Alternative Methods for Handling Iron (Fe) Deficiency of Oil Palms (Elaeis guineensis Jacq.) Grown in Sandy Soil

Iron (Fe) deficiency is one of the general problems that occur in Oil Palms (Elaeis guineensis Jacq.) which are planted on sandy soil. Fe is an important micronutrient in enzyme activities, including cytochromes in the process of electron transfer, chlorophyll synthesis, and in maintaining the chloroplast structure. The research was conducted in 2 series of field observations. The first observation was conducted on the application of clay mineral soils (Plinthic Kanhapludults and Lithic Dystrudepts) in a silt pit, while the second observation was on the application of liquid nano-sized Fe fertilizer through root infusion and trunk injection methods, in addition, Fe chelated fertilizer application were also observed. The observation variable was the level of Fe in leaves 17th. The results showed all treatments significantly increased levels of Fe in leaves with different plant response rates. Root infusion treatment shows a faster response than clay mineral soil treatment, namely at 2 - 4 months after application (50.3 ppm and 38.2 ppm). Meanwhile, clay mineral soil treatment has a slower plant response rate on increasing Fe level in the leaves with a significant difference against the control, precisely at 24 months after application. Normally, regular Fe fertilization is enough to fulfill plant nutrient needs. Application of Fe through trunk injection or root infusion can be an alternative treatment for short-term use, whereas the application of clay mineral (iron-rich) soil can be optional for long-term needs.


Introduction
Oil palm (Elaeis guineensis Jacq.) is an agricultural commodity which important in the Indonesian economy.This is proven by its export value, which reaches US$15.9 billion [1].Based on the data from [2] estimated area of oil palm plantations in Indonesia in 2020 is approximately 14.99 million hectares, which is spread across 25 provinces.Today, the plantations continue to grow, despite not being as massive as in previous years.Other than increasing the amount of limited land, the aforementioned growth also leads to sub-optimal soil use, one of which was sandy soil [3].It refers to coarse-textured soil (sand > 85%), loose structure, possesses high porosity, low nutrient content with low clay and organic matter content [4] -especially when it comes to Fe, as well as having hardpan layers in several areas.Moreover, this type of soil also has a low water holding capacity and highly nutrient leaching [5].In its growth, oil palm requires macro and micronutrients which have their respective roles in supporting plant metabolism.Each of them has a threshold of tolerance based on the standard needs of plants.Each element must be fulfilled to carry out its role in supporting the nutritional needs of a balanced plant.It was different from the law of minimums that have been widely accepted, in which it argues that the final product is determined by the most limiting factors.[6] mentions that the final product is a multiple of the factors which influence it.This explains why crop productivity levels can be lower than expected, due to failure to recognize the effect of several factors on the production.
One of the most important micronutrients for oil palm is Iron (Fe).It is important for several plant enzyme systems, such as cytochrome oxidase (electron transport) and cytochrome (terminal respiration step).Moreover, it is a component offerredoxin protein and necessary for nitrate (NO3-) and sulfate (SO4 2-) reduction, nitrogen (N) assimilation, and energy production (NADP).It functions as a catalyst of an enzyme system, which is related to chlorophyll growth [6].Fe is important for enzyme systems that produce oxidation-reduction reactions and electron transport chains in plants; for synthesizing chlorophyll; maintaining chloroplast structure and enzyme activity; as well as regulating respiration, photosynthesis, nitrate, and sulfate reduction where all of them are crucial for the plant's growth and reproduction [7].Low water holding capacity and availability of Fe in sandy soil cause a bigger risk of Fe deficiency in the plants.The symptoms appear on the youngest leaves, where the areas between the veins will be pale yellow or white (interveinal chlorosis) [7].In oil palm, it is characterized by the symptoms of interveinal chlorosis on the young leaves (1 -3 leaves), as in Figure 1.At a later stage, those young leaves will change color to pale and whitish.Meanwhile, the old leaves will turn yellow and slowly dry up.Finally, in a severe stage, the chlorosis is followed by damage and drying of the fronds, stunted plant growth, and even death [8].[9] argues that several sources of Fe can be used to fulfill plant needs, such as Ferrous sulfate (FeSO4.7H20),Ferrous ammonium sulfate ((NH4)2Fe(SO4)2.6H2O),Fe-chelates (diethylenetriamine penta-acetate -DTPA), ethylene diamine tetraacetate (EDTA), and Ethylenediamine di-2hydroxyphenyl acetate ferric (EDDHA).Generally, Fe fertilizer is given at a dose of approximately 100 grams per plant.Moreover, [10] states that dealing with iron deficiency through the use of the root infusion method and Ferrous sulfate (FeSO4.7H2O)+ citric acid is the most effective and long-lasting way to overcome the aforementioned problem in oil palm plantations.Several research has been done to determine the effectiveness of some treatments in overcoming Fe deficiency in oil palms planted in sandy soil, such as the implementation of mineral soil in a silt pit or "rorak" (some kind of a channel whose function is to serve as temporary storage of water from the surface runoff to be permeated into the soil), as well as the application of FeSO4 by root infusion and trunk injection methods.Clay mineral soil with a reddish-yellow color (such asPlinthic Kanhapludults and Lithic Dystrudepts) generally has a high Fe content than sandy soil (Spodosols).Based on the results of soil analysis around the research site, the average amount of Fe (ppm) in Typic Haplorthods sandy soil is 133.57ppm, whereas, in Plinthic Kanhapludults and Lithic Dystrudepts clay mineral soils, the amount is respectively 1,331.10 ppm and 1,073.20 ppm.That being said, the latter implementation has the potential to substitute Fe deficiency in sandy soil.

