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Article

Preliminary Studies on How to Reduce the Effects of Salinity

by
Yaru Guo
1,2,
Hongguang Liu
1,2,*,
Ping Gong
1,2,
Pengfei Li
1,2,
Rumeng Tian
1,2,
Yao Zhang
1,2,
Yibin Xu
1,2 and
Bao Xue
1,2
1
College of Water Conservancy & Architectural Engineering, Shihezi University, Shihezi 832000, China
2
Key Laboratory of Modern Water-Saving Irrigation of Xinjiang Production & Construction Group, Shihezi 832000, China
*
Author to whom correspondence should be addressed.
Agronomy 2022, 12(12), 3006; https://doi.org/10.3390/agronomy12123006
Submission received: 5 October 2022 / Revised: 11 November 2022 / Accepted: 17 November 2022 / Published: 29 November 2022
Browse Figures
Versions Notes

Abstract

:
Soil salinization is an important factor contributing to the deterioration of soil environment and low crop yield in arable land. In this study, the effects of five fulvic acid applications (0 (CK), 150 (T1), 300 (T2), 450 (T3), and 600 (T4) kg·ha−1) on soil physicochemical properties, humus content of each component, and cotton (Xinluzao No. 82) growth were investigated. It was confirmed that fulvic acid improved soil water-stable macroaggregates, moisture distribution, and desalinization. 0–20 cm soil relative desalinization rate was significantly increased by 2–11.75%. The pH value decreased by 0.09–0.21. The soil organic matter content was significantly increased compared to CK (p < 0.05), with 7.5–26.93% increase in organic matter content in 0–20 cm soil layer. Soil humification was increased to different degrees, with the most significant increase in humic carbon content in T3 treatment (p < 0.05). There was a significant increase in leaf area index (LAI), stem diameter, and plant height of cotton (p < 0.05). Cotton yield increased by 3.64–8.36% compared to CK (p < 0.05). Correlation analysis showed that cotton yield was significantly correlated with the soil textures of saline soils. The best improvement was achieved with 450 kg·ha−1 fulvic acid. The results of this study can provide a theoretical basis for the improvement of saline soils in arid zones to enhance crop growth and yield.

1. Introduction

Soil salinization is a major obstacle to land resource utilization, and saline soils have an altered soil environment and limit plant growth [1,2], thereby reducing agricultural productivity [3]. A quarter of the world’s irrigated land has reduced crop productivity due to excess soil salinity [4]. Xinjiang, located in the hinterlands of northwest China, has a typical dry continental climate [5] and is one of the most important agricultural production areas in the arid and semiarid regions of China [6] and one of the largest cotton-growing area in the world [7]. At the same time, Xinjiang is also the region in China with the widest distribution, most types of saline soils and the most severe salt accumulation [8]. The estimated total area of salinized soil in Xinjiang is 0.8 × 107 ha [9], and 31.1% of currently cultivated land is affected by salinization [10]. Soil salinization decreased the cotton planting area and yield in Xinjiang, which directly caused economic loss to cotton farmers [10]; it threatens the sustainable use of land. Therefore, it is important to study suitable methods to improve saline soil and increase land productivity to stabilize cotton production. Since the last century, researchers have been trying different measures to improve saline soil and improve crop productivity on saline soils, including freshwater flushing, chemical amendments, tillage, and crop diversification [11]. Chemical modifiers are widely used to improve saline soils [12,13]. However, the effects of different modifiers on different soils are also different. For example, gypsum, an inorganic chemical amendment, in soil with poor structure, is easy to cause salt ions to migrate upward to the surface soil through capillarity, thus increasing soil salinity [14]. Soils of different textures have different physical and chemical properties, so the selection of soil conditioners is of great importance [15,16].
Organic acid soil modifiers are natural macromolecular organic compounds formed by microbial decomposition and transformation of animal and plant residues [17], with strong ionic exchange performance and adsorption capacity. These soil modifiers have positive effects on soil structure, plant growth, and nutrient absorption [18,19,20]. Fulvic acid is one of the most active organic acids, rich in organic aromatic substances with hydroxyl, carboxyl, and other active oxygen-containing functional groups [21]. Some researchers had suggested that the addition of fulvic acid to soil can improve soil fertility, nutrient storage, and soil structure maintenance [22,23,24]. Khaleda et al. [25] found that humic acid modifier can be applied in the field to increase soil fertility by increasing soil water holding capacity to enhance plant growth and salt stress tolerance. Nichols and Wright [26] found that humic substances can improve soil self-healing ability and fertility by improving soil physical and chemical properties, soil nutrient cycling to maintain the stable balance of the ecosystem. Bayat et al. [27] used fulvic acid on yarrow in field and greenhouse trials and found a significant increase in leaf nutrient content and plant biomass. Zhang et al. [28] studied the growth characteristics of cotton under salt stress by comparing hydroponics and plot tests and concluded that fulvic acid could improve dry matter quality, reduce the accumulation of salinized harmful substances, and decrease the salt ion content, to improve the salt tolerance of cotton. Many scholars have reached similar conclusions [29,30], pointing out that organic materials such as fulvic acid can increase the humus carbon and nutrient contents in the soil to promote the growth of crops.
The results of previous studies have shown that fulvic acid can improve the properties of saline soils, promote crop growth, and increase yield. However, the improvement effect of fulvic acid on saline soils in Xinjiang, China, is little known, and the optimal application rate need to be further studied. In this study, we investigated the effects of different application rates of fulvic acid on the physicochemical properties, the content of each component of soil humus (extractable humus carbon and humin carbon), and the growth of cotton in saline soils to verify the effect of fulvic acid and determine its optimal application rate, so as to provide some theoretical support and practical reference for improving the saline environment.

