Remediating Severely Salt-Affected Soil with Vermicompost and Organic Amendments for Cultivating Salt-Tolerant Crops as a Functional Food Source

Abstract

Salt-affected soils are a prevalent issue globally, resulting in a severe degradation of soil sustainability and plant productivity, reducing the area of agricultural land, and affecting food security. Therefore, eco-solutions and remediation approaches are needed. The needed remediation for salt-affected soil can be addressed via engineering, physical, chemical, or biological techniques. Salt-tolerant crops are normally used for the remediation of slight and moderate saline soil conditions. However, no crops, including salt-tolerant crops, can be cultivated in areas with extreme salinity levels (ECe 8–16 dS/m). Therefore, the aim of this study was to investigate the effect of vermicompost and organic amendment on the cultivation of salt-tolerant crops (Sesbania rostrata) in severely salt-affected soil under field conditions in order to improve saline soil and crop productivity. The design of the experiment followed a randomized complete block design (RCBD) with three treatments and four replications: T1, severely salt-affected soil (control); T2, severely salt-affected soil + vermicompost at a ratio of 25:75; T3, severely salt-affected soil + vermicompost + rice husk biochar + coconut coir at a ratio of 25:25:25:25. The results found that using vermicompost and organic amendment improved the soil quality, increased the soil fertility (organic matter and plant nutrients N, P, and K), and reduced the soil salinity. Sesbania rostrate could not grow in severely salt-affected soil (T1) alone, but could grow in the treatments with vermicompost and organic amendments (T2 and T3). The percentage of Sesbania survival per plot was also high in the treatments with vermicompost and organic amendments (T2 and T3). The highest growth rate, flower production, biomass, and root morphology of Sesbania rostrata were found in T3, with severely salt-affected soil + vermicompost + rice husk biochar + coconut coir at a ratio of 25:25:25:25 with a statistically significant difference (p < 0.05). Moreover, the Sesbania flowers treated with vermicompost and organic amendments have a higher nutritional value due to their minerals and vitamins than Sesbania flowers grown without using vermicompost and organic amendments. This study’s findings suggest that incorporating vermicompost and organic amendments is a feasible and economical method for enhancing the quality of salt-affected soils in a sustainable manner. The results of this study demonstrate that utilizing vermicompost and organic amendments is a sustainable and economical strategy for enhancing the quality of salt-affected soils and improving yields in severely salt-affected areas, thereby increasing crop production and the nutritional value of the plants as well as helping to increase farmers’ income.

1. Introduction

Salt-affected soils pose a worldwide problem by significantly decreasing soil sustainability and crop productivity, as well as by shrinking the available cultivated land and impacting food security. Soil salinization causes a reduction of approximately 1.5 million hectares in farmland for agricultural production per year. It is estimated that 8.7% of the planet is covered by salt-affected soil, and it is predicted that, in 2050, 50% of all cultivable land will be impacted by salinity [1]. This issue affects an area of 17.81 million hectares in the northeast region of Thailand [2]. The cause of saline soil in northeast Thailand is the salt-bearing rock underground of the region, which is primarily composed of halite from the Mahasarakham Formation [3]. Many areas in northeast Thailand have been experiencing significant constraints on agricultural productivity as a result of salt-affected soils. The expansion of salt-affected soil areas and their impacts on land and water resources are major concerns for farmers residing in affected regions [4].

In northeast Thailand, there are around 9.3 million hectares of farmland, with about 7.9 million hectares dedicated to rainfed agriculture [5]. Around 75% of the area is used for growing rice, with fluctuations in the planted area due to inconsistent water supply and salinity issues [6]. Additionally, the soil types in northeast Thailand are recognized for their poor fertility levels due to their light texture and low inherent nutrient levels [7]. The low fertility levels of sandy soils in the region, along with frequent droughts, soil degradation, and soil salinity, have been linked to this decreased harvest output [8]. The problem of soil salinity is a major challenge faced by farmers in northeast Thailand. The resulting social tension, unemployment, and income decrease experienced by all social classes can be attributed to the diminishing soil productivity caused by salinization. The presence of excessive soil salinity is causing distress to a considerable number of families in the region who have limited land holdings. The combination of agricultural constraints and low productivity has led to poverty, resulting in the region having the lowest per capita income in the country.

Saline soil is characterized by an abundance of salt, particularly sodium chloride (NaCl), in both the soil and root zones. The excessive presence of NaCl results in both ionic (chemical) and osmotic stress, ultimately restricting the growth and productivity of crops. Plant growth is significantly affected by the salinity in the soils and by irrigation, posing a challenge to production [9,10]. Osmotic stress from salt restricts water absorption from soils in plants, leading to high concentrations of Na+ and Cl− ions in plant cells. This ion stress disrupts the tissue balance and hinders nutrient uptake [9]. Drought stress is then induced by osmotic stress, reducing the plants’ water uptake capability. Plants experiences a reduction in chlorophyll content, number of tillers, stover yield, and grain yield for rice under drought stress [11].

The utilization and rehabilitation of salt-affected soils is part of a national policy to ensure food security. Salt-affected soils are classified based on their degree of salinity as slightly, moderately, severely, or very severely affected soils, or as areas with potential salt sources [2]. Limitations on plant growth and crop production are greatly impacted by salinity stress [9,10]. Remediation and sustainable management of salt-affected soils is needed. This remediation can be managed through agronomic management or via engineering approaches, such as the application of organic amendments, drainage and leaching, deep plowing, surface mulching, and the selection of salt-tolerant plant varieties, which would vary according to the amount of salt present and the particular salinization techniques used [2,3,6]. Moreover, a community participatory network is essential for sustainable land management.

However, the management of severely salt-affected soils is difficult and complicated. Severely salt-affected soils are considered waste land. Rehabilitating severely salt-affected soils requires a high investment. Effective strategies to address severely salt-affected soil that take into account ecological and environmental improvements are very important. Previous researchers have developed solutions for the salinity of the soil. They conducted research and applied many methods to rehabilitate salt-affected areas by focusing on reducing salinity and increasing soil fertility, such as utilizing salt-tolerant plants, modifying the microclimate, and using chemical agents and organic matter [2,4,6].

Among the many methods and techniques used in salt-affected soil remediation, the use of organic amendments like animal manure, compost, and vermicompost is becoming more prevalent because these materials are easy to obtain, low in cost, and safe for the environment [4,12]. The attention given to the positive impacts of organic additives for enhancing the characteristics of deteriorated saline soils and promoting plant growth has risen significantly [13,14]. Research has shown that vermicompost, an organic substance made by earthworms decomposing organic matter, can enhance soil quality, increase nutrient availability, and improve water retention. Thus, the implementation of alternative salt-affected soil management practices, such as green manure application, incorporating vermicompost and organic amendments into soils, presents a promising opportunity for enhancing plant growth and crop production [4,12,15,16]. However, most of the previous studies have been conducted in green house conditions with low or moderate salt-affected soil. Moreover, salt-tolerant crops are normally used for the remediation of slight and moderate saline soil conditions. The stem-nodulating legume Sesbania rostrata is recommended for cultivation in salt-affected areas due to its abilities to tolerate salt levels between 4 and 12 dS/m, withstand flooding, and fix nitrogen efficiently [2]. However, salt-tolerant crops like Sesbania rostrata and other similar crops cannot be cultivated in areas with extreme salinity levels (>12 dS/m).

