Effects of Water Irrigation Methods Combined with Mulching Materials on the Growth and Yield of Allium chinense on Acid Sulfate Soil

Rakkyo, also known as oriental onion, sakurab, or scallion, (Allium chinense G. Don) is a commercially important root vegetable (Bah et al., 2012). Rakkyo has been used as food seasoning, food processing and as a folk medicinal herb due to containing steroidal saponins, furostanol saponins, spirostanol saponins and xiebai-saponins as antioxidants (Shahrajabian et al., 2020). According to Pérez-Ortolá and Knox  (2015), edible species of Allium and vegetables with shallow root systems could absorb water from the surface soil layers (18-40 cm) and were exposed to water stress under deficit irrigation. In addition, the available water for annual crops was low, with about 30% of the water amount in saturated soil under field conditions. Therefore, the cultivation of annual crops requires an appropriate irrigation regime to avoid water stress, especially in the dry season. Multiple studies have been carried out to evaluate the effects of mulch materials on improving the water status of dryland crops such as garlic (Allium sativum L.) (Barakat et al., 2020), onion (Allium cepa L.) (Tehulie, 2022), corn (Zea mays L.) (Qin et al., 2015; Sanders et al., 2018; Gao et al., 2019) and soybean [Glycine max (L.) Merr.] (Vincent-Caboud et al., 2019). Mulching also improves crop growth and yield (Han et al., 2022; El-Beltagi et al., 2022). Mulches enhance soil properties by replenishing carbon and nutrients, increasing biological activities and retaining soil moisture (Chalker-Scott et al., 2007).

In Tam Nong district, Dong Thap province, Vietnam where there is acid sulfate soil, farmers used to widely apply rice straw to cover the rakkyo crop. Rice straw has fast decomposition and cannot lift the palanquin leaves, causing poor growth. In addition, farmers tend to use flood irrigation which may induce loss of nitrogen and spread diseases therefore negatively affecting the yield (Fan et al., 2014; Raza et al., 2020). Recently farmers have used Eleocharis dulcis (Burm.f.) Trin. ex Hensch. as mulch and reported positive feedback about the effect of this material on the growth and yield of the rakkyo crop. However, there has been no study about whether using Eleocharis dulcis (ED) as mulch is effective in rakkyo cultivation. Another factor is the irrigation method that the farmers use to water the crop. Such information is beneficial to maximize the production potential while reducing the production cost of the crop. The research was carried out to evaluate irrigation methods combined with mulch materials suitable for growth and productivity in the field.

The field experiment was conducted in Phu Tho commune, Tam Nong district, Dong Thap province, Vietnam in 12 months from January 2021. The experiment was conducted in a randomized complete block design (RCBD) with four replications. Treatments consisted of flooding irrigation combined with rice straw covering (FI-RS), flooding irrigation combined with Eleocharis dulcis covering (FI-ED, control), spray irrigation combined with rice strawcovering (SI-RS) and spray irrigation combined with Eleocharis dulcis covering (SI-ED). The area of each lot was 83.3 m-2 (19.6 m in length, 4.25 m in width and 25 cm in height). The type is planted with a density of 20 ´ 10 cm. After planting, the soil was covered by mulches. Eleocharis dulciswas used to mulch only one time after planting (Fig 1). Rice straw was used to mulch two times after planting (right after planting and four months after planting). Vitamin C content  and soil characteristics were analyzed at the lab of the lab of Faculty of Crop Science, College of Agriculture, Can Tho University. The characteristics of the soil including pH_H2O (4.89), pHKCl (3.76), CHC (5.89%), CEC (12.0 meq/100 g), N_Total (0.182%), P_Total (0.011%), K+ (0.601 meq K+/100) were measured before planting.


 
Data collection
 
Twenty clumps from the inner area of the plot were randomly collected at harvest time. Plant height, root length, number of leaves per plant, bulb weight and Vitamin C content were determined based on the twenty collected clumps. Plant height (cm) was measured from the soil surface to the highest growth peak. The number of leaves of each plant was counted at harvest time. Root length was measured from the base of the clump to the tip of the longest root. The average weight of a bulb was the weight of a clump without leaves and roots divided bythe number of bulbs per clump. Yield was determined by harvesting all the clumps per slot. Vitamin C content in the bulb was determined based on the 2.6 dichlorophenol indophenol titration method which was previously described by Denre (2014). Net profit was calculated according to Jayathilaka (2020). The cost of raw materials (inorganic fertilizers, DPR, plant protection drugs, seeds, water irrigation), labor and product sales were calculated according to 2021 prices.
 
Data analysis
 
The data presented in this paper are the mean values of four replications. All data were analyzed by one-way analysis of variance (ANOVA) using SPSS software package version 13.0 and were compared for significant differences in treatment effects using Duncan’s test at P<0.05. 

