Evaluation of Cobalt Application Combined with Gypsum and Compost as a Regulator of Cabbage Plant Tolerance to Soil Salinity

Abd-Elbaset A. Alkharpotly^1,2, Mohamed A. E. Soliman³, Mohamed A. El-Sherpiny⁴, Ahmed G. Baddour⁴, Dina A. Ghazi⁵, Zinab A. Abdelgawad⁶, Mohamed A. Abdein^7*, Fahad Mohammed Alzuaibr⁸, Abdulrahman Alasmari⁸, Abdulaziz Albogami⁹, Mesfer M. Alqahtani¹⁰, Hebat-Allah A. Hussein¹¹, Fatma M. A. Elkady¹², Mohamed F. M. Abdelkader¹³

¹Horticulture Department, Faculty of Agriculture and Natural Resources, Aswan University, Aswan, Egypt.
²Horticulture Department, Faculty of Desert and Environmental Agricultural, Matrouh University, Mersa Matruh, Egypt.
³Soil and Water Department, Faculty of Agriculture, Damietta University, Damietta, Egypt.
⁴Soil & Water and Environment Research Institute, Agriculture Research Center, Giza, Egypt.
⁵Soil Sciences Department, Faculty of Agriculture, Mansoura University, Mansoura, Egypt.
⁶Botany Department, Women’s College, Ain Shams University, Cairo, Egypt.
⁷Seeds Development Department, El-Nada Misr Scientific Research and Development Projects, Turrell, Mansoura, Egypt.
⁸Biology Department, College of Science, University of Tabuk, Tabuk, Saudi Arabia.
⁹Biology Department, Faculty of Science, Al-Baha University, Alaqiq, Saudi Arabia.
¹⁰Department of Biological Sciences, Faculty of Science and Humanities, Shaqra University, Ad-Dawadimi, Saudi Arabia.
¹¹Biology Department, University College of Nairiyah, University of Hafr Al Batin (UHB), Nairiyah, Saudi Arabia.
¹²Botany Department, National Research Center, Cairo, Egypt.
¹³Department of Plant Production, College of Food and Agriculture, King Saud University; Riyadh, Saudi Arabia.

DOI: 10.4236/oje.2023.1312056                

Abstract

In response to the global food crisis and the imperative to address soil degradation, the international agricultural policy is actively working to alleviate the adverse impacts of soil salinity. As part of this initiative, a field trial spanning two consecutive seasons (2019/20-2020/21) was conducted under saline conditions. The primary objective was to evaluate the influence of various compost sources, including vermicompost at a rate of 0.5 ton·fed^-1 and plant residues compost at a rate of 5.0 ton·fed^-1, as main plots. Subplots were established by applying agricultural gypsum, both in the presence and absence of gypsum requirements. Additionally, sub-subplots were created by externally applying cobalt at a rate of 10.0 mg·L^-1, with one sub-subplot receiving foliar cobalt application and the other not. The trial sought to assess the growth performance, chemical composition, enzymatic antioxidants, yield, and quality of cabbage plants (Brassica oleracea var. capitata L.) cultivated in saline soil. According to the findings, cabbage plants exhibited the most favorable response in terms of plant height, chlorophyll content, carotene levels, leaf area, nitrogen (N), phosphorus (P), potassium (K), head yield, vitamin C, and total dissolved solids (TDS) when treated with vermicompost, followed by plant compost. Conversely, plants grown without compost exhibited the least improvement in performance. Cabbage treated with agricultural gypsum requirements showed better performance than those without gypsum amendment. Moreover, plants subjected to cobalt spray demonstrated the highest growth, yield, and quality parameters compared to those without cobalt foliar application. In contrast, the control group (plants without the studied treatments) displayed the highest levels of enzymatic antioxidants, specifically catalase and peroxidase. This indicates that soil salinity stress led to an increase in catalase and peroxidase production in cabbage plants as a defense against the harmful impact of reactive oxygen species (ROS) resulting from soil salinity stress. The applied treatments (compost, gypsum, and cobalt) led to a reduction in the cabbage plant’s inherent production of catalase and peroxidase. Generally, the combined treatment of vermicompost × gypsum requirements × cobalt proved effective in mitigating the detrimental effects of soil salinity on cabbage plants. These findings hold significance for farmers and policymakers aiming to enhance agricultural productivity in regions affected by soil salinity. Additionally, further research can explore the long-term effects of these treatments on soil health and crop sustainability.

