Assessing the Impact of King Coconut Husk Ash and Biochar, Combined with Chemical Fertilizer Application, on Enhancing Soil Fertility in Coconut Plantations

: Sustainable soil fertility management is crucial for enhancing productivity in coconut plantations. This study investigated the synergistic effects of king coconut husk (KCH) ash, biochar, and chemical fertilizers on soil properties in a coconut plantation over a short period (4 months). Six treatments were applied: control, chemical fertilizers alone (F), fertilizers with ash (FA), fertilizers with biochar (FB), fertilizers with both ash and biochar (FAB), and fertilizers with half ash and biochar (FA 1/2 B). Strongly alkaline KCH ash contained significantly higher total and available potassium content levels than mildly alkaline biochar. Data indicated that KCH ash significantly enhanced soil available potassium, electrical conductivity, and organic carbon content compared to the control and F treatments over a short-term period. Even though biochar application demonstrated initial improvements in soil moisture content, a longer study duration may be required to evaluate its influence on other soil parameters comprehensively. Highlighting the synergistic benefits of KCH ash and biochar, FA 1/2 B treatment exhibited the highest combined index score based on physical, biological, and chemical soil indicators, suggesting its potential for optimizing agricultural outcomes. It emerged as the most promising approach, underscoring the value of exploring sustainable soil amendments derived from agricultural waste streams to promote soil fertility and sustainable coconut production.


Introduction
Coconut is one of the most significant tropical tree crops globally, as well as in local contexts, often called the "tree of life" by many regions in various nations [1].As coconut palms have been cultivated on the same parcels of land for over 60 years, the sustainable utilization of resources and soil fertility have gradually been reduced [2].Along with this, the most formidable challenge facing the coconut industry in the new millennium is achieving a substantial increase in national coconut production to meet the twin objectives of satisfying the domestic demand and catering to the needs of the expanding coconut industry [3].
One effective approach for enhancing soil fertility in this kind of perennial plantation for sustainable agriculture is incorporating organic matter, for example by the methods of composting, crop rotation, green manuring, and intercropping.This not only supplies essential nutrients, but also aids in breaking down clay soils and improving their structure [4][5][6].Another promising strategy is the utilization of biochar and ash in coconut cultivation, as suggested by Rodríguez et al. [7] and Dissanayaka et al. [8].Biochar and ash have emerged as highly promising soil amendments with distinct advantages in recent years.
King coconut husks (KCHs), an abundant agricultural waste material in Sri Lanka from the king coconut water industry, present an opportunity.Converting KCH into biochar and ash mitigates environmental impact and aligns with resource conservation principles [8,9].Numerous studies, including those by Hossain et al. [10] and Knoblauch et al. [11], have highlighted its potential to increase soil organic carbon sequestration while enhancing critical soil health components.These improvements encompass enhanced soil structure, increased soil moisture retention capacity, and improved soil physical quality.Additionally, research has proven that the application of biochar to agricultural fields helps improve airflow through the soil, increases the total pore space available, and leads to an overall enhancement in soil quality and health [12][13][14].KCH ash, as an effective and sustainable soil additive, offers the potential to enhance the productivity of coconut plantations by serving as an economical and eco-friendly source of potassium [8].This, in turn, translates into reduced fertilizer costs and other associated advantages [15,16].Furthermore, the process of KCH transformation into ash contributes to a decrease in waste destined for landfills, aligning with the principles of responsible consumption and production.Moreover, using KCH ash contributes towards improving soil characteristics.It enhances soil structure, augments water-holding capacity, and plays a crucial role in neutralizing acidic soils, rendering the soil more suitable for cultivating coconut trees [17].
It is essential to note that coconut trees thrive in well-draining, potassium-rich soil, a factor vital for their growth and development.However, potassium deficiency is prevalent in many agricultural lands globally, especially in highly weathered tropical soils, where the natural supply of potassium often falls short of the high demand [18].The synergy between KCH ash and biochar, when combined with chemical fertilizers in coconut plantations, can significantly elevate soil nutrient conditions, reduce pollution risks, and promote a more robust soil environment.Nonetheless, the influence of biochar and ash has not consistently yielded positive outcomes, as certain instances have revealed no beneficial effects or even detrimental impacts [19][20][21].Previous studies reported that applying wood biochar to a Typic Kandiudult and rice-straw biochar to Ultiso failed to modify the soil aggregate stability and soil structural stability [22,23].Contrasting these findings, another study indicated that incorporating biochar into the soil does not influence the physical characteristics or water movement and retention capabilities of the soil [24].
A significant portion of the current research findings on the impacts of biochar and ash on soil's physical and hydraulic properties stem from controlled pot experiments using sieved and repacked soils in a regulated, enclosed environment.Therefore, the relevance and applicability of these existing results to actual field conditions in the real world are questionable and subject to scrutiny [25,26].Furthermore, whether this knowledge that applies to king coconut husk biochar and ash applies to real-world farms on a short-term basis is not known.
This experiment aimed to identify the combined influence of KCH ash, biochar, and chemical fertilizer on soil fertility in coconut land, mainly focusing on potassium nutrient replacement.This research addresses critical gaps in our understanding of sustainable soil management and fertility enhancement in coconut lands.Specifically, the objective was to examine the synergistic effects of these soil amendments on key soil characteristics, such as nutrient availability, physical properties, and microbial activity, under actual field conditions in coconut plantations.By examining their synergistic interactions and potential to improve soil properties, this study strives to provide practical insights for farmers, agronomists, and policymakers.Ultimately, the research endeavors to advance coconut agriculture toward greater sustainability by optimizing crop yields while considering broader environmental and economic implications.

