Effect of Rice Husk Biochar and Potassium Humate on the Change of Soil Chemical Properties, Yield and Quality of Cherry Tomato (Lycopersicon esculentum Mill.)

Fertilizers are essential inputs for enhancing yield, product quality and cultivation efficiency in vegetable production, particularly for tomato plants. Biochar, resulting from the thermochemical conversion of organic matter in oxygen-limited conditions (Adekiya et al., 2020; Lehmann and Joseph, 2009), along with potassium humate and appropriate fertilizer selection, can significantly improve soil fertility and plant productivity. According to Adekiya et al. (2020) and Rillig (2009), biochar incorporation enhances various soil properties, including pH, water retention, organic matter content, cation exchange capacity and microbial populations. Furthermore, biochar significantly can adsorb various nutrients, including NH₄⁺, NH₃, nitrate and phosphorus, effectively reducing nutrient leaching from soil (Adekiya et al., 2020; Yin and Xu, 2009).

Potassium humate, a biological substance utilized as a fertilizer, has been shown to enhance soil biological, chemical and physical properties while improving nutrient retention (Izhar Shafi et al., 2020). In rice cultivation, Kumar et al., (2013) reported that applying 100% recommended NPK combined with 10 mg/kg potassium humate significantly increased growth, development and yield compared to treatments without potassium humate supplementation, findings later corroborated by Nassar et al., (2021).

Biochar has recently been proposed as a management strategy to improve crop productivity and mitigate global warming. Biochar may be added to soils with the goal to improve the soil properties (Ramamoorthy et al., 2022).  Besides, Adusu et al., (2020) concluded that biochar can improve the nutrient status of both stockpiled topsoil and degraded mined soil.Its use also enhances plant diversity and improves stockpiled and degraded soils. However, limited information is available regarding the combined use of rice husk biochar and potassium humate in tomato cultivation. Therefore, this study investigated the effects of various potassium humate and biochar application rates on soil chemical properties, cherry tomato development and yield.

Materials varieties

The tomato variety used in the experiment is the F₁ Rado 27 hybrid red cherry tomato produced by Rang Dong Seed One Member Company Limited.

Experimental pot

Experiment using cylindrical terracotta pots (20 cm x 40 cm x 20 cm).

Fertilizers
 
The experiment utilized three primary fertilizers: urea (46% N), diammonium phosphate (DAP; 18% N, 46% P₂O₅) and potassium chloride (60% K₂O). The potassium humate used in the study contained 80% humate, 16% K₂O and 10% moisture. Rice husk biochar was characterized by 24% total carbon content and contained various nutrients, including 0.86% N, 1.358% P₂O₅, 1.5% K₂O, 0.021% Mg, 0.019% Cu and 0.018% Zn.
 
Experimental soil
 
The experimental soil, classified as Gleyic Fluvisols (Alluvial soil group), was collected from vegetable cultivation fields in Thien My commune, Tra On district, Vinh Long province (9^o57'12.9"N 105^o55'57.6"E). Soil samples were obtained from the cultivation layer at 0-20 cm depth (Fig 1). According to the USDA texture triangle, the soil exhibited a clay-silty texture, comprising 1.36% sand, 53.54% silt and 45.10% clay. Chemical analysis revealed slightly acidic conditions (pHH₂O = 5.63), low organic carbon content (2.67% C), low total nitrogen (0.17% N) and high total phosphorus (0.15% P₂O₅).



Research location and time
 
The experiment was conducted at Can Tho University’s net house, Vietnam, over two seasons in 2020-2021. Season 1 (Summer-Autumn 2020) ran from March 15 to July 20, 2020, while Season 2 (Autumn-Winter 2020) extended from September 15, 2020, to January 25, 2021. In both seasons, 14-day-old seedlings were transplanted two weeks after sowing.
 
Soil preparation
 
After the soil is collected, it is returned to the laboratory and dried in the air. After the soil is dry, pound it into small pieces and mix well. A small amount of soil sample (about 5 kg) was collected and pulverized through 0.5 mm and 2 mm sieves to determine pre-experiment soil parameters such as pHH20, %C, total N, total P and soil texture. The remaining was used for the experimental setup. Each pot of experimental soil contains 10 kg (mass of dry soil).
 
Experimental layout
 
The experiment had two factors
 
Four replicates with four plants each, 9 treatments (9 fertilizer formulas) and a completely randomized design (CRD).

+ Factor A (F_A) is 3 levels of biochar rice husk application: 0 tons/ha (B₀), 2.5 tons/ha (B₁) and 5 tons/ha (B₂).  

+ Factor B (F_B) is 3 levels of potassium humate: 0 kg/ha (K₀), 50 kg/ha (K₁) and 100 kg/ha (K₂).The fertilizer combinations applied to NPK fertilizers, as recommended by Ddamulira et al., (2019) is 200 N-150 P₂O₅-200 K₂O kg/ha. Rice husk biochar is entirely applied before sowing and the fertilizer used depends on each treatment. Potassium humate was applied twice at 20 DAS and 40 DAS, each at 50%.
 
