Soil Compaction Mechanism and Improvement in Farmland

Abstract

To improve the quality of sloping soil in farmlands, exploring the mechanism of farmland soil consolidation is vital. This study investigated the improvement of soil pH, exchangeable acid, crop economic characteristics, and the soil compression index under different soil amendments using field experiments and laboratory-simulated cultures. The results show that (1) increasing the pre-consolidation pressure of the soil and reducing the soil compression index and bulk density significantly reduces the risk of soil compaction; (2) soil compression performance improved after the addition of organic and bio-carbon fertilizers and other modifiers to the compacted soil. The comprehensive physical index compression curve (S) value was higher than 0.05; (3) the addition (biomass carbon + chitosan 32 t/hm²) had the best improvement effect, which significantly increased soil pH, reduced soil exchange acid, and prevented the dissolution of soil H⁺ and Al³⁺. Dissolution also reduced the risk of soil compaction stress in rice and significantly increased root thickening and yield. Based on the stagnation mechanism of farmland soil and the improvement principle of physical properties, a comprehensive improvement technology for farmland soil compaction is proposed.

1. Introduction

Soil is fundamental for the sustainable development of humankind and forms the basis for the development of human civilization [1]. Additionally, it is a non-renewable resource [2]. The quality and quantity of soil resources play a decisive role in social and economic development and food security [3]. With the continuous improvement in intensive land management and use in China, various technical measures have been imposed on the soil, including the application of several chemical fertilizers and pesticides, changes in farming methods, adjustment of plant structures, and rotation systems. Thus, potential invisible degradation processes are generally prevalent [4].

In addition, various chemical factors have been invested in farmland soil, causing significant soil pollution, increasing the dependence on chemical substances, and causing imbalances in the farmland’s ecological environment [5]. The quality of farmland soil is indicated by its ability to coordinate water, fertilizer, gas, heat, and resistance to external environmental disturbances [6]. Degradation of the physical properties of soil is a potential cause of soil compaction [7]. Revealing the physical state of farmland soil compaction is vital for scientifically regulating the farmland soil environment, ensuring soil fertility, promoting high yield and quality of crops, rapidly restoring farmland soil vitality, and ensuring sustainable agricultural development. The improvement of acidified soil, saline-alkali soil, and heavy metal-polluted soil is, therefore, of great concern [8].

Therefore, soil compaction has become an integral part of research on soil degradation. Soil compaction refers to a lack of organic matter and poor structure in the soil surface layer. Under external factors, such as irrigation or precipitation, the soil structure is destroyed, the soil material is dispersed and dried, and the soil surface is hardened by cohesive forces [9,10]. In recent years, soil degradation has become an increasing concern in China. In particular, a decline in the arability of farmland soil is typical. As cultivated land hardens, the soil becomes less suitable for crops, resulting in critically degraded crop growth, production, and quality [11,12]. Jing measured the compression and water characteristic curves of brown soil in the Shenyang area and the distribution of the soil hardness profile and water infiltration rate in compacted brown soil [13]. Wang studied the effect of organic matter addition on the acidity and yield of paddy soil [14]. Field tests were conducted to evaluate the actual improvement. Establishing technical specifications for amendment application is an important basis for spreading and applying soil amendments [15]. They also discussed soil compaction mechanisms. Recently, the problem of chemical quality degradation with soil acidification has been studied; however, there are few reports on soil physical quality degradation [16].

This study aims to: (1) dynamically monitor the trend of soil physical state evolution in northeast China; (2) investigate the mechanism of compaction and degradation effects of different soil amendments. Based on the compaction mechanism of farmland soil and the principles of soil improvement, comprehensive improvement technologies, and suggestions for improving farmland soil compaction were proposed.

2. Materials and Methods

2.1. Experimental Matdserials

The main reagent included hydrogen phthalate, disodium hydrogen phosphate, borax, and potassium chloride in a pH 4.01 standard buffer solution. The main instruments included a single-axis compressor, ring knife (100 cm³), balance (0.1 g), thermostatic electric tachometer indicator, dryer, and acidimeter.

