Variation in Crop Yield Depending on the Tractor Tire Contact Area and Clay Loam Soil Moisture

Abstract

As the demand for productivity increases, machines are becoming heavier, and large coverage implements cause the slippage of tractor driving wheels, which initiate soil compaction and displacement. Sources indicate that compaction has an adverse impact on crop growth and grain yield. However, heavy machinery under draft is not always to blame for yield losses. The aim of the six-year research was to establish the relationship between grain yield and the slippage of tractors on heavy clay loam soil under two different moisture conditions. The spring cereals were grown as peas, spring barley, and spring wheat. Each treatment plot was compressed from track-to-track separately, altering the tire contact area on each treatment plot. Different tire contact areas were achieved by adjusting tire pressure and adding double wheels, while simulating a constant draft. Wheeled plots with the smallest tire contact area produced 9% less crop yield, whereas wheeling with the largest tire contact area showed an 8% increase in crop yield compared to the control plot without compaction in the dependency of soil moisture. Wheeling is like rollers breaking down clay loam soil clods and slightly compacting the topsoil, helping to retain moisture, leading to increased yields.

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 of compaction 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 Chan 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]. However, reduction in tire pressure significantly influences the deformation and rolling resistance ratio in various states of soil [16]. A larger contact area is advantageous in loose soils as it helps minimize sinkage and soil deformation. Reducing soil compaction is another critical consideration, as less intensive tillage at shallow harrowing depths minimizes the risk of soil damage [17]. While tractors do compact the soil when pulling implements, the question is whether the heavy machinery is responsible for reducing yields. However, as we will see from our study, compaction is not always detrimental; on the contrary, low soil compaction leads to greater moisture retention and yield improvements.

There are few tests that specifically measure the extent of soil damage caused nor how much crop yields by a slipping tractor pulling an implement are affected by a single pass. It remains unclear whether the soil is damaged to such a degree that plants suffer significant losses, except perhaps at the edges of the field where the tractor turns. To address this gap, a study was conducted to determine the cumulative effect of tractor chassis on crop yield, particularly when the soil is impacted by the slipping wheels of a loaded tractor across plots with too different soil moisture levels.

The aim of the research is to establish the relationship between tractor tire contact area, slippage at constant traction, and grain yield. The study seeks to determine whether increasing the tire contact area and reducing slippage can enhance the environmental friendliness of tractors.

2. Materials and Methods

Field trials were carried out at the Joniškėlis Experimental Station of the Lithuanian Research Centre for Agriculture and Forestry. This study was conducted from 2017 to 2022 as part of the long-term “Productivity and Sustainability of Agricultural and Forest Soils” institutional programme. The soil is Endocalcari Endohypogleyic Cambisol (CMg-n-w-can), typical in the northern part of the Lithuania lowlands (56°21′ N, 24°10′ E). Topsoil (0–25 cm) texture—heavy clay loam (27% of clay, 50% silt, 23% sand). The annual means of the last 40 years’ average temperature and amount of rainfall were 6.1 °C and 547.4 mm. The topsoil is nearly neutral with a pH of 6.4 and a moderate amount (2.9%) of humus. Its composition contained a medium level of phosphorus (P₂O₅ at 190 mg kg⁻¹) and a high level of potassium (K₂O at 260 mg kg⁻¹). Weed control, pest management and fertilization were carried out using machinery, specifically with a sprayer and spreader, according to common technology crop development stages.

Summer crops, including spring wheat, barley, and field peas, were cultivated to mitigate the impact of autumn and winter factors such as frost and prolonged excess moisture. In autumn, the soil was tilled with a chisel and then shallowly harrowed once in spring. The compression of each trial variant follows the scheme, with the entire field sown as a single unit. The crop rotation was the following: field peas “Tinker” (Pisum sativum) was planted in 2017 and 2022, spring wheat cultivars “Vanek” (Triticum aestivum) in 2018 and 2020, and spring barley “Noja” (Hordeum vulgare) in 2019 and 2021. Following soil preparation, the intended plant seeds were sown in mid-April using the Vaderstad Rapid 400C manufactured by Väderstad AB, Sweden seed drill. Each plot of the experiment was harvested with a specially adapted combine harvester “Sampo 500” manufactured in Sampo Rosenlew Ltd, Pori, Finland, and yield was calculated according to Equation (2) and converted to standard moisture (14%).

