Optimization of Nutrient Parameters Inuencing Oil Palm Seedling Growth in Soil Amended with Fly Ash and Sludge

The efficient use of organic fertilisers as a source of nutrient has been a concern in the world oilpalm production. The large amount of Palm Oil Mill Sludge (POMS) and Fly Ash (FA) produced in palm oil industries can effectively be utilized as organic fertilizers and so minimize the amount of waste dispose to the environment. This study is aimed at optimizing the nutrient parameters influencing oil palm seedling growth in soil amended with FA and POMS. The nutrient parameters were optimized using Response Surface Methodology (RSM). The optimization results showed that an increase in the mass of FA and a decrease in the mass of POMS result in a decrease in the growth response of the oil palm seedlings. The responses were best with increase in the mass of POMS and a decrease in the mass of FA. Therefore an indirect relationship exists between the masses of FA and POMS. An empirical quadratic polynomial model was developed to relate the interaction of the POMS and FA. The strong linear correlation between the observed and predicted responses showed that the model reasonably predicted the process using a quadratic polynomial. The maximum plant height of 70.4 cm at 15 % soil amendment was obtained at a mass of 1.46 kg FA and 8.54 kg POMS for the 180 days planting period. Results from this study showed that co application of FA and POMS can be very effective at a maximum of 14.6 % FA and minimum of 85.4 % POMS by mass.

combustion, about 15% by weight of solid waste in the form of ash is produced. The ash produced is of two types which include fly ash and bottom ash.
The accumulation of these wastes is a continuous process as the production increases overtime.
So there is need to put all effort in converting the waste into products (such as organic fertilizer and animal feed) that will be of economic importance to the palm oil industry.
Recycling and applying fly ash and sludge for agriculture purpose have many benefits which have been proven by various studies. Keshet al. (2003) stated that FA serve as a repository of soil nutirent which helps to recover and improve soil properties from alkaline and saline soils. FA amended soils influence all of the soil's physical and chemical characteristics (electric conducting (EC) and particle size distributions) (Pathanet al. 2003;Singh et al. 2011). Singh et al. (2010) reported significant improvements in soil-based nutrient levels (e.g. Na, K, Ca, Mg, and Fe) while Surridgeet al. There is high competition of the Palm Oil Wastes for fuel and animal feeds and only small amounts of these wastes are retained in the soil. In addition, some of these wastes are discharged indiscriminately into the environment and thereby polluting the environment. Meanwhile, because these wastes are biodegradable they could serve as alternative source to soil amendment.
The objectives of this study is to optimize the nutrient parameters influencing oil palm seedlings growth on soils amended with FA and POMS using Response Surface Methodology (RSM) and using a developed empirical quadratic polynomial model to relate the interaction of the POMS and FA and thus be able to predict the response of the oil palm seedlings to the different amendments.

2.1.1Sample Collection and Preparation
Soil samples, FA and POMS were collected from Okomuvillage in Ovia South Local Government Area of Edo State. The soil samples were collected 0 -30 cm at about 2km from Okomu oil palm industry. The soil samples were air dried for a period of two weeks before beingcrushed in porcelain mortal and sieved through 2 mm sieve. The air-dried sieved soil samples were stored in air tight polythene bags and labeled prior to analysis. The fly ash was also sieved through a 2mm sieve to obtain fine sample. The sludge sample was stabilized for a period of 100 days as described by Osa-Iguehide et al. (2021). The prepared fly ash and stabilized sludge were separately and co-applied to the soil at different rate (i.e from 2 -15 %).
Plant samples were analyze according to USEPA guidelines.

2.1.2Physicochemical Characterization
The physicochemical properties of the soil, fly ash and stabilized sludge at day 0 and 100 days were determined before and after amendment using standard methods. The pH and Particle size analysis was determined by the method described by Vogel, (2008). Total nitrogen and phosphorus was determined by Kjeldahl method described by Motsara and Roy, (2008). Flame photometric method was used for the analysis of K and Na while Mg, Ca and heavy metals were analyzed using atomic absorption spectrophotometric method described by Wuanaet al.(2008) The plants biometrics were done 180 days after planting.

