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 properties

    G . 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 a

    aINRA, Unite d'Agronomie de Laon-Peronne, 02007 Laon Cedex, and bINRA, Unite de Science du Sol SESCPF, Centre de

    Recherche d'Orleans, BP 20619, 45166 Olivet Cedex, France


    Tillage and trafc modify soil porosity and pore size distribution, leading to changes in the unsaturated

    hydraulic properties of the tilled layer. These changes are still difcult to characterize. We have

    investigated the effect of compaction on the change in the soil porosity and its consequences for water

    retention and hydraulic conductivity. A freshly tilled layer and a soil layer compacted by wheel tracks

    were created in a silty soil to obtain contrasting bulk densities (1.17 and 1.63 g cm3, respectively). Soil

    porosity was analysed by mercury porosimetry, and scanning electron microscopy was used to distinguish

    between the textural pore space and the structural pore space. The laboratory method of Wind (direct

    evaporation) 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 relation

    between the hydraulic conductivity and the water ratio (the volume of water per unit volume of solid

    phase) 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 the

    matrix. These relict structural pores could be the reason for the change in the hydraulic properties due to

    compaction. They can be used as an indicator of the consequences of compaction on unsaturated

    hydraulic properties. The modication of the pore geometry during compaction results not only from a

    decrease in the volume of structural pores but also from a change in the relation between the textural

    pores and the remaining structural pores.

    Effet du compactage sur la porosite d'un sol limoneux: consequences sur les proprieteshydrauliques en non sature


    Les modications de porosite et de distribution de taille de pores liees aux operations de travail du sol et a

    la circulation des engins agricoles entranent des changements de proprietes hydrodynamiques des sols

    cultives qui restent difciles a prevoir. L'objectif du travail entrepris etait donc d'analyser l'effet du

    compactage severe d'un sol de limon sur l'espace poral de la couche labouree et ses consequences sur les

    courbes de retention d'eau et de conductivite hydraulique. Deux etats structuraux contrastes de la couche

    labouree ont ete crees par un passage de rotobeche (traitement fragmentaire, masse volumique de 1.17 g

    cm3) ou par des passages d'un tracteur de 81.4 kN roue dans roue (traitement compacte, masse

    volumique de 1.63 g cm3). La porosite du sol a ete analysee a l'aide de la porosimetrie au mercure et de

    la 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. Le

    traitement compacte retient moins d'eau que le traitement fragmentaire pour des potentiels hydriques

    inferieurs a 20 kPa. Par contre, la relation entre la conductivite hydraulique et l'indice d'eau n'a pas ete

    affectee par l'etat structural de la couche labouree. L'analyse de la porosite a montre que l'espace poral

    textural n'a pas ete modie par le compactage. Par contre, il a ete mis en evidence dans le traitement

    compacte des pores structuraux reliques, c'est-a-dire des pores structuraux qui ne sont accessibles que par

    Correspondence: G. Richard. E-mail: richard@laon.inra.fr

    Received 4 January 2000; revised version accepted 14 August 2000

    European Journal of Soil Science, March 2001, 52, 4958

    # 2001 Blackwell Science Ltd 49

  • des pores texturaux. La presence de pores reliques explique les differences de proprietes

    hydrodynamiques entre les deux etat structuraux crees. Elle pourrait constituer un indicateur de l'effet

    du compactage sur les proprietes hydrodynamiques d'un sol cultive. La modication de l'espace poral

    liee au compactage concerne donc non seulement la diminution de l'espace poral structural, mais aussi

    les relations entre les pores texturaux et structuraux.


    Compaction, which is what happens when unsaturated soils are

    compressed, affects porosity and related physical properties

    such as mechanical properties, and gas and water transport. For

    most soils, compaction reduces the volume of large pores and

    consequently affects water retention properties and hydraulic

    conductivity in the range of high water potentials. The smallest

    pores affected by compaction are related to soil constitution

    (particle size distribution, organic carbon content and clay

    mineralogy), water content prior to compression and mechan-

    ical pressure applied. Kooistra (1994) studied the porosity of

    tilled sandy-loam soils by optical microscopy of thin sections

    and distinguished macropores (mean diameter > 100m) and

    smaller pores which were considered as micropores. She

    showed that the total porosity was often less decreased by

    compaction than the macroporosity because the microporosity

    would have increased. An approach using porosity analysis

    that takes pore origin into account instead of just pore size was

    proposed by Monnier et al. (1973). They considered pores

    within the soil to be of two types: (i) structural pores which

    result from tillage, trafc, weather and biological activity, and

    thus are affected by compaction, and (ii) textural pores which

    result from the arrangement of the elementary soil particles.

    Textural pores consist of lacunar pores that result from the

    packing of the sand and silt particles with the clay phase and in

    clay pores that result from the packing of clay particles within

    the clay phase. According to that distinction, textural pores are

    unaffected by compaction in the range of mechanical pressure

    applied on the soil surface by trafc. The stability of the

    textural porosity for compaction has been shown for silty,

    clayey-silt and silty-clay soils over large ranges of water

    content and eld compaction conditions by, for example,

    Stengel (1979), Grimaldi (1981), and Guerif (1985).

    Nevertheless, variation of the textural porosity during

    compaction has been recorded for other soils. Coulon &

    Bruand (1989) studied the porosity of a sandy soil by

    combining optical microscopy and mercury porosimetry and

    found that textural porosity was decreased by compaction. In

    contrast, Bruand & Cousin (1995) observed an increase in the

    textural porosity after compacting cores of calibrated loamy-

    clay aggregates in wet conditions in the laboratory. They

    showed that this increase resulted from the formation of relict

    structural pores, i.e. remnants of structural pores that are

    accessible only via the necks of textural pores. Later, Bruand

    et al. (1997) found these relict structural pores in the eld in

    the wheel trafc areas. Consequently, these different studies

    show that lacunar pores and structural pores would interact to

    determine the hydraulic properties of the soil.

    Compared with these numerous studies on the porosity

    dynamics for compaction there are few experimental data on

    the effect of compaction on hydraulic properties. Moreover

    most data concern the water retention properties alone.

    Reicosky et al. (1981) measured the water retention properties

    of cores of compacted loamy aggregates and showed that the

    quantity of water retained varied in proportion to the bulk

    density. Allmaras et al. (1982) did similar experiments on a

    ferralitic clayey soil and also recorded variation of the water

    retention properties with the bulk density. Hill & Sumner

    (1967) expressed water content on a soil dry mass basis and

    discussed the water retention properties. They interpreted

    differences according to the pore size distribution with respect

    to the soil texture mainly. Tamari (1994) studied the water

    retention properties and unsaturated hydraulic conductivity of

    cores of silty-loam aggregates. After compaction he recorded

    an increase in the water retention and unsaturated hydraulic

    conductivity between 20 and 100 kPa, with water content

    expressed on a soil dry mass basis. Tamari (1994) suggested

    that compaction would cause a narrowing in the size of pore

    necks within aggregates without any change of the aggregate


    There is a lack of study combining geometrical analysis of

    the pore geometry during compaction and related hydraulic

    properties. Our aim was to analyse how compaction under eld

    conditions affects both geometrical characteristics of pores and

    hydraulic properties. For that purpose we selected a silty soil in

    two contrasting structural states and we studied (i) the pore

    geometry by using mercury porosimetry and chord size

    distribution on pictures recorded by scanning electron micro-

    scopy, and (ii) the water retention properties and unsaturated

    hydraulic properties using Wind's (1968) evaporation method.

    Materials and methods

    Field experiment