The land quality concept as a means to improve communications about soils
J. Bouma
Member Scientific Council for Government Policy,the HagueandProfessor ofSoils,Lab.
Soil Science andGeology,Wageningen University, theNETHERLANDS
E-mail: [email protected]
Summary
Soilexpertise isnot communicated effectively enough tothepublic atlarge,nortoplanners
andpoliticians. Useoftheland quality (LQ)conceptand emphasis onsoilbehaviorasa
function ofmanagement areexpected tobehelpful in improvingcommunications. Existing
definitions of"soil quality"and"sustainable landmanagement"areanalyzed toderivea
procedure for defining LQindicators ofsustainable land management. Land-ratherthan soil
qualitiesareconsidered toreflect theimpact oftheclimateandthelandscape onsoil behavior.
Land quality isdifferent fordifferent typesoflanduseandattention isarbitrarily confined
heretoagriculture.Simulation modeling ofcropgrowth and solutefluxesisused to define
land quality (LQ)astheratiobetween yield andpotential (orwater-limited)yield (x 100),
whichdefines a"yield gapLQ".Forsoilswithnutrient mining,anutrient-depletion LQis
defined. Theactual agro-ecological condition and itspotential,both expressed byLQ'sfora
given pieceofland,isconsidered hereasindependent input intobroader land-use discussions
which tendtobedominated bysocio-economic andpolitical considerations. Agro-ecological
considerations shouldnotbeheld hostage toactualsocio-economic and political
considerations,which maychange inthenearfuture while LQ'shaveamuch morepermanent
character. Theproposed yield-gap LQreflects yieldsandrisksofproduction as simulations
aremadefor manyyears,and soilandwaterquality associated withtheproduction processis
taken intoaccount.Yields andpollutionrisksareexpressed for Dutchconditions intermsof
theprobability that groundwater ispolluted withnitrates.Theproposedprocedure requiresthe
selection ofacceptableproduction andpollutionrisks,before aLQvaluecanbeobtained.
Existingdefinitions implicitly emphasizethefieldandfarm level.However, LQisalso
important attheregional and higherlevel,which,sofar, hasreceived littleattention.Then,
again,anagro-ecological approach issuggested whendefining LQ'sasinput intothe
planningprocess,emphasizingnotonlyanindependent assessment ofthepotential for
agricultural production,butofnatureconservation aswell.
Keywords:agro-ecosystems, nitratepollution,riskassessment, potential cropyields
Introduction
Thecondition ofsoils isgenerallynotaprime concern when environmental problemsare
discussed insociety.Globalclimate change duetogreenhouse gasses,the"ozon hole",solid
andliquidwastedisposal and, increasingly, waterquality andworldwide watershortageshave
beenmoresuccessful incatchingtheimagination ofthepublic atlarge.These,ofcourse,are
worthyobjectives ofconcern andstudy,butseriousconcernsaboutthe environmental
degradation ofsoilsresulting from intensive forms oflanduseexceedingtheecological
carryingcapacityhavewidelybeenreported butdon't appeartoleadtoalarmoraction(e.g.
Oldeman, 1994).Approximately 15%ofthetotal landsurface oftheworld isdegradedasa
resultofadversehumanaction.Worldwide,about 38% oftheagricultural landisaffected by
significant human-induced soildegradation,ofwhich 56%iscausedbywatererosion,28%
bywinderosionand 7%bynutrientdecline.Substantialareasofprimeagricultural landare,
withoutmuchopposition,permanentlytaken outofproductionwhenused for cityexpansion
andconstruction ofshoppingmallsorroads.Howcanthisrelativeindifference beexplained?
Withoutmakingattemptstospeculateaboutthepsychologyofpublicawareness,wemay
statethatthesoil scienceprofession hasbeenlessthansuccessful incommunicatingits
expertiseeffectively tothepublic,toregulatorsandtopoliticians.InthispaperIwillattempt
tobriefly analyzethisproblem.Ofcourse,thechallengeistoimproveourefforts toshowthat
soilsareimportant.Onewaytodosoistodevelop indicatorsthatcanquicklyexpressthe
valueorqualityofsoils.Suchindicatorsarecrucialinthemodern worldwhereattention
spansareveryshortandwhereattention isincreasinglyattunedtoattractive" soundbites".
Soilorlandqualitycouldbeanideal indicator aswillbefurther explored inthispaper.Our
colleaguesineconomicsand sociologyhaveused attractiveindicators for years(theGross
NationalProductmayserveasanarbitraryexamplehere).Suchindicatorshavesofarnot
beendefined for soils(Pieri etal.,1995).
