Introduction

The genus Eucalyptus (Myrtaceae) includes tall evergreen trees or shrubs, commonly called Eucalyptus trees or blue gum trees and scientifically named Eucalyptus spp. (Coppen 2002; Dhakad et al. 2018). Eucalyptus spp. are native to the states of Tasmania and Victoria in southeastern Australia and include more than 660 species distributed mostly as subtropical latitudes with mild winters (Coppen 2002; Dhakad et al. 2018). Among Eucalyptus spp., mature E. gunnii exhibit a high degree of freezing tolerance such that they can withstand temperatures as low as approximately ‒15°C for short periods, with an excellent cold-acclimation ability such that these trees have been introduced for use by plantations in temperate climate regions (Varelides and Brofas 2000; Coppen 2002; Dhakad et al. 2018). Furthermore, E. gunnii has shown vigorous growth during the first 22 years across a wider range of regions compared to any other Eucalyptus species, and may find applicability to biomass energy production, forestry plantations, and as roadside and ornamental trees. Additionally, the species has highly valuable pharmaceutical and cosmetic ingredients present in its leaf oil (Coppen 2002; Dhakad et al. 2018; Leslie et al. 2018; Ndiaye et al. 2018; Almeida et al. 2024).

Eucalyptus gunnii, E.parvula, and E.pulverulenta have mainly been used in flower arrangements with fresh cut foliage, mostly reared by raw materials cultivated in greenhouses and tree nurseries in the case of South Korea (Gu et al. 2024; Hyun et al. 2024). Therefore, the highest research and development spending levels on Eucalyptus spp. in South Korea have focused on greenhouse experiments dealing with light spectrum effects (Kim and Moon 2006), irrigation methods (Lee et al. 2010), inoculation with edible ectomycorrhizal mushrooms in culture media (Aggangan et al. 2013), and the fertilization of potted seedlings (Cho et al. 2011). The responses of field-grown E.gunnii, E.parvula, and E.pulverulenta to chilling stress were recently examined by assessing their potential for local adaptation and recovery in the southern region of South Korea (Gu et al. 2024; Hyun et al. 2024). However, Eucalyptus spp. trees grow slowly and are susceptible to diseases, fire, and drought, thus requiring effective nutrient management practices to ensure sustainable growth and development, particularly in areas subjected to heat and cold stress with frequent weather changes (Coppen 2002; Acevedo et al. 2021).

Nutrient sufficiency ranges should be determined to promote the growth of juvenile Eucalyptus gunnii based on leaf and soil nutrient analyses (Smethursta et al. 2004; Fernández et al. 2007; Laclau et al. 2010; Cho et al. 2011; Millner and Kemp 2012; Viera et al. 2016; Dhakad et al. 2018; Ferreira et al. 2018; Jiang et al. 2019; Acevedo et al. 2021; Xie et al. 2021). However, to date, there are no reports that focused on the nutrient requirements of young Eucalyptusgunnii commonly planted in greenhouses or in fields in South Korea, where recommended fertilization regimes from tropical or subtropical regions have been widely and blindly adopted for Eucalyptus spp. (Viera et al. 2016; Jiang et al. 2019). The nutrient uptake efficiency in Eucalyptus trees planted in temperate zones of South Korea, with moderately hot and humid summers and cold and dry winters, will most likely differ from those of tree species grown in tropical or subtropical regions (Jiang et al. 2019), resulting in either toxicity or deficiency in the trees grown in South Korea.

Organic fertilization enables a sustainable plantation through the improvement of both the soil quality and plant health and by reducing greenhouse gas emissions (Sharma and Chetani 2017; Zhou et al. 2022). Organic fertilizers that are widely used in a mixture with oil-cake pellets are formulations containing slow-release nutrients, potentially providing a continuous nutrient supply to evergreen trees (Kim et al. 2015; Sharma and Chetani 2017; Choi 2020; Zhou et al. 2022; Park and Choi 2024). Global nutritional research on young Eucalyptus gunnii trees has focused on inorganic fertilization, whereas few studies have evaluated the proper amount of organic fertilizers to improve soil health and tree growth. The objective of this study was to assess the appropriate amount of organic fertilizer to provide locally adaptable standard-fertilization rates for young Eucalyptus gunnii plantations in South Korea for two years.

