Effects on some clay soil qualities following the passage of rubber-tracked and wheeled tractors in...

Effects on some clay soil qualities following the passage ofrubber-tracked and wheeled tractors in central Italy

P. Servadioa,*, A. Marsilia, M. Pagliaib, S. Pellegrinib, N. Vignozzib

aAgricultural Mechanization Research Institute, Via della Pascolare, 16, Monterotondo, Rome, ItalybResearch Institute for Soil Study and Conservation, Piazza D’Azeglio 30 50121 Florence, Italy

Received 28 January 2000; received in revised form 29 January 2001; accepted 22 February 2001

Abstract

There is increased use of rubber-tracked tractors for ploughing on clay soil (Vertic Cambisol) in central, south and insular

Italy instead of metal-tracked tractors, because they allow travel on public roads. Field tests were carried out on arable soil

previously ploughed and harrowed to compare two types of tractors, one rubber-tracked (CAT Challenger Ch 45) and one

wheeled (New Holland 8770) in order to establish the compacting effects resulting from 1 and 4 passes of the tractors in the

same track. The following parameters were studied: soil penetration resistance, bulk density and its increment ratio, soil shear

strength, soil macroporosity and hydraulic conductivity. Multiple passes made by the two tractors induced very similar effects

on the soil in regards to soil penetration resistance. Mean values of penetration resistance (0–0.20 m depth) were 1.15 MPa for

the rubber-tracked tractor and 1.11 MPa for the wheeled tractor; mean values of penetration resistance (0.21–0.40 m depth)

were 1.07 MPa for the rubber-tracked tractor and 1.17 MPa for the wheeled tractor. The decrease in macroporosity, in

particular that of elongated pores in the soil surface layer (0–0.10 m depth) was greater in treatments involving the rubber-

tracked tractor (from 20.2 to 2.7%) than for the wheeled tractor (from 20.2 to 10.3%). Following traffic of the two tractors,

hydraulic conductivity decreased and the following values were found for the five treatments: control, 18.48 mm h�1; wheeled

tractor 1 and 4 passes, 11.15 and 7.45 mm h�1, respectively; rubber-tracked tractor 1 and 4 passes, 3.25 and 1.1 mm h�1,

respectively. Highly significant correlations between shear strength and dry bulk density, and between hydraulic conductivity

and elongated pores and total macroporosity were found. Significant linear relationships between macroporosity and

penetration resistance for 1 and 4 passes of both tractors were found in the soil layers (0–0.10 m). A significant difference was

found between tractors and for correlations of penetration resistance values above control values. However, in the soil layer

(0–0.20 m depth), with respect to the higher degree of macroporosity and low values of penetration resistance, treatments

involving wheeled tractor (1 pass) showed a lower degree of soil compaction than was observed after 1 pass of the rubber-

tracked tractor. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Rubber-tracked tractor; Wheeled tractor; Soil compaction; Soil porosity; Soil penetration resistance; Soil shear strength; Soil

structure; Central Italy

1. Introduction

Metal track-type tractors have the potential for

causing less soil compaction than wheeled tractors

because the tracks usually have a greater surface

area than wheels for tractors with equivalent power

Soil & Tillage Research 61 (2001) 143–155

* Corresponding author. Tel.: þ39-6-906-7917;

fax: þ39-6-906-25591.

E-mail address: [email protected] (P. Servadio).

0167-1987/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 7 - 1 9 8 7 ( 0 1 ) 0 0 1 9 5 - 7

ratings (Brown et al., 1992) and they are used in

many farming areas where large drawbar loads are

required. Other advantages include the large ground

contact area of the track resulting in high tractive

efficiencies, high dynamic traction ratios, low ground

pressures and good stability on steep slopes (Sofiyan

and Maximenko, 1965; Rowland, 1972; Brusentsev,

1967).

