Soil Aggregate Stability

土壤团聚体的经典文章

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Journal of Sustainable Agriculture

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E. Amézketa

a

a

Unidad de Suelos y Riegos, Servicio de Investigación Agroalimentaria, Diputación Generalde Aragón, Apartado 727, 50080, Zaragoza, SpainAvailable online: 22 Oct 2008

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土壤团聚体的经典文章

SoilAggregateAReview

Stability:

E.Amézketa

ABSTRACT.soilandsustainabilitySoilaggregatestabilityisacrucialsoilpropertyaffectingsentedagentsofaggregateandcropformationproduction.andAaggregatebroadoutlinestabilizationoftheprocessesarepre-quantifyandreliableanddiscussedinterpret.inThethisaimreview.ofaggregateAggregatestabilitystabilityisdifficulttoofdeterminewater,descriptionwindandmanagement.andrankingofNumerousthebehaviormethodsofsoilstestshaveunderisbeenthetogiveusedeffectatoodologiesaggregatestabilitywithvaryingsuccess.Thedifferentmeth-iscomplicatethecomparisonamongaggregatestabilitydata.Itstabilityalsodifficulttoobtainaconsistentcorrelationbetweenaggregatecrustingandsurementpotential.otherimportantThispapersoilreviewspropertiesthedifferentsuchassoilmethodserodibilityofortionpreparationtotheofconditionssoilaggregateofsamplestabilitycollectionusedintheinliterature,payingatten-mea-frameworksuggested.includingandtreatmentsthemostinthethefieldandsampleinterestinglaboratory.aspectsAunifiedmethodologicaltimationfeeofThesoilerodibilitypossibilityisofalsousingdiscussed.aggregate[Articlestabilityofexistingcopiesdatamethodsavailableasanfores-isa

address:fromgetinfo@]

TheHaworthDocumentDeliveryService:1-800-342-9678.E-mailKEYWORDS.gregatecro-aggregatestability,Aggregateformation,aggregatestabilization,soilag-stabilitycrusttests,formation,soilerodibility

macro-aggregatestabilitytests,mi-taria,E.AddressDiputaciónAmézketa,UnidadGeneraldedeSuelosAragón,yRiegos,ApartadoServicio727,deInvestigaciónAgroalimen-commentsTheauthorcorrespondencewishestothanktotheDr.authorR.Aragüésat:E-mail:50080espe@

Zaragoza,Spain.ingEnglish.

forimprovementsofthemanuscript.HeforalsoverythankshelpfulL.Padillasuggestionsforcorrect-andJournalofSustainableAgriculture,Vol.14(2/3)1999E1999byTheHaworthPress,Inc.Allrightsreserved.

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土壤团聚体的经典文章

84JOURNALOFSUSTAINABLEAGRICULTURE

INTRODUCTION

ofSoilsourceshumankind.isanaturalMaintainingresourcethatsustainablemustbesoilsustainablyresourcesmanagedorpreservingforthesoilfutureinforlong-termisofprimarynecessity.Thisideaistakenintoaccountre-receivingtheconceptthespecialof‘‘Sustainableattentionagriculture,’’whichcurrentlyisaglobalissue,teeagriculturalcommunity.inAccordingscientificcomunity,toTheTechnicalpolicymakers,andevenintainabletothetoagricultureConsultativeshouldGrouponInternationalAgriculturalAdvisoryResearch,Commit-‘‘sus-ofsatisfychanginghumanneedsinvolvewhilethesuccessfulmaintainingmanagementorofresourcesIntheenvironmentandconservingnaturalresources’’(LalenhancingandPierce,thequality1991).agementthiscontext,soilstructureisakeywordinthefieldofsoilresourceman-ablepointstandardforsustainingofenvironmentalagriculturalquality.productivitySoilwhilemaintaininganaccept-1988).Soilintothecomplexproblemofsustainablestructureagriculture.

isoneimportantentryandSoilstructurecanbedefinedintermsofformandstability(Kayetal.,structurevoidstressesisspacestructuralitsabilitythatexistsformreferstotheheterogeneousarrangementofsolidtoatagiventime,whereasthestabilityofasoil’ssoil(AngersandCarter,retain1996).thisarrangementGoodsoilwhenexposedtodifferentenvironmentalcharacteristicforsustainingagriculturalproductivitystructureisandamostforpreservingdesirablestabilityandcrops.storageofthequality.ofaggregatesItdependsonthepresenceofstableaggregates.Thewater,aeration,andtheerosion,poresbetweenthemaffectsthemovementgeochemicalThus,aggregatestabilityinfluencesbiologicalawiderangeactivityandthegrowthoftainingity,highprocessesinthenaturalandagriculturalenvironments.ofphysicalandMain-bio-pollutionminimizingsoilaggregatederivedsoilaggregatefromerosionstabilitysoildegradationanddegradationisessentialforpreservingsoilproductiv-aswell.andThus,minimizingmaintainingenvironmentalhighsoilsustainablestabilityisarequisiteforthesustainableuseofsoilandfortheasquality.oneoftheagriculture.soilphysicalArshadandCoen(1992)proposedaggregatestabilitydardizationHortensiusaggregateofsoilqualityandWellingpropertiesmeasurements.(1996)thatincludecanserveasanindicatorofsoilInaddition,itinthemeasurementsinternationalofstan-sivestabilitycouldalsoestimateothersoilproperties,suchastheero-soiltime,andworkcrustingandeconomicpotentialofthesoils,whichbythemselvesrequiremuchtrowManymethodsanddifferentproducemayMiller,methodssupport.

existformeasuringsoilaggregatestability(Jas-beexplained1991).The(1)reasonfortheexistenceofsomanydifferentdetermined,destabilization,and(3)bymethodological(2)bybythethedifferentexistencereasons.scalesofdifferentmechanismsthatBasically,atwhichthestabilitymaindestabi-

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lizinghascro-aggregatesbeenmechanismssuggestedinthatthedisintegrationsoilareslaking,ofclaythesoildispersionmacro-aggregatesandclayswelling.Itsion,isthefirststepinthelossofsoilstructure,whileintomi-1987;atime-dependentchemicalprocess,isthesecondstep(Abu-Shararclaydisper-etal.,1989,Quirkgatescalesstability1990;andSoMurray,1991;Shainbergetal.,1992a;LeBissonnais,1988,canandbeAylmore,performed1993).atThedeterminationoffieldsoilaggre-surementtheencesatmethodologythesamescale,isdifferent.differenttheremayEvenbeimportantamongscalesmethods(smallorlarge).Atthesedifferences.ofstabilityThesediffer-mea-tionsmayberelatedtooneormoreofthefollowingaspects:(1)thethetreatmenttest,ofparticularlysamplecollection,(2)thecharacteristicsofthesamplesubjectedcondi-toting,differenttimeappliedofshakingtoitsthestructure,aggregatesize,andmoisturecontent;(3)thethesample,sample,suchasthewettingtechnique,rateofwet-disaggregationdegreesofdisruptiveenergyraindropapplied;impact(4)energytheetc.,whichpresenttheetertreatment,andand(5)dispersion,theexpressionwhichofistheoftenresult,notthatclearlymeasurementis,thedistinguishedofstabilityparam-fromthecomplicatesTheused.

