Monitoring of pH level for maximizing oil palm ffb yields

Depending on the location of estates and types of soils, monitoring of pH changes is an integral part of soil-water management for realizing sustainable Fresh Fruit Bunches (FFB) yields. In Sumber Tani Agung Resources Tbk, fluctuations in pH levels in field blocks as well as nearby drains were monitored since 2019. The usage of portable units of pH meters made it possible for planters to record and monitor the on-site changes in pH. Monitoring of pH was done on two types of soils namely peat (three estates in North Sumatra) and acid sulfate soils (two estates in South Sumatra). Piezometer was established in each block in all five estates and pH reading was taken biweekly, including at nearby watergates for comparison. Lower pH values ranging from 3.5 to 3.7 were observed at piezometers and water gates on blocks established on acid sulfate soils as compared to pH values recorded in peat areas where slightly higher pH values were registered (4.3 to 5.6). The presence of pyrite contributed to a lower range of pH values on acid sulfate soils. Keeping water level at 40-70 cm on peat and at 45-60 cm for acid sulfate soils would have an impact on the improvement of soil physical chemistry and biology properties, besides creating aerobic conditions for roots of the cultivated oil palms. Organic fertilizer such as bunch ash was applied in the 2020-2021 period which also resulted in an increment in pH values. Generally, there were improvements in oil palm growth as well as palm yield performance in areas with higher pH values (>4.50).


Introduction
Generally, the measurement of pH is used to describe the quality of water, especially in terms of the chemical reaction between acidity and alkalinity.The term pH is based on the logarithmic transformation of the hydrogen ion concentration, [H+].The conditions of acidic to very acidic pH would greatly affect the growth of oil palm plants, especially root growth.Generally, plant roots are unable to absorb water that carries nutrient molecules simultaneously in acidic areas.Acidic to very acidic pH makes the roots thick, and thus thick roots make the roots not long, making the root hairs short [1].This will cause the absorption of nutrients and water will be less, causing the growth and development factors of oil palm to be threatened [2].The peat and sulfate soils are also acidic with high active Al and Fe content [1].High 2 Al and Fe content can be toxic to oil palm plants [3].The optimal soil acidity (pH) to support the growth of oil palm is 5.0-6.0 [4].

Averaging of pH Values
Historically many authors calculate average pH values directly.However, averaging of pH values should be initiated by translating pH values into ion Hydrogen concentration [H+].Generally, pH measurements are made for various purposes in almost every field of biology and chemistry.There is confusion regarding the proper method of describing pH, especially in fields as wide-ranging as anesthesiology [5].
Usually, scientists often wish to summarize and describe pH data with a simple measure of central tendency, such as the average.However, pH data are in Negative log10 which is a transformation of concentration [H+].Averaging pH values directly would be an error [6].For instance, the average pH value for pH=6 and pH=1 is pH =3.5.However, when the same pH values translated into concentration [H+], then averaging of pH 1= 10-1mol [H+] /l & pH 6= 10-6 mol[H+] /l would yield a reading of 5 x 10-2 mol[H+]/l or pH = 1.3.As such, pH 1.3 is far different from pH 3.5.It has been extensively argued that pH values cannot be directly averaged but rather must be back-transformed to [H+] and then averaged.The average [H+] may then be re-transformed to the average pH if desired [6].As such, there are two approaches when pH values are averaged for an interpretation of experiments.
Nevertheless, in this study, all pH values were transformed into concentration [H+] and then after averaging, retransformed into pH values.It seems that there would be a mathematically huge difference in pH values if the pH range is big.However, if the pH range is small, the pH readings in both approaches are nearly similar, resulting in mathematically less meaningful.

