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Soil physical changes due to mulching under dry zone conditions: Evidence from Knol-khol (Brassica oleracea var. gongylodes) cultivation in Kilinochchi, Sri Lanka
T. G. S. Methmal*1, T. Sellathurai1, S. Sritharan2
1 – Department of Agricultural Engineering, Faculty of Agriculture, University of Jaffna
2- Farm Unit, Faculty of Agriculture, University of Jaffna
Abstract
Soil physical degradation and rapid soil moisture loss are major constraints to vegetable production in the dry zone of Sri Lanka. Mulching is a proven soil management practice, yet quantitative evidence on its effects on soil physical properties under local dry zone conditions is limited. This study evaluated soil physical changes induced by organic and synthetic mulches during Knol-khol (Brassica oleracea var. gongylodes) cultivation in Kilinochchi District, Sri Lanka. A field experiment was conducted using a Randomized Complete Block Design with four treatments; paddy straw mulch, palmyra dry leaves mulch, polythene mulch, and no mulch (control) with three replications. Soil moisture storage, soil temperature, bulk density, and soil organic matter were monitored throughout the cropping period using standard field and laboratory methods. Results showed that mulching significantly improved soil physical conditions (p ≤ 0.05). Mean soil moisture storage was highest under palmyra dry leaves mulch (23.8 ± 1.9%), followed by paddy straw (21.6 ± 1.7%), polythene mulch (20.2 ± 1.5%), and the control (17.4 ± 1.6%). Organic mulches moderated soil temperature, recording lower mean values under palmyra dry leaves (27.1 ± 0.8 °C) and paddy straw (27.8 ± 0.9 °C) compared to polythene mulch (29.4 ± 1.0 °C) and no mulch (30.2 ± 1.2 °C). Bulk density was lowest under palmyra dry leaves (1.42 ± 0.05 g cm⁻³) and highest under the control (1.56 ± 0.07 g cm⁻³). Soil organic matter content increased under organic mulches, reaching 1.26 ± 0.08% under palmyra dry leaves compared to 1.02 ± 0.05% in the control. The findings demonstrate that organic mulching particularly using locally available palmyra dry leaves substantially improves soil physical quality and moisture conservation and it creates the suitable environmental condition for Knol-Khol cultivation in dry zone areas.
Keywords: Soil physical properties, mulching, soil moisture, bulk density, soil temperature
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Introduction
Soil physical properties govern the movement and storage of water and air in the soil profile and strongly influence root growth, nutrient uptake, and crop productivity (Lal, 2000; Havlin et al., 1990). In dry and semi-arid regions, soils are frequently exposed to high temperatures, intense solar radiation, and irregular rainfall patterns, which accelerate soil moisture loss and structural degradation. Low organic matter content, weak aggregation, and surface crusting are common constraints that limit infiltration and increase runoff, thereby reducing effective water availability for crops.
Sri Lanka’s dry zone is particularly vulnerable to such soil physical degradation due to its prolonged dry spells, high evaporative demand, and increasing pressure from intensive vegetable cultivation. Under these conditions, maintaining favorable soil physical properties is critical for sustaining crop productivity and water use efficiency. Conventional soil management practices that leave the soil surface exposed often exacerbate moisture stress and temperature extremes in the root zone, leading to suboptimal crop growth.
Mulching, defined as the application of organic or synthetic materials on the soil surface, has been widely recognized as an effective soil and water conservation practice (Chalker-Scott, 2007; Kader et al., 2017). By providing a protective cover, mulches reduce direct soil exposure to solar radiation and wind, thereby decreasing evaporative water losses and moderating soil temperature fluctuations. Mulching has also been reported to suppress weed growth, reduce soil erosion, and improve infiltration rates, collectively contributing to improved soil physical quality.
