Use of Cattle Manure as Auxiliary Material to Gypsum to Ameliorate Saline–Alkali Soils

Abstract

Soil salinization is a major threat to agriculture and food security globally. The effectiveness of amendments on soil quality and crop production is management-dependent, and low-cost management practices are essential for developing countries. In this 3-year field study, the effects of cattle manure and gypsum amendments on the physicochemical properties of saline–alkali soil were evaluated. We found that both single gypsum and mixed amendments significantly reduced soil hardness, bulk density, pH, and soil salt content in 20–40 cm in 2015 and 2017. A more significant decrease in soil EC and density was observed with the mixed amendments compared to single gypsum after three years of reclamation. Specifically, applying mixed amendments (M-G15) led to a significant increase in Hordeum yield by 60.94%, whereas the application of single gypsum increased Hordeum yield by 25.20–53.14%. This indicated that co-application of cattle manure can reduce the amount of gypsum needed to achieve similar improvements in soil properties and Hordeum yield, with a long-term cumulative effect. Na⁺/(Ca²⁺ + Mg²⁺) showed the largest negative contribution to Hordeum yield under amendments, while soil bulk density showed the second largest number of negative effects on Hordeum yield under mixed amendments. Single gypsum improved the soil’s physical quality during the early stage of saline–alkali soil remediation, and mixed amendments improved the soil’s physicochemical properties and Hordeum yield during the late stage of remediation. Na⁺/(Ca²⁺ + Mg²⁺) in topsoil was confirmed to be the dominant factor under the mixed amendments affecting Hordeum yield, followed by the soil bulk density. These results confirm that the co-application with cattle manure achieves a similar reclamation effect with a reduced gypsum dosage, thereby lowering the reclamation costs of saline–alkali land in semi-arid areas.

1. Introduction

Soil salinization is one of the critical threats to agriculture and food security globally. The global saline–alkali land area has exceeded 1381 million ha, accounting for about 10.7% of the total land area [1]. According to statistics, there are about 10 million hectares of saline land around the countries bordering the Mediterranean, accounting for about 1% of the total area of the world, of which Algeria has 1 million hm² of saline land [2,3]. Salinization has restricted agricultural development and the utilization of land resources, especially in arid and semi-arid regions of the world [4]. The use of soil degraded by salinization in subsistence agriculture cannot be neglected, and it is necessary to develop economically viable techniques for its remediation, allowing its return to productive agricultural use.

The soil degradation in Algeria is becoming more and more serious, affecting the ecosystem and agricultural production potential and hindering the development of agricultural production. Most of the agricultural soils are facing the problem of salinization, which has become one of the main limiting factors for agricultural production, reducing agricultural production capacity. A relatively high degree of salinization has been observed in the western region, which may be caused by drought, soil clay weight, high salinity of irrigation water from groundwater, and poor drainage systems [2]. Currently, developed countries attach great importance to the improvement of saline–alkali land, and developing countries urgently need support. Therefore, degraded saline–alkaline land that has been targeted for remediation will be an important resource in Algeria. To develop and utilize saline–alkali land, it is necessary to find an effective method for the remediation of saline–alkali land.

In the saline–alkali soil, high salinity could decrease plant growth because of ion toxicity and water deficits caused by osmotic pressure. Therefore, choosing viable and low-cost management practices is essential for the process of recovering these soils [5,6]. Various chemical improvement measures have been investigated for their effectiveness in saline soil remediation. It has been demonstrated that gypsum serves as an effective solution for enhancing saline–alkali soil conditions [7,8]. According to a study over a three-year period, the application of gypsum led to significant reductions in the pH value, alkalinity, and total salt content of alkali soil by 17.20%, 42.63%, and 46.43%, respectively [9].