In two previous studies (Marsili and Servadio,

1996; Marsili et al., 1998), compaction of clay soil

was investigated after the passage of rubber and metal-

tracked tractors. The first study was carried out on

tractors with medium power of 52 kW (rubber tracks)

and 55 kW (metal tracks), and the second study was

carried out on high power tractors of 131 kW (rubber

tracks) and 114 kW (metal tracks). Both tests indi-

cated that metal tracks cause less compaction than

rubber tracks in agreement with Bekker (1969). The

lower compaction caused by metal tracks was attrib-

uted to the more uniform stress distribution on the soil

compared to the rubber track, and this advantage

resulted in a significantly lower bulk density and

penetration resistance in the uppermost soil layers

for a higher number of tractor passes. In the second

test, where soil structure measurements were intro-

duced, it emerged that with respect to the higher

degree of macroporosity, treatments involving the

metal-tracked tractor showed better soil structure

quality than did the rubber-tracked tractor. However,

rubber-tracked tractors have the advantage of being

able to run on concrete and other road surfaces, and

can be made lighter than metal-tracked tractors of

equivalent performance. In this paper, further studies

were carried out using a rubber-tracked tractor in

comparison with a wheeled tractor. Although there

is an almost universal use of wheeled tractors in Italy,

rubber-tracked tractors are now used on clay and

steeply sloped soil, particularly in south, central and

insular areas, in preference to metal-tracked tractors,

because they combine high tractive efficiencies, high

dynamic traction ratios and low soil compaction

attributes of tracked tractors with the high travel speed

and manoeuvrability of 4-wheel drive tractors (Esch

et al., 1990), and they are allowed to travel on public

roads.

Increased awareness of soil conservation problems

has stimulated interest in soil dynamics studies (Scha-

fer and Johnson, 1990) and in soil compaction. Studies

on soil compaction in recent years have included:

laboratory simulation (Koolen, 1974; Burt et al.,

1980; Willatt, 1987; Dawidowski and Lerink,

1990), prediction or statistical correlation between

many of the relevant properties (Ahuja et al., 1984;

Koolen and Vaandrager, 1984; Ohu et al., 1986), and

field experiments (Verma et al., 1976; Greenland,

1977; Raghavan et al., 1978; Russell, 1978; Way

et al., 1995; Wiermann et al., 1999). Other studies

have included field experiments on compaction in

relation to crop production (O’Sullivan, 1992; Lowery

and Schuler, 1994; Soane and Van Ouwerkerk,

1994a,b, 1995; Arvidsson and Hakansson, 1996). In

most of these studies, changes to soil physical proper-

ties following externally applied loads were studied,

but continuous technological progress of the agricul-

tural machines with respect to running gear, axle

loads, etc. have necessitated new field tests and studies

in order to evaluate the significance of mechanical

compaction to soil use and, eventually, to optimize soil

management.

Soil properties which are pertinent to agricultural

soil use, and which are affected during field opera-

tions, are referred to as soil qualities (Koolen, 1987).

Soil qualities was introduced as a generic term for soil

physical and mechanical properties which are relevant

for soil use, and include workability, trafficability,

water and air conductivities, water and air storage

capacities, thermal and strength properties, erodibility,

stability, and root penetrability. Soil physical and

mechanical properties are generally dependent also

on the soil moisture content (Dawidowski and Lerink,

1990).

In this study, measurements were made on the

effects on some clay soil qualities following the

passage of two tractors, one equipped with a rubber

belt track design and one with tires. The primary

objective of this study was to investigate compac-

tion of a ploughed and harrowed clay soil subjected

to successive tractor passes. Parameters measured

included soil penetration resistance, bulk density

and its increment ratio, soil shear strength, soil

porosity and hydraulic conductivity. The second

objective was to develop statistical correlations for

the soil layers tested between soil penetration resis-

tance and porosity, between soil porosity and hydrau-

lic conductivity and between bulk density and soil

shear strength.

144 P. Servadio et al. / Soil & Tillage Research 61 (2001) 143–155

2. Materials and methods

2.1. Soil and treatments

The field tests were carried out using two large trac-

tors, fitted with different types of running gear, one with

rubber tracks (CAT Challenger 45) and the other with

four drive wheels (New Holland 8770). The main

technical characteristics of these tractors and their run-

ning gear are given in Tables 1 and 2. On a plain terrain

30 km north of Rome, compaction tests were carried out

on a well drained clay soil, classified as Vertic Cambisol

(FAO, 1988). The soil was previously ploughed to

0.40 m depth in March and harrowed to 0.20 m depth

with rotary harrow in April 1999. Two months after

harrowing, tractor passes were applied to the soil, 1 and

4 passes on the same track left by the tractor for a total of

four treatments, in a randomised block of eight plots,

each 420 m2. Measurements were also made on a con-

trol area with no traffic adjacent to every plot (Table 3).

