Enhancing NPK Uptake and Biomass of Blueberries in Alluvial Clay Soil Using Biochar and Compost



The Mekong Delta features acidic clay soil of alluvial origin with a soil texture ranging from silty clay to clay. The growth of wild blueberry plants in clay soils requires the addition of materials to enhance soil porosity and aeration.


The objective of this study was to examine the effects of the combined use of biochar and compost on the growth, yield, and NPK uptake of blueberry ( Vaccinium tenellum) cultivated in the Mekong Delta.


The pot experiment had a 3 × 3 factorial design, containing the application of biochar at 0, 10, and 20 t ha -1 (B0, B10, and B20, respectively), compost at 0, 5, and 10 t ha -1 (C0, C5, and C10, respectively), and NPK at 45:20:20 kgha -1 according to the growth stages of blueberries.


The interaction between biochar and compost provided a more efficient response in terms of plant growth, yield, and NPK uptake. Notably, in the combined application of B20 and C10, NPK uptake and biomass of blueberries were significantly increased. However, only the concentration (gkg -1) of N (14.8) and K (3.82), except for P (1.37), in the blueberry leaves were below the Trevett threshold.


This approach effectively mitigates the challenges posed by high clay content in the soil, which results in poorer soil porosity and aeration. The findings emphasize the potential benefits of tailored soil amendment strategies to optimize blueberry cultivation in similar environments.

Keywords: Acidic clay soil, Biochar, Blueberry cultivation, Compost, NPK uptake, Mekong delta.


Blueberry is a plant native to North America that has successfully grown in Vietnam [ 1]. Blueberry fruits contain many nutritional compounds and are a valuable source of raw materials for the pharmaceutical and functional food industries. Blueberries contain a large amount of anthocyanins, flavonols, and flavonoids [ 2], so they are used for medicinal purposes and functional foods [ 3, 4]. Blueberry plants grow best in full sun, requiring a temperature of 20–30 °C for growing, and their optimal growth occurs on well-draining soil with a pH of 4.5–5 [ 5].

The soil in the Mekong Delta (MD) consists of three main types (alluvial, acid sulfate, and saline); alluvial soils are located along the main river, accounting for 31% of the MD area, whereas acid sulfate soils account for 41% of the MD area [ 6, 7]. The average pH and base saturation values (n = 237) in the topsoil of alluvial soil were 5.08 and 66.2%, respectively, and those in acid sulfate soil (ASS; n = 79) were 3.99 and 28.5%, respectively [ 5-8]; this could be attributed to the acidity imparted from the oxidation of pyrite in ASS [ 9]. In MD, blueberry is a widely distributed plant with ornamental flowers. Most soils in the MD contain kaolinite, illite, and montmorillonite in the clay fraction. Over 30% of soil possesses a silty clay-to-clay texture, which is a characteristic inherited from parent materials [ 10, 11], making it poorly drained [ 12, 13]. Consequently, the growth of wild blueberry plants in clay soils requires large amounts of organic matter (OM) [ 14]. A study on alluvial acidic soil in MD showed that both rates of biochar and compost (4 and 6 tons ha -1 year –1, respectively) could improve soil organic matter (SOM) and phosphorus availability, which can contribute to more stems and leaves in blueberry plants after 12 months of planting [ 15]. In MD alluvial soil conditions with high clay content, although blueberries can produce flowers and fruits, fruit density and blueberry yield are low. Therefore, it is necessary to research to improve the disadvantages of clay-rich soils for blueberry growth on alluvial soils in the Mekong Delta.

Biochar is a carbon-rich product obtained from biomass pyrolysis in an oxygen-limited environment [ 16, 17], and nowadays, it attracts significant attention among researchers and farmers due to its various benefits [ 18, 19]. Owing to its high surface area and porous nature, biochar application to soil increases its microbial activity, ion exchange, capacity, soil nutrient availability, and water retention [ 20-22]. Biochar also benefits crops by adding SOM and mineral elements to the soil [ 23], which results in a high rate of nutrient transformation and utilization efficiency [ 24, 25]. A biochar application rate of over 10 tons ha −1 has been recommended to achieve a noticeable effect on P availability in soil [ 26]. It may also have some other benefits for soil microorganisms while suppressing soil parasites [ 27].

Some recent studies have indicated that applying a compost-biochar combination positively synergizes soil physicochemical characteristics and microbiological properties, significantly enhancing crop growth and improving field conditions [ 28].

Although many studies have focused on blueberry plants with individual or combined applications of biochar and compost fertilizers, most of these studies have been conducted on sandy soils. The novelty of the research is growing blueberries on acidic clay soil. Therefore, the aim of the study was to determine the best method of using biochar and compost to achieve high growth and yield for blueberries grown in acidic clay soil.


To achieve the objective, a greenhouse pot experiment was conducted by growing blueberry plants ( Vaccinium tenellum) in the alluvial clay soil of MD, which was then analyzed for the effects of the combination of biochar and compost on improving plant growth and NPK uptake better than compost or biochar alone.

2.1. Study Site, Soil, and Climate

The soil (Gleyic Anthrosols) [ 29] was collected for the pot experiment from a location in Dong Thanh commune, Chau Thanh district, Hau Giang Province, Vietnam (9°51′48.8″N, 105°47′21.8″E), and the physicochemical characteristics of the soil are listed in Table 1.