Thailand is an agricultural country. Large amounts of crop residues or agricultural waste are generated annually as by-products from agricultural plantation and the agroindustry. Rice husk biochar is a common agricultural waste product in Thailand that comes from the rice milling process, which produces around 7.5 million tons per year alongside the production of around 277,901 tons of coconut coir per year [5]. The concept of the circular economy is an economic framework that aims to minimize waste generation and enhance resource utilization. This tactic is in accordance with the Thai government’s flagship project “Bioeconomy, Circular economy and Green economy (BCG)”, which is integral to the country’s social and economic development agendas. Circular economy strategies, such as waste minimization and renewable practices, have been successfully integrated into the agricultural sector. Several studies have been carried out carried out using certain organic residues for saline soil remediation, but there is a lack of studies on the impact of the integration of vermicompost and organic residues—such as rice husk biochar and coconut coir—on the enhancement of soil fertility and the promotion of plant production for functional food in severely salt-affected soil, especially at field sites.

Therefore, the objective of this research was to assess the remediation of heavily salt-affected soil through the application of vermicompost and organic amendments, and to investigate the potential effects of vermicompost, rice husk biochar, and coconut coir—whether used alone or in combination—on soil physicochemical properties, and on the plant growth, biomass, physiological attributes, and nutritional benefits of the flower Sesbania rostrata as a source of functional food when under severe salt stress field saline conditions.

2. Materials and Methods

2.1. Study Area

The study area location was a research field (16°01′58.0″ N 102°41′26.0″ E) at the Ban Hua-Nong village, in the Ban-Phai district of the Khon Kaen province, Thailand, where the soil and the shallow groundwater are salinized (Figure 1). The Khon Kaen province is located in Northeast Thailand in the low-lying Korat Plain, where approximately 336,000 hectares of soil are salinized. The climate is a tropical monsoon climate, the rainy season lasts May–October, and the dry season November–April. The mean annual temperature is 28 °C, andannual rainfall is 1100–1500 mm. Potential evapotranspiration is about 2000 mm.

The study site, the Ban Phai district, is one of the areas in Khon Kaen with many farmlands where salinization has not yet been resolved. Electrical conductivity (EC) and salt distribution in salt-affected areas were surveyed with an EM 38 machine to collect data and make decisions on the experimental design (Figure 2). This study was conducted on areas of very severely salt-affected soils (class 1) and severely salt-affected soils (class 2). These salt-affected soil classes are different from soil salinity classes. The salt-affected soil was classified based on the percentage of salt crust (Figure 2). The soil salinity level was classified via ECe, as follows: non saline: 0–2 dS/m; slightly saline: ≥2–4 dS/m; moderately saline: >4–8 dS/m; highly saline: >8–16 dS/m; severely saline: >16 dS/m. The soil salinity in this study was approximately moderate-to-high salinity.

2.2. Preparation of Soil, Vermicompost, and Organic Amendments

The effects of vermicompost and organic amendment (rice husk biochar + coconut coir) on the soil properties of saline soil and the growth of Sesbania were investigated through an experiment conducted in the field research area at Hua-Nong village, Ban-Phai district, Khon Kaen province, Thailand. Before the experiment began, information on the initial physical and chemical properties of the soils was gathered. After air-drying, the soil was sieved using a 2 mm mesh to remove any stones and debris, making it suitable for soil analysis.

Vermicompost was prepared at The VermiTech@KKU (Research Developing and Learning Center on Earthworm for Agriculture and Environment), Department of Soil Science and Environment, Faculty of Agriculture, Khon Kaen University. Bedding materials were prepared using a blend of soil, cow dung, rice husk ash, and vegetable waste in a ratio of 4:3:2:1 based on dry weight. This study utilized the epigeic earthworm Eudrilus eugeniae (density of about 20 pcs/kg) for the vermicomposting procedure due to its resilience to a broad range of environmental conditions, such as pH, temperature, and moisture content. Soil, cow dung, rice husk ash, and vegetable waste were mixed together to create the bedding material, which was pre-composted for two weeks. To prevent earthworms from being exposed to high temperatures during the initial thermophilic phase, they were added to the mixture on the 14th day of decomposition. The initial chemical characteristics of the raw materials used are shown in

Table 1. The basic chemical properties of bedding material vermicompost and organic amendments used in this study (N = 4).

Parameter	Bedding Materials	Vermicompost	Coconut Coir	Rice Husk Biochar
pH (1:10)	7.99 ± 0.04	7.61 ± 0.01	5.50 ± 0.22	9.80 ± 0.67
EC (dS/m)	1.268 ± 0.01	1.68 ± 0.01	3.70 ± 0.08	1.64 ± 0.09
Organic carbon (%)	13.01 ± 0.01	16.37 ± 0.09	75.5 ± 2.65	46.40 ± 1.12
Total nitrogen (%)	1.38 ± 0.12	2.40 ± 0.01	0.24 ± 0.06	0.54 ± 0.11
Total phosphorus (%)	1.42 ± 0.02	4.70 ± 0.00	0.09 ± 0.02	0.20 ± 0.02
Total potassium (g/kg)	2.85 ± 0.01	4.10 ± 0.01	1.70 ± 0.03	0.73 ± 0.03
C/N ratio	29.65 ± 0.59	12.32 ± 0.49	77.00 ± 1.83	53.50 ± 1.29

. The rice husk biochar and coconut coir used in this study were locally available in the Khon Kaen area. Rice husk biochar, obtained from paddy husks, was created by its undergoing slow pyrolysis at a controlled temperature range of 400–500 degrees Celsius.

2.3. Vermicompost Sampling and Analysis

Following a 30-day vermicomposting period, the vermibeds were mixed thoroughly, and roughly 400 g of samples was gathered from each bed. These samples were dried at 65 °C in an oven and stored in air-tight polythene bags for the evaluation of their pH, electrical conductivity (EC), organic carbon (OC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), and C/N ratios. Physicochemical analyses of the vermicompost sample were performed in a laboratory. The electrical conductivity (EC) and pH levels of the samples were determined in a water suspension (1:10 dry sample: water ratio) with the use of a conductivity meter and a Mettler Toledo Desktop pH meter. Following agitation with a mechanical shaker at 230 rpm for 30 min, the suspension was left to stand for an hour before conducting EC and pH measurements. The determination of organic carbon (OC) was carried out. Total nitrogen (total Kjeldahl nitrogen/total N) was determined using a modified, semi-micro Kjeldahl method following the protocol outlined by [18]. Moreover, the C/N ratio was determined by calculating the ratio of carbon to nitrogen percentages. The total phosphorus, potassium, magnesium, and calcium were determined after digesting 0.2 g of dry sample in 70% sulfuric acid and 65% hydrogen peroxide (AOAC, 2000). Total phosphorus (TP) was measured via the molybdo–vanado–phosphate method with a UV spectrophotometer (UV–VIS Spectrophotometer: LABOMED, Spectro UV-2550, LABOMED Inc., Los Angeles, CA, USA) at 420 nm. Total potassium (TK) was determined using a flame photometer (Flame photometer PFP7, Jenway, Cole-Parmer, Stone, Staffordshire, UK).