calculationliterature.thecomparisonandexpressionamongofaggregateaggregatestabilitystabilityindifferentwaysaggregateItalsomakesdifficulttoobtainaconsistentdatacorrelationpresentedinthetheresionsareapparentstabilityandsoilerodibilityorcrustingpotential.Inthisbetweensense,datamineareaboutmadethewithoutcorrelationcontradictionsintheliterature,duetothefactthatconclu-consideringbetweentheaggregatedifferentstabilitymethodologiesdataandusederodibilitymethodologybothparameters.Consequently,itisimportanttopayattentiontodeter-totheastandardizationusedofwhenthemethodologycomparingstabilityresults.Toavoidtheseproblemsconfusion;Ontheotherhand,thetermsusedwouldinthebenecessary.

literaturecanleadmacro-aggregatesomeusereferstabilitytheterm‘‘aggregatestability’’toreferindistinctlytosometodetermineitonlytomacro-aggregateand/ormicro-aggregatestability.Forexample,stability,thewhereasothersityusuallytests,’’stabilitywhileofmacro-aggregatesareusuallycalled‘‘aggregatetestsusedstabil-toarecalled‘‘dispersionthetestsusedtests.’’todetermineWater-stablestabilityandofmicro-aggregatesareusedcommonlyAggregates,’’parameterusedtermstorefertomacro-aggregatewet-aggregatestability.ThestabilitymostcludethisstabilityWSA.todescribeofmacro-Wethink,macro-aggregateandmicro-aggregates;however,thatstability‘‘aggregateisthe‘‘WaterStableso,inthisstability’’paperwemayin-ingtermasasynonymofsoilstructuralstabilityorsoilaggregation,willinclud-useandthemicro-aggregatestabilityofallstability.

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andThispaperdiscussesthemainprocessesandagentsaggregatestabilization,andreviewsthemainmethodologiesofaggregateusedtoformationdeterminecharacteristicsstability.influencesdatathestabilityoftheThemethodspaperemphasizesresults.Theused,theimportanceofconsideringthepossibilityduetotheoffactthatthemethodologyreviewedasanunifiedliterature,ingtheaggregatestabilityandexperienceFinally,basedonthepectsrangeofmethodologicalframework,whichincludesthemostinterestingweproposeas-aofthedifferentexistingsoils,methods.fromweaklyThisunifiedtostronglymethodaggregatedcouldbesoils.

usedforawideAGGREGATEFORMATIONANDSTABILIZATION

joinedSoilincludesandaggregationheldistheprocessbywhichaggregatesofdifferentsizesarecessesquently,occurtheprocessestogethermoreorlessofbycontinuously,formationdifferentorganicandandinorganicmaterials.Thus,itandstabilization.Inthefieldthesepro-less,theireffectsinteract,complicatingeventheiractdiscrimination.atthesametime.Conse-assomeauthorsreportthattheformationofsoilaggregatesoccursNeverthe-mainlyproducedaresultinorganicbyofphysicalforces,whilethestabilizationofsoilaggregatesis1993;andaorganicnumberoffactors,inparticularthequantityandqualityofforwettingsoilDalalaggregationandBridge,stabilizing(Dexter,1996).1988).However,agents(LynchandBragg,1985;Oades,Theclayphysicalflocculationforcesincludeisapre-requisite(temperateandtheandmechanicalregions),drying(semi-aridalimentarythecompressiveandsub-humidregions),freezingandalternatethawingactionofaswellasdryingactionofroots,andmetalants.Al,cationsThesuchinorganicasCa2+stabilizing,Fe3+,andagentssoilfauna,Al3+includemainlymainlyearthworms,clays,polyvalenttermitesagentscalciumtioncanbeandconsideredmagnesiuminthreecarbonatesmaingroupsand,oxidesgypsum.andhydroxidesofFeandbasedonThetheorganicstabilizing(Tisdalloftheorganicmatter:transient,temporaryandpersistentagebindinganddegrada-rapidlycharides.byandmicroorganismsOades,1982).ThetransientbindingagentsaredecomposedagentsarbuscularTemporarybindingandagentsincludearemicrobial-roots,hyphae,andplant-derivedparticularlypolysac-consistcations,ofresistantmycorrizalaromatichyphae,andsomefungi.Persistentbindingvesicular-agentshumicandstronglysorbedhumicpolymersmaterial(TisdallassociatedandwithOades,polyvalentmetalaggregatesmaterialsappeartobephysicallyprotectedwithinclay-1982).andsilt-sizedThesefragmentsThereareof(Skjemstadroots,severalhyphae,etal.,1993).Theyarederivedfromtheresistantmodelsbacteriaofaggregationcellsand(Edwardscolonies.

andBremner,1967;

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TisdalldifferenceandmodelsamongOades,them1982;ismainlyElliott,the1986;numberOadesofstagesandWaters,ofaggregation.1991),andthechicalparticlesorderconfirm(Clay-P-OM).attachedismicro-aggregatesthehierarchicalorderofsoilaggregation.Thelowesthierar-Thesetoorganicmoleculeslessthan2(OM)mdiameter,byconsistingofclayaggregatesThenexthierarchicalorderisthecombinationpolyvalentofthesecationsmicro-(P)(<250mdiameter.(<2Them)nextintomicro-aggregates((Clay-P-OM)x)y,whichare<250bindm)intomacro-aggregateshierarchical(>250orderism).theFinally,bondingmacro-aggregatesofmicro-aggregatesthatbondsmicro-aggregatesintoclods(severalmmorevencm).TisdallandOades(1982)proposedwillhierarchyateachameter.were:stage.<0.2Inthemselvesarebuiltupinstageswithdifferenttypesofmtheir 0.2-2model,m thestagesofaggregationoraggregatestagesTheseofOadesaggregationandWaterswere:(1991)<20modified2-20m 20-250m >2000mdi-m 20-90themmodel, 90-250suggestingthatthemainmodelscanbeappliedgenerallytosoilswhereorganicm matter>250m.ceptbindingagent.Ingeneral,thesemodelsconfirmtheutilityoftheiscon-thethanoftheofhierarchical250micro-m,respectively.andmacro-aggregatesordersareGoodtoseparateaggregateslessandhigherwell-developedsoilstructureandisaredescribedstableas,‘‘onewhereallnaturewateroperateofandexternalmechanicalstresses’’(Dexter,1988).againstThehierarchicaltheactionsgreaterforsoildifferentstructuresizeindicatesclassesthatofdifferentaggregates.mechanismsofaggregationappearthan250m,andspeciallythosegreaterThus,than2macro-aggregatesmmindiameter,gateser20-250tobeheldmtogetherlargelybyfinerootsandfungalhyphae.Aggre-oxidesbyvariouscementsconsistincludinglargelyofpersistentparticles2-20mdiameterbondedtogeth-Recentand1995)studieshighly(Oadesdisorderedaluminosilicatesorganic(TisdallmaterialsandandOades,crystalline1982).mechanismshowedexample,inthattheformationencrustationandWaters,andof1991;Golchinetal.,1994;Golchinetal.,stabilizationplantdebrisofbythesemineralmicro-aggregates.particlesisanotherdiametermicro-aggregatesbristerencrustedbondedwithdiametersof2-20mconsistofparticles<2Formwithtogetherinorganicbycompounds).persistentorganicUnitsbondssmaller(plantandfungalde-andconsistofclayparticlesheldtogetherbyinorganicandthan2mindiame-willelectrostaticticleshavedifferentbonds.stability.Consequently,Dexteraggregatesofdifferentorganicsizecementsclassesstrengthoflowerhierarchicalorderare(1988)moredenseconcludedandhavethatcompoundpar-thethanparticlesofhigherhierarchicalorder.Healsoasuggestedhigherinternalthatifcallowestsoilordershierarchicalaggregatearesimultaneouslyorderofsoilstructureisdestroyed,theotherhierarchi-stabilityrequiresdestroyed.theanalysisConsequently,ofthebehaviorcharacterizationofparticlesofof

