Rainer Horn

Soil compaction processes and their effects on the structure of arable soils and the environment

Soils are three-phase systems which undergo changes as soon as the external stresses exceed the internal soil strength, defined by the precompression stress value. The three-dimensional stress propagation induces corresponding volumetric soil strain. Soil compaction can result either in a higher bulk density or, when soil loading is attended with retarded water fluxes and high dynamic forces, in a completely homogenised soil characterised by a lower bulk density and a predominance of fine pores. While in natural soils the structure can be described as macroscopically homogeneous, less careful mechanical treatment or reduced addition of organic substances results in less favourable types of soil aggregates. As a result of applied external stresses, physical and chemical processes, such as mass flow and diffusion of water, ions and gases, are at least retarded or even completely altered. Both increased bulk density and homogenisation cause decreased aeration and increased penetration resistance, which results in impeded root development. Reduced water permeability may result in soil erosion, with serious negative effects on the environment. Compacted soil may also contribute to global atmospheric warming due to increased emission of CO2, CH4 and N2O from such soils. Anthropogenic changes in soil structure and soil functions remain constant for extended periods of time and efforts to restore deteriorated soil structure very often fail because of excessive loosening and homogenisation, cultivation of too wet soil or, afterwards, ill-adapted soil management practices, resulting in even worse soil properties. The present paper gives a summary of relevant work performed by the authors.

Soil physical properties related to soil structure

Abstract The aim of this paper is to clarify the effect of soil aggregation on soil physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Such aggregates are, most of all, more dense than the aggregated bulk soil. The intra-aggregate pore distribution consists not only of finer pores but these are also more tortuous. Thus, water fluxes in aggregated soils are mostly multidimensional and the corresponding water fluxes in the intra-aggregate pore system are much smaller. Furthermore, ion transport by mass flow as well as by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. The chemical composition of the percolating soil solution differs even more from that of the corresponding homogenized material the stronger and denser the aggregates are. The rearrangement of particles by aggregate formation also induces an increased apparent thermal diffusivity as compared with the homogenized material. The aggregate formation also affects the aeration and the gaseous composition of the intra-aggregate pore space. Depending on the kind and intensity of aggregation, the intra-aggregate pores can be completely anoxic, while the inter-aggregate pores are already completely aerated. The higher the amount of dissolved organic carbon in the percolating soil solution, the more pronounced is the difference between the gaseous composition in the inter- and in the intra-aggregate pore system. From the mechanical point of view, the strength single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. The more structured soils are, the higher the proportion of the effective stress on the total stress is, but even in single aggregates positive pore water pressure values can be revealed. Dynamic forces e.g. due to wheeling and/or slip processes can affect the pore system as well as the composition of the soil by: (1) a rearrangement of single aggregates in the existing inter-aggregate pore system resulting in an increased bulk density and a less aerated and less rootable soil volume, (2) a complete homogenization, i.e. aggregate deterioration due to shearing. Thus, the smaller texture dependent soil strength coincides with a more intensive soil compaction due to loading. (3) Aggregate deterioration due to shearing results in a complete homogenization, if excess soil water is available owing to kneading as soon as the octahedral shear stresses and the mean normal stresses exceed the stress state defined by the Mohr-Coulomb failure line. Consequently, normal shrinkage processes start again. Thus, the rearrangement of particles and the formation of well defined single aggregates even at the same bulk density of the bulk soil both affect, to a great extent, various ecological parameters. Environmental aspects can also be correlated, or at least explained with the processes in soils, as a major compartment of terrestial ecosystems, if the physical and chemical properties of the structure elements and their composition in the bulk soil are understood.

