Review of Root-Stubble Characteristics and Root-Stubble Crushing and Clearing Technologies for Conservation Tillage

Abstract

Conservation tillage (CT) is an agricultural technology for sustainable soil use, and clearing the root-stubble remaining in the seedbed and nursery bed is a core aspect of CT. In this paper, the characteristics and the testing methods of root-stubble and their growth environments were reviewed, which could provide a reference for the setting of parameters in numerical simulations and the design of stubble treatment devices. The methods for the restoration of the environment in CT are described. Moreover, the root-stubble crushing and clearing technologies and the methods for the evaluation of device performance are summarized. Furthermore, the prospects for the physical properties of the root-stubble soil, the reduction of soil adhesion when a cutter crushes the roots, the tracking of the long-term effects of different methods on soil, intelligent devices for the crushing of root-stubble, and challenges and strategies regarding the application of new root-stubble crushing and clearing technologies are discussed. This paper provides a reference for the development of devices for the crushing of root-stubble from the perspective of CT.

1. Introduction

After harvest, significant amounts of the main beam crops, such as rice, corn, wheat root-stubble and straw, remain in the field [1,2]. Root-stubble include both roots and stubble, while straw is the general term for the remaining stems and leaves in the field after harvesting grains. Traditional methods of treating root-stubble primarily include burning, composting and plowing. Burning involves igniting the corn stubble, which produces dense smoke that causes smog, reduces outdoor visibility and affects traffic safety [3]. The emission of carbon dioxide and particulate matter will not only reduce the air quality, but also contribute to global warming. The yields and quality of crops will be impacted by climate change, which will have a great impact on agricultural production by affecting the growth processes of crops, the suitable planting areas and environmental factors [4]. Composting involves mixing root-stubble with other waste materials for anaerobic fermentation, positively impacting environmental preservation [5]. Plowing mixes are used to provide soil inversion, which helps to bury weeds and crop residues, bury the top layer of non-wetting soil and bring to the surface soil that is more suitable for plant growth. However, the degree of soil disturbance is relatively large, and the soil is exposed for a long time; it lacks the coverage or mixing of straw residues and reduces the soil moisture, which is not conducive to the protection of the agricultural and ecological environment to a certain extent [6]. Conservation tillage (CT) is a novel agricultural farming system and technology centered on ensuring soil quality. It primarily incorporates surface root-stubble and straw cover, reduced or no-till planting and pest and weed control [7]. CT can enhance the soil organic matter content and reduce soil erosion. The device for the crushing of root-stubble is one of the main implements used in CT, typically employed in conjunction with no-till planters for root-stubble treatment. This device effectively addresses the issues of root-stubble and straw clogging during sowing by crushing and clearing root-stubble with minimal soil disturbance, thereby accelerating the decomposition process. Additionally, it can clear straw from the ridges, providing an optimal seedbed environment for subsequent sowing operations. The principles of CT emphasize minimal soil disturbance, maximum root-stubble and straw cover and the effective promotion of biological activity. This approach improves the soil properties, enhances the soil fertility, conserves water, retains soil moisture and mitigates wind and water erosion [8]. Additionally, it reduces greenhouse gas emissions, lowers energy consumption and prevents land degradation. For instance, one of the conservation tillage technologies, the straw mulching and returning method, enhances soil moisture retention by covering the soil surface with straw. Additionally, the nutrients released during the decomposition of straw effectively replenish the soil nutrients, increase the organic matter content and promote the growth and activity of soil microorganisms. This creates a more favorable growth environment for plant roots. Compared to traditional methods such as burning, composting, plowing and rotary tillage, CT results in less environmental pollution, offers higher work efficiency, minimizes soil disturbance, positively impacts soil moisture retention and requires less power consumption for machinery. CT has positive effects on the soil, crops and the environment. It can reduce the decrease in the soil pH. Furthermore, CT contributes to the retention and long-term stability of soil organic carbon, which is significant in achieving carbon neutrality in agricultural production. In summary, as a crucial technology for green and low-carbon agriculture, CT not only helps to improve the agricultural production efficiency and economic benefits, but also plays an important role in protecting soil health and environmental sustainability. The promotion and application of CT are significant for the rational utilization of agricultural renewable resources and the advancement of sustainable agricultural development [9].

Root-stubble treatment is a crucial part of CT before no-tillage and reduced-tillage sowing. Improper treatment can lead to problems such as seeding machine clogging, seed sifting and seed bridging [10,11]. The annual global production of straw and root-stubble is approximately 20 billion tons, with more than half remaining in the field. Asia, Africa and Latin America have the largest amounts of straw and root-stubble, where the agricultural production methods are relatively traditional, the utilization rate of straw and root-stubble is low and the root-stubble treatment technology is underdeveloped. Crop root-stubble, consisting of aboveground stubble and underground roots, form a root-soil aggregate as the root system spreads and consolidates the surrounding soil. The combination of thick stubble with soil makes the mechanical treatment of root-stubble challenging [12]. The main methods of crop root-stubble treatment currently include: (1) stubble side throwing, which cuts off the main roots and throws the stubble to the side, causing minimal soil disturbance but not crushing the stubble completely; (2) stubble rotary mixing, which typically mixes and buries the stubble and soil through strip rotary tillage [13,14], causing significant soil disturbances, destroying the ridge platform and potentially leading to soil moisture loss [15]; (3) root-stubble crushing, which uses horizontal tools to crush the surface residual stubble effectively, but requires high tool rotational speeds, consumes a lot of power and may result in missed cuts when the stubble is tall, as the tool does not enter the soil and leaves the root-soil aggregate untreated. Due to the combination of soil and roots, it is difficult to accurately measure the physical properties. Consequently, the design of soil-contacting root-crushing parts and the setting of stubble-crushing simulation parameters cannot be effectively supported. The traditional rotary tillage technique, which mixes root-stubble and soil, and the crushing stubble technique (which does not address underground roots), present problems, such as soil adhering to cutter, significant soil disturbances and a poor root-crushing effect [16,17]. Therefore, the core issue in root-stubble treatment technology is how to achieve the high-quality crushing of aboveground stubble and underground roots with minimal soil disturbance.

