^(0
"llitqGCj
CODEN; IBBRAH ( 7 - 8 6 ) 1-116 ( 1 9 8 6 )
I N S T I T U U T
VOOR
ISSN 0 4 3 4 - 6 7 9 3
B O D E M V R U C H T B A A R H E I D
RAPPORT 7 - 8 6
ROOTSTUDIESONATROPICALULTISOLINRELATIONTONITROGENMANAGEMENT
ReportoffieldworkatUTA'shighrainfallsubstationatOnne(Port
Harcourt,Nigeria)in1985
by
KurniatumHAIRIAH andMeineVANNOORDWIJK
* FakultasPertanian,UniversitasBrawijaya,Malang,Indonesia
1986
InstituutvoorBodemvruchtbaarheid,Oosterweg 92,Postbus30003,
9750RA Haren(Gr.)
Inst.Bodemvruchtbaarheid,Sapp.7-86 (1986)116pp.
andGreenland,1960;Odu,1977;Vitouseketal.,1983).
bushhasbeenclearedyieldlevelsdeclinerapidlyduringsubsequent
yearsofcropping.
Althoughdecompositionintropicalforestsisfastcomparedwiththat
intemperateregions,nutrientreleaseisspreadoveranumberofmonths
atthestartoftherainyseason.Bernhard-Reversat (1972)showedthat
decayoffreshorganicmatter(forestlitter)wasapproximatelyexponential,butcoupledtoanalmostlinearnutrientrelease,completedin
about5months.Differencesindecompositionratesofleavesofdifferent
treespeciescannotbedirectly relatedtotheirC/Nratio,becauseapparentlyleafsurfacestructure(Bartholomewetal.,1953)andsecondary
plant substancesastanninsplayaroleaswell.Madge(1965)founddecompositionofAlchornealeavestobeslow.Diversityinthevegetation
ofarainforestmaycontributetoagradualnutrientreleasefromdecayingdeadorganicmatter.Heterogeneity ofdeadorganicmatterinputmay
haveaseasonal,structural(leaves,fruits,twigsandbranches)andchemicalaspect.(Madge(1965)andHopkins(1966)publisheddatafrom
Nigeriansemideciduousforestonseasonalaspectsoflitterfall.
Therapiddecompositionofsoilorganicmatterinthecroppingphase
formsthemajorsourceofnutritiontothecrops,butleadstoexhaustion
ofsoilorganicmatteratagreaterspeedthanthatatwhichitisbuild
upinthefallowphase.Theabsenceofaprotectivecoveronthesoil
duringconsiderablepartsofthecroppingphaseleadstoanincreasedmineralisationofsoilorganicmatterwhichusuallyisnot(completely)in
phasewithnutrientuptakebythecrops.Ifthenutrientsupplyis(temporarily)inexcessofcropdemand,nutrientscaneasilybelostfromthe
rootzonebyleaching.
Whencompared totheforestvegetationwithitsclosednutrientcycle,
croppedlandmayexperiencelossesfromthenutrientbalance,duetoa
numberoffactors:
* topsoilerosion,
* removalofnutrientsinthecropsharvested,
* highmineralisationratewhichexceedscropdemandsinthefirstyear
ofcropping,
*absenceofacontinuousrootsystemanduptakecapacitybycrops,
* lackofseasonalcorrespondencebetweenmineralisationanduptakerequirementsofthecrops,mineralisationcanbetoofastearlyinthe
rainyseasonandtooslowlateron(Swiftetal.1981;Andersonand
Swift,1983),
* lackofdiversityinorganicmatterinputthroughcropresidues,
givingsharppeaksinmineralisation (Woodmansee,1984;Swift,1984)
7:,
* probabledeclineofnon-symbioticNfixationasaconsequenceofreducedcarbonsupplytothesoil.
Althoughtherelativeimportanceofeachofthesefactorscannotyetbe
assessed,themineralisationprocessandthewayitisinfluencedbytopsoilmicroclimateprobablyisakey-factorinthenutrientuseefficiency
oftropicalfarmingsystems.Researchaimedatimprovingsoilmanagement
practiceshastotakealltheseaspectsintoaccount,toreduce"leakage"
fromthesystemandtoincreaseinput.Theproblemcanbetackledindifferentways:synchronisationofsupplyanddemandcanbeimproved,storageorbuffercapacityofthesoil/rootsystemcanbeincreasedchemicallyorphysicallybyincreasingtherootingdepth.
Bartholomew(1977)reviewedevidencefortherapidlossesofnitrogen
fromthesoilorganicmatterpoolinthefirsttwoyearsofcropping(100
- 700kgNperhaperyear)andthecomparatively fastbuildupofNin
vegetationinthefirsttwoyearsoffallowing(200-350kgperhaper
year).RatherthanconsideringallNlossestobepermanentandallN
gainstobefixedN,hesuggeststhatNmaybeleached tothesubsoilin
thecroppingphaseandsubsequently recoveredbyadeep-rootedfallowvegetation.Obviously thedepthofleachingwilldependonclimateandsoil
type.Bartholomewquotesdatashowingthatwithintherangeof500-1000
mmannualrainfall,1mmofrainfallroughlycorrespondsto1mmofdownwardmovementfornon-adsorbednutrients.Inregionsofhigherrainfall
thedownwardmovementincreasesinrelativeaswellasabsoluteterms
(about4000mmdisplacementat1600mmrainfall).Bartholomewcouldfind
hardlyanydatatocorroborateorrejecttheassumeddifferencein
rootingdepthbetweencropsandfallowvegetation.
IMProvingnitrogenmanagement
Toimprovebushfallowsystems,twooptionsare1)tomaintainanalternationofcropandfallowphaseandtoimprovetheeffectivenessofthe
fallowvegetationinbuildingupsoilorganicmatter,inimprovingsoil
structureand/orinsuppressingweeds(whicheveristhecriticalfunction
offallowinginthelocalsituation)and2)tointegraterestorative
functionsofthefallowvegetationintothecroppingperiodinaformof
permanentfarming.
