Improvement of water and nutrient efficiencies oil palm through bio-silicic acid application Peningkatan efisiensi penggunaan air dan hara pada kelapa sawit melalui aplikasi bio-asam silikat

Laksmita P. SANTI, Adhy ARDIYANTO, Agung KURNIAWAN, Lilik A. PRABOWO, & Ian SEBASTIAN 1) Indonesian Research Institute for Biotechnology and Bioindustry, Jl. Taman Kencana No.1, Bogor 16128, Indonesia 2) PT Bumitama Gunajaya Agro, Jl. Melawai Raya No.10, Melawai, Kebayoran Baru, Jakarta Selatan, DKI Jakarta 12160, Indonesia 3) PT Labodia Prima, Jl. Boulevard Barat Raya Kelapa Gading Jakarta Utara 14240, Indonesia


Abstract
Crop water use efficiency is critical for high yields in conditions of limited water supplies. This study aims at determining the effect of bio-silicic acid (BioSilAc) application on water use efficiency and nutrient availability for immature (2 years after planting) and mature (5 years after planting) oil palms in sandy soil during a period of low rainfall. A field experiment was conducted on sandy soil at an oil palm plantation in Central Kalimantan. The experiment was arranged in a randomized block design with seven treatments and three replicates using a combination of composted empty fruit bunches of oil palm (CEFBOP) and BioSilAc applications. The treatments (tree -1 year -1 ) were as follows (tree -1 year -1 ): (T1) 100% NPK standard dosage; (T2) T1 + 1.5 kg quartz sand; (T3) 75% (T1) + 1.5 kg quartz sand; (T4) T1+ 4 tablets BioSilAc; (T5) 75% (T1) + 4 tablets BioSilAc; (T6) T1 + 50 kg CEFBOP + 2 tablets BioSilAc; and (T7) 75% (T1) + 50 kg CEFBOP + 2 tablets BioSilAc. The parameters observed were soil and leaf nutrient contents, average weight, and number of fresh fruit bunch (FFB), and daily water usage and water potential using a sap flow meter and stem psychrometer to calculate water use efficiency in T1 (control) and T5 which represents the application of BioSilAc. The results indicated that the application of 75-100% NPK + 4 tablets BioSilAc tree -1 year -1 in mature oil palm was capable of improving yield of 11.9% (T5) and 12.1% (T4) and water use efficiency of 31.3% (mature) and 50.4% (immature) of the control treatment.

