A salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere, its preparation method and application

Abstract

This invention relates to the field of soil conditioning materials technology, and more particularly to a salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere, its preparation method, and its application. By selecting earthworm castings as a filler, sodium alginate as an encapsulating agent, and desulfurized gypsum as a crosslinking agent, this invention constructs salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres with high mechanical strength and high microbial encapsulation rate. These gel microspheres can effectively improve saline-alkali soils by increasing soil nutrient and organic matter content, improving microbial survival rate, and reducing soil pH and salinity. They possess advantages such as safety, high efficiency, low cost, and simple operation, and have broad application prospects.

Description

Technical Field

[0001] This invention relates to the field of soil conditioning materials technology, and in particular to a salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere, its preparation method, and its application. Background Technology

[0002] Saline-alkali soil is an important type of salt-induced soil degradation. In recent years, soil salinization caused by climate change has become a key issue of global soil degradation and an obstacle to agricultural development. Globally, salt affects approximately 935 million hectares of land in arid and semi-arid regions, accounting for more than 20% of the total global irrigated area.

[0003] Currently, many soil conditioners are used to mitigate the negative effects of saline-alkali soils. These include materials or fertilizers such as gypsum and biochar. However, improper use of these conditioners can lead to soil acidification, nitrogen and phosphorus nutrient imbalances, and decreased agricultural product quality. In recent years, the use of salt-tolerant microorganisms has shown promising prospects in buffering and improving soil conditions. *Azotocinobacter glomeratus*, a salt-tolerant free-living nitrogen-fixing bacterium, can use nitrogenase to reduce atmospheric nitrogen molecules into ammonium nitrogen, which can be absorbed and utilized by plants, thus enriching soil nitrogen nutrients. However, the life activities of *Azotocinobacter glomeratus* are still limited by the soil environment. Lack of soil nutrients and high pH levels can significantly reduce the number and function of the microbial population, affecting the improvement effect. To ensure the survival of *Azotocinobacter glomeratus* introduced into the soil, a carrier is needed to assist it in resisting adverse external environmental factors. Sodium alginate is a polysaccharide extracted from brown algae. It possesses excellent biodegradability and is a perfect substrate for encapsulating microorganisms. Its rich carboxyl groups can cross-link with calcium ions in solution to form water-absorbing gel microspheres with a 3D network structure, effectively alleviating soil moisture stress and addressing problems such as soil moisture shortage caused by water stress and soil salt accumulation caused by capillary action. Furthermore, it effectively utilizes calcium... 2+ It provides a source of nutrients and reduces soil salinity. However, sodium alginate microspheres have low mechanical strength and are easily damaged during application, leading to the rapid release and loss of microorganisms.

[0004] Therefore, how to provide a gel microsphere with high mechanical strength, high encapsulation rate of microorganisms, and effective ability to improve saline-alkali soil has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned technical challenges, this invention provides a salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere, the raw materials of which include: earthworm castings, sodium alginate, desulfurized gypsum, microbial inoculants, and co-solvents.

[0006] Earthworm castings are the product of earthworms ingesting and digesting organic matter. They are a natural organic fertilizer rich in nutrients such as nitrogen, phosphorus, potassium, protein, and humus. They have a large specific surface area, good aeration, drainage and high water holding capacity, and are also rich in a large number of beneficial microorganisms.

[0007] This invention constructs a more stable double-network hydrogel by selecting earthworm castings as a filler, sodium alginate as an encapsulating agent, and desulfurized gypsum as a crosslinking agent, thereby improving the mechanical strength of the gel microspheres and their encapsulation efficiency for microorganisms. Furthermore, earthworm castings, as a nutrient source for microorganisms, can effectively improve their survival rate. When applied to soil, the gel microspheres can also swell by absorbing water and salt, releasing some of the nutrients from the earthworm castings into the soil, improving soil nutrient conditions, providing a more favorable environment for microorganisms, and enhancing their survival rate and bioavailability.

[0008] Using industrial waste desulfurization gypsum instead of commonly used calcium chloride as a crosslinking agent can realize the resource utilization of waste and reduce preparation costs.

[0009] In some embodiments, the microbial agent includes microorganisms and Mg2SO4 solution.

[0010] In some implementations, the microorganism is *Azotobacter brownii*.

[0011] In some embodiments, the microbial agent is prepared by culturing microorganisms (preferably Azotobacter brownii) in a nutrient medium (preferably LB medium) and then resuspending the microorganisms in Mg2SO4 solution.

[0012] In some embodiments, the concentration of microorganisms in the microbial agent is 3.8 × 10⁻⁶. 8 CFU / ml or higher.