Method of crop sowing and care
 
Tomato seeds were sown in nursery pots made from banana leaves, using rice husk ash as the substrate. After 30 days, seedlings with 2-3 leaves and a height of 12-15 height were transplanted at a density of 25,000 plants per hectare, with 100 cm between rows and 40 cm between plants (from the center of the pot), one plant per pot (Fig 1).

Plant care was conducted in accordance with Vietnamese Standards (QCVN 01-63: MARD, 2011). Selected plants were healthy and non-malformed, exhibiting white roots, well-developed tops and no signs of infection.
 
Collection and monitoring indicators
 
Soil sampling
 
When the tomato crop was harvested at the end of the season, soil samples were taken. Soils were collected by hand drill (0-20 cm), 4 points on each pot at each treatment and each replicate. Samples were collected, dried at room temperature and then powdered through a 0.5 mm filter to determine the soil’s pH, EC, exchangeable K, available N and P.
 
Collecting agronomic indicators
 
Crop growing criteria
 
Plant height and stem diameter were recorded 30 days after snowing, 60 DAS and 90 DAS.
 
Yield and factors affecting yield
 
At each sampling period, fruit yield (kg/plant), fruit number per plant and diameter (cm) were recorded.
 
Indicators related to fruit quality
 
% Brix and nitrate content in fruit were measured at the time of complete harvest.
 
Methods of soil sample analysis and evaluation of criteria related to growth, yield and quality of cherry tomato plants
Methods of evaluating agronomic and yield parameters
 
Crop growth criteria
 
A graduated ruler and an electronic caliper measure crop height and stem diameter.
 
Criteria for evaluating fruit quality
 
% Brix measured by a hand refractometer, measures 10 fruits/plant of equivalent ripeness. The nitrate content in the fruit was analyzed by colorimetric method at 410 nm with phenolsulfonic acid reagent in the alkaline environment according to TCVN 8742:2011.
 
Indicators related to yield and yield components
 
Fruit diameter (cm), fruit number/plant, average fruit weight (g/fruit) and yield/plant (kg/plant) are recorded and received in batches by counting, measuring and weighing methods.
 
Soil analysis
 
Soil sample analysis followed standard procedures (Houba et al., 1995). Soil pH (pHH₂O) and electrical conductivity (EC) were measured at a 1:2.5. Organic the Walkley-Black method determined matter content (%C), while available phosphorus (P) was assessed using the Bray II method. Available nitrogen (N), including ammonium and nitrate, was extracted with a 1:10 (w/v) solution of 2M KCl and quantified colorimetrically at 650 nm for ammonium and 540 nm for nitrate. Exchangeable potassium was extracted with 1M BaCl₂ and measured using atomic absorption spectroscopy at 766 nm.
 
Data processing
 
The data were synthesized, calculated using Excel 2013 software and tested for ANOVA using Minitab 16.0 statistical software, Duncan’s test at 1% and 5% significance levels.

Rice husk biochar and potassium humate affect soil chemistry over two cherry tomato crops (1^st and 2^nd season)
 
pHH2O
 
Table 1 shows that biochar can improve soil pH levels. 5 tons/ha of biochar yielded the highest soil pH value, followed by 2.5 tons/ha combined with 50-100 kg/ha of potassium humate and 2.5-5.0 tons/ha of biochar significantly improved soil pH in the second season. It is a sustainable and effective way to enhance soil health.


 
Nutrient availability (available N, P and K)
 
Table 2 shows that applying potassium humate or biochar alone significantly increases soil nutrient availability, particularly N, P and K. Fertilizing the soil with 5 tons/ha of biochar and 50-100 kg/ha of potassium humate significantly enhances nutrient bioavailability throughout the growth of cherry tomatoes in both cropping seasons.


 
Effects of biochar rice husk and potassium humate on the growth of cherry tomatoes through two cropping seasons (1^st and 2^nd season)
 
Plant height
 
Plant height is crucial for growth and development. Biochar rice husk showed no effect on plant height, but potassium humate made tomato plants significantly taller (p< 0.05), regardless of the amount applied (50 kg or 100 kg of potassium humate /ha). The interaction analysis also revealed that biochar made from rice husk and potassium humate did not significantly affect plant height during both tomato growing seasons (Table 3).


 
Stem diameter 
 
According to experimental findings, potassium humate and biochar dosage did not affect tomato stem diameter during either growing season (Table 4). According to the interaction analysis, the amount of biochar and potassium humate applied to cherry tomato plants did not impact how much their stems grew in diameter.


 
Effect of rice husk biochar and potassium humate on yield and yield components of cherry tomatoes grown in pots under greenhouse conditions over two growing seasons
 
Tomato yield comprises the following factors: fruit diameter, fruit number/plants and fruit weight. The results presented in Table 5 show that:


 
Fruit diameter
 
Adding biochar did not increase tomato fruit diameter in both cultivation seasons. However, potassium humate helped increase the fruit diameter in the second season. The analysis revealed no interaction between biochar husk dose and potassium humate on cherry tomato fruit diameter.
 