The experimental site was located in the Liaozhong District, Shenyang City, Northern China, where paddy soil was selected for testing. The physical and chemical properties of soil before testing were as follows: pH 5.70, organic matter content 13.24 g/kg, total nitrogen content 1.83 g/kg, effective phosphorus content 66.1 g/kg, and available potassium content 175 mg/kg. First, the soil sample was naturally air dried, and fresh plant and animal residues, such as plant roots and other foreign bodies, were removed. Next, the soil sample was sieved using a filter with a 2 mm pore size and mixed thoroughly. Finally, the soil sample was placed in a moisturizing bag prior to use [17].

2.2. Experimental Design

2.2.1. Test Sample Preparation and Production

The test soil was collected from the Liaozhong District, Shenyang City, Northern China (N41.542650, E122.91099), with a sampling depth of 0–20 cm, which comprised paddy soil with a loam texture. Five treatment groups were established in the experiment: CK, A1, A2, A3, A4, and A5. Their basic physical and chemical properties are presented in

Table 1. Basic physical and chemical properties of the tested soil samples.

Test Soil	pH	Organic	Bulk Density	Mechanical Composition (%)
				>0.02 mm	0.02–0.002 mm	<0.002 mm
CK	5.71	13.24	1.36	64.53	9.81	25.66
A1	6.12	13.31	1.32	65.32	9.77	24.91
A2	6.29	13.26	1.33	65.56	9.75	24.69
A3	5.82	13.56	1.28	65.59	9.73	24.68
A4	6.50	13.22	1.25	65.68	9.66	24.66
A5	6.77	14.11	1.26	65.33	9.12	25.55

. The treatment settings for the soil samples are listed in

Table 2. Types and dosages of soil amendments for different treatments.

Numbering	Treatment	Amount (t/hm2)
CK	No modifier added	-
A1	adding biochar	32
A2	adding PAM	32
A3	adding CTS	32
A4	adding fly ash	32
A5	adding biochar-CTS	32

.

2.2.2. Soil Compression Experiment

A uniaxial compression test was performed to measure the compression curve of the test soil sample, and the pressures applied to each test soil sample were 10 (P₁), 50 (P₂), 80 (P₃), 100 (P₄), 150 (P₅), 200 (P₆), 300 (P₇), and 400 kPa (P₈) [18,19]. At the beginning of the compression experiment, the test soil samples were placed in the sample chamber. After the position was adjusted, increasing pressure was continually applied to the test soil samples. A certain pressure was applied for 5 min, after which the soil sample was allowed to stand for 2 min. The soil volumetric variable (vertical displacement) was recorded, and pressure was subsequently applied until 400 kPa, and the vertical displacement (deformation) of the soil volume obtained after each pressure application was recorded separately as S₁, S₂, S₃, S₄, S₅, S₆, S₇, and S₈, which corresponded to the pressure values.

2.2.3. Soil Improvement Experiment

The tested soil amendments were commercially available. The biomass charcoal was purchased from a biomass charcoal company in Wuhan, chitosan from a biotechnology company in Shanxi, fly ash was obtained from a thermal power plant in Gansu, and polyacrylamide was purchased from Yiwu City Environmental Protection Technology Co., Ltd. (Yiwu, China).

The experiment consisted of six treatments, each covering an area of 21 m². Different management groups had the same management measures, except for the addition of modifiers. The improvers were applied in strips approximately 10 cm deep, which were covered with soil approximately 5 cm thick and then transplanted with rice. On 10 April 2018, tillage was completed after the application of the improver. On April 28, the seedlings were transplanted. Jinfeng seedlings were used for the test. The harvest period lasted from 20 September 2018 to 10 October 2018.

