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Article

Soil Fertility and Plant Growth Enhancement Through Compost Treatments Under Varied Irrigation Conditions

by
Subanky Suvendran
1,
Miguel F. Acevedo
2,
Breana Smithers
2,
Stephanie J. Walker
3 and
Pei Xu
1,*
1
Department of Civil Engineering, New Mexico State University, Las Cruces, NM 88003, USA
2
Department of Electrical Engineering, University of North Texas, Denton, TX 76203, USA
3
Department of Extension Plant Sciences, New Mexico State University, Las Cruces, NM 88003, USA
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(7), 734; https://doi.org/10.3390/agriculture15070734
Submission received: 19 February 2025 / Revised: 24 March 2025 / Accepted: 24 March 2025 / Published: 28 March 2025
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

:
Global challenges such as soil degradation and water scarcity necessitate sustainable agricultural practices, particularly in regions where saline water is increasingly used for irrigation. This study investigates the effects of four compost treatments, including surface-applied mulch compost (MC), Johnson–Su biologically active compost incorporated into soil (JCI), mulch compost incorporated into soil (MCI), and no compost as control (NC), on soil fertility, microbial activity, and Capsicum annuum (chili pepper) growth. Greenhouse experiments were conducted using soil from two different sites (New Mexico State University’s (NMSU) agricultural research plots and agricultural field-testing site at the Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, New Mexico) and two irrigation water salinities (brackish at ~3000 µS/cm and agricultural at ~800 µS/cm). The Johnson–Su compost treatment demonstrated superior performance, due to its high soil organic matter (41.5%), nitrate (NO3−) content (82.5 mg/kg), and phosphorus availability (193.1 mg/kg). In the JCI-treated soils, microbial biomass increased by 40%, and total microbial carbon reached 64.69 g/m2 as compared to 64.7 g/m2 in the NC. Plant growth parameters, including chlorophyll content, root length, and wet biomass, improved substantially with JCI. For instance, JCI increased plant height by 20% and wet biomass by 30% compared to NC treatments. The JCI treatment also effectively mitigated soil salinity, reducing Na+ accumulation by 60% and Cl− by 70% while enhancing water retention and soil structure. Principal Component Analysis (PCA) revealed a distinct clustering of JCI treatments, demonstrating its ability to increase nutrient retention and minimize salinity stress. These results indicate that biologically active properties, such as fungi-rich compost, are critical to providing an effective, environmentally resilient approach for enhancing soil fertility and supporting sustainable crop production under brackish groundwater irrigation, particularly in regions facing freshwater scarcity.

1. Introduction

Water scarcity significantly challenges global agriculture, driving the increased reliance on alternative irrigation sources, notably brackish groundwater. While reducing the demand for freshwater, brackish groundwater irrigation introduces salinity issues detrimental to soil health and crop productivity [1]. Salt-affected soils naturally occur worldwide and often result from irrigation practices, primarily due to sodium salts [2]. Salinization degrades soil fertility and diminishes crop productivity [3]. Organic amendments like compost have been proposed as environmentally sustainable solutions that improve soil structure, enhance water retention, and stimulate microbial activity, particularly under saline irrigation conditions [4]. However, compost efficacy varies based on compost type and irrigation water quality; thus, understanding their interactions is crucial for optimizing soil health and plant growth benefits [5].
Compost enhances soil fertility by increasing organic matter, improving soil structure, water retention, and nutrient cycling [6]. The organic matter in compost serves as a slow-release source of essential nutrients such as nitrogen (N), phosphorus (P), and potassium (K), as well as secondary macronutrients like calcium (Ca), magnesium (Mg), and sulfur (S) [7]. These nutrients are fundamental for promoting robust plant growth and development. Research suggests that incorporating compost can boost soil organic matter by 30–50%, which enhances soil structure and increases its capacity to retain moisture [8]. In addition, compost increases soil cation exchange capacity (CEC), enhancing nutrient retention, reducing leaching, and improving long-term nutrient availability to plants [9], for example, Yüksel and Kavdır [10] demonstrated that compost treatments increased soil CEC by 25%. Under brackish groundwater irrigation, compost can play a critical buffering role and facilitate the removal of excess salts from the root zone, thus helping to enhance soil structure and prevent the accumulation of harmful Na+ levels [11]. This is crucial for mitigating soil degradation caused by brackish groundwater irrigation and fostering beneficial microbial communities essential for nutrient cycling and organic matter decomposition [12,13]. Even under saline stress, these microbial populations help maintain soil fertility and plant productivity [14,15].
Soil health depends on microbial activity as microorganisms facilitate organic matter decomposition and nutrient cycling. Compost amendments can enhance microbial activity by supplying essential organic matter and nutrients to beneficial microbes like bacteria, fungi, and protozoa [16]. Increased microbial activity supports key soil processes such as nitrogen mineralization, phosphorus solubilization, and organic matter decomposition, all of which are vital for maintaining soil fertility [17]. Various studies have highlighted the positive impact of compost on enhancing soil microbial biomass and diversity. For example, Sayara et al. [18] reported that compost treatments increased microbial biomass by 20–40%, leading to improved nutrient cycling and plant growth. Similarly, Xu et al. [19] found that compost amendments increased microbial activity and enzymatic processes, thereby enhancing nutrient availability in compost-treated soils. Certain composts promote fungal growth, which contributes to soil aggregation and water infiltration, both of which are vital for maintaining soil structure under brackish conditions [20]. Fungi offer unique advantages over bacteria in agriculture by forming extensive hyphal networks that improve soil aggregation, water infiltration, and nutrient cycling, particularly in saline or low-fertility soils [21]. Their symbiotic relationships, especially with arbuscular mycorrhizal fungi (AMF), enhance nutrient uptake and plant resilience to stressors like salinity and drought [22]. Additionally, fungi synthesize bioactive compounds that promote plant growth and act as biocontrol agents, making them vital for sustainable agriculture [21].
Sodium (Na+) and Cl− in brackish groundwater can accumulate in the soil, reducing the availability of critical nutrients like N and P [23]. Moreover, high salt concentrations impair the soil’s ability to retain water, reducing plant growth and yields [24]. In addition, brackish groundwater irrigation negatively impacts soil microbial activity as salt-sensitive microorganisms involved in nitrogen fixation and organic matter decomposition decline, reducing microbial diversity and nutrient availability [25]. Addressing these challenges requires strategic soil management to mitigate brackish groundwater irrigation impacts while sustaining soil fertility and productivity. Fungi, being more salt-tolerant than bacteria due to osmolyte production and extensive hyphal networks, play a crucial role under the brackish water irrigation conditions [21,26]. The enhanced salt tolerance of fungi likely explains the effectiveness of biologically active compost in mitigating the negative impacts of brackish groundwater irrigation.
The interaction between compost amendments and irrigation water quality is critical for managing soil salinity and crop productivity, as compost improves soil structure, water infiltration, and alleviates salinity effects by increasing organic matter, enhancing beneficial microbial activity, and improving nutrient availability [27,28,29]. Additionally, compost aids in leaching excess salts from the soil, reducing salt accumulation in the root zone and making it an effective solution to mitigate the adverse impacts of irrigation in water-scarce regions reliant on low-quality water.
Compost application under brackish conditions significantly improves plant growth metrics, such as root length, shoot height, biomass, chlorophyll content, and nutrient uptake, by enhancing soil structure, water retention, and nutrient supply, even in saline soils exceeding electrical conductivity (EC) levels of 4000 µS/cm [30]. Abbas et al. [31] showed that compost amendments increased grain yield and biomass when compared to non-amended saline soils [32]. Furthermore, compost has been linked to a substantial increase in chlorophyll content, which correlates with improved photosynthetic efficiency and plant vigor under salt stress [33].
Despite previous studies highlighting the benefits of compost, there is a lack of systematic research on the underlying mechanisms, and thereby understanding these processes is crucial for advancing sustainable agriculture. Hence, this study aims to investigate the interactions between compost treatments and irrigation water quality, soil type, and their combined effects on soil fertility, microbial activity, and plant growth. Chili pepper was selected as the test crop due to its economic importance and sensitivity to soil and water quality. The specific objectives of this research are as follows:
  • Analyze nutrient retention, focusing on Na+, Cl−, NO3−, and P transport and availability in soil under different irrigation, compost, and soil conditions.
  • Assess microbial biomass and activity in response to compost treatments under irrigation, compost, and soil conditions.
  • Examine plant growth parameters, including chlorophyll content, biomass, plant height, root length, and ion composition of plant tissues under irrigation, compost, and soil conditions.