Total fruit/plant
 
Using Biochar for two seasons increased fruit yield/plant significantly compared to control. There is no difference in fruits/plants between 2.5 and 5 tons of biochar/ha. Treatments with potassium humate had higher fruit/plant, with a dose-dependent effect-no interaction between potassium humate and biochar on fruit/plant increase.
 
Yield
 
Crop yield is essential for farmer profitability and is influenced by genetic makeup, environmental factors, nutrition and cultivation techniques. Effective management of these variables is crucial for maximizing profits. Fig 2 illustrates that applying 5 tons of biochar and 100 kg of potassium humate per hectare significantly enhances tomato yield, yielding the highest results. Interaction analysis, however, indicated no significant interaction between the biochar and potassium humate doses.


 
Effects of rice husk biochar and potassium humate on cheery tomato quality at two cropping seasons in net house conditions
 
The study found that the recommended NPK application,  biochar and potassium humate can increase tomato yield without nitrate accumulation (Fig 3). Nitrate content was lower than the VietGap standard (<150 mg/kg). The doses of biochar and potassium humate did not affect the nitrate content of fresh tomatoes.

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Biochar and potassium humate significantly increased tomato fruit’s sugar content (brix level). The optimal treatment was 5 tons of biochar per hectare and 100 kg of potassium humate per hectare, resulting in the highest brix content (Fig 4). It suggests that these additives can be a powerful tool in boosting the nutritional value of tomato fruit.

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Fertilizing rice husk biochar can significantly improve soil pH and nutrient availability (N, P, K) in soil compared to biochar. Due to the alkaline nature of biochar, soil pH is raised and a better environment for plant growth is created. According to Tan Huiyi et al., (2021), pH plays a crucial role in plant growth and nutrient availability. Biochar contains organic functional groups like C C, C=C,  OH,  COOH and alkaline elements like Ca²+, Mg²+, Na+ and K+. It is why biochar has a high pH (Mandal et al., 2019). Therefore, adding biochar to the soil can change its physicochemical properties, add more nutrients and significantly increase the content of K+ (Hassan et al., 2020; Adekiya et al., 2022). At the same time, adding biochar to the soil improves soil quality and reduces nutrients’ leaching, thereby increasing crop yields (Diatta et al., 2020).

Potassium humate is the salt of humic acid, derived from lignite brown coal and rich in hydroxyl (-OH), carboxylic (-COOH) and phenolic (-COH) groups (El-Beltagi et al., 2023). Potassium humate is a natural fertilizer that resembles humus in soil. It enhances soil properties and nutrient accessibility for plants, improving growth by enhancing nutrient uptake and cellular activities. Recent studies have shown that it is a highly concentrated form of humus with strong cation adsorption capacity. Potassium humate increases tomato plant yield, biomass and harvest index and improves plant biochemicals.
  
Potassium humate has been shown to significantly enhance crop performance and physiological parameters in various studies. Research results of Rahi et al., (2021) demonstrated that tomato crop yield, total plant Biomass and harvest index increased proportionally with potassium humate application rates. Similarly, El-Bassiouny et al., (2014) reported dramatic increases in proline content, total soluble sugars, total amino acids and photosynthetic pigments in response to potassium humate treatments.

The positive effects of potassium humate on plant growth and crop yield can be attributed to several mechanisms. According to Abdeen (2020), potassium humate enhances water retention, nutrient availability, hormonal activity and microbial growth and promotes  organic chemical minerals’ transformation during plant growth. Furthermore, Baldotto et al., (2009) observed that potassium humate application led to increased soil N, P and K levels, enhanced root development, improved plant nutrient uptake and consequently, higher crop yields.

The combined application of biochar and potassium humate significantly increased the oBrix content of tomato fruit flesh compared to the non-fertilized control. This improvement in fruit quality parameters can be attributed to enhanced nutrient uptake and utilization efficiency. The synergistic effect of biochar and potassium humate increases the availability of both macro (N, P and K) and micronutrients and improves the plant’s nutrient absorption capacity. This enhanced nutrient uptake leads to better accumulation of metabolites in leaves and fruits, ultimately resulting in improved fruit quality characteristics. These findings align with the results reported by Spehia et al. (2020).

Applying rice husk biochar and potassium humate positively affected soil properties and cherry tomato production. This treatment improved soil pH and enhanced the availability of essential nutrients (N, P and K) in the soil. Consequently, significant improvements were observed in plant growth parameters, fruit yield and the number of fruits per plant. The combined application also resulted in increased °Brix content of fruits without causing excessive nitrate accumulation. Notably, the nitrate levels in fruits remained within acceptable limits, ranging from 23.45 to 24.94 mg NO₃-/ kg fresh weight, below the regulatory threshold.

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