Organic mulches such as crop residues, straw, and leaf litter play a dual role by modifying the soil microclimate and contributing organic matter to the soil through decomposition. The addition of organic residues enhances soil aggregation, increases porosity, and promotes biological activity, leading to long-term improvements in soil structure and stability (Duiker and Lal, 1999; Paustian et al., 1997). Several studies have demonstrated that organic mulching reduces soil bulk density and increases soil water-holding capacity, particularly in coarse-textured and degraded soils common to dry environments (Saroa and Lal, 2003).
In contrast, synthetic mulches such as polythene primarily influence soil moisture and temperature regimes without adding organic carbon to the soil. Plastic mulches have been shown to effectively reduce surface evaporation and increase early-season soil temperature, which can be beneficial in cooler climates. However, under tropical and dry-zone conditions, excessive soil heating beneath polythene mulch may adversely affect root growth and soil microbial activity (Chakraborty et al., 2008). The long-term implications of synthetic mulches on soil physical health are therefore less favorable compared to organic alternatives.
Previous research across tropical and semi-arid regions has consistently reported improvements in soil moisture retention, moderated soil temperature, and reduced bulk density under mulched conditions (Adekalu et al., 2007; Ali and Talukder, 2008). However, the magnitude and persistence of these effects vary widely depending on mulch material, thickness, climatic conditions, and soil type. Despite the recognized benefits of mulching, locally relevant quantitative data on soil physical changes under different mulching materials remain limited for northern Sri Lanka.
Knol-khol (Brassica oleracea var. gongylodes) is a short-duration, shallow-rooted vegetable crop that is highly sensitive to soil moisture stress and elevated soil temperatures. Its growth and yield are therefore closely linked to root-zone physical conditions, making it an appropriate indicator crop for evaluating soil physical responses to mulching in dry-zone environments. A clearer understanding of how organic and synthetic mulches influence soil physical properties during Knol-khol cultivation is essential for developing climate-resilient soil management strategies for vegetable production systems in Sri Lanka’s dry zone.
Accordingly, this study aimed to quantify soil physical changes specifically soil moisture storage, soil temperature, bulk density, and soil organic matter resulting from the application of organic and synthetic mulches during Knol-khol cultivation under dry-zone conditions of Sri Lanka. -
Materials and Methods
2.1 Study area
The experiment was conducted at the Faculty of Agriculture, University of Jaffna, Kilinochchi, located in Sri Lanka’s dry zone. The area receives approximately 1325 mm of annual rainfall, with nearly 75% occurring during the Northeast monsoon (September–December). Mean daily temperatures range from 25 to 30 °C, and potential evapotranspiration frequently exceeds rainfall during the dry season. The soils are predominantly red-yellow latosols and regosols, characterized by low organic matter content and moderate water holding capacity.
2.2 Experimental design and treatments
A Randomized Complete Block Design (RCBD) was used with four treatments and three replications. The treatments were: (T1) paddy straw mulch, (T2) no mulch (control), (T3) palmyra dry leaves mulch, and (T4) polythene mulch. Each experimental plot measured 1.0 m × 0.7 m. Organic mulches were applied at an approximate thickness of 10 cm one week prior to transplanting. Knol-khol seedlings were transplanted following national agronomic recommendations, and all plots received uniform irrigation and nutrient management.
2.3 Soil sampling and analysis
Soil samples were collected at regular intervals during the cropping period. Soil moisture content was determined gravimetrically and expressed as percentage moisture storage. Soil temperature was measured at 10 cm depth using a soil thermometer during mid-day hours to capture peak thermal conditions. Bulk density was determined using the core sampler method, and soil organic matter content was analyzed using the Walkley–Black wet oxidation method.