[0013] In some embodiments, the cosolvent is gluconolactone.

[0014] In some implementation schemes, the weight ratio of earthworm castings, sodium alginate, and microbial inoculant is (0.5~2):1:50.

[0015] In some implementations, the weight ratio of desulfurized gypsum to co-solvent is 1.5:3.

[0016] Furthermore, the present invention provides a method for preparing the salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres, comprising: mixing sterile water, earthworm castings, sodium alginate and microbial inoculant to obtain a suspension; mixing sterile water, desulfurized gypsum and co-solvent to obtain a crosslinking agent solution; and adding the suspension dropwise to the crosslinking agent solution to undergo a crosslinking reaction to obtain salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres.

[0017] In some implementation schemes, the weight ratio of sterile water, earthworm castings, sodium alginate, and microbial inoculant is 50:(0.5~2):1:50.

[0018] In some implementations, the weight ratio of sterile water, desulfurized gypsum, and co-solvent is 300:1.5:3.

[0019] In some embodiments, the preparation method further includes washing the salt-tolerant earthworm feces-based dual-network microbial sustained-release gel microspheres with water after the crosslinking reaction.

[0020] Furthermore, the present invention provides the application of the salt-tolerant earthworm castings-based dual-network microbial sustained-release gel microspheres or the preparation method thereof in at least one of the following aspects:

[0021] (1) Preparation of soil conditioner;

[0022] (2) Improve saline-alkali soil.

[0023] In some implementation schemes, the goal of improving saline-alkali soil is achieved by increasing soil nutrient and organic matter content, improving microbial survival rate, and reducing soil pH and salinity.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] This invention constructs salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres with high mechanical strength and high microbial encapsulation rate by selecting earthworm castings as filler, sodium alginate as encapsulating agent, and desulfurized gypsum as crosslinking agent. These gel microspheres can effectively improve saline-alkali soil by increasing soil nutrient and organic matter content, improving microbial survival rate, and reducing soil pH and salinity. They have the advantages of safety, high efficiency, low cost, and simple operation, and have broad application prospects. Attached Figure Description

[0026] Figure 1 The graph shows the swelling properties of different gel microspheres under acidic / alkaline conditions.

[0027] Figure 2 This is the release curve of salt-tolerant earthworm feces-based dual-network microbial slow-release gel microspheres IV within 28 days.

[0028] Figure 3 This is a graph showing the test results for the viable count and mortality rate of Azotobacter brownii.

[0029] Figure 4 This is a graph showing the effect of different gel microspheres on soil pH.

[0030] Figure 5 This is a graph showing the effect of different gel microspheres on the total salt content of soil.

[0031] Figure 6 This is a graph showing the effect of different gel microspheres on the total nitrogen content of soil.

[0032] Figure 7 This is a graph showing the effect of different gel microspheres on soil organic matter content. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. In the following embodiments, "parts" refers to parts by weight.

[0034] In the embodiments provided in this specification, unless specific techniques or conditions are specified, the techniques or conditions described in the literature in this field, or the product instructions, shall be followed. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased from legitimate channels.

[0035] Example 1

[0036] This embodiment provides a salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere, prepared as follows:

[0037] (1) The brown azotocin bacteria were cultured in nutrient medium (LB medium containing 10 g / L sodium hydroxide, 10 g / L tryptone, and 5 g / L yeast extract) for 16 h, and then the OD of the bacterial culture was measured using a 0.03% Mg2SO4 solution. 600 The value was adjusted to 1.532, and *Azotobacter chrysogenum* was resuspended to obtain a microbial inoculum. The concentration of microorganisms in the inoculum was 3.8 × 10⁻⁶. 8 cfu / ml;

[0038] (2) Air dry the earthworm castings (filler) and pass them through a 100-mesh sieve. Take 0.5 parts of filler and 1 part of sodium alginate (embedding agent) and mix them with 50 parts of microbial agent (50 mL) and 50 parts of sterile water and stir for 1 hour to form a uniform microbial agent-embedding agent-filler suspension;

[0039] (3) Mix 1.5 parts of crosslinking agent (desulfurized gypsum), 3 parts of cosolvent (gluconolactone) and 300 parts of sterile water and stir for 1 hour. Filter and collect the liquid to obtain a crosslinking agent solution.

[0040] (4) Add the above microbial agent-embedding agent-filler suspension to a 50ml syringe and slowly drip it into the cross-linking agent solution to cross-link into gel microspheres;

[0041] (5) After cross-linking and curing at room temperature for 1 hour, the synthesized gel microspheres were washed three times with deionized water and dried overnight at room temperature. The resulting salt-tolerant earthworm feces-based double-network microbial slow-release gel microspheres were named Gel Microsphere II and stored in centrifuge tubes for later use.

