldest cultivated tree in approximately seventy-seven countries worldwide (El Yamani and Cordovilla, 2024), covering an area of over 11 million hectares (IOC. International Olive Council, 2022). In Europe, it spans the entire Mediterranean region where it originated and remains the dominant cultivated tree (IOC. International Olive Council, 2003).

These vast cultivation areas and conventional agricultural practices have led to intense soil degradation processes (Porta et al., 1999), particularly due to water erosion (Lima et al., 2023), excessive tillage (Aguilera-Huertas et al., 2022), and overuse of chemical fertilizers (Sutton et al., 2013). These processes produce significant losses of organic carbon, nitrogen (Abid and Lal, 2008), soil biodiversity (Morgado et al., 2020; Morgado et al., 2022), fertility (Porta et al., 2008; Domouso et al., 2024), and degradation of soil properties (García-Orenes et al., 2012). Some authors agree that the use of vegetative covers improves soil health (Espejo-Pérez et al., 2013; Sastre et al., 2017).

Integrated production is an agricultural system for food production that optimizes resources and natural production mechanisms to ensure long-term sustainable agriculture. The goal is to use cultivation techniques compatible with social needs, environmental protection, and agricultural production (BOJA, 2008). The Integrated Olive Production Regulation in Andalusia (Junta de Andalucía, 2016) outlines a set of agronomic practices classified into various areas, including fertilization and soil management. In the fertilization section, it is stated that mineral fertilization should take into account crop extractions, soil fertility levels, plant nutritional status, and contributions from other sources (water, organic matter, etc.). To meet these objectives, foliar analyses are required in the first half of July.

Leaf nutrient analysis, which is used to determine the status and fertilization needs of plants, has been widely accepted as a good method for diagnosing deficiencies such as nitrogen (Fernández-Escobar et al., 2011). However, this method is not effective for detecting excess nutrients caused by over-fertilization (Weinbaum et al., 1992). Moreover, changes in crop management can affect nutrient dynamics and reference values. Therefore, integrated production requires periodic reviews of the analyses methods and reference values.

Sap is considered a precise medium for determining plant nutrients (Esteves et al., 2021). However, its use has not been widely adopted because of limitations in the extraction and measurement methods (Esteves et al., 2021). In recent years, various studies (Carella et al., 2016) have helped establish nutrient levels in the xylems of different plants, identifying dynamics related to nutrient supply, plant water status, and soil type. Some authors (Do Amarante et al., 2006) studied nitrogen and amino acid content in the xylem of various legumes under controlled conditions; Cabañero and Carvajal (2007) studied K, Mg, and Ca levels in xylem samples from Capsicum annuum L. using atomic absorption spectroscopy. More recently, Larbi et al. (2010) used xylem samples to study Fe levels in B. vulgaris L. plants. Guérin et al. (2007) used xylem samples from L. ovalifolium Hassk collected in spring to study nutrient mobility and the interaction of N and C compounds in plants with and without fertilization. In olives, xylem morphology is associated with plant–water availability (Bacelar et al., 2007; Rousseaux et al., 2009).

The aim of this study was to assess the effectiveness of sap extraction using a modified Scholander-Hammel chamber for quickly determining the nutritional status of olive trees depending on soil management, as a complementary method to foliar and soil analyses. To achieve this goal, we compared the nutrient concentrations in sap, leaves, and soil from Picual and Hojiblanca olive trees, which are cultivated under integrated production in different provinces of southern Spain, over several years.

To understand plant nutrition in general, and olive tree nutrition in particular, it is essential to understand the availability of nutrients in the soil, their concentration in the leaves, and the mobility of elements in the sap. The transport of water and solutes in the tree occurs through the xylem and phloem systems (White and Ding, 2023). Differentiating between xylem and phloem is not straightforward because the exchange of solutes between them is crucial for regulating long-distance transport (White and Ding, 2023). Therefore, comparing the evolution of nutrient concentrations in the soil and leaf is fundamental to understanding what happens in the sap and the overall nutrition of trees.

The pH in sap is very stable, and although it is sensitive to changes related to the application of different anions or cations, these changes are small and quickly reversed (Larbi et al., 2003). In this study, the pH of sap was higher in the first study period (2018–2019) than in the second period (2022–2024), although the values were around 7. In contrast, electrical conductivity was higher in the second study period (2022–2024), coinciding with a drop in pH (with minimum values of pH reaching 5.5) at times. The continued decrease in precipitation from the first (2018–2019) to the second study period (2022–2024) forced farmers to reduce irrigation doses and use lower-quality water. Additionally, some farmers added amino acids and algae as supplementary treatments to strengthen the trees against drought. This also explains the changes in pH and electrical conductivity in soil, which is also a buffered medium (Porta et al., 1999).

The evolution of leaf and sap nutrients was estimated during the first study period (2018–2019), when precipitation was abundant and the trees were in full production. This allowed the study of changes in nutrient concentrations throughout the activity periods of the plantations. The relationship between sap conduction and periods of drought has been reported in other studies (Terral et al., 2025).

