Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol53.Issue2.Jan2000:
Soil & Tillage Research 53 (2000) 117±128
Stress/strain processes in a structured unsaturated silty loam Luvisol
under different tillage treatments in Germany
C. Wiermanna,*, D. Wernerb, R. Horna, J. Rosteka, B. Wernerc
a
Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University of Kiel, Olshausenstr. 40, D-24118 Kiel, Germany
b
Thuringian Institute for Agriculture, Naumburger Str. 98, D-07743 Jena, Germany
c
Institute for Diagnostic and Interventional Radiology, Friedrich-Schiller-University of Jena, Bachstr. 1, D-07743 Jena, Germany
Received 29 September 1998; received in revised form 17 August 1999; accepted 12 October 1999
Abstract
In agriculture, the degradation of soil structure by tillage and ®eld traf®c is an adverse process causing a reduction in
productivity of arable land. In order to manage this problem, various kinds of traf®c and tillage systems have been developed.
Relatively few studies have examined changes of soil mechanical properties induced by conservation tillage systems. The
objectives of this study were to determine the effect of long term reduced tillage on soil strength properties. A ®eld experiment
was conducted in Germany on a silty loam Luvisol derived from loess, tilled differently by conventional (ploughed) and
conservation (rotary) tillage systems for more than 25 years. In the spring of 1995, plots were compacted by increasing
dynamic loads (number of passes wheel load: 2 2.5, 2 5 and 6 5 Mg) and the soil physical, and mechanical
properties were determined by ®eld and laboratory techniques. The repeated deep impact of tillage tools in conventionally
treated plots resulted in a permanent destruction of newly formed soil aggregates. This led to a relatively weak soil structure of
the tilled horizons as dynamic loads as low as 2.5 Mg induced structural degradation. In the conservation tillage plots, in
contrast, a single wheeling event with 2.5 Mg was compensated by a robust aggregate system and did not lead to structural
degradation. Thus a higher soil strength due to the robust aggregate system was provided by reduced tillage. Increasing wheel
loads and repeated tire passes resulted in an increasing structural degradation of the subsoil in both tillage systems. Since
preserved fragments of channels were observed in depth greater than 30 cm in conservation tillage plots, traf®cked by
6 5 Mg, the conditions for structural recovery are expected to be more favourable with this tillage system than conventional
tillage. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil structure degradation; Soil tillage systems; Soil strength; Soil stress; Soil displacement; Computer tomography
1. Introduction
*
Corresponding author. Present address: Landwirtschaftskammer Schleswig-Holstein, Bildungs-und Beratungszentrum
Bredstedt, Theodor-Storm Str. 2, 25821 Bredstedt, Germany. Tel.:
49-4671-913428; fax: 49-4671-913411.
Soil physical, chemical and biological properties
can be changed by natural as well as by anthropogenic
impacts. In agriculture, the in¯uences of tillage and
®eld traf®c by heavy machinery have been identi®ed
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 0 9 0 - 2
118
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
as harmful processes reducing the productivity of
arable land (Schafer et al., 1992). Thus in any attempt
to improve systems of soil management, the components of both tillage and traf®c systems should be
considered (Raghavan et al., 1990; Soane and van
Ouwerkerk, 1994).
Soil structural degradation induced by heavy
machinery results in altered soil conditions and
may result in yield losses and environmental
impacts. For example diminished permeabilities of
the soil pore system to water and air often result in
decreased soil aeration and in®ltration rates, with the
consequences of soil erosion and the loss of fertile soil
layers (Voorhees et al., 1986; Ball et al., 1997). Yield
losses from soil compaction have been observed over
decades especially on clay soils (Hakansson et al.,
1987).
A variety of traf®c systems have been developed to
overcome the problem of soil compaction. The bene®ts of reduced axle loads, low ground pressure tires,
tracked vehicles and controlled traf®c systems were
investigated by different authors (Chamen et al., 1992;
Erbach, 1994; Hakansson and Petelkau, 1994; Vermeulen and Perdok, 1994). Since soil strength is
changing with the prevailing site conditions, none
of these systems provided a universal solution to
the problem of soil compaction. The applied tillage
system was identi®ed as a main factor, determining the
soil properties and therefore there is a strong correlation between soil compaction and soil management
practices. Ehlers and Claupein (1994) reviewed the
effect of various tillage systems (conventional, conservation, and zero tillage) on soil properties. Ploughless tillage treatments increased soil bulk density and
penetration resistance in lower topsoil layers. Macroporosity and pore continuity increased under conservation tillage treatments. Aggregate stability is
signi®cantly increased by reduced tillage practices
due to increased biological activity and repeated soil
swell and shrink cycles (SchjoÈnning and Rasmussen,
1989; Ehlers, 1997).
The effect of conservation tillage systems on soil
strength parameters and the traf®ckability of arable
land has not been studied in detail. The objectives of
this study were (1) to determine the effect of continuous aggregation processes under long term conservation tillage management on soil mechanical
properties, and (2) to investigate the possibilities of
applying conservation tillage systems to improve soil
strength and prevent soil compaction.