2.4. Experimental Design

This study was conducted at the aforementioned field study sites (A: experimental area for severely salt-affected soil and B: experimental area for very severely salt-affected soil) (Figure 3). The experimental plan for severely salt-affected soil (moderate soil salinity) was a randomized complete block design (RCBD) with five treatments and four replications, namely, T1, severely salt-affected soil (control); T2, severely salt-affected soil + vermicompost ratio of 25:75; T3, severely salt-affected soil + coconut coir ratio of 25:75; T4, severely salt-affected soil + rice husk biochar ratio of 25:75; and T5, severely salt-affected soil + vermicompost + rice husk biochar + coconut coir ratio of 25:25:25:25.

After obtaining the results from the study in severely salt-affected soil, the treatments for the best results were chosen for the experiment in very severely salt-affected soil (high soil salinity). The experiment plan was a randomized complete block design (RCBD) with three treatments and four replications, namely T1, severely salt-affected soil (control); T2, severely salt-affected soil + vermicompost at a ratio of 25:75; T3, severely salt-affected soil + vermicompost + rice husk biochar +coconut coir ratio of 25:25:25:25 from February to June 2022. The ratio of the treatment 3 (severely salt-affected soil + vermicompost + rice husk biochar + coconut coir at a ratio of 25:25:25:25) came from the previous study on the green house experiment [19].

Seeds of Sesbania (Sesbania rostrata) from the Department of Land Development of Thailand were soaked in warm water for 24 h to obtain the viability rate and ensure consistent survival, and the seeds were planted in trays for 14 days. Furrows were plowed and raised into sub-plot sizes of 2 × 4 m per plot in severely and very severely saline soil plots. The plot was raised to a height of 20 cm. The three types of organic materials, vermicompost, rice husks biochar, and coconut coir, were mixed with severely saline soil in different ratios according to each corresponding experimental method and put in a bamboo basket container woven into a cylindrical shape, with a width of 28 cm and a height of 22 cm. A hole was dug 28 cm wide and 22 cm high for the woven basket to fit snugly into the hole. The distance between the holes was approximately 60 cm. The seedlings that were around 10–15 cm tall were taken and planted in the prepared container. Canal water from the field study site was used to irrigate via drip water hose.

2.5. Soil Analyses

Before and after the experiment, soil samples were collected at a depth of 0–15 cm by extracting soil cores from each plot. Analysis of soil physical and chemical properties was conducted on the soil samples at the laboratory of the Department of Soil Science and Environment, Faculty of Agriculture, Khon Kaen University. The saturated paste extracts were analyzed for electrical conductivity (ECe) following the guidelines set by the United States Department of Agriculture (USDA, 1954). Total nitrogen (TN) levels were determined via the Kjeldahl method. Available phosphorus (Avail. P) was determined using the Bray II method. Exchangeable potassium (Exch. K⁺), sodium (Exch. Na⁺), calcium (Exch. Ca²⁺), and magnesium (Exch. Mg²⁺) were extracted with 1 N ammonium acetate (pH 7.0). The concentrations of K⁺ and Na⁺ from these extracts were analyzed via flame photometry, while those of Ca²⁺ and Mg²⁺ were analyzed via atomic absorption spectrometry.

2.6. Growth and Biomass Attributes

The growth and survival rates of Sesbania plants were evaluated by measuring plant height and survival every 2 weeks post-sowing until the end of the experiment. The experiment was concluded at the 3-month mark after sowing. The roots and shoots of Sesbania plants were isolated for the purpose of measuring growth variables. The fresh weight of the root and leaves was determined by weighing them with an analytical balance. The Sesbania plants, along with their roots, were uprooted and subsequently oven-dried to a constant weight at 70 °C for 48 h using a hot air oven to determine their dry weights.

2.7. Root Morphology Study

Plant root samples were collected, and root morphology was measured with a root scanner via flatbed scanners (EcoFABs: Manufacturer: EcoFABs Inc., based in Toronto, ON, Canada) (EPSON Perfection V700: Manufacturer: Epson, based in Nagano, Japan). The area within which the samples were placed was 20 × 25 cm. The root morphology values—length (cm), surface area (SurfArea) (cm²), average diameter (AvgDiam) (mm), and root volume (cm³)—were measured and analyzed with a precision of 0.005 mm (4800 dot per inch).

2.8. Flower Production and Nutrition Analysis

The flower production of Sesbania rostrate was measured during the study, and the nutritional attributes and minerals of Sesbania’s flowers were analyzed as follows: Protein, total carbohydrate, total fat, dietary fiber [20]; calcium (Ca), iron (Fe), and phosphorus (P) [21]; Vitamin A (B-Carotene), Vitamin C, Vitamin B1 (Thiamine), Vitamin B2, Vitamin B3 (Niacin) [22].

2.9. Statistical Analysis

The data shown in this study represent the average values ± standard error (SE) from four repeated experiments. Statistical analysis of the collected data was performed using ANOVA techniques. A comparison of treatment means was conducted using the least significant difference (LSD) method with a 5% significance level. All the data analyses were conducted with the use of Statistix 10 (Analytical Software, 2013). The Pearson correlation coefficient test was used to assess the correlation between the growth, survival rate, and flower production of Sesbania and the soil chemical properties tested.

3. Results

3.1. The Results of Using Vermicompost and Organic Amendments on Soil Salinity and Soil Physiochemical Properties

The results at the field study sites with severely salt-affected soil (moderate soil salinity).

The soils at the study sites all had a sandy loam texture, and their electrical conductivity (ECe) varied significantly in the saturation paste extracts, with values ranging from 3.40 to 5.98 dS m⁻¹. Salinity was lower in the wet season than in the dry season. Soil ECe values differed significantly among treatments. The application of vermicompost and organic amendments resulted in lower ECe than with the control treatments in severely saline soil, with the lowest values found in the rice husk biochar application (0.30 dS m⁻¹) (Figure 4).

The total N content was significantly different among treatments, but there were no differences among T2, T2, and T5 (

Table 2. The soil chemical properties in each treatment before planting Sesbania in severely salt-affected soil at the field study site.

