The effect of humic acid on the desalinization of coastal clayey saline soil Open Access

The area around the Yellow River Delta is large, has low terrain and poor drainage. Its soil evaporates and salt accumulates in spring and autumn (He et al. 2021), leading to high soil salinity. Salinized soil poses serious problems, such as high soil salinity, low organic matter content, difficult crop growth, and low crop yield, which restrict local agricultural production and socio-economic development (Zhang et al. 2021). As a result, the improvement and treatment of coastal saline-alkali soil are of great significance to the development and construction of the Yellow River Delta.

Adding amendments is an especially important measure to improve saline-alkali soil (Zheng 2018). Many scholars have conducted research and gained valuable results. At present, materials in the soil are divided into inorganic (such as gypsum, desulfurized gypsum, aluminum sulfate, calcium silicate, etc.) and organic materials (such as biochar, organic fertilizer, humic acid, etc.) (Zhang et al. 2019).

Armstrong & Tantonn (1992) used gypsum to improve salinized alkaline soil and studied and analyzed the change in the surface soil salinity. They found that desulfurized gypsum can cause soil desalinization, but its high cost restricted its application in practical production. Sun et al. (2011) found that adding aluminum sulfate to the heavy saline-alkali soil in the Songnen Plain can effectively reduce the soil's PH value and cause soil leaching and desalting. However, they also found that the heavy metal ions contained in desulfurized gypsum cannot be controlled, posing a great threat to the soil environment. Yue et al. (2015) used biochar as a modifier and studied its influence on the leaching effect of saline-alkali soil through the indoor soil column test. The results showed that the soil column with biochar would not only shorten the leaching time, but also speed up desalination. Nevertheless, its cost was high, and it was not suitable for large-scale research. When Li (2018) studied the combined effect of calcium silicate and a bio-organic fertilizer on saline-alkali soil, they found that the excessive use of the organic fertilizer increases the soil's PH value, despite it causing soil desalinization to a certain extent. Although the soil fertilizer, farm organic fertilizer, organic waste residue, and other organic substances can significantly improve the fertility of saline-alkali soil and promote salt leaching, these studies (Melillo et al. 2002; Jalali & Ranjbar 2009; Sahin et al. 2011) uncovered that long-term use pollutes groundwater.

Modified materials can improve soil texture and effectively reduce the soil salinity because they also have low salinity, long survival time in the soil, an appropriate PH value, and cause no secondary damage to the environment (Mahdy 2011; Yazdanpanah & Mahmoodabadi 2013). At present, both organic and inorganic materials can effectively improve soil structure but are costly and cause many problems in practical application, such as secondary damage to the soil and the environment. Likewise, they cannot be applied in a large area. Therefore, a common concern of scholars is to find efficient, cheap, environmentally safe, and practical soil ameliorants.

As a macromolecular organic matter, humic acid is accumulated in the remains of animals and plants through a series of chemical processes, i.e., the decomposition and transformation of microorganisms. Because it is also an organic colloid, it is highly active and has a strong adsorption and condensation capacity. It can bond dispersed soil particles, form soil aggregates with high stability, and improve the physical and chemical properties of the soil (Wang et al. 2019). Application of humic acid in soil has attracted the attention of Chinese and foreign scholars. Gao et al. (2020) found in studying the influence of humic acid on sunflower yield and soil salt-alkali index that humic acid has significant effects in reducing soil salinity, soil SAR (Sodium Adsorption Ratio) and improving sunflower yield. Chai et al. (2021) carried out an investigation experiment on the influence of humic acid on soil chemical characters and grape fruit quality, and found that soil exchangeable Ca²⁺, Mg²⁺ and organic matter contents were significantly increased, and fruit quality was significantly improved. Xiong (1979) found that adding humic acid can improve the soil organic matter content and soil water retention ability, which can reduce the content of sodium ions and the loss of potassium in the soil. Studies (Chen et al. 2002; Wang & Zhu 2021) believed that humic acid could promote seed germination and root growth. Previous studies (Liu & Wang 2020; Yang et al. 2021) found that humic acid can help increase the amount of phosphorus in the soil, improve physical and chemical properties, aggregate the stability of the soil, and regulate the soil microbial community.

