## 4 Discussion
As another result, the accumulated soil upheaval and the soil mounds on both sides of the furrow may become larger. However, they partially fall into the furrow after the pass of the subsoil, or these mounds are removed by subsequent surface leveling operations. The use of power-driven disc tools has also shown that soil transportation (displacement) can reduce draught force [24]. Finally, the belt motion changes soil movement and deformation patterns. The tool advance causes soil compression, and the draught force increases [6; 42]. The soil deforms and moves continuously until the soil is no longer under the influence of the tool [6; 42; 51; 52]. The failing soil in compression develops high draught forces and is inefficient in energy use per unit volume of soil disturbed, despite the fact that most modern passive subsoiling tools do so. This is a fundamental flaw in current design concepts for soil-loosening tools [21; 53; 54]. The results in Section 3.4 show that the belt motion increases upward soil movement, reduces soil compaction, and thus reduces draught force by \"lifting\". The results are in agreement with the analysis of [PERSON] and [PERSON] [55] that no compaction will occur providing the vertical component of movement is sufficiently great. This occurs when the upward movement is unrestricted leading to a state of lower compressive stress in the soil. This is also consistent with the viewpoint of critical state soil mechanics that the only soil loosening process is shearing at relatively low compressive stresses [21; 56]. On the other hand, the belt motion may induce some tensile stress in the unsupported upper soil in front of the tool [54]. It is common knowledge that the inherent tensile strength of soils is several orders of magnitude lower than its shear strength induced by compressive stresses. Therefore, such a condition leads to easier soil failure and lower draught force [54]. Draught reduction has many beneficial effects. High tractor power is usually required to overcome large draught resistance, while forced operation would result in deformation and wear of the tool, as well as incorrect working conditions, lower working efficiency, and unacceptable tillage quality. However, by reducing the draught resistance, it is possible to use lighter tractors with less traction power, and higher speed, and avoid various problems above caused by excessive resistance. These further helped to reduce the manufacturing cost of the tractor and implement [24]. However, the risk of applying the above principle to subsoilers is a possible controversy in practical application. First, moving parts (e.g., chains) in contact with soil may cause mechanical failure, and the complex structure may also increase the manufacturing cost. Second, incorrect belt width and speed may transport more subsoil resulting in oversized furrows, which is unacceptable. Third, additional energy is required to drive the belt, which makes the energy consumption performance uncertain. This requires further study. Despite these controversies, this should not prevent the use of the principle. These controversial negative effects can be avoided by careful mechanical design. Considering the successful application of belt-like devices with different structures and engineering purposes, such as chain trenchers, potato or onion diggers, belt soil throwers, coal saws, etc., subsoiling with an upcutting belt motion would be feasible. The excellent draught reduction capability solves the most pressing problem of large draught requirements in subsoiling or other types of tillage, which deserves more attention. This technology has wide application prospects. It should be noted that this study is not exhaustive due to space limitations. First, only one soil particle diameter was used in the simulation, which obviously cannot represent the widespread changes in soil type and condition in nature. However, the simulation clearly showed the draught reduction performance of the belt tools under the same given soil conditions.