Sustainable cocoa intensification: evaluating biochar placement methods and MKP application rate on yield in degraded tropical soilsAbstract
Cocoa productivity in degraded tropical soils is frequently constrained by low nutrient retention, high leaching losses, and poor soil structural stability, particularly in highly weathered acidic soils. In smallholder cocoa systems, the limited use of soil amendments such as biochar further reduces nutrient use efficiency and crop performance. This study evaluated the effects and interactions of cocoa pod husk biochar placement techniques and mono potassium phosphate (MKP) application rates on the physiological characteristics and productivity of 10-year-old cocoa trees in a smallholder plantation in East Kalimantan, Indonesia. A split-plot design was employed, with three biochar placement methods (ring placement, biopore insertion, and surface broadcasting) assigned to the main plots and four MKP rates (0, 10, 20, and 30 g tree−1) to the subplots. Biochar placement significantly influenced stomatal density, whereas MKP application rate significantly affected most reproductive and yield-related traits. Significant interaction effects between biochar placement and MKP rate were observed for the number of flower clusters, yield per tree, and yield per hectare. Orthogonal polynomial analysis revealed predominantly quadratic dose-response relationships, indicating the existence of optimal MKP rates under each biochar placement method. Surface broadcasting consistently produced the strongest yield response, with coefficients of determination reaching R2 = 0.95. Correlation analysis showed that yield per hectare was strongly associated with the number of harvested pods and yield per tree, whereas 100-seed weight and stomatal density were not significantly correlated with final productivity. These findings suggest that yield improvement in cocoa is primarily driven by enhanced fruit retention rather than seed size. Overall, the integration of strategically applied cocoa pod husk biochar with optimized MKP fertilization represents a promising strategy to improve nutrient use efficiency and productivity in smallholder cocoa systems.
Keywords:
biochar placement; cocoa yield; cocoa pod husk biochar; mono potassium phosphate; nutrient use efficiency
Resumo
A produtividade do cacau em solos tropicais degradados é frequentemente limitada pela baixa retenção de nutrientes, elevadas perdas por lixiviação e baixa estabilidade estrutural do solo, sobretudo em solos ácidos altamente intemperizados. Em sistemas de produção de cacau de pequenos produtores, o uso limitado de condicionadores de solo, como o biochar, reduz ainda mais a eficiência no uso de nutrientes e o desempenho das culturas. Este estudo avaliou os efeitos e as interações entre técnicas de aplicação de biochar de casca de cacau e diferentes taxas de aplicação de fosfato monopotássico (MKP) sobre as características fisiológicas e a produtividade de cacaueiros com 10 anos de idade, em uma plantação de pequenos produtores em Kalimantan Oriental, Indonésia. Foi adotado um delineamento em parcelas subdivididas, com três métodos de aplicação de biochar (aplicação em anel, inserção em bioporos e distribuição superficial) como parcelas principais, e quatro taxas de MKP (0, 10, 20 e 30 g planta−1) como subparcelas. A aplicação de biochar influenciou significativamente a densidade estomática, enquanto as taxas de MKP afetaram significativamente a maioria das características reprodutivas e relacionadas à produtividade. Efeitos significativos de interação entre a aplicação de biochar e as taxas de MKP foram observados no número de inflorescências, produtividade por planta e produtividade por hectare. A análise por polinômios ortogonais revelou padrões predominantemente quadráticos de resposta à dose, indicando a existência de níveis ótimos de MKP sob cada método de aplicação de biochar. A distribuição superficial apresentou, de forma consistente, a maior resposta produtiva, com coeficientes de determinação atingindo R2 = 0,95. A análise de correlação indicou que a produtividade por hectare esteve fortemente associada ao número de frutos colhidos e à produtividade por planta, enquanto o peso de 100 sementes e a densidade estomática não apresentaram correlação significativa com a produtividade final. Esses resultados sugerem que o aumento da produtividade do cacau é impulsionado principalmente pela maior retenção de frutos, e não pelo tamanho das sementes. De modo geral, a integração do biochar de casca de cacau aplicado estrategicamente com a adubação otimizada de MKP representa uma estratégia promissora para melhorar a eficiência no uso de nutrientes e a produtividade em sistemas de produção de cacau de pequenos produtores.