Influences and Changes of pH values
Soil acidification can occur under oil palm plantations [7,8].The soil pH changes very slowly and there are no visible symptoms of soil acidification other than declines in crop production which may be dramatic in serious cases [8,9].A study on the characteristics of commonly used eight soil types for oil palm in Southeast Asia, [9] revealed that these soils have a pH of less than 5.0 on the topsoil (0 -30 cm).
[9] also showed that these soils have low to very low contents of nitrogen (N), available phosphorus (P), and exchangeable potassium (K) in six out of eight soil types.Therefore, the availability of soil nutrients especially N, P & K becomes less as soils become more acidic.
However, K uptake by oil palm did not seem to be influenced by low pH levels as reported by [10].A study by [10], revealed that fertilizer application with acidifying fertilizers such as ammonium sulfate had increased soil acidity.[10] also reported a decrease in soil pH values from 4.2 to 3.8 after seven years of NK fertilizer applications.[11] highlighted in their paper that ammonium-based nitrogen fertilizers in excess rates, leaching of nitrate nitrogen, and continual removal of plant & animal waste products could accelerate the change of pH.Many researchers highlighted that oil palm can tolerate well to low pH values in the range of 4 and 5 for commercial oil palm production especially in Southeast Asia [3,7,8,12].[13] reported that (i) experiments carried out in four sites in Indonesia have shown that high yields are possible on soils with low pH and (ii) best management practices can increase pH.[3] also reported that a high FFB yield above 35 tons per ha per year is possible on acid sulfate soils with proper nutrient inputs as well as good water management.

Influences of pH Values on Oil Palm FFB Yields
There are many references published by various researchers on the FFB production in oil palm grown in acidic environments, particularly in acid sulfate and peat soils [7], 12,15,[17][18][19]21].A summary of FFB yields obtained on acid sulfate and peat soils is listed in Table 1.Maximum yields recorded for acid sulfate soils were generally higher as compared to those obtained generally on peat (Table 1).This is due to more compounded factors involved in oil palm cultivation on peat such as low bulk density, palm leaning, and termite attacks [12].Agronomic and management practices such as minimizing the leaching of nutrients, especially K fertilizers, maintaining water level at 50 to 75 cm from the peat surface, detecting and treating termites are among the key aspects to achieving high yield in peat planting [3,12,17].As such, besides pH values, many factors determine high FFB production in oil palm.On acid sulfate soils, Al toxicity and excess sulfates are the major constraints to FFB production in oil palm [3,22].By maintaining water-table at 45 to 60 cm, accelerated pyrite-oxidation was avoided to obtain maximum oil palm yields [3,15,18].[13] highlighted that in one of the treatments, bunch ash application had an ameliorative effect on pH as compared to treatment of ammonium sulfate fertilizer application.However, [13] had advocated to replace bunch ash with Empty Fruit Bunches (EFB).Long term application of EFB in oil palm fields had increased soil pH as well as oil palm FFB yields [23,24].Application of EFB over time generally had improved soil moisture, soil structure, nutrients availability especially N, P, K & Mg as well as pH values [24,25].This overall improvement had contributed in better FFB production in oil palm.
Although numerous information on the improvement of FFB production due to an increase in soil pH is available, there is still a lack of references on the improvement of pH values at commercial scale study.Analytical results on pH changes in soil and water are confined to green house, pot experiments, or smallscale on-site trial plots.Therefore, an attempt was made to monitor water pH levels on a commercial scale in five selected estates in Sumber Tani Agung Resources Tbk, Indonesia.This paper summarizes the results of pH recorded mainly at the piezometer as well as in watergates.A simple correlation analysis was also conducted to estimate the relationship between the recording of pH values and the FFB yield production in the respective field/block.