2. Theoretical considerations
When applying external forces to the soil surface,
soils react in different modes, according to the distribution, orientation and magnitude of the generated
internal stresses. Thus the extent of soil deformation
can be described by the stress and strain relationship
(Koolen, 1994; Guerif, 1994).
In order to de®ne the stress state at a considered
point, three normal stresses (x, y, z) and three
shearing stresses ( x, y, z) must be determined
according to Nichols et al. (1987), Horn et al.
(1992) and Harris and Bakker (1994). Every change
in the stress state induces a change of the strain state.
Thus plastic (irreversible) soil deformation strongly
increases if the induced stresses exceed the internal
soil strength. Analogous to the stress theory, the strain
state at a theoretical point in the soil can be designated
as normal strain x, y, z and shear strain xy, xz, yz
(Koolen, 1994).
3. Material and methods
The experiment was conducted at Reinshof near
GoÈttingen (Lower Saxony/Germany) on a silty
loam Luvisol derived from loess. Since 1971 two
different tillage systems have been applied to the
experimental site. The conventional plots (CT) were
moldboard ploughed to a depth of 25 cm (wheels
in the furrow) every autumn, while on the conservation tillage plots (CS) the mechanical impact
of the tillage tools was restricted to a depth of
10 cm, using a rotary tiller. Thirty per cent of the
organic plant residues were left upon the soil surface,
thus this tillage system is classi®ed as conservation
using the de®nition of the Soil Conservation Service
(1983).
In the spring of 1995, at a water content of ÿ6 kPa,
plots of both tillage systems were compacted track
by track (5% wheel slip) using the following
dynamic loads: 2 2.5, 2 5 and 65 Mg. A quarter
of each tillage treatment remained uncompacted
(0 Mg).
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
3.1. Field methods
Following to the stress theory described by Koolen
(1994), the stress state 10 cm below wheel tires was
determined by using one stress state transducer (SST)
per plot. The SST consisted of six pressure cells,
orientated at an angle of 54.738 to each other as
reported by Nichols et al. (1987). Thus, the complete
stress state in one theoretical point could be calculated. The stress state was characterised by the mean
normal stress (MNS) and the octahedral shear stress
(OCTSS) at the peak of the major principal stress.
Furthermore, the ratio (OSR) between OCTSS to
MNS was calculated, since this parameter was
expected to re¯ect the presence of soil failure beneath
a moving tire (Bailey et al., 1988).
In order to measure the movement of the transducer,
induced by the passing tires, the SST was connected to
a displacement transducer system (DTS) as described
by KuÈhner (1997). Using this system the vertical and
horizontal soil displacement was determined, as a
dynamic load was applied to the soil surface. The
combined SST and DTS measurement device was
installed as described by Wiermann et al. (1999).
119
friction) were derived from the results of a ring shear
test at a forward speed of 0.2 mm sÿ1 using the same
loads as already mentioned for the precompression
test.
Larger samples, which contained a more representative part of soil structure were used for demonstrating the spatial arrangement of macropore systems.
Therefore, 100 100 mm soil cores (plastic cylinders) taken twice as mentioned above were used for Xray computer tomography. The equipment used in this
investigation was a Siemens Somatom 4 Plus. The
principle of a tomograph is to measure the attenuation
of X-rays through the sample in 2 mm thick layers and
to calculate the density dependent attenuation coef®cient. The spatial resolution for high-contrast objects
like macropores is about 750 mm. Therefore images
show only the largest air or water®lled pores. Based on
this three-dimensional reconstruction of separation,
macropores were visualised by a special software
(Somaris VB 40).
4. Results
4.1. Soil parameters
3.2. Laboratory methods
Undisturbed soil samples with a volume of 100, 235
and 250 cm3 (diameter 6, 10 and 8 cm) were taken
vertically at 10, 30 and 60 cm depth before and
immediately after wheeling. The pore size distribution
was determined according to the method of Hartge and
Horn (1992) using tension plates and pressure chambers on four replicates per depth and treatment of the
100 cm3 soil cores. By applying the constant head
method, the saturated hydraulic conductivity (ksat) was
measured on six replicates at 30±36 cm depth per
treatment of the 250 cm3 soil cores.