Introduction
Silicon (Si) has been found in significant concentrations in plants. Many studies have shown the beneficial effects of Si in increasing drought tolerance in plants by maintaining leaf water potential, photosynthetic activity, stomatal conductance, leaves erectness, and structure of xylem vessels under high transpiration rates. All these parameters have been widely used as physiological indicators for the selection of drought-tolerant plant materials (Saud et al., 2014). Mono-silicic acid (H4SiO4) in the form of Si used by plants is found both in liquid and adsorbed phases of Si in soils. The concentration of the H4SiO4 in soil solution is influenced by the soil pH, clay, minerals, organic matter, and Fe/Al oxides/hydroxides, which are collectively related to the geological age of the soil (Tubana & Heckman, 2015). Bio cycling of Si in the soil also occurs through microbial activities that involve fungi, bacteria, and actinomycetes. Thus, through their intricate interplay with soil minerals, plants and microbes contribute appreciably to the ecological Si cycle .
Oil palm (Elaeis guineensis Jacq.) is one of the most important oil crop species in the world and is widely cultivated in Southeast Asia, especially in Indonesia. According to Cha-um et al. (2013), fibrous root system of oil palm quickly absorbs water from the soil in areas under irrigation (0.7 m in depth), while deep roots (1.1 m) develop in palms growing in rainfed areas. In mature oil palms, the daily transpiration rate (T) depends on the season, which is approximately 3.3-6.5 mm d -1 in the rainy season and 1.3-2.5 mm d -1 in the dry season (Carr, 2011). Also, the level of precipitation (> 1,500 mm year -1 ) is one of the most important factors in oil palm cultivation. Oil palm is a commodity that is quite sensitive to drought (Putra et al., 2015;Rodríguez & Romero, 2016).
Water availability plays an important role in many aspects of growth, development, and oil palm yield. Several studies indicate that drought lowered the yield of fresh fruit bunches up to 26.3% (Al-Amin et al., 2011). The decline of oil palm yield is a direct effect of inhibition of the photosynthetic activity of oil palm. Drought causes stomatal conductance of oil palm leaves to decline rapidly because of stomatal closure. The stomatal conductance decreases the rate of CO2 diffusion, so that the photosynthesis activity of oil palm is inhibited by drought in the middle period of the life cycle of the tree (Henson & Harun, 2007). Other factors that inhibit the rate of photosynthesis in drought conditions are the relative water content (Zain et al., 2014), chlorophyll content, and the content of nitrogen and phosphorus in leaf tissue of plants which are low (Ashraf & Harris, 2013). It is pertinent to investigate optimum water supply that has been identified as a major cost of oil palm production (Fereres & Soriano, 2007). According to Safitri et al. (2019), the various negative issues associated with the absorption of water by oil palm plants are inversely proportional because water uptake through the roots of the oil palm is relatively low compared to wheat, rice, maize, soybean, and barley. The maximum level of water absorbed in the upper root zone also shows that oil palm plants absorb a lot of rainwater, which is fast circulation compared to groundwater.
Many studies according to Meena et al. (2013), Santi et al. (2018), and Katz et al. (2021) have claimed that Si application can increase the tolerance to drought in plants. The mechanisms for this protection include physiological, biochemical, and biophysical aspects. Silica fertilizer has a double effect on the soil-plant system under abiotic stresses, i.e.: (i) improved plant-Si nutrition reinforces plant-protective properties against diseases, insect attack, and unfavorable climatic conditions, (ii) soil treatment with biogeochemically active Si substances optimizes soil fertility through improved water, physical and chemical soil properties, and maintenance of nutrients in plant-available forms. The plant absorbs Si from the soil solution in the form of mono silicic acid, also called orthosilicic acid (H4SiO4) (Zargar et al., 2019). Furthermore, there were no significant differences in greenhouses experiment regarding the effect of silicic acid and silicate solubilizing microbe (SSM) application on drought resistance of oil palm seedlings (Amanah et al., 2019).
To improve plant-available Si in the soil, silicate solubilizing fungi (SSF) are potentially important in solubilizing insoluble forms of silicate (Santi, 2020). Silicate solubilizing microorganisms have recently attracted great interest. Maleva et al, (2017) reported that silicate solubilizing microbe could play an efficient role not only in solubilizing insoluble forms of silicates but also potassium and phosphates, hence increasing soil fertility and thereby enhancing plant productivity. This study aims at determining water use efficiency and nutrient uptake on immature and mature oil palm in sandy soil, Central Kalimantan, using NPK fertilizer in combination with CEFBOP and BioSilAc.

Bio-Silicic acid preparation
The silica source was quartz type (325-mesh) collected from Bangka-Belitung province, Indonesia. Silica extract was prepared using the method reported by  and . The extract then was mixed with 5% (w/w) selected SSF (Santi, 2020), i.e., Trichoderma polysporum (10 6 propagules). The slow release-tablet form of silica sources is referred to as Bio Silicic Acid (BioSilAc).