Table 1.
Initial physicochemical characteristics of the experimental soil collected from Dong Thanh commune, Chau Thanh district, Hau Giang Province, Vietnam
Parameters Unit Depth (cm)
0–20 - 20–40
pH H2O - 5.62 - 6.01
EC mS cm −1 0.13 - 0.14
Available phosphorus mg kg −1 19.1 - 8.87
Soil organic matter % 3.11 - 1.32
CEC cmol c kg −1 21.9 - 23.9
Exchangeable cations - - - -
K + cmol c kg −1 0.58 - 0.47
Ca 2+ 10.2 - 10.7
Mg 2+ 4.66 - 6.95
Bulk density g cm −3 1.13 - 1.1
Soil texture - - - -
Sand % 0.3 - 0.7
Silt 43.2 - 33.7
Clay 56.5 - 65.6
Soil textural class - silty clay - clay

The average monthly temperature during 2020–2022 was 28 °C, with May usually having the highest temperature (29 °C) and January having the lowest temperature (26 °C). The average monthly rainfall during 2019–2021 was 175 mm, with the highest rain in September (300 mm) and the lowest in January (10 mm). The meteorological data for the study locations are shown in Fig. ( 1).

2.2. Varieties of Blueberry

Blueberry varieties used in the experiments were sourced from the Center of Agricultural Crops, Vietnam National University of Agriculture [ 1]. Some varieties have been evaluated for suitable growth in greenhouses, produced from the micropropagation project in the Laboratory of Biochemistry of Can Tho University.

Three-year-old blueberry plants ( Vaccinium tenellum) used in this greenhouse experiment (Fig. 2a, b) were obtained from a micropropagation project. To identify the genus name of the blueberries, young leaves of the genus Blueberry were used to extract DNA based on Rogers and Bendich [ 30]. DNA was extracted at the Laboratory of Molecular Biology, Biotechnology Research and Development Institute, Vietnam. Using the Basic Local Alignment Search Tool, the Internal transcribed spacer (ITS) gene sequence of the blueberry samples was compared with the gene sequences of other species in the National Center for Biotechnology Information’s GenBank database. The results showed that the ITS gene sequence of the blueberry sample was similar to the sequence of the species AF273709.1 [ 31], as determined by Quynh, i.e., 96.84% [ 32]. Vaccinium tenellum is a species of shrub in the family Ericaceae [ 33].

Fig. (1). Climatic conditions between 2020 and 2022 in Can Tho City, Vietnam.
Fig. (2). ( a) Three-year-old blueberry plant was used in the greenhouse experiment. ( b) Leaf samples of the blueberry plant used in the experiment were identified as belonging to the genus Vaccinium tenellum at The Faculty of Agriculture, Can Tho University.

2.3. Experimental Design

The pot experiment started in June 2021, was performed in the greenhouse of Can Tho University, and was laid out in a factorial randomized block design (RBD) with two alterations: (a) three levels of biochar (0, 10, and 20 t ha -1) and (b) three levels of compost (0, 5, and 10 t ha -1) (Table 2). Each treatment was replicated four times. The distance between the pots was 30 cm × 30 cm.

N, P, and K fertilizers were applied as described by Marty et al. [ 34] as follows: 45 kg N ha -1, 20 kg ha -1 P 2O 5, and 20 kg ha -1 K 2O. The single fertilizers used were ammonium sulfate (21% N), triple superphosphate (45% P 2O 5), and potassium sulfate (52% K 2O). Ammonium sulfate was used to fertilize blueberries because its ammonia form is absorbed by plants more efficiently than nitrate [ 35, 36].

Table 2.
Two-factor factorial design with the application of biochar and compost fertilizer to the blueberry plants.
Factor Levels Amount of Compost Applied (t ha -1)
Amount of biochar applied (t ha -1) 0 5 10
0 TRT-1 TRT-2 TRT-3
TRT- 4

As recommended by Rutgers University [ 37], in this study, the application of N fertilizer was split into three timings: first during 3–4 weeks after bud-break (April 10 th, 2023), second during 7–8 weeks after bud-break (May 10 th, 2023), and third during 11–12 weeks after bud-break (June 10 th, 2023). P and K fertilizers were applied in January or February (prior to bud-break) [ 38].

For each pot, 10 kg of soil was applied (20 cm × 12 cm/10 L), and the corresponding amounts of biochar at 10 and 20 t ha -1 for each pot were 50 and 100 g, respectively. Before planting blueberries in the pots, the soil at the plant's root zone was mixed with biochar and compost, with a root zone size of approximately 10 cm × 5 cm/2 L.

2.4. Soil Sampling and Analysis

The soil sampled for the greenhouse experiment was located in Hau Giang Province (9°54′30.3″N, 105°51′06.7″E). The chemical properties of the soil were determined. Based on many observation points for exploration, the most representative soil profile for the land was selected [ 39]. Soil sampling in the soil profile was carried out separately at depths of 0-20cm and 20-40cm. Representative soil samples for each soil layer were mixed from 05 representative positions in the soil layer. The total number of soil samples was 2 (1 soil sample/layer × 2 layers).

All soil samples were air-dried to dryness, then crushed and sieved with a 2 mm sieve for laboratory analysis.

The soil pH was determined using a digital pH meter (Metrohm 744) for a 1:2.5 soil-to-water suspension. Soil organic carbon was determined by the Walkley and Black method [ 40]. Soil total N was established through the semi-micro-Kjeldahl method after digesting the samples in H 2SO 4. The total and available P levels in the soil samples were estimated using the ammonium phosphomolybdate and Bray-2 methods, respectively.

2.5. Plant Sampling and Analysis

During harvesting, lowbush blueberries were separated into stems, leaves, and fruit, and then the fresh stem and leaf tissues were oven-dried at 70 °C for 3 days to obtain a constant weight. After the dry weight was recorded, the dried plant tissues were ground in a pulverizer and sieved through a 2 mm sieve. A freeze-drying method was used to prevent fruit degradation due to thermal decomposition to obtain a dried sample of fresh blueberry fruit. Five grams of each of the 20 fresh fruits, cut in half lengthwise, were placed in special freeze-drying bottles and frozen at −40 °C for 1 h, followed by immediate freeze-drying for 72 h in an ultra-freezer at −40 °C. A Labconco FreeZone 4.5 Liter Benchtop Freeze Dry System (7750020) was used at a pressure of 0.002 mBar. Once the fruit was freeze-dried, it was placed in a vacuum plastic bag for its mineral composition (N, P, and K) analysis.