A method to predict the mechanical strength of agricultural soils

Abstract During a 3-year period the physical and mechanical properties of 37 typical, differently textured and structured agricultural soils in Bavaria were determined in order to predict their mechanical compressibility and trafficability. The soil physical properties (bulk density, pore size distribution, saturated hydraulic conductivity, air permeability and penetration resistance) and the soil mechanical properties (pre-consolidation load and the shear strength parameters angle of internal friction and cohesion), were determined on undisturbed, differently pre-dried soil samples (60 and 300 hPa water tension). In order to quantify the changes in soil physical properties affected by loading, all soil physical parameters were measured before and after loading by the confined compression test (load range 10–800 kPa). It was found that in homogeneous, non-structured soils, such as sands and silts with low clay content ( 15%, w/w), stability increased with increasing degree of aggregation (coherent > prismatic > blocky > subangular blocky), due to higher values of the angle of internal friction between the aggregates and higher values for the cohesion within the dense, stable aggregates. The mechanical soil strength, which is determined as the value of the pre-consolidation load, could be predicted by multiple regression analysis with a high degree of significance, when the shear parameters angle of internal friction and cohesion (load range 0–400 kPa) were included as independent variables.

A method for assessing the impact of load on mechanical stability and on physical properties of soils

Methods to quantify the mechanical strength of agricultural soils in order to assess the trafficability are presented. The pedotransfer functions relating the precompression stress as a measure of soil strength and the depending soil parameters are also shown. By using cohesion and angle of internal friction values, the precompression stress can be calculated using the multiple regression equations. Horizon specific values on the mechanical stability of arable soils is determined at various moisture suctions. Changes in dependence of gravel contents are also given. The stress transmission for specific soil horizons is calculated by using classified values of the concentration factor. The mechanical stability for the soil profile is then determined by comparing the actual pressures in a specific soil horizon with the corresponding value of the precompression stress. Stress dependent changes of soil physical properties only occur when applied stress exceeds the precompression stress. These changes in soil physical properties are dependent on soil suction, texture, structure and applied stress. Regression equations presented in this paper can be used to calculate the changes in soil physical and mechanical properties due to loading. The proposed method is a useful tool towards fulfilling the soil protection law in the Federal Republic of Germany.

Dynamics of soil aggregation in an irrigated desert loess

Abstract Experiments were made on an irrigated loess in the Negev Desert, Israel. Lysimeters were filled with disturbed, homogenized soil, and a single almond tree was planted in each of them. The soil in these lysimeters was sampled 1.5 and 2.5 years after the start of the experiment. Additionally, some samples of older, undisturbed soil were examined. Aggregate tensile strength was measured by an indirect tension (crushing) test, and the dry bulk density of the aggregates was determined. It was found that aggregate tensile strength increased progressively with time after disturbance such that the old undisturbed soil was approximately three times stronger than the soil 1.5 years after homogenization. Higher levels of root density and more intensive drying increased aggregate strength. Aggregate density first increased with time after homogenization, but then appeared to decrease steadily towards a low equilibrium value. Mechanisms are proposed to explain these observations.

Aggregate characterization as compared to soil bulk properties

Abstract The aim of this paper is to clarify the effect of soil aggregation on the physical and chemical properties of structured soils and as compared with the homogenized material. Aggregation and aggregate strength do not only depend on biological activity and organic exudates, but also on the intensity, number and time of swelling, and drying events. Such aggregates are not only more dense than the structured bulk soil, the intra-aggregate pore distribution consists not only of finer pores, but they are also more tortuous. Thus, water and ion fluxes by mass flow as well as ion transportation by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. Futhermore, the chemical composition of the percolating soil solution differs more from that of the corresponding homogenized material the stronger and denser the aggregates are. From the mechanical point of view, the strength of single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. However, internal soil parameters, like grain size distribution or chemical composition, further affect the strength. The more structured the soils are, the higher is the proportion of the effective stress on total stress, but even in single aggregates neutral stresses can be revealed. This is true because of the relationship to the smaller value of the hydraulic conductivity and higher tortuosity. Finally, some dynamic effects on aggregation and aggregate deterioration are discussed.

