Study on the Effects of Irrigation Quotas and Amendments on Salinized Soil and Maize Growth

Abstract

Salt damage affects crop yields and wastes limited water resources. Implementing water-saving and salt-controlling strategies along with amendments can enhance crop productivity and support the development of salinized soils towards. In this study, we used “Jia Liang 0987” maize as the test material, and a two-factor split block design was executed to investigate the effects of synergistic management of irrigation volume (W1: 360 mm, W2: 450 mm, and W3: 540 mm) and amendments (T1: microbial agent 816.33 kg·hm⁻², T2: humic acid 6122.45 kg·hm⁻², T3: microsilica powder 612.25 kg·hm⁻²) on water, salt and soil indices, and growth characteristics. The combination of 450 mm of irrigation with humic acid (W2T2) or with microsilica powder (W2T3) significantly lowered the groundwater level by 0.24 m and 0.19 m, respectively. The soil mineralization was significantly reduced by 2.60 g/L and 1.75 g/L with W2T2 and 540 mm of irrigation combined with humic acid (W3T2), respectively. The soil moisture content increased with depth and over time, showing the greatest improvement with W2T2. This combination also showed optimal results for pH and total salt, organic matter, available phosphorus, quick-acting potassium, Cl⁻, and SO₄²⁻ contents. W2T2 and W3T2 improved soil field capacity and HCO₃⁻ contents, and significantly increased total nitrogen and phosphorus content, improving the soil nutrient grade. W2T2 showed the greatest maize plant height (323.67 cm) and stem thickness (21.54 mm for diameter), enhancing above-ground dry biomass (72,985.49 kg·hm⁻²) and grain yield (14,646.57 kg·hm⁻²). Implementing water-saving and salt-controlling strategies with amendments effectively improved soil fertility and crop yield in salinized soils, and the amendments factor played a major role. In saline–alkali soils in the northwest of China, 450 mm of irrigation combined with humic acid is especially helpful for enhancing soil fertility and maize productivity.

1. Introduction

Salinization and water resource shortages are the two major obstacles to sustainable agriculture in arid and semi-arid regions [1], and they are also global challenges [2]. The high salinity and sodium levels in the soil cause the soil to inflate, disperse, and lose its structural integrity. This has a detrimental effect on the permeability coefficient, water infiltration, and porosity of the soil [3]. Thus, this causes the disintegration of the soil structure, loss of organic matter, and a nutrient deficit, which results in decreased soil fertility [4]. It also hinders soil water conductivity and air permeability. Furthermore, the high pH and electrical conductivity and low permeability potential of saline–sodic soil have a direct impact on the rate at which soil enzymes participate in biochemical processes and on microbial activity. This has a detrimental effect on crop growth, because it further lowers the nutrient content of the soil and prevents element cycling within the soil system [5]. According to incomplete statistics from the United Nations Educational, Scientific, and Cultural Organization and the Food and Agriculture Organization of the United Nations (FAO), the global area of saline–alkali soils is approximately 0.833 billion hm² [6]. The structural qualities of the soil are harmed by excessive soil salinity and alkalization, which also hinder biological activity and microbial diversity and lower fertilizer application efficiency [7]. These elements have a negative impact on soil-dwelling plants, reducing corn and wheat crop yields. This severely restricts the sustainable development of irrigated agriculture, which in turn, threatens human food security [8].

Salinized and secondary salinized soils are important land resources. Rational development and improvement of these soils into arable land meet current requirements and agricultural demands, providing researchers with a clear direction for exploration [9]. The essence of salinized soil improvement is to use technical measures to enhance soil fertility, improve the soil, and make it suitable for crop growth and development, thereby achieving high crop yields [10]. This will be an important strategy for the sustainable development of irrigated agriculture in China, and alleviate the status quo of “more people and less land, and tight land resources” [11]. Such improvements will transform adverse factors into favorable conditions; improve the ecological environment; and play a significant role in promoting the sustainable development of regional economies, society, and ecology.

In recent years, strategies for reclaiming saline–alkali land have been studied and developed [12]. Such strategies mainly involve the cultivation of salt-tolerant crops [13], water-saving irrigation [10], back filling of guest soil [11], modifying agent application [14], and water conservation measures [15]. Particularly, water-saving irrigation (WSI) is widely used in arid and semi-arid regions. In China, a number of WSI regimes have been implemented, including controlled irrigation, drip irrigation, and rain-gathering irrigation [16,17,18]. It is a highly efficient technology that modifies the physical state, microbial activity, and crop growth of soil, resulting in reduced soil salinization risk and water consumption [19,20]. By optimizing the amount of irrigation, WSI has been shown to be effective at saving water and increasing corn yield. It is also thought that WSI practice helps lower runoff loads from non-point nitrogen and phosphorus contamination [21]. Therefore, it is crucial to determine the optimal WSI quota for maximum crop yield and minimum water consumption. Applying organic amendments is a useful strategy for enhancing saline soil structure since they can provide sufficient organic matter [22]. Organic amendments, which contain organic acids, act as soil aggregate binders and give microorganisms a source of carbon [23]. Past research has confirmed that the availability and accessibility of organic matter to microorganisms is strongly correlated with the particulate organic matter and the C/N ratio of the organic amendment [24]. Applying nutrients can decrease the stability of soil aggregates and speed up the process through which microorganisms break down organic amendments. The second most common mineral element in the crust of the earth, silicon (Si), improves a plant’s ability to respond to both biotic and abiotic stressors, hence reducing salinity [25]. Crop plants generate resilience to stress in the presence of microsilica by scavenging the harmful effects of reactive oxygen species, lowering excessive Na ion absorption, and reducing electrolyte leakage and malondialdehyde concentration [26]. However, there are few studies showing that a combination of improvement methods is more conducive to saline soils.