2. Materials and Methods

2.1. Experimental Site

The experiment was conducted from April to September 2021 in Shihezi City, Xinjiang (86°03′ E, 44°18′ N; altitude 451 m). The site has a temperate continental climate. The annual average temperature of the study site is 6–8 °C, and the annual average sunshine time is 2865 h (2300–2700 h). The frost-free period is 160–170 d, with an annual average rainfall of 180–270 mm, and annual evaporation of the 1000–1500 mm. The average daily temperature and rainfall during the cotton growing period are shown in Figure 1.

2.2. Experimental Design

A bucket planting experiment was conducted, using plastic buckets 0.45 m × 0.35 m × 0.60 m in size (top inner diameter × bottom inner diameter × height). The treatments comprised tested five application levels of fulvic acid: 0, 150, 300, 450, and 600 kg·ha−1 (respectively denoted as CK, T1, T2, T3, and T4; and calculated the required amount per barrel by area). Fulvic acid was provided by Xinjiang Heise Eco-tech Co., Ltd., Ürümqi in China, with fulvic acid ≥ 50% and water solubility ≥ 95%. Each treatment was repeated five times. Several annular holes were excavated in the study site, then holes were punched in the bottom of the buckets and the buckets were buried in the holes (see Figure 2 for details). The soils for each treatment were taken from Beiquan, Shihezi City. The soils were taken in layers of 0–20 cm, 20–40 cm, and 40–60 cm and put into buckets according to the original layers by the soil bulk density of 1.38 g·m−3. Fulvic acid was evenly mixed into the surface (0–20 cm). The basic physical and chemical properties of the studied soils are listed in Table 1.
As a pillar industry and an important source of income for farmers in Xinjiang, cotton (“Xinluzao No. 82”), which can adapt to drought to a certain degree, was chosen as the research object. Two to three cottonseeds were sown in the center of the test bucket; thinning was carried out at the seedling stage to one plant per bucket. A drip irrigation belt was used for water supply (single-wing labyrinth drip irrigation belt, produced by Xinjiang Tianye Co., Ltd., Ürümqi in China, with distance between drip holes of 30 cm, and flow rate of 2.0 L·h−1), with two drip holes per bucket. A pressure-regulating valve, a pressure gauge, a water meter, and a fertilizing tank were installed at the interface of each drip irrigation belt to control the amounts of water and fertilizer. Drip irrigation with well water was used in the study site, and the salinity of the well water was about 1.27 g·L−1. On 20 April 2021, “dry sowing and wet out” method was used to sow seeds to a depth of 3–4 cm. Based on local practice, sowing, film mulching, and drip irrigation belt laying were completed simultaneously.
The fertilizers used in the experimental group were CO(NH2)2 (including N: 46%) and KH2PO4 (including P2O4: 51.5%). Drip irrigation with water and fertilizer was carried out using a small fertilizer tank and applied after dripping water for about 1 h each time, and then applying the fertilizer with water. After fertilization, water was dripped well again to the current irrigation quota. The cotton was irrigated 11 times during the growth period, the irrigation quota was 4000 m3·ha−1, and the fertilizer was applied with water nine times (as shown in Table 2). During the growth period, spraying for pest control and growth regulators (abbreviated as amine) and other agronomic practices were the same for each treatment.

2.3. Sample Collection and Measurement

Soil samples were collected at cotton maturity in October 2021. For each treatment, the same side and the same position of the drip irrigation belt were selected to collect soil samples at the 0–10, 10–20, 20–40, 40–50, and 50–60 cm soil layers with soil drills. Each treatment was repeated three times and the average value was taken.

2.3.1. Soil Water Stable Macroaggregates Determination

Wet screening method was used [31].

2.3.2. Soil Moisture Determination

The soil moisture content (mass moisture content) was determined by the drying method [32].