Palavras-chave:
aplicação de biochar; produtividade do cacau; biochar de casca de cacau; fosfato monopotássico; eficiência no uso de nutrientes
1. Introduction
Cocoa (Theobroma cacao L.) is a strategic plantation crop that plays a central role in the global chocolate industry and serves as a primary source of income for millions of smallholder farmers across tropical regions (Garcia et al., 2024; Imran, 2025). Indonesia is recognized as one of the world’s leading cocoa producers, with the majority of production derived from smallholder plantations (Widhiyoga and Wijayati, 2022; Rheavanya et al., 2024). Despite its importance, national cocoa productivity remains relatively low and has stagnated in recent years. This limitation is closely associated with declining soil fertility, low nutrient use efficiency, and limited adoption of site-specific nutrient management practices in degraded tropical soils.
Cocoa-growing areas in the tropics are typically dominated by highly weathered soils characterized by low pH, low cation exchange capacity (CEC), and high susceptibility to nutrient losses through leaching (Arévalo-Hernández et al., 2022; Awazi et al., 2025; Steeley et al., 2026). Under such conditions, phosphorus (P) is readily fixed by aluminum (Al) and iron (Fe), while potassium (K) is prone to leaching, particularly in coarse-textured soils (Michael, 2023). Both nutrients, however, are essential during the reproductive phase of cocoa development (Kaba et al., 2022; Rosyady et al., 2023). Phosphorus plays a key role in energy transfer and flower differentiation, whereas potassium regulates photosynthate translocation, fruit development, and seed filling (Rawat et al., 2022; Jiaying et al., 2022; Khan et al., 2023). Insufficient availability or inefficient utilization of these nutrients often leads to high levels of flower and cherelle drop, ultimately reducing yield.
Biochar has been widely recognized as a promising soil amendment for restoring the fertility of degraded tropical soils. It is a stable carbon-rich material produced through biomass pyrolysis, characterized by a high surface area and porous structure that enhance soil water and nutrient retention (Kapoor et al., 2022; Khan et al., 2024). Biochar application has been shown to increase soil pH and CEC, while reducing nutrient losses through leaching, thereby improving soil quality and crop performance (Omara et al., 2023). In cocoa production systems, cocoa pod husk residues are abundantly available and represent a valuable feedstock for biochar production, supporting circular agriculture and sustainable waste management practices.
Nevertheless, most biochar studies in plantation crops have primarily focused on application levels, with comparatively limited attention given to application techniques. In perennial systems such as cocoa, the method of biochar placement is a critical factor influencing its spatial distribution within the root zone and its interaction with plant roots and applied fertilizers. Different placement strategies, such as surface broadcasting, soil incorporation, or localized application, may result in distinct agronomic outcomes. Therefore, evaluating biochar placement techniques is essential not only for improving crop performance but also for enhancing input-use efficiency.
In addition to improving soil properties through biochar, the rapid supply of nutrients during the reproductive phase can be achieved through the application of highly soluble fertilizers. Mono potassium phosphate (MKP) is a readily soluble source of phosphorus and potassium that does not contain nitrogen, making it particularly suitable for supporting reproductive development without promoting excessive vegetative growth (Wirajaya et al., 2022; Chtouki et al., 2024; Rohcahyani et al., 2025). However, the effectiveness of MKP in acidic tropical soils is strongly influenced by soil chemical properties and nutrient retention capacity (Awwal et al., 2025). The integration of biochar and MKP may therefore create a synergistic effect, whereby biochar enhances nutrient retention and use efficiency, while MKP supplies readily available P and K.
To date, studies examining the interaction between cocoa pod husk biochar placement techniques and MKP application levels in cocoa systems remain limited. Few studies have comprehensively evaluated how these combined strategies influence soil chemical properties, plant physiological responses, and yield components under smallholder conditions. Such understanding is crucial for developing precise, efficient, and sustainable nutrient management strategies for cocoa production.
Therefore, this study aims to evaluate the effects of cocoa pod husk biochar placement techniques and MKP application levels, as well as their interaction, growth, and productivity of cocoa. The findings are expected to contribute to the development of nutrient-efficient and sustainable cocoa intensification strategies based on locally available resources.
2. Methods
2.1. Study site and experimental period
The field experiment was conducted from January to May 2025 in a farmer-managed cocoa plantation located in Dusun Danau Redan, Teluk Pandan District, East Kutai Regency, East Kalimantan, Indonesia (0°04’06” N; 117°22’29” E; 31 m above sea level). The site represents typical lowland tropical conditions of smallholder cocoa production systems, with alluvial soil type.
During the experimental period, total rainfall was 969.0 mm, with monthly rainfall ranging from 29.0 to 300.0 mm. The number of rainy days varied from 2 to 20 days per month, indicating humid tropical conditions. Rainfall data for May were recorded until May 8. The mean air temperature in the region typically ranges from approximately 26 to 28 °C, reflecting humid lowland tropical conditions.
