Effects of the Changes of Particle Surface Electric Field and Interaction Force on the Reclaimed Soil Aggregate Structural Stability under the Application of Different Soil Conditioners

Abstract

Aggregate stability is a key factor in the evaluation of soil structure and erosion resistance, which is largely influenced by soil electric field and particle interaction. However, there are few studies on how different organic and inorganic soil conditioners change the surface electric field and interaction force of reclaimed soil to improve the aggregate stability. Therefore, a five-year field experiment was conducted to quantitatively study the effects of FeSO₄ (TM), organic fertilizer (TO), fly ash (TF), maturing agent + organic fertilizer (TMO) and fly ash + organic fertilizer (TFO), compared with control (CK) treatment, on the reclaimed soil internal force and the aggregate crushing strength. The results showed that the reclaimed soil surface potential and electric field intensity increased after 5 years of application of organic and inorganic soil conditioners. Under the same electrolyte concentration and electric field conditions, the crushing strength of aggregates (<5 μm) treated with TFO, TMO, TO, TF and TM decreased by 43.70%, 35.51%, 25.97%, 8.28% and 5.49%, respectively, compared with the control treatment, and the combination of organic and inorganic treatment (TFO and TMO) had a better effect on improving the aggregate crushing resistance. With the application of soil conditioners, the reclaimed soil DLVO force and net resultant force gradually decreased, and the order of magnitude was TFO < TMO < TO < TF < TM < CK, indicating that the application of organic and inorganic soil conditioners enhanced the van der Waals attractive force and net attractive force between reclaimed soil particles, and reduced the net repulsive force between particles. The theoretical calculation results of the reclaimed soil internal force well explain the experimental results of aggregate stability against crushing, and the relationship between aggregate crushing strength and net resultant force is exponential (p < 0.01). Generally speaking, the soil conditioners increase the net attractive force between particles, reduce the possibility of violent crushing of aggregates due to the increase of electric field intensity and improve the aggregate structural stability, among which the combined application of organic and inorganic soil conditioners has a better improvement effect. The results of this study will lay a theoretical foundation for clarifying the improvement of different soil conditioners on the reclaimed soil structural stability and erosion resistance.

1. Introduction

In agricultural production practice, soil aggregates will expand, disperse and break to a certain extent when they are wet after rainfall and irrigation; this is a key step leading to soil erosion [1,2], soil resource degradation and ecological environment problems such as soil nutrient loss, water infiltration obstruction, crop yield reduction and farmland non-point source pollution, restricting food security and healthy and sustainable agriculture development [3,4]. Therefore, it is of great significance to deeply understand the mechanism of soil aggregate stability and fragmentation and maintain and enhance the aggregate structural stability for preventing soil erosion, promoting soil fertility and grain yield and improving ecological environment.

As an important part of soil, the soil aggregate stability directly affects soil fertility characteristics, pore space status, water infiltration characteristics and surface structure changes, and is an important index to evaluate soil erosion resistance and carbon sequestration capacity [5,6]. The more stable the aggregate structure, the stronger the resistance to raindrop impact and water erosion, and its crushing strength greatly affects the intensity of soil erosion. At present, there are four main reasons for the aggregate fragmentation and dispersion under water environment: raindrop impact, uneven expansion of minerals, dissipation and physical and chemical dispersion [7]. Physical and chemical dispersion emphasizes the influence of the soil solution property changes on the aggregate structural stability, especially in the process of modern agricultural production practice. With the extensive use of chemical fertilizers, pesticides, plant growth conditioners and more foreign substances entering the soil, the change of soil solution concentration and properties induced will inevitably affect the soil pore space, swelling and shrinkage and aggregate structural stability [8,9]. Holthusen et al. pointed out that an increase of ion concentration in the soil solution would lead to an improvement of the aggregate structural stability. Hu et al. found through the study of the interaction between ions in purple soil and montmorillonite that the particle surface potential was quantitatively regulated by the solution electrolyte concentration adjustment. When the concentration of KCl solution decreased from 1 mol·L⁻¹ to 10⁻⁵ mol·L⁻¹, the surface electric field of soil particles increased sharply, which led to a rise in the net repulsive force between soil particles, thus increasing the aggregate crushing strength [10].

In recent years, it has been found that the surface electric field and surface potential of soil particles affect the interaction between soil particles, and the main driving forces for the fragmentation and dispersion of soil aggregates are electrostatic repulsive force and hydration repulsive force under the interaction of particles [11]. Accordingly, the main factor for maintaining and improving the aggregate stability and preventing them from being broken and dispersed is the van der Waals attractive force between soil particles [12]. Organic fertilizer, fly ash and FeSO₄ are important soil conditioners. Single or combined applications of them can increase the soil organic matter and cation exchange capacity and improve the soil surface electrochemical properties such as specific surface area and charge quantity, thus affecting the van der Waals attractive force between soil particles, and promoting the cementation and agglomeration of aggregates, which plays an important part in enhancing the soil structure stability [13,14,15]. Therefore, as the changes of particle surface properties and internal forces will affect the formation, stability and dispersion characteristics of soil aggregates, it is of great significance to study how different soil conditioners affect the surface electric field and interaction forces of soil particles to promote the soil structural stability and prevent soil erosion.

