Effectiveness of Various Foliar Fertilizer on the Growth and Performance of Oil Palm (Elaeis guineensis Jacq.) Seedlings in Main Nurseries

Fertilizer can be applied through soil and leaves. Fertilizer application through leaves is more effective than soil application due to faster nutrient absorption. This study aims to determine the effectiveness of applying various foliar fertilizer compositions on the growth and performance of oil palm seedlings. The research was conducted from April 2020 to September 2020 at the Oil Palm Nursery Unit at Politeknik Negeri Lampung. A single factor in a completely randomized design with four replications was used in the experiment. The treatment involved nutrients composition of foliar fertilizer consisting of five levels, namely control (no fertilizer), NPK 20-1515, NPK 27-18-9, NPK 11-8-6, and NPK 27.5-5.5-4.8. Measurement were made on seedling height, stem diameter, number of leaves, leaf greenness, rachis length, and leaflet length. The data were analyzed by means of variance, followed by orthogonal contrast if the result was significantly different. The results showed that the application of foliar fertilizers could increase the growth of seedling height, stem diameter, number of leaves, leaf greenness index, rachis length, and leaflet length. Generally, a foliar fertilizer application gives better results than without a foliar fertilizer application (control). There was no difference in the powder and liquid foliar fertilizer effect on increasing the growth of oil palm seedlings. The formulation of NPK 20-15-15 and NPK 11-8-6 foliar fertilizer had a better effect on the leaf greenness index of oil palm seedlings.


Introduction
The production and productivity of oil palm plantations are determined by the quality of seedlings and the technical cultural cultivation technique from the beginning of planting until the plants enter the mature crop phase. The basic problem of low oil palm productivity is the lack of quality and performance of planting material. The determinant of the quality and performance of oil palm seedlings is the nutritional intake provided during the nursery period. Fertilizer treatment in oil palm nurseries is generally provided through the soil in the form of compound fertilizers. Certain conditions can inhibit the uptake of fertilizers that are given through the soil; for example, low soil moisture causes the absorption of fertilizers by the roots to be inhibited (Wang et al., 2018;Xu et al., 2020). On the other hand, the relatively long maintenance period for oil palm seedlings, ranging from 9-12 months, makes maintenance costs high enough so that efforts are needed to shorten the maintenance time without reducing the quality and performance of oil palm seedlings. Fertilizer through leaves can be applied in addition to fertilization through the soil to accelerate plant growth (Muktamar et al., 2017;Niu et al., 2021).
Administering fertilizers through leaves is a technique of applying liquid fertilizer directly to the leaves. Plants can absorb essential nutrients through their leaves (Patil and Chetan, 2018). The absorption of foliar fertilizer occurs through the stomata and leaf epidermis (Butler et al., 2020;Zhu et al., 2020). Foliar fertilizer spraying technique is commonly used for micronutrients. Macronutrients can also be administered via this method if there is insufficient topsoil moisture to support fertilizer uptake. Spraying foliar fertilizers is not to replace the application of fertilizers through the soil but as an addition. Fertilizers given through the leaves contain both macro-and micronutrients with different concentrations. Fertilizer application through leaves is more effective than through soil because the absorption of leaf nutrients needed for growth occurs faster (Galagi et al., 2018;Joshi et al., 2016;Shahrekizad et al., 2015). Several conditions in the field, such as high rainfall levels, which cause high loss of nutrients from the soil and nutrient deficiency, are the reasons for spraying foliar fertilizers. Leaves can absorb about 90% fertilizer, whereas roots can only absorb about 10% fertilizer (Iswanto, 2002). Nitrogen, phosphorus, and potassium are mobile in plant system so application through the leaves makes it possible to provide these three elements quickly (Das and Avasthe, 2018;Kathpalia and Bhatla, 2018).
Nitrogen deficiency is the most important nutritional problem in oil palm production (Broschat, 2009). Planting media containing organic material derived from plant stems and other wood-derived materials are susceptible to N bonds because this material breaks down (Nikiyuluw et al., 2018). Although P deficiency is generally not a problem in oil palms, it is a severely limiting factor in acid soils in the tropics, where oil palms are grown commercially. Potassium deficiency is by far the most common deficiency in various plants from the family Arecaceae (Broschat, 2009). However, in the nursery phase, potassium deficiency is less common than nitrogen deficiency. Symptoms vary according to species and severity. In many species, the earliest symptoms consist of yellow-orange and necrotic spots on the oldest leaves (Broschat, 2009). Sufficient nutrient content in the soil does not guarantee that oil palm roots in sufficient quantities will absorb these nutrients. Previous studies on various plant commodities showed that application of foliar fertilizers was able to accelerate and increase plant growth. In general, the concentration of foliar fertilizers that produces the best growth on various plants ranges from 0.1-0.4% . The aim of this study was to determine the effectiveness of applying various foliar fertilizer compositions on the growth and performance of oil palm seedlings by comparing seedling height, stem diameter, number of leaves, leaf greenness, rachis length, and leaflet length.

