Soil & Tillage Research 120 (2012) 92–98

Agricultural traffic: Motion resistance and soil compaction in relationto tractor design and different soil conditions

G.F. Botta a,b,*, A. Tolon-Becerra c, M. Tourn a, X. Lastra-Bravo c, D. Rivero d

a University of Buenos Aires, School of Agriculture, 1427 Buenos Aires, Argentinab National University of Lujan Technology Department, 6700 Lujan, Argentinac University of Almerıa, Ctra Sacramento s/n, La Canada de San Urbano, 04120 Almerıa, Spaind National University of La Pampa, School of Agriculture, 6300 La Pampa, Argentina

A R T I C L E I N F O

Article history:

Received 9 May 2011

Received in revised form 15 November 2011

Accepted 18 November 2011

Available online 9 December 2011

Keywords:

Tractor traffic

Soil bearing capacity

Rut depth

Ground pressure

A B S T R A C T

Farmers may desire a high cone index soil for tractive purposes or a low cone index (CI) soil for root

penetration and seedling emergence. The function of any agricultural tractor is to provide mobility for

itself and to power an implement. The aim of this paper was to (a) assess the impact of two tractors with

different tyre sizes and axle loads on motion resistance (MR) and on the CI for three different soil

mechanic conditions and (b) determine the existing relationships between MR and ground pressure

parameters and tyre sinkage. Traffic was simulated with one pass on clay soil for a front-wheel assist

tractor (FWA, load = 77.7 kN) and a four-wheel drive tractor (4WD, load = 98.01 kN) on three soil

conditions: direct sowing systems, ploughed and seedbed. The outlined hypotheses were as follows: (1)

there is a direct relationship between the subsoil compaction and the MR force of the FWA and 4WD

tractors, and (2) the power loss produced by the MR depends on the soil mechanics. The experiment was

conducted in the eastern section of the Rolling Pampa region of Argentina at 348360S, 588400W. MR, rut

depth (RD) and CI were measured. The MR mean values of the 4WD were 9.30, 6.59 and 2.31 kN for

ploughed, seedbed and direct sowing soil, respectively, whereas the values for the FWA were 10.41, 7.91

and 4.67 kN, respectively. For the three soil conditions, no significant differences were found in the RD

between the 4WD and FWA. For the topsoil level (0–150 mm), one FWA pass caused mean values in the

CI of 2150, 1835 and 1780 kPa for direct sowing, seedbed and ploughed soil, respectively, whereas for

4WD the values were 1890, 1640 and 1587 kPa, respectively. For the subsoil (150–600 mm), 4WD

caused higher CI values than the FWA. The CI mean values of the 4WD were 2477, 2240 and 1890 kPa for

direct sowing, seedbed and ploughed soil, respectively, whereas the values for the FWA were 2240, 1870

and 1770 kPa, respectively. For the different soil conditions, the subsoil compaction increased as the total

axle load increased, independently from the ground pressure. Moreover, for both tractors, a greater MR

force was observed in the soil with the lowest bearing capacity. The smallest power loss ratio due to MR

and engine power was found with the 4WD.

� 2011 Elsevier B.V. All rights reserved.

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1. Introduction and literature review

Considering modern Argentina’s farm machinery equipment,agricultural soils receive about 20 Mg km ha�1 of traffic intensityduring seeding operations. This represents only 20% of the totaltraffic of a conventional tillage system and occurs after the primaryand secondary farm works, just at the moment of minimummechanical stability (Botta et al., 2008).

Function of any agricultural tractor is to provide mobility foritself and to power an implement. During tractor work on

* Corresponding author at: University of Buenos Aires, School of Agriculture,

1427 Buenos Aires, Argentina. Tel.: +54 2323 422350; fax: +54 2323 422350.

E-mail address: sad@s6.coopenet.com.ar (G.F. Botta).

0167-1987/$ – see front matter � 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2011.11.008

agricultural soil, three types of losses occur: transmission loss,losses due to motion resistance and losses due to slippage. Anoperator has minimal influence on the first loss; however, there ismuch that can be done to reduce the other two. In the Pampasregion of Argentina, there are two systems of working the land:direct sowing (22 million hectares) and conventional tillagesystems (16 million hectares) (Botta et al., 2010). Within thislandscape are soils with high bearing capacity (those underconstant direct sowing) and soils with medium-to-low bearingcapacity (seedbed and ploughed soils).

Over the last 20 years, this reality, with its two extremes, hasresulted in the steady incorporation of four-wheel-drive tractors(4WD > 90 kN, total load and equal wheels) and front-wheel-assisttractors (FWA > 70 kN, total load and smaller front wheels) intothe Argentine market. These two basic tractor designs account for

G.F. Botta et al. / Soil & Tillage Research 120 (2012) 92–98 93

nearly all of the agricultural tractors in Argentina used to performtasks for both direct sowing and conventional tillage (primary andsecondary tillage) (Botta et al., 2007).