Location and Biochar/Ash Production
The experiment was conducted at the Rathmalagara research station, Madampe (Low Country Intermediate Zone; IL1a), at a 15-20-year-old coconut plantation between February 2023 and July 2023.After spending two months on biochar and ash preparation, the field study was conducted over four months.The soils in the study area were classified as part of the Andigama series, which falls under the broader category of Red-Yellow Podzolic soil.Andigama series soils contain more nutrients due to the high CEC (7.4 cmolc kg −1 soil) [12].
Initially, the king coconut husks were sun-dried to reduce their moisture content to approximately 20%.For biochar and ash production, a double-chamber pyrolyzer which was modified by the Coconut Research Institute in Sri Lanka was used (Figure 1).It was developed to produce biochar and ash using king coconut husk as the feedstock, and to produce the fuel through slow pyrolysis at a temperature of 450-600 • C. It was a combination of three main parts: a large outer chamber, a small inner chamber, and a chimney.The operation was based on auto combustion, i.e., after the initial ignition was created at the top of the pyrolyzer, the combustion fire front moved from the top to the bottom of the chamber.The required air for wood combustion entered through primary air holes at the bottom of the chamber.Dried king coconut husk, which was used as a fuel to supply heat by combustion in the pyrolysis process, turned into ash in the presence of air and was collected at the bottom of the pyrolysis unit.The inner chamber produced good-quality biochar.The biochar and ash production process is shown in Figure 2.

Treatments and Experimental Design
Subsequently, six treatment applications were assigned and distributed following Randomized Complete Block Design (RCBD), with four replicates of each treatment.A single palm was used as a replicate.The treatment mixtures were prepared manually.For some treatments, biochar and ash were mixed with conventional chemical fertilizers of coconut (Table 1).Without additional steps, following the treatment plan as in Table 2, the amendments, in conjunction with inorganic fertilizer, were manually and evenly incorporated into the soil within the designated manure circles, which extended up to a radius of 1.8 m from the base of the palm.This is the area in which 79.9% of coconut roots are present [27].The treatment was applied at a depth of approximately 30 cm, as this depth is optimal for targeting the majority of the plant's root system [28].Following the treatment application, dried coconut fronds were applied as mulch to maximize the efficacy of the fertilizer and derive its full benefits.Supplementary irrigation was not practiced.This approach facilitated interaction with the soil matrix and root zone.The field map is shown in Figure 3.  * ERP: Eppawala rock phosphate; MOP: muriate of potash.Control = no application of inorganic fertilizer, biochar or ash; F = application of 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite; FA = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 3.2 kg of ash; FB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite + 30 kg of biochar; FAB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 3.2 kg of ash; FA 1/2 B = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 1.6 kg of ash.

