Assessment of the Impact of Energy-Saturated Tractor Movers on Soil

Implementation of agricultural technologies for crop cultivation is not possible without the use of energy-saturated tractors. As a result of the passage of tractor movers, they have a negative effect on the soil, leading to its compaction and lower crop yields. The aim of the research is to determine the influence of movers of various running systems of energy-saturated tractors on the physical and mechanical properties of the soil. To assess the impact of tractor movers on the soil, mathematical dependencies were presented to determine the soil index characterizing the physical and mechanical property of the soil, which is changed as a result of the impact and its influence when assessing the complex indicator of soil impact. The results of field studies of the effects of movers of the tractors K-701 and K-744 on the soil are presented. As a result of the research, it was found that the serial single mover of an energy-saturated tractor exerts a strong effect on the soil, creating a high voltage in it leading to excessive soil compaction. The use of double wheels in comparison with single wheels makes it possible to reduce the negative impact on the soil; in particular, a decrease in stresses occurring in the soil to 10.1% and a decrease in soil density along the tracks of movers to 13.8% were recorded. The results of the calculation of a comprehensive indicator of the negative impact on the soil by different tractors are presented. The proposed methodology allows optimal manning of the enterprise’s vehicles providing minimal impact on the soil and crop preservation.


Introduction
Growing crop products and meeting the requirements of agricultural technologies for their cultivation are not possible without the use of machine-tractor units based on energy-saturated tractors. The advantages of energy-saturated tractors are great traction, good maneuverability and high productivity; however, mass of tractors produced in Russia and foreign countries has increased significantly in recent years (Keller et al., 2019). Currently there are widely used energy-saturated tractors "Kirovets" (K-701, K-744P) in the Saratov region, the mass of these tractors exceeds 13 tons. When the tractor moves across the field, due to the large mass, high contact pressures are created under the wheeled undercarriage, which leads to soil deformation and compaction. As a result of soil compaction along the trail of tractor wheels, soil structure changes, its porosity decreases, water-air and thermal conditions of the soil are disturbed, density increases, which ultimately leads to a decrease in crop yield. It was established that the optimal soil density for normal growth and development of an agricultural plant is 1.1-1.3 g/cm 3 (Konishchev et al., 2019;Szatanik-Kloc et al., 2018), and after the passage of a wheeled tractor its value in the tracks reaches 1.4-1.6 g/cm 3 (Szatanik-Kloc et al., 2018;Bogunovic et al., 2017). An increase in density above the optimum by 0.1 g/cm 3 leads to a deterioration in plant growth and a decrease in the yield of grain crops to 12% (Slyusarenko et al., 2016). Performing the full cycle of operations of the applied agricultural technology, movers of energy-saturated tractors subject more than half of the field area to over-compaction, which negatively affects the yield and profit of the enterprise. In this regard, the question arises of the need to reduce the negative impact of wheeled energy-saturated tractors on the soil.

Methods
The aim of the research is to determine the influence of propulsion systems of various running systems of energy-saturated tractors on the physical and mechanical properties of the soil (densityas the main compaction parameter). Research was conducted on the fields of the Novouzenskiy district of the Saratov region. Soil type is heavy loam. In the course of the research, the serial layout of the movers of the tractors K-701 and K-744P and the chassis system with dual tires were considered.
In particular, along the passage tracks of the tractor movers, there were measured the stresses occurring in the soil during the passage of the mover, as well as residual consequences in the form of changes in the density and hardness of the soil. Measurement of stresses was carried out by the method of strain gauging with the help of DPU sensors, on which a sensitive element was installed (Buylov et al., 2020). The sensors were previously laid in the soil at different horizons from 0 to 100 cm and connected to the strain gauge station MIC-018.
Measurement of soil density after the passage of tractor movers was carried out using the cutting cylinder method. The method is based on the introduction of a predetermined volume of a cutting cylinder into the soil with subsequent soil extraction and weighing.