The determination of the mechanical parameters
was conducted at a pore water pressure of ÿ6 kPa. In
order to determine the precompression stress values,
seven soil samples (235 cm3) of each depth and treatment were subjected to static loads of 20±800 kPa
using a uniaxial con®ned compression test (Kezdi,
1969; Horn, 1981). From the changes of the void ratio,
the precompression stress was calculated according to
the method described by Lebert and Horn (1991). The
shear parameters (cohesion and angle of internal
Table 1 gives information on the soil physical,
mechanical and ecological parameters of both tillage
systems before the dynamic loads were applied. In the
CT plots a strong increase in bulk density values from
10 to 30 cm depth was found, while the macroporosity
(pF -oo to 1.8) also decreased. The ksat was lowest in
the CT tillage system at 10 cm depth and increased in
the deeper soil layers. The CS plots showed in contrast
only slight differences in bulk density. The macroporosity at the conventional tillage system showed a
signi®cant difference between the seed bed and the
underlying horizons throughout the soil pro®le,
although the ksat values revealed differences between
these layers. High values of plant available water
(25%) were calculated for both tillage systems.
The soil mechanical parameters also showed clear
differences between the two tillage systems. In general, higher values for the precompression stress were
determined in the conventionally treated plot. The
increased soil strength of CT compared to CS at
30 cm depth was indicated by higher cohesion and
precompression stress values.
120
Soil depth
(cm)
Bulk density
(g/cm3)
Pore size distribution at pF (vol.%)
-oo
1.8
2.5
4.2
ksat
(cm/d)
pH
Texture 50 mm) and ksat under conventional (CT) and conservation tillage (CS) as a function of the
applied dynamic loads: 0, 2 2.5, 2 5 or 6 5 Mg. Site: Reinshof/Germany.
CS (Fig. 4b). A near complete loss of coarse pores
for the 6 5 Mg treatment is shown in Fig. 4c and d
for the loaded plots of both tillage systems. Only a few
vesicles remained undisturbed, probably stabilised by
water as the dynamic load was applied, since the pore
water suction was at ÿ6 kPa. There were no differences to observe between tillage systems in this soil
layer (10±20 cm).
In deeper soil layers (30±40 cm) the comparison of
CT and CS showed clear differences between both
tillage systems (Fig. 5). In the conventionally tilled
plot deformation was similar at both depths (Fig. 4c
vs. Fig. 5a). But in the CS treatment, while the extent
of soil deformation in the 10±20 cm layer was similar
to that in the CT plot, in the 30±40 cm depth layer
more fragments of channels, formed by roots and
worms, were preserved (Fig. 4d vs. Fig. 5b).
5. Discussion
5.1. Soil physical conditions
A strong impact of tillage tools in continuous
conventionally treated plots result in a repeated
destruction of soil aggregates, formed during the
growing season by physical and biological processes.
Thus an isotropic arrangement of weak soil units is
produced (Horn et al., 1997). The soil physical
properties of tilled layers (0±30 cm depth) are
usually characterised by a tortuos pore system, mostly
with a high amount of macropores (Lal, 1995). Consequently a pronounced tortuosity results in low ksat
values, which is con®rmed by the tomographic
images. At 30 cm depth there was a sharp decline
of the macroporosity with a simultaneous increase of
the mechanical soil strength. Thus at this depth a
typical plough pan with horizontally oriented aggregates was formed by the kneading effect of wheeling
tires in the furrow.
In contrast, long term application of conservation
tillage systems usually induces an anisotropic arrangement of the solid particles. Before reaching a dynamic
¯ow balance characterised by an equilibrium of the
soil properties, a transitional stage of about 5±6 years
has to be passed (Voorhees and Lindstrom, 1984; Horn
et al., 1997). Amongst others, Ehlers and Claupein
(1994) reported an increasing amount and continuity
of macropores on CS sites, resulting in increased
ksat values. Thus an increased ability to compensate
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
125
Fig. 4. 3D visualisation of computer tomographic data at 10±20 cm depth of conventional (left) and conservation (right) tilled plots.
Houns®eld values: ÿ1024 and 0. Site: Reinshof/Germany. Treatments: (a) CT 0 Mg, (b) CS 0 Mg, (c) CT 6 5 Mg, and (d) CS 6 5 Mg.
external forces to a certain extent may be expected
on conservation tilled plots.
5.2. Soil displacement
In general, there was more pronounced soil displacement in CT as compared to CS. Structural degradation, as indicated by the precompression stress values
and the ksat vs. macropore relationship, was initiated in
the conventionally tilled plot at the lowest applied load
(2 2.5 Mg). In contrast in the conservation tillage
system, a wheeling event with 2 5 Mg was compensated by a robust aggregate system developed
beneath the tilled horizon. Since the precompression
stress and the ksat vs. macropore relationship was not
in¯uenced by this loading event, there was a higher
compensatory potential with respect to mechanical
impacts for CS compared to CT.
The wheeling events with the 2 5 Mg treatments
induced irreversible structural degradation on both
126
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
Fig. 5. 3D visualisation of computer tomographic data in 30±40 cm depth of conventional (left) and conservation (right) tilled plots.