2.3.3. Soil Salinity Determination

The soil samples were air-dried indoors, before even pulverization. The soil was passed through a 1 mm soil sieve, and a clarified solution of soil and water was prepared with a mass ratio of 1:5 [32]. A digital conductivity meter, Lei Magnetic DDS-11A (Shanghai INESA Scientific Instrument Co., Ltd., Shanghai, China) was used to measure the electrical conductivity (EC, μS/cm), and the calibration equation between the conductivity of the soil leaching solution and the soil salt content is as follows:
S = 0.0036 EC + 0.4769, (R2 = 0.9859)
where S is the soil salinity (g/kg) and EC is the electrical conductivity (μS/cm).

2.3.4. Soil pH Value Determination

The retrieved soil samples were prepared with a soil-water ratio of 1:5, and the pH value of the soil sample leaching solution was measured using a HACH-pHC101 portable pH meter (Shanghai Running Industrial Co., Ltd., Shanghai, China).

2.3.5. Determination of Soil Organic Matter Content

The content of soil organic matter was determined by the potassium dichromate volumetric method [32].

2.3.6. Determination of Carbon Content in Each Component of Soil Humus

To determine the carbon content in each component of soil humus, the modification method of humic substances was adopted, mainly referring to the method of Kumada et al. [33] and Yu et al. [34]. Briefly, a mixed solution of 0.1 mol/L sodium pyrophosphate and 0.1 mol/L sodium hydroxide (pH = 13) was used for extraction. The carbon content of humic acid (CHA), fulvic acid (CFA), and humin (CHM) were determined by the potassium dichromate volumetric method [32]. PQ value = CHA/(CHA + CFA) [35].

2.3.7. Determination of Leaf Area Index (LAI), Stem Diameter and Plant Height in Cotton

At the seedling stage (10 June), budding stage (1 July), blossoming stage (20 July), bolling stage (15 August), and boll opening stage (5 September) of cotton, three cotton plants of uniform growth were selected in each treatment, and plant height (cm) and stem diameter (mm) were measured with a tape measure and vernier caliper, respectively; and then leaf area and leaf area index (LAI) were calculated by measuring the length and width of each leaf with a tape measure [36].

2.3.8. Cotton Yield Index

The number of cotton bolls per plant, boll quality, and seed cotton yield were determined as described previously [37].

2.4. Statistical Analysis

Microsoft Excel 2019 (Microsoft, Redmond, WA, USA) was used to record the data. SPSS 23.0 software (IBM, Armonk, NY, USA) was used for statistical analysis. The one-way analysis of variance (ANOVA) and LSD method for difference analysis was performed with a confidence level of 0.05. Origin2021 (OriginLab, Northampton, MA, USA) was used for drawing the figures.

3. Results

3.1. Effects of Fulvic Acid on Soil Physicochemical Properties

3.1.1. Water-Stable Macroaggregates (>0.25 mm) (R0.25)

The water-stable macroaggregates (>0.25 mm) in profile decreased down the soil layer and the application of fulvic acid increased the macroaggregates in the soil (Figure 3a). The water-stable macroaggregates in the soil surface (0–20 cm) accounted for only 20.20–27.28% of the total aggregates, indicating that the structure of saline soil needs to be further improved. Compared with CK, the content of water-stable macroaggregates (0–20 cm) under T1, T2, T3, and T4 treatments increased by 2.59%, 9.52%, 14.71%, and 13.06%, respectively, in the order of T3 > T4 > T2 > T1. In particular, T3 resulted in the biggest increase of the water-stable macroaggregates in the soil. The results show that the application of fulvic acid in saline soil can increase the number of water-stable macroaggregates and improve the stability of soil water-stable macroaggregates. The effect of increasing dosage on soil water-stable aggregates first increased and then decreased.

3.1.2. Soil Moisture Distribution

The soil moisture distribution in each treatment profile first increased and then decreased. The 0–20 cm soil layer, which is greatly affected by soil evaporation, and the 0–10 cm soil layer, which is significantly affected by rainfall and evaporation, showed the largest variations in water content. The 30–60 cm layer, which is least affected by rainfall and evaporation, showed no obvious change in water content. The water content of the saline soil treated with fulvic acid was significantly higher than that of the control, and with increased application of fulvic acid, the effect on soil water content first increased and then decreased (Figure 3b). Compared with CK, the average water content of soil profile under T1, T2, T3, and T4 treatments increased by 10.92%, 20.99%, 26.47%, and 8.96%, respectively, with the most obvious effect of fulvic acid under T3 treatment. Compared with CK, the soil moisture content of the 0–20, 20–40, and 40–60 cm soil layers under the T3 treatment increased by 25.59%, 20.28%, and 33.81%, respectively. The results showed clearly the application of fulvic acid in saline soil effectively increased the soil water content, but too much application reduced the effect on soil water retention capacity.