Mildly salinized soil is present in the northwest irrigation area of the Hetao Plain in the upper reaches of the Yellow River, located in Liuzhong Village, Pingluo County, in the northern part of the Yinchuan Plain, Ningxia. This area experiences drought and low rainfall throughout the year, with high inherent soil salinity [27]. In the early 21st century, local policies promoting ecological migration and irrigation from the Yellow River led to extensive land reclamation for farming. However, the lack of rainfall to wash away salts, combined with irrigation only reaching the topsoil, caused frequent water and salt movement, resulting in salinization [28]. Coupled with drought and water scarcity, there remains an urgent need to conserve water and control salt while ensuring crop yields, an important way to promote high-quality development in the Yellow River Basin [29]. Using soil salt-barrier-targeting technology and directional regulation technology, in combination with agronomic practices and water and fertilizer management techniques, it is possible to develop a comprehensive improvement plan for mildly salinized land that enhances both yield and quality. Therefore, this study compared different irrigation quotas and types of amendments to analyze the interactions within the salinized soil–water–salt system. It aimed to precisely select and assemble irrigation techniques and integrate soil fertility improvement methods to enhance soil productivity. This research sought to improve salinized soils and increase crop yields, providing technical insights and data support for water-saving and soil fertility enhancement in mildly salinized soils in Ningxia. It holds significant importance for promoting the sustainable economic, social, and ecological development of the Yellow River irrigation region.

2. Materials and Methods

2.1. Overview of the Experimental Site

The study area is located in Liuzhong Village, Pingluo County, in the northern part of Yinchuan Plain, Ningxia. It lies downstream of the Qingtongxia Irrigation District, with Helan Mountain to the west and the Yellow River to the east, with the geographic coordinates, 38°9′04″ N, 106°5′45″ E. The region has a temperate arid continental climate, at an altitude of approximately 1090 m. It experiences drought and low rainfall throughout the year, with an annual average precipitation of 173.2 mm, mostly occurring from June to August. The annual average temperature is 9.0 °C, with a large diurnal temperature variation. The region receives more than 2387.9 h of sunshine annually, with a frost-free period of 171 d. The total annual amount of sunshine exceeds 2800 h, and the frost-free period ranges from 150 to 170 d. Evaporation is intense, and the surface layer consists of salinized irrigated alluvial soil, with a thickness of approximately 0.92 m and a relatively clayey texture [30]. The characteristics of the soil before amendments are listed in

Table 1. Basic properties of the soil in the study area.

Soil Layer	pH	Total Salt	SOM	AH-N	AP	AK	SO42−	HCO3−	Cl−
cm		g/kg	g/kg	mg/kg	mg/kg	mg/kg	g/kg	g/kg	g/kg
0~20	8.52	2.25	13.63	36.4	15.85	149.44	0.05	0.04	0.03
20~40	8.47	2	12.87	30.1	8.7	133.49	0.04	0.04	0.02
40~60	8.45	1.09	8.15	21.12	0.29	120.37	0.08	0.04	0.02
60~80	8.46	1.14	7.23	21.12	0.87	123.16	0.09	0.04	0.02
80~100	8.47	1.13	6.85	21.12	0.87	121.7	0.11	0.04	0.02

.

2.2. Experimental Materials and Design

The test crop was “Jia Liang 0987” maize, and the study period was from March to November 2022. This study used a two-factor split block design, as detailed in Table 1. The main plots were designated for irrigation quotas (W), using tap water drip irrigation with three irrigation levels: W1: 360 mm, W2: 450 mm, and W3: 540 mm. The subplots were designated for soil amendments (T), using three types of amendments: no treatment (T0), microbial agent (T1), humic acid (T2), and microsilica powder (T3). Each treatment had three replicates, totaling 36 treatment plots. The specific application rates of the amendments are detailed in

Table 2. Field trial arrangement.

Main Plot/Irrigation Quota	Subplot/Amendments
W1	T0	T1	T2	T3
	T2	T3	T0	T1
	T0	T1	T2	T3
W2	T2	T3	T0	T1
	T0	T1	T2	T3
	T2	T3	T0	T1
W3	T0	T1	T2	T3
	T2	T3	T0	T1
	T0	T1	T2	T3

. Each treatment plot was enclosed with 50 cm wooden stakes and lines, with labeled tags for differentiation. Each plot was 7 m long and 7 m wide, with an effective planting area of 49 m². The field trial arrangement is detailed in

Table 3. The dosage of the amendments.

Amendments	Dosage/kg·hm−2
microbial agents	816.33
humic acid	6122.45
microsilica	612.25

.

Microbial agent: The formulation was primarily composed of Bacillus subtilis, and contained lactic acid bacteria and yeast, with a concentration of 2.00 × 10⁷ colony-forming units (CFU) per gram. It was produced by Ningxia Qiyuan Biotechnology Co., Ltd., Yinchuan, China.

Humic acid: The total humic acid content was ≥50%, the N content was ≥3%, and the moisture content was ≤2%. It was produced by Ningxia Qiyuan Biotechnology Co.

Microsilica powder: The microsilica powder was a porous material with a particle size of 0.1–10 μm and a silica content of more than 90%. It was produced by Ningxia Qiyuan Biotechnology Co.