In the latter system, secondary tillage follows the completion ofprimary tillage, and this requires low-intensity vehicle traffic. Thistraffic must take place, however, on soil with low bearing capacityand high compactability. In loose soils, a significant increase intopsoil bulk density and tyre sinkage occur (Taylor et al., 1982).This results in the compaction of the soil and leads to motionresistance, one of the primary causes of loss of power in thedrawbar pull. The direct consequence of increased motionresistance is higher fuel consumption and greater soil compaction.Motion resistance implies that fuel must be expended to form atrack (the product of soil deformation) and to push the soil in frontand to the sides of the tyre, leading to the deformation of the tyreitself (Koolen and Kuipers, 1983).

Motion resistance is defined as the difference between grosstraction and drawbar pull. Gross traction is the ratio of the drivewheel torque to the drive wheel rolling radius, where the rollingradius was determined on concrete at zero drawbar pull. All lossesdue to soil and tyre deformation and any other internal resistanceof the tractor have been combined into the term motion resistance(Bashford, 1985).

Motion resistance force is closely linked to topsoil compactionbecause, according to Botta et al. (2002), independent of the soilmechanical condition where the traffic takes place, the more atractor weighs, the greater the force required for self-propulsionand the greater the increase in soil compaction. In this respect, it isimportant to note that typical tillage depths in Argentina areapproximately 150 mm, so the Ap horizon is considered in thisexperiment, 0–150 mm as the topsoil layer, and the subsoil can bedefined as the soil below the tillage layer (Botta et al., 2002). Thisdefinition of the subsoil includes the pan layer, caused by tractors,as the upper part of the subsoil. The pan layer is the layer below theannually cultivated layer. It will vary in thickness depending on thetype and severity of compaction created by either the implementsor wheels or both. In some instances, it is loosened on a regularbasis. The unloosened subsoil is the layer that normally remainsundisturbed by tillage operations. It is also at a depth where tillageoperations would be considered undesirable and often uneco-nomical, and if carried out, would create the potential for damage.This layer may, however, be disturbed during drainage operationssuch as subsurface drainage and mole ploughing as well as soilimprovement operations (Alakukku et al., 2003).

Both Perdok and Tijink (1990) and Wood and Burt (1987)performed soil–bin trials with 18.4-38 tyres on both firm andploughed soils. They utilised dynamic loads of 10 and 20 kN andinflation pressures between 110 and 140 kPa to vary the groundpressure. These studies demonstrate that tyre pressure controlsthe magnitude of the horizontal components in the centre of thesoil–tyre contact area and the dynamic weight at the edge of thearea. They also demonstrate that the motion resistance force isgreater on ploughed soil, as sinkage, in this case, produced spacesbetween the soil under-tread interface that contributed toincreasing the horizontal components of the normal forces. Mrhar(1995) conducted trials on three different soil conditions (hard,medium and soft soil) utilising a 99 kW tractor with static weightdistribution of 26.71 kN and 33.83 kN on the front and rear axles,respectively, and the rear tyre contact area on concrete was0.27 m2. This study quantified both the horizontal and vertical soildeformation as well as slip and motion resistance and determinedthat the highest motion resistance forces on the softest soilcorresponded to the highest ground pressures and to the greatestsoil sinkage.

Other specific references to traffic on non-consolidated tilledsoils were shown by Van den Akker (1998). Their experimental

variable was wheel size, and consequently, different groundpressures were obtained to match different tyres to the sametractor. The treatments were: (a) tyre SR 20.0/70-20 (80 kPa, lowerpressure) and (b) SR 16.0/70-20 (240 kPa, higher pressure). From a0–150 mm depth, the Low Ground Pressure (LPG) system showed a40% reduction in maximum pressure peaks, but at a 550-mmdepth, the differences between these treatments became insignifi-cant.

From the large amount of empirical data on soil–tyreinteraction currently available, some general guidance can begiven to quantify the compactive capability of running gear. Tijink(1994) concluded that the following factors help to reduce thecompactive effect of a single pass of a tyre: (1) low inflationpressure, (2) low tyre load, (3) low average ground pressure (i.e.,the ground pressure measured on a rigid surface), (4) low tyrestiffness, (5) radial tyre construction, (6) low wheel slip, and (7)low lugs. Among these factors, the average ground pressure on arigid surface, which under common conditions is related to the tyreinflation pressure, is the most important. The number of passes ofrunning gear over the same track is an important additional factor.Limitation of the average ground pressure and the wheel load canbe considered to be the major engineering tools for the control ofsoil compaction.