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differentsizemicro-aggregates.

hierarchicalorders,suchasmacro-aggregates,andclay-andsilt-FACTORSAFFECTINGSOILAGGREGATESTABILITYsidered:Twomainfactors(1)soilgroupsprimaryoffactorscharacteristicsaffectingorsoilinternalaggregatefactors,stabilitycanbecon-discusstotivity,thetheinfluencesoil.Amongofthethesoilprimarycharacteristics,weandwill(2)reviewexternalandmineralogy,EC;typeaggregateCaCOofcations,electrolytesodiumadsorption(concentration,ratio,thusSAR;electricalpH,etc.),conduc-clay3andgypsum,theoneage,stability.Amongtheexternalorganicfactors,matter,weandwillFediscussandAltheoxidesclimate,onwithmustthetheothernotbiologicalconsiderfactors,andtheagriculturalmanagement.However,parameters.oneparameterwithoutconsideringtheinteractionsInternalFactors

andItvariablesdispersiblehasbeenshown(Goldbergclaythatet(micro-aggregatewetaggregateal.,1988).stability)stability(macro-aggregatearerelatedtosimilarstability)soilElectrolyte

micro-aggregatesIthasbeensuggesteddropisproducedthatdisintegrationprimarilyofthesoilmacro-aggregatesintoetNa/(Caal.,impact,1992a)beingbythewettingactionandbywater-andcompositionindependentofelectrolyteconcentration(EC)(Shainbergnoted+Mg)0.5]).Barzegaret(speciallyal.(1994b)sodiumandLevyadsorptionandTorrentoratio,[SAR(1995)=bility,howeveranadverseeffectofsodicity(SAR)onmacro-aggregatesta-(>250showing(<250m)thatsodicitydecreasedtheamountofmacro-aggregatesstabilitym).ESPfromnotedTheand0.3tobyslightconsequentlyincreasedtheamountofmicro-aggregates1.3%,LevyadverseeffectofexchangeableNainmacro-aggregatewhileandTorrentotherewas(1995)noeffectwasonlyinthenoticeablerangeatincreasinglyteThe(Quirkconcentrationmainfactors(EC),controllingthesolutionclaydispersion/flocculationfrompHandthesodiumarethe1.5electro-to5%.Amézketa,and1992;Schofield,Amézketa1955;andShainbergAragüés,and1995a,Letey,b).

1984;Suarezadsorptionetal.,1984;ratiosionLowandelectrolyteswellingandconcentrationsconsequentlyandthehighlossSARofvaluessoilstructure.produceTheclayadverse

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effectsareofthesefactorsonthehydraulicconductivityFrenkelwellestablishedQuirk,et(QuirkandSchofield,1955;McNealand/orandColeman,infiltration1966;rateer,Several1990;1986;al.,RengasamyAbu-Sharar1978;Alperovitchand1985;etal.,1981;Shainbergetal.,1981a,b;Olsson,Abu-Sharar1991;Amézketaetal.,1987;andShainbergAragüés,andSing-are1984;presentstudiesKaplanCurtinevenhavealsoshownthatoccurrenceofpoorphysicalconditions1995b).etal.,at1995;lowSARvalues(Osteretal.,1980;Rengasamyetal.,ticles,etal.,1996).SodiumCrescimannocausesswellingetal.,1995;and/orLevyandTorrento,1995;thresholdandslakingofunstableaggregates(Crescimannodispersionetal.,of1995).claypar-AusefulfrommustthosetodefineconcentrationunderchemicalcurvewhichconditionsdefinedfromSARandECvaluesisverystructureis(SAR,stable.EC)However,thatdestabilizesinglestabilitysoilstructureablesbealsodevelopedforeachsoil(PrattandSuarez,1990),sinceotherlinesvari-(1984),Guptaetaffectal.soil(1984),structuralArorastability.

andColeman(1979),ShainbergandLeteyChoromSuarezsoilthatpH.Theetal.etal.(1984),Chiangetal.(1987),Kerenetal.(1988)andimportance(1994)observedofthethatpHclaydispersionincreaseswithincreasingequivalentdifferencesinrelativehydrauliceffectconductivityisdemonstratedbetweenbypHthe6observationandAmézketapHandtodifferencesbetweenSAR20and40(Suarezetal.,91984).wereelectrolytevaluesontheAragüésenhancement(1995a)foundofthatthenegativeeffectofincreasingwithSomeauthorsconcentrationsclaydispersionwasonlyimportantforhavebelowtheflocculationvalue(FV).

al.a(1987),thepresenceHeilandofSpositoCafoundorMg.that,foragivenSAR,claydispersiondiffers(1993a),AbderrahmanandCurtinandetal.Rowell(1979),YousafetAllperovitchgivenSAR,CaismoreeffectivethanMginflocculating(1994b)foundthat,forgibleetal.(1986)observedthatthisdifferentialeffectsoilclaymayparticles.benegli-hancesindispersion.dissolutionsoilshighintheThus,AmézketaofcalciumCaCOcarbonate,becausetheexchangeableMgen-and3andAragüésthereleased(1995a)electrolytesfoundthatCapreventedandclayexchangeableOfsametheeffectfourmajoronthecationsdispersionfoundofinclaysoilsin(Ca,severalMg,calcareousNa,andK),soils.

Mghadtheeffectofleastand(1995)Schofieldclear.ReportedKonsoil(1955)resultsphysicalconsidervaryKorandhydraulicpropertiesisprobablytheasconflictdeleteriouswithrespectasNa.tothiseffect.Quirkmacro-aggregatefoundKasnotdeleterious,limitingclaydispersionLevyandandmaintainingTorrentoofandadsorbedK,whichstability.is72%TheythatattributedthistothelowerhydrationenergyandVanNa.Moreover,DerWatt(1990)theHofmeister’sobservedofanadsorbedlyotropicintermediateNa.Ontheotherhand,LevyserieeffectestablishesofKbetweenthatthede-

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creasingorderofcationspromotingflocculationisCa+2(VanOlphen,1977).>Mg+2>K+>Na+ClayMineralogy

differentClayisoneoftheaggregatingfactorsinthesoil.However,theacteristicsdependingtionofclays,onitsmineralogy.Consideringthephysiochemicaleffectchar-isExchangethanothertionCapacityclaysthesmectiticclaysshouldbemoreefficientonaggrega-(CEC),becauseandoftheirconsequently,largespecificsurfacearea,highCationsubjectcapacity.opedtoslakingInaddition,thankaoliniteEmersonor(1964)illitebecauseshowedhighthethatphysiochemicalpressureswellingwhichclaysinterac-isaredevel-lessmorillonitebyentrappedairisreleasedbyswelling.However,fissuringofmont-swelling(1966)ofmayoccur,duetothecombinationofstressofentrappedairandandble,lowestablishedaggregateselectrolytethatconcentrations,under(LeBissonnais,unstablesoilsconditions,1996a).Conversely,McNealetal.highinmontmorillonitesuchashighSARarevaluestivelywhereasamountsstable,soilsandsoilswithhighhighcontentsin2:1clayinkaoliniteminerals,andbutsesquioxideswithoutorwithareunsta-rela-lowassociationofstrongstabilityofmontmorillonitekaoliniteoftheiraggregateswithhaveironanintermediatebehavior.Thefrequenttooxidesslaking.