The loess tableland is an important agricultural production area in the Chinese Loess Plateau region. In recent years, the development of urbanization and rural construction has caused the serious waste and hollowing out of rural homesteads, which threatens regional cultivated land resources and food security [16,17]. Therefore, the reclamation of abandoned homesteads and the improvement of soil quality provide a practical and feasible path for supplementing the reserve cultivated land resources and ensuring healthy development of soil in the loess tableland. However, due to serious damage to its physical structure and to its deteriorated nutrient status, the reclaimed homestead soil has lost some functions and properties, with low productivity and utilization. It is an urgent need to add exogenous materials to improve its structure and fertility [18]. The climate type of the study area is categorized as a warm temperate sub-humid continental monsoon climate. And the previous researches on the effects of different soil conditioners on the properties of reclaimed soil mainly focused on the changes in conventional nutrient elements, organic carbon, aggregate components and bulk density, but there were few studies on how soil conditioners changed the surface electric field and interaction forces of reclaimed soil particles, which limited the understanding of the internal force mechanism on the improvement of reclaimed soil aggregate stability and quality. Therefore, this paper takes the reclaimed soil of abandoned homestead in the loess tableland as the research object, and quantitatively analyzes the influence of particle surface electric field and interaction force on the soil aggregate structural stability under the treatment of different soil conditioners through combining experiment and theoretical calculation, with a view to provide a theoretical basis for improving the structural stability of reclaimed soil in loess tableland and preventing soil erosion.

2. Materials and Methods

2.1. Field Experimental Area

Built in June 2015, the test plot is set up in the outdoor experimental area of a key laboratory of the Ministry of Natural Resources, and located in Chuyuan Village, Fuping County, Weinan City, Shaanxi Province, China (34°42′ N, 109°12′ E) (Figure 1). The reclamation experimental plot is situated in the Weibei Loess Tableland, with an average annual temperature of 13.3 °C, an average annual precipitation of 513.5 mm, an average annual frost-free period of 225 days and an annual total solar radiation of 135.44 kcal cm⁻², where the natural conditions meet the needs for the normal growth of food crops. The main purpose of the experiment is to improve the reclaimed soil structure and fertility in abandoned homesteads and increase land productivity. Reclaimed soil is from the demolition and backfill of adobe houses in the abandoned homesteads in hollow villages of the loess tableland, with removal of impurities such as gravel and wood in the soil. After the land is leveled, the backfill depth is not less than 30 cm and different organic and inorganic soil conditioners are applied to improve the reclaimed soil structure and nutrient characteristics and enhance the soil fertility. The reclaimed soil tested is raw soil that has not been cultivated for many years. Prior to the experiment, the initial attribute values of the surface reclaimed soil were as follows: the pH value (water–soil mass ratio of 1:2.5) was 8.53 and the soil texture was silty loam, in which the clay (0.002 mm) content was 10.15%, silt (0.05~0.002 mm) content 77.82%, sand (0.05~2 mm) content 12.03%, soil bulk density 1.40 g cm⁻³, soil organic matter content 4.5 g kg⁻¹, total nitrogen content 0.16 g kg⁻¹, available phosphorus content 3.1 mg kg⁻¹, rapidly available potassium content 61.4 mg kg⁻¹ and the proportion of water-stable aggregates with particle size less than 0.25 mm 93.27%. Overall, the soil type is loessial soil with a texture of silty loam, and poor overall level of structure and nutrient contents. For the general situation of the experimental area and the soil’s basic properties, please refer to the literature [14].

2.2. Experiment Design

Aiming at the problems of poor structure and fertility in reclaimed raw soil of abandoned homestead, a five-year field experiment was conducted, and the types and application amount of the tested soil conditioners were selected based on survey of their own characteristics and comparison of results from the literature. Before the experiment, all the soil conditioners were tested to meet the requirements of soil environmental quality in China [13,14,15]. The experiment adopted 6 treatments (

Table 1. Experimental treatments of organic and inorganic soil conditioners for the reclaimed soil improvement.

Number	Treatment	Organic and Inorganic Soil Conditioners	Application Rates (t·ha−1)
1	CK	Control	0
2	TM	Maturing agent (FeSO4)	0.6
3	TF	Fly ash	45
4	TO	Organic fertilizer (chicken manure)	30
5	TMO	Maturing agent + organic fertilizer	0.6 + 30
6	TFO	Fly ash + organic fertilizer	45 + 30

), which are arranged in random plots, with 3 replicates for each treatment. Each treatment plot is a square field of 2 m × 2 m, with a total of 18 plots. In order to avoid the mutual influence between treatment plots, an isolation belt with a width of 80 cm was set in each group of treatment plots. A two-year triple cropping system of winter wheat-summer maize rotation was employed for crops planting. The experiment was conducted in the growth period of summer maize, and the summer maize for the test was sown in early June with 65,000 plants per hectare and harvested in early October. Before planting crops, different soil conditioners were uniformly applied to the surface soil with a depth of 0–30 cm at one time according to the experimental settings, and the nutrient deficiency was supplemented by compound fertilizer, applying 1500 kg ha⁻¹ of compound fertilizer with nitrogen, phosphorus and potassium contents of 15%, 10% and 20%, respectively. The daily management measures such as irrigation amount, fertilizer amount and pest control in the six treatments were consistent. See Table 1 for the application amount of soil conditioners in each treatment [14].