Victor and Cartwright (1993) address the influence of groundpressure on topsoil compaction and the performance of anagricultural tractor. These authors concluded that the low groundpressure exerted by wide tyres on soil produces a decrease intopsoil compaction, an improvement in traction and less motionresistance. On the other hand, a review of the tyres used on tractorssold in Argentina, followed by a comparison of the tyres specifiedby the NIAE wheels manual for different situations (Dwyer andFebo, 1987), demonstrated that the tyres widely in use are smallerin size than those specified. This incorrect compatibility of thetyres with the tractor weight increases the ground pressure, which,in turn, causes greater tyre sinkage, an increase in motionresistance and greater topsoil compaction. This paper seeks tofind a harmonious combination of tractor tyres and ballast, therebyreducing the ground pressure, soil compaction and motionresistance force.

The specific objectives are as follows: (a) assess the impact oftwo tractors with different tyre sizes and axle loads on motionresistance and the cone index of three different soil mechanicconditions, and (b) determine the existing relationships betweenmotion resistance, ground pressure parameters and tyre sinka-ge.The outlined hypotheses were as follows:

Hypothesis 1. There is a direct relationship between subsoil com-paction and the motion resistance force of FWA and 4WD tractors.

Hypothesis 2. The power loss due to motion resistance depends onsoil mechanical conditions.

2. Materials and methods

2.1. The site

The experiment was conducted in the eastern portion of theRolling Pampa region, Buenos Aires State, Argentina, located at348360S, 588400W; altitude 22 m above sea level; slope type 1 withgradient 0.5%; well drained, drainage class 4; no stone class 0. Thesoil was a fine clay, illitic, thermic Typic Argiudol (Soil Conserva-tion Service, 1994), with an organic matter content ranging from3.6% (w/w) on the surface to 1.4% at a 0.6-m depth. The physicaland mechanical properties of the soil are provided in Table 1.

The experimental traffic was imposed on three differentsoil conditions: (a) soil in direct sowing condition: the soil

Table 1Soil physical and mechanical properties.

Depth (mm)

0–150 150–300 300–450 450–600

Proctor

Optimum water content (%, w/w) 22.3 � 0.18 23.0 � 0.15 24.4 � 0.11 25.2 � 0.11

Maximum dry bulk density (Mg m�3) 1.49 � 0.05 1.53 � 0.04 1.68 � 0.03 1.71 � 0.03

Soil organic carbon (kg�1) 16.5 � 4.6 7.00 � 2.1 5.10 � 1.1 4.3 � 1.0

Total nitrogen (g kg�1) 1.80 � 0.08 0.90 � 0.02 0.70 � 0.00 0.8 � 0.00

C/N ratio 9.10 7.70 7.20 5.4

Clay (<2 m) (g kg�1) 230 � 3.37 263 � 2.30 330 � 2.88 372 � 2.63

Silt (2–20 m) (g kg�1) 308 � 4.81 299 � 4.01 309 � 2.31 239 � 1.89

Silt (20–50 m) (g kg�1) 454 � 4.51 433 � 3.46 357� 4.01 385 � 3.86

Fine sand (100–250 mm) 8 � 1.38 5 � 1.10 4 � 0.96 4 � 1.12

pH in H2O (1:2.5) 6 � 0.02 5.6 � 0.01 6.3� 0.03 6.2 � 0.02

G.F. Botta et al. / Soil & Tillage Research 120 (2012) 92–9894

management history consisted of 14 years of direct sowing, with acommon regional crop rotation of winter wheat (Triticum aestivum

L.) followed by soybean (Glycine max L.), (b) seedbed condition:primary tillage with a mouldboard plough to a depth of 180 mm,followed immediately by two passes with a tandem disk harrow(490 N/disk, 40 disks) and one pass of a shaped spring-toothharrow. This treatment represents a tillage system commonly usedin the region, and (c) ploughed soil: primary tillage with amouldboard plough with a 6-furrow plough operating at a 180-mm depth.

The water content (w/w) during trafficking on the three soilconditions, on average, was 17.5% dry basis in the surface (0–150 mm), 18.0% at 150–350 mm and 21.1% at 350–600 mm. Theaverage moisture content of this profile was 18.8%, which is 4.9%below the moisture content of the maximum compaction for thissoil as determined by the standard Proctor compaction test,ASTM.D.698 (1933) (see Table 1). It is important to note that thedifferences in the soil water content between the soil conditions atthe time of traffic were generally not significant (P < 0.01).

2.1.1. Experimental treatments and layout

Each experimental soil condition was trafficked with only onepass by the tractors with their rear tyres travelling over the tracksof the front wheels. This was done to recreate the conditions of thesoil following primary and secondary tillage work. The tractorsused in the field trial had all of the basic design characteristicscommon to vehicles currently on the market in Argentina (Table 2).The model John Deere 7515 (FWA) is identified as TR1 and theZanello 500 (4WD) as TR2. The John Deere 3530 tractor was onlyused to tow TR1 and TR2 to measure the motion resistance force.

