Bionic Design of Furrow Opener Based on Muskrat Claw-Toe Structure to Improve the Operating Performance of Deep Application of Liquid Fertilizer in Paddy Fields in Cold Region of China

Abstract

The deep application of liquid fertilizer in paddy fields is a fertilization technique that applies liquid fertilizer deep near the root system of paddy field crops, which can effectively improve the absorption rate of the crops and reduce the amount of fertilizer applied. In the cold regions of China, the soil return rate of the furrowing operation of the deep application of liquid fertilizer in paddy fields is low, which can easily cause the excessive liquid leakage of fertilizer and affect crop growth. Therefore, it is difficult to popularize in large areas. According to the characteristics of paddy soil in the cold regions of China and the operating requirements of a high backfill rate and low disturbance rate of the soil of the deep application of liquid fertilizer, this paper designed a bionic liquid fertilizer deep application furrow opener based on the claw-toe structure of the muskrat. In this study, an indoor soil bin test was conducted by constructing a deep application environment for the liquid fertilizer in paddy fields. The results of the soil bin test showed the effects of the key operating parameters of the bionic design of the liquid fertilizer deep application furrow opener, spraying pressure of the liquid fertilizer and operating speed on the furrowing resistance, soil disturbance rate and the leakage amount of liquid fertilizer. The bionic design of the liquid fertilizer deep application furrow opener has a low soil disturbance rate and leakage amount of fertilizer when the operating speed is 0.8 m s⁻¹, and the spraying pressure is 0.2 MPa. This furrow opener significantly improves the operating performance of the deep application of liquid fertilizer in the cold regions of China and is suitable for the deep application of liquid fertilizer in the paddy fields of this region.

1. Introduction

Rice is one of the major food crops [1,2]. Heilongjiang Province is an important national grain production base in China with grain output accounting for approximately a quarter of China’s national output [3]. Due to the unreasonable application of chemical fertilizers in the rice production and planting process in recent years, some black soil resources have been seriously degraded, resulting in a decrease in black soil layer thickness and organic matter content, soil acidification and a decline in soil fertility and productive potential [4]. Therefore, it is necessary to carry out scientific and reasonable fertilization operations to protect and utilize these valuable black soil resources.

Liquid fertilizer deep application technology in paddy fields is a fertilization technology that applies liquid fertilizer deep near the root system of paddy field crops, which is an ideal fertilization mode for rice planting in the cold regions of China, and the study of liquid fertilizer deep application technology is of great significance for rice planting and black land conservation [5]. Advanced fertilization techniques can improve grain yield, reduce fertilizer application amounts, save costs and reduce pollution to the ecological environment [6]. Liquid fertilizer has many advantages that solid fertilizer does not have, such as easy absorption, high fertilizer efficiency, low cost, low pollution and improved grain quality [7]. Alam et al. [8] in Bangladesh compared the fertilization of rice with liquid fertilizer and solid fertilizer with the same nitrogen content. Compared with solid fertilizer, the application of liquid fertilizer increased the yield by 10.5% and saved 25% of the nitrogen fertilizer. The high soil disturbance rate caused by the traditional liquid fertilizer deep application technology furrow opener in paddy fields resulted in a high leakage amount of liquid fertilizer. The high furrowing resistance will cause excess power consumption, and the excessive soil disturbance will lead to excessive leakage of liquid fertilizer and negative effects on the growth of paddy field crops. For the above reasons, the deep application of liquid fertilizer in paddy fields cannot be popularized and applied in rice planting in the cold regions of China. The deep application of nitrogen in the root zone can effectively solve the problem of fertilizer loss caused by conventional liquid fertilizer application [9]. Chen [10] et al. designed the overall structure of a pneumatic ejection type liquid fertilizer deep application device according to the characteristics of the paddy field operational environment and the technical requirements of mechanical fertilization. At the same time, they designed an imitation sliding knife type furrow opener suitable for paddy field environments, which reduced the fertilizer jam phenomenon, the furrowing resistance of the furrow opener, and energy consumption, and improved the reliability of the work of the machine compared with the traditional fertilizer deep application machine in paddy fields. However, the device cannot realize the liquid fertilizer deep application back to the soil after mulching, which can easily cause liquid fertilizer leakage, resulting in low liquid fertilizer absorption efficiency.

