Use of the Diagnosis and Recommendation Integrated System (DRIS) for Determining the Nutritional Balance of Durian Cultivated in the Vietnamese Mekong Delta

Abstract

Durian is one of the most valuable and expensive fruits in Vietnam and Southeast Asia. Leaf nutritional disorders are considered to be causes of reductions in fruit yield and quality. However, studies on the nutritional balance in durian leaf are limited. In this study, we used the DRIS method for leaf limitation nutrient diagnosis at the stage 2 months before durian flowering. Our objectives were to (i) establish DRIS norms for the macronutrients (N, P, K, Ca, Mg, and S) in durian leaf and (ii) determine nutrient value ranges that are insufficient or excessive in durian production. A total of 180 leaf samples were collected and examined from 90 durian orchards cultivated on alluvial soils in the Vietnamese Mekong Delta. The results indicate that DRIS establishment for durian was highly reliable due to the significant positive correlation (r > 0.5) among nutrient indices. The limiting nutrients in durian leaf at the investigation stage were S, Mg, Ca, and P, where S and Mg were the most deficient. The optimal ranges of nutrients in durian leaf were determined and recommended in this study. Further studies are necessary to validate the efficiency of DRIS using nutrient omission trials under durian cultivation.

1. Introduction

Durian (Durio zibethinus Murr.) is a fruit originating from Borneo, Southeast Asia (SA) [1]. It is widely cultivated in the tropical regions of SA, such as Indonesia, Thailand, Vietnam, Malaysia, and the Philippines [2]. It is known as the ‘king of fruits’ due to its unique taste and nutritional value [3]. Therefore, it is one of the most expensive and high-value fruits for both domestic markets and export [4]. In the Vietnamese Mekong Delta (VMD), the area of durian cultivation was 30,000 ha in 2020, and this is estimated to increase rapidly in the future. This is a significant concern for local authorities due to farmers lacking experience in durian growing. Durian cultivation has numerous requirements (farming technique, nutrient management, etc.) because these factors are the most effective in determining fruit productivity and quality [5]. Farmers who cultivate durian in the VMD usually undertake nutrient management based on their experience. They apply fertilizers in excess of the actual mass requirement of the plants, harming the growth and development of durian. In addition, nutrient disorders and imbalances in durian leaf cause increasing fruit physiological disorders, resulting in a reduction in flesh quality and, therefore, decreasing the value and price of durian [6]. Thus, leaf nutrient balancing is considered the best strategy for sustainable and effective durian cultivation.

Macronutrients, including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), are considered the main elements that significantly affect durian fruit yield and quality (smell and taste) [6]. N plays many vital roles in growth, flowering, fruit setting and development, and physiological processes in the flesh of durian [6]. N also contributes primarily to the photosynthesis and respiration of the plant [7]. P directly affects energy storage and transfer, and enzyme activation [8]. P is necessary for nutrient uptake, flower initiation, and fruit growth [9]. K aids in the synthesis of proteins and carbohydrates; it also improves the size, taste, and color of fruits [10]. Ca is essential for cell walls, cell division, and creating new roots; it also affects carbohydrate translocation and N absorption [10]. Mg plays an important role in chlorophyll structure, regulating the uptake of other essential elements and carbohydrate translocation [11]. S acts in the biosynthesis of sulfur-containing amino acids and various secondary metabolites [12]. Therefore, deficiencies or excesses of these elements adversely affect durian cultivation and production. In many countries, fertilizer recommendations and nutritional status diagnosis in fruit plants are determined based on leaf analysis [13,14,15] because foliar analysis reveals the actual uptake of the plant from fertilization and soil. This is considered a measurement method for the evaluation of the current nutritional status of crops, which might be replaced by soil analysis [16].

