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DIVISION S-1—SOIL PHYSICS

Time Dependence of Soil Mechanical Properties and Pore Functions for Arable Soils

Institute for Plant Nutrition and Soil Science, Christian Albrechts Univ., 24098 Kiel, Germany

^* Corresponding author (rhorn{at}soils.uni-kiel.de).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Progressive soil degradation due to tillage operations affects crop production and its yield uncertainty, soil erosion by wind and water, and gas emission. The possibility of soil regeneration due to natural or anthropogenic processes is of major concern to sustain soil functions. Material properties like precompression stress, shear strength, and hydraulic conductivity are time dependent, while bulk density is less sensitive to time effects. The hypothesis that the change from conventional to conservation tillage affects mechanical soil strength and pore functions was tested for a stagnic Luvisol derived from glacial till in northern Germany. For more than 7 yr precompression stress, shear strength, and hydraulic conductivity were determined at three depths down to 60 cm. Approximately 3 yr after starting with the Horsch system (annual shallow chiseling in autumn down to an 8-cm depth) as a kind of conservation tillage, soil strength, and the hydraulic conductivity increased in the topsoil by more than 50 kPa and 500 cm d^–1, respectively, even at a higher bulk density as compared with the corresponding values for the conventionally tilled plots. Within the following 2 to 3 yr, these changes were also detected at the 30- to 35-cm soil depth. At the depth of 55 to 60 cm the same trend started after around 7 yr. These changes can be only detected, if the tillage systems were applied continuously and if all tillage and soil operations were performed with light machines, which enable the preservation of newly formed pores and the rearrangement of soil particles creating a more stable soil structure. Thus, the susceptibility to soil deformation will be reduced due to an increased shear strength, which in turn improves pore functioning.

	INTRODUCTION

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THE EFFECTS OF TILLAGE systems on crop yield, water and nutrient uptake efficiency have been repeatedly described and have resulted in a very detailed description of the minimum time required to adjust site properties. Ehlers et al. (1980) and Baeumer (1992) have shown that after approximately 7 yr of continuously applied no tillage (NT) an identical net yield was obtained for no-tilled compared with conventionally tilled sites as a consequence of the increased saturated hydraulic conductivity and pore continuity in the root zone. Horn (1986) determined the effect of long-term tillage on soil mechanical properties and found that, in general, soils subject to conservation tillage are stronger and therefore less susceptible to deformation induced changes in physical properties compared with conventionally tilled soils. However, in those investigations no seasonal changes were analyzed. Wiermann et al. (2001) measured the effect of wheel traffic on changes in physical properties in a Luvisol derived from loess (Reinshof/Goettingen) and in a Mollisol derived from loess (Gross Obringen). They found for the conservation tillage system an increase in soil strength of about 50 to 100 kPa at comparable soil depths and further showed that a decline in saturated hydraulic conductivity or air permeability occurred only if mechanical stresses applied by machines exceeded the soils precompression stress. At the virgin compression load range, the saturated hydraulic conductivity as well as the air permeability decreased linearily if plotted on a lognormal scale with increasing applied mechanical stress.

Van Ouwerkerk and Soane (1994) summarized much of the present knowledge about the effects of wheel traffic on soil physical properties and functions, but as they described mainly short-term effects of conventional tillage systems using more general parameters, no recommendations have been derived for example, for producers. The same is true for the papers of Håkansson et al. (1987), Håkansson and Reeder (1994), and Håkansson and Lipiec (2000) because they dealt primarily with soil bulk density (i.e., mass per volume) and apart from that restricted their research on the plowed topsoil layers.

Or and Ghezzehei (2002) defined mechanical differences between tillage treatments by calculating the elastic modulus (Hooke's Law), including data taken from mechanical measurements of Wiermann (1998) and Kühner (1997). Both the latter authors investigated soils under conventional and conservation tillage systems and classified the conservation tillage systems as more sustainable. Horn et al. (2000) addressed the subject of "Subsoil compaction—processes, consequences and distribution" and focused on the mechanical processes, which can be used for a more detailed analysis of soil deformation. Wiermann et al. (2001), Kühner (1997), Fleige et al. (2002) pointed out that the determination of shear strength parameters and soil precompression stresses, as indicators for the maximum internal soil strength, aids the quantification of pore functions such as hydraulic conductivity and air permeability. Data sets have yet to be developed that can be used to correlate tillage effects on bulk density to soil hydraulic properties. Baumgartl et al. (1995) and Baumgartl and Horn (2001) demonstrated a similarity between the stress strain and the water-retention curve, which may facilitate the derivation of one from the other. Schjønning et al. (1999) determined the effects of tillage on several soil parameters but did not report pronounced time dependent differences between treatments.