3.1.3. Distribution of Soil Salinity

With the end of irrigation, the soil moisture content in the surface layer decreased due to evaporation, hence gradual salt migration from the deep layer to the top caused the profile salinity to increase slightly. The salt content was higher in the 0–20 cm soil layer and lower in the 30–60 cm layer, with most of the salt accumulating in the 0–20 cm layer (Figure 3c). The surface salinity of the saline soil treated with fulvic acid was significantly lower than CK. The ratio of the difference in the salinity of the soil treated with fulvic acid and the salinity of CK was defined as the relative desalination rate (%) (Table 3). Compared with CK, the relative desalination rate of soil profiles under T1, T2, T3, and T4 treatments increased by 4.35%, 8.34%, 9.23%, and 7.10%, respectively, and T3 treatment exhibited the greatest effect of desalination. Compared with CK, the desalination rate of soil layers 0–20 cm, 20–40 cm, and 40–60 cm increased by 11.77%, 10.65%, and 7.70% under T3 treatment. The results showed that the application of fulvic acid to saline soils could reduce soil salinity, with increasing and then decreasing effects with increasing fulvic acid application.

3.1.4. Distribution of Soil pH

After the cotton growth period ended, there was no significant variation in soil pH in the profile, but soil subjected to fulvic acid application was significantly less alkaline and had a significantly lower pH compared to CK (Figure 3d). Compared to CK, the average pH under T1, T2, T3, and T4 treatments decreased by 0.09, 0.18, 0.21, and 0.09, respectively. When the application rate was less than the T3 treatment, the effect on saline soil pH increased and the alkalinity of the soil weakened with increasing amount of fulvic acid. The effect on soil pH decreased when the application amount was greater than that of the T3 treatment. The results indicate that the effect of fulvic acid on soil pH first gradually increased and then decreased as the amount of fulvic acid applied increased. The T3 treatment showed the most significant reduction of pH in saline soils.

3.1.5. Soil Organic Matter

The application of fulvic acid significantly increased the organic matter content (p < 0.05) of the soil (Figure 4a). The content in the 0–20 cm soil layer increased by 7.17, 17.28, 26.80, 26.09%, and that in the 20–40 cm soil layer by 7.83, 31.69, 24.20, and 27.78% under T1, T2, T3, and T4 treatments, respectively, compared to CK. The organic matter content in the 40–60 cm layer of saline soils with fulvic acid application accumulated to different degrees comparable to CK, but there was not significant difference among treatments with fulvic acid application. The organic matter content in different soil layers was in the order 0–20 > 20–40 > 40–60 cm, with significantly higher organic matter content in the 0–20 and 20–40 cm layers than that the 40–60 cm layer. The content increased with the increases in fulvic acid application and then leveled off, while it decreased with increasing soil depth.

3.2. Effects of Fulvic Acid on Soil Content of Humus Component

3.2.1. Content of Humic Acid (CHA)

The fulvic acid significantly increased the amount of CHA (p < 0.05), with significant increases at the 0–20 and 20–40 cm soil layers (Figure 4b). The amount of CHA in the 0–20 cm layer increased by 5.97% under T1, 8.83% under T2, 11.89% under T3, and 14.15% under T4 treatments, and by 5.28, 7.78, 9.03, and 15.8% respectively, at the 20–40 cm layer. The CHA in the 40–60 cm layer with fulvic acid increased relative to CK, but the difference was not significant. The content of CHA among different soil layers varied in the order 0–20 > 20–40 > 40–60 cm. With increased application of fulvic acid, the difference in CHA between the 0–20 and 20–40 cm soil layers gradually decreased.

3.2.2. Content of Fulvic Acid (CFA)

The CFA increased across soil layers of the treated saline soils, but there were no significant differences among the fulvic acid treatments (p < 0.05) (Figure 4c). The content of CFA increased by 9.26%, 10.57%, 10.32%, and 11.07% in the 0–20 cm soil layer and 10.91%, 11.66%, 12.66%, and 11.43% in the 20–40 cm layer under T1, T2, T3, and T4 treatment, respectively, compared with CK. The content of CFA among different soil layers varied in the order 0–20 > 20–40 > 40–60 cm. The application of fulvic acid to saline soils significantly increased the amount of CFA in each soil layer, the amount remained stable in the 0–20, 20–40, and 40–60 cm soil layers under T1, T2, T3, and T4 treatments with increased fulvic acid application, and had significantly more CFA in the 0–20 and 20–40 cm layers than the 40–60 cm layer.

3.2.3. Content of Humin (CHM)

The fulvic acid significantly increased the CHM at each soil layer (p < 0.05) (Figure 4d). Compared with CK, the amount of CHM increased by 14.88, 17.21, 20.53, and 23.57% under T1, T2, T3, and T4 treatments, respectively, in the order of T4 > T3 > T2 > T1. The amount of CHM increased by 102.29% at the 0–20 cm layer under T4 treatment. The amount of CHM was also increased in both 20–40 and 40–60 cm layers compared to CK, but there was no significant difference among fulvic acid treatments. The amount of soil CHM showed a general trend of decreasing with increased of soil depth. There was significantly more CHM at the 0–20 cm than at the 20–40 and 40–60 cm soil layers, and the amounts at the 20–40 and 40–60 cm soil layers did not significantly differ.

