Experimental Study on Optimal Recycling Mechanical Parameters of Cotton Field Mulch film based on Small Soil Trough System

Abstract

Film mulching agriculture in arid areas is faced with pollution caused by film mulching, and currently mainly adopts the mechanized recycling of mulch film. However, residual mulch film in the soil will bind with soil under the farming environment, which affects the recycling effect. The main factors affecting the recycling of mulch film in the soil are not clear. In order to find out the specific factors, the actual dry-wet cycle water environment was simulated by using a small soil trough system based on the film lifting, separation and recycling problem of residual mulch film in the soil. The film lifting force and recycling efficiency of the residual mulch film under the action of wet-dry cycle were studied. The following results were obtained: soil compaction, film lifting angle, and the dry-wet cycle had a significant influence on the film lifting force value, indicating that the dry-wet cycle including water fertilizer had an impact on the soil structure. After mechanical loosening, the film lifting force decreased and the recycling rate of residual mulch film increased obviously. The optimal film recycling effect could be obtained under the following conditions, namely, a film lifting angle of 21.37–45.37°, the number of dry-wet cycles <3.8, a soil moisture of 22.43–23.18%, a soil compaction of 132.51–144.06 KPa, and a residual mulch film area of 45.85–64.5 cm². The experimental results can provide technical reference for residual mulch film pollution control and mechanized recycling.

1. Introduction

Mulch film technology can increase soil temperature and preserve soil moisture. It is the lifeline of agriculture in arid areas, and one of the technical guarantees of food security in arid areas [1]. Drip irrigation technology under mulch is adopted for cotton plantations in arid areas [2]. It can greatly save water, preserve soil moisture, and restrain weed growth [3]. Additionally, it is of a high degree of mechanization. Thus, it can effectively increase crop yield in arid areas. However, the continuous mulch used in drip irrigation cannot be fully effectively recycled, which easily causes the residual pollution of mulch and affects the quality of farmland soil [4]. In the cotton plantation areas of northwest China, the amount of mulch film input is generally 87.00–94.50 kg·ha⁻¹, and the recycling rate is about 75–90% [5]. Due to long-term continuous plantation, the residue of mulch film will accumulate day by day, and finally cause soil pollution. According to statistics [6], the average residual amount of mulch film is about 206.46 kg·ha⁻¹. Therefore, effective recycling treatment is urgently needed. Among the existing recycling technologies [7], mechanized recycling is considered to be one of the lowest cost and best treatment methods before biodegradable mulch film is effectively used. However, because the mechanical relationship between residual mulch film separation from soil is still unclear, the factors affecting the separation of residual mulch film from soil during mechanical recycling are still unclear in the current mechanized recycling. As a result, mechanized recycling has poor effect and lower recycling rate.

In order to facilitate the recycling of mulch film in the soil, the researchers divided the recycling of residual mulch film into residual mulch film recycling of seasonal mulch film and residual mulch film recycling of plough layer over the years [8]. The recycling of seasonal mulch film refers to the recycling of residual mulch film after the current autumn season; the recycling of residual mulch film of a plough layer over the years refers to the recycling of residual mulch film accumulated in soil over the years due to the failure of effective recycling at the current season.

Mulch film becomes a foreign material to the soil after residing in the top layer of soil [9]. Under the action of a water cycle, the fragmented mulch film interacts with soil [10], forming a small mulch film environment system. The stack effect of the micro mulch film environmental system has an impact on the moisture migration and soil structure of the whole soil system. In order to change the soil structure and reduce the pollution effect of residual mulch film on the soil, the residual mulch film is separated from the soil by mechanized operation, and the residual mulch film in the soil is removed. In these relationships, soil adhesion to mulch film, pressure and other resistance are the keys to affecting the removal of residual mulch film from soil [11]. Soil adhesion is a very complex comprehensive mechanical behavior [12]. It is affected by different humidity levels, water and fertilizer transport, and soil compactness. Many scholars argue [13,14] that solid materials such as mulch film in soil form an interface adhesion relationship with soil particles under a soil dry-wet cycle. Thus, the number of dry-wet cycles also affects the soil adhesion force to mulch film separation, and increases the difficulty of mulch film recycling. Therefore, the interaction between soil and mulch film is a complex process. The contact between residual mulch film and soil is also a combination of solid material and soil, which is influenced by the dry-wet cycle and surface soil pressure. Additionally, the change in pore and humidity in soil affects the water film state at the contact interface. To assess the force change in residual mulch film in soil under a dry-wet cycle is the key problem to realize the effective recycling of mulch film under mechanical actions, and to complete the mechanical damage to soil structure and the residual mulch film separation and recycling.