Whendiscussing soilqualityindicatorsinthecontext ofachievinganeffective external
communication,wemust linksoilsandtheirpropertieswithfunctioning anduse.Thisnot
onlyrelatestoagriculture,butalsotonature areasandrecreational facilities. Usepatternsand
managementpracticesareonlyacceptablewhentheyaresustainableinthelongrun.Wewill
therefore discussthesoilqualityaspectinthecontextofsustainable landuseand
management.
Sustainablelandmanagement hasbeendiscussed widelywithin soilscience andagronomy
andguidelines havebeenproposed byFAO(1993).Theseguidelineslist four criteria for
sustainable landmanagementrelatingtoagriculture:(1)production shouldbemaintained;(2)
risksshould notincrease;(3)qualityofsoilandwatershouldbemaintained,and(4)systems
shouldbeeconomically feasible andsociallyacceptable.Soilqualityispartofthis definition
andthistermhas,inturn,beendefined inanother contextas:"thefitness ofaspecific kindof
soiltofunction within itscapacityandwithinnaturalandmanaged ecosystemboundaries,to
sustainplantandanimalproductivity, maintainairandwaterqualityandsupporthuman
healthandhabitation"(Karlenetal, 1997).Eventhough itistemptingtodiscussthese
definitions assuch,wewouldratherfocus onajoint analysisofbothdefinitions, leadingto
descriptionsofsoilquality indicatorsthathaverelevancefor sustainable land management.
Overall,theobjective istodevelopexpressionsfor soilqualitythatmayservetoimprove
communication ofsoilexpertisetousersandstakeholders.Attention inthispaperwill
therefore bepaidtodiscussionson:(1)creatingawarenessaboutsoils;(2)sustainable land
management; (3)soil quality and soil quality indicators,and (4)useofthequalityconceptto
improvecommunication. The focus willbeonthefieldlevel,theusualarea ofactivityof
farmers. However, thequalityconcept isalsoimportant atotherspatial scales,which haveto
beconsidered whendealingwithpolicy issues inregions orcountries. Inthisbroader context,
soilqualityhasnotyetbeen analyzed andthisissuewilltherefore alsobeexplored inthis
paper.
Creating awareness about soils
Eventhough manypeople haveastrong affinity with"the land"theyliveon,itisarather
abstract affinity. Soilsoccur indarknessbelow the surface oftheearth and, incontrastto
weatherandwater, arenotdirectlyvisibleandcannot beexperienced bythesensesunless
exposed inaholeorinanexcavation.Thevertical succession oflayers,which is
characteristic for soils,canbeshown inmonoliths andcanbedisplayed onwallsof offices,
schoolsand otherbuildings.IntheUSA, everyStateoftheUnion hasa"StateSoil",justlike
a"StateBird","StateAnimal"etc.Thevertical succession oflayers inany soil isoften quite
beautiful, certainly when awide arrayofcolorsisexposed asinpodzols and gleysoils.
Successful exhibitions ofsoilmonoliths havebeenheld inart galleries!Butsoilsrepresent
morethanjust prettypictures!Toreallyunderstand thesignificance ofsoils,a functional
approach isneeded which demonstrates functioning within alandscapecontext asgoverned
byinteracting physical,chemical andbiological processes.Morespecifically, functioning
relatestosupplyingwaterandnutrientstoagricultural cropsand varioustypesofnatural
vegetation,purification ofpercolatingwastewaterandcarrying loads.Allthese functions
takentogetherdeterminethevalueofsoils for society.
Howcan the functioning of soilsbecharacterized? Yields ofcropsareoften known,asare
typesofnaturalvegetation thatoccurondifferent soiltypes.Chemical,physicaland
hydrological properties areoften known toacertain extentbutoften inarather qualitative,
descriptive andstaticmanner.Howcanwecatch theeffects ofinteractingphysical,chemical
and biological processes,which determine thedynamic character ofsoilsandtheir
functioning inecosystems?
Manymonitoring techniques areavailablenowtomeasurevarying soilproperties overtime.
Useoftransducers, recorders andproximal andremote sensingtechniques allowsusto"take
thepulse ofmotherearth"(e.g.Bouma, 1999).Suchtechniquesare,however, costlyand
widespread application istherefore unlikely.Theadvance ofcomputer simulation ofsoil
processeshas,however,proved tobequitehelpful indocumenting effects of soilprocesses
andthe impact ofman. (e.g.Alcamo, 1999).Someexampleswill becited later inthispaper.