Soil Compactability and Compressibility

Summary The strength of structured soils during loading depends on both effective stresses and neutral stresses. Thus, soil compaction affects the inter- as well as the intra-aggregate pore size distribution and results in changes of several parameters of the effective-stress equation. Furthermore, differences in the hydraulic conductivity of the bulk soil and in single aggregates, and also differences in the pore continuity must be considered. Therefore, determination of compressibility and compactability requires physically and mechanically well-defined measurements and methods and a detailed analysis on both macro- and micro-scale, in order to deal adequately with the complex relationships between the requirements of growing plants and the soil physical characteristics as affected by loading.

Stress/strain processes in a structured unsaturated silty loam Luvisol under different tillage treatments in Germany

In agriculture, the degradation of soil structure by tillage and field traffic is an adverse process causing a reduction in productivity of arable land. In order to manage this problem, various kinds of traffic 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 field 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 field 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, trafficked 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.

Effect of repeated tractor wheeling on stress/strain properties and consequences on physical properties in structured arable soils

The discussion about the effect of repeated short time wheeling on long-term changes in soil structure and pore functioning reveals a great uncertainty. On the one hand it is told that soil structure elements are rigid and do not undergo intense changes in pore functions as a consequence of the short loading interval during each single wheeling. On the other hand, the complete deterioration of the structure elements and pore functions is assumed to occur, which also results in changes of the shrinkage pattern, soil strength including even strength regain. Consequently, the effect of wheeling on soil deformation and stress/strain distribution was investigated in a soil bin which contained Hiwassee clay at the NSDL, Auburn. If the soil is very strong due to aggregation, plow pan formation or dryness, soil stress applied by repeated wheeling results in an increased primarily vertical soil particle displacement in the Hiwassee clay soil while during repeated wheeling (up to 10×) a more pronounced displacement linked with a more intense movement of particles can be proofed. With increasing number of wheeling events, new platy or again coherent structure elements are formed, which create a very different pore system. The more intense is soil wheeling, the smaller is the saturated hydraulic conductivity and the higher is the unsaturated one at a given pore water pressure value. Such changes are the more pronounced the more completed is the rearrangement of the still existing aggregates into new units like plates. Due to shear because of the three-dimensional soil displacement even under dry conditions such aggregates can be redisturbed and a coherent but very compacted soil horizon can be formed. Under those conditions the values of bulk density are even higher than the Proctor density.

Introduction to the special issue on experiences with the impact and prevention of subsoil compaction in the European Union

Abstract The papers in this special issue present results of the European Union (EU) concerted action “Experiences with the impact of subsoil compaction on soil crop growth and environment and ways to prevent subsoil compaction”. The results and conclusions of earlier research on subsoil compaction are memorized and it is emphasized that the conclusions are still sound: high axle load traffic on soils of high moisture content causes deep and persistent subsoil compaction. The concerted action on subsoil compaction in the EU and an almost identical concerted action on subsoil compaction in central and eastern Europe are briefly introduced. This special issue presents a selection of papers of the concluding workshop of the concerted action on subsoil compaction in the EU. It includes three papers on modeling the impact of subsoil compaction on crop growth, water availability to plants and environmental aspects; three papers on modeling of subsoil compaction by heavy machinery; four papers on measurement of soil mechanical and physical properties in relation to subsoil compaction and four papers on methods to determine the risk of subsoil compaction and to identify prevention strategies. The trends in agriculture in relation to subsoil compaction are discussed. A positive trend is that policy makers in the EU and worldwide recognize soil as a vital and largely non-renewable resource increasingly under pressure. A negative trend is that wheel loads in agriculture are still increasing causing severe damage to subsoils. The conclusion is that European subsoils are more threatened than ever in history. Manufactures, agricultural engineers and soil scientists should collaborate and research should be initiated to solve this problem and find solutions. Subsoil compaction should be made recognized by all people involved from farmer to policy maker. Therefore an assessment of the existence and seriousness of subsoil compaction throughout Europe should be initiated.