In this paper, based on the separation and recycling of residual mulch film in soil, the interference factors and mechanical change process in the process of mulch film removal were revealed from the perspective of the agricultural engineering application. The actual dry-wet cycle environment of water and fertilizer was simulated by a small soil trough experiment system. The variation in film lifting force and recycling efficiency under the dry-wet cycle were studied. The optimal value of film lifting force and the optimal recycling condition were obtained. The research results provide design basis and basic data for mechanized recycling.

2. Stress Analysis of Residual Mulch Film at Plough Layer

According to the literature [15], the residual mulch film in the soil of cotton field can be divided into flake, strip, wire, etc., and it is distributed in a 0–300 mm soil layer in a vertical and inclined manner. It is difficult to recycle small residual mulch film particles by mechanized operation. The flake film shrinks after strip expansion, and the adhesion force is the lowest. Lamellar residual mulch film is the most residual form in soil and has the most representative characteristics. The two surfaces of the whole residual mulch film are in contact with the soil, so it a large stress area and is difficult to recycling. However, it has the greatest impact on crop growth and is the key operation object to be recycled by mechanization. According to the distribution characteristics and mechanical recycling mode, the recycling mainly is targeted at residual mulch film debris with an area of more than 4 m². In the actual production process, the residual mulch film and soil are mixed together. With the action of drip irrigation under the mulch film, water and fertilizer penetrate into the soil through drip irrigation and then evaporate continuously, thus forming a dry-wet cycle and increasing the adhesion of soil to the surface of residual mulch film. The cotton field in Xinjiang mainly adopts the double-row planting mode of 660 + 100 mm. The soil surface is covered with mulch film, and water and fertilizer are carried out by drip irrigation belt. The residual mulch film and soil are mixed together and combined under the interaction between the upper and lower soil [16], as shown in Figure 1.

The irrigation of cotton fields consists of multiple drip irrigation cycles. At the end of each drip irrigation cycle, the soil changes from wet to dry, which forms a dry-wet cycle. Under the action of the soil environment, such as a dry-wet cycle, the residual mulch film, as a foreign substance, binds with soil particles to form a polymerization structure. The morphological characteristics of residual membrane are shown in Figure 2a. Figure 2b depicts the main forms of residual film in soil. When the film is recycled, the external force of mechanized soil loosening is needed to destroy its agglomeration force and realize the separation and recycling of residual mulch film. However, the change in mechanical properties of mulch film separation from soil still needs demonstrating by experimental research results. Additionally, the mechanical changes of mulch film recycling caused by the interaction between water and fertilizer, soil and mulch film need to be verified by experiments.

According to the distribution characteristics of residual mulch film, a mechanical model of residual mulch film in soil as shown in Figure 3 was established. In order to solve the mechanical distribution of residual mulch film more effectively, the mechanical model assumed that the adhesion force of residual mulch film was consistent with that of soil in each contact area. Residual mulch film was distributed in the soil, and the total mass of the upper soil in the residual mulch film area produced a pressure F_y on the mulch film. The contact area of residual mulch film had adhesion function ∑F_n with both the upper and lower soil layers, which was related to the structure of the dry-wet soil environment. As for the calculation of ∑F_n, soil moisture, soil adhesion and other factors should be comprehensively considered. When the mechanical force F_z of residual mulch film fragments worked upward according to the lifting film angle λ of mechanical film lifting device, the mechanical pulling force of residual mulch film itself must overcome soil pressure and adhesion force to complete the film lifting and realize effective recycling of residual mulch film.

When the mechanical force F_z of residual mulch film fragments worked upward according to the lifting film angle λ of mechanical film lifting device, it should be satisfied the following condition:

$$F_l>F_z\times \sin \lambda >F_y+{\displaystyle \sum F_n}$$

(1)

In Equation (1), F_l is the pulling force of plough layer residual mulch film, with the unit of N; λ is the lifting film angle; ∑F_n refers to the comprehensive influencing factors related to soil and environmental conditions, and its changes are mainly related to soil moisture, soil compactness, number of dry-wet cycles, residual mulch film and effective contact area of soil [17]. As seen from Equation (1), compared with the recycling of residual mulch film on the surface layer in the current season, the recycling of residual mulch film on the surface layer was influenced by the pressure F_y from the soil mass, and also related to the soil environmental condition ∑F_n. When the film was lifted, the soil pressure F_y was related to the residual mulch film depth, that is, the depth of mechanical operation. According to the film lifting Equation (1), the film lifting force was not only greater than the resistance of soil to the film lifting, but also less than the pulling force F_l of the film lifting, making film lifting more difficult.