2.3. Sample Collection and Detection

Prior to the experiment, mixed soil samples were randomly collected from the test area on 9 April 2018. Subsequently, they were randomly collected from each rice-cultivating test plot on 18 April 2018. When the rice was harvested (12 October 2018), mixed soil samples were randomly collected from each test plot. All the soil samples were tested for pH, total exchangeable acid (EA), and exchangeable Al³⁺. The test method was based on soil agrochemical analysis [20]. Each plot was counted during the rice harvest period, and ten rice bags were randomly selected from each plot to quantify their economic traits.

2.4. Compression Curve

Based on the vertical displacement (deformation) of the soil volume obtained after each pressure application, the corresponding void ratio e was calculated using the following formula:

$$e=\frac{{\rho }_s}{{\rho }_d}\frac{H-d}{H}-1$$

(1)

where ρ_s is the soil particle density of the sample (g/cm³), ρ_d is the initial bulk density of the soil sample (g/cm³), H is the initial height of the soil sample to be tested, and d is the decrease in the height after pressure was applied.

The slope of the straight-line segment (Cc) between the void ratio and the effective pressure logarithm of the compression test is expressed as follows:

$$Cc=\frac{e1-e2}{logp1-logp2}$$

(2)

where p1 is the first vertical self-weight stress on the soil (kPa), p2 is the sum of self-weight stress and additional stress on the soil (kPa), e1 is the pore ratio after compression stabilization under the action of p1, and e2 is the pore ratio after compression stabilization under the action of p2.

The pre-consolidation pressure value was the applied pressure value corresponding to the maximum absolute value of the curvature k of the compression curve. The curvature calculation formula is as follows:

$$K=\frac{d^{2}e/d{\left(log\mathsf{ϭ}\right)}^2}{\left[1+\left(de/d{\left(log\mathsf{ϭ}\right)}^2\right)\right]}.$$

(3)

3. Results

3.1. Compression Curve Results

Considering the second derivative of the above formula was zero, the enthalpy at the maximum curvature of the soil compression curve, which is the pre-consolidation pressure value, can be obtained [21]. The compression test results for each test soil sample are listed in

Table 3. Statistics of test results of soil samples tested.

Test Soil	(P1 S1)	(P2 S2)	(P3 S3)	(P4 S4)	(P5 S5)	(P6 S6)	(P7 S7)	(P8 S8)
CK	0.028	0.028	0.028	0.028	0.023	0.023	0.023	0.022
A1	0.032	0.032	0.032	0.032	0.026	0.026	0.026	0.025
A2	0.024	0.023	0.022	0.022	0.0219	0.021	0.0217	0.019
A3	0.055	0.055	0.055	0.055	0.055	0.055	0.055	0.048
A4	0.066	0.066	0.066	0.065	0.065	0.064	0.066	0.060
A5	0.073	0.072	0.072	0.071	0.071	0.069	0.069	0.066

.

Usually, when soil is subjected to an external force, the shape changes and the curve denoting the relationship between the external force (pressure) and volume change of the soil volume is termed the compression curve of the soil [22]. A typical compression curve is divided into two parts, namely elastic and plastic. When the pressure of the soil is within the elastic stage, the soil can, to a certain extent, recover when the external force is removed [23]. When the pressure of the soil exceeds the elastic curve and enters the plastic stage, it cannot be automatically restored and can rarely be recovered, even if the external force is removed. In the plastic stage, the mechanical properties of the soil change significantly; that is, the soil is “consolidated” [24].

The pre-consolidation pressure value of the soil is the pressure required for the soil to change from the “elastic” stage to the “plastic” stage. This value indicates how difficult soil compaction will be. The compression index is assumed to be the slope of the plastic portion of the compression curve, which can be used as a parameter to indicate the sensitivity of the soil to compression. Increasing the pre-consolidation pressure of the soil and lowering the compression index significantly reduces the risk of soil compaction [25]. The compression curves of the test soil samples were measured using a uniaxial compression test.

Figure 1 shows the compression curve of brown soil treated with different soil amendments, where the points are measured values, and the curve is the fitting result. Figure 1 shows that the compression curves of the tested soil samples had similar shapes, where the initial compression of the external force minimally changed, but when the external force exceeded a threshold value, the compression amount decreased rapidly, and then the force increased. The rate of change of the compression amount first increases then decreases and gradually stabilizes; that is, the compression curve is a horizontally inverted “S” type.