Materials and Methods

Site condition

Using a mixture of silt loam as a growth substrate, 90-day-old Eucalyptusgunnii Hook.f. seedlings were planted on May 10, 2022 at a spacing of 1.0 m × 1.7 m using a traditional planting density (Forrest and Moore 2008) in 6.6 m × 12.7 m experimental plots at a university farm station in Gyeongsan-si, South Korea (35°N, 127°E). The trial was established with 16 treatment plots in a randomized block design with four replicates per treatment. Each treatment was randomly allocated to the experimental units within each plot comprising a total of five trees. The trees located at the center of each experimental plot were used as data trees to estimate the means of the tree variables selected for measurement. Meanwhile, the other four trees in each experimental plot were used as guard trees or potential data trees if the original data tree was damaged.

To improve soil fertility, a basic fertilizer was applied 30-d before transplant at a rate of approximately 1,000 kg·ha^-1 of manure compost (30% organic matter (OM) and 2.1% total nitrogen (T-N); 30% poultry manure compost, 30% cow manure compost, 25% mushroom waste medium, and 15% sawdust; Taesan Farming Association Co., Ltd., Gyeongju, Korea). A farm-made organic liquid fertilizer in irrigation water was applied with 9.38 mL of fish amino acid [pH = 4.4; electrical conductivity (EC) = 3.3 dS·m^-1; 0.09% (w/w) T-N; 0.02% (w/w) P; 0.14% (w/w) K; 0.02% (w/w) Ca; and 0.013% (w/w) Mg] to each tree, along with 2.38 mL of soil microorganisms [pH 5.2; EC = 0.8 dS·m^-1; 0.13% (w/w) T-N; 0.07% (w/w) P; 0.09% (w/w) K; 0.02% (w/w) Ca; and 0.007% (w/w) Mg] for fifteen times. Black landscape fabric was laid on the soil surface around each tree trunk at 0 days after transplant (DAT) to suppress weeds and conserve soil water and temperature levels. Hose pipes were used to water the trees one to three times at weekly intervals during 90 DAT.

An organic insecticide (Solbitchae, B.t subsp. aizawai GB413, 1.0 ×10⁷ cfu·mL^-1, Greenfarmer Co., Seoul, Korea) and a chemical insecticide (Guidance, lufenuron 2.5% and emamectin benzoate 0.7%, Syngenta Co., Yeoju, Korea) were sprayed onto the trees every week to control beet armyworm (Spodoptera exigua (Hübner)), whose larvae were frequently found on the trees during the summer of 2022. Low numbers of the Salix leaf beetle (Chrysomela vigintipunctata (Scopoli)), Baccarum (Dolycoris baccarum L.), and an active leaf roller, (Archippus breviplicanus Walsingham), did not cause any serious damage to the trees and were naturally controlled in July and August of 2023.

Windbreak barriers made of a mesh polypropylene fabrics were installed to a height of 1.0 m at the experimental site at the end of November of 2022 to reduce cold wind and to protect the trees from freezing injury (Fig. 1). To compare treatments with the windbreak and without the windbreak, wind speeds were classified into the following three categories: slow (< 0.6 m·s^-1), intermediate (0.6–1.0 m·s^-1), and fast (< 0.6 m·s^-1).

Fig. 1.

Photo view of an installed windbreak fence with white non-woven fabric fencing (panel A), cold damage to trees (panel B), and the effect of the windbreak fence against wind (panel C) in a Eucalyptus gunnii plantation in Gyeongsan-si on November of 2022. *** indicates a significant difference between outcomes with and without the windbreak net at p < 0.001.

Average minimum temperatures and cumulative precipitation from 0 (June 2022) to 450 (October 2023) DAT are shown together with the average minimum temperature and cumulative precipitation over the last 30 years (KMA 2024; Fig. 2).