[0042] Example 2

[0043] This embodiment provides a salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere. The only difference between this embodiment and Example 1 is that the amount of earthworm castings is adjusted to 1 part. The resulting salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere is named Gel Microsphere III.

[0044] Example 3

[0045] This embodiment provides a salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere. The only difference between this embodiment and Example 1 is that the amount of earthworm castings is adjusted to 2 parts. The resulting salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere is named Gel Microsphere IV.

[0046] control group

[0047] This control group provides a microbial sustained-release gel microsphere, the preparation method of which differs from Example 1 only in that the amount of earthworm castings is adjusted to 0 parts, and the resulting microbial sustained-release gel microsphere is named Gel Microsphere I.

[0048] Comparative Example

[0049] The comparison is based on published literature, as shown in Table 1.

[0050] Experimental Example 1

[0051] This experimental example tests the swelling and mechanical properties of the gel microspheres prepared in the different embodiments described above. The steps are as follows:

[0052] 1. The swelling performance test method is as follows:

[0053] Swelling analysis was performed using the gravimetric expansion method. Dried gel microspheres (weight W0) were immersed in a 0.9% sterile sodium chloride solution for 24 hours and then removed. Excess water was wiped off with a paper towel, and the microspheres were weighed (weight W1). The swelling rate was calculated as: Swelling rate (ER, %) = (W1 − W0) / W0 × 100%. Each treatment was performed in triplicate.

[0054] The swelling performance test results are as follows:

[0055] The test results of swelling properties are as follows: Figure 1As shown, A1~A4 represent the control group, Example 1, Example 2, and Example 3 gel microspheres, respectively. The swelling capacity of the gel microspheres was significantly higher under alkaline conditions than under acidic conditions, but the swelling rate decreased from 6992.72% to 4580.43% with increasing earthworm castings content. The experimental results indicate that salt and alkaline environments have a synergistic effect on the swelling of the gel microspheres. Under acidic conditions, the unreacted hydrophilic groups -COO- inside the gel spheres can bind water molecules, causing the material to swell. Under alkaline conditions, OH-... - The presence of [a specific substance] accelerates the conversion between -COOH and -COO-, promoting the expansion of the water adsorbent polymer network chains, while the release of microorganisms depends on the swelling or relaxation of the gel polymer system. However, with the increase of filler content, the binding between the filler and the encapsulating agent becomes tighter, leading to a reduction in hydrophilic groups, thereby reducing the swelling performance of the material.

[0056] 2. The test method for mechanical strength is as follows:

[0057] The dried gel microspheres were immersed in a 0.9% sterile sodium chloride solution for 24 hours and then removed. Excess water was wiped off with a paper towel, and the mechanical strength was judged by the appearance.

[0058] The test results for mechanical strength are as follows:

[0059] When the gel microspheres were removed from the 0.9% sterile sodium chloride solution, the gel microspheres without filler were soft, fragile and inelastic, while the gel microspheres with 0.5-2 parts of filler had a certain elasticity and were not easily broken. Moreover, the mechanical strength was the highest when the amount of earthworm castings added was two parts (Example 3).

[0060] Experimental Example 2

[0061] The microbial encapsulation efficiency, sustained release effect of *Azotobacter chrysogenum*, viable count, and mortality rate of the gel microspheres prepared by the above different methods were determined.

[0062] 1. The test method for microbial encapsulation efficiency is as follows:

[0063] Encapsulation efficiency is the number of actual microorganisms in the gel microspheres. Plate counts were used to calculate the number of viable bacteria (N0) in the sodium alginate mixture and the number of unencapsulated viable bacteria (N1) in the cross-linking agent. Encapsulation efficiency was calculated using the following formula: Encapsulation efficiency (EE, %) = (N0 − N1) / N0 × 100%.

[0064] The type of filler significantly affects the microbial encapsulation efficiency. Table 1 shows the test results of the microbial encapsulation efficiency of the gel microspheres and the comparison with existing studies. Previous studies have shown that adding trehalose or starch as fillers to gel microspheres can significantly improve the microbial encapsulation efficiency, with maximum encapsulation efficiencies reaching 84.91% and 70.83%, respectively. In this invention, the gel microspheres prepared using earthworm castings as filler achieve an encapsulation efficiency of up to 90% for microorganisms. Compared to gel microspheres without earthworm castings, adding earthworm castings significantly increases the microbial encapsulation efficiency from 63.61% to over 90%, and when the amount of earthworm castings added is 2 parts, the microbial encapsulation efficiency reaches 100%. This indicates that earthworm castings as a filler can significantly improve the microbial encapsulation efficiency, with the optimal addition amount being 2 parts (Example 3). Therefore, subsequent measurements of the microbial slow-release effect and viable cell count are based on Example 3.