Thus, a better understanding is needed of the soil deformation processes following changes in tillage systems to document time effects on changes in soil strength and pore functioning. This paper presents the results from a long-term experiment to provide evidence to support the following hypotheses:

—The increases in soil stability and pore continuity develop slowly requiring a number of years.

—The increase in soil strength depends on the shear stress per particle contact point, which results from the rearrangement of particles during swelling and shrinkage processes.

—The particle rearrangement leads to an increased hydraulic conductivity and/or air permeability provided that conservation tillage systems have been applied over several years employing light agricultural machinery to not exceed the soils precompression stress.

—Soil strength, defined as precompression stress, must exceed the external stress applied to sustain the rearrangement of particles irrespective of wheel traffic during swelling and shrinkage.

	MATERIALS AND METHODS

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Soils and Experimental Setup
The experiments were performed at the experimental farm of the University in Hohenschulen/Kiel where the main soil type is a stagnic Luvisol (according to World Reference Base [WRB]) derived from weichselian glacial till. From 1991 until 1999, plots, which were originally conventionally plowed, were subdivided into conservation (= Horsch System = chiseled to an 8-cm depth) and conventionally tilled plots. Both treatments received mineral fertilizers and/or manure at various times. Only the 240 kg N ha^–1 fertilized plots under the two tillage treatments were studied. The selected sites were either conventionally tilled to a 25-cm depth each year in autumn or they were chiseled to an 8-cm depth (= conservation tillage plot) at the same period. These tillage systems (four row moldboard plow or chisel) were identical throughout the experiment and the machine mass used for all treatments was only 49.1 kN (Fendt tractor: type Farmer 308 LSA, Marktoberdorf, Germany), which resulted in an average contact area pressure of 90 kPa with a tire inflation pressure of 200 kPa. The lug height was 30 to 40 mm. The tire type was Pirelli TM 200 S 13.6 R24 (front) and Pirelli TM 300 S 16.9 R34 (rear) (Pirelli, Milan, Italy). All required field operations were performed at the same time and the soil water content was below field capacity especially for moldboard plowing or chiseling. The chisel "type Horsch" retained during the operation all residues of the conservation tillage plots on the surface to further improve the biological activity.

We determined changes in soil properties on both conventional and conservation tillage treatments over 9 yr up to three times per year at three depths. The depths were 10 to 15 cm (plowed seedbed or just below the chiseled soil layer), 30 to 35 cm (the actual or former plow pan layer), and 55 to 60 cm (clay enriched Bt horizon). Investigated soil properties were the stress strain behavior using a confined compression test (i.e., the horizontal stress is not defined because of the wall stiffness) and the shear strength under consolidated drained conditions (frame shear test) at a constant pore water pressure of –30 kPa.

The precompression stress value, that is, the transition from the reloading curve to the virgin compression line, was defined by the method of Casagrande (cited in Horn, 1981). Casagrande proposed an empirical construction to obtain from the void ratio–log stress curve the maximum vertical stress for soil samples that had acted on it in the past, referred to as the precompression stress. If the precompression stress value is exceeded by higher vertical stresses, soil samples show virgin compression behavior. The transition from the recompression to the virgin compression line (= precompression stress value) was derived from stress strain measurements with 10 undisturbed soil cylinders (diameter 100 mm, height 30 mm) per depth, which were equilibrated to a pore water pressure of –30 kPa and stressed for 23 h with one defined stress. This pore water pressure value defines the hydraulic stress situation encountered in summer within the upper 1 m of the soil profile.

The shear box tests were performed on all prestressed samples immediately thereafter and the parameters of the Mohr Coulomb failure line were derived from the shear strength as a function of the applied vertical stresses. The shear strength was originally expressed by Coulomb as a linear function of the normal stress on a plane at the same point by:

where c and are the shear strength parameters: cohesion (= intercept) and angle of internal friction, that is, the slope of the Mohr Coulomb failure line.

Changes in the saturated hydraulic conductivity were measured to quantify the changes in pore functions over time.

The bulk density was determined at all depths and times on undisturbed soil cores as an average value of 12 replicates. The particle density was determined by submerse measurement. The particle-size distribution and chemical properties were also measured. More detailed descriptions of these tests are given in Hartge and Horn (1992), Horn and Baumgartl (1999), and Schlichting et al. (1995).

Statistical Analysis
Analysis of variance (ANOVA) was performed for the effect of land use on bulk density, soil strength, and hydraulic conductivity (SPSS11.0). The differences of means between land uses were assessed by least significant difference (LSD) tests.

