Magnetized Saline Water Drip Irrigation Alters Soil Water-Salt Infiltration and Redistribution Characteristics

Abstract

Magnetization constitutes an efficacious physical treatment technique applicable to saline water. The new spiral flow magnetizer, in conjunction with the cyclic magnetization process, has the effect of maximizing effective magnetization time and thereby achieving the optimal magnetization results. Based on this, saline water (0.27, 3, 6, and 10 g L⁻¹) was treated with different levels of magnetization (0, 0.2, 0.4 and 0.6 T), and the effects of magnetized saline water (MSW) drip irrigation on loamy-sand soil moisture, soluble salt infiltration, and redistribution characteristics were studied through a vertical soil column simulation experiment. The results showed that the wetting front migration in MSW drip irrigation experiments exhibited minimal variation during soil water infiltration, and a notable change during redistribution with the experimental duration of 0.27 and 3g L⁻¹ saline water treatments being significantly different (p < 0.05). Treating saline water with different mineralization levels with magnetization demonstrated water retention (0.27 g L⁻¹ excluded) and salt drainage characteristics; calculated soil water storage increased by 1.58–14.19% and salt storage decreased by 0.22–7.66%. The optimal magnetization intensity for low-mineralization (0.27 and 3 g L⁻¹) saline water was 0.2 T and for high-mineralization (6 and 10 g L⁻¹) it was 0.6 T. The adsorption and exchange of cations (19.58–32.12%) by the optimum MSW treatments was greater than that of anions (9.46–14.15%); specifically, the relative exchange capacity of Ca²⁺ and Mg²⁺ in cations was more than K⁺ and Na⁺, while HCO₃⁻ and SO₄²⁻ in anions was more than Cl⁻. This study provides theoretical and technical support for the irrigation of farmland with poor-quality water, as well as for the development of magnetized water irrigation technology.

1. Introduction

In arid or semi-arid regions with limited water resources for agriculture, farmers are forced to use highly mineralized groundwater or seawater for irrigation, with mixed or rotational fresh and saline water [1]. However, these irrigation methods also result in higher soil salinity, an increased risk of salinization, abnormal crop growth, and reduced yields or quality [2,3]. Saline water drip irrigation effectively reduces the irrigation amount and prevents the entry of extra soluble salts into the soil through water [4]. In addition, saline water treatment measures are essential for the sustainable use of water and land resources [5].

Magnetization is a physical treatment technology used to enhance water activity [6]. When saline water is treated with a magnetic field, the hydrogen bonds between water molecules are broken, causing the molecular clusters to shrink, leading to changes in several macroscopic properties, including density, volatility, solubility, conductivity, surface tension, and osmotic potential [7,8,9]. Activity-enhanced magnetized salt water (MSW) has been widely used in agricultural production in various countries as it has a stronger penetration and dissolution capacity in the soil, which promotes the dissolution of soil-soluble salts and the uptake by crop roots [10,11].

A widely used method for magnetizing irrigation water involves placing permanent magnets or electromagnets with opposing magnetic poles on either side of the water pipe [12]. The magnetizing effect of water is largely determined by the duration of its exposure to a magnetic field, regardless of whether it is fresh or saline water [13]. Cai et al. [14] defined the effective magnetization time (EMT) as the cumulative time for a single water molecule to pass vertically through the effective magnetic field width (EMFW) of the magnetizer, which is proportional to the number of times the molecule passes through the magnetic field. Therefore, in both field and laboratory experiments, researchers have attempted to improve the EMT of magnetization treatments through the use of multiple magnetization [15], cyclic magnetization [16], and an increase in the number of magnet pairs [17], although these methods increase irrigation time and input costs. Pang and Deng [18] discovered that the magnetic field has a non-linear effect on water; the longer the magnetic field is applied, the stronger the influence on the properties of the water. When the duration of the action is sufficient, the effect becomes saturated (e.g., after 60 min the viscosity of the water no longer falls). Once the magnetic field is removed, the impact gradually diminishes until it disappears. Unfortunately, it is difficult to determine whether or not the magnetization of irrigation water is saturated from existing studies due to the lack of a definitive EMT for the magnetizer.

The magnetization effect is related to the strength of the magnetizer’s magnetization (the magnetic induction strength at the surface of the magnet), as well as the mineralization and ionic composition of the irrigation water [19]. Wang et al. [20] investigated the effects of 0.3 T magnetization on NaCl solutions of different concentrations, finding that the parameters of Philips and Green-Ampt infiltration equations varied, and that the relative desalination of a 3 g L⁻¹ NaCl solution was improved during fixed-head infiltration. Mohamed and Ebead [21] used 0.1 T magnetization to treat different irrigation water sources (0.33–3.81 dS m⁻¹) and found no significant leaching of soluble potassium salts during the infiltration process, but significant leaching of insoluble phosphorus. Al-Ogaidi et al. [22] reviewed the effects of 0.2 T and 0.4 T magnetization on tap water treatment, showing an increasing surface wetting radius and decreasing vertical wetting depth for homogeneous sandy soil percolation experiments, but an opposite effect for sandy-clay soil percolation experiments. The movement of irrigation water through the soil is a continuous process that involves infiltration, redistribution, drainage, and evaporation [23]. Infiltration is the process with the shortest duration in an irrigation cycle, and it is incomplete to study only the water-salt transport characteristics of magnetized saline water (MSW) during soil infiltration.

