Effect of compaction on the porosity of a silty soil: influence on unsaturated hydraulic properties

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Effect of compaction on the porosity of a silty soil:inuence on unsaturated hydraulic propertiesG . R I C H A R D a , I . C O U S I N b , J . F . S I L L O N a , A . B R U A N D b & J . G U E R I F aaINRA, Unite d'Agronomie de Laon-Peronne, 02007 Laon Cedex, and bINRA, Unite de Science du Sol SESCPF, Centre deRecherche d'Orleans, BP 20619, 45166 Olivet Cedex, FranceSummaryTillage and trafc modify soil porosity and pore size distribution, leading to changes in the unsaturatedhydraulic properties of the tilled layer. These changes are still difcult to characterize. We haveinvestigated the effect of compaction on the change in the soil porosity and its consequences for waterretention and hydraulic conductivity. A freshly tilled layer and a soil layer compacted by wheel trackswere created in a silty soil to obtain contrasting bulk densities (1.17 and 1.63 g cm3, respectively). Soilporosity was analysed by mercury porosimetry, and scanning electron microscopy was used to distinguishbetween the textural pore space and the structural pore space. The laboratory method of Wind (directevaporation) was used to measure the hydraulic properties in the tensiometric range. For water potentials< 20 kPa, the compacted layer retained more water than did the uncompacted layer, but the relationbetween the hydraulic conductivity and the water ratio (the volume of water per unit volume of solidphase) was not affected by the change in bulk density. Compaction did not affect the textural porosity(i.e. matrix porosity), but it created relict structural pores accessible only through the micropores of thematrix. These relict structural pores could be the reason for the change in the hydraulic properties due tocompaction. They can be used as an indicator of the consequences of compaction on unsaturatedhydraulic properties. The modication of the pore geometry during compaction results not only from adecrease in the volume of structural pores but also from a change in the relation between the texturalpores and the remaining structural pores.Effet du compactage sur la porosite d'un sol limoneux: consequences sur les proprieteshydrauliques en non satureResumeLes modications de porosite et de distribution de taille de pores liees aux operations de travail du sol et ala circulation des engins agricoles entranent des changements de proprietes hydrodynamiques des solscultives qui restent difciles a prevoir. L'objectif du travail entrepris etait donc d'analyser l'effet ducompactage severe d'un sol de limon sur l'espace poral de la couche labouree et ses consequences sur lescourbes de retention d'eau et de conductivite hydraulique. Deux etats structuraux contrastes de la couchelabouree ont ete crees par un passage de rotobeche (traitement fragmentaire, masse volumique de 1.17 gcm3) ou par des passages d'un tracteur de 81.4 kN roue dans roue (traitement compacte, massevolumique de 1.63 g cm3). La porosite du sol a ete analysee a l'aide de la porosimetrie au mercure et dela microscopie electronique a balayage, en caracterisant les volumes de pores texturaux et structuraux.Les proprietes hydrodynamiques ont ete estimees entre 80 et 5 kPa a l'aide de la methode de Wind. Letraitement compacte retient moins d'eau que le traitement fragmentaire pour des potentiels hydriquesinferieurs a 20 kPa. Par contre, la relation entre la conductivite hydraulique et l'indice d'eau n'a pas eteaffectee par l'etat structural de la couche labouree. L'analyse de la porosite a montre que l'espace poraltextural n'a pas ete modie par le compactage. Par contre, il a ete mis en evidence dans le traitementcompacte des pores structuraux reliques, c'est-a-dire des pores structuraux qui ne sont accessibles que parCorrespondence: G. Richard. E-mail: richard@laon.inra.frReceived 4 January 2000; revised version accepted 14 August 2000European Journal of Soil Science, March 2001, 52, 4958# 2001 Blackwell Science Ltd 49des pores texturaux. La presence de pores reliques explique les differences de proprieteshydrodynamiques entre les deux etat structuraux crees. Elle pourrait constituer un indicateur de l'effetdu compactage sur les proprietes hydrodynamiques d'un sol cultive. La modication de l'espace poralliee au compactage concerne donc non seulement la diminution de l'espace poral structural, mais aussiles relations entre les pores texturaux et structuraux.IntroductionCompaction, which