Referring to the tensile force experiment of residual mulch film [18,19], the tensile force of residual mulch film varied significantly according to different residual time. For the residual mulch film with a cycle <2 years, its average pulling strength was 8.27 N, and its maximum value was 14.31 N. For the residual mulch film of plough layer with a longer residual time, its pulling strength was smaller, and film lifting became more difficult. Thus, soil loosening device should be designed in the process of mechanized operation to reduce the soil hindrance to the mulch film.

In order to test the mechanical properties of the residual mulch film and film recycling parameters, and reveal the range of mechanical force for top layer of residual mulch film, the mechanical process of film lifting of the top layer residual mulch film separated from soil was experimented. The change in film lifting mechanical force under the action of dry-wet circulation of different farming cycles was figured out. Finally, the optimal film recycling operation conditions were obtained.

3. Materials and Methods

3.1. Experiment Device and Materials

3.1.1. Experiment Device of Small Soil Trough System Film Lifting

According to drip irrigation conditions under mulch film in cotton field, a small cuboid soil trough system with size of 2000 × 1000 mm was designed, as shown in Figure 4. The soil trough was filled with 400 mm deep soil layer. A water and fertilizer supply column was designed above the soil trough. Water and fertilizer were supplied to the soil through the circulating drip pipe at the bottom of the water column to simulate the drip irrigation process under mulch film in farmland. The residual mulch film in the soil was embedded, the surface of the residual mulch film was pasted with SPATO micro sensor (SPATO SBT630 micro S-shaped tension pressure sensor, produced by Guangzhou SPATO Electronic Technology Co., Ltd., Guangzhou, China; range: 0–20 N, accuracy: ±0.001 N) in the direction of the mulch film; the other end of the sensor was pasted on the inner wall of the soil trough. Additionally, the signal line was connected with the amplifier and output to the display screen. In this way, the force variation value of the residual mulch film in the soil under the action of drying and wetting cycle was measured in real time. The film lifting test system of small soil tank system is shown in Figure 5.

3.1.2. Treatment of Experiment Samples

The soil and residual mulch film fragments for the experiment were collected from the farmland of 8th Company, 10th Regiment, Alar City, 1st Division, Xinjiang Production and Construction Corps. The residual mulch film of a 0–300 mm soil layer was obtained. A total of 100 representative residual mulch film pieces were selected after removing the soil, and set horizontally for the experiment. The soil was collected from the surface layer of a 0–300 mm of cotton field. After removing the residual mulch film and sundry in the soil, the soil texture was determined as sandy loam, and the moisture value was 18.5%. The soil was evenly packed into the soil trough. Water and fertilizer were prepared by cotton field liquid fertilizer and loaded into the water and fertilizer supply column for dry-wet cycle experiment.

3.2. Uniformity Experiment Method

Considering a lot of factors and levels have an impact on soil environment, experiment parameters were optimized through uniform experiment design method. The Uniform experiment method, proposed by Chinese mathematician Fang Kaitai and Academician Wang Yuan in 1978 [20], is an experimental optimization method for multi-factor and multi-level complex experimental factors. The residual mulch film samples were evenly buried 50 mm in the soil layer, and multiple groups of repeated samples were set up. The SPATO micro sensor on the surface of residual mulch film was connected, and then the water and fertilizer supply column was opened for uniformity experiment.

Experiment Factor Level

According to the stress analysis of residual mulch film, soil moisture, soil compactness, residual mulch film area size, film lifting angle, and dry-wet cycle times were selected as the experiment factors [21,22]. The actual field conditions of level simulation for each factor were set up, as shown in

Table 1. Design scheme for uniformity test of residual film lifting force.

Test Number	Soil Moisture/%	Soil Compactness/KPa	Residual Mulch Film Area/cm2	Film Lifting Angle/°	Number of Dry-Wet Cycles/n
1	14	100	10	45	5
2	16	140	40	70	12
3	18	180	70	20	4
4	20	220	4	50	11
5	22	260	20	75	3
6	24	90	50	30	9
7	26	120	80	60	2
8	28	160	6	80	7
9	30	200	30	40	1
10	32	240	60	65	6

.

(1)

Soil moisture

The level value of soil moisture varied according to the soil moisture condition before sowing. The interval value of 14–32% was set, and 10 level values were set up with 2% spacing between each level value.

(2)

Soil compactness

Soil compactness was used to simulate the soil environment of farmland. The soil compactness of residual mulch film embedment position was set up through compaction and other methods. And the value of soil compactness was taken and then measured by soil compactness instrument. According to actual measurement, the interval value of soil compactness in the field covered by mulch film was 90–240 Kpa, so 10 level values were set accordingly.