Inthepastmuchefforthasbeendirectedatthefirstoption,by
searchingfor"plantedfallow"orleguminouscovercropsasalternative
fornaturalbushregrowth,withrathervariableresults.NyeandHutton
(1957)found thataplantedAcioacoppicegivesahigherorganicmatter
input thananaturalbushfallow,althoughtheamountofnutrientsstoredinthevegetationisslightlylower.Inyieldeffectonoilpalms
intercroppinganaturalbushfallowvegetationprovedtobe aspositive
asaplantedleguminouscovercrop(Pueraria).Singh(1961)gaveadditionaldataforthesameexperiments,showingthattwoyearsofbushfallow
orPennisetum (elephantgrass)maybesufficient tomaintainmaizeyield
levelsinsomecases.Elephantgrassappearstobethemosteffective
fallowreplacement,especially inrestoringsoilstructure.Jaiyeboand
Moore (1964)showed thatanaturalbushfallowofAcioaandAlchorneais
effectiveasalegumeandmuchmorethananaturalgrasscoverinrestoringsoilfertility.Incontrast,Sanchezetal.(1982)concludedthat
a leguminouscovercrop(Pueraria)for1-2yearshasthesamerestorative
effectonthesoilas25yearsofforestfallow.
Theoptionofimprovingfalloweffectivenesshastobecombinedwith
efficientreclamationtechniquesatthestartofthecroppingperiod,as
majorlossesofnitrogenoccurduringcroplandreclamationbyfire;
nearlyallNintheplantmaterialislostand,dependingontheburning
technique,apartofsoilorganicNaswell(AndriesseandKoopmans,
1984;Stromgaard, 1984).
Thesecondoption,permanentfarmingtechniques,hasrecentlyreceived
mostattention.Researchonimproved farmingpractices,e.g.atthe
InternationalInstituteofTropicalAgricultureinNigeria(UTA,1984a
andb;KangandVanderHeide,1985)iscurrentlydirected toimproved
nitrogenmanagementinpermanentfarmingthrough:
a.Protectingthesoilfromerosionbymaintainingacontinuous
coverofcropsand/ormulchandreducingsoiltillageoperations;manyof
thetechniquestestedrelyheavilyonherbicidesforweedcontroland/or
killingcovercrops.Effectsofsuchpracticesonmineralisationrates
havenotbeeninvestigated indetailasyetbutorganicmatterontopof
thesoildecomposesratherslowly,possiblyinphasewithcropdemand.
Soilcompactionbymachineryinmechanisedfarming,whichcreatesanapparentneedformoreintensivesoiltillageoperations,remainsanunsolvedproblem.
b.mixedcroppingpractices.Awidely recognisedcriteriafortheusefulnessofmixedcroppingisthatthecropsusedshouldbecomplementary
toeachother(atleastpartially)inusingenvironmentalresources
(light,water,nutrients).Undertheseconditionsthe"RelativeYieldTotal" (RYT)andthe"LandEquivalenceRatio"(LER)canbeabove1.0
(Willey,1979;Steiner,1982).Recentlyavoidanceofriskshasbeenquantifiedaswellasausefulcriterion(Vandermeer,1984).Inecological
terminology positiveeffectsontotalyieldcanbeexpectedasfaras
thereis"niche-separation"betweenthecropsused.Themainnicheaspectswithrespecttonitrogenuseare:Nfixation,seasonofmainuptakeactivity,verticalrootdistributionandhorizontalspreadofthe
rootsysteminrelationtothecropspacingused.Asaspecialcaseof
mixedcropping,"weeds"canincertaincasesbepositiveinpreventing
nutrientsfromleachinginperiodsoflowuptakecapacityofthecrops
andmaintaining theminthenutrientcycle(MishraandRamakrishnan,
1984).
c.Introducing treesintomixedcroppingsystemstoincorporatesomeof
thefavourablecharacteristicsofthefallowvegetationintothecropping
cycle.Deep-rooted trees,suchasAcaciaalbidainaridregions(Charreau
andVidal,1965;FreemanandFricke,1980),donotcompetefornutrients
andwaterwithcropsandmayactas"nutrientpumps"recoveringleached
nutrientsandsoilweatheringproductsfromlowerdepths;throughleaf
fallorbydeliberateuseofloppedtreebranchesasmulch,thesmallannualyieldofsoilweatheringprocessescanthusbeadded tothetopsoil;
recently"alley-cropping"hasreceivedmuchinterestinthehumidtropics
(Kangetal.,1981;Getahunetal.,1982;Nairetal.,1984;Kangetal.,
1985).
d.IncreasingnitrogeninputbyincludingN-fixingplants inthecrop
rotationorinmixedcroppingsystems(OssewaardeandWellensiek,1948;
Whyteetal.,1953;AgboolaandFayemi,1972;AyanabaandDart,1977;
Nnadietal.,1981;GrahamandHarris,1982;Atkins,1984).SymbioticN
fixationgenerallyleadstoacidificationofthesoil;e.g.Kowaland
Tinker(1957)foundsoilpHtobeloweredbyalongperiodofPueraria
coverwhileitremainedstableunderweeds/bushcover.Deep-rootedleguminoustreesmaycombinetheadvantagesofnitrogenfixationwithsufficientcationrecirculationfromdeeperlayerstomaintainsoilpH.
SubstantialbacterialN-fixation (about 100kgNperhaperyear)can
takeplaceintherhizosphereofcertaingrasses,e.g.Pennisetua(elephantgrass)(AyanabaandDart,1977).
10
e.Mutuallyadjustingtherateofmineralisationandplantuptakerequirements,byapplyingmulchesofhighC/Nratioinperiodswhenmineralisationmaybetoofastotherwiseandbyestablishingnitrogendemanding
cropsassoonaspossibleafterthestartoftherains(PrabhakaranNair
andGhosh,1984;vanFaassenandSmilde,1985).Thesepossibilitieshave
beenlittleexploitedsofar(DommerguesandDiem,1982;Swift,1984).The
factthatmultiplecroppingsystemswillgiveamoreheterogeneousinput
oforganicmattertothesoilresultinginamoreevenrateofmineralisationduringthecroppingseasonhasbeenlittlestudiedasyet.