Leaf diagnosis is a complex method because it is affected by foliar age and other interactions, which directly influence nutrient uptake and distribution [14]. Various approaches are utilized to interpret the nutritional status of plants using leaf analysis, such as the identification of deficiency symptoms, determining the ratio of nutrients, or using the diagnosis and recommendation integrated system (DRIS) [17,18]. DRIS was introduced and developed by Beaufils (1957, 1971, and 1973) as a solution for tackling the difficulties inherent in diagnostic methods utilizing leaves [19]. Currently, DRIS is an interesting method because it helps in the identification of nutrient imbalances, insufficiencies, or excesses and ranks them in order of importance to determine remedial steps [14,17,20]. DRIS has been successfully used for the evaluation of nutritional imbalances in fruit plants, such as mango [21], guava [22], bananas [23], and pineapple [17]. According to Villamil-Carvajal et al. [24], DRIS norms must be developed for each crop. This is because the accuracy of DRIS norms is affected by the specific cultivar, climate, soil, plant nutrient management, etc. Presently, research concerning the application of DRIS in assessing leaf nutritional status for durian remains lacking and unclear. Therefore, to fill this knowledge gap, we undertook a study on 90 farms that cultivate durian in the VMD. The objectives of this study were to (i) establish DRIS norms for the macronutrients (N, P, K, Ca, Mg, and S) of durian leaf at the stage 2 months before durian flowering and to (ii) determine the ranges of optimal nutrients for durian leaf.

2. Materials and Methods

2.1. Study Description, Soil, and Climate

To carry out the study, we collected durian leaves from 90 orchards in Can Tho city and Hau Giang province (Figure 1). These locations occupy the largest area of durian cultivation, which is considered to have high potential for durian production in the VMD region [25]. The average annual precipitation and air temperature in the period 2020–2022 were 1818 mm and 27.6 °C, respectively [26]. Our previous studies [26,27] indicated that soils are classified as alluvial soils, with high fertility at the soil depth of 0–20 cm, rich soil organic matter (25–30 g kg⁻¹) and total N content (1.0–1.5 g kg⁻¹), and optimal soil bulk density (~1.0 g cm⁻³); data were shown on a dry weight soil. The pH value of soils was 4.5–5.0, which was extracted in distilled water with a ratio of soil/distilled water (1:2.5). According to Amran et al. [28], who reported that durian thrives well in soil pH ranges from 5.5 to 6.5. Therefore, soil pH of the study locations was lower than that in optimal threshold of approximately 1.0 unit.

2.2. Leaf Sampling and Analysis

In this study, the ‘Ri 6’ durian cultivar was investigated because it is popular and comprises approximately 70% of the durian varieties cultivated in the VMD [29]. Before collecting leaf samples, we surveyed the fruit yield of durian in 2022 at 200 orchards. To establish DRIS norms, data should be divided into two groups, including high-yielding and low-yielding [30]. The high-yielding group was calculated using the mean of the fruit yield population + (0.5 × SD) [30]. We successfully separated two fruit yield groups: low yield (<17.6 Mg ha⁻¹) and high yield (≥17.6 Mg ha⁻¹). Next, we selected 90 durian orchards for leaf collection, with 45 low and 45 high fruit yields. Two leaf samples were taken from each orchard. A total of 180 leaf samples were collected in this research.

Durian trees in the study area received an annual application of N–P–K chemical fertilizers; in the high fruit yield group, the applied N, P, and K fertilizers ranged from 1.53 to 1.81 N, 1.16 to 1.41 P, and 0.99 to 1.33 K (kg tree⁻¹ year⁻¹) [25]. In the low fruit yield group, the applied N, P, and K fertilizer applications were 0.94–1.31 N, 0.41–0.82 P, and 0.56–0.80 K (kg tree⁻¹ year⁻¹) [25]. Farmers applied both single and complex fertilizers for their orchards, such as urea (46% N), diammonium phosphate (18% N–46% P), N–P–K (20% N–20% P–10% K), superphosphate (7%P), and potassium chloride (50% K). Most of the durian orchards did not supply any fertilizers containing elements such as Ca, Mg, and S [25].

To be eligible for sampling, each orchard area had to be greater than 3000 m², and the durian plant had to be at least 6 years old. In addition, farming techniques such as irrigation, cultivation, planting density, flower induction, and insect and disease management were similar among orchards. We conducted leaf sampling in August 2023, 2 months before durian flowering. According to Morales et al. [30], 2 months before the flowering stage is considered important due to plants requiring high amounts of nutrients for sprouting, flowering, and fruit setting; therefore, this stage is suitable for leaf collection.