The effects of BioSilAc application in immature and mature oil palm
The field experiment was conducted in Central Kalimantan from December 2017 to December 2019. Socfindo varieties were used as immature (2 years after planting) and mature (5 years after planting) oil palms. The experiments were arranged in a randomized block design with seven treatments and three replications. The treatments comprised a combination of CEFBOP and BioSilAc application as well as a reduction of dosages of NPK fertilizers.
Soil samples were air-dried and passed through a 2 mm sieve and analyzed for the following parameters: pH, soil texture, organic-C, nitrogen (N), phosphorus (P), potassium (K), calcium carbonate (CaCO3), cation exchange capacity (CEC), boron (B), and exchangeable aluminum (Al), hydrogen (H), silica (SiO2), and available SiO2. The available SiO2 in the form of silicic acid (H2SiO4) was measured using a Spectronic 21 spectrophotometer at 660 nm wavelength using the method reported by Rinder & Oelkers (2014). Leaf nutrient analysis was carried out to determine the content of N, P, K, Mg, and SiO2 in oil palm leaf. Soil chemical characteristics of the immaturemature-plant plot were as follow in Table 1.

Sap flow measurement
Water-saving in plants mainly refers to the effective use of water resources in the growth and development of plants, thereby increasing crop water use efficiency (WUE). The heat ratio method (Hölttä et al., 2015) was used to measure the sap flow (water usage) with the SFM1 instrument from ICT International (Australia). This instrument consisted of three needles. The middle needle generates a heat pulse, and the other two are each fitted with two extremely sensitive thermocouples. Three needles were installed on oil palm fronds (9 th frond for immature trees and 17 th frond for mature trees) before the rudimentary leaflets. The measurements were real-time recorded at 30 minutes intervals for a cycle of 7 days for immature and mature trees, respectively; the period of time for the frond to change positions in the oil palm tree's phyllotaxy. The heat pulse used was 30 Joules (data not shown). These sap flow meters' measurements were combined with Stem Psychrometer (PSY1), Soil Moisture Meter (SMM1), and Soil Oxygen Meter (SOM1). A solar panel can be directly connected to the non-polarized charging ports to trickle charge the internal battery for continuous field operation. For this measurement, only two treatments were taken, i.e., each from T1 as a control treatment and T5 as the representative of BioSilAc application. During the months of the study (2019), the average rainfall in the site was 158 mm month -1 . According to Oldeman's rainfall type classification, the site belongs to B1 agroclimatic zone with 7-9 consecutive wet months and 0-1 dry months (Table 2).

Chemical characteristics of BioSilAc
Chemical characteristics of BioSilAc was analyzed to determine total SiO2, available Si, heavy metals contents and other parameter based on Kepmentan No.209/Kpts/SR.320/3/2018 standardization of the product of silica an-organic fertilizer in Indonesia. The results showed a high pH and very low heavy metals content (Table 3) found in the BioSilAc. This product had highly available silica in H4SiO4 form at the level of 7.2%, which is higher than that of similar commercial product of silica fertilizer. According to USPTO (2001) about "silica-based fertilizer and method for using the same", claimed that the silica gel could be used as the main component of the fertilizer of the invention contains all stages of silica gel from silica hydrogel to silica xerogel. The silica gel or silica sol to have such a property that the silicic acid (H4SiO4) concentration becomes equal to 5-15 ppm. Furthermore, BioSilAc has a specification parameter in pH and amount of SSF, i.e., 9.6 (pH) and 10,000 -1,000,000 propagule g -1 of T. polysporum. As a result of this study, it is obvious that silica fertilizers in crop plantations are still uncommon and can be regarded as cutting-edge technology in combination with bio-fertilizers. Silica-based fertilizers are commonly made from rice husk, sugar cane bagasse, fly ash, and basic slag, while BioSilAc was prepared from quartz sand extract.

