Effects of Mulching on Soil Properties and Yam Production in Tropical Region

Abstract

Mulching plays a pivotal role in modern sustainable agriculture, offering a versatile solution to enhance soil quality, improve soil health, conserve resources, and optimize crop performance. This study examined the effects of various mulching materials on soil properties, seasonal variations in soil and environmental variables, and yam production in a tropical environment, with a focus on sustainable agricultural practice. We applied a range of mulch treatments, including black polythene, weedmat, sugarcane straw, organic compost, cowpea-live, juncao grass, sawdust, and a control with no mulch. The results indicated that the organic compost mulch significantly increased soil pH and soil electrical conductivity (EC). The control treatment resulted in the highest soil moisture content, while the highest soil temperature were recorded for the black polythene and organic compost mulch treatments. The organic compost mulch enhanced the soil organic carbon (SOC) content, soil available phosphorus (SAP) content, and soil exchangeable calcium (SECa) content. The weedmat mulch showed the highest soil exchangeable potassium (SEK) content, and the control treatment exhibited the highest soil exchangeable magnesium (SEMg) and sodium (SENa) content. In terms of micronutrients, the sawdust mulch and black polythene mulch significantly increased soil exchangeable iron (SEFe) and copper (SECu) levels, respectively. Notable seasonal variations in soil pH, temperature, and environmental humidity were observed during the crop period. The soil pH fluctuated from slightly acidic levels in August 2023 to neutral levels in October, and then decreased to slightly acidic levels in early 2024 before stabilizing by March 2024. The soil temperature peaked in November and dropped in January, while the environmental humidity ranged from 48.25% in December to 76.33% in February. The study demonstrated that the organic compost mulch stood out as an advantageous choice because of its capacity to enhance the soil’s properties and offer a balanced nutrient mix, making it particularly beneficial for yam cultivation. It also proved to be a reliable and balanced option to enhance soil quality with stable soil quality indices (SQIs). The weedmat mulch proved to be highly effective in enhancing yam growth and productivity. The weedmat mulch is the most profitable and cost-effective option for yam cultivation, providing the highest net returns and strong financial viability. This study emphasizes the value of choosing the right mulching materials to support soil quality, crop productivity, and economic returns in tropical settings, making strides toward more sustainable farming practices.

1. Introduction

Yam (Dioscorea alata L.) is a staple crop in many tropical regions, holding significant agronomic and economic importance. It is considered as a staple food in traditional diets and is an important source of income for many communities; so, enhancing yam production is vital for ensuring food security and supporting rural livelihoods [1]. Soil quality and soil health are the key determinants of crop productivity. Soil quality and soil health both emphasize the soil’s ability to function within its ecosystem boundaries, supporting plant and animal productivity, maintaining air and water quality, and promoting overall health. Soil is viewed as a finite, living resource, essential for sustaining biological productivity and the environment [2]. The soil’s physico-chemical properties contribute to soil quality and soil health by influencing soil functionality and ecosystem sustainability. The key soil properties such as structure, texture, and pH influence nutrient availability and soil health. A well-structured soil with good aggregation facilitates better water infiltration and root penetration, while a balanced pH ensures that essential nutrients like N, P, and K are available to plants [3]. Soil nutrients play a crucial role in supporting plant growth and microbial activity. Adequate nutrient levels enhance plant health and productivity, which in turn promotes soil structure through root systems and increases the soil organic matter (SOM) content [4]. The presence of nutrients also supports microbial life, which is essential for nutrient cycling and the decomposition of organic matter (OM), thereby maintaining soil fertility and enhancing soil biodiversity [5]. Nutrient management helps prevent soil degradation and reduces the risk of environmental issues such as nutrient runoff and erosion [6]. Thus, optimal soil properties and nutrient levels are fundamental for sustaining soil quality and health, promoting both agricultural productivity and environmental stewardship.

Mulching is a widely adopted practice that significantly influences soil properties, thereby affecting crop performance [7]. It involves the covering of the soil surface with organic or inorganic materials. This practice mimics the natural forest floor, which is continuously covered with leaf litter and organic residues. Mulches regulate microbial diversity and soil aeration, creating a conducive environment for microbial activity, which in turn enhances nutrient cycling [8]. The choice of mulch is critical, as different types have varied effects on the soil’s physico-chemical and biological properties. Organic mulches such as crop residues, compost, and straw decompose over time, which adds OM to the soil. This process improves soil structure and aggregate stability, reduces soil compaction, and promotes soil biological activity [9]. Also, due to their porous nature, organic mulches enhance infiltration, buffer soil pH, and conserve water, which is particularly beneficial during the dry periods of tropical climates [10]. In contrast, inorganic mulches, such as plastic films, do not decompose and primarily function by conserving soil moisture and reducing weed competition. However, they may have limited benefits towards soil quality [11]. Inorganic mulches create barriers that limit moisture loss; however, this can lead to increased surface runoff if not managed properly [12]. Both types of mulches act as insulators, moderating soil temperature fluctuations, which is crucial for yam tuber development [13]. Organic mulches contribute to the improvement of the soil structure by increasing the SOC content. These mulches lead to soil aggregation, reduced bulk density, and improved porosity, which facilitate root growth and development [14]. The physical barrier provided by mulches protects the soil surface from the erosive forces of wind and water, preserving the topsoil and preventing nutrient loss [15].

The decomposition of organic mulches releases essential nutrients such as N, P, and K, thereby enriching the soil nutrient pool [16]. Inorganic mulches do not contribute to soil fertility directly, but they enhance nutrient uptake efficiency by maintaining optimal moisture and temperature regimes [17]. Microorganisms play a crucial role in OM decomposition and nutrient cycling, leading to improved soil health [18]. The enhanced microbial activity also promotes the formation of humus, which further improves soil structure and fertility. Research in tropical regions have demonstrated the various impacts of different mulches on soil properties and crop yields. For instance, a study in Nigeria showed that yam plots mulched with organic materials such as straw and compost had a significantly higher soil moisture content and lower soil temperatures compared to un-mulched plots, which led to improved tuber yield and its quality [19]. Similarly, in Ghana, the use of rice straw mulch resulted in better soil structure and increased yam productivity [20]. A study by [21] revealed that mulching with black plastic film led to higher soil temperatures by about ~2 °C, low soil bulk density, increased volumetric water content, and enhanced cocoyam yield by 72%, providing the most favorable environment for yam production. The authors of [22] revealed that organic mulches, such as Tithonia and Chromolaena, increased SOM and nutrient concentrations in both the soil and leaves, and enhanced the growth and yield of yams. A study by [23] revealed that mulching materials significantly promoted the emergence and development of yam setts, leading to an increased tuber yield. In Fiji, the traditional practice of using organic mulches like coconut husks, rice straw, sugarcane trash, and palm leaves has been effective in enhancing soil fertility and crop performance. However, with changing climatic conditions and increased pressure on land resources, there is a growing interest in exploring the use of inorganic mulches such as plastic films. These mulches, despite their wide benefits, pose challenges related to environmental sustainability and disposal [24].

Despite the benefits of mulches in yam cultivation, there is limited research on their effects on soil properties and yam productivity [25]. The impact of various mulch materials on overall soil quality will influence their future adoption and use in agriculture. Soil quality affects not only the productivity of farmland but also regulates the biological balance of agroecosystems [26]. Soil quality can be assessed using various indicators, including physical aspects (such as bulk density, texture, and water holding capacity), chemical properties (like pH, SOC, available N, P, and K), and biological factors (including enzymatic activities, microbial activity, and microbial biomass) [27]. Therefore, in this study, we investigated the impact of different mulch materials on soil properties, seasonal variations in soil and environmental variables, soil quality, and productivity in yam cultivation. We hypothesized that different mulches will significantly affect soil properties and yam production. The study assessed the impact of organic and inorganic mulches on various soil properties and yield production to offer a theoretical foundation and practical guidance for their wider application. This is essential for enhancing soil quality and advancing sustainable agricultural practices.

2. Materials and Methods

2.1. Experimental Site Description

During the dry season (19 July) of 2023 in the austral winter, a field study was conducted at the Dobuilevu Research Station (DRS) experimental field located in Dobuilevu, situated east of Viti Levu, Fiji. The site is situated approximately 111 km away from the capital city of Suva, positioned at an elevation of 85 m above mean sea level (amsl). The location is situated between 17°33′39″ S latitude and 178°14′42″ E longitude (Figure 1). The DRS spans approximately 53.04 hectares (ha) in the northeastern part of Viti Levu, Fiji, located in the island’s intermediate climatic zone. The study area has a warm temperate climate with seasonal variations influenced by a tropical maritime climate. The area undergoes distinct wet and dry seasons, with warm summers and hot winters. The summer season spans from November to April. These months feature warm days, brief afternoon showers, vibrant sunsets, and increased humidity, making hydration essential. Daily temperatures reach up to 31 °C, with an average of 25–26 °C. The warmest months are January and February. The winter season brings shorter but sunnier days, with cooler evenings. Temperatures during this season are slightly lower than in the wet season, averaging 23–24 °C, with most days ranging between 19 and 28 °C. The coolest months are July and August, and rainfall decreases significantly, with only 50–75% of the precipitation seen in the wetter months. The average relative humidity ranges from 76.0 to 85.0% (Figure 2).