The analysis of the soil physical properties is very important for agricultural production, the ecological environment and other aspects. These properties not only affect the soil fertility and soil and water conservation capacity but also can be used to assist the design of soil-engaging components to improve the performance of these components. However, there may be some problems, such as insufficient accuracy and complicated operation in the measurement of the soil physical properties, and there are some errors when measuring them. Affected by the different measurement methods and techniques, some characteristics, such as the soil moisture content, have obvious timeliness. For example, the drying method is more accurate, but it cannot reflect the change in the soil moisture content in time. Sensor technology can monitor the soil moisture content in real time, but it may be affected by factors such as the environment and equipment accuracy. In addition, methods for the analysis of soil properties are not universally applicable to all soil types. For instance, when measuring the soil permeability, conventional methods such as constant head permeability tests can effectively determine the permeability in sandy soils due to their larger particle size and more abundant pore spaces. However, in clay soils, where the particles are fine and the pore structures are complex, applying the same methods may yield inaccurate results. Therefore, in practical applications, it is essential to consider these factors comprehensively to select the most appropriate method. The Northeast Black Soil Protective Tillage Action Plan (2020–2025) has explicitly outlined the scope of physical property application analysis in CT. Utilizing soil parameter measurement methods, it is possible to assess the soil quality and understand the soil fertility status, providing a scientific basis for the implementation of CT. It aims to improve and enhance the monitoring and evaluation mechanism for arable land quality, serving as a foundational reference for the implementation and refinement of CT. Numerous studies on crop root-stubble crushing and clearing technologies have been conducted in China, leading to the innovative design of both active and passive root-stubble crushing devices. Although many data indicate that the performance of these devices could be excellent, research on various factors, such as the operational effect of the device under different environmental conditions, is lacking, which has hindered their further development [18]. In this study, various soil types and characteristics have been summarized, most of which cover the main soils around the world. The related devices are mainly classified and summarized from the perspective of crushing and clearing. The root-stubble crushing and clearing devices are mainly used for staple crops, including corn, rice, wheat and so on. Some economic crops, such as peanuts and sweet tea, and other root-stubble treatment methods need to be reviewed. The effective utilization methods, such as feed, fuel, power generation and anaerobic fermentation, are not considered in this article. The summary of the root-stubble crushing and clearing devices is based on the published literature. The ongoing and unpublished results need to be supplemented later. The purpose of this review is summarized as follows: (1) to explore the characteristics of crop root-stubble and their growth environment and to propose a reference for device innovation; (2) to summarize the mechanical root-stubble treatment technologies and device performance; (3) to summarize the test methods for the evaluation of device performance; and (4) to predict the physical properties of the root-stubble-soil and wear-resistant parts in soil-engaging stubble cutting, describe the reduction in soil adhesion during the root crushing of the cutter and propose an intelligent device for root-stubble crushing. This study can provide a basis for the selection of root-stubble crushing and clearing devices based on different soil types, environments, regions and other factors. It offers a method for the determination of the soil physical properties and the verification of the performance of root-stubble treatment devices. Additionally, it provides technical support for the innovation of CT machine structures and offers scientific and technological assistance in improving CT systems.

2. Study on the Characteristics of Root-Stubble Their Growth Environment

2.1. Study on Morphological Characteristics

The mechanical design for root-stubble crushing is based on the physical property parameters of crop.root-stubble Corn, rice and wheat are considered the three main grains, with a significant amount of stubble left in the field after harvest each year. Mechanized cleaning faces challenges, such as the large quantity of root-stubble as well as the strong bonding of roots to soil, which are difficult to address [1]. Through a survey of 852 farms, we found that the sowing processes of certain plants, such as rice, wheat and soybeans, could be carried out at a shallow depth without encountering obstacles due to their roots. However, maize roots are robust. If the roots are not completely cleared, the seeds sown afterward could come into direct contact with the roots, affecting the yield [19]. The growth environment elements include stubble, roots and soil. The morphological parameters of typical crop root-stubble(corn, rice and wheat) are summarized in Table 1.

Table 1. Typical crop root-stubble shape parameters of root-stubble.

Corn [20,21,22,23,24]	Cutting Height	Diameter	Depth of Root Upper Segment	Main Root		Vice Root
				Diameter	Number	Diameter	Number
	30–124 mm	19–31 mm	50–80 mm	4–6 mm	11–17	3–5 mm	11–14
Rice [25,26,27,28]	Cutting Height	Diameter	Depth of Root Upper Segment	Root Length		Root Density
	136–223 mm	36–84 mm	63–144 mm	300–500 mm		100,000–150,000 mm2
Wheat [29,30]	Cutting Height	Diameter	Depth of Root Upper Segment	Total Root Length	Root Surface Area	Total Root Volume
	150–200 mm	2.69–3.85 mm	30–50 mm	2446.8–4269.6 mm	23,590–35,900 mm2	220–290 mm3

2.2. Study on Physical Properties

The physical property parameters of soil include the shear modulus, Poisson’s ratio, elastic modulus, static friction coefficient, collision recovery coefficient and so on. Accurate physical property parameters for crops and their environment are crucial in selecting models and setting parameters in numerical simulations of root crop interactions. They also provide a valuable reference for the design of key components. The methods and content involved in determining the physical property parameters of root-stubble and their growth environment are detailed in Table 2.

Table 2. Study on environmental and physical properties.

Object	Mechanical Characteristic Parameter	Test Instrument	Research Content and Results
Loess	Shear strength	ZJ strain-controlled direct shear apparatus	Shear stress: 8.25 kPa (0–60 mm, moisture content: 17.59%), 2.47 kPa (60–120 mm, moisture content: 22.98%) and 9.01 kPa (120–180 mm, moisture content: 20.96%) [31].
Loam 65 Mn	Static friction coefficient and dynamic friction coefficient	MXD-01 friction coefficient measuring instrument	The rolling friction factors for 65 Mn soil particles in different layers are as follows: loam tillage layer (moisture content: 7.54%): 0.107, loam plough bottom (moisture content: 11.45%): 0.130, loam core layer (moisture content: 14.6%): 0.078. The static friction factors are as follows: loam tillage layer: 0.313, loam plough bottom: 0.639, loam core layer: 0.427 [32].
Sandy soil 65 Mn stubble			The friction factors are as follows (moisture content: 14.6%): 65 Mn and stubble: static 0.42, dynamic 0.394; stubble with itself: static 0.383, dynamic 0.263; sandy soils and stubble: static 0.378, dynamic 0.308; sandy soils and 65 Mn: static: 0.340, dynamic 0.299 [33].
Corn stubble steel grade 45	Sliding friction angle	Sliding friction test bench (self-made)	The sliding friction angles are as follows: Corn stalk—45 steel: 23° to 31°; Corn stubble—45 steel: 15.2° to 15.8°; Corn roots—45 steel: 30° to 38° [22].
Rice	Shearing force	FUDOH texture analyzer	The cutting limit forces of the upper and lower sections of rice are 206.5 N and 156.8 N, respectively (moisture content: 27.85%) [34].
Rice	Sliding cutting force	TMS-PRO texture instrument	At a sliding cut angle of 45°, the sliding shear forces per unit diameter are 4.32 N/mm for the upper section and 3.16 N/mm for the lower section of paddy (moisture content: 27.85%) [35].
Rice and loam mixture, wheat and loam mixture			At the soil moisture content of 10.2%, the shear strength of both rice and wheat mixed with loam is 7.92 MPa. At 20.5% moisture content, the shear strength is 19.85 MPa for rice and 13.45 MPa for wheat. At 29.6% moisture content, the shear strength increases to 21.56 MPa for rice and 15.27 MPa for wheat [36].
Corn	Shearing stress	Universal material tester	At a shear speed of 240 mm/min, the ultimate shear value of the corn system is 83.59 N [37].

The types and physical properties of soil around the world are shown in Table 3.

Table 3. Types and physical properties of soil around the world.