Experimental Design
The research was conducted from April 2020 to September 2020 at the Oil Palm Nursery Unit of Politeknik Negeri Lampung, Lampung Province, Indonesia. The study site is located at 5° 21' 6.3" S and 105° 13' 40" E. Based on the Agricultural Meteorology Station data of Politeknik Negeri Lampung, the average daily temperature and rainfall during this research were 26.2 o C and 247.77 mm month -1 , respectively. The experiment included a single factor in a completely randomized design with four replications. The treatment involved nutrients composition of foliar fertilizer consisting of five levels, namely control (no fertilizer), NPK 20-15-15 (powder), NPK 27-18-9 (powder), NPK 11-8-6 (liquid), and NPK 27.5-5.5-4.8 (liquid). There were 60 seedling samples in the experiment; each experimental unit had four oil palm seedlings measured.

Application of Treatment
Before receiving treatment, oil palm seedlings were arranged in the main nursery with a distance of 70 cm x 70 cm. Oil palm seedlings were arranged into 12 groups and labeled according to treatment. The location for placing the seedlings was on the land of the Oil Palm Nursery Unit. Oil palm plants were sprayed using a hand sprayer that has been filled with foliar fertilizer solution based on the composition of the foliar fertilizer according to the predetermined treatment. Foliar fertilizer solution was made with a concentration of 0.2% and spraying volume was ± 3.5 ml per plant. Foliar fertilizer was sprayed on the top and bottom surface of the leaves every two weeks in the morning when there was no rain.

Measurement and Data Analysis
Plant characteristics, such as seedling height, stem diameter, number of leaves, leaf greenness, rachis length, and leaflet length were recorded. Measurement of the growth of oil palm seedlings was carried out to 60 observation units at 9 month after treatment. The data obtained were analyzed using ANOVA, followed by orthogonal contrast if the result was significantly different, using RStudio. The correlations were illustrated using the ggpairs function in the GGally R Package (Schloerke et al., 2020).