Bionic design has many advantages. With the deepening of research on agricultural design, bionic design has been widely used in agricultural machinery, and good results have been achieved. Zhao [11] et al. established a mathematical model of the fitting curve according to the streamlined curve of the sailfish head and designed a curve-shaped furrow opener imitating the sailfish head, which reduced the operating resistance of the seeding furrow opener and reduced its disturbance to the soil. Zhang [12] et al. designed a bionic furrow opener by extracting the biological profile curve of the claw toe of the soil species mole cricket and fitting the curve, and then conducted simulation tests. The maximum stress of the bionic furrow opener acting on the soil was reduced by 56.32%, which improved the efficiency of the furrow opener. Wang [13] et al. designed a bionic sturgeon liquid fertilizer deep application furrow opener based on the flow curve of the sturgeon body combined with bionics and verified the mechanism of low furrowing resistance and effective soil backfill of the bionic structure with indoor soil bin tests. The above scholars simulated the appearance features of organisms through curve extraction and fitting but did not achieve a comprehensive replication of the geometric features of the biological structures. The resistance and friction reduction performance of low-resistance animals in soil have always been the focus of research on furrow openers. Sun [14] et al. designed a new type of furrow opener based on the low-resistance characteristics of the bear claw and verified the accuracy of the model through EDEM. They obtained the motion characteristics of the bionic furrow opener of the bear claw and verified that the design of the furrow opener has low resistance. Xue [15] et al. took the head convex hull of dung beetles and the ridged hump of the back of pangolins as the bionic prototype, studied and analyzed the drag reduction characteristics of the non-smooth morphology of its surface and constructed the bionic coupling element combination applied to the surface of the furrowing disc to reduce the operating resistance of the furrowing disc during the furrowing process. The above research is all about the furrowing operation of a dry field, but there is limited research on the furrowing operation of paddy fields.

In this study, the claw-toe structure of the muskrat, which are widely distributed in the cold regions of China, was used as the bionic prototype. The muskrat is the only member of the genus muskrat, which is widely distributed in the cold northeast region of China and has a body length of 35~40 cm and tail length of 23~25 cm. It has short limbs, 4-toed forefeet, and sharp claws, and is good at building holes in water; its claws and toes have the characteristics of good drag reduction and low disturbance [16]. The outer profile of the claw-toe of the muskrat was extracted with a 3D contactless reverse scanner, and a 3D digital design was carried out. The surface structure of the organism with its low resistance and low disturbance characteristics were completely extracted and optimized, which was then manufactured through 3D-printing technology. A new bionic liquid fertilizer deep application furrow opener was designed for the deep application of liquid fertilizer in the paddy water environment. Meanwhile, the soil bin environment of the paddy field (126°58′31″ N, 45°32′29″ E) in Acheng District of Harbin City, Heilongjiang Province, China was constructed, and the indoor soil bin test was carried out to explore the interaction between the bionic liquid fertilizer deep application furrow opener and the soil and water layer; single-factor tests were conducted with spraying pressure and the operating speed of the liquid fertilizer as the test factors and soil disturbance and the liquid leakage amount of fertilizer as the test indicators, respectively, and the internal mechanism of the bionic structure affecting the operating performance of the liquid fertilizer deep application furrow opener was analyzed.

In this study, we designed an opening and fertilizer application device through a muskrat paw-toe bionic and used the low disturbance rate of the bionic furrow opener and high backfill rate of the soil to improve the absorption of liquid fertilizer, reduce fertilizer loss, improve rice production advantages and increase rice yield. At the same time, it can promote liquid fertilizer deep application technology, reduce the amount of chemical fertilizer applied in local areas and protect the precious black land resources.

2. Materials and Methods

The paw toe of the muskrat was selected as the bionic prototype to extract biological features, three-dimensional reverse processing was carried out, and the design was optimized. According to the agronomic requirements of rice fertilization in Heilongjiang Province, the size of the openers was set, and the structure of the openers was designed. The indoor soil bin and the test bench were built. The operating speed and liquid fertilizer spraying pressure were selected as the test factors, and the ditching resistance, soil disturbance rate and liquid fertilizer leakage were used as the test indicators to carry out the indoor soil bin test.

2.1. Shape Design of the Bionic Liquid Fertilizer Deep Application Furrow Opener

2.1.1. Extraction of Bionic Prototype

The claw-toe structure of the muskrat is shown in Figure 1a. In this study, naturally dead muskrat from the artificial breeding of Shahezi Xinke farm in Wuchang City, Harbin, Heilongjiang Province, China was used. The biological features were extracted using an ATOS Ⅱ SO non-contacting 3D scanner manufactured by COM, Germany, as shown in Figure 1b. After obtaining the claw-toe point cloud data, the Geomagic Design X software was used to carry out the point cloud smoothing and miscellaneous point elimination optimization, and finally, an editable 3D model of the muskrat claw toe was obtained, as shown in Figure 1c.