Leaf samples collected from the plant showed no symptoms of nutritional deficiency and pest infestations. Leaves were picked from the 5th and 6th positions from the bud according to the method of Suarta et al. [31]. Each leaf sample included 40 leaves collected from 5 durian trees. After collection, durian leaves were rinsed with distilled water 4 times to remove dust and insects. Subsequently, they were put in an oven for drying at 65 °C for 4 days and then ground and stored in vials to analyze mineral nutrients.

The contents of mineral nutrients (N, P, K, Ca, Mg, and S) in the leaves were analyzed based on the method of Houba et al. [32]. Firstly, leaf samples were converted to inorganic forms using a mixture of 12 g of salicylic acid, 36 mL of water, and 200 mL of 96% H₂SO₄. Secondly, a solution of hydrogen peroxide (H₂O₂, 30%) was added during the leaf digestion process. Finally, N was determined using the Kjeldahl method; P was analyzed via a UV spectrophotometer (UV-1800, Shimadzu, Tokyo, Japan); K, Ca, and Mg were measured via atomic absorption spectrophotometry (iCE 3500, Thermo Scientific, Waltham, MA, USA) at wavelengths of 766, 422, and 285 nm, respectively; and S was determined using the colorimetric method.

2.3. Establishment of DRIS Norms

The DRIS method was described by Beaufils [33] and modified/improved by other agronomists [19,34]. DRIS norms should be conducted in 3 steps, including (1) nutrient ratio establishment (X₁/X₂, X₂/X₁, X₁/X₃, X₃/X₁, … X₁/X_n, X_n/X₁); (2) calculation of mean values, standard deviation (SD), variance (σ²), and coefficient of variation (CV); and (3) statistical testing (t-test) using nutrient concentrations between high- and low-productivity groups. Next, DRIS norms should be established based on the results of nutrient ratios that have significant differences (p < 0.05) in nutrient concentrations among yielding groups.

2.4. DRIS Indices

DRIS indices are the values calculated after DRIS norms are established. According to Khuong et al. [17], DRIS indices should be determined based on the following equation:

$$\mathrm{I}\mathrm{X}1=\frac{\left[\mathrm{f}\left(\frac{\mathrm{X}1}{\mathrm{X}2}\right)+\mathrm{f}\left(\frac{\mathrm{X}1}{\mathrm{X}3}\right)\dots +\mathrm{f}\left(\frac{\mathrm{X}1}{\mathrm{X}\mathrm{n}}\right)\right]}{\mathrm{Y}}$$

(1)

where IX1 is the DRIS index for the X1 nutrient. $\mathrm{f}\left(\frac{\mathrm{X}1}{\mathrm{X}2}\right)$ is the function calculated for the X1 and X2 nutrient ratio; $\mathrm{f}\left(\frac{\mathrm{X}1}{\mathrm{X}2}\right)=\left[\left(\right(\mathrm{X}1/\mathrm{X}2)/(\mathrm{x}1/\mathrm{x}2\left)\right)-1\right]\times (1000/\mathrm{C}\mathrm{V})$ if (X1/X2) ≥ (x1/x2) and $\mathrm{f}\left(\frac{\mathrm{X}1}{\mathrm{X}2}\right)=\left[1-\left(\right(\mathrm{x}1/\mathrm{x}2)/(\mathrm{X}1/\mathrm{X}2\left)\right)\right]\times (1000/\mathrm{C}\mathrm{V})$ if (X1/X2) < (x1/x2). X1/X2 is the nutrient ratio for diagnosis and x1/x2 is the value of nutrient ratio that is determined from DRIS norms. CV is the coefficient of variation, and Y is the number of functions used for the calculation of the total nutrients.

2.5. Establishing Durian Leaf Nutrient Optimum Ranges

In this study, nutrient optimum ranges of leaves were established using the average nutrient concentration of the high-productivity group [30]. The value ranges of durian leaf nutrients were calculated by taking into account the standard deviation (SD). Leaf nutrient classifications of durian were in accordance with the procedure of Morales et al. [30], including excessive (>$\frac{4}{3}$ SD), high ($\frac{2}{3}$ < SD < $\frac{4}{3}$), optimum ($-\frac{2}{3}$ < SD < $\frac{2}{3}$), low ($-\frac{4}{3}$ < SD < $-\frac{2}{3}$), and deficient (<$-\frac{4}{3}$ SD). In our study, a total of five classes for durian foliar diagnosis were suggested.