3.2. Maximum Curvature of the Compression Curve

The maximum curvature is the slope of the transition from the “elastic” to the “plastic” stage of the compression curve. Its numerical value represents the speed of elasticity loss under the action of the external force of the soil, representing the speed at which the knot occurs [26]. The maximum curvature (K) of brown soil under different initial bulk densities is displayed in the

Table 4. Statistics of test results of soil samples tested.

Test Soil	10 kPa K	50 kPa K	80 kPa K	100 kPa K	150 kPa K	200 kPa K	300 kPa K	400 kPa K
CK	1.88	1.84	1.82	1.77	1.32	1.31	1.32	1.36
A1	1.77	1.74	1.72	1.67	1.32	1.32	1.33	1.40
A2	1.72	1.67	1.55	1.40	1.36	1.32	1.32	1.35
A3	1.31	1.31	1.32	1.31	1.32	1.31	1.39	1.51
A4	1.362	1.36	1.362	1.38	1.39	1.42	1.41	1.58
A5	1.39	1.39	1.40	1.41	1.44	1.46	1.48	1.60

.

There was a positive correlation between the maximum curvature of the compression curve, the initial void ratio, and the void ratio difference of the brown soil sample. A large initial void ratio indicates loose test soil, resulting in a larger maximum curvature and, thus, a more rapid transition to the plastic phase. The compression test results for the soil samples in different treatment groups are shown in

Table 5. Statistics of compression test results.

Test Soil	10 kPa e	50 kPa e	80 kPa e	100 kPa e	150 kPa e	200 kPa e	300 kPa e	400 kPa e
CK	1.20	1.19	1.18	1.16	0.80	0.78	0.76	0.68
A1	1.16	1.15	1.14	1.12	0.76	0.74	0.72	0.64
A2	1.14	1.12	1.06	0.96	0.91	0.85	0.75	0.69
A3	0.79	0.78	0.77	0.77	0.76	0.76	0.65	0.56
A4	0.68	0.68	0.68	0.66	0.65	0.62	0.63	0.52
A5	0.65	0.65	0.64	0.63	0.61	0.59	0.58	0.51

.

3.3. Pre-Consolidation Pressure Value

The pre-consolidation pressure value is the applied pressure value corresponding to the maximum curvature of the compression curve. Under different soil treatments, the maximum pressure of the compression curve and the applied pressure corresponds to values in

Table 6. Statistics of pre-consolidation pressure values of soil samples.

Test Soil	10 kPa	50 kPa	80 kPa	100 kPa	150 kPa	200 kPa	300 kPa	400 kPa
CK	158.00	156.68	155.37	152.73	105.33	102.70	100.06	89.53
A1	152.73	151.42	150.10	147.47	100.06	97.43	94.80	84.26.
A2	150.10	147.47	139.57	126.40	119.81	111.91	98.75	90.85
A3	104.01	102.70	101.38	101.38	100.06	100.06	85.58	73.73
A4	89.53	89.53	89.53	86.90	85.58	81.63	82.95	68.46
A5	85.58	85.58	84.26	82.95	80.31	77.68	76.36	67.15

.

The pre-consolidation pressure values of the samples increased with initial soil bulk density, and the difference between the bulk density treatments of each treatment group reached the 1% significance level. This result may be because an increase in bulk density leads to an increase in the friction between the soil particles, thereby increasing the pre-consolidation pressure value of the soil. Under the same initial water content and initial bulk density conditions, the pre-consolidation pressure values of different treatment groups were determined by adding different soil amendments to adjust the bulk densities of the same soil sample, where A2 > A4 > A3 > A1 > A5. Although differences in the value of pre-consolidation pressure are present, the same amount of soil conditioner adjusts the bulk density of the same soil sample. Compared with other soil amendments, the pre-consolidation pressure of A5 was the smallest, which was the same as that of the soil conditioner, and the volumetric capacities were consistent.