Region	Type [38]	Soil Cohesion (kPa) [39,40,41]	Angle of Internal Friction (°) [42,43]	Dry Density (g/cm3) [44]	Wet Density (g/cm3) [45]	Poisson’s Ratio for Soil [46]	Shear Modulus (MPa) [47,48,49]
Asian	Clay soils	20~100 kPa	15°~25°	1.2–1.4 g/cm3	1.5–1.7 g/cm3	0.30~0.45	1~20 MPa
	Sandy soils	0~10 kPa	30°~45°	1.5–1.7 g/cm3	1.8–2.0 g/cm3	0.25~0.35	10~100 MPa
	Loam soils	10~40 kPa	25°~35°	1.3–1.5 g/cm3	1.6–1.8 g/cm3	0.30~0.40	5~50 MPa
	Silty soils	15~30 kPa	25°~35°	1.4–1.6 g/cm3	1.7–1.9 g/cm3	0.30~0.40	5~50 MPa
	Phaeozem	20~30 kPa	15°~25°	1.1–1.3 g/cm3	1.4–1.7 g/cm3	0.30~0.40	10~25 MPa
European	Peat soils	1~10 kPa	5°~20°	0.1–0.8 g/cm3	0.2–0.9 g/cm3	0.10~0.30	1~10 MPa
	Sandy soils	5~20 kPa	30°~40°	1.5–1.7 g/cm3	1.6–1.8 g/cm3	0.2~0.35	10~50 MPa
	Loam soils	10~30 kPa	20°~35°	1.2–1.6 g/cm3	1.3–1.7 g/cm3	0.25~0.35	20~80 MPa
	Silty soils	5~25 kPa	25°~35°	1.5–1.7 g/cm3	1.6–1.8 g/cm3	0.25~0.35	15~50 MPa
	Clay soil	20~100 kPa	20°~35°	1.1–1.4 g/cm3	1.2–1.5 g/cm3	0.4~0.45	10~30 MPa
	Phaeozem	20~40 kPa	25°~35°	1.2–1.4 g/cm3	1.5–1.7 g/cm3	0.30~0.35	10~50 MPa
Americas	Sandy soil	10~50 kPa	30°~40°	1.5–1.7 g/cm3	1.6–1.8 g/cm3	0.25~0.35	5~50 MPa
	Loam	30~100 kPa	25°~35°	1.2–1.4 g/cm3	1.3–1.5 g/cm3	0.25~0.40	10~100 MPa
	Clay soil	50~200 kPa	20°~30°	1.1–1.3 g/cm3	1.2–1.4 g/cm3	0.35~0.45	30~300 MPa
	Peat soil	5~30 kPa	10°~20°	0.1–0.2 g/cm3	0.2–0.3 g/cm3	0.05~0.15	0.1~1 MPa
	Rocky soil	50~300 kPa	30°~60°	2.0–2.5 g/cm3	1.8–2.3 g/cm3	0.15~0.35	50~200 MPa
	Phaeozem	15~30 kPa	25°~35°	1.1–1.4 g/cm3	1.4–1.7 g/cm3	0.20~0.30	50~200 MPa
Oceania	Sandy soil	100~200 kPa	30°~40°	1.5–1.8 g/cm3	1.6–2.0 g/cm3	0.25~0.35	5~50 MPa
	Loam	20~300 kPa	25°~35°	1.2–1.5 g/cm3	1.3–1.6 g/cm3	0.25~0.40	10~100 MPa
	Clay soil	200~1000 kPa	15°~30°	1.0–1.4 g/cm3	1.1–1.6 g/cm3	0.35~0.45	30~300 MPa
	Peat soil	10~100 kPa	5°~15°	0.1–0.3 g/cm3	0.2–0.5 g/cm3	0.05~0.15	0.1~1 MPa
Africa	Sandy soil	50~150 kPa	30°~40°	1.5–1.8 g/cm3	1.6–2.0 g/cm3	0.25~0.35	5~40 MPa
	Loam	100~250 kPa	25°~35°	1.2–1.5 g/cm3	1.3–1.6 g/cm3	0.25~0.40	10~80 MPa
	Clay soil	500~1000 kPa	15°~30°	1.0–1.4 g/cm3	1.1–1.6 g/cm3	0.30~0.45	20~200 MPa
	Peat soil	5~100 kPa	5°~15°	0.1–0.3 g/cm3	0.2–0.5 g/cm3	0.05~0.20	0.1~1 MPa

The physical properties of different types of the soil around the world are presented. There are various soil types in Asia. The angles of internal friction, as well as the dry and wet densities of the black soil in Asia, are low. The cohesion of the soil at the contact points may be weak when the roots are crushed. The cohesion and internal friction angles of peat soils in Europe are between 1 and 10 kPa and 5° and 20°, respectively, which are low. For the soil, the rotational speed of the cutter unit should range from 1200 to 1600 r/min, and the power required by the cutter unit should be relatively low at 14.7–29.4 kW [50]. In contrast, the cohesion and internal friction angles of rocky soils in the Americas range from 50 to 300 kPa and 30° to 60°, respectively, indicating high values. The density and cohesion of rocky soils are also high. Consequently, the higher power of the driven stubble crushing unit would be necessary. A higher rotational speed in the cutter unit, between 2000 and 2500 r/min, is also required [51], and the power consumption of the unit could reach between 73.5 and 110.25 kW [52]. The cohesion and shear modulus of the loam soils in Oceania exhibit the largest range of variation, with values between 20 and 300 kPa and 10 to 100 MPa, respectively. Therefore, the design of the cutter unit should account for operating parameters that can be maintained within a wide range and are adjustable, ensuring excellent adaptability. The cohesion of the clay soils in Africa ranges from 500 to 1000 kPa and the angle of internal friction is between 15° and 30°. For the soil, the rotational speed of the cutter unit should also be 2000–2500 r/min, and the power of the cutter unit should range from 73.5 to 110.25 kW. In summary, reference values for the design of the operating parameters of the cutter unit in different regions around the world can be provided [53].

The physical properties of the soil, stubble and roots were studied using a mechanical property test instrument. The study of the soil physical properties mainly focused on determining the mechanical properties between the soil and soil-contacting parts. From the perspective of efficiently crushing stubble, the research centered on the cutting force, the shear modulus and the static and dynamic friction forces between the cutting parts, with key mechanical parameters including the cohesive force between the soil and the roots of the underground root-soil complex, the cutting force, the shear stress and the shear strength of roots at different soil depths. The crushing effect of severing the root system, the traction effects of the root-soil complex and the disturbance behavior of the soil were determined by these parameters. Clarifying these parameters is of great significance in constructing an accurate soil simulation environment and for the structural design of stubble treatment components.

2.3. Application of Characteristic Parameters of Crop Root-Stubble and the Environment in Numerical Simulations

Numerical simulation technology has become a primary method in the study of particles in the agricultural field, with the discrete element method (DEM) being a typical representative [54]. EDEM, a widely used type of discrete element software, constructs parametric models of particles’ solid characteristics. The shapes and characteristics of particles are represented by physical and mechanical parameters. By using various particle contact models, the velocities, accelerations and other parameters of particles at different times are calculated to simulate and predict the particle motion behavior [55]. The application model and characteristics are presented in Table 4.

Table 4. Characteristics and application of model in numerical simulation of root-stubble.