2.6. Data Analysis

We used Microsoft Excel (ver. 16) and IBM SPSS Statistics (ver. 20) software for data analysis. Student’s t-test was used to compare averages of nutrient concentration between low- and high-productivity groups. Pearson’s correlation was applied to evaluate the relationships between nutritional variables. Data were interpreted on a dry-weight basis.

3. Results

3.1. The Concentration of Leaf Nutrients under Low and High Durian Fruit Yields

Table 1. Nutrient parameters for DRIS establishment (n = 180).

Nutrient (g kg−1)	Productivity Group	Minimum	Mean	Maximum	SD	CV (%)	t-Test
N	High	13.3	23.0	32.2	4.72	20.6	***
	Low	11.2	19.9	28.3	4.91	24.7
P	High	1.11	2.51	4.20	0.66	26.3	***
	Low	1.05	1.58	3.10	0.35	22.0
K	High	11.6	21.1	32.4	5.51	26.2	***
	Low	10.5	17.0	29.1	4.05	23.8
Ca	High	13.9	26.8	52.2	8.91	33.3	**
	Low	10.2	22.9	39.1	7.52	32.9
Mg	High	2.65	5.69	13.2	2.62	46.0	***
	Low	1.22	4.01	8.64	1.77	44.1
S	High	1.40	3.34	9.01	1.67	50.0	***
	Low	1.01	2.36	6.01	0.90	38.1

Low productivity (<17.6 Mg ha⁻¹); high productivity (≥17.6 Mg ha⁻¹). SD: standard deviation; CV: coefficient of variation; ** and *** indicate p < 0.01 and p < 0.001, respectively.

shows the nutrient concentrations for low and high durian fruit productivity. There was a significant difference in nutrients between the low- and high-productivity groups. The mean of the N concentration in the high-productivity group was higher by 3.10 g kg⁻¹ than that in the low-productivity group. Similarly, the average contents of P, K, Ca, Mg, and S of the high-productivity group were 0.93, 4.10, 3.90, 1.68, and 3.00 g kg⁻¹ higher than those in the low-productivity group, respectively. CV in each nutrient population ranged from 20.6% to 50% in both groups.

Figure 2 shows the relationship in respect of nutrient concentration between high- and low-yielding groups. There was a strong positive (r = 0.78 ***) relationship between P and K nutrients in the high-yielding group (Figure 2a). Similarly, Ca was positively correlated with Mg (r = 0.56 ***), and Mg was also positively correlated with S (r = 0.52 ***). Negative correlations were observed between the nutrient concentrations in the high-productivity group, such as the following: P and Ca (r = −0.47 ***), P and Mg (r = −0.61 ***), P and S (r = −0.45 ***), K and Ca (r = −0.61 ***), K and Mg (r = −0.72 ***), and K and S (r = −0.47 ***). However, there was no significant correlation among nutrients in the low-yielding group (Figure 2b).

3.2. DRIS Norms for Durian

Table 2. DRIS norms establishment for durian leaf.