However, the widespread use of gypsum in developing countries is hindered by the low-cost management requirement and its limited availability [6]. Therefore, it is necessary to find new ways to increase crop yield and reduce gypsum application in saline–alkali land. Organic amendments, such as manure, are recognized as an important tool for reversing soil salinization. Research conducted in developed countries has revealed that the coordinated application of organic materials, such as animal manure and maize straw, could promote a reduction in soil salinity and an increase in crop yields [6]. While gypsum is the fastest, its cost may be a barrier, justifying the search for alternatives, such as combining it with manure. Although most studies have focused on the effects of gypsum or organic materials on soil properties and crop yield [7,8,9], there is a lack of long-term follow-up analysis. In addition, the key factors hindering the remediation of saline–alkali soil by gypsum are unclear.

Thus, the objective of this study was to explore the impact of applying gypsum in combination with manure on restoring the productivity of Hordeum and remediation of saline–alkali soil in the semi-arid areas of northeastern Algeria.

2. Materials and Methods

2.1. Overview of Test Area

The field experiment was conducted for three years in Hamadena Experimental Station, located in a primary salinization zone in Relizane province (35°54′00″ N, 0°47′00″ E), Algeria. The area has a typical Mediterranean climate, with annual precipitation of 150–350 mm, average evaporation of 1000–1400 mm, an altitude of 48 m, and an average maximum temperature of 28 °C and minimum of 12 °C. The precipitation is unevenly distributed, with 70% of the total annual rainfall occurring in winter and spring, and little rain in summer. In this area, high evaporation and uneven distribution of rainfall lead to a serious salinization of the soil. As typical of saline soil, the electrical conductivity (EC, soil: H₂O ratio = 1:5) was 2.42 ms∙cm⁻¹, pH (soil: H₂O ratio = 1:2.5) was 8.25, soil organic matter (SOM) content was 2.31%, total salt content was 5.30 mg∙kg⁻¹, exchangeable sodium ions content was 5.47 cmol∙kg⁻¹, and soil alkalinity was 16.10%.

2.2. Experimental Design

The experiment was carried out with a micro-plot area of 720 m² from April 2015 to 2017. A total of eight treatments were laid out as a completely randomized block design with three replicates, and they were arranged in 24 plots (5 × 6 m for each plot). The treatments could be described as the following: (1) G0 (gypsum 0 t ha⁻¹), (2) G7.5 (single application of gypsum 7.5 t ha⁻¹), (3) G15 (single application of gypsum 15 t ha⁻¹), (4) G30 (single application of gypsum 30 t ha⁻¹), (5) M-G0 (single application of manure 75 t ha⁻¹), (6) M-G7.5 (manure 75 t ha⁻¹ + gypsum 7.5 t ha⁻¹), (7) M-G15 (manure 75 t ha⁻¹ + gypsum 15 t ha⁻¹), and (8) M-G30 (manure 75 t ha⁻¹ + gypsum 30 t ha⁻¹). Gypsum and organic amendments were applied to the soil surface annually and then incorporated to a depth of 25–30 cm by plow and rotary tillage, and compound fertilizers were applied before sowing. Hordeum was planted for three consecutive years with sprinkler irrigation at a rate of about 1200 m³ ha⁻¹. The main component of gypsum in the test was CaSO₄·1/2H₂O, with a pH value of 8.61, soil organic matter content of 0.86 g kg⁻¹, Na content of 3.03%, Ca content of 36.34%, and Mg content of 0.23%. The composition of the manure in the test was a moisture value of 2.78%, pH of 6.21, soil organic matter content of 34.7%, N content of 3.34%, P₂O₅ content of 2.48%, K₂O content of 0.75%, and Na⁺ content of 1.9%. These values correspond to a laboratory analysis of the samples used.