Houns®eld values: ÿ1024 and 0. Site: Reinshof/Germany. Treatments: (a) CT 6 5 Mg, and (b) CS 6 5 Mg.
tillage systems as indicated by the vertical and horizontal soil displacement. These aggregates are more
or less reduced in size and changed in particle arrangement (Horn et al., 1994). Also the values for the ratio
of OCTSS to MNS revealed soil failure beneath the
wheeling tires on the conventionally treated plot. Thus
soil deformation processes induced by divergence
and/or shear were induced on this tillage system. In
contrast, in the CS site this ratio did not show the
observed structural degradation processes, indicated
by the displacement and the precompression stress
measurements.
Further traf®cking with a 5 Mg wheel load resulted
in additional structural degradation on both tillage
systems. In the CT plot soil degradation processes
obviously proceed to deeper soil layers, since the
precompression stress values at 60 cm was increased
by the third pass. Also in the CS site repeated wheeling
events induced a rearrangement of solid particles in
deeper soil horizons and therefore altered soil physical
conditions. A strong decrease of macropores was
observed at 30 cm depth with a simultaneous increase
of the precompression stress value. For deeper soil
layers (>30 cm), the extent of structural degradation
was reduced with CS as compared to CT, since
tomographic images showed a still intact vertically
orientated pore system. A preservation of pre-existing
vertically orientated macropores following soil compression has already been observed by Hartge and
Bohne (1983).
5.3. Topsoil vs. subsoil compaction
Soil stresses exceeding the internal soil strength
induced a rearrangement of the solid particles. Dexter
(1988) described the following three steps of structural
degradation: First, compression of the soil induces a
denser packing of lower porosity, but the soil aggregates are still intact. Second, weak aggregates collapse
to smaller structural units and ¯ow into the interstices
of the remaining intact aggregates. Third, the structural units collapse and plastic ¯ow occurs, resulting in
an unstructured soil matrix.
In these experiments all these steps were observed.
For the conventionally tilled plot the isotropic soil
structure of the topsoil was not able to compensate for
any of the applied dynamic loads. Thus a denser
packing of the structural units, resulting in a decreased
macroporosity and increased soil strength, was
induced by single wheeling events. On this tillage
system, stress attenuation occurred ®rst in the compacted plough pan at 30 cm depth. A strong decrease
of stresses beneath a hard pan layer was reported by
Taylor and Burt (1987). Thus structural degradation
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
processes are restricted to the topsoil layers until the
horizontally orientated aggregates are crumbled.
Induced by repeated dynamic wheeling events, the
structural units of the plough pan collapsed and the
soil strength decreased. The compensatory effect of
the hard pan layer therefore declined by kneading
processes and stresses were directly transmitted to the
subsoil as described by Horn et al. (1998). Since the
subsoil was subjected to further soil degradation processes, the topsoil layer was expected to act as a rigid
body with elastic soil properties, the compacted topsoil layer translated and rotated to deeper soil horizons
without further changes of its soil properties.
In the conservation tillage treatment in contrast
stresses were attenuated at 10 cm depth by a soil layer
of high mechanical strength, formed by tillage tools
and ®eld traf®c. A soil layer of similar properties was
reported by SchjoÈnning and Rasmussen (1989). They
observed the development of a ``rotovator-pan'' with
increased soil strength on shallow tilled sites. This
hard pan in combination with a robust soil structure of
the subsoil was probably responsible for the observed
stress compensation when traf®cking with 2.5 Mg
wheel load. With increasing wheel loads and repeated
tire passes the strength of the hard pan decreased by
kneading processes induced by the dynamic impact of
the wheeling tires (slip). Stresses of further passes
were therefore transmitted to deeper soil layers and
structural degradation processes started at depths
>10 cm. Since preserved fragments of channels were
detected in soil layers beneath 30 cm depth in the
6 5 Mg treatment, the effects of biological and/or
physical processes, which can diminish the negative
effects of soil compaction, are expected to be more
favourable in CS compared to conventionally tilled
sites.
6. Conclusion
Soil strength of the untilled layers in CS was clearly
higher than that of the topsoil structure observed in the
conventional tillage system. Thus stresses induced by
a single wheeling event with a dynamic load of 2.5 Mg
were better resisted than in the conventional till soil.
On sites suitable for conservation tillage, the reduction
of tillage intensity is therefore a useful method to
improve soil strength and reduce subsoil compaction.
127
In both tillage systems repeated wheeling events
resulted in pronounced structural degradation of the
subsoil. However, in CS at depths >30 cm preserved
fragments of macropores were detected, while a completely homogeneous soil matrix was observed at the
ploughed plots. Thus, the conditions for a structural
recovery by physical and/or biological processes are
expected to be more favourable with conservation
tillage.
Soil layers, of high mechanical strength, were
observed below the tillage depth with both soil management systems. These layers obviously supported
stress attenuation and therefore protected deeper soil
layers from structural degradation.
The displacement measurements in combination
with computer tomographic images are useful techniques for assessing the extent of structural degradation due to dynamic tire passes of agricultural
machinery.
Acknowledgements
The authors are highly indebted to the German
Research Foundation (DFG) which ®nancially supported the research.