The nutrient (N, P, and K) content of the plant parts was determined at the Laboratory of Soil Science Department, College of Agriculture, Can Tho University. The dried samples were digested in an H 2SO 4 and H 2O 2 mixture, and then the N concentration of air-dried samples was determined by the Kjeldahl method. The P concentration was determined spectrophotometrically as the molybdenum blue color was formed by the reaction of ammonium molybdate with ascorbic acid [ 41], and the K concentration was determined photometrically using an air-acetylene flame.

At harvest, different plant parts from treatments four (TRT-4), five (TRT-5), and six (TRT-6) (Table 2) were collected for plant nutrient (N, P, and K) analysis.

2.6. Plant Growth and Fruit Yield

At the time of fruit harvest, the number of leaves and stems on the plant was counted, and the plant’s height was measured. Each treatment was repeated four times, and the number of plants in each was measured in four replicates.

To determine the leaf and stem dry weights, the leaf and stem fresh weights were measured and then dried in an oven for 24 h.

The blueberries in the trees did not ripen simultaneously. To determine fruit yield, the total number of fruits was the cumulative number on the tree of the treatment calculated in each harvest; on average, there were four fruit harvests. Each time, all the fruits on the tree were weighed. The mean fruit weight was determined based on the total weight (kg) of fruits per plant divided by the total number of fruits.

2.7. Statistical Analysis

The mean with standard deviation ( ± SE) was calculated for each treatment. SPSS software (version 20.0) was used to conduct a two-way analysis of variance, and Duncan’s multiple range test was used for multiple comparisons with statistical levels at 5% and 1%. The relationships among plant dry weight, nutrient (N, P, and K) content, and nutrient uptake were determined using Pearson’s correlation coefficients.


3.1. N Application and the Growth Stages of Blueberries

The N application to blueberries was based on the bud-break stage of the plant. In the greenhouse experiment of 2022, some noticeable stages of fruit development were recorded: (i) Bud-break on March 11 th; (ii) Early bloom on April 01 st; (iii) Green fruit on April 08 th; and (iv) Late green fruit on April 18 th.

Fig. (3). Stages of Vaccinium tenellum fruit development. Greenhouse experiments were conducted at the College of Agriculture, Can Tho University, Vietnam, in April 2022.

N was applied to the blueberry approximately two months after bud-break. As the bud of Vaccinium tenellum broke in the second week of March (Fig. 3), the green fruit stage occurred one month later; therefore, the N fertilizer application rate was divided into the following: first N in late April, second in mid-May, and third in mid-June.

3.2. Effect of Combined Biochar and Compost Application on Blueberry Growth and Yield

3.2.1. Plant Growth and Yield

Increasing the amount of biochar applied to the soil (0, 10, and 20 t ha -1) resulted in increased numbers of leaves and fruit per plant, plant height, and fruit yield. Specifically, the fruit yield at B20 was the highest (9.3 g plant -1) (Table 3). Similarly, plant growth increased with increasing compost application, with the highest fresh blueberry fruit yield of 9.1 g plant -1 at C10.

The numbers in parentheses represent the standard deviation. B0, no biochar applied; B10, biochar applied at 10 t ha -1; B20, biochar applied at 20 t ha -1; C0, no compost applied; C5, compost applied at 5 t ha -1; C10, compost applied at 10 t ha -1. The blueberry plants were treated with 45 kg N ha -1, 20 kg P 2O 5 ha -1, and 20 kg K 2O ha -1.

Table 3.
Effect of biochar and compost application on plant growth and blueberry fruit yield
Treatments Leaf Number Shoot Number Plant Height (cm) Fruit Number/Plant Fruit Weight (g/fruit) Fruit Yield (g/plant)
Biochar - - - - - -
B0 154 33 45 13.7 0.243 3.5
- (±103) (±17) (±12.5) (±5.01) (±0.02) (±1.61)
B10 298 54.0 62.0 31.7 0.247 7.80
- (±92.5) (±15.9) (±8.2 0) (±8.92) (±0.02) (±2.53)
B20 446 66.0 68.0 36.4 0.254 9.30
- (±87.2 0) (±11.8) (±14.20) (±8.19) (±0.01) (±2.37)
Compost - - - - - -
C0 207 38.0 46.0 18.6 0.229a 4.30
- (±97.0) (±14.9) (±14.6) (±8.61) (±0.01) (±2.0)
C5 312 51.0 64.0 28.0 0.257b 7.30
- (±12) (±17.7) (±12.2) (±10.6) (±0.02) (±2.94)
C10 379 61.0 65.0 35.5 0.259b 9.10
- (±134) (±15.6) (±15.9) (±12.0) (±0.02) (±2.96)
CV(%) 47.5 35.9 26.9 45.2 9.2 47.8

Due to the interaction between biochar and compost (Table 3), the number of leaves, number of fruits, and fruit yield exhibited significant coefficient of variation (CV) values (Table 3) of 47.5%, 45.2%, and 47.8%, respectively.

In this study, multiple regression analysis was used to predict fruit yield by considering leaf number, stem number, and stem height as the three important independent variables, which are presented in the following equation (Eq. 1):


Significant at: *** p = 0, ** p = 0.001, and * p = 0.01; R 2 = 0.89***

Where fruit yield (Y) is expressed in tons of fresh fruit per hectare, Leaf # is the leaf number per plant, Stem # is the stem number per plant, and Stem H is plant height measured in cm. Among all factors, Leaf# is most significant in increasing the fruit yield.

