Table Of ContentREACTIONS WITH VARIABLE-CHARGE SOILS
Thesoilwasfor alongtimeregardedasacollection
ofinsolubleinert mineralfragments admixedwith
smallquantitiesofmoresolublesubstances .
This viewhadtheadvantageofsimplicity .
Butunfortunately itleads to inaccurateconclusions
Russell& Prescott,Journal ofAgriculturalScience 1916
Developments in Plant and Soil Sciences
I. J. Monteith and e. Webb, eds.,
SoilWaterand Nitrogen in Mediterranean-typeEnvironments. 1981. ISBN90-247-2406-6
2. J.e. Brogan, ed.,
Nitrogen LossesandSurface Run-offfrom LandspreadingofManures. 1981. ISBN 90-247-2471-6
3. J.D. Bewley, ed.,
Nitrogen and Carbon Metabolism. 1981. ISBN90-247-2472-4
4. R. Brouwer, I. Gasparikova, J. Kolek and B.e. Loughman, eds.,
Structureand Functionof Plant Roots. 1981. ISBN90-247-2510-0
5. Y.R. Dommerguesand H.G. Diem,eds.,
MicrobiologyofTropical Soils and Plant Productivity. 1982. ISBN90-247-2624-7
6. G.P. Robertson, R. Herreraand T. Rosswall, eds.,
Nitrogen Cyclingin EcosystemsofLatin Americaand the Caribbean. 1982. ISBN 90-247-2719-7
7. D. Atkinson etaI., eds.,
Tree Root Systemsand their Mycorrhizas. 1983. ISBN90-247-2821-5
8. M.R. Sari':and B.e. Loughman, eds.,
Genetic Aspects ofPlant Nutrition. 1983. ISBN90-247-2822-3
9. J.R. Freneyand J.R. Simpson,eds.,
Gaseous LossofNitrogen from Plant-Soil Systems. 1983. ISBN90-247-2820-7
10. United Nations EconomicCommission for Europe.
Efficient UseofFertilizers in Agriculture. 1983. ISBN90-247-2866-5
II. J.TinsleyandJ.F. Darbyshire,eds.,
BiologicalProcessesandSoilFertility. 1984. ISBN90-247-2902-5
12. A.D.L.Akkermans, D. Baker,K. Huss-DanellandJ.D.Tjepkema,eds.,
FrankiaSymbioses. 1984. ISBN90-247-2967-X
13. W.S.SilverandE.e.SchrOder,eds.,
PracticalApplicationofAzollafor RiceProduction. 1984.ISBN90-247-3068-6
14. P.G.L. Vlek,ed.,
MicronutrientsinTropicalFoodCropProduction. 1985.ISBN90-247-3085-6
15. T.P. Hignett,ed.,
FertilizerManual. 1985.ISBN90-247-3122-4
16. D. VaughanandR.E.Malcolm,eds.,
SoilOrganicMatterandBiologicalActivity. 1985.ISBN90-247-3154-2
17. D. PasternakandA.SanPietro,eds.,
BiosalinityinAction: BioproductionwithSalineWater. 1985.ISBN90-247-3159-3
18. M. Lalonde,e.Camireand1.0.Dawson,eds.,
FrankiaandActinorhizal Plants. 1985. ISBN90-247-3214-X
19. H. Lambers,J.J. NeetesonandI.Stulen,eds.,
Fundamental,EcologicalandAgriculturalAspectsofNitrogenMetabolisminHigherPlants. 1986.
ISBN90-247-3258-1
20. M.B. Jackson,ed.
NewRootFormationinPlantsandCuttings. 1986. ISBN90-247-3260-3
21. F.A.SkinnerandP. Uomala,eds.,
NitrogenFixationwithNon-Legumes. 1986. ISBN90-247-3283-2
22. A.Alexander,ed.
FoliarFertilization. 1986.ISBN90-247-3288-3
23. H.G.v.d. Meer,J.e. RydenandG.e. Ennik,eds.,
NitrogenFluxesinIntensiveGrasslandSystems. 1986.ISBN9O-247-3309-X
24. A.U. MokwunyeandP.L.G. Vlek,eds.,
ManagementofNitrogenand PhosporusFertilizersinSub-SaharanAfrica. 1986.