Base fertilizer: urea, total nitrogen (TN) content of ≥46.0%, particle size range: (Φ) 0.85–2.80 mm, produced by Inner Mongolia Ordos United Chemical Co. All treatments used conventional base fertilizer (914.29 kg·hm⁻² of urea applied), consistent with other field management practices.

2.3. Measurement Items and Methods

2.3.1. Physical and Chemical Properties of Soils and Their Measurement Methods

Soil samples were collected before (1 April), during (1 July), and after (1 October) soil improvement at the study site in 2022 to determine various physical and chemical properties. Physical properties included soil water content and field capacity. Chemical properties included the following soil fertility indicators: pH and total salts, organic matter, total nitrogen, alkaline hydrolyzable nitrogen, available phosphorus, and available potassium content. The soil salt ions measured included HCO₃⁻, Cl⁻, and SO₄²⁻.

Soil samples were collected from 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm depths using the ring knife method before soil improvement (1 April). Soil samples from a depth of 0–20 cm were collected using the ring knife method during soil improvement (1 July) and after soil improvement (1 October). A portion of the fresh soil sample was placed in a covered aluminum box, weighed, and then dried in an oven at 105 °C until it reached a constant weight, after which it was weighed again to calculate the soil moisture content. The soil field capacity was measured using the ring knife method. Other collected soil samples were dried naturally, ground finely, and sieved through 1 mm and 0.25 mm sieves. With a 5:1 water-to-soil ratio to extract the soil water solution, soil salt content was measured by a conductivity meter [31]. The pH value was measured using a PHS-25 precision acidity meter, and electrical conductivity was measured using a conductivity meter, which was then converted to total soil salt content. Soil organic matter (SOM) content was measured using potassium dichromate with external heating; total nitrogen (TN) content was measured using the Kjeldahl method; total phosphorus (TP) content was measured using the molybdenum-antimony colorimetric method; and alkaline hydrolysable nitrogen (AH-N) content was measured using the alkali diffusion method. Available phosphorus (AP) content was measured using the sodium bicarbonate extraction-molybdenum-antimony colorimetric method. Available potassium (AK) content was measured using the ammonium acetate-flame photometry method [32]. Soil salt ion content was measured as follows: SO₄²⁻ content was measured using the EDTA indirect chromogenic titration method, HCO₃⁻ contents were measured using the double indicator titration method, and Cl⁻ content was measured using the AgNO₃ titration method [33].

2.3.2. Groundwater Level and Mineralization, and Measurement Methods

In 2022, soil samples were collected before soil improvement (1 April), during soil improvement (1 July), and after soil improvement (1 October). A sectional rotary soil auger with a scale was used to drill from the surface downward until water was reached. The depth at which the auger submerged was recorded as the groundwater level. The muddy water extracted was collected using an automatic water suction pump and stored in 500 mL polyethylene bottles, which were then taken to the laboratory for mineralization measurements [34].

2.3.3. Maize Growth Characteristics and Their Measurement

Plant height, stem thickness, aboveground dry matter mass, and maize kernel yield were determined at the time of maize harvest. After maturity, three representative maize plants with uniform growth were selected. Plant height was measured using a telescopic 5 m tape measure, and stem thickness was measured using a vernier caliper. In the study plots, six representative maize plants with uniform growth (three from the inner rows and three from the outer rows) were selected. Yield measurement was conducted using the high-yield creation method of the Ministry of Agriculture and Rural Affairs, measuring grain yield for each treatment. Six representative plants (three plants in each inner and outer row) with uniform growth were selected. Samples of the whole plant above ground were taken back to the laboratory for weighing and rinsing with distilled water. They were then placed in an oven to inactivate at 105 °C for 30 min, followed by drying at 70 °C until a constant weight was achieved. The final weight measured was recorded as the above-ground dry biomass.

2.4. Data Analysis

The study data were organized using Excel 2010 software, plotted using Origin2021 (OriginLab Corporation, Northampton, MA, USA). ANOVA, correlation analysis and principal component analysis were executed with SPSS 25.0 software (IBM, Armonk, NY, USA). All data were expressed as the mean ± standard deviation (SD) of three replicates and were compared using the least significant difference (LSD) method (p < 0.05). Using the method of factor analysis with SPSS software, total explained variance and the core coefficient matrix of each principal component was determined. The comprehensive score of each treatment was calculated as follows:

$$\mathrm{F}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{N}}_{\mathrm{i}}\times {\mathrm{F}}_{\mathrm{i}}$$

$${\mathrm{N}}_{\mathrm{i}}={\displaystyle \frac{{\mathsf{\beta }}_{\mathrm{i}}}{\sum _{\mathrm{i}=1}^{\mathrm{n}}\mathrm{i}}}$$

where F is the comprehensive score of each treatment, N_i is the weight of each principal component, F_i is the average core of each component, and β_i is the variance contribution rate of each component.

The soil nutrient level was determined by the ‘Technical specifications for assessment and rating criteria of cultivated land quality’ (DB 33/T 895-2013) [35].

3. Results

3.1. Characteristics of Groundwater Level Movement in Salinized Soils

After applying different irrigation quotas and amendments, the overall groundwater level showed a decreasing trend, with an annual reduction of 0.06 m to 0.24 m (Figure 1). Under the same amendment, the W2 irrigation quota showed a greater reduction in the groundwater level, while W1 had the least effect. At the same irrigation quota, the effect of different amendments on soil groundwater level reduction was T2 > T3 > T1 > T0. After amendment, the W2T2 treatment had the most significant effect on groundwater level reduction, with a 0.24 m reduction, followed by W2T3, with a 0.19 m reduction.