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of a Norfolk sandy loam. Soil Till. Res. 50, 127±135.
Stress/strain processes in a structured unsaturated silty loam Luvisol
under different tillage treatments in Germany
C. Wiermanna,*, D. Wernerb, R. Horna, J. Rosteka, B. Wernerc
a
Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University of Kiel, Olshausenstr. 40, D-24118 Kiel, Germany
b
Thuringian Institute for Agriculture, Naumburger Str. 98, D-07743 Jena, Germany
c
Institute for Diagnostic and Interventional Radiology, Friedrich-Schiller-University of Jena, Bachstr. 1, D-07743 Jena, Germany
Received 29 September 1998; received in revised form 17 August 1999; accepted 12 October 1999
Abstract
In agriculture, the degradation of soil structure by tillage and ®eld traf®c is an adverse process causing a reduction in
productivity of arable land. In order to manage this problem, various kinds of traf®c and tillage systems have been developed.
Relatively few studies have examined changes of soil mechanical properties induced by conservation tillage systems. The
objectives of this study were to determine the effect of long term reduced tillage on soil strength properties. A ®eld experiment
was conducted in Germany on a silty loam Luvisol derived from loess, tilled differently by conventional (ploughed) and
conservation (rotary) tillage systems for more than 25 years. In the spring of 1995, plots were compacted by increasing
dynamic loads (number of passes wheel load: 2 2.5, 2 5 and 6 5 Mg) and the soil physical, and mechanical
properties were determined by ®eld and laboratory techniques. The repeated deep impact of tillage tools in conventionally
treated plots resulted in a permanent destruction of newly formed soil aggregates. This led to a relatively weak soil structure of
the tilled horizons as dynamic loads as low as 2.5 Mg induced structural degradation. In the conservation tillage plots, in
contrast, a single wheeling event with 2.5 Mg was compensated by a robust aggregate system and did not lead to structural
degradation. Thus a higher soil strength due to the robust aggregate system was provided by reduced tillage. Increasing wheel
loads and repeated tire passes resulted in an increasing structural degradation of the subsoil in both tillage systems. Since
preserved fragments of channels were observed in depth greater than 30 cm in conservation tillage plots, traf®cked by
6 5 Mg, the conditions for structural recovery are expected to be more favourable with this tillage system than conventional
tillage. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil structure degradation; Soil tillage systems; Soil strength; Soil stress; Soil displacement; Computer tomography
1. Introduction
*
Corresponding author. Present address: Landwirtschaftskammer Schleswig-Holstein, Bildungs-und Beratungszentrum
Bredstedt, Theodor-Storm Str. 2, 25821 Bredstedt, Germany. Tel.:
49-4671-913428; fax: 49-4671-913411.
Soil physical, chemical and biological properties
can be changed by natural as well as by anthropogenic
impacts. In agriculture, the in¯uences of tillage and
®eld traf®c by heavy machinery have been identi®ed
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 0 9 0 - 2
118
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
as harmful processes reducing the productivity of
arable land (Schafer et al., 1992). Thus in any attempt
to improve systems of soil management, the components of both tillage and traf®c systems should be
considered (Raghavan et al., 1990; Soane and van
Ouwerkerk, 1994).
Soil structural degradation induced by heavy
machinery results in altered soil conditions and
may result in yield losses and environmental
impacts. For example diminished permeabilities of
the soil pore system to water and air often result in
decreased soil aeration and in®ltration rates, with the
consequences of soil erosion and the loss of fertile soil
layers (Voorhees et al., 1986; Ball et al., 1997). Yield
losses from soil compaction have been observed over
decades especially on clay soils (Hakansson et al.,
1987).
A variety of traf®c systems have been developed to
overcome the problem of soil compaction. The bene®ts of reduced axle loads, low ground pressure tires,
tracked vehicles and controlled traf®c systems were
investigated by different authors (Chamen et al., 1992;
Erbach, 1994; Hakansson and Petelkau, 1994; Vermeulen and Perdok, 1994). Since soil strength is
changing with the prevailing site conditions, none
of these systems provided a universal solution to
the problem of soil compaction. The applied tillage
system was identi®ed as a main factor, determining the
soil properties and therefore there is a strong correlation between soil compaction and soil management
practices. Ehlers and Claupein (1994) reviewed the
effect of various tillage systems (conventional, conservation, and zero tillage) on soil properties. Ploughless tillage treatments increased soil bulk density and
penetration resistance in lower topsoil layers. Macroporosity and pore continuity increased under conservation tillage treatments. Aggregate stability is
signi®cantly increased by reduced tillage practices
due to increased biological activity and repeated soil
swell and shrink cycles (SchjoÈnning and Rasmussen,
1989; Ehlers, 1997).
The effect of conservation tillage systems on soil
strength parameters and the traf®ckability of arable
land has not been studied in detail. The objectives of
this study were (1) to determine the effect of continuous aggregation processes under long term conservation tillage management on soil mechanical
properties, and (2) to investigate the possibilities of
applying conservation tillage systems to improve soil
strength and prevent soil compaction.
