Combatting Soil Degradation
Soil Compaction
28
Laura Alakukku
University of Helsinki, Finland
Introduction
Definition of Soil Compaction Processes
More than one hundred years ago, Wollny (1898) described the importance of a favourable soil structure for
crop growth and yield. Compaction of the soil is one of
the major ways in which treatments affect soil structure,
and soil compaction problems plague agricultural, horticultural and forest crop production everywhere in the
world. For this reason, soil compaction has been defined
as one of the five threats to sustained soil quality by the EU
Soil Framework Directive (Commission of the European
Communities, 2006). Globally, soil compaction accounts
for 4% (68.3 million hectares) of anthropogenic soil degradation (Oldeman et al., 1991). In Europe, compaction
accounts for about 17% of the total degraded area. Soil
compaction is a complex problem in which machine/
soil/crop/weather interactions play an important role and
may have economic and environmental consequences for
world agriculture (Soane and Ouwerkerk, 1995).
The focus of this chapter is on soil compaction due to
field traffic. The harmful compaction of arable and forestry soils is mainly attributable to wheel or track traffic
with heavy machines under unfavourable soil conditions.
Intensification of cropping practices is also accompanied
by diminished soil structure stability and the expansion
of intensive cultivation to new land areas leads to soil
compaction. This chapter reviews reasons for arable soil
compaction and the effects of compaction on crop production and the environment.
Soil compression refers to the decrease in porosity or
increase in bulk density of soil when it is subjected to
externally or internally applied loads. External static
and dynamic loads may be caused by rolling, trampling
or vibration, while the internal loads are associated with
water suction or water pressure due to a hydraulic gradient (Horn and Lebert, 1994). Soil consolidation is a
process by which a saturated soil is compressed under a
long-term load accompanied by a reduction in porosity
with expulsion of water. In contrast, soil compaction is a
process in which an unsaturated soil is compressed by a
load applied for a short time with no expulsion of water.
Short-time loads are applied by field traffic, tillage implements and trampling by livestock. Soil may also become
compressed naturally under its own weight and by rain,
or by shrinking due to drying of clayey soil
In agriculture, soil compaction is usually accompanied
by deformation since besides compression, lateral movement occurs during field operations and animal trampling
(Koolen and Kuipers, 1983). Thus, soil compaction results in a decrease in porosity but also causes non-volumetric changes in soil structure.
217
Combatting Soil Degradation
Figure 28.1. Field traffic factors and soil properties affecting the soil compaction process and the effects of soil compaction on soil properties, crop
production, environment, economy and soil workability.
Effects of Soil Compaction on Soil, Crop
Growth and Environment
Virtually all physical, chemical and biological soil properties and processes are affected to varying degrees by soil
compaction (Figure 28.1). Many studies have found that
compaction modifies the pore size distribution of mineral
soils, mainly by reducing the porosity and especially the
macroporosity (diameter > 30 μm, e.g. Eriksson, 1982;
Ehlers, 1982). Besides the volume and number of macro-
218
pores, compaction also modifies the pore geometry, continuity and morphology, which is very important since in
wet soil rapid water and air movement occurs in continuous macropores.
Soil compaction can have positive impacts, for instance by increasing the plant-available water capacity
of sandy soils (Rasmussen, 1985) or by reducing nitrate
leaching (Kirkham and Horton, 1990). However, soil
compaction has often been found to have harmful effects on many soil properties relevant to soil workability,
Combatting Soil Degradation
Figure 28.2. Autumn-ploughed heavy clay soil in the following spring.The soil was compacted with a 21 tonne tandem axle load (tyre inflation pressure 700 kPa) four months before autumn ploughing (left) or kept uncompacted (right). Photo: L. Alakukku.
drainage, crop growth and the environment (Figure 28.1).
Compaction due to field traffic increases the dry bulk density (Arvidsson, 1998), shear strength and penetrometer
resistance (e.g. Blackwell et al., 1986) of different soils,
limiting root growth and increasing the draft requirement in tillage (Figure 28.2). Soil compaction has been
found to reduce water infiltration (Pietola et al., 2005) and
saturated hydraulic conductivity (Alakukku et al., 2003).