The evolution of leaf nutrients studied in 2018 was repeated without significant differences in the same quarters of 2019. The yields during these 2 years (data not presented) were very similar. Some elements such as P, B, and K show different dynamics compared to those observed by other authors (Fernández-Escobar et al., 1999; Nieto et al., 2017). However, other elements such as N, Mg, Ca, Mn, and Zn maintain the same dynamics. This is primarily due to changes in olive fertilization management in recent years. For instance, for N, the evolution of its concentration in leaves has not changed because it was the element that was most carefully applied and dosed. Other elements like K or P no longer decrease in concentration throughout the year, as previously indicated in earlier studies, but their concentrations remain high in leaves for most of the year, with dynamics similar to those of N. On one hand, K plays a fundamental role in nitrate transport in the xylem (Van Beusichem et al., 1988; White, 2012), which is evident in our study by the high similarity of N and K dynamics in leaves (Figure 2); on the other hand, these nutrients are currently applied in various and continuous ways: to the soil, in the center of the row, as foliar fertilizers in March, May, June, and December, and in small doses via fertigation throughout most of the year. Additionally, foliar applications prevent K sequestration by soil colloids, especially clays, enhancing its immediate availability throughout the year (Rodrigues et al., 2012). Similarly, studies that have increased P application in the soil have frequently concluded that there is no response from trees or production to these additions (Fernández-Escobar et al., 2017). The type of soil plays a crucial role in this element (Porta et al., 1999). In this study, the farm with the lowest K and P values in the soil (Osuna) also had the lowest concentrations of both sap and leaves during both study periods (Figure 5). However, very high K and P values in the soil, as seen in Luenga and Guadiana (Table 2; Figure 5), do not correlate with higher concentrations in sap and leaves (Figure 5). The high CaCO₃ and/or clay content in these three farms (Table 2) were the factors highlighted by various authors to explain the availability of these elements. Recently, other authors (Ferreira et al., 2018) have reported that when experiments are carried out in inert substrates, a response to P application in the soil is observed.

One element that well reflects this management change is B, whose evolution in previous studies has been described as a progressive decrease in leaf concentration with age, moving toward flowers and fruit (Nieto et al., 2017). However, it is now commonly applied as complexes of B, Mg, and S or as sodium borate via foliar application, which maintains more stable concentrations of B, S, and Na (Figure 2). The three elements had their lowest concentrations in January, February, and March of 2019 (point 4, Figure 2) and in the following quarter of both years (points 1 and 5, Figure 2), after harvest, and before the spring foliar application. The transport of B from the leaf to the fruit has been noted by several authors (Rodrigues et al., 2012; Başar and Gürel, 2016).

Ca and Mg have not changed their application as fertilizers or alongside other fertilization practices, and they share similar dynamics throughout both study periods. These elements had the highest concentrations in leaves and sap, as well as in soils where their concentrations were also higher (Figure 5; Table 2), such as Luenga (L), Guadiana (G), and Osuna (O). No significant differences were observed between the two study periods for either element, but in general, the standard deviations were higher in the second period (2022–2024, Figure 5), especially on the three previously mentioned farms, likely due to differences in precipitation and irrigation water quality during the two periods on these farms.

The availability of Fe, Mn, Cu, and Zn is influenced by soil characteristics such as pH and calcium carbonate content (Marschner, 1993). The dynamics of these elements are very similar to each other (Figure 2) and, in general, higher in the second study period (2022–2024) in leaves, and especially in sap (Figure 6), compared to the first period (2018–2019). Başar and Gürel (2016) noted that these micronutrients, particularly Zn, are transferred from the leaf to the fruit, which could explain the decrease in Zn concentration in leaves in the third and fourth quarters of both years, when the fruit requires it most (Figure 2). This would also explain the decreases in Mn and Zn in the leaves and the increase in sap during the same period (Figure 6). Furthermore, the similarity in Cu dynamics in both years (Figure 6), which is only applied to leaf and sap, seems to confirm these results.

The correlations between nutrient concentrations, pH, and EC in leaf, sap, and soil are presented in Supplementary Figures S1–S3 (Supplementary Material). In general, the most frequent correlations are observed within the same matrix (leaf-to-leaf, sap-to-sap and soil-to-soil), especially in sap. Nutrient correlations between sap and soil are positive but statistically insignificant (R² < 0.5). Additionally, the correlations between pH, EC, Mn, and Cu in soil are negative with respect to other elements, both in soil and sap.

These results are likely influenced by the sampling design, where fertilization practices by farmers were not considered. A study with more rigorously controlled conditions would likely yield more robust correlations.

In order to know the nutrition of olive trees and to carry out a fertilisation that generates an adequate production while protecting the environment, it is convenient to know the availability of nutrients in the soil, their concentration in the leaves and the mobility of the elements in the sap. Water and nutrients in the sap are transported from the soil by the xylem and then distributed through the phloem according to the nutritional needs related to the phenological stages of the tree.

The dynamics of the concentrations of many elements in leaf and sap, for example, phosphorus, potassium, calcium or magnesium, follow inverse patterns: in periods when concentrations are increasing in the leaves, they are decreasing in the sap and vice versa, as corresponds to a transport medium such as the sap and a nutrient store such as the leaf.

Nutrient concentrations in soil, leaf and sap varied seasonally with tree phenology, fertiliser application and climate. Nutrient concentrations in leaf were higher than in soil and in soil higher or equal than in sap, except for K, the most abundant element in sap, with concentrations exceeding those in soil.

In the first study period (2018–2019) values with smaller standard deviations were observed than in the second study period (2022–2024). Low rainfall and poor irrigation water quality affected the availability and mobility of nutrients. Elements that are not usually applied as fertilisers, such as Ca and Mg, showed higher stability in the two study periods.

Tree sap showed sensitivity to changes in nutrient concentrations. These results indicate that sap extraction using a modified Scholander-Hammel chamber is an effective method for providing additional information to foliar and soil analyses. Therefore, it can contribute to improving the determination of the nutritional status of the olive tree.