3.2.4. Changes in Soil Humus Composition

The trends of CHA/CFA and PQ values of soil humus were similar (Figure 5). Compared with CK, the CHA/CFA and PQ values of the 0–20, 20–40, and 40–60 cm soil layers treated with fulvic acid showed increasing trends. The effect of fulvic acid on soil CHA/CFA and PQ values were in the order of T4 > T3 > T2 > T1, but the difference between T3 and T4 treatments was not significant. Under T4 treatment, the CHA/CFA values at the 0–20, 20–40, and 40–60 cm layers increased by 31.78, 36.05, and 33.49%, and PQ values increased by 14.09, 10.58, and 6.18%, respectively, compared with CK. The profile distribution of CHA/CFA and PQ values in the soil was generally in the order of 0–20 > 20–40 > 40–60 cm. In the CK treatment, the CHA/CFA value of each soil layer was less than 1 and the PQ value was less than 0.5, indicating that further improvement of humic acid carbon content is needed. Overall, the results showed that the application of fulvic acid to saline soils significantly increased the CHA/CFA and PQ values, increased the proportion of humic acid in humus, and improved the soil humus quality. The T4 treatment improved the effect significantly, with no significant difference in effect between T3 and T4.

3.3. Effects of Fulvic Acid on Growth

3.3.1. Leaf Area Index (LAI), Stem Diameter and Plant Height

There was no significant difference in stem diameter and leaf area index among the experimental treatments at the beginning of cotton growth, but the plant height under T3 was significantly higher than that of CK (Figure 6). The leaf area index (LAI) first of increased and then decreased as the growth period advanced, and the peak LAI was during S4, while some leaves began to fall off during S5 and LAI showed a decreasing trend. The LAI of fulvic acid treatments was significantly higher than that of CK treatment from S3 to S5. The LAI increased by 1.44, 5.75, 29.14, and 13.31% under T1, T2, T3, and T4 treatments, respectively, compared to CK. As the cotton grew, the stem diameter increased continuously for each treatment, reaching a maximum at the S4 period. From the S4, the difference in stem diameter between treatments gradually increased. After the S3, plant growth under T1, T2, T3, and T4 treatments was accelerated much faster than CK. Until the end of the growth period, the variation in plant height was in the order T3 > T4 > T2 > T1 > CK. From the last measurement, the T3 treatment had the tallest (p < 0.05) plants (51.8 cm). This was followed by T2 (49.1 cm) and T4 (50.3 cm) treatments, with no significant difference between T2 and T4 treatments. Overall, the LAI, stem diameter, and plant height were superior under T3 treatment.

3.3.2. Cotton Yield

Fulvic acid less than T2 treatment, showed no significant effect on seed cotton yield per boll (p < 0.05), but the T2, T3, and T4 treatments significantly improved seed cotton yield per boll, with the greatest effect at the T3 treatment level, and little difference between T4 and T3 treatments (Table 4). The fulvic acid treatments also significantly increased the number of bolls per plant and T2, T3, and T4 treatments had comparable effects. The fulvic acid increased the seed cotton yield considerably by 3.64% under T1, 5.78% under T2, 7.96% under T3, and 6.22% under T4. The increased the yield of seed cotton following fulvic acid application was due to the significant increase in the number of bolls per plant. There was some positive effect of fulvic acid treatment on the quality of seed cotton yield per boll, but this effect was only significant at the appropriate application rate.

3.3.3. Correlation of Cotton Yield with Soil Properties

The relationships between cotton seed yield and soil aggregates, water content, salinity, pH, soil organic matter content, and soil humification were highly significant or significant (Table 5). Cotton seed yield was positively correlated with aggregates, water content, organic matter content, and soil humus (p < 0.05) and negatively correlated with soil salinity and pH (p < 0.05). These results show that the reductions in soil salinity and pH and an improvement in soil fertility can help increase cotton production.