Fig. 2.

Average and minimum air temperatures and cumulative precipitation in a Eucalyptus gunnii plantation in Gyeongsan-si at 60 d intervals from 0 to 240 days after transplant (DAT) in 2022–2023 and over the period of 1991–2020.

Fertilizer treatments

Four fertilization treatments were designed for young Eucalyptus spp. trees based on the soil application methods recommended by Ferreira et al. (2018) and Smethursta et al. (2004). A single application of a chemical fertilizer was used at a rate equivalent to 25 g of actual T-N per tree per year, with conventional fertilization at 0 DAT in 2022 and at 300 DAT in 2023. Additionally, three oil-cake treatments were applied annually to the soils: 0 g T-N (0% oil cake), 25 g T-N (100% oil cake), and 50 g of actual T-N per tree (200% oil cake). Five equal splits of oil cakes were made on each treated tree at 0 DAT, 30 DAT, 60 DAT, 90 DAT, and 120 DAT in 2022, with three equal splits of oil cakes made at 300 DAT, 330 DAT, and 360 DAT in 2023.

The hemical fertilizer (Mugeobio Co., Youngcheon, South Korea) treatment was prepared with 18.0% soybean corn gluten feed, 14.0% urea, 13.0% molasses, 10.0% fishmeal, 10.0% bone meal, 8.0% potassium sulfate, 8.0% blood meal, 5.0% ammonium sulfate, 4.5% potassium chloride, 3.5% magnesium oxide, and 1.0% borax (pH 7.0, EC = 2.0 dS·m^-1; 6.7% (w/w) T-N; 4.0% (w/w) P; 4.6% (w/w) K; 6.4% (w/w) Ca; and 1.8% (w/w) Mg. The mixed oil cake pellets (Mugeobio Co., Youngcheon, South Korea) used for the treatments were obtained from the residue of 58.0% castor bean, 22.0% soybean, and 20.0% rice bran after mechanically pressing from the various seeds, containing the following: pH 7.0, EC = 6.8 dS·m^-1; 35.9% (w/w); T-C; 4.7% (w/w) T-N; 2.1% (w/w) P; 1.6% (w/w) K; 6.8% (w/w) Ca; and 0.7% (w/w) Mg.

Measurements

Topsoil (0–20 cm layer) samples were collected at half points between the tree trunks and branch tips of adjacent trees using an auger to analyze soil essential mineral nutrients at 0 and 450 DAT, according to agricultural science and technology methods of the RDA (2000). Soil pH and EC levels were measured in a 1:5 soil: distilled water mixture using an FE20 pH meter (Mettler Toledo CO., Jiangsu, China) and an HI-2315 EC meter (Hanna Co., Seoul, South Korea). The soil OM content was determined by the oxidation and dry combustion method; T-N concentrations were determined by the Kjeldahl method, P₂O₅ by the Lancaster method, and exchangeable K, Ca, and Mg concentrations using a 1.0 M CH₃COONH₄ (pH = 7.0) extraction method.

Approximately 150 leaves per tree were sampled at 450 DAT in 2023 to analyze the essential mineral-nutrient contents. Leaf samples were dried at 65°C for 60 h to a constant mass; their dry weights were then measured on a precision digital balance (KERN PFB 2000-2, KERN-SOHN GmbH CO., Balingen, Germany). Leaf samples were then analyzed for T-N by a micro-Kjeldahl instrument, for P by the vanadate method, and for K, Ca, and Mg by atomic absorption spectrometry based on the Standards of Research, Survey, and Analysis in Agricultural Science and Technology of the RDA (2000).

The tree trunk thickness, tree height, number of shoots, number of leaves, and SPAD values were measured for each fertilization treatment at 30, 90, 150, 210, 270, 330, 390, and 450 DAT. The SPAD readings, chlorophyll contents, were obtained from a handheld device (SPAD-502; Minolta Co., Ltd., Tokyo, Japan). All treated trees were excavated at 510 DAT, divided into three parts, roots, shoots, and leaves to determine fresh weight (FW) on each tree organ, and then oven-dried in a dry oven at 65°C for 160 h prior to measuring the dry weight (DW) using an electronic balance (PFB2000-2, Kern-Sohn GmbH, Balingen, Germany).