Useofcomputersimulation techniques todemonstrate thedynamic behavior ofsoilsasa
function ofmanagement byman ismuchmoreattractive for themodernsoil userthan classic
soil interpretations, aspresented insoil surveys,which listrelative suitabilities ofsoilsfora
seriesof landuses.Thesophisticated modernuserwantsarangeofoptionstochoosefrom
whencopingwith land-useproblems.Heisusedtomakinghisownchoices anddoesnotlike
tobepresented withsingle"best"solutionstoproblems,developedbyotherswithouthis
activeparticipation.Hewantsresearchers todefine a"windowofopportunity"for anygiven
soil,whilehe(orshe)makestheappropriatechoices.
Weexpectthatgeneralsoilawareness can increasewhensoilqualities aredefined inthe
contextofcharacteristic "windowsofopportunity"for anygivensoil,andwhenthesesoil
qualitiesarecommunicated effectively tovarioususersofsoil information.
Sustainable land management
Intheirdefinition ofsustainableland management, FAO(1993)clearlyfocused on
agricultural production.Thiscanbeachoice,butweshouldrealizethat landmanagement has
broaderimplicationsthanagricultural production alone.Thisisexpressed inthedefinition of
soilqualitywherenaturalecosystemsarementioned aswellashumanhealth andhabitation.
Bethatasitmay,itshouldberecognized thatFAO(1993)defines sustainable land
management andnotsustainabilityassuch.Emphasis istherefore onspecific actionbyman
andnotonvagueconceptual definitions, whichisattractive.Theelementsofthe definition
arelogicalbuttheyshould, forbetter understanding,begrouped intotwocategories.The first
threedefine plausibleagro-ecological aspects:productivity,risksduringproduction and
qualityofsoilandwater.Thefourth category isdifferent: economic feasibility (whichalso
stronglyeffects social acceptability) islargelybeyondthecontrol ofthe landmanager.Even
thoughamanagement schememaybesustainable from anagro-ecological pointofview,it
canbeeconomicallyunsustainablebecauseofpoorprices for agriculturalproduce.This
shouldcertainlybeconsidered butitisunwisetonotexplorevarious agro-ecological
management options,evenwhenatthispoint intimetheseoptionsarenoteconomicallyor
sociallyfeasible. Timesmaychange:doublingoftheworldpopulation, exhaustion of fossil
fuels requiringgrowthofenergycropsandneedsforrawplantmaterialtobegrown in future
byfarmers assourcematerials for industry, arelikelytochangewhatmayatthispointintime
beanegativeeconomicoutlook for agriculturalproduction.Agro-ecological research, focused
ondeveloping future sustainable landmanagementpractices,shouldnotbeheldransomto
currenteconomicconditions.
How,then,tocharacterizeagriculturalproduction,productionrisksandthequalityofsoiland
water?Monitoringofagriculturalproduction systemscanproviderelevant information but
proceduresaretediousand costlyandmonitoringoverperiods ofmanyyearsisneededto
coverunusualweatherconditions.Thelatterareneeded toadequatelyexpressrisksof
production,which areassociatedwithadverseconditionsthatonlyoccur occasionally.
Monitoringoversuchextendedperiods oftimeissimplynotfeasible. Fortunately, inrecent
years,systemsanalysisofagriculturalproduction systems,usingcomputersimulation ofcrop
growth,hasdevelopedtotheextentthatmethodsareoperational andveryuseful forthe
analysis ofsustainable management systems. Examples willbeprovided inalatersection of
thispaper.
Soil quality
Manystudieshavebeen made about soil quality (e.g. Doran andJones, 1996)but there isas
yetnot awell defined, universal methodologytocharacterize soilquality andtodefine aset
ofclear indicators.Doran and Jones(1996)present four physical, four chemical andthree
biological indicators which, according totheauthors,togetherrepresent aminimal datasetto
characterize soil quality. But noexamples areprovided. Gomesetal.(1996)define six
indicators andthreshold values for measuring sustainabilityofagricultural production systems
atthe farm level. Implicitly, higherdegreesofsustainability correspond with higher soil
qualities.Otherexamplesof soilquality studies asreported byDoran andJones(1996) list
seriesofsoilcharacteristics as indicators ofsoilqualitybutnonereally addressthe broad
spirit orscopeofthe Karlen et al.(1997) definition.
Anumberofgeneral considerations canbe madewhendefining soil quality:
Any"fitness ofaspecific kind ofsoilto function" depends stronglyonclimatic conditions,
which varyamongclimatic zoneswhile theweatheralsovariesatanygiven location during
theyear.Aside from this,soilsareparts of landscapes and their functioning isstrongly
affected bytheirposition inthese landscapes.Toconsider"soil"without theclimaticand
landscape context,when defining quality, isnotrealistic.Weshouldtherefore speakabout "
landquality"ratherthan "soil quality"(Pieri etal, 1995),astheterm" land"expresses the
broadercontext inwhich thesoil functions.