Particle Contact Model	Model Characteristic	Application Range	Model Characteristic Parameters	Getparms Method	Applications
Hertz Mindlin	Normal and tangential forces of particles include damping components, related to the coefficient of restitution between particles.	Root-stubble (regarded as a rigid body) mixed with buried soil is simulated.	Crash restitution coefficient, static friction coefficient, dynamic friction coefficient (all conventional).	Velocity of the object before and after collision is measured using a flat plate, high-speed camera and wire rope suspension, among other methods; calculations are conducted. Static and dynamic friction coefficients are measured via the inclined plane method [56].	This model was employed to analyze the effects of the parameters of the rotary cutter-burying finger combination machine on the root-stubble rotary-burying quality. The structure of the device was optimized. A field experiment was carried out to verify the accuracy of the model used to simulate root-stubble incorporation. Compared with the traditional rotary tiller, the combined machine enhanced the depth of root-stubble incorporation and reduced the proportion of the root-stubble in the 0–5 cm soil layer by 12.47% after operation [57].
Hertz Mindlin with Bonding	Bonding bonds are employed to connect particles. These bonds resist tangential and normal movement and break when stress exceeds a threshold.	The crushing effects of stubble, roots and soil are simulated.	Normal stiffness coefficient, tangential stiffness coefficient, critical normal stress, critical tangential stress.	A shear test of the measured object by a texture analyzer is performed to obtain the normal and tangential shear forces, normal stiffness per unit area and tangential stiffness per unit area. Four key parameters are derived using a relational formula [58].	This model was employed to simulate the effect of spring-rake teeth on the movement of the stubble along the ridge, and the optimal structural combination of spring-rake teeth was determined. A new machine with spring-rake teeth was processed, and a field experiment was conducted to measure the rate of root-stubble clearing of the machine. The discrepancy between the rate of root-stubble clearing in the simulation and that in the field experiment was only 1.1%, verifying the accuracy of the simulation results. Compared with the traditional horizontal stubble cleaner, the new machine showed a 3.7% increase in the rate of clearing root-stubble.
Hertz Mindlin with JKR	The contact model accounts for cohesion between particles, considering the influence of van der Waals forces in the contact zone.	The adhesion effects between soil particles, the soil root system and soil–soil components are simulated.	Surface energy.	The surface energy of the object is tested using three types of liquids and a camera with the droplet method, followed by calculations to obtain the results [59].	The model was employed to analyze the interactions among different soil layers in heavy clay. The performance of the driven disc plow and double-edged rotary tillage combined machine was investigated on stubble returning. The combined machine was processed, and a field experiment was conducted. Under heavy clay conditions, the rate of returning stubble of the combined machine reached 97.7%, with an error of just 2.01% between the simulation and the actual results, which verified the accuracy of the simulation results. Compared with the traditional rotary cultivator, the stubble returning rate of the new machine increased by 24.23% [60].

3. Mechanized Treatment Technology for Crop Root-Stubble

3.1. Technology of Passive Crushing and Side Throwing of Root-Stubble

3.1.1. Technology of Crushing and Clearing Root-Stubble

The typical disc cutter is one of the traditional passive tools for the cutting of root-stubble, primarily relying on the machine’s gravity to penetrate the soil and utilizing the friction between the machine and the soil to rotate passively, thus achieving root-stubble cutting and soil crushing [61]. The operational principle of this device for the crushing and clearing of stubble passively is illustrated in Figure 1.

Figure 1. Operational principle of device for passive crushing of root-stubble.

For the passive root-stubble cutter component, the weight of the machine must be sufficient for the cutter to effectively crush the root-stubble into the soil. Additionally, factors such as the angle of attachment of the operating machine to the power machine, the structural characteristics of the cutting disk [62], the disc cutter mounting angle [63] and the suitable machine operation mode [64] will all impact the soil penetration of the implement. The depth of penetration of the disc cutter can be controlled by an often agronomically appropriate hook-up angle, and the soil penetration of the disc cutter can be improved by selecting a mounting inclination that matches the type of disc cutter, designing the disc cutter edge based on a rent-reducing structure and selecting a suitable angle of penetration gap (usually less than 10°). This method is predominantly used in large no-till planters. The types and characteristics of the typical disc cutter are detailed in Table 5.

Table 5. Types and characteristics of the typical disc cutter.

Type	Structure	Application Environment	Characteristics
Angle-notched disc		Zones of high root-stubble intensity and coverage	The edge of the disc has some gaps, resulting in a small contact area with the soil and strong trafficability. The gap structure has a gathering effect on the root-stubble, which is beneficial in cutting off the stem. However, the cutting resistance is significant, and the power consumption is high [65].
Corrugated disc		Arid and semi-arid zones	The surface of the disc is corrugated, relying on the corrugated grooves to disturb the soil. It has a strong ability to loosen the soil, and the disturbance to the soil is considerable. The number of corrugated grooves can be reasonably selected according to the working object’s condition. The root-stubble are mainly cut by sliding, which provides a certain ability to cut stubble [66].
Planar disc		Zones of low root-stubble intensity and coverage	The disc is a complete circle, which causes low soil disturbance during operation, low working resistance and good cutting performance for root-stubble. However, the effectiveness of cutting stubble is not optimal when the root straw coverage is extensive [67].

Innovative design and research on the traditional disc cutter structure and its parameters have been conducted by many experts and scholars, as shown in Table 6.

Table 6. Types and characteristics of new disc cutter.

Name	Structure	Application Environment	Content	Comparison
Concave disc for cleaning of root-stubble on the ridge		Row crop area in the Northern Temperate Zone	A concave disc for the cleaning of root-stubble on the ridge was designed. The device is composed of double concave discs inclined symmetrically. Through the flipping effect of the concave disc, the root-stubble could be cut off and pushed into the furrows on both sides by using the device. The rate of disturbed soil of the unit was 22%, and the rate of covered straw and root-stubble after operation was only 12.7%, which was 75.9% lower than that before operation [68].	The ground temperature of the seed strip at 5 cm was increased by 0.6–2.4 °C compared to a no-till planter for corn rows not equipped with concave discs.
Self-driven arc tooth disc stubble-crushing device		Northern Temperate Zone	Based on the multi-tooth structure, a passive arc-shaped tooth disc stubble-crushing device was designed. Using its ground wheel effect, combined with the power of the soil-crushing device during operation, the stubble cutting, rotary tillage and ridging processes were completed in one step. The rates of root-stubble and soil crushing of the unit are 93.5% and 92.4%, respectively [69].	Compared with traditional tillage equipment and imported combines, the soil breaking rate is increased by 2.4–6.9%, the stubble breaking rate is increased by 6.3–12.9% and the fuel consumption is reduced by 26.3–40.4%.
Combined curve stubble-crushing disc cutter		Phaeozem region of the Northern Temperate Zone	The combined curve disc cutter was designed by combining the sliding blade edge with the arc blade curve. Experiments proved that the combined curve disc cutter had a good stubble-cutting ability and improved the no-till planter’s passing ability through black soil corn stubble. The root-stubble could be crushed completely when the rate of the covered straw was 62.77% [70].	——
Archimedes spiral notch disc stubble cutter		Corn row crop area in the Northern Temperate Zone	By utilizing the Archimedes spiral, the structure of the notch disc stubble cutter was optimized, the stubble resistance of the disc cutter was reduced and the quality of the disc cutter’s stubble trenching was improved. The power consumption and the rate of root-stubble crushing of the cutter are 0.86 kW and 95.7%, respectively [71].	——
Passive disc stubble-crushing cutter with automatic cutting edge angle		No-tillage corn rows under straw crushing and returning to the field	A passive disc stubble-crushing cutter with an automatic cutting edge angle was designed. The cutting edge angle could be adjusted according to the characteristics of the stubble and soil at different depths, reducing tillage resistance and increasing the stubble-crushing ratio. The rate of root-stubble crushing of the cutter is 96.8% [72].	Reduces tillage resistance by 13.3% and 20.6%, reduces fuel consumption by 19.3% and 35.3% and improves stubble breakage by 16.1% and 4.6% relative to disc and gap disc cutters.
Anti-blocking device of no-tillage planter		Zones with inter-row intercropping patterns and alternating wide and narrow row recreation patterns	A combined blade suitable for inter-row interaction and wide–narrow row modes was designed. It reduced the operating power consumption and improved the straw-cutting rate. The rate of root-stubble cutting and the power consumption of a single cutter are 90.9% and 8.15 W, respectively [73].	——
Concave arc-shaped lateral blade notched single-disc circular blade		No-tillage corn row field	A concave arc-shaped lateral blade notch single-disc circular blade was designed, which has a disruptive effect on the soil below the ground surface, can cause the secondary cutting of stubble and enhances the stubble-cutting efficiency. When the level of straw covering is large, the cutter at the forward speed of 6 km/h can achieve root-stubble crushing and side-throwing under the condition of less disturbed soil [61].	Compared with circular blades, the average forward displacement of stubble under the action of lateral blades is 568 mm, 639 mm and 502 mm, respectively, increasing by 1.15 times, 1.12 times and 1.21 times. This enhances the working duration of the blades on the stubble.

