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Full Research Article

Physiological Indicator for Egg Plant Growth in Red Yellow Podzol Soil with Beneficial Reuse of Pollutant Materials

H
Hasan Basri JUMIN1,*
0000-0002-5996-3421
M
Mardaleni1
E
Ernita1
S
Sulhaswardi1
S
Selvia Sutiana1
1Faculty of Agriculture, Islamic University of Riau, Indonesia.

ABSTRACT

Background: Red-yellow Podzol soil is generally regarded as infertile and covers approximately 45.7 million hectares in Indonesia, accounting for about 29.7% of the national land area. Despite these limitations, this soil type can be utilized for agricultural production through the application of appropriate soil amendments and alternative nutrient sources.

Methods: A two-factor experiment was conducted at the experimental station of the Faculty of Agriculture, Islamic University of Riau, Indonesia. The first factor examined the effect of Pekanbaru city wastewater on eggplant (Solanum melongena L.), while the second factor was the pplication rate of palm kernel shell ash. Observed parameters included net assimilation rate (NAR), mean relative growth rate (MRGR), dry matter weight and total plant biomass.

Result: Net assimilation rate and mean relative growth rate served as physiological indicators of dry matter accumulation. The combined application of city wastewater and palm kernel shell ash showed a significant interactive effect on eggplant growth. Increases in NAR were consistently followed by increases in MRGR, resulting in a significant enhancement of dry matter production. The application of palm kernel shell ash contributed to the amelioration of excessive iron and aluminum availability in red-yellow Podzol soil, thereby improving nutrient uptake efficiency. Overall, the integrated use of city wastewater and palm kernel shell ash significantly increased eggplant dry matter yield on red-yellow Podzol soil.

INTRODUCTION

Red-yellow Podzol soils are widely utilized for rice cultivation, plantations, rubber gardens, coffee and oil palm production. In Indonesia, these soils are extensively distributed across West Java, Sumatra, Kalimantan, Sulawesi, Nusa Tenggara and Papua. They cover approximately 45.7 million hectares, accounting for about 29.7% of the country’s total land area (Badan Pusat Statistik Indonesia, 2023).
       
Red-yellow Podzol soils are characterized by strong acidity, low organic matter content and poor availability of macro nutrients, particularly phosphorus, which typically ranges from 2 to 10 mg kg-1. The low availability of phosphorus is mainly caused by its fixation through binding with Al3+ and Fe3+ ions. These soils generally have a cation exchange capacity of less than 16 cmol kg-1, base saturation below 35%, low organic carbon content and high concentrations of aluminum and iron, resulting in a high phosphorus fixation capacity. Soil pH commonly ranges from 3.10 to 5.00, classifying these soils as strongly to very strongly acidic. Such acidity is primarily attributed to intensive leaching of essential base cations, which reduces soil fertility and limits agricultural productivity (Prasetyo and Suriadikarta, 2006; Sulaiman et al., 2018; Adlim et al., 2020).
       
Pekanbaru is a rapidly developing city on the island of Sumatra, where population growth is largely driven by economic expansion. The rapid establishment of new industries and increased local transmigration have contributed significantly to urban development. However, liquid waste generated from traditional markets in Pekanbaru is poorly managed and poses a serious threat to environmental quality. This wastewater produces unpleasant odors, degrades urban aesthetics and contaminates ground water as well as surface water bodies such as rivers and lakes, which are widely used for drinking water, domestic activities and other essential purposes.
       
Waste derived from oil palm fruit processing, particularly palm oil shells (endocarps), may negatively affect agricultural land if not properly managed. Because palm oil shells decompose slowly, farmers commonly burn them to produce charcoal or ash. Previous studies have reported that palm oil shell ash contains essential nutrients, including calcium, magnesium and other minerals (Jumin et al., 2021; 2024). In addition, ash application can enhance the availability of key soil nutrients such as nitrogen, phosphorus, calcium and magnesium. The presence of calcium and magnesium is particularly important, as these elements can reduce the toxic effects of excessive aluminum and iron in acidic soils (de Oliveira et al., 2013).
       
Palm oil shell ash has long been used by farmers in Sumatra and preliminary observations indicate that its application can improve vegetative growth and yield of oil palm plants. These observations highlight the potential of palm oil shell ash as an alternative organic soil amendment. Moreover, palm oil shell ash may help alleviate aluminum and iron toxicity in swampy or strongly acidic soils, thereby improving soil chemical properties and crop performance (Costa et al., 2004). In Red-yellow Podzol soils, the effectiveness of Rhizobium is often limited due to low calcium and magnesium availability (Adlim et al., 2020). Reduced nutrient availability and soil acidity can suppress photosynthetic activity, which is reflected in lower mean relative growth rate (MRGR) and net assimilation rate (NAR). Net assimilation rate, which represents the rate of dry matter accumulation, is strongly influenced by leaf area, as leaves capture solar radiation and facilitate the absorption of carbon dioxide (CO2) and water for photosynthesis. The availability of CO2, water and oxygen is closely associated with light interception by the plant canopy.
       