2.1.2. Design of the Bionic Liquid Fertilizer Deep Application Furrow Opener

To realize the deep application of liquid fertilizer in paddy fields, the secondary design of the obtained 3D model of the claw toe of the muskrat was carried out. In order to ensure that the fertilizer spraying pipe could spray liquid fertilizer in time during the fertilization operation, the structure of the fertilizer spraying pipe was designed so that it could be attached to the furrow opener, and the adjustable disassembling and assembling rack was designed. The mounting hole of the bionic liquid fertilizer deep application furrow opener was used to install the whole furrow opener on the load measuring rack, and the depth of the furrow opener could be adjusted up and down. The connecting hole was fixed to fix the furrow opener at the lower end of the share shaft. The quick-plug connector of the fertilizer spraying pipe was used to connect the fertilizer hose, and the fertilizer spraying pipe was used to spray the liquid fertilizer. The overall height of the furrow opener was 109 mm, the left and right width was 82 mm and the design working height of the furrow opener was 70 mm [17]. The actual height of the furrow could be adjusted through the mounting hole. The structure diagram of the bionic liquid fertilizer deep application furrow opener is shown in Figure 2. The bionic ditch opener adopted 3D-printing technology, and its main material was photosensitive resin. The quick plug and the fertilizer spraying tube at the lower end were made of stainless steel with a mass of 112 g and a gravity of 1 N. The gravity brought by the opener’s own weight had little effect on the soil disturbance rate compared with the resistance. At the same time, the position of the opener on the horizontal plane remained basically unchanged during the trenching process, so it could be ignored. The influence of the weight on the soil disturbance rate during ditching was examined.

2.2. Indoor Soil Bin Tests

The indoor soil bin environment was built, and rice seedlings with good growth for transplanting were selected so that the furrowing and fertilization environment was consistent with the actual field operation environment. The test bench was built, the soil bin test was conducted, the instruments were selected, and the chemical reagents to measure the test indicators were selected.

2.2.1. Soil Bin Construction

In order to explore the operating performance of the bionic liquid fertilizer deep application furrow opener, an indoor soil bin was constructed for the bench performance test at the Agricultural and Animal Husbandry Machinery Laboratory of Northeast Agricultural University, Harbin, Heilongjiang Province, China (126.72′62″ N, 45.74′16″ E). The soil bin was formed by gluing acrylic plates with structural dimensions of 1600 × 450 × 500 mm and a wall thickness of 8 mm. The depth of the soil in the soil bin was 500 mm, and the height of the water layer was 300 mm [18]. The travel of the bionic liquid fertilizer deep application furrow opener in the soil bin was 1200 mm, including a 350 mm operating preparation area, a 450 mm soil disturbance rate measurement area and liquid leakage amount of fertilizer measurement area, and a 350 mm stop area, as shown in Figure 3. The soil in the soil bin was selected from the experimental paddy field of Acheng Base in Harbin, Heilongjiang Province, China. Longjing 35 rice seedlings were selected and artificially transplanted into the test soil bin, and the spacing between the rows of rice and plants was ensured to be 300 mm and 150 mm, respectively. The tested rice seedlings were tillering stage seedlings with good growth and no pests and diseases.

The test bench was mainly composed of the test bench car, the bionic liquid fertilizer deep application furrow opener, the hydraulic system for the liquid fertilizer deep application and the acrylic soil bin. The test bench for the liquid fertilizer deep application is shown in Figure 4.

2.2.2. Soil Bin Test Design

The main factors affecting the operating performance of liquid fertilizer deep application are operating speed and spraying pressure [19]. The experimental factors in this study were selected according to the actual situation of the 6-row, high-speed rice transplanter commonly used for side-deep fertilization in the paddy fields of the cold area of Northeast China with an operating speed of 2.52–4.32 km h⁻¹ [20,21] as well as the agronomic requirements of rice fertilization amount and relevant scholars of the South China Agricultural University research on liquid fertilizer jet fertilization [10,22,23]. In this study, the operating speed (2.16~3.60 km h⁻¹) and liquid fertilizer spraying pressure (0.04~0.2 MPa) were selected as experimental factors, and the single-factor test was carried out under the operating environment of 30 mm at the root side and 50mm deep [24], and the influence rule was analyzed based on the test results.