Simojoki et al. (1991) found that soil compaction reduces
CO2 and O2 exchange. The likelihood of drainage problems increases when compaction reduces the permeability
of soil, especially subsoil, and may lead to waterlogging
problems in rainy years. Poorly drained soil may also dry
slowly, reducing the number of days available for field operations and hampering crop growth due to soil wetness.
The reduction in drainage rate attributed to soil compaction can be expected to increase the emissions of greenhouse gases from soil (Ball et al., 1999), for instance by
increasing denitrification. Furthermore, compaction may
increase surface runoff and topsoil erosion by impeding
water infiltration (Fullen, 1985). The effects of compaction on soil properties are reviewed by Soane et al. (1982),
Lipiec and Stępniewski (1995) and Alakukku (1999).
Environmental and soil workability responses have been
reviewed by, among others, Soane and Ourwerkerk (1995)
and Chamen et al. (1990), respectively.
By affecting soil properties and processes, soil compaction influences crop growth, yield and the use efficiency
of fertilisers (Figure 28.1). After tillage the tilled layer
is often too loose and moderate recompaction of topsoil
improves crop growth. Harmful soil compaction has been
reported to reduce yield (e.g. Schjønning and Rasmussen,
1994; Arvidsson and Håkansson, 1996; Hanssen, 1996),
crop water use efficiency (Radford et al., 2001) and nutrient uptake (Arvidsson, 1999; Alakukku, 2000). Crop
responses and the reasons for the effects of soil compaction on crop growth, yield and nutrient uptake have been
widely discussed by Lipiec and Stępniewski (1995) and
Håkansson (2005).
Persistence of Soil Compaction
Compaction induced by field traffic has both short- and
long-term effects on soil and crop production. Short-term
(1–5 years) effects are mainly associated with topsoil (0–
30 cm) compaction, which is largely controlled by tillage
operations, field traffic and the way in which these operations are adapted to soil conditions. Topsoil compaction
is alleviated by tillage and natural processes of freezing/
thawing, wetting/drying and bioactivity.
Normal tillage does not loosen the subsoil (below about
30 cm). The effects of subsoil compaction may persist
for a very long time. In spite of cropping and deep frost,
the effects of heavy machine traffic have been detected
in mineral soils more than 10 years after application of
the load (Blake et al., 1976; Etana and Håkansson, 1994;
Wu et al., 1997). In all these investigations, the effects of
compaction persisted for the duration of the experiment.
219
Combatting Soil Degradation
Subsoil compaction has also been found to decrease
grain yield (Håkansson and Reeder, 1994) and nitrogen
uptake (Alakukku, 2000) several years after subsoil compaction. The long-term effect on crop growth and yield
has been found to depend on the climatic conditions and
is most evident in rainy growing seasons (e.g. Alakukku,
2000). Håkansson and Petelkau (1994) suggested that subsoil compaction tends to be highly persistent, and in nonswelling sandy soils and tropical areas it may be permanent
immediately below tillage depth. Thus, subsoil compaction
is a severe invisible and cumulative problem which is difficult to correct by, for instance, deep loosening (Kooistra
and Boersma, 1994; Olesen and Munkholm, 2007).
Prevention of Field Traffic-induced Soil
Compaction
Factors influencing the compaction capability of machinery traffic can be divided into two main variables: soil
bearing capacity and soil stress caused by field traffic
(Figure 28.1). Soil bearing capacity or strength means the
capability of a soil structure to withstand stresses induced
by field traffic without changes in the soil structure. Soil
strength varies temporally, spatially and vertically owing
to differences in soil properties (Figure 28.1). In this section the major soil properties and traffic factors relevant
to avoiding soil compaction are discussed. The causes and
prevention of soil compaction have been reviewed in more
detail by Alakukku et al. (2003), Chamen et al. (2003),
Håkansson (2005) and Hamza and Anderson (2005).
Influence of Soil Moisture Content on Soil
Compaction
Working the soil at the wrong moisture content increases
the probability of soil compaction. Soil moisture content/or potential is the dominant property affecting soil
strength during field traffic (Hamza and Anderson, 2005).
As the moisture content increases, the strength of an unsaturated soil drops. Thus, the same stress compacts a soil
more when it is moist than when it is dry (e.g. Arvidsson,
2001). Saturated soil does not technically compact without the water draining out from the soil. However, wet
soil is in a very weak state and may smear, with resultant
220
puddling disturbing pore continuity and causing soil compaction when the homogenised soil dries (Guėrif, 1990).