2.3. Investigate Items and Methods

Soil samples of 0–20 cm and 20–40 cm from each plot were collected after the harvest of the barley, and then air dried and passed through a 2 mm sieve in the laboratory. Soil pH and EC were analyzed in a suspension with a 1:2.5 soil: water ratio (PHS-3C, REX, Shanghai, China; DDSJ-319L, REX, Shanghai, China). Soluble cations (Na, Ca, Mg) were determined by plasma–atomic emission spectroscopy (ICAP–6300, Thermo Fisher Scientific Inc., Waltham, MA, USA). The contents of anions SO₄²⁻ and Cl⁻ in soil samples were determined by the EDTA Volumetric Method and silver nitrate titration method, respectively. Total salt content in soil was analyzed according to the gravimetric method. Soil bulk density was determined using the core method. The hardness and porosity of soil were measured by a dynamic cone penetrometer and a Three-Phase Power Analyzer (DIK-1150, Daiki Rika Kogyo Co., Ltd., Saitama, Japan), respectively. The soil organic matter content was analyzed using an elemental analyzer (C/N Flash EA 112 Series-Leco Truspec, LECO Inc., St. Joseph, MI, USA). N was analyzed using the Kjeldahl nitrogen determination method, P was determined by a UV-visible spectrophotometer (UV-2450, Shimadzu, Kyoto, Japan), and K₂O content was measured by the ammonium acetate extraction and flame photometric method, respectively.

2.4. Statistical Analyses

Statistical analysis was carried out with SPSS 30.0 Statistical Analysis System software, and Illustrations were prepared via Origin 2024. The significance of differences between the treatments was analyzed using one-way ANOVA and LSD at the 5% level of significance (p < 0.05) with the software of SPSS 30.0. Structural equation modeling (SEM) analysis was performed using R 4.2.3 to evaluate the responses of Hordeum yield and soil physical and chemical properties to variations in gypsum and manure application.

3. Results

3.1. Variation in Soil Physical Properties Induced by Gypsum and Manure

According to the ANONA test, there were significant differences in soil hardness of the 0–20 cm soil layer between the treatments with and without gypsum. In 2015, the soil hardness in 7.5 t ha⁻¹, 15 t ha⁻¹, and 30 t ha⁻¹ gypsum treatments decreased by 19.15%, 32.79%, and 44.11%, respectively, compared with no gypsum application. With the increase in gypsum amounts, the soil hardness showed a decreasing trend. Different amounts of gypsum combined with manure treatment significantly decreased soil hardness (Figure 1a). The effect of single manure applications on soil hardness was not observed in 2015. Differently, soil hardness was reduced in 2017 by 22.06% and 62.92% in treatment M-G0 and M-G15, respectively. In the case of manure applied in combination, significant differences in soil hardness could be observed both in 2015 and 2017.

Simultaneously, the soil bulk density of the 0–20 cm soil layer also decreased by 17.86% and 25.01% under the application of single gypsum in 2015 and 2017, respectively (Figure 1b). No significant differences were observed among the treatments of different amounts of gypsum. In 2015, there was no significant difference in soil bulk density between the treatments with and without the application of manure. After two years, manure combined with 7.5–15 t ha⁻¹ gypsum could decrease soil bulk density by 23.48–24.53%.

In 2015, there was no significant difference in soil porosity of the 0–20 cm soil layer among all these treatments. In 2017, manure combined with different amounts of gypsum treatments significantly increased soil porosity (Figure 2). With the increase in the proportion of gypsum combined with manure, the soil porosity showed an increasing trend (M-G15 > M-G7.5 > M-G0) after two years of application, and the soil porosity was most significantly improved by 29.88% in treatment M-G15. Compared to a single application of gypsum, the soil porosity in manure combined with different amounts of gypsum treatments increased by 17.17%, 29.02%, and 20.25%, respectively.

3.2. Soil Chemical Properties’ Response to Gypsum and Manure Amendment

The soil pH value was reduced to a greater extent in 2017 under the gypsum application combined with manure than in 2015 (Figure 3a). There was no significant difference between treatments with and without manure (p > 0.05) in 2015. In 2017, manure combined with gypsum significantly reduced the soil pH value by 0.18–0.23.

Two years of gypsum application combined with manure significantly reduced the soil EC value. In 2015, the application of manure and gypsum led to a significant increase in EC, and a higher soil EC value was observed in manure treatments than in gypsum treatments alone (Figure 3b). In 2017, the soil EC value was significantly increased by 18.41–27.91% under single gypsum application and reduced by 5.62–19.19% under the application of manure and gypsum.