is what happens when unsaturated soils arecompressed, affects porosity and related physical propertiessuch as mechanical properties, and gas and water transport. Formost soils, compaction reduces the volume of large pores andconsequently affects water retention properties and hydraulicconductivity in the range of high water potentials. The smallestpores affected by compaction are related to soil constitution(particle size distribution, organic carbon content and claymineralogy), water content prior to compression and mechan-ical pressure applied. Kooistra (1994) studied the porosity oftilled sandy-loam soils by optical microscopy of thin sectionsand distinguished macropores (mean diameter > 100m) andsmaller pores which were considered as micropores. Sheshowed that the total porosity was often less decreased bycompaction than the macroporosity because the microporositywould have increased. An approach using porosity analysisthat takes pore origin into account instead of just pore size wasproposed by Monnier et al. (1973). They considered poreswithin the soil to be of two types: (i) structural pores whichresult from tillage, trafc, weather and biological activity, andthus are affected by compaction, and (ii) textural pores whichresult from the arrangement of the elementary soil particles.Textural pores consist of lacunar pores that result from thepacking of the sand and silt particles with the clay phase and inclay pores that result from the packing of clay particles withinthe clay phase. According to that distinction, textural pores areunaffected by compaction in the range of mechanical pressureapplied on the soil surface by trafc. The stability of thetextural porosity for compaction has been shown for silty,clayey-silt and silty-clay soils over large ranges of watercontent and eld compaction conditions by, for example,Stengel (1979), Grimaldi (1981), and Guerif (1985).Nevertheless, variation of the textural porosity duringcompaction has been recorded for other soils. Coulon &Bruand (1989) studied the porosity of a sandy soil bycombining optical microscopy and mercury porosimetry andfound that textural porosity was decreased by compaction. Incontrast, Bruand & Cousin (1995) observed an increase in thetextural porosity after compacting cores of calibrated loamy-clay aggregates in wet conditions in the laboratory. Theyshowed that this increase resulted from the formation of relictstructural pores, i.e. remnants of structural pores that areaccessible only via the necks of textural pores. Later, Bruandet al. (1997) found these relict structural pores in the eld inthe wheel trafc areas. Consequently, these different studiesshow that lacunar pores and structural pores would interact todetermine the hydraulic properties of the soil.Compared with these numerous studies on the porositydynamics for compaction there are few experimental data onthe effect of compaction on hydraulic properties. Moreovermost data concern the water retention properties alone.Reicosky et al. (1981) measured the water retention propertiesof cores of compacted loamy aggregates and showed that thequantity of water retained varied in proportion to the bulkdensity. Allmaras et al. (1982) did similar experiments on aferralitic clayey soil and also recorded variation of the waterretention properties with the bulk density. Hill & Sumner(1967) expressed water content on a soil dry mass basis anddiscussed the water retention properties. They interpreteddifferences according to the pore size distribution with respectto the soil texture mainly. Tamari (1994) studied the waterretention properties and unsaturated hydraulic conductivity ofcores of silty-loam aggregates. After compaction he recordedan increase in the water retention and unsaturated hydraulicconductivity between 20 and 100 kPa, with water contentexpressed on a soil dry mass basis. Tamari (1994) suggestedthat compaction would cause a narrowing in the size of porenecks within aggregates without any change of the aggregateporosity.There is a lack of study combining geometrical analysis ofthe pore geometry during compaction and related hydraulicproperties. Our aim was to analyse how compaction under eldconditions affects both geometrical characteristics of pores andhydraulic properties. For that purpose we selected a silty soil intwo contrasting structural states and we studied (i) the poregeometry by using mercury porosimetry and chord sizedistribution on pictures recorded by scanning electron micro-scopy, and (ii) the water retention properties and unsaturatedhydraulic properties using Wind's (1968) evaporation method.Materials and methodsField experimentWe did the eld experiment from September 1994 to June1995 at Mons-en-Chaussee (Somme, Northern France) (Sillon,1999). The soil is an Orthic Luvisol (FAOUNESCO, 1975)developed on loess. The experiment was done after a wheatcrop, and the straw was burned after harvesting. Two plots,8 m wide and 20 m long, were tilled in September with arotovator (tillage depth = 0.3 m) to obtain a ne soil structure.A fully compacted tilled layer was produced in one of the two50 G. Richard et al.# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 4958plots in March by mechanical compaction under wetconditions (water potential around 5 kPa) using a heavytractor (81.4 kN with tyres 65 cm width inated at 200 kPa).The tractor was driven across the plot to cover the whole soilsurface. An uncompacted tilled layer with a loose structure andlarge porosity was produced in the other plot by digging thesoil (tillage depth = 0.3 m) in early April (the zones under thewheel tracks were not considered in this study). Table 1 givesthe physical properties of the ploughed layer of each plot.Bulk density measurementWe measured the bulk density at 5, 10, 15, 20, 25 and 30 cmdepth using a gamma probe (Bertuzzi et al., 1987) at sixlocations in each plot. The porosity was expressed using thevoid ratio to give changes in porosity for the same mass of soilwhatever the bulk density. The total void ratio (e, the porevolume per unit volume of solid phase) was calculated bye = (p /) 1, (1)where is the bulk density and p the particle density. Theparticle density was measured using a pycnometer on fourreplicates per plot (Table 1). The textural void ratio, et, wasmeasured on sieved aggregates between 2 and 3.15 mm indiameter using the kerosene method, which is based upon theArchimedes' principle (Monnier et al., 1973). These aggre-gates were produced by hand and sieving and were smallenough to allow measurement of the textural void ratiobecause they contained only textural pores. The structural voidratio, es, was calculated from the total void ratio and thetextural void ratio byes = e et . (2)The thickness of the ploughed layer was estimated bymeasuring the distance between the bottom of the ploughedlayer and the soil surface every 5 cm over two 5 m-wide soilproles. The aggregate size distribution was assessed withinthe uncompacted ploughed layer by sieving wet (water contentof 0.2 g g1) soil samples of 14 dm3 (six replications).Hydraulic propertiesWe determined the water content and the hydraulic con-ductivity in the laboratory at various water potentials, , forwater potentials varying between 5 and 80 kPa by usingWind's (1968) evaporation method.The water content was expressed as the water ratio (#,volume of water per unit volume of solid phase) by# = ( /w) w , (3)in which w is the gravimetric water content (mass of water perunit mass of oven-dried soil), and w is the density of waterassumed equal to 1 g cm3. Thus as for e, # gives the watercontent for a constant mass of oven-dried soil, whatever thebulk density. The samples were undisturbed soil cores (15 cmdiameter and 7 cm high) from the 522 cm layer. We collectedve replicates from the compacted layer and four from theuncompacted layer. The cores were saturated using the methodof Richard et al. (2001) to reproduce the eld rewettingconditions as accurately as possible. The saturated cores werethen subjected to evaporation, and the water potentials wererecorded. In such situations, Richard et al. (2001) showed thatthe laboratory measurements of the hydraulic properties are ingood agreement with the ones obtained from eld experiments.For a soil core, the Wind method gives a tted waterretention curve, from which we calculated a set of (, #) pairsof values, and a set of (#, K) and (, K) pairs of values, whereK is the unsaturated hydraulic conductivity. The mathematicalprocedures used to t (#) and to calculate K from Darcy'slaw are described in Morath et al. (1997). For each treatment,we calculated the mean water ratio # for every increment of5 kPa for , the mean hydraulic conductivity K for everyincrement of 2 kPa for , or for every increment of 0.005 m3m3 for #.Mercury porosimetryWe investigated the pore size distribution by mercuryporosimetry. The mercury, as a non-wetting liquid, was forcedinto a dry porous sample under pressure, and the relationshipbetween the equivalent pore diameter (D, inm) and theapplied