Stress/strain processes in a structured unsaturated silty loam Luvisol
under different tillage treatments in Germany
C. Wiermann
a,*, D. Werner
b, R. Horn
a, J. Rostek
a, B. Werner
caInstitute for Plant Nutrition and Soil Science, Christian-Albrechts-University of Kiel, Olshausenstr. 40, D-24118 Kiel, Germany bThuringian Institute for Agriculture, Naumburger Str. 98, D-07743 Jena, Germany
cInstitute 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 passeswheel load: 22.5, 25 and 65 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
65 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
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 *Corresponding author. Present address:
Landwirtschafts-kammer Schleswig-Holstein, Bildungs-und Beratungszentrum Bredstedt, Theodor-Storm Str. 2, 25821 Bredstedt, Germany. Tel.: 49-4671-913428; fax:49-4671-913411.
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 ben-e®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; Ver-meulen 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 correla-tion between soil compaccorrela-tion and soil management practices. Ehlers and Claupein (1994) reviewed the effect of various tillage systems (conventional, con-servation, and zero tillage) on soil properties. Plough-less tillage treatments increased soil bulk density and penetration resistance in lower topsoil layers. Macro-porosity and pore continuity increased under conser-vation 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 contin-uous aggregation processes under long term
conserva-tion tillage management on soil mechanical
properties, and (2) to investigate the possibilities of
soils react in different modes, according to the dis-tribution, 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 strainx,y,z and shear strainxy,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 con-servation 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).
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 calcu-lated. 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).
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 cham-bers 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 cm3soil cores.
The determination of the mechanical parameters was conducted at a pore water pressure ofÿ6 kPa. In order to determine the precompression stress values, seven soil samples (235 cm3) of each depth and treat-ment 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
friction) were derived from the results of a ring shear test at a forward speed of 0.2 mm sÿ1using the same loads as already mentioned for the precompression test.
Larger samples, which contained a more represen-tative part of soil structure were used for demonstrat-ing the spatial arrangement of macropore systems. Therefore, 100100 mm soil cores (plastic cylin-ders) taken twice as mentioned above were used for X-ray 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 750mm. 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
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. Theksatwas 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 macro-porosity at the conventional tillage system showed a signi®cant difference between the seed bed and the underlying horizons throughout the soil pro®le, although theksatvalues revealed differences between these layers. High values of plant available water (25%) were calculated for both tillage systems.
Table 1
Physical and chemical soil properties of a Luvisol derived from loess, tilled differently by conventional and conservation tillage systems since 1971 at Reinshof, Germany
Soil depth (cm)
Bulk density (g/cm3)
Pore size distribution at pF (vol.%) ksat
(cm/d)
pH Texture <2mm (g/kg) Mechanical properties atÿ6 kPa
-oo 1.8 2.5 4.2
Clay Silt Sand Precompression stress (kPa)
Cohesion (kPa)
Angle of internal friction (8)
Conventional tillage
10 1.49 43.2 36.0 31.5 10.0 0.7 6.7 150 729 121 150 5.8 33.5
30 1.63 38.4 35.1 30.6 10.5 23.0 5.9 150 741 109 200 35.0 33.0
60 1.52 41.1 35.9 31.1 18.3 205.0 5.5 230 668 102 100 11.5 30.0
Conservation tillage
10 1.55 41.0 35.5 28.6 10.3 5.0 6.4 150 728 122 70 16.1 31.9
30 1.53 42.4 34.6 27.2 10.3 148.0 6.2 150 743 107 100 8.8 32.1
60 1.52 42.5 38.2 34.3 17.7 160.0 5.9 220 678 102 70 20.1 26.5
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4.2. Soil displacement under wheel load
Fig. 1 shows the vertical and horizontal soil dis-placement at 10 cm depth for both tillage systems, traf®cked twice by a 2.5 Mg wheel load. A more pronounced vertical and horizontal soil displacement on CT compared to CS could be observed. Further-more a distinct elastic soil behaviour was observed on the conventionally treated plots between the rear and front wheel as well as after removing the load.
Similar results were obtained when applying a dynamic load of 5 Mg (Fig. 2). On the conventional tillage system the ®rst pass induced a soil
displace-ment of nearly 100 mm in vertical and 20 mm in horizontal direction. For the CS in contrast, a vertical transducer movement of only 25 mm was observed. Further soil displacement in both tillage systems was induced by repeated wheeling events. The third pass on the CT plot resulted in a similar soil move-ment as described for the ®rst pass. For the CS plots in contrast, there was increasing vertical and horizontal transducer displacement for the third pass compared to the ®rst pass. Thus with increasing passes, the induced soil movement on the CS treated plot became closer to that of the conventional tillage system.