3.2. Variation Patterns of Groundwater Salinity in Salinized Soil

The groundwater salinity in the study area ranged from 5.88 to 11.43 g/L. Irrigation leaching mitigated the degree of soil salinization to a certain extent, and the groundwater salinity showed an initial increase followed by a decrease (Figure 2). W2 had the greatest effect on groundwater mineralization reduction, W3 had a slightly smaller effect than W2, and W1 had the smallest effect. At the same irrigation quota, the reduction in groundwater mineralization was more significant with T2. As a result, W2T2 had the most significant reduction in groundwater mineralization, with a reduction of 2.60 g/L.

3.3. Characteristics of pH and Salinity Changes in Salinized Soils

During and after the improvement, both soil pH and total salt content showed a decreasing trend (

Table 4. Variation characteristics of soil pH and total salt content in salinized soil.

Treatment	pH			Total Salt (g/kg)
	Earlier Stage	Medium Stage	Late Stage	Earlier Stage	Medium Stage	Late Stage
W1T0	8.52	8.43 a	8.14 b	2.25	2.07 a	1.50 a
W1T1	8.52	8.23 ef	7.92 c	2.25	2.04 a	1.36 bc
W1T2	8.52	8.20 ef	7.63 ef	2.25	1.73 abc	1.21 de
W1T3	8.52	8.37 ab	7.96 c	2.25	1.93 efg	1.36 bc
W2T0	8.52	8.33 bc	8.16 ab	2.25	1.97 ab	1.48 a
W2T1	8.52	8.27 cde	7.75 d	2.25	1.89 bcd	1.27 cd
W2T2	8.52	8.17 f	7.54 f	2.25	1.62 efg	1.07 f
W2T3	8.52	8.35 ab	7.72 de	2.25	1.73 g	1.22 de
W3T0	8.52	8.33 bcd	8.25 a	2.25	1.77 cde	1.43 ab
W3T1	8.52	8.25 ef	7.81 d	2.25	1.76 def	1.21 de
W3T2	8.52	8.17 f	7.64 e	2.25	1.67 def	1.12 ef
W3T3	8.52	8.25 de	7.93 c	2.25	1.82 fg	1.25 d
Irrigation quota	-	15.37 **	48.47 **	-	54.92 **	32.77 **
Amendments	-	86.23 **	458.35 **	-	55.61 **	131.98 **
Interaction	-	6.82 **	11.17 **	-	8.40 **	3.19 *

Notes: W1: 360 mm irrigation quota, W2: 450 mm irrigation quota, W3: 540 mm irrigation quota; T0: no conditioning, T1: microbial agent, T2: humic acid, T3: microsilica powder. Lowercase indicates significant difference in soil properties among treatments at the 0.05 level. * indicates significant impact at the 0.05 level and ** indicates significant impact at the 0.01 level. Earlier stage: 1 April was before the improvement period. Medium stage: 1 July was during the improvement period. Late stage: 1 October was after the improvement period.

). During the mid-improvement period, the W2 irrigation quota combined with the T2 amendment showed the best results for reducing soil pH, achieving reductions of 0.26 to 0.35. This effect continued into the late-improvement period. The pH of W2T2 was reduced by 0.98 from the pre-improvement level and significantly reduced by 0.10 to 0.71 from all other treatments. The soil total salt content was reduced from 8.00% to 28.00% and from 33.33% to 52.44% during and after the amendment, respectively, compared to the pre-amendment period. Under the combined influence of irrigation quotas and amendments, the W2T2 treatment showed the greatest reduction, significantly lowering the total salt content by 21.74% and 28.67% compared to that of W1T0. The direct improvement effect of the amendments was more significant than irrigation and the interactions.

Irrigation quotas combined with amendments effectively reduced the content of various soil anions. The reduction effect on anions was more pronounced in the late-improvement period compared to the mid-improvement period (

Table 5. Anion migration in salinized soil.

Treatment	SO42− (mg/kg)			HCO3− (mg/kg)			Cl− (g/kg)
	Earlier Stage	Medium Stage	Late Stage	Earlier Stage	Medium Stage	Late Stage	Earlier Stage	Medium Stage	Late Stage
W1T0	54.40	52.36 a	26.56 ab	39.35	35.78 ab	21.99 b	35.76	33.52 a	26.87 ab
W1T1	54.40	50.39 a	35.41 ab	39.35	27.90 bc	20.73 bcd	35.76	28.99 bcd	24.89 bc
W1T2	54.40	46.20 abc	32.32 ab	39.35	18.49 d	16.40 cdef	35.76	23.89 e	15.39 gh
W1T3	54.40	49.51 a	19.52 b	39.35	32.43 abc	21.01 bc	35.76	28.22 bcd	20.16 de
W2T0	54.40	52.45 a	20.11 b	39.35	26.65 c	11.93 fg	35.76	31.09 ab	16.45 fg
W2T1	54.40	50.60 a	31.41 ab	39.35	17.02 d	12.45 efg	35.76	29.49 b	22.62 cd
W2T2	54.40	41.79 c	19.29 b	39.35	14.92 d	10.86 g	35.76	25.03 de	13.28 h
W2T3	54.40	48.81 ab	40.67 ab	39.35	17.37 d	12.48 efg	35.76	28.80 bcd	26.45 ab
W3T0	54.40	52.07 a	49.65 a	39.35	37.62 a	34.42 a	35.76	29.19 bc	28.57 a
W3T1	54.40	46.80 abc	38.83 ab	39.35	29.99 abc	17.67 bcde	35.76	27.79 bcde	23.17 c
W3T2	54.40	42.48 bc	39.76 ab	39.35	29.70 bc	11.11 fg	35.76	25.43 cde	18.66 ef
W3T3	54.40	46.85 abc	43.65 ab	39.35	32.10 abc	15.41 defg	35.76	30.40 ab	22.20 cd
Irrigation quota	-	3.65 *	12.09 **	-	1.29	1.29	-	0.40	37.33 **
Amendments	-	22.18 **	0.61		78.07 **	78.07 **	-	36.43 **	138.94 **
Interaction	-	1.02	3.34 *	-	21.53 **	21.53 **	-	3.96 **	51.36 **

