# Variation in Crop Yield Depending on the Tractor Tire Contact Area and Clay Loam Soil Moisture
## 1 Introduction  
Soil protection programs often recommend improving soil health using plant-based methods, such as diversifying crop rotations and growing cover crops. However, the negative impact of heavy machinery on soil is sometimes overlooked. Thus, improving tractor chassis design is also an important soil protection measure [1]. More powerful machinery is expected to be used to meet the growing demand for agricultural efficiency in the future [2, 3]. The modernization of agricultural technology is by combining implements to perform several tasks simultaneously, thus increasing the number of working parts and requiring greater traction force. Consequently, the mass of tractors has increased to meet these power demands [3, 4]. In agriculture, wheeled tractors are the most used and have the greatest impact on soil. As the tractor pulls an implement across the field, the driving wheels engage with the soil, and the tire grips sink into the ground, creating shear and frictional forces [5, 6]. With the expansion of farmland areas, there are cases where it is challenging to complete tasks on time, leading to operations being carried out without considering soil conditions. Soil compaction tends to be more pronounced in regions characterized by colder and moist climates [7]. The issue of soil compaction is particularly significant in heavy clay soils, as they are often susceptible to compression [8]. The degree ofcompaction is influenced by factors such as tire dimensions, air pressure, and the draft exerted by tractors. The compression of soil is not solely attributed to the mass of machinery but also results from the skidding of driving wheels [9]. It is more effective to prevent soil compaction beforehand than to address the issue after it has occurred, as this approach reduces environmental impacts [10]. This finding is supported by [PERSON] et al. [8], who observed a significant reduction in the root growth of canola and wheat in the layer beneath wheel tracks. There was no noticeable difference in wheat yield on the wheel track (5.3-5.5 t/ha); canola grain yield on the wheel track was only 34% of that between the tracks (1.1 t/ha versus 3.2 t/ha). When the front and rear driving wheels run on the same track, the tire grips disturb and replace the soil, leading to both vertical compaction due to the tractor's weight and horizontal compaction from traction. The extent of compaction depends on the tire dimensions, air pressure, and the load carried by the tractor [11]. Because of the substantially large size of the tire support areas on heavy machinery, the pressure applied per unit of tire contact area is sometimes more critical than the machine's weight. Studies have shown that the contact length of tire 18.4 R38 increases from 0.64 m at pressure 124 kPa to 0.76 m at 41 kPa tire pressure [12]. This adjustment allows more tire grips to interact with the soil, facilitating soil movement and enhancing the involvement of soil layers. The expansion of the tire contact area is influenced by increases in wheel radius and width, coupled with a decrease in tire pressure. Consequently, this results in a larger contact area and consistently achieves a higher net traction ratio [13]. Tire pressure and dynamic load influence contact area with the ground. The experiments show that with an increase in an 18.4R38 tire contact area, the slip can reduce by 10% on loose soil and by 4% on hard soil [14]. Although tire contact area is an approximate measure that does not account for specific soil conditions, correlations exist between tire performance and draft. The tractor, slipping at about 30%, develops the maximum traction force; above that, it does not increase anymore because the tire grippers completely cut the soil layer [15]. While tire contact area adequately represents its influence on traction, changes in soil conditions have a much more pronounced impact on traction performance than changes in tire dimensions [6].