Result and Discussion
Based on measurement of palm seedlings, foliar fertilizer application promotes the increase in seedling height, stem diameter, and number of leaves over control. The results of further orthogonal contrast tests showed that the height of the seedlings, stem diameter, and number of leaves on the oil palm seedlings treated with foliar fertilizer were significantly larger than those without foliar fertilizer (control) ( Table 1). Certain types of foliar fertilizer have no effects on seedling height, stem diameter, and number of leaves ( Figure 1). Different foliar fertilizer formulations affect seedling height, stem diameter, and number of leaves. These results show that the application of foliar fertilizer (in powder and liquid form) with high N content effectively enhances the growth of oil palm seedling. Foliar fertilizer is fertilizer in liquids or solids generally dissolved in water and sprayed onto the plants, especially leaves. The foliar fertilizer application is commonly used in horticultural crops, including ornamental and fruit crops. Foliar fertilizer can also be sprayed to plant seedlings. Some types of foliar fertilizers circulating in the market are enriched with micronutrients. The nutrients are entering the leaf through the stomata or by diffusion through the epidermis (Su et al., 2019). This mechanism allows nutrients to enter the plant more effectively and is subsequently utilized by the plant.
The results also show that provision of foliar fertilizer with high nitrogen content can increase the greenness index of leaves and the length of young leaves (Table 1 and Figure 2). The ratio between nitrogen-phosphorus content of 1:1.3 and nitrogenpotassium content of 1:1.3 to 1:1.8 is the composition of nutrients in foliar fertilizers that produces the highest leaf greenness index. The leaf greenness index correlates with the nitrogen levels in the leaves and contributes to photosynthesis. The results of previous studies also showed a high positive correlation between leaf greenness index and leaf chlorophyll concentration (Jhanji and Sekhon, 2018;Neilson et al., 2015). Chlorophyll is a photosynthetic pigment that is important for converting light energy into chemical energy, so leaf chlorophyll content is an essential variable from a physiological perspective. Nitrogen deficiency or water limitation suffered by oil palm seedlings can lead to a low greenness index.
Nitrogen, as well as phosphorus, potassium, and magnesium, are mobile elements. Under conditions of deficiency, the plant can extract N from the oldest leaf in the head and transplant it to a new leaf that is growing (spear leaf) to allow for continued growth. Therefore, nutrient deficiency symptoms occur first on the oldest leaves, and as the deficiency increases, the younger leaves will be affected. Nitrogen stains appear to be characterized by a uniform light green to bright yellowish-green color on the oldest leaves. However, they will usually affect the entire title except for the spear leaf, which is generally slightly greener (Broschat, 2009). When all the mature leaves have run out of mobilizable N, the oil palm's growth will slow down and eventually stop altogether. Severe N deficiency does not cause tissue necrosis. Nitrogen stains can be overcome immediately by fertilizing nitrogen, and the leaves of plants that have rust will turn green again. Factors that affect the effectiveness of foliar fertilizer sprays include the physicochemical properties of the formulation, the environment in which the spray is applied, and the characteristics of the plant to which the spray is applied (Fernández and Brown, 2013). The physicochemical properties of spray formulations including molecular size, solubility, pH, surface tension, retention, and dispersion of the formulation play a major role in determining the effectiveness of foliar absorption of fertilizer solution.
The deficiency of phosphorus and potassium nutrients is relatively difficult to identify at the oil palm nursery phase. The earliest symptoms consist of yellow-orange and necrotic spots on the oldest leaves (Broschat, 2009). As deficits develop, necrosis spreads to the edges and ends of the leaves (von Uexkull and Fairhurst, 1991). In some species, spotting does not appear, and necrosis of the leaf tip is the only visible symptom. As with the case of N, K is a very mobile element in the phloem, and the most severe symptoms are on the oldest leaf and towards the end of each affected leaf (Maillard et al., 2015). In other conditions, some Arecaceae plants lacking K show leaflets necrosis only (Broschat, 2009).
The growth and yield of a plant are related to each other. Correlation analysis is needed to determine a relationship between plant growth and yield. Correlation analysis was used to determine the close relationship between the growth and growth components and The results also show that provision of foliar fertilizer with high nitrogen content can increase the greenness index of leaves and the length of young leaves (Table 1 and Figure 2). The ratio between nitrogen-phosphorus content of 1:1.3 and nitrogen-potassium content of 1:1.3 to 1:1.8 is the composition of nutrients in foliar fertilizers that produces the highest leaf greenness index. The leaf greenness index correlates with the nitrogen levels in the leaves and contributes to photosynthesis. The results of previous studies also showed a high positive correlation between leaf greenness index and leaf chlorophyll concentration (Jhanji and Sekhon, 2018;Neilson et al., 2015). Chlorophyll is a photosynthetic pigment that is important for converting light energy into chemical energy, so leaf chlorophyll content is an essential variable from a physiological perspective. Nitrogen deficiency or water limitation suffered by oil palm seedlings can lead to a low greenness index. The results also show that provision of foliar fertilizer with high nitrogen content can increase the greenness index of leaves and the length of young leaves (Table 1 and Figure 2). The ratio between nitrogen-phosphorus content of 1:1.3 and nitrogen-potassium content of 1:1.3 to 1:1.8 is the composition of nutrients in foliar fertilizers that produces the highest leaf greenness index. The leaf greenness index correlates with the nitrogen levels in the leaves and contributes to photosynthesis.
The results of previous studies also showed a high positive correlation between leaf greenness index and leaf chlorophyll concentration (Jhanji and Sekhon, 2018;Neilson et al., 2015). Chlorophyll is a photosynthetic pigment that is important for converting light energy into chemical energy, so leaf chlorophyll content is an essential variable from a physiological perspective. Nitrogen deficiency or water limitation suffered by oil palm seedlings can lead to a low greenness index.  Crop Science Vol. 9 No. 1, February 2022 www.j-tropical-crops.com yield and yield components. Seedling height has a high positive correlation with other observation variables. The high positive correlation was also shown in the measured variables of seedling height, stem diameter, number of leaves, and leaf greenness index. Based on these results, it can be seen that the measured variables of seedling height (R 2 = 0.630-0.782), stem diameter (R 2 = 0.519-0.765), number of leaves (R 2 = 0.792), and leaf greenness index (R 2 = 0.513-0.568) are essential variables of the growth and morphology of oil palm seedlings, especially in fertilization experiment. The results of the correlation analysis shown in Figure 3 indicate that the number of leaves has a very significant positive correlation with the greenness index of the leaves (R 2 = 0.792, p < 0.001). Leaves are the primary organ that function in photosynthesis because, in the leaves, there are pigments that play a role in absorbing sunlight. Chlorophyll is a bioactive molecule that receives sunlight and utilizes its energy to synthesize carbon dioxide and water into carbohydrates through photosynthesis.