Therefore, this study aims to evaluate the interaction between urban wastewater and palm oil shell ash on the mean relative growth rate and net assimilation rate of eggplant (Solanum melongena L.) grown in Red-yellow Podzol soil.

MATERIALS AND METHODS

Experimental design and plant material
 
The study was conducted from August 2023 to July 2025 at Agricultural Laboratory and the Experiment Station of the Islamic University of Riau, Indonesia using eggplant (Solanum melongena L.) as the experimental crop. The experiment employed a two-factor factorial design arranged in a completely randomized design (CRD). The first factor was city wastewater, applied at four levels: 0.0 mL L-1, 500 mL L-1, 750 mL L-1 and 1000 mL L-1. The second factor was the dose of palm kernel shell ash (Fig 1), consisting of four levels: 0.0 g plant-1, 250 g plant-1, 500 g plant-1 and 750 g plant-1. Plants were grown under natural light conditions with a photoperiod typical of the local environment.

Fig 1: Experiment materials.


 
Mean relative growth rate (MRGR)
 
Dry weight (W) data of eggplant biomass were collected over four sampling periods at 7-day intervals, starting from 7 days after planting (DAP). The mean relative growth rate (MRGR) was calculated using the equation proposed by South (1995).

     
Where,
W1 and W2= Plant dry weights at times t1 and t2, respectively.
 
Net assimilation rate (NAR)
 
Net assimilation rate (NAR) was calculated based on total dry weight per unit leaf area. Sampling was carried out four times at 7-day intervals, beginning at 7 DAP. Data were analyzed using the equations proposed by Briggs and Whittinghamf (1952)  and Vernon and Willson (1963), as follows:

       
Where,
W1 and W2= Total plant dry weights. A1 and A2= Leaf areas at times t1 and t2, respectively.
 
Leaf area and growth analysis
 
Leaf area was measured from subsamples using a leaf area meter and image analysis software following the method of Briggs and Whittinghamf (1952). Leaf area (L) was recorded four times during the observation period. The mean values of dry weight (W) and leaf area (L) were then used to calculate the efficiency of dry matter production (EM), representing the mean assimilation efficiency for each time interval (t2-t1), as proposed by Vernon and Allison (1963).

 
Net assimilation rate (NAR) represents the increase in total plant dry weight per unit leaf area over a given period of time.
 
Wastewater analysis
 
The chemical composition of the city wastewater was analyzed using standard laboratory procedures at the Laboratory of Agricultural Technology. Parameters analyzed included nitrogen, phosphorus, calcium, pH, metals, heavy metals and other potential pollutants. The characteristics of city wastewater in Pekanbaru, as in many urban areas of Indonesia, are influenced by population density, domestic activities and industrial inputs. Although wastewater characteristics may vary depending on sampling location and time, both physical properties (color, temperature and odor) and chemical properties were determined using appropriate standard analytical methods.
 
Dry weight measurement
 
Plant dry weight was measured four times during the growth cycle. Samples were collected at 7, 14, 21 and 28 days after planting (DAP). Dry weight was used as a key parameter for evaluating MRGR and NAR, following the method described by Jumin et al., (2019).

RESULTS AND DISCUSSION

Characteristics of domestic wastewater in pekanbaru
 
Domestic wastewater in Pekanbaru City, Riau Province, Republic of Indonesia, originates mainly from household activities such as toilets, kitchens and laundry. Wastewater samples were collected from four different areas of Pekanbaru City, namely the southern, northern, eastern and western zones. Additional contributions to wastewater originate from commercial activities, including restaurants, traditional markets and small workshops, as well as minor industrial discharges such as food processing units and car-washing facilities. In several drainage systems, stormwater run off mixes with domestic sewage (Fig 1),  further influencing wastewater characteristics.
       
The analysis of wastewater quality indicated high levels of organic pollution. Biological Oxygen Demand (BOD) values were high, reflecting a substantial organic load, while Chemical Oxygen Demand (COD) values were typically two to three times higher than BOD, confirming the dominance of biodegradable organic matter. Dissolved oxygen (DO) concentrations were very low, indicating intensive microbial decomposition of organic substances (Tabel 1). The pH of the wastewater ranged from slightly acidic to near neutral, which is typical for urban domestic wastewater.
 
Effect of treatments on plant growth
 
Vegetative growth of eggplant continued into the juvenile stage, during which physiological growth parameters, namely net assimilation rate (NAR) and mean relative growth rate (MRGR), were evaluated. The responses of eggplant to the combined application of city wastewater and palm kernel shell ash are presented in Fig 2 and 3. The observed variations in NAR and MRGR indicate that plant growth dynamics were strongly influenced by nutrient availability and environmental conditions during the vegetative phase. These parameters provide a sensitive measure of the efficiency of photosynthesis and biomass accumulation under different treatment combinations. 

Fig 2: Relationship between net assimilation rate × (mg. cm-2 day-1) and mean relative growth rate × (0.01 g. day-1) under the influence of palm kernel shell ash and city wasterwater in the egg plant.