Forward speed was 0.50 m s�1 for both tractors.

The initial soil conditions are given in Table 4. Soil

moisture content was measured at a depth from 0 to

0.20 m by taking samples of soil immediately outside

the track left by the tractors and in the control areas

using a corer sampling ring. These samples were

weighed and dried until they reached a constant

weight. Soil compaction was quantified by measuring

cone penetration resistance, dry bulk density, and

shear strength and soil structure was determined by

measuring porosity and hydraulic conductivity.

2.2. Penetration resistance and dry bulk density

Soil penetration resistance was measured in the

tracks left by each tractor after 1 and 4 passes and

Table 1

Main technical characteristics of the rubber-tracked tractor and its track used in the traffic studies

Characteristics CLAAS-CAT Ch 45 tractor

Measured mass with ballast (kg) 11895

PTO power (kW) 179

Track tread (m) 1.895

Overall width (m) 2.525a

Overall length (m) 5.40

Cabin height max (m) 3.5

Height above soil of implement hitch (m) 0.48

Type of track Two reinforced rubber tracks

Total track length (m) 8.60 external

Track thickness (m) 0.028

Lugs per track (number) 36 � 2

Distance between centres of lugs (m) 0.24

Lug height (m) 0.063

Lug width (m) 0.30

Radius of steering area (m) 2.80

Supporting wheels (number) 3b

Diameter of driving wheel (m) 1.455

Diameter of support wheels (m) 0.36

Diameter of track wheel (m) 0.80

Theoretical ground contact length (m) 2.20

Track width (m) 0.635

Total area of support of the two tracks on firm soil (m2) 1.61c

Average ground contact pressure (kPa) 72.5

Track pre-tension load (kN) 59

a Version used for the tests.b Drive wheels not included.c Total area of support of the two tracks on firm soil (Ta) was estimated as follows: Ta ¼ 0:3ðEddw þ Edtw þ 3EdswÞ � Tw � 2, where

Eddw is the external diameter of driving wheel; Edtw the external diameter of track wheel; Edsw the external diameter of support wheels; Tw

the track width (external diameter is diameter of wheel þ two tracks and lugs thickness).

P. Servadio et al. / Soil & Tillage Research 61 (2001) 143–155 145

on the control areas, which had no traffic, using an

Eijkelkamp penetrologger (electronic penetrometer)

with a 608 cone and base area of 100 mm2 driven into

the soil at a constant rate (5 cm s�1). For each plot,

including the control areas, 20 penetrometer readings

were taken in increments of 1 cm at depths of 0–

0.40 m. Dry bulk density was measured by taking

samples of soil below the tracks left by each tractor,

Table 2

Main technical characteristics of the wheeled tractor and its tires used in the traffic studies

Characteristics New Holland 8770 tractor

Measured mass with ballast (kg) 10128

PTO power (kW) 140

Wheel tread (m) 1.780

Overall width (m) 2.500

Overall length (m) 4.970

Cabin height max (m) 3.070

Height above soil of implement hitch (m) 0.500

Type of tires Front Good Year Rear Good Year

Identification initials DT 820 600/65 R28 DT 820 710/70 R38

Open centre Yes Yes

Wheel rim DW 20 A DW 23 A

Rim diameter (m) 0.711 0.965

Section of tire (maximum width) (m) 0.622 0.715

Aspect ratio (height/width) 0.650 0.700

Rolling radius (m) 0.670 0.855

External diameter (m) 1.491 1.932

Lugs number (number) 38 38

Lugs height (m) 0.047 0.053

Lugs width (m) 0.043 0.060

Lugs angle (8) 45 40–45

Load on the two tires (kN) 45.57 55.70

Total contact area of the two tires on firm soil (m2) 0.56a 0.83a

Theoretical ground contact pressure (kPa) 70.9 71.8

Inflation pressure (kPa) 140 160

a Total contact area of the two tires on firm soil (Ta) was estimated as follows: Ta ¼ St � 0:3 � Ed � 2, where St is the section of the tire;

Ed the external diameter.

