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COMPACTION J J H van den Akker, Alterra Wageningen UR, Wageningen, The Netherlands B Soane, Formerly with Scottish Agricultural College, Edinburgh, UK ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Soil compaction is a form of physical degradation in which soil biological activity and soil productivity for agricultural and forest cropping are reduced, resulting in environmental consequences away from the immediate area directly affected. Compaction is a process of densification and distortion in which total and airfilled porosity and permeability are reduced, strength is increased, soil structure partly destroyed, and many changes induced in the soil fabric and in various characteristics. The term ‘compaction’ is used to identify a process and should be distinguished from the term ‘compactness,’ which indicates for a given time and position the state of packing of the solid soil constituent. The compaction process can be initiated by wheels, tracks or rollers, traffic of cultivation machinery, and passage of draft or grazing animals. Soil densification can also be caused by heavy overburdens of ice and snow and by illuviation of clay from the A-horizon into the B-horizon. These processes may take place slowly over long periods of time and result in the B-horizon having much higher bulk density and strength than the A-horizon. Densification also occurs during structural collapse near the soil surface due to the impact of rain, resulting in formation of surface crusts. In arable land with annual ploughing, both topsoil and subsoil compaction should be considered. We define the subsoil as the soil below the loosened layer (about 20–35 cm thick). This definition of the subsoil includes the panlayer as the upper part of the subsoil. This panlayer is, in many cases, less permeable for roots, water, and oxygen than the soil below it and is the bottleneck for subsoil functions. In contrast to the topsoil, the subsoil is not loosened annually, compaction is cumulative and, in the long run, a more or less homogeneous compacted layer is created. The resilience of the subsoil for compaction is low, and subsoil compaction is at least partly persistent. Problems of compaction are widely distributed throughout the world, but tend to be most prevalent where heavy machinery is used in agriculture or forestry, in both temperate and tropical areas. Soils that are naturally fragile in structure, such as soils of the
humid tropical forest and light-textured soils in areas of low but erosive rainfall, are particularly prone to problems arising from compaction and subsequent high risks of erosion due to reduction of permeability. Compaction is now included in surveys of soil degradation, and preliminary estimates have suggested that the area of degradation attributable to soil compaction may equal or exceed 33 Mha in Europe and 18 Mha in Africa. A project on mapping of Soil and Terrain Vulnerability in Central and Eastern Europe (SOVEUR) by the United Nations Food and Agriculture Organization (FAO) and International Soil and Reference Information Centre (ISRIC) showed that compaction is the most widespread kind of soil physical soil degradation in these countries. About 25 Mha proved to be lightly and about 36 Mha moderately compacted.
Factors and Processes Affecting Distribution and Intensity of Compaction Compaction under Running Gear
Compaction under wheels and caterpillar tracks is a dynamic process in which a soil volume under the wheel undergoes normal and shearing stresses resulting in normal and shear strains. Neighboring soil volume elements at a certain depth under a wheel are considered in Figure 1. During a wheel pass volume element 1 at a certain depth endures not only compaction but also deformation, as depicted by the volume elements 2–26. The impact of the wheel load on the physical properties of the soil depends on the strength of the soil (see Stress–Strain and Soil Strength). If the exerted soil stresses exceed the precompression stress and shear strength of the soil, then compaction will be accompanied by large deformations and macropores and structure will be remolded, resulting in degradation of soil physical qualities. Degradation of soil physical qualities will be less severe if only the precompression stress is exceeded, because then the soil will mainly compact with limited deformations, and the remnants of the biomacropores and the intra-aggregate space will still retain a certain continuity. Loading Characteristics of Individual Wheels or Caterpillar Tracks
For practical purposes a useful parameter is the average ground contact pressure P (kPa), which is defined as the wheel load divided by the ground contact area.
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Figure 1 Displacement and deformation in a vertical section of a soil volume element under a wheel. Reproduced with permission from Koolen AJ and Kuipers H (1983) Agricultural soil mechanics. Advanced Series in Agricultural Sciences 13, Springer-Verlag, Heidelberg.
The average ground contact pressure can also be calculated as the sum of tire inflation pressure Pi and a pressure Pc for carcass stiffness: P ¼ c Pi þ Pc
½1
The range for c is 0–1.25 and for Pc is 0–50 kPa. The factor c also depends on the carcass stiffness, and therefore eqn [1] is mostly reduced to P ¼ c Pi or P ¼ Pi þ Pc. At inflation pressures larger than 200–300 kPa, the influence of carcass stiffness diminishes and the factor c can become smaller than 1. The higher the inflation pressure, the more the strength and firmness of the soil determine the ground pressure. The peak stresses in the ground contact area determine the peak stresses in the soil that result in compaction and distortion of the soil structure if the soil strength is exceeded. Peak stresses under lugs can be two to five times higher than the average ground contact pressure. However, the resulting soil stresses decrease rapidly with depth because the ground contact area is small. If soil stresses at a depth of 0.2–0.3 m are considered, then the peak stresses under the lugs can be neglected and a parabolic stress distribution in the ground contact area with a peak stress of one-and-a-half to two times the average ground contact pressure can be assumed. Peak stresses under caterpillar tracks are two to four times the average ground contact pressure and depend strongly on the firmness of the soil and design of the track system. Examples of load characteristics for various vehicles and animals are shown in Table 1. Compaction by agricultural machinery extends well into the subsoil (Figure 2). An important cause of subsoil compaction is moldboard plowing in which the furrow-side tractor wheels apply appreciable loads directly to the upper surface of the subsoil, causing a
Table 1 Maximum loads and ground contact pressures applied by various sources
Source
Large-wheeled tractor (120 kW) Small-wheeled tractor (40 kW) Sugarbeet harvester (loaded) Slurry tanker (loaded) Track-laying tractor Horse Cow
Total load (kN)
Load per wheel/track/ hoof (kN)
Average ground contact pressure (kPa)
100
50
250 – 350
40
20
200 – 300
300 –600
50 –120
200 – 400
100 –300
25 – 60
200 – 500
140 8 4–5
70 2–8 1–4
40 75 – 300 120 – 480
plow pan. Attempts have been made to specify maximum recommended average ground contact pressures in order to minimize compaction, especially in the subsoil. Suggested maximum values range from 80 kPa for wet soils in spring to 200 kPa for dry soils in summer, but progress in gaining official acceptance for such standards has been slow. Field Traffic Intensity and Distribution
The overall incidence of soil compaction within a given field depends upon the distribution of traffic for each field operation and the cumulative value throughout the life of the crop. The weight of the crop to be transported from the field is an important factor in the compaction risk. The traffic intensity and compaction risk for a potato crop and a sugar beet crop are, respectively, more than twice and almost twice that of a winter wheat crop. As the number and width of wheels fitted to vehicles increase, so does the overall proportion of the field area covered
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Figure 2 Increase of bulk density to considerable depth when a bulldozer was used for forest clearing in Surinam (- - - no vehicle traffic; — bulldozer traffic). Reproduced with permission from Van der Weert R (1974) Tropical Agriculture (Trinidad) 51(2): 325–331.