Fig 3: Net assimilation rate × (0.001 mg. cm-2. day-1) of egg plant under control of palm kernel shell ash and cirty waste water.


       
Main Sources in Pekanbaru Domestic wastewater (toilets, kitchens, laundry) as contributor at the Pekanbaru city Riau province, Republikc of Indonesia. Wasterwater  collected from wasterwater  Pekanbaru city in  four place; south, north, east and west. Commercial activities (restaurants, markets, small workshops). Minor industrial discharge (food processing, car washing). Storm water runoff mixing with sewage in some drainage systems. Biological Oxygen Demant (BOD), Indicates high organic load,  Typically 2-3 × BOD value and chemical Oxygen Demant (COD Very low due to organic pollution, Dissolved Oxygen (DO) Very low due to organic pollution and pH slightly neurtal to acid.
       
The vegetative growth continued into the juvenile stage, during which   NAR and  MRGR were analyzed. The results of the treatments are presented in (Fig 2 and 3). The combined application of palm kernel shell ash and city wastewater significantly enhanced the net assimilation rate (NAR) and mean relative growth rate (MRGR) of eggplant. A strong positive linear relationship was observed between palm kernel shell ash and city wastewater with respect to NAR (y = 0.9004x + 11.073; R² = 0.8515) (Fig 2), indicating a substantial improvement in photosynthetic efficiency associated with increased nutrient availability.
       
Under control conditions, the relationship between NAR and MRGR remained positive but was comparatively weaker (y = 0.6221x + 2.6887; R² = 0.5435) (Fig 3), suggesting limited physiological response in the absence of external nutrient inputs. In contrast, egg plants treated with 1000 mL L-1 city wastewater combined with 750 g plant-1 palm kernel shell ash at 60 days after planting exhibited a very strong linear relationship between NAR and MRGR (y = 0.979x + 0.7562; R² = 0.9692) (Fig 4). This indicates highly efficient biomass accumulation driven by enhanced photosynthetic activity.

Fig 4: Relationship between net assimilation rate and mean relative growth rate of eggplants (g/plant) after treated with city waster water and eggshell ash of palm oil at 60 days age.


       
These results clearly demonstrate a synergistic interaction between city wastewater and palm kernel shell ash. The combined application likely improved nutrient uptake, metabolic activity and carbon assimilation, thereby promoting vegetative growth and overall plant performance. The observed linear interaction pattern indicates that an increase in one factor was followed by a proportional increase in the effect of the other. In this study, simultaneous increases in palm kernel shell ash dosage and city wastewater concentration consistently enhanced MRGR.
       
The relationship between NAR and MRGR under the influence of palm kernel shell ash and city wastewater was predominantly linear, indicating that an increase in NAR was consistently accompanied by an increase in MRGR. Although NAR and MRGR represent distinct physiological parameters, they are closely linked through their shared dependence on photosynthetic efficiency and biomass allocation processes (Poorter and Nagel, 2000).
       
Based on the present findings, appropriate increases in the concentration of city wastewater and palm kernel shell ash positively affected both NAR and MRGR (However, excessively high application rates of palm kernel shell ash may pose potential risks to agricultural land, particularly in red yellow podzol soil or poorly drained soils, due to possible nutrient imbalance or alkalinity stress.
       
The results further indicate that plants became more tolerant of Red-Yellow Podzol soil conditions following treatment with city wastewater and palm kernel shell ash. This response is expected, as Red-Yellow Podzol soils are characterized by low pH and deficiencies in calcium and magnesium (Table 1). Palm oil shell ash is known to contain calcium, potassium and magnesium compounds (Hutahaen, 2007; Herman and Rolly, 2018). Calcium and magnesium are essential macronutrients; however, their availability in Red-Yellow Podzol soils is often restricted due to high iron and aluminum concentrations. Nutrient inputs from city wastewater significantly improved soil fertility and plant growth, consistent with previous findings (Kouadio, 2018; Al-Shamary et al. 2025).

Table 1: The chemical composition of waster water at pekanbaru 2024.


       
Plants grown in soils with varying mineral compositions, including calcium, magnesium and other essential and non-essential elements, tend to adjust their NAR and MRGR to optimize growth and adapt to specific ecological conditions (Beyer and Onken, 2004). This adaptive response was clearly reflected in the improved physiological performance observed in this study.
       
The influence of wastewater application  growth is presented in Fig 5. Although palm oil shell ash is generally regarded as an agricultural waste product (Su et al., 2018), in this study it functioned effectively as a nutrient source due to its beneficial mineral composition. Wastewater-derived nutrients enhanced soil productivity and egg plant growth by increasing soil pH and reducing the toxic effects of excessive iron and aluminum, thereby improving calcium and magnesium availability.

Fig 5: Relationship between seed dray weight, net assimilation rate and mean relative growth of egg plants after treated with city waster water and palm kernel shell ash (series 1 fruit weight (g/fruit), series 2 is MRGR × (0.01 mg/day), series 3 is NAR × (0.003 mg/cm2/day).