Zlotopolski [24] showed that the salt migration law in soil irrigated with MSW can be effectively demonstrated using laboratory soil columns. Based on this, we utilized a self-developed utility model magnetizer [25] with a spiral water hose and cyclic magnetization treatment for 60 min to maximize the EMT. This allowed us to further illustrate the effect of magnetized saline drip irrigation on water and salt transport in loamy sand. The objectives of the research are: (1) to provide precise EMT and calculation formulae for magnetization devices for similar research purposes; (2) to investigate the role of MSW, particularly in the redistribution process in drip irrigation; and (3) to determine the differential effects of magnetization treatments on common soil ions when the ionic composition of the irrigation saline water is complex. It is intended to provide a theoretical basis for using poor-quality water safely and effectively by magnetization treatment technology.

2. Materials and Methods

2.1. Experimental Supplies

2.1.1. Soil Sample

The soil sample was collected from the 10th Company (87°32′6.49″ E, 44°32′1.35″ N) of the 103rd Regiment, Wujiaqu City, Sixth Division of Xinjiang Production and Construction Corps, China. It was derived from an aeolian sand farmland (0–60 cm plough layer) on the southern margin of the Gurbantunggut Desert and had a bulk density of 1.55 g cm⁻³. To maintain a consistent and low salt content in the soil sample, we repeatedly washed soil columns with laboratory tap water until the conductivity of the leakage liquid remained constant. The washed soil sample was then air-dried, ground, and passed through a 2 mm sieve after removing impurities.

According to the international classification standard of soil texture, the sample belonged to loamy sand (4.34% clay, 9.46% silt, 86.20% sand), which is determined by a laser particle size analyzer (LS-POP-VI type, OMEC Instrument Co., Ltd., Zhuhai, China). The soil’s initial mass moisture content was determined using the drying method. The foil sampler weighing method was used to measure the soil saturated mass moisture content and field moisture capacity, which were then converted into volume moisture content. The initial volume moisture content was 0.0013 cm³ cm⁻³, while the saturated volume moisture content was 0.4120 cm³ cm⁻³. The field moisture capacity was 0.2317 cm³ cm⁻³.

The initial soil pH was measured with a Mettler-Toledo FE28 pH meter and recorded as 8.66. The electrical conductivity (X) of the soil extract (soil–water ratio 1:5) was measured using a Shanghai Lei Ci DDSJ-308F electrical conductivity meter. The initial salt content (Y) of the soil was 0.39 g kg⁻¹, calculated using the conversion formula Y = 0.0002X^1.1007 (R² = 0.9963 p < 0.01 N = 20). The contents of CO₃²⁻ and HCO₃⁻ in the soil were 0 and 232.25 mg kg⁻¹ by double indicator titration, Cl⁻ was 35.49 mg kg⁻¹ by the Mohr method, Ca²⁺ and Mg²⁺ were 67.42 and 21.36 mg kg⁻¹ by EDTA complexometric titration, SO₄²⁻ was 103.70 mg kg⁻¹ by EDTA indirect complexometric titration, and K⁺ and Na⁺ were 29.01 and 17.11 mg kg⁻¹ by flame spectrophotometry.

2.1.2. Saline Water

The saline water used in the experiment was prepared by fully mixing sodium chloride, magnesium sulfate and calcium chloride reagent (analytical reagent, content ≥ 99.5%) according to the mass ratio of 2:1:1, and dissolving in tap water. The pH value of laboratory tap water was 6.60, the salinity was 268.68 mg L⁻¹, and the contents of CO₃²⁻, HCO₃⁻, Cl⁻, SO₄²⁻, Ca²⁺, Mg²⁺, K⁺ and Na⁺ were 0, 136.90, 29.10, 75.24, 54.34, 6.10, 22.29 and 13.15 mg L⁻¹, respectively.

2.2. Experimental Design and Implementation

2.2.1. Control Variables and Laboratory Treatments

The magnetization intensity and mineralization degree of saline water are two factors that can be easily controlled in agricultural production. Therefore, they were each set at four levels. The magnetization levels were 0, 0.2, 0.4 and 0.6 Tesla (T), and the mineralization levels were 0.27, 3, 6, and 10 g L⁻¹. The experiment was completely randomized with 16 treatments (

Table 1. Summary of the designed combinations in the laboratory experiment.

Mineralization of Irrigation Water (g L−1)	Magnetization (T)
	0 (M0)	0.2 (M2)	0.4 (M4)	0.6 (M6)
0.27 (S1)	M0S1	M2S1	M4S1	M6S1
3.00 (S3)	M0S3	M2S3	M4S3	M6S3
6.00 (S6)	M0S6	M2S6	M4S6	M6S6
10.00 (S10)	M0S10	M2S10	M4S10	M6S10

), and each treatment was repeated three times. The experiments began immediately after magnetizing saline water with varying levels of mineralization.

2.2.2. Magnetizing Device

An independently developed water magnetization treatment device was used in this study for agricultural irrigation, as depicted in Figure 1. The device measures 98 cm in length, 24 cm in width, and 26 cm in height. The magnetizer’s internal water flow channel consists of a 32 mm-diameter, 2 mm-thick acrylic spiral tube. The spiral pipe has fixed plates in the center and on the outside top and bottom, allowing for the installation of permanent magnets. The magnetizer can be mounted on any level of piping within a drip irrigation system using threaded connections. A total of 30 sintered NdFeB magnets, each measuring 160 mm × 30 mm × 20 mm (L × W × H), are positioned within the fixed plate. The study measured the effective magnetic course (EMC) at 628.32 cm and the effective magnetic field area (EMFA) at 8.04 cm² while the spiral tube was in the magnetic field. The experiment involved replacing permanent magnets with varying magnetization strengths, and the magnetic field strength of each permanent magnet was calibrated using a TD8620 handheld digital Tesla meter (Tianheng Measurement and Control Technology Co., Ltd., Changsha, China, with an accuracy of 1%).