mayberesponsiblefortheverypendsAnotherColemanonclaymechanismmined(1979),mineralogy.ofOsteretal.El-Swaifyaggregatebreakdown,chemicaldispersion,de-(1980),and(1976),ShainbergVanOlphen(1977),Aroraandtioncomparisonvalues).thatillitesThisareisthedueclaystothemostsmallersensitiveedge-to-facetodispersionandLetey(1984)deter-attraction(higherforcesfloccula-inoftotheotherclaysbecauseoftheirregularandterracedsurfacesflocculationtheilliteparticles.smectitebyAroraandColeman(1979)foundthatthesensitivitytothemicaceous.soil>dispersionkaolinite.NaHCO3wasinthedecreasingorder:illite>vermiculite>orderNevertheless,was:montmorilloniticVelasco-Molina>etal.(1971)foundthatthanHowever,atlowESPvalues,themicaceoushalloysitic-kaoliniticsoildispersedmore>cultClaythemineralogyhalloysitic-kaoliniticinfluencessoil.

soilaggregatestability,buttheeffectistheirtoassessbecausesoilsusuallycontainamixtureofclayminerals,diffi-sosense,behaviormontmorilloniteFrenkelisetmodifiedbytheassociationwithotherminerals.InthisThisnitebehavioristoal.(1978)reportedthattheadditionofsmallamountsofexplainedkaolinsoilsbythepromotesthedispersionofkaolininflocs.preventingparticles(faces)areadsorbedfactonthatthenegativelypositivelychargedchargedmontmorillo-1978).

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claysMoreover,previousinvestigationsconcludedthatthedispersion(AroracouldberelatedtothedispersionofthecorrespondingreferenceofclayssoilandflocculationForsterandColeman,referencevalue(1990)(FV)reported,1979;Yousaf,ofsoilhowever,1983).Rengasamy(1983)andGoldbergclaysuspensionsthatthecolloidalweregreaterstabilityandthetherefore,claymatter,(MillerFenotsuspensions.Extrapolationfromreferenceclaythanresultsthoseis,ofandpossibleAloxides,andadditionalfactorssuchasthecontentoforganicbeInconclusion,etal.,1990;theFrenkeloreffectofetCaCO3affectthecolloidalstabilityofsoilsclayal.,1992;mineralogyKretzschmaronaggregateetal.,1993).

tiesambivalent.However,theinteractionofthisfactorwithotherstabilitysoilseemstoForstermaytests(1990)beresponsibleandfortheapparentlycontradictoryresults.Goldbergproper-andate(micro-aggregateAmézketastabilityandtests)Aragüésusingreference(1995a)reportedclaysthatdispersional.,Claytodescribecontentthearenotappropri-hasflocculation-dispersionbehaviorofsoilclays.

However,1992a;aloweraggregateTernanBrubakerbeenetstability.al.et(1996)al.,also1992;consideredasacementingagent(ShainbergetfoundCurtinthatetsoilsal.,1994a;withahigherLeBissonnais,claycontent1996a).hadCaCO3andGypsum

calcareousThebeneficialal.soilshaseffectbeenofreportedcalcitedissolutiononthestructuralstabilityofand(1981a,preventSharmab),(1986),YadavandwhoGirdharconcluded(1981),byAlperovitchetal.(1981),ShainbergetthatBen-Hurthereleasedetal.(1985),CaandMinhasofclaydispersion.Onthecontrary,soilinfiltrationratewasisindependentenoughtosodicitythepresenceofcalciteorgypsuminthesoil,dependingbasicallyontheappliedonparticlesaggregatetoofthethesoil.soilsurfaceandontheelectricalconcentrationofthewaterstabilityLeBissonnais(1996a)concludedthattheeffectofCaCO3clayVerymustandbeonpresenttheclayprobablyandcontent.dependsonthesizedistributionofCaCO3CaCOForabeneficialaggregatingeffectsufficient3particlesmustforwell-crystallizedtheirfinesolubilityparticlesoftosoilbecalcitehigherhavebesmallerthanthesizeofsilt.thansufficientlythetheoreticallyhighsurface-freepredictedvaluesenergiesenrichingReclaimingtionandtoirrigationsodiccalciteacontinuouswatersoils(InskeepsupplywithwithCOnaturallyandBloom,occurring1986).

forCaCO3isenhancedbyofCa2,which2+thatleadsremovestoaNaconstant+limedissolu-sitesofEfficiency(Nadleretfromtheexchangeofal.,reclamation1996a).

ofsoilswithofthethepercolatingwaterfilmaroundsolution.theIncreasinggypsumparticles,flowgypsumrates(2)(1)dependsreducesreducesontheflowratethethecontactthicknesstime

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betweenandConsequently,(3)reducesaunitvolumetheopportunityofwaterandfortheCa-Nasurfaceareaofthegypsumparticles,forreclamation.increasingflowratesreducesexchangetheefficiency(Nadlerofetusingal.,gypsum1996a).OrganicMatter

controversial.

Theroleoforganicmatter(OM)withregardtoaggregatestabilityisstilllates,Additiondispersionaspartates,oforganiclactates,anionsand(fulvates,acetates)tocitrates,soilsuspensionsoxalates,tartrates,increasessalicy-clayPiccolo1990;KretzschmarFrenkeland(ShanmuganathanMbagwu,etal.,1992;1989;andOades,1983;DurginandChaney,1984;HeilKosmasandandMoustakas,1990;Goldbergetal.,1995).fulvicVandenetal.,Broek1993;(1989)ItamiobservedandSposito,Kyuma,1993a,b;Tarchitzkyetal.,1993;that1995;andHeilandSposito,whereasandcitricacidstoclaysuspensionsnotablyadditionofsmallamountsofeffect.aromaticacids(salicylicandp-hydroxybenzoic)increasedhaveclayaflocculatingdispersion,substances.

VisserandCaillier(1988)alsoshowedthedispersiveeffectofhumictweenOnHamblinorganictheothermatterhand,therehavebeennumerouspositivecorrelationsbe-FortúnFierrosandandFortún,Davies,and1989);1977;waterChaneyHamblinstableandandaggregates,WSA((Greenland,1971;Swift,Greenland,1984;1977(allthreecitedinsonThisViqueira,apparentcontradiction1989;MbagwuonandtheeffectsPiccolo,of1989;BenitoOMcomesandFortúnRuedaandDíaz-frometal.,1989).ConsideringofOMwithstabilityparameterssuchasWSA,andDispersedthecompari-concludedthatincreasingWSAincreasesaggregatestability,Clay,itcanDC.beclaydecreasesdispersionthatOMseparateaggregatedecreasesincreasesstability.aggregateaggregateTheparadoxstability,stability.ConsideringthatincreasingcanitbecanbeconcludedthatOMstability,aggregatestabilityinmacro-aggregatestabilityunderstoodandmicro-aggregateinpartifwewhereasDCaswereflectsproposeinthispaper.WSAreflectsmacro-aggregatestabilitytradiction.Therearemorelikelymicro-aggregatethreemainhypothesisstability.

toexplaintheapparentstability,effectmacro-aggregateFirst,bothstabilitystabilityparametersemphasizedifferentaspectscon-ofclay,anionanddependsinparticular,onthetypeontheofsizeunionandofbetweenmicro-aggregatestability.Second,thethethehumicsubstancesandtheparticlesislongerandandbindthanthemtheclayedge,itwillorganicattachtoanions.theedgesOnlyofifseveraltheorganicclaylevels,Shanmuganathanmacro-aggregatesandtogetherOades,(Durginandmicro-aggregates.1983).Third,andChaney,OM1984;Emerson,1977;Thus,actsdifferentlyorganicbondsatthestabi-