2.2. Experimental Design and Management

An experiment was set up with three replications and eight treatments in a Randomized Complete Block Design (RCBD), viz., black polythene mulch (BPM), weedmat mulch (WMM), sugarcane straw mulch (SSM), organic compost mulch (OCM), cowpea-live mulch (CLM), juncao grass mulch (JGM), sawdust mulch (SDM), and a control (CON). The study plot sizes each measured 4 m by 2 m with an area of 8 m², with 2 rows per plot and 5 plants per row. The total study area size was 323 m². Organic mulches were applied to a thickness of 5 cm. The BPM, with a thickness of 20 microns, was purchased from Vinod Patel, a local hardware store in Rakiraki, Fiji, while the WMM, with a thickness of 200 microns, was obtained from Marco Polo, another local store in Suva, Fiji. All the plots were fully covered with mulch to ensure a 100% coverage ratio (Figure 3). This study design was chosen to maintain a manageable scale for focused observation and data collection. By reducing the number of rows, we can concentrate on the direct effects of the different mulching treatments on soil properties and yam yield, minimizing the additional variability that larger plot sizes might introduce. Using smaller, uniform plots helps in reducing environmental variability, such as soil differences and microclimatic conditions, allowing us to obtain more accurate and reliable results that can be attributed directly to the treatments rather than external factors. The standardized conditions, such as uniform soil preparation, equal spacing between plants, and management practices across all treatments, helps mitigate these edge effects, ensuring that the observed differences were primarily due to the mulch treatments rather than plot positioning. The planting patterns were ridge planting patterns. A local yam variety, Voli, was planted over a period from 19 July 2023 to 12 April 2024, and grown over a 9-month period, with harvesting taking place on 14 April 2024. The seed yams were cut to an average weight of 220 g and planted using a spacing of 1 m (between ridge) by 0.8 m (within plants). Mulching was carried out immediately after planting the prepared seed yams. The field was prepared by tractorization and brought to a fine tilth before sowing. Preparation included two deep ploughs and tine harrows, field rotavating, and the formation of ridges. The ridge dimensions were 45 cm in height and 1 m in width. A balanced organic fertilizer, i.e., poultry manure (PM), containing approximately 3.3% N, 2% P₂O₅, and 4% K₂O, was applied at a rate of 10 tons (t) per hectare (t ha⁻¹) during land preparation two weeks prior to planting. Single super phosphate (SSP), muriate of potash (MOP), and urea were applied at a rate of 200 kg ha⁻¹. SSP containing about 16% P₂O₅ and MOP about 60% K₂O were applied as a full basal dose during planting, while urea with 46% N was applied at three intervals, viz., 8, 12, and 16 weeks after planting (WAP). Yams were harvested on 14 April 2024, following the conclusion of the growth period.

2.3. Soil Physico-Chemical Properties

Composite soil samples were collected at a depth of 0–15 cm prior to the application of fertilizers and manures to assess the soil’s physico-chemical properties. The samples were air-dried at room temperature, crushed, sieved through a 2 mm mesh, and stored in zip-lock bags. Laboratory tests were conducted to evaluate the soil characteristics and initial fertility status.

Table 1. Soil physico-chemical properties.

S. No	Soil Properties	Value	Rating
1	Soil pH	5.77	Slightly acidic
2	EC (mS cm−1)	0.05	Very low salinity
3	BD (g cm−3)	1.09	Optimal
4	Temperature (°C)	27.33	Warm
5	SOC (%)	2.27	High
6	SAN (%)	0.08	Low
7	SAP (mg kg−1)	12.33	Medium
8	SEK (me/100 g)	0.40	Low
9	SECa (me/100 g)	18.52	High
10	SEMg (me/100 g)	11.30	High
11	SENa (me/100 g)	0.04	Very low
12	SEFe (mg/kg)	30.62	High
13	SEMn (mg/kg)	45.20	Very high
14	SECu (mg/kg)	6.55	High
15	SEZn (mg/kg)	1.50	Medium

Soil pH, soil reaction; EC (mS cm⁻¹), electrical conductivity (millisiemens per centimeter); BD (g cm⁻³), bulk density (grams per cubic centimeter); Temperature (°C), temperature (degrees celsius); SOC (%), soil organic carbon (percentage); SAN (%), soil available nitrogen (percentage); SAP (mg kg⁻¹), soil available phosphorus (milligrams per kilogram); SEK (me/100 g), soil exchangeable potassium (milliequivalents per 100 grams); SECa (me/100 g), soil exchangeable calcium (milliequivalents per 100 grams); SEMg (me/100 g), soil exchangeable magnesium (milliequivalents per 100 grams); SENa (me/100 g), soil exchangeable sodium (milliequivalents per 100 grams); SEFe (mg/kg), soil exchangeable iron (milligrams per kilogram); SEMn (mg/kg), soil exchangeable manganese (milligrams per kilogram); SECu (mg/kg), soil exchangeable copper (milligrams per kilogram); SEZn (mg/kg), soil exchangeable zinc (milligrams per kilogram).

presents the soil’s physico-chemical properties in the study area. The soil had a slightly acidic pH (5.77) with very low salinity (0.05 mS cm⁻¹), optimal bulk density (1.09 g cm⁻³), and a warm temperature (27 °C). The SOC content was high (2.27%), while the SAN content was low (0.08%). The SAP content was medium (12.33 mg kg⁻¹) with the SEK content being low (0.40 me/100 g). The soil had high levels of SECa (18.52 me/100 g) and SEMg (11.30 me/100 g), with very low SENa levels (0.04 me/100 g). The soil had a high content of SEFe (30.62 mg/kg), a very high content of exchangeable manganese (SEMn) (45.20 mg/kg), a high content of SECu (6.55 mg/kg), and a medium content of exchangeable zinc (SEZn) (1.50 mg/kg).

2.4. Soil and Environment Variable Analysis

The various physico-chemical properties of the soil were measured after the crop was harvested. This was selected as the best time to assess the cumulative impact of the different mulching treatments on soil characteristics throughout the entire growing season.

2.4.1. Basic Soil Properties

Soil pH was measured by mixing soil samples with distilled water in a 1:5 ratio, allowing the mixture to equilibrate, and then measured using a calibrated pH meter [28]. The soil water suspension used for pH determination was allowed to settle down and the conductivity of the supernatant liquid was determined using a Solubridge conductivity meter [29]. Soil bulk density (BD) was determined using the core method, where a known volume of soil was extracted, dried at 105 °C, and weighed; the BD was calculated as the dry weight of soil divided by the volume of the core [30]. Soil moisture content was determined gravimetrically by weighing soil samples to obtain the wet weight, drying them in an oven at 105 °C for 24 h to obtain the dry weight, and then calculating the moisture content as the percentage difference between the wet and dry weights [31]. Soil temperature was measured using a soil thermometer inserted into the soil at a depth of 10 cm [32].

2.4.2. SOC and Essential Soil Nutrients

The method described by [33] was used to determine the SOC content. A total of 1 g of 0.5 mm of the sieved soil sample was digested using 10 mL 1 N potassium dichromate (K₂Cr₂O₇) and 20 mL of concentrated sulfuric acid (conc. H₂SO₄) in a 500 mL conical flask. The samples were cooled for 30 min (room temperature), followed by the addition of 200 mL of distilled water (dH₂O), and 10 mL of conc. phosphoric acid (H₃PO₄), 1 mL of diphenylamine indicator; then, the samples were titrated against 0.5 N ferrous ammonium sulfate (FAS). The use of chromic acid (K₂Cr₂O₇ + H₂SO₄) was used to oxidize the SOM, and the leftover K₂Cr₂O₇ was subsequently back-titrated against FAS (redox titration). The Kjeldahl method was used to measure SAN by first digesting a 10 g sample of air-dried, sieved soil with concentrated sulfuric acid in a Kjeldahl digestion flask. This process converts organic N compounds into ammonium sulfate. Subsequently, the released ammonium ions were distilled from the digestate using steam, and the ammonia gas was absorbed and titrated with a standard acid solution to quantify the N content [34]. The Bray P1 method was used to determine SAP. Soil was extracted with an ammonium fluoride and hydrochloric acid solution at pH 2.5; then, the mixture was shaken and filtered. The extracted P in the filtrate was measured colorimetrically by forming a blue complex and assessing its intensity with a spectrophotometer. The SAP concentration was quantified by comparing the sample’s absorbance to known P standards [35]. SEK was determined by extracting soil with a neutral 1 M ammonium acetate solution and measuring the K concentration using atomic absorption spectroscopy (AAS). A 10 g sample of air-dried, sieved soil was mixed with 20 mL of the ammonium acetate solution, shaken for 1 h, and then filtered. The filtrate, containing dissolved K ions, was analyzed using AAS to quantify K concentrations based on specific absorption wavelengths [36]. SECa, SEMg, and SENa levels in soil were extracted using a 1 M ammonium acetate solution and subsequently measured by AAS. A total of 10 g sample of air-dried sieved soil was mixed with 20 mL of the ammonium acetate solution in a 50 mL extraction bottle. The mixture was shaken for 1 h and soil suspension was filtered to separate soil particles from the solution containing the dissolved cations. The filtrate was then analyzed using AAS, where specific wavelengths were used to quantify Ca, Mg, and Na concentrations [36].