2.2.1Central CompositeDesign (CCD) of Soil Amendment Process
RSM was used to assess the influence of various process parameters to maximize plant height of oil palm seedlings. The experimental design for this reaction was carried out by employing a full factorial rotatable central composite design (RCCD) for two factors with replicates at the centre point, and star points were used in the investigation by the statistical software package Designexpert ® (version 8.0.6; stat-ease, Inc., Minneapolis, USA). The variables used were mass of FA( ) and mass of POMS (B) each at alpha (-1) and alpha (+1) coded levels. The actual levels of the variables for CCD experiments were selected based on the initial levels as the center points. A total of 13 experimental trials that included 4 trials for factorial design, 4 trials for axial points (two for each variable) and 5 trials for replication of the central points were performed.
The response of the amendment process is plant height incentimeter(cm). Table 1 shows the CCD experimental range of values for the process variables considered for the soil amendment process.
where Y is the response, i.e. the plant height (cm), Xi and Xj represent the independent variables, β0 is constant, βi is linear term coefficient, βii is the quadratic term coefficient, βij is cross-term coefficient and 'n' is the number of process variables studied and optimized during the study.
ANOVA was carried out to estimate the effects of process variables and their possible interaction effects on the maximum plant height in the response surface regression procedure. The goodness and best fit of the model was evaluated by a regression coefficient R 2 . The response surface and contour plots are obtained using the fitted quadratic polynomial equation generated from regression analysis by keeping one of the independent variables at central value (0) and varying the other.

Pot Experiments
A greenhouse pot experiment was adopted in this research. Sprouted palm seedlings obtained from Nigeria Institute for Oil Palm Research (NIFOR) were carefully sown in blackcylindrical polythene bags, each containing 2kg soil mixture (soil + FA + POMS as suggested by RSM).
Prior to planting, the soil mixture was stabilized for a period of two weeks while watering once a day with about 200 cm 3 of water. One sprouted palm seedling was planted per pot and each experiment was performed in triplicate. Surface irrigation was done with borehole water based on the water holding capacity of the soil once a day for the 180 days planting period while plant analysis was done 60, 120 and 180 days after planting (Osa-Iguehie et al. 2021).   Table 2 shows the physicochemical parameters of the soil, fly ash and stabilized sludge. The pH of the soil was 7.70 which is slightly above normal pH for plant growth, even though most soils have pH values between 3.5 and 10 (Queensland Department of Environment and Heritage Protection, 2016). Soil pH is one of the important parameters in soil analysis as it affects most chemical processes. It specifically affects the availability of nutrients to plant by contributing significantly to the chemical forms of the different nutrients thereby affecting the chemical reactions which these nutrients undergo in soil. The fly ash was alkaline with a pH of 10.92; pH value of fly ash has close correlation with its CaO content. High CaO content in fly ash results in higher pH value (Antoni, 2016 The optimum soil pH range for most plants is between 5.5 and 7.5 (Queensland Department of Environment and Heritage Protection, 2016). This is assumed to be the normal pH range for ordinary soils that favour plant and microorganisms. However, many plants have adapted to thrive at pH values outside this range. The particle size analysis (table 3) of the soil shows that, the parent soil has a clay content of 9%, silt 6% and sand 85% which makes the soil to be classified as loamy sand.

3.1RESULTS AND DISCUSSION
To study the individual and interactive effects of variables, a 3 point Rotated Central Composite Design (RCCD) was used for the 2 variables. The factorial point shows the values that are within the range of the coded value from the design of experiment as shown in Table 3. Orders 1, 4, 9 -13 showed the same growth pattern. The similar growth pattern showed by these orders could be attributed to the 50/50 ratio in which these orders were mixed. Order 1 had an FA mass of 1.46 kg and POMS mass of 1.46 kg, order 4 had a FA mass of 8.54 kg and POMS mass of 8.54. Order 9 -13 had FA mass of 5 kg and POMS mass of 5 kg. These orders were used to determine the variation in the optimization process (Table 5). Response A = 2 % Amendment, Response B = 5 % Amendment, Response C = 10 %

