HowDoMineralogyandSoilChemistryImpactHowCloselyPotassiumSoilTestChangesAreRelatedToMassBalance?
MichelD.Ransom,[email protected]
AngelaFlorence,[email protected]
MichaelThompson,[email protected]
RandalSouthard,[email protected]
KnownFactsAboutSoilK• Kis7th mostabundant
elementinearth’scrust• Kisenrichedinsoilduring
weathering• Kexistsinsolution,
exchangeable,mineral,andfixedforms
• SoiltestKvaluescanchangeseasonally
• Inabilitytounderstandandpredictthesechangesisacriticalissueinnutrientmanagement
KdeficiencyinsoybeaninKansas,USA.PhotocourtesyofD.R.Diaz,K-StateResearchandExtension
Objectives
• RevieweffectsofmineralogyonKfixation
• DiscusstheroleinKfixationoftheoxidation-reductionofFeinstructuralpositionsoflayersilicates
• PresentamodelofKfixationthatcanaccountforsoiltestresultsanddeterminationsofmassbalanceofsoilK
KdeficiencyinsoybeaninKansas,USA.PhotocourtesyofDMengel,K-StateResearchandExtension
STRU
CTURA
LK
Micasand
Feldspars
K Minerals
Primarymineralsaremineralsthatcrystallizefromthemagmaofvolcaniceruptions
ThosethatcontainKare• Micas (e.g.,biotiteandmuscovite)• Feldspars (e.g.,microclineandorthoclase)
Exchangeable K
Soil Solution K
Fixed KStructural K
K
K
K
KK
K
K
KK K KK K K
K K
K K
KK KK K
K
http://legacy.belmont.sd62.bc.ca/teacher/geology12/photos/minerals/K-feldspar.jpghttp://skywalker.cochise.edu/wellerr/mineral/mica/6muscovite-peel.jpg
What is potassium fixation?
Li+
Be2+
Na+
Mg2+
K+
Ca2+
Rb+
Sr2+
Cs+
Ba2+
-3000
-2000
-1000
0
0 1 2Hy
dra
tion
ener
gy (k
J*m
ol-1
)
Ionic radius (Å)
Why is potassium fixed?
Source data:Faure, G. 1991. Principles and applications of geochemistry. 2nd ed. Prentice Hall, New Jersey. Burgess, J. 1978. Metal Ions in Solution. Ellis Horwood: Chichester, England.
How does clay mineralogy affect potassium fixation?
Building blocksSi tetrahedron ( ) à Tetrahedral sheet ( )
Al octahedron ( ) à Octahedral sheet ( )
Clay mineral classification criteria1. Layer type (1:1, 2:1, 2:1:1)
1:1 2:1 2:1:1
WeatheredMicas
ImagesbyMLThompson
• Micasare2:1layersilicateswithK+ ionsoccupyingtheditrigonal cavitiesbetweentetrahedralandoctahedrallayers
• Duringmicaweathering– Interlayersexpandduringweathering– Kisreleased– CECincreases
• WhenKisfixedbetweenthelayersofexpanded2:1layersilicates– Interlayerscontract– K+ ionsaretrappedinditrigonal voids– CECdecreases
AdaptedfromSchulze(1989)
Smectitesmayrangeincharge,butthehigherthechargethemorelikelytheyareto“fix”K.
Vermiculite isahigh-chargemineral,mainlyderivedfrombiotite.
AdaptedfromSchulze(1989)
Inacidic soils,vermiculite andsmectite canbe“pillared”withhydroxy-Alpolymers.Thesematerialsarecalledinterlayeredvermiculiteandsmectite.
← Al(OH)x + K
AdaptedfromSchulze(1989)
Smectite Vermiculite Illite/MicaLayercharge(charge/half unitcell)
dejure criterion-0.2to-0.6 -0.6to-0.9 -0.75to-1
Howdoes2:1claymineralogyaffectpotassiumfixation?
Smectite Vermiculite Illite/MicaLayercharge(charge/half unitcell)
dejure criterion-0.2to-0.6 -0.6to-0.9 -0.75to-1
Expansibilitydefacto criterion
High Moderate None
Vali, H., R. Hesse, and R. F. Martin. 1994. A TEM-based definition of 2:1 layer silicates and their interstratified constituents. Am. Miner. 79: 644-653.
TEM images composed of isolated 2:1 layer silicates (smectite-group phases) and packets of discrete illite from Vali et al. (1994).
2:1 mineral classification criterion
Expansibility = f(layer charge,
charge distribution)
Expansibility = imperfect proxy for layer charge
Howshouldstructuralironredoxstateaffectpotassiumfixation?
Chen, S. Z., P. F. Low, and C. B. Roth. 1987. Relation between K fixation and the oxidation state of octahedral iron. Soil Sci. Soc. Am. J. 51:82-86.Khaled, E. M. and J. W. Stucki. 1991. Iron Oxidation State Effects on Cation Fixation in Smectites. Soil Sci. Soc. Am. J. 55(2): 550-554.Shen, S. and J. W. Stucki. 1994. Effects of iron oxidation state on the fate and behavior of K in soils. p. 173-185. In J. L Havlin, J. Jacobsen, P. Fixen, G. Hergert (ed.) Soil testing: prospects for improving nutrient recommendations. SSSA Special Publication 40, Madison, WI.