On the whole, it is a simple and feasible measure to use humic acid materials to improve saline-alkali soil. However, most of the current studies on the improvement of saline-alkali soil by humic acid focus on soil nutrients, soil desalting, soil ion changes, organic carbon and microbial activities, etc. The study on the adsorption and leaching effect of humic acid on saline soil is very limited.

Therefore, this study takes the coastal saline soil of the Yellow River Delta as the research object. Given the problems of large saline-alkali land area, high salinity, poor structure and low fertility, we choose high-quality, environmentally friendly and practical humic acid as improved material. Through the simulated soil column leaching and adsorption experiment and a field test, we mainly study the effect of humic acid on desalination of coastal clay saline soil. The main objectives are as follows:

• (1)

  The effect of humic acid on the adsorption characteristics of water and salt in coastal clay saline soil under saline leaching was analyzed through laboratory tests. And study the mechanism and law of humic acid on soil desalting effect under fresh water leaching.

• (2)

  By analyzing the effect of humic acid on the variation of water and salt in the field and wheat yield, the suitable dosage of humic acid for the improvement of clay saline soil was determined. The relationship and effect of application amount of humic acid on saving water and reducing salt in the improvement of clay saline soil were verified.

The ratio of saline soil and humic acid in the simulated soil column is shown in Table 1. CK was used as control treatment and did not contain humic acid. Soil was collected in spring for this laboratory experiment, and the salinity of the initial soil sample was 14.92 g/kg. Due to the small amount of humic acid, its salinity was ignored. CK and T1-T8, all soil columns, underwent the same treatment procedure. The simulated soil column in the laboratory test was divided into 9 groups of treatments, with 3 parallel treatments for each group. Humic acid and saline soil were mixed evenly according to the determined ratio, then filled into a PVC pipe with a diameter of 7 cm at a constant bulk density of 1.49 g/cm³. The simulated soil column was pressed to a height of 6.5 cm. A total of 27 soil columns were simulated.

Saltwater from the drainage channel and distilled water made in the laboratory were used to simulate leaching soil columns. Table 2 shows the salinity and volume of water used for the two leaching sessions.

The first leach water contained saltwater with a salinity of 7.47 g/L (SW), while the second one contained freshwater with a salinity of 0 g/L (FW)(125 ml is formulated according to irrigation quota of Shandong Province, simulating normal field irrigation). The simulated soil column was rinsed with distilled water for 24 hours after the first rinsing with saltwater. A glass bottle is set under the soil column to collect the filtrate after each leaching, so as to determine its salinity and volume.

The field test was conducted in the Wudi County in Binzhou City and located at a longitude of 117 °45′29.84″–117 °49′47.19″ E and a latitude of 37 °51′10.01″–37 °53′32.02″ N. The area consists of clay saline soil, while the main crop cultivated there is wheat.

The field experiment was divided into 5 treatments: the control treatment (CK), treatment 1 (T1), 2 (T2), 3 (T3), and 4 (T4). Each treatment was further divided into 3 groups of repeated experiments. The amount of humic acid in different treatments is shown in Table 3 (Under CK,T1-T4 treatment, the ratio of humic acid used in the field was the same as that in the laboratory test).

Soil samples were taken from plots 30–5 in the Bohai Granary Project test area. The soil sampling depth was 0–40 cm. After retrieving soil samples, they were air-dried and cleaned of impurities in the laboratory. After the soil sample is fully mixed, it is screened by a 2 mm sieve for reserve use.

The water samples used in the laboratory test were taken from the drainage channel in the test area and the salinity of the water samples were measured prior to each test.

Humic acid used in the laboratory and field tests was provided by commercial companies. In the soil column adsorption and leaching simulation test, the leach water was deionized water made in the laboratory.