Tree survival rates (%) were visually assessed at 510 and 690 DAT by determining the symptoms of leaf discoloration, wilting, yellowing, and browning in more than 50% of each treatment tree group.

Additionally, 1,000 leaves were collected from each treatment tree at 410 DAT and were extracted for 2 h at 100°C using a steam-distillation process to obtain the leaf hydrosol content (ES 20L, Dongbu Co., Gwangju, South Korea).

Four 20–30-cm-long branches with fully expanded leaves were randomly sampled from each treatment tree at 410 DAT and kept in a vase with or without tap water at room temperature to examine the longevity of the cuttings (Gu et al. 2024). Cuttings weights and foliar SPAD values were recorded at 3-d intervals from day 0 to day 15 after harvest (DAH) using a SPAD-502 chlorophyll meter (Minolta Co., Tokyo, Japan). Cutting weight loss was calculated as the difference in the cutting weight between days 0 and 15 after the harvest day.

Statistical analysis

The experiment was laid in a randomized block design with four blocks (i.e., replicates) for each treatment to reduce variability among the treatment conditions. A statistical analysis was conducted by means of a one-way analysis of variance with repeated measures of the same variables in SAS software 14.1 (SAS Inc., Cary, USA) with the procedure Proc GLM. The mean differences among the treatments were compared using Duncan’s multiple range test at a 5% probability level.

Results and Discussion

Air and soil conditions

Although the minimum air temperatures in both years, 2022 and 2023, were higher than the corresponding averages of the last 30 years from 40 to 100 DAT, the minimum air temperature mostly tended to decrease significantly from 200 to 270 DAT to approximately ‒14.2°C at 258 DAT, on January 25, 2023 (KMA 2024; Fig. 2). Heavy precipitation was observed frequently during the entire season, reaching 100.8 mm at 382 DAT, on May 29, 2023. Significant exposure to environmental stress and larger fluctuations in microclimate conditions reportedly lead to reduced growth and survival of juvenile Eucalyptus trees during early tree establishment (Coppen 2002; Dhakad et al. 2018); this affected tree vigor and survival in this study.

The soil pH in the chemical fertilizer plots at 450 DAT decreased to 5.87, below the optimum pH range of Eucalyptus tree sites (Smethursta et al. 2004; Viera et al. 2016; Ferreira et al. 2018; Salekin et al. 2019), compared to other plots which ranged from 6.80 to 6.95 (Table 1). This was presumably caused by the high nitrification of NH₄⁺ and the high loss of NO₃^- observed in conjunction with low concentrations of NO₃^- in the soils treated with the chemical fertilizer, as to the high S₂O₃^2- oxidation level caused by urea and ammonium sulfate in the fertilizer (Kissel et al. 2020). The 0% oil-cake plots had the lowest soil EC of 0.03 dS·m^-1 and soil OM of 26.9 g·kg^-1, with the highest values observed for the 200% oil-cake plots. Additionally, the soil P₂O₅ and K₂O concentrations increased significantly with the application of the chemical fertilizer, followed by the oil-cake treatments at 200%, 100%, and 0% T-N. In turn, the CaO and MgO concentrations in the soil decreased in the chemical fertilizer plots, presumably due to the high concentration of dissolved K⁺ in the soil solution, leading to antagonistic effects on the absorption of Mg and Ca ions (Jakobsen 1993). However, the soil nutrient status in all treatment plots satisfied the optimum annual soil-nutrient requirements to sustain high fertility and young tree growth in Eucalyptus spp. plantations (Smethursta et al. 2004; Fernández et al. 2007; Laclau et al. 2010; Cho et al. 2011; Millner and Kemp 2012; Viera et al. 2016; Ferreira et al. 2018; Jiang et al. 2019; Acevedo et al. 2021).