Details on Study Areas
Details on the location of estates as well as number of piezometers on estate basis are given in Table 2.This study was carried out in five estates in Sumber Tani Agung Resources Tbk (STAR).Studies on changes of pH values in the oil palm grown areas especially on peat soils (number 1,2 &3 in Table 2) and acid sulfate soils (number 4 & 5 in Table 2) were monitored for three-year period 2020-2022.In this study, pH values were also recorded at two different locations, (i) piezometer and (ii) watergates.The watergates are located nearby to piezometer, approximately 100-140 meters away from the piezometer in each block.Efforts are taken to ensure the readings of pH values from the piezometer as well as from watergates are averaged in accordance with the concept of averaging of [H+] in the mixtures/solutions.The water level was kept at 40-70 cm on peat and at 45-60 cm for acid sulfate soils throughout the monitoring period as the water level has an impact on the improvement of soil physical chemistry and biology properties, besides creating aerobic conditions for roots of the cultivated oil palms.
Generally, ratios of land to piezometer were within the range of 24 to 26 ha per piezometer in all estates except for Kuala Puntian Estate (number 4 in Table 2) where a higher ratio of 36 was recorded.Basically, Kuala Puntian Estate was converted from an ex-coconut plantation into the current oil palm plantation.Therefore, the slightly high ratio of land to piezometer in Kuala Puntian Estate was attributed to the bigger block sizes of the ex-coconut plantations era.The same block sizes of ex-coconut were adopted as field blocks for the cultivation of oil palms in Kuala Puntian Estate.
The main soil types in peat areas as in number 1, 2 & 3 in Table 2 are basically Tropohemists/Troposaprists/Tropofibrists, Meanwhile, the dominant soil types in Kuala Puntian and Upang Jaya estates are mainly acid sulfate soils with the family of Sulfaquents,Tropaquepts, and Hydraquents.One of the dominant soil series is the Jawa series.This type of soil is a fine, mixed, isohyperthermic and brown family of Sulfic Tropaquept with poorly drained, brown-colored B horizons and a sulfuric horizon between 50 to 100 cm depth [22].

Details on Study Areas
For this study, pH value from each piezometer was recorded on bi-weekly basis (once in two weeks).A battery-powered Hanna Instruments portable unit was used for reading pH.All pH measurement was taken from the water samples collected at the piezometer in the field (Figure 1).Commercial recording FFB production on block basis was used to compare pH values registered for each block thoroughout three-year period (2020-2022).Data were collected from all five estates as in Table 2 and analysed for correlation study to detect any relationship between pH values and FFB yield production.
pH studies involving various types of soils were mostly made with green house, pot experiments or small-scale on-site trial plots.However in this study, commercial oil palm blocks are involved and total land hectarages from five estates are of above 14,451.6ha as shown in Table 2.As such a method was adopted by taking readings of high pH values, based on the observations for three years i.e., from 2020 to 2022.pH value for each block is recorded and monitored bi-weekly by estate personnel in each study site.

6
After observing blocks with highest pH values, a frequency of occurance of same block throughout the 12 months is calculated and colour-coded for easy identification of blocks.Five field-blocks that have high frequency of highest pH values throughout 12 months (for instance for Batu Mundom Estate in 2022) were identified.Then these high-ranking blocks were selected for correlating with respective block FFB yield recording.The same procedure was repeated for blocks with lowest pH values for the period 2021-2022.An example of selection of blocks with high and low pH values is shown in Table 3.

Table 3: Monthly summary on blocks with five highest and five lowest readings of pH values in 2022 for Batu Mundom Estate. Frequency of occurrence is listed at four columns at far right
There are two calculation methods used in the experiments that involves readings of pH.At first pH values of mixtures are transformed into concentration of [H+] in the mixtures/solutions.According to [6] the concept of calculating the pH leads to the common conclusion that pH data cannot be averaged directly but rather must be transformed to [H+] before the mean is calculated and then retransformed to obtain the average pH.The same method was used in this study whenever average pH values are calculated from population pool.This means all average pH values are based on calculating the amount of H+ in each solution rather than calculation based on direct averaging of pH values.In fact, [5] reported that [H+] averaging (rather than pH averaging) was the best measure of central tendency by making solutions of various pH values with either HCl or NaOH, mixing the solutions,and comparing the calculated pH with the actual pH measurement.

Results and Discussion
Results of pH values recorded with Hanna Instruments for the period of 2020-2022 are given in Table 4.In this study, pH values were recorded at two different locations, (i) piezometer and (ii) watergates which are located nearby to piezometer, approximately 100-140 meters away from piezometer.For the 2020-2022 period, lower pH values ranging from 3.5 to 3.7 were observed at piezometer and water gates on blocks established on acid sulfate soils (Table 4).Meanwhile on peat soil, pH values ranging from 4.3 to 5.6 were registered at piezometer and water gates.Lower pH values on acid sulfate soils are attributed to the presence of the pyrite layer.This pyrite layer can generate excessive acidity, resulting in a pH drops to below 3.5 [3,14,15].Palms will suffer from hyperacidity symptoms.This would result in a poor yield, if oil palm continuously grown on acidity conditions [3,15].At Batu Mundom, Sikapas and Selat Beting estates, water pH values at piezometer were generally lower than those recorded for watergates.However, there was no distinct changes or trend seen for blocks/estates established on acid sulfate soils, probably attributed to (i) presence of pyritic layers within water-controlled horizons and (ii) smooth & continuous removal of H-ion rich-water by practicing a good water management [15,18].