The measurement items mainly include soil salinity and water content before and after humic acid treatment, the volume and salt content of soil leaching filtrate, and wheat yield.

• (1)
  Soil salinity: To measure the soil salinity, soil samples screened by a 1 mm sieve were mixed with deionized water in a ratio of 1:5. The soil extract was obtained after centrifugation. The filtrate of soil was then filtered, and the conductivity value of the filtrate was measured by the DDB-2 conductivity meter. After recording the electrode constant, the soil salinity was calculated according to the standard curve formula determined by laboratory technicians. The electrode constant was 1.028. The conversion formula between the soil conductivity measured by the electrode and the soil salinity is shown in Formula (1).

  

  (1)

  where y₀ is the soil salinity, g/kg; X is the conductivity value at 25 °C, DS m⁻¹
• (2)
  Desalination rate:

  

  (2)

  where r is the desalination rate (%); y₁ is the soil salinity before leaching; y₂ is the soil salinity after leaching.
• (3)
  Soil water content is determined through the drying method. The aluminum box was weighed and marked as M_0. Afterward, the weight of the aluminum box and air-dried soil was weighed and marked as M₁. The aluminum box was then placed into the oven at 105 °C. After 4 hours, it was taken out, weighed, and marked as M₂. According to Formula (3), the water content θ was obtained:

  

  (3)
• (4)

  Soil bulk density: To determine the soil bulk density, samples were taken with a soil ring knife from soil at a depth of 0–20 and 20–40 cm at each sampling point (100 cm³). Three parallel samples were taken from each test plot, sealed, stored, and brought back to the laboratory. The soil bulk density was calculated after drying the soil in an oven at 105 °C until it achieved a constant weight.

• (5)
  Salinity of leaching solution and soil salinity after leaching: The electrode constant of 0.977 was used to determine the salinity in the leached liquid. The conversion formula between the conductivity measured by the electrode and the salinity is described in Formula (4). The salinity of the filtrate and the leached liquid product were calculated according to this formula, Then the soil salinity of simulated soil column after leaching is calculated according to the volume of leaching liquid, and the calculation formula is as follows (5) and (6):

  

  (4)

  

  (5)

  

  (6)

  where M is the salinity of the filtrate (g/L); is the conductivity value at 25 °C (μs/cm⁻¹) · y₁ and y₂ are the soil salinity of the simulated soil column after each leaching, respectively, (g/kg); V₁ and V₂ are the volume of filtrate collected after each wash, respectively, (L); M₁ and M₂ are respectively the salinity values (g/L) of the filtrate collected after each leaching, which are calculated according to Equation (4).

The specific determination method is as follows:

• (1)

  Two pieces of circular filter paper with a diameter of 7 cm were placed at the bottom of the soil column to prevent the loss of soil particles during the test. At the same time, two screens were placed on the surface of each soil column to avoid soil depression caused by a water impact.

• (2)

  The installed soil column was placed horizontally on the test table, the leach water was slowly and evenly poured into the soil column through the glass rod, and the upper water layer in the soil sample was kept stable when adding more water.

• (3)

  The leachate was collected in the beaker after the leaching process ended. The leachate volumes V₁, V₂ and the conductivity value of the leachate were measured.

• (6)
  Salt removal efficiency or desalting efficiency refers to the amount of salt discharged per unit of water volume.

  

  (7)

  where P is the salt removal efficiency (g/L); M is content in the filtrate (g/kg); V is the amount of leaching.
• (7)

  Wheat yield: Six different representative points were selected for each treatment according to the ‘S’ type. The effective panicle number and the 1,000-grain weight and average panicle number of wheat per mu were calculated, after which the wheat yield of different treatments was calculated in the field test.