Data Collection and Analysis
Soil samples were accurately obtained manually using a soil augur from the designated manure circle of each palm after the treatment application at the 0-45 cm depth.This sampling process occurred at one-month intervals after the treatment application, spanning a four-month timeline.Laboratory analysis of the collected soil samples was completed following standard analysis protocols.EC measurements were conducted by preparing suspensions of the samples in distilled water, with a ratio of 1 part ash/biochar sample to 20 parts water by weight/volume (w/v) for evaluating ash/biochar samples, and a ratio of 1 part soil sample to 6 parts water (w/v) for soil sample evaluating.These suspensions were measured using EC meters (edge meter, Hanna, Romania) at a temperature of 25 • C, after being shaken for 30 min [29].For measuring pH, the same procedure was used with a 1:10 (w/v) ratio of soil samples.The Kjeldahl method, KCl extraction, acetic acid extraction and UV spectroscopy, and ammonium acetate extraction and atomic absorption spectrophotometry were used to determine the total nitrogen, available nitrogen, available phosphorus, and available potassium, calcium, and magnesium content of the samples [30].Microbial activity was assessed by the sealed incubation of soil samples with NaOH to trap the evolved CO 2 , which was then measured by titration [31].Other than that, moisture content was found by weighing a soil sample, drying it in an oven at 105 • C for 24 h, and calculating the weight loss.Bulk density was determined by taking an undisturbed core soil sample, drying it, and calculating the dry weight per unit volume.Soil temperature was measured using a soil thermometer (WatchDog 2400 with Waterscout SMEC 300 sensor produced by Spectrum Technologies, 3600 Thayer Court, Aurora, IL 60504, USA).Precipitation and atmospheric relative humidity (RH) were also recorded throughout the research period.However, significant fluctuations or variations in the precipitation and RH throughout the study period were not observed.
The statistical analyses in this study were carried out using MINITAB 19 software.Descriptive statistics were employed to provide a comprehensive overview of the dataset.Finally, data were subjected to statistical comparison utilizing one-way analysis of variance (ANOVA) at a 5% significance level, followed by Tukey's pairwise comparison test to identify significant differences between treatment groups.The values of the initial samples were given as covariates in the ANOVA.The correlation coefficient matrix was drawn using the R software package (v.4.0.2) (R Development Core Team, 2014).

Characterization of KCH Ash and Biochar
The electrical conductivity (EC) and pH of both end products showed a significant difference.KCH ash had a notably higher pH (11.31) compared to the KCH biochar (9.19).Additionally, the EC of the biochar (1719.33 µS/cm) was significantly higher than that of the ash (19.70 µS/cm).Lower electrical conductivity indicates fewer dissolved salts.
The measured chemical properties of the KCH biochar and ash offer insights into their suitability for soil amendment and plant growth (Table 3).Mildly alkaline biochar contains significantly lower levels of total potassium (K) content than strongly alkaline KCH ash.The same results were observed for available forms of phosphorus (P) and potassium (K).Although its EC is relatively high, its other available nutrient levels were not significantly different when compared to KCH ash.The choice between these materials depends on soil conditions; biochar can help correct acidity and improve long-term fertility, while king coconut ash is particularly useful for neutralizing highly acidic soils and providing essential nutrients for plant growth [32,33].