by wheels. However, this effect does not counteract the benefits from a reduction in compaction due to reduced ground contact pressure. Soil Compactibility
Soils can vary from being sufficiently strong to resist all likely applied loads (low compactibility) to being so weak that they are compacted by even low loads (high compactibility). Well-structured soils combine good physical soil properties with high strength. Sandy soils with a single-grain structure and compacted massive soils can be very strong. Rootability and soil physical properties are then often poor. Roots have a binding action and increase the elasticity and resistance of a soil to compaction. Soil moisture content and soil water suction have a dominant influence on soil compactibility. In soils where capillarity is the main cohesive force, strength increases with drying until it reaches a maximum, and then again decreases upon further dehydration because the cross-sectional area of the menisci decreases more than capillary suction between grains increases. This cohesive force based on soil water suction is called apparent cohesion. Drying also increases the true cohesion. Soil water suction increases the cohesion and so the strength and resistance to compaction of soils vary considerably. In dry structured (aggregated) soils, soil water concentrates in the aggregates, and cohesion and soil strength in the aggregate increases considerably. Dry structured soils shrink and turn into an assembly of individual aggregates that fit
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together rather neatly. This assembly of aggregates has a very high interaggregate angle of internal friction and moderate interaggregate cohesion. This soil is strong and has a low compactibility. However, if such a soil is overloaded and compacted, the aggregates will be crushed and the interaggregate space will be filled up, resulting in a dense compact soil. Dry soils resist loads readily. However, extremely dry sandy soils can be deformed and compacted rather easily. As the moisture content increases, soil compactibility increases until the moisture content is approximately at the field capacity point, when a condition known as the optimum moisture content for compaction is reached. At still higher moisture contents, the soil becomes increasingly incompactible as the moisture tends to fill ever more of the total porosity and further loss of air-filled porosity becomes impossible. However, although the compaction may be minimal, the plastic flow of a wet soil results in a complete destruction of soil structure and macropores with accompanying diminishment of soil physical qualities. The reaction of dry and strong soils may be largely elastic, whereas at high moisture content and low strength the reaction may be plastic flow. Increases in organic matter content tend to reduce soil compactibility and to increase elasticity. For example, peat soils are quite resistant to compaction. The value of the optimum moisture content for compaction increases as the organic matter content increases.
Effects on Soil Physical and Mechanical Properties Bulk Density, Porosity, and Packing State
Bulk density (the mass of soil solids per unit volume) is the most direct and easy-to-measure indicator of changes in compactness, but changes in packing state can be better quantified by total porosity or void ratio. The relation between porosity (especially macroporosity) and soil physical properties such as saturated and unsaturated hydraulic conductivity and gas diffusivity is much closer than the relation of these soil physical properties with bulk density. Compaction causes major reductions in macroporosity (>50 m), reduction of total porosity, and often an increase in microporosity, resulting in a major impact on soil physical properties. Relative terms for packing state (e.g., relative density, degree of compactness) enable the same threshold values to be found for overcompaction for soils with different texture. Hydraulic Properties
Saturated hydraulic conductivity is very sensitive to the compaction process by wheels that includes
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shearing and kneading of the soil. Figure 3 shows that wet soils are easier to compact and that the effect on the saturated hydraulic conductivity is more than proportional. The destruction of soil structure under wheels explains the greater degradation of soil qualities than observed in uniaxial tests. In the process of compaction, the macroporosity decreases, whereas the microporosity often increases. This results in larger water contents for a wide range of matric potentials in compacted versus uncompacted soil. This results in a higher unsaturated hydraulic conductivity of a compacted soil versus uncompacted soil. However, near saturation mesoand macropores are also filled and contributes to the hydraulic conductivity, and thus the hydraulic conductivity of an uncompacted soil becomes higher than that of a compacted soil.
Aeration Characteristics
An essential feature of compaction is reduction in the air-filled pore space. This property is an indication of the aeration status of the soil for plant growth and microbial activity. At air-filled porosity values less than 10%, oxygen deficiency is likely, especially in warm weather, while values less than 5% probably indicate incipient anaerobiosis. Analogous to saturated hydraulic conductivity, aeration status depends strongly on structure and continuous macropores. In a poorly structured compact soil, the threshold values of 10% and 5% should be doubled. In a well-structured soil, these threshold values may be halved. Other indices of aeration status, such as oxygen diffusion rate, oxygen content of soil air, and air permeability, provide more precise indicators of the aeration level. Strength Characteristics
Figure 3 Changes in (a) total porosity and (b) saturated hydraulic conductivity of a sandy clay loam subjected to field traffic with low ground pressure (..... LGP, tire inflation pressure 80 kPa) or high ground pressure (- - - HGP, tire inflation pressure 240 kPa) or subjected to uniaxial stresses of 0.1, 0.2, 0.4, 0.6, and 0.8 MPa at various water contents. Solid lines refer to compression of aggregated mixtures. (After Dawidowski and Lerink, 1990.) Reproduced with permission from Horton R, Ankeny MD, and Allmaras RR (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier.
In most cases, the strength of a soil is increased by compaction. However, a well-structured soil is stronger than a poorly structured soil and a wet soil is weaker than a dry soil. In periods with wet-weather conditions, compacted poorly structured soils tend to become wetter than uncompacted well-structured soils due to limited infiltration capacity, lower saturated hydraulic conductivity, and high microporosity of compacted soils. In these circumstances, the strength and trafficability of compacted soils become lower than that of well-structured soils. After a wet period, a compacted soil will stay wet longer with limited workability. After a dry period, the strength of a compacted soil will increase considerably and result in high trafficability. Soil tillage then requires more powerful equipment and a higher energy input to loosen the soil and reduce clods to acceptable sizes. Compaction increases the penetration resistance for roots, resulting in limited rooting depth and reduced crop growth. At penetration resistances (measured with a cone with a diameter of 12.7 mm and top angle of 30 degrees) of 1.5 MPa and 3.0 MPa, root growth rates are reduced to 50% and 0% respectively. However, in well-structured soils, roots can make use of continuous macropores to penetrate deeply into the soil. In drying soils, strength and penetration resistance increase.