The data processed in the experiment were standardized by Microsoft Excel 2010 and the data in the chart were the mean ± standard deviations of the three repetitions. The curve fitting of the mathematical models was plotted using Origin 2017(IBM, USA). A one-way analysis of variance (one-way ANOVA) followed by LSD (Least Significant Difference) test was used to determine the statistical significance of each parameter in SPSS (IBM, USA). In the figures for each assay, values followed by different letters are significantly different at p ≤ 0.05.

As can be seen from Figure 3(a), the soil water content under T1-T8 was higher than that under the CK treatment. It showed a trend of first increasing and then decreasing with the increase of the humic acid content. The soil water content gradually increased under T1 and T2 and decreased under T3-T8. Compared to the CK treatment, the soil water content under T1-T8 increased by 0.7%, 1.0%, 0.8%, 0.9%, 0.7%, 0.3%, 0.3%, and 0.1%, respectively. The difference between T1-T5 and the CK treatment was significant (p < 0.05). However, no significant difference was observed in the soil water content between T6-T8 and the CK treatment. When the soil was treated with humic acid, the soil water content was the highest in T2 (37.75%), followed by T4 (37.72%), but it was the lowest in T8 (37.43%).

It can be seen from Figure 3(b) that the soil salinity had the concentration of 14.9 g/kg and 6.5 g/kg before and after salt leaching, respectively. Thus, the soil salinity was greatly reduced. After leaching, the soil salinity showed a multivariate change because it first increased, then decreased, and then again increased (Figure 3(b)). Under the CK and the T1-T2 treatments, the soil salinity rose, but gradually decreased under T3-T6. The change in the soil water content in the early stage of the experiment was consistent with that in the soil. Compared to the CK treatment, the salinity in T1-T8 increased by 2.19%, 3.80%, 0.88%, 1.82%, 0.91%, −0.79%, 1.27%, and 3.74%, respectively. In addition, there were significant differences between treatments T2-T8 and the CK treatment (p < 0.05), while no significant difference was observed in the salinity between treatments T1 and T3-T7 and the CK treatment. When the soil was treated with humic acid, the soil salinity under T2 was the highest (6.52 g/kg), followed by T8 (6.51 g/kg). It was the lowest under T6 (6.23 g/kg), even lower than under the CK treatment. However, the two treatments showed no significant difference between each other.

The soil salinity had a concentration of 6 g/kg before leaching and 4 g/kg after leaching. It significantly decreased under all treatments in which desalting occurred to a certain extent, too. However, the desalting effect was different due to the different amounts of humic acid. As shown in Figure 4, the desalting rate under the CK and the T1-T8 treatments decreased by 31.01%, 34.24%, 35.16%, 35.23%, 35.28%, 36.38%, 36.76%, 35.46%, 34.07%, respectively, when increasing the amount of humic acid. Overall, the rate of desalination first increased and then decreased. The difference in the desalting rate between the T1-T8 and the CK treatments reached a significant level (p < 0.05). T6 had the highest desalting rate, followed by T5, while T8 had the lowest desalting rate.

As shown in Figure 5, the salt removal efficiency first increased and then decreased with an increase in humic acid. The soil salt removal efficiency was concentrated at 6–7 g/L in 125 mL freshwater leach. Compared to the CK treatment, the salt removal efficiency under the T1-T8 treatments increased by 3.23%, 7.55%, 6.36%, 6.76%, 5.10%, 3.20%, 2.31%, and 1.95%, respectively. The results indicate that adding humic acid can improve the efficiency of soil salt removal, which was the highest in T2, followed by T4, and the lowest in T8.

On one hand, the T2-T4 treatments showed a significant difference in the salt removal efficiency compared to T1. The salt removal efficiency increased with the increase of humic acid at T1-T4 treatments. On the other, the salt removal efficiency in the T4-T8 treatments gradually decreased. The difference in the salt removal efficiency in the T6-T8 treatments reached a significant level compared to T4. At T6-T8 treatments, the soil salt removal efficiency gradually decreased with the increase of humic acid.