Table 3

Treatments determined by type of tractor and number of passes

Treatments Type of tractor Number of passes

on same track

RT-1 Rubber-tracked 1

RT-4 Rubber-tracked 4

WT-1 Wheeled 1

WT-4 Wheeled 4

Control No passes

Table 4

Soil physical properties of the Vertic Cambisol used in the traffic

studies

Property g kg�1

Soil organic carbona (0–0.50 m depth) 19

Particle size distributionb

Sand (2000–50 mm) 120

Silt (50–2 mm) 320

Clay (<2 mm) 560

Plastic limitc 265

Liquid limitc 525

Plastic indexc 260

Moisture content

At depth of 0–0.10 m 300

At depth of 0.11–0.20 m 350

a Determined by Walkley and Black (1934).b Determined by hydrometer method (Cestelli Guidi, 1987).c Determined by Atterberg limits (Cestelli Guidi, 1987).


after 1 and 4 passes and from the control areas using a

corer with a 100 cm3 volume sample ring (internal

diameter 5 cm, length 5.1 cm and wall thickness of

0.15 cm) at depths of 0–0.05, 0.06–0.10, 0.11–0.15

and 0.16–0.20 m. These samples were weighed and

dried until they reached a constant weight. In addition,

the increment ratio of density (Gn) was used as a

compaction criterion (Fujii, 1992) and is defined as

Gn ¼ gn � g0

g0

¼ gn

g0

� ��1

(1)

where g0 is the initial density (control) and gn the

density after the nth (1 and 4) tractor passes.

2.3. Shear strength measurement

Soil shear strength, as a parameter to measure

vehicle–soil interaction (Wong, 1993), was measured

in the tracks left by each tractor after 1 and 4 passes

and on the control areas, which had no traffic, using a

Stahlwille Manoskop vane shear test meter with a

torsion wrench from 8 to 32 Nm, vane diameter

70 mm and vane height 30 mm driven into the soil

at a constant rate. For each plot, including the control

areas, six measures were taken at 35 mm increments

within depths of 0–0.35 m. Shear resistance (t)

(N m�2) was calculated with the Terzaghi and Peck

(1979) relationship to applied torque.

2.4. Macroporosity and saturated hydraulic

conductivity measurements

The pore system was characterized by image ana-

lysis on thin sections from undisturbed soil samples.

For this, six undisturbed samples were collected in the

surface layer (0–0.10 m) and in the 0.10–0.20 m layer

of control plots and in the areas compacted by 1 and 4

passes of each tractor.

Samples were dried by acetone replacement of

water (Murphy, 1986), impregnated with a polyester

resin and cut into 6 cm � 7 cm, vertically orientated

thin sections (Murphy, 1986). Such sections were

analysed by means of image analysis techniques

(Pagliai et al., 1984), using the IMAGE PRO-PLUS

software produced by Media Cybernetics (Silver

Spring — USA). The analysed image covered

4:5 � 5:5 cm2 of the thin section, avoiding the edges

where disruption can occur. Total porosity and pore

distribution were measured according to their shape

and size. In this experiment, the instrument was set up

to measure pores larger than 50 mm (macroporosity).

Pores were measured by their shape, which is

expressed by the shape factor (perimeter2/(4p�area))

and divided into regular (more or less rounded) pores

(shape factor 1–2), irregular pores (shape factor 2–5)

and elongated pores (shape factor >5). These classes

correspond approximately to those used by Bouma

et al. (1977). Pores of each shape group were further

subdivided into size classes according to either the

equivalent pore diameter for regular and irregular

pores, or the width for elongated pores (Pagliai

et al., 1983, 1984). Thin sections were also examined

using a Zeiss ‘‘R POL’’ microscope at 25� magnifica-

tion to observe soil structure.

To measure saturated hydraulic conductivity, six

cores (5.68 cm diameter and 9.5 cm high) were col-

lected from the 0 to 0.10 m layer of each plot in areas

adjacent to those sampled for thin section preparation.