       
Furthermore micro organisms bio;ogical prosess  improved the soil micro environment around root hairs, enhancing biological fot nutrient uptake ofthe plants and increasing the availability of nutrients essential for peanut growth (Choudhary and Verma, 2024). Palm oil shell ash, which contains nitrogen, magnesium and other minerals, further conpensated for nutrient deficiencies commonly found in red yellow podzol soil or acidic soils, making such environments more suitable for horticultural crops (Ohanaka et al., 2025).
       
The relationship between NAR and MRGR for eggplant (Fig 2) revealed strong interdependence, with increases in NAR consistently followed by increases in MRGR. This positive relationship resulted in greater dry matter accumulation and ultimately higher yields, particularly in peanut plants (Fig 5). Leaf expansion plays a critical role in light interception and photosynthetic efficiency. Reduced leaf expansion limits light absorption, while palm kernel shell ash soil pollutants may induce chlorophyll deficiency, thereby suppressing photosynthetic rates (Kumar et al., 2025; Kumar and Singh, 2022). The application of palm oil shell ash to Red-Yellow Podzol soils increased calcium and magnesium availability while reducing iron and aluminum uptake by plants (Ewelike et al., 2021).
       
Chlorophyll acts as a photosynthetic antenna, capturing light energy for photochemical reactions that convert solar energy into carbohydrates such as glucose (Pezeshki, 2018; Kumar and Singh, 2022); The mean relative growth rate (MRGR) reflects overall plant growth over a given period without directly accounting for leaf-level efficiency (South, 1995), whereas net assimilation rate (NAR) provides a more precise estimate of photosynthetic activity per unit leaf area per day. Thus, MRGR serves as a rapid indicator of whole-plant growth dynamics, while NAR offers insight into physiological efficiency (Vernon and Allison, 1963, Shipley, 2006). Simultaneous evaluation of these parameters is essential for early detection of growth limitations.
       
Organic matter present in city wastewater can reduce excessive iron and aluminum availability by limiting their uptake, particularly in acidic and peat soils (Ungureanu et al., 2021; Kouadio et al., 2018; Khamidah and Saputra, 2020). In Red-Yellow Podzol soils, palm oil shell ash further immobilizes iron and aluminum, rendering them less available to plants. Consequently, the interaction between city wastewater and palm kernel shell ash helps regulate metal concentrations and improve soil chemical balance. The calcium, magnesium and supplied by both inputs increase soil pH toward near-neutral conditions, thereby enhancing nutrient availability and plant growth (Al-Shamary et al., 2025).
       
According to Gage (2004), soil microorganisms play a crucial role in decomposing organic matter and regulating the soil microclimate, which facilitates nitrogen absorption through plant–microbe interaction in the rhizosphere. Microbial activity benefits both partners: bacteria supply nitrogen to plants through biological prosess (Table 2.1 and Table 2.2), while plants provide carbon substrates to microorganisms. During organic matter decomposition, micro nutrients also become available, further supporting eggplant growth (Brewin, 2010, Boyle, 2020 and Etasami, 2022).

Table 2.1: Chemical characteristic pekanbaru city wastewater 2024.



Table 2.2: Biological characteristics.


               
Overall, the patterns of NAR and MRGR were closely aligned, as dry matter accumulation reflects sustained photosynthetic activity. NAR represents the efficiency of chlorophyll in capturing solar energy, while MRGR reflects cumulative biomass production over time. Under healthy growth conditions, leaves efficiently absorb light and synthesize carbohydrates. However, under stress conditions such as disease or pest infestation, photosynthetic efficiency declines and resulting in reduced NAR. In such cases, MRGR may not decrease proportionally, leading to deviations from linearity between these two parameters.

CONCLUSION

The interactive treatment of Pekanbaru city wastewater and palm kernel shell ash produced significant differences in the net assimilation rate and mean relative growth rate of egg plants grown in red-yellow podzol soil. 
       
This study suggests that although palm kernel shell ash is commonly regarded as a pollutant that can contaminate soil and water bodies, it also has potential as a valuable source of organic nutrients.  In healthy plants—those not affected by disease or pest infestation-the relationship between net assimilation rate and mean relative growth rate typically linear. However, when leaves are infected by bacteria or fungi, the net assimilation rate tends to decline, while the mean relative growth rate may remain relatively stable. Under such conditions, the relationship between net assimilation rate and mean relatave growth rate becomes non-linear.
       
Conclusion as the combined application of city wastewater and palm kernel shell ash represents a sustainable soil management strategy for improving crop productivity on marginal red-yellow podzol soils, while simultaneously promoting the beneficial reuse of urban and agro-industrial waste.

ACKNOWLEDGEMENT

We thank to Rector Islamic University of Riau for Financial support to publishing of our paper the Agricultural Science Digest ARCC journal, The special thank to vice Rector in Administrative and Finance support. 
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.