2.1. The Meteorological Conditions

Environmental conditions post-sowing play a crucial role in the growth of summer crops, influencing seed germination and early sprout development [18]. In past years, it has been noticeable that in Northern Europe, after sowing spring crops, there is often a lack of rain and heat, the warm period only starting at the end of May. In May, there is usually more rainfall, but the higher temperature evaporates most of the moisture. If the seeds germinate, the seedlings have time to take root because, in the following months, the temperature rises, and the rainfall is not always sufficient. In mid-summer, drought and excessive temperatures prevail. Proper seedbed preparation is crucial to mitigate these issues [19].

During the experiment years from 2017 to 2021, the weather from late March to early April was typically dry and cool, often experiencing frosts until late May, with increased rainfall starting in June. The lack of winter snow and dry periods after spring sowing led to uneven seed germination and sparse seedlings. Only in 2018 and 2022 was the post-sowing spring period reasonably wet and maintained moisture throughout the summer. April and May of 2018 were adequately wet and warm (Figure 1 and Figure 2).

2.2. Experimental Design and Treatment Applications

The wheeling treatments by the slipping wheels of a loaded tractor were carried out every spring for six years on non-ploughed soil, where only shallow tillage was performed to a 10 cm depth with a spring tine cultivator every autumn (Figure 3). The soil disturbance was expected to remain at the surface and accumulate the damage effect over several years. A soil with volumetric moisture of 22% is considered a wet soil, while 17% is considered a relatively dry soil. A compaction of soil was performed by a draft-loaded tractor on a single pass, compressing the soil once track by track on the whole surface of the treatment plot (Figure 4), adjusting the tire contact area (A) from A2 to A5 according to a randomization scheme.

All wheels of the tractor were engaged in drive mode, meaning all wheels were slipping. After a few days, the soil moisture content dropped from 22% to 17% at a depth of 0–10 cm, and then the nearby soil plot surface was compressed the same as before. The tire contact area (A) was adjusted by changing the air pressure in the tires and adding dual wheels of the same dimensions. Two air pressure settings of all tires were used: the nominal recommended pressure based on the technical characteristics of the tires (160 kPa) and the minimum permissible air pressure on loose soil (80 kPa). The contact area (A) of the tractor tires varied as follows: A2—0.60 m², A3—0.88 m², A4—1.08 m², and A5—1.44 m². Tire contact area was measured in the soil of the test field. The non-compacted control plot was marked as A1. The total number of treatment plots in both soil moistures was 40 (Figure 4).

Soil compaction and slippage test was performed using a “CASE MX135” manufactured in Doncaster, England tractor loaded with a constant draft, which matched the maximum operational load. The tractor’s drawbar was kept around 20 kN and deliberately chosen to induce slipping and exert a significant impact on the soil. The tractor, as its draft load, was towing another tractor with a tine cultivator. The investigative tractor was equipped with a data acquisition system used to measure draft and speed (

Table 1. Specifications of measurement devices.

Instrumentation	Measurements	Range	Accuracy
Draft sensor	KMB series 30	±40 kN	±1.5%
Ground speed sensor	Ground speed	0.89–19.67 m s−1	±3%
Force meters with external dynamometric cell, PCE-FB 50k	Force	1–50 kN	±0.1%
Moisture sensor, ML2x-UM-1.21	Soil moisture	m3.m−3 or %vol.	±0.5 m3.m−3

). The static weight distribution was 45% on the front axle and 55% on the rear axle, with a total static weight of 49.3 kN, according to the tractor specification chart (

Table 2. Technical data of tractor Case MX 135.

Manufacturer	Case IH
Engine: Cummins/CDC 5.9 L, 6-cyl. diesel
Engine type	Turbocharged diesel 6-cylinder 12-valve Liquid-cooled inline 359 [5.9 L]
Power	135 hp [100.7 kW] 2200 min−1
Transmission 2C03B
Chassis:	MFWD
Transmission	Type:	partial power shift
	Gears:	16 forward and 12 reverse
	Four power shift gears in four synchronized ranges. Three ranges in reverse.
Clutch	wet disc
Front tires	c
Rear tires	520/65 R38 Firestone
Trust Sensor	KMB, ser. 30
Speed radar	DJ RVS III
Weight:	12,460 lbs [5651 kg]
Wheelbase:	106.3 inches [270 cm]
Hydraulics type	closed center pressure flow compensated (PFC) (pressure: 200 bar)

).