For the experiment, 100 L of saline water was used each time. The MSW was circulated for 60 min in a closed loop using a self-priming centrifugal pump (the maximum flow rate was 2 m³ h⁻¹ and the maximum suction range was 8 m). EMT was calculated to be 181.86 s using the follow formula:

$$\mathrm{EMT}={\displaystyle \frac{\mathrm{EMC}\times \mathrm{EMFA}}{1000\times \mathrm{V}}}\times \mathrm{T}$$

(1)

where EMT is the effective magnetization time (s); EMC is the effective magnetic course (cm), which is typically the length of the water pipe in the magnetic field; EMFA is the effective magnetic field area (cm²), which is generally the cross-sectional area of the water pipe in the magnetic field; V is the volume of the circulating MSW (L); and T is the time of circulating magnetization (s).

2.2.3. Soil Water-Salt Transport Experimental System

The system (Figure 2) was used to investigate the dynamic changes in soil moisture content, conductivity, and salt ion content during infiltration and redistribution under MSW drip irrigation. The system consisted of a device for magnetizing saline water circulation, soil columns, and a simulated self-pressure drip irrigation device.

(1)

Saline water circulation magnetization treatment device: The artificially prepared saline water was passed through the magnetizer multiple times to achieve complete circulation of magnetization. The device consisted of a water storage tank, an electric stirrer, a self-priming pump, magnetizers, a pipeline, and pipe fittings. The water storage tank utilized a 150 L polyethylene plastic container, with an industrial electric stirrer installed at the water inlet. The magnetizer’s spiral pipe diameter, as well as the PVC pipe and all connecting fittings, were of the branch pipe size (32 mm) commonly used in drip irrigation systems for agricultural production.

(2)

Soil columns: These were constructed using a 70 cm plexiglass pipe with an inner diameter of 20 cm and a wall thickness of 5 mm. The bottom was sealed with a 10 mm thick acrylic plate, and five holes were added to ensure proper exhaust and drainage. Additionally, six sampling holes were arranged in a honeycomb pattern at 10 cm intervals from the bottom up, each with an inner diameter of 2.5 cm. The sampling holes were evenly spaced on the same horizontal plane to facilitate soil sample collection. Rubber plugs were used to block the holes during the experiment, as described by Zhao et al. [26]. The soil columns were placed on an iron frame with a hollow upper end and a certain height. An excessive droplet collection box was arranged in the middle of the iron frame.

(3)

Simulated self-pressure drip irrigation device: The water supply equipment used a cylindrical Marriotte bottle with an inner diameter of 15 cm and a height of 70 cm. The bypass scale pipe allowed for easy reading of the water level, and the water supply head of the telescopic frame was adjusted to be controlled at 93 cm. The Marriotte bottle was connected to a modified medical infusion set to drip MSW into the soil column. The infusion set’s drip pot was inserted into a pressure-compensated barb dripper (designed to have a flow rate of 40 L h⁻¹, NETAFIM Agricultural Science and Technology Co., Ltd. Guangzhou, China). The dripper’s flow rate was controlled by the flow regulator to be (2.36 ± 0.08) L h⁻¹.

2.2.4. Soil Column Filling and Infiltration-Redistribution Test

Before filling the soil column, a 5 cm layer of quartz sand was placed at the bottom as a filter layer. On top of the filter, a 5 mm sponge with the same cross-section as the soil column was placed to prevent fill infiltration. Spare soil samples were arranged in sections based on initial bulk density and filled to a height of 60 cm (total filling weight 29.915 kg). Each layer of fill was compacted to a thickness of 5 cm using a hammer. Artificial roughness was added between the layers to ensure good contact. Special attention was given to compacting the fill at the edges of the column to prevent rapid water infiltration along the column’s edge and to avoid the occurrence of edge effects. After filling, the soil column was left at room temperature for 12 h. Prior to conducting the infiltration test, a splash-proof filter paper was placed in the centre of the topsoil layer.

During the infiltration and redistribution experiments, the position of the wetting front and the water level of the Marriotte bottle were recorded at different time points. The wetting front was not obvious during the first 20 min, and then recorded at intervals of 5 min between 20 and 90 min, at intervals of 10 min between 90 and 150 min, and at intervals of 30 min after 150 min. The maximum and minimum distances of wetting front migration were recorded at each time point. When the accumulated infiltration amount reached 3.6 L (water supply time was 93 min), the water supply was stopped. The pipe orifice of the soil column was then immediately sealed with plastic wrap to prevent the water from evaporating naturally. Once the wetting front had fully reached the upper edge of the filter layer, the process of soil water redistribution came to an end. The soil columns were then placed horizontally and sampled by layers through sampling holes. The sampling positions were 5, 15, 25, 35, 45 and 55 cm away from the soil surface, with a sampling depth of 3 cm. The soil samples were tested for mass moisture content, electrical conductivity, and the content of eight ions, following previously described methods.

2.3. Calculation and Statistical Analysis

2.3.1. Calculation of Wetting Front Migration, Water-Salt and Ion Storage

The wet front migration was quantified by measuring the infiltration distance of MSW in the soil columns. The data from three replicate soil columns for each experimental treatment were averaged (D_μ). The following equation was used to calculate the mean distance of wetting front migration for each soil column:

$$\mathrm{D}={\displaystyle \frac{{\mathrm{D}}_{\max }+{\mathrm{D}}_{\min }}{2}}$$

(2)

where D is the mean distance of wetting front migration (cm); D_max is the maximum distance of wetting front migration (cm); and D_min is the minimum distance of wetting front migration (cm).