2.4.3. Soil Micronutrients

Micronutrients (SEFe, SEMn, SECu, and SEZn) were extracted from soil using a DTPA (diethylenetriaminepentaacetic acid) solution, which was prepared with calcium chloride and triethanolamine and adjusted to pH 7.3. A 10 g soil sample was mixed with 20 mL of DTPA solution, shaken for 2 h, and then filtered. The concentrations of the micronutrients in the filtrate were measured using AAS [37].

2.4.4. Time Series of Soil pH, Temperature, and Environmental Humidity

The time series data on soil pH, temperature, and environmental humidity were collected monthly from August 2023 to March 2024 using a digital 5-in-1 soil test kit. These measurements are crucial for assessing soil quality and health. Soil pH directly affects nutrient availability and microbial activity, which are vital for maintaining soil fertility and supporting plant growth. Soil temperature influences root development and the efficiency of biological processes in the soil, while environmental humidity impacts moisture levels and the risk of soil degradation. By monitoring these variables over time, the study can identify seasonal trends and fluctuations that affect soil quality, allowing for targeted adjustments in soil management practices. This approach aids in optimizing soil health, enhancing nutrient uptake, and improving overall soil conditions, ultimately leading to better yam productivity and sustainable land management.

2.4.5. Soil Quality Index Calculations

Three distinct soil quality indices (SQIs) were developed and analyzed using the Principal Component Analysis (PCA)-based SQI, regression-based SQI, and linear scoring-based SQI to evaluate the impact of various mulching treatments on soil quality, which were adopted from [38]. The study examined the 8 treatments and the physico-chemical properties of the soil. The PCA-based SQI was used to simplify the soil quality data and identify the most influential indicators of soil quality. This method allowed us to weight these indicators based on their importance, creating a composite SQI score for each treatment. We then normalized these scores on a scale from 0 to 1, where higher scores indicate better soil quality. The regression-based SQI was developed using multiple regression analysis, which linked soil quality indicators with the soil’s physico-chemical properties. By applying the regression coefficients to the soil indicators for each treatment, we generated an SQI that predicts the condition of the soil quality. These scores were also normalized to a 0 to 1 scale. For the linear scoring-based SQI, we assigned scores to each soil quality indicator based on established thresholds for optimal soil conditions. Each indicator was rated from 0 (poor quality) to 1 (optimal quality). The final SQI for each treatment was the weighted average of these individual scores, offering a clear and interpretable measure of soil quality.

2.5. Crop Growth and Yield Observations

The growth and yield characteristics of yams were systematically observed throughout the study. For the biometrical analysis, three plants were randomly selected and tagged in each replication. Observations were recorded monthly and at harvest, with the mean values calculated for each trait. The vine length of the three tagged plants was measured monthly using a meter ruler. The stem diameter of the three tagged plants was measured monthly using a millimeter ruler. The tubers from each plot were collected and counted, and the average number of tubers per plot was calculated. The weight of the tubers from the three tagged plants was measured in kilograms using an electronic balance, and their average was calculated as the tuber weight plant⁻¹. The weight of the tubers from each plot was measured in kilograms using an electronic balance, and the average was calculated as the tuber weight plot⁻¹. Plants from each plot were manually harvested using a spade, and the tubers were thoroughly cleaned before their fresh weights were recorded. The tuber yield for each plot was then converted to kilograms per hectare (kg ha⁻¹).

2.6. Statistical Analysis

The data collected for different parameters were statistically evaluated using the method proposed by [39]. The data were subjected to an analysis of variance (ANOVA), with a mean comparison using Duncan’s Multiple Range Test (DMRT) at a significance level of 5%. The data were presented as a mean plus minus standard errors with alphabets. The letters mentioned in the table indicated its significance. The soil quality analysis was performed using SQI R package. All the statistical analyses were performed using the R statistical software, version 4.4.1.

3. Results and Discussion

3.1. Influence of Mulch Material on Soil Properties

A substantial influence of different mulch materials on basic soil properties in yam cultivation was demonstrated (

Table 2. Basic soil properties as affected by different mulch material.

Treatments	Soil pH	EC (mS/cm)	Moisture (%)	Temperature (°C)
BPM	5.73 ± 0.09 b	0.05 ± 0.003 ab	10.20 ± 0.96 ab	28.80 ± 0.44 a
WMM	5.83 ± 0.12 ab	0.05 ± 0.010 b	10.10 ± 1.07 ab	28.50 ± 0.29 ab
SSM	5.60 ± 0.06 b	0.07 ± 0.003 a	9.50 ± 1.57 b	28.50 ± 0.01 ab
OCM	6.37 ± 0.38 a	0.07 ± 0.012 a	12.50 ± 0.32 ab	28.80 ± 0.17 a
CLM	5.83 ± 0.07 ab	0.04 ± 0.007 b	12.70 ± 0.21 ab	28.70 ± 0.17 ab
JGM	5.60 ± 0.01 b	0.05 ± 0.003 ab	10.80 ± 2.02 ab	28.30 ± 0.17 ab
SDM	5.63 ± 0.23 b	0.03 ± 0.003 b	12.90 ± 0.29 ab	28.20 ± 0.17 ab
CON	5.87 ± 0.15 ab	0.04 ± 0.003 b	13.50 ± 0.45 a	28.00 ± 0.01 b
CD (p ≤ 0.05)	1.69	0.06	10.12	2.12
SE (d)	0.26	0.01	1.58	0.33

BPM, black polythene mulch; WMM, weedmat mulch; SSM, sugarcane straw mulch; OCM, organic compost mulch; CLM, cowpea-live mulch; JGM, juncao grass mulch; SDM, sawdust mulch; CON, control (no mulch); EC, electrical conductivity; CD (p ≤ 0.05), critical difference at 5% level of significance; SE (d), standard error of difference. Values with different letters in the same column indicate significant differences between treatments (p ≤ 0.05, n = 8). Data are means ± standard error.

). The application of different mulch materials significantly influences soil properties, which are crucial for the enhancement of soil quality [40]. Soil pH was significantly affected by the different mulch treatments. The highest soil pH was recorded with organic compost mulch (6.37), which was significantly (p ≤ 0.05) higher (8.52%) than the control treatment (5.87). In contrast, the lowest was observed with sawdust and juncao grass mulch (5.60), and these were significantly (p ≤ 0.05) lower (12.09%) as compared to the organic compost mulch. The high soil pH with organic compost mulch could be attributed to the buffering capacity of OM, which neutralizes soil acidity [41]. This result aligns with previous studies where organic amendments have been shown to raise soil pH due to the decomposition process releasing basic cations [42]. In contrast, low pH in sawdust and juncao grass mulch may be due to decomposition of OM, which produces organic acids that lowers soil pH levels [43].

Soil EC (mS/cm) was significantly (p ≤ 0.05) higher (40%) with sugarcane straw and organic compost mulch (0.07) compared to black polythene, weedmat, and juncao grass mulch (0.05). The lowest EC was recorded with sawdust mulch (0.03), which was 57.14% lower compared to the highest values. The increased EC levels with sugarcane straw and organic compost mulch could be due to the higher decomposition rates of these materials, which release soluble salts into the soil [44]. This finding is in consistent with [45], who observed elevated EC levels in soils amended with organic compost due to increased mineralization rates. The low EC levels in sawdust mulch could be due to sawdust decomposing more slowly than other organic mulches. With slow decomposition, fewer nutrients and soluble salts are released into the soil over time, thus causing a low EC level.

The moisture content (%) varied among the treatments. The highest soil moisture content was observed in the control treatment (13.50), which was significantly (p ≤ 0.05) higher (4.65%) compared to sawdust mulch (12.90). The lowest was recorded with sugarcane straw mulch (9.50), which was 29.63% lower than the control treatment. The higher moisture content with the control and sawdust mulch treatments may be due to reduced soil evaporation rates [46]. Sawdust mulch is particularly effective at conserving soil moisture due to its ability to form a protective barrier over the soil surface, reducing direct evaporation [47]. In the control treatment, the soil surface was directly exposed to environmental conditions. While this exposure typically increases evaporation rates, it can also create a microclimate where the soil surface cools rapidly at night, leading to condensation and moisture retention. This microclimate effect may have contributed to higher moisture readings [48]. The control treatment may also have benefitted from natural soil conditions, which have better moisture retention properties, as the control treatment did not involve additional amendments that could disrupt the soil’s natural balance [49]. The low moisture with sugarcane straw can be attributed to the high absorptive capacity of sugarcane straw, meaning it retains a significant amount of water within the mulch layer itself, thus reducing the amount of water that infiltrates the soil below [50]. Additionally, the slower decomposition rate of sugarcane straw forms a thick mat on the soil surface that can hinder water infiltration during rain or irrigation events, further contributing to lower moisture levels [51].