The tractors were towed at a speed of 5 km h�1. The tyres of thetowed tractor rolled along a different line than that of the towingvehicle (Fig. 1). This was done so that the towed tractor did nottraffic over the tracks of the towing tractor and therefore did not

Table 2Tractor characteristics.

Tractor

FWA

John Deere 7515

4WD

Zanello 500

Engine power (CV/kW) 140/102.6 194/143

Front tyres 18.4-26 23.1-30

Rear tyres 24.5-32 23.1-30

Inflation pressure, front tyre (kPa)a 100 140

Inflation pressure, rear tyre (kPa)a 80 80

Total weight (kN) 77.7 98.01

Front weight (kN) 31.10 60.27

Rear weight (kN) 46.60 37.74

Forward speed (km h�1) 5 5

a Inflation pressure with according to Firestone agricultural tyre handbook.

affect the measurements of motion resistance (MR) force. Thevalues obtained for MR force were corrected by the angle ofmovement between the two tractors. Twenty readings were takenalong each test plot using a dynamometer device.

The MR force was measured using a dynamometer deviceprovided by the Agricultural Machinery class (Agronomy Faculty,Buenos Aires University, Argentina). A cab-mounted unit collectedthe draft force data with a sampling frequency of 200 Hz. The datawere stored in a data logger and downloaded to Excel files foranalysis.

The tyre inflation pressure and tractor ballast were notmodified for the trial, assuming that the tractors are preparedfor tillage with implements that demand large pulling force(mouldboard plough, chisel plough, heavy discs, etc.). Cross-plytyres are usually utilised on commercial farms in Argentina.

Prior to the trial, the two tractors were weighed to obtain theirtotal and individual axle loads (Fig. 2). The experimental designconsisted of a randomised block (traffic with each tractor) withthree replications. The plot size was 20 m wide and 75 m long(1500 m2). The tyre/soil contact area was measured by reversing ordriving the tractor into the virgin soil of the experimental field andspraying the area around the tyre with paint. A hydraulic lift wasthen used to raise the tractor so that the tyre track could betransferred onto a sheet of glass, printed from there onto paper,and measured with a planimeter. The average ground pressure wasestimated as the total axle load divided by the tyre/soil contactarea for both tyres on the axle. Finally, the tyre widths weremeasured in the field under working conditions (Botta et al., 2008).

Statistical analyses were performed utilising the Statgrafprogram v 7.1. An analysis of variance (ANOVA) was performedon the data, and Duncan’s multiple range test was used to analysethe means.

Fig. 1. Motion resistance measured for 4WD tractor on ploughed soil.

Fig. 2. The tractors were weighed on public scales.

G.F. Botta et al. / Soil & Tillage Research 120 (2012) 92–98 95

2.1.2. Survey parameters monitored

The soil water content (w/w), cone index (CI) and rut depth (RD)were measured on the same day, after the traffic treatments wereapplied.

The soil water content was measured with a gamma probe(Troxler, 3440) at different depth ranges as follows: 0–150, 150–300 and 30–450 mm taken at intervals of 50 mm. Each quotedvalue of the soil water content is the average of ten measurements,all of which were verified by cylinder (100 mm in high, 50 mm indiameter) because in soils with >3% organic matter, the gammaprobe can substantially overestimate the water content.

Table 4Ground pressure and soil tyre/contact area for rear and front tyres measured in three

Tyre Direct sowing Seedbed

Contact area (m2) Ground pressure (kPa) Contact area

4WD tractor

Rear tyre 23.1-30 0.379 49.8 0.401

Front tyre 23.1-30 0.520 57.2 0.533

FWA tractor

Rear tyre 24.5-32 0.420 56.6 0.430

Front tyre 18.4-26 0.250 61.8 0.260

Table 3Cone index values (kPa) at five depth ranges after the passage of the tractors in three

Depth range (mm) Increment of CI (%) with

respect to control plot

for FWA tractor

FWA Tractor

John Deere 7515

Direct sowing

0–150 20.1 2150 c

150–300 8.90 2190 a

300–450 5.10 2208 a

450–500 2.80 2260 a

500–600 1.67 2302 a

Seedbed soil

0–150 68.3 1835 b

150–300 12.0 1680 b

300–450 10.5 1780 b

450–500 16.4 1980 b

500–600 8.51 2040 b

Ploughed soil

0–150 125.3 1780 c

150–300 36.3 1500 b

300–450 29.9 1689 b

450–500 33.4 1881 b

500–600 33.5 2010 b

Values with different letters (horizontally) are significantly different at each depth (P <

The CI was determined using a Scout 900 recording penetrom-eter S 313 (ASAE Standards S313.2, 1993) at the centre line of thetyre track. Each datum is the average of 25 samples for each plot inthe depth range of 0–600 mm, taken at intervals of 25 mm. Thisparameter was measured on the bottom of the RD, in the centrelines of the tyre tracks, because the compressive effects tend toconcentrate in this zone (Sohne, 1958).