Soil Physical Properties
In this study, a significant difference in soil moisture content was observed among various treatments at 4 and 12 weeks after treatment application (p < 0.05), with the highest moisture content recorded in FAB, where a combination of urea, ERP, dolomite, KCH ash, and biochar was applied (Table 4).Nonetheless, the treatments did not exert a statistically significant influence on the soil moisture content measured at 8 and 16 weeks after their application.Biochar application and fertilizer ash were crucial in increasing soil water content.Previous research by Wang et al. [34] supports this finding, indicating that biochar can result in an average 26% improvement in soil moisture in the top 20 cm of soil.Moreover, previous research reported that biochar can enhance moisture retention through improved sorption properties [35,36].These findings underscore the suitability of biochar and fertilizer ash application to impact soil moisture levels significantly.However, this may be confirmed with long-term investigations.Complex soil processes and environmental variability could explain the lack of differences in soil moisture observed at intermediate time points, even when amendments modified moisture levels shortly after treatment application.* Means that do not share a letter are significantly different at p < 0.05 in each column.Control = no application of inorganic fertilizer, biochar or ash; F = application of 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite; FA = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 3.2 kg of ash; FB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite + 30 kg of biochar; FAB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 3.2 kg of ash; FA 1/2 B = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 1.6 kg of ash.
Even though a significant difference was initially obtained among the mean values (p < 0.05) of soil bulk density in the different treatments, any statistically significant impact on soil bulk density was not evident, as evidenced by the subsequent sampling conducted at 8 weeks, 12 weeks, and 16 weeks after treatment application (Table 4).Biochar application has been shown to reduce bulk density, enhancing soil structure and porosity, as supported by studies from Toková et al. [37] and Luo et al. [38].This reduction in bulk density may be more pronounced in the first month after application, but stabilizes over time.Blanco-Canqui [39] highlights that the long-term effects of biochar on bulk density can vary based on interactions with factors like soil type and climate conditions.Similarly, applying fertilizer and ash can influence soil properties, including bulk density, but the magnitude and duration of its effects may differ.Therefore, the observed pattern of bulk density changes over time can be attributed to the synergistic effects of these amendments and their interactions with soil and environmental factors.Also, no significant difference was recorded in the soil temperature at 0-15 cm depth of the manure circle where different treatments were applied at different sampling intervals (Figure 4).This suggests that at the shallow depth range and within the manure circle for a short time period, the applied treatments did not appreciably alter factors that affect soil temperature, like moisture content, thermal conductivity, heat capacity, etc.The research findings indicated that the average soil pH values did not exhibit any statistically significant variations across the different treatment applications within the manure circle areas (Table 5).The ash and biochar applications did not affect the pH, which was expected.According to Faye et al. [40], the study found that a single application of 5-10 t ha −1 rice husk biochar or Typha australis plant biochar and/or manure led to a significant buffering in soil pH from 5.5 to 6.3 in sandy soils in Senegal.These enhanced soil properties persisted and were reported for a minimum of eight years following the biochar and/or manure amendment.This finding suggests that the effect on soil pH may require a more protracted time frame to manifest and that the four-month period under consideration may not be adequate to discern significant alterations in this soil parameter.* Means that do not share a letter are significantly different at p < 0.05 in each column.Control = no application of inorganic fertilizer, biochar or ash; F = application of 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite; FA = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 3.2 kg of ash; FB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite + 30 kg of biochar; FAB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 3.2 kg of ash; FA 1/2 B = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 1.6 kg of ash.
Soil EC values observed in the treatments with biochar and ash were significantly higher compared to the control and F treatments (Table 5).The electrically charged surfaces of biochar particles can adsorb and hold onto plant nutrients, reducing nutrient losses through leaching, chemical fixation, and volatilization [41].This retention of nutrients by biochar occurs because of electrostatic attractions between the charged biochar surfaces and ionic nutrients [42].Furthermore, the porous nature and high surface area of biochar enables it to adsorb fertilizer compounds.The increased retention and availability of fertilizer ions lead to an accumulation of salts that raise EC [12].This aligns with the findings of Cao et al. [43], who recorded that biochar, when combined with fertilizer, can increase soil EC.Notably, research conducted by Kumar et al. [44] demonstrated that biochar amendment resulted in elevated soil EC values, alongside other improvements in soil properties like pH, total carbon (TC), and total phosphorus (TP).Another study, led by Zhaoxiang et al. [45], found that biochar generally enhanced soil properties, including EC, particularly when used at higher rates in conjunction with increased nitrogen (N) fertilization.This boost in EC following biochar application is often associated with alterations in soil pH, as biochar tends to elevate soil pH toward a more alkaline range [37].These collective findings suggest that the combination of biochar with fertilizer can enhance the electrical conductivity of soil, potentially leading to improved nutrient availability and overall soil health.However, this may require confirmation with long-term investigations.

