INVESTIGATION AND ASSESSMENT OF SOIL DENSITY CHANGED BY REPEATED PASSES OF MIDDLE-WEIGHT TRACTORS WITH DIFFERENT DRIVE SYSTEMS
DOI:10.30825/5.EJPAU.192.2020.23.4

Zbigniew Błaszkiewicz
Department of Environmental and Mechanical Engineering, Faculty of Biosystems Engineering, Poznan University of Life Sciences, Poland

ABSTRACT

This work presents the investigation of the influence of repeated passes of medium-weight tractors with single wheels and additional wheels on the soil density to the depth of 0.4 m. The experiments were carried out in the Greater Poland Region (Poland), on light soil (Luvisol, loamy sand). The soil density was analysed in the arable layer at the two depths of 0.08–0.12 m and 0.18–0.22 m and in the plough pan at a depth of 0.30–0.34 m. The tractors weighing 52.1, 62.8 and 71.8 kN equipped with single wheels (standard wheels) and tractors weighing 52.1 and 71.8 kN equipped with additional wheels were used in the experiments. The research proved that repeated passes of the tractors with standard and addition wheels caused a linear or non-linear (logarithmic) increase in the soil density in the arable layer. Only light tractor 52.1 kN with dual wheels caused soil density increase in the hard pen. The above soil density changes inconsistent depend on the weight tractors and mean tractor pressure. Repeated passes of tractors with additional wheels resulted in lower soil density in the arable layer especially by second and third pass. A larger number of passes of middle-weight tractors with standard wheels as well with additional wheels increases the risk of reduced yield of cultivated plants.

Key words: medium tractors, repeated passes, soil density.

1. INTRODUCTION AND AIM

Intensive tractors travelling on farmland cause decrease the yield of crops, increases the amount of energy that is necessary to cultivate soil, intensifies soil erosion [12, 23] and emission of greenhouse gases (CO2) to the atmosphere [16]. The risk of excessive soil compaction varies according to local soil and weather conditions, technological and technical factors of farming machinery, as well as farming traffic techniques [23]. This implicates investigations conducted under different conditions and allowing for new factors so as to achieve better results and make simple practical use of them.

The soil density is the most frequently researched indicator of critical soil compaction. The investigation of the intensive traffic on the soil was realized mainly in arable layer. The authors indicate a linear increase in soil compaction in the arable layer caused by tractors with single wheels or non – linear [4, 6]. Author of this work shows that this irregularity can cause tractors with single wheels with various weights in the deeper layer.

There are few available publications on the influence of repeated passes of tractors with additional wheels on soil density. According to Błaszkiewicz [4], multiple passes of lighter tractors with dual wheels weighing 19.4 kN and 27.5 kN cause a non-linear increase in soil density in the arable layer. In the hard pen the increase of the soil density depend on the weight of this tractors. Apart from that, dual wheels in the very light tractor resulted in lesser soil compaction at all the depths up to 0.4 m, especially after the first and second pass. Dual wheels in the heavier tractor (27.5 kN) reduced soil compaction only in the arable layer. We do not know if these regularities were also observed in medium-weight tractors with additional wheels.

In Poland the traditional farming technology is still predominant on light soils occupying about to 50% of the total farmland area. Middle-weight tractors with power of 40–80 kW are chiefly used in farming. They usually have single and increasingly addition wheels. In consequence, different wheel systems in tractors may cause different soil compaction during repeated passes along the same ruts. The literature provides divergent results of experiments of repeated pass effect conducted with different tractors and under different soil conditions. The causes of increment in the soil density after repeated passes of tractors with various running gears have not been comprehensively determined.

The aim of this study was to investigate of the influence of repeated passes of selected medium-weight tractors with single wheels and with additional wheels on light soil density to the depth of 0.4 m.

2. RESEARCH MATERIAL AND METHODS

The experiments were carried out in the Greater Poland Region, on common soil in the Central Polish Lowland, in the Leszno stadial of the Weichselian glaciation. The investigation was conducted in the traditional cultivation system on loamy sand soil (Luvisol) [13] loosened by ploughing. For many years every year the soil in the experiment places was cultivated by ploughing to a depth of about 0.24 m, without loosening the plough pan. The soil characteristic during field experiments is presented in Table 1.