2. Materials and Methods

2.1. Experimental Design and Treatment Setup

A greenhouse study was conducted for over 70 days in 2023 to assess the combined effects of irrigation water quality, soil type, and compost treatment on chili pepper growth. The experimental design followed a full factorial approach, testing three factors: (1) irrigation water: brackish (BW) at ~3000 µS/cm and agricultural water (AW) at ~800 µS/cm, (2) soil type: agricultural soil (AS) from NMSU’s agricultural research plots and BGNDRF soil (BS) from its agricultural field-testing site, and (3) compost: no-compost (NC), Johnson–Su compost incorporated into the soil (JCI), mulch compost incorporated into the soil (MCI), and surface-applied mulch compost (MC). The BW treatment had an EC of 2958 ± 51 µS/cm, generated by blending brackish groundwater from BGNDRF Well 1 and reverse osmosis (RO) concentrate, while the AW treatment had an EC of 796 ± 7 µS/cm, produced by mixing BGNDRF well 1 water and RO permeate. The compost and soil characteristics are presented in Table S1, while the major ion concentrations are shown in Figure S1. Each of the 16 treatment combinations was replicated three times (Table 1).

2.2. Soil Collection and Column Preparation

Soils from both sites were classified as clay loam and samples were collected from three depths: 0–15 cm, 15–30 cm, and 30–45 cm. After air-drying and sieving through a 3 mm mesh, both soils were manually packed into PVC columns (50 cm height × 10 cm diameter) by carefully weighing pre-determined amounts of air-dried soil to replicate field bulk density conditions. The volume of each PVC column was calculated, and the required soil mass to achieve the target bulk density (1.35–1.40 g/cm3) was determined accordingly. The bottom of the columns was designed to permit adequate drainage, thus minimizing secondary soil salinization.
Two compost types were utilized: (1) commercial mulch compost (MC) from GRO-WELL Brands Inc., Tempe, AZ, USA, consisting of aged bark and composted plant matter to enhance soil structure, and (2) Johnson–Su compost (JC), produced from yard waste in a bioreactor under aerobic, undisturbed conditions for one year. During the JC composting process, moisture was maintained at 70% (w/w) through daily irrigation, and earthworms (Eisenia fetida) were introduced once the temperature dropped below 28 °C to enhance microbial activity and soil health [34]. For the JCI treatment, JC was incorporated into the topsoil (0–15 cm) at a 10% weight-to-weight ratio, approximately 20.25 kg wet compost per m2 for 15 cm soil depth. The MCI columns were treated with the same compost application rate as JCI, whereas 12.74 kg of wet compost per m2 was applied on the surface of the topsoil for the MC columns.

2.3. Plant Growth Conditions and Irrigation Management

Irrigation was applied daily at a volume of 300 mL per column until the soil columns reached saturation. Subsequently, irrigation volumes were adjusted to maintain soil moisture levels between 0.200 and 0.350 m3/m3 volumetric water content.
Chili pepper was selected for this study due to its economic importance in the southwestern United States. As a moderately salt-tolerant crop, it can thrive under controlled salinity conditions, provided EC remains below 300 µS/cm with an appropriate leaching fraction [35]. Twelve seeds were sown in each column on the seventh day of irrigation after saturating the soil columns, and four healthy seedlings were selected per column after germination. Plants were grown under AgroMax F54T5HO high-output fluorescent lamps (HTG Supply, Callery, PA, USA), each rated at 54 watts and featuring a full daylight spectrum of 6400 K to ensure consistent light exposure, and the greenhouse was maintained at a minimum temperature of 18 °C for optimal growth where the relative humidity ranged from 20 to 30%.

2.4. Plant Growth and Biomass Measurements

At harvest, wet biomass weight was recorded using a DYMO M25 balance (DYMO, M25, Tarzana, CA, USA). Chlorophyll content was measured using a SPAD 502DL Plus Chlorophyll Meter (Spectrum Technologies, Aurora, IL, USA), expressed as SPAD units, and plant height was recorded. After careful extraction and thorough washing to remove soil particles, root lengths (cm) were measured manually using a measuring tape.

2.5. Plant Tissue and Soil Analysis

Plant tissue analysis for major cations was conducted using composite samples of dried and ground leaf and stem biomass. The samples were digested (Ethos UP microwave, Milestone Inc., Shelton, CT, USA), and total nitrogen was determined using the Kjeldahl method (Technicon AutoAnalyzer, Tarrytown, NY, USA). Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES; Optima 4300 DV, PerkinElmer, Waltham, MA, USA) was performed to quantify total metals and trace elements using EPA method 200.7. All analyses were conducted at the Soil and Environmental Science Laboratory at NMSU.
Initial and post-harvest soil samples were analyzed at WARD Laboratories (Kearney, NE, USA) following standard extraction and detection protocols. The pH and soluble salts were measured using a 1:1 soil-to-water suspension method. Organic matter (%) was determined using the Loss-on-Ignition (LOI) method. Nitrate (NO3−) (mg/kg) was analyzed using the Cadmium Reduction Method via Flow Injection Analysis (FIA). Phosphorus (P) (mg/kg) was extracted using the Olsen Sodium Bicarbonate Method (SOP 1008) and quantified using an FIA at 880 nm. Exchangeable cations (mg/kg), including potassium (K+), sodium (Na+), calcium (Ca2+), and magnesium (Mg2+), were extracted using 1N Ammonium Acetate and analyzed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Chloride (Cl−) (mg/kg) was determined using potentiometric titration, while sulfate (SO42−) (mg/kg) was extracted using the Mehlich III Sulfur and Phosphorus Method and measured via ICP-OES. Cation exchange capacity (CEC) or sum of cations (meq/100 g) was calculated based on exchangeable cations. Soil microbial analysis was conducted to evaluate the impact of different treatments on soil biological activity. Following the final harvest, topsoil samples were carefully extracted, homogenized, and placed in sterile sampling bags to prevent contamination. Samples were stored in insulated shipping containers at 4 °C and shipped overnight to Earthfort Laboratory (Corvallis, OR, USA) for microbial analysis. Total fungi and bacteria were quantified following standard laboratory protocols, including fluorescein diacetate (FDA) hydrolysis for microbial activity assessment. Active bacterial and fungal populations were determined using direct staining methods, while protozoan communities, including flagellates, amoebae, and ciliates, were enumerated using the Most Probable Number (MPN) method.
These analyses provided critical insights into the biological activity and health of the soil under different treatments at the end of the experiment. All instruments were calibrated according to the manufacturer’s guidelines before use to guarantee measurement accuracy. All analyses were conducted following the standard validated protocols of the respective laboratories.