Nutrient Ratios	High Fruit Yield (n = 90)				Low Fruit Yield (n = 90)				σ2L/σ2H	Selected Ratio
	Mean	SD	CV (%)	Variance (σ2H)	Mean	SD	CV (%)	Variance (σ2L)
N/P	9.84	3.57	36.3	12.6	13.1	4.34	33.2	18.7	1.48 ***	✓
P/N	0.11	0.03	30.7	0.00	0.08	0.03	31.2	0.00	0.58
N/K	1.16	0.39	33.3	0.15	1.23	0.40	32.4	0.16	1.06
K/N	0.95	0.28	29.7	0.08	0.92	0.35	38.1	0.12	1.55 **	✓
N/Ca	0.96	0.38	39.8	0.14	0.97	0.43	43.8	0.18	1.24 **	✓
Ca/N	1.23	0.55	44.8	0.30	1.22	0.48	39.6	0.23	0.77
N/Mg	4.77	1.98	41.6	3.89	6.09	3.37	55.3	11.2	2.89 ***	✓
Mg/N	0.26	0.14	55.3	0.02	0.21	0.11	49.5	0.01	0.53
N/S	8.60	4.45	51.8	19.6	9.72	4.52	46.5	20.2	1.03 ***	✓
S/N	0.15	0.09	57.0	0.01	0.13	0.06	44.2	0.00	0.40
P/K	0.12	0.02	16.8	0.00	0.10	0.03	34.6	0.00	2.83
K/P	8.54	1.71	20.1	2.90	11.3	3.69	32.7	13.5	4.64 ***	✓
P/Ca	0.11	0.05	47.7	0.00	0.08	0.03	43.5	0.00	0.43
Ca/P	12.1	6.85	56.8	46.5	15.0	5.75	38.3	32.7	0.70	✓
P/Mg	0.54	0.27	50.2	0.07	0.50	0.31	62.1	0.09	1.29 **	✓
Mg/P	2.67	2.08	77.7	4.27	2.64	1.28	48.3	1.61	0.38
P/S	0.95	0.51	53.3	0.25	0.78	0.36	46.8	0.13	0.51 *	✓
S/P	1.54	1.25	80.9	1.54	1.55	0.67	43.3	0.44	0.29
K/Ca	0.92	0.46	50.6	0.21	0.83	0.34	40.5	0.11	0.52 ns	✓
Ca/K	1.45	0.82	56.5	0.66	1.41	0.56	39.9	0.31	0.47
K/Mg	4.60	2.29	49.8	5.20	5.37	3.34	62.1	11.0	2.12 ***	✓
Mg/K	0.32	0.23	73.4	0.05	0.25	0.13	51.5	0.02	0.30
K/S	7.98	4.14	51.9	17.0	8.37	4.03	48.1	16.0	0.94 **	✓
S/K	0.18	0.14	77.0	0.02	0.15	0.07	46.2	0.00	0.23
Ca/Mg	5.22	1.95	37.3	3.75	6.94	4.12	59.4	16.8	4.48 ***	✓
Mg/Ca	0.22	0.09	41.9	0.01	0.19	0.12	60.3	0.01	1.58
Ca/S	9.77	5.48	56.1	29.7	11.3	6.46	57.2	41.3	1.39 ***	✓
S/Ca	0.14	0.08	57.0	0.01	0.12	0.06	54.0	0.00	0.65
Mg/S	1.94	1.11	56.9	1.21	1.92	1.12	58.1	1.23	1.02
S/Mg	0.63	0.28	44.7	0.08	0.72	0.47	65.3	0.22	2.78 ***	✓

Low productivity (<17.6 Mg ha⁻¹); high productivity (≥17.6 Mg ha⁻¹). SD: standard deviation; CV: coefficient of variation; *, **, and *** indicate p < 0.05, p < 0.01, and p < 0.001, respectively; ns: not significant. ✓, nutrient pair ratio selected for DRIS indices.

shows the values of the mean, SD, CV, and σ² of nutrient ratios in two durian fruit productivity groups (low and high). After the calculation of the nutrient ratios in leaves, 15 nutrient ratios were selected from the DRIS norms to determine DRIS indices. These were N/P, K/N, N/Ca, N/Mg, N/S, K/P, Ca/P, P/Mg, P/S, K/Ca, K/Mg, K/S, Ca/Mg, Ca/S, and S/Mg.

3.3. DRIS Indices of Durian Leaf

The nutrient indices for durian were successfully established from the DRIS norms (

Table 3. Nutrient index (n = 180) for durian leaf.

Nutrient Index	Minimum	Mean	Maximum	SD
N	−27.7	2.17	24.2	8.07
P	−23.7	−0.71	19.3	7.24
K	−30.7	0.76	33.2	11.3
Ca	−26.0	−1.09	31.0	6.23
Mg	−55.6	−2.36	54.9	17.0
S	−53.7	−3.71	29.9	11.0

SD: standard deviation.

). Among the five nutrients investigated, we observed that the N and K concentrations in durian leaves were quite high. In particular, the K index was slightly abundant (0.76), and the N nutrient index was excessive (2.17). Meanwhile, the concentrations of P, Ca, Mg, and S in durian leaf were deficient. The means of those nutrient indices (P, Ca, Mg, and S) were −0.71, −1.09, −2.36, and −3.71, respectively. S and Mg contents were extremely deficient compared with those of Ca and P. The limited nutrients in durian leaf at the stage 2 months before flowering followed the pattern S > Mg > Ca > P.