2.3. Sample Preparation and Determination of Basic Properties

After the summer corn was harvested in early October 2020, the physical and chemical samples of reclaimed soil tillage layer with a depth of 0–20 cm were collected according to the experimental treatments, and three replicates were randomly taken for each treatment to analyze the mechanism of different soil conditioners to change the surface electric field and internal force of reclaimed soil particles. After the soil samples were naturally air-dried indoors, impurities such as plant roots and gravels were removed. In order to quantitatively characterize the influence of surface electric field and internal force of soil particles on the aggregate structural stability, it is necessary to prepare single-ion Na⁺ saturated samples from reclaimed soil under different treatments [10,19,20]. The research shows that the polarizability of Na⁺ is low under the electric field, and that the effect of electric field strength and internal force on the soil aggregate structural stability can be more directly reflected by preparing soil samples into Na⁺ saturated samples [8,10]. The specific preparation process of Na⁺ saturated samples is as follows [10]: Firstly, 1500 g air-dried soil samples under different treatments are weighed in a 5000 mL beaker, 0.5 mol L⁻¹ NaCl hydrochloric acid solution is added by 5000 mL and the supernatant is centrifuged after stirring and balancing for 24 h. Repeat this step three times, then wash the soil with deionized water to remove excess salt and finally dry it at 60 °C, grind and sieve, collect 1~5 mm saturated soil samples and bag them for later use.

The basic physicochemical properties of reclaimed soil were determined using the conventional classical analysis method, the content of soil organic matter was determined using the K₂Cr₂O₇ heat capacity method [21], the pH was determined using the electrode method(soil: water ratio of 1:2.5), the soil texture was determined using a MS3000 laser particle size analyzer and the soil specific surface area (SSA) and cation exchange capacity (CEC) were determined using the combined determination method of material surface properties [22].

2.4. Determination of Aggregate Breaking Strength

In this experiment, the aggregate crushing strength under different treatments was measured by sedimentation method to characterize the stability of reclaimed soil aggregates. The aggregate crushing strength is the mass percentage of micro-aggregates and single particles with diameters of less than 10 and less than 5 μm released by soil aggregate crushing in the total aggregate [23,24]. The specific experimental process is as follows: accurately weigh 20 g of saturated aggregate samples with a particle size of 1~5 mm, slowly pour them into a 500 mL measuring cylinder filled with NaCl solutions with the concentrations of 10⁰, 10⁻¹, 10⁻², 10⁻³ and 10⁻⁵ mol L⁻¹, seal it with plastic wrap and let it stand for two minutes, then gently turn the measuring cylinder upside down four times, with an interval of 30 s each time, so that the crushed aggregate fragments are evenly distributed in the suspension. Then, calculate the standing time according to Stokes’ Law, and extract the suspension of soil fine particles with different particle sizes by still water sedimentation siphon method and then move them to an aluminum box for drying and weighing. Then calculate the mass percentage of single particle and micro-aggregate with this particle size in the total aggregate according to the formula. The greater the aggregate crushing strength, the more unstable the aggregate is at such solution concentration, and the smaller the aggregate crushing strength, the more stable the aggregate is.

2.5. Calculation of Soil Surface Potential and Surface Electric Field

According to the classic double layer theory, the surface electric field of reclaimed soil in this experiment was quantitatively regulated by the bulk solution electrolyte concentrations, and the electrolyte concentration of NaCl solution was set to 1, 10⁻¹, 10⁻², 10⁻³ and 10⁻⁵ mol L⁻¹. In the NaCl solution system, the soil surface potential (φ₀) can be calculated by Equations (1)–(3) [12,24]:

$${\phi }_0=\frac{-2RT}{ZF}\ln \frac{1-a}{1+a}$$

(1)

$$\frac{\kappa C}{S{C}_0}=1+\frac{4}{1+a}-\frac{4}{1+e^{-1}a}$$

(2)

$$\kappa =\sqrt{\frac{8\pi F^2{Z}^2{c}_0}{\epsilon RT}}$$

(3)

where φ₀ (mV) is the surface potential; R (J K⁻¹ mol⁻¹) is the universal gas constant; T (K) is the absolute temperature; F (C mol⁻¹) is the Faraday constant; Z is cation valence; a is the intermediate variable; C₀ (mol L⁻¹) is the electrolyte concentrations in the bulk solution; ε is the dielectric constant for water (8.9 × 10⁻⁹ C² J⁻¹ dm⁻¹); κ (dm⁻¹) is the Debye–Hückel parameter; c₀ (mol L⁻¹) is the equilibrium concentration of the cation in bulk solution; c (cmol kg⁻¹) is the cation exchange capacity and S (m² g⁻¹) is the specific surface area.

The soil surface potential and surface electric field distribution at x from the soil particle surface can be calculated with the Equations (4)–(6) [8,24]:

$$E\left(x\right)=-\sqrt{\frac{8\pi RT}{\epsilon }}\left[c_0\left(e^{\frac{-ZF\phi \left(x\right)}{RT}}-1\right)\right]$$

(4)

$$\phi \left(x\right)=\frac{4RT}{F}\tan h^{-1}\left(b{e}^{-kx}\right)$$

(5)

$$b=\tan h\left(\frac{ZF{\phi }_0}{4RT}\right)$$

(6)

where φ(x) is the potential at x distance from the particle surface, V; x is the distance between two adjacent particles in the double electric layer, nm; b is the intermediate variable and E(x) is the electric field strength at x distance from the particle surface, V m⁻¹.