The samples were slowly saturated and saturated

hydraulic conductivity was measured using the fall-

ing-head technique (Klute and Dirksen, 1985).

3. Results and discussion

3.1. Penetration resistance and dry bulk density

Mean values of soil penetration resistance at various

depths (from 0 to 0.40 m in increments of 0.05 m),

standard deviations of the mean and coefficients of

variation for the various plots are shown in Table 5,

subdivided by tractor type (RT and WT) and number

of passes of the same track (1 and 4) in addition to

those for the control plots. Table 6 shows dry bulk

densities and the increment ratio of density (Gn) after 1

and 4 passes on the same track left by the tractors.

Results shown in Tables 5 and 6 indicated that for both

tractors, soil penetration resistance and dry bulk den-

sity increased with the number of passes on the same

track and this finding agrees with that of other studies

(Marsili and Servadio, 1992, 1996; Marsili et al.,

1998).

Examining the values of penetration resistance for

different soil layers (Table 5) and comparing the RT-1

and RT-4 passes with the control plot, it appears that

differences were statistically significant only in the


upper layers (0–0.15 m depth), while differences

between WT-1 and WT-4 passes and the control plot

were statistically significant only in the upper two

layers (0–0.10 m depth).

A comparison of the mean penetration resistance

for 1 pass of the two tractors shows that in the layers

there was a statistically significant difference between

the RT-1 pass and WT-1 pass in favour of the WT-1

pass only in the upper layers (0–0.15 m depth), while

in the deeper layers (0.26–0.40 m) there was a statis-

tically significant difference between WT-1 pass and

the RT-1 pass in favour of the RT-1 pass.

A comparison of the treatments for 4 passes reveals

that there was a statistically significant difference

between the RT-4 passes and WT-4 passes, with less

compaction in the WT-4 passes only in one layer

(0.11–0.15 m). A statistically significant difference

between the RT-4 passes and WT-4 passes in favour

of the RT-4 passes existed only from 0.36 to 0.40 m

depth. This suggests that multiple passes made by the

two tractors cause very similar effects in the soil.

The values of dry bulk density between 0 and

0.20 m depth increased with the increase in the num-

ber of passes on the track, and all the differences

between the traffic treatments showed increases over

the control (Table 6). After 1 pass of the tractor, the

density increment ratio (Gn) over the control was the

same for wheeled and rubber-tracked tractors (4.8%),

while the highest values of density increment ratio was

found for rubber-tracked after 4 passes (17.7%) and

the difference between RT-4 and WT-4 was statisti-

cally significant in favour of WT treatment.

In spite of the fact that the rubber-tracked tractor

and wheeled tractor had a similar average ground

contact pressure, the greater penetration resistance

caused by the rubber-tracked tractor, especially in

the upper layers (Table 5) can be attributed both to

the lug shape and to the influence of the track forces on

the soil, the contact pressure distribution, the contact

pressure under the track supporting wheels, the track

roller reactions on links, and the track tension (Sofiyan

and Maximenko, 1965; Brusentsev, 1967; Rowland,

1972; Wong, 1993).

3.2. Soil shear strength

Soil shear strength results, measured at depths of

0–0.30 m in increments of 35 mm are shown in

Table 5

Mean values of soil layers of penetration resistance (MPa) as a result of 1 (RT-1; WT-1) and 4 passes (RT-4; WT-4)

Depth (m) Mean resistancea (MPa)

Control RT-1 RT-4 WT-1 WT-4

0–0.05 0.85 b 1.38 a, c 1.48 a, c 1.25 a, b 1.44 a, c

0.06–0.10 0.79 b 1.19 a 1.10 a, b 1.01 a, b 1.09 a, b

0.11–0.15 0.85 b 0.99 a 1.04 a 0.92 b 0.96 b

0.16–0.20 0.94 a 0.87 a 0.98 a, b 0.84 a 0.95 a, b

0.21–0.25 1.00 a 0.91 a 0.96 a 0.92 a 0.97 a

0.26–0.30 1.11 a 0.91 a 1.00 a, b 1.18 a, b 1.12 a

0.31–0.35 1.29 a 1.00 a 1.12 a, b 1.47 a, b 1.20 a, c

0.36–0.40 1.53 a 1.11 a 1.22 a 1.78 a, b 1.38 a, c

a Standard ANOVA test conducted, at the same depth of sample, between tractors for various number of passes and to the control plot and

between the two tractors for the same number of passes. Mean results are flanked on the same line by letters. Each two means which share a

letter do not differ significantly, level of significance �0.01 (Gomez and Gomez, 1976).