3.2.2. Plant Dry Weight

In the B20+C10 treatment, the dry weight of stems and leaves and fresh fruit yield were 16, 7.0, and 10.1 g ha -1, respectively (Fig. 4), higher than those in the combined treatment at B20+C5. However, the dry weights of the stems, leaves, and fruits in the biochar B20 treatment were significantly lower at 9.34, 3.60, and 5.99 g ha -1, respectively.

Fig. (4). The combined effect of biochar and compost on the dry weight of blueberry plant parts. Greenhouse experiments were conducted at the College of Agriculture, Can Thơ University. The dry weight of the stems and leaves was measured, whereas for the fruit, the fresh weight was measured in May 2022. Standard deviation (vertical bar) is used to represent the difference among columns of the same color.

3.3. The Combined Effect of Biochar and Compost Fertilization on N, P, and K Contents and Uptake of Blueberry Plants

3.3.1. N, P, and K Contents in Blueberry Plants

Evaluating the nutritional status of leaves using published thresholds, along with observations of plant growth, is the best method for developing fertilizer management programs. The N, P, and K contents in the blueberry stems, leaves, and fruits were lowest when using biochar alone (B20) (Table 4). Using combined biochar at B20+C5 helped improve the N, P, and K contents. Moreover, at B20+C10, the N, P, and K contents in the plant parts increased markedly. The leaves had greater N, P, and K accumulation than the stems and fruits (Table 4).

Different letters indicate significant treatment differences (* p < 0.05 and ** p < 0.01). B10, biochar applied at 10 ha -1; B20, biochar applied at 20 ha -1; C0, no compost applied; C5, compost applied at 5 t ha -1; C10, compost applied at 10 t ha -1. The blueberry plants were treated with 45 kg N ha -1, 20 kg P 2O 5 ha -1, and 20 kg K 2O ha -1.

According to Trevett (1972) [ 42], the sufficiency ranges of N, P, and K in leaves of lowbush blueberry were 16.0–23.8, 1.2–2.2, and 4.0–9.0 g kg -1, respectively. However, in the present study, the leaf N and K contents were found to be 14.8 and 3.82 g kg -1, respectively (Table 4), for B20, which were less than those of Trevett.

At B20+C5, the N, P, and K contents of leaf tissues were 17.5, 1.25, and 4.62 g kg -1, respectively, whereas those of B20+C10 were 19.2, 1.48, and 5.23 g kg -1, respectively. Both combined treatments exhibited higher values than those of Trevett, indicating that compost played a positive role in improving mineral nutrient uptake by plants.

The CV (%) of N and K contents in stems, leaves, and fruits was approximately 10%–15% (Table 4), except for P content in stem and fruit, which was 34.5% and 26.2%, respectively.

3.3.2. N, P, and K Uptake by Blueberry Plants

Total plant N, P, and K uptake, considered the total N, P, and K contents of the plant stems, leaves, and fruits, was measured after the destructive harvest on May 28 th, 2022. The highest N, P, and K uptake was recorded for B20+C10 treatment plants at 400 mg N, 29 mg P, and 237 mg K per plant (Fig. 5), whereas the lowest uptake was for B20 treatment at 175 mg N, 10 mg P, and 102 mg K per plant.

Fig. (5). The combined effect of biochar and compost on the NPK uptake of different plant parts of blueberries. A greenhouse experiment was conducted at the College of Agriculture, Can Thơ University, in May 2022. Standard deviation (vertical bar) is used to represent the difference among columns of the same color.
Table 4.
Combined effect of biochar and compost application on N, P, and K contents in stems, leaves, and fruits of blueberries
Effect of Biochar and Compost Application on N, P and K Content in Stem, Leaf and Fruit of Blueberry
Treatment Stem Mineral Content (g kg -1) - Leaf Mineral Content (g kg -1) - Fruit Mineral Content (g kg -1)
- N P K - N P K - N P K
B20 6.33 0.27 6.7 - 14.8 1.17 3.82 - 10.5 0.65 4.29
- ±0.49 ±0.05 ±0.76 - ±0.61 ±0.09 ±0.26 - ±0.54 ±0.07 ±0.72
B20 + C5 7.08 0.31 8.07 - 17.5 1.37 4.62 - 11.8 0.78 5.12
- ±0.25 ±0.04 ±0.49 - ±1.12 ±0.06 ±0.18 - ±0.79 ±0.07 ±0.32
B20 + C10 7.79 0.46 8.65 - 19.3 1.48 5.23 - 13.3 1.08 5.79
- ±0.42 ±0.12 ±0.83 - ±1.7 ±0.05 ±0.44 - ±0.92 ±0.19 ±0.25
CV% 10.2 34.4 13.7 - 12.8 11.1 14.5 - 11.5 26.2 15.2

3.4. Correlation Matrix Between Blueberry Plant Dry Weights and Mineral Nutrients

A correlation matrix was built to reflect the interrelationships among different plant parameters. A positive correlation was observed among the dry weight of the plant parts (stems, leaves, and fruits) (Pearson’s r ranged from 0.81 to 0.88). Furthermore, the correlation was strong among fruit’s N ( r = 0.80), P (r = 0.90), and K (r = 0.83) uptake (Fig. 6).

Fig. (6). Correlation matrix among plant parts dry weight, NPK content, and uptake in blueberry. July 11 th, 2023.

Among the correlation coefficients presented in Fig. ( 6), correlations between N, P, and K uptake and dry weight were significant. The positive correlations between N uptake and stem (r = 0.95), fruit (r = 0.92), and leaf (r = 0.93) weights (Fig. 6) were highest compared to the correlation relationship between P uptake and stem (r = 0.88), fruit (r = 0.91), and leaf (r = 0.84) weights. Similarly, the K uptake capacity was also strongly correlated with stem, fruit, and leaf weights at r = 0.95, 0.89, and 0.94, respectively.


4.1. N Application and the Growth Stages of Blueberries

In this study, N fertilizer was applied according to the growth stages of the blueberry plants; in particular, the first N fertilization was applied 3–4 weeks after bud break.