2.6. Quantitative Calculation of Reclaimed Soil Particle Internal Forces

The reclaimed soil particle interaction forces include electrostatic repulsive force, van der Waals attractive force and hydration repulsive force, and the net pressure (P_net) of soil internal forces is the sum of electrostatic repulsive force pressure (P_E), hydration repulsive force (P_hyd) and van der Waals attractive force (P_vdW). The reclaimed soil particle interaction force and net pressure were calculated according to Equations (7)–(11) based on the calculation results of surface electrochemical properties such as soil specific surface area and surface charge density [12,24]:

$$P_E=\frac{2}{101}RT{c}_0\left\{\cosh \left[\frac{Z_{i}F\phi (d/2)}{RT}\right]-1\right\}$$

(7)

$$P_{hyd}=3.33\times {10}^{4}ex{p}^{-5.76\times {10}^{9}d}$$

(8)

$${\theta }_{\mathrm{m}}=-{\rho }_{w}SSA\left(3\sqrt{\frac{A_{eff}}{6\pi {\rho }_{w}g\psi }}\right)\times 1000$$

(9)

$$P_{vdW}=-\frac{A_{eff}}{0.6\pi }{\left(10d\right)}^{-3}$$

(10)

$$P_{net}=P_E+P_{hyd}+P_{vdW}$$

(11)

where A_eff (J) is an effective Hamaker constant which was estimated by analyzing the dry end of the soil water characteristic curves with a dew-point potentiometer. θ_m is the gravimetric water content (kg kg⁻¹), ρ_w is the density of water (kg m⁻³), g is acceleration due to gravity (m s⁻²), ψ is the matric potential head (m H₂O), Z_i is cation valence, d (dm) is the distance between two adjacent particles and φ(d/2) (V) is the potential at the middle of the overlap of the electric double layers of two adjacent particles.

2.7. Data Statistical Analysis

Data collation and statistical analysis were performed using Microsoft Excel 2016 and SPSS22.0. Figure generation was performed using Origin software 2019, error analysis of soil aggregates breaking strength was carried out and regression analysis was performed on the reclaimed soil net pressure and aggregates breaking strength.

3. Results

3.1. Effects of Different Organic and Inorganic Soil Conditioners on Reclaimed Soil Surface Potential and Surface Electric Field

According to the electric double layer theory, different electrolyte solutions will affect the thickness of the electric double layer of soil particles, and then affect the soil surface potential and the electric field intensity around particles [8,24]. According to the measured specific surface area and cation exchange capacity, the soil surface potential at different electrolyte concentrations can be quantitatively calculated by Formula (1) to Formula (3). It can be seen from

Table 2. Reclaimed soil surface potential under different soil conditioners application rates and bulk solution electrolyte concentrations.

Electrolyte Concentration	Surface Potential (mV)
(mol·L−1)	CK	TM	TF	TO	TMO	TFO
1	−142.14	−174.96	−161.75	−155.5	−165.86	−162.91
0.1	−198.08	−232.29	−218.61	−212.1	−222.87	−219.81
0.01	−256.03	−290.79	−276.93	−270.32	−281.86	−278.16
0.001	−314.77	−349.72	−335.79	−329.14	−340.14	−337.02
0.00001	−432.85	−467.89	−453.94	−447.27	−458.29	−455.17

that with a decrease of electrolyte concentration in the bulk solution of reclaimed soil and the application of soil conditioners, the absolute value of the surface potential of the soil particles tends to increase. This is due to the fact that a low electrolyte concentration cannot sufficiently compress the particle electric double layer, which weakens its ability to shield the surface charges of particles. When the electrolyte concentration is 1 mol·L⁻¹, the surface potentials of reclaimed soil particles under CK, TM, TF, TO, TMO and TFO treatments were −142.14, −174.96, −161.75, −155.5, −165.86 and −162.91, respectively. When the electrolyte concentration is reduced to 0.00001 mol·L⁻¹, the absolute values of surface potential increased significantly, which were −432.85, −467.89, −453.94, −447.27, −458.29 and −455.17, respectively. After the electrolyte concentration decreased from 1 mol·L⁻¹ to 0.00001 mol·L⁻¹, the absolute values of soil surface potential under CK, TM, TF, TO, TMO and TFO treatments increased by 2.04, 1.67, 1.81, 1.87, 1.76 and 1.79 times, respectively. The addition of soil conditioners slowed down the influence of solution concentration change on soil surface potential to some extent. At the same solution concentration, the soil surface potentials varied under different soil conditioners. The reason is that the organic matter content and particle composition of reclaimed soil are changed due to the differences in the characteristics of different soil conditioners, which are the most important charged particles in the soil [24,25].