2.2. Plant material and experimental design
The study utilized 10-year-old productive cocoa trees (clone MCC01) planted at a spacing of 3 × 3 m. Uniform trees in terms of vigor and canopy structure were selected prior to treatment application. The experiment was arranged in a split-plot design with three replications, which is consistent with standard field experiments in perennial crops where environmental variability is relatively controlled within experimental blocks.
Biochar application technique was assigned as the main plot factor, while mono potassium phosphate (MKP) application rate served as the subplot factor. The biochar placement treatments consisted of ring placement, biopore insertion, and surface broadcasting. MKP was applied at four levels: 0, 10, 20, and 30 g tree−1. The combination of treatments resulted in 12 treatment combinations, which were replicated three times, yielding a total of 36 experimental plots. Each experimental plot consisted of two cocoa trees; therefore, a total of 72 trees were used in the experiment. In this study, the experimental unit was defined as a plot consisting of two trees. A schematic diagram of the experimental layout is presented in Figure 1 to illustrate the split-plot arrangement.
Schematic layout of the split-plot experimental design showing the arrangement of treatments across three replications. Biochar placement techniques were assigned as main plots, where b1 = ring placement, b2 = biopore insertion, and b3 = surface broadcasting. Mono potassium phosphate (MKP) application levels were assigned as subplots, where p0 = 0 g tree−1, p1 = 10 g tree−1, p2 = 20 g tree−1, and p3 = 30 g tree−1.
2.3. Field preparation and crop management
Prior to treatment application, trees were pruned to remove unproductive branches and improve light interception. Field sanitation was conducted by removing weeds, plant residues, and infected fruits to reduce pest and disease incidence. Pest and disease management was performed preventively through routine sanitation practices; however, no quantitative monitoring or scoring of pest and disease incidence was conducted during the experimental period. Basal fertilization was applied one week before biochar application using 1 kg cattle manure and 50 g urea per tree.
2.4. Biochar production and application
Biochar was produced in situ from cocoa pod husk residues collected from the plantation, without the addition of external materials. The feedstock was sun-dried and pyrolyzed in a closed drum kiln under limited oxygen conditions for 4-6 h at temperatures ranging from 200 to 400 °C. After pyrolysis, water was applied to terminate combustion and prevent excessive ash formation. The resulting biochar had a pH of 8.0, organic carbon content of 45%, organic matter of 77%, total nitrogen of 2.3%, available phosphorus of 183 mg kg−1, exchangeable potassium of 641 mg kg−1, and ash content of 9.47%.
Biochar was applied at a rate equivalent to 10 t ha−1 using the designated placement techniques. Ring placement involved incorporation into a shallow circular trench (5 cm depth, 1.5 m diameter) around the canopy projection. The biopore method consisted of four vertical holes per tree (50 cm depth and 15 cm diameter). In the surface broadcasting treatment, biochar was evenly distributed on the soil surface surrounding the tree. All applications were performed at approximately 60 cm from the trunk.
2.5. MKP application
MKP fertilizer was dissolved in water according to the respective treatment levels and applied as a foliar spray using a backpack sprayer, with approximately 5 L of solution per tree.
2.6. Soil sampling and analysis
Composite soil samples were collected before treatment application and at the end of the experiment from five sampling points per plot at a depth of 0-20 cm. Samples were analyzed for soil chemical properties in accredited soil laboratories.
2.7. Observed variables and data analysis
Observed variables included number of flower clusters, fruit set, retained fruits, harvested pods, 100-seed weight (at 8% moisture content), stomatal density, yield per tree, and yield per hectare. Data were subjected to analysis of variance (ANOVA) based on a split-plot design. When significant effects were detected, means were compared using the Least Significant Difference (LSD) test at the 5% significance level. Pearson correlation analysis was performed to evaluate relationships among variables. In addition, orthogonal polynomial analysis was conducted to assess dose-response relationships of MKP application levels.
3. Results
3.1. Analysis of variance
The analysis of variance showed that biochar application techniques had a highly significant effect on stomatal density (SD) but did not significantly influence the other observed traits (Table 1). In contrast, MKP application levels significantly affected most variables, except the weight of 100 dried seeds at 8% moisture content (W100) and stomatal density. Significant interaction effects between biochar placement and MKP application rate were observed for most variables, except for the number of fruit set (NFS), number of harvested pods (NHP), and stomatal density.
Mean squares from split-plot ANOVA showing the effects of biochar application techniques and MKP fertilizer doses on cocoa traits.
The coefficient of variation (CV) for biochar treatments ranged from 4.38% to 47.72%, with the highest variability observed in the number of retained fruits (NRF) and the lowest in W100. Similarly, CV values for MKP treatments ranged from 6.21% to 41.60%, with the highest variation in harvested pods and the lowest in W100.