Available Nitrogen Content
In the study, the analysis of available nitrogen content initially showed no significant differences among the treatments, but significant variations emerged at 16 weeks.Significantly, the highest available nitrogen content was recorded in the FA treatment, where a combination of inorganic fertilizer and KCH ash was applied (Figure 5a).The available nitrogen is low in KCH biochar-applied treatments compared to KCH ash-applied treatments.This may be because of the inherent physical and chemical characteristics of the biochar raw material.As shown by Li et al. [46], biochar with higher cation exchange capacity (CEC) reduces ammonification and nitrification rates, leading to lower nitrogen availability.Additionally, Pokharel and Chang [47] found that biochar with higher ash content reduces nitrous oxide (N 2 O) production, which can result in decreased nitrogen availability.Means that do not share a letter are significantly different at p < 0.05.Control = no application of inorganic fertilizer, biochar or ash; F = application of 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite; FA = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 3.2 kg of ash; FB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite + 30 kg of biochar; FAB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 3.2 kg of ash; FA 1/2 B = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 1.6 kg of ash.

Available Potassium Content
In terms of available potassium levels, the study identified notable differences among the treatments.Specifically, treatments that involved the application of KCH ash displayed significantly higher available potassium values compared to both the control and treatments, where only inorganic fertilizers were applied (Figure 5b).Interestingly, a reduction in available potassium values was observed when comparing the treatments involving inorganic fertilizer alone (F) and fertilizer-biochar application (FB) to those that included KCH ash.This suggests that KCH ash, either alone or combined with other amendments, has a notable effect on increasing available potassium levels in the soil, while other treatments resulted in lower available potassium levels in comparison.
The application of KCH ash has proven effective in enhancing the available potassium content in the soil.Notably, a study by Herath and Wijebandara [15] found that a nutrient-rich supplement derived from KCH contained substantial amounts of macro and micronutrients, with potassium being one of them.The potassium content of the nutrient (Supplementary Materials) was approximately three times higher than other locally available organic manures.When tested in field conditions, the nutrient-incorporated treatment (Supplementary Materials) exhibited significantly higher soil K content than the other treatments.This highlights the potential of KCH ash as a valuable source of potassium for coconut cultivation [8].KCH ash can be utilized as a nutrient (Supplementary Materials) in coconut cultivation, effectively boosting potassium levels in the soil.However, when biochar is applied as the sole amendment, it may not provide a sufficient K content to significantly raise overall potassium levels, which could explain the reduction in available potassium content observed in FB [48].

Available Phosphorus Content
The available phosphorus content exhibited no initial significant differences, but showed a significant effect in the different treatments at 16 weeks, with higher values in the fertilizer-ash mixed treatment (FA) (Figure 5c).Treatments involving biochar application also showed a decrease in available phosphorus content.The available phosphorus values were high in the king coconut husk ash treatment with fertilizer because the ash contains significant amounts of phosphorus (Table 3).According to Herath and Wijebandara [15], applying bagasse ash and rice husk ash to agricultural land has been shown to maximize the chemical health of the soil, particularly in terms of available phosphorus.
The retention of nitrogen and phosphorus by biochar can contribute to the reduction in available nitrogen and phosphorus in the soil, slowly releasing them over time [49].According to Das and Ghosh [50], the slow-release behavior of biochar-based fertilizers helps reduce nutrient leaching and improve nutrient use efficiency.Biochar also has the potential to retain and absorb major nutrients, suppressing their leaching potential [8].