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lizebrokenaggregatesculantanddisaggregationagainstslakinghasandoccurred,disaggregation,theorganicbutmatteronceactstheseasbondsaareIneffectthis(Emerson,1983;Goldbergetal.,1990;andItamiandKyuma,defloc-1995).soilofsense,organicGoldbergmatteretonal.soil(1990)structureandNadlerisafunctionetal.(1996b)ofthesizesuggestscalethatofthetheoverparticlesaggregates,theparticleanalyzed.charge(GoldbergThus,inclay-sizedetaggregates,organicmatteracts(TisdallroaggregationandorganicOades,matter1982).actsThus,asaOMbindingal.,1990),wouldagent,whereashavethroughincoarsesand-sizeddifferentrootseffectsandonhyphaemac-Kretzschmaraggregates(changing(colloidaletthanal.(1993)onmicroaggregation.stability)foundHeilandSposito(1993b)andwasthatnottheeffectcausedofbyorganicmatterinclay-sizedconsistpolymers.oftheapproachThestericparticlecharge),butbystericmechanisms.electrostaticStericmechanismsmechanismspolymerrepulsioncoatingresultingactsasfromatheoverlapofadsorbedorganicreducingoftwocoatedparticlestotwicehardthethicknesssurface,andlimitstheclosestbetweentheeffectivenessofattractiveshort-rangevanofanderadsorbedWaalsforceslayer,al.,Othermineralmore1994),workssuggestinghaveparticlesfound(HeilthatsomenocorrelationandSposito,componentsbetween1993b).

oftheOMorganicandWSAcarbon(Carterpoolareet1990).activelyfractionRobersoninvolvedtotal(HF)soiletinstabilizingaggregatesthanothers(PerfectandKay,carbohydrateal.(1991)anicCandtotalcarbohydratewerenotsignificantlysoilstructure,correlatedwhilewithwere(1995)postulatedHot-waterbyextractableHaynesandcarbohydrates,Swift(1990)whichandhavemicrobialorigin,andtobeinvolvedinsoilaggregatestabilitization.GijsmanGolchinandetal.Thomas(1994)closelyPugetetal.(1995)indicatedthatthestabilityofsoilstructureismorecontent.

relatedtotheyoungandactiveSOMfractionthantototalSOMmacro-aggregationInsummary,theingandcontradictoryorganicmatterresultsarefoundduetointhecorrelationsamongaggregation,reasons:(1)further(2)onlytherepartisaofcontenttheorganicoforganicmatteroneormoreofthefollow-carbonisresponsibleabovewhichforwater-stablemajorincreaseinwater-stableaggregation,(3)organicmaterialstherearenotisthenoamountbindingofagents,and(4)itisthedispositionratherthanthetypeortransientThestabilizingorganicporarilyaggregatingeffectmattereffectofwhichorganicisimportantofpolysaccharidesmatterresults(TisdallandOades,1982).onfrommicro-aggregates,thecombinationtheoftem-thepersistantstabilizingSeveralworkseffecthaveofpolymerseffectofrootsandhyphaeonmacro-aggregates,andtheconfirmedandthesearomaticconclusionscompoundsstudyingonmicro-aggregates.thedistributionof

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organicElliott,matteratdifferentsoilaggregatesizefractions(Christensen,1993;PugetAngers1986;etGuptaal.,andGermida,1988;CambardellaandElliott,1986;1992,fractionsetal.,Hassink,ofsoil1995)1993a,1995;Schultenetal.,1993;Beareetal.,1994b;organicaswellmatterasthedecompositionratesofsizeanddensityeffectgates,of1995).organicInmatteraddition,istosome(Golchinformauthorsetal.,1994;Golchinetal.,1995;ahydrophobichavereportedcoatingthatanotherpositivequentlyreducingsoilwettability,slowingdownthewettingaroundrate,andtheaggre-conse-van,Caron1990;reducingBlackman,thesensitivity1992;ZhangtoslakingandHartge,(Mbagwu1992,andcitedPiccolo,inZhang,1989;Sulli-anisms:Theet1994;dispersiveal.,1996).

negatively(1)theblockingeffectoforganicmatteristheresultofthefollowingmech-ionsofadsorbedbyorganicchargedorganicmatter,organicofpolymerand(3)anions,positivelylayersthesteric(2)thechargededgesofclaymineralsby(HeilrepulsioncomplexationandSposito,resultingof1993a,frompolyvalentcat-b).theoverlapFeandAlOxides

controversyTheinfluenceofsesquioxidesonsoilaggregationisstillamatterofEl-Swaify(1985),1990,Chiangandinliterature.Emersonetal.(1987),(1975),McNealandColeman(1966),McNealetal.(1968),ShainbergEl-Swaifyetal.(1976),(1987),DuPlessisandShainberg(1996)1991),Goldbergetal.(1990)andPinheiro-DickKerenandandSchwertmannSinger(1989,areandverystatedthatsoilshavingahighcontentofironandaluminumoxidesnegligible.Bartolistable.ity,Thisetal.However,Borggaard(1983,citedinvanDenBroek,1989)apparent(1991)contradictionshowedthatistheirrelatedeffectonaggregatestabilityisasmainlytoparticlethelevelsizeatandwhichdistributionoxidesact.oftheTheoxidemajor(vantoDenthevariationBroek,1989),incrystallin-aswellswelling.onsoilmicro-aggregation,andbasicallyeffectlimitingoftheseclaydispersioncompoundsismechanisms:Thequioxides(1)stabilizingironandeffectofthesesquioxidesarethroughdifferentandcanbindclayparticlesaluminumtotheinsolutionactasflocculants,(2)ses-(1976),Elprecipitateorganicmolecules,and(3)sesquioxidesRayahandasgelsonclaysurfaces.

KerenFrenkelandRowellShainberg(1973),(1980),El-SwaifyGoldbergandEmerson,and(1975),El-SwaifyeffectiveandSinger(1989,1990)establishthataluminumGlaubigpolymers(1987),aremoreandaluminumin(GoldbergaandpolymersaggregatingGlaubig,isduethanironpolymers.Thehigherefficiencyofthe1987,toand(1)Kerentheirhigherpointofzerocharge(pzc)tionhigherforces;chargeanddensity,(2)theplanarandconsequentlyandshapeofthetoSinger,Alstronger1989,precipitates,clay-polymers1990),leadingtodifferingattrac-tothe

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sphericalahigherbindingshapeofsurfacetheFetopolymers;whatmeansahighertouchingsurface,sopolymersHowever,that(asMcNealFeCl)wereetal.claybetter(1968)particles.

stabilizingandShainbergthanAletpolymers,al.(1987)foundthatFe3thethestabilizingeffectwasrelatedtothechargeonthepolymersandconcludedandnottoidesIntotalconclusion,amountextracted.

thereisaluminumonmicro-aggregateresolved.ismoreeffectivestability,goodevidenceofthepositiveeffectofthesesquiox-inaggregationeventhoughofsoiltheparticlesdebateofiswhethernotcompletelyironorExternalFactorsClimate

typeClimateizeandisoneofthesoil-formingfactorsthatstronglyinfluencesthesoilorganicandaggregatemattersummarizethedegreeofsoilaggregation.CarterandStewart(1996)character-storage.theBlackmanimpactofvaryingclimateonsoilaggregationandrelationtostabilitytheweatherandconditionsBullocket(1992)thatal.(1988)observedaseasonalvariationintheaffectaggregatepresentedseasonalprocessesining-thawingWatercontentincyclesadynamiccycles)changesway.Ininfluence(wetting-dryingcycles)andairstability.