"Fitness to function" willalwaysbeconsidered inpractice inrelation toothersoils.
Production levelsinyearswith favorable weather maynot betoodifferent among different
soilsbuthighquality soilstend toproduce wellunderadverseconditionswhenothersoils
don't deliver.Ofcourse,production figures alonearenotenough:cost andquality ofproduce
mustbeconsidered aswell.When similaryieldsarereached atlowercostsorwith ahigher
quality, thequalityofthesoil maybeconsidered to behigheraswell.Butherethe
management factor is introduced.
Apotentiallyhigh-quality soil can still have lowyields,resulting from poormanagement,
while low-quality soilsmay havehigh yields duetoexcellent management. When defining
landquality for agiventypeofsoil,attention shouldbefocussed oneffects ofawiderangeof
management typesasapplied for agiven growingseason. Studyingasinglegrowingseason
onagiven farm will notprovideuseful information toderive representative land quality
indicators. Infact, ahigh landquality impliesthathigh yieldsmaybeobtained even under
adverse conditions andundermediocremanagement. High quality landhasahigh resilience,
which isbestdescribed astheabilitytobouncebackafter theeffects ofpoormanagement or
poorweatherconditionshavebeen suffered. Lowquality landdoesnothavesuchresilience.
Theabovepointdescribed short-term effects ofmanagement during agrowingseason.For
instance, compaction of structure whendrivingoverwet landwhichqualifies aspoor
management. When the farmer would havewaited afew days,compaction mightnothave
occurred. Butthereisalsoalong-term effect ofmanagement. Whenpoormanagement leads
to,for instance,erosion orstrongsubsoilcompaction,changes inthesoilarepermanent. Lost
soilwillnotreturn and subsoil compaction isdifficult toremove. Long-term management
mayalsobe favorable, for instancewhen increasingtheorganicmattercontentofthesoilby
organicmanuring.Droogers andBouma(1997)usedthetermgenoform todescribethe
genetic soiltypeandphenoform todescribelong-term effects ofmanagement inthe samesoil
type.
"Fitness tofunction" iscloselytied tolandutilization type.The function isquitedifferent in
naturalecosystems orinagricultural production systems.When functioning isdirected
towards:"human health andhabitation"onecould also thinkabouthousingdevelopmentsand
recreation facilities. Here,attention will,arbitrarily,beconfined toagricultural production
systems,which isinlinewiththeearlierdiscussion onsustainablemanagementsystems.
Thedefinition ofsoilqualitymentions:"aspecific kind ofsoil",which isnot further
explained. Webelievethatitwouldbewisetousesoilsurveysandsoiltaxonomy systems
heretodefine specific soiltypes(genoforms, asmentioned above)thatarewelldefined, also
intermsoftheirpositions inlandscapesasshownonsoilmaps.IntheUSA,thesoilseries
wouldbeaproper "carrier"ofinformation. Inthecontext ofthelandqualityconcept,
however, emphasiswouldbeonland behavior intermsofcropproduction,itsrisksand
environmental sideeffects. Theimplicithypothesiswould bethateachsoilseries,occurring
inagivenagro-ecological zonewithacharacteristic climate,hasacharacteristicrangeof
productionrates,risksandsideeffects. Wemaycallthisacharacteristic"windowof
opportunity". Wellexpressed phenoforms ofagivengenoform mayhave significantly
different windows!Also,different genoforms havenotalwaysdifferent rangesofproperties:
twosoilsmaybegenetically different butmayfunction identically. Inallcasessuchranges
shoulddefine landquality.Again,highqualitylandperforms welleven underadverse
conditions intermsofweatherandmanagement. Lowqualitylandperforms poorlyeven
undergoodconditions ofweather andmanagement.Thebigchallengenowistoquantify such
broaddescriptions.
The landqualityconceptshouldalsoindicatewhetherornotquality canbeimproved by
management, eithershort-orlongterm.Isthereanabsolutetheoretical upperlevelof
production? Ifso,howcan itbereached? Canitbereached withoutadversity affecting
environmental quality?Howdoesthatlevelcomparewiththelevelofothersoilsinother
areas? Suchquestionsarelikelytobeaskedbyfarmers andother landusersandtheyneedto
beaddressed.