3.1.2. Technology of Side-Throwing Stubble

As the device advances, the parts (primarily a staggered arrangement of wheel fingers) are passively operated. The main function of the device is to throw soil and stubble on both sides. Additionally, the stubble also can be crushed to a certain extent during the throwing process. The operational principle of the device for the passive throwing of stubble is shown in Figure 2.

Figure 2. Operational principle of device for passive throwing of stubble.

The root straw is cut by the key components and diverted to both sides of the seed belt. Many studies have been conducted by experts, and the main findings are shown in Table 7.

Table 7. Devices for side throwing of stubble.

Name	Structure	Application Environment	Content	Comparison
Archimedes spiral serrated notched disc stubble-crushing cutter-star ridge-cleaning wheel-combined stubble-cleaning device		Row crop area in the Northern Temperate Zone	By integrating the Archimedes spiral serrated notched disc stubble-crushing cutter with the star-shaped ridge-clearing wheel, the anti-winding stubble-crushing ridge-clearing device was designed, enhancing the passability of the corn ridge no-tillage planter. The rates of root-stubble clearing and crushing of the unit are 93.49% and 92.21%, respectively [21].	——
Concave claw-type stubble-cleaning tooth plate		Thick corn stalk zone	A concave claw-type straw stubble-cleaning disc was designed. The wheel grab was used to grasp the stubble on the side and cast it behind, improving the stubble-cleaning rate of the device. The improved rate of root-stubble clearing of the unit is 83.61% [74].	Compared with the traditional plane stubble-cleaning wheel, the average lateral throwing speed of the root straw of the concave stubble-cleaning wheel was increased by 31.02%.
Star-tooth concave disc straw-cleaning anti-blocking device		Row crop area in the Northern Temperate Zone	A star-tooth concave disc straw-cleaning anti-blocking device was engineered. The stubble and straw could be side-thrown to both sides of the machine along the tangential direction of the star-tooth concave surface. The device operated stably and enhanced the straw cleaning rate. The rate of clearing root-stubble and the operating resistance of the unit are 92.2% and 142.6 N, respectively [75].	Compared with the flat claw wheel straw-cleaning and anti-blocking device, the straw-cleaning rate of the seedling belt was increased by 27.5%, and the working resistance was reduced by 23.1%.
Side cutter and stubble tooth plate combined stubble-cleaning device		Zones with high straw cover	A combined stubble-cleaning device composed of a side cutter and stubble tooth plate was designed. The straw on one side of the seed belt was cut by the notched stubble-cutting disc, and the straw on the seed belt was removed by the stubble tooth plate, thus improving the stubble-cleaning rate of the seed belt. The rate of root-stubble clearing of the unit is 91.4% [76].	——
Involute tooth-shaped stubble-separating grass disc		Zones with large quantities of straw and different cropping patterns	An involute tooth-shaped stubble-separating grass disc suitable for finger-type corn no-tillage precision seeders was designed, thereby improving the cleaning effect of stubble in the seedbed and nursery bed. The unit is suitable for use with finger-clip seeders and could achieve no-till seeding of corn at a forward speed of 9 km/h [77].	——
Bionic shifting and diffluence straw and anti-blocking device		Arid areas covered with a large amount of wheat straw	Based on the bionic prototype of the white star flower scarab line, a bionic shifting and diffluence straw and anti-blocking device was designed. The straw was lifted and thrown out to the side and rear by the bionic grass wheel, and then the straw was shunted to both sides by the bionic grass distribution baffle. The soil disturbance and the actual straw removal rate were 20.00% and 90.58%, respectively [78].	——
Grass soil separation device with combination of stubble cutting and grass guiding		Zones covered with wheat straw stubble	The combination device of the concave notched disc cutter and the opener with a grass-guiding plate was used to cut, throw and guide the straw, root-stubble and weeds, which enhanced the passing ability. The rate of clearing root-stubble and straw was 90.16%, and the rate of disturbed soil was 20.85% [79].	——
Device for cleaning of straw in sowing strip		No-tillage sowing strip in drip irrigation area	A device for wide–narrow maize no-tillage sowing strips in drip irrigation areas was designed, which could move the straw residues in the narrow row to the adjacent wide row, effectively avoiding the occurrence of winding blockages. The rate of stubble and straw clearing was 87.61% [80].	——

The passive stubble-crushing device is used to cut or separate root-stubble either by its own weight or by utilizing the structural characteristics of the device. The structure of the device is relatively simple, often comprising a combination of a stubble-crushing cutter and a ridge-clearing wheel. It primarily targets the cutting of stubble on the surface and does not involve the root soil combination. Under certain conditions, the stubble-cleaning effect is satisfactory; however, the effectiveness of stubble cutting can be suboptimal in the presence of soil crusting, field stubble and other challenging conditions.

3.2. Technology of Active Crushing of Root-Stubble

This method primarily employs the high-speed rotation of the stubble-cutting parts to achieve root cutting, using the surface as support. The structure is more complex than that of the passive stubble-crushing device [81]. The operational principle of the device for the active crushing of root-stubble is shown in Figure 3.

Figure 3. Operational principle of device for active crushing of stubble.

When faced with extensive root straw coverage, the seed belt cleaning effect is superior, and numerous related studies have been conducted by experts and scholars. The main findings are shown in Table 8.

Table 8. Devices for active crushing of root-stubble.