Growth and survival

Foliar T-N concentrations significantly increased by 2.49% in the 200% oil-cake treated plots at 450 DAT, followed by 2.47% in the 100% oil-cake trees, 2.33% in the chemical fertilizer trees, and 2.06% in the 0% oil-cake-treated trees (Fig. 3). Chemical fertilization did not significantly increase the foliar T-N concentration, which was reflected by the low soil EC and OM contents and by the leaching of soluble synthetic fertilizers into the soil under humid weather with abundant rainfall in 2023 (KMA 2024). Foliar P concentrations were not significantly different among the treated trees, ranging from 0.43% to 0.52%. The 0% oil-cake treatment significantly reduced foliar K concentrations while increasing Ca and Mg concentrations owing to the low K concentrations in the soil, which seemingly promoted an antagonistic effect of K on Ca and Mg in root absorption and transport within the trees (Xie et al. 2021). All foliar macronutrient concentrations, except for the 0% oil-cake-treated trees with 1.47% Ca, far exceeded the optimum levels (0.5–1.0%) shown for Eucalyptus spp. cultivated in Australia, New Zealand, Chile, and Brazil (Millner and Kemp 2012; Viera et al. 2016; Acevedo et al. 2021).

Fig. 3.

Foliar macronutrient concentrations in Eucalyptus gunnii trees supplied with 0% oil-cake (0 oil cake), 100% oil-cake (100 oil-cake), 200% oil-cake (200 oil-cake), and a chemical fertilizer (chemical fert.) at 450 days after transplant in 2023 in Gyeongsan-si. Different lowercase letters above each data point indicate significant treatment differences as per Duncan’s new multiple-range test at p < 0.05.

Tree height and trunk thickness values were smallest in the 0% oil-cake-treated trees from 30 to 450 DAT (Fig. 4). Chemical fertilization increased the tree height considerably at 90 DAT as it quickly released high amounts of nutrients into the soil (Laclau et al. 2010), with a similar extension of the tree height in the 100% and 200% oil cake-treated trees occurring from 150 to 450 DAT. Similarly, the tree trunk thickness increased significantly in the 200% oil cake-treated trees from 150 to 450 DAT, as mineralization progressed gradually in the organic fertilizers (Laclau et al. 2010). Sustained development of tree vegetation, previously regarded as a predictor of trunk thickness during the growth of young Eucalyptus spp. trees, may lead to an increase in the number of shoots and leaves (Butnor et al. 2019; Gu et al. 2024; Hyun et al. 2024). The 200% oil-cake treatment increased the number of shoots from 90 to 270 DAT with a further increase, although a significant difference was not observed over the period from 330 to 450 DAT (Fig. 5A), which influenced the increase in seasonal leaf production (Fig. 5B). All trees grew slowly from 210 to 330 DAT and were exposed to shorter days and low temperatures during the winter, which would have reduced the cytokinin contents associated with leaf degradation and biomass (Coppen 2002; Dhakad et al. 2018). Leaf SPAD readings of the chlorophyll contents present in the leaves (Xiong et al. 2015) increased under the chemical fertilizer treatment from 30 to 150 DAT (Fig. 5C), mostly due to the rapid release of essential nutrients into the soil and the readily available form for the trees early in the growing season. As the trees grew larger, the lowest SPAD readings were mostly observed over the period from 90 to 450 DAT with the 0% oil-cake treatment without nutrient supplementation. The seasonal SPAD readings increased for all treated trees from 0 to 330 DAT, in contrast to deciduous fruit trees (Xiong et al. 2015), whereas decreasing values were found from 330 to 390 DAT as old leaves were replaced by newly formed leaves.

Fig. 4.

Tree height (panel A) and trunk thickness (panel B) of Eucalyptus gunnii trees supplied with 0% oil-cake (0 oil cake), 100% oil-cake (100 oil-cake), 200% oil-cake (200 oil-cake), and a chemical fertilizer (chemical fert.) at 60 d intervals from day 0 to 450 after transplant (DAT) in Gyeongsan-si in 2022–2023. Different lowercase letters above each phase indicate significant treatment differences determined as per Duncan’s multiple-range test at p < 0.05.