Compaction in Crop Production Compacted soils usually have unsatisfactory physical conditions for plant growth, but the extent of loss of productivity depends on soil type, plant species, and weather conditions.
COMPACTION Effects on Germination and Establishment
Compact soils result in cloddy seedbeds after primary tillage, poor soil/seed contact, and reduced germination. Soils may develop a compact surface layer due to crusting after heavy rainfall, which has sufficient strength to restrict or even to inhibit seedling emergence, especially of dicotyledonous species. Effects on Root Growth and Distribution
Macropores (>50 m diameter), through which roots can generally proliferate readily, are much reduced in compacted soil. As a result, root growth is restricted or even inhibited. In Figure 4 the limitations to root growth are conceptualized as relations between soil porosity and soil water potential at which soil aeration and mechanical resistance meet specified root requirements. In a soil with a certain pore volume, the soil water suction determines whether root growth is limited by too high mechanical resistance, by aeration problems, or is not limited. In Figure 4 the two thin lines show the situation in the case of a poorly structured soil. A poorly structured soil with few continuous macropores needs more air-filled pores
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for aeration than a well-structured soil. Also much lower mechanical resistances are allowed in a poorly structured soil than in a well-structured soil because roots can follow the macropores in the well-structured soil. Effects on Plant Growth and Yield
The level of compaction (often expressed in dry bulk density) influences the growth, yield, and quality of crops, depending on crop species, soil type, and weather conditions (Figure 5). Optimum level of compaction tends to be higher for sandy soils, in dry seasons, and for monocotyledonous species. At a compaction level less than optimum, crops suffer from reduced soil/root contact, reducing germination and nutrient transfer, while at a compaction level higher than optimum, root growth and aeration are restricted and denitrification can lead to N losses. Soil biota and biological processes are influenced by soil compactness. This is partly because of the influence of pore size distribution on spatial habitats for bacteria and fungi and partly because compacted soils may suffer from anaerobiosis, which in turn affects microbial metabolism markedly. The burrowing abilities of soil fauna, particularly earthworms, are reduced in compacted soils. Interactive Crop Responses to Compaction and Fertilizer Application
Where farmers perceive the growth of crops to be adversely affected on compacted soils, a common action is to apply additional N fertilizer. However, the growth responses obtained may be less significant than those that would accrue at lower levels of compactness (Figure 6), and the additional nitrogen applications may be inefficient economically and detrimental to the environment. Crop Responses to Subsoil Compaction
Figure 4 A conceptual relationship between soil porosity and soil water potential in which soil aeration and mechanical resistance (PR, penetration resistance) meet specified root requirements. Root growth is insufficient in the shaded areas and impossible beyond a soil water potential of 1600 kPa. Added to the figure are the same relationships if the structure is deteriorated resulting in a strong reduction of macropores. PR too high; too wet, aeration too low; rootable; PR limiting; poorly structured soil; aertoo dry. (After Boone, 1988.) Reproduced ation limiting; with permission from Lindstrom MJ and Voorhees MB (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier.
Amelioration of subsoil compaction is much more expensive and less effective than loosening compacted topsoils. Avoidance of compaction in the subsoil is therefore a much greater need than in the topsoil. Seventeen years after a single full-field compaction action, crop and nitrogen yield losses can still be significant on the compacted sites (Figure 7). After this compaction experiment only moderate axle loads were allowed on the fields. In practice, most subsoil in agricultural fields are partly compacted every year, and the subsoil quality is often worse than in these long-term experiments. Notice that the compaction effect was more pronounced in all years for harvested nitrogen than for grain dry matter. The strong decline of the effect in the first few years was mainly due to
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Figure 5 Conceptual relationship of crop response to level of soil compaction in relation to weather, soil texture and (a) crop type and (b) crop sensitivity. Reproduced with permission from Lindstrom MJ and Voorhees WB (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier.
Modeling of Crop Responses to Soil Compaction
Figure 6 Dry-matter yields of ryegrass (first cut, mean of 3 years) in response to applied nitrogen fertilizer at three levels of bulk density (3 –12 cm depth). Adapted from data in Douglas JT and Crawford CE (1993) Grass Forage Science. Oxford: Blackwell Science.
the recovery of the topsoil. The effect was influenced by rainfall during the subsequent growing seasons. In dry seasons a moderately compacted subsoil may result in yield advantages due to reduced loss of soil water percolating below and out of the reach of the root zone, whereas in wet seasons, the reduced permeability of the subsoil can lead to anaerobic conditions within the topsoil, resulting in direct damage to the crop and loss of nitrogen by denitrification.
Crop growth is less than potential when the uptake of water, oxygen, or nutrients is less than the demand of the crop. Potential crop growth is determined considering the prevailing weather conditions. Reduced crop growth may be caused by reduction of the length of the growing period, low temperature, limited supply from the soil of water, oxygen, and nutrients to the root system, and a limited activity of the root system. Soil water plays a central role in these limiting factors, and effects of soil compaction on crop growth and biological functioning should be modeled in relation to water. In Figure 8 a scheme is presented of the interrelationships among soil tillage, field traffic, soil structure and soil physical, chemical, and biological properties. In Figure 9 a part of this scheme is considered in more detail. To simulate crop responses to soil compaction all aspects presented in Figures 8 and 9 should be included in the model. However, up to now no model exists that includes all aspects, such as limitations of root growth, the role of macropores, reduced availability of nutrients (e.g., loss of nitrogen by denitrification), and effects of reduced biological activity. The existing models in general underestimate the impact of compaction on crop growth.
Effects on Environmental Components Soil compaction influences a number of environmental parameters even at considerable distance from the original location at which the compaction occurred (Figure 10). Compaction may change the fluxes of greenhouse gases from the soil to the atmosphere
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Figure 7 Mean grain and nitrogen yields of annual crops in control treatment (=100%), and relative to the control in loading treatment of four passes with a 50 kN axle load in 1981 on clay soil, for 17 successive years after the loading. L, lodging; S, sprouting; relative grain yield (%); relative nitrogen yield (%). Reproduced with permission from Alakukku L (2000) Advances in GeoEcology 32. Reiskirchen: Catena Verlag.