By taking the surface potential values in Table 2 into Formula (4) to Formula (6) in turn, the distribution curves of soil surface electric field under different treatments are obtained (Figure 2). The addition of soil conditioners slightly increased the surface electric field intensity between particles, and with the increase of the distance between soil particles, the electric field intensity decreased in varying degrees (Figure 2). At different electrolyte concentrations, the electric field intensities of soil surface treated with different soil conditioners were all as high as 10⁸ V m⁻¹, and such a strong electric field would have a great impact on the soil surface properties and the interaction between particles. With the increase of electrolyte solution concentration, the electric field intensity at the same distance on the surface of soil particles decreased under different treatments, and the action distance of surface electric field was shortened sharply. When the electrolyte concentration was less than or equal to 0.001 mol·L⁻¹, the action distance of the electric field could reach more than 100 nm, and the electric field intensity was still as high as 10⁵−10⁶ V·m⁻¹. Such a strong electric field is bound to have a great impact on the aggregate structural stability. However, when the electrolyte concentration increased to 1 mol·L⁻¹, the action distance of electric field was shortened to less than 10 nm, and the electric field intensity would rapidly attenuate to zero, which would effectively weaken the influence of electrostatic repulsive force on the aggregate structural stability. For example, when the distance between soil particles under CK and TO treatments was 10 nm, with the decrease of electrolyte concentration or the increase of absolute value of surface potential, the electric field intensity first increased sharply and then tended to be stable, and with other soil conditioners treatments, it presented similar changing trends (Figure 3). It can be seen from Figure 3 that the electric field intensity decreased sharply with the increase of electrolyte concentration, indicating that high electrolyte concentration can significantly shield and reduce the electric field around soil particles. However, when the electrolyte concentration was less than 10⁻² mol·L⁻¹, the electric field intensity on the particle surface did not vary obviously with the decrease of the concentration. To sum up, the electric field intensity and magnitude around soil particles can be changed by regulating the electrolyte concentration of the bulk solution.

3.2. Effects of Different Organic and Inorganic Soil Conditioners on Reclaimed Soil Aggregates Breaking Strength

As an important part of soil structure, aggregate stability enhancement is of great significance for improving soil physical and chemical properties, regulating soil water, nutrients, air and heat and reducing soil erosion [5]. In order to quantitatively evaluate the effect of the soil conditioner application on soil aggregate stability, the quantity of micro-aggregates with diameters less than 10 μm and more than 5 μm released from the soil aggregate crushing under different soil conditioner treatments was measured at varying electrolyte concentrations (Figure 4 and Figure 5). For all the soil conditioner treatments, with the decrease of electrolyte concentration, the surface potential and electric field intensity increased significantly, and the percentage of micro-aggregates with diameters less than 10 μm and more than 5 μm released from the broken reclaimed soil showed a sharp increase at first and then tended to be stable. When the electrolyte solution concentration is 0.00001 mol L⁻¹, the electric field intensity is the strongest, and the quantity of micro-aggregates released by aggregate crushing is the largest. When the concentration of electrolyte solution is 1 mol L⁻¹, the electric field intensity is overall weak, and the quantity of micro-aggregates released by aggregate crushing is the least (Figure 4 and Figure 5). The above results show that when the electrolyte concentration of the reclaimed soil bulk solution is diluted, the surface potential and electric field intensity of soil particles increase, the crushing strength of soil aggregates increases rapidly. After crushing, more small particles will be released, and the aggregate stability will decrease. When the solution concentration drops to 0.01 mol L⁻¹, the aggregate crushing strength gradually slows down, and the structural stability will sharply decrease at first and then stabilize.

Under the same electrolyte concentration and electric field conditions, the aggregate crushing strength decreased continuously with the application of organic and inorganic soil conditioners compared with the control treatment. When the solution electrolyte concentration was 10⁻¹ mol L⁻¹, the percentages of micro-aggregates with a diameter less than 10 μm released by aggregate crushing under TFO, TMO, TO, TF, TM and CK treatments were 9.87%, 15.13%, 18.16%, 25.66%, 30.35% and 32.59%, respectively, and the aggregate crushing strength decreased by 69.73%, 53.57%, 44.26%, 21.26% and 6.87%, respectively, compared with the control treatment; the percentages of micro-aggregates with a diameter less than 5 μm released by aggregate crushing were 3.97%, 5.31%, 6.69%, 8.58% and 9.21%, respectively, and the aggregate crushing strength decreased by 60.73%, 47.47.0%, 33.77%, 15.03% and 8.72%, respectively, compared with the control treatment. When the solution concentration was reduced to 10⁻⁵ mol L⁻¹, the aggregate crushing strength was significantly increased. The percentages of micro-aggregates with a diameter less than 5 μm released by aggregate crushing under TFO, TMO, TO, TF and TM treatments were reduced by 43.70%, 35.51%, 25.97%, 8.28% and 5.49%, respectively, compared with the control treatment, which indicates that the addition of soil conditioners significantly weakened the aggregate crushing strength. These results indicate that at a low electrolyte concentration (high electric field intensity), the addition of soil conditioners such as organic fertilizer and fly ash reduces the aggregate crushing strength, helps release a small amount of micro-aggregates, and enhances the aggregate structural stability. At a high electrolyte concentration (low electric field intensity), the effect is more significant, and the aggregate crushing strength is significantly weakened, only weak swelling or crushing, which provides some theoretical basis for the reclaimed soil structure improvement and soil and water conservation. Through comparison, it was found that the effect of different conditioners in promoting the formation and stability of reclaimed soil aggregates was different and that the overall order was TFO > TMO > TO > TF > TM > CK, with organic conditioners better than inorganic conditioners, and that the organic and inorganic combined treatments (TFO and TMO) have a better effect on improving the structural stability and erosion resistance of reclaimed soil aggregates. However, there is a question here. Theoretically, with the addition of the conditioners, the electrostatic repulsive force between reclaimed soil particles would increase on the basis of a slight increase in the surface potential and electric field intensity, thus reducing the aggregate stability. But in fact, the crushing strength of the reclaimed soil aggregates was weakened, and the reason was that the soil aggregate stability is determined by the net resultant force of electrostatic repulsive force, hydration repulsive force and van der Waals attractive force between particles jointly, not just by one of them [19,24]. Therefore, we will further introduce the DLVO force and net interaction force next.