Considering theabove,wedecidedtousesimulation modelingofcrop growth,asa function
ofwaterandnutrient regimes,asameanstocharacterize agriculturalproduction ofagiven
typeofsoil, located within agivenclimaticzone.Thismaybeeitheragenoforni orwell
defined phenoforms ofagivengenoform. Ofcourse,suchsimulations should bevalidatedby
field measurements. Models effectively integrate soilandweather astheycalculate daily
production values.Indeed,data arethusgenerated tocharacterize " land"qualities.By
calculatingpotential productions,anabsoluteupper yield level isobtained for anygivensite
(atleast for agivenplantvariety).Potential productions arebased onclimatic dataonlyas
wellasplant parameters specific for agivenspecies.Water andnutrient supplyaresupposed
tobeoptimalwhilepestsand diseases donotoccur. Water-limited yields canalsobe
calculated,takingintoaccount thewaterthatcanbesupplied undernatural conditions tothe
cropatthegivensitebythesoil,again assuming thatpestsanddiseases donotoccur.Such
yieldsare lowerbecause watersupplyisnotalways optimal.Ofcourse,pestsanddiseases
mayoccurbuttheyoccur independently ofthe siteconditionsthat areusedtocalculate waterlimited yields.Onemajor advantage ofsimulation models isthat calculations canbemade for
manyyears,providingexpressions fortheeffect ofthevaryingweatherconditions thatcould
neverbeobtainedbymonitoring.This,however, isonlytrue whenmodelshavebeen
independently validated usingfieldmeasurements! Someexamples willnowbepresented to
illustratetheproposed procedure.
Land quality indicators
Exploratory studiesfor seven major tropical geno-and phenoforms
Boumaetal.(1998)studied sevenmajor tropicalsoilsto illustrateuseofthe land quality
concept for exploratorypurposes (Table 1).Potential andwater-limited productionswere
calculated intermsof" grain equivalents",andsoil dataneeded for themodel included
estimated infiltration rates,depthofrootingandavailablewater. Thereaderisreferred tothe
sourcepublication for details(Bouma etai, 1998;Penning deVriesetal., 1995a, 1995b).
Landquality(LQ)wasdefined as:
LQ=(yield /potential yield)x 100.
Yieldwasexpressed hereasacalculated water-limited yield,butreal yieldscould beusedas
well.Threehypotheticalphenoforms weredefined inthisexploratorystudyfor eachofthe
seven genoforms inTable 1.Effects oferosionwereexpressed byremoving theupper40cm
to50cmofsoil,depending onsoilproperties.Compaction was expressed byrestricting
rootingto40cm,adepthatwhich aplowpan mayoccur. Limingexpresses apotentially
favorable effect ofmanagement. Acidsubsoilsthatrestrict rootingcanbeopenedupbydeep
liming.Potential productions,aspresented inTable 1,varybetween 8and23tons/ha,
illustratingclimatic effects intermsofradiation andtemperature. LQvalueswererelatively
high, indicatingrelativelyhighrainfall ratesleadingtoproduction valuesthatarerelatively
closetopotential values.TheZambia soil,however, occurringinadryerclimatehadaLQof
only50.Real yieldscanalsobeused intheequation,rather thanwater-limited yields.Then,
animpression isobtained oftheyieldgap forthegiven siteand fortheactual landqualityof
themanagement systembeingused. Erosion clearly leadstoreductions ofLQvalues,
particularlyintheOrthicAcrisol inChinawhereerosionresults intheoccurrence ofanacid
subsoil nearthe soilsurface whichrestrictsrooting.Theeffects ofcompaction arestronger
thanthoseoferosion undertheassumed conditionshere,whileliminghasastrongeffect due
todeeperrooting.Valuesreported arerelativeLQvaluesinrelationtothevalueforthesite
based onwater-limited yields.Anabsolute LQvaluecanalsobedefined when calculated
yieldsarecomparedwiththehighestpotentialproductionthatispossibleintheworld
(estimated tobe41.1tons/ha).Then, LQvaluesarerelativelylowintherangeof20-39.
Theexploratory analysispresentedhereallowsaroughestimateoftherelativeeffects of
somemanagement measures,usingasimplesimulation model andestimatesofchangesin
soilparametersthat areassociatedwithcertaineffects ofmanagement. Also,averageclimatic
datawereusedhere.Runningthemodelfor anumberofyearswithreal-timeweatherdata
would havegivenabetterimpression aboutyield stabilityandrisks.Still,theexploratoryLQ
analysispresented herecan,inouropinion,beuseful whendiscussingpossibleeffects of
different typesofmanagementandincomparingdifferent soils.
Table 1.Potentialyields,intermsofgrain-equivalents,forsevenmajorsoiltypesofthetropicsand
derivedrelativeandabsolutelandquality(LQ)values,basedonwater-limitedandpotentialyields.