Treatment	pH	EC dS/m	OM (%)	N (%)	P (ppm)	Exchangeable
						K	Na	Ca	Mg
						Cmol/kg
T1 saline soil (control)	5.63 c	3.11 a	1.86 b	0.09 b	6.49 d	0.22 c	1.580 a	9.79 a	3.20 a
T2 saline soil + vermicompost (25:75)	6.65 a	0.33 bc	5.96 a	0.30 a	55.00 a	0.66 a	0.003 b	0.04 b	0.01 b
T3 saline soil + coconut coir (25:75)	6.00 b	0.40 b	6.56 a	0.33 a	36.00 c	0.43 b	0.01 b	0.04 b	0.01 b
T4 saline soil + rice husk biochar (25:75)	6.70 a	0.26 c	3.31 b	0.17 b	47.00 b	0.62 a	0.003 b	0.04 b	0.01 b
T3 saline soil + vermicompost + biochar from rice husk + coconut coir (25:25:25:25)	6.48 a	0.28 c	5.68 a	0.29 a	43.00 b	0.51 ab	0.004 b	0.03 b	0.01 b
F-test	**	**	**	**	**	**	**	**	**
CV (%)	1.86	5.51	19.39	19.05	26.32	14.63	14.6	20.53	11.68

Note: Values in the same column followed by the same letter are not significantly different at the 5 percent level as per the least significant difference test; **: significantly different at p ≤ 0.01.

and

Table 3. The soil chemical properties in each treatment after planting Sesbania in severely salt-affected soil at the field study site.

Treatment	pH	EC dS/m	OM (%)	N (%)	P (ppm)	Exchangeable
						K	Na	Ca	Mg
						Cmol/kg
T1 saline soil (Control)	5.35 d	3.59 a	1.93 d	0.10 d	6.65 c	0.25 b	2.46 a	11.31 a	3.04 a
T2 saline soil + vermicompost (25:75)	6.93 bc	0.44 bc	6.45 bc	0.32 bc	39.00 b	0.23 b	1.27 bc	0.14 b	0.03 b
T3 saline soil + coconut coir (25:75)	7.68 a	0.35 bc	5.85 c	0.29 c	34.00 b	0.30 b	1.04 c	0.11 b	0.03 b
T4 saline soil + rice husk biochar (25:75)	6.60 c	0.54 b	9.25 ab	0.46 ab	44.00 a	0.44 a	1.44 b	0.15 b	0.05 b
T3 saline soil + vermicompost + biochar from rice husk + coconut coir (25:25:25:25)	7.23 ab	0.30 c	10.45 a	0.52 a	41.00 b	0.29 b	1.40 b	0.13 b	0.05 b
F-test	**	**	**	**	**	**	**	**	**
CV (%)	3.88	9.75	19.43	19.39	61.15	20.35	10.29	35.57	10.59

Note: Values in the same column followed by the same letter are not significantly different at the 5 percent level as per the least significant difference test; **: significantly different at p ≤ 0.01.

), which had the highest total N content (0.30%), whereas the control saline soil treatment without vermicompost or organic amendment had the lowest values (0.09%). There were significant differences in extractable P between soils and among treatments. The vermicompost application treatment had the highest soil extractable P among the different treatments (55.00 ppm).

The results at the field study sites with very severely salt-affected soil (high soil salinity).

The effects of using vermicompost together with organic amendment on the soil EC in high soil salinity during Sesbania production are shown in

Table 4. Effects of vermicompost and organic amendment on soil EC in high salt-affected soil during Sesbania production.

	DAP	Electrical Conductivity (dS/m)
Treatment		14	28	42	56	70	84	98	112	126
T1 saline soil (control)		3.67 a	6.21 a	6.23 a	11.39 a	11.48 a	13.82 a	16.88 a	8.27 a	9.64 a
T2 saline soil + vermicompost 25:75		0.39 b	0.73 b	0.55 b	1.06 b	0.87 b	0.60 b	0.48 b	0.66 b	0.71 b
T3 saline soil + vermicompost + biochar from rice husk + coconut coir 25:25:25:25		0.51 b	0.50 b	0.59 b	0.68 b	0.47 b	0.80 b	0.32 b	0.73 b	0.79 b
F-test		**	**	**	**	**	**	**	**	**
CV (%)		8.04	8.20	8.65	7.05	6.39	7.23	8.82	7.13	7.01

Note: DAP = day after planting, Values in the same column followed by the same letter are not significantly different at the 5 percent level as per the least significant difference test; **: significantly different at p ≤ 0.01.

. The electrical conductivity, pH, and sodium content were found in decreasing trends in T2 (saline soil + vermicompost at a ratio of 25:75) and T3 (saline soil + vermicompost + rich husk ash +coconut coir at a ratio of 25:25:25:25) when compared to T1 (saline soil (control)), with an increase in the fertility of the soil in terms of the organic matter, nitrogen content, and the amount of exchangeable potassium (

Table 5. The soil chemical properties in each treatment before planting Sesbania in very severely salt-affected soil at the field study site.

Treatment	pH	EC dS/m	OM (%)	N (%)	P (ppm)	Exchangeable
						K	Na	Ca	Mg
						Cmol/kg
T1 saline soil (control)	6.63 b	16.23 a	1.36 b	0.07 b	2.43 a	0.15 b	80.50 a	12.96 b	1.55 b
T2 saline soil + vermicompost 25:75	6.93 ab	8.73 b	7.95 a	0.40 a	5.80 b	0.32 a	11.97 b	17.45 a	4.66 a
T3 saline soil + vermicompost + biochar from rice husk + coconut coir 25:25:25:25	7.18 a	6.01 b	6.65 a	0.33 a	5.00 b	0.34 a	25.88 b	21.05 a	3.44 a
F-test	*	**	**	**	**	*	**	**	**
CV (%)	3.31	8.84	8.02	6.28	7.63	8.44	7.11	6.57	9.51

Note: Values in the same column followed by the same letter are not significantly different at the 5 per cent level as per the least significant difference test; * and **: significantly different at p < 0.05 and p < 0.01, respectively.

and

Table 6. The soil chemical properties in each treatment after planting Sesbania in very severely salt-affected soil at the field study site.

Treatment	pH	EC dS/m	OM (%)	N (%)	p (ppm)	Exchangeable
						K	Na	Ca	Mg
						Cmol/kg
T1 saline soil (control)	6.53	15.26 a	1.24 b	0.06 b	2.50 b	0.14 b	79.6 a	17.45 b	2.70 b
T2 saline soil + vermicompost 25:75	6.98	1.12 b	12.43 a	0.62 a	5.60 a	0.70 a	11.9 b	22.96 a	3.35 b
T3 saline soil + vermicompost + biochar from rice husk + coconut coir 25:25:25:25	6.70	1.96 b	7.42 ab	0.37 ab	6.30 a	0.60 a	25.8 b	21.05 ab	6.67 a
F-test	ns	**	*	*	*	**	**	**	**
CV (%)	3.61	8.82	8.46	8.42	7.82	8.17	7.03	8.13	8.58

Note: Values in the same column followed by the same letter are not significantly different at the 5 percent level by the least significant difference test; ns: not significant; * and **: significantly different at p < 0.05 and p < 0.01, respectively.

).