Inalltheseresearchtopicsagriculturalresearchsofarismainly
followingratherthanguidinglocalfarmingpractices (Richards,1985);
e.g.alley-cropping techniquesweredevelopedonthebasisoflocalfarmingpractice,coppicingtreesofthebushfallowvegetationratherthan
removingthembeforegrowingcrops.Thereisscopefor"learningfromthe
forest"byanalysingtheecologicalprocessesbywhichnaturalvegetationsdealwiththelocalenvironment,aswellas"learningfromthefarmer". Byintegratingbothtypesofknowledgewemayhopetodevelopagriculturalsystemswhichcandealwiththeincreasedproductionlevelsrequired,atmoderatelevelsofexternalinputs,usingallavailableresourcesasefficientlyaspossible.
Rootingdepthandnitrogennanagement
Efficientuseofnitrogenunderconditionsofhighrainfallandhence
severeleachingdependsoneitheradequatesynchronisationofplantdemandfornitrogenwiththemineralisationrateorontheabilitytodevelopdeeprootsystems,abletorecoverleachednutrientsfromdeepersoil
layers.Figure1.2showsthattheremaybevariationinthedemandcurve
ofthecropandinthemineralisationrate(e.g.duetodifferencesin
soilmicroclimateasaffectedbysoilcover),butcompletesynchronisationishardlypossible.Thelargerthegapbetweensupplyandimmediate
demand,thedeepernutrientswillleachintothesoilandthedeeperthe
plantrootsystemwillhavetobetorecoverthem.Quantificationofthis
relationshipdependsonlocalclimateandsoilcharacteristics.
11
kg N ha"
cumulative
1
Icrop demandT _ n ^ A
supply by
required storage,
inrooted soil volume
start of rains
exact scale
depends onrainfall \
time
\
\
—n
requiredrooting
depth
Figure1.2.Schematicinteractionbetween(lackof)synchronisationof
supplyofnutrientsbymineralisationandcropdemandfornitrogen,
withtherootingdepthrequired forfullrecoveryofleachednitrogen.
AtUTA'shighrainfallstationinOnne(PortHarcourt,S.E.Nigeria)
thesoilisatypicalultisol.Efficiency ofnitrogenuse,bothoffertiliserandorganicorigin,asmeasuredinlysimetersandwith
Ninthe
field,islow(VanderHeideetal., 1985).Theefficiencyoffertiliser
Nuse(apparent recovery rate)inthecurrenttrialsis30-40%(ata
moderateN-inputlevelof90kgN/ha).Physicalandchemicalaspectsof
leachingonthissoiltypehavebeendescribedbyPleysierandJuo(1981)
andAroraandJuo(1982)andhavebeenincorporatedwithmicrobiological
processesinadynamicsimulationmodelfornitrogenleachingby
deWilligen (1985).
Figure1.3 showstheresultofsomecalculationswiththemodelin
whichrootingdepthwasvariedandpossibilitiesforuptakebythecrop
werecalculated.Clearlyadeeperrootsystemcantakeupaconsiderably
largershareofthenitrogenwhichbecomesavailablebymineralisation.
Anymeasuretoimproverootingdepthundertheseconditionswillbehelpfulinimprovingnitrogenuseefficiency.
12
crop uptake,
kgN/ha
100
30 40 50 60
rootingdepthincm
Figure1.3.Resultsofasimulationmodelofnitrogendynamicsforsoil
andclimaticconditionsofOnne(S.E.Nigeria).CropNuptakehasbeen
calculated forafertiliserNdosisof150kgNperha,giveninthree
applications,forthreesoilpHlevelsandwithvarying rootingdepth
(DeWilligen,1985).
Factorsaffectingrootingdepth
Highly leached tropicaluplandsoilsoftenhavealowfertilityandare
characterised byalowbasesaturation,highlevelsofexchangeableAl
andalowpH.SanchezandSalinas (1981)reviewed lowinputtechnology
formanagingacidsoilsinthetropics.Twooptionsarepossible:to
adaptthefarmingsystembyselectingacid-tolerant crops(andcrop
varieties),ortoadaptthesoiltostandardfarmingconditionsbyapplying(largeamountsof)lime.
Inlargepartsofthehumid tropicsacidsoilconditionshinderthedevelopmentofdeeprootsystemsandthereforeleadtoalowefficiencyof
Nuse.Limingisawellknownproceduretoimproverootdevelopmentin
acidsoillayers,althoughithastobecontinued foralongtimeif
deepersoillayershavetobeimprovedwithoutsoiltillage(Aroraand
Juo, 1982).Effectsoflimingonnitrogenmanagementmaybeofamixed
nature,however,asabeneficialeffectofdeeperrootdevelopmentwill
coincidewithanincreased riskofleaching.Theriskofnitrogenlosses
duetoleachingisincreasedbyliming,becausenitrateratherthan
ammoniumbecomesthedominantformofnitrogen,nitrificationbeing
13
stimulatedathighersoilpHlevels(DeWilligen,1985,figure1.3).In
largepartsofthehumid tropicslimeisnotlocallyavailableand
transportationcosts(consideringthelargeamountsnecessary)willlimit
theuseoflimeundertheseconditions.Selectionofacid-tolerant
cultivarsandcropcombinationsseemstobetheonlyobviousapproach
here.Informationonrootdevelopmentunderfieldconditionsshouldbea
firststep.
Themainlimitationtorootdevelopmentonacidsoilsprobablyarises
fromthehighavailabilityofaluminiumandmanganeseions.Considerable
variationintoleranceforaluminiumandmanganeseexistsbetweencrops
andcultivarsofcrops(Foyetal., 1978).Toleranceofdifferentcrops
toacidsoilsand tohighAllevelsisbasedonvariousphysiological
principles.TheseincludefixationofAlionsinrootcellwalls,interactionswithcalciumandphosphateuptakeandalocalchangeofpHaround
theroot.RhizospherepHdependsonthebalancebetweenanionsandcationstakenupbytheroots.Themainvariablecomponentofthisbalance
isnitrogen,leadingtoarhizospherepHwhichiseitherhigher(inthe
caseof predominantnitrateuptake)orlowerthanbulksoilpH(incase
ofpredominantammoniumuptake)(RileyandBarber,1971;Nye,1981).