Literature Review
The problem of negative impact of the energy-saturated tractor movers on the soil is valid not only for Russia, but also for a number of foreign countries with developed agricultural production. The recorded constant increase in the mass of tractors (Keller et al., 2019) causes excessive soil compaction, exceeding the permissible values, which leads to a decrease in the yield of agricultural crops. Lamandé and Schjønning (2018) note that an increase in the mass of the axis of the moving wheel in the soil leads to an increase in stresses created in the soil. The resulting stresses have values that exceed the bearing capacity of the soil, which causes its deformation and compaction. A similar situation was noted for caterpillar movers (Mudarisov et al., 2020).
In addition to the mass attributable to its axis, the internal air pressure in the tire also affects the increase in soil density after the passage of the tractor movers. Holthusen et al. (2018) prove that increasing air pressure in a tire leads to an increase in soil density along the tracks of tractors and combines. Reducing the air pressure in the tire helps to increase the bearing area of the wheel and reduce soil compaction.
Undoubtedly, an increase in soil density contributes to a decrease in crop yield. So, Szatanik-Kloc et al. (2018) proved that increasing the density of the soil from 1.29 g/cm 3 to 1.61 g/cm 3 reduces the development of the root mass of plants and, as a result, the development of the plant itself due to lack of nutrients.
The process of compaction of the soil is aggravated by numerous passes of energy-saturated tractors across the field as a result of the implementation of a full cycle of agricultural cultivation technology. The authors of the studies (Obour et al., 2017;Obour et al., 2018;Silva et al., 2018) note that soil compaction depends on the timing and time of tillage operations, while soil compaction reaches the lower layers to a depth of 1 m.
To implement the research, the above authors developed and presented a methodology for determining the degree of influence of the parameters of tractor propulsion on the soil compaction process. The considered methods well describe only the process of compaction of the soil by tractor engines under the considered conditions presented by the authors. But with changing soil and tractor operating conditions, the proposed methods require adjustment and further study.

Results
To determine the complex degree of negative impact of the movers of energy-saturated tractors on the soil, we propose to make a general assessment of the changes in the physical and mechanical properties of the soil as a result of this effect. This is possible if there is measurement of changes in the indicator of the soil reflecting its physical and mechanical property, taking into account the impact of the tractor movers on the soil. The proposed methodology for assessing the impact of tractor movers on the soil is based on the fact that the soil has the initial value of the physical and mechanical property indicator before the onset of action and its increase after the tractor mover is exposed. Then this condition can be written as: The most common way to reduce the negative impact of wheeled tractor movers on the soil is to increase the contact area of the mover by installing additional wheels (doubling). To determine the degree of influence of a single and dual mover of the K-701 and K-744R tractor on the soil, we conducted studies to determine the stresses that occur in the soil during the movement of the mover.
In the course of studies, it was established (Figure 1) that stresses equal to 225 kPa -a single wheel and 148 kPa -double wheels arise in the contact track of the wheel of the K-701 tractor with soil.
The stresses in the contact point created by the single wheel of the K-744P tractor are by 6.1% less compared to the K-701 tractor, and for double wheels this parameter is lower by 7.2%. An increase in depth leads to a decrease in the propagation of stresses arising in the soil during the passage of the K-701 and K-744P tractors with both single and double wheels (Figure 1). Comparing the results obtained, it can be seen that the stresses generated in the soil during the passage of the K-744P tractor with single wheels are on average by 10.1% lower compared to the stresses generated in the soil during the passage of the wheels of the K-701 tractor. A similar trend is observed with double wheels.

Conclusion
The presented mathematical dependences and the results of the studies allow determining the degree of negative impact of the energy-saturated tractor movers on the soil. Focusing on the tractor with the least degree of impact of movers on the soil, it is possible to carry out the optimal selection of tractors and technical devices providing the least compaction of the soil and, as a result, preservation of the crop yield.