The effects of vermicompost and organic amendments on soil physiochemical properties and soil salinity characteristics are presented in Table 4. In the saline soil, the pH levels were consistently above 6. Vermicompost application produced pH values that were close to neutral (7.0). Soil ECe values differed significantly among treatments. Vermicompost and organic amendment application resulted in lower ECe levels than that found in saline soil, with the vermicompost treatment showing the lowest values (1.12 dS/m). The treatment with vermicompost resulted in the highest soil organic matter content (7.95% before and 12.43% after), whereas the treatment with no vermicompost and organic amendment resulted in the lowest value (1.36% in very severely saline soil).

More organic matter was accumulated in the soil upon applying vermicompost compared to the control severely saline soil and to that treated with vermicompost and organic amendment (rich husk ash + coconut coir). The total nitrogen and phosphorus content followed a similar pattern. The results displayed in Table 5 and Table 6 highlight significant variations in exchangeable cations across different treatments. The presence of vermicompost and organic amendments resulted in greater accumulation of exchangeable K+. There was a significant difference in the exchangeable sodium levels among treatments in saline soil. Treatments that did not include vermicompost and organic amendments showed elevated levels of exchangeable Na⁺ (80.50 cmol/kg). The application of vermicompost led to a higher accumulation of exchangeable Ca²⁺.

3.2. The Use of Vermicompost and Organic Amendment on the Growth of Sesbania in Severely Salt-Affected Soil

The heights of Sesbania plants at various stages post-sowing, from 2 to 10 weeks, are illustrated in Figure 5. At the beginning of growth, there were no significant differences in Sesbania plant heights between the treatments. Conversely, the presence of vermicompost and organic supplements resulted in a significant elevation in plant height as opposed to the control group (saline soil with no fertilizer application) at 4, 6, 8, and 10 weeks following sowing (Figure 5). The results indicate that vermicompost and the combination of vermicompost, rice husk ash, and coconut coir were more effective compared to the individual use of rice husk biochar or coconut coir.

This study showed that Sesbania rostrate was unable to thrive in highly saline soil.

Table 7. Effects of vermicompost and organic amendment on the growth of Sesbania in very severely salt-affected soil.

	DAP	Height (cm)
Treatment		14	28	42	56	70	84	98	112	126
T1 saline soil (Control)		10.25	0.00 b	0.00 b	0.00 b	0.00 b	0.00 b	0.00 b	0.00 c	0.00 c
T2 saline soil+ vermicompost 25:75		10.25	18.75 a	50.75 a	65.25 a	74.50 a	79.45 a	87.00 b	88.50 b	88.50 b
T3 saline soil+ vermicompost + biochar from rice husk +coconut coir 25:25:25:25		10.25	18.25 a	52.25 a	73.38 a	75.50 a	78.00 a	94.50 a	94.50 a	94.50 a
F-test		ns	**	**	**	**	**	**	**	**
CV (%)		5.10	11.31	10.79	11.37	11.39	11.83	10.23	9.81	9.81

Note: Values in the same column followed by the same letter are not significantly different at the 5 per cent level by the least significant difference test; ns: not significant; **: significantly different at p < 0.01; DAP: day after planting.

illustrates the varying heights of Sesbania plants recorded from 2 to 18 weeks post-sowing. There were no significant differences in Sesbania plant heights among treatments during the early growth stages. On the other hand, utilizing vermicompost and organic amendments led to a notable increase in plant height compared to the treatment without vermicompost application at 6, 8, 10, and 12 weeks after sowing (WAS). Salt-tolerant crops exhibited the most significant growth rate when grown in T3, which included saline soil, vermicompost, rich husk biochar, and coconut coir at a ratio of 25:25:25:25, followed by T2, namely, saline soil + vermicompost at a ratio of 25:75.

The results show that the survival of Sesbania in severely salt-affected soil was observed in every treatment, but the highest survival rate of Sesbania was associated with the application of vermicompost, followed by the application of vermicompost and organic amendments. The survival rate of Sesbania was the lowest in severely salt-affected soil without vermicompost and organic amendments (Figure 6).

The results show that the survival of Sesbania in severely salt-affected soil was realized in T2 and T3. Salt-tolerant crops (Sesbania) could not survive in T1, namely severely saline soil only. The highest survival rate of Sesbania was found for the application of vermicompost and organic amendments followed by the application of vermicompost only (

Table 8. Effects of vermicompost and organic amendment on the number of survivals of Sesbania plants in very severely salt-affected soil.

	DAP	Percent of Survival (%)
Treatment		14	28	42	56	70	84	98	112	126
T1 saline soil (control)		100.00	0.00 b	0.00 b	0.00 b	0.00 b	0.00 b	0.00 b	0.00 b	0.00 b
T2 saline soil+ vermicompost 25:75		100.00	100.00 a	100.00 a	100.00 a	80.00 b	71.60 b	71.60 b	66.70 b	66.70 b
T3 saline soil+ vermicompost + biochar from rice husk +coconut coir 25:25:25:25		100.00	100.00 a	100.00 a	100.00 a	91.60 a	88.30 a	88.30 a	83.30 a	83.30 a
F-test		ns	**	**	**	**	**	**	**	**
CV (%)		0.00	0.00	0.00	0.00	12.91	12.89	12.89	15.71	15.71

Note: Values in the same column followed by the same letter are not significantly different at the 5 percent level as per the least significant difference test; ns: not significant; **: significantly different at p < 0.01; DAP: day after planting.

).

The results show that the flower production of Sesbania in severely salt-affected soil was found in every treatment, but the flower production rate of Sesbania was found to be the highest in the application of vermicompost followed by the application of vermicompost and organic amendments. The flower production rate of Sesbania was the lowest in severely salt-affected soil without vermicompost and organic amendments (Figure 7).

The flower production rate of Sesbania in highly salt-affected soil was found to be higher in T3, with saline soil + vermicompost + rich husk biochar + coconut coir fiber at a ratio of 25:25:25:25, followed by that in T2, namely, with saline soil + vermicompost at a ratio of 25:7 (

Table 9. Effects of vermicompost and organic amendment on the flower production of Sesbania in very severely salt-affected soil.

	DAP	Flower per Plant (gram)
Treatment		14	28	42	56	70	84	98	112	126
T1 saline soil (control)		0.00	0.00	0.00 c	0.00 c	0.00 c	0.00	0.00	0.00	0.00
T2 saline soil + vermicompost 25:75		0.00	0.00	2.70 b	3.80 b	2.40 b	0.00	0.00	0.00	0.00
T3 saline soil + vermicompost + biochar from rice husk +coconut coir 25:25:25:25		0.00	0.00	3.90 a	4.20 a	3.80 a	0.00	0.00	0.00	0.00
F-test		ns	ns	**	**	**	ns	ns	ns	ns
CV (%)		0.00	0.00	5.85	3.06	8.98	0.00	0.00	0.00	0.00

Note: Values in the same column followed by the same letter are not significantly different at the 5 percent level as per the least significant difference test; ns: not significant; **: significantly different at p < 0.01; DAP: day after planting.

).