Spring is the most intense season of evaporation in the Yellow River Delta region, and also the most obvious time of soil resalinity, which is the key period of soil salinity control in the improvement of saline-alkali soil. We took the soil from April 30, 2017, to study the effect of humic acid on soil water and salinity under natural conditions in the field, and to explore the effect of humic acid on soil resalinity in spring. From 15 April to 30 April, there was no irrigation or rainfall, and soil water content was in a constant state of evaporation. The results indicate that the field was not irrigated, there was no rain, and soil water content constantly evaporated from April 15 to 30, 2017.

• (1)

  Changes in soil water content and salt salinity at a depth of 0–20 cm

Crops are cultivated at the depth of 0–60 cm, so it is significant to study the changes in the water and salinity in the soil at this depth. At this depth, the soil water content under each treatment had a concentration of 25%, higher than that at a 0–20 cm depth. Compared to the CK treatment, the water content of T1-T4 treatments increased by 6.40%, 7.14%, −1.04%, and 5.34%, respectively. T1 and T2 treatments were significantly different from the CK treatment. The highest water content was in T2 and the lowest in T3. At a 0–60 cm depth, the soil salinity under the CK, T1, T2, T3, and T4 treatments was 2.34 g/kg, 1.79 g/kg, 1.72 g/kg, 2.11 g/kg, and 2.09 g/kg, respectively. The soil salinity in T1 and T2 decreased by 23.76% and 26.71%, respectively, with a significant difference compared to the CK treatment. The soil salinity in T3 and T4 decreased by 9.82% and 10.68%, respectively, with no significant difference (Figure 7).

• (3)

  Changes in soil water content and salinity at a depth of 0–1 m.

With the increase of soil depth, the variation in soil water content treated with different humic acids was basically the same since it first increased and then decreased (Figure 8(a)). At a 0–40 cm depth, the soil water content gradually increased with the increase of soil depth, while at a 40–100 cm depth, it gradually decreased with the increase of soil depth. The maximum water content had a concentration of 30–35% at a 40–60 cm depth, while the minimum water content was concentrated 15–18% at a 0–20 cm depth. The soil water content under the humic acid treatment was higher than that under the CK treatment.

With the increase of soil depth, the soil salinity under each treatment first decreased, then increased, and then decreased again (Figure 8(b)). The soil salinity gradually decreased at a 0–20 cm depth, then it increased at a 20–40 cm depth, and then again decreased and continued to do so at a 40–100 cm depth. The salinity on the soil surface was mainly concentrated at a 0–20 cm depth, followed by a 40–60 cm depth. The salinity in each layer under the humic acid treatment was lower than that under the CK treatment.

Wheat is one of the main crops cultivated in the Yellow River Delta. However, soil salinization is seriously affecting the delta, so most of the land consists of medium-to-low-yield fields. Research on the effect of humic acid on yield is another indicator that can reflect the improvement of saline-alkali soil under the humic acid treatment.

Soil water is the solvent and carrier of salt. As it flows, it carries the salt with it (Zhang et al. 2015). To study the change in soil salinity, we must first see how the water content changes.

This research found that humic acid can improve soil adsorption, increase the saturated water content in the soil, and improve the soil's capacity to hold water (Figure 3(a)), which is consistent with the findings reported by Gu Xin et al. (Li 2012; Gu et al. 2017). This phenomenon can be interpreted as the combination of humic acid dissolved in water with soil minerals or other interactions that change the original structure of the soil. In addition, the soil salinity in the Yellow River Delta was mainly NaCl, accounting for 70% of the total salinity. As a soil dispersant, Na⁺ will lead to the dispersion of aggregates and clay particles, resulting in the collapse of soil macropores and small voids, and hinder the movement of gas and water. The decrease of Na⁺ can increase the number and stability of soil aggregates (Li et al. 2004). Some studies (Zhang et al. 2019; Gao et al. 2022) have shown that, Ca²⁺ and Mg²⁺ contained in humic acid can displace Na⁺ into soil solution, which significantly reduces the adsorption of Na⁺ to soil. In addition, humic acid as an organic colloid, produces a certain number of colloids, binds dispersed soil particles, forms a soil granular structure with high stability, and helps increase the content and proportion of soil pores, thus enhancing the soil's capacity to hold water. With the continuous increase of humic acid, the soil water content gradually increased under T1 and T2. It reached the maximum in T2, but decreased in T3-T8, possibly because the humic acid increased soil pore content and improved soil water permeability. Although humic acid can somewhat improve the soil's capacity to hold water, adding it excessively can cause water loss. Likewise, the humic acid would only be wasted, and the optimal effect would not be achieved.