(3)

Residual mulch film area

According to the actual residual mulch film, 10 level values were set up for the residual mulch film area, namely, 4 cm², 6 cm², 10 cm², 20 cm², 30 cm², 40 cm², 50 cm², 60 cm², 70 cm² and 80 cm².

(4)

Film lifting angle

According to the film starting angle of residual mulch film recycling machinery, a level change value every 10° was set up from 0 to 90°. In order to simulate the mechanical tooth film lifting process, the upper end of each piece of broken film surface was connected with the SPATO micro sensor, and various types of film were set up from 0 to 90° perpendicular to the soil surface.

(5)

Number of dry-wet cycles

In accordance with the irrigation amount of cotton plantation in Xinjiang, it is necessary to conduct irrigation once (130.5 m³/ha) at the time of spring seed emergence, and afterwards irrigation twice a month (364.125 m³/ha, 12 times in total). The water supply column of water and fertilizer was prepared according to the actual situation of the farmland. Based on the actual situations, water was supplied to the soil trough from the soil surface through drip irrigation belt, with the irrigation amount of 364.125 m³/ha, and 12 times in total. The soil trough was placed in outdoor sunlight, being consistent with the farmland environment. It was considered a dry-wet cycle when water supply reached the bottom of the soil trough and the moisture in the soil was reduced to 12% of the initial water at the end of each irrigation cycle. After removing the interference times of late irrigation, the level values of 1, 2, 3, 4, 5, 6, 7, 9, 11 and 12 dry-wet cycles were set up.

(6)

Film lifting force experiment after loosening soil by cutter tooth

The residual mulch film in the soil bonded with soil after the dry-wet cycle. Due to the circulation of water, particles in the soil and the surface of the mulch film combined. As a result, it was repeatedly broken in the film lifting experiment, failing to achieve the recycling of the mulch film. Therefore, according to the film lifting mode of the residual mulch film recycling machines, the film lifting experiment of soil in the soil trough was conducted to destroy the cohesion or surface cohesion of soil formed under the action of dry-wet cycle and tractor pressure, so as to realize the film lifting recycling of residual mulch film.

The experiment was conducted in the soil trough. The sensor and residual mulch film were also set up in accordance with the above experimental method. After 1–12 dry-wet cycles, a toothed mechanical tip was designed to simulate rotary tillage and destroy the soil structure around residual mulch film. The toothed mechanical tip entered into the soil for 200 mm, and covering the range of soil 50 mm around the edge of the residual mulch film. The surrounding soil was loosened, and then measured for the film force, and the film pulling force was measured by the sensor. Finally, the recycling value when the mulch film could be normally pulled up without breaking was tested.

3.3. Experiment Indexes

(1)

Film lifting force

The effective values of film pulling force and film recycling were selected as experiment indexes. To be specific, the pulling force of film lifting was the pulling force of mulch film after the mulch film was separated from the soil according to the angle of film pulling obtained by SPATO micro sensor, with the unit of N. based on the experiment, it could be divided into film pulling force changed under unloosened soil condition and loosened soil condition.

(2)

Residual mulch film recycling rate

The residual mulch film recycling rate refers to the percentage of fully-recycled samples by film lifting force after the separation of residual mulch film in the total samples of residual mulch film in the experiment cases. The complete film lifting could be defined as follows: the complete recycling process of mulch film under film lifting force without being broken, falling off and tearing; it was considered as ineffective recycling if residual mulch film was pulled off. The recycling rate of residual mulch film could be calculated by the Equation (2):

$${\mathrm{M}}_0=\frac{m_2-m_l}{m_2}\times 100\%$$

(2)

where, M₀ is the recycling rate of residual mulch film,%; m₂ is the total number of residual mulch film before the experiment, pieces; m_l is the number of residual mulch film that was pulled off, pieces.

4. Results

4.1. Influence on the Unloosened Soil Condition on the Change in Film Lifting Force and Residual Film Recycling Rate

Table 2. Uniformity test results.

Test Number	Soil Moisture ${\mathit{x}}_1/\%$	Soil Compactness ${\mathit{x}}_2/\mathbf{KPa}$	Residual Mulch Film Area ${\mathit{x}}_3/{\mathbf{cm}}^2$	Film Lifting Angle ${\mathit{x}}_4$/°	Number of Dry-Wet Cycles ${\mathit{x}}_5/\mathbf{n}$	Mean Value of Film Lifting Force$\mathbf{y}$/N	Recycling Rate of Residual Mulch Film${\mathbf{M}}_0/\%$
1	14	100	10	45	5	8.553	90.43%
2	16	140	40	70	12	15.386	22.48%
3	18	180	70	20	4	6.748	94.61%
4	20	220	4	50	11	13.518	28.86%
5	22	260	20	75	3	6.382	95.68%
6	24	90	50	30	9	10.902	65.63%
7	26	120	80	60	2	5.784	100%
8	28	160	6	80	7	9.712	38.79%
9	30	200	30	40	1	5.665	100%
10	32	240	60	65	6	9.313	71.54%