The RD was measured using a profile meter consisting of a set ofvertical metal rods (length 500 mm and diameter 5 mm), spaced at25 mm horizontal intervals, sliding through holes in a 1-m longiron bar. The bar was placed across the wheel tracks perpendicularto the direction of travel, and the rods were positioned to conformto the shape of the depression.

3. Results and discussion

The original condition of the soil in each plot was the result ofdifferent farming systems. The values of the CI (without traffic) werealways higher for direct sowing soil, followed by seedbed and,finally, ploughed soil (see Table 3). These results were predictable.

In general, there were no significant differences found in the soilwater content between the soil conditions when the CI wasmeasured, and correction or allowance for this was not considerednecessary.

The inflation pressure of all the tyres utilised fell within thepressure intervals recommended in the Firestone tyre manufac-turer manual (Table 2).

The ground pressure data (Table 4) showed smaller values onsoil under the mouldboard plough because this soil condition

soils conditions.

Ploughed soil

(m2) Ground pressure (kPa) Contact area (m2) Ground pressure (kPa)

47.0 0.470 40.1

56.5 0.590 46.3

54.1 0.550 42.2

59.7 0.320 47.4

soils condition.

4WD Tractor

Zanello 500

Increment of CI (%) with

respect to control plot

for 4WD tractor

Control plot

1890 b 11.8 1690 a

2300 b 14.4 2010 a

2499 b 19.0 2100 a

2509 b 14.1 2198 a

2600 b 14.8 2264 a

1640 c 50.4 1090 a

2200 c 46.6 1500 a

2275 c 41.3 1610 a

2405 c 41.4 1700 a

2480 c 31.9 1880 a

1587 b 100.8 790 a

1690 c 53.6 1100 a

1945 c 49.6 1300 a

2004 c 42.1 1410 a

2121 b 40.9 1505 a

0.01, Duncan’s multiple range test).

Fig. 3. Relationship between motion resistance (kN) and cone index (kPa) in topsoil (0–150 mm) for three soil conditions.

G.F. Botta et al. / Soil & Tillage Research 120 (2012) 92–9896

supports the wheel load lesser than the soil under direct sowingand seedbed conditions, according to Botta et al. (2006). The largestcontact area measured for each tyre on ploughed soil logicallycorresponded to a lower ground pressure in the trials done on thissoil mechanic condition. The FWA tractor used narrower fronttyres than the 4WD tractor, causing a higher ground pressure in allsoil conditions compared to the 4WD tractor. For ploughed soil, thecontrasts were 47.4 and 46.3 kPa; for seedbed soil, they were 59.7and 56.5 kPa; and for direct sowing, they were 61.8 and 57.2 kPafor the FWA and 4WD tractors, respectively. In addition, the rearFWA tractor tyres caused higher ground pressure than the 4WDtractor tyres because the rear axle of the FWA tractor was heavierthan that of the 4WD tractor.

In the three soil conditions, for the range of 0–150 mm (topsoil),all treatments showed significant differences in the increase of CIvalues after traffic. These data (Table 3) confirm that tractor trafficsignificantly (P < 0.01) increased the CI compared to the controlplot that had no traffic. It is also important to note that for all soilconditions, the FWA caused greater increases in CI values at thisdepth level than the 4WD tractor. Fig. 3 shows that for all the soilconditions, there is a strong positive relationship between MR andtopsoil compaction CI under the narrow range of soil watercontents encountered. Additionally, in the quoted figure, it can beseen that the MR is higher in all tillage regimes for the FWA tractorthan for the 4WD tractor. These results are corroborated by theresults of each trial for RD, as can be observed in Fig. 4. The RD wasgreater for the FWA tractor, a fact that is directly related to theground pressure and topsoil compaction. Although there were nostatistically significant differences, they did establish a tendency.

Finally, the results of RD measurements found that the RD wasgreater in ploughed soil than in other mechanical soil conditions.The low-bearing capacity of the ploughed soil resulted in greatertyre sinkage, which was seen in the formation of deeper and morepronounced ruts (Fig. 4), coinciding with the findings of Perdok andTijink (1990).

Fig. 4. Mean of rut depth values produced by tractors for three soil conditions.

The CI results in the subsurface for the three soil conditions(below 150 mm) were in accordance with the results of Carman(2002). Table 3 shows that the CI values correspond directly to thetotal tractor weight. For the 0–600 mm depth range, the 4WDtractor produced higher CI values than the FWA tractor.