Research Site
Field research was conducted at Selangkun Estate, PT Sawit Sumbermas Sarana Tbk., precisely in an area that has sandy soil (TypicHaplorthods).Meanwhile, soil and plant tissue analysis were conducted at Sulung Research Station Laboratory.

Research Method
This research is done in 2 series of field observations, which are the application of clay mineral soil in silt pit, as well as the implementation of liquid Fe through root infusion and trunk injection methods, and also Fe chelated fertilizer application.Data were analyzed descriptively, and an advance test is conducted by Duncan Multiple Range Test (DMRT) 5%

Clay Mineral Soil Application in Silt Pit
The field observation was conducted by three treatments, which are the implementation of Plinthic Kanhapludults (MLA), Lithic Dystrudepts (MLB), and control (without clay mineral soil).Each treatment has three replicates on several plots, consisting of three oil palms.Furthermore, Plinthic Hapludults and Typic Dystrudepts Fe-rich mineral soils are obtained from the open land area around the plantation.This implementation is done through two silt pits with dimensions of 1.0 m x 0.7 m x 0.3 m (lxw xh).They are built at a distance of around 1.5 m from the base of the oil palm trunk, as in Figure 2. The amount of clay mineral soil added to each silt pit is 250 kg -based on the treatment.For other inputs related to fertilization and technical culture (weed control, budding, and harvesting), in accordance with the standard (estate practices).

Figure 2. Clay mineral soil application in silt pit
The variables observed are the dynamics of the iron in the leaves and vegetative plants in the form of rachis length and Petiole Cross Section (PCS).The sampling for the leaves on the 17 th frond before application, 12 months after application, as well as 24 months after application.The determination of the 17 th frond is based on the leaf arrangement pattern (phyllotaxy) of the oil palm, as in Figure 3.The 1 st frond is the youngest -with an open condition.Furthermore, the position of the 17 th frond is two levels below the 1 st .The leaf samples are taken compositely from each plant in each treatment plot.Afterwards, the samples are prepared to leaf nutrients analyzed in the laboratory.As for the measurement of Fe content, it analyzed with flamephotometry-atomic absorption spectroscopy (AAS) method.The data are analyzed statistically with Duncan Multiple Range Test (DMRT) 5%.

The Application of Liquid Fe through Root Infusion and Trunk Injection Methods
The observation is done with 2 treatments, which are root infusion and trunk injection.The liquid Fe used is a commercial product of liquid Fe fertilizer, with nano-sized particles containing 4.0% Fe at a dose of 0.75 ml/plant.For each treatment, 37.5 ml of liquid Fe is combined with up to 2.5 liters of water (1.5% solution concentration) and applied to 50 plants -based on the treatment (15 of them serve as observation samples).

Root Infusion
Fe solution is given by root infusion method to plants with iron deficiency.As much as 50 ml of Fe solution is put in a small thick plastic.The search for active primary roots is done by making a hole of 20 cm x 20 cm x 20 cm (l x w x h).The root has a diameter of 0.6 -0.8 cm, with a white root crosssection (not brown).Then, the tip of the active primary root is inserted into the Fe solution plastic until it reaches the bottom of the plastic.Afterward, it will be tied with rubber, hence protecting the solution from being spilled, as in Figure 4.The examination of the infusion holes is conducted three to six days after application.If the Fe solution does not decrease after the second examination, it must replace the primary root with a similar method.To ease the evaluation, the treated oil palm can be marked.

Trunk Injection
Trunk Injection is done by making holes by using a drill in the trunks of oil palms with Fe deficiency, precisely at a height of one meter from the ground (with a slope of 35 0 -45 0 downwards and a depth of 30 -40 cm, as in Figure 5).Furthermore, as much as 50 ml of Fe solution is put in the hole, and then covered with clay.