Furrowing resistance, soil disturbance rate and leakage amount of liquid fertilizer were selected as the test indicators to evaluate the energy consumption of the furrowing, soil return effect and operating quality and efficiency of the whole liquid fertilizer deep application system [25,26,27].

• Furrowing resistance P

The furrowing resistance P is an important indicator of the energy consumption of the machine operation [28], and the DS2 digital display push–pull tester (Dongguan Zhiqu Precision Instrument Co., LTD., Guangzhou, Guangdong Province, China) was selected in this study to measure the furrowing resistance of the bionic furrow opener.

• Soil disturbance rate ρ

The soil disturbance rate ρ is an important indicator of the soil disturbance behavior of the furrow opener [29], and a larger disturbance rate indicates a greater amount of soil disturbance during operation. To measure the gully profile and soil disturbance rate ρ after the operation of the bionic furrow opener, 21 tracer blocks in 3 colors with dimensions of 10 × 10 × 8 mm and a similar density to the soil were placed in the soil [30,31]. Each tracer block was numbered and arranged equally and uniformly in 3 layers in a measurement area with a soil disturbance rate of 100 × 100 × 60 mm. The coordinates of the origin (0, 0, 0) were set, the initial coordinates (x, y, z) of each layer of the tracer blocks were recorded, and the position coordinates of each tracer block after disturbance at the end of the furrowing operation were measured, as shown in Figure 5, to draw a gully profile map [32,33]. The soil disturbance rate is the ratio of the disturbed soil area in the gully profile to the product of the furrowing width and furrowing depth, which was calculated according to Equation (1),

ρ = (A₁ + A₂)/dh × 100%

(1)

where A₁ is the gully collapse area, mm²; A₂ is the gully profile area, mm²; h is the gully depth, mm; and d is the gully width, mm.

• Liquid leakage amount of fertilizer amount V

The leakage amount of liquid fertilizer V refers to the amount of liquid fertilizer that leaks in the water layer after the furrowing operation without being buried in the gully soil by the soil backfill [34,35,36]. The leakage amount of fertilizer was indicated by measuring the content of nitrogen in the water. After the furrowing operation was completed, a syringe was used to inhale the water solution in the water layer directly behind the furrow opener, the same volume of mixed water solution was injected into a test tube, the urea-detection reagent was added [37] (Hangzhou Luheng Biological Company, Jinhua City, Zhejiang Province, China), and it rested for 10 min; then, it was compared with the urea content color card, which can measure the urea content in the water layer after furrowing more accurately, and the leakage amount of fertilizer was determined. At the same time, after each operation, the water solution in the soil bin was cleaned and replaced with a new water solution while keeping the volume of the soil and water in the soil bin basically unchanged.

2.3. Indoor Soil Bin Test Design

The indoor soil bin test was based on the analysis of the operation process of the fertilizer spraying system with the liquid fertilizer spraying pressure (0.04 MPa, 0.08 MPa, 0.12 MPa, 0.16 MPa and 0.20 MPa) and operating speed (2.16 km h⁻¹, 2.52 km h⁻¹, 2.88 km h⁻¹, 3.24 km h⁻¹ and 3.60 km h⁻¹) as the test factors. The furrowing resistance P, soil disturbance rate ρ and liquid leakage amount of fertilizer V were the test indicators, and each group of horizontal tests was repeated three times; the average value was taken as the test result to evaluate the operating performance of the bionic liquid fertilizer deep application furrow opener.

3. Results and Discussion

Through the indoor soil bin test, the ditching resistance at different operating speeds was recorded, the curves of the operating speed and ditching resistance were drawn, and the influence of the operating speed on the ditching resistance was discussed; according to the changes in the front and rear spatial displacement of the tracer block at different speeds [38,39], its contour curve on the YOZ plane was drawn, and the effect of operation speed on the soil disturbance rate was discussed; after ditch fertilization, the fertilizer leakage at the same speed and different pressures and the same pressure and different speeds was measured, the fertilizer leakage curves with speed and pressure changes were drawn, and the influence of the operating speed and liquid fertilizer spraying pressure on liquid fertilizer leakage was discussed.

3.1. The Effect of Operating Speed on Furrowing Resistance

Figure 6 shows the resistance of the bionic liquid fertilizer deep application furrow opener during operation at different speeds. With an increase in speed, the resistance of the furrow opener increases. When the speed is between 2.16 km h⁻¹ and 2.88 km h⁻¹, the resistance of the furrow opener tends to increase smoothly, but when the speed exceeds 2.88 km h⁻¹, the resistance of the furrow opener increases sharply with the increase in speed. The furrowing resistance between adjacent operating speeds has not been measured experimentally, so the relationship between the operating speed and ditching resistance is a straight and dotted line.