The mechanization degree of agriculture in the study area is relatively high. The wheel pressure range of tractors in this area is estimated at roughly 1.06–9.5 Mg. When agricultural machinery with wheel pressures exceeding 3.3 Mg is used, it produces a pressure of 50 kPa, thus affecting the soil layer up to depths of 40 cm. When agricultural machinery with wheel pressures of over 6.5 Mg are used, it produces a pressure of 100 kPa, thereby affecting the soil layer up to depths of 60 cm. Studies have shown that soil deformation occurs after the first rolling of heavy agricultural machinery; therefore, to reduce the risk of soil compaction and the number of mechanical rollings, the weight of the machine and the pressure applied should be decreased [27].

3.4. Analysis of Soil Improvement Effect

3.4.1. Effect of Different Improvers on Soil pH

Changes in soil pH are one of the most commonly used indicators for measuring changes in soil acidity [28]. Analysis results indicated (Figure 2) that the treatment group was able to adjust the soil pH when compared to the blank control.

After one quarter of the study period, the applications of modifiers A5, A1, A4, A2, and A3 resulted in an increase of 0.24–1.47, 0.72–1.34, 0.43–0.96, 0.23–0.58, and 0.24–0.98 units, respectively, when compared with the control group. This result indicates that A3 had a notable effect on acidic soil. Comparing the treatments of each improver, treatment group A5 had the best impact, with a pH of 6.77, which was 1.06 units higher than the blank control, with an 18.56% increase rate. The pH of the soil treated with A2 (PAM32 t/hm²) was 6.29, which was 0.58 units higher than that of the blank control, with an increased rate of 10.15%.

Although the soil conditioner A3 (CTS) had a significant effect on improving the soil, it can only increase alkalinity for a short duration. Biomass carbon + CTS treatment can effectively improve the soil pH for a longer time. Comparing biomass carbon, CTS single application, and the compound treatment, we concluded that both single biomass carbon and CTS can improve soil pH. After one season of application, the effect of a single application of the biomass carbon on soil pH was better than that of CTS. However, the treatment of biomass carbon + CTS showed more improvement and a longer duration. Application of CTS to biomass charcoal can enhance the effect on soil pH.

3.4.2. Effects of Different Soil Amendments on Soil Exchangeable Acids

Soil exchangeable acids, including latent and active acids, are capacity indicators of soil acidity and are essential references for soil acidity regulation [29]. As depicted in

Table 7. Effect of different soil amendments on exchangeable acid in plough layer (0–20 cm).

Processing Number	Exchangeable Al3+		Exchangeable Acid
	April 2018	October 2018	April 2018	October 2018
CK	3.15	2.21	2.31	1.54
A1	2.81	1.93	2.10	1.32
A2	2.89	1.91	2.18	1.42
A3	2.20	1.92	1.71	1.55
A4	1.38	1.09	0.97	0.56
A5	1.31	1.05	0.89	0.43

, the molar concentration of total exchangeable acid in the soil of different treatments was between 0.43 and 1.54 cmol/kg after a season of application. Compared to the blank control, the application of soil amendments reduced the total amount of exchangeable acid in the plowed soil and improved soil acidification.

After one season of application, treatment A5 (32 t/hm²) was the most successful. The molar concentration of total exchangeable acid was only 0.43 cmol/kg, which was 0.46 cmol/kg less than that in the blank control, and the decrease was as high as 51.68%. Followed by A1 (biomass charcoal) and A4 (fly ash 32 t/hm²), the total molar concentrations of exchangeable acids were 1.32 and 0.56 cmol/kg, respectively. Treatments A1 and A4 were 0.78 and 0.41 cmol/kg lower than that of the blank control, which comprised reductions of 35.77% and 42.26%, respectively. This result is inversely related to the pH of the plowed soil.

