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ProfessorofGeology

SonomaStateUniversity

Box279,Forestville,California95436

USA

wrightw@sonoma.edu.

SUMMARY

WinegrowersinSonomaCounty,California,arelearningtouseknowledgeofsoilpropertiestoproduceworldrenownedwines.Highlyvariablesoiltexture,mineralogyandchemistryreflectthecomplexunderlyinggeology,whichdictatessoilcharacteristics.Highqualitygrapesgrowonlyundercertainoptimumsoilconditions,includingabalanceofnutrientswithaCa:

Mg:

Kratiobyweightofabout6:

1:

1,andclayswithalowcationexchangecapacity（CEC）.LowCECclayisalsodesirableforminimumwaterretentionandslownutrienttransfertograpevineplants.

__ThehighestqualitygrapesgrowonsandstonesoftheWilsonGroveFormation,andextrusiverhyoliticlavas/volcanicashoftheSonomaVolcanics.Theseformationstendtoproducesoilsthatareclosetoperfectlybalancedinnutrientcontent,andhavelowcationexchangecapacities.Alluvialdepositsproducesoilsofvariablequalityandsuitabilityforwinegrapegrowth,owingtotheirvariablesourcecompositionandchemistry.SoilsdevelopedonorfromFranciscanComplexbedrockalsomaybesuitableforwinegrapegrowth,butmaycontainthemagnesium-andnickel-richmineralserpentinewhichcancausemagnesiumimbalanceinthesoil,ornickeltoxicity.Becausemagnesiumisahighlymobilechemicalelement,itspresenceinalluviumcanchangesoilsuitabilityforwinegrowthbothdownslopeanddownstreamfrommagnesiumsourcessuchasserpentine-bearingbedrock.Franciscangreywackesandstoneproducesidealsoilsforqualitygrapesinhighcoastalclimatezones.

Somemodificationoflocalsoilconditions（e.g.additionoflime,ornutrients）canbeundertakentoimprovelocalsoilconditionsforwinegrapegrowthand/orrootstockscanbechosentomatchsoilcharacteristicstoovercomesoildeficienciesorotherproblems.InSonomaCounty,considerationsofgeologyandsoilarecriticalintheproductionofhigh-qualitywines.Increasingly,theworldclasswinesofSonomaCountyareaproductofcarefulstudyofthesoilandclimateaspectsofterroir,bothofwhichcombinetomakethisaspecialplaceforwinegrowing.

INTRODUCTION

Qualitywinemakingstartsinthevineyard.Startingfromthe1500s,theFrenchexperimentedwithdifferentplantingsindifferentareasand,throughtrialanderror,commonlyfoundtheperfectmatchbetweengrapevarietyandenvironment（e.g.Pomerol,1989;

Wilson,1999）.InSonomaCounty,California,thefactorsthatdefineaqualityvineyardarebeingrevealed,onebyone.WhetherthewinesaretheflavourfulChardonnaysofCarneros,thewonderfullyfruityZinfandelsofDryCreek,ortherich,deepberryflavoursofRussianRiverPinotNoirwines,winegrowershavebeenworkingtoperfectthefinalproductbymatchingvarietalandrootstocktoanumberoffactorsinthevineyardenvironmentastheFrenchlearnedtodosomanyyearsago.Winegrowersnowrealizethat,inordertoproduceasuperiorwine,theyneedtobepartplantphysiologist,partsoilscientist,andpartmeteorologistandclimatologisttoachievetheirgoal.

Theconceptofterroirincorporatesthenotionofsiteandlocationwhichincludesallfactorsthatworktogethertocreatearegionwithparticularcharacteristicstomatchtheneedsofwinegrapesthatwillproducequalitywine.Thesefactorsstartwiththerockandresultingsoilthroughgeologicprocesses,continuethroughclimateandvineyardpracticeandthewinemakersart,andendwiththeconsumingpublic.Nowmanyvinophilesareawareofthescientificbasisforourexperiencewithterrior（e.g.Haynes,1999;

MeinertandBusacca,2000）.Inthispaperweconsider,fortheSonomaCountyAVA（AmericanViticulturalArea）,themajorgeologicfactorsofterroir:

theparentrockandthesoilmineralogyandtexture,andtheirimportanceinqualitygrapegrowthandwineproduction.

Unfortunately,manywinegrowersinCalifornia（andelsewhere）sometimescatertoomuchtothemajor“endmember”ofterroir,theconsumingpublic.Winecolumnistsandreviewers___seemtopreferbig,aromaticCabernetsaboveeverythingelse.Thispreferencehastendedtochangethewinemakingprocesstogethigherratingnumbers,tosomedegreemaskingtheimportanteffectsofterroir.

GEOLOGYANDTERROIR

__Thefoundationofterroiristheunderlyinggeology,includingbedrockandthesoilsthatdevelopfrombedrock.Wehaveallheardvineyardlegendsabouttherelationshipbetweencertainrocktypes,soils,andgrapequality.OnexaminationofthegeologyofsomerepresentativeterroirsintheSonomaCountyAVA,wecanexplaintheevolutionandcharacterofthesoilsandtheireffectonwinequality.Whencombinedwithwinegrowerexperienceswiththeseterroirs,knowledgeofsoilcharacterandevolutionisprovingtobeextremelyusefulinformationforall.

