# Variation in Crop Yield Depending on the Tractor Tire Contact Area and Clay Loam Soil Moisture
## 3 Results  
With an increasing tire contact area, slippage (\(\delta\)) decreases, and as the working speed (\(v_{W}\)) increases, the actual travelled distance (\(L_{S}\)) approaches the theoretical distance (\(L_{T}\)) according to the wheel revolutions. The actual travelled length decreases relative to the slip percentage. By increasing the tire contact area from A2 to A5, the grip on the soil improved, reducing slippage by 23% without adding excess weight to the tractor. Slippage was reduced from 30% to 10% and from 33% to 13% in different soil plots with varying volumetric moisture levels, respectively (Figure 6). The greatest reduction in slippage was achieved by adding double wheels and lowering tire pressure. The results indicate that increasing the contact area of tractor tires reduces soil top layer displacement (Figure 7). It was found that as the tire contact area increased, the displacement of the dry soil layer decreased from 34.65 cm to 16.23 cm, and in wet soil, it decreased from 38.25 cm to 21.25 cm. The smallest soil displacement occurred in dry soil using double wheels with reduced tire pressure. Conversely, the maximum soil  
Figure 6: Slippage dependence on tire contact area. displacement was observed in wet soil when using single wheels at nominal tire pressure. It was also noted that greater soil displacement correlates with greater topsoil moisture. The depth of the ruts left by tractor wheels is influenced by the slippage, tire contact area, soil hardness, and moisture levels. Slippage of the tractor's driving wheels adversely affects the topsoil layer, causing the tire grips to dig into the soil, creating deep and compressed ruts [20]. To properly cultivate these ruts and level the soil surface, it is necessary to increase the working depth of the implement, which is used for loosening and levelling the tracks [21,22]. This process leads to increased draft and higher fuel consumption [23]. Increasing the contact area of the tractor tires, particularly by adding double wheels, reduced the depth of ruts. Figure 8 presents rut depth measurements (cm) as affected by changes in tractor tire contact area (A) under two distinct soil moisture conditions: dry (17%) and wet soil (22%). This reduction ranged from 11.8 cm to 7.89 cm in wet soil and from 11.26 cm to 7.43 cm in dry soil, corresponding to a rut-depth reduction of approximately 30% in dry soil, and resulting in a 33.1% reduction in wet soil relative to A2. By increasing the tire contact area, the tractor's mass is distributed over a larger surface, which results in reduced soil deformation and compaction. This effect is especially noticeable when the driving wheels slip, particularly in wet soil conditions. It was also concluded that the reduction in ruts depth had a positive effect on yield. Figure 8: Rut depth dependence on tire contact area. Figure 7: Soil displacement dependence on tire contact area. The formation of a dry crust is more noticeable in clay loam soil, leading to poor soil contact with seeds [24]. This rough soil aggregate structure results in poor seed germination, which was observed in the unwheeled control plot A1, where the number of sprouts was low. Tractor wheels pulverized the soil aggregates, revealing significant structural differences among the treatments with different tire contact area combinations. It was observed that during the dry period after sowing, seeds germinated more quickly in areas where the soil was rutted by a chassis with a smaller tire contact area. In the control plots where the soil was not pulverized, seed germination was weak, resulting in a lower number of plants. Although rutting and soil compaction helped retain moisture, driving with the smallest tire contact area created deeper ruts, causing the moist soil to be displaced more profoundly. Maintaining a soil structure that is not excessively rough is crucial for proper seed germination. To ensure adequate seed-to-soil contact, it is important that the soil is not overly compacted.