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A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D

Vol. 8 (1999): 333–351.

© Agricultural and Food Science in FinlandManuscript received June 1999


Vol. 8 (1999): 333–351.

Review

Subsoil compaction due to wheel trafficLaura Alakukku

Department of Agricultural Engineering and Household Technology, PO Box 27, FIN-00014 University of Helsinki,Finland. Current address: Agricultural Research Centre of Finland, Crops and Soil, FIN-31600 Jokioinen, Finland,

e-mail: [email protected]

The article reviews those major soil properties and traffic factors, which together influence subsoilcompaction resulting from the passage of agricultural vehicles. Likewise, the effects of subsoil com-paction on soil properties, processes and crop growth are discussed on several levels, from methodsof measuring to the persistence of compaction effects. The risk of subsoil compaction exists whenev-er moist soils are loaded with heavy axle load and moderate to high ground contact stress. Subsoilcompaction tends to be highly persistent. To avoid the risk of long-term deterioration, limits for theinduction of mechanical stresses in the subsoil should be established through international team-work.

Key words: axle load, ground contact stress, long-term effects, soil physical properties

Introduction

Soil compaction has become a problem of ma-jor proportions in agriculture to day (Soane andOuwerkerk 1994a) causing reduced yield, eco-nomic and environmental damage and poorer soilworkability. According to Håkansson (1994a),the harmful compaction of arable soils is main-ly attributable to wheel traffic where heavy ma-chines are used in unfavourable soil conditions.As mechanization has intensified, machinerypower and weight and implement size have in-creased. As an example, the increase in power

and weight of newly sold tractors in Finlandduring 1976—1996 is shown in Fig. 1. In Ger-many, the proportion of newly registered trac-tors larger than 44 kW jumped from 33% in 1976to 77% in 1992 (Renius 1994). In the UnitedStates the average power of new tractors in-creased from 60 to 85 kW between 1973 and1986 (Eradat Oskoui and Voorhees 1991), andthe gross weight of a 85 kW tractor was 6 Mg.Today in the United States, agricultural tractorscommonly have a total weight of 20 Mg. Like-wise, in Western Europe, fully loaded combineand sugar beet harvesters may weigh more than20 and 35 Mg, respectively, and slurry tankers

mailto:[email protected]

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Alakukku, L. Subsoil compaction due to wheel traffic

may weigh as much as 25 Mg (Håkansson andPetelkau 1994).

The continuous increase in the weight of farmmachinery has increased the potential for pro-gressive subsoil damage. In the present paper,subsoil refers to soil below normal primary till-age depth. Although Hadas (1994) argues thatsubsoil compaction constitutes a problem with-in the realm of soil compaction in general andshould not be a special field of research, someimportant differences between topsoil and sub-soil compaction nevertheless exist. Normal till-age does not loosen the subsoil and the effectsof compaction are difficult to correct. Thus, theeffects of subsoil compaction often persist for along time. The present paper reviews, subsoilcompaction due to field traffic with the follow-ing objectives:

(1) to determine the internal and external param-eters that influence the capability of wheeltraffic for subsoil compaction

(2) to evaluate existing stress, axle load andfarming recommendations for avoiding sub-soil compaction

(3) to discuss the long-term effects of subsoilcompaction on agriculture

Compaction of subsoils bywheel traffic

According to Koolen and Kuipers (1983), soilcompaction in agriculture is usually accompa-nied by deformation in addition to compressionlateral movement. The machinery traffic thatcauses soil compaction, which may be definedas an increase in bulk density, also causes non-volumetric changes in soil structure. Factors in-fluencing the compaction capability of wheeltraffic include the soil conditions during the traf-fic, the type and intensity of forces applied, the

Fig. 1. Tractors of different powerand weight as proportion of newtractors sold in Finland in 1976–1996. Calculated from the statis-tics of annual tractor sales com-piled by the Institute of Agricul-tural Engineering of the Agricul-tural Research Centre.

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number of loading events and the duration ofloading (Fig. 2). In this section we look at someof the major soil properties (internal parameters)and traffic factors (external parameters) relevantfrom an agricultural engineering point of view.

Internal soil strengthInternal topsoil and subsoil strength resists theexternal stresses induced by wheel traffic. Inter-nal soil strength varies temporally, horizontallyand vertically owing to differences in soil tex-ture, content and kind of organic matter, struc-ture, root density and especially moisture con-tent/potential (Fig. 2). The effects of these dif-ferent parameters on the compressibility andcompaction behaviour of soils have been re-viewed among others by Horn and Lebert (1994).

Soil moisture content and/or potential is thedominant property affecting soil strength duringcompaction (Dawidowski and Lerink 1990). Assoil moisture content increases, the strength ofan unsaturated soil drops rapidly, increasing therisk of soil compaction. Thus, the same stresscompacts a subsoil more when it is moist thanwhen it is dry (Salire et al. 1994). Pure sandysoil may be weak when either dry or saturated,however, since soil resists loading best when thecapillary cohesion is highest. According toAkram and Kemper (1979), the soil compactiondue to a given stress is greatest when the soil isat field capacity (ψ

m = –10 kPa). Saturated soil

cannot be compacted without the water drainingout from the soil. Saturated soil may smear, how-ever, with resultant puddling and subsequent soilcompaction when the homogenized soil dries(Guérif 1990).

Fig. 2. Field traffic factors and soil properties affecting the soil compaction process (Canarache 1991,Soane and Ouwerkerk 1994b).

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Stresses induced by forcesapplied to subsoil

During field traffic, downwards acting forces dueto dynamic wheel load (vertical normal forces),shear forces imposed by wheel slip and vibra-tion forces transmitted from the engine throughwheel or track are exerted on the soil. A towedwheel applies mainly vertical normal forces tothe soil, while the driven wheel also exerts shearforces. Downwards acting forces have been themain concern in connection with subsoil com-paction. Raghavan and McKyes (1977) never-theless found in laboratory studies that up to 50%of topsoil compaction was attributable to shearstress owing to wheel slip. Likewise, at highslips, smearing damages the soil. According toKoolen et al. (1992), the shear effect vanishesrelatively rapidly with depth. However, Kirby(1989) concluded from a model evaluation thatshear damage may extend to a depth of 1.5 to 2times the tyre width in a soil profile of uniformstrength. Even though the shear effect may van-ish rapidly, it can damage the subsoil markedly,especially during ploughing, owing to furrowwheel slipping. Davies et al. (1973) suggest aslip maximum of 10% to avoid topsoil damageowing to shear. The same limit is probably ap-propriate for subsoils.

The contribution of vibration effects to thecompaction below pneumatic tyres has seldombeen documented for arable soils (Soane et al.1981). Wong and Preston-Thomas (1984) pro-pose that a tracked tractor compacts topsoil morethan expected because of the vibration transmit-ted through the track to the soil. With powerfultracked tractors in increasing used in agriculture,study is needed of the extent of the vibration theytransmit to the soil and the possible effects ofthe vibration on subsoil.

Average ground contact stressThe average ground contact stress (wheel loaddivided by ground contact area between tyre andsoil) estimates the vertical stress exerted on thesoil surface by wheel or track. The contact stress

is often evaluated from the tyre inflation pres-sure. The relationship between the averageground contact stress and the inflation pressureof a tyre depends, however, on tyre stiffness andsoil conditions. For stiff agricultural tyres, tyrewalls carry a considerable proportion of the to-tal load, and on rigid surfaces the contact stressis higher than inflation pressure (Plackett 1984).Burt et al. (1992) found that the dynamic aver-age ground contact stress below an 18.4R–38tractor tyre on rigid soils was closely approxi-mated by the inflation pressure, whereas on un-compacted soils the contact stress was clearlylower than the inflation pressure. Burt et al.(1992) and Tijink (1994) offer detailed introduc-tions for the determination of contact stress.

The average ground contact stress denotes thecalculated average value of the vertical stress inthe tyre/track- soil contact area. The stress is not,however, uniformly distributed over the contactarea. Stress distribution beneath the wheel iscomplex because of the tyre lug patterns andbecause the tyre itself can be deformed. Thus,the maximum ground contact stress under lugsor stiff tyre walls on a firm soil or at the centreof the contact area of a hard tyre on a soft soilmay be twice or even four times the estimatedaverage ground stress (Smith 1985, Burt et al.1992). Likewise, the ground contact stress un-der a track will concentrate under the roadwheels(Wong 1986). The effect of uneven ground con-tact stress distribution on soil compaction hasbeen little investigated. It is believed that theeffect of the unevenness is limited to the upperpart of the soil profile. However, Akker et al.(1994) found that at a depth of 0.35 m the peakstress with a low pressure tyre was about 60%of that with normal tyre even though the infla-tion pressure of the low pressure tyre (80 kPa)was only 33% that of the normal tyre (240 kPa).They proposed that the ground contact stress wasconcentrated below the stiff sides of the low pres-sure tyre.

Extent of stressesIn unsaturated soils, external stresses are trans-mitted three-dimensionally via solid, liquid and

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gaseous phases. Most of the analytical modelsfor the propagation and distribution of stressesin the soil profile describe the stress distribu-tion under a point load acting on a homogene-ous, isotropic, semi-infinite, ideal elastic medi-um. The theoretical solution was proposed byBoussinesq (1885, ref. Söhne 1953). Fröhlich(1934) later modified the original solution byintroducing a concentration factor (ν) to accountfor the increase in Young’s modulus with soildepth due to overburden stress. Söhne (1953,1958) and Gupta and Raper (1994) review theequations that describe stresses on a soil elementas a result of a point load.

According to the theory, a vertical normalstress (σ

z) under a point load (P) at a given depth

(z) can be expressed as

[1]

where R is the radial distance from the pointload to the subsoil point under consideration andβ is the angle between the vertical line from theloaded point and R (Gupta and Raper 1994).

An analytical solution for the soil verticalnormal stress beneath the centre of a uniformlyloaded circular ground contact area can be cal-culated as a function of depth using the equa-tion given by Taylor et al. (1980):

[2]

where p is the average ground contact stress(tyre load/ground contact area) acting on the soil-tyre contact area, and α is the half aperture an-gle between the point at depth z and the edge ofthe contact area.

The stress described in the equations corre-sponds to that in an elastic (Poisson ratio 0.5),isotropic body if the concentration factor n isequal to 3 (Gupta and Raper 1994). Söhne (1953,1958) assumed the values 4, 5 and 6 for hard,average and soft soil conditions, respectively.Soil firmness he described as a combination oflooseness (bulk density) and moistness. Horn etal. (1987) report more fully that the concentra-tion factor depends on the moisture content, den-sity, load history, structure and texture of the soil.

According to Horn (1980), ν varies from 1 forstrong material to 10 for soft soil, and it is lowerin aggregated than in non-aggregated soil (Horn1986). In summary it can be stated that in strongand/or dry soils with low ν, vertical normal stressis distributed more horizontally, in a shallowersoil layer, but in weak and/or wet soils with highν it is transmitted to deeper depths.

The analytical solution has limitations. Esti-mation of the concentration factor is problemat-ic and expensive (Gupta and Raper 1994). Fur-thermore, the soil conditions assumed above areseldom encountered in the field since soil is morea plastic medium than an elastic one, and theconditions vary spatially. Likewise, any non-uniformity in the soil profile tends to distort thestress distribution. Thus, Taylor et al. (1980)found that a plough pan in the soil profile, whichis a common situation in agriculture, causedhigher stress above the pan and lower stress be-low it than was the case for a uniform soil pro-file. Hard-pan tends to act like an elastic bridge,spreading the load over a wider area by reduc-ing the concentration factor (Dexter et al. 1988).Thus, finite-element models are more often used,to take into account the heterogeneity of a soilprofile (Kirby 1999). Likewise, the analyticalsolution described above is based on static stress.Hadas (1994) states that the results may be ex-pected to be different when the stress is dynam-ic (short duration stress changing during driv-ing). Nor is stress ever uniformly distributed overthe ground contact area, as equation [2] assumes(see sect. Average ground contact stress).

The analytical solution shows that the stressin the soil under a loaded wheel decreases withdepth (Fig. 3). From this, a highly simplifiedconclusion can be drawn: the stress in the top-soil depends on the average ground contactstress, but the stress in the subsoil is determinedmostly by the wheel load (Söhne 1958, Carpen-ter et al. 1985). The same has been found in fieldand soil bin experiments (Danfors 1974, Tayloret al. 1980, Bolling 1987, Fig. 3). Hadas (1994)and Olsen (1994) criticize the generalization,however. On the basis of analytical calculations,Olsen (1994) concludes that the decrease of in-

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duced vertical normal stress with depth in theupper subsoil (0.10–0.30 m to 1 m depth) de-pends on both ground contact stress and wheelload, and below 1 m solely on wheel load. Thecriticism is justified since, based on equation (2),reduction in the average ground contact stressreduces the magnitude of stresses transmitted tothe soil. The wheel load determines the normalstress level deep in the soil profile, but the stresslevel will never exceed the maximum groundcontact stress level.

From the analytical solution and experimen-tal results the following conclusion can be drawnon the effects of wheel load and contact stresson the soil stress and compaction. As the wheelload is increased with the same tyre and contact

area, the stress at a specific depth increases anda given stress is transmitted deeper (Danfors1974). When the wheel load is increased, eventhough the contact stress is kept unchanged byincreasing tyre dimensions or the number of tyres(dual, triple), a given isostress is transmitteddeeper into the soil and a greater soil volume isstressed (Fig. 3, Hadas 1994). When the numberof tyres is increased, the extent of a given isos-tress may, however, be reduced by spacing thetyres widely apart to avoid any interaction be-tween them (Olsen 1994). Lebert et al. (1989)found in in situ stress transmission measurementsat the same wheel load but increasing contact areathat vertical stresses can be reduced in the top-soil by using larger tyres, but in the subsoil the

Fig. 3. Calculated isostress lines as a function of wheel load (WL) with constant tyre inflation pressure(TIP) in a homogeneous soil (left, redrawn from Söhne 1953), and measured vertical stress distribution asa function of depth at constant wheel load in a silt soil (right, Lebert et al. 1989). A represents contact areaand p represents average ground contact stress. Figures reprinted with kind permission from Institut fürBiosystemtechnik, Bundesforschungsanstalt für Landwirtschaft and Kluwer Academic Publishers.

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reductions are less significant (Fig. 3). Likewise,Danfors (1994) reported that a reduction in in-flation pressure from 150 to 50 kPa (axle load >8 Mg) reduced the compaction of moist clay soilsonly down to a depth of 0.30–0.40m, not in thedeeper layers. In summary it can be stated thatrisk of subsoil compaction exists whenever amoist soil is loaded by a high wheel load andmoderate to high ground contact stress.

Number of wheel passesThe number of passes affects the number of load-ing events and the coverage, intensity and dis-tribution of wheel traffic. In field experimentson mineral soils, an increase in the number ofpasses in the same track increased subsoil com-pactness (Gameda et al. 1987, Schjønning andRasmussen 1994, Alakukku and Elonen 1995a)and the depth of the compacted layer (Sommerand Altemüller 1982, Alakukku 1996a, b). Anincrease in the number of random passes wouldnormally increase the spatial coverage of trafficin a field. Heavier machines usually reduce thenumber of passes per unit of land area per oper-ation because of increased working width. Olfeand Schön (1986) report that a doubling in trac-tor power from 60 to 120 kW clearly reducedthe area of soil covered by wheel tracks in cere-al cultivation. At the same time, however, theintensity of field traffic increased from 176 to216 Mg km ha-1. Due to the use of heavier ma-chines. Thus, the risk of subsoil damage mayincrease, even though the percentage of the fieldcovered by wheel traffic is reduced by increasedworking width.

Driving speedWhen the velocity of a machine is increased, theduration of the loading is reduced. Bolling (1987)measured the effects of velocity on the maximumsoil stress below a wheel centre with two wheelloads (0.82 Mg, tyre inflation pressure 160 kPaand 1.5 Mg, 170 kPa) on sandy loam soil and

found that an increase in velocity from 2 km h-1

to 10 km h-1 decreased stress at 0.30 m depthbelow the wheel centre. The effect of velocitywas greater on loose than on dense soil. Similarresults were reported by Horn et al. (1989). Ear-lier, however, Danfors (1974) found that highervelocity caused bouncing, which transmittedhigh point stresses to the subsoil. An increase invelocity would seen to reduce the stress trans-mitted to upper subsoil layers. The effects ofvelocity on the stress in deeper layers and thepractical importance of velocity to subsoil com-paction have seldom been documented, howev-er. The highest velocity tested has been 8–12 kmh-1, which is the normal speed in field operations.

Extent of subsoil compactionThe most serious source of subsoil compactiontends to be tractor wheels running in the openfurrow during mouldboard ploughing becausethe tractor wheels then run directly on the upperpart of the subsoil. As summarized in Table 1,field traffic on the soil surface with an axle loadhigher than 6 Mg and with tyre inflation pres-sure greater than 50 kPa compacts moist miner-al soils to 0.50 m depths or deeper. Under unfa-vourable soil conditions, an axle load less than5 Mg also compacts the subsoil. The depth ofcompaction in organic soils has seldom been in-vestigated because the effects of organic soilcompaction have not been considered harmful.An axle load higher than 6 Mg has been foundto compact organic soils to 0.40–0.50 m depths(Table 1). In view of the persistence of subsoilcompaction (see sect. Long-term effects of com-paction in agriculture), Håkansson and Petelkau(1994) regard compaction to depths more than0.40 m as unacceptable.

Stress, axle load and farmingrecommendations

With the aim of avoiding subsoil compaction,recommendations have been given for maximum

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values of average ground contact stress and stressat 0.50 m depth (Table 2) and for axle loads. Formoist soils, it is sometimes recommended that

ground contact stress should not exceed 50 kPa(Table 2). Carpenter and Fausey (1983) suggestthat the maximum ground contact stress with

Table 1. Maximum depth of soil compaction due to field traffic with different axle loads and soil conditions.

Axle Depth ofSoil load TIP1) compaction

Type Moisture (Mg) (kPa) (m) Reference

Coarse sandSand

SandSandSandLoamy sandLoamLoamSilt loamSilt loamSilt loamSilt loamSilty loamSiltSandy claySilty clay loamSilty clayClay loam

Clay loam

Clay loamClay loamClay

ClayClay

ClayClayFenMullSedge peat5)

1) TIP = tyre inflation pressure, FC = field capacity, PW = permanent wilting point, SS = saturation2) tandem axle3) average ground contact stress4) total load5) sedge peat mixed with clay from 0.20 m to 0.40–0.50 m depths, and underlain by gythia

Schjønning and Rasmussen (1994)Danfors (1974)

Håkansson (1985)Akker (1988)Dumitru et al. (1989)Lipiec et al. (1990)Gameda et al. (1984)Lipiec et al. (1990)Schuler and Lowery (1984)Blackwell et al. (1986)Hammel (1988)Wood et al. (1993)Stewart and Vyn (1994)Alakukku (1997)Gameda et al. (1984)Voorhees et al. (1978)Schuler and Lowery (1984)Voorhees et al. (1986)

Danfors (1994)

Alakukku and Elonen (1995a)Alakukku (1997)Danfors (1974)

Eriksson (1976)Aura (1983)

Håkansson (1985)Alakukku (1996a, b)Schmidt and Rohde (1986)Pietola (1995)Alakukku (1996a, b)

At FC1)

Near FC

Near FC––60% of FC0.22 g g-1

78% of FC0.16 g g-1

–14– –60 kPaNear FC72% of SS1)

Near FC< 0.3 m FC0.23 g g-1

–0.20 g g-1

> 0.3 m PW1)

Near FCNear FC

Near FC85–90% of FCNear FC

–Near FC> FCNear FCNear FC–> FC> FC

222)

162)

46

162)

6.42.51.7

10/201.7

12.513.2

10/2015.21211

10/204.5

12.59/189/18

8102)

5212)

162)

46

404)

33

162)

192)

63

162)

2203)

400400400300240300

60414/414

60220118

200/330210145600

414/4142503)

220150/2003)

150/2003)

50/15050/150

150/300800400400400

–140140300700

–1203)

700

0.600.600.200.400.500.800.40

0.20–0.400.60

0.20–0.400.400.500.750.400.350.500.600.450.400.300.500.500.500.35

0.40–0.450.600.200.401.000.200.400.500.500.500.35

0.40–0.50

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high wheel loads should not exceed the stressallowed in the subsoil. Few data exist, however,to allow assessment of the maximum allowablesubsoil stress in different conditions, and thisarea should be addressed in future studies.

From a practical point of view it is relevantthat the recommendations for ground contactstress are close to the recommendations for themaximum tyre inflation pressures given by Dw-yer (1983, 50 kPa for moist soil, 100 kPa fordry soil) and Perdok and Tijink (1990, 50 kPa

for moist soil, 250 kPa for dry soil). When thecontact stress is to be minimized, the technicalsolution will depend on the demands of the op-eration in which a machine is used. The tyreinflation pressure should, however, always bethe lowest allowable in the prevailing situation(tyre loading capacity, velocity, traction), andthe tyre -soil contact should be uniform. For adetailed discussion of tyre factors see, amongothers, Tijink (1994); for track factors see Er-bach (1994).

Table 2. Recommendations for maximum average ground contact stress and vertical soil stress at 0.50 m depth in differentsoil conditions to prevent soil compaction.

Ground contact Stress at 0.50 mstress (kPa) depth (kPa)

Summer/ Summer/Soil Spring autumn Spring autumn Reference

SandLoamClayArable land

ClayMC > 90% of FC2)

MC 70–90% of FCMC 60–70% of FCMC 50–60% of FCMC < 50% of FCSand, Sandy loamMC > 90% of FCMC 70–90% of FCMC 60–70% of FCMC 50–60% of FCMC < 50% of FC

Silts, 14–27% clayψm –30 kPa2)

ψm –100 kPa

Mineral soilsMC ≥ FC

1) moisture content < 70% of field capacity2) moisture content (MC) of field capacity (FC), ψ

m is matric potential

3) the stress at which irreversible deformation occurs at 0.45 m depthSoil with high bulk density has higher value than soil with low bulk density

4) cited by Lipiec and Simota (1994)

3030354550

3030354550

Petelkau (1984)

Vermeulen et al. (1988)

Rusanov (1994)

Salire et al. (1994)

Bondarev et al. (1988)4)

5080

15050

80100120150180

95120145180215

40–50

801)

1501)

2001)

100

100120140180210

120145170215250

2525303535

2525303535

64–2713)

119–356

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To avoid soil compaction below normal pri-mary tillage depth (0.2–0.3 m), single axle loadsnot exceeding 4 to 6 Mg have been recommend-ed for moist mineral soils (Danfors 1974, 1994,Voorhees and Lindstrom 1983, Petelkau 1984)even when the tyre inflation pressure is 50 kPa(Danfors 1994). For tandem axle loads on moistsoils Danfors (1974, 1994) proposes a limit of8–10 Mg. On dry soil, the axle load may be highwithout causing subsoil compaction (Table 1).Farmers cannot, however, easily estimate mois-ture conditions in the subsoil. Moreover, heavyfield traffic on moist fields often cannot be avoid-ed owing to the time constraints of given fieldoperations. Thus, recommendations need to beset with a view to the wettest conditions prevail-ing during the normal use of a machine.

While stress and axle load limits may be con-sidered to be the engineering tools for the con-trol of soil compaction, it should be rememberedthat the farming system, and the way of usingmachinery as part of that system, will markedlyinfluence the effects of the field traffic on soilsand crops. For instance, if the drainage does notwork, a larger tractor or larger tyres will not es-sentially reduce the workability or trafficabilityproblem and may even intensify the compactionproblem. Moreover, spring application of slurrymanure before seedbed preparation is recom-mended as the most effective way of conservingplant nutrients. Yet slurry tankers exert substan-tial axle loads on soils which are usually wet,and may induce significant compaction. Thus,Håkansson and Danfors (1988) calculated thatthe compaction cost of spreading liquid manureon wet soil may exceed the economic value ofthe nutrients supplied by the manure. Further-more, the area covered by wheel traffic can bereduced without increasing working width byusing combined implements or linked operationsor by concentrating field traffic to the sametracks. An example is combined seedbed prepa-ration, fertilization and sowing. Likewise, thetransport traffic on moist soil should be concen-trated to as few tramlines as possible, with theselocated perpendicular to the subsurface drainsor to headland roadways.

Long-term effects of compactionin agriculture

Compaction induced by field traffic has bothshort- and long-term effects on soil and crop pro-duction. Short-term (1–5 years) effects are main-ly associated with topsoil (0–0.30 m) compac-tion, which is largely controlled by tillage oper-ations, field traffic and the way in which theseoperations are adapted to soil conditions. Thetopsoil compaction is alleviated by tillage andnatural processes of freezing/thawing, wetting/drying and bioactivity. Aura (1983) found thatthe compaction of clay soils within the ploughlayer due to traffic with an axle load of 3 Mg inspring was alleviated by ploughing and frost bythe following spring. When the plough layer isseverely compacted, the recovery of heavy claysmay take three (Alakukku 1996b) or even fiveyears (Arvidsson and Håkansson 1996) despiteannual ploughing and frost. This section discuss-es the effects of wheel traffic on subsoil proper-ties and on crop production in field conditions.

Subsoil properties and behaviourMany studies have found that compaction mod-ifies the pore size distribution of mineral sub-soils mainly by reducing the macroporosity (>30 µm) (Eriksson 1976, Ehlers 1982, Blackwellet al. 1986, Alakukku 1996a, 1997). Besides thevolume and number of macropores, compactionmay also affect their continuity. Modificationsin soil macroporosity are very important sincethey affect other soil properties and soil behav-iour.

Subsoil compaction has been found to haveharmful effects on many soil properties relevantto soil workability, drainage, crop growth andenvironment. Subsoil compaction due to heavyfield traffic increased the dry bulk density, vaneshear resistance and penetrometer resistance ofsubsoils with clay content 0.06–0.85 g g-1 (Ta-ble 3). Compacted subsoils had a more massive

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and coarse structure (Pollard and Webster 1978,Alakukku 1997). Likewise, compaction reducedthe saturated hydraulic conductivity (Table 3),CO

2 and O

2 exchange (Simojoki et al. 1991) and

air permeability (Horn et al. 1995) of subsoils.The likelihood of drainage problems increas-

es when compaction reduces the permeability ofthe subsoil and may lead to waterlogging prob-lems in rainy years. Poorly drained soil may alsodry slowly, reducing the number of days availa-ble for field operations. Furthermore compactionmay increase surface runoff and topsoil erosionby impeding water infiltration (Fullen 1985).Likewise, the reduction in drainage rate attrib-uted to subsoil compaction can be expected toincrease the emission of green house gases fromthe soil, for instance, by increasing denitrifica-tion. The environmental consequences of soilcompaction have been reviewed by Soane and

Ouwerkerk (1995). However, there is little in-formation as yet on the implications of machin-ery induced subsoil compaction for the hydrau-lic processes in the soil profile and the qualityof the environment. These subjects are in needof further study.

Normal tillage does not loosen the subsoil(below about 0.30 m). The effects of subsoilcompaction may persist for a very long time. Forexample, in spite of cropping and deep frost, theeffects of subsoil compaction due to high axleload traffic were measurable in soils with claycontents of 0.06–0.85 g g-1 three to 11 years af-ter application of heavy loading (Table 3, Gault-ney et al. 1982, Voorhees et al. 1986, Gameda etal. 1987, Etana and Håkansson 1994, Lowery andSchuler 1994, Wu et al. 1997). Subsoil compac-tion still persisted six years after the traffic in acropped sandy loam (Pollard and Webster 1978)

Table 3. Change in dry bulk density (BD), total porosity (TP), penetrometer resistance (PR), vane shear resistance (VR),macroporosity (MP), saturated hydraulic conductivity (Ksat) and number of cylindrical pores (NB) in the subsoil of mineralsoils after compacting treatments. Results are expressed relative to the control.

Change in soil properties due to loading (%)

Soil Loading BD TP PR VR MP Ksat NB Reference

Sandy loamSandy loam

Loam

Loam

Silt loamSilty clay loamSilt

Clay loam

Clay loam

Clay loam

Clay loamSilty clayClay

Heavy clay

6 years earlier by tractor4 passes 38 Mg sugarbeet harvester9 years earlier, 4 passes16 Mg tandem axle load4 passes 38 Mg sugarbeet harvester1 pass 13.2 Mg axle loadTraffic during one season3 passes 10 Mgsingle axle load9 years earlier, 4 passes,750 kPa contact stress7 years earlier, 4 passes18 Mg axle load3 passes 20 Mgtandem axle load4 passes 18 Mg axle load4 passes 12.5 Mg axle load9 years earlier, 4 passes16 Mg tandem axle loadSeveral years 40 Mg

–32

–17

–7

–67

–38

Pollard and Webster (1978)Arvidsson (1998)

Alakukku (1996b)

Arvidsson (1998)

Blackwell et al. (1986)Voorhees et al. (1978)Alakukku (1997)

Blake et al. (1976)

Logsdon et al. (1992)

Alakukku (1997)

Voorhees et al. (1986)Lowery and Schuler (1994)Alakukku (1996b)

Eriksson (1976)

103

4

205

11

3

42

6

–7

–8

200

12400

15

–1

252318

63

12

–33

–15

–45

–8

–14

–42

–64

–67

–84

–80

–78

–90

57

13–48

–14

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and ten years after the traffic in a cropped clay(Duval et al. 1989). In all these investigations,the effects of compaction persisted for the dura-tion of the experiment. Håkansson and Petelkau(1994) concluded that subsoil compaction tendsto be highly persistent, and in non-swelling sandysoils and tropical areas it may be permanent im-mediately below the tillage depth.

As shown above, subsoil compaction can bequantified by measuring many different soilproperties. A common approach is to determinethe changes in bulk density. Bulk density is alsoincluded in most models predicting subsoil straindue to external stresses (Smith 1985). The use-fulness of bulk density determinations for agri-cultural purposes is limited, however, since thebulk density does not take into account the non-volumetric changes of subsoil strcture due to theloading and it does not determine the behaviourof subsoils. Horn et al. (1996) demonstrated un-der idealized sandy loam soil bin conditions thatup to 60% of soil deformation can be determinedas a volumetric compression and up to 40% hasto be defined as volume-constant deformation.Changes in soil behaviour properties, such as

mechanical behaviour, water retention and wa-ter and air flow, better express the effects of com-paction on soil than does bulk density (Gupta etal. 1989, Carter 1990).

Crop responseThe harmful effects of subsoil compaction arereflected not only in soil properties but in cropgrowth and yield. Soil compaction to depths of0.50 m or more by axle load traffic greater than9 Mg decreased the yield of grain (Håkansson etal. 1987, Fig. 4) and nitrogen uptake (Alakukkuand Elonen 1995b), and delayed (Gaultney et al.1982, Gameda et al. 1987) or accelerated(Alakukku and Elonen 1995b) the ripening ofcrops up to several years after the loading. Thegreatest decrease in yield occurred in the firstthree years after the compaction (Fig. 4), proba-bly mainly owing to plough layer compaction(Håkansson and Reeder 1994). By degrees, an-nual ploughing and freezing alleviated theplough layer compaction, and after the firstfew years the reduction in yield became less(Fig. 4).

It may be that the subsoil compaction is notalleviated, however, and its long-term effect onthe crop growth and yield will then depend onthe climatic conditions of the growing season.Voorhees (1992) found that subsoil compactionresulting from loading with an 18 Mg axle loaddecreased maize yield significantly on clay loamin relatively dry and rainy years but not in yearswith normal precipitation. Lowery and Schuler(1994) reported that the subsoil compaction dueto traffic with a 12.5-Mg axle load reduced maizeyield considerably only in a rainy year. Likewise,Alakukku and Elonen (1995b) found that, whenthe beginning of the growing season was dry,subsoil compaction in clay soil had little effecton the grain and nitrogen yields of annual cropswhereas in rainy years the yields were clearlyreduced. They also observed that extendeddrought reduced the yields in all plots. Further-more, subsoil compaction may have little effecton crop yield if the crop does not need to extract

Fig. 4. Crop yields relative to control treatment (=100%)for 12 successive years (1–12) after loading with four pass-es by vehicles having a single axle load of 10 Mg ortandem axle load of 16 Mg and tyre inflation pressure> 200 kPa. Mean values from field experiments in sevencountries in Europe and North America. Data for the lastfour years (Alakukku 1999) are included in the figure, whichis redrawn from Håkansson and Reeder (1994).

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water from the subsoil. Montaga et al. (1998)reported that the root growth of vegetable cropsin the subsoil was reduced by compaction, butthere was no direct effect on crop shoot growthor yield when water and nutrients were adequate-ly supplied to the soil.

A common approach in examinins the effectsof subsoil compaction is to determine the quan-titative changes in yields. Yield quantity alonedoes not, however, always show how harmfulthe influence of soil compaction on crops maybe. Interestingly, nitrogen yield has been foundto be a more sensitive measure of the influenceof topsoil and subsoil compaction than the grainyield. Douglas et al. (1995) found that the aver-age dry matter yield and nitrogen uptake of grasssilage over a period of four years were 13 and18% lower, respectively, in conventional trafficplots than in zero traffic plots. Likewise, takenas a mean of the first nine years after a clay soilwas compacted to 0.50 m depth by a single heavyloading, the grain yield of annual crops in com-pacted plots was reduced by 3% but the nitro-gen yield harvested in grain yield by 7%(Alakukku and Elonen 1995b). The long-termeffects of subsoil compaction on crop quality andnutrient uptake have seldom been studied, how-ever.

The reasons for yield reduction are not al-ways well quantified. Yet in any attempt to mon-itor the consequences of compaction it is impor-tant to quantify the effects on the subsoil/plant/weather system. Likewise, mainly annual graincrops were grown in field experiments. Yet cropsensitivity to topsoil compaction varies withplant species (Brereton et al. 1986) and evenvariety. In summary it should be emphasized thatfield experiments on the effects of subsoil com-paction on crop production in different crop ro-tations need to be carried out over a sufficientlylong period of time to account for iteractionsbetween variations in weather conditions andcrop response. Moreover, to better quantify sub-soil/plant/weather interactions, measurements ofplant development, water and nutrient uptake,root growth and soil and weather conditions needto be made during the growing season.

Cumulation of subsoil compactionSince the alleviation of a severe subsoil com-paction takes many years, if it occurs at all, aheavy loading repeated in the same place eachyear may increase soil compactness and yieldlosses year by year. The area of compacted soilmay also increase if the heavy field traffic is notconcentrated annually in the same tracks. Soilcompaction may thus become more harmful astime goes on, even though the effect of a singlepass by a heavy vehicle tends to be rather small(Håkansson 1994a). Arvidsson and Håkansson(1996) report that repeated loading of 350 Mgkm ha-1 on wet clay soils before ploughing in-creased the yield losses of spring cereals annu-ally during the first four years. After that, theyields levelled out. They evaluated, however,that the effects of compaction were mainlycaused by compaction in the topsoil. Gameda etal. (1987) did not observe cumulative yield losseswith maize even though heavy field traffic withaxle loads 10–20 Mg increased the maximum soildensity each year during the first three years.Repeated field traffic does not always increasesoil compaction cumulatively. Alblas et al.(1994) loaded sandy soils with an axle load of10 Mg twice a year for four years but did notfind any cumulative subsoil compaction or cu-mulative losses in silage maize yield.

In separate studies, Håkansson et al. (1996)and Fenner (1997) quantified the cumulativecompaction effects on the subsoil in farmersfields induced by machinery traffic in recentdecades. Håkansson et al. (1996) found that soilpenetration resistance at 0.40 m depth was anaverage 40% higher in 17 fields (subsoil claycontent 0.01–0.58 g g-1) with intensive potato andsugar beet production and where large quanti-ties of slurry were spread than in the controlfields under permanent grass without field traf-fic in the past 35 years. In eight fields (subsoilclay content 0.35–0.60 g g-1) in a cereal produc-tion region with less intensive machinery traf-fic, the corresponding increase in penetrationresistance was 10%. Håkansson et al. (1996) es-timated persistent crop yield reductions caused

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by subsoil compaction in the 17 and eight fieldsto be 6% and 1.5%, respectively. According tothese two studies the state of compactness ofarable subsoils and soil productivity are depend-ent on the intensity of heavy field traffic. Fen-ner (1997) reported similar results for penetra-tion resistance on loess soil in two neighbour-ing fields with maximum axle loads of 4 Mg and8.9 Mg during the past 20 years.

General discussion andconclusions

Results reviewed in the present paper show thatheavy machinery traffic increases the risk of longlasting soil physical degradation and crop yieldlosses, particularly in temperate, moist zones.Threats to soil productivity need to be minimizedin future. As world population is forecasted togrow 8 billion and the demand for food to in-crease by 64% during the next 25 years (World-watch Institute 1996), world food productionmust necessarily be increased. Yet little addition-al arable land is available. In general, this meansthat food production per unit area will have tobe increased rather than be allowed to decrease.Furthermore, the impact of agriculture on theenvironment must be diminished. Likewise, indeveloping countries, improved and modernfarming systems are needed to enhance crop pro-duction. Thus, it is necessary to continue devel-oping technical solutions to avoid detrimentaltopsoil and subsoil compaction induced by fieldtraffic and to find methods to improve alreadycompacted subsoils.

The risk of subsoil compaction is high whenmoist soils are loaded with high axle load trafficwith moderate to high ground contact stress.Håkansson (1994b) notes that the gradually in-creasing weight of agricultural machinery rais-es concern about a possible permanent deterio-ration of the subsoil. With the aim of avoidingsubsoil compaction, some recommendations

have been given for axle loads and for maximumpermissible ground contact stress and stress at0.50 m depth. To avoid long-lasting soil physi-cal degradation below normal tillage depth, in-ternational limits should be established for me-chanical stresses in the subsoil. Håkansson andReeder (1994) suggest that these could be sim-ple maximum axle load limits, or combined axleload -ground contact stress limits. Since agricul-tural machinery and farming practices are simi-lar world-wide, the recommendations for maxi-mum mechanical stress limits should be arrivedat through international teamwork. Our under-standing of the subsoil compaction process isincomplete, however, and our modelling of com-paction in soil/machine/plant systems imperfect.Thus, before appropriate stress limits can be set,areas such as the bearing capacity of subsoilsunder different conditions should be investigat-ed.

While large and heavy machinery is oftenblamed for soil compaction problems, it shouldbe noticed as well that the farming system is notusually changed when the machinery size in-creases. Larger machines, often equipped withlarge, low-pressure and/or dual tyres, are usedin the same soil conditions (macroporosity,drainage) and in the same tillage and other work-ing practices as smaller ones. Added to this, in-creased engine power and/or enlarged tyre con-structions make it possible to work in worse con-ditions. Large machines do not, however, com-pensate for the poor drainage or other misman-agement of soil structure. Thus, in examining thepossible ways to avoid subsoil compaction dueto field traffic, the whole farming system needsto be considered, not just the machinery.

Soil compaction affects virtually all physi-cal, chemical and biological soil properties andprocesses by modifying soil macroporosity. Yetthe effects of heavy loading on subsoils and theeffects of subsoil compaction on drainage, cropgrowth and environment are typically investigat-ed with a limited number of soil, crop and envi-ronmental parameters. When these effects arebeing evaluated, it is relevant to take the eco-nomics into account as well. Voorhees (1991) has

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pointed out that even though a given machinerysystem decreases crop yield a little, it still maymake economic sense to use it. Eradat Oskoui etal. (1994) reviewed the economics of modifyingmachinery to minimize soil compaction. The ef-fects of subsoil compaction on the farm econo-my have seldom been documented, however.Further studies will be needed to obtain suffi-cient information on the long-term and cumula-tive effects of machinery induced subsoil com-paction on soil hydrology, crop productivity,farm economics and environment. The risks ofsubsoil compaction should be analysed on thebasis of the findings of these studies, and theprobabilities of harmful consequences of subsoilcompaction in different situations should be eval-uated.

Subsoil compaction has been documented tobe long-lasting and difficult to correct. Deeploosening is expensive and will seldom amelio-rate the compacted structure completely. More-

over, the loosened soil is often recompactedwithin two or three years, with even worse phys-ical properties (Kooistra and Boersma 1994).Thus, it is better to avoid subsoil compaction inthe first place than to rely on alleviating the com-pacted structure afterwards. The alleviation ofsoil compaction through biological tillage byearthworms and plant roots is in need of furtherstudy. Whalley and Dexter (1994) note that bi-opores are more stable than the macropores pro-duced by mechanical tillage.

Acknowledgements. This writing of this paper was finan-cially supported by the Commission of the European Com-munities, Agriculture and Fisheries (FAIR) specifiec RTDprogramme, FAI5–CT97–3589, “Experiences with the im-pact of subsoil compaction on soil, crop growth and envi-ronment and ways to prevent subsoil compaction”. In prin-ciple the presented data do not necessarily reflect its viewsand in no way anticipates the Commission’s future policyin this area.

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Vol. 8 (1999): 333–351.

SELOSTUSRaskaan peltoliikenteen aiheuttama pitkäaikainen maan tiivistyminen

Laura AlakukkuMaatalouden tutkimuskeskus

Maataloudessa käytettävien koneiden tehot ja painotovat kasvanut viime vuosikymmenien aikana jatku-vasti. Tämä on lisännyt pohjamaan tiivistymisriskiä.Tässä kirjoituksessa tarkastellaan, miten maan omi-naisuudet ja peltoajoon liittyvät tekijät vaikuttavatpohjamaan tiivistymiseen. Lisäksi selvitetään, mitenpohjamaan tiivistymisen vaikuttaa maahan, peltovil-jelyyn, satoon ja ympäristöön.

Pohjamaan tiivistymisriski on suuri, kun koste-alla pellolla ajetaan raskaalla kalustolla, jonka pin-tapaine on kohtalainen tai suuri. Tiivistymisen eh-käisemiseksi on annettu yksittäisiä suosituksia akse-lipainon, pintapaineen ja maassa 0,50 metriin ulot-

tuvan jännityksen ylärajaksi. Tekniset suoditukset pit-käaikaisten tiivistymishaittojen ehkäisemiseksi tuli-si kuitenkin laatia kansainvälisenä yhteistyönä. En-nen suositusten laatimista on tutkittava mm. pohja-maiden kuormituksen kestävyyttä vaihtelevissa olo-suhteissa.

Pohjamaan tiivistyminen vaikuttaa lähes kaikkiinmaan fysikaalisiin, kemiallisiin ja biologisiin ominai-suuksiin ja prosesseihin. Se on todettu pienentävänmyös kasvien satoa ja typenottoa. Tutkimusten mu-kaan pohjamaan tiivistymisen vaikutukset säilyvätpitkään. Karkeissa maissa ne voivat olla jopa pysy-viä.

Wood, R.K., Reeder, R.C., Morgan, M.T. & Holmes, R.G.1993. Soil physical properties as affected by graincart traffic. Transaction of American Society of Agri-cultural Engineers 36: 11–13.

Worldwatch Institute 1996. State of the World. World-

watch Institute, Washington D.C., USA. 209 p.Wu, L., Allmaras, R.R., Gimenez, D. & Huggins, D.M.

1997. Shrinkage and water retention characteristicin a fine-textured mollisol compacted under differentaxle loads. Soil & Tillage Research 44: 179–194.

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