Notes: W1: 360 mm irrigation quota, W2: 450 mm irrigation quota, W3: 540 mm irrigation quota; T0: no conditioning, T1: microbial agent, T2: humic acid, T3: microsilica powder. Lowercase indicates significant difference in soil properties among treatments at the 0.05 level. * indicates significant impact at the 0.05 level and ** indicates significant impact at the 0.01 level. Earlier stage: 1 April was before the improvement period. Medium stage: 1 July was during the improvement period. Late stage: 1 October was after the improvement period.

). After the improvement, the concentrations of SO₄²⁻, HCO₃⁻, and Cl⁻ decreased by 64.54% to 87.32%, 12.53% to 72.40%, and 20.11% to 62.86%, respectively, compared to before the improvement. In contrast, in the mid-improvement period, the decrease was only by 4.40% to 33.19%. At the same irrigation quota, the effect of each amendment on the reduction in soil anions was as follows: T2 > T1 > T3 > T0. Under the same amendment, the W2 irrigation quota had the greatest effect, followed by W3. Overall, the W2T2 treatment showed the most significant reduction in SO₄²⁻, HCO₃⁻, and Cl⁻. Amendments and their interaction with irrigation quotas reduced soil anion content more significantly than irrigation factors alone.

3.4. Effects of Water and Salt Transport on Soil Fertility

Soil organic matter, alkaline hydrolysable nitrogen, available phosphorus, and available potassium contents showed an overall increasing trend during the improvement period. The effect of soil fertility enhancement was more pronounced in the late-improvement period (

Table 6. Effects of water and salt migration on soil organic matter, alkaline hydrolysable nitrogen, available phosphorus contents and available potassium content.

Treatment	SOM (g/kg)			AH-N (mg/kg)			AP (mg/kg)			AK (mg/kg)
	Earlier Stage	Medium Stage	Late Stage	Earlier Stage	Medium Stage	Late Stage	Earlier Stage	Medium Stage	Late Stage	Earlier Stage	Medium Stage	Late Stage
W1T0	13.63	13.84 de	17.54 d	149.44	149.35 efgh	173.38 cde	15.85	10.20 e	14.20 h	149.44	149.35 efgh	173.38 cde
W1T1	13.63	14.56 bcde	21.58 bc	149.44	152.84 efg	161.58 def	15.85	25.16 c	41.46 d	149.44	152.84 efg	161.58 def
W1T2	13.63	15.41 abc	22.77 ab	149.44	168.89 abc	192.40 abc	15.85	43.40 a	45.22 cd	149.44	168.89 abc	192.40 abc
W1T3	13.63	14.62 bcde	20.64 c	149.44	145.56 fgh	155.16 ef	15.85	21.29 cd	23.86 fg	149.44	145.56 fgh	155.16 ef
W2T0	13.63	13.30 e	18.32 d	149.44	141.05 gh	148.34 f	15.85	16.90 de	31.75 e	149.44	141.05 gh	148.34 f
W2T1	13.63	14.49 bcde	21.85 bc	149.44	158.63 cde	176.43 abcd	15.85	34.08 b	54.89 ab	149.44	158.63 cde	176.43 abcd
W2T2	13.63	16.50 a	24.19 a	149.44	171.41 ab	195.92 a	15.85	25.46 c	57.32 bc	149.44	171.41 ab	195.92 a
W2T3	13.63	15.52 abc	20.88 c	149.44	139.38 h	168.36 def	15.85	16.53 de	29.34 a	149.44	139.38 h	168.36 def
W3T0	13.63	13.73 de	22.09 bc	149.44	146.12 fgh	171.41 cde	15.85	15.34 de	17.69 ef	149.44	146.12 fgh	171.41 cde
W3T1	13.63	14.15 cde	21.84 bc	149.44	155.90 def	190.85 abc	15.85	27.25 bc	48.95 gh	149.44	155.90 def	190.85 abc
W3T2	13.63	15.94 ab	22.63 b	149.44	179.79 a	195.03 ab	15.85	26.95 bc	44.64 cd	149.44	179.79 a	195.03 ab
W3T3	13.63	15.06 abcd	20.75 c	149.44	167.23 bcd	174.06 bcde	15.85	26.84 bc	19.16 gh	149.44	167.23 bcd	174.06 bcde
Irrigation quota	-	1.41	17.30 **	-	61.99 **	34.78 **	-	1.29	115.57 **	-	19.19 **	10.21 **
Amendments	-	33.65 **	96.66 **	-	70.58 **	182.41 **	-	78.07 **	449.05 **	-	78.46 **	33.48 **
Interaction	-	1.94	21.15 **	-	9.11 **	115.08 **	-	21.53 **	7.14 **	-	9.84 **	6.39 **

Notes: W1: 360 mm irrigation quota, W2: 450 mm irrigation quota, W3: 540 mm irrigation quota; T0: no conditioning, T1: microbial agent, T2: humic acid, T3: microsilica powder. Lowercase indicates significant difference in soil properties among treatments at the 0.05 level. ** indicates significant impact at the 0.01 level. Earlier stage: 1 April was before the improvement period. Medium stage: 1 July was during the improvement period. Late stage: 1 October was after the improvement period.