shows the results of the uniformity experiment. Obviously, in the 50 pre-embedded residual mulch film samples, the mean lifting force of residual mulch film force all changed from 5.553 N to 13.313 N, showing an increasing trend. By contrast, the recycling rate of residual mulch film was decreasing, indicating that a larger number of residual mulch film failed to be fully pulled out, and the residual mulch film was pulled off and torn with the change in soil conditions, affecting the recycling rate of residual mulch film. The uniformity regression analysis was conducted on the results in Table 2. Additionally, the regression equation of film lifting force and film residue recycling was established through MATLAB software. Then, the optimal combination of various factors were obtained and solved according to the significance degree of partial regression coefficient, and finally the optimal parameters were obtained.

4.1.1. Analysis of Influence of Various Factors on the Change in Film Lifting Force

The data in Table 2 were imported into the MATLAB uniformity regression calculation model. The nonlinear regression model of various factors on the film lifting force was obtained, as shown in Equation (3).

$$\mathrm{Y}=78.31-8.6848x_1-0.0381x_2-0.1616x_3+0.0801x_4+5.6961x_5+0.2174x_1{}^2+0.00015x_2{}^2+0.0013x_3{}^2-0.5160x_5{}^2$$

(3)

Table 3. Variance analysis results of influence of various factors on film lifting force index.

Source of Variance	Sum of Squares	Degree of Freedom	Variance	F Value	Fα Value	Significance Level
$x_1$	0.0071	1	0.0071	0.0224	7.7086	non-significant
$x_2$	3.0847	1	3.0847	9.7063	21.1977	significant
$x_3$	0.0051	1	0.0051	0.0160		non-significant
$x_4$	3.3949	1	3.3949	10.6825		significant
$x_5$	26.1444	1	26.1444	82.2670		highly significant
Regression	113.2151	5	22.6430	71.2494	6.2561	highly significant
Surplus	1.2712	4	0.3178		15.5219
Sum	114.4863	9

shows the variance analysis results of the influence of various factors on the film lifting force index. Among them, the regression coefficient R value was 0.9944, and the equation had high fitting degree. From the aspect of the significance of the influence, the regression equation was highly significant, and there was high correlation, so it could be used to predict the influence of the film force change. Based on the significance results, the order of the influence of the factors on the film lifting force was as follows: the number of dry-wet cycles > film lifting angle > soil compactness > residual mulch film area > soil moisture. In other words, the number of dry-wet cycles was the most influential factor, followed by the film lifting angle, and soil moisture was the least influential factor. In the simulation experiment, the most common interval value of soil moisture in the spring ploughing and sowing season was selected, showing that soil moisture had the least influence on the film lifting force when the soil moisture value was 14–32%. The number of dry-wet cycles had the largest binding force for the soil and the mulch film. When the number of dry-wet cycles reached a certain value, the cohesion of soil and film was enhanced. As a result, the difficulty of film raising was increased, and the residual mulch film tended to tear and break. In order to further obtain the optimal film lifting conditions under the influence of film-lifting force, the partial derivative of regression Equation (3) was obtained, then:

$$\{\begin{array}{c}-8.6848+2\times 0.21740x_1=0\\ -0.0381+2\times 0.00015x_2=0\\ -0.1616+2\times 0.0013x_3=0\\ 5.6961-2\times 0.5160x_5=0\end{array}$$

(4)

By solving Equation (4), the optimal condition could be obtained, namely, $x_1$ = 19.97, $x_2$ = 127, $x_3$ = 62.15, $x_4$ = 39.95 and $x_5$ = 5.5.

In order words, the optimal film lifting force was obtained under the conditions of soil moisture of 19.28%, soil compactness of 127 KPa, residual mulch film area of 62.15 cm², film lifting angle of 39.95°, and the number of dry-wet cycles ≤5.5.

4.1.2. Analysis of Influence of Various Factors on the Change in Residual Mulch Film Recycling Rate

The residual mulch film recycling data in Table 2 were imported into the MATLAB homogeneity regression calculation model. And the nonlinear regression model of various factors on residual mulch film recycling was obtained, as shown in Equation (5).

$$\mathrm{Y}=73.2579+0.6915x_2-1.2381x_3-0.2863x_4-47.7024x_5+0.0184x_1{}^2-0.0024x_2{}^2+0.0135x_3{}^2+0.0067x_4{}^2$$

(5)

Table 4. Influence of various factors on residual film recovery index variance analysis results.