Slippage was calculated using Equation (1) by measuring the distance traveled with and without load. The soil displacement caused by the front and rear wheels was determined using chalk and measured with a ruler (Figure 5). Rut depth was measured by ruler from the surface level.

The draft was kept the same in all tests as a constant load, but the slippage changes dependent on the tire contact area, as we will see later in the results. The slip of a tractor is described as speed loss, slippage calculated as the ratio between the actual speed $v_W$ and the theoretical speed $v_T$, or the travelled distance losses in the same number of revolutions of rear wheels, and slippage is estimated by the expression:

$$\delta ={\displaystyle \frac{v_T-v_W}{v_T}}={\displaystyle \frac{L_T-L_S}{L_T}}$$

(1)

where L_T is a distance length without load at variant of contact area (rolling radius), the actual distance with load and slip is denoted by L_S corresponding tire contact area variant (A), $v_T$ is a theorical speed, and $v_W$ is an actual speed.

In late spring, the number of emerged seedlings was recorded every second day in four 0.25 m² plots in the center of each field. The data for grain yield, including the number of ears, grain number per ear, and grain mass for spring wheat, barley, and field peas, were collected manually from each variant plot, covering an area of 0.25 m² in four replicates. The mass of one thousand grains was measured from each treatment plot of the harvested crop using a combine harvester. Yield results for each crop averaged for two years.

The grain yield (t·h⁻¹), approximated according:

$$Yield={\displaystyle \frac{N_e·N_g·G}{{10}^5}}$$

(2)

where N_e—number of ears, N_g—grain number in each ear, G—mass of thousand grains.

The statistical analysis was carried out using the RStudio programming software, RStudio/2023.12.0+369 Chrome/116.0.5845.190 Electron/26.2.4 Safari/537.36 package. The results were considered significant when p < 0.05.

3. Results

With an increasing tire contact area, slippage (δ) 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 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.

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. Well-structured soil helps maintain a balanced air–water ratio [25]. In heavy loamy soils, creating a suitable seedbed can be challenging, as the soil tends to form clods that dry out quickly, leading to moisture loss at the seed depth. In unprepared soil, seeds struggle to germinate, resulting in sparse crops and empty spaces often colonized by weeds. Compressing the soil can increase and maintain soil moisture under dry conditions. However, uncompacted soil at a depth of 0–5 cm retained more moisture compared to soil with single-wheel drive at nominal tire pressure. In other words, when single wheels slipped approximately 30%, the soil was extensively dug up and dried. As climate conditions shift towards dryness, it becomes evident that seeds do not germinate as well as they should in tractor tracks. It was observed that the seed germination rate was higher in treatments with wet soil compared to those with dry soil. The soil water in the layers beneath the seedbed serves as a primary moisture reserve for successful seed emergence during dry post-sowing periods.

Large seeds of field peas require more water for emergence than cereal grains. Figure 9 illustrates the cumulative effect of tractor tire contact area on field pea germination (plants per m²) under two distinct soil moisture conditions. In both soil conditions, germination increases with larger tire contact areas. This suggests that increased tire contact may improve germination, making it crucial to preserve the moisture of the upper seedbed layer. Increasing from A2 to A5 in dry soil, germination was about 19%; on wet soil was better by 11%. Peas that germinated later, across all contact areas, emerging only after rain, caused unripped peas at harvest.

Figure 10 illustrates the cumulative effect of tractor tire contact area on barley germination (plants per m²) under two distinct soil moisture conditions. Under dry soil conditions, barley germination ranged from 376.02 plants·m⁻² (control, A1) to 406.67 plants·m⁻² (largest contact area, A5), showing an 8.2% increase in germination seedling density. Incremental increases in tire contact area consistently resulted in increased germination of barley compared to the non-compacted control. In wet soil conditions, barley germination increased more noticeably, rising from 388.67 plants·m⁻² in the control (A1) to 408.62 plants·m⁻² at A5. A steady upward trend was observed as the tire contact area increased, resulting in improved barley germination, particularly noticeable in wetter soil conditions.