pressure (P, in kPa) was obtained from the JurinLaplace law:Table 1 Physical properties of the compacted and uncompacted tilled layersParticle size distributionThickness Clay (< 2m) Silt (250m) Sand (502000m) Organic C CaCO3 Particle density/cm ______________________________________________________ /g kg1 _____________________________________________________ /g cm3Uncompacted 34 141 807 52 10 5 2.69Compacted 26 155 794 51 10 5 2.68Soil compaction, pore geometry and hydraulic properties 51# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 4958D = 4(cos) /P , (4)where is the surface tension of mercury, and is the contactangle between soil and mercury. The value for was0.484 N m1, and was 130 (Fies, 1984). We operated atpressures P from 4 to 2000 kPa to obtain the pore sizedistribution from 360 to 0.006m. Three clods (35 cm3 involume) from each layer were studied. They were oven-driedat 105C for 24 h prior to measurements.Scanning electron microscopyWe obtained polished thin sections of samples from thecompacted and uncompacted layers by the method of Bruandet al. (1996). The thin sections were examined in a scanningelectron microscope using backscattered electron emission.The backscattered electron scanning images (BESI) wererecorded as grey-level images at 3 20 and 3 200 magnica-tion to examine the structural pores and textural pores.We quantied the porosity on binary images, obtained bythresholding the grey-level BESI, at 3 20 for 7 BESI and at3 200 for 5 BESI. The porosity was expressed as thesurface:void ratio, which is the surface area of void per totalsurface area. The 3 20 BESI were used to calculate thestructural void ratio, and the 3 200 BESI to calculate thetextural void ratio. We also delineated the silt and sandparticles on one 3 200 BESI per type of layer. We drew binaryimages with only the silt and sand fraction as solid phase andwithout the clay fraction. These images were used to study thechange in the skeleton packing before and after compaction.We calculated the chord distributions in the skeleton phase andthe star volume, corresponding to the rst moment of the chorddistribution (Cousin et al., 1996). The star volume charac-terizes the spacing of particles, since it gives the distribution ofthe mean distance between the particles.ResultsHydraulic propertiesThe compacted layer retained less water than the uncompactedlayer at water potentials greater than 20 kPa (Figure 1), whileit was the opposite for a water potential less than 20 kPa. Thedifference in water ratio between the compacted and theuncompacted layer was 0.05 at a water potential of 5 kPaand 0.05 at a water potential of 80 kPa.The hydraulic conductivity was greater in the compactedlayer than in the uncompacted layer for water potentialssmaller than 15 kPa (Figure 2a). The ratio between thehydraulic conductivity of the compacted layer and that of theuncompacted layer was 2.1 at 80 kPa. The relations betweenthe hydraulic conductivity and the water ratio for thecompacted and uncompacted layers were similar (Figure 2b).Structure and porosityThe structure of the compacted ploughed layer was massive,with very few visible pores, whereas the uncompactedploughed layer had a fragmented structure with distinctaggregates and clods. Small aggregates with a diameter< 0.2 cm accounted for 27% (in mass), clods with a diameter> 4 cm accounted for 8%.The bulk density changed little with depth in the compactedand the uncompacted layers (coefcients of variation smallerthan 5%), indicating little difference in porosity with depthwithin each type of layer. There was a great difference in totalvoid ratio between the compacted (0.64, corresponding to atotal porosity of 0.39 m3 m3) and uncompacted (1.30,corresponding to a total porosity of 0.57 m3 m3) layers(Table 2). This difference resulted mainly from a decrease instructural void ratio during compaction (Table 2).Mercury porosimetryThe cumulative mercury intrusion volumes for each treatmentare shown in Figure 3. We used the subhorizontal inexionpoints of the cumulative curves as proposed by Bruand & Prost(1987) to identify three classes of pores A, B and C (Table 3).The threshold diameter between pores A and B was 40m,and 0.05m between pores B and C. They were similar foreach treatment. These threshold values were used to calculatethe three corresponding void ratios (eA, eB and eC) for thecompacted and uncompacted clods by adding together theintruded pore volume for each class of pore (Table 3). Thetotal void ratio for the clods from the compacted layer was0.531 and 0.655 for the clods from the uncompacted layer.Void ratios eA and eB were smaller in the compacted clodsFigure 1 Water retention curve of the compacted (d) and uncom-pacted (s) layers (the water content is expressed by the water ratiowhich is the volume of water per unit volume of the solid phase).Horizontal bars indicate 6 standard errors.52 G. Richard et al.# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 4958than in the uncompacted ones. Compaction led to a decrease ineA of 69% and in eB of 12%.Figure 4(a) shows the pore volume histograms as a functionof the equivalent pore diameter, and for each range ofdiameters, the difference in intruded pore volume betweenclods from the compacted and uncompacted layers (Figure 4b).There was no difference for the pore diameter < 0.05m (eCvolume), while the pore volume was always greater in theuncompacted clods than in the compacted clods for porediameters > 40m (eA volume). The eB volume wassubdivided into three pore volumes. For equivalent porediameter from 0.05 to 1m, pore volumes were similar in thetwo types of clods. Pore volumes were greater in compactedclods than in uncompacted clods for pore diameters between 1and 4m, but they were smaller for pore diameters between 4and 40m.Backscattered electron scanning imagesFigure 5(a)(d) shows the structural pores within thecompacted and uncompacted layers. The mean structural poreratio was 0.125 (standard deviation = 0.076) in the uncom-pacted clods. The pores had complex, diverse shapes withvarying total curvatures and numerous biological features(Figure 5a,c). The mean structural pore ratio in the compactedclods was 0.032 (standard deviation = 0.026). Pores werealways circular or elongated, with a constant total curvaturesign (Figure 5b,d).Figure 5(e,f) shows the packing of silt particles with theclayey phase, which formed pores of 1050m diameter.These textural pores were lacunar, as designated by Fies &Bruand (1990). The textural pores resulting from the packingof the clay particles within the clayey phase could not bedistinguished on backscattered electron scanning imagesbecause they were too small (Attou et al., 1998). The lacunarpore ratio measured on the images without any structural poreswas 0.24 (standard deviation = 0.080) for the compacted clodsand 0.29 (standard deviation = 0.086) for the uncompactedclods. These two mean values were not statistically different(Student's test, 5% level).The chord distribution functions in the skeleton phase werecalculated for the two types of clod. The two distributions canbe superimposed (Figure 6), and the star volumes (16.36 3 106m2 for the compacted clods and 16.42 3 106 m2 for theuncompacted clods) were nearly equal.Figure 2 The logarithm of hydraulic conductivity measured in m s1plotted against the water potential (a) and the water ratio (b) in thecompacted (d) and uncompacted (s) layers. Vertical bars indicate6 standard errors.Table 2 Water ratio (volume of water per unit volume of the solid phase) and total, textural and structural void ratios calculated using themeasurements of eld bulk density. Standard errors in parenthesesWater ratio, # Bulk density /g cm3 Total void ratio, e Textural void ratio, et Structural void ratio, esUncompacted 0.62 (0.01) 1.17 (0.01) 1.30 (0.02) 0.69 (0.01) 0.61 (0.02)Compacted 0.57 (0.01) 1.63 (0.01) 0.64 (0.01) 0.62 (0.01) 0.02 (0.01)The water ratio was measured at the same time as the bulk density.Soil compaction, pore geometry and hydraulic properties 53# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 4958DiscussionEffect of compaction on pore morphologyThe overall difference between total pore volumes of clodsbefore and after compaction was 19% as measured by mercuryporosimetry. As proposed by Bruand & Prost (1987), the threevoid ratios eA, eB and eC identied from mercury porosimetrymeasurements can be attributed to the structural, lacunar andclay pores, respectively. Thus the 40m threshold is the limitbetween the structural and lacunar pores, and the 0.05mbetween the lacunar and clay pores. Table 3 shows similar claypore volume for the two types of layers and shows thatcompaction greatly reduces the structural pore volume (by69%) and has much less inuence on the lacunar pore volume(by 12%). Nevertheless, compaction did not affect the spacingbetween skeleton particles, as shown by the chord distributioncalculation (Figure 6). This means that the textural pores werenot affected by compaction. A more detailed analysis of themercury