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    Full Research Article

    Physiological Indicator for Egg Plant Growth in Red Yellow Podzol Soil with Beneficial Reuse of Pollutant Materials

    H
    Hasan Basri JUMIN1,*
    0000-0002-5996-3421
    M
    Mardaleni1
    E
    Ernita1
    S
    Sulhaswardi1
    S
    Selvia Sutiana1
    1Faculty of Agriculture, Islamic University of Riau, Indonesia.

    ABSTRACT

    Background: Red-yellow Podzol soil is generally regarded as infertile and covers approximately 45.7 million hectares in Indonesia, accounting for about 29.7% of the national land area. Despite these limitations, this soil type can be utilized for agricultural production through the application of appropriate soil amendments and alternative nutrient sources.

    Methods: A two-factor experiment was conducted at the experimental station of the Faculty of Agriculture, Islamic University of Riau, Indonesia. The first factor examined the effect of Pekanbaru city wastewater on eggplant (Solanum melongena L.), while the second factor was the pplication rate of palm kernel shell ash. Observed parameters included net assimilation rate (NAR), mean relative growth rate (MRGR), dry matter weight and total plant biomass.

    Result: Net assimilation rate and mean relative growth rate served as physiological indicators of dry matter accumulation. The combined application of city wastewater and palm kernel shell ash showed a significant interactive effect on eggplant growth. Increases in NAR were consistently followed by increases in MRGR, resulting in a significant enhancement of dry matter production. The application of palm kernel shell ash contributed to the amelioration of excessive iron and aluminum availability in red-yellow Podzol soil, thereby improving nutrient uptake efficiency. Overall, the integrated use of city wastewater and palm kernel shell ash significantly increased eggplant dry matter yield on red-yellow Podzol soil.

    INTRODUCTION

    Red-yellow Podzol soils are widely utilized for rice cultivation, plantations, rubber gardens, coffee and oil palm production. In Indonesia, these soils are extensively distributed across West Java, Sumatra, Kalimantan, Sulawesi, Nusa Tenggara and Papua. They cover approximately 45.7 million hectares, accounting for about 29.7% of the country’s total land area (Badan Pusat Statistik Indonesia, 2023).
           
    Red-yellow Podzol soils are characterized by strong acidity, low organic matter content and poor availability of macro nutrients, particularly phosphorus, which typically ranges from 2 to 10 mg kg-1. The low availability of phosphorus is mainly caused by its fixation through binding with Al3+ and Fe3+ ions. These soils generally have a cation exchange capacity of less than 16 cmol kg-1, base saturation below 35%, low organic carbon content and high concentrations of aluminum and iron, resulting in a high phosphorus fixation capacity. Soil pH commonly ranges from 3.10 to 5.00, classifying these soils as strongly to very strongly acidic. Such acidity is primarily attributed to intensive leaching of essential base cations, which reduces soil fertility and limits agricultural productivity (Prasetyo and Suriadikarta, 2006; Sulaiman et al., 2018; Adlim et al., 2020).
           
    Pekanbaru is a rapidly developing city on the island of Sumatra, where population growth is largely driven by economic expansion. The rapid establishment of new industries and increased local transmigration have contributed significantly to urban development. However, liquid waste generated from traditional markets in Pekanbaru is poorly managed and poses a serious threat to environmental quality. This wastewater produces unpleasant odors, degrades urban aesthetics and contaminates ground water as well as surface water bodies such as rivers and lakes, which are widely used for drinking water, domestic activities and other essential purposes.
           
    Waste derived from oil palm fruit processing, particularly palm oil shells (endocarps), may negatively affect agricultural land if not properly managed. Because palm oil shells decompose slowly, farmers commonly burn them to produce charcoal or ash. Previous studies have reported that palm oil shell ash contains essential nutrients, including calcium, magnesium and other minerals (Jumin et al., 2021; 2024). In addition, ash application can enhance the availability of key soil nutrients such as nitrogen, phosphorus, calcium and magnesium. The presence of calcium and magnesium is particularly important, as these elements can reduce the toxic effects of excessive aluminum and iron in acidic soils (de Oliveira et al., 2013).
           
    Palm oil shell ash has long been used by farmers in Sumatra and preliminary observations indicate that its application can improve vegetative growth and yield of oil palm plants. These observations highlight the potential of palm oil shell ash as an alternative organic soil amendment. Moreover, palm oil shell ash may help alleviate aluminum and iron toxicity in swampy or strongly acidic soils, thereby improving soil chemical properties and crop performance (Costa et al., 2004). In Red-yellow Podzol soils, the effectiveness of Rhizobium is often limited due to low calcium and magnesium availability (Adlim et al., 2020). Reduced nutrient availability and soil acidity can suppress photosynthetic activity, which is reflected in lower mean relative growth rate (MRGR) and net assimilation rate (NAR). Net assimilation rate, which represents the rate of dry matter accumulation, is strongly influenced by leaf area, as leaves capture solar radiation and facilitate the absorption of carbon dioxide (CO2) and water for photosynthesis. The availability of CO2, water and oxygen is closely associated with light interception by the plant canopy.
           