Name	Structure	Application Environment	Content	Comparison
Driving disc-ditcher anti-blocking unit body		Regions with corn stubble cover twice a year	A driving disc-ditcher anti-blocking unit body was designed. This increased the cutting rate of wheat stubble and reduced the soil disturbance. The rates of root-stubble cutting and soil disturbance of the unit are 92.1% and 8.4%, respectively [82].	Compared to the strip crushing no-tillage wheat seeder, the driven disc-type no-tillage wheat seeder increased the rate of stubble cutting by 11.2% and reduced the soil disturbance by 58.8%.
Dynamic imitation locust mouth part stubble-crushing cutter		Regions with stout corn stalks	Bionic prototypes based on the structure and movement of locust mouth parts were used to design a dynamic bionic stubble-crushing cutter. It demonstrated an improved cutting rate in root straw compared to the driving gap disc stubble-crushing cutter. The rate of root-stubble cutting and torque of the unit are 92.9% and 54.1 N·m, respectively [83].	Compared to passive notch disc stubble cutters, the active notch disc stubble cutter can increase the stubble cutting rate by 22.6% to 27.4%. Compared to driven notch disc stubble cutters, it can increase the stubble cutting rate by 8.6% to 13.5% while reducing the torque output by 19.5%.
Double cutter power-cutting stubble-crushing device		Regions with two growing seasons for wheat and corn per year	By combining active and passive disc cutters, a root-stubble crushing device for passive soil crushing and active stubble crushing was developed. It enhanced the passability of the no-tillage planter when dealing with large wheat root straw coverage. The root-stubble crushing and cutting rates of the unit are 98.7% and 96.2%, respectively [29].	——
Stubble-crushing device with trapezoidal combined cutter head		Regions with higher soil moisture content	A trapezoidal tooth-shaped cutter head was designed, with double cutter heads arranged at a specific angle and axially parallel in the circumferential direction. The combined cutter head improved the machine’s anti-blocking performance against stubble. The unit is suitable for crushing root-stubble in conditions where the amount of covered straw is less than 1.2 kg/m2 [84].	——
Driven stubble-crushing cutter for stripless tillage		The maize intercropping area in the North Temperate Zone	The strip-driven stubble-crushing cutter was designed based on the principle of sliding drag reduction. The blade curve was determined to reduce both the stubble-crushing resistance and the amount of soil moved. The tillage resistance of the driven stubble-crushing cutter is 1042.81 N. Compared to the driven disc blade assembly, the tillage resistance decreased by 19.78% [85].	Compared to the driven disc blade assembly, the tillage resistance decreased by 19.78% and the amount of soil thrown up decreased by 13.95%.
Strip-type inter-row side-throwing straw anti-blocking device		Regions with two growing seasons for wheat and corn per year	A strip-type inter-row side-throwing straw anti-blocking device was designed to throw straw to the inter-rows on both sides of the seedbed and nursery bed using a side-tipping cutter, thereby improving the straw-cleaning performance of the seeding belt. The rate of clearing root-stubble and the torque of the unit are 82.7% and 298.2 N·m, respectively [86].	Compared to traditional rotary tillers and flat rotary cutters, the lateral cutting blade bed cleaning rate was increased by 22.7% and 26.1%, respectively. The operating torque was decreased by 21.3% and 3.3%, respectively.
Strip-type rotary cutting and post-throwing anti-blocking device		Regions with two growing seasons for wheat and corn per year	A strip-type rotary cutting and throwing anti-blocking device was designed. By integrating the opener with the rotary cutting cutter, the soil disturbance and machine power consumption were reduced, enhancing the no-tillage planter’s passability in heavily straw-covered stubble. The rate of soil disturbance and power consumption of the unit are 25.0% and 12.08 kW, respectively [87].	Compared to traditional rotary tillers, the designed rotary cutting blade reduced the power consumption by 13.83%, decreased the soil disturbance by 37.5% and improved the groove depth stability coefficient by 8.2%.
Lateral stubble cutter for original stubble field		Regions with a large amount of straw coverage	Lateral sliding cutting blade teeth were designed to cut, transport and throw corn stubble via rotary blade with teeth aligned vertically to the forward direction. This improved the stubble clearance rate and effectively cleaned the original stubble bed. Under the conditions of 1.28 kg/m2 straw coverage and a 273 mm average height of the stubble, the power consumption of the unit was 4.9 kW without root-stubble entanglement and the blocking of the cutters during the operation [88,89].	After optimization, the two types of structured blade teeth exhibit a reduction in the vibration intensity of 46.5% and 41.8% compared to the existing blade tooth combinations, with the power consumption decreasing by 29.7% and 35.9%, respectively.
Driving drum-type active anti-blocking mechanism		Regions with two growing seasons for wheat and corn per year	A driving drum-type active anti-blocking mechanism was designed from the perspective of the ground wheel’s power drive drum rotation. Based on fluid mechanics principles, the stubble-cleaning effect was improved. The rate of soil disturbance of the unit is 9.6% when root-stubble crushing is performed [90].	Compared to the strip rotary tillage maize no-tillage planter, its soil disturbance is reduced by nearly 54%.
Anti-blocking device straw-cleaning unit structure		Regions with a large amount of straw coverage	An active stubble-crushing device for a no-tillage planter was designed for large ridge corn stubble fields to address the issue of frequent blockages during no-tillage seeding in fields covered with root straw, particularly in northern cold regions. The rate of clearing root-stubble, the power consumption and the rate of soil disturbance of the unit are 93.94%, 4.36 kW and 31.9% [91].	——
Vertical shaft rotary straw-crushing device		Regions with large straw coverage and two harvests per year	A vertical shaft stubble-crushing straw-crushing device was proposed to crush surface straw and weeds and evenly distribute them on the surface, thus improving the machine’s passing ability. The qualified rate of the length of the root-stubble after crushing is more than 90% [92].	——
Spiral split seed belt cleaning device		Regions with a large amount of straw coverage	A spiral split seed belt cleaning device was designed by combining a notched disc with a helically arranged stubble cutter. It cuts the root straw and transports it to the sides of the seed belt. The rate of clearing root-stubble of the unit is 92.55% [93].	——
Anti-blocking device of rotary cutting with slide plate pressing straw		The region in the Northern Temperate Zone with crops having two harvests per year	A new type of anti-blocking device was developed for rotary cutting with a slide plate pressing straw. The support cutting of root straw by the skate and cutter was used to enhance the cutting rate of root straw [94].	Compared with the belt rotary tiller, the rate of soil disturbance and the fuel consumption are reduced by 25% and 7.04%, respectively.
Seed belt stubble-cleaning device		The region in the Northern Temperate Zone with crops having one harvest per year	A combined seed belt stubble-cleaning device consisting of a cleaning belt cutter and a stubble cutter was designed to improve the cleanliness of the seed belt and reduce soil disturbance. The rates of root-stubble clearing and soil disturbance of the unit are 86.59% and 8.83%, respectively [95].	Compared to traditional strip stubble elimination devices, the cleanliness of the planting strip after operation is increased by 26.89%, while the soil disturbance is decreased by 17.95% compared to the average.
Fixed flail anti-winding returning straw-crushing machine		Tropical and subtropical regions	A fixed flail anti-winding returning straw-crushing machine was designed. The cooperative action of the flail cutter and fixed cutter supported and crushed the stubble, solving the winding problem of the crushing cutter roller. The rate of banana root-stubble crushing of the unit is 95.1% [96].	Compared to the existing horizontal banana straw shredder for field incorporation, the qualified rate of straw shredding is increased by 1.7%.
Leaf round baler picking crushing cutter		Tropical and subtropical regions	A symmetrical vertical oblique cutting pick-up crushing mechanism was designed. By adjusting the number of blade limit washers, the limit position of each crushing cutter on the connecting pin shaft was regulated, as well as the number of crushing cutters, to enhance the crushing effect on the stubble. The qualified rate of cane root-stubble crushing of the unit is 85.43% [97].	——
Cutting and throwing combined anti-blocking device for wide-seedbed seeding of wheat		Corn root-stubble covering the ground	A combined cutting and throwing device was designed for the clearing of crop residues and prevention of blockages. It utilizes vertically arranged inclined notch discs in conjunction with a spinning throwing mechanism to effectively clear crop stubble and straw. The rates of root-stubble cutting and soil disturbance of the unit are 96.2% and 6.9%, respectively [98].	The cleanliness of the seedbed is 90.1%, the stubble-crushing rate is 96.2%, the width of soil disturbance is 127 mm and the soil disturbance volume is 6.9%.
Seed belt vertical crushing root-stubble device		Corn root-stubble covering the ground	Vertically rotating straight cutters were used to plow the soil and snapping fingers were used to clean the root-stubble around the tilled strip. The rates of root-stubble clearing and soil crushing of the unit are 97.7% and 96.4%, respectively [33].	——