Keywords

Vermicompost, Plant Compost, Gypsum Requirements, Cobalt, Soil Health and Crop Sustainability

1. Introduction

While humanity lacks control over environmental stressors, individuals must comprehend how stressors impact plants and other organisms. This understanding enables the implementation of preventive measures [1] . Salinity stress stands out as a highly damaging abiotic stress factor that negatively influences the growth, productivity, and physiology of plants [2] . Saline soil, characterized by the accumulation of excessive salts, typically manifests visibly on the soil surface [3] . Natural capillary action transports salt to the soil surface, loading it from saline groundwater, and subsequent accumulation occurs through evaporation. Human activities can further contribute to the density of salinity in the soil [4] . As soil salinity intensifies, the detrimental effects of salt increase, potentially leading to the degradation of both soil and plants [5] . The elevated salinity hinders the plant’s absorption of certain elements, and an increased intake of salts can result in ionic poisoning of the cell. Additionally, the rise in salt levels leads to a reduction in water absorption by the plant due to the high osmosis of soil water [6] . Soil salinity exerts a harmful impact, resulting in a reduction in the yield of all crops [7] . This influence adversely affects plant growth by diminishing leaf water potential, triggering morphological and physiological changes, disrupting biochemical processes, generating reactive oxygen species (ROS), and escalating ion toxicity and osmotic stress [8] [9] .

To safeguard higher plants from the detrimental impact of saline soil, an effective approach involves the use of soil organic fertilizers. These fertilizers have the potential to supply essential nutrients to plants thriving in salt-affected soil [10] . Additionally, they play a role in enhancing and improving soil properties, which in turn positively influence the nutritional components of vegetable crops [11] . Vermicompost, a nourishing organic fertilizer, contains beneficial soil microbes such as “nitrogen-fixing bacteria” and mycorrhizal fungi, along with substantial levels of nitrogen (2% - 3%), phosphorus (1.55% - 2.25%), potassium (1.85% - 2.25%), humus, and micronutrients [12] . The production of vermicompost involves the biodegradation of organic substances through interactions between earthworms and microorganisms [13] . It has been known that sodium and magnesium possess a negative impact on soil’s attributes when their levels are relatively high compared with calcium [14] . Agricultural gypsum has become an efficient soil amendment for reclaiming salt-affected soil of poor aggregation or soil structure [10] . Its application increases soluble Ca²⁺ in the soil solution to substitute the adsorbed Na⁺, thus overcoming the dispersion impacts of Na⁺ and improving the soil structure in the dispersed saline soils [7] . Another protective method against soil salinity stress involves the exogenous application of cobalt [15] . This application has the potential to enhance various plant tolerances to salinity conditions [16] , thereby positively impacting the growth performance of higher plants and mitigating the adverse effects of salinity stress [17] . While cobalt was previously considered solely beneficial for higher plants, it is now officially classified as an essential element for higher plants, as indicated in REGULATION (EU) 2019/1009 by the Official Journal of the European Union [18] . Cabbage (Brassica oleracea var. capitata L.) holds significance as a crucial leafy vegetable cultivated globally and belongs to the Brassicaceae family [19] . It is known for its rich nutritional profile, containing vitamins A, B1, B2, and C, along with antioxidants, riboflavin, carotenoids, thiamine, minerals, and polyphenols [20] .

So, the specific objectives of the current study were to evaluate the cobalt application combined with gypsum and compost as a regulator of cabbage plant tolerance to soil salinity.

2. Materials and Methods

2.1. Experimental Location

The current research work was executed during two successive seasons (2019/20-2020/21) in the Experimental Farm of the Faculty of Agriculture, Mansoura Univ., El-Dakahlia Governorate, Egypt (31˚03'00"N 31˚22'59"E).

2.2. Initial Soil Analysis

Before transplanting the seedlings, a composite initial soil sample was taken for analysis depending on Dane and Topp [21] and Sparks et al., [22] . Table 1 indicates its characteristics.