intrusion curve (Figure 4) shows that the volume oflacunar pores of 440m decreased, whereas the volume oflacunar pores with diameters of 14m increased. The BESIshowed that some of the pores cannot be easily classied aslacunar or structural pores (Figure 5c,d). The mercuryintrusion curves and BESI indicate that some of the poresthat were structural pores prior to compaction were accessibleonly through the textural pores after compaction. Suchstructural pores thus behave as textural pores even thoughthey may have looked like structural pores on thin sections.They were called relict structural pores by Bruand & Cousin(1995), and they were recorded under pressures of 50 and200 kPa at a water potential of 1 kPa, and under a pressure of600 kPa at a water potential of 63 kPa. The eld conditionswhich produced the compacted layer were within the range ofwater potentials and mechanical pressures described byBruand & Cousin (1995). The water potential was 5 kPaunder the tractor wheels, and the mean applied pressure at thesoil surface under a rear tyre of the tractor was assessed at 8090 kPa, since the load on each rear tyre was 27 kN and thecontact surface between the soil and the tyre was 0.30.35 m2.Inuence of compaction on hydraulic propertiesOutside the saturation range, compacted soils retain more waterthan loose soils, the water content being expressed on a soilvolume basis. Such a difference is due mainly to the decrease inthe soil volume during compaction because of reduction of theFigure 3 Cumulated mercury intrusionvolumes of samples collected in the com-pacted (d) and uncompacted (s) layers.Table 3 Pore volumes measured by mercury porosimetry on clods from each layer. Standard errors in parenthesesVoid ratio for pores diameter ranging fromTotal void ratio (e) 0.006 to 0.05m (eC) 0.05 to 40m (eB) 40 to 360m (eA)Uncompacted 0.655 (0.005) 0.030 (0.002) 0.531 (0.012) 0.094 (0.014)Compacted 0.531 (0.028) 0.035 (0.001) 0.467 (0.016) 0.029 (0.011)54 G. Richard et al.# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 4958structural porosity which retains very little water, as can be seenfrom the data of Reicosky et al. (1981) or Assouline et al.(1997). However, water content has to be expressed as a waterratio, as done by Hill & Sumner (1967), to analyse the change inwater retention due to variation in pore geometry.The differences in water retention shown on Figure 1 are theconsequences of a change in the pore size distribution forcompaction in a range of sizes corresponding to pores thatcontribute to the water retention. Our results showed thatcompaction affects the water retention properties for waterpotentials ranging between 5 and 80 kPa. There was adecrease in water retained between 5 and 20 kPa, whichcorresponds to a reduction of the volume of pores withequivalent diameter between 60 and 15m. Conversely, theincrease in water content between 20 and 80 kPa corre-sponds to an increase in the volume of pores with equivalentdiameter between 4 and 15m. Mercury porosimetry showed adecrease in pore volume with diameters larger than 4m and aincrease in pore volume for pore diameters ranging between 1and 4m. The description of the pore size distributionrecorded by mercury porosimetry cannot be used directly asa description of the pore geometry of the wet soil (Bruand &Prost, 1987). However, the decrease in the pore volumeretaining water between 5 and 20 kPa would correspond tothe decrease in pore volume larger than 4m as determined bymercury porosimetry. In the same way, the increase in porevolume retaining water at potentials ranging between 20 and80 kPa would correspond to the increase in pore volumebetween 1 and 4m as measured by mercury porosimetry. Thepore diameters calculated from water retention data andmercury porosimetry data are similar. The agreement betweenwater retention and mercury porosimetry data suggests that theincrease in water retained at potentials between 80 and20 kPa is due to the formation of relict structural pores withinthe textural network.The reduction in the pore space between aggregates and thedeformation of the aggregates at contact points that occurduring compaction of an aggregated soil increases the contactsurface area between the aggregates (Gupta et al., 1989).Consequently, there is a greater continuity between the poreslled with water in a compacted soil than in a loose one, asshown by De Cockborne et al. (1988) for nitrate diffusion.Thus the hydraulic conductivity would be increased bycompaction when water is mainly within