A high soil temperature (°C) was recorded in the black polythene and organic compost mulch treatments (28.80); this value was significantly (p ≤ 0.05) higher (0.35%) compared to the cowpea-live mulch treatment (28.70). The lowest soil temperature was observed in the control treatment (28.00), which was 2.78% lower than the black polythene and organic compost mulch treatments. The increased soil temperature under the black polythene mulch treatment can be attributed to the material’s ability to absorb and retain heat [10]. Organic compost mulch can also increase temperature by creating a microenvironment that traps heat [52]. These findings are consistent with those of [53], who reported higher soil temperatures under plastic mulch treatment. The low soil temperature in the control treatment may be due to the lack of a surface cover, as the soil is directly exposed to ambient environmental conditions. Mulch serves as an insulating layer that reduces heat loss during cooler periods and minimizes heat gain during the hotter periods [54].

3.2. Influence of Mulch Material on SOC and Essential Soil Nutrients

Table 3. SOC and essential soil nutrients as affected by different mulch material.

Treatments	SOC (%)	SAN (%)	SAP (mg/kg)	SEK (me/100 g)	SECa (me/100 g)	SEMg (me/100 g)	SENa (me/100 g)
BPM	2.63 ± 0.03 ab	0.19 ± 0.02 b	12.30 ± 2.03 b	0.61 ± 0.10 ab	26.10 ± 6.04 a	14.70 ± 0.47 a	0.08 ± 0.03 a
WMM	2.60 ± 0.01 ab	0.21 ± 0.01 ab	17.70 ± 0.33 b	1.07 ± 0.27 a	20.70 ± 1.42 a	14.10 ± 0.75 a	0.05 ± 0.02 a
SSM	2.60 ± 0.15 ab	0.29 ± 0.06 a	18.70 ± 6.69 b	0.80 ± 0.10 ab	21.80 ± 0.77 a	14.60 ± 0.31 a	0.10 ± 0.02 a
OCM	2.90 ± 0.31 a	0.20 ± 0.02 ab	38.00 ± 16.30 a	0.63 ± 0.12 ab	27.30 ± 4.77 a	14.60 ± 0.70 a	0.06 ± 0.02 a
CLM	2.47 ± 0.18 ab	0.21 ± 0.01 ab	11.00 ± 1.00 b	0.50 ± 0.10 b	21.10 ± 0.93 a	14.60 ± 0.60 a	0.11 ± 0.04 a
JGM	2.50 ± 0.21 ab	0.18 ± 0.01 b	10.70 ± 3.28 b	0.90 ± 0.20 ab	17.40 ± 0.74 a	13.80 ± 0.60 a	0.10 ± 0.03 a
SDM	2.53 ± 0.09 ab	0.22 ± 0.01 ab	11.30 ± 2.19 b	0.46 ± 0.11 b	20.70 ± 0.46 a	13.60 ± 1.63 a	0.10 ± 0.02 a
CON	2.37 ± 0.18 b	0.24 ± 0.02 ab	7.67 ± 2.91 b	0.46 ± 0.08 b	21.70 ± 2.18 a	15.20 ± 0.12 a	0.12 ± 0.01 a
CD (p ≤ 0.05)	1.23	0.23	57.37	1.32	27.84	5.02	0.22
SE (d)	0.19	0.04	8.98	0.21	4.36	0.79	0.03

BPM, black polythene mulch; WMM, weedmat mulch; SSM, sugarcane straw mulch; OCM, organic compost mulch; CLM, cowpea-live mulch; JGM, juncao grass mulch; SDM, sawdust mulch; CON, control (no mulch); SOC, soil organic carbon; SAN, soil available nitrogen; SAP, soil available phosphorus; SEK, soil exchangeable potassium; SECa, soil exchangeable calcium; SEMg, soil exchangeable magnesium; SENa, soil exchangeable sodium; CD (p ≤ 0.05), critical difference at 5% level of significance; SE (d), standard error of difference. Values with different letters in the same column indicate significant differences between treatments (p ≤ 0.05, n = 8). Data are means ± standard error.

highlights the influence of different mulch materials on the SOC content and essential soil nutrients. The highest SOC (%) content was recorded with organic compost mulch (2.90), which was significantly (p ≤ 0.05) higher (10.27%) as compared to black polythene mulch (2.63), while the lowest was in the control treatment (2.37), which was 18.28% lower than the organic compost mulch. The highest SOC content in the organic compost mulch treatment is due to the nature of the material itself. Organic compost is rich in OM, and when it decomposes, the OM is integrated into the soil, enhancing its SOC content [55]. This is consistent with findings by [56], who noted that organic amendments significantly increase the SOC content due to the high amount of decomposable OM they add to the soil. Moreover, organic mulches have been reported to improve soil microbial activity, which contribute to increased SOC levels [57]. The SOC content in the black polythene mulch treatment was second highest, demonstrating that even non-organic mulches can positively influence the SOC content, albeit to a lesser extent than organic compost. This could be due to the polythene mulch’s ability to reduce soil erosion and maintain soil moisture, creating a microenvironment that slows the decomposition of the already existing OM in the soil [58]. However, it does not contribute additional OM to the soil, which explains the low SOC content as compared to organic compost mulch. In contrast, the low SOC content in the control treatment is due to no mulch additions to these plots. Without the addition of mulch materials, there is a minimal input of OM into the soil, leading to lower SOC levels. This aligns with the general understanding that bare soil or untreated plots tend to have lower SOC levels due to the lack of additional organic inputs and the higher rates of erosion and decomposition of existing OM [59]. Our findings are in line with those of [60], who observed increased SOC levels with the application of organic mulches compared to inorganic or non-mulched treatment.

The highest SAN (%) was recorded with sugarcane straw mulch (0.29), which was significantly (p ≤ 0.05) higher (20.83%) as compared to the control treatment (0.24). The lowest was observed with juncao grass (0.18), which was 37.93% lower than the sugarcane straw mulch. The high SAN in sugarcane straw mulch could be due to its high OM content and its slower rate of decomposition, which steadily releases nutrients into the soil [61]. This slow and steady release is beneficial in maintaining a higher SAN level over time. Similar findings have been reported by [62], who observed that sugarcane straw mulch improved the soil nutrient status, including available N. Interestingly, the control treatment, which had no mulch, showed a relatively high SAN level, close to that of sugarcane straw mulch. This could be due to the natural soil N content and minimal interference from mulches that might otherwise immobilize N. The SAN level for black polythene mulch, organic compost, and juncao grass were lower than that of sugarcane straw, suggesting that while these mulches provided benefits in terms of the SOC content, they may not be as effective in enhancing available N. Black polythene mulch, for example, does not add OM to the soil, which might explain its lower SAN level. Organic compost mulch, while high in SOC content, may have initially immobilized N as it decomposes, reducing the SAN level [63]. The low SAN level in the juncao grass mulch treatment might be due to the composition of the grass, which may decompose more quickly or have a lower N content compared to other mulch types. Rapid decomposition can lead to initial N immobilization, where microbes use available N for their own growth, temporarily reducing the amount available for plants [64]. Our findings are in consistent with studies by [56,64], who have shown that the nutrient content of the mulch material, its decomposition rate, and its impact on soil microbial activity are critical factors influencing the SAN level.

The highest SAP level (mg/kg) was recorded with organic compost mulch (38.00), which was significantly (p ≤ 0.05) higher (103.21%) than sugarcane straw mulch (18.70). The lowest was observed with control (7.67), which was 79.82% lower than the organic compost mulch. The notable increase in the SAP level with organic compost can be attributed to the high nutrient content and the gradual release of nutrients during decomposition [65]. This finding is consistent with previous studies that have demonstrated the benefits of organic mulches in increasing the soil nutrient content [66]. The slow decomposition of organic compost not only supplies essential nutrients over time but also improves soil structure and microbial activity, which are crucial for nutrient cycling and availability [67]. In comparison, the low SAP level in the control treatment could be attributed to the lack of organic or mineral inputs. Without these additions, P tends to become less accessible to plants over time. This is partly because P can bind with soil particles, making it less soluble and harder for plants to absorb. Additionally, without OM, there is less microbial activity in the soil, which is crucial for breaking down organic P compounds into forms that plants can use [68]. This is in line with research by [69], who reported that non-mulched soil has low soil P levels due to the absence of protective covering that allows nutrients to be washed away more easily through leaching. Additionally, mulch supports microbial communities that play a key role in breaking down OM and releasing P in a form that plants can absorb. There were no significant differences (p ≤ 0.05) observed between organic compost mulch with weedmat and sugarcane straw mulch, suggesting that both weedmat and sugarcane straw mulches also play effective roles in maintaining or enhancing soil P levels, potentially due to their ability to conserve soil moisture and suppress weed growth, thereby indirectly supporting nutrient availability.