In the climate of the Nordic countries soils are often wet
in spring after snow melt and in autumn. This creates critical conditions for traffic in manure/slurry/sewage sludge
application, tillage and crop harvesting. A good drainage
system is of critical importance to limit the periods during which the soil is wet and therefore reduces the risk
of soil compaction damage. Likewise, adapting practices
and cropping to avoid field traffic during moist soil conditions reduces or avoids soil compaction.
Stresses Applied to Soil by Machines
Stresses on soil can be limited by controlling surface
contact stress and wheel loads (Figure 28.1). The average ground contact stress (wheel load divided by contact
area between tyre and soil surface) estimates the average value of the vertical stress in the contact area. The
contact stress is often evaluated from the tyre inflation
pressure. Tijink (1994) offers a detailed examination of
the determination of ground contact pressure. Stress on
the soil can be reduced by lowering the ground contact
stress by decreasing the tyre inflation pressure, through
larger tyres with the same load, lower wheel load or lower inflation pressure or a combination of these. With the
aim of avoiding soil compaction, recommendations have
been given for maximum values of average ground contact stress, inflation pressure and stress at 50 cm depth
(subsoil compaction). For wet and loose soils (extremely
vulnerable) Spoor et al. (2003) recommend a maximum
ground contact stress of 65 kPa (tyre inflation pressure 40
kPa), while for hard (not particularly vulnerable) soils the
recommended maximum is 200 kPa (160 kPa). Technical
solutions to reduce ground contact stress (e.g. number
of wheels, tyre construction, tracks) and inflation pressure are discussed by Chamen et al. (2003), Hamza and
Anderson (2005) and Ansorge and Godwin (2007).
In unsaturated soils, external stresses are transmitted
three-dimensionally via solid, liquid and gaseous phases.
As far as the extent of stress in the soil and the probability of subsoil compaction are concerned, surface contact stress and wheel load are the dominant influences.
Contact stress determines the initial level of stress at the
surface, but wheel load decides the rate at which the stress
decreases with depth (e.g. Chamen et al., 2003). From
Combatting Soil Degradation
analyses and experimental results, the following conclusions can be drawn: for a particular surface contact stress,
larger tyres or tracks (with larger wheel/track load) transmit stress deeper than smaller tyres or tracks with lower
load (e.g. Lebert et al., 1989), while a higher moisture
content decreases the strength of the soil and increases
the stress transmitted deeper into the soil (Arvidsson et
al., 2001). In summary, it can be stated that the risk of
subsoil compaction exists whenever a moist or weak soil
is loaded by a moderate to high surface contact pressure
on a large contact area, i.e. with a high wheel load.
compaction is high when the stresses exerted are higher
than the strength of the soil. Soil wetness decreases the
soil strength. To prevent soil compaction the machines
and equipment used on fields in critical conditions should
be adjusted to the actual strength of the soil by controlling
wheel/track loads and using low tyre inflation pressure.
Traffic management should also be planned to minimise
the amount of unnecessary field traffic.
Number of wheel/track passes
The number of passes affects the number of loading events
and the coverage, intensity and distribution of wheel traffic. When a vehicle has been converted to low wheel load
and ground pressure by increasing the number of wheels
that follow in the same track (tandem axle-concept), average ground contact pressure is lower, but the number of
wheel passes in the same track is higher. Because of the
multi-pass effect, tandem axle construction is less efficient
in avoiding high levels of compactness in the topsoil than
wide tyres and dual wheel arrangements. The first pass
of a wheel/track causes most compaction, but repeated
wheeling can still increase the compactness of soil. The
repeated number of wheel passes may also increase the
risk of subsoil compaction. Annually repeated traffic may
cause cumulative effects if the effects of earlier subsoil
compaction have not disappeared before new loading.
Unnecessary field traffic can be avoided e.g. by adapting
the size of implements well to the size of the tractor used
and by combining field operations. In a controlled traffic
system, all field traffic is concentrated to temporary or
even permanent wheel tracks (tramlines) (Chamen et al.,
2003; Hamza and Anderson, 2005). Håkansson (2005)
discusses the planning of traffic pattern to minimise the
area covered by wheels/tracks.
Conclusions
Compaction is one of the major ways in which agricultural treatments affect soil structure, threatening soil quality
and productivity everywhere in the world. The risk of soil
221
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