2.3. Preparation of the Substances Studied

Compost: Plant residues i.e., maize stock, soybean, wheat, and chickpea residues) was obtained and then composted at the site of the experiment depending on El-Hammady et al., [23] .

Vermicompost: It was prepared using Earthworm (Eisenia fetida) depending on Wako, [24] using the same previous plant residues. The collected substrates were chopped and added to the worm bin.

Agricultural gypsum (CaSO₄. 2H₂O): It was obtained from El Shafeey company, Giza, Egypt, and has a calcium content of 22.9 g/100g⁻¹, sulfur content of 17.9 g/100g⁻¹, pH value of 7.8 and EC value of 2.5 dSm⁻¹ (1:5 agricultural gypsum amendment: water). Gypsum requirements (GR equivalent 2.80 ton·fed⁻¹) were measured according to FAO and IIASA [25] as follows:

$\begin{array}{c}\text{GR}=\frac{\text{InitialsoilESP}\left(13.9\text{\%}\right)-\text{RequiredsoilESP}\left(10\text{\%}\right)}{100}\\ \times \text{CEC}\left(42.0\text{cmol}\cdot {\text{kg}}^{-1}\right)\times 1.72\times \text{CEC}\left(42.0\text{cmol}\cdot {\text{kg}}^{-1}\right)\times 1.72\end{array}$

Cobalt: Cobalt sulfate (CoSO₄, 38.022% Co) was obtained from El-Gamhoria Company, Egypt, and then its solution was prepared by dissolving a known mass of the compound in the solvent, and then prepared the studied rate (10.0 mg·L⁻¹).

Table 2 shows the properties of both studied compost sources.

2.4. Application time of the Substances studied

Compost and vermicompost: They were thoroughly mixed with the surface

Table 1. Characteristics of the initial soil taken at a depth of 0.0 - 30 cm before transplanting.

Table 2. Properties of both studied compost sources.

of the studied saline soil layer (0 - 30 cm) in a single application one month before transplanting.

Agricultural gypsum (CaSO₄. 2H₂O): It was thoroughly mixed with the surface of the studied saline soil layer (0 - 30 cm) in a single application four before transplanting. During the four months before transplanting, the studied saline soil was irrigated after gypsum soil addition up to the saturation limit every 20 days to get rid of Na⁺. The height of added irrigation water above the studied saline soil surface was about 10 cm.

Cobalt: Foliar application of cobalt solution was sprayed at three stages (20, 40 and 60 days after transplanting) by hand sprayer until saturation point with the volume of 450 L·fed⁻¹.

2.5. Experimental Setup

A field trial was conducted under a split-plot design with three replicates aiming to evaluate the impact of different compost sources i.e., vermicompost at rate of 0.5 ton·fed⁻¹ and plant residues compost at rate of 5.0 ton·fed⁻¹ as main plots as well as agricultural gypsum [once in the presence of gypsum requirements (2.80 ton·fed⁻¹) and other in the absence of gypsum requirements] as subplots and exogenous application of cobalt as sub-subplots at rate of 10.0 mg·L⁻¹ [once with foliar application of cobalt and other in the absence of cobalt] on the growth performance, chemical constituents, enzymatic antioxidants, yield and quality of cabbage plant grown on saline soil.

Cabbage seedlings (cv. Brunswick, 45 days old) took place at the field on 25^th of December during both growing seasons with a planting distance of 0.85 m apart within rows and 75 cm. between rows, where the sub subplot area was 20.0 m².

Irrigation process, mineral fertilization (N, P and K) and the other common agricultural practices for cabbage production were carried out as recommended by The Ministry of Agriculture.

2.6. Harvest

The Harvest process was done after 75 days after transplanting.

2.7. Measurements

At harvesting time, cabbage plant samples were taken randomly from each experimental sub sub-plot to record the following criteria:

1) Growth parameters: Plant height (cm) and leaf area (cm²).

2) Photosynthetic pigments: Chlorophyll (SPAD) and carotene content (mg·g⁻¹) were determined spectrophotometrically by the procedure postulated by Ranganna [26] .