4. Discussion

The results showed that the content of macroaggregates, soil moisture, and organic matter increased, and the salt content and pH decreased in the saline soil treated with fulvic acid, which was consistent with previous findings in the literature [38,39,40]. With increased application amount, the soil improvement effect showed a trend of increasing first and then decreasing. This may be because fulvic acid is an organic compound [41]. The application of fulvic acid to saline soils can significantly increase soil organic matter content and soil carbon pool reserves, facilitate soil organic matter sequestration, and promote soil fertility conversion and crop uptake [42]. Fulvic acid can be applied to soil as an organic colloid in humus [43], and under the action of cementation, soil particles and microaggregates bind to promote the formation of soil aggregate structure [44]. The accumulation of organic carbon in the soil also promotes the formation of soil aggregates [45]. Increased content of soil aggregates improves soil structure and pore space, slows soil water consumption, increases soil water content, allows salt ions in soil to dissolve more fully in water, increases the downward transport of salt with water, and allows more water to participate in the salt washing process for an enhanced salt pressing effect [46]. Fulvic acid is an acidic organic substance that can neutralize some bases in the soil to reduce the soil pH [47]. In this study, increased amount of fulvic acid application did not always cause an increased effect on soil properties. This may be because the trend of increasing the mass fraction of water-stable macroaggregates with increasing organic carbon slows after a certain mass fraction of organic carbon in the soil [48,49]. A too high application of fulvic acid will result in water absorption and swelling of the soil, the expansion of soil pores, and soil water loss [50]. The application of exogenous materials has a relative saturation point in terms of crop yield and soil function, which is also in line with the law of diminishing returns in the principles of scientific fertilization [51].
Increased carbon content of humus components is important for soil carbon stabilization, as these components constitute stable soil carbon pools [52]; thus, these components are considered effective indicators of carbon sequestration in soils. Humus acid is generally dominated by fulvic acid [42]. After the application of fulvic acid, nonstructural products are first formed in the process of soil humification, and with increased treatment time, some nonstructural products are converted into humic acid first or decomposed to CO2 by microorganisms, with dynamic conversion of fulvic acid into humic acid [42]. The results of this study showed that treatment with the fulvic acid conditioner significantly increased the CHA and gradually increased the CHA with increased application level. The CFA of the 0–20 cm and 20–40 cm soil layers of the treated saline soil was maintained at a stable level. This indicates that a large part of the fulvic acid formed by nonstructural substances is converted into humic acid and that further application of fulvic acid maintained a dynamic balance. Especially for the 20–40 cm soil layer, the soil with fulvic acid was stabilized. Previous work showed that with increased application rate of fulvic acid, the CHA/CFA and PQ values gradually increased, indicating that the higher the degree of soil humification, the more complex the molecular structure in the soil [53]. As shown previously, the application of fulvic acid improved the fertility state of the soil. Liu et al. [42] showed that straw returning can increase the CHA and CHM in cotton soil and can also significantly increase CHA/CFA and PQ values. These findings using straw instead of fulvic acid are similar to the results of this study but with a different effect on the humus [54].
Humic substances promote crop growth by alleviating salt stress and improving the soil environment [55,56,57]. Wang et al. [58] found that treatment with fulvic acid can significantly increase the organic matter content of saline soil in arid areas, reduce soil salinity content, and significantly increase cotton yield in saline cotton fields in Xinjiang. Scholars [59,60] also found that fulvic acid can promote soil properties and plant growth. The correlation analysis in this study showed that cotton yield was positively correlated with aggregates, moisture content, organic matter content, and soil humus (p < 0.05) and negatively correlated with soil salinity and pH (p < 0.05) (Table 4). Cotton can adapt to drought [61], but cotton root growth can be severely inhibited with increasing soil salinity [62]. Salt stress also inhibits boll production and promotes boll shedding, resulting in lower cotton yields [63]. Application of the fulvic acid can improve the soil structure and soil physical properties, promote the chelation of various elements, and facilitate the release of soil nutrients to increase interaction between soil, crops, and microorganisms to improve the soil and improving the growth and development of crops [55]. Despite the limited time of the study, there was obvious improvement of saline soil quality and cotton seed yield after the application of fulvic acid. These results have guiding implications for agricultural production. Our future research will investigate the effects of long-term application of fulvic acid in standardized saline cotton fields and the dynamics of each index.

5. Conclusions

Soil salinization and increased crop productivity are pressing problems in semiarid and arid regions. Based on the results of the bucket planting experiment in 2021, our main conclusions are as follows: applying fulvic acid to saline soil can improve soil macroaggregate structure and water content, decreased soil salinity and pH, and increased soil organic matter content. With increased application amount, the soil improvement effect showed a trend of increasing first and then decreasing. The best effect of fulvic acid on soil properties was obtained at the rate of 450 kg·ha−1. Fulvic acid can improve the quality of soil humus; it significantly increased the CHA/CFA and PQ values, increased the proportion of humic acid in the humus, and improved soil humus quality. With increased application, its effect on each component of soil humus first showed an increasing trend and then slowed. Fulvic acid treatment improved leaf area index (LAI), stem diameter, and plant height; it also increased the seed yield of cotton by 3.64–6.22%. The correlation analysis further showed that the better soil texture was helpful to plant growth (Table 5). In summary, the 450 kg·ha−1 was the optimal amount of fulvic acid in this study. Further study is needed to evaluate the effect of long-term application of fulvic acid in standardized saline cotton fields and provide management principles and practices for improving the saline environment.