Source of Variance	Sum of Squares	Degree of Freedom	Variance	F Value	Fα Value	Significance Level
$x_1$	1.1637	1	1.1637	0.0276	7.7086	non-significant
$x_2$	1.9559	1	1.9559	0.0464	21.1977	significant
$x_3$	25.5716	1	25.5716	0.6068		non-significant
$x_4$	122.0977	1	122.0977	4.8974		significant
$x_5$	253.29	1	253.29	60.1054		highly significant
Regression	794.93	5	158.99	37.7280	6.2561	highly significant
Surplus	168.5611	4	42.1403		15.5219
Sum	811.79	9

shows the variance analysis results of the influence of various factors on the residual mulch film recycling index. Among them, the regression coefficient R value was 0.9896, and the equation had a high fitting degree. From the perspective of the significance of the influence, the regression equation was highly significant and highly correlated, so it could be used to analyze the influence of the change in residual mulch film recycling. Based on the significance results, the order of influence of various factors on residual mulch film recycling was as follows: number of dry-wet cycles > Film lifting angle > Soil compactness > Soil moisture > Residual mulch film area.

The partial derivative of regression Equation (5) was solved, and the following optimal solution was obtained: $x_1$ = 22.43, $x_2$ = 144.06, $x_3$ = 45.86, $x_4$ = 21.37, and $x_5$ = 4.26.

In order words, the optimal recycling rate of residual mulch film was obtained under the conditions of soil moisture of 22.43%, soil compactness of 144.06 KPa, residual mulch film area of 45.85 cm², film lifting angle of 21.37°, and the number of dry-wet cycles ≤4.26.

4.2. Analysis of Toothed Loosened Soil Condition on the Change in Film Lifting Force and Residual Film Recycling Rate

The influence test of film lifting force and recovery rate under the condition of cutter tooth scarification is shown in Figure 6.

Table 5. Results of variance analysis of influence of various factors on residual film recovery index after soil loosening.

Test Number	Soil Moisture ${\mathit{x}}_1/\%$	Soil Compactness ${\mathit{x}}_2/\mathbf{KPa}$	Residual Mulch Film Area ${\mathit{x}}_3/{\mathbf{cm}}^2$	Film Lifting Angle ${\mathit{x}}_4$/°	Number of Dry Wet Cycles ${\mathit{x}}_5/\mathbf{n}$	Mean Value of Film Lifting Force$\mathbf{y}$/N	Recycling Rate of Residual Mulch Film ${\mathbf{M}}_0/\%$
1	14	100	10	45	5	4.036	100%
2	16	140	40	70	12	4.228	97.64%
3	18	180	70	20	4	4.521	100%
4	20	220	4	50	11	5.191	100%
5	22	260	20	75	3	7.594	84.12%
6	24	90	50	30	9	6.357	94.29%
7	26	120	80	60	2	6.788	98.37%
8	28	160	6	80	7	7.143	89.18%
9	30	200	30	40	1	8.952	90.94%
10	32	240	60	65	6	6.021	100%

shows the influence on film lifting force and film recycling rate after toothed loosening soil. When the soil was loosened, the film lifting force was significantly reduced, with the lowest value of 4.036 N, and the highest value of 8.952 N. The film lifting force value was mostly within the range of the pulling force of the residual mulch film in the plough layer, so that the residual mulch film could be completely recycled without being pulled off, and the recycling rate of residual mulch film was greatly improved. Under experimental conditions, the recycling rate of residual mulch film ranged between 89.18% and 100%. Obviously, after the adhesion between residual mulch film and soil was destroyed by mechanical action, the soil cohesion was greatly reduced [23]; the mean value of film pulling force was reduced; and the recycling rate of whole film was increased. The uniformity regression analysis was conducted on the results in Table 5. The regression equation of film lifting force and film residue recycling was established through MATLAB software. Then, the optimal combination of various factors was obtained and solved according to the significance degree of partial regression coefficient, and finally the optimal parameters were obtained.

4.2.1. The Change in Film Lifting Force after Soil Loosening

The residual mulch film recycling data in Table 5 were imported into the regression calculation model of MATLAB uniformity by using MATLAB. And the nonlinear regression model of each factor on residual mulch film recycling was obtained, as shown in Equation (6).

$$\mathrm{Y}=3.5551+0.0732x_1-0.0124x_2+0.0715x_3+0.0187x_4-0.4909x_5+0.0004x_1{}^2-0.0005x_3{}^2+0.0642x_5{}^2$$

(6)

Table 6. Variance analysis results of the influence of various factors on the film lifting force after soil loosening.