Results of pH Values in Piezometer
Results of a simple correlation analysis to estimate the relationship between the recorded water pH levels and the FFB production for peat and acid sulfate soils are shown in Table 5 and 6 respectively.Outlier values were eliminated after a careful study over the linear correlation relationship.A total of 44, 102, 27, 45 and 90 recordings of pH values were used for obtaining coefficient of determination (R 2 ) for Batu Mundom, Sikapas, Selat Beting, Kuala Puntian and Upang Jaya estates respectively.Based on the results, it is observed that high correlation between pH values and FFB yield recordings was at Batu Mundom Estate in 2021 with R 2 value at 0.7487.Satisfactory R 2 values (> 0.5) were obtained in nine set of data out of fifteen, representing 60% from total set of recordings.
The value of coefficient of determination (R 2 ) was obtained after a simple linear correlation study involving all recorded pH values against the respective FFB production records.R 2 values ranging from 0.0002 to 0.7487 and 0.0235 to 0.7286 were recorded for estates established on peat and acid sulfate soils respectively.Generally, the value of coefficient of determination (R 2 ) in 2022 for blocks established on peat soils was lower than those registered in the years of 2020 and 2021.Lowest R 2 value was recorded in Selat Beting Estate for pH values versus cumulative FFB yields over three years i.e. from 2020 to 2022.This shows that there is other than pH values which has significant influence on the FFB yields of cumulative years.This also suggesting that there are other contributing factors for FFB yields such as palm age, harvesting interval, water management and balanced nutrient inputs.Contribution of such agronomic practices have significant influences on the FFB productions as stated by various researchers [7,8,10,16,18,27].
In Kuala Puntian and Upang Jaya estates, the value of coefficient of determination (R 2 ) in 2020 was relatively lower as compared to those readings from 2021-2022.One of the agronomic practices that have high influence on the changes in soil pH values is application of bunch ash.Application of bunch ash in 2021 in Kuala Puntian and Upang Jaya might have contributed towards an increase in pH values.This corelate well with the findings of [13] where application of bunch ash had increased soil pH compared to application of ammonium sulfate fertilizer.In this study, bunch ash was applied at the rate of 1.75 kg/palm up to 2.5 kg/palm in all estates.As such, application of bunch ash in the fields might had contributed towards the pH increment in the water/piezometer.This needs further verification.2. In 2020, a negative linear relationship was observed for Selat Beting Estate (Figure 2.i).Eventually, this negative linear relationship also had an influence on the cumulative results of pH values versus FFB productions in the period 2020-2022 (Figure 2.l).Further monitoring will be carried out to witness the changes in the coming years.

Figure 1 .
Materials used for pH measurement (a) Battery-powered Hanna Instruments unit for reading pH/EC/TDS, (b) samples of at the piezometer in the field and (c) recording of pH values with Hanna Instruments

Table 1 .
A summary on FFB yields on Acid Sulfate and Peat soils recorded and published by various researchers in oil palm industry

Soil Type/Series Maximum FFB Yield (mt/ha/year) Year of harvestin g at Sources and Remarks Acid Sulfate soils
[12]mercial scale data; no liming was carried out[12].

Table 2 .
Details on Estates where the study on pH values were carried out during 2020-2022

Table 4 .
Summary on changes in water pH values at piezometer & watergates on peat and acid sulfate areas in 2020,2021 & 2022

Table 5 .
Linear relationship of pH values at piezometer versus oil palm FFB yields on peat areas (2020-2022)

Table 6 .
Linear relationship of pH values at piezometer versus oil palm FFB yields on acid sulfate soils,