ISBN9O-247-3312-X
25. Y.ChenandY. Avnimelech,eds.,
TheRoleofOrganicMatterinModernAgriculture. 1986. ISBN9O-247-336O-X
26. S.K.DeDattaandW.H. PatrickJr.,eds.,
NitrogenEconomyofFloodedRiceSoils. 1986. ISBN90-247-3361-8
27. W.H.GabelmanandB.e. Loughman,eds.,
GeneticAspectsofPlantMineralNutrition. 1987.ISBN90-247-3494-0
28. A.vanDiest,ed.,
PlantandSoil:InterfacesandInteractions. 1987.ISBN90-247-3535-1
29. UnitedNations,ed.,
TheUtilizationofSecondaryandTraceElementsinAgriculture. 1987. ISBN90-247-3546-7
30. H.G.v.d. Meer, R.J. Unwin,G.e. EnnikandT.A.vanDijk,eds.,
AnimalManureonGrasslandandFodderCrops. FertilizerorWaste? 1987. ISBN90-247-3568-8
31. N.J. Barrow,
ReactionswithVariable-ChargeSoils. 1987. ISBN90-247-3589-0
Reactions with Variable-Charge
Soils
By
N.J. BARROW
CSIRO, Division ofAnimal Production
Wembley, Western Australia
1987 MARTINUS NIJHOFF PUBLISHERS ~.
a member of the KLUWER ACADEMIC PUBLISHERS GROUP "
DORDRECHT / BOSTON / LANCASTER
Distributors
for the UnitedStatesandCanada: Kluwer Academic Publishers, P.O. Box 358,
Accord Station, Hingham, MA 02018-0358, USA
for the UK and Ireland: Kluwer Academic Publishers, MTP Press Limited,
Falcon House, Queen Square, Lancaster LAI lRN, UK
forallothercountries: KluwerAcademicPublishersGroup, DistributionCenter,
P.O. Box 322, 3300 AH Dordrecht, The Netherlands
Library of Congress Cataloging in Publication Data
Barrow. N. ..l.
Reactions with variable-charge soIls I N...l. Barrow.
p. c•• -- (Develop.ents In plant and soil scIences)
ISBN 9024735890
1. Variable charge sotls--Mathe.atlcal .odels. 2. VarIable charge
solls--Data processing. 3. Soil solutlons--Mathe.atlcal .odels.
4. Soil solutlons--Data processing. 5. Soil absorption and
adsorptlon--Mathe.atlcal .odels. 6. Soil absorption and adsorptlon
-Data processing. 7. Ions--Mathe.atlcal .odels. 8. Ions--Data
processing. I. Title. II. Series.
S592.53.B37 1987
631.4·1--dc19 87-17785
CIP
ISBN-13:978-94-010-8141-2 e-ISBN-13:978-94-009-3667-6
DOl: 10.1007/978-94-009-3667-6
Copyright
© 1987 by Martinus Nijhoff Publishers, Dordrecht.
Softcoverreprintofthehardcover1stedition1987
All rights reserved. No part ofthis publication may be reproduced, stored in a
retrieval system, or transmitted in any form or by any means, mechanical,
photocopying, recording, or otherwise, without the prior written permission of
the publishers,
Martinus Nijhoff Publishers, P.O. Box 163, 3300 AD Dordrecht,
The Netherlands.
