Sustainable
Agriculture
Editor: Christine Jakobsson
Sustainable Agriculture
Introduction
More than one hundred years ago, Wollny (1898) de-scribed 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, hor-ticultural 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 deg-radation (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 for-estry 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 pro-duction and the environment.
Definition of Soil Compaction Processes
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 gra-dient (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 imple-ments 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 move-ment occurs during field operations and animal trampling (Koolen and Kuipers, 1983). Thus, soil compaction re-sults in a decrease in porosity but also causes non-volu-metric changes in soil structure.
Soil Compaction
Laura Alakukku
University of Helsinki, Finland
Effects of Soil Compaction on Soil, Crop
Growth and Environment
Virtually all physical, chemical and biological soil proper-ties 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-pores, compaction also modifies the pore geometry, con-tinuity and morphology, which is very important since in wet soil rapid water and air movement occurs in continu-ous macropores.
Soil compaction can have positive impacts, for in-stance 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 ef-fects on many soil properties relevant to soil workability,
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.
drainage, crop growth and the environment (Figure 28.1). Compaction due to field traffic increases the dry bulk den-sity (Arvidsson, 1998), shear strength and penetrometer resistance (e.g. Blackwell et al., 1986) of different soils, limiting root growth and increasing the draft require-ment 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 prob-lems 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 op-erations and hampering crop growth due to soil wetness. The reduction in drainage rate attributed to soil compac-tion can be expected to increase the emissions of green-house 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 compac-tion 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 compac-tion 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 nu-trient uptake (Arvidsson, 1999; Alakukku, 2000). Crop responses and the reasons for the effects of soil compac-tion 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 opera-tions are adapted to soil condiopera-tions. 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.
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 pres-sure 700 kPa) four months before autumn ploughing (left) or kept uncompacted (right). Photo: L. Alakukku.
grain yield (Håkansson and Reeder, 1994) and nitrogen uptake (Alakukku, 2000) several years after subsoil com-paction. 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 sub-soil compaction tends to be highly persistent, and in non-swelling 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 dif-ficult 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 machin-ery 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 sec-tion 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 con-tent/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 un-saturated 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 with-out the water draining with-out from the soil. However, wet soil is in a very weak state and may smear, with resultant
paction 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 crit-ical 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 dur-ing 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 con-ditions 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 aver-age ground contact stress (wheel load divided by contact area between tyre and soil surface) estimates the aver-age 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 low-er inflation pressure or a combination of these. With the aim of avoiding soil compaction, recommendations have been given for maximum values of average ground con-tact 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 pres-sure 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 prob-ability of subsoil compaction are concerned, surface con-tact 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
analyses and experimental results, the following conclu-sions can be drawn: for a particular surface contact stress, larger tyres or tracks (with larger wheel/track load) trans-mit 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.
Number of wheel/track passes
The number of passes affects the number of loading events and the coverage, intensity and distribution of wheel traf-fic. 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), aver-age 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 agricultur-al treatments affect soil structure, threatening soil quagricultur-ality and productivity everywhere in the world. The risk of soil
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.
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