3.3. DLVO Force between Reclaimed Soil Particles under Different Organic and Inorganic Soil Conditioners

According to classical DLVO theory, DLVO force refers to the resultant force of electrostatic repulsive force and van der Waals attractive force [25]. Figure 6 shows the distribution of DLVO force of reclaimed soil with the change of distance between particles under the treatment of different organic and inorganic conditioners, where the positive value indicates that the DLVO force between soil particles is a repulsive force, and the negative value indicates an attractive force. For reclaimed soil at any solution concentration, the DLVO force shows a net attractive force when the distance between soil particles under CK treatment is less than about 0.2 nm; meanwhile, it shows a net repulsive force when the distance is greater than about 0.2 nm, indicating that 0.2 nm is the critical distance between particles of control soil. The critical distance between particles of reclaimed soil under the treatments of TO, TM and TF is about 0.3 nm, that under TMO treatment is about 0.5 nm, and TFO treatment about 0.7 nm. It can be concluded from the critical distance between particles of reclaimed soil treated with different conditioners that it is more likely to show a repulsive force under the control treatment, and the addition of organic and inorganic conditioners significantly increases the van der Waals attractive force between particles, with its increase greatly higher than that of electrostatic repulsive force, thus reducing the DLVO force between reclaimed soil particles and increasing the crushing critical distance between particles of reclaimed soil aggregates; it is more likely to show an attractive force in the reclaimed soil under the organic and inorganic treatment (TFO), with better cementation.

For the same conditioner treatment, the critical distance between particles of reclaimed soil at different solution concentrations will also change. As the concentration increases, the crushing critical distance between particles of reclaimed soil will increase. For TFO treatment, if the solution concentration is increased to 1 mol L⁻¹, there is always an attractive force when the distance between particles of reclaimed soil is within 8 nm. In addition, when the DLVO force is a repulsive one, its strength also varies at different solution concentrations and under different organic and inorganic conditioner treatments. With the increase of solution concentration and the application of conditioners, its repulsive strength gradually decreases. The order of strength of DLVO force is shown as CK > TM > TF > TO > TMO > TFO given the comparison of treatments with different conditioners, indicating that the application of conditioners to some extents reduces the possibility of violent crushing of aggregates due to the increase of electric field intensity after the solution dilution. When the DLVO force is a net attractive force, theoretically the dry soil aggregates would not expand and decompose when exposed to water, while in fact, the dry soil breaks and decomposes to a certain extent after wet, which indicates that there is a huge short-range acting force between particles against the DLVO force and thus a repulsive force prevails [11,22]. The interaction force affecting the fragmentation and stability of aggregates also includes the hydration repulsive force, and its influence on the fragmentation of aggregates will be discussed on the basis of the classical DLVO force in the following.

3.4. Net Interaction Force between Reclaimed Soil Particles under Different Organic and Inorganic Soil Conditioners

The net resultant force between particles of reclaimed soil is the sum of electrostatic repulsive force, van der Waals attractive force and hydration repulsive force. Given the hydration repulsive force, the distribution diagram of net resultant force at the distances of 1.6 nm and 2 nm between particles of reclaimed soil treated with different conditioners is drawn (Figure 7), where the positive values indicate that the net resultant force between reclaimed soil particles is a repulsive force, while the negative values indicate an attractive force, and the strength order of net resultant force under different treatments is as follows: TFO < TMO < TO < TF < TM < CK, manifesting that the application of organic and inorganic conditioners enhances the net attractive force of reclaimed soil particles, among which the TFO and TMO treatments can facilitate the formation and stability of reclaimed soil aggregates. Given the fixed electrolyte concentration and distance between soil particles, the net resultant force between reclaimed soil particles gradually decreases with the application of conditioner, which weakens the influence of repulsive force on the aggregate stability. As the distance between soil particles increases from 1.6 nm to 2 nm, the net resultant force between reclaimed soil particles gradually decreases, and the hydration repulsive force decreases from 3.31 atm to 0.33 atm, whose influence is very weak. When the distance between reclaimed soil particles is greater than 2 nm, the van der Waals attractive force and electrostatic repulsive force shall exert a major influence on the structural stability of aggregates.

When the electrolyte concentration increases from 0.01 mol L⁻¹ to 1 mol L⁻¹, the net resultant force of reclaimed soil under the six treatments significantly decreases and a net attractive force is showed gradually, and the agglomeration and stability of the reclaimed soil aggregates continue to increase. When the electrolyte concentration decreases from 0.01 mol L⁻¹ to 0.00001 mol L⁻¹, the net resultant force of reclaimed soil varies gently, indicating that the structural stability of aggregates no longer significantly changes when the electrolyte concentration is less than 0.01 mol L⁻¹. This is instructive for improving the soil structural stability and preventing soil erosion by adjusting soil solution concentration [12,26]. Meanwhile, when the electrolyte concentration is 1 mol L⁻¹, the net resultant force of the reclaimed soil under TFO, TMO, TO, TM and TF treatments is negative at the distance of 2 nm. Compared with the net resultant force of 0.063 atm under the CK treatment, the application of different conditioners greatly increases the attractive force under the same conditions, and enhances the agglomeration and structural stability of reclaimed soil particles, which is of important theoretical value for the selection of proper soil conditioners and the guidance for improvement of structural stability and erosion prevention of newly reclaimed soil.