Table 1. The soil characteristics during field experiments

Soil properties	Units	Tractor with weight 62.8 kN	Tractors with weight 52.1 kN	Tractors with weight 71.8 kN
Soil density, at the depth (m):     – 0.08–0.12     – 0.18–0.22     – 0.28–0.32 Soil water content at the depth (m):     – 0.08–0.12     – 0.18–0.22     – 0.28–0.32	Mg·m-3 m3·m-3	1.331 1.401 1.580 0.076 0.080 0.081	1.364 1.376 1.630 0.109 0.097 0.099	1.360 1.408 1.659 0.120 0.115 0.116

The influence of multiple passes (i) of tractors on the density (ρ) was determined in the arable soil layer (A. l.) at depths of 0.08–0.12 m (Depth I) and 0.18–0.22 m (Depth II) and in the plough pan at a depth of 0.30–0.34 m (Hard pan). The tractors weighing 52.1, 62.8 and 71.8 kN equipped with single wheels (standard wheels) and tractors weighing 52.1 and 71.8 kN equipped with additional wheels were used in the experiments (Table 2). The wheel pressure of the wheels was determined using methods proposed by Mc Kyes [15] and Grečenko [11].  The wheel pressure is the ratio between its load and the area of its contact with soil. The contact area was calculated applying the method proposed by Grečenko [11]. It is the product of the tyre width, diameter and the empirical coefficient of 0.245 proposed for flexible tyres and soft soil. The mean tractor pressure is the quotient of the tractor weight and the sum of contact area of its all wheels [25]. The tractor weighing 52.1 kN had dual wheels in the front and at the back, the tractor weighing 71.8 kN had triple wheels on either side of the rear axle. Table 2 shows the parameters of the tractors used in the experiments.

Each tractor with a particular running gear drove one way on the same ruts as many times as had been assumed. After pass of a tractor with single wheels along the same ruts four soil samples were randomly collected with 100 cm³ cylinders from investigated soil layers along the ruts axis. In the investigation of the tractors with additional wheels soil samples were collected from the ruts formed by standard and additional wheels. Then the mean soil density was estimated. The experiment was replicated three times at randomly selected places in the field. The density (ρ) was measured by means of the gravimetric (weight-dryer) method and expressed as Mg·m^-3. The results present the Figs 1–5. In the investigations the soil density was determined for selected passes of tractors ranging from 1–7 (Fig. 2) and ranging from 1–8 (Figs. 1, 3–5).

Table 2. The characteristics of tractors.

Specification	Units	Tractors (4WD) with weight:
		62.8 kN	52.1 kN	71.8 kN
Engine power Construction weight of tractor Front axle load Rear axle load Tyre of front wheels Tyre of rear wheels Internal pressure in front tyre Internal pressure in rear tyre Pressure (mean) of tractors with single wheels Pressure (mean) of tractors with additional wheels Pressure of rear single wheel Pressure of rear dual wheels Pressure of rear triple wheels Pressure of front single wheel Pressure of front dual wheels Fitting for additional wheels	kW kN kN kN MPa MPa kPa kPa kPa kPa kPa kPa kPa	87.6 46.10 28.06 34.72 12.4–24 18.4–34 0.14 0.10 107.8 – 88.0 – – 150.1 – –	67,1 37.51 23.64 27.47 13.6R24 16.9R34 0.18 0.10 90,6 45,3 79.0 39.5 – 104.0 52.0 MD-PLUS	114,0 49.35 32.37 38.45 18.4R34 14.9R24 0.18 0,10 108,2 47,7 91.3 – 34.0 133.5 – AW-Quick

The results of the experiment were analysed using professional software STATISTICA 12. The data were tested with Factorial ANOVA. The significant effects were observed for the p < 0.05. The obtained dependences were described with regression equations and coefficients of determination shown at Figs. 1–5.

3. RESULTS AND DISCUSSION

3.1. The influence of repeated passes of tractors with single wheels on soil density

Soil arable layer
As results from Figs. 1–3, repeated passes of tractors with single wheels caused linear or non-linear increases in soil density in ruts at two depths of the arable layer. The dependence of r(i) describe very good the linear regression equation for the tractors weighing 52.1  kN and 71.8 kN. The dependence of r(i) describe non – linear regression for the tractors weighing 62.8 kN. The determination rate R2 for these regression equations amount more than 90%. The investigated passes of the tractors weighing 52.1 kN and 71.8 kN increased the soil density to about 0.1 Mg·m^-3 (100 kg·m^-3) while the tractor of 62.8 kN increased the soil density two times more to about 0.2 Mg·m^-3 (200 kg·m^-3). 

Generally, the tractors in all range of investigated passes cause similar changes at the two depth of arable layer (Figs 1–3). Only the lightest tractor (52.1 kN) caused at greater depths soil densities significantly lesser (p < 0.05), by about 0.07 Mg·m^-3 (70 kg·m^-3) (about 3% of total porosity), especially after the first and second pass of the tractor.