Active units generally rely on power from sources such as tractor rear output shafts, electric motors or hydraulic motors, which are driven through a power transmission system to cut the root-stubble and clear them to the sides of the seedbed. Improving the effectiveness of crushed root-stubble can be achieved through the innovative design of the structure and the arrangement of the cutters. When the cutters do not engage the soil, they can maintain effective performance. However, if the cutting parts contact the soil and continue to work, soil adhesion to the tool may occur. This issue highlights the need for an innovative breakthrough in the technology for the crushing of stubble in combination with soil.

For the comparative study of different root-stubble treatment methods, it is crucial to investigate not only their abilities in terms of root-stubble fragmentation and soil disturbance but also the effects of various practices on the soil physicochemical properties, particularly the long-term adverse impacts. For instance, Li et al. found that long-term no-tillage operations led to increased soil bulk density and decreased pH values. The former can result in soil compaction, affecting crop emergence and the increasing subsequent tillage resistance, while the latter can cause soil acidification, accelerating organic matter decomposition [99]. Rotary tillage operations, compared to no tillage, cause greater soil disturbance, thus exerting larger impacts on the soil. For example, Lal et al. discovered that rotary tillage led to the loss of soil organic carbon, and the disturbance of the surface soil exacerbated soil erosion by wind and water [100]. Bronick et al. summarized extensive research data indicating that rotary tillage disrupts the soil structure, reduces the soil aggregate stability and affects the soil permeability and aeration [101]. Malik et al. found that rotary tillage results in nutrient loss in the surface soil layers, likely due to soil mixing caused by tillage [102]. Therefore, considering all aspects to minimize the impact on the soil’s physical and chemical properties and exploring different root-stubble treatment methods in suitable areas can provide a theoretical basis for the improvement of the soil quality and seedbed quality.

3.3. Methods to Test the Performance of the Root-Stubble Crushing Device

The performance test is conducted using various methods and test types. The structural parameters of the tool and the working parameters of the device are carefully selected to enhance the quality of stubble crushing, reduce the soil disturbance and lower the power consumption. Numerous experts and scholars have conducted studies in this area, with the main findings presented in Table 9.

Table 9. Methods to test the performance of the root-stubble crushing device.

Test System	Test Method	Type of Test	Test Content
	Discrete element numerical simulation	Single-factor test, comparative validation	DEM was employed to simulate the effect of a deep loosening shovel and disc combination unit on root-stubble crushing. A reasonable assembly between a deep loosening shovel and disc was obtained through a single-factor experiment in the simulation. The rate of root-stubble crushing of the combined machine increased by 19.38% compared with the disc cutter and by 8.86% compared with the notched cutter, demonstrating the possibility of optimizing the cutter structure using discrete element simulation [103].
	Discrete element numerical simulation	Multi-factor test	DEM was employed to simulate the effect of the disc on clearing stubble. When the rotary radius of the disc, the radius of curvature and the length of the toothed disc were 163 mm, 350 mm and 52 mm, respectively, the rate of clearing stubble of the disc was 91%, which was obtained through a multi-factor test in the simulation. A physical experiment was conducted, and the rate of clearing stubble of the new machine was 91.4%. The accuracy of the simulation test was verified [76].
	Discrete element numerical simulation	Single-factor test, multi-factor test	DEM was employed to determine the form and number of the cutters with forward reverse rotation through a one-factor test. The parameters of the cutter were optimized through a multi-factor test in the simulation. A physical experiment demonstrated that the rate of root-stubble crushing of the machine composed of the cutter and planter was 95.72%, which proved the feasibility of optimizing the structure of the cutter through the multi-factor test in the simulation [104].
	High-speed photography test, discrete element numerical simulation, field experiment	Multi-factor test, high-speed photography test	DEM was employed to analyze the motion of the stubble after being thrown by the cutter. A multi-factor test was conducted to optimize the structure of the cutter. The optimized cutter was processed, and a high-speed camera test was conducted, demonstrating the consistency of the trajectory of cutting and throwing stubble between the simulation and physical experiments. A physical experiment demonstrated that the rate of root-stubble clearing and the power consumption of the machine were 93.2% and 2.1 kW, respectively, meeting the agronomic requirements [105].
	Test in soil bin, discrete element numerical simulation	Multi-factor test	DEM was employed to investigate the effect of the structure of an active cutter on the motion of the stubble after being thrown. The optimized structure of the active cutter was obtained through a multi-factor test in the simulation. A physical experiment demonstrated that the rate of root-stubble clearing of the machine was 91.85%, the stability of the furrow depth was 86.67% and the rate of soil disturbance was 26.47%. The performance of the strip no-tillage planter proved the feasibility of optimizing the structural parameters of the cutter by DEM [106].
	Test in soil bin, discrete element numerical simulation	Multi-factor test, comparative validation	DEM was employed to conduct a multi-factor test, and the structure of the cutter was optimized. a physical experiment was conducted, and it was shown that the rate of root-stubble clearing, the rate of soil disturbance and the power consumption of the new machine were 92.5%, 29.6% and 1.51 kW, respectively. Compared to the performance of the original machine (where the inclination angle of the cutters was 0°), it was found that the rate of root-stubble clearing of the new machine was improved by 14.7% [107].
	High-speed photography test	Comparison validation	Stubble movement was monitored using a high-speed camera. The working speed was the comparative factor, with the tillage resistance, fuel consumption and stubble ratio as the test indices. The performance of the passive disc cutter and the traditional disc cutter at different working speeds was compared and analyzed [72].
	Test in soil bin	Multi-factor test	The bionic disc cutter’s parameters were optimized through a multi-factor test, using the stubble cutting rate and cutting torque as indices. The wear resistance of the bionic disc was also assessed using a confocal laser scanning test [83].
	Field experiment	Multi-factor test	A multi-factor test was used to find the optimal combination of working parameters for the stubble-crushing anti-blocking device, with the stubble cutting rate and tool power consumption as indices [73].