3.5. Relationship between Net Interaction Force and Aggregate Stability of Reclaimed Soil Particles under Different Organic and Inorganic Soil Conditioners

To further analyze and evaluate the relationship between the net resultant force between reclaimed soil particles and aggregate stability with the application of conditioner, the fitted relationship curve of the aggregate crushing strength and the net resultant force at the distance of 2 nm between particles under different treatments is established (Figure 8), as can be seen from which, there is an exponential functional relationship between the crushing strength of reclaimed soil aggregates and the net resultant force (p < 0.01) that with the decrease of net resultant force, the crushing strength of aggregates decreases exponentially. When the electrolyte solution concentration is 1 mol L⁻¹, the net resultant forces under CK, TM, TF, TO, TMO and TFO treatments are 0.063, −0.389, −0.415, −1.217, −2.018 and −4.456 atm, respectively, and the percentages of microaggregates (<5 μm) released by aggregate crushing are 7.03%, 6.28%, 5.97%, 3.59%, 2.45% and 1.55%, respectively, indicating that with the application of conditioner, the net resultant force of reclaimed soil decreases and turns to be an attractive force, and the crushing strength of aggregates is lower. Among them, TFO and TMO are the better options for the improvement of structural stability of soil aggregates.

When the net resultant force between reclaimed soil particles is between −5 atm and 15 atm, the aggregate crushing strength rises slowly with the increase of the net resultant force; While the net resultant force is between 15 atm and 20 atm, the aggregate crushing strength rises sharply with the increase of the net resultant force, and the structural stability of reclaimed soil decreases sharply, which is easy to be damaged. To summarize, the structural stability of aggregates is strongly affected by the internal forces between soil particles. Compared with the control treatment, with the application of organic and inorganic conditioners, the net attractive force between reclaimed soil particles is increased while the soil organic matter and specific surface area are increased, the crushing strength of aggregates is significantly reduced, and the agglomeration and stability of reclaimed soil structure are enhanced. Among them, TFO and TMO treatments are of better resistance to aggregate crushing. This is consistent with the studies by Hu et al., who revealed that the incorporation of biochar reduced the crushing strength of loessial soil aggregates, and increased the attractive force between particles and the stability of aggregates [12].

4. Discussion

4.1. Responses of Reclaimed Soil Surface Electric Field and Aggregates Breaking Strength to Different Organic and Inorganic Soil Conditioners

The aggregate structural stability is an important factor affecting soil fertility, water conduction and soil erosion [27,28]. During the production practice, when the irrigation water or rainwater permeates into the soil, the bulk solution electrolyte concentration of relatively dry soil will be diluted immediately. In this study, as the electrolyte concentration of the reclaimed soil solution decreases, the thickness of diffused double layer of particles increases, and the absolute value of the surface potential and the electric field intensity of soil particles increase (Table 1, Figure 2), indicating that the solution concentration and soil properties of reclaimed soil are the main factors affecting the soil surface potential and electric field intensity [23,24,25]. With the addition of different conditioners, the absolute value of surface potential and electric field intensity between reclaimed soil particles show a slight increase, and the reason behind this is largely that there is a particular difference in these conditioners themselves, and their addition increases the charge density and cation exchange capacity of reclaimed soil, thus affecting the surface potential and electric field intensity between particles. This has been verified by the previous research by Liu et al. [14,27]. In this study, with the decrease of electrolyte concentration or the increase of surface potential, the electric field intensity shows a sharp increase at first and then a stability. When the electrolyte concentration is less than 0.01 mol·L⁻¹, the electric field intensity of particle surface no longer changes significantly with the decrease of concentration, indicating that the electrolyte concentration of 10⁻² mol·L⁻¹ in this study is the key critical solution concentration affecting the electric field intensity of reclaimed soil (Figure 3). This is similar to the findings by Hu et al., who studied the change of the electric field intensity of purple soil with solution concentration variation through the still water sedimentation method and found that the electrolyte concentration of 10⁻² mol·L⁻¹ is the key critical value affecting the change of electric field intensity of purple soil [10].

The crushing strength of reclaimed soil aggregates increases sharply at first and then tends to be stable with the decrease of electrolyte concentration of bulk solution. The electric field intensity is related to the crushing strength of aggregates. When the solution electrolyte concentration is higher and the electric field intensity is weaker, the soil aggregates only expand or break weakly, with fewer fine particles released. When the electrolyte concentration of reclaimed soil bulk solution is lower, the surface potential and electric field intensity of soil particles increase, the crushing strength of soil aggregates increases rapidly, with more fine particles released after crushing, and the stability of the aggregates decreases significantly. When the solution concentration is less than 0.01 mol L⁻¹, the crushing strength of aggregates is weakened tends to be stable gradually (Figure 4 and Figure 5). This is consistent with the results by Liu et al., who evaluated the effect of electrolyte concentration regulation on the aggregate stability of loessal soil and Lou soil, and revealed that the soil surface electric field is a key factor for dry aggregates crushing when exposed to water, and the electrolyte concentration of 0.01 mol·L⁻¹ is an important threshold that affects the aggregate stability [5,19]. The addition of soil conditioners such as organic fertilizer, fly ash and FeSO₄ increases the organic matter content and the specific surface area of reclaimed soil, and the important cementing materials such as organic matter and iron oxide enhance the cementation and agglomeration of soil particles, thus forming highly stable aggregates and reducing the crushing strength of reclaimed soil aggregates [29,30]. When the solution electrolyte concentration is 0.1 mol L⁻¹, the crushing strength of aggregates (<10 μm) under the treatments of TFO, TMO, TO, TF, TM and CK decreases by 69.73%, 53.57%, 44.26%, 21.26% and 6.87%, respectively, compared with the control treatment, and the organic and inorganic treatments (TFO and TMO) can better improve the crushing resistance of reclaimed soil aggregates (Figure 4). The application of different conditioners to a certain extent reduces the possibility of violent crushing of aggregates due to the increase of electric field intensity after the electrolyte solution dilution.