2.6. Statistical Analysis

Three-way ANOVA was conducted to compare the effect of the three treatment factors after confirming normality. Homogeneity of variance was assessed using Levene’s test, ensuring the assumption of equal variances was met. Based on these confirmations, standard ANOVA was applied for statistical analysis. Data were also statistically analyzed using Principal Component Analysis (PCA) to assess the influence of irrigation water type, soil quality, and compost treatment on soil salinity, plant growth, and nutrient leaching. This analysis helped identify key factors driving variations and interactions within the dataset. Tukey’s pairwise test was applied for comparisons where significant differences (p < 0.05) were observed. All statistical analyses were performed using MINITAB version 17.0 (Minitab, LLC, State College, PA, USA) with additional checks for normality to ensure valid inferences.

3. Results

3.1. Physicochemical Properties of Compost and Soil

Table 2 presents the physicochemical parameters of compost and soil at various depths. Agricultural soil (AS) showed a consistent pH of around 8.0 across depths, with higher organic matter (3.0–3.2%) than BS, which exhibited a slightly alkaline pH (7.9–8.0) and organic matter of 2.3–2.6%. Soluble salts were significantly higher in BS (1.25–1.3 mS/cm) compared to AS (0.16–0.31 mS/cm), indicating higher salinity levels in BS. Nitrate (NO3−) content in AS ranged from 23.2 mg/kg at 0–15 cm depth to 36.9 mg/kg at 30–45 cm depth, while in BS, it varied from 9.6 mg/kg at 0–15 cm to 1.2 mg/kg at 30–45 cm. In AS, P content ranged from 21.7 mg/kg at 30–45 cm depth to 24.4 mg/kg at 0–15 cm depth. The BS showed lower P levels, varying from 6.6 mg/kg at 0–15 cm to 3 mg/kg at 30–45 cm. For potassium (K+), AS contained 679–734 mg/kg across depths, while BS levels declined from 713 mg/kg at 0–15 cm to 531 mg/kg at 30–45 cm. Agricultural soil (AS) showed relatively low SO42− levels across depths (ranging from 47 mg/kg to 52 mg/kg), while BS exhibited extremely high SO42− levels, particularly at deeper depths (ranging from 3442 mg/kg at 0–15 cm to 4657 mg/kg at 15–30 cm) due to local geological conditions. These high SO42− concentrations in BS, coupled with the elevated salinity levels, can potentially hinder plant growth by causing sulfate toxicity and disrupting the uptake of essential nutrients such as nitrogen [36].
Sodium (Na+) levels were consistent across depths in AS columns, ranging from 97 mg/kg to 108 mg/kg. In contrast, BS exhibited higher Na+ levels, increasing from 140 mg/kg at 0–15 cm depth to 219 mg/kg at 30–45 cm depth. The high Na+ content in BS highlights the need for effective compost treatments with lower Na+ or soluble salt to mitigate soil salinity. Chloride (Cl−) content followed a similar trend of Na+ for BS and AS quality. The observed increase in Na+ and SO42− concentrations in BW treatments highlight potential risks to soil health and plant growth. For chili peppers, Na+ levels exceeding 115–230 mg/L in irrigation water can negatively impact growth, causing ion imbalances, leaf chlorosis, and reduced biomass accumulation due to osmotic stress [37]. Additionally, high Na+ levels can deteriorate soil structure, leading to reduced water infiltration, increased compaction, and lower microbial activity due to restricted aeration [38]. Sodium (Na+) toxicity in plants can cause leaf chlorosis, stunted growth, and reduced biomass accumulation due to ionic imbalance and osmotic stress [39]. Sulfate (SO42−) levels commonly range from 0 to 960 mg/L in irrigation water, and excessive concentrations may disrupt nutrient uptake, particularly NO3− and P, leading to competitive inhibition and potential sulfur toxicity under prolonged exposure [37]. The shift in soil chemistry may also select for salt-tolerant microbial species, potentially altering microbial diversity, terrestrial carbon cycling, and nutrient cycling [40,41].
These factors underscore the importance of soil amendments such as compost, which can help buffer salinity effects by improving soil structure, water retention, and microbial resilience. In AS, Ca2+ content was consistently high, ranging from 5568 mg/kg at 30–45 cm depth to 5762 mg/kg at 0–15 cm depth. The BS exhibited extremely high Ca2+ levels, increasing from 11,550 mg/kg at 0–15 cm to 17,960 mg/kg at 30–45 cm depth. In AS, Mg2+ content ranged from 397 mg/kg at 15–30 cm depth to 412 mg/kg at 0–15 cm depth, while, in BS, Mg2+ levels ranged from 214 mg/kg at 0–15 cm to 267 mg/kg at 30–45 cm depth. Copper (Cu) levels were higher in AS (0.9–1.29 mg/kg) than in BS (0.66–0.72 mg/kg). Iron (Fe) was also higher in AS (5.9 mg/kg) than in BS (2–2.4 mg/kg). Manganese (Mn) levels were the highest in AS (18.9 mg/kg at 0–15 cm), decreasing with depth, while BS ranged from 2.6 to 6.5 mg/kg. In AS, CEC values ranged from 33.4 meq/100 g (meq-milliequivalent) at 30–45 cm depth to 34.6 meq/100 g at 0–15 cm depth. The BS showed significantly higher CEC values, ranging from 62 meq/100 g at 0–15 cm to 94.3 meq/100 g at 30–45 cm depth. The high CEC in BS indicates a greater nutrient retention capability, essential for supporting plant growth under various compost treatments.
The JC exhibited a slightly basic pH (7.7) and high organic matter content (41.5%), indicating its rich nutrient profile, which contrasts with the more alkaline pH (8.4) and lower organic matter (24.5%) of the (MC). Nitrate (NO3−) levels were significantly higher in JC (82.5 mg/kg) compared to MC (0.7 mg/kg). These findings indicate that JC contributes significantly to NO3− content, enhancing soil fertility. Phosphorus (P) levels were higher in the JC (193.1 mg/kg) than in the MC (36.5 mg/kg). Johnson-Su compost (JC) significantly boosts P levels, which are crucial for plant growth and development. Mulch compost (MC) contained significantly more K+ (1451 mg/kg) compared to JC (163 mg/kg). The high potassium and iron content observed in MC may be attributed to the composition of its raw materials, particularly decomposed plant matter and aged bark, which naturally contain these nutrients.
The sodium (Na+) content was notably higher in MC (830 mg/kg) compared to JC (107 mg/kg), which exhibited significantly lower SO42− levels (116 mg/kg) compared to MC (344 mg/kg). Chloride (Cl−) levels were significantly higher in MC (919.7 mg/kg) compared to JC (44.2 mg/kg). The high Cl− content in BS, along with the contribution from MC, necessitates efficient treatments to effectively manage salt buildup. Calcium (Ca2+) levels were significantly higher in JC (4417 mg/kg) than in MC (2832 mg/kg). Similarly, Mg2+ levels were higher in JC (420 mg/kg) compared to MC (274 mg/kg). High levels of Ca2+ and Mg2+ can enhance soil aggregation, thereby reducing soil dispersion and improving permeability, which is critical in managing salinity [42]. This is particularly important in relation to the sodium adsorption ratio (SAR), which measures the potential for Na+ to adversely affect soil structure. High concentrations of Ca2+ and Mg2+ can lower SAR values, thereby mitigating the negative impacts of Na+ on soil structure [43]. This makes compost treatments like JC beneficial in enhancing soil structure and mitigating the adverse effects of salinity, especially in soils irrigated with brackish water. Mulch compost (MC) had more Cu (1.07 mg/kg) than JC (0.16 mg/kg). Copper (Cu) is essential for enzyme activity, photosynthesis, and lignin synthesis, contributing to plant structure and resistance to diseases [44]. However, excess Cu can lead to toxicity, negatively impacting root development. Mulch compost (MC) contained substantially more Fe (41.9 mg/kg) than JC (4.7 mg/kg). Iron (Fe) is crucial for chlorophyll synthesis and electron transfer in photosynthesis, ensuring healthy plant growth [45]. Mulch compost (MC) contained more Mn (4.4 mg/kg) compared to JC (1.7 mg/kg). Cation Exchange Capacity (CEC) was higher in JC (26.5 meq/100 g) than in MC (23.8 meq/100 g). The combination of high organic matter, Ca2+, and Mg2+ content, and CEC makes JC an ideal treatment for improving soil health and plant growth under brackish water conditions, aligning with previous findings that highlight the benefits of organic composts in saline soil management [9].