Figure 8 Interrelationships among climate, soil management, soil properties, and crop growth. Reproduced with permission from Boone FR (1988) Soil and Tillage Research. Amsterdam: Elsevier.
through mechanisms associated with effects on soil permeability, aeration, and crop development. Compaction increases CO2 emissions because cultivation of compacted soils requires appreciably more energy than cultivation of uncompacted soils. Approximately 90% of the global N2O emissions to the atmosphere comes from soils. Compacted soils tend to be wetter than noncompacted soils and denitrification is enhanced. The flux of N2O increases rapidly as air-filled porosity declines (Figure 11). Compaction from vehicle traffic prior to the establishment of cereal
crops can cause marked increases in the N2O flux during the early growth period in spring (Table 2). Due to reduced permeability, compacted soils usually show greater runoff and hence greater erosion than noncompacted soils. Surface rills and even gullies are sometimes directly associated with wheel tracks, particularly over seedbeds following periods of highly erosive rainfall. Surface waters may thus carry additional burdens of clay and silt, fertilizer, and pesticides when runoff occurs from areas of compacted soil.
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Figure 9 Relationships among soil macropores, soil physical, chemical, and biological properties, rhizosphere and root system. a, surface boundary; b, storage; c, transport; d, sink or source aspects. Reproduced with permission from Boone FR (1988) Soil and Tillage Research. Amsterdam: Elsevier.
Figure 10 A conceptual diagram showing the major pathways whereby compacted soil conditions may influence components of the environment. Reproduced with permission from Soane BD and Van Ouwerkerk (1995) Soil and Tillage Research. Amsterdam: Elsevier.
Figure 11 Increased emission of nitrogen by denitrification as air-filled porosity decreases. Reproduced with permission from Sextone AJ, Parkin TB, and Tiedje JM (1988) Soil Biology and Biochemistry. Amsterdam: Elsevier. Table 2 Influence of vehicle traffic (zero, light, heavy) prior to the establishment of wheat and spring barley on N2O flux during the early spring growth period.
Crop
Spring barley Winter wheat
Period (dates and days)
16 May–14 July ¼ 60 days 8 March–8 May ¼ 62 days
Cumulative N2O flux (g N2O-N ha1) Zero
Light
Heavy
320
310
401
245
210
578
Adapted from data in Ball BC, Parker JP, and Scott A (1999) Soil and Tillage Research. Amsterdam: Elsevier.
Techniques for the Reduction of Compaction The compactive capability of vehicles can be minimized by reductions in overall mass, increases in ground contact area, and reduction of ground contact pressure. Tires of greater width and diameter will increase ground contact area, as will reduction in inflation pressure. Dual wheels, multiple axle systems, especially tandem axles for trailers and tankers with low-pressure tires, provide extra contact area.
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Vehicle traffic can be reduced by combining different operations into a single-pass operation. Toolcarriers (gantries), up to 12 m width, have been found capable of providing traffic-free zones in which, because of the lack of compaction, tillage requirement is reduced and crop yield and quality may be improved. Trafficking and working soil when it is too wet should be avoided. Tillage should be reduced as much as possible to optimize biological and physical processes that improve soil structure, in particular macroporosity. Improving drainage will result in drier and stronger soils. Increases in soil organic matter, either as a surface mulch or incorporated, will increase soil elasticity and reduce compaction.
Amelioration of Compacted Soils Natural weathering due to freezing and thawing is usually restricted to the top few centimeters of the surface soil and rarely penetrates to the subsoil. Swelling and shrinking arising from changes in water content can cause the loosening of compacted soils. Roots of certain species can penetrate compacted layers and soil biota can slowly increase macroporosity. In this way, gradual improvements can be made in the soil physical quality of compacted layers. However, highly compacted parts, in particular layers that cannot be penetrated by roots, will not or only slowly recover. Compacted topsoils can be loosened by tillage. However, loosened compacted soils are cloddy and require additional secondary cultivation to achieve suitable seedbed tilth. Loosening subsoils requires high-draft special equipment and high traction, destroys still existing continuous macropores, and weakens soil structure and strength. Subsoiling should be done when the subsoil is dry with a minimum of loosening and with the aim to form cracks (see Subsoiling). Many loosened subsoils recompact within a few years with worsened soil physical properties and
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rootability. The result is that subsoils that have been loosened once must be loosened regularly every 4–5 years. Therefore the subsoil should be inspected beforehand to determine rootability, aeration status, and drainability, to avoid unnecessary subsoiling. See also: Conservation Tillage; Cultivation and Tillage; Stress–Strain and Soil Strength; Structure; Subsoiling
Further Reading Ha˚kansson I (ed.) (1994) Subsoil compaction by high axle load traffic. Special Issue of Soil Tillage Research 29: 105–306. Horn R, van den Akker JJH, and Arvidsson J (eds) (2000) Subsoil compaction: distribution, processes and consequences. Advances in GeoEcology 32. Koolen AJ and Kuipers H (1983) Agricultural soil mechanics. Advanced Series in Agricultural Sciences 13. Larson WE, Blake G, Allmaras RR, Voorhees WB, and Gupta S (eds) (1989) Mechanics and related processes in structured agricultural soils. NATO ASI Series E: Applied Sciences 172. Monnier G and Goss MJ (eds) (1987) Soil Compaction and Regeneration. Rotterdam, Netherlands: A. A. Balkema. Oldeman LR, Hakkeling RTA, and Sombroek WG (1991) World Map of the Status of Human-induced Soil Degradation, an Explanatory Note. Wageningen, Netherlands: ISRIC. Pagliai M and Jones RJA (eds) (2002) Sustainable land management–environmental protection: a soil physical approach. Advances in GeoEcology 35. Soane BD and Van Ouwerkerk C (eds) (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier. van den Akker JJH, Arvidsson J, and Horn R (eds) (2003) Experiences with the impact and prevention of subsoil compaction in the European Community. Special Issue of Soil Tillage Research 73: 1–186. Van Ouwerkerk C (ed.) (1988) Tillage and traffic in crop production. Special Issue of Soil Tillage Research 11: 197–372. Van Ouwerkerk C and Soane BD (eds) (1995) Soil compaction and the environment. Special Issue of Soil Tillage Research 35: 1–113.