From the analyse of the Figures 1–3 and the Table 2 results from that, in general, the increases in soil density caused by repeated passes of tractors were neither related with the weight of these tractors nor with their mean pressures. The lightest tractor (52.1 kN) with the lowest pressure and the heaviest tractor (71.8 kN) with the highest pressure caused a similar linear increase in soil density in the arable layer (significantly density increase from the third pass). On the other hand, repeated passes of the tractor with a mean weight of 62.8 kN, whose wheel pressure was comparable to that of the heaviest tractor (71.8 kN), caused a non-linear (logarithmic) increase in soil density (significantly density increase from third pass). The increase was much greater than those caused by the investigated lighter or heavier tractors. Consecutive passes of the tractor with a mean weight of 62.8 kN caused the greatest increase in soil density in the entire arable layer (significantly density increase from the second pass). The soil density exceeded even 1.8 Mg·m^-3 (1800 kg·m^-3). This non-linear increase in soil density was also reported in another study by Błaszkiewicz [4], but it was observed only in a very light tractor (17.4 kN).

Fig. 1. The influence of the repeated passes of the tractor weighing 52.1 kN equipped with single wheels on soil density

Fig. 2. The influence of the repeated passes of the tractor weighing 62.8 kN equipped with single wheels on soil density

Fig. 3. The influence of the repeated passes of the tractor weighing 71.8 kN equipped with single wheels on soil density in the arable layer

The observation that tractors of different pressures caused a similar increase in soil density (Figs. 1 and 3) does not confirm the results of the studies by Botta et al. [6], who reported that increases in the density of the soil arable layer were clearly related with higher pressures of tractor wheels. However, the observation made in this paper is in agreement with the results of the study by Chigarev and Loydata [7], who found that unfavourable soil properties caused by consecutive passes of light tractors could be the same as after passes of heavier tractors.

Soil plough pan
In general, the study proved that repeated passes of all tractors with single wheels did not cause significant changes in plough pan density (Figs. 1–3). These dependencies ρ[i] describe horizontal linear regression lines for the 62.8 and 71.8 kN tractors and for the 71.8 kN weight tractor the line only slightly increased. This observation indicated that repeated passes increased soil density, but the increase was not statistically significant (p < 0.05). The soil density in the ruts left by the above tractors of 62.8 and 71.8 kN was high – it ranged from about 1.68 to about 1.74 Mg·m^-3 (1680 to about 1740 kg·m^-3). When the tractor weighing 62.7 kN passed, soil density in the plough pan was slightly greater, i.e. about 1.8 Mg·m^-3 (1800 kg·m^-3).

3.2. The influence of repeated passes of tractors with additional wheels on soil density

Arable layer
In general, repeated passes of tractors weighing 52.1 and 71.8 kN equipped with additional wheels caused a non-linear increase in soil density (Figs. 4 and 5). The significantly density increase (p < 0.05) from the second and third pass at both depths of the arable layer under study is observed. The dependences ρ[i] for the range 1–8 passes can be described with logarithmic equations (R2 > 84%). As can be seen in the Figures, the non-linearity of these dependences resulted from much greater increases in soil density after the second and third pass then after the following passes.

Fig. 4. The influence of the repeated passes of the tractor weighing 52.1 kN equipped with dual wheels on soil density

Fig. 5. The influence of the repeated passes of the tractor weighing 71.8 kN equipped with triple wheels on soil density

As results generally from Figures 4 and 5, the tractors weighing 52.1 and 71.8 kN with additional wheels caused lesser soil densities than the tractors with single wheels within the whole range of repeated passes.

But this soil density decrease at the deeper layer is not the same for the investigated tractors. The 52.8 kN tractor with additional wheels after 8 passes caused the increase in soil density to about 50% greater (about 0.15 Mg·m^-3 (150 kg·m^-3)) than the tractor with single wheels. However, this effect was not observed with the heavier tractor weighing 71.8 kN. Both running systems of this tractor caused similar increases in soil density. It seems that greater increases in soil density caused by the lighter tractor with additional wheels resulted from lesser soil density due to smaller pressure during the first pass.

There is different influence of repeated passes of these tractors with additional wheels on the soil density at the depth in the arable layer (Figs. 4 and 5). At the lesser depth of the arable layer both tractors of different weights caused a similar density within the whole range of repeated passes (p > 0.05). At this deeper depth of the arable layer the lighter tractor with dual wheels on its axles caused a significantly lesser compaction (p < 0.05) within the whole range of repeated passes.