Source of Variance	Sum of Squares	Degree of Freedom	Variance	F Value	Fα Value	Significance Level
$x_1$	0.4297	1	0.4297	2.5156	7.7086	significant
$x_2$	0.0223	1	0.0223	0.0785	21.1977	non-significant
$x_3$	0.0054	1	0.0054	0.0189		non-significant
$x_4$	0.0037	1	0.0037	0.0130		non-significant
$x_5$	4.6212	1	4.6212	16.2991		significant
Regression	21.9465	5	4.3893	15.4809	6.2561	significant
Surplus	1.1341	4	0.2835		15.5219
Sum	23.0806	9

shows the variance analysis results of influence of various factors on film lifting force index under the loosen soil condition. Among them, the regression coefficient R value was 0.9751, and the equation has a high fitting degree. From the aspect of significance of influence, the regression equation was significant and highly correlated. Based on the significance results, the number of dry-wet cycles and soil moisture had a significant influence on the film lifting force index in the experiment, while the other indexes had no significant effects. For these reasons, after the soil was loosened, the soil compactness, soil moisture, and the size of the residual film had fewer constraints on the film lifting effect, making film lifting easier. With the difference in the number of dry-wet cycles, however, there were different degrees of clumps in the soil. The soil clumps still had a certain bonding with the mulch film after loosening the soil, so a certain significant relationship was formed. The adhesion between soil area with high moisture and residual mulch film increased after soil loosening, which also affected the change in film lifting force.

The partial derivative of regression Equation (6) was solved, and the following optimal solution was obtained: $x_1$ = 21.82, $x_2$ = 137.52, $x_3$ = 71.5, $x_4$ = 40.5 and $x_5$ = 3.8.

In order words, the optimal recycling rate of residual mulch film was obtained under the conditions of soil moisture of 21.82%, soil compactness of 137.52 KPa, residual mulch film area of 71.5 cm², film lifting angle of 40.5°, and the number of dry-wet cycles ≤3.8.

4.2.2. Effective Recycling Rate after Soil Loosening

The residual mulch film recycling data in Table 5 were imported into the regression calculation model of MATLAB uniformity by using MATLAB. The nonlinear regression model of each factor on residual mulch film recycling was obtained, as shown in Equation (7).

$$\mathrm{Y}=85.2956+1.9770x_1-0.1047x_3-0.4207x_4-0.0240x_1{}^2-0.0003x_3{}^2+0.0038x_4{}^2-0.1436x_5{}^2$$

(7)

Table 7. Variance analysis results of the influence of various factors on the recovery rate of residual film after soil loosening.

Source of Variance	Sum of Squares	Degree of Freedom	Variance	F Value	fα Value	Significance Level
$x_1$	2.3412	1	2.3412	0.5389	7.7086	non-significant
$x_2$	7.8105	1	7.8105	1.7978	21.1977	non-significant
$x_3$	4.249	1	4.249	0.9780		non-significant
$x_4$	32.5466	1	32.5466	7.4915		significant
$x_5$	132.9778	1	132.9778	30.6084		significant
Regression	268.1219	5	53.6244	12.3431	6.2561	significant
Surplus	17.3780	4	4.3445		15.5219
Sum	285.4998	9

shows the variance analysis results of the influence of various factors on the residual mulch film recycling index. Among them, the regression coefficient R value was 0.9691, and the equation had a high fitting degree. From aspect of the significance of the influence, the regression equation was significant, which could be used to analyze the influence of the change in residual mulch film recycling. Based on the significance results, the order of influence of various factors on residual mulch film recycling was as follows: number of dry-wet cycles > film lifting angle > soil firmness > residual mulch film area size > soil moisture.

The partial derivative of regression Equation (7) was solved, and the following optimal solution was obtained: $x_1$ = 23.18, $x_2$ = 132.51, $x_3$ = 64.5, $x_4$ = 45.35 and $x_5$ = 3.89.

In other words, the optimal recycling rate of residual mulch film was obtained under the conditions of soil moisture of 23.18%, soil compactness of 132.51 KPa, residual mulch film area of 64.5 cm², film lifting angle of 45.35°, and the number of dry-wet cycles of ≤3.89.

5. Discussion

5.1. Analysis of the Influencing Factors of Film Lifting Force

The film lifting force is the key index of residue film recycling. The expectation of the design recycling was that the lifting force could realize the separation of the residual mulch film from the soil [24], overcome the soil mass gravity and soil adhesion, and also ensure that the value of the lifting force was not too large because too large of a lifting force could cause the residual mulch film to be pulled off, and unable to realize the recycling.