In 2015, the results of soil salt content in the 0–20 cm soil layer (Figure 4a) showed that there was a higher increase in treatment M-G15 and M-G30 than in other treatments. In contrast, the soil salt content of the 20–40 cm layer soil was significantly reduced in treatments of gypsum and manure application than in that of single gypsum (Figure 4b). In 2017, there were no significant differences in the soil salt content between the treatments of combined and single amendments. In addition, the soil salt content of the 20–40 cm soil layer was lower by 5.43–58.06% in 2017 than in 2015 under gypsum and manure application. It was observed that the application of single gypsum significantly reduced Na⁺/(Ca²⁺ + Mg²⁺) by 40.01–82.59% during the three years (Figure 5a). Under the gypsum application combined with manure, there was no significant difference in Na⁺/(Ca²⁺ + Mg²⁺) value among all the treatments. Simultaneously, the Cl⁻/SO₄²⁻ value was reduced by 21.46–71.59% in 2017 compared to the initial soils (Figure 5b). Under a single gypsum application, the Cl⁻/SO₄²⁻ value decreased rapidly after three years of reclamation. In the treatments of the combined gypsum and manure application, significant reductions in Cl⁻/SO₄²⁻ were observed both in 2015 and 2017.

3.3. Effect of Gypsum and Manure on Yield

The difference in Hordeum yield under each treatment was analyzed (Figure 6). There was no significant difference in Hordeum yield between G7.5 and M-G7.5 in both 2015 and 2017. The Hordeum yield significantly increased by 16.76–60.94% under 2 years of mixed application of gypsum and manure. In 2017, 15 t ha⁻¹ gypsum combined with manure led to a significant increase in Hordeum yield by 60.94%, whereas the application of single gypsum increased Hordeum yield by 25.20–53.14%. The Hordeum yield in M-G30 was significantly higher than in G30 in 2017. The results of Hordeum yield in M-G15 treatment were significantly higher, by 7.65–18.97%, than in M-G7.5 and M-G30 (p < 0.05).

3.4. Relationships Between Soil Physicochemical Properties and Hordeum Yield

The results of correlation analysis among the soil physicochemical properties are shown in Figure 7. Soil bulk density, which represents soil physical properties, has a significantly negative (−) correlation with soil porosity and a positive (+) correlation with soil hardness. Total salt content in the 0–20 cm layer also has a significantly positive (+) correlation with Hordeum yield, and it was significantly correlated with most soil physicochemical properties in the 20–40 cm layer. In addition, Na⁺/(Ca²⁺ + Mg²⁺) value also has a significantly positive (+) correlation with Cl⁻/SO₄²⁻ value and a negative (−) relationship with Hordeum yield.

The SEM revealed that the application of single gypsum exerted direct effects on Hordeum yield, while gypsum combined with manure exerted indirect effects (Figure 8). This could explain 67% and 80% of the variance in Hordeum yield of single gypsum and mixed amendments, respectively. Na⁺/(Ca²⁺ + Mg²⁺) had a strong negative effect on Hordeum yield in model 1 (−2.43) and model 2 (−0.80). Soil bulk density negatively affected Hordeum yield in model 2 (−0.46). Na⁺/(Ca²⁺ + Mg²⁺) showed the largest negative contribution to Hordeum yield in both models, while soil bulk density showed the second largest negative effect on Hordeum yield only in model 2 (gypsum combined with manure).