The water storage of the soil column was defined by two distinct values: theoretical and calculated. The theoretical water storage (W1) was equal to the actual irrigation amount (V₀), which was 3.6 L. The calculated water storage (W2) was the total water content of the 60 cm fill soil, calculated as a 10 cm layer in six layers. The mean value of the three replicate soil columns for each treatment was calculated (W2_μ). The following equations were employed to calculate the calculated water storage and absolute error rate for each soil column:

$$\mathrm{W}={\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathsf{\gamma }}_{\mathrm{i}}{\mathsf{\theta }}_{\mathrm{i}}{\mathrm{V}}_{\mathrm{i}}}$$

(3)

$${\mathrm{P}}_{\mathrm{W}}={\displaystyle \frac{\left|\mathrm{W}1-\mathrm{W}2\right|}{\mathrm{W}1}}\times 100\%$$

(4)

where W is the calculated water storage (L); P_W is the absolute error rate of water storage (%); n is the number of soil layers in the calculated depth; γ_i is the volumetric weight of the soil in layer i (g cm⁻³); θ_i is the moisture content of the soil in layer i (g g⁻¹); and ${\mathrm{V}}_{\mathrm{i}}$ is the volume of the soil in layer i (dm³).

The salt storage of the soil column was similarly characterized by theoretical and calculated values. When the mineralization of the irrigation water was the same, the theoretical salt storage (S1) was identical for each treatment of the experiment. The calculated salt storage (S2) was the total salt content of the 60 cm fill soil and was calculated in the same way as the water storage. The data from the three replicate soil columns for each experimental treatment were averaged to obtain the mean value of the calculated salt storage (S2_μ). The following equations were employed to calculate the theoretical salt storage, calculated salt storage, and absolute error rate for each soil column:

$${\mathrm{S}1=\mathrm{S}}_{\mathrm{W}}+{\mathrm{S}}_0$$

(5)

$${\mathrm{S}}_{\mathrm{w}}=\mathrm{M}\times {\mathrm{V}}_0$$

(6)

$${\mathrm{S}}_0={\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}\mathrm{Y}\times {\mathrm{m}}_{\mathrm{i}}}$$

(7)

$$\mathrm{S}2={\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{W}}_{\mathrm{i}}}\times \mathrm{M}+{\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{Y}}_{\mathrm{i}}}\times {\mathrm{m}}_{\mathrm{i}}$$

(8)

$${\mathrm{P}}_{\mathrm{S}}={\displaystyle \frac{\left|\mathrm{S}1-\mathrm{S}2\right|}{\mathrm{S}1}}\times 100\%$$

(9)

where S1 is the theoretical salt storage (g); S_w is the salinity of the irrigation saline water (g); S₀ is the initial salt storage of the soil column fill (g); S2 is the calculated salt storage (g); P_S is the absolute error rate of the salt storage (%); M is the mineralization of the irrigation saline water (g L⁻¹); V₀ is the volume of irrigation water (L); n is the number of soil layers in the calculated depth; m_i is the mass of soil in layer i (kg); Y is the initial salinity of the soil column fill (g kg⁻¹); W_i is the calculated water storage of layer i (L); and Y_i is the salinity of soil in layer i (g kg⁻¹).

Soil column salt ion storage was calculated in a manner analogous to the calculation of salt storage, as demonstrated in the following:

$${\mathrm{C}}_{\mathrm{X}}={\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{y}}_{\mathrm{i},\mathrm{X}}}$$

(10)

where C_X (X = CO₃²⁻, HCO₃⁻, Cl⁻, SO₄²⁻, Ca²⁺, Mg²⁺, K⁺ and Na⁺) is the storage of salt ions (mg kg⁻¹); and y_i,X is the content of salt ions in layer i (mg kg⁻¹).

2.3.2. Statistical Analysis

The ANOVA was performed using the Data Processing System 18.10 Advanced Edition [27], and significant differences between treatments were compared with Duncan’s new multiple range test (DMRT) at the 0.05 level, with different lowercase letters indicating significant differences in the same group of data (p < 0.05). The graphics were generated by Microsoft PowerPoint 2021 and Origin 2021.

3. Results

3.1. Effects of Magnetized Saline Water Drip Irrigation on Wetting Front Migration

The mean distance of wetting front migration (D_μ) over time for each treatment in the experiment is shown in Figure 3. It can be observed that D_μ for MSW drip irrigation increases with time. Nevertheless, D_μ exhibits minimal variation during soil water infiltration, while displaying a pronounced disparity in the redistribution process.

The experimental duration of 0.27g L⁻¹ saline water treatments (Figure 3A) exhibited a statistically significant difference (p < 0.05). M2S1, M4S1 and M6S1 showed a reduction in duration of 8.57%, 6.19% and 2.38%, respectively, compared to M0S1. At the same experimental time, D_μ in M2S1 was the greatest, while that in M0S1 was the smallest. For the sake of illustration, when the experimental time reached 150 min, D_μ of the remaining magnetized treatments increased by 8.02%, 7.51% and 7.13% in comparison to M0S1. Consequently, 0.27 g L⁻¹ saline water treated with magnetization was demonstrated to enhance the rate of infiltration wetting front advancement.

The experimental duration of 3g L⁻¹ saline water treatments (Figure 3B) were significantly different (p < 0.05). M2S3, M4S3 and M6S3 demonstrated an increase in duration of 13.81%, 7.73% and 3.31%, respectively, compared to M0S3. D_μ in M0S3 was the largest, while that in M2S3 was the smallest, when the experimental time was held constant. At 150 min, D_μ of the remaining magnetized treatments decreased by 6.34%, 1.62% and 1.62% in comparison to M0S3. It was demonstrated that the velocity of the wetting front could be reduced to varying degrees following the magnetization of 3g L⁻¹ saline water. Of the magnetization treatments tested, the 0.2 T treatment was found to have the most pronounced effect.