RocksandSoils

Rocksrepresentallkindsofchemistryasreflectedinthecrystalsormineralsofwhichtheyaremade,andthetextureswhichdescribethephysicalrelationshipsamongminerals.ThedominantchemicalcomponentofrocksfoundatthesurfaceisSiO2（=silica）,morecommonlyknownasthemineralquartz.Quartziscomposedofacovalently-bondedmoleculewhichisamongthemostdurableofnaturalmaterials:

itdoesnotbreakdowneasilyandisinertchemically.Mostsandysoilscontainquartz.Othermineralswithsilicacompoundsinvolvedindifferentstructureshaveweakerbondsandmostcommonlyareamixtureofmagnesium（Mg）,calcium（Ca）,potassium（K）,sodium（Na）andotherminorelementswhichcanbereleasedtotheenvironmentbymechanicalorchemicalweathering,andthuscanprovidethenutrientsessentialtothegrowthofwinegrapevines.This“bigfour”groupofnutrients（Mg,Ca,K,Na）invariouscombinationsisanimportantfactorindeterminingthecontributionofsoilchemistrytotheproductionofflavourfulwinegrapes.BelowwewillexplorehowthesenutrientsaffectvineplantsandwhatcombinationandratioofnutrientsworksbestforpremiumwinegrowinginSonomaCounty.

Themineralcontentofrocksisalsoimportantindeterminingsoiltexture-thenature,size,shapeandorientationandarrangementofparticles-whichinturndictatestherootgrowthdepthandtheabilityofthevineplanttoobtainrequirednutrients.Accesstonutrientsislargelyafunctionofwateravailabilityinthesoil,asitiswaterthatisneededtotransportnutrientstovineplantroots.Claymineralsplayacriticalrolehere,astheycanretainwaterandactasharborsfornutrientsbecauseoftheircationexchangecapacity（CEC）.TheabundanceandtypeofclaymineralsdeterminesCEC.KaoliniteandillitearelowCECclayswhileMontmorillinitehashighCEC.Nutrientions,whicharedominantlyelectricallypositivecations,aretrappedbynegativefringechargesonclaymineralsandhumus（decayedorganicmatter）.Claymineralsandhumushavelargesurfaceareasperunitweight,whichmakethemeffectivenutrientharbors.

Asandy,well-drainedsoilwithlittleornoclaymineralcontentandthuslowCECmayresultinalocaldeficiencyofnutrients,reducingwinegrapequalityproducingvegetaltasteinwine.Aclay-richsoilwithahighCECmayhavelocallyavailablenutrients,butcanalsocausetherootstobeimmersedinwater（havewetfeet）,thusexcludingoxygenwhichisneededforthenitrogencycleandotherprocessesthatfeedthevineplant.Deeprichsoilscreatehighvigorgrowthproducinglargewaterygrapes.AmoderatecontentofclaymineralswithalowCECseemstobeoptimum,withjustenoughtexturalandnutrientbenefits,andwater,tokeepthegrapesgrowingthroughthegrowthstage,andnaturallyslackingoffaftergrowthstopsandripeningbegins（Fig.1）.MikePorter,aSonomaCountyvineyardconsultant,hasrankedsoilsaccordingtoCEC,andinvariablyfindsthatalowCEC（3-14）insampledSonomaCountyvineyardsbegetsthehighestqualitygrapes（Porter,1994,unpublished）.

Clayandhumusalsocontrolormodifythephysicalpropertiesofthesoil.Theymayformflexibleelasticbridgesbetweensoilparticlestomaintainsoilstructureandpreserveporosity,evenafterbeingcompactedbyheavyequipment.Pebblesandrocksinthesoilseemtobeamajorfactorinwatersupply:

inclay-richsoils,pebblesandrockstendtobreakupthesoil,providingavenuesforwaterpercolationandrootpenetration.Ifpresentonthevineyardsurface,pebblesandrockscanabsorbheatduringthedayandpromoteindolentslowcoolingintheevening.

SurfaceProcesses

Thefirstgeologicprocess___insoilformationisthebreakdownofmineralsinplaceatthesurface.Thiscanhappenbythephysicalfracturingandseparationofmineralsormechanicalweathering:

orbychemicalchange–chemicalweathering-instigatedbydiluteacidsinwaterfromtheatmosphere.

Followthehistoryofasoil:

theparentmaterialbreaksdownbymechanicalweathering,fallingfromaclifforerodedfromastreambedandformingfragmentsofvarioussizesincludingsilt,sandandgravel.Asthesefragmentsaccumulateonmoreorlesslevelsurfaces,waterattacksthemwithvariousdiluteacidsandthisrearrangesthemolecules,ejectingsomeionsandaddingnewones,andchangingthestructure.Quartzmaintainsitschemistryandstructure,butacloseandveryabundantcousin,feldspar（K,Ca,Na,Alsilicates）canbeeasilybrokendownchemicallyandalteredtoyieldclayminerals,amongthemostcommonandimportantcomponentsofsoil.Chemicalweatheringreleasesnutrientcationswithtypeandabundancedictatedbytheoriginalchemistryoftheparentrock.Organicmaterialderivedfromdecayedpl