3) Chemical constituents: N (Kjeldahl method), P (spectrophotometer method), K (flam photometer method) were estimated in cabbage leaves according to Walinga et al., [27] . The oven-dried samples were wet digested by a mixture of perchloric and sulfuric acids (1:1) according to the method of Peterburgski, [28] .

4) Enzymatic antioxidants: catalyze and peroxidases (ΔA·min⁻¹·0g⁻¹, FW) were determined as described by Alici and Arabaci, [29] .

5) Head quality attributes: Head length and diameter (cm), No of wrapper leaves, average head weight (kg) and head yield (ton·ha⁻¹).

6) Quality traits: Using a hand Refractometer, TDS (total dissolved solids, %) was determined according to AOAC, [30] . Vitamin C (VC, mg/100g⁻¹) was estimated via titration with 2.6 diclorophenol indophenol blue dye according to AOAC, [30] . Samples were oven-dried at 60˚C until constant weight, and then dry matter percent was calculated.

2.8. Statistical Analysis

Statistical analysis of the obtained data was done according to Gomez and Gomez [31] .

3. Results

Data tabled show the individual effect impact of two types of compost sources i.e., vermicompost and plant residues compost as well as gypsum and cobalt and their interactions on the performance of cabbage plants grown on salt-affected soil during seasons of 2019/2020 and 2020/2021. Growth performance i.e., plant height (cm) and leaf area (cm²) and photosynthetic pigments i.e., chlorophyll (SPAD) and carotene content (mg·g⁻¹) were shown in Table 3, while leaves chemical constituents i.e., N, P and K were illustrated Table 4. Table 5 indicates head quality attributes i.e., head length and diameter (cm), No of wrapper leaves, average head weight (kg) and head yield (ton·h⁻¹), while Table 6 indicates quality traits i.e., vitamin C (mg/100g⁻¹), total dissolved solids (TDS, %) and dry matter (DM, %). Finally, Table 7 indicates enzymatic antioxidants i.e., catalyze and peroxidase (ΔA·min⁻¹·g⁻¹, FW).

3.1. Growth Performance, Quality, Yield, and Its Components

Individual effect of different compost sources: The results indicated that the cabbage plant responded best to vermicompost compared to plant compost, while the control treatment (plants grown without compost) had the least response

Table 3. The Effect of cobalt application combined with gypsum and compost on the performance of cabbage plants expressed in plant height, chlorophyll and carotene contents and leaf area during seasons of 2019/2020 and 2020/2021.

Table 4. The Effect of cobalt application combined with gypsum and compost on leaves chemical constituents of cabbage during seasons of 2019/2020 and 2020/2021.

Table 5. The Effect of cobalt application combined with gypsum and compost on head physical qualities of cabbage and yield during seasons of 2019/2020 and 2020/2021.

Table 6. The Effect of cobalt application combined with gypsum and compost on the quality of cabbage during the seasons of 2019/2020 and 2020/2021.

Table 7. The Effect of cobalt application combined with gypsum and compost on the cabbage plant’s self-production from enzymatic antioxidants during the seasons of 2019/2020 and 2020/2021.

and performance enhancement. Specifically, the plants treated with vermicompost had the highest values of plant height (cm), leaf area (cm²), chlorophyll (SPAD), carotene content (mg·g⁻¹), N, P and K, head length and diameter (cm), No of wrapper leaves, average head weight (kg) and head yield (ton·h⁻¹),vitamin C (mg/100g⁻¹), total dissolved solids (TDS, %) and dry matter (DM, %). The plants treated with plant residues compost had slightly lower values than those treated with vermicompost, but still showed better performance compared to the control treatment.

Individual effect of agricultural gypsum: The findings presented in Tables 3-6 demonstrate that cabbage plants treated with agricultural gypsum had the highest values of plant height (cm), leaf area (cm²), chlorophyll (SPAD), carotene content (mg·g⁻¹), N, P and K, head length and diameter (cm), No of wrapper leaves, average head weight (kg) and head yield (ton·h⁻¹), vitamin C (mg/100g⁻¹), total dissolved solids (TDS, %) and dry matter (DM, %). On the other hand, the lowest values were recorded for plants grown without gypsum amendment.