Figure 11 presents spring wheat germination results (plants·m⁻²) depending on tractor tire contact area (A) under two soil moisture conditions. The control plot without tractor compaction (A1) served as a reference for comparison. Adjustments in tire contact area were achieved by altering tire air pressure and adding dual wheels. The tire contact area rose from A1 (397.89 plants·m⁻²) to a maximum at A5 (419.77 plants·m⁻²), representing a 5.2% increase in wheat germination under dry soil conditions. As tire contact area increased in moist soil circumstances, germination improved progressively and peaked at A5 (434.89 plants·m⁻²), a 6.3% increase over the control (A1, 408.98 plants·m⁻²). Germination was 2.6% higher than in the control plot, even at the least adjusted contact area (A2). These findings underscore the positive impact of improved seed contact on germination rates, particularly in wet soil conditions, by showing that increasing tractor tire contact area consistently improves wheat germination. Crop productivity relies on favorable moisture conditions after sowing; a good start of sprout germination accompanies later during the whole growing period as a result of soil compaction level. The contact area of the tractor tires with the soil had a significant influence on the number of productive stems of spring wheat and barley. In the treatment of the dry soil plot, the number of productive stems of spring wheat and barley was lower compared to their number in the wet soil variant. It was found that the biomass of spring wheat and field peas per square meter measured lower in the A2 and A3 plots than in the A4 and A5.

Agricultural machinery with commonly used wheeled chassis significantly impacts germination, which can lead to reduced crop yields [25]. In less damaged soils rutted by double wheels, grain yield was higher compared to the control plot, considering the plant growth period. Ahmad et al. [7] determined that wheat grain yield was significantly reduced by soil compaction. In the first harvest year, maximum wheat grain yield (395.7 g·m⁻²) was obtained from T1 (no compaction), which progressively decreased to the minimum (278.9 g·m⁻²) from T4 (six passes). A similar trend of grain yield was observed in the second harvest year when the maximum grain yield (432.6 g·m⁻²) was recorded for T1 (control) and the minimum (323.0 g·m⁻²) from T4 (six passes). It was established that from compacted soil, wheat grain yield was lower than control for both years [7].

The results of our research investigated the compaction of a single pass, on the basis that tractors do not make multiple passes during sowing, but rather a single pass showed that the yield of spring wheat and peas negatively responds to the soil compaction under displacement by tractor slippage. Yield losses were approximately like stated by Ahmad et al. [7]. However, the lowest yields were obtained for peas in an uncompacted plot, which was coarse, cloddy soil. It was determined that there were no meaningful differences in grain yield mass between options when the soil was rutted at different tire pressures. However, significant differences were observed between options when the soil was rutted by single versus double wheels. Dry weather conditions led to fluctuations and common losses in grain yields, which are summarized and presented in the graphs. Soil compaction with double wheels had a positive effect on grain yield.

Figure 12 illustrates field pea yield results (t·ha⁻¹) as influenced by varying tractor tire contact areas (A) under two soil moisture conditions. Under dry soil conditions, the highest yield was observed at A5—2.75 t·ha⁻¹, which was approximately 9.5% higher compared to the non-compacted control plot A1—2.51 t·ha⁻¹. However, intermediate tire contact areas A2 and A3 resulted in decreased yields compared to A1, by 9.6% and 3.2%, respectively. Under wet soil conditions, increasing the tire contact area led to progressively higher yields. The highest yield was also achieved at the largest tire contact area, A5—3.0 t·ha⁻¹, representing a substantial yield increase of 8.6% compared to the control plot, A1—2.76 t·ha⁻¹. The smallest contact area (A2) showed a 7.3% yield loss compared to the control under wet soil conditions.

Figure 13 illustrates barley yield results (t·ha⁻¹) as influenced by varying tractor tire contact areas (A) under two soil moisture conditions. Under dry soil conditions, the highest yield was observed at A5—3.75 t·ha⁻¹, which was approximately 8.38% higher compared to the non-compacted control plot A1—3.46 t·ha⁻¹. However, intermediate tire contact areas A2 and A3 resulted in decreased yields compared to A1, by 9.83% and 6.07%, respectively. In contrast, under wet soil conditions, increasing the tire contact area led to progressively higher yields. The highest yield was also achieved at the largest tire contact area, A5—4.11 t·ha⁻¹, representing a substantial yield increase of 17.09% compared to the plot A2—3.51 t·ha⁻¹. Even the smallest tire contact area (A2) showed an 8.26% yield loss compared to the control under wet conditions.