Compared to the CK treatment, the soil salinity significantly increased under the humic acid treatment (Figure 3(b)).This may be because salt is dissolved in water and salt moves with water, forming a transient salt retention due to the increased water-holding capacity of the soil. With the increase of humic acid, the soil salinity gradually increased under T1 and T2 and decreased under T2-T6, which was similar to the change in the soil water content.This phenomenon further indicates that the change of soil salinity is closely related to water movement.The increase of the soil salinity under T6-T8 indicated that the change in the soil salinity was more complicated than that in the water content. At the same time, studies (Nong et al. 2022) have also shown that when the salinity of water is high (greater than 6 g/L), organic matter cannot play an effective role in reducing salinity. Moreover, humic acid itself will carry a small amount of salt, salt water irrigation, especially high salinity water will also introduce a large number of Na⁺ and Cl⁻ and other ions. All of these may lead to the retention of salt in humic acid-treated soil.

However, it can be confirmed that this does not affect the active desalting effect of humic acid under freshwater leaching. The desalting efficiency (Figure 5) and desalting rate (Figure 4) of soil under humic acid treatment are higher than those under CK treatment. However, further studies will be conducted to investigate the specific internal ion changes, the internal interaction between brackish water irrigation and humic acid, and whether the effect of humic acid on soil salinity is negative under high salinity brackish water leaching.

The desalting of soil is the result of the complex reaction of humic acid in soil under the washing of fresh water. However, it is well known that organic matter promotes the structure of saline soil. As for the mechanism of organic matter on soil structure, more people believe that organic ions can improve the composition of exchangeable ions through complexation, reduce the number of dispersed ions, and increase the number of soil clay and aggregate stability (Barzegar & Nelson 1997; Li et al. 2002). In laboratory experiment, salt removal efficiency was the highest under T2 and the lowest under T8 (Figure 5). Not only was the adsorption of water and salt relatively high under T2, but the leaching effect was high, as well. Generally, salt removal efficiency was relatively good.

This research found that the soil salinity decreased significantly under highly salinized saltwater leach (Figure 3(b)). This indicates that when soil salinity is high, salt water with high salinity can have a certain leaching effect on salt in the soil and reduce its salinity and the salt stress on crops. Because the research area is located at the end of the irrigation area in the Yellow River Delta, it faced the problem of insufficient quantities of fresh and saltwater. How to effectively realize the desalination of clay saline soil and improve the efficient utilization of freshwater is particularly important for the improvement of saline-alkali soil in this area. Therefore, this research proposes treating the saline-alkali wasteland containing large amounts of salt with highly salinized saltwater to effectively improve the wasteland, turn it into a medium-to-low-yield farmland, and increase the available land area in the Yellow River Delta.

The humic acid dosage of CK, T1-T4 in field experiment and CK, T1-T4 in laboratory experiment were consistent. The laboratory experiment is a simple simulation of the field experiment to explore the law of soil desalting and salt adsorption by humic acid under fresh and salt water leaching. The field experiment is the application of laboratory test and verify, exploring in complex nature condition, the effect of humic acid on soil water and salt and production change. Differently from the laboratory experiment, the variation of water and salt in the field is affected by climate, rainfall, groundwater and many other factors. In the laboratory test, the leaching of fresh water and salt water was only carried out, and the simple mechanism analysis was carried out from the perspective of water salt. Therefore, the specific changes of field results may be slightly different from those of laboratory experiments.