temperature(freez-thetheformationanddestructionofsoilstructurelarytheactionasafield,soilissubjectedtomultiplewettinganddryingandresultevaporation.oftheprocessesTheseofcycles,condensation,affectedbyrainfall,airtemperature,irrigation,capil-tureAfterfreezing-thawinginduceBraggaboutyearstheofinvestigation,cycles.

thereisstilldiversityofopinioninthelitera-isingattributed(1985)effectofthesecyclesonsoilstructuralstability.LynchandtoandmainlyOades(1993)mentionedthatthesoilaggregateformationsuspensionandthawing.Grantofclay,Syntheticphysicalforcessuchaswettinganddryingandfreez-siltandaggregateswereformedbywettinganddryingasomethesoils,andBlackmoresand(Singeretal.,1992;Amézketaetal.,1995).usuallyvertisols,(1991)presentedandWenkeandGrant(1994)observedthat‘‘self-mulching.’’

courseofwettinganddrying.Thethebehaviorabilitytoofre-aggregatethesesoilstheirwasclaycalledinandSoulidesandAllison(1961),Tisdallmacro-aggregateMullaetal.(1988),stability.(1992)foundetal.(1978),Lehrschetal.(1991),However,thatUtomowettingandandDexterdrying(1982),cyclesdecreasedstablemacro-aggregation.andBarzegaretal.Singer(1995)etfoundal.(1992)thatthesesuggestedcyclesresultedDexteretal.thatwettinginwater-and

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dryingeffectcycleshadaneffectonthestabilityofsoil’’Utomowasanddependentartificialaggregates,buttheDexteron(1982)theclaytype.

thetoexplaintheapparentinconsistenciesintroducedthementionedconceptofabove.‘‘equilibriumstatesofhistory,hypothesisstabletherethatforanygivensoilcompositionandphysicalTheyenvironmentintroducedumappropriatevaluemacro-aggregates.isanequilibriumwillincreasetheirSoilsproportion,whichproportionhaveorrangeofproportionsofwaterandlesssoilsthanwhichtheappropriateequilibri-thevaluewilldecreasetheirproportion.Theyalsohaveattributemorethanparttheofbiologicalinconsistenciestothefailuretodistinguishbetweenphysicalandmicro-encesHowever,aggregatingfactors.

aggregateinsoilsomeatsize,type,ofthecontradictoryresultsmayalsoarisefromdiffer-methodordifferencesandrateofwettinginexperimentalconditions,suchassoilexplainingthetimeofsampling.Wewilldiscusssomethesamples,andsoilwatercontenttheExperimentaltheapparentofthesepossiblereasonsforconditionsinconsistencies.

suchasthemethodgregatewettingratesinfluencetheeffectsofwetting-dryingofwettingcyclesandinparticular(Kemperstability.1996a).andRosenau,1986;Aggregateswettedveryslowlywillmaintaintheironmacro-ag-structureslowpletelyenough.However,Duringthewetting,wettingReichertprocessandtheaggregatescanNorton,bemayhighly1994;Amézketaetal.,eitherdisruptivedisintegrateifitiscom-notness(slaking)orremainintactwithonlylooseningatthepointsofweak-uniform(mellowing)shearordifferential(Barzegarhydrationetal.,and1996).Rapidwettingproduces:(1)non-betweenplanes;and(2)airentrapmentinsideswellingcapillaryofthepores.clayfraction,causingplanesdifferentstructuralunitsarestrongenoughtoovercomeWhenthethebondsshearsoil(McKenziematrix,andthereducingpressureofentrappedair,onlymicrocracksareformedintheOades,andDexter,the1985;cohesivenessLeofparticlesandhencethesoilstrengthtainproducesintact.1993;OnandtheBarzegarotherhand,etal.,when1995).Bissonnais,thebondsInthese1989;LeSouderetal.,1991;arecases,theaggregatesmain-smallerapressureexceedingtheircohesivestrength,weakaggregatesandtheentrappedslakeairGäthincreasesandstructuralFrede,units(KemperandRosenau,1986;GrantandDexter,1990;intoduringsoilfriability1995).andMellowingreducesisthedesirableenergyinofagriculturaldraftforcessoilsoftillagebecausetoolsitSomeStabilitycultivationalso(Barzegaretal.,1996).

creasingauthors1990a,soilwaterhavevariescontentfoundwiththatthesoilwatercontentatthetimeofsampling.(Gerard,macro-aggregate1987;Cootestabilityetal.,decreasedwithin-etal.,1994),c;Gollanyandetclayal.,dispersion1991;Rasiahincreasedetal.,1992;withCaron1988;Perfectetal.,increasingetal.,water1992b;content

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(Rasiahobservedet(Kemperthatal.,1992;structuralCaronstabilityetal.,1992b;increasedWattswithetal.,increasing1996a),whereaswatercontentothersCaronandRosenau,1986;MonroeandKladivko,1987;Angers,1992;ofal.,anAnetal.,1996).

aggregateincreaseintothethedisruptinginitialwatercontentconditionincreasestheresistance(1993b)1989;andnotedGollanythatetaal.,positive1991;relationshipLeforcesBissonnais(Trumanetal.,1990;LeBissonnaisetbetweenandsinger,macro-aggregate1992).Angersetal.(directsoilwatercontentisobservedwhentheaggregatesareallowedstabilitytoslakeexistsminimizedwhenimmersiontheaggregatesofdry-aggregatesarewettedinunderwater),whereasanegativecorrelationthatslakingstresses.LeBissonnaisandvacuumSingeror(1992)tensionalsoandthereforeobservedstabilitythedueisapparentduetopositiveeffectofincreasingwatercontentonaggregateaggregates,tothefactductionsothatareductioninslaking,whiletheapparentnegativeeffectiswetshearaggregatesstrengthareofwetaggregatesislowerthanthatofdryattributedintionbycohesiveWattsstrengthetal.(1996a)ofallparticlelessresistantinparttobondstoraindropimpact.There-osmoticathighereffectswatercontentswas(1994)ofstabilizingmaterial.Shainbergetal.(1992a)andReichertplustheanddissolu-NortonandtheaddedclaysurfacesthatformakessmectiticsoilsthehydrationofexchangeablecationsisTherelationshipbetweenwetstructuralaggregatesstabilityweakerparametersthandryandaggregates.watercontentAggregatecomplexnalstabilitybecauseisdifferentafunctionprocessesinteractoverdifferenttimescales.ruptivebondwaterforcesstrengthfromwithinrapidthewettingaggregateofbothareinversely(Gollanyexternaldisruptiverelatedetal.,forcesandinter-to1991).theantecedentExternaldis-soilwatercontent.resistantcontent.InternalthetoslakingWithevengreaterforcesbindingaggregatesarealsoinverselyrelatedtothoughsoilcohesionwatercontent,isreduced.soilAggregateaggregatesaremorebetweenresultofthebalanceofbothtendencies.Inaddition,therelationshipstabilityistory,Kaysoilaggregatetypeetc.Kemperstabilityetandwatercontentisdependentoncroppinghis-moderate(1992)pullsoilobservedwatertensions,thatmaximumal.(1987),probablyratesDexteretal.(1988),andCaronandbecauseofcohesionsomerecoverytensionisoccurredneededatessentialtheparticlesintodirectcontact,butacontinuouswaterphaseisalsoto(1988),optimumDextertoallowetdiffusional.(1988),ofbondingagentstothecontactpoints.DexterLimitwatercontentforthixotropicandWattshardeningetal.(1996a)ismentionedthatthe(1996a)In(PL)addition,ofthearoundthelowerPlasticEmersonsoil.