4.2. Responses of Reclaimed Soil Net Pressures and Structural Stability to Different Organic and Inorganic Soil Conditioners

The internal forces between soil particles are affected by surface electrochemical properties such as the quantity of soil surface charge, specific surface area and solution concentration, which are closely related to soil organic matter content, particle composition and clay minerals [12,31]. In this study, with the application of organic and inorganic conditioners, the van der Waals attractive force between particles rises significantly, with its increase greatly higher than that of electrostatic repulsive force, thus reducing the DLVO force between reclaimed soil particles and increasing the crushing critical distance between particles of aggregates. It is more likely to show an attractive force in the reclaimed soil under the combined organic and inorganic treatment (TFO). The greater the crushing critical distance between particles, the better the agglomeration and cementation of soil (Figure 6). This is consistent with the results by Yu et al. and Hu et al. The former revealed that after 240 days of straw cultivation, the DLVO force significantly decreased with the increase of wet soil organic matter, which improved the cohesiveness of soil particles. The latter, through a two-year study on the improvement of loessial soil, showed that with the application of different amounts of biochar, the increase of van der Waals attractive force in loessial soil particles was significantly higher than that of electrostatic repulsive force due to the increase of organic and inorganic complex caused by the rise of clay particles and organic matter, which offsets the adverse effects of the increase of electrostatic repulsive force, and the DLVO force generally decreases [12,25].

This study found that with the application of different soil conditioners, the net resultant force between reclaimed soil particles gradually decreases, with an overall size order of TFO < TMO < TO < TF < TM < CK (Figure 7). The application of organic and inorganic amendments reduces the net repulsive force between particles and enhances the cohesion and attractive force of reclaimed soil particles. The effect of organic amendments to promote the cementation and stability of aggregates is better than that of inorganic amendments, and the combined organic and inorganic treatments (TFO and TMO) have a better effect on improving the net attractive force of reclaimed soil structure. This is similar to the research results of Huang et al., who showed that the addition of conditioners such as humus and biochar increased soil organic matter, while reducing the net resultant force between soil particles and increasing the net attractive force between particles [5,32,33]. With the application of soil conditioners such as organic fertilizer, fly ash and FeSO₄, the organic matter content and cation exchange capacity of the reclaimed soil increase, the van der Waals attractive force and net attractive force between the reclaimed soil particles enhance, and the crushing strength of aggregates is reduced, thus improving the stability of aggregates, which is conducive to improving the fertility and erosion resistance of the reclaimed soil. The theoretical calculations of soil internal forces are very consistent with the experimental results of the aggregates stability against crushing, indicating that with the decrease of net resultant force (the increase of net attractive force), and the aggregate crushing strength decreases exponentially (Figure 8). The findings of Annabi et al. and Yu et al. also revealed similar results that with the addition of compost materials and straws, the amount of organic cementing substances that promote the agglomeration of soil aggregates was improved, organic and inorganic complexes were increased, soil hydrophobicity was enhanced, soil wetting rate and solution concentration dilution rate were slowed down, and ultimately van der Waals attractive force and net attractive force between particles were increased, and the cohesion and stability between soil aggregate particles were enhanced [25,34]. Therefore, the interaction force between soil particles plays a key role in the stability of aggregates. The combined application of organic and inorganic conditioners increases the net attractive force between reclaimed soil particles, and promotes the cementation and stability of soil aggregates, which provides a new idea of regulation through internal forces for the structural stability improvement and erosion prevention of the reclaimed soil.

5. Conclusions

The surface electric field and interaction forces of soil particles play a crucial role in the stability and fragmentation of aggregates. The addition of conditioners and the decrease in electrolyte concentration both increase the surface potential and electric field intensity of reclaimed soil. At a solution concentration of 10⁻⁵ mol L⁻¹ (high electric field intensity), the crushing strength of aggregates (<5 μm) under TFO, TMO, TO, TF and TM treatments was decreased by 43.70%, 35.51%, 25.97%, 8.28% and 5.49%, respectively, compared with the CK treatment. The combined organic and inorganic treatments (TFO and TMO) have a better effect on improving the crushing resistance of reclaimed soil aggregates. With the application of organic and inorganic amendments, the net resultant force of reclaimed soil gradually decreases, in the order of TFO < TMO < TO < TF < TM < CK, indicating that the application of soil conditioners enhances the van der Waals attractive force and net attractive force between reclaimed soil particles and reduces the net repulsive force between them. The experimental results of aggregates stability against crushing are consistent with the theoretical calculations of interaction forces. As the net attractive force increases, the crushing strength of aggregates decreases exponentially. To sum up, the application of soil conditioners increases the net attractive force between particles and reduces the possibility of violent crushing of aggregates due to the increase of electric field intensity. In particular, the coupling application of organic and inorganic conditioners has a better effect, which provides a scientific basis for improving the structural stability of reclaimed soil and preventing water and soil erosion.