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COMPACTION J J H van den Akker, Alterra Wageningen UR, Wageningen, The Netherlands B Soane, Formerly with Scottish Agricultural College, Edinburgh, UK ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Soil compaction is a form of physical degradation in which soil biological activity and soil productivity for agricultural and forest cropping are reduced, resulting in environmental consequences away from the immediate area directly affected. Compaction is a process of densification and distortion in which total and airfilled porosity and permeability are reduced, strength is increased, soil structure partly destroyed, and many changes induced in the soil fabric and in various characteristics. The term ‘compaction’ is used to identify a process and should be distinguished from the term ‘compactness,’ which indicates for a given time and position the state of packing of the solid soil constituent. The compaction process can be initiated by wheels, tracks or rollers, traffic of cultivation machinery, and passage of draft or grazing animals. Soil densification can also be caused by heavy overburdens of ice and snow and by illuviation of clay from the A-horizon into the B-horizon. These processes may take place slowly over long periods of time and result in the B-horizon having much higher bulk density and strength than the A-horizon. Densification also occurs during structural collapse near the soil surface due to the impact of rain, resulting in formation of surface crusts. In arable land with annual ploughing, both topsoil and subsoil compaction should be considered. We define the subsoil as the soil below the loosened layer (about 20–35 cm thick). This definition of the subsoil includes the panlayer as the upper part of the subsoil. This panlayer is, in many cases, less permeable for roots, water, and oxygen than the soil below it and is the bottleneck for subsoil functions. In contrast to the topsoil, the subsoil is not loosened annually, compaction is cumulative and, in the long run, a more or less homogeneous compacted layer is created. The resilience of the subsoil for compaction is low, and subsoil compaction is at least partly persistent. Problems of compaction are widely distributed throughout the world, but tend to be most prevalent where heavy machinery is used in agriculture or forestry, in both temperate and tropical areas. Soils that are naturally fragile in structure, such as soils of the
humid tropical forest and light-textured soils in areas of low but erosive rainfall, are particularly prone to problems arising from compaction and subsequent high risks of erosion due to reduction of permeability. Compaction is now included in surveys of soil degradation, and preliminary estimates have suggested that the area of degradation attributable to soil compaction may equal or exceed 33 Mha in Europe and 18 Mha in Africa. A project on mapping of Soil and Terrain Vulnerability in Central and Eastern Europe (SOVEUR) by the United Nations Food and Agriculture Organization (FAO) and International Soil and Reference Information Centre (ISRIC) showed that compaction is the most widespread kind of soil physical soil degradation in these countries. About 25 Mha proved to be lightly and about 36 Mha moderately compacted.
Factors and Processes Affecting Distribution and Intensity of Compaction Compaction under Running Gear
Compaction under wheels and caterpillar tracks is a dynamic process in which a soil volume under the wheel undergoes normal and shearing stresses resulting in normal and shear strains. Neighboring soil volume elements at a certain depth under a wheel are considered in Figure 1. During a wheel pass volume element 1 at a certain depth endures not only compaction but also deformation, as depicted by the volume elements 2–26. The impact of the wheel load on the physical properties of the soil depends on the strength of the soil (see Stress–Strain and Soil Strength). If the exerted soil stresses exceed the precompression stress and shear strength of the soil, then compaction will be accompanied by large deformations and macropores and structure will be remolded, resulting in degradation of soil physical qualities. Degradation of soil physical qualities will be less severe if only the precompression stress is exceeded, because then the soil will mainly compact with limited deformations, and the remnants of the biomacropores and the intra-aggregate space will still retain a certain continuity. Loading Characteristics of Individual Wheels or Caterpillar Tracks
For practical purposes a useful parameter is the average ground contact pressure P (kPa), which is defined as the wheel load divided by the ground contact area.
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Figure 1 Displacement and deformation in a vertical section of a soil volume element under a wheel. Reproduced with permission from Koolen AJ and Kuipers H (1983) Agricultural soil mechanics. Advanced Series in Agricultural Sciences 13, Springer-Verlag, Heidelberg.
The average ground contact pressure can also be calculated as the sum of tire inflation pressure Pi and a pressure Pc for carcass stiffness: P ¼ c Pi þ Pc
½1
The range for c is 0–1.25 and for Pc is 0–50 kPa. The factor c also depends on the carcass stiffness, and therefore eqn [1] is mostly reduced to P ¼ c Pi or P ¼ Pi þ Pc. At inflation pressures larger than 200–300 kPa, the influence of carcass stiffness diminishes and the factor c can become smaller than 1. The higher the inflation pressure, the more the strength and firmness of the soil determine the ground pressure. The peak stresses in the ground contact area determine the peak stresses in the soil that result in compaction and distortion of the soil structure if the soil strength is exceeded. Peak stresses under lugs can be two to five times higher than the average ground contact pressure. However, the resulting soil stresses decrease rapidly with depth because the ground contact area is small. If soil stresses at a depth of 0.2–0.3 m are considered, then the peak stresses under the lugs can be neglected and a parabolic stress distribution in the ground contact area with a peak stress of one-and-a-half to two times the average ground contact pressure can be assumed. Peak stresses under caterpillar tracks are two to four times the average ground contact pressure and depend strongly on the firmness of the soil and design of the track system. Examples of load characteristics for various vehicles and animals are shown in Table 1. Compaction by agricultural machinery extends well into the subsoil (Figure 2). An important cause of subsoil compaction is moldboard plowing in which the furrow-side tractor wheels apply appreciable loads directly to the upper surface of the subsoil, causing a
Table 1 Maximum loads and ground contact pressures applied by various sources
Source
Large-wheeled tractor (120 kW) Small-wheeled tractor (40 kW) Sugarbeet harvester (loaded) Slurry tanker (loaded) Track-laying tractor Horse Cow
Total load (kN)
Load per wheel/track/ hoof (kN)
Average ground contact pressure (kPa)
100
50
250 – 350
40
20
200 – 300
300 –600
50 –120
200 – 400
100 –300
25 – 60
200 – 500
140 8 4–5
70 2–8 1–4
40 75 – 300 120 – 480
plow pan. Attempts have been made to specify maximum recommended average ground contact pressures in order to minimize compaction, especially in the subsoil. Suggested maximum values range from 80 kPa for wet soils in spring to 200 kPa for dry soils in summer, but progress in gaining official acceptance for such standards has been slow. Field Traffic Intensity and Distribution
The overall incidence of soil compaction within a given field depends upon the distribution of traffic for each field operation and the cumulative value throughout the life of the crop. The weight of the crop to be transported from the field is an important factor in the compaction risk. The traffic intensity and compaction risk for a potato crop and a sugar beet crop are, respectively, more than twice and almost twice that of a winter wheat crop. As the number and width of wheels fitted to vehicles increase, so does the overall proportion of the field area covered
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Figure 2 Increase of bulk density to considerable depth when a bulldozer was used for forest clearing in Surinam (- - - no vehicle traffic; — bulldozer traffic). Reproduced with permission from Van der Weert R (1974) Tropical Agriculture (Trinidad) 51(2): 325–331.