The root morphology of Sesbania in highly salt-affected soil was investigated. The maximum root length of Sesbania was found in T3, with vermicompost + rich husk biochar + coconut coir fiber at a ratio of 25:25:25:25, followed by in T2, namely, with saline soil + vermicompost at a ratio of 25:75. Salt-tolerant crops could not survive in T1 saline soil only. The maximum surface area of Sesbania roots was found in T3, with vermicompost + rich husk biochar + coconut coir fiber at a ratio of 25:25:25:25, followed by T2, namely, with severely saline soil + vermicompost at a ratio of 25:75. The maximum average diameter was found in T3, with vermicompost + rich husk biochar + coconut coir fiber at a ratio of 25:25:25:25, followed by T2, namely, with saline soil + vermicompost at a ratio of 25:75. The maximum root volume was found in T3, with vermicompost + rich husk biochar + coconut coir fiber at a ratio of 25:25:25:25, followed by T2, namely, with severely saline soil + vermicompost at a ratio of 25:75 (

Table 10. Effects of vermicompost and organic amendment on root morphology of Sesbania plants in very severely salt-affected soil.

Treatment	Length (cm)	Surface Area (cm2)	AvgDiam (mm)	RootVolume (cm3)
T1 saline soil (control)	0.00 c	0.00 c	0.00 c	0.00
T2 saline soil + vermicompost 25:75	1816.39 b	214.39 b	0.49 b	2.04 b
T3 saline soil + vermicompost + biochar from rice husk + coconut coir 25:25:25:25	4961.95 a	496.19 a	0.56 a	3.98 a
F-test	**	**	**	**
CV (%)	8.60	8.14	7.68	9.05

Note: Values in the same column followed by the same letter are not significantly different at the 5 percent level as per the least significant difference test; **: significantly different at p < 0.01.

, Figure 8).

The maximum fresh biomass was found in T2, namely, with severely saline soil + vermicompost at a ratio of 25:75, followed by T3, with severely saline soil + vermicompost + rich husk biochar + coconut coir fiber at a ratio of 25:25:25:25. The maximum dry biomass was found in T2, followed by T3 (

Table 11. Effects of vermicompost and organic amendment on the biomass of Sesbania in very severely salt-affected soil.

Treatment	Fresh Biomass	Dry Biomass
	g/Plant
T1 saline soil (control)	0.00 b	0.00 b
T2 saline soil + vermicompost 25:75	66.62 a	29.81 a
T3 saline Soil + vermicompost + biochar from rice husk + coconut coir 25:25:25:25	66.07 a	29.60 a
F-test	**	**
CV (%)	7.16	5.71

Note: Values in the same column followed by the same letter are not significantly different at the 5 per cent level as per the least significant difference test; **: significantly different at p < 0.01.

).

3.3. Nutritional Value of Sesbania rostrata Flower

The results of the analysis of the flowers of the Sesbania flower show that the flowers of Sesbania grown in severely salt-affected soil with the application of vermicompost and organic amendment had more nutritional value than those grown in normal soil (

Table 12. Nutritional value of Sesbania rostrata flower grown with and without vermicompost and organic amendment in severely salt-affected soil for Sesbania production.

Nutritional Value	Flower
	Grown in Vermicompost and Organic Amendments	Grown in Normal Soil	Unit
Protein	3.60	3.35	g/100 g
Total carbohydrate	8.15	5.60	g/100 g
Total fat	0.47	0.40	g/100 g
Calcium (Ca)	337.70	51.00	mg/kg
Iron (Fe)	13.96	8.20	mg/kg
Phosphorus(P)	528.60	56.00	mg/kg
Dietary fiber	5.78	3.90	g/100 g
Vitamin A (B-Carotene)	427.56	34.30	ug/100 g
Vitamin B1 (Thiamine)	0.22	10.20	mg/100 g
Vitamin B2	0.04	0.33	mg/100 g
Vitamin B3 (Niacin)	0.02	2.80	mg/100 g
Vitamin C	0.04	24.00	mg/100 g
Zinc (Zn)	3.69	ND	mg/kg
Iron (Fe)	56.24	ND	mg/kg
Selenium (Se)	ND	ND	mg/kg
Silicon (Si)	24.40	ND	mg/kg

Note: ND = not detected.

).

4. Discussion

4.1. Vermicompost and Organic Amendments & Soil Salinity and Soil Physiochemical Properties

In this study, the application of vermicompost and an organic amendment like rice husk biochar + coconut coir resulted in decreased salinity and enhanced soil properties in severely salt-affected soils. Soil physical, chemical, and biological properties can be enhanced by incorporating vermicompost and organic amendments like rice husk ash and coconut coir materials. The soil pH can go down or up based on the original pH of the organic material added. It is possible that rice husk biochar and coconut coir play a role in boosting soil porosity and aeration [23,24]. Additionally, the vermicompost utilized in this research contains calcium and magnesium, functioning as bases in the form of carbonates, oxides, and hydroxides when introduced to highly saline soil. As the Ca²⁺ concentration in the soil solution rises, there is an increase in Na⁺–Ca²⁺ exchange at the cation exchange sites in the soil, leading to more leaching of exchanged Na⁺ in percolating water and a consequent decrease in soil salinity, as indicated in this study [4]. Soil organic matter is not only a source of essential plant nutrients, but also impacts the physical, chemical, and biological aspects of soil, all of which significantly contribute to maintaining soil fertility [12,14]. This study found that vermicompost and organic amendments were beneficial for increasing soil organic matter to severely saline soil. Soils treated with vermicompost and organic amendments, such as rich husk ash and coconut coir materials, showed significantly higher levels of total nitrogen and extractable phosphorous when compared to the severely saline soil treatment.

4.2. Vermicompost and Organic Amendments & Sesbania Growth and Biomass Attributes

Vermicompost is recognized for its exceptional quality as a nutrient-rich organic fertilizer, featuring good physical structure, high microbial activity, and abundant humic substances [12]. The results of this study are similar to those of other studies according to which using vermicompost addition was found to be a successful strategy in rehabilitating salt-affected soils and mitigating the detrimental effects of salt stress on plant growth by improving soil aggregation and reducing the harmful effects of salt stress on plant growth, enhancing the supply of nutrients and organic material for plants, controlling the microbial population, and promoting salt leaching [15,19,23,24,25,26]. It has been demonstrated that applying rice husk biochar as a treatment could improve the growth environment for soil bacteria, resulting in a notable enhancement of soil phosphorus availability and related enzymes [27]. The examination of principal component analyses has indicated that biochar has a tendency to impact the structure of bacterial communities in degraded soils like saline soil. The presence of biochar leads to an increase in the relative abundance and distribution of Thiobacillus, Pseudomonas, and Flavobacterium in soils to different extents; all three of these are recognized as genera of phosphate-solubilizing bacteria. In summary, biochar’s impact on phosphate-solubilizing bacteria is positive, ultimately increasing phosphorous availability [27,28].

The positive outcomes of using biochar to reclaim salt-affected soil can be understood through its enhancement of water-holding capacity and its organic matter content, bulk density, and salt sorption capacity, which are facilitated by its high cation exchange capacity, specific surface area, and porosity. This results in the mitigation of the detrimental effects of soil salinity, an improvement in soil quality, and the facilitation of plant growth [14,15,29,30]. The advantage of coconut coir has been explained as helping with the physical properties of soil thanks to its good drainage, high water-holding capacity, ability to accelerate root growth and development, and relatively affordable cost.