With experimental durations of 6g L⁻¹ (Figure 3C, 185.17 ± 6.75 min) and 10g L⁻¹, saline water treatments (Figure 3D, 179.92 ± 2.91 min) were not significantly different (p > 0.05). In the process of soil water redistribution, D_μ in 6g L⁻¹ saline water treatments was M0S6 > M2S6 > M4S6 > M6S6. Compared with M0S6, D_μ in the remaining magnetized treatments decreased by 1.70%, 2.50% and 7.19% at 150 min. At the same experimental time, D_μ in M0S10 was the largest, while that in M6S10 was the smallest, and there was little difference between M0S10 and other magnetized treatments. The results indicated that the magnetization intensity of 0.6 T was the most effective in reducing the wetting front propulsion of 6 and 10g L⁻¹ saline water. In order for the 10 g L⁻¹ saline water to be more effective in reducing wetting front propulsion during soil water redistribution, it is recommended that the intensity of the magnetization be greater than 0.6 T.

3.2. Effects of Magnetized Saline Water Drip Irrigation on Water-Salt Content in Soil

Subsequent to the soil water redistribution process, the distribution of water content in the 0–60 cm soil layer for each treatment of the experiment is depicted in Figure 4. Once the mineralization of the irrigation water was confirmed, the calculated water storage (W2_μ) was found to be significantly different among the magnetization treatments, especially for the 0.27 and 3 g L⁻¹ treatments. The mean W2_μ of each treatment was 3.4654 L, with a standard deviation of 0.1584 L. The average absolute error rate of water storage, when compared with the actual irrigation capacity of 3.6 L, was found to be 3.74%, which met the experimental accuracy requirements.

As illustrated in Figure 4, with an increase in soil depth, there was an initial increase in soil moisture content, which then decreased. At a depth of 35 cm, the soil moisture content reached its maximum. The soil moisture content of the different soil layers with 0.27 g L⁻¹ MSW was less than that of the non-magnetized, and M2S1 was found to be the smallest. W2_μ of M2S1, M4S1, and M6S1 decreased by 9.32%, 7.88%, and 6.32%, respectively, in comparison to M0S1 (Figure 4A).

Figure 4 also demonstrated that the soil moisture content of the shallow layer (soil depth 0–40 cm) with 3, 6 and 10 g L⁻¹ MSW was higher than that of the non-magnetized, while the deep layer (soil depth 40–60 cm) exhibited the opposite. When the magnetization was 0.2 T, the change in soil moisture content in 3 g L⁻¹ MSW was the most pronounced, with W2_μ increasing by 14.19% compared with M0S3 (Figure 4B). When the magnetization was 0.6 T, the change in soil moisture content in 6 and 10 g L⁻¹ MSW was the most obvious. In comparison to M0S6, W2_μ of M6S6 increased by 5.05% (Figure 4C). Similarly, W2_μ of M6S10 increased by 4.67% in comparison to M0S10 (Figure 4D). Therefore, 0.27 g L⁻¹ saline water treated with magnetization could reduce soil moisture content and water storage capacity. In contrast, 3, 6 and 10 g L⁻¹ saline water treated with magnetization could increase the soil moisture content in the shallow layer, and decrease that of the deep layer. Moreover, 3 g L⁻¹ MSW with 0.2 T, 6 and 10 g L⁻¹ MSW with 0.6 T had a clear water retention effect.

The salt input and output items of the soil column were analysed, and a salt balance calculation was carried out utilizing the variation in salt storage in different soil layers, as shown in Figure 5. A notable discrepancy was observed in the calculated salt storage (S2_μ) for distinct magnetization intensity treatments, despite the uniformity in the mineralization of the irrigation water. The average absolute error rate of salt storage, when S2_μ was compared with the theoretical, was found to be 2.03%, which also met the experimental accuracy requirements.

Figure 6 presents the distribution of soil salinity in the 0–60 cm soil layer for each treatment of the experiment following the soil water redistribution process. The distribution of salt in different soil layers under MSW drip irrigation was found to be consistent, with the salt migrating in accordance with the law of ‘salt travels with water’. This migration was observed to occur from the shallow to the deep, ultimately resulting in the accumulation in the vicinity of the wetting front.

The soil salinity of the shallow layer was less than that of non-magnetized treatments for MSW (3, 6 and 10 g L⁻¹), but greater in the deep layer. MSW of 0.27 and 3 g L⁻¹ treatments, the most dramatic change in soil salinity was 0.2 T magnetization. In comparison to the non-magnetized, S2_μ of M2S1 and M2S3 in the shallow layer decreased by 10.40% and 14.33%, respectively, in direct contrast to the deep layer, which increased by 10.69% and 4.59%, respectively. The most violent change in soil salinity was 0.6 T magnetization for the 6 and 10 g L⁻¹ MSW treatments. Compared to the non-magnetized, S2_μ of M6S6 and M6S10 in the shallow layer decreased by 7.58% and 13.40%, respectively, while in the deep layer increased by 2.42% and 6.51%. The results showed that the soil salinity of the shallow layer could be reduced and that of the deep layer could be elevated following the magnetization of saline water. Low salinity water (0.27 and 3 g L⁻¹) treated with 0.2 T magnetization and high salinity water (6 and 10 g L⁻¹) treated with 0.6 T magnetization exhibited remarkable salt draining effects.