Oxid
ized
smec
tite
Red
uced
smec
tite
Fe3+ + e- à Fe2+
é negative layer chargeé layer contraction
é fixation
Relationship between Fe reduction and é fixation well established for smectites containing
octahedral Fe.
Reductions of structural Fe in smectites and vermiculites causes an increase in layer charge
and an increase in K fixation.
SoilRedoxEffectsonSoilTestK
• Soilmineralogicalcharacteristicsarenotdynamicduringgrowingseason.
• SoilredoxconditionscanbedynamicovershorttimeperiodsleadingtochangesinoxidationstateofstructuralFeinK-fixingminerals.
• Reducingconditions:• Kfixation↑
• Oxidizingconditions:• Krelease↑
PhotocourtesyofK-StateDepartmentofAgronomy
K Fixation Model
K+
Interlayer potassium + (Forces of contraction > Forces of expansion)=
Potassium fixation
KFixationModel– KMassBalance• CorrelationofsoiltestmeasurementswithmassbalanceofKisneededformaximumKefficiency
• DifficultbecauseKfixationvarieswith– Mineralogyoflayersilicates– OxidationstateofFe– PresenceofhydroxylAlinterlayers
• VariationofKfixationcausedbythesefactorscausesspatialandtemporalvariationsofsoiltestK
• ForsoilswithKfixation,traditionalsoiltestKmeasurementsdonotrepresenttheseasonalandspatialvariationsoccurringinthefield
QuestionsandDiscussion
STRU
CTURA
LK
Micasand
Feldspars
Cropresidue
Rootuptake
Harvestremoval
Plantbiomass
grain
Leachingandlateralmovement
ExchangeableKinorganicmatter
ExchangeableKinclayminerals
(smectite,vermiculite)
Evapo-transpiration
Redox processes,pH
pH,siteselectivity
Advection,dispersion
Diffu
sion
K/√(Ca+Mg)
K/√(Ca+Mg)
K in Soil – A beautiful mystery
Diffu
sion
pH,Eh,organicanions
Fertilizeramendments
Dissolution
Thompson– ISU– Dec.2016
FixedKEdgesites:Di-vermiculiteRedoxsites:Tri-verm andHCsmectiteInterlayersites:HIVandHIS
ExtrapolateMineralogicalandK-FixationDataUsingSoilSurveys
• SoilsurveysareavailableinelectronicformatforvirtuallyallofUS• CanusesoilsurveystoextrapolatemineralogicalandK-fixationdatafor
horizonsataspecificsitetootherlandareasbydevelopingaprocedurethatdoesnotmakemajorchangestoSoilTaxonomyorrequiresdevelopmentofnewlaboratoryprocedures
• Requiresrecognitionofparticlesize,mineralogy,andCECclassesthatexhibitKfixationusingdataobtainedwithUSDA-NRCSSoilCharacterizationQueryInterface– http://ncsslabdatamart.sc.egov.usda.gov/querypage.aspx
• Particlesizeclassesneednoadditionalmodification– ≥18%clay:loamyandclayeyparticlesizeclasses(fine-loamy,fine-silty,fine,
andveryfineparticlesizesubclasses)• ExistingCECclasses(superactive,active,semiactive,andsubactive)need
morework– NeedtomeasureCECusingKasanindexcation inadditiontoregularindex
cation (e.g.,Ca,NH4,orNa)– Determine%CECreduction(Ransometal.,1988)andincludeindatabase– AddreductivesubclassestoCECclassforsoilswithCECreduction>~10%
SoilPropertiesAffectingKFixation• Kfixationoccurswithvermiculiteand
transitionalmineralsformedfromweatheringofmica(Hartleyetal.,2014;Tran,2012)– Layerchargeoccursinbothtetrahedraland
octahedralsheets(Feinoctahedralpositions)– Vermiculite,transitionalvermiculite/smectite
(TVS),andregularlyinterstratifiedmica-smectiteormica-vermiculite(RIMS/RIMV)
• Mineralogyclassesneedrevision– Currentclassesaretoobroad,e.g.,smectitic– Mineralogyoffine-loamyandfine-silty classes
arebasedonsandandsiltfractionmineralogy– ExpandcontrolsectiondepthtoincludeA
• Addnewk-fixic mineralogysubclasswhereK-fixingminerals>~5– 10%inclayfractionregardlessofparticle-sizeclass
• Eram series(Site10fromHartley,2010)
LayerSilicate
Mg-EG(Å)* Mg-GLY(Å)
Mica 10 10
Smectite(lowcharge)
17 18
Vermiculite 14 14
TVS 17 14 - 15
RIMS/RIMV(Hydrobiotite) ~24(Mg-25)
Fine,mixed,active,thermicAquic Argiudolls
Fine,k-fixic-mixed,reductive-active,thermicAquic Argiudolls
CECsubclassK-fixingmineralogysubclass *Indicatestypicalproceduredoneinlabs