4. Discussion

4.1. Effect of Gypsum and Manure on Soil Properties and Crop Yield

Numerous studies have confirmed that the application of gypsum and organic amendments can modify soil properties [10] and enhance crop yield, such as rice [11], wheat [12], sugarcane [13], cotton [14], and tomatoes [8,15]. As we all know, soil bulk density, hardness, and soil porosity are important indicators that symbolize soil structure [16]. Gypsum has been reported to reduce soil bulk hardness and increase soil porosity [17], which was confirmed in our study. With the increase in gypsum amounts, the soil hardness showed a decreasing trend in 2015. Although there were no significant differences in soil porosity among the treatments with different amounts of gypsum in 2015, the amended soil had a higher porosity than the CK treatment after 2 years of gypsum application. The Ca²⁺ ions released from gypsum could act as soil aggregation agents by directly participating in the formation of organic–inorganic complexes in the soil, which enhances the porosity and air permeability of the topsoil layer [18]. This finding is consistent with other studies, which reported that as the gypsum application amount increased, the content of water-stable soil aggregates rose significantly, and the total soil porosity improved by 0.16% to 10.52% compared to the untreated soil [19].

In 2015, gypsum had a very significant effect on reducing soil pH and Na⁺/(Ca²⁺ + Mg²⁺), but it also significantly increased the total salinity and EC of 0–20 cm soil. The overuse of gypsum may result in soil salt buildup and an increase in the overall salt concentration. Previous studies have shown that the application amount of gypsum was not as much as possible through indoor soil column experiments [20]. After two years of gypsum application, the EC and pH decreased, and the Na⁺/(Ca²⁺ + Mg²⁺), Cl⁻/SO₄²⁻, and total salt content were significantly reduced. As a calcium-based ameliorant, Ca²⁺ released from gypsum could replace Na⁺ in soil colloids and combine with CO₃²⁻/HCO₃⁻ in saline–alkali soil to form CaCO₃, which reduces the salt content and pH of the soil [21,22]. The findings demonstrated that gypsum significantly enhances the physicochemical properties of saline–alkali soil. However, it is important to note that the ameliorative effect on saline–alkali soil did not exhibit a substantial increase with a higher gypsum application amount.

Consistent with the results from the single gypsum application experiment, the incorporation of manure did not significantly alter the physicochemical properties of the soil. Moreover, it led to an increase in EC and salt ion concentrations in 2015. This effect may be related to low crop germination and vegetation cover, and high soil evaporation in the first year, caused by the quality of manure. Consequently, the application of manure resulted in an increase in the soil’s EC value. In this study, the application of manure as an amendment in the early stage of saline–alkali soil remediation needs to be combined with diffuse irrigation to optimize its effectiveness. After two years of application of manure, pH, EC, and Na⁺/(Ca²⁺ + Mg²⁺) were significantly reduced. The findings revealed that the application of manure over a three-year period facilitated the leaching of salts below the 40 cm depth, suggesting that the ameliorative effects of manure on soil porosity and structure are revealed gradually over time. Consequently, the ability of manure to enhance soil porosity and improve soil aeration is only observable with long-term application in saline soils. In summary, gypsum is more suitable than manure for the early stage of amendment in saline–alkali soil.

In this study, the co-application of manure and gypsum over a two-year period significantly increased Hordeum yield in saline–alkali soil. The results demonstrated that the combination of manure with any dosage of gypsum was more effective than the application of gypsum alone in enhancing soil porosity and mitigating unfavorable soil chemical properties. The treatment M-G15 exhibited the highest efficiency on Hordeum yield, as illustrated in Figure 6. The application of gypsum and manure could directly increase the number of organic colloids and participate in the process of soil agglomeration, thereby optimizing soil pore structure and benefiting the growth of crop roots [23]. Previous research also reported that the combined application of gypsum and humic acid as a mixed amendment increased the total soil porosity by 0.16% to 10.52% compared with the basic soil [24]. Both manure and gypsum may function as salt-ion chelating agents in soil and react with salt ions, especially Na⁺ and Cl⁻, thereby reducing the salt content in soil [25,26]. Consequently, the enhanced soil pore structure and reduced salinity greatly contribute to crop growth in saline soils. Furthermore, the application of gypsum provided sufficient nutrients beneficial to crop resistance to harsh environments, including Ca, Si, S, and other elements, which is conducive to the growth and yield of crops in saline–alkali soil [27]. In this study, the integrated application of gypsum at 15 t ha⁻¹ and manure at 75 t ha⁻¹ was effective.