The effects of the 4WD tractor load on the CI were significant(P < 0.01). Based on the findings of Hakansson and Petelkau(1994), this probably occurred because in current farmingpractices, extreme loading of the upper part of the subsoil mayoccur while driving in the open furrow during ploughing and alsowhen high-loaded wheels combined with high tyre inflationpressures are applied while the soils are still wet. Therefore, thegreatest increases in the CI in the subsoil were produced by the4WD tractor. This increase was always high and revealedstatistically significant differences (P < 0.01) even though therewas only one tractor pass. This means that there is no legitimateevidence to validate Hypothesis 1, but rather there is sufficientdata to reject it. Additionally, in accord with Botta et al. (2002) andHakansson (2005), this tends to confirm that the pressureinfluences the soil strength in the surface layers and the wheelload provides more influence deeper in the profile.

Soil compaction in the subsoil tends to be cumulative becauseconventional tillage is seldom carried out at that depth. Subsoilingoperations are the only mechanical means able to solve thisproblem. Despite the fact that the maximum subsoiling depthattainable by conventional equipment is about 600 mm, we agreewith Hakansson and Reeder (1994), who stated that when subsoilcompaction is induced below the Ap horizon, mechanicalloosening to alleviate this compaction is very difficult, alwaysexpensive and eventually impossible. In addition, these authorsestablished that subsoil compaction can cause lasting reductions incrop yield.

The MR force was greater on the ploughed soil and diminishedfor the direct sowing soil, revealing statistically significantdifferences (P < 0.01) between the different mechanical soilconditions (Table 5 and see Fig. 3). This coincides with the resultsof Wood and Burt (1987) and Randall et al. (1985). These authorsstate that almost 90% of the RD values were caused by the first pass.They also state that the MR force of the first traffic accountsbetween 70 and 80% of the values registered from the second pass.The highest MR values on ploughed soil may have been due to thevery soft soil condition because the tyre sinkage was such that the

Table 5Mean of motion resistance values (kN) for both tractors in three soils condition.

Soil conditions FWA John Deere 7515 4WD Tractor Zanello 500

Direct sowing 4.67 b 2.31 a

Seedbed 7.91 c 6.59 c

Ploughed soil 10.41 d 9.30 d

Values with different letters within each soil conditions shows significant

differences among tractors (P < 0.01, Duncan’s multiple range test).

Fig. 5. Mean of power losses due to motion resistance (kW).

G.F. Botta et al. / Soil & Tillage Research 120 (2012) 92–98 97

tyre lugs produced a deeper mark, contributing to a greater MRforce than the seedbed.

These results are in accordance with Stranks (2006), who foundthat the increase in the MR in soft soil (soil bin) was caused by thetyre under load sinking into the soil as it passed over it. This led toan increase in MR because the amount of soil in front of the tyreincreased along the run. All the plots of MR on the low CI soil showa similar trend. The plots for the DS soil show a more linear tracewith little or no increase in rolling resistance over the run resultingfrom the soil being able to withstand the applied load.

In accord with Wood and Burt (1987) is the consideration thatthis sinkage resulted in an increase in the opposing force of the soilto the advance of the wheel, which, according to these authors, isthe cause of the greatest loss on soft soils.

It is important to note that this test did not account for anychange in the front–rear weight distribution that occurs in thespecial working situation of the FWA tractor. During the test, thefront–rear weight transfer for this tractor was estimated for thethree soil conditions. The results show that the weight transfer inthis trial (towed tractor) is low. When the FWA tractor was towedon direct sowing, seedbed and ploughed soil, the weight transferswere 0.77 kN, 1.30 kN and 1.71 kN, respectively.

The greater MR force for all mechanical soil conditionsregistered by the FWA tractor (Table 5) could be explained bythe elevated soil build-up in front of the rear tyre in particular. Thissituation would put it on the same level as the 4WD. In addition, itcan be seen that there is not a harmonious relationship betweenthe tyre width, 24.5-32, and the 45.08 kN of axle weight for theFWA tractor. This fact is clearly seen in the highest MR force valuesfor the three soil conditions. This can probably be attributed to thegreater RD on soft soils. Tyre 23.1-30 with 37.74 kN appears to bethe best combination of width and axle weight because itcorresponds to the lowest value of motion resistance on ploughedsoil for the 4WD tractor (Table 5). Tyre 23.1 R 30, which has thesmallest diameter utilised in the rear axle in this study, may havebeen favoured by its low inflation pressure, providing a betterresult than may be expected using a higher pressure. This isconfirmed by the value in the tyre soil contact area, which is muchhigher than could be calculated using existing models, e.g., Woodand Burt (1987).

Another factor that could influence the results is the interactionof the tandem wheels matched in the 4WD tractor. In this tractor,the front and rear tyres are 23.1-30. Therefore, the rear tyresfollowed completely in a compacted path. This could be the causeof the lower motion resistance values produced by the 4WDtractor.