Fig. 5.

Number of shoots (panel A), number of leaves (panel B), and leaf SPAD values (panel C) of Eucalyptus gunnii trees supplied with 0% oil-cake (0 oil cake), 100% oil-cake (100 oil-cake), 200% oil-cake (200 oil-cake), and a chemical fertilizer (chemical fert.) at 60 d intervals from day 0 to 450 after transplant (DAT) in Gyeongsan-si in 2022–2023. Different lowercase letters above each phase indicate significant treatment differences determined as per Duncan’s multiple-range test at p < 0.05.

The total tree dry-mass level was substantially partitioned into shoots, leaves, and roots at 450 DAT (Table 2). Little information on biomass allocation has been obtained by means of the destructive sampling of Eucalyptus trees due to the large volume of trees with high amounts of woody vegetation in the structure and leaves and the large woody root systems (Bartelink 1999; Laclau et al. 2010). The 200% oil-cake treatment produced significantly high root- shoot- and leaf-dry masses, as well as whole tree dry-mass levels, which ranked in the following order: 100% oil cake > chemical fertilizer > 0% oil cake. A single application of the chemical fertilizer reduced the nutrient uptake efficiency of treated trees receiving higher amounts of rainfall in 2023 relative to the uptake efficiency recorded in a normal year (KMA 2024), ultimately resulting in limited resource levles as well as the top parts of the trees competing with root growth as shown by the high R:whole tree ratio.

High (> 53%) tree survival rates were observed for trees under the chemical fertilizer treatment at both 510 and 690 DAT, whereas the lowest ( > 27%) survival rates were recorded for the 0% oil-cake-treated trees at the same sampling time points (Fig. 6). Meanwhile, the 200% oil-cake-treated trees showed a 40% survival rate at 690 DAT, while a lower (30%) survival rate was observed for 100% oil-cake-treated trees. This at least partially resulted from the rapid growth and production of more foliage, the extended flush, and fewer roots, which reduced the cold-hardening process in acclimated trees relative to those under the chemical-fertilizer treatment (Sakai and Larcher 1987).

Fig. 6.

Tree survival rates of Eucalyptus gunnii trees supplied with 0% oil-cake (0 oil cake), 100% oil-cake (100 oil-cake), 200% oil-cake (200 oil-cake), and a chemical fertilizer (chemical fert.) at 510 (panel A) and 690 (panel B) days after transplant in Gyeongsan-si in 2023. Different lowercase letters above each data point indicate significant treatment differences as per Duncan’s new multiple-range test at p < 0.05.

The survival rate of young Eucalyptus gunnii was mostly enhanced by high foliar concentrations of T-N (r² = 0.383) and K (r² = 0.327), with few correlations observed for foliar SPAD, P, Ca, and Mg (Fig. 7). High foliar T-N and K concentrations reportedly ameliorate freezing stress on Eucalyptus spp. by increasing the osmotic potential of the cell solution and preventing electrical leakage (Karimi 2017). The application of T-N along with P and K increased nutrient uptake and enhanced hardening in the nursery, thereby increasing frost tolerance of E. globulus Labill (Fernández et al. 2007). Tree survival was positively affected by the trunk thickness (r² = 0.727; Fig. 8), as previously observed for greater trunk cold-hardiness and defensive structures with increasing carbon contents in young apple trees compared to other tree compartments (Billing et al. 2022). Tree survival probability was also closely related to the tree height (r² = 0.151), number of shoots (r² = 0.601), total leaf DW (r² = 0.503), and average leaf DW (r² = 0.387), all of which were largely dependent on an increase in the trunk thickness (Butnor et al. 2019; Gu et al. 2024; Hyun et al. 2024).

Fig. 7.

Linear regression analysis of the relationships between tree survival rates and foliar nutrient concentrations of Eucalyptus gunnii trees in Gyeongsan-si in 2023: ns, not significantly different; and ***, significantly different at p < 0.001.