by wheels. However, this effect does not counteract the benefits from a reduction in compaction due to reduced ground contact pressure. Soil Compactibility
Soils can vary from being sufficiently strong to resist all likely applied loads (low compactibility) to being so weak that they are compacted by even low loads (high compactibility). Well-structured soils combine good physical soil properties with high strength. Sandy soils with a single-grain structure and compacted massive soils can be very strong. Rootability and soil physical properties are then often poor. Roots have a binding action and increase the elasticity and resistance of a soil to compaction. Soil moisture content and soil water suction have a dominant influence on soil compactibility. In soils where capillarity is the main cohesive force, strength increases with drying until it reaches a maximum, and then again decreases upon further dehydration because the cross-sectional area of the menisci decreases more than capillary suction between grains increases. This cohesive force based on soil water suction is called apparent cohesion. Drying also increases the true cohesion. Soil water suction increases the cohesion and so the strength and resistance to compaction of soils vary considerably. In dry structured (aggregated) soils, soil water concentrates in the aggregates, and cohesion and soil strength in the aggregate increases considerably. Dry structured soils shrink and turn into an assembly of individual aggregates that fit
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together rather neatly. This assembly of aggregates has a very high interaggregate angle of internal friction and moderate interaggregate cohesion. This soil is strong and has a low compactibility. However, if such a soil is overloaded and compacted, the aggregates will be crushed and the interaggregate space will be filled up, resulting in a dense compact soil. Dry soils resist loads readily. However, extremely dry sandy soils can be deformed and compacted rather easily. As the moisture content increases, soil compactibility increases until the moisture content is approximately at the field capacity point, when a condition known as the optimum moisture content for compaction is reached. At still higher moisture contents, the soil becomes increasingly incompactible as the moisture tends to fill ever more of the total porosity and further loss of air-filled porosity becomes impossible. However, although the compaction may be minimal, the plastic flow of a wet soil results in a complete destruction of soil structure and macropores with accompanying diminishment of soil physical qualities. The reaction of dry and strong soils may be largely elastic, whereas at high moisture content and low strength the reaction may be plastic flow. Increases in organic matter content tend to reduce soil compactibility and to increase elasticity. For example, peat soils are quite resistant to compaction. The value of the optimum moisture content for compaction increases as the organic matter content increases.
Effects on Soil Physical and Mechanical Properties Bulk Density, Porosity, and Packing State
Bulk density (the mass of soil solids per unit volume) is the most direct and easy-to-measure indicator of changes in compactness, but changes in packing state can be better quantified by total porosity or void ratio. The relation between porosity (especially macroporosity) and soil physical properties such as saturated and unsaturated hydraulic conductivity and gas diffusivity is much closer than the relation of these soil physical properties with bulk density. Compaction causes major reductions in macroporosity (>50 m), reduction of total porosity, and often an increase in microporosity, resulting in a major impact on soil physical properties. Relative terms for packing state (e.g., relative density, degree of compactness) enable the same threshold values to be found for overcompaction for soils with different texture. Hydraulic Properties
Saturated hydraulic conductivity is very sensitive to the compaction process by wheels that includes
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shearing and kneading of the soil. Figure 3 shows that wet soils are easier to compact and that the effect on the saturated hydraulic conductivity is more than proportional. The destruction of soil structure under wheels explains the greater degradation of soil qualities than observed in uniaxial tests. In the process of compaction, the macroporosity decreases, whereas the microporosity often increases. This results in larger water contents for a wide range of matric potentials in compacted versus uncompacted soil. This results in a higher unsaturated hydraulic conductivity of a compacted soil versus uncompacted soil. However, near saturation mesoand macropores are also filled and contributes to the hydraulic conductivity, and thus the hydraulic conductivity of an uncompacted soil becomes higher than that of a compacted soil.
Aeration Characteristics
An essential feature of compaction is reduction in the air-filled pore space. This property is an indication of the aeration status of the soil for plant growth and microbial activity. At air-filled porosity values less than 10%, oxygen deficiency is likely, especially in warm weather, while values less than 5% probably indicate incipient anaerobiosis. Analogous to saturated hydraulic conductivity, aeration status depends strongly on structure and continuous macropores. In a poorly structured compact soil, the threshold values of 10% and 5% should be doubled. In a well-structured soil, these threshold values may be halved. Other indices of aeration status, such as oxygen diffusion rate, oxygen content of soil air, and air permeability, provide more precise indicators of the aeration level. Strength Characteristics
Figure 3 Changes in (a) total porosity and (b) saturated hydraulic conductivity of a sandy clay loam subjected to field traffic with low ground pressure (..... LGP, tire inflation pressure 80 kPa) or high ground pressure (- - - HGP, tire inflation pressure 240 kPa) or subjected to uniaxial stresses of 0.1, 0.2, 0.4, 0.6, and 0.8 MPa at various water contents. Solid lines refer to compression of aggregated mixtures. (After Dawidowski and Lerink, 1990.) Reproduced with permission from Horton R, Ankeny MD, and Allmaras RR (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier.
In most cases, the strength of a soil is increased by compaction. However, a well-structured soil is stronger than a poorly structured soil and a wet soil is weaker than a dry soil. In periods with wet-weather conditions, compacted poorly structured soils tend to become wetter than uncompacted well-structured soils due to limited infiltration capacity, lower saturated hydraulic conductivity, and high microporosity of compacted soils. In these circumstances, the strength and trafficability of compacted soils become lower than that of well-structured soils. After a wet period, a compacted soil will stay wet longer with limited workability. After a dry period, the strength of a compacted soil will increase considerably and result in high trafficability. Soil tillage then requires more powerful equipment and a higher energy input to loosen the soil and reduce clods to acceptable sizes. Compaction increases the penetration resistance for roots, resulting in limited rooting depth and reduced crop growth. At penetration resistances (measured with a cone with a diameter of 12.7 mm and top angle of 30 degrees) of 1.5 MPa and 3.0 MPa, root growth rates are reduced to 50% and 0% respectively. However, in well-structured soils, roots can make use of continuous macropores to penetrate deeply into the soil. In drying soils, strength and penetration resistance increase.