The highest SEK level (me/100 g) was recorded with weedmat mulch (1.07), which was significantly (p ≤ 0.05) higher (18.89%) than juncao grass (0.90). The lowest was observed with sawdust and control (0.46), both of which were 57.01% lower than the weedmat mulch. The high SEK level in the weedmat treatment can be attributed to its effectiveness in reducing soil erosion and leaching, which helps to maintain higher levels of K [70]. Weedmat mulches, by minimizing weed competition and maintaining a stable soil environment, ensure that more K remains available for plant uptake. This is in line with findings by [71]. Juncao grass mulch also recorded a relatively higher SEK level, likely due to its organic nature, which contributes to soil fertility and nutrient availability as it decomposes. This higher SEK content might be due to the specific nutrient composition of juncao grass, which may release K more effectively. The low SEK content in the sawdust mulch and the control treatments suggest that these conditions are less effective in maintaining soil K levels. Sawdust mulch may initially immobilize K, as it decomposes, a phenomenon noted by [72]. The control treatment, with no mulch, lacks the protective and nutrient-adding benefits of mulches, leading to lower SEK values due to nutrient leaching. The weedmat mulch treatment significantly increased the SEK level compared to the cowpea-live mulch, sawdust, and control treatments, highlighting its effectiveness in preserving soil K. These findings align with other research projects, which indicate that mulching significantly affects soil nutrient dynamics [73]. For instance, Ref. [46] found that mulches, particularly those that minimize soil disturbance and erosion, enhance soil nutrient availability.

The various mulches showed no meaningful difference (p ≤ 0.05) in the SECa content. The highest SECa content (me/100 g) was recorded in the organic compost mulch treatment (27.30), which was 4.60% higher than black polythene mulch (26.10). The lowest was observed in the juncao grass treatment (17.40), which was 36.27% lower than the highest value. The high SECa content in the organic compost mulch treatment may be due to its high Ca content and the capacity to preserve soil nutrients. Organic compost is known to add substantial amounts of OM and essential nutrients to the soil as it decomposes [74]. This process releases Ca and other nutrients, thereby enhancing soil fertility. This is supported by a study from [75], which indicates that organic amendments, such as compost, can significantly increase soil nutrient levels. Black polythene mulch also showed a relatively higher SECa value, which can be explained by its role in reducing soil compaction, weed suppression, and leeching losses, which helps to preserve the soil nutrient levels [76]. The low SECa value in the juncao grass treatment might be due to its lower inherent Ca content and faster decomposition rate, which might initially immobilize Ca as microbes break down the OM. This can temporarily reduce the availability of Ca to plants. Similar findings have been reported by [77], who noted that certain organic mulches can lead to initial nutrient immobilization, affecting the immediate availability of essential nutrients.

The highest SEMg content (me/100 g) was recorded in the control treatment (15.20), which was 3.40% higher than black polythene mulch (14.70). The lowest was observed in the sawdust mulch treatment (13.60), which was 10.53% lower than the control treatment. The high SEMg content in the control treatment suggests that natural soil conditions without any mulch intervention can retain more exchangeable Mg. This may be due to the absence of any mulch-related processes that might immobilize Mg temporarily or interfere with its availability. A study by [56] indicates that natural soil processes can sometimes maintain higher nutrient availability, as there are no additional factors altering the nutrient dynamics. The low SEMg content in the sawdust mulch treatment might be due to its higher rate of nutrient immobilization. Sawdust, being a high C-rich material, often requires N and other nutrients for microbial decomposition, temporarily reducing the availability of these nutrients in the soil [78]. This initial immobilization phase can lead to lower SEMg levels. Our findings align with other studies showing that mulching can influence soil nutrient dynamics, but the specific impacts can vary based on the type of mulch and other contextual factors [79]. Despite mean variations in the SEMg content, no significant difference (p ≤ 0.05) was found among the different treatments.

The highest SENa content (me/100 g) was recorded in the control treatment (0.12), which was 9.09% higher than the cow-pea live mulch treatment (0.11). The lowest was observed in the weedmat treatment (0.05), which was 58.33% lower than the control treatment. The control treatment exhibited the highest SENa level, suggesting that the absence of mulch might allow for the natural retention of Na in the soil. The soil’s existing Na levels are likely preserved without any external interference from mulch materials, which can sometimes alter nutrient dynamics through processes such as immobilization or enhanced leaching. The author of [80] highlighted that undisturbed soils maintain their structure and nutrient balance more effectively, as there are no additional factors altering the nutrient dynamics. Cowpea-live mulch also demonstrated a relatively high SENa value, which can be attributed to the N-fixing ability of cowpea. Being a leguminous plant, cowpea enhances soil fertility by fixing atmospheric N, thus improving the overall nutrient status of the soil. The authors of [81] highlighted that N-fixing plants contribute to soil fertility by adding OM and nutrients, which can have a positive impact on the availability of other essential elements. The low SENa content in the weedmat treatment might be due to its non-organic nature, which does not contribute additional nutrients to the soil and affects nutrient dynamics differently compared to organic mulches. Despite these variations, no statistically significant difference (p ≤ 0.05) was observed among the treatments.

3.3. Influence of Mulch Material on Soil Micronutrients

Table 4. Soil micronutrients as affected by different mulch material.

Treatments	SEFe (mg/kg)	SEMn (mg/kg)	SECu (mg/kg)	SEZn (mg/kg)
BPM	81.70 ± 3.53 a	76.70 ± 4.91 a	9.00 ± 0.01 a	2.00 ± 0.01 b
WMM	70.70 ± 1.20 ab	70.30 ± 4.91 a	8.33 ± 0.33 a	2.00 ± 0.01 b
SSM	76.70 ± 10.70 a	79.30 ± 4.67 a	7.67 ± 0.67 a	2.00 ± 0.58 b
OCM	52.00 ± 2.52 b	51.00 ± 6.11 b	7.67 ± 0.33 a	4.13 ± 1.95 a
CLM	62.70 ± 8.99 ab	65.00 ± 2.00 ab	8.33 ± 0.67 a	1.67 ± 0.33 b
JGM	60.00 ± 9.50 ab	67.00 ± 6.00 ab	7.67 ± 0.88 a	1.67 ± 0.33 b
SDM	82.00 ± 1.00 a	78.00 ± 9.07 a	9.00 ± 0.01 a	2.00 ± 0.01 b
CON	64.00 ± 9.29 ab	69.70 ± 7.88 a	7.67 ± 0.88 a	1.67 ± 0.33 b
CD (p ≤ 0.05)	59.47	47.22	4.28	6.35
SE (d)	9.31	7.39	0.67	0.99

BPM, black polythene mulch; WMM, weedmat mulch; SSM, sugarcane straw mulch; OCM, organic compost mulch; CLM, cowpea-live mulch; JGM, juncao grass mulch; SDM, sawdust mulch; CON, control (no mulch); SEFe, soil exchangeable iron; SEMn, soil exchangeable manganese; SECu, soil exchangeable copper; SEZn, soil exchangeable zinc; CD (p ≤ 0.05), critical difference at 5% level of significance; SE (d), standard error of difference. Values with different letters in the same column indicate significant differences between treatments (p ≤ 0.05, n = 8). Data are means ± standard error.

demonstrates the influence of different mulch materials on soil micronutrients. The highest SEFe value (mg/kg) was recorded in the sawdust mulch treatment (82.00), which was significantly (p ≤ 0.05) higher (0.37%) than the black polythene mulch treatment (81.70). The lowest value was observed in the organic compost treatment (52.00), which was 36.58% lower than the sawdust mulch. The high SEFe value in the sawdust mulch treatment suggests that sawdust may create favorable conditions for the retention or mobilization of Fe in the soil [82]. Sawdust, being an organic material, decomposes and releases organic acids, which can help to chelate Fe, making it more available to plants. This process can lead to higher concentrations of exchangeable Fe in the soil [83]. The study by [84] supports this, indicating that organic mulches can enhance micronutrient availability through OM decomposition and subsequent nutrient release. The second highest SEFe value was recorded in the black polythene mulch treatment, suggesting that it can likely contribute to a better microenvironment, which can enhance the microbial activity responsible for nutrient cycling. Although it does not add OM like sawdust, it still influences the physical properties of the soil that can indirectly affect the nutrient availability required for root growth and plant nutrient uptake [85]. Despite organic compost generally benefitting by adding nutrients to the soil, it appeared that in this case, the compost may have bound Fe in forms less available to plants, or other factors such as pH changes caused by compost could have affected Fe availability, leading to low levels [86]. Organic compost can sometimes lead to the formation of stable organic–Fe complexes that are not readily exchangeable [87]. This phenomenon, discussed by [88], indicated that OM can sometimes immobilize certain nutrients. The present study’s findings align with other studies showing that mulch type can significantly influence soil nutrient dynamics [89]. For instance, [90] found that different mulch treatments could lead to varying levels of micronutrients in the soil, depending on their composition and decomposition rates.