Amendment, Response D = 15 % Amendment
During the First 60 days it was observed that the plant height for these orders increased from 2 -from 2 -10 % amendment and a decrease in 15 % amendment. This growth pattern can be attributed to the fact that the FA was more dominant in affecting the plant height due to the slight increase in the pH of these orders as the rate of amendment increased and as such the higher pH led to the slower growth rate during the first 120 days of planting. However after 180 days of planting there was a slight change in the growth pattern as 10 % amendment was observed to have led to increase in the plant height as compared to the growth during 120 days of planting.
The increase in plant height at 10 % amendment could be attributed to decomposition of the organic matter making nutrients available for plant uptake. But this increase did not extend to the plant height at 15 % amendment which may be due to the fact that the higher pH could not have due to poor aeration, depletion of plant-available water leading to the soils becoming hard and cloddy. There was also a gradual decrease in concentration of nitrogen and phosphorus as the rate of amendment increased with orders 2 and 7. The least plant height of 19.8 cm was observed for 15 % order 7 with FA mass of 5.00 kg and 0.00 kg POMS.
The plant height of Orders 3, 5, and 8 increased from 2 -5% amendment and then decreased from 10 -15 % amendment in the first 60 days of planting. This could be attributed to slow decomposition of the organic matter which increased as the rate of amendment increased from 10 -15 % and as such mineralization was slower in these amended samples. This led to the lower nutrient availability of these soils when compared with the 2 and 5 % amendment which has better growth performance due to lower organic matter and as such faster rate of decomposition within the first 60 days. There was a change in this pattern 120 days after planting.
The highest plant height of 58.4 cm was recorded in 15 % order 3. There was a slight increase in the plant height as the rate of amendment increase from 2 -15 % with order 3 and 5 which showed that after 120 days of planting the organic matter has decomposed to an appreciable extent and hence the nutrient is now available for plant uptake. This was also evidence in the nutrient analysis of the plant (Tables 6 -8). However after 180 days of planting, the plant heights were observed to increase tremendously with increase in the rate of amendment. This showed that the rate of mineralization increased with the decomposition of organic matter leading to greater availability of nutrient for the plants and hence the better growth performance with increase in rate of amendment.
The optimum plant height of 70.4cm was observed for 15% order 3 with FA mass of 1.46 kg and POMS mass of 8.54kg.
Presented in Tables 6 -8 is the nutrient analysis of the plant during the planting period.   During the first 60 days, the level of nitrogen, phosphorus and potassium in the plant ranged from 0.077 -0.219 %, 0.058 -0.137 % and 0.023 -0.077% respectively. Orders 3 and 5 showed more nitrogen and phosphorus absorption at 2 and 5 % amendment. It was observed that the absorption of nitrogen and phosphorus increased from 2 -5 % amendment and then decreased gradually from 10 -15 % amendment. The decrease in the absorption of nutrient from 10 -15 % in orders 3 and 5 could be attributed to the high organic matter contents in the orders, this tend to temporarily immobilize these nutrients before the complete mineralization of the organic matter. The higher nutrients absorption at 2 and 5 % of orders 3 and 5 can therefore be attributed to lower organic matter content which decomposed faster and the nutrients was readily release for plant uptake. The absorption of nutrients by the plant was also evident in the plant growth.
There was also a gradual decrease in concentration of nitrogen and phosphorus as the rate of amendment increased in orders 2 and 7. The decrease in absorption of these nutrients in these orders can be attributed to low concentration of these nutrients in FAbecause these orders have higher percentage of FA (85.4 and 100 % for orders 2 and 7 respectively) and thus extremely high pH. Queensland Department of Environment and Heritage Protection (2017) reported that the optimum pH range for most plants is between 5.5 and 7.5. When pH exceeds this range nutrient availability tends to decrease.
After 120 days of planting the absorption of nutrients was similar to what was observed for the 60 days planting period, there was slight increase in the absorption of nutrient by the plants. The highest concentration of nitrogen and phosphorus were found in orders 3 and 5 while the lowest concentration nitrogen and phosphorus were found in orders 2 and 7.
After 180 days of planting it was observed that nutrient absorption in orders 3 and 5 increased with increase in the rate of amendment showing that the organic matter has decomposed and hence the nutrient is now available for plant uptake.This is evident in the growth of the oil palm seedlings.
The highest percentage of potassium was found in orders 2 and 7 through-out the planting period, the high concentration of potassium in these orders could be attributed to the higher percentage of fly ash present in these orders. Orders 3 and 5 have the least concentration of potassium due to lower concentration of fly ash in them. Generally there was a gradual increase in the absorption of potassium as the rate of amendment increased from 2 -15 %.
The plots shown in Figure 1 is the representation of the predicted plant height versus the actual plant height for the various amendments. Tables 9 -12 shows model summary statistics for the plant height.   show that the model (Equations 3 -6) reasonably predicted the process using a quadratic polynomial.
The data obtained were subjected to analysis of variance (ANOVA) at a significant level of 5 %.
In equations (3 -6) above a positive coefficient means that an increase in the variable will cause an increase in the response while a negative coefficient will result in a decrease in the response (Russel, 2009;Ocholiet al. 2018

4.1CONCLUSION
This research attempted to assess the effect of optimizing nutrient parameters on the growthof oil palm seedlings in soil amended with fly ash (FA) and palm oil mill sludge (POMS).The optimization of results using RSM showed that an increase in the mass of FA and a decrease in the mass of POMS resulted in a decrease in the growth response of the oil palm seedlings and vice versa. The result obtained from the developed empirical quadratic polynomial model showed that strong linear correlation existed between the observed plant responses and predicted responses and thus the model reasonably predicted the process using a quadratic polynomial. The maximum plant height of 70.4 cm at 15 % soil amendment was obtained at a mass of 1.46 kg FA and 8.54 kg POMS for the 180 days planting period.
The data obtain from this research revealed that response surface methodology can be used to optimize nutrient parameters influencing oil palm seedling growth in soil amended with fly ash and sludge and that the co-application of FA and POMS can be very effective at a maximum of 14.6 % FA and minimum of 85.4 % POMS by mass for planting oil palm seedlings.

ACKNOWNLEDGEMENT
We are most grateful to the Department of Chemistry University of Benin for providing an enabling environment for the research work.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors,