Optimizing Cabbage Cultivation in Paddy-Converted Fields Using Discarded Coir Substrates and Controlled Irrigation

Abstract

This study aimed to reuse discarded coir substrates and optimize irrigation as a low-cost solution for addressing waterlogging in paddy-converted farmland. We employed a 2 × 4 factorial design, with two cultivation methods consisting of paddy soil (PS) and coir substrates (CS), and four irrigation levels (IL) set as 140% crop evapotranspiration (ETc140), 100% ETc (ETc100), 60% ETc (ETc60), and non-irrigated control (ETc0). We evaluated the growth and physiological characteristics of cabbage (Brassica oleracea L. var. Capitata), including the outer leaf growth, yield components, water use efficiency (WUE), photosynthetic parameters, chlorophyll content, proline content, malondialdehyde (MDA) content, and glucosinolates (GLs) content. The results indicated that the interaction between the CS and IL significantly improved cabbage growth, photosynthetic activity, and stress resistance compared with PS. Notably, when CS was combined with ETc100 and ETc60 irrigation levels, cabbage exhibited optimal growth parameters, and CS-ETc60 achieved the highest WUE. This study indicated that using discarded coir substrates combined with appropriate irrigation levels offers an effective and low-cost solution for mitigating waterlogging problems.

1. Introduction

Due to dietary diversity, urbanization, and a growing preference for quick meals made from wheat or other coarse grains, the consumption of rice has gradually decreased in South Korea [1]. According to data from Statistics Korea, per capita rice consumption in South Korea decreased from 132.7 kg in 1980 to 56.4 kg in 2023, representing a 57.6% decline. Therefore, the Korean government is making efforts to control rice overproduction and maintain stable domestic rice prices through paddy field rotation practices [2]. The government also implemented policies that encourage the multi-purpose utilization of paddy fields to improve farmland management [3]. The policy advocates converting paddy fields into dry fields for 2 to 3 years to grow upland crops and then converting them back into paddy fields [4]. However, the conversion of paddy fields for the cultivation of upland crops presents significant challenges, particularly in terms of drainage. The low bulk density, small particle size, and high groundwater table characteristics of paddy soils can result in inadequate drainage, potentially leading to lower crop yields and quality [3,4,5].

Currently, open ditch and subsurface drainage systems are commonly used to address farmland drainage issues, and they are often combined to enhance drainage efficiency [6]. An open ditch is a drainage channel excavated around agricultural fields to remove excess water; however, its effectiveness is limited due to susceptibility to weed growth and sediment blockage [7]. Subsurface drainage involves burying pipes underground to effectively improve drainage by lowering the water table. Subsurface drainage is prone to pipe blockages, so filter layers such as non-woven fabric, gravel, and rice husks are typically added around the pipes [8,9,10]. However, the construction materials for underground drainage systems, such as PE pipes and filtering materials, not only add to the financial burden on farmers but also fail to meet the requirements of sustainable development [11]. In recent years, to reduce the installation costs of drainage systems and improve the durability of pipes, many studies have focused on improving pipe materials and minimizing the use of heavy machinery during installation [12,13,14,15]. According to the Rural Development Administration of Korea, the cost of installing underground drainage pipes is approximately 6.5 million won per hectare, while the cost of installing drainage pipes using the traditional excavation method is as high as 13.7 million won [16]. Although the installation costs of drainage systems have been optimized to the greatest extent possible, there remains an urgent need to develop cost-effective and efficient drainage technologies [17].

In recent years, as the area of facility horticulture in Korea has expanded, the generation of horticultural waste has been steadily increasing [18,19,20]. Coir substrates have gained widespread use in controlled environment agriculture due to their excellent drainage properties and suitability for plant growth, making them highly favored by farmers [21]. However, the extensive use of coir substrates has also introduced challenges related to waste disposal. The accumulation of used coir substrates not only occupies space but also poses potential environmental pollution due to its plastic bag packaging [22]. Although the Korean government enforces strict policies on waste segregation and recycling, facility horticulture wastes such as rockwool, plant residues, and coir substrates have not yet been clearly classified within the legal framework for waste management [19]. Our field investigations revealed that some facility horticulture farmers distribute discarded coir substrates to field crop farmers for use as soil amendments. However, this practice may not comply with current waste management regulations. This observation highlights the urgent need for clear legal guidelines and the promotion of sustainable disposal strategies to address the challenges associated with managing discarded substrates more effectively. Coir substrates are primarily composed of crushed coconut particles and fibers and are considered a good organic growing medium [21,23]. Based on this characteristic, we have advocated for classifying discarded coir substrates as plant waste (51-17-29) in our discussions with the environmental department. This would allow them to be incorporated into a recycling system as a part of facility horticulture waste management regulations. However, due to the lack of systematic scientific research and supporting data, the advancement of such policies currently faces significant obstacles.

Therefore, exploring the recycling and reuse of discarded coir substrates not only contributes to responding to the 3R policy (reduce, reuse, and recycle) [24] but also promotes the sustainability of agricultural production by repurposing resources. Given the excellent drainage properties of coir substrates, we believe that their application in converted paddy fields for upland crop cultivation can effectively mitigate waterlogging issues. Furthermore, coir substrates can serve as an economical and environmentally friendly alternative to traditional subsurface drainage methods. To verify this hypothesis, we conducted a soybean planting experiment in 2022, achieving preliminary results and successfully applying for a related cultivation technique patent [3]. However, the previous research lacked scientific irrigation management practices. This study aims to utilize discarded coir substrates in combination with optimal irrigation levels for cultivating cabbages in paddy-converted fields. By thoroughly analyzing the crop’s growth, photosynthetic characteristics, and physiological traits, we seek to optimize this cultivation technique. This approach not only effectively mitigates waterlogging stress but also reduces farmers’ production costs, achieving a win–win scenario for both economic and ecological benefits.

2. Materials and Methods

2.1. Experimental Site

This experiment was conducted at Chungnam National University (36°22ʹ N and 127°21ʹ E) in Daejeon, Korea, between 22 April and 10 June 2023. The study site was characterized by a humid subtropical climate, with the following meteorological data: annual average temperature of 13.1 °C, relative humidity of 67.9%, wind speed of 1.7 m/s, and total precipitation of 1351.2 mm. Daily data on rainfall, temperature, wind speed, sunlight hours, and relative humidity were collected from the automated synoptic observing system (ASOS) of the Korea Meteorological Administration during the 50-day cabbage growth period. The ETo calculator (Version 3.2, September 2012, FAO) was employed to calculate the daily reference evapotranspiration (ETo). This software was based on the calculation procedures outlined in the FAO Irrigation and Drainage Paper No. 56 [25]. The weather data and daily ETo during the total cabbage growth stage are presented in Figure 1.

2.2. Application of Coir Substrates

To investigate the influence of using discarded coir substrates on drainage efficiency after converting paddy fields to upland, two cultivation methods (CMs) were compared: one with only paddy soil (PS) and the other with discarded coir substrates (CS) on ridges. The coir substrates used in this experiment were in slab form (100 × 20 × 10 cm, Daeyoung GS Co., Ltd., Daegu, Republic of Korea) and recycled after being used for one year in the hydroponic cultivation of Capsicum annuum L. Before the experiment, soil samples and discarded coir substrates were collected and analyzed for their physical and chemical properties. The topsoil (0–20 cm) of PS was identified as a sandy loam (66% sand, 28% silt, and 6% clay) according to USDA Taxonomy. Based on the soil color, profile characteristics, physical properties (

Table 1. Physical properties of paddy soil (PS) and coir substrates (CS) used in this study.

	PD (g/cm3)	BD (g/cm3)	WHC (%)	FC (%)	TP (%)
PS (0~20 cm)	2.13	1.21	44	39	43.1
CS	1.37	0.08	70	57	78.6

PD—bulk density; BD—bulk density; WHC—water holding capacity; FC—field capacity; TP—total porosity; PS—paddy soil; CS—coir substrates.

), and chemical properties (

Table 2. Chemical properties of paddy soil (PS) and coir substrates (CS) used in this study.

	pH (1:5)	EC (dS·m−1)	Total N (%)	OM (g·kg−1)	AP (mg·kg−1)	Exch. Cations (cmol+·kg−1)
						K+	Ca2+	Mg2+
PS	5.4	0.4	0.30	17	127	0.52	3.4	0.6
CS	5.5	4	1.89	831.9	6070.56	1.52	78.95	19.43

Total N—total nitrogen; OM—organic matter; AP—available P₂O₅; PS—paddy soil; CS—coir substrates.

) of the experimental site, it corresponds to the classification of Cambic Arenosol, as described in [26]. The CS consisted of a 50:50 mixture of chips and dust. The physical properties of PS and CS were evaluated using the methods described by Elliott et al. [27]. Particle density was determined using the pycnometer method by measuring the mass of oven-dried soil (W1) and the volume (V) of displaced water. A soil core with a volume of 100 cm³ was used for sampling. Each core was dried at 105 °C until reaching a constant weight (W2). The cores were then carefully filled with either PS or CS to preserve their original structure. The samples were saturated with distilled water, and the saturated weight was recorded as W3. Gravitational water was allowed to drain until no visible water remained, and the drained weight was recorded as W4. All physical characteristics were calculated using the formulas below, and the results are presented in Table 1 (n = 5).

$$\mathrm{Particle} \mathrm{density} (\mathrm{g}/{\mathrm{cm}}^3)={\displaystyle \frac{\mathrm{W}1}{\mathrm{V}}}$$

(1)

$$\mathrm{Bulk} \mathrm{density} (\mathrm{g}/{\mathrm{cm}}^3)={\displaystyle \frac{\mathrm{W}2}{100}}$$

(2)

$$\mathrm{Water} \mathrm{holding} \mathrm{capacity} (\%)={\displaystyle \frac{\mathrm{W}3-\mathrm{W}2}{100}}$$

(3)

$$\mathrm{Field} \mathrm{capacity} (\%)={\displaystyle \frac{\mathrm{W}4-\mathrm{W}2}{100}}$$

(4)

$$\mathrm{Total} \mathrm{porosity} (\%)=(1-{\displaystyle \frac{\mathrm{B}\mathrm{u}\mathrm{l}\mathrm{k} \mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}}{\mathrm{P}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e} \mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}}}) \times 100$$

(5)

The chemical properties analysis methods for PS and CS were based on the procedures outlined by Rebecca Burt [28] with some modifications, and the results are presented in Table 2 (n = 5). The pH and electrical conductivity (EC) of the PS and CS samples were determined using the 1:5 soil/water extraction method with a pH and EC meter (HI 5522; Hanna instruments, Seoul, Republic of Korea). The total nitrogen (N) content was determined using a Primacs analyzer (SNC-100, Skalar, Breda, The Netherlands) following the Dumas methodology, with detection by thermal conductivity (TCD). The organic matter (OM) content was determined using the chromic acid-sulfuric acid digestion method with a spectrophotometer (Thermo Genesys 20, Thermo Fisher Scientific, Waltham, MA, USA). Available phosphorus (P₂O₅) was extracted with 0.5 M HCl and measured using the molybdenum blue spectrophotometric method with a spectrophotometer (Thermo Genesys 20, Thermo Fisher Scientific, Waltham, MA, USA). Exchangeable potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) were extracted with ammonium chloride and determined by ICP-OES (iCAP^TM 7400, Thermo Fisher Scientific, Waltham, MA, USA).

The fertilization amounts were based on the fertilization guidelines provided by the Daejeon Agricultural Technology Center based on the soil analysis results. The initial fertilization included 129 kg∙ha⁻¹ of nitrogen (N), 395 kg∙ha⁻¹ of phosphorus (P₂O₅), and 61 kg∙ha⁻¹ of potassium (K₂O), which were applied as a base fertilizer during soil preparation. For topdressing, 239 kg∙ha⁻¹ of nitrogen and 41 kg∙ha⁻¹ of potassium were applied in three stages after cabbage transplanting: on days 10, 20, and 30. Topdressing involved foliar spraying with a water-based solution containing dissolved fertilizer. Pest and disease management for all treatments followed local cultivation standards.

2.3. Irrigation Application

Near the experimental site, a one-ton water tank was installed, and each plot was equipped with drip irrigation PVC pipes with a flow rate of 2.5 L·h⁻¹. Based on the crop evapotranspiration (ETc) values, four distinct irrigation levels (IL) were established: 140% ETc (ETc140), 100% ETc (ETc100), 60% ETc (ETc60), and a non-irrigated control (ETc0). The irrigation frequency was set at once every three days, with the irrigation volume determined by the cumulative ETc over the three-day period. Each irrigation treatment was replicated three times under both PS and CS conditions, resulting in a total of 24 experimental plots. Each plot had a width of 1 m, a length of 10 m, and a total area of 10 m². To prevent mutual interaction between plots and avoid the influence of soil moisture on the results, plastic boards were used for separation. The ’KoKoMa’ cabbage (Brassica oleracea L. var. Capitata) was cultivated using standard agricultural practices, with a plant density of 60 × 30 cm and a ridge height of 40 cm. Following ridge formation, we mulched the treatment plots with black plastic film to suppress weed growth. The entire coir substrate slabs were positioned bottom-side up on the ridges, with 10 × 10 cm openings cut into the black film to align with the original planting holes. This setup allowed plant roots to extend into the underlying soil to absorb moisture and nutrients. Meanwhile, new openings were cut to match the cabbage planting densities (Figure 2). We conducted a three-day continuous irrigation flush of the substrate bags to mitigate any potential negative effects from nutrient residues in the discarded coir substrates.

Using the calculated ETo and crop coefficient (Kc), the crop evapotranspiration (ETc) was determined by the following Formula (6):

ETc = ETo × Kc

(6)

Here, ETc refers to daily water requirements (mm·day⁻¹) and Kc to the crop coefficient. With plastic film mulching, the average Kc decreased by 10–30%. In this experiment, the Kc values for cabbage, based on Allen [25], were reduced by 20%. Thus, the adjusted Kc values were 0.56, 0.84, and 0.76 for the initial, mid, and late growth stages, with durations of 10, 20, and 20 days, respectively. The total irrigation amounts (ET) for ETc140, ETc100, ETc60, and ETc0 were 208.21 mm, 148.72 mm, 89.23 mm, and 0 mm, respectively (

Table 3. The total irrigation amounts (ET) for different irrigation levels.

Irrigation Level	ETc140	ETc100	ETc60	ETc0
ET (mm)	208.21	148.72	89.23	0

).

2.4. Mearsurement of Growth, Yield, and Water Use Efficiency

Five representative samples (n = 5) from each treatment were randomly selected at the cupping (30th day) and harvesting stages (50th day) of cabbages. Investigations were conducted on outer leaf number, outer leaf area, outer leaf fresh weight, and outer leaf dry weight during the cupping and harvesting stages. Additionally, measurements of diameter and fresh and dry weights of cabbage head parts were conducted at the harvesting stages. The dry weight was determined by drying the samples at 105 °C for one day, followed by heating to a constant weight at 85 °C. The weight was measured using an electronic scale (MW-2N, CAS Co., Ltd., Yangju, Republic of Korea). Leaf area was measured using a leaf area meter (LI-3100; LI-COR Co., Ltd., Lincoln, NE, USA). The yield for each treatment was calculated from the fresh weight of cabbage heads, adjusted by planting density, and then converted to yield per hectare.

The water use efficiency (WUE) was calculated using the following Formula (7):

$$\mathrm{WUE} (\mathrm{kg}·{\mathrm{m}}^{-3})={\displaystyle \frac{\mathrm{Y}}{\mathrm{E}\mathrm{T}\mathrm{}\times \mathrm{}10}}$$

(7)

where Y is the cabbage yield (kg·ha⁻¹), ET is the total irrigation amount (mm), and 10 is the conversion factor from millimeters to cubic meters per hectare (m³·ha⁻¹).

2.5. Mearsurement of Photosynthetic Characteristics and Pigments

The photosynthesis of cabbage mainly relies on the outer leaves, so the photosynthetic and pigment measurements were conducted using the outer leaves as a standard. On sunny days during the cupping and harvesting stages, three uniformly growing cabbage plants were selected for each treatment (n = 3). The third fully expanded outer leaf was chosen for photosynthetic measurements, which were conducted using a LI-6800 photosynthesis system (Licor. Inc., Nebraska, NE, USA). The operational parameters for the photosynthesis system were set as follows: temperature at 25 ± 3 °C, CO₂ concentration at 400 μmol·mol⁻¹, air-flow rate at 600 μmol·mol⁻¹, and photosynthetic photon flux density at 1000 μmol·m²·s⁻¹. The photosynthetic parameters measured included the net photosynthetic rate (P_n; µmol·CO₂·m⁻²·s⁻¹), intercellular CO₂ concentration (C_i; μmol·CO₂·mol⁻¹), stomatal conductance (g_s; mol·H₂O·m⁻²·s⁻¹), and transpiration rate (T_r; mmol·H₂O·m⁻²·s⁻¹).

The determination of chlorophyll in the outer leaves followed the method proposed by Lichtenthaler and Buschmann [29] with some modifications. Each treatment was conducted in triplicate (n = 3). The collected samples were rapidly frozen in liquid nitrogen, then freeze-dried using a freeze dryer (TFD5503, IlshinBioBase Co., Ltd., Dongducheon, Republic of Korea) and ground into powder. Approximately 20 mg of the powdered samples were taken, mixed with 2 mL of 90% methanol, and thoroughly blended. Subsequently, the mixture underwent ultrasonication for 20 min, followed by centrifugation at 2000 rpm for 10 min. The absorbances of chlorophyll a, chlorophyll b, and carotenoid were measured at wavelengths of 665.2 nm, 652.4 nm, and 470 nm. The chlorophyll a, chlorophyll b, and carotenoid contents, and the chlorophyll a/b ratio were calculated using the following formulas:

ca (μg·mL⁻¹) = 16.82A_665.2 − 9.28A_652.4

(8)

cb (μg·mL⁻¹) = 36.92A_652.4 − 16.54A_665.2

(9)

c_(x+c) (μg·mL⁻¹) = (1000A₄₇₀ − 1.91ca − 95.15cb)/225

(10)

chlorophyll a/b ratio = ca/cb

(11)

In the equations, ca represents chlorophyll a, cb represents chlorophyll b, and c_(x+c) represents carotenoids (x + c = xanthophylls and carotenes). The variable A signifies the absorbance at the respective wavelengths.

2.6. Evaluation of Proline Content

To assess cell osmotic pressure, the proline content of three samples from each treatment was measured and followed the method proposed by Ábrahám et al. [30] with some modifications (n = 3). Approximately 50 mg of freeze-dried samples were homogenized in 500 μL of 3% sulfosalicylic acid. The homogenate was centrifuged at 2000 rpm for 10 min. The supernatant was then mixed with an equal volume of glacial acetic acid and acid-ninhydrin reagent and incubated at 96 °C for 1 h. After cooling, the reaction mixture was extracted with toluene, and the absorbance was measured at 520 nm. The proline concentration was calculated using a standard curve prepared with proline concentrations of 0, 11.5, 23, 34.5, 46, and 57.5 μg/mL.

2.7. Evaluation of Malondialdehyde Content

To evaluate cell membrane damage and stress, the malondialdehyde (MDA) content of three samples from each treatment was determined following the method proposed by Jambunathan [31] with some modifications (n = 3). About 200 mg of freeze-dried samples were homogenized in 4 mL of 0.1% trichloroacetic acid (TCA). The homogenate was centrifuged at 10,000 rpm for 15 min. A total of 1 mL of the supernatant was mixed with 4 mL of 0.5% thiobarbituric acid in 20% TCA, and the mixture was heated at 95 °C for 30 min, then quickly cooled on ice. After centrifugation at 10,000 rpm for 10 min, the absorbance of the supernatant was measured at 532 nm and 600 nm. The MDA concentration was calculated using its extinction coefficient (155 mM⁻¹·cm⁻¹).

2.8. Glucosinolate Analysis

To evaluate the quality of cabbage under different treatments, we analyzed the glucosinolate content in the heads of the cabbage after harvest (n = 3). The glucosinolate (GL) content was analyzed using high-performance liquid chromatography (HPLC) (1260 Infinity Series, Agilent Technologies Inc., Santa Clara, CA, USA), following the modified method described by Yang [32]. Freeze-dried samples (100 mg) were extracted with 1.5 mL of methanol (70% v/v) and heated at 70 °C for 5 min. After centrifugation at 12,000 rpm for 10 min at 4 °C, the supernatant was collected. This extraction was repeated three times. The combined supernatant was applied to a DEAE-Sephadex A-25 column (Sigma-Aldrich Korea Co., Ltd., Seoul, Republic of Korea), which was then washed three times with 1 mL of distilled water. The bottom of the DEAE-Sephadex A-25 was sealed with parafilm, and 75 μL of 50 mM sulfatase solution was added. The top of the column was also sealed with parafilm, and the column was incubated at room temperature for 16 h. After 16 h, the parafilm was removed, and 0.5 mL of distilled water was passed through the column three times to collect the GL extract. The extract was then filtered through a 0.45 μm syringe filter and used for HPLC analysis.

The GL content was analyzed using an Inertsil ODS-3 column (150 × 3.0 mm, 3 μm) (GL Science Co., Ltd., Tokyo, Japan), and the mobile phases consisted of distilled water (A) and acetonitrile (B, 100% v/v). The gradient program was executed as follows: 0% solvent B at the start (0 min); 0% solvent B from 0 to 2 min; 10% solvent B from 2 to 7 min; 31% solvent B from 7 to 16 min; maintaining 31% solvent B between 16 and 19 min; returning to 0% solvent B from 19 to 21 min; and holding at 0% solvent B for the remainder of the run, from 21 to 27 min. After removing bubbles from the system using distilled water, the entire process was conducted at a flow rate of 0.4 mL/min with an injection volume of 10 μL, a detection wavelength of 227 nm, and a temperature maintained at 40 °C. The individual GLs (PRO: progoitrin; SIN: sinigrin; 4HGBS: 4-hydroxy glucobrassicin; 4MGBS: 4-methoxy glucobrassicin and NGBS: neoglucobrassicin) were quantified by comparing the HPLC peak areas to response factors, following ISO 9167-1 (1992) guidelines [33], with sinigrin used as an external standard.

2.9. Statistical Analysis

Data from each treatment in this experiment were analyzed using ANOVA in SPSS (Version 26.0.0, SPSS Inc., IL, USA), and the means were compared using Tukey’s multiple range test (p ≤ 0.05). Graphs were generated using OriginPro (Ver. 2021, OriginLab Corporation, Northampton, MA, USA).

3. Results

3.1. Temperature, Rainfall, and Reference Evapotranspiration (ETo)

Figure 1 shows the records of the changes in temperature, rainfall, and reference evapotranspiration (ETo) from cabbage transplanting to harvest. The accumulated total ETo for the entire growing period was 205.9 mm. Within the first 30 days from cabbage transplantation to cupping, the cumulative rainfall reached 132.6 mm. From cupping to harvest, the total cumulative rainfall was 70.9 mm. Throughout the growth period, the daily maximum temperature ranged from 11.5 to 31.5 °C, while the daily minimum temperature ranged from 2.5 to 18.9 °C.

3.2. Growth Parameters, Yield, and Water Use Efficiency

Figure 3 shows the effects of four irrigation levels (ETc140, ETc100, ETc60, and ETc0) and two different cultivation methods (paddy soil (PS) and coir substrates (CS)) on the outer leaf number, outer leaf area, outer leaf fresh weight (FW), and outer leaf dry weight (DW) of cabbage. At the cupping stage, no significant differences were observed in the outer leaf number, outer leaf FW, and outer leaf DW between PS and CS under the same irrigation level. In the PS treatment, the PS-ETc140 showed a significant increase of 22.08% in the outer leaf number and 59.3% in the outer leaf FW compared to the PS-ETc0. In the CS treatment, the CS-ETc140 showed significant increases of 41.4% in the outer leaf area, 47.2% in the outer leaf FW, and 43.5% in the outer leaf DW compared to the CS-ETc0. At the harvesting stage, there were no significant differences in the outer leaf number between PS and CS under the same irrigation level. However, CS showed a significantly higher outer leaf area, FW, and DW compared to PS. In the PS treatment, the PS-ETc60 showed significant increases of 42.7% in the outer leaf area and 26.1% in the outer leaf FW compared to the PS-ETc0. In the CS treatment, the outer leaf FW increased significantly by 23.8% and 26.8% in the CS-ETc100 and CS-ETc60, respectively, compared to the CS-ETc0.

Table 4. The yield components of cabbage (Brassica oleracea L. var. Capitata ‘KoKoMa’) at the harvesting stage under different irrigation levels in coir substrates (CS) and paddy soil (PS).

Treatments	HD (cm·Plant−1)	HFW (g·Plant−1)	HDW (g·Plant−1)	Yield (t·ha−1)	WUE (kg·m−3)
CM
PS	13.19 b	970.24 b	64.81 b	54.33 b	49.13 b
CS	15.91 a	1581.14 a	99.37 a	87.95 a	77.74 a
IL
ETc140	14.43 a	1238.65 a	83.46 a	69.37 a	38.07 b
ETc100	14.63 a	1301.97 a	82.16 a	72.91 a	56.02 b
ETc60	14.99 a	1341.98 a	85.94 a	75.15 a	96.23 a
ETc0	14.13 a	1190.17 a	77.29 a	66.65 a	-
CM × IL
PS-ETc140	13.32 bc	1014.31 cd	71.30 b	56.80 cd	31.17 e
CS-ETc140	15.54 ab	1462.99 b	95.62 a	81.93 b	44.96 c
PS-ETc100	12.80 c	895.35 d	59.56 b	50.14 d	38.52 d
CS-ETc100	16.46 a	1708.58 a	104.76 a	95.68 a	73.51 b
PS-ETc60	14.16 abc	1083.74 c	71.46 b	60.69 c	77.71 b
CS-ETc60	15.82 ab	1600.22 ab	100.42 a	89.61 ab	114.74 a
PS-ETc0	12.46 c	887.56 d	56.90 b	49.70 d	-
CS-ETc0	15.80 ab	1492.77 b	97.68 a	83.60 b	-
CM	***	***	***	***	***
IL	NS	NS	NS	NS	***
CM × IL	NS	***	***	***	***

HD—head diameter; HFW—head fresh weight; HDW—head dry weight; WUE—water use efficiency; CM—cultivation methods; IL—irrigation levels. All data are expressed as mean ± standard error (n = 5). Different letters indicated significant differences (p < 0.05) according to Tukey’s multiple range test and t-test. NS, *, **, and *** represent non-significant or significant at p < 0.05, 0.01, and 0.001, respectively.

compares the effects of different cultivation methods (CMs) and irrigation levels (IL) on the cabbage yield components. When comparing the CM, the CS treatment showed an increase of 20.8% in head diameter, 62.8% in head FW, 53.3% in head DW, and 60.4% in yield compared to the PS treatment. When comparing IL, these factors did not show any significant differences. Further analysis of the interaction of the cultivation methods and irrigation levels (CM × IL) showed that, at the same irrigation level, the CS treatment had a significantly higher head FW, head DW, and yield than the PS treatment. In the PS treatment, the PS-ETc60 showed the highest values for these yield components. Similarly, in the CS treatment, the CS-ETc100 showed the highest values for these yield components, but no significant differences were observed between the CS-ETc100 and CS-ETc60. In addition, the PS-ETc60 showed the highest WUE among the PS treatments, while the CS-ETc60 showed the highest WUE among the CS treatments.

3.3. Photosynthetic Characteristics and Pigment Contents

At the cupping stage, in the PS treatment, the PS-ETc100 showed a significant increase in P_n, g_s, and T_r, while its C_i was significantly lower than that of the other PS treatments (Figure 4). In the CS treatment, the CS-ETc140 tended to have higher P_n, g_s, and T_r. At harvest, at the same irrigation level, P_n, g_s, and T_r were significantly higher in the CS treatment compared to the PS treatment, while C_i was significantly lower. In the PS treatment, the PS-ETc60 tended to exhibit higher Pn, gs, and Tr, while the C_i was the lowest. In the CS treatment, the CS-ETc100 tended to exhibit higher Pn, gs, and Tr, while the C_i was the lowest.

At the cupping stage, chlorophyll a, b, and carotenoids tended to be higher in the CS treatments than in the PS treatments at the same irrigation level (Figure 5). In the PS treatment, chlorophyll a significantly increased by 23.3% in the PS-ETc140 and 22.7% in the PS-ETc100 compared to the PS-ETc0. Chlorophyll b significantly increased by 23.9% in the PS-ETc140 and 18.3% in the PS-ETc60 compared to the PS-ETc0. Total carotenoids significantly increased by 52% in the PS-ETc140, 40% in the PS-ETc100, and 28% in the PS-ETc60 compared to the PS-ETc0. There was no significant difference in the chlorophyll a/b ratio under the PS-ETc140, PS-ETc100, and PS-ETc60 compared to the PS-ETc0. In the CS treatment, chlorophyll a significantly increased by 35.2% in the CS-ETc140, 28.3% in the CS-ETc100, and 21.4% in the CS-ETc60 compared to the CS-ETc0. Chlorophyll b was significantly increased by 26.0%, 16.9%, and 27.3% in the CS-ETc140, CS-ETc100, and CS-ETc60, respectively, compared to the CS-ETc0. Carotenoids were significantly increased by 35.7% in the CS-ETc140, 39.3% in the CS-ETc100, and 28.6% in the CS-ETc60 compared to the CS-ETc0. There was no significant difference in the chlorophyll a/b ratio under the CS-ETc140, CS-ETc100, and CS-ETc60 compared to the CS-ETc0. At harvest, chlorophyll a tended to exhibit higher in the CS treatments compared to the PS treatments at the same irrigation level. In the PS treatment, chlorophyll a, b, and carotenoids did not show significant differences in the PS-ETc100 and PS-ETc60 compared to the PS-ETc0, while these parameters were significantly reduced in the PS-ETc140 compared to the PS-ETc0. The chlorophyll a/b ratio was significantly higher in the PS-ETc140 and PS-ETc100 compared to the PS-ETc0. In the CS treatment, chlorophyll a was significantly increased by 19.2% in the CS-ETc100 compared to the CS-ETc0. Chlorophyll b was significantly increased by 39.6%, 68.8%, and 97.9% in the CS-ETc140, CS-ETc100, and CS-ETc60, respectively, compared to the CS-ETc0. Carotenoids were significantly increased by 56.3% in the CS-ETc140 compared to CS-ETc0. The chlorophyll a/b ratio was significantly lower in the CS-ETc140, CS-ETc100, and CS-ETc60 compared to the CS-ETc0.

3.4. The Proline and MDA Contents

When comparing cultivation methods (CMs) at the cupping stage, there was no significant difference in the proline content between the CS and PS treatments (

Table 5. The proline and malondialdehyde (MDA) contents of cabbage (Brassica oleracea L. var. Capitata ‘KoKoMa’) at the harvesting stage under different irrigation levels in coir substrates (CS) and paddy soil (PS).

Treatments	Proline (mg·g−1 DW)		Malondialdehyde (μmol·g−1 DW)
	Cupping	Harvesting	Cupping	Harvesting
CM
PS	1.21 a	1.54 a	270.47 a	479.39 a
CS	0.94 a	0.82 b	240.23 b	460.34 a
IL
ETc140	1.13 b	1.27 a	258.48 a	505.49 a
ETc100	0.97 b	0.91 a	259.10 a	447.42 b
ETc60	0.68 c	0.89 a	243.93 a	428.58 b
ETc0	1.54 a	1.65 a	259.89 a	497.98 a
CM × IL
PS-ETc140	1.20 b	1.47 b	284.00 a	532.91 a
CS-ETc140	1.05 bc	1.06 bc	232.97 c	478.08 bc
PS-ETc100	1.13 bc	1.11 bc	278.21 a	478.35 bc
CS-ETc100	0.81 d	0.71 c	239.99 bc	416.49 d
PS-ETc60	0.89 cd	1.11 bc	248.51 abc	411.18 d
CS-ETc60	0.46 e	0.66 c	239.36 bc	445.97 cd
PS-ETc0	1.62 a	2.45 a	271.17 ab	495.13 b
CS-ETc0	1.45 a	0.84 bc	248.61 abc	500.82 ab
CM	***	***	***	**
IL	***	***	NS	***
CM × IL	*	**	NS	***

CMs—cultivation methods; IL—irrigation levels. All data are expressed as mean ± standard error (n = 3). Different letters indicated significant differences (p < 0.05) according to Tukey’s multiple range test and t-test. NS, *, **, and *** represent non-significant or significant at p < 0.05, 0.01, and 0.001, respectively.

). However, at harvest, the proline content was significantly reduced by 25% in the CS treatment compared to PS. When comparing the different irrigation levels (IL), the proline content was significantly reduced by 26.7%, 33.3%, and 53.5% during the cupping stage in the ETc140, ETc100, and ETc60, respectively, compared to the ETc0. At harvest, the proline content showed no significant difference. Further analysis of the interaction between cultivation methods and irrigation levels (CM × IL) revealed that, at the same irrigation level, the proline content in the CS treatment tended to exhibit lower than in the PS treatment during both the cupping and harvesting stages. When comparing CMs, the MDA content of CS showed a significant reduction of 11.19% at the cupping stage, while there was no significant difference between the PS and CS treatments at harvest. When comparing IL, the MDA content did not vary significantly across different irrigation levels during the cupping stage. However, at harvest, the MDA content was significantly reduced by 10.16% and 13.93% in ETc100 and ETc60, respectively, compared to ETc0. Further analysis of the interaction between CM × IL revealed that, at the same irrigation level, the MDA content in the CS treatment tended to exhibit lower than in the PS treatment during the cupping stage.

3.5. Glucosinolate Contents

As shown in

Table 6. The glucosinolate contents of cabbage (Brassica oleracea L. var. Capitata ‘KoKoMa’) head at the harvesting stage under different irrigation levels in coir substrates (CS) and paddy soil (PS).

Treatments	Glucosinolate Concentration (μmol·g−1 DW)
	PRO	SIN	4HGBS	4MGBS	NGBS	Total GLs
CM
PS	0.28 a	4.02 a	0.04 a	1.29 a	0.01 a	5.63 a
CS	0.27 a	3.62 a	0.03 a	1.84 a	0.01 a	5.77 a
IL
ETc140	0.36 a	4.57 a	0.04 a	2.27 a	n.d.	7.23 a
ETc100	0.34 a	4.59 a	0.05 a	1.65 a	0.02 a	6.64 a
ETc60	0.28 a	4.17 a	0.03 b	1.75 a	n.d.	6.22 a
ETc0	0.13 b	1.94 b	0.02 c	0.61 b	0.02 a	2.70 b
CM × IL
PS-ETc140	0.37 a	4.55 ab	0.05 a	1.30 de	n.d.	6.26 bc
CS-ETc140	0.35 a	4.59 ab	0.03 bcd	3.23 a	n.d.	8.20 a
PS-ETc100	0.27 ab	4.09 bc	0.04 abc	1.47 cd	0.03	5.92 c
CS-ETc100	0.40 a	5.08 a	0.05 ab	1.83 bc	n.d.	7.36 ab
PS-ETc60	0.30 ab	4.52 ab	0.03 cd	1.49 cd	n.d.	6.34 bc
CS-ETc60	0.26 ab	3.82 c	0.03 cd	2.00 b	n.d.	6.10 c
PS-ETc0	0.17 bc	2.90 d	0.02 de	0.91 e	n.d.	4.00 d
CS-ETc0	0.08 c	0.97 e	0.01 e	0.31 f	0.03	1.40 e
CM	NS	**	**	***	NS	NS
IL	***	***	***	***	NS	***
CM × IL	*	***	*	***	NS	***

CMs—cultivation methods; IL—irrigation levels; PRO—Progoitrin; SIN—sinigrin; 4HGBS—4-Hydroxyglucobrassicin; 4MGBS—4-Methoxyglucobrassicin; NGBS—neoglucobrassicin; n.d.—The concentration of compound was not detected in samples. All data are expressed as mean ± standard error (n = 3). Different letters indicated significant differences (p < 0.05) according to Tukey’s multiple range test and t-test. NS, *, **, and *** represent non-significant or significant at p < 0.05, 0.01, and 0.001, respectively.

, several major glucosinolate compounds were quantified, including Progoitrin (PRO), sinigrin (SIN), 4-Hydroxyglucobrassicin (4HGBS), 4-Methoxyglucobrassicin (4MGBS), neoglucobrassicin (NGBS) and total glucosinolates (GLs). When comparing cultivation methods (CMs), the results indicate that there are no significant differences in the total GL or individual GL concentrations between the PS and CS. Across different irrigation levels (IL), compared to the ETc0, the PRO, SIN, 4HGBS, 4MGBS, and total GL concentrations were significantly higher at ETc140, ETc100, and ETc60. The interaction analysis (CM × IL) reveals that in the PS treatment, the contents of PRO, SIN, and 4HGBS are highest in the PS-ETc140, while NGBS is only detected in trace amounts in the PS-ETc100. In the CS treatment, the contents of PRO, SIN, and 4HGBS are highest in the CS-ETc100 treatment, while NGBS is not detected in any of the CS treatments.