Individual effect of cobalt: Tables 3-6 demonstrate that the cabbage plants treated with foliar spray of cobalt exhibited higher values for growth, yield, and quality parameters than the plants grown without cobalt application.

Interaction effect: Also, as shown in Tables 3-6, the plants treated with the combined treatment of vermicompost × gypsum requirements × cobalt was useful in reducing harmful effect of soil salinity on cabbage plant and recorded the highest values of plant height (cm), leaf area (cm²), chlorophyll (SPAD), carotene content (mg·g⁻¹), N, P and K, head length and diameter (cm), No of wrapper leaves, average head weight (kg) and head yield (ton·h⁻¹), vitamin C (mg/100g⁻¹), total dissolved solids (TDS, %) and dry matter (DM, %).

3.2. Enzymatic Antioxidants

Table 7 displays that the control cabbage plants (grown without any treatments) exhibited the highest levels of enzymatic antioxidants, specifically catalase and peroxidase. This suggests that soil salinity stress triggered an increase in the production of catalase and peroxidase in cabbage plants, as a defense mechanism against the harmful effects of ROS resulting from salinity stress. On the other hand, the application of the studied treatments (compost, gypsum, and cobalt) led to a decrease in the plant’s self-production of catalase and peroxidase.

Individual effect of different compost sources: Contrary to the previous results, the highest values of catalyze and peroxidase (ΔA·min⁻¹·g⁻¹, FW) were recorded with control treatment (without organic soil addition) followed by soil addition of plant residues compost then the vermicompost treatment.

Individual effect of agricultural gypsum: Table 7 shows that the highest values of aforementioned enzymatic antioxidants were recorded with control treatment (without gypsum soil addition) followed by gypsum treatment

Individual effect of cobalt: The same Table also, illustrates that the highest values of catalyze and peroxidase (ΔA·min⁻¹·g⁻¹, FW) were achieved with control treatment (without foliar application of cobalt) followed by cobalt treatment.

Interaction effect: Also, as shown in Table 7, the cabbage plants treated with the combined treatment of vermicompost × gypsum requirements × cobalt recorded the lowest values of catalyze and peroxidase (ΔA·min⁻¹·g⁻¹, FW).

4. Discussion

Generally, it can be said that both organic sources had a positive impact on improving the growth, quality, and yield of cabbage plants grown in saline soil, due to their ability to supply essential macro and micronutrients. Moreover, both sources may aid in increasing soil aggregates, facilitating leaching of salts away from the root zone. The superiority of vermicompost over plant residue compost may be attributed to its lower C/N ratio and higher nutrient content. Additionally, vermicompost may have a faster and more immediate effect compared to plant residue compost. Furthermore, the nutrients in vermicompost may dissolve easily in irrigation water, whereas those in plant residues compost may not be as readily available for the cabbage plant to utilize.

The superior performance of vermicompost may be attributed to its ability to facilitate and chelate solid elements in the soil, making it easier for the plant to absorb and benefit from them. Vermicompost not only supplies the plant with necessary major and minor elements but also provides a diverse range of bacteria that have multiple important functions for the plant. This means that it provides the soil with the ability to manufacture and create nutrients, growth regulators, and materials to resist soil pests, which helps to restore the soil’s vitality. Vermicompost also contains antibiotics and fungi such as Actinomyces, which can raise the plant’s biological resistance against insects and diseases, reducing the need for pesticide spraying. These findings are consistent with those of Rekha et al., [12] ; Ceritoğlu et al., [13] ; Abo El-Ezz et al., [10] and Ghazi et al., [11] .

Agricultural gypsum may have been superior due to its ability to increase soil aggregates by providing Ca²⁺ ions that facilitate the formation of stable soil aggregates. This, in turn, leads to the leaching of salts with continuous soil irrigation before planting, as a result of the soil addition of gypsum. Moreover, the addition of agricultural gypsum might improve the properties of saline soil, leading to an increase in cabbage tolerance against salinity conditions. Agricultural gypsum has a Ca²⁺ content of 23%, which enables the displacement of Na⁺ ions on the cation exchange sites of the saline soil colloids. Therefore, the application of agricultural gypsum to saline soil could accelerate Na⁺ leaching, subsequently increasing the percentage of exchangeable calcium and decreasing the percentage of exchangeable sodium. The obtained results are in accordance with those of Ghazi et al., [9] . The foliar application of cobalt resulted in more robust plant growth and superior performance, quality, and yield, owing to its ability to stimulate growth during various physiological stages. These findings are consistent with those of Baddour et al. [15] , who reported that cobalt can mitigate the adverse effects of salinity and enhance the plant’s ability to withstand it.