N is the most important nutrient for maintaining vegetative growth, fruit development, and flower sprout formation and differentiation [ 43]. Sufficient N content in tissues is required to grow the following year’s crop. However, the plant took up less than 2% of N before the end of April [ 3] because blueberry growth in this stage relied on the amount of N stored at the beginning of the season. Stored N was used to maintain reproductive and vegetative growth. Flowering, fruit formation, and initiation of fruit development depend on stored nutrients [ 44].

Excess N at the beginning of a crop can reduce fruit quality, increase the risk of fruit disease, and delay ripening. Excess N at the end of the crop leads to excessive vegetative growth, late budding, and reduced flower bud development [ 38].

N fertilization for blueberries in this study was based on research by Rutgers University, which was based on the appearance of bud-break because, at this time, less fertilizer is needed, and active fertilization after the leaves and buds is ten times more effective than fertilization before bud-break. P and K are not absorbed by plants immediately after application but are absorbed later; therefore, P and K should not be applied before planting [ 37].

4.2. The Combined Effect of Biochar and Compost Application on Blueberry Growth and Yield

As shown in Table 3, the growth and fruit yield of blueberries increased with increasing biochar application rates at 10 and 20 t ha -1, which is consistent with previous studies showing that biochar alone can increase soil nutrient content and crop production [ 45]. According to Steiner et al. [ 46], grain yield can be doubled when biochar is used together with mineral fertilizers, but its effect fades after a number of crop seasons. However, considering the cost of these materials, excessive application of biochar or compost fertilizer may reduce net income [ 47]. Farhangi et al. [ 48] indicated that a high rate of biochar application (>30 t ha -1) did not substantially affect crop yield. Moreover, over-fertilizing biochar does not improve crop yields; even over-fertilizing at 20 t ha -1 reduces plant yield [ 49]. Gao et al. determined that biochar application at 10–20 t ha -1 is the most suitable application rate [ 50].

A previous study [ 51] revealed that plant growth increases with increasing compost application (Table 3) because compost contains the major nutrients required by all plants, including N, P, K, Ca, Mg, S, and essential trace elements considered essential for plants [ 52, 53]. Incorporating compost into the soil before planting a blueberry crop is recommended to improve plant growth and yield [ 54].

In blueberry soils with a high clay content, incorporating organic matter into the root zone improves aggregate stability, pore volume, hydraulic conductivity, and soil field capacity [ 55]. This would improve aggregate stability, pore volume, hydraulic conductivity, and soil field capacity [ 56].

Application of organic fertilizers is also related to increased soil total N, which promotes the growth of beneficial soil microorganisms, decomposes biomass, and indirectly provides NPK and other nutrients to plants through the crop rhizosphere [ 57]. One efficient method for increasing SOM levels is compost application, which is an effective way to treat waste, improve soil quality, and promote a better environment [ 56]. According to Amlinger et al. [ 50], the requirements of farmland using SOM can be fulfilled by an annual application of 7–10 tha -1 of dry matter compost; further decomposition of organic compounds results in stable humus, which is an organic product with slow decomposition rate [ 56].

The results in Table 3 show the interactive effects of biochar and compost on the leaf number, fruit number, and fruit yield of blueberries. Amlinger et al. [ 52] indicated that the combination of compost and biochar could enhance the material quality, leading to long-term stability of biochar and increased C sequestration potential. Therefore, the application of biochar-organic amendment mixtures or mineral fertilizers could sustainably and economically increase crop yield [ 58].

The crop yield model (Equation 1) observed in this study showed that crop yield was strongly associated with and dependent on plant growth and development (r 2 = 0.89). This result is consistent with that of a study by Fournier et al. [ 59], where more flowers had a good correlation with stem height and the number of leaves, which resulted in higher fruit biomass. Therefore, the proposed model can be used to predict wild blueberry yields.

4.3. The Combined Effect of Biochar and Compost Fertilization on Npk Content and Uptake of Blueberries

Trevetts’s sufficiency ranges (g/kg) of N, P, and K contents (16.0–23.8, 1.20–2.20, and 4.0–9.0, respectively) in the lowbush blueberry leaves [ 42] were compared with those of Hart et al. (14–22, 0.8–2.0, and 4.0–5.5, respectively) [ 38], which showed that the N and P ranges of Trevett were higher than those of Hart et al.; however, there was an insignificant distinction between the K range of Trevett and Hart et al.

As shown in Table 4, in the B20 treatment, the concentrations of N (14.8 g N kg -1) and K (3.82 g K kg -1) in the blueberry leaves were below the Trevett threshold. Since K plays an important role in photosynthesis and subsequent carbohydrate translocation and metabolism [ 60], the leaf number and size were reduced due to K values lower than the Trevett threshold [ 42]. However, the results in Table 4 show that in the B20 treatment, the concentration of P in the blueberry leaves (1.37 gP kg -1) was above the Trevett threshold [ 42], which is consistent with the conclusion of a previous study [ 26] that the addition of biochar significantly increases the availability of P in agricultural soil by a factor of 4.6.

Soil nutrient levels are not always the same as those of leaves; for example, sometimes, P content in soil is low, not in leaves. Thus, fertilizer recommendation for lowbush blueberry is mainly based on testing leaf tissue; nonetheless, soil testing is complementary [ 61]. Therefore, David and Kevin [ 39] also monitored the N, P, and K contents in leaf tissues. Many studies have shown that blueberries can be fertilized with N, P, and K, but K application has minimal or no effect on the lowbush blueberry fruit yield [ 62].

When K was applied to blueberry plants [ 63] based on soil K content in Michigan, the yield of ‘Bluecrop’ blueberry increased at low soil K (0.001–0.03 g kg -1) [ 64], but the yield of ‘Jersey’ blueberry had no effect at high soil K (0.03–0.08 g kg -1) [ 65].