3.2. Impact of Compost Treatments on Soil Microbial Biomass, Biological Nutrients, and Protozoa Populations

The total fungi content was significantly higher in the BW-AS-MC treatment (301.0 mg/kg) compared to the BW-AS-NC treatment (12.3 mg/kg). Similarly, total bacteria were the highest in BW-AS-MC (399.0 mg/kg) and the lowest in BW-AS-NC (141.3 mg/kg). The AW-BS-JCI treatment also showed high fungi (204.8 mg/kg) and bacteria (320.3 mg/kg) levels, indicating that JCI enhances microbial biomass (Table 3). The initial conditions in Table S1 reveal that JC had higher total fungi (2951 mg/kg) and bacteria (1589 mg/kg) compared to MC (1386 mg/kg and 682 mg/kg, respectively). The F:B ratio was the highest in BW-AS-JCI (0.8), indicating a balanced microbial community. These trends suggest that compost treatments, especially JC, significantly boost microbial populations. This result aligns with previous findings showing that organic composts, especially those rich in organic matter like JC, promote microbial diversity and abundance by improving the soil’s physical and chemical properties [12,13]. In contrast, treatments like BW-AS-NC (0.1) and BW-BS-MC (0.1) showed lower F:B ratios, suggesting bacterial dominance, which can sometimes lead to a faster depletion of organic matter without sufficient fungal decomposition [46]. A balanced F:B ratio is essential for healthy soil ecosystems, as fungi contribute to organic matter breakdown, while bacteria are important for nutrient mineralization [46]. Biologically active compost, enriched with fungi and other beneficial microbial communities (F:B = 1.86 for JC), combined with high organic matter, Ca2+ and Mg2+, and enhanced CEC, provides JC as an ideal solution for sustainable agriculture.
Biological carbon was the highest in BW-AS-MC (86.23 g/m2) and lowest in BW-BS-NC (18.05 g/m2). Similarly, biological nitrogen was the highest in BW-AS-MC (11.07 g/m2) and the lowest in BW-BS-NC (3.37 g/m2). These increases in biological carbon and nitrogen in compost-treated soils, particularly in the MC and JCI treatments, are consistent with studies that link organic amendments to increased soil organic matter, carbon sequestration, and nitrogen availability [8]. The initial biological carbon and nitrogen contents were higher in JC (559.34 g/m2 and 51.27 g/m2, respectively) compared to MC (254.77 g/m2 and 22.50 g/m2, respectively), highlighting the superior nutrient enrichment potential of JCI. This enrichment is critical for improving long-term soil fertility, especially in degraded soils. Aerobic fungi and bacteria were the highest in AW-BS-MCI (33.1 mg/kg and 7.9 mg/kg, respectively) and the lowest in treatments like BW-BS-JCI (3.6 mg/kg and 7.2 mg/kg, respectively). The ratio of aerobic fungi to aerobic bacteria was the highest in BW-BS-MC (6.2) and the lowest in BW-BS-JCI (0.5). These results suggest that MCI, with its higher fungal biomass, may be better suited for soils where fungal activity is crucial for organic matter decomposition, while JCI promotes a more balanced microbial community, which is beneficial for nutrient cycling [4,15]. This balance is important because fungi contribute to long-term soil structure stability by forming stable aggregates, while bacteria are critical for rapid organic matter turnover [46,47].
Flagellate, amoebae, and ciliate populations were the highest in AW-AS-MCI, with flagellates reaching 17,467 MPN/g and amoebae at 7250 MPN/g. In contrast, AW-BS-NC showed no detectable levels of protozoa. These findings suggest that compost treatments, especially MC and MCI, significantly enhance protozoa populations, which play a crucial role in nutrient cycling and soil fertility. Protozoa play a key role in nutrient cycling by feeding on bacteria, thereby releasing nitrogen and other nutrients into the soil in a form that is readily available to plants [48,49]. These findings align with previous studies that emphasize the critical role of organic amendments in improving microbial diversity, soil health, and productivity [50].
The observed increases in microbial biomass and diversity in compost-treated soils can be attributed to the role of compost in providing organic carbon, essential nutrients, and favorable microbial habitats. Johnson-Su compost (JCI), in particular, supports a fungal-dominant environment, which enhances soil aggregation, promotes long-term organic matter stability, improves carbon sequestration, and reduces nutrient leaching. The differences observed between compost types suggest that surface-applied mulch compost (MC) fosters bacterial dominance, leading to rapid organic matter turnover, while JCI promotes microbial balance, ensuring sustained nutrient availability. These findings highlight the importance of selecting compost amendments based on the desired microbial and soil health outcomes. Future studies should investigate the long-term impacts of different compost applications on microbial succession and their interactions with soil physicochemical properties to optimize compost use in sustainable agriculture.

3.3. Effects of Compost Treatments on Soil NO3−, P, and Organic Matter Content

Figure 1 and Table S2 present the differences in mean ± standard deviation for the mass of NO3−, P, and organic matter (OM) percentages, as well as the mass of P in the soil at the end of the experiment. Nitrate (NO3−) levels varied significantly across treatments and depths. The BW-BS-JCI treatment had the highest NO3− content at 0–15 cm (21.74 ± 4.1 mg) and 15–30 cm (3.35 ± 2.8 mg), indicating that JCI significantly enhances NO3− availability and retention in the soil (Figure 2a and Table S2). In contrast, BW-BS-MC showed the lowest NO3− levels at 0–15 cm (0.67 ± 0.2 mg) and 15–30 cm (1.05 ± 0.4 mg), suggesting less effective NO3− retention. These results are consistent with the initial soil conditions (Table 2 and Table S3), where JC had a higher NO3− content than MC, underscoring the effectiveness of JCI in maintaining soil fertility. Nitrite (NO2−) was not observed in soil but was detected in leached water (Figure S2). This suggests that nitrite is either being rapidly converted to other forms of N or that its presence is minimal due to efficient N cycling facilitated by the compost treatments [51].
Organic Matter (OM) was the highest in AW-AS-MC (5.90 ± 0.2% at 0–15 cm), highlighting the effectiveness of MC in enhancing soil organic matter. In contrast, BW-BS-NC showed the lowest OM content (2.61 ± 0.1% at 0–15 cm), indicating the necessity of compost application for improving soil organic content (Table S2). The lack of significant variation in organic carbon content across soil depths in BS may be attributed to previous plowing, which likely resulted in a more uniform distribution of OM. The comparison in Table 2 shows that JC initially had a higher organic matter percentage (41.5%) than MC (24.5%), which translated into higher OM retention in treatments using JC. The soil factor (S) showed a significant influence on the OM and P content across all the layers of varying treatments (p < 0.05). The soil factor (S), compost (C), and interaction of S × C played a significant role in the mass of NO3− in the top layer of the soil (p < 0.05).
Phosphorus (P) levels varied significantly across treatments and depths. The highest P content was detected in AW-AS-JCI (52.9 ± 5.8 mg at 0–15 cm, 45.1 ± 8.8 mg at 15–30 cm, and 35.5 ± 2.7 mg at 30–45 cm), again emphasizing the efficacy of JCI in enhancing soil P levels across all depths (Table S2 and Figure 2c). This is critical, as P is a limiting nutrient in many soils, and its availability is crucial for plant growth, root development, and energy transfer processes [52]. Conversely, BW-BS-NC had the lowest P levels (11.8 ± 3.1 mg at 0–15 cm, 8.9 ± 1.0 mg at 15–30 cm, and 9.2 ± 1.9 mg at 30–45 cm), indicating the critical role of compost in P enrichment. The initial high P content in JC (193.1 mg/kg) compared to MC (36.5 mg/kg) is reflected in the enhanced P levels in JCI-treated soils. The comprehensive analysis of physio-chemical and biological parameters indicates that JCI consistently improves soil health by enhancing nutrient retention and microbial activity due to high OM, higher nutrient content, and microbial composition than NC and MC. These findings align with extensive research on the benefits of organic composts in improving nutrient availability and microbial diversity in soil ecosystems [27].