Analysis of Furrowing Resistance at Different Speeds

During the operation of the bionic liquid fertilizer deep application furrow opener, the furrow opener is subjected to the resistance of the soil as well as the water layer. The furrowing resistance at different speeds was analyzed to set the operating speed of the bionic liquid fertilizer deep application furrow opener, reduce energy consumption and improve the efficiency of the furrowing operation.

When the furrow opener works in the field, it can be considered to be in uniform linear motion. Observed from the overhead direction, there is a relative velocity v between the bionic furrow opener and soil particles, which can be decomposed into v_x and v_y along the coordinate axis, as shown in Figure 7, where v_x is all converted into horizontal resistance F₁, which hinders the forward motion of the furrow opener.

$$F1=ma=m\frac{v_x}{t}=m\frac{v\cos \alpha }{t}$$

(2)

The bottom profile curve is differentiated to obtain the function of cosα with respect to x [40],

$$\tan \alpha =\frac{dx}{dy}$$

(3)

$$\cos \alpha ={\left(\frac{1}{{(\frac{dy}{dx})}^2+1}\right)}^{\frac{1}{2}}$$

(4)

When soil particles are in contact with the surface of the furrow opener, α is the tangential inclination angle at the point where the soil particles are located, that is, tanα is the differential of the function at dy, and the resistance F₁ of a single soil particle on the bionic furrow opener is

$$F_1=m\frac{v}{t}{\left({\left(\frac{dy}{dx}\right)}^2+1\right)}^{-\frac{1}{2}}$$

(5)

During the operation of the furrow opener, the resistance applied in the horizontal direction is the sum of the forces applied at each point, and F₁ is integrated within the value range of y. ${\sum }^{ }{F}_1$ is the resultant force of the soil particles on one side of the bionic furrow opener,

$${\displaystyle \sum F_1=}{\displaystyle {\int }_0^y{F}_1}dy={\displaystyle {\int }_0^{y}m\frac{v}{t}{\left({\left(\frac{dy}{dx}\right)}^2+1\right)}^{-\frac{1}{2}}dy}$$

(6)

where t is the time required for the velocity change in the soil particle, s; a is the acceleration of the soil particle, m s⁻²; v is the velocity of the soil particle along the surface of the furrow opener, m s⁻¹; and m is the mass of the soil particle, kg.

The forward resistance of the furrow opener is mainly composed of the front soil resistance, the water layer resistance and the bottom soil friction resistance [41]. The resistance of the furrow opener at different speeds is shown in the figure. Under the condition of 50 mm depth, the speed increases from 2.16 km h⁻¹ to 3.60 km h⁻¹. The test results show that when the operating speed increases, the horizontal resistance of the furrow opener increases with the increase in operating speed. For the front soil resistance, as the speed increases, the soil reaction force also increases. At the same time, the furrow opener working in the soil bin cement mixture is equivalent to that in the fluid. When the relative speed v is small, the magnitude of resistance f is proportional to v:

$$f=kv$$

(7)

where f is the friction force, N; and k is the drag coefficient.

In the equation, the proportionality coefficient k is determined by the size and shape of the object and the properties of the fluid. In this test, the size and shape of the furrow opener and the properties of the fluid did not change, so when the furrow opener moves in the fluid, the resistance it receives increases with the increase in the speed [42]. Increasing speed increases the pressure between the bottom of the furrow opener and the soil, which in turn leads to an increase in friction. Therefore, as the speed increases, the resistance of the furrow opener gradually increases.

3.2. The Effect of Operating Speed on the Gully Profile and Soil Disturbance Rate

The spatial position of the tracer block before and after ditching at different speeds was recorded, and the gully contour curve on the YOZ plane was drawn. According to the soil disturbance rate formula, the soil disturbance rate of the opener at different speeds was calculated, the curve of the soil disturbance rate following the speed change was drawn, and the soil disturbance behavior of the opener was theoretically analyzed.

According to the analysis in Figure 8, with an increase in operating speed, the furrow width gradually increases, and the furrow profile slightly expands. The reason is that with an increase in operating speed, the disturbance behavior of the furrow opener on the soil is dominated by soil lateral throwing. At low speed, the soil disturbance behavior of the furrow opener is mainly to squeeze the soil and compress and bond the soil. The soil–water mixture during furrowing is subjected to less force, and small displacement occurs.