(1967),KayandDexterwatercontent.addedthatTheythefounddispersivethattherebehaviorisaofcriticalsoils(1990)isminimumalsoaandfunctionWattsetal.watercontent

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necessary(1996a)wasandforforclaysomedispersion.ofthesoilsForoftheEmersonthreesoils(1967)studiedthebyWattsetal.minimumclosetotheplasticlimit(PL).However,forothercriticalsoilstheminimumcriticalconcludedwasequaltoorthatmuchhigherthanthePL.Ontheotherhand,Rasiah(1994)greaterthethanmaximumclaydispersionoccurredatsoilwatercontentstheDryingofthesoilincreasessaturation.

tensionornegativepressureinthewater,andcreasingretreatingdryingmoleculescontinues,thenumbermeniscisolubleofpullcontactsmallcompoundspointssuspendedmineralparticlestogether,in-suchatwhichassilica,bondingcarbonates,cantakeplace.Astoareconcentratedintheliquidphase.Thissolutionphaseandcontractsorganicthesethe1986;contactimmediatepoints,vicinitystrengtheningofthecontactsandsolutesareprecipitatedaroundal.,of1992;KemperandLehrschetal.,1987;andBrown,Bullockthe1995).etcohesional.,1988;(KemperandRosenau,1984;Thus,Dexteraetal.,1988;Mullaetpointscolloidsandprecipitationofslightlysolublemineralscombinationaroundofthedepositioncontactsoilagecohesion.betweenDextersoilparticles(1988)andstabilizeOadestheaggregateswhendry,increasingageondevelopmentoccursdryinganddevelopscreatessoiltensilestructure,stressescreating(1993)whichaggregates.addthatinsomesoilsshrink-willeventuallyOndrying,leadshrink-totheCracksiswettest.

appearofwherecracksthealongsoilhasplaneslowoftensileweaknessstrengththuswhichcreatingiswhereaggregates.thesoilwettingOntheothergressivearedevelopmentnothand,entirelyHusseinofreversedandAdey(1995)reportedthatchangesduetostructuralonunitsdryingthroughandthissequentialhysteresisleadstopro-andLehrschetal.(1991),Edwards(1991),Mullaetal.(1992)wet/dryandcycles.Starickatosoilfreezing-thawingBenoit(1995)foundcycles,thatwhilemacro-aggregatePerfectetstabilitydecreasedinresponsecontentstabilityBenoitattheupontimeofthesefreezingcycles.theAggregateal.soil.Lehrschstability(1990b)etal.(1991),dependsfoundanandonincreasesoilwaterinwithfreezingincreasing(1995)observedtimeoffreezingaggregateswaterwasproducedcontentthatatmacro-aggregatestabilitydecreasedStarickalinearlyandgreaterthantheirfreezing.0.20disruptionBullockkgkg onlyet1ifal.the(1988)waterobservedcontentatthatthetweenFreezingaggregatesresultsintheexpansion.

oficecrystalsinporesbe-thebetweenaggregatesparticles,intobreakingsmallerparticle-to-particleaggregates.Duringbonds,thawingandlittleeffectivelysplitting(Kayetal.,micro-aggregates,andtheporescreatedbyexpandingbondingicecollapseexiststheThesoilproperties1985;Bullockisbrokenofupcompactedetal.,andloosened,clays1988).

therebytendtoraisingbeimproveddrainagebyandfrostventilation

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conditions,willtation,bedeterioratedwhereasbythefreezing,propertiesbecauseofuncompactedmicrofabricandchangeshighly(particlesaturatedorien-soilsthawdaetsettlementreductioncanineffectiveleadtoacompactionporespace)andbyicedecreasepressure,inpermeabilityconsolidation(Czur-andstabilityAiral.,and1995).

coolingtureofthroughsoiltemperaturealsohaveanindirecteffectonsoilaggregatethesoiltheireffectonwatercontentofthesoilsurface.Rapidetal.,tothesurfacesurfacefromdeeperinducessoil,gradientsincreasingwhichthecauseswatermigrationcontent(Bullockofmois-soilFinally,1988).

windvelocityaffectsalsosoilerodibility.InlossandlimitingWindagriculturalerosionisasustainabilityserioussoilsoilstructuraldegradationstability,andespeciallyinmanyplacesprocessofleadingtospeciallyagriculture,theriskofwinderosionisincreasedduringfallowtheperiods,world.erosionifresidueissparse(Larneyetal.,1994).TheseverityofwinderistheandfractionRosenau,dependsontheaggregatesizedistributionofthesoilsurface(Kemp-of1986).particlesIthassmallerbeenthanconsidered0.84mmthat(Campbellthewind-erodibleetal.,1993a,fractionb).Time(Aging)

bility.TimeparedIncreasesisanotherinrelativesoil-formingstabilityfactor,withwhichtimealsoinfluencesaggregatesta-addition,aggregatespactionincreasesheldinstrengthatconstantofwaterclaysovercontentweremonths(Blakefoundinartificiallypre-orandyearsGilman,1970).Inasorremoldinghavebeenobservedbycivilengineersfollowingandrecognizedcom-known‘‘thixotropicseemascuring,agehardening’’(Molopeetal.,1985a).Thiseffectisalsoreorientationtoberesponsibleorstrengthregain(Dexteretal.,1988).Physicalprocessesaofplatyclayforparticlesthesestabilizations.byexternallyMitchellapplied(1960)hypothesizedplateletsmoreuniformlyparallelarrangement.‘‘Inthismoreorderedshearingstructureforcesclayinto(leastrest,negative).canslippastoneanother,andmatricwaterpotentialismaximumticles.thermalWhentheshearingforceisremovedandthesystemcomestotime,Theresultingoscillationsdisordertendtorandomizetheorientationoftheclaypar-hydrationmatricsurroundingprocessespotentialanddecreasesimpartsbyredistributionastherigiditywatertothesystem,andatthesameofionsstructureinbecomesmodifiedbyassociatedtheclaysurfaces.’’Thus,orientationtheofelectricwatermoleculesdoublelayersandpositionsstabilityofcationloweredequilibrium,potentialandspontaneousshiftingofclayparticlestomentationduringthroughaging.thedryingDexterenergyeffectetal.could(1988)accountforthebuild-upinaggregatealsoaddedcontributethatparticle-particletoage-hardening.

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aggregatesThisthixotropicproducedagebyhardeningcultivationof(Molopeclaysalsoetal.,contributes1985a;tostabilizesoilingSeasonalMullaseasonhavevariationsalsobeenand/orobservedvariations(Perfectinaggregateetal.,1990a;stability1987).