The highest SEMn value (mg/kg) was recorded in the sugarcane straw mulch treatment (79.30), which was significantly (p ≤ 0.05) higher (1.67%) than the sawdust treatment (78.00). The lowest value was observed in the organic compost treatment (51.00), which was 35.69% lower than the sugarcane straw mulch. The highest SEMn value in the sugarcane straw mulch treatment suggests that sugarcane straw creates favorable conditions for the retention or mobilization of Mn in the soil. Sugarcane straw, being an organic material, decomposes and releases organic acids that can help to chelate Mn, making it more available to plants [91]. A high SEMn value was also recorded in the sawdust mulch treatment, similar to the sugarcane straw treatment. With time, OM decomposition releases nutrients and other organic acids such as humic and fulvic acids, which improve Mn availability. This decomposition process contributes to the nutrient pool, enhancing the levels of Mn in the soil [92]. The study by [93] corroborates the positive impact of organic mulches on micronutrient availability. The low SEMn value in the organic compost treatment implies that despite the compost’s general benefit of adding nutrients to the soil, it appears that the compost might have bound Mn in forms less available to plants, or other factors such as changes in soil pH caused by compost application could have affected Mn availability [94]. This is consistent with findings by [95], who indicated that OM can sometimes immobilize certain nutrients, potentially explaining the lower SEMn value observed with compost mulch.

The highest SECu value (mg/kg) was recorded in both the black polythene and sawdust mulch treatments (9.00); this value was 8.04% higher than the weedmat and cow-pea live mulch treatments (8.33). The lowest value was observed in the sugarcane straw, organic compost, juncao grass, and control treatments (7.67); this value was 14.78% lower than the higher values. Despite mean discrepancies, no significant difference (p ≤ 0.05) was observed among the different treatments. The effectiveness of black polythene mulch in maintaining high SECu levels can be attributed to its ability to minimize water runoff and soil erosion, which helps prevent the leaching of Cu from the soil, leading to increased SECu concentrations. Sawdust adds OM to the soil as it breaks down. This OM can bind to Cu, making it more stable in the soil and less likely to be lost, thereby maintaining higher SECu levels [54]. Similar findings have been reported by [96]. Weedmat influences the soil microclimate, which can indirectly affect nutrient dynamics. Weedmat helps in moderating soil temperatures by providing a barrier against extreme heat and cold. This stable temperature range supports microbial activity, which is crucial for the decomposition of OM and nutrient cycling. Increased microbial activity leads to more efficient nutrient mineralization and availability to plants [97]. Cowpea-live mulch has symbiotic relationships with N-fixing bacteria (rhizobia) in its root nodules. These bacteria convert atmospheric N into forms usable by plants, increasing the N content in the soil. Enhanced N levels can stimulate the growth of plants and soil microorganisms, promoting the more robust decomposition of OM and release of nutrients, including Cu [98]. The low SECu value in the sugarcane straw, organic compost, juncao grass mulch, and control treatments may result from a variety of factors, including nutrient immobilization or lower decomposition rates compared to other mulches. For example, while organic compost generally adds nutrients to the soil, it can sometimes bind certain elements in forms less available to plants. Despite these variations, no statistically significant difference (p ≤ 0.05) was observed among the different treatments.

The highest SEZn value (mg/kg) was recorded in the organic compost mulch treatment (4.13), which was significantly (p ≤ 0.05) higher (106.50%) than the black polythene, weedmat, sugarcane straw, and sawdust mulch treatments (2.00). The lowest value was observed in the cowpea-live, juncao grass mulch, and control treatments (1.67); this value was 59.56% lower than the organic compost mulch treatment. The high SEZn value in the organic compost treatment can be attributed to the substantial amount of OM it encompasses. The microbial decomposition of organic compost releases Zn and other micronutrients into the soil [98]. This is consistent with findings by [75], who indicated that organic amendments can significantly increase micronutrient availability through improved soil biological activity and OM decomposition. Organic compost can help buffer soil pH, maintaining it within an optimal range for nutrient availability. Micronutrients such as Zn are more available in slightly acidic to neutral pH levels, which organic compost can help maintain [99]. The same SEZn levels were recorded with the black polythene, weedmat, sugarcane straw, and sawdust mulch treatments. Black polythene and weedmat mulches primarily affect the soil’s physical environment by conserving moisture and moderating temperature, which can indirectly support nutrient retention and uptake. The organic nature of sugarcane straw and sawdust mulch contributes to the gradual release of Zn as they decompose, improving its availability in the soil. Similar findings have been reported by [46]. The low SEZn value observed in the cowpea-live mulch, juncao grass, and control treatments might be due to less effective Zn release or retention mechanisms compared to other mulches. While cowpea-live mulch benefits from N fixation, it may not contribute as significantly to Zn availability. Juncao grass mulch might decompose at a slower rate, resulting in lower micronutrient release. These findings align with observations by [90], who noted that the effectiveness of different mulches in nutrient release varies based on their composition and decomposition rates.

3.4. Seasonal Variations in Soil pH, Temperature and Environmental Humidity

The study portrays seasonal variations in soil pH, temperature, and environmental humidity over an eight-month period from August 2023 to March 2024. The results indicate significant fluctuations, which are crucial for soil health and agricultural productivity. Soil pH measurements over the study period displayed notable fluctuations (Figure 4a). Initially, on 17 August 2023, the soil pH was recorded at 6.08, indicating slightly acidic conditions. A subsequent increase was observed, with pH values reaching 6.81 on 15 September 2023, and peaking at 7.00 on both 13 October and 10 November 2023, reflecting neutral soil conditions. This stability continued until 8 December 2023, when the soil pH measured 6.98. A significant decrease in soil pH was noted in early 2024, with values dropping to 6.56 on 5 January 2024, and further to 6.40 on 2 February 2024, indicating a return to slightly acidic conditions. However, the soil pH recovered to 6.98 on 1 March 2024, and remained stable at this level through 29 March 2024. Soil pH exhibited notable seasonal fluctuations. Initially, soil pH increased from 6.08 (slightly acidic) in August 2023 to 7.00 (neutral) by October 2023. This rise can be attributed to higher temperatures and moderate humidity levels during the late summer and early fall, which likely enhanced microbial activity and OM decomposition, leading to increased soil alkalinity [100]. The stability in soil pH around neutral levels from October to December 2023 can be linked to the consistent environmental conditions that supported steady-state microbial and chemical processes [99]. However, a significant drop in soil pH was observed in early 2024, coinciding with the winter months. The decrease to slightly acidic levels (6.56 to 6.40) in January and February 2024 may be due to increased soil moisture from higher humidity and lower temperatures, which can slow down microbial activity and alter the chemical equilibrium of the soil. The recovery of soil pH to near-neutral levels in March 2024 suggests a resurgence in microbial activity and nutrient cycling as temperatures and humidity stabilized, supporting the soil’s natural buffering capacity.

The soil temperature data showed seasonal variations (Figure 4b) during the crop cycle. On 17 August 2023, the soil temperature was 26.79 °C. This value increased slightly to 25.13 °C on 15 September 2023, and then rose substantially to 29.54 °C by 13 October 2023. The highest temperature recorded during the study period was 32.88 °C on 10 November 2023. Following this peak, a slight decrease was observed, with temperatures dropping to 31.38 °C on 8 December 2023. During the winter months, temperatures fell significantly to 25.75 °C on 5 January 2024, and 26.08 °C on 2 February 2024. By 1 March 2024 and 29 March 2024, temperatures had stabilized at 27.17 °C. The data showed a clear seasonal trend in soil temperature, with the highest values recorded in November 2023 (32.88 °C) and the lowest in January 2024 (25.75 °C). These fluctuations are typical of temperate climates, where soil temperatures closely follow atmospheric temperatures [101]. The high temperatures in late summer and fall likely promoted rapid OM decomposition and nutrient mineralization, contributing to the increase in soil pH during this period. In contrast, the winter months brought cooler soil temperatures, which can slow down microbial processes and OM decomposition, potentially contributing to the observed decrease in soil pH. The stabilization of soil temperatures around 27.17 °C in March 2024 indicates the transition to spring, which is conducive to renewed microbial activity and nutrient cycling [102].

Environmental humidity exhibited considerable variation throughout the study period (Figure 4c). Initial measurements on 17 August 2023 recorded a humidity level of 59.25%. This value increased to 64.79% by 15 September 2023, and then slightly decreased to 62.38% on 13 October 2023. A significant drop in humidity to 52.33% was observed on 10 November 2023, followed by a further decline to 48.25% on 8 December 2023. A marked increase in humidity was noted in early 2024, with values rising to 71.75% on 5 January 2024, and peaking at 76.33% on 2 February 2024. By 1 March 2024 and 29 March 2024, humidity levels had stabilized at 70.75%. Environmental humidity showed substantial variation throughout the study period, with the highest values observed in February 2024 (76.33%) and the lowest in December 2023 (48.25%). The increased humidity in winter can lead to a higher soil moisture content, influencing soil pH levels [103]. The drop in humidity during the fall may have contributed to the stability of soil pH around neutral levels by limiting excessive soil moisture that can lead to acidic conditions [104]. The significant increase in humidity during winter, combined with lower temperatures, likely contributed to the observed decrease in soil pH by enhancing soil moisture levels and reduced microbial activity. As humidity stabilized in early spring, the recovery of soil pH suggests a return to more favorable conditions for microbial processes and nutrient cycling.