2. Theoretical considerations
When applying external forces to the soil surface,
soils react in different modes, according to the distribution, orientation and magnitude of the generated
internal stresses. Thus the extent of soil deformation
can be described by the stress and strain relationship
(Koolen, 1994; Guerif, 1994).
In order to de®ne the stress state at a considered
point, three normal stresses (x, y, z) and three
shearing stresses ( x, y, z) must be determined
according to Nichols et al. (1987), Horn et al.
(1992) and Harris and Bakker (1994). Every change
in the stress state induces a change of the strain state.
Thus plastic (irreversible) soil deformation strongly
increases if the induced stresses exceed the internal
soil strength. Analogous to the stress theory, the strain
state at a theoretical point in the soil can be designated
as normal strain x, y, z and shear strain xy, xz, yz
(Koolen, 1994).
3. Material and methods
The experiment was conducted at Reinshof near
GoÈttingen (Lower Saxony/Germany) on a silty
loam Luvisol derived from loess. Since 1971 two
different tillage systems have been applied to the
experimental site. The conventional plots (CT) were
moldboard ploughed to a depth of 25 cm (wheels
in the furrow) every autumn, while on the conservation tillage plots (CS) the mechanical impact
of the tillage tools was restricted to a depth of
10 cm, using a rotary tiller. Thirty per cent of the
organic plant residues were left upon the soil surface,
thus this tillage system is classi®ed as conservation
using the de®nition of the Soil Conservation Service
(1983).
In the spring of 1995, at a water content of ÿ6 kPa,
plots of both tillage systems were compacted track
by track (5% wheel slip) using the following
dynamic loads: 2 2.5, 2 5 and 65 Mg. A quarter
of each tillage treatment remained uncompacted
(0 Mg).
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
3.1. Field methods
Following to the stress theory described by Koolen
(1994), the stress state 10 cm below wheel tires was
determined by using one stress state transducer (SST)
per plot. The SST consisted of six pressure cells,
orientated at an angle of 54.738 to each other as
reported by Nichols et al. (1987). Thus, the complete
stress state in one theoretical point could be calculated. The stress state was characterised by the mean
normal stress (MNS) and the octahedral shear stress
(OCTSS) at the peak of the major principal stress.
Furthermore, the ratio (OSR) between OCTSS to
MNS was calculated, since this parameter was
expected to re¯ect the presence of soil failure beneath
a moving tire (Bailey et al., 1988).
In order to measure the movement of the transducer,
induced by the passing tires, the SST was connected to
a displacement transducer system (DTS) as described
by KuÈhner (1997). Using this system the vertical and
horizontal soil displacement was determined, as a
dynamic load was applied to the soil surface. The
combined SST and DTS measurement device was
installed as described by Wiermann et al. (1999).
119
friction) were derived from the results of a ring shear
test at a forward speed of 0.2 mm sÿ1 using the same
loads as already mentioned for the precompression
test.
Larger samples, which contained a more representative part of soil structure were used for demonstrating the spatial arrangement of macropore systems.
Therefore, 100 100 mm soil cores (plastic cylinders) taken twice as mentioned above were used for Xray computer tomography. The equipment used in this
investigation was a Siemens Somatom 4 Plus. The
principle of a tomograph is to measure the attenuation
of X-rays through the sample in 2 mm thick layers and
to calculate the density dependent attenuation coef®cient. The spatial resolution for high-contrast objects
like macropores is about 750 mm. Therefore images
show only the largest air or water®lled pores. Based on
this three-dimensional reconstruction of separation,
macropores were visualised by a special software
(Somaris VB 40).
4. Results
4.1. Soil parameters
3.2. Laboratory methods
Undisturbed soil samples with a volume of 100, 235
and 250 cm3 (diameter 6, 10 and 8 cm) were taken
vertically at 10, 30 and 60 cm depth before and
immediately after wheeling. The pore size distribution
was determined according to the method of Hartge and
Horn (1992) using tension plates and pressure chambers on four replicates per depth and treatment of the
100 cm3 soil cores. By applying the constant head
method, the saturated hydraulic conductivity (ksat) was
measured on six replicates at 30±36 cm depth per
treatment of the 250 cm3 soil cores.
The determination of the mechanical parameters
was conducted at a pore water pressure of ÿ6 kPa. In
order to determine the precompression stress values,
seven soil samples (235 cm3) of each depth and treatment were subjected to static loads of 20±800 kPa
using a uniaxial con®ned compression test (Kezdi,
1969; Horn, 1981). From the changes of the void ratio,
the precompression stress was calculated according to
the method described by Lebert and Horn (1991). The
shear parameters (cohesion and angle of internal
Table 1 gives information on the soil physical,
mechanical and ecological parameters of both tillage
systems before the dynamic loads were applied. In the
CT plots a strong increase in bulk density values from
10 to 30 cm depth was found, while the macroporosity
(pF -oo to 1.8) also decreased. The ksat was lowest in
the CT tillage system at 10 cm depth and increased in
the deeper soil layers. The CS plots showed in contrast
only slight differences in bulk density. The macroporosity at the conventional tillage system showed a
signi®cant difference between the seed bed and the
underlying horizons throughout the soil pro®le,
although the ksat values revealed differences between
these layers. High values of plant available water
(25%) were calculated for both tillage systems.