AbsoluteLQvaluesarederivedfromthemaximumpotentialyieldintheworldof41. 1 ton/ha/yr.
Valuesarederivedforthesevengenoformsandforthreehypotheticalphenoforms,expressingthe
effects oferosion,compactionandliming,(after Boumaetal., 1998).
Soil
Name
Type
Prot.Prod.
Rel.
Absolute
Tons
Land
Land
Dry
Quality
Quality
Matter/
Water
Erosion
Comp
Liming
Water
1
Ferric Acrisol
China
Ha*Yr
13
Limited
96
85
action
75
100
Limited
32
2
Orthic Ferralsol
Indonesia
18
90
75
70
100
39
3
Cambic
Colombia
12
96
80
72
100
31
Arenosol
4
FerricLuvisol
Nigeria
14
90
75
55
90
32
5
Ferralic
Nigeria
14
85
70
50
85
30
Arenosol
6
Orthic Acrisol
China
8
90
50
85
100
20
7
Orthic Ferralsol
Zambia
23
50
40
30
50
27
Yieldgapandsoil nutrientbalances in Africa
InaWorld-Bank funded study, Bindraban etal.(2000) studied land quality indicators for
African soilsalsousingpotential,water-limited, nutrient-limited andactual yields.Incontrast
totheexploratory studyofBoumaet al.(1998),theyused specific data forfieldsites.The
difference between actual andpotential yield expresses theyieldgap,which isconsidered to
representtheLQindicator. This isnotunlikethe approach followed byBouma et al.(1998)
exceptthat thelatterauthors expressed LQasanumberbetweenzeroandonehundred and
thattheyalsodefined anabsolute LQ.Thedifference betweenpotential and water-limited
yieldprovidesan indication what might beachieved byimprovedwatermanagementwhile
thedifference between water-limited andnutrient-limited yield indicatesthepotential impact
ofimproved fertilization. Bindraban etal.(2000) also added asecond LQ,which defines the
soilnutrient balance.Thisishighly relevant for Africa wheresoilminingisrampant.Thesoil
nutrient balance isthenetdifference betweengrossinputsandoutputs ofnutrients tothe
systemand isexpressed inrelation tothesoilnutrient stock.Ofcourse,effects ofdepletion
aremuchworseinsoilswithalowstock ofnutrients ascomparedwith soilshavingahigh
stock.Combiningfiveclasses fornutrient stockwithfive classesofN-Pdepletion resultsin
sixclasses forthenutrient depletion LQ.
Theattractiveaspect ofthisstudyis itsemphasisontheintegrated characterization ofthe
entireland systemusingquantitative and specifically defined techniques,suchas simulation
modeling andnutrient budgeting.
Balancingproduction andenvironmental requirements in aprimeagricultural soilin
theNetherlands
Application ofsimulation techniques for aseriesofgrowing seasons,expressing climate
variability, wasrealized inamoredetailed Dutch case studyinaprime agricultural soil
(DroogersandBouma, 1997;Bouma andDroogers, 1998).Threephenoforms were identified
inthefieldandwerestudied.Here,onlyconventional arable land isshown (CONV)and land
affected bybiodynamic farming for aperiod of60years(BIO).The lattersoilhada
significantly higherorganicmattercontent (appr.4%versus2%) asaresult ofbiological
management. Amoredetailed simulation modelwasused inthisstudy,allowing calculations
ofcropgrowth and associated water andnitrogen regimes.Calculationsweremadefor a 15yearperiod usingreal-timeweatherdata andalargesetofnitrogen fertilization rates.Results
couldtherefore beexpressed asprobability curves(Figure 1)listingtheprobability thata
certain yieldwould beexceeded onthe vertical scale andtheyield itselfonthe horizontal
scale.Threeyieldcurvesareshown inthefigureandtheyexpresstheprobability (never,3%
and 10%oftheyears)thatnitrate leachingwillexceed theenvironmental threshold value for
nitratecontentofgroundwater. Thus,risksareexpressed inquantitativetermsby combining
theuncertainty ofyieldsduetovariableweather indifferent yearswith the associated
leachingofnitrates.Onemorecurve isshown:thedarksolid,andalmostvertical, line
representspotential yield.
'O
2500 5000 7500 10000
uw
0
2500 5000 7500 10000
1
Yield (kgha'1)
Yield (kgha )
ExceedingN-leaching
never
3%
10%
;
|
Potential
production
1
|
/. / y
/
Figure 1.Probabilitiesthatyieldsofwheatareexceededasafunction ofthreeprobabilitiesthatthe
thresholdvaluefornitrateleachingtothegroundwaterisexceededaswell.Dataarebasedon
simulationsfora30-yearperiodforaprimeagriculturalsoilintheNetherlands(after Droogersand
Bouma,1997).