Botta (1997) found that the use of tyres up to 0.58 m wide wasbest for conventional tractors (2WD – 102.6 kW engine power).Above this width, an increase in motion resistance force and lowtopsoil compaction occur on low-bearing capacity soils. However,occasionally, as for the direct sowing soil, the value of the MR forcefor the FWA almost doubled that of the 4WD tractor. This could beexclusively due to the greater axle load of the tractor and theabsence or decrease of a moving wave of soil in front of the tyre inthis soil condition. It is important to note that the tractor weighttransfer in this tests is rear-to-front not front-to-rear (which occursin practice) causing a slight increase in ground pressure at the FWAtractor because its front tyres are narrower than the 4WD. Thiscould explain why the FWA tractor produces greater motionresistance losses than the 4WD.

The power loss (kW) due to MR was always greater for the FWAtractor (Fig. 5). Additionally, it must be highlighted that the FWAtractor has a 102.6 kW power engine and the 4WD has a 143 kWpower engine, which means that proportionally (%) the poweravailable for traction will be affected more in the FWA tractor.Taking this into consideration and based on past research, these

tractors could reduce the power loss due to MR if correctlymatched radial tyres are used. The improvement could be due tothe greater flexibility of the radial-ply, giving a lower internal MR.Additionally, the sidewalls of the radial tyre are too supple tosupport any amount of the load compared with the amountsupported by the much stiffer cross-ply sidewalls. At low loads andcorresponding low inflation pressures, the radial has an advantageover the cross-ply, reducing as the loading increases, so that at highloads and therefore high inflation pressures, both tyres adoptsimilar characteristics. Additionally, according to Botta et al.(2008), radial tyres reduced topsoil compaction and RD comparedto cross-ply tyres. However according to Taylor et al. (1976), in asoft soil, much of the total soil/tyre deformation takes place in thesoil, which decreases the difference between the shape of a radialtyre and a cross-ply tyre. In a firm soil, most of the deformationmust occur in the tyre, which maximises the difference in theshape of radial and cross-ply tyres.

According to Dwyer (1978), the overall Tractive Efficiency of a4WD is in the range of 0.78–0.8, and for a FWA, it is between 0.66and 0.68. This author was working with early FWA tractors withmuch smaller front wheels than the current generation. In fact,Marquez Delgado (2011) considers that an FWA tractor wouldhave a tractive efficiency of 0.74, a figure close to 4WD tractors’value in the worst tractive conditions. The overall TractiveEfficiency is defined as the ratio between the power provided tothe engine and the power effectively utilised in traction. In the caseof the two tractors tested, a higher MR force was found in the soilwith the lowest bearing capacity (ploughed soil). The power lossdecreased for both tractors when the soil bearing capacityincreased, thereby providing, thanks to the results obtained,sufficient evidence to validate Hypothesis 2.

4. Conclusions

Within the limits of our experimental conditions, we can arriveat the following conclusions.

In the case of both tractors on the three soil conditions, a greaterMR force was observed in the soil with the lowest bearing capacity(ploughed soil). This loss decreased for both tractors as the soilbearing capacity increased.

For the three different soil conditions, the results are consistentwith the notion of topsoil damage related to ground pressure, andsubsoil damage being related to total axle load.

One pass of the tractors with high total weight (up to 90 kN) onthe soil under the direct sowing system produced subsoilcompaction.

The smallest power loss ratio due to MR and engine power wasfound with the 4WD tractor on the three soil conditions.

References

Alakukku, L., Weisskopf, P., Chamen, W.C.T., Tijink, F.G.J., van der Linden, J.P., Pires,S., Sommerf, C., Spoor, G., 2003. Prevention strategies for field traffic-inducedsubsoil compaction: a review. Part 1. Machine/soil interactions. Soil Till. Res. 73,145–160.

G.F. Botta et al. / Soil & Tillage Research 120 (2012) 92–9898

ASAE Standards, 1993. Soil Cone Penetrometer S 313.2. American Society ofAgricultural Engineering, 611 pp.

Bashford, L.L., 1985. Axle power distribution for a front-wheel assist tractor. TransASAE 28 (5), 1385–1388.

Botta, G., Tolon-Becerra, A., Lastra-Bravo, X., Tourn, M., 2010. Tillage and trafficeffects (planters and tractors) on soil compaction and soybean (Glycine max L.)yields in Argentinean pampas. Soil Till. Res. 110, 167–174.

Botta, G., Rivero, D., Tourn, M., Bellora Melcon, F., Pozzolo, O., Nardon, G., Balbuena,R., Tolon Becerra, A., Rosatto, H., Stadler, S., 2008. Soil compaction produced bytractor with radial and cross-ply tyres in two tillage regimes. Soil Till. Res. 101,44–51.

Botta, G., Pozzolo, O., Bomben, M., Rosatto, H., Rivero, D., Ressia, M., Tourn, M., Soza,E., Vazquez, J., 2007. Traffic alternatives in harvest of soybean (Glycine max L.):effect on yields and soil under direct sowing system. Soil Till. Res. 96, 145–154.