Fleming (1983)showedAltoleranceofwheatcultivarstobebasedonpreferentialnitrateuptakefromammonium/nitratemixtures.Boubele(1984),
studyingAltoleranceofagroforestry trees(Leucaena,Gliricidia,
SesbanlaandCalliandra),foundtheslow-growingCalliandratobeleast
affectedbyAlinculturesolution,probablybecauseofitsabilityto
maintainadequateCalevelsintheshoot(Sesbanlawastheleasttolerant
inthiscomparison). Generally leguminouscropswithfunctioningsymbioticNfixationwillacidifytheirrootenvironment (vanBeusichem,1984).
SomeLeguminosae,suchasLupinus,form"proteoid",thickenedrootsin
whichprotonexcretionisconcentrated,whichishelpfulinphosphorus
uptakefromneutralsoils(Gardneretal.,1982).RhizosphereacidificationraisesthequestionwhetheritispossibleatalltocombineefficientNfixationandAltoleranceinthelongrun.
Leguminouscropsdiffermarkedlyintheirtolerancetoacidsoilconditions(and/orhighlevelsofavailablealuminium)andsomeprogressin
selectingtolerantvarietieshasbeenmade.Stylosantheshumilisisknown
tobeatolerantcrop(PinkertonandSimpson,1983),abletoexploitacidicsoillayersunderconditionsoflowPsupply.Leucaenaisconsidered
tobenotverytoleranttoacidsoilconditions(SanchezandSalinas,
14
1981),althoughrecentlysomenewgenotypeswithimproved tolerancehave
becomeavailable.
Apartfromsoilacidity,rootdevelopmentmaybeinfluencedbysoil
structureandtheavailabilityofporesleftbyotherrootsand/orby
soilorganisms (e.g.KowalatidKassam(1978)mentionedtermitesasimportantorganismsforrootdevelopmentinNigeriansavannasoils).Littleis
knownofsucheffectsinacontextofshiftingcultivation.Itmaywell
bepossiblethatrelativelydeeprootdevelopmentunderafallowvegetationpermitsrelativelydeepcroprootdevelopment inthefirstyears
ofcropping,withasubsequentdeclinewhenthestructureofthesoil
deteriorates.Sucheffectswouldcontributetothedeclineofcropyields
generallyascribed todecreasingchemicalsoilfertility.Informationon
rootdevelopmentofshrubsand treesofthefallowvegetationisrelevant
inthisrespect (NyeandGreenland,1960)
Purposeofpresentresearch
Knowledgeoftherootsystemofallcropsinthefarmingsystemcanbe
helpfulinimprovingnutrientuseefficiency.Majoraspectsare:
* depthofrootpenetration,
* horizontalspreadoftheroots,timelyestablishment ofcompleteexploitationoftopsoil,inrelationtoplantingscheme,
* compatibilityofcropsinintercropping,
* nodulationofleguminouscrops,
*mycorrhizaformation(mainlyinrelationtoP-uptake efficiency).
ThisreportwillcoverfieldobservationsonrootdevelopmentatOnne
in1985.Anewexperimentwaslaidouttostudyrootdevelopmentofleguminouscovercrops,asinfluencedbyliming.Inthreeexistingexperimentsobservationsonrootdevelopmentweremade:upland ricewithand
withoutNPKfertilisation,maize/cassavaintercroppingandanalleycroppingexperiment.Twosemi-quantitativetechniquesforrootobservation
werechosen(SchuurmanandGoedewaagen,1972andBöhm, 1979):
A.Pinboard technique:thisgivesanoverallviewofrootdistribution,
rootbranchingofasingleplant,allowsdeterminationofrootdryweight
androot/shootratiosandallowsseparationofarootsystemintermingled
withthatofneighbouringplants.
15
B.Rootmappingtechnique:thisallowsadetaileddescriptionofroot
distribution.Mapscanbemadeinaverticalplanetostudydistribution
ofrootsinrelationtothesoilprofileandinahorizontalplaneto
studywhetherornotasoillayerishomogeneously exploitedbyroots.
Forallcropsstudiedsomepreliminary observationsweremadeon
mycorrhizalinfectionofroots.
rootat40cmdepth (compareplateIV.d);B.rootsgrowinginahorizontalantchannel.
m g copreventsnaamg orcrops^bwennen,i^ö^;.xneortenratnerShallow
rootsystemsofleguminouscropsmakethemmoresuitableforintercroppingwithperennialtreecrops,thoughnotasacompletereplacementof
thebushfallowintermsofrestorationofsoilfertility.Moreland
16
2.ROOTPENETRATIONINTOTHESOILATONNE
20
Rain(R)
mm/day
20
r
22
Quantinstated thatsownPuerariastands,characterisedbyshallowstolons,adenserootmatinthetop5cmandveryfewdeeprootsarereplacedbygrassesinnaturalsuccessionafterabout5years.Theeffect
ofPuerariaandStylosanthesontheregenerationofsoilstructureis
restricted tothetop-soil,therelativelydeep-rootedCajanusindicusis
thebestleguminouscropinthisrespect(theirdatarelatetothe1500
mmrainfallzone).NyeandHutton(1958)foundacomparatively lowroot
massofPuerariastandsinthetop25cmoftheprofilecompared tobush
falloworelephantgrassintherainforestzoneofNigeria.
TropicalLeguminosaecommonlyusedbyfarmers,suchasCentrosema
pubescens,Stylosanthesgracilis,Calopogoniummucunoides,Pueraria
phaseoloidesandVignasinensisarecapableofnodulationandnitrogen
fixationundernormalconditions (Adeboyejo,1976).Centrosemapubescens
isknowntoproducegoodgrowthandfixappreciablequantitiesofnitrogenintheabsenceoffertiliser.Otisi(1979)studiednodulationof
CentrosemaandPsophocarpusinapotexperiment,usingsoilfromUTA's
highrainfallsubstationatOnne.Centrosemagaveapoornodulation,regardlessofinoculation.Psophocarpusshowedareasonabledegreeof
nodulation,whichwasnotimprovedbyinoculation.Psophocarpusbelongs
tothecowpeacross-inoculantgroup.
Liminginsmallamountsisknowntoimproverootdevelopmentinacid
soils(AroraandJuo,1982).Kang(1970)foundthatlimingimprovednodulationofcowpeaandcorrectedNdeficiencyinanacidsoilfromOnne.