Plough pan density
Repeated passes of the tractors weighing 52.1 and 71.8 kN with additional wheels did not have similar influence on plough pan density (Figs. 4 and 5). Repeated passes of the heaviest tractor weighing 71.8 kN with triple wheels did not cause changes in soil density (p > 0.05). The dependence ρ[i] was characterised by a straight horizontal line and equation (Fig. 5). The absence of increase in soil density may have been caused by very high density after the first pass. The high soil density caused its higher strength and reduced soil deformation under wheels. On the other hand, consecutive passes of the lightest tractor weighing 52.1 kN with dual wheels caused parabolic increases in soil density in the ruts. The significantly density increase (p < 0.05) from the five pass is observed. This may have resulted from much lesser soil density and strength after the first pass (Figs. 4 and 5). It is noteworthy that after the first pass of the heavier tractor weighing 71.8 kN the plough pan density was much greater than after the first pass of the lighter tractor weighing 52.1 kN although both tractors exerted similar pressures (Table 2). This fact might confirm the opinion expressed by Botta et al. [6] and Arvidsson and Keller [3] that the density of subsoil layers is influenced by the weight of tractors. According to Olsen [18], increased soil density may be caused by the fact that although wheels exert similar pressures, greater weights cause greater strength in the subsoil layer.

3.5. Agronomic assessment of repeated soil compaction in ruts

This study provides a simplified assessment of the influence of repeated passes of tractors and their running systems on the yield of selected crops. Specific soil densities in ruts were compared with other authors’ empirical findings concerning optimal light soil densities for the highest yield of crops. According to the literature, the optimal soil density for winter wheat is about 1.49 Mg·m^-3 (1490 kg·m^-3) [10, 21], for sugar beet – about 1.51 Mg·m^-3 (1510 kg·m^-3) [19], for winter barley – 1.4–1.5 Mg·m^-3 (1400–1500 kg·m^-3) [20, 21] and 1.43 Mg·m^-3 (1430 kg·m^-3) [8], for rye [9] and maize grown for seeds [17, 21, 22] – 1.35–1.45 Mg·m^-3 (1350–1450 kg·m^-3). Other authors make general statements that increased density in soil prepared by tractors for sowing causes significantly reduced yield of crops [1, 14].

As results from the comparison, standard wheels of all the tractors caused unfavourable soil compaction after the first pass, i.e. more than 1.6 Mg·m^-3 (1600 kg m-3) (Figs. 1–3). As a result, the yield of all of the aforementioned crops might be reduced by consecutive passes of tractors. As results from Figs. 4 and 5, the additional wheels in the tractors weighing 52.1 and 71.8 kN caused considerable reduction in soil density within the whole range of passes under study. The density was reduced to 1.45–1.54 Mg·m^-3 (1450–1540 kg·m^-3), on average by about 5–8 % (i.e. 3–5% of total porosity) (Figs. 1–5). However, only after the first and second pass soil density was close to the optimal value only for winter wheat and sugar beet, whereas in the ruts left by the lighter tractor soil density was close to the optimal value for winter barley (Figs. 4 and 5). A larger number of passes of tractors with additional wheels increases the risk of reduced yield.

The empirical dependences ρ[i] of this research may be used in further prognostic research to calculate the local distribution of soil density in fragments of farmlands undergoing repeated compaction by tractors with standard wheels and with additional wheels. The dependences may also be used for improvement in prediction of yield, e.g. with the methods proposed by Van de Zande [5, 24].

4. CONCLUSIONS

The findings of this study and results published in the literature led to the following conclusions concerning the conditions of experiments. Repeated passes of the tractors with single wheels and with additional wheels caused a linear or non-linear (logarithmic) growth in soil density at the investigated depths in arable layer. Totally only light tractor with dual wheels caused soil density increase in the hard pen. The quantity of the above soil density changes inconsistent depend on the weight tractors and mean tractor pressure. In general, multiple passes of tractors with additional wheels resulted in lower soil density in the arable layer than passes of tractors with single wheels especially by second and third pass. However, this regularity was not always observed in the plough pan.

Totally one can concluded that the yield of all of the analysed crops might be reduced by the first and consecutive passes of middle-weight tractors with standard wheels. The tractors with additional wheels only after the first pass leave soil density close to the optimal value only for selected cultivated plants.

The study was financed by the Ministry of Science and Higher Education in Poland.

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Received: 31.08.2020
Reviewed: 8.09.2020
Accepted: 8.11.2020

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Zbigniew Błaszkiewicz
Department of Environmental and Mechanical Engineering, Faculty of Biosystems Engineering, Poznan University of Life Sciences, Poland
Wojska Polskiego 50
60–637 Poznań
Poland
email: zbigniew.blaszkiewicz@up.poznan.pl

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