Some studies have indicated that P application does not improve crop productivity despite low soil P content [ 66].

According to Parent et al. [ 67], based on growth-limiting nutrient concentrations, nutritive variables are multivariate in nature, and components should be understood as a whole. Pereira [ 68], based on the concept of nutritional balance, establishes limits on nutrients according to the plant’s requirements, facilitating a nutritional balance between nutrients in the leaf.

4.4. Correlation Matrix of Blueberry Plant Dry Weights and Mineral Nutrients

N, P, and K contents in stems, leaves, and fruits were positively correlated (Pearson’s r range of 0.67–0.85 in stem, 0.73–0.88 in leaf, and 0.68–0.71 in fruit) (Fig. 6). Kamprath [ 69] revealed that leaf P content was positively correlated with fruit P content, and total P accumulation was also positively correlated with total N accumulation [ 69]. Under traditional and organic farming methods, N, P, and K contents in leaves are all positively correlated with each other [ 70]; this reflects the dilution of those elements with increasing leaf and stem biomass. It has been suggested that soil N availability and plant N status greatly influence P and K uptake by plants. In the case of low P and K content, the N content was correspondingly low; thus, an increase in crop yield is mainly associated with N fertilization [ 67].

The results of this study are in agreement with those of a study by Quesnel et al. [ 71], in which N, P, K, Ca, and Mg contents in leaves are positively correlated with fruit yield. Many factors were assumed to have caused the differences between the values published in earlier studies and those obtained by Quesnel et al. However, the most important factor is SOM, as demonstrated by Trevett et al. [ 72], and a positive correlation exists between the leaf nutrient concentration and SOM.


Although alluvial soil is acidic, which is favorable for blueberry growth, the soil has poor porosity and aeration due to its high clay content. The use of biochar did not improve nutrient uptake for blueberries, but compost showed a positive role in improving this uptake.

In the combined application of biochar and compost, the interaction between these two materials results in a more effective response in terms of plant growth, yield, and NPK uptake. This approach effectively mitigates the challenges posed by high clay content in the soil. The findings emphasize the potential benefits of tailored soil amendment strategies to optimize blueberry cultivation in similar environments.


Ngo Ngoc Hung and Ngo Phuong Ngoc conceptualized and designed the experiment. Le Ngoc Quynh, Le Van Dang and Tran Hoang Em conducted the experiments and collected and analyzed samples. Le Van Dang, Le Minh Ly, and Pham Thi Phuong Thao analyzed the data and wrote the paper-original draft. Ngo Ngoc Hung and Ngo Phuong Ngoc reviewed, edited, and approved the final manuscript. All authors have read and agreed to the published version of the manuscript.


MD = Mekong Delta
ASS = Acid sulfate soil
OM = Organic matter
SOM = Soil organic matter
RBD = Randomized block design
SE = Standard deviation
B0 = No biochar applied
B10 = Biochar applied at 10 t ha -1
B20 = Biochar applied at 20 t ha -1
C0 = No compost applied
C5 = Compost applied at 5 t ha -1
C10 = Compost applied at 10 t ha -1
Wt = Weight
Y = Fruit yield


Not applicable.


All data generated or analyzed during this study are included in this published article.


This project was supported by the Ministry of Education and Training, Vietnam, with grant number B2021-TCT-10.


There is no conflict of interest among the authors.


The authors would like to thank Ms. Huynh Ngoc Truyen, a graduate student, for providing support in the measurements of the agronomic parameters.