As shown in Figure 9, the furrowing operation was carried out at speeds of 2.16 km h⁻¹ to 3.60 km h⁻¹, and the gully profile curve was plotted using the displacement changes in the tracer block before and after the operation in the YOZ plane. Figure 9 shows the effect of the operating speed on the soil disturbance rate. When the speed is between 2.16 km h⁻¹ and 2.88 km h⁻¹, the disturbance rate of the furrow opener to the soil decreases with the increase in speed. The disturbance rate of the furrow opener to the soil increases with the increase in speed when the speed is between 2.88 km h⁻¹ and 3.24 km h⁻¹, and decreases with the increase in speed when the speed is 3.24 km h⁻¹ to 3.60 km h⁻¹; when the speed is 2.88 km h⁻¹, the soil disturbance rate is the lowest, which is 63%. The soil disturbance rate between adjacent operating speeds has not been measured and calculated experimentally, so the relationship between the operating speed and soil disturbance rate is a straight and dotted line.

Soil Backfilling Rate at Different Speeds

When the furrow opener is fully buried during operation, the soil is disturbed mainly by in situ lifting and soil out-throwing [43,44]. As shown in Figure 10, S₁ and S₂ are the cross sections of the furrow formed by the out-thrown soil, S₃ is the cross section of the soil lifted in situ, and S₄ is the cross section of the fertilizer furrow. At this time, the disturbed soil will not flow back due to the obstruction of the furrow opener.

The volume of soil is conserved during the operation of the furrow opener [45], so the cross-sectional area of the soil is conserved, and the cross-sectional soil before returning to the soil satisfies:

$$S_4=S_1+S_2+S_3$$

(8)

where S₄ represents the cross-sectional area of the gully profile, mm²; S₁ and S₂ represent the cross-sectional area of the furrow ridge, mm²; and S₃ represents the cross-sectional area of the soil in situ lifted area, mm².

The soil reflux is shown in Figure 10. As the furrow opener travels, the lifted soil slides down the slope to the rear, filling the fertilizer furrow and burying the liquid fertilizer. The outthrown soil will accumulate according to the natural accumulation angle of the soil, and some of the outthrown soil will flow back to the fertilizer furrow [46].

Combining Figure 10 and Figure 11, it can be obtained that after soil reflux, the cross-sectional soil should satisfy:

$$S_7=S_3+S_5+S_6$$

(9)

where S₇ represents the cross-sectional area of the soil in the backfilled area, mm²; S₅ and S₆ represent the cross-sectional area of the reflux soil of the furrow ridge, mm²; and S₈ represents the cross-sectional area of the soil in the water erosion area after furrowing and fertilization, mm².

The soil disturbing behavior of the bionic liquid fertilizer deep application furrow opener is inevitable. According to the analysis of its disturbing behavior and the soil returning principle, the returned soil is mainly in the S₃, S₅ and S₆ areas [47], and the filled area is mainly the S₇ area, but it is difficult to quantify the returned soil. Therefore, the return depth h₂ in the figure is defined as the indicator to measure the soil returning performance of the efficient return liquid fertilizer furrow opener. When the return depth h₂ is higher than the depth h₃ of the lower fertilizer furrow, that is h₂ > h₃, the soil return performance of the furrow opener meets the design requirements. The soil in the S₈ area was migrated due to the flow of water after furrowing.

With an increase in the speed, the initial velocity of the soil lateral throwing movement increases, the horizontal distance of the soil lateral throwing becomes larger, and the soil–water mixture is subjected to a large force during the furrowing process and bigger displacement occurs, resulting in the soil in the cross-section of the furrow ridge formed by the soil throwing away from S₁ and S₂ difficult to fall back. After the furrowing is completed, since the gravity of the soil–water mixture itself and the pushing force of the water are far less than the force on the soil during the furrowing process, most of the soil deviates from the original position when the soil flows back, and the cross-sectional area of the gully profile increases. However, due to the increase in the operating speed of the furrow opener, the relative speed with the soil and water layer increases, the reaction force on the soil and water layer increases, the water flow speed and soil flow speed increases, and the soil backfill speed accelerates. Therefore, with the increase in the speed, the soil disturbance rate increases, and the furrowing backfill effect gets steadily worse.

3.3. The Effect of Operating Speed and Liquid Fertilizer Spraying Pressure on the Leakage Amount of Liquid Fertilizer

The fertilizer leakage at the same speed and different pressures and the same pressure and different speeds was measured, the curve of the fertilizer leakage with speed and pressure was drawn, and the influence of the operating speed and liquid fertilizer spraying pressure on the liquid fertilizer leakage was analyzed.