3.3. Effects of Magnetized Saline Water Drip Irrigation on Soil Salt Ion Content

After the soil water had been redistributed, the distribution of salt ions in different soil layers under MSW drip irrigation exhibited a consistency with the salt salinity, which increased with depth. Taking the magnetization treatment with the most dramatic change in soil salinity at the same mineralization level as an example, the content of the eight ions in the 40–60 cm soil layer at the end of the experiment is shown in

Table 2. Content of soil soluble salt ions (40–60 cm) under magnetized saline water (0.27 and 3 g L⁻¹) drip irrigation. Values are the means ± standard deviation. DMRT was used for mean comparisons at p < 0.05.

Soluble Salt Ions		Content (mg kg−1)
		M0S1	M2S1	M0S3	M2S3
Cation	K+	18.80 ± 0.95 b	21.42 ± 1.22 a	22.70 ± 1.07 b	26.24 ± 1.11 a
	Na+	19.13 ± 2.63 a	21.42 ± 1.29 a	32.11 ± 1.52 b	37.01 ± 1.80 a
	Ca2+	62.47 ± 3.15 b	79.96 ± 3.49 a	96.86 ± 4.57 b	118.44 ± 3.04 a
	Mg2+	11.03 ± 1.52 b	24.44 ± 3.36 a	14.58 ± 0.69 b	34.15 ± 4.69 a
	∑	111.44	147.23	166.25	215.84
Anion	Cl−	121.65 ± 7.88 a	127.18 ± 6.48 a	257.90 ± 12.18 a	264.46 ± 12.29 a
	HCO3−	359.28 ± 9.92 b	435.75 ± 19.14 a	461.12 ± 11.89 b	499.52 ± 16.44 a
	CO32−	0.00	0.00	0.00	0.00
	SO42−	232.74 ± 5.53 a	251.72 ± 8.12 a	595.32 ± 28.11 b	664.39 ± 31.74 a
	∑	713.67	814.65	1314.34	1428.37

Note: Different lower case letters indicate significant differences in data across treatments for the same mineralization.

and

Table 3. Content of soil soluble salt ions (40–60 cm) under magnetized saline water (6 and 10 g L⁻¹) drip irrigation. Values are the means ± standard deviation. DMRT was used for mean comparisons at p < 0.05.

Soluble Salt Ions		Content (mg kg−1)
		M0S6	M6S6	M0S10	M6S10
Cation	K+	26.25 ± 1.27 a	28.06 ± 3.10 a	36.92 ± 1.60 b	44.27 ± 2.22 a
	Na+	92.18 ± 4.47 b	110.61 ± 4.49 a	184.85 ± 9.14 b	214.64 ± 5.00 a
	Ca2+	184.83 ± 8.96 b	237.00 ± 11.06 a	294.95 ± 15.60 b	358.70 ± 15.89 a
	Mg2+	50.12 ± 2.43 b	77.69 ± 8.59 a	53.93 ± 2.34 b	64.77 ± 4.12 a
	∑	353.39	453.36	570.64	682.39
Anion	Cl−	510.31 ± 4.91 b	554.38 ± 15.41 a	734.67 ± 6.55 b	772.21 ± 10.36 a
	HCO3−	620.62 ± 30.10 b	680.91 ± 21.5 a	840.77 ± 38.04 b	963.73 ± 14.72 a
	CO32−	0.00	0.00	0.00	0.00
	SO42−	538.91 ± 26.13 b	654.59 ± 32.85 a	1037.70 ± 29.07 b	1124.55 ± 24.98 a
	∑	1669.83	1889.87	2613.15	2860.48

Note: Different lower case letters indicate significant differences in data across treatments for the same mineralization.

.

The total content of soil anions in the 0.27 g L⁻¹ saline water treatments was 6.40 and 5.53 times that of cations, and a comparison between treatments M0S1 and M2S1 revealed a 14.15% increase in soil anions and a 32.12% increase in soil cations in the latter. The total content of soil anions in the 3 g L⁻¹ saline water treatments was 7.91 and 6.62 times that of cations, and a comparative analysis of M0S3 and M2S3 revealed an 8.68% increase in soil anions and a 29.83% increase in soil cations in the latter. The total content of soil anions in the 6 g L⁻¹ saline water treatments was 4.73 and 4.17 times that of cations; there was a 13.18% increase in soil anions and a 28.29% increase in soil cations in M6S6 relative to M0S6. The total content of soil anions in the 10 g L⁻¹ saline water treatments was 4.58 and 4.19 times that of cations. In comparison to M0S10, the soil anions and cations in M6S10 exhibited an increase of 9.46% and 19.58%, respectively. The findings indicated that despite the markedly higher soil anion content in each experimental treatment, the cations showed a more substantial rise than anions in the soil irrigated with optimal magnetization intensity-treated saline water when compared to non-magnetized treatments, at varying mineralizations. This further suggested that the impact on the adsorption and exchange of soil cations was more pronounced.

The K⁺ in the soil column fill was primarily derived from the soil background and laboratory tap water. The Na⁺, Ca²⁺ and Mg²⁺ were predominantly derived from the chemical agents utilized in the preparation of saline water. In comparison to the non-magnetized, the relative increment of soil Mg²⁺ in 0.27, 3 and 6 g L⁻¹ saline water with optimum magnetization intensity treatments was the largest, at 1.21, 1.34 and 0.55 times, respectively. The relative increment of soil Ca²⁺ in 10 g L⁻¹ saline water with an optimum magnetization intensity treatment was the greatest, reaching 21.61%. The findings demonstrated that the impact of optimal magnetization intensity treatment on the soil Ca²⁺ and Mg²⁺ of saline water with varying mineralization was more pronounced than that of soil K⁺ and Na⁺.