TABLE OF CONTENTS
Introduction VII
ChapterAI The ionicspecies present in soilsolutions I
Dissociationconstants I
Values for dissociationconstants I
The behaviourofsometypical anions 2
The behaviourofsomecations 4
Chapter A2 Variablecharge oxides as soil constituents and as models ofsoil
constituents 6
The oxides presentin soils 6
Synthetic oxides as models ofsoil oxides 7
The reaction ofcationswith oxides 8
The reaction ofanions with oxides IO
Changes incharge resultingfrom adsorption 12
Summary 14
Chapter A3 Describingand explainingthe adsorption behaviourofoxides 16
Thedifferences betweendescribingand explaining 16
Thesingle-planemodel 16
Thethree-plane model 21
Thefour-plane model 24
Summary 28
ChapterA4 The rate ofreaction with oxides 30
The rate ofthe adsorption reaction 30
The rate ofthe second reaction 31
Description ofdate usingthe modelfor rate ofreaction 32
Consequences ofthe model ofthe rate ofreaction 35
Summary 36
Chapter A5 Thefour-plane model and how it works 37
Varyingthe charge parameters 37
Varying the parametersthatdescribe adsorption 40
Varyingthe dissociationcharacteristics 42
Varyingthe background electrolyte 44
Varying the amount ofadsorption 47
Summary 48
ChapterA6 Fixed charge and variablecharge components insoil 49
Thedefinition ofsurface charge 49
Theeffects ofadsorbed ions 51
Separatingpermanentand fixed charge 51
Is itimportant to distinguish permanentand variable charge? 52
V
ChapterA7 The reactionofanions and cations with soil 54
Theeffects ofconcentration on sorption 55
Effects oftime onsorption 57
Describingthe rate ofthe reaction 58
Theeffect oflevel ofapplication 64
Effects oftemperature 64
Repeated additions ofa nutrient and desorption 65
Effects ofpH and ofelectrolyteconcentration 71
Summary 77
Chapter A8 Modellingthe reaction ofanions and cationswith soil 81
The threeassumptionsofthe model 81
Theformal equations ofthe model 85
Modellingthe characteristics ofsorption 87
The individualeffects ofthe parameters ofthe model 96
Summary 99
Chapter Bl Speciationin solution 101
Contains BASICprograms MOLLYand ZNOHPto
calculateion species
Chapter B2 Solvingsimultaneousequations - thefour-plane model 108
Contains BASICprogram BOWDEN whichsolvesthe
simultaneousequations ofthe model
Chapter B3 Derivingequationsto describe adsorptionand rate ofadsorption 118
Presents algebrato showthesolutions ofrateequationsin some
practicalcases
Chapter B4 Modifications to thefour-plane model 124
Contains BASICprogram ZNCL2and RATEOX which apply
the four-plane model to metaladsorption and to the rate
ofthe reaction
Chapter B5 Fittingmodels to data 142
Considers the problems offinding the"best"value ofparameters
and presents BASICprogram FITPINITwhich
fits datato thefour-plane model
Chapter B6 Applying models tosoil 156
Presents BASICprogramsTHETA and DIFPLUS which
simulatethe behaviourofsoiland FITTHETA and FITRATE
whichfit date to models
ChapterB7 Describingreaction ofions withsoil 178
Considersthe problemswhich arise wheresorptioniscalculated
from the change in solutionconcentration. Program PREDX
solves simultaneousequationsto predictsorption. FITFUNCfits
variousfunctions to the observed variable.
VI
Introduction
Thisbookisaboutions,aboutvariable-chargesurfacesandaboutmodels.Jti~aboutions
because most ofthe substances in which soil scientists are interested occur in the soil
solution as ions. This applies to both plant nutrients and pollutants. Thus the,reilction
between soil and say phosphate does not involve a substance called "phosphate"; it
involves phosphate ions. Ions are charged particles. When thesecharged particles react
with charged surfaces, the outcome ofthe reaction is affected by boththe charge onthe
particlesandthechargeonthesurfaces. Hencewealsoneedto understandthechargeon
the surfaces. Ifour understanding is adequate, we should be able to express our ideas
precisely- ideallybywritingequations. Unfortunatelytheequationsturnouttobequite
complexand to interrelatewitheachotherinacomplexway.Ifwewanttoenvisagethe
effect ofvarying some ofthe conditions we have to include the equations in,computer
programs. Becausethese programsdescribe physicalsystems, they are called models.
Awordabouttheroleandfunctionofmodelsisinorder.Acomputermodelisnomore
than a precisely expressed hypothesis. Like all hypotheses, it can be used to make
predictions. The predictions from some hypotheses turn out to be better than those of
othersandwearetoprefertheonethatmakesthebestpredictions. However,inpractice,
oldideasarenotabandonedreadily.Weclingtothemuntilthenewideasbecomefamiliar.
Forexample,someofthemodelsthathavebeenusedtodescribeionreactionwithsurfaces
haveapleasingfamiliarityaboutthem. Theyseemtoinvolveonlysmallmodificationsof
ideasaboutreactionsofionsinsolution. Perhapsthatiswhytheyhavebeenenthusiasti
cally adopted - eventhough they are notas good as alternative models. An important
roleforcomputermodelsistohelpustobecomefamiliarwiththeideasbehindthemodel.