Compaction in Crop Production Compacted soils usually have unsatisfactory physical conditions for plant growth, but the extent of loss of productivity depends on soil type, plant species, and weather conditions.
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Compact soils result in cloddy seedbeds after primary tillage, poor soil/seed contact, and reduced germination. Soils may develop a compact surface layer due to crusting after heavy rainfall, which has sufficient strength to restrict or even to inhibit seedling emergence, especially of dicotyledonous species. Effects on Root Growth and Distribution
Macropores (>50 m diameter), through which roots can generally proliferate readily, are much reduced in compacted soil. As a result, root growth is restricted or even inhibited. In Figure 4 the limitations to root growth are conceptualized as relations between soil porosity and soil water potential at which soil aeration and mechanical resistance meet specified root requirements. In a soil with a certain pore volume, the soil water suction determines whether root growth is limited by too high mechanical resistance, by aeration problems, or is not limited. In Figure 4 the two thin lines show the situation in the case of a poorly structured soil. A poorly structured soil with few continuous macropores needs more air-filled pores
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for aeration than a well-structured soil. Also much lower mechanical resistances are allowed in a poorly structured soil than in a well-structured soil because roots can follow the macropores in the well-structured soil. Effects on Plant Growth and Yield
The level of compaction (often expressed in dry bulk density) influences the growth, yield, and quality of crops, depending on crop species, soil type, and weather conditions (Figure 5). Optimum level of compaction tends to be higher for sandy soils, in dry seasons, and for monocotyledonous species. At a compaction level less than optimum, crops suffer from reduced soil/root contact, reducing germination and nutrient transfer, while at a compaction level higher than optimum, root growth and aeration are restricted and denitrification can lead to N losses. Soil biota and biological processes are influenced by soil compactness. This is partly because of the influence of pore size distribution on spatial habitats for bacteria and fungi and partly because compacted soils may suffer from anaerobiosis, which in turn affects microbial metabolism markedly. The burrowing abilities of soil fauna, particularly earthworms, are reduced in compacted soils. Interactive Crop Responses to Compaction and Fertilizer Application
Where farmers perceive the growth of crops to be adversely affected on compacted soils, a common action is to apply additional N fertilizer. However, the growth responses obtained may be less significant than those that would accrue at lower levels of compactness (Figure 6), and the additional nitrogen applications may be inefficient economically and detrimental to the environment. Crop Responses to Subsoil Compaction
Figure 4 A conceptual relationship between soil porosity and soil water potential in which soil aeration and mechanical resistance (PR, penetration resistance) meet specified root requirements. Root growth is insufficient in the shaded areas and impossible beyond a soil water potential of 1600 kPa. Added to the figure are the same relationships if the structure is deteriorated resulting in a strong reduction of macropores. PR too high; too wet, aeration too low; rootable; PR limiting; poorly structured soil; aertoo dry. (After Boone, 1988.) Reproduced ation limiting; with permission from Lindstrom MJ and Voorhees MB (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier.
Amelioration of subsoil compaction is much more expensive and less effective than loosening compacted topsoils. Avoidance of compaction in the subsoil is therefore a much greater need than in the topsoil. Seventeen years after a single full-field compaction action, crop and nitrogen yield losses can still be significant on the compacted sites (Figure 7). After this compaction experiment only moderate axle loads were allowed on the fields. In practice, most subsoil in agricultural fields are partly compacted every year, and the subsoil quality is often worse than in these long-term experiments. Notice that the compaction effect was more pronounced in all years for harvested nitrogen than for grain dry matter. The strong decline of the effect in the first few years was mainly due to
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Figure 5 Conceptual relationship of crop response to level of soil compaction in relation to weather, soil texture and (a) crop type and (b) crop sensitivity. Reproduced with permission from Lindstrom MJ and Voorhees WB (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier.
Modeling of Crop Responses to Soil Compaction
Figure 6 Dry-matter yields of ryegrass (first cut, mean of 3 years) in response to applied nitrogen fertilizer at three levels of bulk density (3 –12 cm depth). Adapted from data in Douglas JT and Crawford CE (1993) Grass Forage Science. Oxford: Blackwell Science.
the recovery of the topsoil. The effect was influenced by rainfall during the subsequent growing seasons. In dry seasons a moderately compacted subsoil may result in yield advantages due to reduced loss of soil water percolating below and out of the reach of the root zone, whereas in wet seasons, the reduced permeability of the subsoil can lead to anaerobic conditions within the topsoil, resulting in direct damage to the crop and loss of nitrogen by denitrification.
Crop growth is less than potential when the uptake of water, oxygen, or nutrients is less than the demand of the crop. Potential crop growth is determined considering the prevailing weather conditions. Reduced crop growth may be caused by reduction of the length of the growing period, low temperature, limited supply from the soil of water, oxygen, and nutrients to the root system, and a limited activity of the root system. Soil water plays a central role in these limiting factors, and effects of soil compaction on crop growth and biological functioning should be modeled in relation to water. In Figure 8 a scheme is presented of the interrelationships among soil tillage, field traffic, soil structure and soil physical, chemical, and biological properties. In Figure 9 a part of this scheme is considered in more detail. To simulate crop responses to soil compaction all aspects presented in Figures 8 and 9 should be included in the model. However, up to now no model exists that includes all aspects, such as limitations of root growth, the role of macropores, reduced availability of nutrients (e.g., loss of nitrogen by denitrification), and effects of reduced biological activity. The existing models in general underestimate the impact of compaction on crop growth.
Effects on Environmental Components Soil compaction influences a number of environmental parameters even at considerable distance from the original location at which the compaction occurred (Figure 10). Compaction may change the fluxes of greenhouse gases from the soil to the atmosphere
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Figure 7 Mean grain and nitrogen yields of annual crops in control treatment (=100%), and relative to the control in loading treatment of four passes with a 50 kN axle load in 1981 on clay soil, for 17 successive years after the loading. L, lodging; S, sprouting; relative grain yield (%); relative nitrogen yield (%). Reproduced with permission from Alakukku L (2000) Advances in GeoEcology 32. Reiskirchen: Catena Verlag.