Under the condition of unloosened soil, soil compactness, film lifting angle and the state of the dry-wet cycle all had significant influence on the film lifting force. According to the solution of regression equation, the optimal film lifting force was obtained under the condition of soil firmness of 127 KPa, film lifting angle of 39.95°, and the number of dry-wet cycles of 5.5. Under the condition of loosen soil, the dry-wet cycle and soil moisture had significant influence on the film lifting force, while the other factors did not have significant influence. For these reasons, the soil compactness, soil moisture and the size of residual mulch film had less constraint on the film lifting effect after loosening soil, which made it easier to lift residual mulch film. With the difference in the number of dry-wet cycles, however, there were different degrees of clumps in the soil. The soil clumps still had a certain bonding with the mulch film after loosening the soil, so a certain significant relationship was formed. Under loose soil conditions, the optimal film lifting force was obtained under the conditions of soil moisture of 21.82%, film lifting Angle of 40.5°, and the dry-wet cycles were less than 3.8 times. The film forming force was affected by many factors, including soil moisture, soil compactness and the number of wet-dry cycles. Among them, the soil condition under the influence of wet-dry cycles was the biggest factor limiting the film lifting force. Therefore, in the process of mechanical recycling, it is suggested to first design a soil-loosening machine according to the situation to destroy the soil condition, and then to recycle the mulch film.

5.2. Analysis of the Optimal Interval Value of Residual Mulch Film Recycling Machinery

The recycling rate of residual mulch film is the most direct index to evaluate the recycling effect of residual mulch film. In the regression analysis of soil trough experiment, the dry-wet cycle and film-raising angle had the most significant influence on the recycling rate of residual mulch film. According to the results of the uniformity regression equation, the optimal recycling rate of residual mulch film for unloosened soil was obtained under the conditions of film lifting angle of 21.37° and the number of dry-wet cycles of 4.26; the optimal recycling rate of residual mulch film for loosened soil was obtained under the conditions of film lifting angle of 45.35° and the number of dry-wet cycles of less than 3.89. The recycling rate of residual mulch film increased and the film lifting force decrease after the soil was loosened. The optimal film lifting effect could be obtained when film lifting angle range was 21.37–45.37°. The number of dry-wet cycles affected soil moisture and soil compactness. The optimal film recycling effect could be obtained when soil moisture was 22.43–23.18% and soil solidity was 132.51–144.06 Kpa. In addition, according to the calculation, a better residual mulch film recycling could be obtained when the residual mulch film area was 45.85–64.5 cm². All the above was based on the experiment results the limited soil condition. Due to many test factors, compared with the orthogonal test design adopted by Xuenong Wang [25] and the response surface regression test method adopted by Jianhua Xie [26], the uniformity test method adopted in this paper reduces the number of tests and shortens the test cycle. However, larger fragments of mulch film were more advantageous to recycling in the actual process of recycling. For these reasons, in the experiments, the residual mulch film was precisely lifted through the sensor, and only the situation after film pulling off was considered in the mulch film recycling. When mechanical recycling was adopted for the residual mulch film, it was more difficult for the mechanization blade to precisely lift the smaller fragments of mulch film. In order to obtain a better film recycling effect, it is necessary to study the relationship between film recycling and rotary tillage with cutter tooth arrangement density.

6. Conclusions

According to the distribution characteristics of residual mulch film, an experimental device of residual mulch film raising soil trough is designed, and the homogenization design experiment of film lifting force is conducted. The analyses of the experiment results are as follows: (1) Under the condition of unloosened soil in the soil trough experiment environment, soil firmness, film lifting angle, dry-wet cycle and other factors have a significant influence on the film lifting force. The dry-wet cycle and film lifting angle have a significant influence on the recycling of residual mulch film, indicating that the dry-wet cycle affects the soil structure, increasing the difficulty of film lifting. Under the condition of loose soil, the dry-wet cycle and soil moisture have significant influence on film lifting force and film recycling rate. Therefore, in the design of residue film recycling machine of a plough layer, a soil lifting machine and tools should be designed according to the situations. The soil conditions should be destroyed first before conducting the film lifting recycling. (2) The experimental results show that the film lifting force decreases and the residual mulch film recycling rate increases significantly after soil loosening. According to the uniformity regression analysis, the optimal film recycling effect can be obtained under the condition of film lifting angle of 21.37–45.37°, and the number of dry-wet cycles <3.8, soil moisture of 22.43–23.18%, soil solidity of 132.51–144.06 KPa, and residual mulch film area of 45.85–64.5 cm². (3) Mechanical film is different from the film lifting method through sensors to connect broken films of different sizes in the experiment. If mechanical film lifting method is used, it is difficult for cutter teeth to precisely obtain the smaller broken mulch film. Thus, it is necessary to rely on the arrangement density of cutter teeth to obtain higher recycling rate of residual mulch film.