[0065] Table 1 Results of Microbial Encapsulation Efficiency Test

[0066]

[0067] 2. The test method for the sustained-release effect of Azotobacter brownii is as follows:

[0068] 0.5 g of gel microspheres were immersed in 10 mL of sterile sodium chloride solution (0.9%) and stored at room temperature for 28 days. Samples (0.1 mL) were collected at 1, 3, 5, 7, 14, and 28 days, and the number of viable bacteria in the solution was determined by the dilution plating method. All experiments were performed in triplicate.

[0069] Taking the gel microspheres of Example 3 as an example, the results of testing the sustained-release effect of Azotobacter brownii are as follows: Figure 2 As shown in the image, the gel microspheres exhibit a sustained-release effect, with a significant increase in viable bacteria count during the first 5 days, reaching 10. 7 cfu g -1 The viable bacterial count peaked on day 14, at approximately 10. 8 cfu g -1 .

[0070] 3. The test methods for viable count and mortality rate of Azotobacter brownii are as follows:

[0071] Viable bacteria count: 1.0 g of dried gel microspheres were immersed in 10 mL of sterile phosphate buffer (pH 7.0) for 1 hour. The microspheres were then ground into the solution, resulting in the release of bacteria embedded or covered by the microspheres. After serial dilutions, all viable bacteria were counted for colony counting.

[0072] The microbial mortality rate is calculated as follows: Mortality rate = (Total number of bacteria - Viable number of bacteria) ÷ Total number of bacteria × 100%.

[0073] Taking the gel microspheres of Example 3 as an example, the results of the determination of viable count and mortality rate of Azotobacter brownii are as follows: Figure 3 As shown, on day 1, the number of viable *Azotobacter chrysogenum* in the salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres was more than twice that of the original bacterial solution. After 28 days, the number of viable bacteria in the original bacterial solution decreased significantly, with a mortality rate of 98.8% for *Azotobacter chrysogenum*, while the number of viable bacteria in the salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres decreased by only 36.7%. These results indicate that the salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres can provide a carbon source for *Azotobacter chrysogenum*, increase microbial activity, and effectively prevent microorganisms from being disturbed by external environmental factors.

[0074] Experimental Example 3

[0075] This experimental example uses gel microspheres prepared in the above-described examples and the control group to treat moderately saline-alkali coastal soil, including the following steps:

[0076] The experiment employed a randomized experimental design, comprising: a control group (CK, blank soil), treatment group 1 (gel microspheres I, A1), treatment group 2 (gel microspheres II, A2), treatment group 3 (gel microspheres III, A3), and treatment group 4 (gel microspheres IV, A4), with four replicates per group. For each replicate, 300g of air-dried soil (passed through a 2mm sieve) was weighed, mixed thoroughly with the gel microspheres, and then placed in flowerpots. Throughout the experiment, the soil moisture content was maintained at 60% of field capacity.

[0077] 1. Effects on soil pH

[0078] After 30 days of treatment, the soil pH values ​​of different treatment groups were tested, and the results are as follows: Figure 4 As shown in the figure, the soil pH values ​​of different gel microsphere treatments ranged from 8.10 to 7.12. Compared with the control group, the soil pH values ​​of soils treated with 0 parts (A1), 0.5 parts (A2), 1 part (A3), and 2 parts (A4) of earthworm castings generally showed a decreasing trend, with pH values ​​decreasing by 3.7%, 2.0%, 5.34%, and 4.6%, respectively. Except for the A4 and A3 treatment groups, which showed no significant difference in soil pH, the other treatments showed significant differences. The buffering effect of gel microspheres on soil pH may be related to the acidic functional groups they carry. Under high pH conditions, the acidic functional groups will undergo deprotonation (-COOH at pH>4; -OH at pH>8), initiating a neutralization reaction and thus lowering the soil pH. In addition, under high salinity and alkalinity conditions, the gel microspheres swell, and the organic acids contained in the earthworm castings are released into the soil through the pores, reacting with alkaline substances such as carbonates and bicarbonates in the soil and lowering the soil pH.