As shown in Figure 12, with an increase in the pressure of the liquid fertilizer deep application system at the same speed, the overall leakage amount of fertilizer shows a decreasing trend. The greater the pressure, the smaller the leakage amount of fertilizer. The liquid fertilizer leakage between adjacent fertilizer spraying pressures at the same operating speed was not measured, so the relationship curve between the same operating speed and different liquid fertilizer spraying pressures and liquid fertilizer leakage is a straight and dotted line.

As shown in Figure 13, under the same pressure with an increase in speed, the overall leakage amount of fertilizer shows a decreasing trend, and the higher the speed, the smaller the leakage amount of fertilizer. The liquid fertilizer leakage between adjacent operating speeds at the same liquid fertilizer spraying pressure was not measured, so the relationship curve between different operating speeds and liquid fertilizer leakage at the same liquid fertilizer spraying pressure is a straight and dotted line.

Leakage Amount of Fertilizer under the Same Pressure and Different Speeds

During the furrowing operation, the liquid fertilizer jet strikes the soil surface at a certain inclination angle and loses part of its momentum, and this part of the energy is transferred to the damaged soil surface in the form of action force [48], as shown in Figure 14. When the pressure at the outlet of the fertilizer spray tube is small, the liquid fertilizer is sprayed out and cannot destroy the soil surface due to the blocking effect of the water flow and the soil, and the liquid fertilizer remains in the water layer. As the pressure increases, part of the jet is shot into the soil and retained in it. Therefore, the analysis of the size of the outlet pressure of the fertilizer spray pipe and the critical ground-breaking pressure is of guiding significance for the measurement of the leakage amount of fertilizer [49]. The force of the continuously submerged liquid fertilizer jet on the soil surface during the furrowing operation is F, and this force is stable.

According to the conservation of momentum law, we get

$$F\mathrm{\Delta }t=m{v}_0-mv{\prime }_0$$

(10)

where m is the mass of the liquid fertilizer, kg; v₀ is the flow rate of the liquid fertilizer jet before impacting the soil, m s⁻¹; and v’₀ is the flow rate of the liquid fertilizer jet after impacting the soil, m s⁻¹.

Assuming that the angle of change in the forward direction before and after the impact of the liquid fertilizer jet on the soil is φ, and the size is constant and equal to v, the force of the liquid fertilizer jet acting on the soil surface is

$$F=\rho Q{v}_0-mQv{\prime }_0\cos \theta =\rho Q{v}_0（1-\cos \phi ）$$

(11)

$$Q=\pi R^2{v}_0$$

(12)

where ρ is the density of the liquid fertilizer, kg m⁻³; Q is the flow rate of the liquid fertilizer, m³ s⁻¹; Φ is the angle of change in the forward direction before and after the impact of the liquid fertilizer jet on the soil, (°); and R is the diameter of the liquid fertilizer pipe, mm.

For the continuous jet, the relation between the nozzle outlet pressure p₀ and jet velocity v follows

$$p_0=\frac{1}{2}\rho v_0^2$$

(13)

Therefore, it is easy to obtain the soil per unit area force $\overline{F}$ of the soil at the fertilizer spraying outlet

$$\overline{F}=\frac{F}{S_{}}=2p_0（1-\cos \phi ）$$

(14)

where p₀ is the nozzle outlet pressure, MPa.

The erosion effect of the submerged water jet on the soil layer where the rice roots are located is not only related to the parameters of the water jet but also closely related to the physicochemical properties of the eroded soil itself. According to the literature [50], the critical failure pressure F_cr of the soil per unit area is related to parameters, such as soil shear strength, permeability, soil particle size, and soil density, that is

$$F_{cr}=\varsigma {\tau }_f^2{(\frac{d_{60}}{k})}^{-2}{\gamma }_d^{-1}$$

(15)

where ζ is the correction coefficient, determined with the test, ζ = 1.8 × 10¹⁰; τ_f represents the soil shear strength, kPa; d₆₀ represents the limited particle size of the soil particles, mm; γ_d represents the soil dry bulk weight, N m⁻³; k is the soil permeability coefficient, ms⁻¹; and d₆₀ k⁻¹ represents the erosion resistance strength of the soil.