4. Discussion

4.1. Effects of Cultivation Methods and Irrigation Levels on Growth Parameters, Yield, and Water Use Efficiency

In paddy-converted farmlands, crop growth depends heavily on effective soil drainage. Therefore, this study aimed to explore the potential of discarded coir substrates as a low-cost alternative to subsurface drainage systems. Poor drainage typically reduces the oxygen diffusion rate (ODR) in the soil, causing root systems to experience hypoxia. This condition disrupts metabolic processes, leading to the accumulation of toxic substances such as ethanol and lactic acid in the rhizosphere [34]. In agricultural production, mechanical tillage combined with fertilization is commonly used to improve aeration in the root zone and mitigate the negative effects of poor drainage on crops [35]. In this study, the discarded coir substrates had a chip-to-dust ratio of 50:50 and exhibited a total porosity of up to 78.6% (Table 1). The fiber component enhances the gas diffusion capacity of the substrate, while the dust component improves its water retention ability. This balance of air and water regulation ensures that the root zone maintains optimal oxygen supply and moisture conditions, providing an ideal environment for healthy root growth [36]. Studies have shown that cabbage seedlings are particularly sensitive to water stress, and poor drainage can significantly limit their growth and development [37]. In this experiment, severe water accumulation was observed in the furrows of the experimental field after heavy rainfall, making drainage difficult (Figure 6a). During the cupping stage of cabbage, wilting symptoms were observed in the aboveground parts of the PS treatment. After digging up the roots, signs of significant rot were observed in the paddy soil (PS) treatment, but no similar signs were found in the discarded coir substrates (CS) treatment (Figure 6b). In the CS treatment, root growth was healthy, with a greater number of roots concentrated in the coir substrate (Figure 6c). The coir fiber substrate provides abundant nutrients and water for the roots. Its high porosity promotes root expansion in the substrate, thereby supporting healthy root development and increasing root density [38]. Our results also showed that compared to PS, cabbage growth under CS treatments exhibited better outer leaf and head growth, higher dry matter accumulation, and a 61.5% increase in yield (Figure 3 and Table 4). Similarly, a related study has shown that waterlogging stress significantly reduces cabbage leaf area, shoot fresh weight, root fresh weight, and dry matter accumulation [39]. Fei-Baffoe et al. [40] found that using sewage sludge to improve soil nutrients and aeration resulted in an 88% increase in cabbage yield. These results further confirmed the superior drainage performance of the coir substrates. In our study, compared to PS, CS not only exhibited superior nutritional content, such as higher total nitrogen and organic matter levels but also demonstrated a higher EC and a rich cation exchange capacity, indicating a significant advantage in providing essential nutrients for plant growth (Table 2). Research has indicated that potatoes can die within a few days under hypoxia conditions, while winter wheat, although able to survive in low-oxygen environments, experiences a significant reduction in growth rate [41]. Excessive soil moisture often leads to nitrate leaching and intensified denitrification processes by soil microorganisms, resulting in a decrease in soil nitrogen content [42]. Therefore, effective drainage not only promoted plant growth but also encouraged plants to absorb and store nutrients more efficiently [43].

To respond to the multi-purpose utilization of paddy fields and to explore the cultivation potential of crops with low self-sufficiency, we conducted a second crop cycle experiment with soybeans from 15 June to 30 October 2023 (Supplementary Material). The climate conditions and reference evapotranspiration (ETo) during the soybean growing season are shown in Figure S1. During the experiment, concentrated rainfall from late June to early August highlighted the drainage issues in the farmland. The results showed that CS significantly impacted soybean growth compared to PS. Specifically, CS significantly improved the leave number, leaf area, flower number, as well as fresh and dry weight of the shoot growth during the soybean R2 growth stage (Table S2). Studies have shown that waterlogging during the soybean flowering stage negatively affects pod formation and grain development, leading to yield reductions [44]. In this experiment, the use of discarded coir substrates significantly enhanced the soybean flower number, pod number, and yield (Figure S2, Tables S2 and S3). These results suggest that the excellent drainage properties of discarded coir substrate played an important role during the crop’s growing period.

In central Korea, cabbage is typically grown in two seasons each year, in spring and autumn, with most of the rainfall occurring in summer [45]. To prevent issues such as premature bolting caused by insufficient water, proper irrigation management is crucial, especially in the context of increasing climate change and water resource scarcity [46,47]. Combining appropriate irrigation strategies is essential to fully leverage the advantages of coir substrates and optimize cabbage growth outcomes. Nowadays, about 75% of agricultural land worldwide relies on rain-fed irrigation, while other methods like sprinkler, furrow, basin, and drip irrigation are used [48,49]. Although these traditional methods are widely used, they often lack the precision needed for efficient water use, especially under water scarcity conditions [50]. According to the Food and Agriculture Organization (FAO), the water requirement for cabbage throughout its growing period generally ranges from 380 mm to 500 mm [48]. Additionally, Shinde’s [47] research showed that with a microsprinkler irrigation system, the best cabbage yield occurred at 1 ETc (100% ETc), with a total irrigation volume of 428 mm. For drip irrigation, the optimal treatment also used 1 ETc (100% ETc), and the total irrigation volume was 290 mm. However, in our experiment, the irrigation amount for the CS-ETc60 was only 89.23 mm (Table 3), significantly lower than the traditional requirements. According to Sandhu’s survey [51], the seasonal ETc values of soybeans typically range from 450 mm to 700 mm in different climatic conditions. In contrast, the irrigation amount for the CS-ETc60 in our study resulted in a total irrigation volume of 299.19 mm throughout the entire growing season (Table S1). Moreover, due to the shallow root system of cabbage, over-irrigation can cause nutrient leaching from the topsoil layer, leading to nutrient deficiencies and water wastage [52]. In this experiment, a low irrigation level (ETc60) not only effectively maintains a water supply but also enhances the utilization of residual nutrients in coir substrates. Turner and Kramer [53] found that a moderate water deficit might enhance growth and yield due to the physiological adjustments that crops make under limited water conditions. Under water deficit irrigation conditions, maintaining irrigation levels between 60% and 100% of ETc generally did not significantly impact plant growth. At the same time, it reduces water use and improves WUE [54]. Research has shown that as irrigation intensity increases, the overall porosity of the topsoil gradually decreases, particularly macropores and elongated pores. This reduction can adversely affect the drainage and aeration properties of the soil [55]. This experiment was conducted in a paddy-converted field. To control weeds, the entire area was covered with plastic film, which effectively reduced soil water evaporation [56]. At the same time, we also adjusted the Kc value to achieve water savings. Moreover, the coir substrate not only has excellent aeration properties, but its superior water retention further enhanced moisture retention and promoted root development and water use efficiency [57,58].

