subsoil compaction: risk, avoidance, identification and alleviation

Soil & Tillage Research 73 (2003) 175–182

Subsoil compaction: risk, avoidance, identificationand alleviation

G. Spoora,∗, F.G.J. Tijinkb, P. Weisskopfca Model Farm, New Road, Maulden, Bedford MK45 2BQ, UK

b Institute of Sugar Beet Research (IRS), P.O. Box 32, 4600 AA Bergen op Zoom, The Netherlandsc Eidgenössische Forschungsanstalt für Agrarökologie und Landbau (FAL), Reckenholzstrasse 191, CH-8046 Zurich, Switzerland

Abstract

This paper aims to provide guidance for field practitioners on the vulnerability of different subsoils to compaction underdifferent field conditions and on the tyre pressures necessary to reduce or avoid damage. It also indicates ways of identifyingsituations where some compaction alleviation may be necessary to improve subsoil conditions and methods for alleviatingsubsoil compaction problems, without increasing the risk of more extensive compaction damage in the future.© 2003 Elsevier B.V. All rights reserved.

Keywords:Subsoil; Compaction avoidance; Compaction alleviation; Subsoiling

1. Introduction

The continuing trend towards the use of larger andheavier equipment for field operations increases therisk of compaction damage occurring in subsoils.Compaction damage anywhere in the soil system cre-ates problems, but once it extends to the subsoil it ismuch more difficult and costly to alleviate. There is,therefore, every reason to adopt appropriate machineryand soil management practices to ensure it does notoccur. In situations where problems have arisen, alle-viation is necessary, but this must be achieved withoutmaking the soil more susceptible to re-compactionin the future. Guidance is required on these issues ifsubsoil problems are to be minimised or avoided.

Different subsoils vary in their ability to supportgiven loads without suffering compaction damage.This ability is very dependent upon the more stablesoil properties of soil type and the packing arrange-ment of soil particles and aggregates. It also depends

∗ Corresponding author.

upon the soil moisture status and the protection thesubsoil receives from the soil above at the time ofloading. The degree of protection depends upon thefirmness of the soil above and on the presence orabsence of any stronger soil layers. In most soilswhich have been under arable cropping at some timein the past, there is frequently a stronger soil layerjust below cultivation depth, formed as a result ofprevious implement action, horse’s hooves ploddingand tractor wheels running in the open furrow duringploughing operations. This layer, frequently referredto as a pan, is usually relatively strong and morecompact. Providing this pan layer does not restrictroot development, gas exchange and drainage, itcan play an important role in absorbing compactionstresses before they reach deeper sections of the sub-soil. Disrupting any such non-impeding layer wouldonly make the subsoil as a whole more vulnerable tocompaction without improving crop production andhence it would be inadvisable to disturb it. A frequentproblem encountered, however, is in rapidly identify-ing whether such a pan layer is causing impedance,

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176 G. Spoor et al. / Soil & Tillage Research 73 (2003) 175–182

so that sound decisions on the possible need for itsdisruption can be made.

Where the pan layer and/or the soil below is caus-ing impedance problems, alleviation measures will benecessary. These measures must not create excessiveloosening, otherwise the subsoil will become evenmore prone to re-compaction in the future. The typeof disruption procedure adopted is, therefore, verycritical for success.

It would be advantageous when planning field op-erations, if direct measurements could be made ofthe support capacity of the subsoil. This informationwould allow the loading regimes imposed on thesubsoil to be kept within acceptable limits to avoidcompaction. Unfortunately, no rapid quantitativefield tests to provide such information are currentlyavailable. There is, therefore, a requirement for analternative approach to estimate the vulnerability ofthe subsoil to compaction at the time of the proposedoperation, together with guidance on the required in-flation pressures to minimise or avoid subsoil damageat the particular level of vulnerability.

This paper aims to address these issues and providea practical guide for field managers and equipmentoperators and manufacturers, to assist in avoidingand where necessary overcoming subsoil compaction.The approaches and guidelines suggested draw uponcurrent research information and extensive field expe-rience in northern Europe. They represent what is con-sidered to be the best information currently available,but it is expected they will be updated and strength-ened, as further research information and improvedtechniques emerge. If detailed supporting researchinformation is required on these aspects together withfield operating practices to minimise subsoil com-paction problems, the reader is referred to the papersof Alakukku et al. (2003)andChamen et al. (2003).