Thebestwaytodothisistoplaywiththemodel.Theword"play"isusedquitedeliberately
becauseoftherelationbetweenplayingandexploring. Ifamodelisto beplayedwith,it
shouldnotbetoofrighteninganditshouldbereadilyaccessible.Inthejargonofcomputer
users, it should be "user friendly". These considerations have influenced my choice of
presentation. Onepossibilitywastotrytopresentgiantmodelsthatattempttocaterfor
everypossiblemethodofuse.This"blackbox"approachdoesnotinvolvetheuserinmuch
understanding and, since the possible uses are many, the model can become unwieldy.
InsteadIhavetried topresentprogramsthatfitsomecommonusagesandwhichcanbe
changed bythe user to suit other purposes. This is one reason for using BASICfor the
programs - it is fairly easy to make changes and to get them running. The main
disadvantage ofBASIC is that it has several'dialects'and this can restrict usage. I have
therefore used Microsoft-BASIC as this is probably the most common dialect. The
programswereinitiallydevelopedonothermachinesbutwereconvertedandrunusingan
IBM-compatible personal computer equipped with a "Trump card". This card permits
verysimpleeditingofprogramsandveryrapidcompiling.Theprogramsusedhererunon
itabouttentimesfasterthanonanIBM-compatiblemachine.Itthereforeovercomesthe
maindisadvantageofBASIC- itsslowness.TheTrumpcardwaspurchasedfromSweet
Micro Systems, Inc. 50 Freeway Drive, Cranston, RI 02920, USA.
Thisbookisdividedintotwosections.Thefirst sectiondescribestheobservationsthat
are to be explained and then outlines the models that may be used to explain them. It
movesfromtheparticulartothegeneral-fromthebehaviourofsubstances(suchasiron
oxides),thathavebeenused as modelsofsoilconstituents,tothebehaviourofsoilitself.
VII
ThissequencewaschosenbecauseIthinkthatmodelsatanylevelshouldbebasedon,an~
be compatible with, knowledge at a moredetailed level. Thus models ofsoilshould be
based on knowledge ofthe behaviour ofsoil constituents. Ofcourse, models become
impossiblycomplexifeverydetailofthebehaviourofaconstituentisincludedinmodels
ofthe behaviour ofthe whole. So, as we movefrom the particularto the generalsome
abstractingandsimplificationisneeded.Thesecondsectionisconcernedwithmakingthe
models work. It begins by discussing the problems ofsolving the sets ofsimultaneous
equations that comprise many models. It continues by discussing the problems of
allocatingvaluesto parameterstotestwhetheramodelcancloselydescribeasetofdata.
Thesetwoproblemsaresimilarbecausebothinvolverepeatedcalculationsusingdifferent
testvaluesuntilthebestsetofvaluesisfound. Finallylistingsarepresentedforseveralof
the modelsdescribed in thefirst sectionofthe book.
This book is mainlydirected towards those research workers in soil science who are
interested in either the supply ofnutrients to plants from the soil, or in the control of
pollutantsthathavereactedwiththesoil.Howeveritwillalsobeofinterestinawiderange
ofapplicationsinwhichionsreactwithsurfaces.Althoughafairproportionofthebookis
devotedtocomputerprograms,itisnotabookforcomputerprogrammers.Iamonlyan
amateurprogrammerandsotherewillbenoflashinglightsorblinkingmessages.Ihope
this means that the programs will be useful to, and usable by, the other very amateur
programmerswho want to explorethissubject.
VIII
Chapter At
The ionic species present in soil solutions
Itisabasicdoctrineofthisbookthatneither"phosphate",nor"zinc",noranyotherplimt
nutrient or pollutant reacts with soil. Rather, individual species - mostlyions - react
with soil. Most ofthe readers ofthis book will be familiar with the species present in
solutionforsomenutrientssuchasforphosphateandsulfate- buttherangeofbehaviour
ofthe various elementsofinterest is large. Understanding this behaviourisessential to
laterarguments and so will be summarized in this chapter.