4.2. Effects of Cultivation Method and Irrigation Levels on Photosynthetic Characteristics and Pigment Contents

Chlorophyll contents and photosynthetic characteristics are reflective of plant health [59]. Better root-zone aeration significantly improves plant growth, photosynthetic efficiency, and chlorophyll content [60]. The results of the photosynthesis analysis showed that at the same irrigation levels, P_n, g_s, and T_r in the Cs treatment were significantly higher than those in the PS treatment, both during the cupping and harvesting stages. In contrast, C_i was significantly lower in the CS treatment (Figure 4). This suggested that cabbage grown in the coir substrates could more efficiently utilize CO₂ during photosynthesis, demonstrating better stomatal regulation. Sarkar et al. [61] found that coir substrates significantly enhanced the growth of Red Leaf Lettuce (Lactuca sativa L.), as well as significantly improved photosynthetic efficiency and te pigment content. The results of this experiment indicated that the chlorophyll content in the CS treatment was significantly higher than that in the PS treatment, both at the cupping and harvesting stages (Figure 5). Chlorophyll a is a key pigment in photosynthesis, capable of absorbing light energy and converting it into chemical energy to support plant growth and development [62]. Thus, the coir substrates were more conducive to increasing the photosynthetic capacity and growth rate of cabbage due to a more suitable moisture content and better aeration.

The differences in performance between treatments at different irrigation levels highlighted the importance of appropriate irrigation levels. In the CS treatment, at the cupping stage, the CS-ETc140 treatment provided sufficient moisture to significantly improve the photosynthetic parameters. However, at harvest, the photosynthetic parameters of the CS-ETc100 performed even better (Figure 4). As cabbage required a considerable amount of water during the cupping stage, the CS-ETc140 met this demand and promoted photosynthesis. However, excessive irrigation can reduce root aeration, which subsequently affects water and nutrient uptake and increases physiological stress [63]. In contrast, the CS-ETc100 provided an optimal amount of water during the harvest period, avoiding stress responses and maintaining high photosynthetic efficiency. Although the PS-ETc60 showed higher values of P_n, g_s, and T_r under the PS treatment, its overall efficiency did not reach that of the CS treatments (Figure 4). This indicated that although paddy soil can support a certain level of cabbage photosynthesis, its performance was limited compared to that of the CS treatments.

4.3. Effects of Cultivation Methods and Irrigation Levels on Proline and MDA Contents

Proline is a key osmotic regulator in plants facing stress conditions. Its accumulation often indicates the plant’s response to environmental stress [64]. Malondialdehyde (MDA) is a lipid peroxidation product generated under oxidative stress in plants, and its concentration serves as an indicator of oxidative damage [65]. Studies suggested that the excellent water retention and aeration properties of coir substrates may help alleviate the stress caused by excess moisture or hypoxia, thereby reducing proline accumulation [66]. In this study, the proline content in the CS treatment was lower than that in the PS treatment during the cupping stage, although the difference was not significant. However, the proline content in the CS treatment was significantly lower than in the PS treatment at the harvesting stage (Table 5). This suggested that the superior aeration and water retention of coir substrates helped plants avoid severe water stress during the cupping stage, resulting in lower proline accumulation [67,68]. In contrast, the poorer aeration of paddy soil might result in some stress, leading to a slight increase in proline levels. At the harvesting stage, the poor aeration of paddy soil might lead to increased stress accumulation, increasing the proline levels. For the MDA content in cabbage leaves, the CS treatment had significantly lower levels compared to the PS treatment during the cupping stage. Although the difference was not significant at harvest, the MDA content in the CS treatment remained lower than in the PS treatment (Table 5). Yue’s [69] study showed that using coconut coir powder mixed substrates to cultivate Dalbergia odorifera T. Chen seedlings can effectively alleviate oxidative stress and maintain cell membrane integrity during plant growth. Additionally, the analysis of the MDA content in the paddy soil treatment indicated a significant increase in the MDA levels of the PS-ETc140 treatment at the harvesting stage, suggesting more severe oxidative damage to plant cell membranes (Table 5). This further confirmed the negative impact of poor soil aeration in paddy-converted fields on cabbage growth, while highlighting the advantages of reusing coir substrates for improved plant development.

4.4. Effects of Cultivation Methods and Irrigation Levels on Glucosinolate Contents

Glucosinolates (GLs) are a class of biologically active and nutritionally valuable secondary metabolites that are widely found in cruciferous plants such as cabbage [70,71]. GLs not only provide defensive benefits to plants but also have potential health benefits for humans, including antioxidant, anticancer, and anti-inflammatory effects [72]. In terms of GL content, the irrigation levels significantly affect both the total GLs and individual GL compounds, which is consistent with the findings of Radovich et al. [73] (Table 5). Meanwhile, in all treatments, SIN and 4MGBS were the main individual GLs. Additionally, under the ETc140 and ETc100 irrigation treatments, the total GL content in the CS was higher than that in the PS. This may be related to the abundant residual nutrients in the discarded coir substrates. GLs are derived from amino acids and are rich in sulfur, so the uptake of nitrogen and sulfur is critical for their synthesis [74].

5. Conclusions

This study demonstrates that combining discarded coir substrates with a 60% ETc irrigation amount significantly improves cabbage growth performance, yield, and water use efficiency (WUE). Additionally, the second cycle experiment confirmed the broad applicability of using discarded coir substrates in paddy-converted fields. The use of discarded coconut coir not only provides an economically efficient drainage solution for paddy-converted fields but also addresses the issue of waste disposal in facility horticulture. These findings offered scientific evidence for improving agricultural water resource use efficiency and provided new insights for sustainable agricultural development. Especially in the context of climate change and increasing water scarcity, the reuse of discarded coir substrates holds significant practical value. We recommend further promoting the reuse of coir substrates in future agricultural practices, combined with precise irrigation management, to optimize crop growth and achieve effective water resource utilization.

6. Patents

Method for cultivating upland crops in paddy soil using waste organic substrates. Patent No. 102649085. Granted to Jongseok Park, Jiwoo Park, and Xin Wang, the Republic of Korea, Filing Date: 30 November 2022, Grant Date: 14 March 2024.