3.2. Correlation analysis
Correlation analysis revealed that most observed variables were positively associated with yield per hectare (YH) (Table 2). Significant positive correlations were found between YH and the number of flower clusters (r = 0.65*), fruit set (r = 0.61*), retained fruits (r = 0.70*), harvested pods (r = 0.97**), and yield per tree (YT) (r = 1.00**).
Pearson correlation coefficient matrix among observed traits in relation to cocoa yield per hectare.
In contrast, 100-seed weight (r = 0.02ns) and stomatal density (r = −0.45ns) were not significantly correlated with YH.
3.3. Orthogonal polynomial analysis
3.3.1. Flowering and fruit development
Figure 2 illustrates the response of the Number of Flower Clusters (NFC), Number of Fruit Set (NFS), and Number of Retained Fruits (NRF) to increasing MKP application rate under different biochar application techniques. The response of NFC to increasing MKP application rate exhibited distinct patterns across treatments. Under ring placement (b1), the relationship followed a quadratic model (R2 = 0.213; r = 0.46), indicating a weak association and suggesting that increasing MKP application rate did not consistently stimulate flower cluster formation. In contrast, biopore insertion (b2) showed a very strong positive linear response (R2 = 0.9529; r = 0.98), demonstrating that higher MKP application rate were closely associated with increased NFC. Surface broadcasting (b3) exhibited a strong quadratic pattern (R2 = 0.8617; r = 0.93), indicating the presence of an optimum dose beyond which NFC declined.
Response of Number of Flower Clusters, Number of Fruit Set and Number of Retained Fruits to MKP application rate in various biochar application techniques based on orthogonal polynomial analysis.
A similar variation was observed for NFS. Under ring placement (b1), the quadratic model (R2 = 0.70; r = 0.84) suggested a strong relationship with evidence of an optimum dose. The response under biopore insertion (b2) was slightly weaker (R2 = 0.6543; r = 0.81) but still indicated a positive trend up to a certain threshold. The strongest response was recorded under surface broadcasting (b3), with a quadratic model showing R2 = 0.9922 and r = 0.99, reflecting a very strong association between MKP application rate and fruit set formation and clearly indicating an optimum level.
The NRF exhibited quadratic responses across all application techniques. Under ring placement (b1), the relationship (R2 = 0.6586; r = 0.81) indicated a strong positive trend up to an optimum dose followed by a decline. A comparable but slightly stronger response was observed under biopore insertion (b2) (R2 = 0.7816; r = 0.88). The most pronounced response occurred under surface broadcasting (b3), where the quadratic model (R2 = 0.9224; r = 0.96) demonstrated a sharp increase in retained fruits up to the optimum dose and a subsequent reduction at higher MKP levels.
3.4. Yield per Tree and Yield per Hectare
Figure 3 presents the response of Yield per Tree (YT) and Yield per Hectare (YH) to increasing MKP application rate under different biochar application techniques. The response of YT exhibited quadratic patterns across all treatments, with varying strengths of association. Under ring placement, the relationship was weak (R2 = 0.2784; r = 0.53), indicating limited responsiveness of yield to increasing MKP application rate despite the presence of an apparent optimum trend. In contrast, the biopore system showed a strong quadratic response (R2 = 0.8061; r = 0.90), suggesting a more consistent increase in yield up to the optimum dose. The strongest response was observed under surface broadcasting, where the quadratic model (R2 = 0.95; r =0.98) demonstrated a very strong and highly consistent increase in yield followed by a decline at higher MKP levels.
Response of Yield per Tree and Yield per Hectare to MKP application rate in Various Biochar Application Techniques Based on Orthogonal Polynomial Analysis.
A similar trend was observed for YH, which also followed quadratic dose-response patterns across treatments. The weakest association occurred under ring placement (R2 = 0.2784; r = 0.53), whereas the biopore system produced a strong response (R2 = 0.8061; r = 0.90). Surface broadcasting again generated the most pronounced relationship (R2 = 0.9508; r = 0.98), confirming a clear optimum MKP application rate and a highly consistent production response at the hectare scale.
4. Discussion
The present study demonstrates that cocoa responses to nutrient management are governed not only by fertilizer application levels but also by the spatial placement of soil ameliorants, particularly biochar within the root zone. The significant interaction between biochar placement techniques and MKP application across most reproductive and yield-related variables indicates that the effectiveness of phosphorus (P) and potassium (K) fertilization is strongly mediated by rhizosphere conditions. In acidic tropical soils with low cation exchange capacity and high leaching potential, biochar improves soil physicochemical properties, thereby enhancing nutrient retention and fertilizer-use efficiency (Huang et al., 2023; Liu et al., 2025). These findings confirm that biochar primarily functions as a soil conditioner that facilitates more efficient utilization of applied nutrients rather than acting solely as a nutrient source.