The primary source of HCO₃⁻ in the soil column fill was same as K⁺, while the sources of the Cl⁻ and SO₄²⁻ were analogous to other cations. The relative increment of soil HCO₃⁻ in 0.27 and 10 g L⁻¹ saline water with optimum magnetization intensity treatments was the largest, at 21.28% and 14.62%. The relative increment of soil SO₄²⁻ in 3 and 6 g L⁻¹ saline water with an optimum magnetization intensity treatment was the greatest, reaching 11.60% and 21.47%. The findings demonstrated that the impact of optimal magnetization intensity treatment on the soil HCO₃⁻ and SO₄²⁻ of saline water with varying mineralization was more pronounced than that of soil Cl⁻.

4. Discussion

4.1. Relationship between Magnetization Mechanism and Wetting Front Migration

The accepted mechanism for the magnetization of saline water is the “hydrogen bond fracture theory”. This theory is based on two key principles: the influence of a magnetic field on water itself, and the influence of a magnetic field on salt in water [28].

Research on the influence of water itself indicates that the hydrogen bond is an intermolecular force that is not as firm as a chemical bond. In liquid water, the hydrogen bond is in a dynamic balance of constantly disconnecting and combining [29,30]. The applied magnetic field provides the energy for the thermal movement of the water molecules, thereby disrupting the equilibrium and shifting it to the right; polar water molecules are directional aligned by the Lorentz force, resulting in a change in the orientation of their dipoles; the angle of the hydrogen-oxygen bond decreases from 104.5° to approximately 103°, the direction of the magnetic moment between atoms changes, the value increases, and the hydrogen bond breaks abnormally [31,32].

Research on the influence of salt in water indicates that the dissolved ions in water combine with the surrounding water molecules to form hydrated ions, which then form a cluster structure with a certain size through hydrogen bonds with other water molecules in the solution. Hydrated ions are subjected to the action of the Lorentz force of a magnetic field, which results in their spiral circular motion, while positive and negative ions rotating in opposite directions leads to the twisting of the hydrogen bonds between them [33]. In addition to the action on water molecules, cations of soluble salts in the magnetic field perform a clockwise spiral motion for cutting magnetic lines of force, while anions perform a counterclockwise spiral motion for cutting magnetic lines of force; when this motion reaches a certain intensity, it can also break the hydrogen bond chain between the water molecules [34,35].

Irrespective of the theory used, the hydrogen bonding in MSW is disrupted, changing the mechanism of interaction between water molecules and between water molecules and soluble salt ions. This, in turn, affects the important characterization parameters of soil water infiltration (e.g., surface tension and viscosity). Wang et al. [36] observed that the surface tension was reduced by less than 4% following the magnetization treatment of low mineralization saline water (0.1 g L⁻¹), with a reduction by 9.14% to 13.84% of high mineralization saline water (5 g L⁻¹). Their results indicate that by increasing the mineralization of saline water, the greater the relative reduction in surface tension of magnetization treatment, and the better the magnetization effect.

The results of our experiments conducted indicate that the magnetization treatment of low-mineralization saline water (0.27 g L⁻¹) during soil water infiltration and redistribution increases the rate of propulsion of the wetting front. On the one hand, the relatively weak effect of magnetization treatment on low-mineralization saline water on infiltration characterization parameters is evident. Despite the decrease in surface tension of MSW and the reduction in contact angle at the surface of the hydrophobic soil colloid [37], infiltration ability remained unaltered. On the other hand, due to the imposition of a set of experimental conditions, the soil column filling exhibits a low initial salt content (0.39 g kg⁻¹) following repeated washing. In the dry soil (initial volume water content 0.0013 cm³ cm⁻³), the soil water suction is high, resulting in an infiltration duration of only 93 min, which is considerably shorter than that observed in similar research [38]. The Markov bottle continuously feeds the soil column, and the loamy sand is characterized by a high number of large pores, which facilitate the flow of saturated water and thereby reduce the length of the water flow path. These factors collectively resulted in the retardation effect of the magnetization treatment not being reflected, and thus the magnetization of low-mineralization saline water increased the advance rate of the wetting front by a marginal amount, which was consistent with the research findings in Niaz et al. [39]. Conversely, the magnetization treatment of high-mineralization saline water (3, 6 and 10 g L⁻¹) was found to result in a reduction in the rate of wetting front propulsion. This phenomenon can be attributed to the magnetization treatment reducing the surface tension of the saline water while simultaneously increasing the viscosity [40]. It is proposed that more MSW entered into the small pores of the soil matrix increased the adsorption and capillary action of the soil on MSW. This resulted in a lengthening of the circulation paths and time, which in turn reduced the rate of wetting front propulsion. These findings are consistent with the existing literature, as evidenced by the research results from Yi et al. [41], which corroborate the same conclusions.

In studies of the same mineralization saline water with different magnetization treatments, it has been demonstrated that the relative reduction value of surface tension is a single peak curve, with an optimal magnetization. In instances where the saline water mineralization was less than 5 g L⁻¹, the relative reduction value of surface tension resulting from 0.2-0.3 T magnetization is the greatest [36]. The results of our experiments, namely that 0.27 and 3 g L⁻¹ saline water with a magnetization treatment of 0.2 T exhibited the greatest effect on wetting front migration, were verified. The optimal magnetization of saline water with a mineralization greater than 6 g L⁻¹ remains to be determined; however, our research findings suggest that this is 0.6 T, while that of 10 g L⁻¹ saline water should exceed 0.6 T, which can be validated in future experiments.