Author Contributions

Conceived and designed the experiments: Y.G. and H.L. Performed the experiments: Y.G., H.L., P.G., P.L., R.T., Y.Z., Y.X. and B.X. Analyzed the data: Y.G. Contributed materials/analysis tools: Y.G. and H.L. Wrote the paper: Y.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (52069026), the Southern Xinjiang Key Industry Innovation and Development Support Plan Project (2020DB001), and the Xinjiang Production and Construction Crops International Science and Technology Cooperation Project (2021BC003).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors have declared that no competing interest exist.

References

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Agronomy 12 03006 g001 550
Figure 1. Average daily temperature and precipitation during the cotton growth period in 2021.
Figure 1. Average daily temperature and precipitation during the cotton growth period in 2021.
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Figure 2. Schematic diagram of barrel cotton planting. CK is 0 kg·ha−1 fulvic acid, T1 is 150 kg·ha−1 fulvic acid, T2 is 300 kg·ha−1 fulvic acid, T3 is 450 kg·ha−1 fulvic acid, and T4 is 600 kg·ha−1 fulvic acid.
Figure 2. Schematic diagram of barrel cotton planting. CK is 0 kg·ha−1 fulvic acid, T1 is 150 kg·ha−1 fulvic acid, T2 is 300 kg·ha−1 fulvic acid, T3 is 450 kg·ha−1 fulvic acid, and T4 is 600 kg·ha−1 fulvic acid.
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Figure 3. The effect of fulvic acid on soil water-stable macroaggregates (>0.25 mm) (R0.25) (a), moisture content (b), salinity content (c), and pH value (d) in the 0–60 cm soil layer. CK is 0 kg·ha−1 fulvic acid, T1 is 150 kg·ha−1 fulvic acid, T2 is 300 kg·ha−1 fulvic acid, T3 is 450 kg·ha−1 fulvic acid, and T4 is 600 kg·ha−1 fulvic acid.
Figure 3. The effect of fulvic acid on soil water-stable macroaggregates (>0.25 mm) (R0.25) (a), moisture content (b), salinity content (c), and pH value (d) in the 0–60 cm soil layer. CK is 0 kg·ha−1 fulvic acid, T1 is 150 kg·ha−1 fulvic acid, T2 is 300 kg·ha−1 fulvic acid, T3 is 450 kg·ha−1 fulvic acid, and T4 is 600 kg·ha−1 fulvic acid.
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Figure 4. The effect of fulvic acid on soil organic matter (a), the carbon content of humic acid (b), the carbon content of fulvic acid (c), and the carbon content of humin (d) in the 0–60 cm soil layer. Error bars represent standard errors of the means (n = 3). The least significant differences (LSD0.05) are at 5% level of significance. Different letters on top of each bar show significant differences between treatments, same below.
Figure 4. The effect of fulvic acid on soil organic matter (a), the carbon content of humic acid (b), the carbon content of fulvic acid (c), and the carbon content of humin (d) in the 0–60 cm soil layer. Error bars represent standard errors of the means (n = 3). The least significant differences (LSD0.05) are at 5% level of significance. Different letters on top of each bar show significant differences between treatments, same below.
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Figure 5. The effect of fulvic acid on the ratio of humic acid to fulvic acid (CHA/CFA) (a) and the ratio of humic acid to the sum of humic acid and fulvic acid (PQ value) (b) in the 0–60 cm soil layer.
Figure 5. The effect of fulvic acid on the ratio of humic acid to fulvic acid (CHA/CFA) (a) and the ratio of humic acid to the sum of humic acid and fulvic acid (PQ value) (b) in the 0–60 cm soil layer.
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Figure 6. Changes in leaf area index (a), stem diameter (b), and plant height (c) of cotton during the growing period. S1, S2, S3, S4, and S5 represent the seedling stage, budding stage, blossoming stage, bolling stage, and boll opening stage, respectively. Different letters on top of each bar show significant differences between treatments.
Figure 6. Changes in leaf area index (a), stem diameter (b), and plant height (c) of cotton during the growing period. S1, S2, S3, S4, and S5 represent the seedling stage, budding stage, blossoming stage, bolling stage, and boll opening stage, respectively. Different letters on top of each bar show significant differences between treatments.
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Table