Largeresponsestolimingwereobservedwithratesbetween0.25and 1t
CaCO perha,whichhavelittleeffectonsoilpHbutmaytemporarilyre3
duceAl-levelsinthesoil.Murphyetal.(1984)foundthatconcentrationsinsoilsolutionofover25micromolaraluminiumdelayedtheappearanceofnodules,reducedthepercentageofplantswhichnodulatedand
decreased thenumberanddryweightofnodulesproducedbyCentrosema
pubescens,MacroptlllumlathyroidesandStylosanthesgulanensls.
RhizobiumseemstobemoresensitivetoAltoxicitythanthehostplant.
Horst(1984)developedascreeningtestforAltoleranceofcowpeavarietiesbasedongerminationinsoilcontaining2.2meqAlper100gsoil
bycomparinggerminationwithplantperformanceinalong-termpotexperiment,usingasoilfromOnne.Inboththeshort-andthelong-termexperimentthesamegenotypeswereclassifiedasmosttolerantandmost
sensitive.Theperiodofcropestablishmentapparently isacritical
stageforleguminouscropsundertheseconditionsandsmallamountsof
23
limemayhelptoovercomeproblemsintheinitialstages.
Thepresentstudyconcentratedonrootdevelopmentandnodulationof
acid-tolerant leguminouscovercrops,asaffectedbyliming (750kg
Ca(OH) perhaequivalentto1tCaCO perha).Rootingdepthwasstudied
aswellasrootdryweight,whichisameasureofinputoforganicmatter
intothesoil.
3.2.Methods
Plotswithleguminouscovercropsforrootobservationsweresownat
IITA'shighrainfallsubstationatOnne(PortHarcourt),S.E.Nigeria.
Theexperimentalsitewaslaidoutonapartofthefarmwhichwasmechanically clearedofforestandtreestumpsmorethan10yearsagoandhas
beenundermechanisedfarmingsincethen.Beforethecovercropsexperimentwasstartedtheareawascoveredwithanaturalgrassfallow,cut
regularly.Somepropertiesofthesoilbeforetheexperimentwasstarted
areshownintable3.1.Limewasbroadcaston27April,3daysbefore
planting.
Sixspeciesofleguminous (cover)cropswereused:Centrosemamacrocarpum(CIATno5062,lot82.234,Quilichao),Desmodiumovalifolium(CIAT
3784,lot82.228),PuerariaphaseoloidesandPsophocarpuspalustris(both
fromseedssuppliedbyDr.BTKang (UTA)),MucunautilisandVigna
unguiculata(fromseedsavailableinOnne).Foreachspeciesfourplots
wereused(0and750kgCa(OH) perha,equivalentto1tCaCO perha,
2
32
induplicate)inarandomizedblockdesignwithplotsof6x5m (total
experimentalarea28x46m).
Threeseedswereplantedpercluster (hill);theseedlingswerelater
thinnedtoone.Seedswereplantedbetween30Apriland2May,ataplantingdistanceof30x20cm.Weedingwasdoneinplotswithcropsthat
areslowintheir establishment:onceforCentrosemaandPueraria,twice
forPsophocarpusandthreetimesforDesmodium.Duringtheinvestigation
thecowpea(Vigna)plotwassprayedweeklywithGameline20forinsect
control.Lightinterceptionwasmeasuredweeklywithalightmeter(BBC
GoerzMetrawattM 1507.3717.k52),wherebylightintensityatfivepositionsineachplotwascomparedwiththeimmediately followingmeasurement
outsidetheplot.
24
TABLE3.1.Soilpropertiesofthefieldusedforcovercropexperimentat
Onne.
Soil
depth
(cm)
pH
Org.Mat. Sand
(H20) (%)
(%)
0-5
5-15
15-25
25-40
4.5
4.5
4.5
4.5
Soil
Depth
(cm)
Total P-Bray
N (%) (ppm)
1.78
1.31
0.98
0.73
83
83
77
73
SiltClay
(%)
13 4
13 4
12 11
10 17
0.111
0.071
0.061
0.082
78.75
54.05
47.70
46.80
Bulk,density
g/cm (n=4)
average+ s.d.
loamy
loamy
sandy
sandy
1.40+ 0.14
1.50+ 0.08
1.48+ 0.05
1.40+ 0.08
sand
sand
loam
loam
NH.OAc- Exch.cations
(me/100g)
Ca4
Mg Mn
0-5
5-15
15-25
25-40
Texture
(%)
1.03
0.64
0.74
0.56
0.460.210.05mm
—
Totalacid.CEC
(me/100g) (me/100g]
K
Na
0.17
0.21
0.19
0.16
0.24
~"
1.642
1.798
2.007
2.048
Fourtimes-2,5,8and 14weeksafteremergence-samplesoftheroot
systemweretakenwithapinboard (SchuurmanandGoedewaagen,1972)of40
x 60x 12cm,eachsamplecontaining2plants(60=2x 30).Afterwashingawaythesoilaphotographwastakenoftherootsystem.Thesample
wassubsequently cutin10-cmlayersandsubdividedaccordingtodistance
totheplant(0-5cmand5-15cm,seefigure3.1).
Foreachsubsamplerootdryweightandnoduledryweightwasrecorded
aftercleaningtheroots.Atthesamedatesabove-groundbiomasswas
sampledfromthree1-mstripsofplantsineachplot.Samplesofplant
o
topsweredriedat65 C,weighedandmixedforNanalysis.
Acompositesoilsample(forthefourlayersasintable3.1)wascollectedonthesamedatesastherootexaminationsandanalysedat U T A
(Ibadan)forpH,CEC,exchangeableAI,Ca,KandMg.
Onthelastdateofrootsamplingsomefinerootsofeachspecieswere
collectedandstoredinalcoholformycorrhizaanalysisattheInstitute
forSoilFertility;stainingtechniqueswereusedforrootsandthe
proportionofinfectedrootswasdeterminedasdescribedbyGiovanetti
andMosse(1980).
Additionalobservationsweremadeona2-yearoldPuerariafieldonthe
samepartoftheexperimentalstation.