4.2. Relationship between the Magnetization Mechanism and Water-Salt-Ion Content in Soil

The magnetization of saline water has a direct impact on the migration process of the wetting front, and an indirect influence on the change in water, salt and ion content in the soil following the redistribution of soil water.

In the course of our experiment, the water supply to the Markov bottle was terminated (i.e., the infiltration process was brought to an end), at which point the wetting front advanced to a depth of approximately 40 cm. Subsequent to the process of soil water redistribution, the rate of advancement of the wetting front gradually slowed. This facilitated the infiltration and flow of MSW into soil micro-pores. The unsaturated flow movement was intensified, resulting in a higher water content in the shallow soil (soil depth 0–40 cm). When the water storage capacity in the shallow layers is considerable, the water in the layer flows slowly from top to bottom in the soil pores, and the water potential gradient becomes smaller, resulting in less water entering the deeper layer (soil depth 40–60 cm) of the soil. This reflects that MSW increases the water holding capacity of the soil, which is consistent with the results obtained in the laboratory soil column experiments of Abdelghany et al. [42] and the field drip irrigation plot experiments of Khoshravesh et al. [43]. Specifically, low-mineralization saline water (0.27 g L⁻¹) magnetization treatment increased the rate of propulsion of the infiltrating wetting front, with the water flowing only in the soil macropores, which resulted in a reduction in the soil moisture content and soil water storage capacity.

Soluble salts migrate with the movement of soil water, undergoing either partial adsorption by soil particles or segregation and precipitation when the concentration exceeds the solubility of water. The magnetized treatment has been shown to improve the solubility of inorganic salt (in particular, salts that are insoluble), increase the binding force of salt ions to surrounding water molecules, and reduce the likelihood of salt ions undergoing a reaction that results in the formation of precipitates [44]. Consequently, during the redistribution of soil water, MSW caused the dissolution of more soluble salt in the shallow layer, which was transported to the deep layer due to its own concentration gradient. This illustrates the remarkable capacity of MSW to drain salt. The findings of our study are consistent with those reported by Hamza and Al-Sulttani [45]. Furthermore, Bu et al. [46] also achieved a comparable result in a three-year field experiment utilizing under-film dripping irrigation.

GUO et al. [47] employed a molecular dynamics simulation to investigate the impact of a magnetic field on the microstructure and kinetic properties of salt ions. Their findings indicate that when a chloride solution is subjected to a magnetic field strength of 0.2 T, the force between ions and water is diminished, resulting in an increase in contact ion pairs and a reduction in solvent separation ion pairs. This phenomenon enhances the diffusion coefficient of the cation and reduces that of the anion. These findings support the conclusions of our study regarding the greater impact of saline water magnetized treatment on cations compared to anions, and Mostafazadeh-Fard et al. [48] also draw similar conclusions.

The surface chemical microscopic study of soil colloids demonstrated that the rate, quantity, and strength of ion adsorption on soil colloids are contingent upon factors such as the surface potential, valence, and radius of the colloids. In natural conditions, soil colloids were predominantly negatively charged and adsorbed a range of positively charged cations. The order of affinity between cations with varying valence states and soil colloids was Ca²⁺ > Mg²⁺ > K⁺ > Na⁺ [49]. The hydration radius of Ca²⁺ and Mg²⁺ in saline water was relatively small, and their contact angle on the surface of hydrophobic soil colloids following magnetization treatment was also restricted, resulting in high wettability [37]. Accordingly, in the course of our experiment, the exchange of calcium and magnesium ions in the soil solution and colloidal surfaces was more pronounced during the redistribution of soil water, resulting in a greater exchange. Similarly, Liu et al. [50] reached the same conclusion through the application of MSW to Populus×euramericana ‘Neva’ through irrigation.

The adsorption of anions by colloidal particles in soil was less than that of cations, and the negative adsorption order of anions with different sodium salts was SO₄²⁻ > Cl⁻ [49]. It is possible that the HCO₃⁻ in tap water used in our experiment may undergo ionization, forming CO₃²⁻; this, in turn, may react with the Ca²⁺ and Mg²⁺ present in the soil solution, forming slightly soluble or even insoluble salts. The utilization of saline water with a magnetized treatment has the potential to reduce the probability of salt ions forming precipitation whilst simultaneously enhancing the solubility of slightly soluble or insoluble salts [44]. Consequently, magnetization treatment exerts a more pronounced impact on the anions SO₄²⁻ and HCO₃⁻ in saline water.

5. Conclusions

The combination of magnetized water treatment technology and drip irrigation represents a novel high-tech irrigation technology. Our laboratory soil column simulation experiments have yielded intriguing conclusions when salt water of varying mineralization levels is subjected to cyclic magnetization via spiral flow magnetization devices of distinct magnetization strengths, and subsequently dripped into soil columns filled with loamy sand. During the infiltration and redistribution of MSW in the soil column, the characteristics of soil moisture, salts and ions underwent alteration, with the majority of these changes proving advantageous (including the velocity of wetting front migration, soil moisture content, soil salinity, and water-salt-ions storage within the entire soil column). The utilization of drip irrigation with magnetized low-mineralization saline water serves to expedite the advancement of wetting fronts, which may be employed for the enhancement of salinized loamy sand or the salt washing of spring- and winter-irrigated soils. The utilization of drip irrigation with magnetized high-mineralization saline water maintains soil water retention and facilitates salt drainage, which effectively mitigates the adverse effects of saline irrigation on soil habitats. Additionally, the relationship between the mineralization of saline water and the optimal magnetization treatment intensity is elucidated. Awareness of this information would facilitate the advancement and deployment of MSW irrigation technology.