3.4. Impact of Compost Treatments on Soil Cation and Anion Content Under Different Irrigation Regimes

Figure 2 and Table S4 present the differences in mean ± standard deviation for the mass of major cations (Ca2+, Mg2+, Na+) and anions (Cl−) of soil at the end of the experiment from columns of different treatments, comparing BW and AW irrigation along with four types of compost treatment and two soil types.
Calcium (Ca2+) levels varied significantly across treatments and depths (p < 0.05) due to the soil properties, where BS had significantly higher Ca2+. The highest Ca2+ content was observed in the AW-BS-MCI treatment, with values of 15,972 ± 1054 mg at 0–15 cm, and, in the BW-BS-MC treatment, with values of 22,130 ± 1078 mg at 15–30 cm and 25,105 ± 237 mg at 30–45 cm. In contrast, the BW-AS-MCI treatment had the lowest Ca2+ levels, with 6562 ± 103 mg at 0–15 cm, 6694 ± 66 mg at 15–30 cm, and 6983 ± 165 mg at 30–45 cm for BW-AS-NC treatment columns. The initial conditions (Table 2) showed that BS had a much higher Ca2+ content (11,550–17,960 mg/kg) compared to AS (5568–5762 mg/kg), indicating that compost treatments, particularly JCI, enhance Ca2+ retention in high Ca2+ soils like BS (Table S4). The C, S, and interaction of S × C indicated significant effects (p < 0.05) on Ca2+ content across all layers. This highlights the importance of organic compost in calcium retention [11]. This is crucial for maintaining soil structure and nutrient availability, particularly under high-salinity conditions where Ca2+ helps mitigate the negative effects of sodium on soil aggregation [53].
The magnesium (Mg2+) content also showed significant differences (p < 0.05) due to compost and soil between all layers. The highest levels were in AW-AS-NC (604 ± 8 mg at 0–15 cm, 604 ± 49 mg at 15–30 cm), whereas the AW-BS-JCI treatment had the lowest Mg2+ content. The initial Mg2+ levels were lower in BS (214–267 mg/kg) compared to AS (397–412 mg/kg), suggesting that BW irrigation combined with JCI helps maintain higher Mg2+ levels in AS treatment columns (Table S4). This finding aligns with previous studies that emphasize the role of organic compost in improving cation retention, which is critical for maintaining nutrient availability in brackish conditions [54]. Magnesium (Mg2+) is essential in chlorophyll production and plant enzyme activation, making its retention vital for plant health and growth [55].
Sodium (Na+) levels were the highest in the BW-AS-NC treatment, with values of 1438 ± 144 mg at 0–15 cm, and the highest values for 15–30 cm and 30–45 cm measured for the JCI-treated BW columns, indicating significant Na+ leaching to deeper layers without accumulation in the top layer. The AW-BS-MCI treatment showed the lowest Na+ levels, with 249 ± 35 mg at 0–15 cm, 180 ± 9 mg at 15–30 cm, and 174 ± 2 mg at 30–45 cm. The initial Na+ content in BS was higher (140–219 mg/kg) than in AS (97–108 mg/kg), demonstrating the importance of compost treatments, especially JCI, in mitigating Na+ accumulation. Sodium (Na+) leaching to deeper layers is important for reducing soil salinity at the root zone, which is crucial for plant health, particularly in brackish water environments where high Na+ concentrations can impair soil structure and water infiltration [56]. Significant influences on the mass of Na+ were observed for compost (p = 0.000), soil (p = 0.000), and water (p < 0.05).
Chloride (Cl−) levels were significantly higher in BW treatments, particularly in BW-BS-JCI (159.0 ± 90.5 mg at 0–15 cm and 112.4 ± 26.7 mg at 15–30 cm) (Figure 2d and Table S4). The lowest Cl− levels were found in the AW-BS-MCI treatment, with values of 15.7 ± 2.7 mg at 0–15 cm, 11.6 ± 2.4 mg at 15–30 cm, and 12.6 ± 1.9 mg at 30–45 cm. The initial Cl− levels were significantly higher in BS (44.4–114.2 mg/kg) compared to AS (21.3–38.3 mg/kg) (Table 2), suggesting that the compost treatments are crucial to leaching out Cl− to deeper layers, especially under brackish water irrigation. These results are consistent with the literature, where organic composts have been shown to improve the leaching of excess salts, such as Cl−, thereby reducing the risk of salt accumulation in the root zone as observed by Rezapour et al. [57]. The p-values for Cl− content showed a significant impact due to irrigation water type (p = 0.000) across all layers and compost in the middle and bottom layers (p < 0.05).
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Figure 2. The mass of (a) Ca2+, (b) Mg2+, (c) Na+, and (d) Cl− of soil at the end of the experiment from columns of different treatments in each layer of soil for two types of irrigation water and soil with four types of compost treatment.
Figure 2. The mass of (a) Ca2+, (b) Mg2+, (c) Na+, and (d) Cl− of soil at the end of the experiment from columns of different treatments in each layer of soil for two types of irrigation water and soil with four types of compost treatment.
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3.5. Principal Component Analysis (PCA) of Soil Ion Dynamics Under Different Treatments