Table 6

Mean values of soil layers (0–0.20 m) of dry bulk density and its

increment ratio (Gn)

Treatments Dry bulk densitya (Mg m�3) Gn (%)

RT-1 1.3 a 4.8

RT-4 1.5 b 17.7

WT-1 1.3 a 4.8

WT-4 1.4 c 16.3

Control 1.2 d 0

a Standard ANOVA test conducted, at the same depth of

sample, between tractors for various number of passes and to the

control plot and between the two tractors for the same number of

passes. Mean results are flanked on the same line by letters. Each

two means which share a letter do not differ significantly, level of

significance �0.01 (Gomez and Gomez, 1976).


Fig. 1. In agreement with the results of penetrometer

and dry bulk density tests, the values of shear strength

increased with the increase in the number of passes on

the track, and for all treatments there were increases in

the upper soil layers (0–0.06 m).

A comparison of the mean shear strength for the two

tractors shows that in the upper layers (0–0.10 m), the

increases were higher for treatments WT both for 1

and 4 passes, in the layers (0.10–0.20 m), the increases

were particularly high for treatments RT-4 passes and

in the deeper layers (0.20–0.30), the increases were

particularly high for treatments WT-1 pass. This

proves that soil strength in these layers (from 0 to

0.30 depth in this case) for these types of tractors in the

same field conditions should be considered as being

significantly influenced.

A significant correlation ðp < 0:05Þ was found

between soil shear strength and dry bulk density for

single and multiple passes of tractors equipped with

both rubber tracks and with tires, both from 0 to

0.10 m and from 0.10 to 0.20 m depth (Figs. 2 and 3).

3.3. Macroporosity

Total macroporosity, expressed as a percentage of

area occupied by pores larger than 50 mm per thin

section, in the control and compacted areas is shown in

Fig. 4. In agreement with soil penetration resistance

results, the decrease of macroporosity in the surface

layer (0–0.10 m) was different after the passes of the

two tractors. The rubber-tracked tractor caused a

significant reduction of macroporosity after only 1

Fig. 1. Effects of soil compaction, caused by 1 (RT-1; WT-1) and 4 passes (RT-4; WT-4) of the two tractors (RT and WT), on shear strength in

the soil layer (0–0.30 m).

Fig. 2. Correlation between dry bulk density and shear strength in the soil surface layer (0–0.10 m).


pass, in comparison to the control area. After 4 passes,

the deleterious effect on macroporosity was more

evident. In fact, the total macroporosity was less than

10% and, according to the micromorphometric

method, such a soil is considered dense of compact

(Pagliai, 1988).

The wheeled tractor caused a significant reduction

of macroporosity only after 4 passes, and the porosity

was higher, even though not significantly, than that

measured in plots under 1 pass of rubber-tracked

tractor. The decrease of macroporosity after a single

pass of the wheeled tractor was not significant in

comparison to control areas.

The compacting effect of traffic by the two high-

power tractors seemed to be limited to the surface

layer: in fact, the macroporosity in the 0.10–0.20 m

layer did not show significant differences between

uncompacted areas and those compacted by 1 and 4

passes of the two tractors.

3.4. Pore shape and size distribution

Soil compaction following the passage of the two

tractors not only reduced total macroporosity but also

modified the soil pore system, i.e. modified the shape

and the size distribution of pores. Pore shape and size

Fig. 3. Correlation between dry bulk density and shear strength in the soil subsurface layer (0.10–0.20 m).

Fig. 4. Effects of soil compaction, caused by 1 (RT-1; WT-1) and 4 passes (RT-4; WT-4) of the two tractors (RT and WT), on soil

macroporosity expressed as a percentage of area occupied by pores larger than 50 mm per thin section. Average of six replicates. Each two

means which share a letter do not differ significantly ðp � 0:05Þ.


distribution in the 0–0.10 m layer of the areas com-

pacted by the passes of the two tractors showed large

differences compared with uncompacted areas (Fig. 5).

The reduction of porosity following the compaction by

1 and 4 passes of the tractors was due to the decrease in

the proportion of elongated pores. After 1 pass of the

wheeled tractor, the total elongated porosity value was

not significantly different in comparison to control

areas, but the size distribution was quite different. The

proportion of the elongated pores in the larger size

classes decreased and, in particular, pores of the 300–

500 mm size classes completely disappeared. The

elongated pores are the most important, because many

of these pores directly affect plant growth by easing

root penetration and storage and transmission of water

and gases. For example, according to Russell (1978)

and Tippkotter (1983), feeding roots need pores ran-

ging from 100 to 200 mm to grow into. According to

Greenland (1977), pores of equivalent pore diameter

ranging from 50 to 500 mm are the transmission pores

(elongated and continuous pores) which are important

both in soil–water–plant relationships and in main-

taining good soil structure conditions.

As was the case of total macroporosity, in the 0.10–

0.20 m layer, the pore shape and size distribution did

not show significant differences between the uncom-

pacted areas and those compacted by 1 or 4 passes of

the two tractors, thus confirming that the compacting

effect of machinery traffic was limited, in this soil, to

the surface layer.

3.5. Soil structure

Macroporosity, pore shape and size distribution

variations due to compaction by machinery traffic

were reflected in the type of soil structure. Micro-

scopic examination of thin sections showed that in the

uncompacted areas, a subangular blocky structure was

homogeneously present within the entire 0–0.20 m

layer, while in the 0–0.10 m layer of the compacted

areas the structure was more dense.

In this soil, there was no evidence of platy structure

formation even in the areas compacted by 4 passes of

both tractors, as generally found in compacted soils

(Marsili et al., 1998). However, damage to soil struc-

ture can be recognized by decreases in the proportion

of transmission pores, i.e. elongated and continuous

pores with equivalent diameters ranging from 50 to

500 mm (Greenland, 1977; Pagliai et al., 1983). A

massive structure was evident for samples subjected

to 4 passes of RT tractor, characterized by few and thin

elongated pores. Furthermore, these pores were scar-

cely connected, so hindering water infiltration and

increasing the risks, already high for this soil, for

water stagnation or surface runoff. In the 0.10–0.20 m

layer, there was always a massive structure; micro-

scopic examination of this layer in comparison to the

surface layer of uncompacted areas showed that the

differences were mainly due to the reduction of elon-

gated pores.

3.6. Correlation between soil macroporosity and

saturated hydraulic conductivity

Saturated hydraulic conductivity of the 0–0.10 m

layer decreased following the traffic of the two tractors

(Table 7) and the lowest values were found after the

pass of the rubber-tracked tractor. In the 4 passes of

this tractor (RT-4), the hydraulic conductivity was

drastically reduced in agreement with the low pre-

sence of elongated pores which were very thin and not

continuous in a vertical direction.

Fig. 6 shows a highly significant correlation

ðp < 0:01Þ between hydraulic conductivity and elon-

gated pores and a significant correlation with total

macroporosity. Since the elongated pores represented

the highest proportion of total macroporosity in the

control soils and the variation after compaction mainly

Table 7

Effect of soil compaction, caused by 1 (RT-1; WT-1) and 4 passes

(RT-4; WT-4) of the two tractors, on saturated hydraulic

conductivity in the surface soil layer (0–0.10 m)

Treatments Saturated hydraulic

conductivitya (mm h�1)

RT-1 3.3 a

RT-4 1.1 b

WT-1 11.2 c

WT-4 7.5 d

Control 18.5 e

a Standard ANOVA test conducted, at the same depth of

sample, between tractors for various number of passes and to the

control plot and between the two tractors for the same number of

passes. Mean results are flanked on the same line by letters. Each

two means which share a letter do not differ significantly, level of

significance �0.01 (Gomez and Gomez, 1976).