Figure 3 presents the relationship between the nutrient indices (N, P, K, Ca, Mg, and S) established from DRIS norms. A strong positive correlation was observed between DRIS indices for durian leaf. In particular, there was a positive correlation between IN and IP (r = 0.69), IN and IK (r = 0.55), IN and ICa (r = 0.60), IN and IMg (r = 0.48), and IN and IS (r = 0.54). IP correlated positively with IK, ICa, IMg, and IS, with r = 0.80, 0.89, 0.59, and 0.77, respectively. Similarly, IK correlated positively with ICa (r = 0.65), IMg (r = 0.68), and IS (r = 0.59). There was a strong positive correlation between ICa and IMg, ICa and IS, and IMg and IS (r = 0.68, 0.79, and 0.59, respectively).

3.4. Nutrient Optimal Range for Durian Leaf

The classification of nutrient-optimal ranges for durian leaf cultivated in the VMD was undertaken (

Table 4. Durian leaf nutrient optimum ranges for the stage 2 months before flowering.

Nutrient (g kg−1)	Excessive	High	Optimum	Low	Deficient
N	>29.3	26.1–29.3	19.8–26.0	16.7–19.7	<16.7
P	>3.30	2.91–3.30	2.11–2.90	1.60–2.10	<1.60
K	>28.3	24.7–28.3	17.4–24.6	13.7–17.3	<13.7
Ca	>38.6	32.7–38.6	20.8–32.6	14.9–20.7	<14.9
Mg	>9.10	7.31–9.10	3.81–7.30	2.20–3.80	<2.20
S	>5.50	4.41–5.50	2.21–4.40	1.10–2.10	<1.10

). There were five classes (excessive, high, optimum, low, and deficient) established for each nutrient in durian leaf at the stage 2 months before flowering. The optimal ranges of nutrients (N, P, K, Ca, Mg, and S) for durian leaf were 19.8–26.0, 2.11–2.90, 17.4–24.6, 20.8–32.6, 3.81–7.30, and 2.21–4.40 g kg⁻¹, respectively. Deficient nutrients were determined as follows: N < 16.7 g kg⁻¹, P < 1.60 g kg⁻¹, K < 13.7 g kg⁻¹, Ca < 14.9 g kg⁻¹, Mg < 2.20 g kg⁻¹, and S < 1.10 g kg⁻¹.

4. Discussion

We observed that there was a significant difference in nutrient concentrations between low and high fruit productivity (Table 1). In this study, we collected leaves from orchards that had similar farming techniques (see Section 2.2). However, fertilization management and use were different in the two productivity groups. The orchards of the high-yielding group received more than approximately 50% N–P–K chemical fertilizers than the low-yielding group. This is considered one reason for the concentrations of nutrients in the high-productivity group being higher than those in the low-productivity group. The results of our study are in agreement with Morales et al. [30], Khuong et al. [17], and Xuan et al. [20]. They reported that nutrient concentrations are higher in high-yielding groups compared with low-yielding groups. A previous study indicated that differences in the amount of fertilizer applied significantly affect leaf nutritional contents [35].

Imbalanced fertilizer application also affected leaf nutrients. We realized that farmers growing durian often do not use or apply fertilizers containing elements such as Ca, Mg, and S in the long term. This leads to a decrease in or depletion of these nutrients in the soil and, thus, a deficiency in the leaves. In contrast, N and P are applied in large amounts by farmers because they believe that these nutrients will increase fruit yield quality. Various studies have indicated that the use of imbalanced fertilizers leads to decreased soil fertility and nutrition insufficiency in leaves, resulting in reductions in plant growth and productivity [36,37]. According to Ong et al. [38], 10 tons of durian fruit removed approximately 4 P–3 Ca–5 Mg (kg). Unfortunately, the authors did not reveal the data in respect of S in their study. In this study, the average durian fruit yield was 16 tons year⁻¹; therefore, around 6.5, 5.0, and 5.0 kg (P, Ca, and Mg, respectively) were used by fruit each year. Hence, farmers should return these elements via fertilizer (granular or foliar) application to offset the rate of nutrient loss from fruits. Other factors affecting the concentration of nutrients in leaves are low soil pH and high iron and aluminum contents, resulting in a decrease in the concentrations of exchangeable Ca and Mg in the soil. A previous study [39] reported that durian orchards in the VMD had low soil pH (pH < 5) and exchangeable cations (Ca²⁺ and Mg²⁺). Moreover, most durian trees are cultivated in raised beds in which the surface soil is always higher by 0.5 m compared with the water in the canal. This reduces the exchangeable cations because these cations runoff due to rainfall and irrigation [26]. Although growers applied a certain amount of P for their gardens, P is a limited nutrient in durian leaf. P use efficiency decreased due to P being precipitated by Al and Fe under low pH conditions, resulting in increasing insoluble hydroxide compounds that are unavailable for plant uptake [26]. Therefore, soil pH regulation is necessary for improving the availability of P in soil.