). During the improvement process, the optimal irrigation quota for improving each soil fertility indicator varied. In contrast, the effect of the W2 irrigation rating on soil fertility enhancement was pronounced in the late-improvement period. Additionally, the T2 amendment consistently outperformed other amendments at enhancing soil fertility throughout the entire improvement process. Under the W2T2 treatment, the increases in soil organic matter, alkaline hydrolysable nitrogen, available phosphorus, and available potassium contents were 77.48%, 86.90%, 173.31%, and 31.10%, respectively. Other treatments showed significant increases ranging from 0.46% to 32.08%. The direct effects of the amendments were more significant than those of the irrigation factors and their interactions with the amendments.

3.5. Effects of Water and Salt Transport on Maize Growth Characteristics

The plant height and stem thickness of maize plants were highly significantly affected by irrigation rates, amendments and the interaction of irrigation rates and amendments (

Table 7. Effects of water and salt regulation on maize growth characteristics.

Treatment	Plant Height /cm	Stem Diameter /mm	Aboveground Dry Matter Mass/kg·hm−2	Grain Yield /kg·hm−2
W1T0	294.11 ± 4.67 c	17.45 ± 0.35 e	49,697.35 ± 2550.86 e	9620.59 ± 191.95 d
W1T1	315.22 ± 2.71 abc	19.40 ± 0.37 bcd	69,816.50 ± 4128.54 ab	13,393.24 ± 901.72 abc
W1T2	319.33 ± 9.02 ab	20.55 ± 0.30 ab	70,228.57 ± 1012.36 ab	13,964.46 ± 765.25 ab
W1T3	306.00 ± 3.48 abc	20.32 ± 0.60 ab	58,497.13 ± 1933.73 cd	11,987.76 ± 369.51 c
W2T0	260.22 ± 4.91 d	18.11 ± 0.67 de	47,274.97 ± 1037.14 e	10,220.06 ± 357.93 d
W2T1	297.42 ± 8.98 bc	19.85 ± 0.88 bc	62,423.95 ± 2013.26 c	10,204.58 ± 464.11 d
W2T2	323.67 ± 2.85 a	21.54 ± 0.24 a	72,985.49 ± 632.73 a	14,646.57 ± 165.15 a
W2T3	309.00 ± 4.18 abc	20.75 ± 0.13 ab	64,378.98 ± 1221.55 bc	13,462.67 ± 368.84 abc
W3T0	257.33 ± 19.88 d	18.46 ± 0.35 cde	46,977.16 ± 5067.40 e	9005.85 ± 454.86 d
W3T1	301.00 ± 2.00 abc	20.40 ± 1.19 ab	52,012.99 ± 1473.69 de	12,874.92 ± 697.46 bc
W3T2	293.78 ± 9.16 c	20.15 ± 0.27 ab	64,986.51 ± 566.29 bc	12,037.73 ± 139.68 c
W3T3	300.78 ± 1.58 abc	19.81 ± 0.18 bc	61,136.75 ± 2025.60 c	12,198.71 ± 776.92 c
Irrigation quota	20.42 **	3.97 *	21.73 **	2.38 *
Amendments	51.28 **	42.73 **	125.96 **	89.17 **
Interaction	5.95 **	2.77 *	11.82 **	19.58 **

Notes: W1: 360 mm irrigation quota, W2: 450 mm irrigation quota, W3: 540 mm irrigation quota; T0: no conditioning, T1: microbial agent, T2: humic acid, T3: microsilica powder. Lowercase indicates significant difference in soil properties among treatments at the 0.05 level. * indicates significant impact at the 0.05 level and ** indicates significant impact at the 0.01 level. Earlier stage: 1 April was before the improvement period. Medium stage: 1 July was during the improvement period. Late stage: 1 October was after the improvement period.

). As the irrigation quota increased, the plant height and stem thickness of maize initially increased and then decreased. The W1 and W2 irrigation quotas resulted in higher values, with the W2 quota being the highest. Under the same irrigation quota, the T2 amendment had the most significant effect on increasing maize plant height. Therefore, the W2T2 treatment achieved the highest values, with plant height and stem thickness reaching 323.67 cm and 21.54 mm, respectively, which was significantly higher than the other treatments by 1.36% to 24.38% and 4.82% to 23.44%, respectively. The trends in above-ground dry biomass and grain yield were similar to those observed for plant height and stem thickness. The yield increase effect of different amendments was T2 > T3 > T1 > T0, with the W2 irrigation quota showing the best results. Consequently, the W2T2 treatment achieved above-ground dry biomass and grain yields of 72,985.49 kg·hm⁻² and 14,646.57 kg·hm⁻², respectively, representing increases of 3.93% to 55.36% and 4.85% to 62.63% compared to the other treatments. The singular effect of the amendments was still higher than that of the irrigation quota and their interaction.