Clump height
 
At harvest time, the plant height of the SI-ED treatment (47.2 cm) was significantly higher than that of the SI-RS treatment and FI-RS (Fig 2). However, there was no statistically significant difference between the two irrigation methods (unpublished data). Spray irrigation and flood irrigation using Eleocharis dulcismulch for higher plant height than rice straw mulch irrigation methods because the structure of the mulch material was more durable than straw mulch materials, helping the soil retain moisture well and avoid weeds. In comparison with ED mulch, rice straw mulch was applied and faster weed growth required higher costs for weed control. In addition, rice straw decomposed so fast that it must be applied 2-3 times per crop season, costed more, showed lower land coverage and moisture retention, reduced biodiversity and increased the risks of pests and diseases. Rice straw mulching quickly collapses and made the soil less ventilated than ED mulching (Fig 3).




 
Length of roots
 
Fig 2 showed that the root length of rakkyo in flood irrigation methods combined with Eleocharis dulcismulching was significantly longer than those in SI-RS treatment. However, there was an insignificant difference in the root length of rakkyo in FI-RS and FI-ED. At harvest time, the root length of rakkyo in FI-ED treatment (10.5 cm) was significantly higher than that in SI-RS but insignificantly differed from the root length in other treatments. According to Seifu et al., (2017), the mulch material retained moisture, maintained stable soil temperature and improved the development of garlic’s root system. Therefore, the FI-ED treatment could retain moisture and maintain stable soil temperature better than the rest treatments.

Number of bulbs/clump
 
The number of bulbs (stems) per clump at harvest time in the two straw-covered treatments was higher than in the other treatments (Fig 2). The number of bulbs (stems) per clump of rakkyo was consistent with the research results. According to the study of Bah et al., (2012) on four large genera of the same family (Amaryllidaceae) grown in sandy soil, the average number of bulbs developed after eight months of planting ranged from 10 to 25 shoots per clump.
 
Number of leaves per bulb
 
The number of leaves per bulb in the two Eleocharis dulciscovering treatments was higher than in the other two at harvest time (Fig 2). Because Eleocharis dulciswas covered only 1 time, it made the rakkyo leaf grew more favorably than rice straw that had to be covered 2 times. When mulching for the 2^nd time, straw negatively affected leaf growth. 
 
Weight of bulb
 
The average bulb weight in the SI-ED treatment was 4.38 g, higher than the two treatments of SI and FI combined with rice straw mulch (2.60 and 2.63 g, respectively) (Fig 4). There was no significant difference between the bulb weights (2.60-3.48 g bulb^-1) of SI-RS, FI-ED and FI-RS treatments. Thus, rakkyo cultivation using SI combined with Eleocharis dulcismulching effectively increased average bulb weight compared to other treatments.


 
Vitamin C content
 
Fig 4 shows that the vitamin C content in rakkyo between different treatments was not significant. The average vitamin C content of 7.64 mg 100 g^-1, 3 times lower than the vitamin C content in 100 g of garlic (Fahmi, 2019) and 4 times lower than in purple cabbage sprouts (Singh et al., 2006).
 
Bulb yield
 
The spray irrigation method combined with Eleocharis dulcismulching yielded 40.9 t ha^-1, 25.9% higher than the FI-ED of 32.5 t ha^-1 (Fig 5). The yield in the FI-RS treatment (30.0 t ha^-1) was 7.7% lower than in the FI-ED (32.5 t ha^-1). Similarly, the yield of rakkyo in SI-RS was 1.9% lower than in the control treatment. This showed that Eleocharis dulcis mulching significantly increased rakkyo production. Similarly, with the same mulching material, the flood irrigation method yielded less than the spray irrigation method. This could be explained by the effectiveness of the mulching material (due to its durable structure, moisture, long use time, limited grass, etc.). The rice straw that was used to mulch decomposed so fast during the rakkyo season that it generated conditions for weed growth and negatively affected productivity. Because the objective of the experiment was not disease evaluation, it was impossible to collect grass samples to see the yield difference of the experiments.


 
Financial efficiency
 
Table 1 indicates that the profit from growing the Allium chinense was high (above 97 million VND ha^-1). Spray irrigation combined with water chestnut mulching for Allium chinense helped increase yield significantly higher than the FI-RS, FI-ED and SI-ED for Allium chinense. Flood irrigation methods combined with Eleocharis dulcis mulching increased the profit up to 25.9% in comparison with the control treatment as well as the treatment of SI-RS also increased up to 8.0%. The profit of the treatment of SI-ED dramatically increased up to 119.5% in comparison with the control treatment.

Rakkyo are grown on alkaline soil in Tam Nong district, Dong Thap province using a spray irrigation method combined with water Eleocharis dulcis was suitable for the growth and development of rakkyo. In the absence of Eleocharis dulcis sources, rakkyo cultivation using spray irrigation combined with rice straw mulching could achieve economic efficiency. In the production of rakkyo, the flood irrigation method needs to be replaced by spray irrigation.

The authors would like to thank Assoc. Prof. Dr. Tran Thi Ba, Can Tho University for her expertise and assistance throughout all aspects of our study and for her help in writing the manuscript.

This research was funded by Dong Thap Department of Science and Technology under Grant No. 99/2020/HĐ-KHCN.

All authors declared that there is no conflict of interest.