the aggregates. InFigure 4 (a) Void ratio calculated by mercury porosimetry as a function of equivalent pore diameter for the compacted (j) and uncompacted(h) layers, and (b) differences in void ratio between the compacted and uncompacted layers as a function of pore diameter.Soil compaction, pore geometry and hydraulic properties 55# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 4958our case, for water potentials less than 15 kPa, the hydraulicconductivity of the compacted layer was greater than that ofthe uncompacted layer (Figure 2a). Moreover, the similarity ofthe relations between hydraulic conductivity and the waterratio in the compacted layer and in the uncompacted layer(Figure 2b) would be due to two opposite inuences ofcompaction on hydraulic conductivity, increasing the contactsurface between aggregates on one hand and decreasing theproportion of the water that contributes to water transfer on theother hand. Indeed, the water retained in the relict structuralpores of the compacted layer would not contribute to the watermovement because these pores are accessible only through thenecks of lacunar pores and behave as reservoirs. As aconsequence, a smaller proportion of the water contributes tothe water movement in the compacted layer than in theuncompacted layer.Figure 5 Backscattered electron scanning images of the two types of soil layers: (a) and (c), structural pores in the uncompacted layer at 3 20magnication; (b) and (d), structural pores in the compacted layer at 3 20 magnication; (e) and (f), lacunar pores at 3 200 magnication inthe uncompacted and compacted layers, respectively.56 G. Richard et al.# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 4958ConclusionWe examined a silty soil in two structural states to elucidatethe relation between the unsaturated hydraulic properties andthe change in pore geometry induced by compaction. Ourresults show that such an analysis can be improved bycombining measurements of the hydraulic properties and dataon the origin, morphology and accessibility of pores. They alsoshow that more importance should be attached to the unit usedto express the water content. Results showed that the relictstructural pores described by Bruand & Cousin (1995) for aclayey loam soil are present also in a silty soil with a differentmineralogical composition. These relict structural pores arestructural pores that have been distorted by compaction in theeld during trafc and that are accessible only through thenecks of the lacunar pores. The change in the soil hydraulicproperties caused by compaction would be related to theformation of such relict structural pores. Thus water retainedby soil would increase in a compacted soil because of thecontribution of a volume of relict structural pores to waterretention in a range of water potentials where lacunar porescontribute to water retention alone.Evidently the dynamics of the pore geometry duringcompaction cannot be considered as resulting from a decreasein the volume of structural pores alone but can also becharacterized by a change in the relation between the texturalpores and the remaining structural pores. According to the soilcomposition characteristics, a better knowledge of the range ofwater potential and mechanical pressure leading to theformation of relict structural pores should enable us to usethe volume of relict structural pores as an indicator of soilcompaction and of the consequences of compaction onhydraulic properties.ReferencesAllmaras, R.R., Ward, K., Douglas, C.L. & Ekin, L.G. 1982. Longterm cultivation effects on hydraulic properties of a walla silt loam.Soil and Tillage Research, 2, 265279.Assouline, S., Tavares-Filho, J. & Tessier, D. 1997. Effect ofcompaction on soil physical and hydraulic properties: experimentalresults and modelling. Soil Science Society of America Journal, 61,390398.Attou, F., Bruand, A. & Le Bissonnais, Y. 1998. Effect of clay contentand silt-clay fabric on stability of articial aggregates. EuropeanJournal of Soil Science, 49, 569577.Bertuzzi, P., Bruckler, L., Gabilly, Y. & Gaudu, J.C. 1987.Calibration, eld testing and error analysis of a gamma-ray probefor in situ measurements of dry bulk density. Soil Science, 144,425436.Bruand, A. & Prost, R. 1987. Effect of water content on the fabric of asoil material: an experimental approach. Journal of Soil Science,38, 461472.Bruand, A. & Cousin, I. 1995. Variation of textural porosity of aloamy-clay soil during compaction. 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