3.6. Influence of Water, Salt, and Soil Fertility on Crop Yield

Based on the correlation analysis of water and salt content and fertility indicators in salinized soil with the above-ground dry biomass and grain yield of maize under different irrigation quotas and amendments, it was found that an increase in pH and total salt contents significantly inhibited the above-ground dry biomass and grain yield of maize. In contrast, an increase in the water content, field capacity, and soil organic matter and alkaline hydrolyzable nitrogen contents significantly increased the above-ground dry biomass and grain yield of maize (p < 0.05).

To further explore the impact of different irrigation modes on the above-ground dry biomass and grain yield of maize, principal component analysis was conducted on related factors. The eigenvalue of the first principal component (PC1) was 12.67, with a variance contribution rate of 60.34%. The eigenvalue of the second principal component (PC2) was 2.70, with a variance contribution rate of 12.88%. Together, they explained 73.22% of the variance. This indicates that the two principal components largely represent the influence of various indicators on the above-ground dry biomass and grain yield of maize. As shown in Figure 3 (right), available phosphorus content, available potassium content, and field capacity had the most significant positive effects on promoting the above-ground dry biomass of maize. The soil moisture content and available phosphorus content had the most significant positive effects on promoting maize grain yield. The pH and total salt content had the greatest negative impact on the formation of maize above-ground dry biomass and grain yield.

After a comprehensive evaluation of crop yield indicators, the highest score was achieved by W2T2, followed by W3T2, with W1T0 having the least effective outcome (

Table 8. Evaluation value and ranking of each component.

Treatment	Score	Ranking
W1T0	−4.13	12
W1T1	−0.69	8
W1T2	2.51	3
W1T3	−1.58	9
W2T0	−3.06	11
W2T1	0.94	4
W2T2	4.49	1
W2T3	0.38	6
W3T0	−2.55	10
W3T1	0.88	5
W3T2	2.72	2
W3T3	0.09	7

Note: W1: 360 mm irrigation quota, W2: 450 mm irrigation quota, W3: 540 mm irrigation quota; T0: no conditioning, T1: microbial agent, T2: humic acid, T3: microsilica powder.

). Under the same conditions, the application of humic acid improved salinized soil more effectively than the application of microbial agents, microsilica powder, or no amendments. The 450 mm irrigation quota had a much greater effect on water-saving and salt control in saline soil compared to the 540 mm and 360 mm irrigation quotas. That is, the 450 mm irrigation quota with humic acid was most conducive to water conservation and salt control, improvement of salinized soils, and regulation to enhance the productivity of maize.

4. Discussion

In saline–alkali soil improvement, soil water acts as both a solvent and carrier for soil salts, making the study of water–salt balance and precise irrigation essential. Accurate irrigation based on water–salt balance and the application of amendments are key to water conservation, reducing salinity, improving soil texture, and enhancing crop yields [36]. This has become an important method for improving saline–alkali soils. Drip irrigation can inhibit the upward movement of groundwater salts, reduce salinity near the crop root system by leaching, loosen the soil to reduce evaporation and provide sufficient water for good crop growth [37]. The findings of the current study support a similar view that drip irrigation with different irrigation quotas can increase soil moisture content, effectively reduce total soil salt content and the content of other salt ions, and decrease the sodium adsorption ratio and alkalinity. These effects are more significant as the irrigation quota increases. A previous study [38] found a negative correlation between groundwater level and mineralization, where an increase in salinity driven by a rise in groundwater level triggered an increase in mineralization, and vice versa. This study supports a similar view, as dynamic monitoring of water and salt revealed that changes in salinity in saline–alkali soils corresponded to groundwater level changes. When the groundwater level rose (from April to July), salinity increased, and when the groundwater level fell (from July to October), salinity significantly decreased. The rise in groundwater levels was due to the high rainfall in June and July. The combined effect of rainfall and drip irrigation caused the groundwater level to rise, resulting in the leaching of salts into deeper soil layers and the accumulation of salts in the surface soil, which increased the salinity of the soil [39]. As rainfall later decreased, salinity also decreased relatively. Drip irrigation effectively promotes deep soil water infiltration, increasing moisture content in the 0–40 cm soil layer. Previous studies found that field capacity decreases with increasing soil depth [40]. In this study, a slight decrease in field capacity was observed over time and depth, possibly due to the reduction in rainfall.

The soil plays a crucial role in determining the suitability of water-saving irrigation techniques and amendments. In China, sandy soils account for only 14.05% of paddy fields, which are partly distributed throughout Ningxia Province [17]. Given the low soil fertility and high penetration rate of sandy soils, it is inappropriate to design irrigation regimes that solely conserve water [41]. In the soil aggregate-forming and stabilizing process, soil carbon serves a crucially important role. By promoting fungal growth, organic amendments with a high C/N ratio have been shown to be crucial in the formation of water-stable aggregates [42,43]. Along with the use of soil amendments, it loosens the soil’s aggregate structure, enhancing its water retention capacity and directing moisture to accumulate around the maize root zone. Previous findings showed that the main elements supporting the quicker formation and longer stability of coastal saline–alkali soil aggregates are the characteristics of organic amendments, which can alter microbial communities by raising soil C/N ratio and effective chemical compositions of solid-state SOM. Additionally, the inclusion of inorganic amendments can enhance the stability and development of microaggregates as opposed to macroaggregates [44]. Our results revealed that both amendments and different drip irrigation quotas effectively reduced the total salt content and the contents of Cl⁻, SO₄²⁻, and HCO₃⁻. The 540 mm irrigation quota showed the best results, with humic acid being the most effective amendment. Combining irrigation with amendments can reduce soil salinity and salt ions [45], effectively mitigating soil barrier factors [36], and ensuring a favorable water–salt environment for crop growth. This study also found that the main salt ions were Cl⁻, while K⁺, SO₄²⁻, and HCO₃⁻ were present in smaller amounts, and CO₃²⁻ was almost nonexistent. The levels of Cl⁻ decreased after the application of amendments. This may be due to the ions being leached by drip irrigation and the amendments conditioning the soil, as well as interactions between ions leading to their precipitation and removal.