Figure 3 and Table S5 present the PCA results, illustrating the clustering of treatment groups based on the multivariate impact of irrigation water, soil, and compost treatments on soil ion dynamics. The PCA was conducted to reduce the dimensionality of the data and to identify the key variables contributing to the variability in soil ion content. The PCA identified five principal components (PCs) with eigenvalues greater than 1, explaining 100% of the variance in the dataset. The PC1 and PC2 accounted for most of the variance, with 53.8% and 28.4%, respectively. This indicates that the first two components capture the most significant patterns in data related to soil ion dynamics. The PC1 was primarily associated with the total mass of OM, Ca2+, and P, with loadings of 0.471, −0.503, and 0.489, respectively. This suggests that PC1 represents a gradient of soil fertility and nutrient content, with higher values indicating greater OM and P but lower Ca2+. The PC2 was mainly associated with NO3−, Na+, and Cl− content, with loadings of 0.264, −0.680, and −0.686, respectively, indicating that PC2 represents a gradient of soil salinity for lower values.
The PCA score plot clusters the treatment groups based on irrigation water type, soil, and compost treatment. Treatments using AW generally clustered separately from those using BW, highlighting the distinct impact of water quality on soil ion dynamics. Agricultural water (AW) treatments such as AW-AS-JCI showed the highest separation along PC1, indicating enhanced organic matter and P content with reduced Ca2+ compared to other treatments. This separation supports the conclusion that JCI is highly effective in enhancing soil fertility, particularly in terms of OM and P accumulation, aligning with findings from previous studies that highlight the importance of organic compost in improving nutrient retention and reducing salinity [28]. The AW- and BS-treated columns represent high NO3− and Ca2+ and the AW- and AS-treated columns represent high OM, P, NO3− and Mg2+, whereas the BW and BS columns had higher levels of Ca2+, Na+, and Cl−, and the BW and AS columns had higher levels of OM, Mg2+, Na+, and Cl−. The AW-AS-MC and AW-AS-MCI also showed positive PC1 scores, reflecting improved soil fertility with moderate salinity levels. Conversely, the BW treatments, like BW-BS-JCI and BW-BS-MCI, showed higher PC1 scores, indicating better soil nutrient retention compared to other BW treatments. In contrast, BW-AS-NC and BW-BS-NC had negative PC1 and PC2 scores, indicating lower soil fertility and higher salinity, emphasizing the need for compost treatments to mitigate these effects. Comparing the PCA results with Table 3 and Table 4, the enhanced clustering of JCI treatments aligns with the higher NO3−, OM, and P contents observed in Tables S2 and S3, supporting the PCA findings of improved soil quality in these treatments.

3.6. Nutrient Uptake and Plant Performance Under Soil, Compost, and Irrigation Treatments

The bar charts in Figure 4 present the concentrations of various nutrients in chili pepper plant tissues grown under different treatments using BW and AW for irrigation. The treatments include combinations of NC, MC, JCI, and MCI with AS and BS soils. Iron (Fe) levels (Figure 4a) were significantly higher in the BW-BS-MCI treatment, indicating that the combination of BW and MCI effectively increases iron uptake by chili pepper. Studies have shown that organic amendments improve the bioavailability of micronutrients like Fe by releasing organic acids that help chelate these metals [58]. Manganese (Mn) levels (Figure 4b) were highest in the BW-AS-MCI treatment, showing that mulch compost under BW irrigation significantly enhances manganese uptake, suggesting that the combination of BW and compost treatments improves Mn availability by increasing microbial activity and organic matter, which release Mn from soil particles for plant absorption [59]. The calcium (Ca2+) content (Figure 4c) was consistently higher in all the treatments, with the highest levels measured in AW-BS-JCI. This indicates that BW combined with JCI enhanced Ca2+ uptake in chili pepper. This suggests that JCI enhances Ca2+ uptake in plant tissues, possibly by improving soil structure and CEC, which increases the availability of Ca2+ to plants. Composts have been shown to buffer against Ca2+ leaching and enhance the plant’s ability to absorb this important nutrient under brackish water irrigation conditions [11]. Magnesium (Mg2+) levels (Figure 4d) were the highest in BW-BS-MCI, reflecting the effectiveness of MCI in increasing Mg2+ uptake under BW irrigation. Brackish water, which typically contains higher salt concentrations, can enhance Mg2+ uptake due to competitive interactions between cations in the soil solution [60]. Agricultural water (AW) treatments generally exhibited lower Mg2+ levels, with the least amount in AW-BS-MC.
The potassium (K+) content (Figure 4e) varied significantly across treatments, with the highest levels in BW-AS-MCI. This suggests that MCI under BW irrigation was effective in enhancing potassium uptake. The application of mulch compost under BW irrigation likely enhanced K uptake by improving soil structure and increasing microbial activity, which, in turn, improved nutrient cycling and increased the K content in MC (Table 2). Potassium (K+) is a key nutrient involved in osmotic regulation, especially in brackish environments, and organic amendments, like compost, can help plants maintain K homeostasis under salt stress [61].
Sodium (Na+) levels (Figure 4f) were significantly higher in the AS and MCI treatments, particularly in AW-AS-MCI, due to the high level of Na+ in MC (although AW had lower Na+ levels, as shown in Table 2). The BW and JCI treated columns demonstrated lower Na+ levels despite high Na+ levels in BW (Figure S2). Johnson-Su compost incorporation alleviated the Na toxicity of plants by chelating the Na+ with a carboxylic group present in the compost, which is indicated by higher Na+ in the soil (Table S4) [12]. The Na+/K+ ratio (Figure 4h) was the highest in AW-AS-MCI, reflecting the high Na+ content and low K+ availability in this treatment due to high Na+ in MC (Table 2). The BS treatments exhibited lower Na+/K+ ratios, showing the most balanced ratio, ensuring healthy plant growth.
Copper (Cu) levels (Figure 4g) were the highest in BW-AS-MCI. AW treatments generally showed lower copper levels, with the least amount in BW-BS-JCI and AW-BS-JCI. The highest Cu levels in the BW-AS-MCI (34.24 mg/kg) treatment suggest that the combination of BW and MCI (Table 2) enhanced Cu bioavailability in plant tissues, exceeding the toxicity concentration of 20–30 mg/kg [62]. Higher Cu levels can induce toxicity, interfering with iron uptake and causing oxidative stress, root damage, and growth inhibition [63]. Root length was lower for BW-AS-MCI columns compared to other BW columns (Table 4).
The total nitrogen levels (Figure 4i) were the highest in AW-BS-MCI. Brackish water (BW) treatments combined with MC and JCI or AS showed relatively higher N levels, emphasizing the presence of NO3− in compost and soil to maintain N levels. Compost can supply sufficient nitrogen to plants even under BW irrigation by slowly releasing nitrogen from organic matter during decomposition [17].