[0079] 2. Impact on total soil salt content

[0080] After 30 days of treatment, the total salt content of the soil in different treatment groups was tested. The changes in total salt content of the soil under different treatments are as follows: Figure 5 As shown, the total salt content of each treatment ranged from 3.60 to 4.33 g / kg. -1 The total salt content of soils in treatments A1, A2, A3, and A4 was significantly lower than that in the control group, decreasing by 14.0%, 10.8%, 11.6%, and 17%, respectively. The gel microspheres can provide a calcium source for the soil, reducing the content of exchangeable sodium in saline-alkali soils. Soil salts can also permeate into the microspheres under osmotic pressure until osmotic equilibrium is reached. The highest soil salt removal efficiency in treatment A4 is likely due to the highest content of earthworm castings, which provides sufficient carbon source for microorganisms, maximizing the activity of *Azotobacter graminearum* and thus leveraging the synergistic effect between microorganisms and gel microspheres.

[0081] 3. Impact on soil total nitrogen content

[0082] After 30 days of treatment, the total nitrogen content in the soil of different treatment groups was tested. The changes in total nitrogen content in the soil of different treatments are as follows: Figure 6 As shown, compared with the control group, gel microspheres with different amounts of earthworm castings all increased the total nitrogen content of the soil. With increasing earthworm castings content, the total nitrogen content of the soil increased. When the earthworm castings content was 2 parts, the total nitrogen content in the soil was significantly different from the control group, increasing by 14.0%. This may be because the increased earthworm castings content increased the cross-linking density with sodium alginate, achieving a higher encapsulation efficiency of *Azotobacter glomerulosa*, and earthworm castings can provide a carbon source for *Azotobacter glomerulosa*, increasing its activity. Under the action of nitrogenase, *Azotobacter glomerulosa* can fix atmospheric nitrogen into the soil, increasing soil nutrient content.

[0083] 4. Impact on soil organic matter content

[0084] After 30 days of treatment, the soil organic matter content of different treatment groups was tested. The changes in soil organic matter content of different treatments are as follows: Figure 7 As shown, gel microspheres with different amounts of earthworm castings effectively improved soil organic matter content, increasing it by 6.2% to 8.2% compared to the control group. The treatment with two parts earthworm castings showed the best effect in increasing soil organic matter content. Under saline-alkali conditions, the gel microspheres swelled, causing the molecular chains to loosen and promoting the release of *Azotobacter graminearum*. *Azotobacter graminearum* participates in important processes such as soil organic matter decomposition and nutrient transformation, thus increasing soil organic matter content to a certain extent.

[0085] In conclusion, a ratio of two parts vermicompost is optimal, maximizing microbial activity. At this ratio, the salt-tolerant vermicompost-based dual-network microbial slow-release gel microspheres exhibit the best effect in improving saline-alkali soil.

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A salt-tolerant earthworm castings-based dual-network microbial slow-release gel microsphere, characterized in that, Its raw materials include: earthworm castings, sodium alginate, desulfurized gypsum, microbial agents, and solubilizers; The microbial inoculant includes microorganisms and Mg2SO4 solution; the microorganisms are *Azotobacter brownii*. The preparation method of the salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres includes: mixing sterile water, earthworm castings, sodium alginate and microbial inoculant to obtain a suspension; mixing sterile water, desulfurized gypsum and co-solvent to obtain a crosslinking agent solution; and adding the suspension dropwise to the crosslinking agent solution to undergo a crosslinking reaction to obtain salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres.

2. The salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres according to claim 1, characterized in that, The concentration of microorganisms in the microbial agent is 3.8 × 10⁻⁶. 8 CFU / ml or higher.

3. The salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres according to claim 1, characterized in that, The co-solvent is gluconolactone.

4. The salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres according to claim 1, characterized in that, The weight ratio of earthworm castings, sodium alginate, and microbial inoculant is (0.5~2):1:50; and / or the weight ratio of desulfurized gypsum to co-solvent is 1.5:

3.

5. The method for preparing salt-tolerant earthworm castings-based dual-network microbial sustained-release gel microspheres according to any one of claims 1 to 4, characterized in that, include: A suspension was prepared by mixing sterile water, earthworm castings, sodium alginate and microbial inoculant. A crosslinking agent solution is obtained by mixing sterile water, desulfurized gypsum and a co-solvent. The suspension was added dropwise to the crosslinking agent solution to undergo a crosslinking reaction, resulting in salt-tolerant earthworm feces-based dual-network microbial slow-release gel microspheres.

6. The preparation method according to claim 5, characterized in that, Also includes: After the cross-linking reaction, the salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres were washed with water.

7. The use of the salt-tolerant earthworm castings-based dual-network microbial slow-release gel microspheres according to any one of claims 1 to 4, or the product prepared by the method according to claim 5 or 6, in at least one of the following aspects: (1) Preparation of soil conditioner; (2) Improve saline-alkali soil.

Images