It can be said that the cabbage plants grown under control conditions without any treatment had higher levels of self-produced enzymatic antioxidants (catalase and peroxidase (ΔA·min⁻¹·g⁻¹, FW)) in their tissues compared to those grown under compost (either plant residues or vermicompost) combined with gypsum and cobalt to tolerate salt stress. This is likely because cabbage plants have the ability to increase various scavenging mechanisms of free radicals (ROS) to alleviate the damage caused by salinity stress. Salinity stress disrupts the balance between free radical production and scavenging, resulting in oxidative damage to plant cells. Both types of compost can provide nutrients to cabbage plants, thereby helping them to tolerate salt stress. The addition of compost significantly reduced oxidative stress damage in cabbage by reducing the production of activated oxygen species and lipid peroxidation. In other words, the application of vermicompost and plant residues compost might reduce the accumulation of free radicals in compost-treated cabbage plants compared to untreated plants.

The cabbage plants treated with gypsum and cobalt had lower levels of enzymatic antioxidants (catalase and peroxidase (ΔA·min⁻¹·g⁻¹, FW)) compared to those grown without these treatments. The decrease in enzymatic antioxidants in gypsum-treated plants can be attributed to the role of gypsum in increasing soil aggregates and leaching salt, which reduces the need for cabbage plants to produce more catalase and peroxidase. On the other hand, cobalt plays a vital role in boosting plant immunity and scavenging free radicals (ROS) in the chloroplast of cabbage plants, reducing the need for more catalase and peroxidase production. Therefore, it can be concluded that cobalt treatment alleviated the cabbage plant’s self-production of catalase and peroxidase. These findings are consistent with previous studies by [9] [10] [11] [12] [13] [15] [32] [33] ; and [34] .

Finally, it can be said that the studied materials, namely vermicompost, agricultural gypsum, and cobalt, collectively contribute to mitigating salinity stress through several mechanisms. Vermicompost, rich in beneficial soil microbes and essential nutrients, enhances soil fertility, promoting improved plant growth and nutrient absorption. Agricultural gypsum acts by increasing soluble calcium in the soil, displacing adsorbed sodium, thereby mitigating dispersion effects and enhancing soil structure in saline conditions. Cobalt, recognized as an essential element for higher plants, plays a pivotal role in enhancing plant tolerance to salinity conditions. Its exogenous application reflects positively on growth performance, reducing the harmful effects of salinity stress. The combination of vermicompost, gypsum, and cobalt synergistically addresses various aspects of salinity stress, offering a comprehensive and effective approach to improving the resilience of cabbage plants in saline soils.

5. Conclusion

This study underscores the significant role of integrated treatments, specifically the combined use of vermicompost, agricultural gypsum, and cobalt, in mitigating the detrimental effects of soil salinity on cabbage plants. The findings highlight the positive impact of vermicompost on various growth and quality parameters, with cabbage plants treated with agricultural gypsum also showing improved performance. Furthermore, the application of cobalt, now recognized as an essential element for higher plants, demonstrated substantial benefits in terms of enhanced growth, yield, and quality. The study emphasizes the potential of these interventions in alleviating the challenges posed by soil salinity, offering valuable insights for farmers and policymakers seeking to boost agricultural productivity in salinity-affected regions. Moving forward, future research should delve into the long-term effects of these treatments on soil health and the sustainability of crops, providing a more comprehensive understanding of their implications.

Funding

The current research work was funded by the personal efforts of the authors.

Data Availability Statement

Not applicable.

Acknowledgements

The authors would like to thank, Egypt: Aswan, Matrouh, Damietta & Mansoura Universities, Agriculture Research Centre, National Research Center, El-Nada Misr Scientific Research and Development Projects, Saudi Arabia: Tabuk, Al-Baha, Shaqra, Hafr Al Batin & King Saud Universities.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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