The combination of vermicompost with organic amendments such as rice husk biochar and coconut coir could lead to the mutual enhancement of their performance and the offsetting of their individual weaknesses, thereby influencing their overall effectiveness in severely salt-affected soil remediation.

The described treatments with vermicompost and organic amendments have led to an increased Sesbania plant height, survival, fresh and dry biomass yield, and root morphology due to the enhanced physiological growth of the plants. Among the various treatments examined in this study, it was found that combining vermicompost with rice husk biochar and coconut coir was more effective in promoting increased growth, biomass, and quality of Sesbania plants than using rice husk biochar or coconut coir separately. Vermicompost increases plant productivity. It is suggested that microorganisms that play a role in vermicomposting may be responsible for producing plant growth hormones [31]. Nevertheless, multiple studies have pointed out the significant presence of humic acids in vermicompost, which may act as bio stimulants to promote plant growth [32]. The enhancement of the water-holding capacity of the soil, cation exchange capacity, nutrient availability, and physical properties in saline conditions is attributed to the addition of organic matter, the availability of macronutrients as well as micronutrients, and to improved soil physical properties under severely saline conditions. The application of organic amendment can result in positive effects on saline soils, such as the slow release of nutrients and the enhancement of soil biological and physical properties. The addition of organic amendments led to a notable decrease in salt levels and to an enhancement in the availability of nitrogen, phosphorus, and potassium [33]. Soil aggregate stability is increased by organic matter, which is attributed to the bonding or adhesion properties of waste products from bacteria and fungi, as well as to bacterial hyphae. Enhanced soil aggregate stability contributes to better soil porosity, water infiltration, and water-holding capacity [13,34].

It was proposed in this study that biochar has the potential to influence the composition of bacterial communities in degraded soils like saline soil. The relationship between rice husk biochar, soil phosphorus availability, phosphatase activities, and bacterial community characteristics was found. Rice husk biochar was found to elevate the prevalence and dispersion of Thiobacillus, Pseudomonas, and Flavobacterium in soils, which are recognized as phosphate-solubilizing bacteria, leading to an increase in phosphorous availability [35].

Incorporating vermicompost and organic amendments into severely saline soils could help deliver essential nutrients for plant growth, enhance soil permeability and water movement, decrease electrical conductivity, promote the exchange of Na+ with Ca2+, offer habitats and organic carbon for soil microorganisms, and foster the formation of large water-stable aggregates [23,35,36,37,38,39].

The use of vermicompost was reported to be more effective than inorganic amendments in improving the quality of saline–sodic soil and increasing wheat growth and yield [19]. When exposed to saline conditions with 4‰ NaCl w/w, blessed thistle and peppermint plants showed improved growth and a higher K+/Na+ ratio due to the application of 150 g/kg cow manure vermicompost, which boosted the activities of superoxide dismutase, peroxidase, and catalase enzymes [40]. The application of 25 g/kg biochar helped to raise the soil pH and cation exchange capacity in salt-affected soils, stimulating the growth of plants above and below ground, increasing nutrient levels in plant tissues, and ultimately boosting safflower seed and oil production [41]. In addition, biochar has the ability to defend plants from oxidative stress by triggering antioxidant enzymes and reducing the presence of reactive oxygen species (ROS) [42,43]. The presence of biochar in soil has a significant impact on the activity and structure of microbial communities, ultimately controlling nutrient cycling [43,44,45,46]. Coconut coir is a natural fiber extracted from the husk of coconuts. It is a byproduct of the coconut industry and has gained popularity in various horticultural applications due to its versatility and sustainability. Coconut coir is valued for its water retention, aeration properties, and high cation exchange capacity. The combination of vermicompost, rice husk biochar, and coconut may interact in ways that enhance each other’s performance and address their individual shortcomings, thereby improving their overall effectiveness in saline soils remediation.

When compared with severely salt-affected soil, the Sesbania growth results were increased by vermicompost and vermicompost + rich husk biochar + coconut coir fiber. However, the observed heights were not significantly different across both treatments (p > 0.05). These results could be explained by the fact that rice husk biochar can absorb Na+. Salt-affected soils severely hinder agricultural growth due to issues such as sodium ion toxicity, nutrient deficiencies, and soil structure changes [47]. Biochar is a carbon-containing product generated through the high-temperature treatment of organic waste in the absence of oxygen. With its high cation exchange capacity (CEC), adsorption capacity, and C content, it is commonly incorporated as a soil amendment. The presence of biochar often results in a decrease in Na+ levels in soil colloids by either effectively adsorbing it or by utilizing the calcium (Ca) or magnesium (Mg) present on its surface to swap sodium ions (Ex-Na) in soil colloids through cation exchange, enhancing salt leaching during irrigation. Accordingly, the above result using vermicompost and rice husk biochar could be advantageous for organic or sustainable agriculture by supporting soil fertility and the conservation of the ecosystem. Obtained from the husk of coconuts, coconut coir is an organic substance that offers environmental benefits as a renewable resource. It is known for its excellent physical properties and has been successful in supporting the growth of various plants, including strawberries [48]. The root morphology of Sesbania was found to be the best in the treatment 3, with vermicompost and organic amendments (risk husk biochar and coconut coir) in highly salt-affected soil. These results show that organic amendments such as rice husk biochar and coconut coir fiber help improve the physical properties of the soil. The coconut coir property boasts excellent drainage, high water retention, promotes root growth, and is reasonably priced. It is recognized for its impressive cation exchange capacity (CEC), enabling it to attract and retain essential nutrients like potassium, calcium, iron, and zinc. The pH of coir is not as acidic as peat, falling within the slightly acidic-to-neutral range. Its great buffering capacity and perfect air-to-water ratio enable the proper management of the nutrient solution and the implementation of frequent fertigation. Adequate oxygen is necessary for plant roots to absorb water and the nutrients crucial for plant development, growth, and yield. In this research, coconut coir dust was employed as the growth medium due to its organic nature, high cation exchange capacity, and exceptional water retention capacity, rendering it appropriate for a range of irrigation schedules [49,50].

Correlation analysis showed that soil salinity (ECe) did significantly affect the growth index (height, survival, flower production) of Sesbania and decrease soil nutrients, but that soil nutrients such as N, P, or K from vermicompost and organic amendments enhance the growth index (height, survival, flower production) of Sesbania (Figure 9).

4.3. Vermicompost and Organic Amendments & Sesbania Nutrition and Potential as a Functional Food Source

Sesbania rostrata is a short-lived perennial legume that forms nodules on its stems and is found in abundance across tropical Africa and Asia. Its main purpose is for use as a green manure and as a source of forage for small ruminants. Typically, Sesbania has the ability to withstand salinity levels of 4–12 dS/m electrical conductivity (ECe) [2]. In this study area, its soil was severely salt-affected, with the EC values sometimes higher than 12 dS/m. Food quality is not solely dependent on the genetic characteristics of crops, but also on other factors such as environmental conditions and cultivation methods. Plant culture quality may increase under abiotic stress conditions [51].