Available Calcium and Magnesium Content
While prior studies have demonstrated an increase in the available Ca and Mg content in soil through the addition of ash, biochar, or fertilizers, the current research did not observe any statistically significant differences among the mean values (p > 0.05) or any discernible trend.This lack of observable effects might be attributed to the short duration of the study period (Table 6) [51].* Means that do not share a letter are significantly different at p < 0.05 in each column.Control = no application of inorganic fertilizer, biochar or ash; F = application of 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite; FA = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 3.2 kg of ash; FB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite + 30 kg of biochar; FAB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 3.2 kg of ash; FA 1/2 B = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 1.6 kg of ash.

Organic Carbon Content
Even though a significant difference was not initially observed among the mean values, a significant effect was observed on the organic carbon content at 12 and 16 weeks between the different application methods.Significantly higher values were recorded in ash-applied treatments at 16 weeks compared to biochar-applied treatments (Table 6).However, Saletnik et al. [52] found that the soil carbon content increased after the use of biochar and ash combination, compared to the no-amendment plot.Lengthening the duration of the experiment will facilitate obtaining a clearer trend regarding changes in organic carbon content over time.

Soil Biological Properties
Even though previous studies showed higher microbial activity with the biochar amendment, a significant difference at p > 0.005 was not observed among the mean values of soil microbial activity among the different treatments in this study [53][54][55].
This mainly happens due to modified soil properties and habitat provision with biochar addition.When it comes to the impact of ash mixing with the soil, no significant reduction in microbial activities at p > 0.05 was observed, as mentioned by Bang-Andreasen et al. [56].However, there were no significant differences in microbial activity between the treatments after 16 weeks, so it is possible that the short duration of the experiment may not have allowed enough time for significant changes in microbial activity to occur.Additionally, other factors such as soil type, initial microbial community composition, and environmental conditions may have influenced the results.Further investigation and longer-term evaluation periods are needed to understand the behavior of these amendments on microbial activity in soil.
The graphical representation below reveals the positive and negative correlations of the measured variables of varying magnitudes (Figure 6).When the application of soil amendment targets an increment of available soil potassium content, most of the measured parameters can be increased.Other than that, a scree plot was developed to determine the number of principal components to retain in Principal Component Analysis (PCA) depending on the amount of variation they cover (Supplementary Figure S1).Based on its results, PCA plots were designed with identified three major principal components to explain 49.5% of the total variability (Supplementary Figures S2 and S3).A small angle between two vectors implies a positive correlation between two variabilities while the formation of a large angle between two vectors suggests a negative correlation.A 90 • angle indicates no correlation between the two characteristics.Supplementary Figures S4 and S5 summarized the contribution of variables to the principal components.
A combined index derived from the P Index, R Index, and Score Index was used for determining the most effective combination of ash, biochar, and inorganic fertilizer, which resulted in significant improvements across soil physical, biological, and chemical properties.According to the results, FA 1/2 B, which involved the application of 800 g of urea, 900 g of ERP, 1000 g of dolomite, 30 kg of king coconut husk biochar, and 1600 g of king coconut husk ash, shows the highest potential for optimizing agricultural outcomes (Table 7).However, it is important to remember that these are preliminary results that can be verified and possibly confirmed with long-term investigations.Control = no application of inorganic fertilizer, biochar or ash; F = application of 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite; FA = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 3.2 kg of ash; FB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite + 30 kg of biochar; FAB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 3.2 kg of ash; FA 1/2 B = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 1.6 kg of ash.
The findings from this study highlight the potential benefits of integrating biochar and ash derived from king coconut husks with inorganic fertilizers in coconut plantations.The synergistic effects of these amendments can lead to improvements in soil nutrient availability, physical properties, and biological activity, ultimately enhancing soil fertility and potentially increasing crop yields.However, it is essential to note that the observed results are specific to the experimental conditions and soil types studied, and further long-term investigations under diverse field conditions are necessary to validate and generalize these findings.Additionally, it is crucial to consider the economic feasibility and environmental implications of implementing these soil amendment strategies on a larger scale.Factors such as the availability and cost of biochar and ash production, transportation logistics, and potential impacts on greenhouse gas emissions and carbon sequestration should be evaluated.Collaboration between researchers, farmers, and policymakers is essential to develop practical and sustainable soil management practices tailored to local conditions and resources.