The soil mechanical parameters also showed clear
differences between the two tillage systems. In general, higher values for the precompression stress were
determined in the conventionally treated plot. The
increased soil strength of CT compared to CS at
30 cm depth was indicated by higher cohesion and
precompression stress values.
120
Soil depth
(cm)
Bulk density
(g/cm3)
Pore size distribution at pF (vol.%)
-oo
1.8
2.5
4.2
ksat
(cm/d)
pH
Texture 50 mm) and ksat under conventional (CT) and conservation tillage (CS) as a function of the
applied dynamic loads: 0, 2 2.5, 2 5 or 6 5 Mg. Site: Reinshof/Germany.
CS (Fig. 4b). A near complete loss of coarse pores
for the 6 5 Mg treatment is shown in Fig. 4c and d
for the loaded plots of both tillage systems. Only a few
vesicles remained undisturbed, probably stabilised by
water as the dynamic load was applied, since the pore
water suction was at ÿ6 kPa. There were no differences to observe between tillage systems in this soil
layer (10±20 cm).
In deeper soil layers (30±40 cm) the comparison of
CT and CS showed clear differences between both
tillage systems (Fig. 5). In the conventionally tilled
plot deformation was similar at both depths (Fig. 4c
vs. Fig. 5a). But in the CS treatment, while the extent
of soil deformation in the 10±20 cm layer was similar
to that in the CT plot, in the 30±40 cm depth layer
more fragments of channels, formed by roots and
worms, were preserved (Fig. 4d vs. Fig. 5b).
5. Discussion
5.1. Soil physical conditions
A strong impact of tillage tools in continuous
conventionally treated plots result in a repeated
destruction of soil aggregates, formed during the
growing season by physical and biological processes.
Thus an isotropic arrangement of weak soil units is
produced (Horn et al., 1997). The soil physical
properties of tilled layers (0±30 cm depth) are
usually characterised by a tortuos pore system, mostly
with a high amount of macropores (Lal, 1995). Consequently a pronounced tortuosity results in low ksat
values, which is con®rmed by the tomographic
images. At 30 cm depth there was a sharp decline
of the macroporosity with a simultaneous increase of
the mechanical soil strength. Thus at this depth a
typical plough pan with horizontally oriented aggregates was formed by the kneading effect of wheeling
tires in the furrow.
In contrast, long term application of conservation
tillage systems usually induces an anisotropic arrangement of the solid particles. Before reaching a dynamic
¯ow balance characterised by an equilibrium of the
soil properties, a transitional stage of about 5±6 years
has to be passed (Voorhees and Lindstrom, 1984; Horn
et al., 1997). Amongst others, Ehlers and Claupein
(1994) reported an increasing amount and continuity
of macropores on CS sites, resulting in increased
ksat values. Thus an increased ability to compensate
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
125
Fig. 4. 3D visualisation of computer tomographic data at 10±20 cm depth of conventional (left) and conservation (right) tilled plots.
Houns®eld values: ÿ1024 and 0. Site: Reinshof/Germany. Treatments: (a) CT 0 Mg, (b) CS 0 Mg, (c) CT 6 5 Mg, and (d) CS 6 5 Mg.
external forces to a certain extent may be expected
on conservation tilled plots.
5.2. Soil displacement
In general, there was more pronounced soil displacement in CT as compared to CS. Structural degradation, as indicated by the precompression stress values
and the ksat vs. macropore relationship, was initiated in
the conventionally tilled plot at the lowest applied load
(2 2.5 Mg). In contrast in the conservation tillage
system, a wheeling event with 2 5 Mg was compensated by a robust aggregate system developed
beneath the tilled horizon. Since the precompression
stress and the ksat vs. macropore relationship was not
in¯uenced by this loading event, there was a higher
compensatory potential with respect to mechanical
impacts for CS compared to CT.
The wheeling events with the 2 5 Mg treatments
induced irreversible structural degradation on both
126
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
Fig. 5. 3D visualisation of computer tomographic data in 30±40 cm depth of conventional (left) and conservation (right) tilled plots.
Houns®eld values: ÿ1024 and 0. Site: Reinshof/Germany. Treatments: (a) CT 6 5 Mg, and (b) CS 6 5 Mg.
tillage systems as indicated by the vertical and horizontal soil displacement. These aggregates are more
or less reduced in size and changed in particle arrangement (Horn et al., 1994). Also the values for the ratio
of OCTSS to MNS revealed soil failure beneath the
wheeling tires on the conventionally treated plot. Thus
soil deformation processes induced by divergence
and/or shear were induced on this tillage system. In
contrast, in the CS site this ratio did not show the
observed structural degradation processes, indicated
by the displacement and the precompression stress
measurements.