Under salt stress conditions, plants boost primary metabolites, such as sugars, amino acids, and organic acids, to regain osmotic equilibrium [52,53]. Sugars are the primary solutes involved in osmotic adjustment, but their utilization restricts the availability of other physiological processes like growth and production. Additionally, plants must respond to the secondary oxidative stress caused by salinity, and secondary metabolites serve as antioxidants [40].

Sesbania grown with vermicompost and organic amendments under severely saline soil condition had a higher content of protein, minerals, and vitamins than Sesbania grown without adding vermicompost and organic amendments. Vermicompost is an organic fertilizer enriched with microbes, vitamins, enzymes, and hormones, with lower soluble salts, a neutral pH, enhanced ion exchange capacity, humic acid content, and elevated levels of nitrates, calcium, and magnesium. It boosts the soil’s water-holding capacity. The presence of plant hormones like auxins, gibberellins, and enzymes is said to encourage plant growth [53] and deter plant pathogens. Additionally, it improves soil quality by introducing helpful microorganisms that in turn introduce various enzymes, including phosphatases and cellulases. It improves seed germination, plant development, and ultimately boosts crop productivity [54].

The cost of rehabilitating severely salt-affected soil through the application of vermicompost and organic supplements was notably cheaper in comparison to the standard method involving gypsum. This is because the price of inputs (organic amendments) has decreased due to their local availability and because it also involves agricultural residue or waste that needs to be managed. Even if the farmer wants to buy them, the cost for coconut coir and rice husk biochar is THB 3 per kg each, and for vermicompost it is THB 10 per kg. One kg of the vermicompost and organic amendment mixture in this study costs THB 4 per kg for remediation of 1 m² of severely salt-affected soil. Therefore, it costs THB 40,000 for one hectare as compared with using gypsum at THB 12 per 1 m² of severely salt-affected soil. The price of severely salt-affected soil bioremediation through vermicompost mixed with rice husk biochar and coconut coir per hectare (THB 40,000 for one hectare) is lower in comparison with the conventional method of reclamation (Gypsum @ 10 tons/ha (THB 1200 per ton)–total cost THB 120,000) (USD 1 = THB 36.5). Our project also trained the farmers in the severely salt-affected area on how to produce vermicompost from agricultural waste and household waste by themselves. Aiding in the management of agricultural waste, this also leads to a decrease in the price of vermicompost.

From the results of this experiment, it was seen that the severely salt-affected soil bioremediation using vermicompost and organic amendments could improve the physico-chemical properties of soil, markedly decrease soil EC values, and increase soil fertility and Sesbania growth rates. Generally, Sesbania rostrata is known as a common green-manure legume or animal feed. The plant’s effectiveness in fixing nitrogen in waterlogged soil has made it an appealing green manure option for lowland rice agriculture in Asia for its tolerance to soil salinity. However, in Southeast Asia, Sesbania has edible flowers and its leaves are commonly eaten. In Thailand, people use Sesbania for many traditional uses. The flowers of Sesbania are edible and are often used to supplement meals. Flowers with the pistils removed (to reduce bitterness) are edible and are boiled, deep-fried, or used in curries and soups. Young leaves and shoots are eaten in salads, stews, or as potherbs. Sesbania, a medicinal plant that can be eaten, is commonly found across various Asian countries. The medicinal benefits of the Sesbania tree are derived from utilizing all its components in traditional healing practices. It offers benefits in the treatment of diarrhea, dysentery, microbial infections, and inflammatory disorders. It provides high levels of vitamin A, calcium, and phosphorus. Sesbania flowers and leaves contain a wide range of nutrients, such as protein, minerals, and vitamins. Sesbania contains vitamin A, folate, thiamin, niacin, and vitamin C. Its flowers are rich sources of magnesium, phosphorus, potassium, and selenium. According to the National Institute of Nutrition, its leaves are abundant in protein that possesses all eight essential amino acids and has remarkable levels of calcium. The utilization of vermicompost and organic amendments can aid in rehabilitating severely salt-affected soil to support the growth of salt-tolerant crops with improved salt stress tolerance and higher nutritional value, benefiting farmers.

5. Conclusions

The growth of Sesbania was significantly increased by the application of vermicompost and organic amendments, as shown in the above findings. It appears that utilizing vermicompost and organic amendment can be beneficial in mitigating salinity issues and enhancing the soil’s chemical, physical, and biological properties, resulting in improved Sesbania yield in severely saline environments and in greater nutritional value for the plant.

The deterioration of agricultural land and the growing production of agriculture residues or waste has turned into a significant global problem. Agriculture plays a vital role in Asia’s economy, supporting the livelihoods of billions of people. Policy makers and researchers are striving to discover inventive strategies to handle these issues; in light of the growing demand for agricultural products as the global population booms, the role of agriculture and rural areas in food production is more essential than ever. Agriculture and rural areas are vital not only for food production, but also for the maintenance and adaptation of agroecosystems, the knowledge systems of people, the provision of diverse goods and services, and compliance with quality of life. Farmers rely on inorganic fertilizers to solve nutrient problems in the soil. However, this can lead to long-term damage. Turning agricultural waste into valuable products such as vermicompost and soil amendments and using them in the remediation of severely salt-affected soil for the cultivation of salt-tolerant crops as a source of functional food should be an ecological, economic solution. A practical and sustainable strategy for recycling agricultural biomass wastes is presented in this study, offering double benefits in waste treatment and restoring salt-affected environments, with worldwide applicability.

In our study, it was also found that Sesbania grown in severely salt-affected soil remediated with vermicompost and organic amendments has a higher nutritional value than when grown in other soils. The term functional foods is utilized to denote foods that offer health benefits beyond essential nutrition. Sesbania could be promoted to grow in severely salt-affected soil as a functional food. Farmers face challenges when growing crops due to the high salinity of the soil, resulting in barren land. Selling 1 kg of Sesbania flowers can fetch THB 1200, with each square meter yielding 120 g of flowers. With a production of 1193 kg per hectare, a farmer could potentially earn THB 139,000 annually from previously uncultivated land.

The integration of vermicompost with rice husk biochar and coconut coir proved to be an effective method for enhancing severely salt-affected soil, as it boosted major and micronutrients while also nurturing soil microorganisms. At the end of the study, Sesbania showed a survival rate between 81% and 92%. A 1:3.5 cost–benefit ratio analysis was determined by the market demand for Sesbania. The superior physio-chemical, nutritional, and biological attributes of vermicompost and organic amendments render them more appropriate for rehabilitating saline soils in comparison to traditional chemical amendments such as gypsum. In summary, the experiment results established that the unified method for rehabilitating extremely salt-affected soil has proven to be a productive, economical, and environmentally friendly plan for not just Thailand, but also for other semi-arid regions. According to the authors, the benefits of this study can be extended to other severely salt-affected areas in Thailand and other developing countries.