The soil layer of 0–20 cm and 0–60 cm has a great influence on the growth of crops. This time, the soil was collected in the spring without rain for a long time. Under the effect of long-term evaporation, the soil salting effect was the most obvious. The soil salinity in the humic acid treatment group was lower than that in CK treatment group, and the soil water content was higher than that in CK treatment group. This phenomenon indicates that humic acid has a positive effect on the regulation of water and salt under evaporation condition. Humic acid can not only increase the water holding capacity of soil, but also effectively promote soil desalting, which is consistent with the results of laboratory tests that humic acid improves the desalting effect. It is worth mentioning that the results of laboratory salt adsorption test and field soil desalting process are two completely different processes, so it is of little significance to compare the results.

In the field results, we found that there was no significant difference in the variation of surface soil water content between 0 and 20 cm under each treatment (Figure 8(a)). This is mainly because, compared with other soil layers, the 0–20 cm soil is most affected by external factors such as rainfall and evaporation, and the soil moisture changes sharply, and the influence of humic acid dosage is not obvious.

The yield data (Figure 9) showed that compared with CK treatment, the plots under humic acid treatment all obtained higher wheat yield. In humic acid treatment, wheat yield first increased and then decreased with the increase of humic acid content, and the wheat yield under T2 treatment was the highest.Humic acid has a great contribution to increase crop yield, which is consistent with the results of previous studies (Yang 2012; Zhang et al. 2014).Similar to the effect of soil desalting in laboratory experiments, humic acid did not increase yield linearly. Beyond a certain point, the improvement effect of humic acid may gradually decline. Liu (2021) and Lal (2020) reported this similar phenomenon in their experiments. They believed that SOM (soil organic matter content) was closely related to crop yield, but there was a threshold. The threshold value depends on the nature of soil and tillage system, beyond which SOM has no obvious effect on yield.

Generally, gypsum and furfural slag are used in the improvement of clay saline soil (Armstrong & Tantonn 1992; Cheng et al. 2014; Cui et al. 2014). Although these materials are effective in improving soil structure and reducing soil salinity, there are single improvement effects, only salt washing, no fertilizer, or secondary damage to the ecology, which makes it difficult to achieve long-term ecological improvement and sustainable development of the saline-alkali soil. In this research, humic acid was used to treat the coastal clay saline soil. The results indicate that humic acid could not only affect the adsorption of water and enhance the soil's capacity to hold water, but also improve and increase the adsorption and leaching of salt. Humic acid can also effectively reduce the soil salinity, inhibit soil salinization, and improve wheat yield. It was found that humic acid increased the adsorption of soil water, and also increased the adsorption of soil salt.

This research also observed that the soil's capacity to hold water, desalination efficiency, and wheat yield did not increase with the increase of humic acid. In the field experiment, the wheat yield (Figure 9) under T2 treatment was the highest, and in the laboratory experiment, the soil desalting efficiency (Figure 5) under T2 treatment was the highest. Although the desalting rate (Figure 4), of T6 was the highest considering the cost, the dosage of humic acid under T2 treatment was the most reasonable. As a result, the optimal amount of humic acid used in this research was 0.149 g/kg.

In combination with the simulated soil column adsorption and leaching tests, this research conducted the humic acid test on wheat in the field. Likewise, it analyzed the variation and distribution of the soil water content and salinity and wheat yield. The results indicate that humic acid had positive effects on the water content and salinity and wheat yield. It was also found that humic acid can improve the physical and chemical properties of the soil, effectively increase the soil water content, reduce the soil salinity, inhibit soil salinization, and increase wheat yield. Using humic acid to improve saline-alkali soil can realize the comprehensive regulation of water conservation, salt reduction, and yield increase. These findings have important theoretical and practical significance for the improvement of saline-alkali soil in the Yellow River Delta.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.