2. Assessment of subsoil vulnerability tocompaction

The approach suggested for assessing the likelyvulnerability of subsoils to compaction is based, inthe absence of quantitative data, on field experience.This experience has been derived from profile pit ob-servations on a wide range of soils, largely occurringin intensively farmed areas in northern Europe, where

large-scale equipment is employed. The developmentof the approach is described in detail inJones et al.(2003). The assessment procedure comprises of twostages:

1. Assessment of thesusceptibilityof the subsoil tocompaction based on soil texture and its packingdensity.

2. Combining soil susceptibility with moisture statusand topsoil condition data at the time of traffick-ing, to convert susceptibility to compaction into avulnerabilityclass.

2.1. Susceptibility classification

The soil texture classesused in the susceptibil-ity assessment are based on those within the FAO–UNESCO classification system identified inFig. 1.

Subsoilpacking densityis estimated usingEq. (1),from a knowledge of the actual bulk density and theclay percentage

PD = Db + 0.009C (1)

where PD is the packing density in Mg m−3, Db thedry bulk density in Mg m−3 and C the clay content(wt.%).

The susceptibility classes for different soils andpacking state conditions are indicated inTable 1. Nosoil structure item is included directly in this suscepti-bility classification, since in practice subsoil structureand its stability are often closely related to texture andpacking density. The susceptibility classes inTable 1have been assigned on the basis of the following:

1. that subsoil structure within the coarse, mediumand medium fine texture classes is weak in terms ofits potential resistance to subsoil compaction, and

2. strong and coarse structural units are frequentlyfound in the fine and very fine texture classes, theseplaying an important role in resistance to com-paction.

Where deviations from these structure conditionsoccur, due allowance will need to be made throughsusceptibility class adjustment.

2.2. Vulnerability classification

Table 2 classifies the vulnerability of subsoils tocompaction on the basis of soil susceptibility, soil

G. Spoor et al. / Soil & Tillage Research 73 (2003) 175–182 177

Fig. 1. Soil texture classes.

Table 1Susceptibility to compaction according to texture and packingdensitya

Texture class Packing density (Mg m−3)

Low(<1.40)

Medium(1.40–1.75)

High(>1.75)

Coarse VH H Mb

Medium (<18% clay) VH H MMedium (>18% clay) H M LMedium fine (<18% clay) VH H MMedium fine (>18% clay) H M Lc

Fine Md Le Lc

Very fine Md Le Lc

Organic VH H

a Susceptibility classes: L, low; M, moderate; H, high; VH,very high.

b Except for naturally compacted or cemented coarse (sandy)materials that have very low (L) susceptibility.

c These soils are already compact.d These packing densities are usually found only in recent

alluvial soils with bulk densities of 0.8–1.0 Mg m−3 or in topsoilswith >5% organic carbon.

e Fluvisols in these categories have moderate susceptibility.

wetness and subsoil protection from above. The vul-nerability class provides an indication of the degree ofrisk that compaction may occur.

Situations identified as having “significant subsoilprotection” are those where all loads are applied atthe soil surface in the presence of a stronger pan layerat depth and a strong, firm topsoil layer. The more

Table 2Vulnerability to compaction according to soil susceptibility andwetnessa

Susceptibility class Wetness condition

Wet Moist Dry Very dry

VH Eb (E)c E (E) V (E) V (V)H V (E) V (E) M (V) M (M)M V (E) M (V) N (M) N (N)L M (V) N (M) N (N) N (N)

a Classes of vulnerability to compaction: N, not particularlyvulnerable; M, moderately vulnerable; V, very vulnerable; E, ex-tremely vulnerable.

b Classes outside brackets refer to situations with significantsubsoil protection.

c Classes within brackets refer to situations with minimal sub-soil protection.


vulnerable situations, “minimal subsoil protection”,are those where tractors operate in the furrow bottomduring ploughing operations and where surface loadsare applied under loose, weak topsoil conditions. Inthe absence of a stronger pan layer together with a veryloose wet top soil, situations where the soil tends toflow on loading, the vulnerability rating may have to beincreased further. The risks of topsoil structure damagecan also be considerable in this wet topsoil situation.

The soil moisture status inTable 2is described interms of the degree of wetness/dryness prevailing inthe subsoil. Soils at moisture contents close to fieldcapacity correspond approximately to the ‘Wet’ classand those approaching permanent wilting point the‘Dry’ class.

There are some medium fine to very fine texturedlow density, weakly structured subsoils with very lim-ited macro-porosity. In these situations, a small reduc-tion in this porosity would have a very significant ad-verse effect on their physical properties. In such cases,whilst their vulnerability to compaction is unlikely tochange, their sensitivity to the effects of compaction isgreater than soils with greater macro-porosity. In suchsituations, working on a higher vulnerability ratingwould provide a greater margin of safety against dam-age at high moisture contents. In converse situations,such as in dense, strong, coarsely structured subsoils,it may be possible to reduce the vulnerability rating.

These classifications are intended for guidanceonly and adjustments can be made to take account oflocal factors and management aspects as suggested. Itshould be noted that changes have been made to theinitial susceptibility classification and wetness classesas developed byJones et al. (2003), which werebased strictly on the FAO/UNESCO textural classesand climatic wetness considerations. A further textu-ral division has been introduced within the mediumand medium fine texture classes at 18% clay content,to take account of different soil susceptibilities asobserved in the field.

3. Ground contact pressure requirements toavoid subsoil compaction on subsoils of differingvulnerability class

If subsoil compaction is to be avoided, the load-ing pressures applied to soils need to be adjusted

according to the compaction risk, as defined by thevulnerability class rating for that particular situation.The higher the vulnerability rating, the lower the pres-sures needed to avoid damage.

As when developing guidelines for subsoil vulner-ability to compaction, due to the absence of read-ily obtainable quantitative information on subsoilpre-consolidation stresses (stresses to which the soilhas previously been subjected and responded), cur-rent guidelines on recommended pressures have beenbased on more empirical field experiments and fieldexperience. In addition, whilst the ideal information toconsider would be the actual normal and shear stressdistribution within the tyre/soil contact area, againthis information is not readily available for many tyresover the required range of soil conditions. The follow-ing guidelines for tyres are, therefore, based upon tyreinflation pressures which are both readily measurableand adjustable. For tracks, the ground pressure calcu-lated from the vehicle mass and contact area generallysuggests a more optimistic situation than is actuallythe case. Thus, it is advisable as a guide when as-sessing subsoil compaction risk, to at least double thecontact pressure calculated. Experience has shownthat this allows for pressure/stress concentration underthe rollers and for uneven load distribution.

The basic principle adopted in developing the fol-lowing inflation pressure/soil vulnerability guidelinesis that the pressures applied to the subsoil shouldbe less than those applied in the past. As previouslyindicated, most soils in arable situations have beensubjected to the stresses applied by tractor wheelsworking within the open furrow during ploughingoperations. These stresses have been associated withtyre inflation pressures of the order of 150 kPa. Pro-viding, therefore, subsoils and any associated pans,if present, are currently in an acceptable state ofmacro-porosity and compactness, keeping pressuresbelow 150 kPa in the subsoil should avoid undesirablecompaction damage in the future.

The guideline maximum inflation pressures for agri-cultural radial ply tyres to provide appropriate groundpressures for each vulnerability class, are identified inTable 3.

The recommended pressures for the ‘extremely’ and‘very’ vulnerable classes have been derived from fieldexperiments investigating compaction of topsoils indifferent states of compactness and moisture status


Table 3Recommended maximum tyre inflation pressure according to vul-nerability class

Vulnerability class Recommended maximum

Groundpressurea

(kPa)

Tyre inflationpressureb

(kPa)

E (extremely vulnerable) 65 40V (very vulnerable) 100 80M (moderately vulnerable) 150 120N (not particularly vulnerable) 200 160

a The average vertical stress in the soil/tyre area, calculatedfrom the wheel load and the contact area on a smooth rigid surface.

b Estimated fromp = 1.25pi , wherep is the average groundpressure andpi the tyre inflation pressure. At a tyre inflationpressure of 40 kPa the effect of carcass stiffness considered andthe ground pressure is estimated to be 65 kPa.

(Tijink et al., 1995). The 50 kPa value relates to avoid-ing topsoil compaction during the crop growing sea-son when trafficking under weak moist conditions, andthe 100 kPa value when the soil has gained furtherstrength on drying with some settlement.

Where circumstances make it impossible to achievethese recommended inflation pressures, other fieldmeasures, discussed in detail inChamen et al. (2003),need to be taken to at least limit, if not avoid, subsoilcompaction.

4. Identification of potential subsoil pan andcompaction problems causing impedance

The importance of a strong pan or compact soillayer in helping to protect the deeper subsoil fromcompaction has previously been emphasised and pro-vided that such layers are not causing significantimpedance to root development, aeration or drainage,it is most advantageous to leave them undisturbed. Thedegree of impedance present in such layers cannot bereadily recognised from physical measurements suchas bulk density, soil strength or penetration resistance.Whilst these measurements may give some measureof compactness, they provide no indication of themost important property, macro-porosity, which itselfcan be very time consuming to quantify. More ap-propriate measures of impedance are soil rootability,

aeration status and/or drainability. Of the three, roota-bility is the most readily assessed and this assessmentcan best be achieved by visually examining the rootsystem of an established growing crop.

The optimum time to assess rootability and hencewhether impedance is a problem, is when the exist-ing crop should be growing actively without stress.At such a time, the root system should, if there areno problems, be extensive and deep within the soilprofile. Plant roots can be readily seen at this time,even by the untrained eye, and any rooting problemsreadily identified. As there is no absolute measure ofroot development, it is often advantageous for com-parative purposes, to examine cropped areas wherethe crop is growing well and where growth is poor.A comparison of root development in the two sit-uations will indicate whether differences in rootinghave occurred and particularly whether impedance ispresent in one situation but not in the other. Similarroot development to depth through any pan layer inboth, would indicate that impedance is unlikely to bethe cause of the growth differences.

Profile pits are the most practical way of assessingroot development and hence identifying impedance.These can be opened up using either a spade or hy-draulic excavator, the choice being dependent uponthe depth of the suspected problem. Cleaning up theside of the pit after opening to remove any smear,helps in identifying roots, their location and theirdistribution. Further useful information is obtainedby taking out undisturbed spadefuls of soil from theside of the pit in the suspected problem depth ranges.These undisturbed spadefuls when carefully brokenapart by hand, reveal the extent and nature of rootdevelopment, as well as identifying the presence ofmacro-pores and root channels which may be presentwithin the compacted areas. Some of the main visiblesymptoms of severe impedance are tap roots failingto grow downwards, the horizontal layering of roots,roots being squashed against the side of soil structuralunits and thickened rather than slender roots. Rootdensity within the firmer/more compact area is un-likely to be as high as in the soil above or below. Pro-viding, however, roots have been able to move throughthis area and are developing well in the subsoil below,with no signs of prolonged impeded drainage abovethe more compacted layer, then there will be no needfor any loosening or fissuring treatment.


As yet, there is no rapid practical substitute for thehuman eye to assess impedance in these situations.The relatively short time expended in gleaning thisinformation, is well rewarded within the decisionmaking process itself and beyond. A penetrometercan be of some assistance after profile pit examina-tion, in helping to identify the extent of the problemacross the field within the same soil type. It is onlyuseful, however, providing it has been ‘calibrated’against the prevailing soil and rooting condition.This can be achieved by probing in the profile pitarea and relating the penetration resistance valuesobtained to the actual rooting condition observed.Once this link has been established, penetrometerchecks can be made in other parts of the field. Careis, however, needed in the calibration, to ensure theprocedure is exactly the same as will be used for thefield checks. If the penetrometer is not always in-serted from the soil surface to the required depth, thedeeper readings often vary, due to changes in slidingresistance along the shaft of the penetrometer as itpenetrates.

5. Alleviation of soil impedance

Where a significant impedance problem has beenidentified, alleviation measures will be necessary andthe problem must be overcome without significantlyreducing the support capacity of the compacted layer.Whilst in some situations biological activity and natu-ral weathering alone may be sufficient to alleviate theproblem, in others, these processes may be too slowor inadequate. In these latter cases mechanical mea-sures will be necessary, but these may be limited inwhat they can achieve without making the soil evenmore susceptible to future compaction. The prime aim

Fig. 2. Tensile soil failure with subsurface blade.

of these mechanical measures must, therefore, be toimprove conditions with minimal loss of soil support,leaving the natural and biological processes to com-plete the remediation and stabilise the resulting soilcondition.

Subsoiling operations to alleviate soil compactionare frequently associated with considerable loosen-ing, soil rearrangement and loss of bearing capacity.Such a type of disturbance is most inappropriate forfuture subsoil protection from loading stresses. Theprime aim in compaction alleviation operations must,therefore, be the creation of fissures or cracks throughthe damaged zone to restore rooting and drainage, butwith minimum disturbance to the remaining bulk ofthe soil profile. This disturbance is in effect, “fissur-ing without loosening”, allowing the bearing capacityof the soil to be maintained. Such an aim can bestbe achieved by generating a tensile soil failure withinthe damaged area, where fissures are generated, leav-ing the soil mass between the fissures largely intact,unbroken and strong.

Tensile failure can be generated by lifting the soilmass with a subsurface blade and allowing it to flowover the blade so that soil bending occurs, the bend-ing action placing the soil in tension and creatingfissures (Fig. 2). Appropriate cultivation tools forinducing this type of failure (Fig. 3) are winged sub-soilers, subsurface sweeps and Paraplow type angledleg subsoilers (Spoor and Godwin, 1978). In the caseof the latter implement, the soil bends over the tineleg and adjustable flap. The height of soil lift overthe wing, leg or flap, controls the magnitude of thetensile forces induced and hence the degree of fissur-ing achieved. A very small lift may not induce anyfissuring, whereas a large lift can create considerablesoil loosening and rearrangement, as the soil bendsand falls over the wing or leg.


Fig. 3. Soil fissuring implements.

The degree of soil fissuring and the size of fissureproduced during the field operation is dependent notonly upon the lift height of the subsoiler wing orflap, but also on working depth and soil moistureconditions. For a given wing lift height, the greaterthe working depth the smaller the soil disruption, andthe fewer and narrower the fissures created. Simi-larly, with a given implement lift height at a constantworking depth, the closer the soil moisture condi-tion approaches a plastic state, the less fissuring thatwill result. Under plastic soil conditions with highinternal soil cohesion, no fissuring is likely to occur,particularly at greater working depths. The degreeof fissuring produced is, therefore, under the controlof the implement operator. Adjustments to the liftheight of the wing or flap and/or to the subsoilerworking depth will, over a wide range of moistureconditions, allow the desired soil disturbance to beachieved.

The uniformity of fissuring across the field is de-pendent upon tine spacing and this should be adjustedto ensure the complete soil mass in the problem area islifted. Tine spacings between 1.5 and 2.0 times work-ing depth, depending upon soil moisture conditions,are usually required to achieve this and such spac-ings will also leave a level soil surface. Working depthideally should be just below the problem area, unlessslightly deeper working is required to produce the de-sired type of soil disturbance.

Ideally, the fissuring operation should be carried outas late as possible in any sequence of field operations,preferably just before if not after seeding; this reducesthe risk of re-compaction from subsequent wheelings.The longer the time period available for the treatedprofile to stabilise before being subjected to furtherloading, the greater the regain in soil strength and themore permanent and successful the improvement islikely to be. It is important that a vigorous deep rooting


crop, for example, cereals, is established afterwards,to complete the stabilisation process.

6. Conclusions

Subsoil compaction needs to be avoided if at allpossible. This requires a sound knowledge of the com-paction characteristics of the soil in its most vulnera-ble state, so that appropriate machinery planning andrunning gear selection procedures can be followed.Until more rigorous quantitative information becomesavailable, the vulnerability classification and the pres-sure guidelines presented, provide a starting guide asto the degree of compaction risk likely to be present ina given field situation and the tyre pressures requiredto minimise or avoid subsoil damage.

As cultivation pans, when present, play a particu-larly important role in helping protect the subsoil fromcompaction, these pans or compacted layers shouldonly be disturbed if they are significantly impedingroot development, aeration or drainage. The soil pro-file inspection method described, carried out duringthe active crop growing season, has proved very satis-factory in practice in positively determining whetherthere is any need for a disturbance treatment.

Where soil disturbance is needed, the fissuring tech-nique suggested has proved successful in re-establish-ing macro-pore continuity between the uncompactedrootable soil above and the impeding layer below, withminimal loss of bearing capacity. Subsequent biolog-ical and weathering activity complete the remediationprocess.

References

Alakukku, L., Weisskopf, P., Chamen, W.C.T., Tijink, F.G.J.,Van der Linden, J.P., Pires, S., Sommer, C., Spoor, G.,2003. Prevention strategies for field traffic-induced subsoilcompaction: a review. Part 1. Machine/soil interactions. SoilTill. Res., 73, 145–160.

Chamen, W.C.T., Alakukku, L., Pires, S., Sommer, C., Spoor,G., Tijink, F.G.J., Weisskopf, P., 2003. Prevention strategiesfor field traffic-induced subsoil compaction: a review. Part 2.Equipment and field practices. Soil Till. Res. 73, 161–174.

Jones, R.J.A., Spoor, G., Thomasson, A.J., 2003. Vulnerability ofsubsoils in Europe to compaction: a preliminary analysis. SoilTill. Res. 73, 131–143.

Spoor, G., Godwin, R.J., 1978. An experimental investigation intothe deep loosening of soil by rigid tines. J. Agric. Engng. Res.23, 243–258.

Tijink, F.G.J., Doll, H., Vermeulen, G.D., 1995. Technical andeconomic feasibility of low ground pressure running gear. SoilTill. Res. 35, 99–110.

subsoil compaction: risk, avoidance, identification and alleviation

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