Regular Fe Fertilizer
Application of Fe chelated fertilizer were conducted by dose 100 gram/plant.It applied around the feeding roots, surrounding plant circle.Fe chelated fertilizer containing 25% Fe2O3 and has a granular form.

Results and Discussion
Plants possess the mechanisms to absorb Fe under aerobic conditions.First, trivalent iron (Fe3+) will morph into aqueous iron (Fe2+) in which the phytosiderophores are secreted by the roots.In the case of Fe deficiency, disturbances may occur in plants, such as increased formation and activity of carrier cells in the roots, as well as reduced lateral roots [7].Provision of plant nutrient sources must be done in a 4R manner (Right source -Right rate -Right time -Right place) and sometimes it is added with the Right methods, hence becoming 5R.The purpose is to increase crop productivity and create a sustainable farming system [11].The results of analysis of Fe in leaves before the application on observed plants that have symptoms of interveinal chlorosis show a value of 56.54 -72.55 ppm (Table 1).This signifies that the value is below the critical level of Fe in leaves, where it can vary depending on the type of plant and site.The research results on several types of plants show a minimum level of Fe in leaves <70 ppm.Other research concludes that the optimal level of Fe in oil palm leaves is around 50 -250 ppm.However, it depends on the local soil condition [12].Generally, the response to the increased Fe nutrients due to regular Fe fertilizer can be seen after 4 -6 months.

The level of Iron in leaves after the application of Clay Mineral Soil in silt pit
The red or yellow color of the soil is related to the iron-oxide mineral content.The former is influenced by hematite minerals, while the latter is caused by goethite minerals in the soil [13].The amount of Fe in the leaves of the plant in the silt pit treatment group which receive the implementation of Plinthic Kanhapludults (MLA) soil and silt pit, as well as the application Lithic Dystrudepts (MLB) soil, has a significant difference with the control (namely at 24 months after application, as in Table 1).The slow Fe absorption by the plants is assumed caused by the high Fe in the soil is not available for the plants.Moreover, the level of Fe in the control plants also experience a slight increase.It is caused by an increase in the soil organic matter, as a result of OPEFB in the research block.Organic matter may increase the availability of Fe in the soil by forming metal-organic bonds (chelate) -hence reducing chemical fixation or precipitation of Fe in the form of Ferric Hydroxide [9].The results of the reaction with organic matter can increase the redox potential of iron oxide (Fe 3+ ), therefore it shifts into iron hydroxide (Fe 2+ ).This compound can dissolve in water, as well as can be easily taken up by roots [14].

The Level of Iron in Leaves after the Application of Liquid Fe through Root Infusion, Trunk
Injection, and Fe Chelated Methods Based on the results of the analysis of Fe level in the leaves, trunk injection (with up to 4 months after application) shows a faster response rate and higher nutrient increase than root infusion (namely 50.3 ppm / 2 months after application and 38.2 ppm / 4 months after application).However, based on the pattern of Fe increase in the trunk injection treatment group it can only be seen 2 months after application, then it will decrease to 4, 6, 8, and 10 months after application.Meanwhile, the root infusion treatment shows an increase in 2, 4, and 6 months after application, namely 8.6 ppm, 27.0 ppm, and 36.2 ppm -which later decreases in 8 and 10 months after application, namely 2.5 ppm and 10.0 ppm.Fe-chelated treatment showed on 2 months after application Fe levels were the same as before treatment, however on 6 months after treatment Fe levels increased to 74.3 ppm.Based on the trends in the three treatment groups, Fe nutrients in the leaves are considered to be at the optimum level 6 months after application.Nanotechnology can serve as an alternative in dealing with the problems faced by the oil palm industry [15].Through the latest technological approach, trunk injection was applied by a special machine [16].

Conclusion
The results showed that all treatments significantly increased the level of Fe in the leaves with different plant response rates.The fastest response occurs in the plants which belong to the trunk injection and root infusion treatment groups.The implementation of Fe fertilizer through trunk injection was able to increase the amount of Fe in leaves by 50.3 ppm (at 2 months after application), whereas the root infusion method can increase the amount of Fe in leaves by 38.2 ppm (at 4 months after application).It is faster than the regular fertilizer treatment.Meanwhile, the slowest response rate occurs in the soil application treatment.Normally, regular Fe fertilization is enough to satisfy the plant's needs.Application of Fe through trunk injection or root infusion method can each serve as an alternative for short-term use, whereas the application of iron-rich mineral soil can be an option for long-term needs.

Figure 6 .
Figure 6.Fe Level on Leaves 17 th

Table 1 .
Fe level on leaves 17 th on 0,12, and 24 MAT (Month After Treatment)

Table 2 .
Average of Fe level on leaves 17 th in Each Treatment