Discrete element numerical simulations, soil bin tests and field experiments are crucial in assessing the performance of stubble-crushing devices and have been developed significantly. These methods are primarily used to evaluate the wear resistance, stubble-crushing rate, soil disturbance rate and power effect of the device. Research on stubble-crushing devices typically relies on experimental methods. DEM could be employed to simulate the process of crushing and clearing root-stubble. The single-factor test is designed to investigate the effects of the key parameters of the cutter on its performance, while the multi-factor simulation test is conducted to obtain the optimal parameter combinations for the cutter. The optimized cutters could be processed into new machines independently or combined with no-tillage planters, such as active root-stubble crushing and clearing machines, strip no-tillage planters and corn wheat rotation no-tillage planters. Typically, the simulation results are verified using two methods. The first method involves utilizing a high-speed camera test to track the trajectory of the stubble ejected by the cutter and comparing the results with the simulation data. The accuracy of the simulation results can be validated based on a comparison. The second method entails conducting a field experiment, during which indicators such as the power consumption, the rate of root-stubble clearing and the rate of root-stubble crushing are measured. These measurements are then compared with the simulation data to verify the accuracy of the simulation results. To evaluate whether the performance of the new machine is improved, its field experiment performance is compared with that of the original machine, using the rates of root-stubble clearing and root-stubble crushing as indexes. According to references [33,60,64] and [103], the rate of root-stubble clearing, the rate of root-stubble crushing and the rate of root-stubble returning for the cutter optimized by numerical simulation could be increased by 3.7% to 19.38%, 14.7% and 24.23%, respectively. It can be observed that the discrete element method has been widely applied in the study of the structural design and performance of units for the crushing and throwing of root-stubble [108]. This method could significantly shorten the development cycle of the unit and accomplish scientific research with lower cost consumption.

4. Prospects

4.1. Strengthening the Analysis of the Physical Properties of the Root-Stubble Soil

The crop root-stubble soil combination is complex, forming a composite soil consisting of the root-stubble and the tillage soil layer. The inconsistency in the stubble height leads to differences in the nodule diameters and the characteristics of cutting failure in the aboveground stubble. Furthermore, the roots are distributed across various soil layers [109]. The ‘locking effect’ caused by the soil root connection, along with the physical characteristics in different soil layers, such as the water content, shear modulus and other factors [110], complicates the study of root failure characteristics. Understanding the physical and mechanical properties of the root-stubble soil is essential. Such knowledge will provide significant reference value in enhancing the accuracy of numerical simulations of stubble crushing and in optimizing and developing stubble-crushing components.

4.2. Application of Wear-Resistant Parts in Soil Engaging and Crushing of Roots

The continuous high-speed operation of seeders creates high demands for the reliability of stubble-cleaning devices, especially regarding wear resistance. The bottleneck issues of new material and technology requirements are becoming increasingly apparent [111]. Traditional materials like 65 Mn steel and conventional quenching processes are becoming inadequate for the needs of high-performance, cost-effective agricultural machinery. Alternative treatment processes, such as laser phase transformation and flame spraying [112], should be considered. Improving the application materials and process selection standards for soil-engaging parts that correspond to different soil environments, clarifying the reliability test bench for soil-engaging parts under actual conditions and enhancing the failure assessment and evaluation system for parts are crucial steps forward.

4.3. Application of the Reduction of Soil Adhesion to the Cutter during Root Crushing

Reducing the power consumption has always been a critical issue in the field of agricultural machinery. The power consumption of cutting parts is influenced by the robustness of stubble and the consolidation between the roots and soil. It is inevitable that the tool will contact the soil when the root is crushed. During the continuous operation of the machinery, the cutter will adhere to the soil, especially when the soil moisture content is high, which can even prevent the machine from operating normally [113]. To optimize the surface of the cutter [114,115,116,117], methods such as bionic micro-electroosmotic anti-adhesion and mechanical vibration desorption [118] can be employed. In terms of reducing the power consumption of the cutting parts, self-excited vibration, electroosmotic, spring profiling [72], inflatable liquid filling methods, external magnetic fields and other methods could be explored. These methods could not only reduce the power consumption of the cutting tool but also minimize soil adhesion, thus ensuring the optimal performance of the cutter when it contacts the soil and crushes the roots consecutively.

4.4. Tracking the Long-Term Effects of Different Methods on the Soil Physicochemical Properties in Various Regions

Currently, the research on the influence of different root-stubble treatment methods on the soil physicochemical properties is still incomplete [119]. The main deficiencies include the insufficient coverage of the soil physicochemical properties and root-stubble treatment methods studied, a lack of comparative studies under specific climatic and soil conditions [120] and the absence of quantitative evaluation methods to resolve contradictions regarding the superiority of different soil physicochemical properties after the operation of two methods. Addressing these issues is crucial in determining the optimal root-stubble treatment methods under varying geographical and climatic conditions. However, conducting such research requires substantial manpower, material and time, necessitating effective coordination among global researchers for comprehensive and systematic deployment planning.

4.5. An Intelligent Device for Root-Stubble Crushing

The primary challenge for stubble-crushing devices is to achieve the high-quality crushing of both stubble (aboveground) and roots (underground) with minimal soil disturbance. The accurate prediction and acquisition of the depth and distribution range of the target stubble are essential to reduce the disturbance of non-root-involved soil and to precisely sever and destroy the underground soil root junction. However, the sub-surface location of roots adds complexity to the application of intelligent visual monitoring systems. Constructing a big data platform could be a solution [111], where crop information such as the variety and row spacing is recorded during sowing. Collecting stubble samples before the cutting process could help to estimate the distribution of underground roots, providing a reference for the depth and disturbance range of root cutting components. Implementing tracking control technology to plan and deduce the root cutting operation path can increase the operational efficiency [121]. Real-time adjustments to the scheme can be made by the device when deviations occur, enabling efficient root cutting [122]. After harvesting, variations in the stubble length, posture and row spacing require a visual monitoring system at the front end of the power machine to identify the stubble characteristics. Through the sensor device, the information is sorted out and fed back to the cutting tool posture intelligent control efficient crushing system. According to the working environment and operation situation, the operation plan is adjusted in real time to achieve the accurate identification and efficient crushing of the stubble on the ground [123].

4.6. Challenges and Strategies in Promoting the Application of New Root-Stubble Crushing and Clearing Technologies and Machinery

While striving to develop new technologies and machinery for the crushing and clearing of root-stubble, the promotion and application of these innovations face certain challenges. Firstly, farmers’ acceptance of new technologies and equipment is a significant issue. To address this, relevant research and development personnel should conduct demonstrations and moderate publicity in the local area before promoting the machinery. If necessary, they should also perform humanistic inspections [124]. Secondly, due to differences in the natural environments and farming patterns worldwide, it is challenging to comprehensively promote a single type of machinery [125]. Therefore, modular and serial design should be emphasized during the machinery design process to adapt the equipment to different operating environments through machinery selection or simple standardized modifications [126]. Finally, the timeliness of machinery troubleshooting during operation is crucial. For new equipment, especially that with higher levels of intelligence, farmers often struggle to diagnose and repair faults that occur during operation. This situation imposes higher demands on the overall system optimization and after-sales service of new machinery.