Dissociation constants
Considerthe acid HAwhichdissociates:
AU
Forthis reaction, thedissociationconstantis:
Al.2
Where squarebracketsindicateconcentrations. Inspection ofthis equation will show
=
that, when KA [H+], the acid is halfdissociated. The negative logofthe dissociation
constant(pKA) is thus the pH at which the acid is halfdissociated.
Equation A1.2 mayalso beexpressed in terms ofactivities instead ofconcentrations.
Thisformatgivesamore-generalvalue.Itcanbeconvertedtothevalueappropriatefora
givenionicstrength as indicated, forexample,by Lindsay(1979).
Acidsmayalsobemultibasic- thatistheymaydissociatetwoormoreprotons.They
may also- be involved in polymerisation reactions or reactions with other ligands.
Equations to describe some of these reactions are given in Chapter BI together with
computerprograms to caculatethe proportions ofthespecies present.
Valuesfor the dissocation constants
Manytextbooksandhand-bookstabulatevaluesfordissociationconstants. Someof
these are reliable; others are not. For example, a range ofvalues can be found for the
dissociation constants ofhydrofluoric acid and for the second dissociation constant of
selenious acid. Obviously, some ofthe published values are wrong! One good source of
values for elements ofinterestto soil scientists is the book by Lindsay(1979). A much
largerrange ofelementsisconsideredbyBaesand Mesmer(1976). Thisbookisentitled
"The Hydrolysis ofCations", but, even so, it contains useful sections on some anions
includingborates,silicates,selenitesand molybdates. Anattractiveaspe<;tofthebookis
thatitismorethanatabulationofobservedvalues;itisalsoanevaluation.Theauthorsare
notafraid to rejectobservationsiftheyconsiderthemunreliable.TheyalsomakeitclearI
that accurate values ofsome ofthe constants simply do not exist. This is especially so,
wherethebehaviouriscomplex- involvingperhapsseveraldissociationstepsandseveral
I
ChapterAl Theionicspeciespresentinsoilsolutions
polymers.Inthesecases,thebehaviourcansometimesbedescribedalmostequallywellby
severaldifferentschemesandsotheallocationofconstantsisdifficult. Inthesubsequent
section,whichsummarisesthebehaviourofimportantelements,BaesandMesmer(1976)
has been used as a majorsource ofconstants.
Thebehaviourofsometypicalanions
Ofthe ions ofinterest, the behaviourofsulfate and selenate is probablythe simplest. In
bothcases,thepK2isnear2. AtthepH ofsoil,say4and above,theyarethereforefully
dissociatedtogiveSO~-andSeOtThepKofhydrofluoricacidisnear3.Insimplesystems
it is also virtuallyfully dissociated atsoil pHvalues to give F. Howeverfluoride has a
strongaffinityforaluminiumions(Lindsay1979)and,atpHvaluesbelowabout5.5,there
may be appreciable concentrations of aluminium in solution. This can mean that the
fluoride is largely presentas cationiccomplexessuch as AIF2+.
Thebehaviourofphosphateappearstobemorecomplexinthatitistribasic. However
thefirstdissociationconstantisnear2.ThishaslittleinfluenceinthebehaviouratsoilpH
values.Similarlythethirddissociationconstant,atabove12,alsohaslittleinfluence.Thus,
inpractice,wearemainlyconcernedwiththedissociationoftheacidH PO:;togiveHPO~
2
and H+.ThepKforthisdissociationisnear7. Hence,overthe rangeofnormalsoilpH
values,wechangefrommostlyH2PO:;atpH4tomostlyHPO~-atpH8.Overmuchofthis
range, the proportion present as HPO~-increases ten-fold for each unit increase in pH
(Fig.AU). This rate of change falls offas the HPO~- ion begins to dominate. These
changes in the proportions ofthe phosphate ions withchanges in pH are an important
componentofthechemistryofsoilphosphate. Unfortunatelytheyhavesometimesbeen
overlooked in studies onthe effects ofpH onphosphate retention.
1.0,--::------:--------::::::=--
so42-.SeO42-.F-
SeO/
C
(QQJ>>) H4B04
C. '-.H3SiO~
c:
o
~
10-6 Fig. AU Effect of pH on the fraction
~
present as the indicatedspecies at an ionic
strengthof0.03.ThusthelinelabelledSe02;
indicates the r-roportion of the selenite
presentasSeO;.
4 6 8
pH
2