Journal of Tropical
Oil palm is a nutrient-dense plant and requires a complete and balanced supply of nutrients for optimal plant growth, especially nitrogen (Yadegari et al., 2020). All plants use nitrogen in the form of nitrate and ammonium (Edy et al., 2020). It is the most important factor for the proper growth and development of plants by playing an important role in the biochemical and physiological functions of plants. Nitrogen is the macro element most commonly absorbed by plants under normal conditions in the soil. It enhances the green colour of leaves and is a building block of essential cellular components such as amino acids, proteins and nucleic acids. Approximately 80% of the total cations and anions make the plants absorb nitrate and ammonium, which greatly impact the cation and ion balance involved in regulating cellular pH and the pH of the biosphere (Torres-Olivar et al., 2014;Leghari et al., 2016). For this reason and because their high mobility in the soil, nitrogen is also the most deficient nutrient for most crops. Nitrogen deficiency affects plant growth by reducing photosynthesis leaf area and green leaf longevity (Mu and Chen, 2021). This study indicates that the application of foliar fertilizer effectively increases the vegetative growth of oil palm seedlings, especially seen in the growth performance of the seedlings and greenness of the leaves. Applying foliar fertilizers is recommended to quickly correct the mobile nutrient deficiencies in plants. This action is recommended for oil palm nurseries when there is a deficiency of macronutrients and low rainfall conditions so that fertilization through soil cannot immediately solve the problem.

Conclusion
Foliar fertilizer application enhances the growth and performance of oil palm seedlings. Both powder and liquid foliar fertilizer are equally effective in increasing the growth of oil palm seedlings in terms of seedling height, stem diameter, number of leaves, leaf greenness, rachis length, and leaflet length. Among the different types of foliar fertilizers, NPK 20-15-15 and NPK 11-8-6 have significant effects on the leaf greenness index of oil palm seedlings.
leaves (Broschat, 2009). As deficits develop, necrosis spreads to the edges and ends of the leaves (von Uexkull and Fairhurst, 1991). In some species, spotting does not appear, and necrosis of the leaf tip is the only visible symptom. As with the case of N, K is a very mobile element in the phloem, and the most severe symptoms are on the oldest leaf and towards the end of each affected leaf (Maillard et al., 2015). In other conditions, some Arecaceae plants lacking K show leaflets necrosis only (Broschat, 2009  The growth and yield of a plant are related to each other. Correlation analysis is needed to determine a relationship between plant growth and yield. Correlation analysis was used to determine the close relationship between the growth and growth components and yield and yield components. Seedling height has a high positive correlation with other observation variables. The high positive correlation was also shown in the measured variables of seedling height, stem diameter, number of leaves, and leaf greenness index. Based on these results, it can be seen that the measured variables of seedling height (R 2 = 0.630-0.782), stem diameter (R 2 = 0.519-0.765), number of leaves (R 2 = 0.792), and leaf greenness index (R 2 = 0.513-0.568) are essential variables of the growth and morphology of oil