Caronwithinetal.,a1992c;grow-however,etal.,1992;LehrschandJolley,1992).Thesetemporalvariations,tothetimearefactorattributed(aging).toclimaticfactorsandtobiologicalfactorsratherthanBiologicalFactor

roots,Theciallysoilbiologicalmicrobes,systemandsoilisanotherfauna.Theysoil-formingformaggregatesfactor.ItincludesplantAllactivities,biotamacropores,influenceand(3)theirsoilbutstructuretheirmajorby-productsthrough:roleis(Jastrow(1)inthethestabilizationandofsoilpores,structure.espe-andorganismsMiller,themselves,(2)theirstructureThemaineffectsofthemostimportantbiological1991).

componentsonsoilsuchwillbediscussedinturn.However,readersarereferredtoreviewsMillerasLynchandBragg(1985),GuptaandGermida,(1988),Jastrowandsionof(1991),theimportanceOades(1993),ofsoilKandelerbiotaonsoilandstructure.Murer(1993),forawiderdiscus-Roots

(1982).TheyareconsideredinpositivethepresenceSeveralauthorsofrootsevidencedastemporarybindingagentsbyTisdallandOades(Habibetthatmacro-aggregatestabilityisimprovedintolocalizedstableeffectmacro-aggregates,onsoilaggregational.,1990;PojasokandKay,1990b).Theirevenconsistwhenon:the(1)rootenmeshingfineparticlestodecomposabletheaxissoilofenvironmenttherootandarounddrawingtheroots,soilreorientingisdead;clayparticles(2)dryingparallelthelationwormsintherhizosphere;organicresidues(5)providingtosoil;(4)foodsupportingparticlestogether;forsoila(3)supplyinganimals,largemicrobialpopu-1996);insolutionandand(6)thereleasingmesofaunapolyvalent(Tisdallcations,andOades,increasing1982;concentrationsOades,such1993;asTisdall,earth-ofionscrops’Rooting(Pojasok1996).differentpattern,andabilitiesrootKay,functions1990b).

inmodifyingandphysiologysoilstructureare(ChanresponsibleandforthegrassesMonocotyledenousformerarebetterthancerealsplantsinstabilizingaresuperioraggregates,todicotyledenousduetothefactplantsHeenan,thatandthe1993).

containamuchgreaterrootbiomass(LynchandBragg,1985;Oades,Onthecontrary,Caronetal.(1992a)reportedashort-termdecreaseinsoil

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structuralingphysicalbromegrassstabilityestablishment,(decreaseinWSAandtheyandattributedincreaseinthatdispersibledecreaseclay)infollow-(1981)fragmentationofaggregatesbyrootpenetration.ReidpartandtoGosstheclayorganicfromalsoshowedthatthegrowthofrootsofmaizeandtomatodispersedbondsbetweenacids,twoloams.exudedTheincreaseddispersionwasattributedtochelatesandclayandbyorganicroots,whichmatterremovedpolyvalentcationsfromthegregation,However,theirineffectspiteofisinthesegeneralpunctual(Reidpositive.

negativeetal.,effects1982).

ofrootsonsoilag-SoilMicrobes

rhizalLiveand/orareconsideredfungi,anddeadtemporarysaprophyticbacteria,bindingfungihyphae,agentsareparticularthevesicular-arbuscularmycor-bymainTisdallaggregatingandOadesmicrobes.(1982).

TheybiomassManytaKandelerandGermida,andresearcherswater-stablehave1988;Carter,aggregates,foundasignificant1992;Sparlingthisis,macro-aggregatecorrelationbetweenmicrobialetal.,1992;stability(Gup-molecularThemicroorganismsandMurer,1993;Amézketaetal.,1996b).

Bruceetal.,1992;extracellularweightcompoundsproduce(KandelerenzymesandresponsibleMurer,1993),formineralizinghigh1990;TheseChenu,polysaccharides(ChaneyandSwift,1986a;andBaldockalsoreleaseetal.,Oades,mucilages1993)orgumswhicharebindsoilparticles,stabilizingthesoilaggregates.Amézketa1982)becausetheyconsideredaredecomposedtransientrapidlybindingagents(Tisdallandstudyetal.(1995)usedartificialaggregatesinoculatedbymicroorganisms.withbacteriatoSynthetictheeffect40tion_C.Halfaggregatesonaggregateoftheaggregateswerecreatedstabilitywerebyofinoculateddryingthepolysaccharidesawithslurrybacteriaofsand,producedtosiltallowandinproduc-claysituat.tion,ofthanaspolysaccharidescontrol.WSAandtheotherhalfweremaintainedwithoutinocula-bacteriaWSAgregateandofthethebacteriaaggregates-control.oftheaggregatesthemselveswereTheincubatedpolysaccharideswithbacteriawerehigherresponsiblefortheproducedincreasedbythenumberstability.Molopeetal.(1987)suggestedthattherearealimitedag-adhesivesofducedinmicrositesaggregatebetweensoilparticleswherepolysaccharidescanactaslittlebybacteriainexcessformation.ofthatoccupyingTheyaddedallthattheavailableanypolysaccharidesiteswouldpro-addaggregateDoriozornothingtoaggregatestability.

ides.mineralLivingstabilityetal.(1993)organismwasconcludedthattheinfluenceoflivingorganismsonaremoreable,thanbysimplyastrongsecretionofpolysacchar-fective.Thesematrixauthorsunderconditionsemphasizetheinwhichthemselves,importancephysicaltoinduceastructureinaofthemechanismssuctionresultingproveinef-from

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adsorptioncontributingofwaterbytheorganismsandthepromotetofabricdevelopment.Thesemechanismsgrowthdevelopoflivingstressesorganismsthatismpackingcells,areorganizationtheeffect,worksgrowingInaddition,infungihaveageneralobservedmassofDegensthatclaymycorrhizae(Tisdall,1991;slowedHadastheslakingetal.,thefissureandentanglingprocess1994).Thomasintheetal.(1993)tionetal.(1994)reportedthathyphaearenotsoeffectiveinsoil.theHowever,stabiliza-clearer,Nowofsandythatsoils.

theroleofmicroorganismsinsoilthebility.soilthemicrobialquestionpopulationiswhetherorortheirnotby-productsthereisanyprospectaggregationisbecomingtopromoteforaggregatemanipulatingsta-etThisisoneofthetargetsofsoilbiotechnology.Inthissense,productional.(1995)candemonstratedbemanagedthatbymicrobialNsupplyextracellulartoagriculturalpolysaccharidesRobersonsoils.(EPS)SoilFauna

structureEarthworms,materialsbyingestingmanysoilinsectandlarvae,mixinganditintimatelyotherlargerwithfaunamaystabilize1982;intheirguts,andegestingitascastsorpellets(TisdallhumifiedorganicpromoteJastrow1995).infiltrability,andMiller,1991;Oades,1993).EarthwormburrowsandstronglyOades,promoteSomeofsoildegradation.research,gashowever,exchange,Shipitalosuggestsandrootgrowth(PoierandRichter,andProtzthat(1988)earthwormproposeactivitythatcanalsomicro-aggregatessoilbyearthwormsresultsindisruptionofsomeexistingbondsingestionwithinmoreing.dispersiblethanandrealignmentuningestedofclaydomains.Therefore,freshcastsareoccursSignificantwhenwhenincorporatedimprovementorganicinthesoil,debriswatercontributingtosoilerosionandcrust-fromstabilitythefoodoffresh,moistcastsonlyactivitymoistcastsareagedordried.Nevertheless,inthesourcesispresentandbeetlesInaddition,enhancesnematodes,soilaggregatelongterm,castingtermites,stability.

ants,spidersHowever,andtanthemothsaddedcanthat,formingeneral,structurethebymesofaunacreatingandthelarvaeofvariousarebioporesnotconsidered(Oades,1993).moveintheformationofstructureinarablesoilsbecausetheyareimpor-themostsoilparticles.Nevertheless,themesofaunainconjunctiontoosmallwithtoporeslargerreaderForinfaunacouldwellbebeneficialinenhancingandstabilizingthemorewhichinformationtheylive.

ontheeffectoftheactivitiesofanimalsandFosterisreferred(1992),toSchraderHole(1981),etal.Lobry(1995),deandBruynPoierandandConacherinsoils,theRichter(1995).

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