Figure 8 Interrelationships among climate, soil management, soil properties, and crop growth. Reproduced with permission from Boone FR (1988) Soil and Tillage Research. Amsterdam: Elsevier.
through mechanisms associated with effects on soil permeability, aeration, and crop development. Compaction increases CO2 emissions because cultivation of compacted soils requires appreciably more energy than cultivation of uncompacted soils. Approximately 90% of the global N2O emissions to the atmosphere comes from soils. Compacted soils tend to be wetter than noncompacted soils and denitrification is enhanced. The flux of N2O increases rapidly as air-filled porosity declines (Figure 11). Compaction from vehicle traffic prior to the establishment of cereal
crops can cause marked increases in the N2O flux during the early growth period in spring (Table 2). Due to reduced permeability, compacted soils usually show greater runoff and hence greater erosion than noncompacted soils. Surface rills and even gullies are sometimes directly associated with wheel tracks, particularly over seedbeds following periods of highly erosive rainfall. Surface waters may thus carry additional burdens of clay and silt, fertilizer, and pesticides when runoff occurs from areas of compacted soil.
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Figure 9 Relationships among soil macropores, soil physical, chemical, and biological properties, rhizosphere and root system. a, surface boundary; b, storage; c, transport; d, sink or source aspects. Reproduced with permission from Boone FR (1988) Soil and Tillage Research. Amsterdam: Elsevier.
Figure 10 A conceptual diagram showing the major pathways whereby compacted soil conditions may influence components of the environment. Reproduced with permission from Soane BD and Van Ouwerkerk (1995) Soil and Tillage Research. Amsterdam: Elsevier.
Figure 11 Increased emission of nitrogen by denitrification as air-filled porosity decreases. Reproduced with permission from Sextone AJ, Parkin TB, and Tiedje JM (1988) Soil Biology and Biochemistry. Amsterdam: Elsevier. Table 2 Influence of vehicle traffic (zero, light, heavy) prior to the establishment of wheat and spring barley on N2O flux during the early spring growth period.
Crop
Spring barley Winter wheat
Period (dates and days)
16 May–14 July ¼ 60 days 8 March–8 May ¼ 62 days
Cumulative N2O flux (g N2O-N ha1) Zero
Light
Heavy
320
310
401
245
210
578
Adapted from data in Ball BC, Parker JP, and Scott A (1999) Soil and Tillage Research. Amsterdam: Elsevier.
Techniques for the Reduction of Compaction The compactive capability of vehicles can be minimized by reductions in overall mass, increases in ground contact area, and reduction of ground contact pressure. Tires of greater width and diameter will increase ground contact area, as will reduction in inflation pressure. Dual wheels, multiple axle systems, especially tandem axles for trailers and tankers with low-pressure tires, provide extra contact area.
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Vehicle traffic can be reduced by combining different operations into a single-pass operation. Toolcarriers (gantries), up to 12 m width, have been found capable of providing traffic-free zones in which, because of the lack of compaction, tillage requirement is reduced and crop yield and quality may be improved. Trafficking and working soil when it is too wet should be avoided. Tillage should be reduced as much as possible to optimize biological and physical processes that improve soil structure, in particular macroporosity. Improving drainage will result in drier and stronger soils. Increases in soil organic matter, either as a surface mulch or incorporated, will increase soil elasticity and reduce compaction.
Amelioration of Compacted Soils Natural weathering due to freezing and thawing is usually restricted to the top few centimeters of the surface soil and rarely penetrates to the subsoil. Swelling and shrinking arising from changes in water content can cause the loosening of compacted soils. Roots of certain species can penetrate compacted layers and soil biota can slowly increase macroporosity. In this way, gradual improvements can be made in the soil physical quality of compacted layers. However, highly compacted parts, in particular layers that cannot be penetrated by roots, will not or only slowly recover. Compacted topsoils can be loosened by tillage. However, loosened compacted soils are cloddy and require additional secondary cultivation to achieve suitable seedbed tilth. Loosening subsoils requires high-draft special equipment and high traction, destroys still existing continuous macropores, and weakens soil structure and strength. Subsoiling should be done when the subsoil is dry with a minimum of loosening and with the aim to form cracks (see Subsoiling). Many loosened subsoils recompact within a few years with worsened soil physical properties and
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rootability. The result is that subsoils that have been loosened once must be loosened regularly every 4–5 years. Therefore the subsoil should be inspected beforehand to determine rootability, aeration status, and drainability, to avoid unnecessary subsoiling. See also: Conservation Tillage; Cultivation and Tillage; Stress–Strain and Soil Strength; Structure; Subsoiling
Further Reading Ha˚kansson I (ed.) (1994) Subsoil compaction by high axle load traffic. Special Issue of Soil Tillage Research 29: 105–306. Horn R, van den Akker JJH, and Arvidsson J (eds) (2000) Subsoil compaction: distribution, processes and consequences. Advances in GeoEcology 32. Koolen AJ and Kuipers H (1983) Agricultural soil mechanics. Advanced Series in Agricultural Sciences 13. Larson WE, Blake G, Allmaras RR, Voorhees WB, and Gupta S (eds) (1989) Mechanics and related processes in structured agricultural soils. NATO ASI Series E: Applied Sciences 172. Monnier G and Goss MJ (eds) (1987) Soil Compaction and Regeneration. Rotterdam, Netherlands: A. A. Balkema. Oldeman LR, Hakkeling RTA, and Sombroek WG (1991) World Map of the Status of Human-induced Soil Degradation, an Explanatory Note. Wageningen, Netherlands: ISRIC. Pagliai M and Jones RJA (eds) (2002) Sustainable land management–environmental protection: a soil physical approach. Advances in GeoEcology 35. Soane BD and Van Ouwerkerk C (eds) (1994) Soil Compaction in Crop Production. Amsterdam: Elsevier. van den Akker JJH, Arvidsson J, and Horn R (eds) (2003) Experiences with the impact and prevention of subsoil compaction in the European Community. Special Issue of Soil Tillage Research 73: 1–186. Van Ouwerkerk C (ed.) (1988) Tillage and traffic in crop production. Special Issue of Soil Tillage Research 11: 197–372. Van Ouwerkerk C and Soane BD (eds) (1995) Soil compaction and the environment. Special Issue of Soil Tillage Research 35: 1–113.