3.5. Influence of Mulch Material on Soil Quality

Table 5. Soil quality index as affected by different mulch material.

Treatments	PCA-Based SQI	Regression-Based SQI	Linear Scoring-Based SQI
BPM	0.370	0.625	0.379
WMM	0.125	0.875	0.130
SSM	0.000	1.000	0.000
OCM	0.500	0.500	0.503
CLM	1.000	0.000	1.000
JGM	0.500	0.500	0.498
SDM	0.750	0.249	0.748
CON	0.500	0.500	0.501

BPM, black polythene mulch; WMM, weedmat mulch; SSM, sugarcane straw mulch; OCM, organic compost mulch; CLM, cowpea-live mulch; JGM, juncao grass mulch; SDM, sawdust mulch; CON, control (no mulch); PCA, principal component analysis; SQI, soil quality index.

presents the SQI of different mulches. Cowpea-live mulch showed the highest SQI values using PCA and linear scoring methods, both at 1.000, indicating superior soil quality under this mulch. On the contrary, sugarcane straw had the lowest SQI, recording 0.000 for both PCA and linear scoring, although it showed a perfect score of 1.000 in the regression method. This contrast highlights the diverse perspectives provided by different SQI assessment techniques [38,105]. Organic compost and juncao grass mulch showed balanced outcomes across all three methods, each recording an SQI close to 0.500. Similarly, sawdust mulch showed high SQI values with PCA (0.750) and linear scoring (0.748), but had a lower regression-based SQI of 0.249. Weedmat and black polythene mulch exhibited lower SQI values overall, except in the regression-based approach where weedmat mulch scored 0.875, suggesting potential discrepancies in how these methods interpret soil quality under these mulches. The varying SQI values across various mulches highlight the importance of selecting the right mulch to enhance soil quality and enhance crop productivity. Cowpea-live mulch stands out, with both the PCA and linear scoring methods showing that it has the highest SQI. This suggests that cowpea-live mulch might be especially good at improving soil quality, probably because it adds OM and makes nutrients more available to plants. This aligns with earlier studies, which show that organic mulches can improve soil by preserving soil moisture, reducing soil erosion, maintaining soil temperature, augmenting soil structure, improving soil fertility, and improvising soil biological regimes [71,106]. The low SQI values for sugarcane straw mulch in both the PCA and linear scoring methods could indicate some drawbacks with this mulch, like a poorer soil aeration and slower breakdown of OM, which might negatively impact soil structure and nutrient cycling [107]. However, the high score in the regression-based method suggests that sugarcane straw mulch might still offer some benefits that the other methods do not capture, perhaps related to specific soil nutrients or physical properties. Organic compost mulch and juncao grass mulch show moderate and balanced SQI results across all methods, indicating that these mulches create a stable environment for improving soil. Research supports these findings, showing that compost and grass mulches are beneficial because they release nutrients gradually and improve the soil structure over time [54,108]. Organic compost mulch proves to be a reliable and balanced option, consistently showing good performance with stable SQI values across all methods. Although cowpea-live mulch achieves the highest SQI values in the PCA and linear scoring methods, suggesting it may have an edge in enhancing certain soil properties, organic compost should not be overlooked. It offers steady and sustained improvements to soil health, making it a strong choice, especially for those who prefer a mulch that provides gradual nutrient release.

3.6. Influence of Mulch Material on Yam Growth and Production

Table 6. Yam growth and yield as affected by different mulch material.

Treatments	Vine Length Plant−1 (m)	Stem Diameter Plant−1 (mm)	Total Number of Tubers Plot−1	Average Tuber Weight Plant−1 (kg)	Tuber Weight Plot−1 (kg)	Tuber Yield (kg ha−1)
BPM	3.02 ± 0.16 ab	5.64 ± 0.34 ab	16.33 ± 1.20 ab	0.61 ± 0.19 b	3.76 ± 1.01 bc	4703.33 ± 1257.55 bc
WMM	3.39 ± 0.11 a	5.47 ± 0.20 ab	18.00 ± 4.04 ab	1.19 ± 0.25 a	9.51 ± 1.94 a	11,894.17 ± 2419.80 a
SSM	2.79 ± 0.24 abc	5.44 ± 0.39 ab	17.00 ± 0.58 ab	0.54 ± 0.16 b	4.90 ± 1.63 bc	6125.00 ± 2039.80 bc
OCM	2.64 ± 0.32 bcd	6.17 ± 0.13 a	20.33 ± 3.18 ab	0.59 ± 0.14 b	5.06 ± 1.33 bc	6328.33 ± 1663.58 bc
CLM	2.25 ± 0.07 cd	5.44 ± 0.07 ab	14.00 ± 3.21 ab	0.12 ± 0.02 b	1.11 ± 0.42 c	1380.83 ± 528.00 c
JGM	2.00 ± 0.19 d	5.00 ± 0.10 b	25.33 ± 3.71 a	0.59 ± 0.15 b	5.61 ± 0.90 b	7013.33 ± 1121.93 b
SDM	2.00 ± 0.21 bcd	5.03 ± 0.41 b	12.00 ± 1.73 b	0.37 ± 0.06 b	2.37 ± 0.55 bc	2965.00 ± 689.51 bc
CON	1.99 ± 0.16 d	5.25 ± 0.14 b	17.00 ± 8.50 ab	0.22 ± 0.10 b	1.94 ± 0.89 bc	2426.67 ± 1107.19 bc
CD (p ≤ 0.05)	1.87	2.37	33.89	1.41	11.45	14,311.58
SE (d)	0.29	0.37	5.31	0.22	1.79	2240.137

BPM, black polythene mulch; WMM, weedmat mulch; SSM, sugarcane straw mulch; OCM, organic compost mulch; CLM, cowpea-live mulch; JGM, juncao grass mulch; SDM, sawdust mulch; CON, control (no mulch); plant⁻¹, per plant; plot⁻¹, per plot; kg ha⁻¹, kilogram per hectare; m, meter; mm, millimeter; kg, kilogram; CD (p ≤ 0.05), critical difference at 5% level of significance; SE (d), standard error of difference. Values with different letters in the same column indicate significant differences between treatments (p ≤ 0.05, n = 8). Data are means ± standard error.

highlights the significant impact of various mulches on yam growth and yield parameters. The vine length indicates the plant’s photosynthetic capacity and influence on crop growth [109]. The longest vine length plant⁻¹ (m) was recorded in the weedmat mulch treatment (3.39), which was significantly (p ≤ 0.05) longer (12.25%) than the vine length recorded for the black polythene mulch treatment (3.02). The shortest vine length was observed in the control treatment (1.99), which was 41.30% shorter than that of the weedmat treatment. In this study, weedmat mulch resulted in the longest vines, followed by black polythene mulch, suggesting these materials enhance above-ground linear growth. Conversely, the control treatment exhibited shorter vines, possibly due to restricted growth conditions. The extended vine lengths observed with weedmat and black polythene mulches may be attributed to their ability to retain heat in the root zone, thereby increasing root activity and nutrient uptake, which supports vigorous above-ground growth [110]. In contrast, the lack of mulch in the control treatment led to shorter vines, potentially due to greater fluctuations in soil temperature and moisture, which can stress plants and inhibit growth. The author of [111] reported that the yams planted under mulched plots were significantly higher in emergence rate, vine length, number of stem branches, number of leaves, and leaf area index (LAI) than un-mulched plots. Our findings are consistent with earlier reports by [112], which observed significantly higher emergence and growth rates of yam seedlings in mulched plots compared to un-mulched ones.

The stem diameter serves as an indicator of plant vigor and stability [113]. The widest stem diameter plant⁻¹ (mm) was observed in the organic compost mulch treatment (6.17), which was significantly (p ≤ 0.05) larger (9.40%) than that in the black polythene treatment (5.64). The narrowest stem diameter was recorded in the juncao grass treatment (5.00), which was 18.96% smaller than that of the organic compost treatment. The wider stem diameter of yam vines observed in the organic compost mulch treatment is likely due to the gradual release of nutrients from decomposing OM, which supports steady growth and contributes to sturdier stems [114]. Organic mulches also enhance macro-aggregate stability and improve water infiltration rates, which can benefit both the vine length and stem thickness [115]. Conversely, the narrower stems associated with juncao grass mulch may indicate inadequate nutrient support for later growth stages. This could result from limited OM, nutrient imbalances, slower decomposition rates, or reduced microbial activity [116]. Moreover, the type of mulch affects the micro-environment around the yam crop, which collectively influences germination, vine length, and stem diameter [117].