    Therefore, this study aims to evaluate the interaction between urban wastewater and palm oil shell ash on the mean relative growth rate and net assimilation rate of eggplant (Solanum melongena L.) grown in Red-yellow Podzol soil.

    MATERIALS AND METHODS

    Experimental design and plant material
     
    The study was conducted from August 2023 to July 2025 at Agricultural Laboratory and the Experiment Station of the Islamic University of Riau, Indonesia using eggplant (Solanum melongena L.) as the experimental crop. The experiment employed a two-factor factorial design arranged in a completely randomized design (CRD). The first factor was city wastewater, applied at four levels: 0.0 mL L-1, 500 mL L-1, 750 mL L-1 and 1000 mL L-1. The second factor was the dose of palm kernel shell ash (Fig 1), consisting of four levels: 0.0 g plant-1, 250 g plant-1, 500 g plant-1 and 750 g plant-1. Plants were grown under natural light conditions with a photoperiod typical of the local environment.

    Fig 1: Experiment materials.


     
    Mean relative growth rate (MRGR)
     
    Dry weight (W) data of eggplant biomass were collected over four sampling periods at 7-day intervals, starting from 7 days after planting (DAP). The mean relative growth rate (MRGR) was calculated using the equation proposed by South (1995).

         
    Where,
    W1 and W2= Plant dry weights at times t1 and t2, respectively.
     
    Net assimilation rate (NAR)
     
    Net assimilation rate (NAR) was calculated based on total dry weight per unit leaf area. Sampling was carried out four times at 7-day intervals, beginning at 7 DAP. Data were analyzed using the equations proposed by Briggs and Whittinghamf (1952)  and Vernon and Willson (1963), as follows:

           
    Where,
    W1 and W2= Total plant dry weights. A1 and A2= Leaf areas at times t1 and t2, respectively.
     
    Leaf area and growth analysis
     
    Leaf area was measured from subsamples using a leaf area meter and image analysis software following the method of Briggs and Whittinghamf (1952). Leaf area (L) was recorded four times during the observation period. The mean values of dry weight (W) and leaf area (L) were then used to calculate the efficiency of dry matter production (EM), representing the mean assimilation efficiency for each time interval (t2-t1), as proposed by Vernon and Allison (1963).

     
    Net assimilation rate (NAR) represents the increase in total plant dry weight per unit leaf area over a given period of time.
     
    Wastewater analysis
     
    The chemical composition of the city wastewater was analyzed using standard laboratory procedures at the Laboratory of Agricultural Technology. Parameters analyzed included nitrogen, phosphorus, calcium, pH, metals, heavy metals and other potential pollutants. The characteristics of city wastewater in Pekanbaru, as in many urban areas of Indonesia, are influenced by population density, domestic activities and industrial inputs. Although wastewater characteristics may vary depending on sampling location and time, both physical properties (color, temperature and odor) and chemical properties were determined using appropriate standard analytical methods.
     
    Dry weight measurement
     
    Plant dry weight was measured four times during the growth cycle. Samples were collected at 7, 14, 21 and 28 days after planting (DAP). Dry weight was used as a key parameter for evaluating MRGR and NAR, following the method described by Jumin et al., (2019).

    RESULTS AND DISCUSSION

    Characteristics of domestic wastewater in pekanbaru
     
    Domestic wastewater in Pekanbaru City, Riau Province, Republic of Indonesia, originates mainly from household activities such as toilets, kitchens and laundry. Wastewater samples were collected from four different areas of Pekanbaru City, namely the southern, northern, eastern and western zones. Additional contributions to wastewater originate from commercial activities, including restaurants, traditional markets and small workshops, as well as minor industrial discharges such as food processing units and car-washing facilities. In several drainage systems, stormwater run off mixes with domestic sewage (Fig 1),  further influencing wastewater characteristics.
           
    The analysis of wastewater quality indicated high levels of organic pollution. Biological Oxygen Demand (BOD) values were high, reflecting a substantial organic load, while Chemical Oxygen Demand (COD) values were typically two to three times higher than BOD, confirming the dominance of biodegradable organic matter. Dissolved oxygen (DO) concentrations were very low, indicating intensive microbial decomposition of organic substances (Tabel 1). The pH of the wastewater ranged from slightly acidic to near neutral, which is typical for urban domestic wastewater.
     
    Effect of treatments on plant growth
     
    Vegetative growth of eggplant continued into the juvenile stage, during which physiological growth parameters, namely net assimilation rate (NAR) and mean relative growth rate (MRGR), were evaluated. The responses of eggplant to the combined application of city wastewater and palm kernel shell ash are presented in Fig 2 and 3. The observed variations in NAR and MRGR indicate that plant growth dynamics were strongly influenced by nutrient availability and environmental conditions during the vegetative phase. These parameters provide a sensitive measure of the efficiency of photosynthesis and biomass accumulation under different treatment combinations. 

    Fig 2: Relationship between net assimilation rate × (mg. cm-2 day-1) and mean relative growth rate × (0.01 g. day-1) under the influence of palm kernel shell ash and city wasterwater in the egg plant.