3.53
2.90
3.10
2.99
25
distancetonearestplant(cm)
—5 A,. ,5
15
5,,w.5-
Figure3.1.Schematicrepresentation ofapinboard sampleand itsdivisioninsubsamples.Averageroot density inalayer isobtained bya 1:2
weighting ofaverage rootdensity inthe"5-5"and "5-15" subsamples.The
combination of plant densityand pinboard sizeallow foran efficient
sampling of the"unit soilarea"(VanNoordwijket al., 1985).
3.3.Results
Soilanalysis
Soilanalysis during theexperiment (figure3.2) showed that liminghad
no effect onexchangeableMg,K,MnandNaconcentration.Totalacidity,
exchangeableAl concentrationandCECdecreasedwhile soilpHandCaconcentration inthetoplayerincreased,althoughvariationamong samples
islarge.Effects onCaandpHdidnotappearuntil 5-8weeksafterliming,butadecrease ofAlinthetopsoilwasalready evidentafter2
weeks. IfallCa(OH) wouldhavedissolved andhad beenfound inthe
2
soilanalysisof thetop10cmanincreaseofabout 1.4meCaper 100g
couldbeexpected [750
kgCa(OH) perha=2x 10 meCaperha: 1hax
9 3 L
9 2
10cm= 10 cm = 1.4x 10 g (assuming abulkdensity of 1.4);hence
750kgCa(0H )perha= 2x 10 meCaperha=2/l.4- 1.4meCaper 100
g ] . Theobserved increaseisonly about0.4meCaper 100g.
26
O
T
15
0.4
Ca(me/100g)
0.8 1.2
0
0.1
0.2
Mg(me/100g)
0.4
0.5 0.6
0.3
CS]—i—i—r
71
f/
25h
aak+
40L
0
1.0
~~i
"5*1-
CEC(me/100g) L
2.0 3.0 4.0
~i
r—
1r
AAK»
\
o
1
rAr
pH
6
i * .+
%
A nto
15h
m "AI
25
40L
0.5
Al(me/100g)
1.0 1.5 2.0
0
L total acidity (me/100g)
1
2
3
i —
•""•"
40
depth(cm
-lime +lime
A
*2wk
a
« 5 wk
o
« 8 wk
x
+14 wk
Figure 3.2. Soil analysis data during the experiment, as affected by
liming.
27
Shootgrowth
Figure3.3 showsthetimecourseofthelightinterceptionbytheindividualcrops.MucunaandVignahadaquickstartandtookabout4weeksto
reach50%lightinterception,buttheystartedtosenesceafter13weeks.
Centrosema,PuerariaandPsophocarpushadaslowstartandtook7,10and
13weeks,respectively,toreach50%lightinterception,butdidnotshow
anysenescenceduringtheexperiment.Desmodiumwasevenslowerthan
Psophocarpusandhadnotyetreached 50% lightinterceptionafter13
weeks.
light interception
by crop(%)
100
y /• °
Centrosema^ /Pueraria
a'
psophocarpus
o 8
Imodium
8 10 12 U
time(weeks)
Figure3.3.Lightinterceptionbysixcovercrops(averageforwithand
withoutlime)asmeasuredwithalightmetercomparinglightintensities
insideandoutsideplots.
Thedifferencesamongthespeciesinabove-ground productionasreflectedindrymatteryieldsat5,8and 14weeksafteremergenceareshown
infigure3.4.Liminghadnosignificant effectsonplantgrowthexcept
forVignaat8weeks(positive)andMucunaat14weeks (negative).For
drymatterproductionthesixcropscanberankedinthedecreasingorder:Mucuna,Vigna,Centrosema,Pueraria,PsophocarpusandDesnodiua.
28
plant dry weight.
(g/m2)
350
1Centrosema macrocarpum
2Desmodium ovalifolium
3 Pueraria phaseleoides
4Psophocarpus palustris
5Mucuna utilis
JLSD(5%;
6Vignaunguiculata
300I—i-lime
VZ\+ Iime
ILSD(5%]
200-
I LSD(5%)
100
r-B3
i&JL
5wk-
I
J
1«
II
Ir
id1U
iii
14
K
!
-8wk-
-Uwk-
time
Figure3.4.Above-grounddrymatterproductionbysixcovercropsas
affectedbyliming.
Nitrogenuptake
Table3.2givesdataontotalnitrogenconcentrationofabove-ground
biomassofthesixspecies(forDesmodiumnosamplewascollectedonthe
firsttwodatesbecauseofinsufficientplantgrowth).
Limingslightlyincreasedthenitrogenconcentrationofallcrops.Nitrogenconcentrationdecreasedwithtimeinallcrops.LimingconsiderablyincreasedtheCaconcentrationofallcropsandhadapositiveeffect
onKandPconcentrationaswell.
Figure3.5 showsthenitrogenuptakeinabove-ground biomassforthe
sixspeciesasitdevelopedintime.MucunaandVignashowedthefastest
growthanduptakeandgrewatanaverageNconcentrationofabout3.5%.
After8weekstheNconcentrationofthecowpea(Vigna)cropstartedto
decreasewhenmostofitsNhadbeenredistributed internallytothe
seeds.NuptakebyMucunaproceededatalowrateafter8weeks.The
othercropswereconsiderablyslowerintheirNuptakeandgrewatN
concentrationsof2-3%;after14weekstheywerestillintheinitial
partofasigmoiduptakepattern.
29
+ lime
-lime
o Mucuna
A Vigna
• Centrosema
v Pueraria
+ Psophocarpus
QDesmodium
Vigna seeds
timeJweeks)
Figure 3 . 5 . Nitrogen uptake and dry matter production (above-ground) by
s i x cover crops.
30
TABLE3.2.A.Totalnitrogenconcentrationofshoots(%N)forlimed(1)
andunlimed (0)treatmentsatfourharvestperiods.Datagivenareaveragevaluesforduplicatesamples.B.TotalP,KandCaconcentration (%
ofdrymatter)14weeksaftersowing.