Fig. 5. Macropore size distribution, according to the equivalent pore diameter for regular and irregular pores, or the width for elongated pores,

in the surface soil layer (0–0.10 m).

caused a reduction of such pores, this result confirmed

that hydraulic conductivity is directly correlated with

elongated continuous pores. These results stressed that

the compaction is one of the most important aspects,

not only of soil degradation but also of environmental

degradation, since the strong reduction of water infil-

tration may increase the risks of soil erosion.

3.7. Relation between total macroporosity and soil

penetration resistance

Previous studies (Pagliai et al., 1992; Marsili et al.,

1998) on the effects of compaction caused by different

types of mobility system on total macroporosity and

soil structure would appear to have demonstrated a

good correlation in the compacted surface layer (0–

0.10 m) between macroporosity and soil penetration

resistance. Fig. 7 shows the relations ðp < 0:05Þ

between penetration resistance and total macroporos-

ity at 0–0.10 m depth for single and multiple passes of

tractors equipped with rubber tracks (RT-1 pass and

RT-4 passes) and with tires (WT-1 pass and WT-4

passes). The values can be represented by a linear type

regression, namely

y ¼ ax þ c (2)

where y is the penetration resistance in MPa, x the total

macroporosity expressed in %, a a coefficient, and c a

constant which, for single and multiple passes with the

rubber-tracked tractor, amount to a ¼ �0:0314 and

c ¼ þ1:6195, and for single and multiple passes with

the wheeled tractor amount to a1 ¼ �0:0487 and

c1 ¼ þ2:0887.

For the 0.10–0.20 m depth, the differences in por-

osity are not significant enough to find any relation-

ship with soil penetration resistance.

Fig. 6. Correlations between hydraulic conductivity and elongated pores and total porosity in the surface layer (0–0.10 m).

Fig. 7. Correlations between soil penetration resistance and macroporosity after single and multiple passes of tractors equipped with rubber

tracks and with tires in the soil layer (0–0.10 m depth).


These results indicate that in these test conditions,

the wheeled tractor caused less soil compaction and

resulted in a higher degree of macroporosity in the

surface layer than the rubber-tracked tractor.

4. Conclusions

Tests conducted on two high-powered tractors with

two different types of running gear allowed the eva-

luation of the degree of soil compaction by analysing

variation in some physical properties. After single and

multiple passes of the two tractors, statistically sig-

nificant differences in soil penetration resistance in

favour of the wheeled tractor were recorded only in the

surface layers, while a statistically significant differ-

ence in favour of the tracked tractor was found in the

deeper layers. The decrease in macroporosity, in

particular of elongated pores in the soil surface layer,

was greater in treatments involving the rubber-tracked

tractor than for the wheeled tractor. Hydraulic con-

ductivity of the surface layer was relatively low at the

test site as the soil is only slightly permeable, and was

inversely proportional to macroporosity and to elon-

gated pores. However, with respect to the higher

degree of macroporosity, treatments involving the

wheeled tractor showed better soil structure quality.

Tractors equipped with low aspect ratio tires give

rise to less soil damage than that following the passage

of rubber-tracked tractors, particularly on the upper

layers. A tractor equipped with rubber tracks of some-

what greater mass and power than the wheeled tractor

caused effects on the soil substantially equivalent to

those of the wheeled tractor, but with moderate

damage in the upper soil layers (0–0.20 m). This

can be attributed both to the stress action of the lugs

on the rubber track and to the compression caused by

the three track supporting wheels. Within the deeper

layers (from 0.20 to 0.40 m), an improvement of the

soil measured parameters in favour of the rubber-

tracked tractor was observed.

Acknowledgements

This work was carried out under the auspices of a

Concerted Action on subsoil compaction of the Com-

mission of the European Communities, Agriculture and

Fisheries (FAIR) specific RTD programme, FAIR5-

CT97-3589 ‘‘Experiences with the impact of subsoil

compaction on soil, crop growth and environment

and ways to prevent subsoil compaction’’. The authors

wish to thanks Dr. Olga Grasselli forherhelp inplanning

and collecting soil samples for porosity and hydraulic

conductivity measurements and Mr. Andrea Rocchini

for technical assistance in laboratory analysis.

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