    Fig 3: Net assimilation rate × (0.001 mg. cm-2. day-1) of egg plant under control of palm kernel shell ash and cirty waste water.


           
    Main Sources in Pekanbaru Domestic wastewater (toilets, kitchens, laundry) as contributor at the Pekanbaru city Riau province, Republikc of Indonesia. Wasterwater  collected from wasterwater  Pekanbaru city in  four place; south, north, east and west. Commercial activities (restaurants, markets, small workshops). Minor industrial discharge (food processing, car washing). Storm water runoff mixing with sewage in some drainage systems. Biological Oxygen Demant (BOD), Indicates high organic load,  Typically 2-3 × BOD value and chemical Oxygen Demant (COD Very low due to organic pollution, Dissolved Oxygen (DO) Very low due to organic pollution and pH slightly neurtal to acid.
           
    The vegetative growth continued into the juvenile stage, during which   NAR and  MRGR were analyzed. The results of the treatments are presented in (Fig 2 and 3). The combined application of palm kernel shell ash and city wastewater significantly enhanced the net assimilation rate (NAR) and mean relative growth rate (MRGR) of eggplant. A strong positive linear relationship was observed between palm kernel shell ash and city wastewater with respect to NAR (y = 0.9004x + 11.073; R² = 0.8515) (Fig 2), indicating a substantial improvement in photosynthetic efficiency associated with increased nutrient availability.
           
    Under control conditions, the relationship between NAR and MRGR remained positive but was comparatively weaker (y = 0.6221x + 2.6887; R² = 0.5435) (Fig 3), suggesting limited physiological response in the absence of external nutrient inputs. In contrast, egg plants treated with 1000 mL L-1 city wastewater combined with 750 g plant-1 palm kernel shell ash at 60 days after planting exhibited a very strong linear relationship between NAR and MRGR (y = 0.979x + 0.7562; R² = 0.9692) (Fig 4). This indicates highly efficient biomass accumulation driven by enhanced photosynthetic activity.

    Fig 4: Relationship between net assimilation rate and mean relative growth rate of eggplants (g/plant) after treated with city waster water and eggshell ash of palm oil at 60 days age.


           
    These results clearly demonstrate a synergistic interaction between city wastewater and palm kernel shell ash. The combined application likely improved nutrient uptake, metabolic activity and carbon assimilation, thereby promoting vegetative growth and overall plant performance. The observed linear interaction pattern indicates that an increase in one factor was followed by a proportional increase in the effect of the other. In this study, simultaneous increases in palm kernel shell ash dosage and city wastewater concentration consistently enhanced MRGR.
           
    The relationship between NAR and MRGR under the influence of palm kernel shell ash and city wastewater was predominantly linear, indicating that an increase in NAR was consistently accompanied by an increase in MRGR. Although NAR and MRGR represent distinct physiological parameters, they are closely linked through their shared dependence on photosynthetic efficiency and biomass allocation processes (Poorter and Nagel, 2000).
           
    Based on the present findings, appropriate increases in the concentration of city wastewater and palm kernel shell ash positively affected both NAR and MRGR (However, excessively high application rates of palm kernel shell ash may pose potential risks to agricultural land, particularly in red yellow podzol soil or poorly drained soils, due to possible nutrient imbalance or alkalinity stress.
           
    The results further indicate that plants became more tolerant of Red-Yellow Podzol soil conditions following treatment with city wastewater and palm kernel shell ash. This response is expected, as Red-Yellow Podzol soils are characterized by low pH and deficiencies in calcium and magnesium (Table 1). Palm oil shell ash is known to contain calcium, potassium and magnesium compounds (Hutahaen, 2007; Herman and Rolly, 2018). Calcium and magnesium are essential macronutrients; however, their availability in Red-Yellow Podzol soils is often restricted due to high iron and aluminum concentrations. Nutrient inputs from city wastewater significantly improved soil fertility and plant growth, consistent with previous findings (Kouadio, 2018; Al-Shamary et al. 2025).

    Table 1: The chemical composition of waster water at pekanbaru 2024.


           
    Plants grown in soils with varying mineral compositions, including calcium, magnesium and other essential and non-essential elements, tend to adjust their NAR and MRGR to optimize growth and adapt to specific ecological conditions (Beyer and Onken, 2004). This adaptive response was clearly reflected in the improved physiological performance observed in this study.
           
    The influence of wastewater application  growth is presented in Fig 5. Although palm oil shell ash is generally regarded as an agricultural waste product (Su et al., 2018), in this study it functioned effectively as a nutrient source due to its beneficial mineral composition. Wastewater-derived nutrients enhanced soil productivity and egg plant growth by increasing soil pH and reducing the toxic effects of excessive iron and aluminum, thereby improving calcium and magnesium availability.

    Fig 5: Relationship between seed dray weight, net assimilation rate and mean relative growth of egg plants after treated with city waster water and palm kernel shell ash (series 1 fruit weight (g/fruit), series 2 is MRGR × (0.01 mg/day), series 3 is NAR × (0.003 mg/cm2/day).