As shown in Table 7 and Figure 3, the soil exchangeable acid includes H⁺ and Al³⁺, and the soil exchangeable Al³⁺ accounts for 48–93% of the total exchangeable acid in the soil. Thus, exchangeable Al³⁺ is the main component of soil exchangeable acid, not only of the soil. An important indicator of acidification is also used to reflect the presence of aluminum toxicity in the soil, and its content affects the growth and development of crops.

Compared to the blank control, the content of exchangeable Al³⁺ in the plowed soil decreased after the application of the soil amendment, indicating that the modifier inhibited the activation of soil Al³⁺. After one season of application, the content of exchangeable Al³⁺ in the plow layer decreased in the following order: CK < A5 < A4 < A2 < A3 < A1. Treatment A5 (soil conditioner 32 t/hm²) had the best effect, and the molar mass concentration of exchangeable Al³⁺ content was 1.05 cmol/kg, which was 1.16 cmol/kg less than the blank control, and the reduction was 52.48%. Followed by treatments A2 (32 t/hm²) and A4 (fly ash 32 t/hm²), the molar mass concentrations of exchangeable Al³⁺ were 1.91 and 1.09 cmol/kg, respectively, which is a reduction of 0.30 and 1.12 cmol/kg compared to the blank control, with decreased rates of 13.57% and 50.67%, respectively. This result is inversely related to the pH of the plowed soil but positively correlated with the total amount of exchangeable acid in the plowed soil.

3.4.3. Effects of Different Soil Amendments on Economic Traits of Rice

Table 8. Induction and disease resistance of different seed coating agents against mite.

Treatment	Effective Panicle (10 × 104/hm2)	Height (cm)	Base Stem Width (cm)	Thousand Seed Weight (g)	Theoretical Yield (Kg/hm2)	Actual Output (Kg/hm2)	Yield (%)
A1	17.13 ± 2.09 a	19.27 ± 2.57 a	2.15 ± 2.57 a	24.89 ± 0.03 a	12,279 ± 0.15 a	11,051 ± 0.34 a	7.59 ± 0.09 a
A2	16.72 ± 2.09 a	19.66 ± 2.57 a	1.91 ± 2.57 a	23.89 ± 0.03 a	11,503 ± 0.15 a	10,353 ± 0.34 a	0.8 ± 0.09 a
A3	17.11 ± 2.09 b	20.97 ± 2.57 b	2.12 ± 2.57 b	24.01 ± 0.03 b	11,831 ± 0.15 b	10,648 ± 0.34 b	3.6 ± 0.09 b
A4	16.99 ± 2.09 b	19.03 ± 2.57 b	2.05 ± 2.57 b	23.93 ± 0.03 b	11,709 ± 0.15 b	10,538 ± 0.34 b	2.59 ± 0.09 b
A5	17.26 ± 2.09 a	21.15 ± 2.57 a	2.20 ± 2.57 a	25.87 ± 0.03 a	12,859 ± 0.15 a	11,573 ± 0.34 a	12.6 ± 0.09 a
CK	16.05 ± 4.41 a	18.61 ± 2.65 a	1.91 ± 2.57 a	3.80 ± 0.03 a	11,412 ± 0.59 a	10,271 ± 0.23 a	-

Note: The mean difference with the same letter in the same column in the table is not significant (p > 0.05).

shows that different soil amendments can promote rice plant growth to varying degrees. Compared to the blank control, the rice yield increased by 0.8–12.60% after the application of the modifier, and treatment A5 (soil conditioner 32 t/hm²) had the dominant effect on rice yield, which increased by 12.60%. The second most effective was treatment A1 (soil conditioner 32 t/hm²); the increase in rice yield was 7.59%. Compared to the blank control, after applying the modifier, the thousand seed weight gain rate reached 4.35–13.04%. Among them, treatments A5 (soil conditioner 32 t/hm²) and A1 (soil conditioner 32 t/hm²) had the best effect on increasing the thousand seed weight of rice, and the weight gain rate was 13.04%.