Masuoka C, Yokoi K, Komatsu H, Kinjo J, Nohara T, Ono M. Two novel antioxidant ortho-benzoyloxyphenyl acetic acid derivatives from the fruit of Vaccinium uliginosum. Food Sci Technol Res 2007; 13(3): 215-20.
Kalt W, Cassidy A, Howard LR, Krikorian R, Stull AJ, Tremblay F. Recent research on the health benefits of blueberries and their anthocyanins. Adv Nutr 2020; 11(2): 224-36.
Patel S. Blueberry as functional food and dietary supplement: The natural way to ensure holistic health. Med J Nutrition Metab 2014; 7(2): 133-43.
Seedlings of blueberry 2023. Available from: http://viennghiencuucaygiong.com/giong-cay-viet-quat/
Torell M, Salamanca A, Ahmed M. Management of wetland resources in the lower Mekong Basin: Issues and future directions. 2001. Available from: https://api.semanticscholar.org/CorpusID:129419241
Mensvoort V. Soil Knowledge for Farmers, Farmer Knowledge for Soil Scientists. The case of acid sulphate soils in the Mekong Delta, Viet Nam. PhD Dissertation, Wageningen University, Netherlands. 1996.
Hung NN. Natural processes change soil fertility in the Mekong Delta Agriculture Publishing House. Vietnamese 2009.
Ngoc NP, Dang LV, Qui NV, Hung NN. Chemical processes and sustainability of rice-shrimp farming on saline acid sulfate soils in mekong delta. Heliyon 2023; 9(2): e13532.
Van KL. Physical fertility of typical Mekong Delta soils (Vietnam) and land suitability assessment for alternative crops with rice cultivation. PhD Thesis Ghent University Faculty of Sciences. 2003.
Chiem NH. Geo-pedological study of the Mekong Delta. South Asian Stud 1993; 31: 158-86.
Tran BL. Physical Fertility of a Soil under Intensive Rice Cultivation in the Mekong Delta (Vietnam) and Land Suitability Assessment for Alternative Crops with Rice Cultivation. Case Study at Long Khanh Village. Thesis, Ghent University, Free University of Brussels, Belgium. 2004.
Quang PV. Soil degradation of raised-beds on orchards in the Mekong delta-field and laboratory methods. TRITA-LWR PhD Thesis 1073. 2013.
Growth and Development of the Wild Blueberry. Wild Blueberry Fact Sheet A.2.0. 2011. Available from: https://www2.gnb.ca/content/dam/gnb/Departments/10/pdf/Agriculture/WildBlueberries-BleuetsSauvages/a20e.pdf
Ngoc NP, Dang LV, Quynh LN, Hung NN. Enhancing soil fertility and lowbush blueberry (Vaccinium angustifolium) growth using bio-organic fertilizer. IOP Conf Ser Earth Environ Sci 2022; 1087(1): 012077.
Lehmann J, Rillig MC, Thies J, Masiello CA, Hockaday WC, Crowley D. Biochar effects on soil biota – A review. Soil Biol Biochem 2011; 43(9): 1812-36.
Wu P, Ata-Ul-Karim ST, Singh BP, et al. A scientometric review of biochar research in the past 20 years (1998–2018). Biochar 2019; 1(1): 23-43.
Ebrahimi M, Souri MK, Mousavi A, Sahebani N. Biochar and vermicompost improve growth and physiological traits of eggplant (Solanum melongena L.) under deficit irrigation. Chem Biol Technol Agric 2021; 8(1): 19.
Souri MK, Hatamian M. Aminochelates in plant nutrition: a review. J Plant Nutr 2019; 42(1): 67-78.
Backer RGM, Schwinghamer TD, Whalen JK, Seguin P, Smith DL. Crop yield and SOC responses to biochar application were dependent on soil texture and crop type in southern Quebec, Canada. J Plant Nutr Soil Sci 2016; 179(3): 399-408.
Arabi Z, Eghtedaey H, Gharehchmaghloo B, Faraji A. Effects of biochar and bio-fertilizer on yield and qualitative properties of soybean and some chemical properties of soil. Arab J Geosci 2018; 11(21): 672.
Razzaghi F, Obour PB, Arthur E. Does biochar improve soil water retention? A systematic review and meta-analysis. Geoderma 2020; 361: 114055.
Liu Y, Lu H, Yang S, Wang Y. Impacts of biochar addition on rice yield and soil properties in a cold waterlogged paddy for two crop seasons. Field Crops Res 2016; 191: 161-7.
Mizuta K, Matsumoto T, Hatate Y, Nishihara K, Nakanishi T. Removal of nitrate-nitrogen from drinking water using bamboo powder charcoal. Bioresour Technol 2004; 95(3): 255-7.
Glaser B, Lehmann J, Zech W. Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal - a review. Biol Fertil Soils 2002; 35(4): 219-30.
Glaser B, Lehr VI. Biochar effects on phosphorus availability in agricultural soils: A meta-analysis. Sci Rep 2019; 9(1): 9338.
Ebrahimi M, Mousavi A, Souri MK, Sahebani N. Can vermicompost and biochar control Meloidogyne javanica on eggplant? Nematology 2021; 23(9): 1053-64.
Liu J, Schulz H, Brandl S, Miehtke H, Huwe B, Glaser B. Short‐term effect of biochar and compost on soil fertility and water status of a Dystric Cambisol in NE Germany under field conditions. J Plant Nutr Soil Sci 2012; 175(5): 698-707.
Dung TV, Qui NV, Dang LV, Toan LP, Hung NN. Morphological and physico-chemical properties of the raised-bed soils cultivated with Nam Roi pomelo in Chau Thanh district - Hau Giang province. CTUJS 2020; 56: 130-7.
Rogers SO, Bendich AJ. Extraction of DNA from plant tissues. In: Gelvin SB, Schilperoort RA, Verma DPS, Eds. Plant Molecular Biology Manual. Springer Netherlands 1989; pp. 73-83.
Floyd JW. Phylogenetic and biogeographic patterns in gaylussacia (Ericaceae) based on morphological, nuclear DNA, and chloroplast DNA variation. Syst Bot 2002; 27(1): 99-115.
Quynh LN, Ngoc NN, Em TH, Hung NN. Using internal transcribed spacers for blueberry (Vaccinium) species identification. VAAS 2023.
Small Black Blueberry Available from: https://eol.org/pages/582122
Marty C, Lévesque JA, Bradley RL, Lafond J, Paré MC. Lowbush blueberry fruit yield and growth response to inorganic and organic N-fertilization when competing with two common weed species. PLoS One 2019; 14(12): e0226619.
Percival DC, Priv Ã. Nitrogen formulation influences plant nutrition and yield components of lowbush blueberry (Vaccinium angustifolium Ait.). In: Acta Horticulturae. Leuven, Belgium: International Society for Horticultural Science (ISHS) 2002; pp. 347-53.
Doyle JW, Nambeesan SU, Malladi A. Physiology of nitrogen and calcium nutrition in blueberry (Vaccinium sp.). Agronomy 2021; 11(4): 765.
Hart J, Strik B, White L, Yang W. Nutrient management for blueberries in Oregon. Or State Univ Ext Serv EM (Pittsburgh Pa) 2006; EM 8918.