2.4 Statistical analysis
Data were analyzed using analysis of variance (ANOVA). Treatment means were compared using Dunnett’s test at a 95% confidence level (p ≤ 0.05). Results are presented as mean ± standard deviation (SD). - Results and Discussion
3.1 Soil moisture storage
Mulching significantly increased soil moisture storage compared to the no-mulch control (p ≤ 0.05). As shown in Figure 1, the highest mean soil moisture storage was observed under palmyra dry leaves mulch (23.8 ± 1.9%), followed by paddy straw mulch (21.6 ± 1.7%) and polythene mulch (20.2 ± 1.5%). The control treatment recorded the lowest moisture storage (17.4 ± 1.6%). These results indicate that surface mulching effectively reduces evaporative losses and improves in-situ water conservation under dry-zone conditions (Ali & Talukder, 2008; Adekalu et al., 2007).
Organic mulches performed better than polythene mulch, likely due to their ability to enhance infiltration and improve soil structure through organic matter addition. The superior moisture retention under palmyra dry leaves mulch can be attributed to its dense coverage and slower decomposition rate, which prolongs soil surface protection and increases microporosity.
Figure 1: The soil moisture content with days after transplanting
3.2 Soil temperature
Soil temperature was significantly influenced by mulching material. Mean soil temperature at 10 cm depth was lowest under palmyra dry leaves mulch (27.1 ± 0.8 °C), followed by paddy straw mulch (27.8 ± 0.9 °C). Polythene mulch resulted in higher soil temperatures (29.4 ± 1.0 °C), while the no-mulch control recorded the highest temperatures (30.2 ± 1.2 °C).
Organic mulches acted as thermal insulators, reducing heat transfer to the soil and buffering diurnal temperature fluctuations (Chalker-Scott, 2007; Chakraborty et al., 2008). In contrast, polythene mulch amplified soil heating under high solar radiation, which may adversely affect root activity and soil biological processes in tropical dry environments.
3.3 Bulk density
The Figure 2 express the soil bulk density before and after the application of different mulch treatments. Significant differences in soil bulk density were observed among treatments (p ≤ 0.05). The lowest bulk density was recorded under palmyra dry leaves mulch (1.42 ± 0.05 g cm⁻³), followed by paddy straw mulch (1.46 ± 0.06 g cm⁻³). Polythene mulch showed moderate bulk density (1.51 ± 0.05 g cm⁻³), while the control treatment exhibited the highest bulk density (1.56 ± 0.07 g cm⁻³).
Reduced bulk density under organic mulches indicates improved soil aggregation and increased pore space, which enhance root penetration and water movement (Duiker & Lal, 1999; Lal, 2000). The higher bulk density observed in the control reflects soil compaction and structural degradation due to direct exposure to rainfall and irrigation impacts.
Figure 2: The bulk density of soil under different treatment condition
3.4 Soil organic matter content
Soil organic matter content increased significantly under organic mulching treatments (Figure 3). Palmyra dry leaves mulch increased soil organic matter to 1.26 ± 0.08%, while paddy straw mulch recorded 1.18 ± 0.07%. In contrast, polythene mulch (1.05 ± 0.06%) and the no-mulch control (1.02 ± 0.05%) showed minimal changes.
The addition and gradual decomposition of organic residues contributed to carbon inputs and improved soil structure under organic mulches (Paustian et al., 1997; Saroa and Lal, 2003). Enhanced organic matter content is closely linked to improved aggregation, moisture retention, and overall soil physical resilience in dry zone soils.
Figure 3: The organic matter content of soil under different treatment conditions
- Conclusions
This study demonstrates that mulching significantly improves soil physical properties during Knol-khol cultivation under dry-zone conditions of Sri Lanka. Organic mulches; particularly palmyra dry leaves were most effective in enhancing soil moisture storage, moderating soil temperature, reducing bulk density, and increasing soil organic matter content. Compared to polythene mulch and no mulch, organic mulching provided superior soil physical benefits and greater resilience against moisture stress. The use of locally available palmyra dry leaves as mulch is recommended as a sustainable and cost effective soil management practice for improving soil physical quality and water conservation in dry zone vegetable production systems and it creates the suitable environmental condition for Knol-Khol cultivation in dry zone areas.
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