Conclusions
This study evaluated the potential of utilizing KCH ash and biochar, in conjunction with chemical fertilizers, as soil amendments for improving soil health in coconut plantations.The findings demonstrate the significant benefits of integrating KCH ash into the soil, as it markedly increased available potassium levels, electrical conductivity, and organic carbon content compared to the control and fertilizer-only treatments.While biochar application initially improved soil moisture retention, its effects on other soil parameters were less pronounced within the four-month study duration.Notably, the FA 1/2 B treatment, which involved the application of chemical fertilizers, biochar (30 kg), and half of the recommended quantity of KCH ash (1.6 kg), exhibited the highest combined index score based on physical, biological, and chemical soil indicators over this short period.These points suggest that this specific combination of amendments has the potential to optimize agricultural outcomes in coconut plantations.By leveraging these byproducts, coconut cultivation can benefit from enhanced soil fertility, improved nutrient availability, and reduced reliance on synthetic fertilizers.Furthermore, the utilization of KCH ash and biochar aligns with the principles of circular economy and sustainable waste management, mitigating the environmental impact associated with agricultural residues while simultaneously promoting soil health and productivity.Future long-term studies encompassing a broader range of soil types and environmental conditions are recommended to further validate and refine these findings, ultimately paving the way for the widespread adoption of these sustainable soil management strategies in coconut cultivation.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/crops4020017/s1, Figure S1: Scree plot analysis showing the percentage of explained variations in response to the number of principal components for the ap-plication of king coconut husk ash and biochar, combined with chemical fertilizer, on enhancing soil fertility in coconut plantations; Figure S2: PCA for the parameters influencing the application of king coconut husk ash and biochar, combined with chemical fertilizer, on enhancing soil fertility in coconut plantations; the relationship between principal components l and ll; Figure S3: PCA for the parameters influencing the application of king coconut husk ash and biochar, combined with chemical fertilizer, on enhancing soil fertility in coconut plantations; the relationship between principal components l and lll; Figure S4

Figure 4 .
Figure 4. Treatment effects on soil temperature at different sampling weeks.

Figure 5 .
Figure 5. Available macronutrients of soil in various treatments at 16 weeks after treatment establishment; (a) available nitrogen; (b) available potassium; (c) available phosphorus.Means that do not share a letter are significantly different at p < 0.05.Control = no application of inorganic fertilizer, biochar or ash; F = application of 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite; FA = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 3.2 kg of ash; FB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 0.9 kg of muriate of potash + 1.0 kg of dolomite + 30 kg of biochar; FAB = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 3.2 kg of ash; FA 1/2 B = 0.8 kg of urea + 0.9 kg of Eppawala rock phosphate + 1.0 kg of dolomite + 30 kg of biochar + 1.6 kg of ash.

Table 1 .
Major nutrients and their percentages in applied chemical fertilizers.

Table 2 .
Treatments and treatment application rates (per palm).

Table 3 .
Nutrient composition of KCH biochar and ash.
* Means that do not share a letter are significantly different at p < 0.05 in each column.KCH = king coconut husk.

Table 4 .
Treatment effects on soil moisture content and bulk density at different sampling intervals (weeks).

Table 5 .
Treatment effects on soil pH and EC at different sampling intervals (weeks).

Table 6 .
Treatment effects on available calcium, available magnesium, and organic carbon content at 16 weeks after treatment establishment.