The CV values of DRIS norms were low, with ranges of 16.8–80.9% and 31.2–65.3% in the high- and low-productivity groups, respectively (Table 2). Walworth and Sumner [19] indicated that DRIS norms are highly reliable when the CV is low (<100%) and σ²_L/σ²_H is high. This is because a high CV could lead to large deviations in results between nutrient ratios. Our results are consistent with those of Llanderal et al. [40], who successfully determined the limiting nutrients of pepper using DRIS norms, which had a low CV and high σ²_L/σ²_H. Another study showed that DRIS norms established from higher σ²_L/σ²_H and lower CV values in nutrient ratios were more reliable for calculating the DRIS index [17]. A similar study also concluded that DRIS norms were highly accurate and efficient when the nutrient ratios had a low CV and high σ²_L/σ²_H [41,42]. Therefore, the DRIS norms in our study were reliable for DRIS index calculation due to the high σ²_L/σ²_H and low CV of the nutrient ratios.

From the results of the DRIS norms in Table 2, we selected 15 nutrient ratios, which were used to calculate the DRIS indices (Table 3). The results indicate that there was a strong positive correlation (r > 0.5) among nutrient indices (Figure 3). Therefore, these indices have high reliability and could be used widely for the nutritional diagnosis of durian leaf cultivated on the VMD. In addition, we observed that durian leaves had a shortage of P, Ca, Mg, and S (DRIS index < 0). Meanwhile, there was excessive N and K (DRIS index > 0) in durian leaf. According to Beverly [43], the nutrient index is excessive when the value is higher than zero. Further, nutrients are insufficient if the value of the nutrient index is smaller than zero, and nutritional balance is confirmed if the nutrient index is zero [44,45].

We successfully determined the optimal nutrient range in durian leaf cultivated on the VMD (Table 4). In this study, the proposed nutrients (g kg⁻¹) that are optimal for leaf durian are 19.8–26.0 N, 2.11–2.90 P, 17.4–24.6 K, 20.8–32.6 Ca, 3.81–7.30 Mg, and 2.21–4.40 S. These optimum ranges for durian leaves at the stage 2 months before flowering agree with the results of Poovarodom et al. [13], who reported the standard concentrations (g kg⁻¹) of N (20–24), P (1.5–2.5), K (15–25), Ca (17–25), and Mg (2.5–5.0) in durian leaf cultivated in Thailand. Another study [46] in Thailand showed that the average nutrient concentrations (g kg⁻¹) in durian leaf were 19–23, 1.7–2.6, 14–18, 20–35, 4.4–6.9, and 2.4–2.7 (N, P, K, Ca, Mg, and S, respectively). Therefore, the nutrient optimal ranges established in our study are considered an important basis for assessing the nutritional status of durian trees, thereby enabling sustainable and effective durian cultivation in the VMD.

5. Conclusions

DRIS indices were successfully established for the nutritional diagnosis (N, P, K, Ca, Mg, and S) of durian and had high reliability. The concentrations of P, Ca, Mg, and S in leaves were deficient, while the contents of N and K were excessive. Therefore, to ensure sustainable durian development, we recommend that farmers should apply complex chemical fertilizers that contain P, Ca, Mg, and S to replenish their orchards. In addition, enhancing and regulating soil pH should be necessary to increase soil available nutrients (P, Ca, and Mg) for durian. Based on the results of our previous studies [27,47], the applications of lime, rice husk biochar, or compost were not only increasing soil pH but also supplying nutrients for the soil. The use of soil conservation practices (leguminous cover cropping and rice straw mulching) is also considered the best strategy for improving soil pH and avoiding the loss of nutrients or exchangeable cations on the surface of raised soil beds.