Let $\overline{F}$ > $F_{cr}$, it can be obtained that the critical ground-breaking pressure p₀ of the furrowing and fertilizer spraying of the bionic liquid fertilizer deep application furrow opener should meet

$$p_0＞\frac{\zeta {\tau }_f^2{(\frac{d_{60}}{k})}^{-2}{\gamma }_d^{-1}}{2(1-\cos \phi )}$$

(16)

In the process of the furrowing operation, the fertilizer spraying nozzle is in close contact with the soil, and the energy loss at this distance is ignored; therefore, the outlet pressure of the fertilizer spraying pipe is the critical ground-breaking pressure. According to the literature [51,52,53], the parameters of sticky black soil in the typical area of Northeast China were taken and substituted into the above equation: τ_f = 13 kPa, d₆₀ = 0.05 mm, γ_d = 1.43 × 10⁴ N m⁻³, and k = 1.17 × 10⁻⁶ m s⁻¹. Given that the diameter of the nozzle is 0.004 m, it can be obtained after calculation that the critical ground-breaking pressure of furrowing and fertilizer spraying of the furrow opener p₀ ≥ 0.143 MPa.

When the outlet pressure of the fertilizer spray tube is greater than the critical ground-breaking pressure, the liquid fertilizer is washed into the soil; when the bionic liquid fertilizer furrow opener operates, the soil backfill and this part of the liquid fertilizer are buried in the soil. Therefore, at the same speed with an increase in pressure, the liquid fertilizer buried in the soil also increases, the volume of the water layer in the soil bin of each test remains the same size, and the leakage amount of fertilizer in the water layer is reduced. When the outlet pressure of the fertilizer spray tube is less than the critical ground-breaking pressure, after the furrowing operation, the liquid fertilizer sprayed from the fertilizer tube is not shot into the soil layer and flows into the water. When the soil is backfilled, the liquid fertilizer flows with the water into the water layer in the soil bin. At the same time, the flow gap of the liquid fertilizer is small (within 1s), and with the increase in pressure in the same volume of the water layer, there is less leakage amount of liquid fertilizer. Under the same pressure with an increase in speed, the time of furrowing and fertilizing operation gradually decreases, and the volume of liquid fertilizer sprayed during the operation time also gradually decreases, and the concentration of the measured leakage amount of liquid fertilizer gradually decreases under the condition that the volume of the water layer in the soil bin remains unchanged.

4. Conclusions

In this study, we developed a bionic design of a furrow opener based on the muskrat claw-toe structure and carried out an indoor soil bin test. The experimental results show that the operating speed of the furrow opener and the spraying pressure of the liquid fertilizer are the main factors affecting the operating performance of the furrow opener for liquid fertilizer deep application.

In this study, we found that, at the speed of 2.16 km h⁻¹ to 3.60 km h⁻¹, the resistance of the muskrat claw-toe structure bionic liquid fertilizer deep application furrow opener increases gradually with an increase in the speed. When the speed is 3.60 km h⁻¹, the resistance value of the bionic liquid fertilizer deep application furrow opener is 33 N. At the speed of 2.16 km h⁻¹ to 3.60 km h⁻¹, the soil disturbance rate of the bionic liquid fertilizer deep application furrow opener decreases first, then increases and then decreases with an increase in the speed. When the speed is 2.88 km h⁻¹, the soil disturbance rate is the smallest, and its disturbance rate is 63%.

In this study, we found that, under the same speed of the muskrat claw-toe structure bionic design furrow opener, when the spraying pressure of liquid fertilizer is 0.04 MPa to 0.20 MPa, the fertilizer loss rate gradually decreases with an increase in pressure. In the soil bin test, when the speed is 3.60 km h⁻¹ and the pressure is 0.2 MPa, the minimum leakage amount of fertilizer is 3.5 mg L⁻¹. Under the same pressure, when the bionic liquid fertilizer deep application furrow opener operates at the speed of 2.16 km h⁻¹ to 3.60 km h⁻¹, the fertilizer loss rate gradually decreases with an increase in the speed. When the pressure is 0.2 MPa and the speed is 2.88 km h⁻¹, the minimum leakage amount of fertilizer is 2.5 mg L⁻¹.

In this study, we established the interacted coupling model of furrow opener–soil–liquid fertilizer and analyzed the critical ground-breaking pressure at the spray nozzle of the bionic liquid fertilizer deep application furrow opener. When the spraying pressure of liquid fertilizer is greater than 0.143 MPa, the liquid fertilizer will be washed into the soil.

In this study, the bionic furrow opener device of liquid fertilizer deep application of muskrat claw-toe has a low soil disturbance rate, good burying effect of liquid fertilizer, low fertilizer loss rate and stable operation performance of the fertilizer application system, which can meet the requirements of liquid fertilizer deep application.