3.7. Impact of Irrigation Water, Soil, and Compost Treatments on Plant Growth Parameters

Table 4 reports the differences in mean ± standard deviation for various plant growth parameters of chili pepper grown under different treatments. The chlorophyll content was the highest in the AW-BS-JCI treatment (45.2 ± 2.6), indicating that the combination of AW and JCI enhanced chlorophyll production in chili pepper. The lowest chlorophyll content was observed in the AW-AS-NC treatment (26.3 ± 2.5), suggesting that AW without compost supplementation results in lower chlorophyll levels, likely due to the lower availability of essential nutrients, such as nitrogen and magnesium, which are critical for chlorophyll synthesis [55]. The JCI columns significantly increase chlorophyll irrespective of IW and S. The p-values indicate the significant effects of compost (p = 0.000), soil quality (p = 0.011), and the interaction of IW × S and IW × S × C (p < 0.05) on chlorophyll content. Johnson-Su compost incorporation (JCI) consistently improved chlorophyll levels across irrigation and soil quality, supporting the role of organic matter in improving nutrient availability [10]. Plant height was significantly influenced by IW, S, C, and their interaction, with the tallest plants measured in the AW-BS-JCI treatment (27.4 ± 1.5 cm). The shortest plants were found in the AW-AS-NC treatment (8.2 ± 0.4 cm). The results suggest that JCI significantly promotes plant height under AW and BW irrigation. Significant interactions were observed for compost (p = 0.000), soil quality (p = 0.000), water (p = 0.018), and the interaction of IW × S × C (p = 0.006).
The amount of wet biomass was the highest in the AW-BS-JCI treatment (30.0 ± 2.0 g) and the BW-BS-JCI treatment (29.7 ± 1.5 g), indicating the effectiveness of this treatment in enhancing biomass production by JCI irrespective of soil fertility and irrigation water. This suggests that the biologically active and beneficial chemical composition can overcome concerns regarding soil and water quality. The lowest amount of wet biomass was recorded in the AW-AS-NC treatment (1.3 ± 0.2 g), emphasizing the importance of compost application in improving plant biomass despite the use of AW and AS. The p-values for compost (p = 0.000) and soil quality (p = 0.000) and their interactions (p < 0.05) were all significant. The root length was the longest in the BW-BS-JCI, AW-BS-NC, and AW-BS-JCI treatments (45.0 ± 0.0 cm), suggesting that JCI treatment promotes extensive root development along with BS. The shortest root length was found in the AW-AS-NC treatment (10.5 ± 0.9 cm), highlighting the adverse effects of inadequate compost application on root growth. Significant interactions were observed for the factors and their interactions (p < 0.05). The MC treatment had the higher plant growth parameters next to JCI.
High organic content in JCI increases CEC, which improves the retention and availability of key nutrients like nitrogen, potassium, magnesium, and calcium, all of which are critical for plant growth under brackish conditions. This is consistent with findings that organic matter amendments help plants better tolerate salinity by improving soil–water relations and nutrient availability [12,64].
Table 4. Differences in mean ± standard deviation for the plant growth analysis of different treatments (three-way ANOVA) columns with different water, soil, and soil compost treatments for BW and AW irritation.
Table 4. Differences in mean ± standard deviation for the plant growth analysis of different treatments (three-way ANOVA) columns with different water, soil, and soil compost treatments for BW and AW irritation.
TreatmentChlorophyllPlant Height (cm)Wet Biomass (g)Root Length (cm)
BW-AS-NC36.0 ± 1.0 abcd14.7 ± 0.1 def4.7 ± 1.2 ef15.3 ± 0.6 f
BW-AS-MC34.3 ± 2.3 bcde11.0 ± 2.2 efgh4.5 ± 1.3 ef15.5 ± 0.4 f
BW-AS-JCI43.2 ± 1.2 ab23.3 ± 4.3 abc21.7 ± 2.5 b32.3 ± 0.6 c
BW-AS-MCI28.5 ± 3.9 de5.8 ± 0.9 h1.9 ± 0.1 f10.7 ± 1.8 g
BW-BS-NC32.8 ± 2.9 cde12.9 ± 0.8 defg3.7 ± 0.6 ef30.5 ± 1.6 cd
BW-BS-MC37.9 ± 1.1 abc17.8 ± 1.9 bcd9.3 ± 1.2 cd41.2 ± 1.0 b
BW-BS-JCI42.9 ± 3.9 ab23.9 ± 1.1 ab29.7 ± 1.5 a45.0 ± 0.0 a
BW-BS-MCI30.5 ± 3.4 cde10.2 ± 1.2 fgh2.8 ± 1.0 ef20.7 ± 0.3 e
AW-AS-NC26.3 ± 2.5 e8.2 ± 0.4 gh1.3 ± 0.2 f10.5 ± 0.9 g
AW-AS-MC37.6 ± 2.3 abcd17.2 ± 1.3 cde9.3 ± 1.2 cd16.6 ± 0.9 f
AW-AS-JCI37.3 ± 2.3 abcd25.6 ± 2.4 a20.0 ± 3.5 b27.9 ± 2.3 d
AW-AS-MCI30.8 ± 4.2 cde8.0 ± 0.7 gh2.0 ± 0.5 f14.8 ± 1.7 f
AW-BS-NC36.3 ± 6.4 abcd17.6 ± 4.5 cd6.7 ± 1.2 de45.0 ± 0.0 a
AW-BS-MC39.1 ± 2.5 abc17.6 ± 2.2 cd11.3 ± 1.2 c42.3 ± 1.2 ab
AW-BS-JCI45.2 ± 2.6 a27.4 ± 1.5 a30.0 ± 2.0 a45.0 ± 0.0 a
AW-BS-MCI28.5 ± 1.5 de9.9 ± 0.5 fgh2.9 ± 1.1 ef16.2 ± 0.8 f
p-value    IW0.4870.0180.1350.009
             S0.0110.0000.0000.000
             C0.0000.0000.0000.000
                    IW × S0.0410.4470.1220.000
                    IW × C0.1710.0780.0080.000
                 S × C0.4040.3980.0000.000
                          IW × S × C0.0040.0000.0060.000
Mean values indicated by different superscript letters are significantly different from each other (p < 0.05); a—higher mean value; b—lower mean value; n = 3 replicates.

4. Conclusions

This study demonstrated that compost treatments, particularly biologically active JCI, effectively mitigated the negative effects of BW irrigation on soil health and plant growth. Johnson-Su compost incorporation (JCI) significantly enhanced microbial biomass, especially beneficial fungi, aerobic bacteria, and protozoa, contributing to improved soil structure, nutrient retention, and water infiltration.
Differences in compost application methods were also observed. While MC moderately improved microbial activity, MCI negatively affected plant growth, likely due to microbial imbalance and nutrient overload in the root zone. These findings highlight the importance of aligning compost type and application method with specific soil and crop conditions.
In low-fertility desert soils, JCI improved soil organic matter, CEC, and microbial diversity, while facilitating the leaching of harmful salts from the root zone. This resulted in better root development, higher chlorophyll content, and increased biomass, demonstrating JCI’s potential for improving productivity in saline, arid environments.
Overall, this study underscores the role of biologically active compost in enhancing soil fertility and resilience under challenging irrigation conditions. Further research should explore the dynamic interactions between microbial communities and soil chemistry to optimize compost strategies across diverse agricultural systems.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/agriculture15070734/s1, Figure S1: Major ion concentrations in different types of irrigation water; Figure S2: Mass of NO3− and NO2− leached from columns of different treatments for two types of irrigation water and soil with four types of compost treatment; Table S1: Biological parameters of compost and initial soil of each layer; Table S2: Differences in mean ± standard deviation for the mass of NO3−, P, and Organic matter % values of soil at the end of the experiment from columns of different treatments (Three-way ANOVA) for two types of irrigation water and soil with four types of compost treatment; Table S3: Mass/percentage of OM (%) and NO3− (mg)in the topsoil layer (0–15 cm) for different compost treatments and soil: Table S4: Differences in mean ± standard deviation for the mass of major cations and anion of soil at the end of the experiment from columns of different treatments (Three-way ANOVA) for two types of irrigation water and soil with four types of compost treatment; Table S5: Principal Component Analysis: NO3−, OM, Cl−, Na+, Ca2+, Mg2+, and P.

Author Contributions

Conceptualization, S.S., S.J.W., M.F.A., B.S. and P.X.; methodology, S.S. and P.X.; validation, S.S., M.F.A. and P.X.; formal analysis, S.S. and P.X.; investigation, S.S. and P.X.; resources, B.S., M.F.A. and P.X.; data curation, S.S.; writing—original draft preparation, S.S. and P.X.; writing—review and editing, S.S., S.J.W., M.F.A., B.S. and P.X.; visualization, S.S.; supervision, P.X.; project administration, B.S., M.F.A. and P.X.; funding acquisition, B.S., M.F.A., S.J.W. and P.X. All authors have read and agreed to the published version of the manuscript.