Further traf®cking with a 5 Mg wheel load resulted
in additional structural degradation on both tillage
systems. In the CT plot soil degradation processes
obviously proceed to deeper soil layers, since the
precompression stress values at 60 cm was increased
by the third pass. Also in the CS site repeated wheeling
events induced a rearrangement of solid particles in
deeper soil horizons and therefore altered soil physical
conditions. A strong decrease of macropores was
observed at 30 cm depth with a simultaneous increase
of the precompression stress value. For deeper soil
layers (>30 cm), the extent of structural degradation
was reduced with CS as compared to CT, since
tomographic images showed a still intact vertically
orientated pore system. A preservation of pre-existing
vertically orientated macropores following soil compression has already been observed by Hartge and
Bohne (1983).
5.3. Topsoil vs. subsoil compaction
Soil stresses exceeding the internal soil strength
induced a rearrangement of the solid particles. Dexter
(1988) described the following three steps of structural
degradation: First, compression of the soil induces a
denser packing of lower porosity, but the soil aggregates are still intact. Second, weak aggregates collapse
to smaller structural units and ¯ow into the interstices
of the remaining intact aggregates. Third, the structural units collapse and plastic ¯ow occurs, resulting in
an unstructured soil matrix.
In these experiments all these steps were observed.
For the conventionally tilled plot the isotropic soil
structure of the topsoil was not able to compensate for
any of the applied dynamic loads. Thus a denser
packing of the structural units, resulting in a decreased
macroporosity and increased soil strength, was
induced by single wheeling events. On this tillage
system, stress attenuation occurred ®rst in the compacted plough pan at 30 cm depth. A strong decrease
of stresses beneath a hard pan layer was reported by
Taylor and Burt (1987). Thus structural degradation
C. Wiermann et al. / Soil & Tillage Research 53 (2000) 117±128
processes are restricted to the topsoil layers until the
horizontally orientated aggregates are crumbled.
Induced by repeated dynamic wheeling events, the
structural units of the plough pan collapsed and the
soil strength decreased. The compensatory effect of
the hard pan layer therefore declined by kneading
processes and stresses were directly transmitted to the
subsoil as described by Horn et al. (1998). Since the
subsoil was subjected to further soil degradation processes, the topsoil layer was expected to act as a rigid
body with elastic soil properties, the compacted topsoil layer translated and rotated to deeper soil horizons
without further changes of its soil properties.
In the conservation tillage treatment in contrast
stresses were attenuated at 10 cm depth by a soil layer
of high mechanical strength, formed by tillage tools
and ®eld traf®c. A soil layer of similar properties was
reported by SchjoÈnning and Rasmussen (1989). They
observed the development of a ``rotovator-pan'' with
increased soil strength on shallow tilled sites. This
hard pan in combination with a robust soil structure of
the subsoil was probably responsible for the observed
stress compensation when traf®cking with 2.5 Mg
wheel load. With increasing wheel loads and repeated
tire passes the strength of the hard pan decreased by
kneading processes induced by the dynamic impact of
the wheeling tires (slip). Stresses of further passes
were therefore transmitted to deeper soil layers and
structural degradation processes started at depths
>10 cm. Since preserved fragments of channels were
detected in soil layers beneath 30 cm depth in the
6 5 Mg treatment, the effects of biological and/or
physical processes, which can diminish the negative
effects of soil compaction, are expected to be more
favourable in CS compared to conventionally tilled
sites.
6. Conclusion
Soil strength of the untilled layers in CS was clearly
higher than that of the topsoil structure observed in the
conventional tillage system. Thus stresses induced by
a single wheeling event with a dynamic load of 2.5 Mg
were better resisted than in the conventional till soil.
On sites suitable for conservation tillage, the reduction
of tillage intensity is therefore a useful method to
improve soil strength and reduce subsoil compaction.
127
In both tillage systems repeated wheeling events
resulted in pronounced structural degradation of the
subsoil. However, in CS at depths >30 cm preserved
fragments of macropores were detected, while a completely homogeneous soil matrix was observed at the
ploughed plots. Thus, the conditions for a structural
recovery by physical and/or biological processes are
expected to be more favourable with conservation
tillage.
Soil layers, of high mechanical strength, were
observed below the tillage depth with both soil management systems. These layers obviously supported
stress attenuation and therefore protected deeper soil
layers from structural degradation.
The displacement measurements in combination
with computer tomographic images are useful techniques for assessing the extent of structural degradation due to dynamic tire passes of agricultural
machinery.
Acknowledgements
The authors are highly indebted to the German
Research Foundation (DFG) which ®nancially supported the research.
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