In most studies, the application of gypsum and manure was effective in saline–alkali soil improvement [28,29]. In this country, gypsum demonstrates significant efficacy during the early stage of saline–alkali soil remediation, while the combined application of manure and gypsum in the later stage exhibits greater efficacy and advantages compared to a single application of gypsum. Therefore, a strategic approach could involve the exclusive application of gypsum for the first three years during the early stage of saline–alkali soil remediation, followed by the integrated application of manure and gypsum (15 t ha⁻¹) to further enhance remediation outcomes and sustainably increase crop yields.

4.2. Direct Effect of Soil Physicochemical Properties on Hordeum Yield

Manure is widely recognized for its direct influence on the physicochemical properties of soil [30,31]. To elucidate the role of manure during the gypsum amendment process, we conducted a comparative analysis by dividing the study into two groups to identify the driving factors of yield. Our analysis revealed that soil chemical and physical factors exerted indirect negative effects on crop yield, primarily through Na⁺/(Ca²⁺ + Mg²⁺) and soil bulk density. Under single gypsum applications, Na⁺/(Ca²⁺ + Mg²⁺) emerged as the primary regulator of crop yield, whereas under combined applications of gypsum and manure, both Na⁺/(Ca²⁺ + Mg²⁺) and soil bulk density conditions were identified as key determinants.

It is well known that soil bulk density and porosity directly influence the migration of salinity and the rooting capacity of crops [32,33,34]. Elevated soil bulk density reduces soil porosity [33], hindering salt leaching, limiting root aeration and penetration [35], and, consequently, greatly affecting crop yield [36]. The negative correlation between soil porosity and Na⁺/(Ca²⁺ + Mg²⁺) indicated that Na⁺/(Ca²⁺ + Mg²⁺) could adversely impact Hordeum yield by reducing soil porosity. The three-year application of gypsum significantly improved the porosity of the soil, which was conducive to promoting the migration and exchange of Na⁺ and Cl⁻ ions [37]. This is consistent with Mao (2016), who demonstrated that the application rate of 60 Mg/ha of gypsum reduced the exchangeable sodium percentage (ESP) to below 6% and facilitated the exchange of sodium ions (Na⁺, HCO₃⁻ + CO₃²⁻, and Cl⁻) with neutral salt ions (Ca²⁺ and SO₄²⁻) in the soil [38]. The most pronounced effect of the long-term application of manure and gypsum is the reduction in soil bulk density and the enhancement of soil porosity, which greatly facilitates the leaching of salt ions into deeper soil layers [39]. This process directly reduces salt stress on crop emergence and contributes to improvement of crop yield [40]. Therefore, Na⁺/(Ca²⁺ + Mg²⁺) should be prioritized as a critical factor influencing yield under single gypsum application in saline–alkali soil, while both soil bulk density and Na⁺/(Ca²⁺ + Mg²⁺) are identified as key determinants affecting Hordeum yield under the combined application of gypsum and manure.

5. Conclusions

The combined application of gypsum and manure is an effective strategy for the restoration of saline–alkaline soils and increasing Hordeum yield. Gypsum reduced soil bulk hardness and increased soil porosity significantly in the first year of saline–alkali soil remediation. Furthermore, the soil EC value and Hordeum yield were significantly higher with the mixed amendment of gypsum and manure than with a single individual application after three years of remediation. Na⁺/(Ca²⁺ + Mg²⁺) and bulk density in topsoil are identified as critical factors influencing Hordeum yield under the mixed amendments. The recovery effects of manure on saline–alkaline soils are most evident after long-term application (three years). Our results suggest that a promising strategy could be the initial application of gypsum for rapid improvements, followed by co-application with manure for long-term and sustainable benefits. Given the spatial heterogeneity of soil properties across different regions, the application methods of manure must be tailored to local soil conditions. In general, this study describes the effect of single gypsum and gypsum combined with manure on saline–alkali soil reclamation in barley fields over time, and the results provide a factual basis and data for the optimization of saline–alkali soil reclamation practices in Hordeum fields using gypsum and organic amendment.