Funding

Funding support was provided by the Innovations at the Nexus of Food, Energy, and Water Systems (INFEWS) program of the National Science Foundation under award number 1856052 to the University of North Texas (UNT), New Mexico State University (NMSU), and Colorado State University (CSU).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

The authors thank the Brackish Groundwater National Desalination Research Facility (BGNDRF) for hosting the field experiment and David Johnson for providing Johnson–Su compost.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 15 00734 g001
Figure 1. The mass of (a) NO3−, (b) P, and (c) percentage of OM of soil at the end of the experiment from columns of different treatments in each layer of soil for two types of irrigation water and soil with four types of compost treatment.
Figure 1. The mass of (a) NO3−, (b) P, and (c) percentage of OM of soil at the end of the experiment from columns of different treatments in each layer of soil for two types of irrigation water and soil with four types of compost treatment.
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Figure 3. Principal Component Analysis (PCA) score plot displaying the clustering of treatment groups based on the multivariate impact of irrigation water, soil, and compost treatments on soil ion dynamics. Each point represents a treatment combination, with BW and AW treatments distinguished by color.
Figure 3. Principal Component Analysis (PCA) score plot displaying the clustering of treatment groups based on the multivariate impact of irrigation water, soil, and compost treatments on soil ion dynamics. Each point represents a treatment combination, with BW and AW treatments distinguished by color.
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Figure 4. Levels of (a) Fe (mg/kg), (b) Mn (mg/kg), (c) Ca (%), (d) Mg (%), (e) K (%), (f) Na (%), (g) Cu (mg/kg), (h) Na+/K+ ratio, and (i) total nitrogen (mg/kg) in plant tissues under different treatment combinations with different water, different soil, and different soil compost treatments for BW and AW irrigation.
Figure 4. Levels of (a) Fe (mg/kg), (b) Mn (mg/kg), (c) Ca (%), (d) Mg (%), (e) K (%), (f) Na (%), (g) Cu (mg/kg), (h) Na+/K+ ratio, and (i) total nitrogen (mg/kg) in plant tissues under different treatment combinations with different water, different soil, and different soil compost treatments for BW and AW irrigation.
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Table 1. Types of treatment combinations studied in the greenhouse experiments with triplicate soil columns for each condition and the notations for the columns.
Table 1. Types of treatment combinations studied in the greenhouse experiments with triplicate soil columns for each condition and the notations for the columns.
Water (IW)Brackish Water—BW (2958 ± 51 µS/cm)Agricultural Water—AW (796 ± 7 µS/cm)
Soil (S)Agricultural Soil (AS)BGNDRF Soil (BS)Agricultural Soil (AS)BGNDRF Soil (BS)
Compost (C)NCMCJCIMCINCMCJCIMCINCMCJCIMCINCMCJCIMCI
TreatmentBW-AS-NCBW-AS-MCBW-AS-JCIBW-AS-MCIBW-BS-NCBW-BS-MCBW-BS-JCIBW-BS-MCIAW-AS-NCAW-AS-MCAW-AS-JCIAW-AS-MCIAW-BS-NCAW-BS-MCAW-BS-JCIAW-BS-MCI
Note: NC—no compost, MC—surface-applied mulch compost, JCI—Johnson–Su compost incorporated into the soil, MCI—mulch compost incorporated into the soil.
Table 2. Physio-chemical parameters of compost and soil.
Table 2. Physio-chemical parameters of compost and soil.
ParametersCompost TypeASBS
JCMC0–15 cm15–30 cm30–45 cm0–15 cm15–30 cm30–45 cm
pH (1:1)7.78.48.18.28.17.97.98.0
Soluble salts (mS/cm) (1:1)0.360.790.310.160.251.301.251.30
Organic matter (%)41.524.53.23.13.02.62.32.6
NO3− (mg/kg)82.50.723.225.236.99.64.21.2
P (mg/kg)193.136.524.423.321.76.64.63.0
K (mg/kg)1631451734679687713577531
Na+ (mg/kg)1078301089797140213219
Cl− (mg/kg)44.2919.738.325.721.3114.260.244.4
Ca2+ (mg/kg)4417283257625648556811,55017,14017,960
Mg2+ (mg/kg)420274412397403214222267
Cu (mg/kg)0.161.071.290.900.750.720.680.66
Fe (mg/kg)4.741.95.93.74.02.32.02.4
Mn (mg/kg)1.74.418.94.02.46.55.52.6
SO42− (mg/kg)116344524749344246573949
CEC/Sum of Cations (meq/100 g)26.523.834.633.733.462.090.094.3
Note: 1:1—Compost: Water.
Table 3. Biological parameters of different treatment columns after 72 days of irrigation.
Table 3. Biological parameters of different treatment columns after 72 days of irrigation.
Total Fungi (mg/kg)Total Bacteria (mg/kg)Total F: B Biological Carbon (g/m2)Biological Nitrogen (g/m2)Biological Carbon: Biological NitrogenAerobic Fungi (mg/kg)Aerobic Bacteria (mg/kg)Aerobic Fungi: Aerobic BacteriaFlagellates MPN/gAmoebae MPN/gCiliates MPN/g
BW-AS-NC12.3141.30.118.933.535.48.53.92.26206.66206.60.0
BW-AS-MC301.0399.00.886.2311.077.822.810.82.18453.82240.52240.5
BW-AS-JCI120.8152.00.833.614.247.916.26.62.47951.02107.30.0
BW-AS-MCI13.4154.40.120.683.865.420.94.74.4780.42132.70.0
BW-BS-NC11.7134.90.118.053.375.418.25.03.60.0200.80.0
BW-BS-MC14.7148.90.120.163.735.430.24.96.20.0222.10.0
BW-BS-JCI180.1306.70.659.988.307.23.67.00.50.0213.5213.5
BW-BS-MCI134.3285.50.551.727.586.826.57.73.40.0765.519,017.6
AW-AS-NC12.1139.60.118.693.495.422.04.84.67658.4227.6742.7
AW-AS-MC173.9309.10.659.518.337.120.79.62.2741.22025.70.0
AW-AS-JCI48.4253.90.237.246.455.815.16.92.2728.3728.30.0
AW-AS-MCI10.0132.20.117.513.295.327.06.54.217,466.97249.80.0
AW-BS-NC195.9319.70.663.528.687.317.43.55.00.00.00.0
AW-BS-MC10.5139.10.118.443.475.325.82.112.60.0226.80.0
AW-BS-JCI204.8320.30.664.698.747.45.45.71.00.0211.4211.4
AW-BS-MCI89.2229.30.439.236.016.533.17.94.20.0733.27560.1
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Suvendran, S.; Acevedo, M.F.; Smithers, B.; Walker, S.J.; Xu, P. Soil Fertility and Plant Growth Enhancement Through Compost Treatments Under Varied Irrigation Conditions. Agriculture 2025, 15, 734. https://doi.org/10.3390/agriculture15070734

AMA Style

Suvendran S, Acevedo MF, Smithers B, Walker SJ, Xu P. Soil Fertility and Plant Growth Enhancement Through Compost Treatments Under Varied Irrigation Conditions. Agriculture. 2025; 15(7):734. https://doi.org/10.3390/agriculture15070734

Chicago/Turabian Style

Suvendran, Subanky, Miguel F. Acevedo, Breana Smithers, Stephanie J. Walker, and Pei Xu. 2025. "Soil Fertility and Plant Growth Enhancement Through Compost Treatments Under Varied Irrigation Conditions" Agriculture 15, no. 7: 734. https://doi.org/10.3390/agriculture15070734

APA Style

Suvendran, S., Acevedo, M. F., Smithers, B., Walker, S. J., & Xu, P. (2025). Soil Fertility and Plant Growth Enhancement Through Compost Treatments Under Varied Irrigation Conditions. Agriculture, 15(7), 734. https://doi.org/10.3390/agriculture15070734

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