Stepwise regression analyses were also performed and the type of regression between tillage effects and correlations in physical properties with time (days) since this study began were determined using standard statistical procedures.

	RESULTS

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General Soil Description
From the physical and chemical properties of the soil (Table 1) it can be seen that the stagnic Luvisol has a characteristic amount of organic C in the Ap and Al horizons. The soil is free of calcium carbonate down to 150 cm, but the pH values range only between 6.2 and 6.6. The pH decreases with depth in the upper soil horizons (up to the Bgt horizon) resulting from the annual fertilization of the topsoil and increases again in the parent material (Cg). While the Ap horizon is well structured (subangular blocky to crumbly), the Al horizon has a tillage-induced platy structure and a slightly coherent structure at greater depth, which is followed by the Bgt horizon with a blocky to prismatic structure. The C horizon encountered at a depth >150 cm below the Bgt horizon shows a platy structure due to its geological deposition (= glacial preloading); the parent material is calcareous with pH values >7. The initial bulk density values of the various horizons vary between 1.45 g cm^–3 in the topsoil down to 1.8 g cm^–3 in the C horizon, which is typical for this soil. The AlSw horizon shows a platy structure with a slightly increased bulk density of 1.65 g cm^–3, which results from plowing.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Time dependent changes in bulk density at a depth of 10 to 15 cm.

Precompression Stress
The changes in the precompression stress over time and depth show significant differences in between the two treatments for the first and second soil horizon. In the 10- to 15-cm depth, soil strength increases after an intermediate time of smaller precompression stress values for nearly 3 yr, which is significantly different from the trend in the conventionally plowed soil (Fig. 2a). Tillage reduces the internal soil strength of the topsoil (A horizon). The same trend can also be seen at the depth of 30 to 35 cm where strength increases after approximately 4 to 5 yr (Fig. 2b). At the depth of 55 to 60 cm the same trend starts only after around 7 yr (Fig. 2c), however, these trends are not significantly different over time. The time dependent changes in the precompression stress values can, apart from the depth 10 to 15 cm conventionally tilled plot, be highly significantly described by second-order equations (Table 3).

View larger version (28K): [in this window] [in a new window]   Fig. 2. Time dependent changes in precompression stress at a depth of (a) 10 to 15, (b) 30 to 35, and (c) 55 to 60 cm, respectively.

View larger version (21K): [in this window] [in a new window]   Fig. 3. Changes in soil cohesion at constant pore water pressure of –30 kPa with time at a depth of (a) 10 to 15, and (b) 30 to 35 cm, respectively.

View larger version (27K): [in this window] [in a new window]   Fig. 4. Changes in the saturated hydraulic conductivity with time at (a) 10 to 15, (b) 30 to 35, and (c) 55 to 60 cm, respectively.

	DISCUSSION

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Soil structure in general undergoes intense changes due to seasonal swelling, shrinkage, and the reformation of the pore system, which affects hydraulic or gaseous fluxes. In addition, the rigidity of the pore system is affected by the internal rearrangement of particles.

Hartge (1965), Horn (1981), and Junge et al. (2000) have shown that soils have a well-defined internal strength, which may be either derived from drying and wetting or from mechanical processes. Based on the effective stress equation, Toll (1995) proposed a conceptual model for the similarity between mechanical and hydraulic boundary "strengthening" or strength-limiting conditions. Fredlund and Rahardjo (1993) pointed out that the interrelation between the hydraulic and mechanical processes affect the overall strength of three phase systems. Baumgartl and Horn (1999) argued that the effective stress between single particles within or between aggregates could be altered through mechanical or hydraulic (i.e., pore-water pressure) stresses. Additional soil deformation/shrinkage can only be expected after exceeding the internal strength either by higher mechanical stresses or by a more intense drying. Horn (1995) described the stress dependent changes of the aggregation and of the structure dependent soil strength and quantified the boundary stresses for the various structure elements.

The obtained results about the changes in mechanical strength and hydraulic conductivity under conservation tillage can be therefore explained by combining the actual applied mechanical stress, the seasonal changes in swelling and shrinkage as well as the maximum soil dryness:

• While the conservation site was only mechanically stressed at the soil surface, the annual plowing at the 30-cm depth of the conventionally tilled site always resulted in a homogenization of the topsoil and a stress application to the subsoil.
  
• The external forces applied by the agricultural machines are smaller than the maximum internal soil strength at each depth in the conservation tillage site, which therefore does not change the existing pore systems and pore functioning but indirectly enhance the mobility and accessibility of particle surfaces.
  