Logsdon SD. Soil Science: Step-by-step field analysis. ASA-CSSA-SSSA 2008; pp. 137-47.
Walkley A, Black A. An examination of Degtjareff method for determining soil organic matter, and proposed modification of the chromic acid tritation method. Soil Sci 1934; 37(1): 29-38.
Jackson MLSoil. Chemical Analysis. Englewood Cliffs, NJ: Prentice-Hall Inc. 1958.
Trevett MF. A second approximation of leaf analysis standards for lowbush blueberries. Research in the life sciences. Maine Agricultural Experimental Station Bulletin 1972; 5: 15-6.
Percival D, Sanderson K. Main and interactive effects of vegetative-year applications of nitrogen, phosphorus, and potassium fertilizers on the wild blueberry. Small Fruits Rev 2004; 3(1-2): 105-21.
Loescher WH, McCamant T, Keller JD. Carbohydrate reserves, translocation, and storage in woody plant roots. HortScience 1990; 25(3): 274-81.
Doan TT, Henry-des-Tureaux T, Rumpel C, Janeau JL, Jouquet P. Impact of compost, vermicompost and biochar on soil fertility, maize yield and soil erosion in Northern Vietnam: A three year mesocosm experiment. Sci Total Environ 2015; 514: 147-54.
Steiner C, Teixeira WG, Lehmann J, et al. Long term effects of manure, charcoal and mineral fertilization on crop production and fertility on a highly weathered Central Amazonian upland soil. Plant Soil 2007; 291(1-2): 275-90.
Jin Z, Chen C, Chen X, et al. Soil acidity, available phosphorus content, and optimal biochar and nitrogen fertilizer application rates: A five-year field trial in upland red soil, China. Field Crops Res 2019; 232: 77-87.
Farhangi-Abriz S, Torabian S, Qin R, Noulas C, Lu Y, Gao S. Biochar effects on yield of cereal and legume crops using meta-analysis. Sci Total Environ 2021; 775: 145869.
Li C, Zhao C, Zhao X, et al. Beneficial effects of biochar application with nitrogen fertilizer on soil nitrogen retention, absorption and utilization in maize production. Agronomy 2022; 13(1): 113.
Gao Y, Shao G, Yang Z, et al. Influences of soil and biochar properties and amount of biochar and fertilizer on the performance of biochar in improving plant photosynthetic rate: A meta-analysis. Eur J Agron 2021; 130: 126345.
Fischer D, Glaser B. Synergisms between compost and biochar for sustainable soil amelioration. In: Kumar S, Bharti A, Eds. Management of Organic Waste. IntechOpen 2012.
Amlinger F, Peyr S, Geszti J, Dreher P, Karlheinz W, Nortcliff S. Beneficial effects of compost application on fertility and productivity of soils Literature Study. Austria: Federal Ministry for Agriculture and Forestry, Environment and Water Management 2007.
Smith JL, Collins HP, Bailey VL. The effect of young biochar on soil respiration. Soil Biol Biochem 2010; 42(12): 2345-7.
McArthur D. Optimizing nutrient delivery in variable soils for sustainable highbush blueberry production. In: IV International Symposium on Mineral Nutrition of Deciduous Fruit Crops. 2001.
Schmid A, Suter F, Weibel FP, Daniel C. New approaches to organic blueberry (Vaccinium corymbosum L.) production in alkaline field soils. Eur J Hortic Sci 2009; 74: 103-11.
Rawls WJ, Nemes A, Pachepsky Y. Effect of soil organic carbon on soil hydraulic properties. In: Development of Pedotransfer Functions in Soil Hydrology Developments in Soil Science. Elsevier 2004; pp. 95-114.
Trupiano D, Cocozza C, Baronti S, et al. The effects of biochar and its combination with compost on lettuce ( lactuca sativa L.) growth, soil properties, and soil microbial activity and abundance. Int J Agron 2017; 2017: 1-12.
Schulz H, Dunst G, Glaser B. Positive effects of composted biochar on plant growth and soil fertility. Agron Sustain Dev 2013; 33(4): 817-27.
Fournier MP, Paré MC, Buttò V, Delagrange S, Lafond J, Deslauriers A. How plant allometry influences bud phenology and fruit yield in two Vaccinium species. Ann Bot 2020; 126(5): 825-35.
Lu Z, Lu J, Pan Y, et al. Anatomical variation of mesophyll conductance under potassium deficiency has a vital role in determining leaf photosynthesis. Plant Cell Environ 2016; 39(11): 2428-39.
Yarborough DE, Smagula JM. Fertilizing with Nitrogen and Phosphorus. Orono, ME, USA: University of Maine 2013.
Lafond J. Nitrogen, phosphate and potassium fertilization in the production of wild lowbush blueberry. Can J Soil Sci 2020; 100: 99-108.
David R. Benefits of Using Liquid Sources of Potassium Fertilizer in Highbush Blueberry. Fluid Fertilizer Foundation Report 2016.
Paul E. Optimum potassium nutritional level for production of highbush blueberry. J Am Soc Hortic Sci 1983; 108(4): 520-2.
Hancock JF, Nelson J. Leaf potassium content and yield in the highbush blueberry. HortScience 1988; 23(5): 857-8.
Lafond J, Ziadi N. Biodisponibilité de l’azote et du phosphore dans les sols de bleuetières du Québec. Can J Soil Sci 2013; 93(1): 33-44.
Parent SÉ, Parent LE, Rozane DE, Natale W. Plant ionome diagnosis using sound balances: Case study with mango (Mangifera Indica). Front Plant Sci 2013; 4: 449.
Serra AP, Marchetti ME, Bungenstab DJ, et al. Diagnosis and Recommendation Integrated System (DRIS) to Assess the Nutritional State of Plants. In: Biomass Now. IntechOpen 2013.
Kamprath EJ. Enhanced phosphorus status of maize resulting from nitrogen fertilization of high phosphorus soils. Soil Sci Soc Am J 1987; 51(6): 1522-6.
Vilhena NQ, Quiñones A, Rodríguez I, Gil R, Fernández-Serrano P, Salvador A. Leaf and fruit nutrient concentration in rojo brillante persimmon grown under conventional and organic management, and its correlation with fruit quality parameters. Agronomy 2022; 12(2): 237.
Quesnel PO, Côté B, Fyles JW, Munson AD. Optimum nutrient concentrations and CND scores of mature white spruce determined using a boundary-line approach and spatial variation of tree growth and nutrition. J Plant Nutr 2006; 29(11): 1999-2018.
Trevett MF, Carpenter PN, Durgin RE. Seasonal trend and interrelation of mineral nutrients in lowbush blueberry leaves. In: Volume 665 of Experiment station bulletin. Maine Agricultural Experiment Station 1968.