Species
Treatment
0
1
0
1
0
1
0
1
À.Nconcentration
2weeks
5weeks
8weeks
14weeks
Mucuna
4.6 4.9
3.6 3.9
3.6 3.4
3.0
3.5
Vignavegetative
5.4 5.5
3.6 3.6
3.4 3.0
1.8
2.0
pods
•
•
•
3.2
3.0
Centrosema
4.3 4.8
2.8 2.6
2.8 2.7
2.2
1.9
Pueraria
4.7 4.6
3.5 4.4
3.2 3.5
2.5
2.8
Psophocarpus
5.0 5.1
4.1 4.2
4.1 4.7
2.8
2.8
Desmodium
•
•
3.1 3.1
2.0
2.3
•
•
B.Pconcen-
•
•
•
Kconcen-
Caconcen-
tration(Z)
tration(Z)
tration(Z)
Mucuna
0.23 0.23
1.23 1.22
0.21 0.29
Vignavegetative
0.22 0.27
1.00 1.27
0.21 0.30
pods
0.22 0.29
0.43 0.64
0.02 0.02
Centrosema
0.23 0.28
0.82 1.30
0.33 0.46
Pueraria
0.24 0.27
0.95 1.05
0.21 0.34
Psophocarpus
0.26 0.28
0.92 1.25
0.21 0.37
Desmodium
0.16 0.20
0.79 1.04
0.19 0.23
31
Rootobservations
TherootsystemofthesixcropsatfinalharvestisshowninplateII
andIIIandfigure3.6.Detailsofthedistributionofrootdryweight
overtheprofilearegiveninfigure3.7.Dataonnodulationand
mycorrhizaaregiveninfigures3.8.and3.9.Table3.3 summarizesthe
dataasregardstheeffectsofliming.
Althoughtherootsystemofthesixcropshadthesameoverallshape
withaverticaltaprootandabundantbranchrootsinthetopsoil,there
weresomemarkeddifferencesinrootdevelopment.Centrosemarootspenetratedto70cmdepthwhiletheotherspeciesdidnotgrowbeyond50cm.
Allspeciesformedabundantbranchrootsinthetop10cmofthesoil,
buttheproportionoftotalrootdryweightfoundinthetop10cmofthe
profilevariedfrom85%forMucunato57%forCentrosemaandDesnodiunt
(unlimed treatments).Insomecasesatypicalthickeningofpartofthe
rootswasobservedwhichresembles"proteoid roots"asdescribedby
Gardneretal.,1982(figure3.6).SuchrootswereseeninMucuna(+lime,
2weeks,5-10cmdepth),Pueraria(+lime,8weeks,10-15cmdepth)and
Psophocarpus(-lime,2weeks,5-10cmdepth).
32
-Lime
Mucuna u t i l i s
0r
+Lime
Vigna unguicuîata
0
Centrosema macrocarpum
0
PlateII.Rootsystemofcovercrops(Mucunautilis,Vignaungaiculata,
Centrosemanacrocarpum)washedfrompinboardsampleatfinalharvestfor
thesixspeciesasaffectedbyliming.
33
Lime
+Lime
Pueraria
phaseleoides
0+0
Desmodium ovalifolium
PlateIII.Rootsystemofcovercrops(Puerariaphaseleoides,
Psophocarpuspalustris,Desmodiumovatifolium)washedfrompinboardsampleatfinalharvestforthesixspeciesasaffectedbyliming.
34
Figure3.6.Diagramofrootdistributionofthesixspecies,alsoshowing
theshapeandpositionofthenodules.
35
b. Vigna
a. Mucuna
+Lime
-Lime
0 01
0.3
+Lime
root d e n s i t y ( m g / c m 3 )
0.5 „0 0.1
0.3
0.5
0.1
-Lime
root density(mg/cm 3 )
0.5 „ 0 0,1
0.3
0.5
0,3
i
20
30
5wk
5wk
20
30P
II
-i
8wk
8wk
Uwk
Uwk
40F
50
soil depth
(cm)
root density at 5cm distance from plant
15 average
soil depth
(cm)
d. P u e r a r i a
c Centrosema
-Lime
+Lime
-Lime
root density(mg/cm 3 )
, 0.05
0.5
root density(mg/cm3)
U
03
i
r
2wk
8wk
40 I
0.7
0.1
03
05
i
07„
-o
0.1
0.3
H
Uwk
Uwk
i Uwk
40
soil depth
(cm)
70«
soil depth
(cm!
Uwk
05
36
f. Desmodium
e.Psophocarpus
+Lime
-Lime
- Lime
root density(mg/cm 3 )
3
„ 0 0.1
0.3
0.5
root density(mg/cm )
0 0.1
0.3
0.5
0 01
0.5
03
Or
0
1—
t
i —
I
Hi
Uwk
soil depth
(cm)
30
f
Uwk
iO
soil depth
(cm)
Figure3.7.Distributionofrootdryweightofcovercrops.Foreach
layerrootdryweightat0-5andat5-15cmfromthenearestplantis
shownaswellastheaverageforthelayer.
Forallspeciesthehighestrootweightdensity(mg/cm)occurredin
thetoplayerat0-5cmfromtheplant.Rootweightdensityinthetop
layerat5-15cmfromtheplant.Thedifferencebetweenthetwosample
positionswassmallerinthesubsoilthaninthetopsoil(figure3.7).
Nodulationwasrestricted tothetopsoil,exceptforPuerariawhich
formednodulesevenat40cmdepth.Nodulemorphology (figure3.7)varies
fromirregular,cylindricalforMucunatoglobularfortheotherspecies,
withVignaandPuerariaformingthelargestnodulesandDesmodiumbyfar
thesmallest.Noduleshapeconformed tothedescriptionsgivenbyCorby
etal. (1983).PuerariaandPsophocarpushadbyfarthehighestnodule
dryweight.Aroughimpressionofnoduleefficiencycanbeobtainedby
observingthecolourofnodulesunderadissectingmicroscopeaftercuttingthem.NfixationbyRhizobiumisonlypossibleunderanaerobicconditionsandthered-coloured"leghemoglobin"isresponsiblefor
!