Fig. 2. Two-dimensional vertical and horizontal soil displacement at 10 cm depth on conventional (left) and conservation (right) tillage systems induced by the ®rst (25 Mg) and third pass (65 Mg) using a wheel load of 5 Mg. Soil water suction during wheeling:ÿ6 kPa. Site: Reinshof/Germany.
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4.3. Soil stress values
The calculated MNS and OCTSS values for the investigated wheeling events are summarised in Table 2. The stress state of the ®rst pass at CT was clearly dominated by shear stresses, when applying a wheel load of either 2.5 or 5 Mg. In CS plots in contrast, these values were smaller, indicating less soil failure beneath the running tire. While the OSR values for the CT plot declined with the sixth passage of the tire (5 Mg wheel load), the ratio of OCTSS to MNS increased in the CS system. Thus an increasing degree of soil failure at a depth of 10 cm occurred on the CS plot.
A comparison of the tillage systems with regard to the precompression stress values has already been described in Table 1. Further differences between both tillage treatments, subjected to increasing dynamic loads, become obvious in Table 2. On the CT plot the precompression stress at 10 cm depth was clearly increased by a wheel load of 2.5 Mg in the 10 cm depth, while in CS only slight changes occurred. The ®rst pass with 5 Mg wheel load induced an increasing precompression stress value in the CT plot at 30 cm, but on the CS plot in contrast this occurred at the 10 cm depth. Further wheeling events resulted in pronounced precompression stress values of the sub-soil and decreased values of the topsub-soil in both tillage systems. Thus the soil strength declined in the topsoil and increased in the subsoil by repeated wheeling
events. In the CT plots the soil is weakened to depth of 30 cm by the 65 Mg treatment, while in the CS treatment site the soil strength still increased at this soil depth.
4.4. Macroporosity and ksat
Fig. 3 shows the macroporosity vs.ksatrelationship at the 30±36 cm soil depth layer for both tillage systems. The arrangement of pair values in general follows the stress intensity for the CT plots. In contrast a stress depended distribution of pair values could not be derived under the CS treatment. Only the 65 Mg treatment was affected. The volume of macropores decreased, while the hydraulic conductivity did not change at all, even with a dynamic load of 65 Mg.
4.5. Computer tomographic images
The computer tomographic images assisted in the interpretation of soil physical values by a three-dimen-sional reconstruction of the shape, course and branch-ing of the coarse pore systems. Fig. 4 demonstrates a typical con®guration of pore space in the unloaded plots of both tillage systems in the 10±20 cm depth layer. An irregular arrangement of connected inter-aggregate packing pores was found in the ploughed horizon of CT (Fig. 4a) but channels with preferred vertical orientation embedded in a compact soil matrix were evident in the untilled Ap-horizon of
Table 2
Calculated MNS and OCTSS as well as the ratio of OCTSS to MNS (OSR) at peak major principle stress at 10 cm depth in CT and CS wheeled by dynamic loads of 2.5 Mg and 5 Mga
Wheeling 22.5 Mg wheel load Precompression stress (kPa)
25 Mg wheel load Precompression stress (kPa) MNS (kPa) OCTSS (kPa) OSR MNS (kPa) OCTSS (kPa) OSR
Conventional tillage
CS (Fig. 4b). A near complete loss of coarse pores for the 65 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 differ-ences 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). Con-sequently 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 aggre-gates 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 arrange-ment 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
Fig. 3. Relationship between macroporosity (>50mm) andksatunder conventional (CT) and conservation tillage (CS) as a function of the
external forces to a certain extent may be expected on conservation tilled plots.
5.2. Soil displacement
In general, there was more pronounced soil displa-cement in CT as compared to CS. Structural degrada-tion, as indicated by the precompression stress values and theksatvs. macropore relationship, was initiated in the conventionally tilled plot at the lowest applied load
(22.5 Mg). In contrast in the conservation tillage system, a wheeling event with 25 Mg was com-pensated by a robust aggregate system developed beneath the tilled horizon. Since the precompression stress and theksatvs. 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 25 Mg treatments induced irreversible structural degradation on both
tillage systems as indicated by the vertical and hor-izontal soil displacement. These aggregates are more or less reduced in size and changed in particle arrange-ment (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 com-pression 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 aggre-gates are still intact. Second, weak aggreaggre-gates collapse to smaller structural units and ¯ow into the interstices of the remaining intact aggregates. Third, the struc-tural 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 com-pacted 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
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 pro-cesses, the topsoil layer was expected to act as a rigid body with elastic soil properties, the compacted top-soil layer translated and rotated to deeper top-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 65 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.
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 com-pletely 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 man-agement 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 tech-niques for assessing the extent of structural degrada-tion due to dynamic tire passes of agricultural machinery.
Acknowledgements
The authors are highly indebted to the German Research Foundation (DFG) which ®nancially sup-ported the research.
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