Effects of BioSilAc application on immature oil palm Nutrient use
The data from this experiment showed that the application of organic matter in EFBOP had not been able to suppress the loss of nutrients due to evaporation and adsorption by clay particles. Organic carbon (C) level was measured at very low range (T1 and T2) to moderate (T3-T7). Generally, nutrient uptake of K, Ca, and Mg are reduced by the water deficit. The application of BioSilAc in this area can suppress the level of exchangeable Al in the soil of 34.5% to 69.8%, and therefore the availability of P and K can be optimized for oil palm trees. Furthermore, the total and available SiO2 at T5 treatment were higher than in the other treatments, including the control (T1). Tubana & Heckman (2015) reported that despite its abundance, Si is never found in an available from inside the plant tissue but is always combined with other elements, usually in oxides or silicates. Mono silicic acid (H4SiO4) is the only form of Si used by plants, which is found both in liquid and adsorbed phases of Si in soils. BioSilAc application increased the amount of released Si in the soil solution. According to Santi et al. (2018), the deposition of bio-silica occurs in the endodermis part of roots of oil palm seedling. The results of a field experiment revealed that applying BioSilAc to sandy soil for two years can increase the rate of total and available SiO2 in the soil up to 16.3% (SiO 2 ) and 6.9% (H 4 SiO 4 ) of the control (T1) ( Table 4).
Based on the nutrients analyzed in immature oil palm (Table 5), it is observed that the application of BioSilAc in the nutrients content of N and P were not significantly different than the control treatments, but the value of K was generally higher at 4.7% to 33% with BioSilAc treatments, compared to the control treatments. The results of the analysis showed that the nutrient content of N is high-optimum, while P and K levels were optimum. BioSilAc application increased the silica nutrient in the leaves of immature oil palm by about 4.3-13.5%. Furthermore, the adequacy of micronutrients, such as Cu and B in all treatments were optimum to high level and, the action of SSF and silicic acid reactions in the soil make the silica content of oil palm leaf in T4, T5, and T6 treatments significantly different from T1 (control). However, it is necessary to further study the role of SSF and silicic acid (H4SiO4) in increasing total and available silica in the soil and oil palm leaf, respectively.

Water use efficiency
In oil palm, water deficit also affects sex differentiation, reduces the proportion of female inflorescences, promotes inflorescence abortion, and may affect bunch ripening and oil accumulation (Adam et al., 2011). The potential application of BioSilAc to increase oil palm resistance to drought stress was analyzed directly through the determination of water usage on immature and mature oil palm in the field conditions by using a sap flow meter and stem psychrometer. The sap flow meter was used to monitor oil palm's daily water usage at various conditions. The stem psychrometer monitored the oil palm's water potential, which can be correlated to plant resistance to drought stress. The measurement data of the sap flow meter were analyzed using ICT International Data View software, and this showed that the daily rate and accumulation of water usage in the immature crop control treatment (T1) was greater than the T5 treatment, i.e., 18.05 L tree -1 day -1 compared to 8.96 L tree -1 day -1 (Table 6). This means that the application of BioSilAc to immature oil palm for three consecutive years can improve the water use efficiency, i.e., the amount of carbon assimilated as biomass or FFB produced per unit of water used by the oil palm tree with 50.4%.
According to stem psychrometer data, the T5's (green graph) plant water potential was more negative (-1.5 MPa) than T1 (blue graph, -0.97 MPa). By the end of every day, T5 recovered faster (returned to 0 MPa in the absence of sunlight) than T1 (Figure 1). The results of this study suggest that BioSilAc application on T5 can reduce water consumption in oil palm and increase the water potential (the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, and matrix effects such as capillary action of the plant) thus the photosynthesis rate making more effective recovery possible during drought condition compared to the T1 (control). Furthermore, the measurement of frond 9 th by using Durometer showed that BioSilAc application increased the hardness of immature oil palm frond from 50.5 Hd (T1) to 57 Hd (T5).