Table 1. Basic physical and chemical properties of the tested soil.
Table 1. Basic physical and chemical properties of the tested soil.
ItemsValues
Soil textureSandy loam
Soil holding capacity/(%)20.6
Initial soil electrical conductivity/(μS·cm−1)2245.31
Initial soil salt content/(g·kg−1)8.56
Initial soil volumetric water content/(m3·m−3)17.83
Soil organic matter/(g·kg−1)13.75
Humic acid/(g·kg−1)3.77
Fulvic acid/(g·kg−1)2.55
Humin/(g·kg−1)3.97
Total nitrogen/(g·kg−1)0.78
Alkaline hydrolysis nitrogen/(mg·kg−1)51.38
Available P/(mg·kg−1)48.12
Available K/(mg·kg−1)551.29
pH8.55
Cation exchange capacity/(cmol·kg−1)5.08
Table
Table 2. Irrigation and fertilization treatment of cotton at different growth stages.
Table 2. Irrigation and fertilization treatment of cotton at different growth stages.
Growth PeriodDates
(2021)		IrrigationFertilization
Irrigation Quota/(m3·ha−1)Irrigation TimesCO(NH2)2/(kg·ha−1)KH2PO4/(kg·ha−1)Fertilization Times
Seeding04.20–04.303501———
Seeding stage05.01–06.15600230151
Bud stage06.16–07.03730290402
Blossoming and boll stage07.04–08.18207054502305
Boll opening stage08.19–10.162500130151
Whole growth period167 days4000116003009
Table
Table 3. Effects of fulvic acid treatment to desalinate saline soil.
Table 3. Effects of fulvic acid treatment to desalinate saline soil.
TableSoil Depth/(cm)
0–1010–2020–3030–4040–5050–60
T14.10 ± 0.42 d3.04 ± 0.38 d4.19 ± 0.33 d4.75 ± 0.39 b9.72 ± 0.31 a0.37 ± 0.28 d
T211.41 ± 0.40 b8.46 ± 0.46 b8.74 ± 0.40 c5.15 ± 0.30 b7.75 ± 0.67 c8.53 ± 0.48 a
T313.65 ± 0.34 a9.85 ± 0.37 a13.19 ± 0.33 a8.13 ± 0.53 a8.74 ± 0.23 b1.82 ± 0.30 c
T49.63 ± 0.53 c5.04 ± 0.36 c12.18 ± 0.39 b3.23 ± 0.22 c6.85 ± 0.29 d5.58 ± 0.46 b
Note: CK is 0 kg·ha−1 fulvic acid, T1 is 150 kg·ha−1 fulvic acid, T2 is 300 kg·ha−1 fulvic acid, T3 is 450 kg·ha−1 fulvic acid, and T4 is 600 kg·ha−1 fulvic acid. Values in the table are means ± standard errors (n = 3); different letters in the same column indicate significant differences at p < 0.05, same below.
Table
Table 4. Changes in cotton yield and composition under different application rates of fulvic acid.
Table 4. Changes in cotton yield and composition under different application rates of fulvic acid.
TreatmentSeed Cotton Yield
per Boll/(g)		Number of Bolls
per Plant		Seed Cotton Yield/(g/plant)Relative Production
(%)
CK4.83 ± 0.24 c5.22 ± 0.14 c25.25 ± 1.16 d100
T14.88 ± 0.11 c5.36 ± 0.26 b26.17 ± 2.35 c103.64
T24.91 ± 0.22 bc5.44 ± 0.31 a26.71 ± 2.06 b105.78
T34.98 ± 0.16 a5.47 ± 0.22 a27.26 ± 1.21 a107.96
T44.93 ± 0.36 ab5.44 ± 0.13 ab26.82 ± 3.44 ab106.22
Note: Values in the table are means ± standard errors (n = 3); different letters in the same column indicate significant differences at p < 0.05.
Table
Table 5. Correlation analysis of cotton yield and monitoring indexes.
Table 5. Correlation analysis of cotton yield and monitoring indexes.
IndexSeed Cotton YieldAggregatesMoisture ContentSalt ContentpHOrganic Matter
Aggregate0.92 *
Moisture content0.92 *0.98 **
Salt content−0.93 *−0.97 **−0.99 **
pH−0.89 *−0.93 *−0.99 **0.97 **
Organic matter0.97 **0.92 *0.91 *−0.95 *−0.87
Humus composition0.96 **0.810.77−0.79−0.730.90 *
Note: * and ** indicate significant correlation at p < 0.05 and extremely significant correlation at p < 0.01, respectively.
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Guo, Y.; Liu, H.; Gong, P.; Li, P.; Tian, R.; Zhang, Y.; Xu, Y.; Xue, B. Preliminary Studies on How to Reduce the Effects of Salinity. Agronomy 2022, 12, 3006. https://doi.org/10.3390/agronomy12123006

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Guo Y, Liu H, Gong P, Li P, Tian R, Zhang Y, Xu Y, Xue B. Preliminary Studies on How to Reduce the Effects of Salinity. Agronomy. 2022; 12(12):3006. https://doi.org/10.3390/agronomy12123006

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Guo, Yaru, Hongguang Liu, Ping Gong, Pengfei Li, Rumeng Tian, Yao Zhang, Yibin Xu, and Bao Xue. 2022. "Preliminary Studies on How to Reduce the Effects of Salinity" Agronomy 12, no. 12: 3006. https://doi.org/10.3390/agronomy12123006

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Guo, Y., Liu, H., Gong, P., Li, P., Tian, R., Zhang, Y., Xu, Y., & Xue, B. (2022). Preliminary Studies on How to Reduce the Effects of Salinity. Agronomy, 12(12), 3006. https://doi.org/10.3390/agronomy12123006

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