           
    Furthermore micro organisms bio;ogical prosess  improved the soil micro environment around root hairs, enhancing biological fot nutrient uptake ofthe plants and increasing the availability of nutrients essential for peanut growth (Choudhary and Verma, 2024). Palm oil shell ash, which contains nitrogen, magnesium and other minerals, further conpensated for nutrient deficiencies commonly found in red yellow podzol soil or acidic soils, making such environments more suitable for horticultural crops (Ohanaka et al., 2025).
           
    The relationship between NAR and MRGR for eggplant (Fig 2) revealed strong interdependence, with increases in NAR consistently followed by increases in MRGR. This positive relationship resulted in greater dry matter accumulation and ultimately higher yields, particularly in peanut plants (Fig 5). Leaf expansion plays a critical role in light interception and photosynthetic efficiency. Reduced leaf expansion limits light absorption, while palm kernel shell ash soil pollutants may induce chlorophyll deficiency, thereby suppressing photosynthetic rates (Kumar et al., 2025; Kumar and Singh, 2022). The application of palm oil shell ash to Red-Yellow Podzol soils increased calcium and magnesium availability while reducing iron and aluminum uptake by plants (Ewelike et al., 2021).
           
    Chlorophyll acts as a photosynthetic antenna, capturing light energy for photochemical reactions that convert solar energy into carbohydrates such as glucose (Pezeshki, 2018; Kumar and Singh, 2022); The mean relative growth rate (MRGR) reflects overall plant growth over a given period without directly accounting for leaf-level efficiency (South, 1995), whereas net assimilation rate (NAR) provides a more precise estimate of photosynthetic activity per unit leaf area per day. Thus, MRGR serves as a rapid indicator of whole-plant growth dynamics, while NAR offers insight into physiological efficiency (Vernon and Allison, 1963, Shipley, 2006). Simultaneous evaluation of these parameters is essential for early detection of growth limitations.
           
    Organic matter present in city wastewater can reduce excessive iron and aluminum availability by limiting their uptake, particularly in acidic and peat soils (Ungureanu et al., 2021; Kouadio et al., 2018; Khamidah and Saputra, 2020). In Red-Yellow Podzol soils, palm oil shell ash further immobilizes iron and aluminum, rendering them less available to plants. Consequently, the interaction between city wastewater and palm kernel shell ash helps regulate metal concentrations and improve soil chemical balance. The calcium, magnesium and supplied by both inputs increase soil pH toward near-neutral conditions, thereby enhancing nutrient availability and plant growth (Al-Shamary et al., 2025).
           
    According to Gage (2004), soil microorganisms play a crucial role in decomposing organic matter and regulating the soil microclimate, which facilitates nitrogen absorption through plant–microbe interaction in the rhizosphere. Microbial activity benefits both partners: bacteria supply nitrogen to plants through biological prosess (Table 2.1 and Table 2.2), while plants provide carbon substrates to microorganisms. During organic matter decomposition, micro nutrients also become available, further supporting eggplant growth (Brewin, 2010, Boyle, 2020 and Etasami, 2022).

    Table 2.1: Chemical characteristic pekanbaru city wastewater 2024.



    Table 2.2: Biological characteristics.


                   
    Overall, the patterns of NAR and MRGR were closely aligned, as dry matter accumulation reflects sustained photosynthetic activity. NAR represents the efficiency of chlorophyll in capturing solar energy, while MRGR reflects cumulative biomass production over time. Under healthy growth conditions, leaves efficiently absorb light and synthesize carbohydrates. However, under stress conditions such as disease or pest infestation, photosynthetic efficiency declines and resulting in reduced NAR. In such cases, MRGR may not decrease proportionally, leading to deviations from linearity between these two parameters.

    CONCLUSION

    The interactive treatment of Pekanbaru city wastewater and palm kernel shell ash produced significant differences in the net assimilation rate and mean relative growth rate of egg plants grown in red-yellow podzol soil. 
           
    This study suggests that although palm kernel shell ash is commonly regarded as a pollutant that can contaminate soil and water bodies, it also has potential as a valuable source of organic nutrients.  In healthy plants—those not affected by disease or pest infestation-the relationship between net assimilation rate and mean relative growth rate typically linear. However, when leaves are infected by bacteria or fungi, the net assimilation rate tends to decline, while the mean relative growth rate may remain relatively stable. Under such conditions, the relationship between net assimilation rate and mean relatave growth rate becomes non-linear.
           
    Conclusion as the combined application of city wastewater and palm kernel shell ash represents a sustainable soil management strategy for improving crop productivity on marginal red-yellow podzol soils, while simultaneously promoting the beneficial reuse of urban and agro-industrial waste.

    ACKNOWLEDGEMENT

    We thank to Rector Islamic University of Riau for Financial support to publishing of our paper the Agricultural Science Digest ARCC journal, The special thank to vice Rector in Administrative and Finance support. 
     
    Disclaimers
     
    The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.

    CONFLICT OF INTEREST

    The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.

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