Compared to the blank control, the plant height increase rate of rice was 8.96–16.21% after the application of the modifier, and the treatment of A5 (soil conditioner 32 t/hm²) had the greatest effect on increasing the plant height of rice (16.21% increase rate). Treatment A3 (soil conditioner 32 t/hm²) also had a notable effect on increasing plant height (15.99% increase rate). Compared to the blank control, the coarsening rate of rice rhizomes was between 9.46% and 29.73% after the application of a modifier. Among them, treatments A5 (soil conditioner 32 t/hm²) and A1 (soil conditioner 32 t/hm²) increased rice rhizome thickness by 28.38% and 29.73%, respectively.

3.4.4. Effect of Different Soil Amendments on Soil Compression Index

The analysis showed that, compared to the blank control, each treatment group applying the modifier increased the soil pre-consolidation pressure value and reduced the compression index (Figure 1). One quarter after the application of A5, the pre-consolidation pressure value of each treatment group increased by 0.24–1.47 units, and the compression index decreased by 0.1–0.6 units.

One quarter after the application of A1, the pre-consolidation pressure value of each treatment group increased by 0.24–1.47 units, and the compression index decreased by 0.1–0.6 units. One quarter after the application of A4, the pre-consolidation pressure of each treatment group increased by 0.24–1.47 units, and the compression index decreased by 0.1–0.6 units. In the first quarter after the application of A3, the pre-consolidation pressure value of each treatment group increased by 0.24–1.47 units, and the compression index decreased by 0.1–0.6 units.

In the first quarter after the application of A2, the pre-consolidation pressure value of each treatment group increased by 0.24–1.47 units, and the compression index decreased by 0.1–0.6 units; this result is inversely related to the bulk density of the plowed soil, but positively correlated with the organic matter and mechanical composition of the plowed soil.

4. Discussion

Manually adding soil amendments allowed for soil bulk density to be controlled while ensuring that the soil pre-consolidation pressure value and soil compression index were within the controllable range. With a decrease in soil bulk density, the soil compression index decreased, and the interaction between soil initial bulk density and soil amendment content significantly affected the compressive index of the soil. By selecting the weight of the agricultural machinery and the amount of pressure applied, the soil’s pre-consolidation pressure value and compression index were maintained within a normal range. Soil deformation occurred after the first crushing by heavy agricultural machinery. Therefore, to reduce the risk of soil compaction, the weight of the machine, and the amount of pressure applied should be reduced, in addition to reducing the number of mechanical rollings.

Comparing the effects of various improvers, treatment A5 (biomass carbon + chitosan 32 t/hm²) was the most effective; not only can it significantly improve soil pH, reduce soil acid exchange, and prevent soil H⁺ and Al³⁺ dissolution, but it can also reduce the risk of soil compaction stress in rice and significantly increase root diameter and yield. The primary reasons for this analysis are as follows: (1) Treatment with A5 with the appropriate amount of biomass carbon and chitosan can prevent the dissolution of soil H⁺ and Al³⁺ and reduce the acidity of the soil; (2) treatment with A5 comprises an appropriate amount of chitosan, which can increase the organic matter content in the soil, thereby enhancing the buffering capacity of the soil for acidity changes; and (3) the combination of biochar and chitosan can improve soil aggregate structure, increase porosity, reduce soil bulk density, increase soil compressibility index, and improve soil physical compaction properties. Treatment with A5 significantly increased soil pH, reduced exchangeable soil acids, substantially reduced dissolved Al³⁺ content, reduced soil compaction risk, and ensured average crop growth, thereby improving crop economic traits.

Reducing soil compaction in farmland is a systematic project. Comprehensive improvement measures must be taken according to actual conditions. Based on protective measures, soil pre-consolidation pressure can be improved by applying soil amendments, the compression index can be enhanced, and farmland consolidation can be improved. The physical, chemical, and biological properties of the soil maintain a sustainable farmland ecosystem [30]. This study clarified the practical application value and effect of soil improvement from the physical point of view through field experiments. However, this study only explains the causes and solutions of farmland compaction in the physical context. Future studies should improve the chemistry, biological genome, and other microscopic orders to explain farmland soil improvement.