As an independent factor, biochar placement significantly influenced stomatal density but had limited effects on most yield components. This suggests that biochar initially affects plant physiological processes, likely through improvements in soil moisture retention and nutrient availability, before translating into yield responses. Previous studies have shown that enhanced water availability and cation balance in the root zone can regulate stomatal behavior, thereby influencing photosynthetic performance and transpiration dynamics (Behzad et al., 2023; Navarro et al., 2025; Jahan et al., 2025). However, without sufficient nutrient supply, these physiological improvements alone may not result in increased productivity. This explains the relatively weak main effect of biochar when applied alone and the pronounced response observed when combined with MKP, indicating a synergistic interaction between soil amelioration and nutrient supply.
In contrast, variation in MKP application significantly affected most reproductive and yield-related parameters, except for 100-seed weight and stomatal density. The strong responses observed in flower cluster formation, fruit set, fruit retention, and harvested pods highlight the critical role of P and K during the reproductive phase of cocoa. Phosphorus is essential for energy transfer and reproductive organ development, whereas potassium regulates photosynthate translocation and osmotic balance, both of which are key determinants of fruit retention (Rawat et al., 2022; Ganeshamurthy et al., 2023). The absence of a significant effect on 100-seed weight indicates that yield improvement was primarily driven by an increase in fruit number rather than seed size. This suggests that, under the conditions of this study, enhancing reproductive success is more effective for increasing yield than improving individual seed mass.
The interaction between biochar placement and MKP application further indicates that crop response to fertilization depends on the spatial distribution of biochar within the soil. The predominantly quadratic dose-response patterns observed in the orthogonal polynomial analysis suggest the existence of an optimum MKP application rate, beyond which yield declines. This pattern is consistent with the nutrient balance principle, whereby nutrient supply enhances plant performance up to an optimal threshold, after which excess availability may lead to nutrient imbalance or osmotic stress (Delgado et al., 2024; Zhandybayev et al., 2024). Among the evaluated techniques, surface broadcasting consistently produced the strongest responses for most yield parameters, including yield per tree and per hectare. This may be attributed to a more uniform distribution of biochar within the canopy projection zone, facilitating greater interaction between roots, biochar, and applied nutrients. In mature cocoa systems with extensive lateral root distribution, such spatial coverage appears more effective than localized placement methods, although the biopore technique showed a strong response in early reproductive traits.
Correlation analysis further indicated that cocoa productivity is primarily determined by quantitative yield components, particularly fruit formation and retention, rather than physiological traits or seed size. The strong association between yield per hectare and the number of harvested pods reflects the cumulative nature of yield formation in perennial crops (Haque and Sakimin, 2022; Sousa et al., 2022; Subramaniyan et al., 2023). According to the yield component framework described in Marschner’s Mineral Nutrition of Higher Plants (Marschner, 2012), total yield is more strongly influenced by the number of reproductive units than by their individual weight. The lack of correlation between 100-seed weight and final yield suggests that seed size is more closely related to assimilate partitioning during seed filling rather than overall yield formation. Similarly, stomatal density contributes to physiological regulation but does not directly explain yield variation unless it is associated with improved reproductive success.
Although this study provides important insights into the interaction between biochar placement and MKP application, it was conducted over a single production cycle (January-May 2025). The observed variables mainly focused on reproductive and yield components, which are known to respond within a relatively short time frame. However, longer-term studies are required to evaluate the consistency of these responses under varying environmental conditions and across multiple production cycles.
From an agronomic perspective, these findings highlight the importance of integrating soil amelioration strategies with precise nutrient management in cocoa production systems. The use of cocoa pod husk-derived biochar supports circular agriculture while improving soil quality and long-term nutrient retention. When combined with optimized MKP application, this approach enhances fertilizer-use efficiency and reduces nutrient losses through leaching. Therefore, sustainable intensification of cocoa production in degraded tropical soils should not rely solely on increasing fertilizer inputs but rather on optimizing the interaction between soil amendments and nutrient management strategies. Overall, this study demonstrates that improving cocoa productivity in smallholder systems requires a synergistic approach that integrates soil quality enhancement with targeted nutrient supply to support reproductive success and yield formation.
Data Availability Statement
The data supporting the findings of this study are available from the corresponding author upon reasonable request.
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Editor:
Takako Matsumura Tundisi