Soil fertility nutrient indicators are crucial factors for assessing soil fertility [46]. According to the national soil nutrient-grading standards (DB 33/T 895-2013), soil is classified into six levels (as shown in

Table 9. Soil nutrient-grading standards.

Grade	SOM (g/kg)	AH-N (mg/kg)	AP (mg/kg)	AK (mg/kg)	TN (g/kg)	TP (g/kg)
1st	>40	>150	>40	>200	>2	>1
2nd	30~40	120~150	20~40	150~200	1.5~2	0.8~1
3rd	20~30	90~120	10~20	100~150	1~1.5	0.6~0.8
4th	10~20	60~90	5~10	50~100	0.75~1	0.4~0.6
5th	6~10	30~60	3~5	30~50	0.5~0.75	0.2~0.4
6th	<6	<30	<3	<30	<6	<6

Note: SOM: soil organic matter, AH-N: alkali hydrolysable nitrogen, AP: available phosphorus, AK: available potassium, TN: total nitrogen, TP: total phosphorus.

). Salinized soil was improved through irrigation leaching and amendment conditioning, resulting in enhanced soil nutrient content, as detailed in

Table 10. Ratings of improved soil nutrients.

Treatment	SOM (g/kg)	AH-N (mg/kg)	AP (mg/kg)	AK (mg/kg)	TN (g/kg)	TP (g/kg)	Soil Grade
CK	13.63	36.4	15.85	149.44	0.85	0.84	4th
W1T0	17.54	33.40	14.20	173.38	1.00	1.11	4th
W1T1	21.58	48.10	41.46	161.58	1.08	1.27	3rd
W1T2	22.77	62.47	45.22	192.40	1.08	1.80	3rd
W1T3	20.64	42.40	23.86	155.16	0.88	1.40	3rd
W2T0	18.32	45.57	31.75	148.34	0.91	1.11	4th
W2T1	21.85	48.60	54.89	176.43	1.00	1.54	3rd
W2T2	24.19	68.03	57.32	195.92	1.16	2.09	3rd
W2T3	20.88	45.27	29.34	168.36	0.97	1.39	3rd
W3T0	22.09	42.90	17.69	171.41	0.75	1.27	4th
W3T1	21.84	52.40	48.95	190.85	1.07	1.69	3rd
W3T2	22.63	43.73	44.64	195.03	1.17	2.03	3rd
W3T3	20.75	61.77	19.16	174.06	0.90	1.59	3rd

Note: W1: 360 mm irrigation quota, W2: 450 mm irrigation quota, W3: 540 mm irrigation quota; T0: no conditioning, T1: microbial agent, T2: humic acid, T3: microsilica powder. SOM: soil organic matter, AH-N: alkali hydrolysable nitrogen, AP: available phosphorus, AK: available potassium, TN: total nitrogen, TP: total phosphorus.

. Initially, the soil’s nutrient level was at level four. After improvement, the nutrient levels in treatments other than W1T0, W2T0, and W3T0 increased by one level, reaching level three.

Under the same amendment, the 450 mm and 540 mm irrigation quotas were more effective at enhancing soil nutrients and reducing soil pH. This may be because higher irrigation quotas can accelerate the dissolution of amendments and fertilizer, leading to quicker reactions, improved soil texture, and enhanced soil fertility.

Water is essential for crop growth. Amendments such as humic acid, microbial agents, and microsilica powder can improve soil texture, thereby promoting healthy crop growth and increasing crop yield. This study found that drip irrigation with a 450 mm quota combined with humic acid effectively increased maize plant height, stem thickness, above-ground dry biomass, and grain yield. The conclusion of this study is that humic acid significantly improved maize physiology and increased grain yield compared to the application of chemical fertilizers alone, similar to the findings of Ali et al. [47].

5. Conclusions

The application of humic acid or microsilica powder at a 450 mm irrigation quota reduced the water table by 0.24 m and 0.19 m, respectively. The reduction in soil mineralization and HCO₃⁻ at 450 mm and 540 mm irrigation quotas with humic acid treatment was greatest. Soil pH and total salt, Cl⁻, and SO₄²⁻ content was most significantly reduced with the 450 mm irrigation quota combined with humic acid. At the same time, this combination maximized the increase in soil nutrients. Maize growth reached its maximum values under the 450 mm irrigation quota combined with humic acid treatment. Moreover, the improvement effect of the amendments on the soil was significantly higher than that of the irrigation quotas and their interactions. Considering the comprehensive water and salt indicators, total salts and salt ions, soil fertility indicators, and maize growth characteristics, the 450 mm irrigation quota combined with humic acid showed the best results for water-saving, salt control, and enhancing soil fertility and productivity. More efficient amendments’ usage investigation is needed on water-saving irrigation in saline–alkali land to achieve better productivity.