Fig. 8.

Linear regression analysis of the relationships between tree survival rates and various growth parameters of Eucalyptus gunnii trees in Gyeongsan-si in 2023: ns, not significantly different; * and ***, significantly different at p < 0.05 and p < 0.001, respectively.

Leaf hydrosol and longevity of cut foliage

The concentrations of leaf hydrosol, distilled floral, and aromatic water were high in the Eucalyptusgunnii treated with the chemical fertilizer (Fig. 9A). Leaf hydrosols are used in cosmetic preparations, pharmaceutical products, and food-flavoring substances (Ndiaye et al. 2018; Almeida et al. 2024). Approximately 450 mL of hydrosol was obtained from 1,000 leaves from the 200% oil-cake-treated trees (Fig. 9B), extracted from a small amount of essential oil (data not shown), which may provide important information for possible production and marketability in South Korea. The 100% oil-cake treatment resulted in a lower concentration of floral water, presumably owing to the dilution effects on the larger leaf mass of the trees compared to that of the 0% oil-cake-treated trees.

The weight loss and SPAD readings changed slightly when the cut foliage was kept in a vase with water between 0 and 6 DAH (Fig. 10). However, such weight loss rapidly increased to approximately 20% at 9 DAH and to over 40% at 15 DAH for all the cut foliage samples kept in a vase of water. We concluded that a 7-d vase life period for the cut foliage of Eucalyptus is quite viable (Gu et al. 2024). In contrast, weight loss by cut foliage increased significantly from 0 to 6 DAH in the absence of water, as observed in our previous study (Gu et al. 2024). Cut foliage kept without water from 6 to 15 DAH resulted in 60% to 65% weight loss due to desiccation and dark greening of the leaves, as shown by the increase in the SPAD values, particularly when the trees were treated with the chemical fertilizer.

Fig. 9.

Foliar concentrations (panel A) and amounts of floral water (panel B) of Eucalyptus gunnii trees supplied with 0% oil-cake (0 oil cake), 100% oil-cake (100 oil-cake), 200% oil-cake (200 oil-cake), and a chemical fertilizer (chemical fert.) at 450 days after transplant in Gyeongsan-si in 2023. Different lowercase letters above each data point indicate significant treatment differences as per Duncan’s new multiple-range test at p < 0.05.

Fig. 10.

Weight loss and leaf SPAD values of non-watered or watered cut foliage of Eucalyptus gunnii trees supplied with 0% oil-cake (0 oil cake), 100% oil-cake (100 oil-cake), 200% oil-cake (200 oil-cake), and chemical fertilizer (chemical fert.) at three-day intervals from 0 to 15 days after harvest (DAH) in 2023. Different lowercase letters above each data point indicate significant treatment differences as per Duncan’s new multiple-range test at p < 0.05.

Conclusions

The 200% oil-cake treatment would provide a locally adaptable standard fertilization rate to enhance the growth, development, and economic secondary-metabolite contents of juvenile Eucalyptus gunnii trees compared to fertilization with 100% oil-cake or a chemical fertilizer. Based on the results summarized herein for the 200% oil-cake treatment, the maximum levels of essential nutrients to ensure tree vegetation obtained were as follows: 31.6 g·kg^-1 soil OM, 0.05 dS·m^-1 soil EC, and 2.47% foliar T-N, which are similar levels to those reported for Eucalyptus spp. cultivated in Australia, New Zealand, Chile, and Brazil (Millner and Kemp 2012; Viera et al. 2016; Acevedo et al. 2021). However, weather conditions were quite severe, with minimum temperature dropping to approximately ‒14.2°C at 258 DAT, with heavy precipitation frequently observed for the whole season in the years 2022 and 2023, which reduced tree establishment rates. Future studies should focus on the age-dependency of tree responses to increases in fertilization while assessing the economic potential of annual biomass residues, the commercial essential oil in the leaves and seeds, and the cut foliage. This will allow us to provide a reliable nutritional reference for the effective and successful establishment of Eucalyptus gunnii plantations in South Korea.