• Junkersfeld (1995) found in field experiments at the investigated sites in Hohenschulen that water uptake by wheat, rape, and barley down to the 80-cm depth resulted in minimal pore-water pressure values of up to –70 kPa in the conservation tillage plots, while the water uptake in the tilled plot was mostly restricted to the topsoil and never decreased to less than –40 kPa below the plowpan layer. Furthermore, the hydraulic gradients between different soil horizons and/or distances from the plant at the same soil depth were greater in the conservation than in the conventionally tillage plots. Ehlers (1996) also supported these findings, that after several years of conservation tillage the root distribution was more pronounced to deeper depth and the result was a more homogenous water uptake. Consequently, the positive changes in soil structure and soil strength are generated by the long-term effects of repeated swelling and shrinkage, as a structure improving.
  
• Reorientation of soil particles by menisci forces can assume to be more pronounced in the conservation tillage site.
  

Horn and Dexter (1989) described the reorientation of soil particles in structure elements, which coincide with the alteration of aggregate strength and inter- as well as intraaggregate pore functions. They concluded that if soils are repeatedly wetted and dried throughout the years, existing aggregates get dispersed starting from the outer skin, which results in the mobilization of particles. Consequently, the translocation of clay particles and colloids and their repeated rearrangement inside aggregates due to water menisci forces finally result in the formation of stronger, but at the same moment less dense aggregates. Additionally, the pore continuity is improved, which increases the air and water permeability. Furthermore, the mechanical strength of the non-tilled soils was much greater than of the tilled soils (Horn, 1986), which is in full agreement with the described data.

Thus, the presented results and trends reconfirm these findings and are also supported by the well-known processes, that

—Conservation tillage increases the mechanical strength of the soil, which is well known as structure strengthening;

—The formation of finer interaggregate pores due to shrinkage and the rearrangement of particles (Horn, 1995) are the basis for sustaining proper pore functioning and soil mechanical properties with time.

Consequently, increases in soil strength as it is defined by the parameters precompression stress or cohesion are to be expected when shifting to conservation tillage and the alteration of soil physical properties with time after the tillage system was changed are the result of aforementioned processes. The more intense the drying and swelling and the higher their frequency, the earlier can the strengthening and improved functioning of the pore system be detected even down to deeper soil depth.

Up to now all wheel traffic was done with small machines, which did not exceed the internal soil strength during field operations. We postulate that any stress application not accounting for the maximal internal soil strength (i.e., precompression stress value) must prevent or at least further delay any rearrangement of particles and reformation of stronger aggregates and coarser pores. There is no elasticity of this soil, which would be needed for such reformation processes (Horn, 2002). With regard to soil quality, we point out that the melioration of soil structure requires long-term stress release in combination with pronounced swelling and shrinkage. To what extent the climatic conditions in Northern Germany help to improve the site properties in comparison with the enhanced particle mobility under the wetter soil conditions has to be further analyzed in comparison with corresponding experiments performed under different climatic and site conditions.

	CONCLUSIONS

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Under a continuously applied system of conservation tillage, we can expect a more stable and a better functioning pore system at the same time after more than 7 yr down to a depth of 60 cm under the climatic conditions prevailing in northern Germany. These findings, however, can only be obtained if during all tillage operations the internal soil strength is never exceeded by the applied mechanical stresses. Presuming those boundary conditions, both shrinkage due to water uptake by plants and swelling when the soil is rewetted result in the rearrangement of soil aggregates and the creation of newly formed and more continuous pore systems. Aggregates as well as the soil structure are in addition to that much stronger. Hydraulic stresses, induced by the water uptake by plants and the generation of hydraulic gradients, both strengthen the existing soil structure by increasing the effective stresses between soil particles/aggregates. The strengthening through the rearrangement of aggregates and consequent changes in the pore system requires time and can only occur if the applied mechanical stresses are smaller than the internal soil strength, which is defined by the precompression stress.

How far these findings can be also extended to other regions and modified tillage systems have to be evaluated to develop a more general system of recommendations in view of sustainable agriculture.

	ACKNOWLEDGMENTS

 
The author is highly indebted to: The German Research Foundation (DFG), which financially supported the long-term experiments by grants in the Special Research Programme (SFB) 192 (project C1); Mrs. Petra Koester, who carried out the numerous measurements; and Prof. Dr. A.R. Dexter/Poland for the improvements of the English text and the improving comments of the reviewers.

Received for publication January 14, 2003.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (16) Citing Articles via Google Scholar Google Scholar Articles by Horn, R. Search for Related Content PubMed Articles by Horn, R. Agricola Articles by Horn, R. Related Collections Crop Physiology & Metabolism Soil Compaction Hydraulic Conductivity