Effects of BioSilAc application in mature oil palm Nutrient use
In the plot area of BiosilAc application for mature oil palm, N soil nutrient levels were low to moderate range, while P and K were very low to low, and the organic matter was low to moderate. The application of BioSilAc in T3-T6 treatments increased the total SiO2 and available Si in the soil. The available Si in T4 treatment was higher than that of other treatments. Furthermore, P availability for plant in T2-T7 with quartz sand and BioSilAc treatments were higher than that of the T1 (control) while the concentration of exchangeable aluminum in T2-T7 were lower than T1 (Table 7). In the soil system, the release of Si has more significant interaction with P and other nutrients (Meena et al., 2014). Si application reduces soil sorption of P, especially at low pH, and thus increases the plant-available portion of P in the soil (Greger et al., 2018).   Treatments Perlakuan The soil fertility evaluation standard for oil palm according to Fairhurst and Hardter (2003): N (%) = <0.08 (very low), 0.12 (low), 0.15 (medium), 0.25 (high), > 0.25 (very high); P (ppm) = <120 (very low), 200 (low), 250 (medium), 400 (high), >400 (very high); organic-C (%) = <0.8% (very low), 1.2 (low), 1.5 (medium), 2.5 (high), and >2.5 (very high), pH=<3.5 (very low), 4.0 (low), 4.2 (moderate), 5.5 (high), >5.5 (very high)

Water use efficiency
Sap flow meter and stem psychrometer were installed on the 17 th frond of T1 and T5 treatments. Based on the measurements using the sap flow meter, the water usage for T5's frond was 0.26 L day -1 and was lower than T1's (0.38 L day -1 ). This data showed that the daily water usage of T5 (0.75 L day -1 ) was lower compared to T1 (1.01 L day -1 ). The total daily water use of T1 was 30.38 L tree -1 day -1 , while T5 was 20.86 L tree -1 day -1 (Table 10). BioSilAc application can improve water use efficiency by 31.3%. In plants, water deficiency can result from a deficit of water from the soil, an obstacle to the uptake of water, or excess water loss; in these cases, a similar consequence is the limitation of plant growth and crop yield. According to Deshmukh & Belanger (2015) and Chen et al. (2018), silicon (Si) has been widely reported to alleviate the plant water status and water balance under various stress conditions. Si application was found to be involved in the adjustment of root hydraulic conductance by upregulating aquaporin gene expression and concentrating K in the xylem sap. The potential key mechanisms involved in Si-mediated enhancement of plant root water uptake under water deficiency include (1) enhancement of the osmotic driving force via active osmotic adjustment, (2) improvement of aquaporin transport activity at both transcriptional and posttranscriptional levels, and (3) modification of root growth and increasing root/shoot ratio.
Based on the stem psychrometer observation data, the plant water potential in T5 (green graph) was more negative (-2.2 MPa) than T1 (blue graph, -1.6 MPa) and also recovered faster by the end of every day (Figure 2).
Our study suggested that the BioSilAc application (T5) can reduce water consumption in oil palm. Plant water potential and photosynthesis rate were higher than T1. Furthermore, T5 treatment also showed a more effective recovery rate during drought conditions than control (T1). In brief, the more negative potential will improve turgor pressure so that the plant still has higher stomatal conductance and photosynthesis. The H4SiO4 absorbed (T5) is assumed to be transported through the xylem and is deposited in the frond leaves epidermal surfaces of oil palm, increasing the hardness of frond leaves 58.5 Hd compared to control (T1) 55 Hd.  Table 9. Average productivity of mature oil palm, two-years after BioSilAc application on a regular basis Tabel 9. Produksi rata-rata pada TM setelah dua tahun aplikasi BioSilAc secara rutin

Conclusion
The effect of BioSilAc application on fertilizer use and water consumption efficiencies of oil palm grown on zone B climate-field sandy soil were shown to be effective in this study. The application of 4 tablets BiosilAc tree -1 year -1 could save NPK fertilizers dosage up to 25%, improve WUE up to 31.3% (mature) and 50.4% (immature). The highest yield of FFB was 22.38 tons to 22.4 tons ha -1 year -1 obtained from the combination of 75-100%NPK + 4 tablets BioSilAc tree -1 year -1 . BioSilAc fertilization reduced the need for mineral P and N fertilizers for FFB production and increased the hardness of immature and mature oil palm fronds. However, further study is necessary to confirm the results, particularly under a different agroclimatic area.