The highest number of total tubers plot⁻¹ was recorded in the juncao grass treatment (25.33), which was significantly (p ≤ 0.05) higher (24.59%) than that in the organic compost treatment (20.33). The lowest number of total tubers was observed in the sawdust treatment (12.00), which was 52.63% lower than that of the juncao grass treatment. The high number of total tubers plot⁻¹ with juncao grass may be due to its beneficial effects on soil properties, such as improved soil fertility and moisture retention, which are crucial for tuber development [118]. Additionally, the gradual release of nutrients from decomposing juncao grass may have provided a steady supply to support tuber growth. Organic compost mulch is also likely to offer a slow-release nutrient source throughout crop growth, promoting healthier plants. In contrast, the low tuber numbers observed with sawdust may be attributed to its high C-to-N ratio, which can cause temporary N deficiency as microorganisms decompose the sawdust, competing with plants for N [119]. This reduced nutrient availability may have limited tuber development. Furthermore, both juncao grass mulch and organic compost are known for their moisture-retention capabilities, which are vital for tuber formation and growth [71]. Sawdust mulch may have been less effective in retaining moisture, leading to less favorable conditions for tuber development and consequently fewer tubers.

The highest average tuber weight plant⁻¹ (1.19 kg) and tuber weight plot⁻¹ (9.51 kg) were observed in the weedmat mulch treatment, which were significantly (p ≤ 0.05) higher (95.08% and 69.52%) than those in the black polythene (0.61 kg) and juncao grass treatments (5.61 kg), respectively. The lowest average tuber weight plant⁻¹ (0.12 kg) and tuber weight plot⁻¹ (1.11 kg) were recorded in the cowpea-live mulch treatment, which were 89.92% and 88.33% lower than that of the weedmat mulch treatment. Similarly, the highest tuber yield (kg ha⁻¹) was observed in the weedmat treatment (11,894.17), which was significantly (p ≤ 0.05) higher (69.59%) than that in the juncao grass treatment (7013.33), with the lowest yield being 88.39% lower in the cowpea-live mulch (1380.83) than weedmat treatment. Weedmat mulch consistently delivered the best results, achieving the highest average tuber weight plant⁻¹, tuber weight plot⁻¹, and tuber yield ha⁻¹. This performance aligns with [120], who noted that synthetic mulches like weedmat effectively suppress weeds, retain moisture, and maintain optimal soil temperatures for crop performance. By blocking light, weedmat effectively prevents weed growth, reducing competition for essential resources like water and nutrients. It also minimizes moisture loss through evaporation, ensuring consistent water availability, which is crucial for tuber development [71]. Additionally, weedmat helps maintain stable soil temperatures, which supports better root and tuber growth. By protecting the soil from extensive forces, synthetic mulches further enhance nutrient and water uptake, ultimately leading to improved tuber yields [121]. Black polythene mulch also contributed to an increased tuber weight plant⁻¹, although its impact was not as pronounced as that of weedmat mulch. This is because black polythene, like weedmat, effectively suppresses weeds and helps retain moisture, but it may not regulate soil temperature as efficiently or provide the same level of consistent growth conditions [122]. On the other hand, juncao grass mulch, an organic option, significantly improved tuber weight plot⁻¹ and yield per ha⁻¹. This can be attributed to its ability to enhance soil fertility and structure as it decomposes, gradually releasing nutrients and improving soil quality over time. While not as effective at weed control as synthetic mulches, juncao grass mulch supports strong overall growth, which is reflected in the improved yields. Our findings are supported by [123,124], who also noted a significant increase in crop yield with the use of synthetic mulches. The cowpea-live mulch tended to underperform across all metrics, likely due to competition for resources and potential allelopathic effects [125]. This is consistent with research by [126], which identified the negative impacts of live mulches on crop performance.

3.7. Influence of Mulch Material on Economics of Yam Cultivation

The data in

Table 7. Relative economics of yam as affected by different mulch material.

Treatments	Total Cost of Cultivation (FJD ha−1)	Total Yield (kg ha−1)	Gross Returns (FJD ha−1)	Net Returns (FJD ha−1)	Cost-Benefit Ratio (BCR)	Internal Rate of Return (IRR)	Payback Period (Years)	Return on Investment (ROI)	Break-Even Yield (kg ha−1)	Break-Even Price (FJD kg−1)
BPM	$21,907.63	4703.33	$28,219.98	$6312.35	1.29	28.81%	3.47	28.81%	3651.27	$4.66
WMM	$35,857.63	11,894.17	$71,365.02	$35,507.39	1.99	99.00%	1.99	99.00%	5976.27	$3.01
SSM	$27,650.59	6125.00	$36,750.00	$9099.41	1.33	32.90%	3.04	32.90%	4608.43	$4.52
OCM	$27,650.59	6328.33	$37,969.98	$10,319.39	1.37	37.30%	2.68	37.30%	4608.43	$4.37
CLM	$17,750.83	1380.83	$8284.98	−$9465.85	0.47	−53.32%	N/A	−53.32%	2958.47	$12.86
JGM	$27,650.59	7013.33	$42,079.98	$14,429.39	1.52	52.17%	2.18	52.17%	3775.10	$3.94
SDM	$32,650.59	2965.00	$17,790.00	−$14,860.59	0.54	−45.53%	N/A	−45.53%	5441.77	$11.01
CON	$20,840.55	2426.67	$14,560.02	−$6280.53	0.70	−30.14%	N/A	−30.14%	3473.43	$8.59

revealed the economics of different mulch treatments for yam cultivation. Weedmat mulch emerged as the most profitable, with a total cultivation cost of FJD 35,857.63 ha⁻¹. It yielded an impressive gross return of FJD 71,365.02 ha⁻¹, leading to a substantial net return of FJD 35,507.39 ha⁻¹. For every dollar invested with weedmat, there was a return of nearly two dollars (1:1.99), making it a standout choice. The short payback period and high IRR of 99.00% further highlights weedmat’s efficiency in maximizing profitability. The juncao grass and organic compost mulch also performed well, each with a cultivation cost of FJD 27,650.59 ha⁻¹. JGM offered a gross return of FJD 42,079.98 ha⁻¹, translating to a net return of FJD 14,429.39 ha⁻¹, while OCM brought in a net return of FJD 10,319.39 ha⁻¹. This means that for every dollar invested, JGM returned 1.52 dollars, and OCM returned 1.37 dollars. On the other hand, cowpea-live and sawdust mulch did not perform well. Cowpea-live mulch resulted in the lowest net return, losing FJD 9465.85 ha⁻¹, with a poor return ratio of just 0.47. Sawdust mulch was not better, with a net loss of FJD 14,860.59 ha⁻¹ and a return ratio of 0.54. The control treatment also showed a negative return of FJD 6280.53 ha⁻¹, with a return ratio of 0.70. These results are consistent with studies by [54,71], who have noted that mulching, especially with materials like straw and composts, can significantly increase crop yields while conserving resources. The success of weedmat in this study likely stems from its ability to significantly increase yam yields compared to other mulch types and untreated soil. This echoes the findings of [53], who also found that mulch treatments, particularly with inorganic mulches such as polythene, can be highly profitable.

4. Conclusions

In conclusion, this study shed light on how different mulches impact soil properties and yam cultivation, along with how seasonal changes influence soil and environmental variables and overall soil quality. Organic compost mulch proved to be especially effective, improving soiL pH, increasing SOC, and enhancing key nutrients like SAP and SECa, all of which contribute to better soil fertility and C storage. Meanwhile, sawdust and sugarcane straw mulches each had unique effects on soil nutrient dynamics. The control plot, without mulch, retained more moisture but had lower soil temperatures. Seasonal changes also played a significant role in soil quality. Soil pH reached neutral levels toward the end of 2023, became slightly acidic in early 2024, and then stabilized. Soil temperature peaked in November, dipped during winter, and evened out by March, while humidity was highest in early 2024 before stabilizing. Among the mulches, organic compost stood out to be a reliable and balanced option for its ability to enhance soil quality due to consistently showing good performance with stable SQI values. The weedmat mulch was particularly effective in supporting yam growth and productivity, while the organic compost mulch was notable for encouraging strong stem growth and a good number of tubers. It provided a balanced nutrient mix and improved soil structure, making it highly beneficial for yam farming. The relative economics clearly indicate that the weedmat mulch is the most profitable and cost-effective mulching option for yam cultivation, delivering the highest net returns and demonstrating excellent financial viability. The juncao and organic compost mulches also offer solid returns, making them valuable alternatives for farmers seeking sustainable and economically sound practices. Based on these findings, it is recommended that farmers choose mulches that align with their specific soil conditions and seasonal factors to optimize soil quality and crop yields. Organic compost mulch is particularly recommended for its overall benefits in soil quality, along with weedmat mulch for crop productivity optimization and economic feasibility. By selecting appropriate mulches, farmers can enhance immediate productivity and ensure the long-term health and sustainability of their farms.