When driving over dry soil with double wheels and reduced tire pressure, versus single wheels and nominal tire pressure, the grain yield increased by 9.1% for spring wheat (Figure 14), from 4.35 t·ha⁻¹ to 4.75 t·ha⁻¹. Under dry soil, the largest yield losses occur in A2 3.6% compared to the control plot. In wet soil, double wheels and reduced tire pressure versus single wheels and nominal tire pressure gave an increased grain yield of 8.3%. The difference in yields in control plots between dry and wet soil was 10.4%.

The results give conclusions that in both moisture conditions the A5 tire contact area gives the highest yields relative to control. Using double wheels at reduced pressure increases the grain yield (by 8%), whereas wheeling soil by single wheels at nominal pressure causes yield losses (by 9%).

4. Discussion

When a tractor pulls an implement, maintaining traction force, it induces slippage. To achieve a high net traction coefficient, the optimal volumetric soil moisture content is 15–20%. Beyond this range, traction efficiency decreases. Tractors achieve maximum draft at 30–40% slippage. From an energy efficiency perspective, an 8% slip is considered the best. If the slip is lower, more fuel is wasted due to the tractor’s excess weight for countering the slip; if it is higher, the soil is adversely affected [26,27].

The front and rear driving wheels displace the topsoil layer depending on the slippage value. According to Battiato and Diserens [28], a 27% slip can move the soil layer by 35 cm. The soil compaction is greater when the tractor slips than when simply passing through the soil without a draft. The authors also found that as slip increases from 1% to 27%, normal soil pressure rises from 90.6 kPa to 104.4 kPa. Additionally, the maximum shear pressure increases from 19.7 kPa to 42.6 kPa for the front wheels and from 6.0 kPa to 61.6 kPa for the rear wheels at a tire pressure of 160 kPa [28,29].

Swedish researchers suggest that good crop emergence requires soil moisture under the seeds to be at least 5% (w/w) higher than the moisture level at which plants wilt. Hard and dense soil beneath the seedbed layer supports capillary rise of water to the seed and results in good seed germination in the absence of rainfall. However, not all seeds end up in the hard and moist soil layer during sowing [18].

Presumably, shallow soil tillage during the six years of our study produced organic matter on the topsoil, which, contrary to expectations at the beginning of the study, reduced the effect of compaction by slippage. However, at the end of the research, the weeds spread significantly, and soil moisture was lower than in moldboard-ploughed soil. These findings confirm that increasing tire contact area by reducing air pressure and using dual wheels effectively minimizes soil compaction impact, resulting in significantly improved spring cereal yields, particularly under wetter soil conditions.

For future research: contrary to expectations, shallow tillage over a period of 6 years resulted in more surface organic matter and increasingly reduced the impact to the soil of the tractor driving wheels. Shallow tillage, or perhaps the summer-only crop rotation, is thought to be responsible for the higher weed spread, particularly the spread of long-lasting thistles (Cirsium arvense). Contrary to expectations, in the wet weather conditions, the soil moisture was lower in the treatment not ploughed soil than in the moldboard ploughed soil in the neighboring field, presumably due to the formation of water runoff channels.

5. Conclusions

The results of this specific practice study provide helpful indications and appropriate choices of tractor chassis configurations to meet soil structure requirements on heavy clay loam soils, thereby reducing soil compaction. The research determined that the soft wheeling could keep the soil moist and could improve the spring crop yield in dry weather conditions.

• It was settled that by increasing the tire contact area the grip with soil was improved and slippage was reduced. The soil surface displacement also has decreased by increasing tire contact area. The biggest positive effect was achieved by adding double wheels. The scale of the effect was influenced by soil moisture. Increasing tire contact area and reducing slippage enhance the environmental friendliness of tractors.
• It was determined that on wheeled clay soil the soil aggregates were pulverized and showed a significantly better structure than in the control plot. The germination with a better spout count was bigger after rutting the soil with a double tire contact area; in the control plots, the seeds germinated weakly—there were a smaller number of sprouts. Soil compacting kept the soil moist, but wheeling with the smallest tire contact area created deeper ruts and dried top-soil layers.
• It was established that the soil plots that were compressed with the lowest tire contact area gave less crop yield; in the plots that were compressed with the greatest tire contact area, the crop yield increased compared to the control plot without any compression of soil.
• The effect of increasing the tire contact area on the results was less pronounced when working on dry soil. Increasing tire contact area, especially with double wheels, has been proven and recommended for wet soil.