ThegraphsinFigure 1 force theusertomakechoicesaboutriskstotakewhenbalancing
yieldversusnitrate leaching.Thisisarelevantkeyproblem inDutch agriculturewhere
production interestshavetobebalanced against environmental restraints.Again,land quality
canbeexpressedby:(actual/potential yield)x 100.
Assuming,arbitrarily, ayield levelthathasaprobabilityof20%ofbeingexceeded and a 10%
probabilitythatnitrate leachingwill exceedthethreshold,aLQvalueof89isobtained for
BIOand84forCONV.Ifhowever,the leachingprobabilityisreducedto 3%, LQvalues
become73and60.Ifleachingisneverallowed,LQvaluesbecome61and 33, demonstrating
thehigher"quality"oftheBIOsoil.Different LQvaluesareobtainedwhen different
exceedancevalues for yieldprobabilityareused.Theycanallbederived from Figure 1 as
needed.Figures suchasFigure 1 aresuitabletoallowtheusertomakechoices.Tothedismay
ofsome,theydonotprovideclear-cut answersandjudgements.Scienceprovides thetoolsto
userstoallow themtoexercisetheirresponsibility. Sciencedoesnottakeawaytheir
responsibilitybutallowsusers,beit farmers,plannersorpoliticians,tomakemorerational
choices.
Goingbacktothedefinition ofsoilquality, theprocedure illustrated heretoestimateLQ
values,coversyieldsindifferent years,risksinvolved andenvironmental sideeffects interms
10
ofnitratepollution ofgroundwater, which isthe most important problem inDutchsoils.
Elsewhere otherproblems mayfigure.Thiscomesclosetowhat Karlen etal.(1997)must
havehad inmind. LQ's are also indicators for sustainable land management, as defined
above.Theydefine production levelsovertheyearsandrisksinvolved aswell asthequality
ofwater. Management practices,defined inthiscontext, relateonlytonitrogen fertilization
andmanagement includes,ofcourse,much morethan that. AnyLQvalueindicates ayield
gap,beingthedifference between observed yieldsandthepotential one.Alogical question is
howtobridge theyield gap.Toanswer that question all factors, which affect real yields,have
tobe identified.
Land quality at larger scales
Landqualities,discussed sofar, werederived for individual piecesofland corresponding with
acertain soilseriesandareimplicitly focused onthefarmer orlocal land user.Attention was
confined toagricultural production and,intheDutchcase study,on important aspectsof
environmental quality.Thedefinition ofsoilquality (Karlen etal., 1997)isfocused ona
"specific kind ofsoil"andhas,therefore, abuilt-in spatial scaledimension. But theLQ
concept notonlyrelatestoplotsorfields wheresinglekindsofsoiloccur,but alsotolarger
areassuchascommunities,regions,countriesand even largerentitieswheremany different
soilsoccur. Inthatcontext, LQ'sarealsoimportant butquestions arenowasked by
politicians,plannersandrealestatebrokers,ratherthan farmers.Howshouldthe qualityof
landinalargeareabejudged? Asoilmapcanbeused todistinguish allsoilseriesorsoil
associations thatoccurintheareaand individual landqualitiesofthedifferent soilseriescan
beconsidered fordifferent landusecategories.Anaverage land quality, weighted byrelative
areasoccupied bythedifferent soil series,couldthentheoreticallybe calculated.
Functioning oflandwithinnatural ormanaged ecosystem boundaries,sustaining plantand
soilproductivity, maintenance ofsoil andwaterqualityandsupport ofhuman healthand
habitation"aremanybutnotnecessarily allelements ofimportance whendealingwith landin
largerareas.Infrastructure isimportant, but,particularly, socio-economic conditionsthat
determinethepopulation pressure onthe land.Aswiththediscussion onfields and farms,we
advocate anapproach inwhichthequalityofland inaregion isfirstjudged intermsofits
owninherentproperties.Next,theseproperties willbeonlyone(and asitturnsoutinreal life,
aratherminor) factor indetermining themostdesirable landuseinaregionwhichishighly
influenced bysocio-economic andpolitical considerations andwhich willalwaysbetheresult
oftradeoffs between manyconflicting landuseoptions.The relevant question hereis,then,
howthequalityofthe landcanplayarolewhen weighing suchalternativeoptions.