Botta, G., Jorajuria, D., Rosatto, H., Ferrero, C., 2006. Light tractor frequency on soilcompaction in the rolling Pampa region of Argentina. Soil Till. Res. 86 19–14.

Botta, G., Jorajuria, D., Draghi, L., 2002. Influence of the axle load, tyre size andconfiguration, on the compaction of a freshly tilled clayey soil. J. Terramech. 39(1), 47–54.

Botta, G., 1997. Armonizacion del peso y rodado del tractor para reducir lacompactacion del suelo. M.Sc. Tesis. Facultad de Ciencias Agrarias y Forestales,Universidad Nacional de La Plata, La Plata, Argentina, 61 pp.

Carman, K., 2002. Compaction characteristics of towed wheels on clay loam in a soilbin. Soil Till. Res. 65, 37–43.

Dwyer, M.J., Febo, G., 1987. Agricultural Tyre Handbook. National Institute ofAgricultural Engineering, West Park, Silsoe, UK, p. 32.

Dwyer, M.J., 1978. Maximizing agricultural tractor performances by matchingweight, tire size and speed to the power available. In: Proc. 6th ISTVSConference, Vienna.

Firestone Agricultural Tire, 2010. http://www.firestoneag.com/agricultural_tires.asp.

Hakansson, I., 2005. Machinery-induced compaction of arable soils – incidence,consequences–counter-measures. Swedish University of Agricultural Sciences,Department of Soil Sciences, Uppsala. Division of Soil Management. Report No.109, 153 pp.

Hakansson, I., Petelkau, H., 1994. Benefits of limited axle load. In: Soane, B.D.,Ouwerkerk, C. (Eds.), Soil Compactions in Crops Production. Elsevier, Amster-dam, Netherlands, pp. 479–499.

Hakansson, I., Reeder, R.C., 1994. Subsoil compaction by vehicles with high axle loadextent, persistence and crop response. Soil Till. Res. 29, 277–304.

Koolen, A.J., Kuipers, H., 1983. Agricultural Soil Mechanics. Springer-Verlag, Amster-dam, 458 pp.

Marquez Delgado, L., 2011. Field tractors test.(Ensayos de tractores). RevistaAgrotecnica 9, 40–54 (Ed. B&H, Madrid, Espana).

Mrhar, M., 1995. (Use of heavy tractor in forage harvesting from the viewpoint ofthe bearing capacity of the soil.) Aktualni problemi mehanizacije poljoprivrede.undate, 347–354; 5 tab. Zagreb, Yugoslavia.

Perdok, U.D., Tijink, F.G.J., 1990. Developments in IMAG research on mechanizationin soil tillage and field traffic. Soil Till. Res. 16 (1), 121–143.

Proctor, R., 1933. Fundamental principles of soil compaction. Eng. News Rec. 111245–248, 286–289, 348–351.

Randall, K., Wood, E., Wells, L.G., 1985. Characterizing soil deformation by visualinspection of the profile. J. Terramech. 22 (3), 183–184.

Soil Conservation Service, 1994. Keys to Soil Taxonomy, 6th ed. Soil Survey Staff,USDA, Washington, DC, USA.

Sohne, W., 1958. Fundamentals of pressure distribution and soil compaction undertractors tyres. Agric. Eng. 39, 276–281.

Stranks, S.N., 2006. The effects of tyre systems on the depth and severity ofcompaction. Thesis for the Degree of Master of Science by Research. CranfieldUniversity at Silsoe NSRI, UK, 71 pp.

Taylor, J., Burt, E.C., Bailey, A.C., 1976. Radial Tire Performance in Firm and Soft Soils.ASAE, St. Joseph, MI 49085. ASAE Paper No. 76-1036, pp. 1062–1064

Taylor, J., Trouse, C., Burt, E.C., Bailey, A.C., 1982. Multipass behavior of a pneumatictire in tilled soils. Trans. ASAE 25, 1229–1236.

Tijink, F.G.J., 1994. Quantification of vehicle running gear. In: Soane, B.D., vanOuwerkerk, C. (Eds.), Soil Compaction in Crop Production. Elsevier, Amsterdam,Netherlands, pp. 391–415.

Van den Akker, J.J.H., 1998. Prevention of subsoil compaction by defining amaximum wheel load bearing capacity. In: Marlander, B., Tijink, F., Hoff-mann, C., Beckers, R. (Eds.), Soil Compaction and Compression in Relation toSugar Beet Production. Advances in Sugar Beet Research, IIRB, 47, rue Mon-toyer 1000, Brussels, Belgium, pp. 43–54.

Victor, C., Cartwright, A., 1993. Benefits of low pressure tyres in agriculture. WorldAgriculture 151–154.

Wood, R.K., Burt, E., 1987. Thrust and motion resistant from soil tire interface stressmeasurements. Trans. ASAE 30 (5), 1288–1292.