Clearly, restricting theattention toagriculture inthiscontext astheonlyland-usetypeis
unsatisfactory. Asidefrom agricultural issues,regional land-useplans arelikelytodealwith
establishingnatureareas,transport corridorsand locations forhousing and industrial
development. Because oftherelativelyhigh capital expenditure associated withbuilding
11
activities,LQ's for landinagivenareawhenconsidering suchactivities isboundtobe
relativelyunimportant.Drainingofpotential building sitesoraddingthick layersofsandto
increasethesupportcapacityoflandthathasalownaturalcarryingcapacity, isfinancially no
problem.Fornatureareasand, lessso,for agriculture,whicharemuchlesscapital intensive,
thepictureisdifferent. Local conditions ofthelandareveryimportant indetermining water,
nutrientandtemperature regimesthat governoccurrenceofnatural vegetationsbutthatalso
increasinglyhaveanimpactontypesofagriculture thatareecologicallybalanced.The
approach totake,therefore, wouldbetodefine LQ's foragriculture andnature for land
occurring intheareatobeconsidered andtointroducethisintimeintothebroader land-use
planningprocess.LQ's for agriculturehavebeendiscussed above.LQ's fornaturerequirea
separate discussion,whichisbeyondthescopeofthistext.
Modern land-use planning increasingly usessimulation modelinginthecontext ofsystems
analysistoderiveoptimal land-usepatterns inaregion.Thisapproachwasrecently
demonstrated inCostaRica(Bouman etal., 1999).Thus,interests ofagriculture andnature
would,asisoften thecase,notbearest-factor left behindafter land-usedecisionshave
alreadybeentakenbased ondemands from housing,industryandinfrastructure. Rather,these
interestsofagricultureandnaturewouldbesubmitted atanearlytimeallowinga significant
effect onthedecisionmakingprocess bypointingoutwhereprime landislocated for
agricultureandnature.UseofLQindicators canbequitehelpful here!Ofcourse,thepolitical
processmaystill ignorethis,butnobodywouldbeabletoclaimafterwards that theydidnot
know.
Usingthe quality concept toimprove communications about soils
Expression oftheproductive capacity oflandinrelation toenvironmental requirementsin
termsofasingleindicator,which isrelated topotentialproduction, canbehelpful inour
opiniontocommunicatemoreeffectively withstakeholders aboutuseoflandin future as
comparedwithcurrent conditions,whereemphasisismoreonthepropertiesoflandrather
thanonitsbehavior.Theindicator, asproposed,providesaqualitymeasurefor agivensoil
type,butbyrecognizingoccurrence ofgenoforms andphenoforms, it acknowledges
importanteffects ofmanagement.Thus,characteristic "windows ofopportunity"areobtained
for anysoiltypeoccurring inagivenagro-ecological zone.Useofactualyieldsprovidesan
impressionofyield gaps,whiletheabsolutequalitymeasureranksthesoilonaglobalscale.
Wecertainlycannotpreserveallland foragriculturalproduction.However,thequality
measurediscussed heremayhelptomoreeffectively showwhich landisparticularly valuable
anddeservesmore attention.
Studiesdiscussed abovehaveanexploratory character. Before possibleimplementation ofthe
schemeonawiderscale,attention shouldbepaid tounified procedures for determining
potentialyieldsanduseofsimulation models,includingacriticalanalysisofdatademands.
12
Conclusions
Theproblem ofworldwide land degradation and indiscriminate useofprimeagricultural land
fordevelopment isinsufficiently recognized bysocietyat large.Onereason is therather
ineffective mannerinwhich soil scientistspresent theirexpertise.Wesuggest that creative
useoftheland-qualityconcept, leadingtospecific indicators,mayraisepublic awareness of
theimportanceofthe land.
Existing definitions ofsoil quality andsustainable land management haveseveral elementsin
common.Anapproach isproposed heretodefine aland qualityindicator for sustainable land
management focused onagricultural landusewhich integrateselements ofyield, riskand
environmental qualityusing quantitative,reproducibletechniques such assimulation
modelingornutrient budgeting.Well intentioned normative and descriptiveapproaches will
notbeenough.
Socio-economic andpolitical conditions areveryimportant when defining land qualityand
sustainable land management. Weadvocate,however, aseparate assessment oftheagroecologicalpotential oftheland which should,inanearlyphaseofdiscussions withall
stakeholders,beintroduced as independent input intobroader land usediscussions.The future
oftheland cannot beheld hostagetoeconomic conditionsoftheday.
Landqualities,discussed inliterature,havesofar implicitlybeen focused on field and farm
levelandonagricultural landuse,the latterincreasinglywithin anecologically sound context.
Landqualitiesarealso important at largerscales,such astheregional andnational leveland
thisrequiresadditional research becauseasinglefocus onagriculture isnotrealistic inthis
caseasmanyother forms oflandusepresent theirdemands aswell.
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