37
nodule dry weight,
(g/m*)
1.0
0.8
I—i +lime
czn -lime
0.6
0.4
[LSD(5%)
ILSD(5%)
0.2h
•^ilrfe
0 11 2 2 ^ 3
2wk
1.0
ijjdn
5wk
JLSD(5%)
0.8
I
£LJä
JLSD(5%)
z9
0.6
0.4
0.2
11
0
il
na2i
i
IL
8wk
I
«nà
0
i IsH
1i
0 ^ S31fó
Uwk
time
1Centrosema macrocarpum
2Desmodium ovalifolium
3Pueraria phaseleoides
4Psophocarpus palustris
5 Mucuno utilis
6 Vigna unguiculata
Figure3.8.Dryweightofnodulesforthesixspeciesasaffectedby
liming.
maintaininganaerobicconditionsinsideanodule.Noduleswhicharenot
redcanbeconsideredas"non-effective".Thepercentage"non-effective"
nodulesvariedamongthecovercropsandgenerallydecreasedduringthe
season.InVignaandespeciallyMucunaitincreasedagainatthetimeof
thelasttwoobservations(noduleshadturnedbrownandrottenatthe
timeofthelastobservation).NodulesofCentrosemawere"non-effective"
inthefirsttwoobservationsandonlygraduallyimproved (stillabout
40%"non-effective"after14weeks).InPueraria"non-effective"nodules
wereseenmainlyinthefirstobservation;inPsophocarpusabout60%of
thenodulesremained"non-effective"after8weeks.Desmodiumhadthe
38
lowestpercentage"non-effective"nodules(10-20%).
Therewaslittledifferencebetweenmycorrhizainfectionoftheroots
inthetopsoiland thesubsoil(figure3.9).
infectedmycorrhiza
10r (%)
— toplayer
— sup layer
lime
~i+lime
1
centrosema
puerana
mucuna
desmodium
psophocarpus
Figure3.9.PercentageinfectionwithVesicular-ArbuscularMycorrhiza
(V.A.M.)offinerootsofthesixspeciesatfinalharvest;"top-layer"
refersto0-15 cm,"sub-layer"to15-40cm.
Limingdidnothavedramaticeffectsonrootgrowth,buttherewasa
generaltendencyforrootdensitiesinthetopsoiltoincreasewhilerootingdepthwasdecreased.Totalrootdryweightwasincreasedbyliming
inthefast-growing speciesMucunaandVignaaswellasintheslower
growingPsophocarpus;itwas unaffected forCentrosemaandPuerariaand
decreased forDesmodi.ua(table3.3).Liminghadapositiveeffectonthe
maximumnoduledryweightinthefast-growing speciesMucuna,Vignaand
Centrosemaandanegativeeffectinthethreeslowergrowingspecies.LimingseemstohaveaclearlynegativeeffectonV.A.M.inDesmodium,a
weaklynegativeoneinPsophocarpusandnooraweaklypositiveeffectin
theothers.
39
TABLE3.3.Effectsoflimingonrootparameters(0=unlimed,1=limed).
Species
Treatment
max.dry max.root
weight
2
(g/m)
%root
max.nodule max.V.A.M.
depth(cm) weightin dryweight
2
top10cm (g/m)
%infection
Mucuna
24.133.050
40
84
86
4.5
6.0
3.5
2.5
Vigna
22.331.650
40
78
54
4.9
8.0
-
6.3
Centrosema
56.655.570
70
58
60
0.5
1.4
7.4
9.2
Pueraria
45.844.350
40
73
88
10.3
6.8
2.9
6.6
Psophocarpus
28.337.350
50
77
86
7.2
4.0
4.7
3.2
Desmodium
29.421.840
30
57
76
3.4
1.8
5.8
0.3
Koot/shootratio
Figure3.10comparesabove-groundandbelow-groundbiomassproductionfor
thesixspecies.Intheinitialstagesallspeciesconformed toa
root:shootratioof1:4, butlaterVignaandMucunahardlyformedmore
rootswhileshootdryweightincreased,leadingtoroot:shootratiosof
about 1:12,whiletheotherspeciesmaintainedaroot:shootratioof 1:4
orevenincreased to1:2.Maximumrootdryweight (Centrosema,Pueraria
andPsophocarpus)didnotcoincidewithmaximumshootproduction(Mucuna,
VignaandCentrosema).
Observationsona2-yearoldPuerariafield
Someadditionalobservationsweremadeina2-yearoldPuerariafieldat
thestartoftherainyseason.PlateIVshowsrootsystemsaswashedfrom
pinboardsamples.Table3.4givessomedata.Puerariaformsmanystolons
(creepingstems justabovetheground)whichformadensemat.Individual
plantscannolongerberecognised.Nconcentrationandroot:shootratio
aresimilartothevaluesforthe14-weeksoldplantsintheexperiment.
40
root dry weight,
(g/m2)
60
/r/s=1/2
, r / s =1/4
r/s=1/12
xCentrosema
•Desmodium
aPueraria
APsophocarpus
oMucuna
oVigna
BO 100
120 UO ISO 180 200 220 240 2S0 280 300 320
340 360 380 j400 IZ_J
plantdryweight,(g/m2)
Figure3.10.Relationbetweenrootandshootgrowthforcovercrops(averageforlimeandnolime).
TABLE3.4.QuantitativedataonthePuerariasamplesshownin-PlateIV.
Shootdryweightwasdetermined forthreesamplesfrom0.6m.
Plantdry
weight(g/m2)
Nconcentration
shoot(n=3)
stolon
190
189
2.52+ 0.08
root
48
20
5.7
6.1
2.9
1.0
83
0-10
10-20
20-30
30-40
40-50
50-60
total 0 - 6 0
root:shootratio
0.21
(%)
Mycorrhizal
infection (%)
5.2
41
3.4.Discussion
Thesixspeciesusedinthisexperimentshowedobviousdifferencesin
lifecycle,rapidityofcoveringthesoil,necessity ofweedinganddepth
ofrootdevelopment,whichmayaffecttheirsuitabilityasacovercrop.
Twospecies(MucunaandVigna)exhibited fastgrowth,coupledwithearly
senescence,alowroot:shootratioandarelativelyhighNconcentration.
Thesecropsmaybenefitfromtheflushinmineralisationatthestartof
therainsthroughrapiduptake;theymaybesuitableasashort-term
covercrop,releasingnitrogenfromdecayingorganicmatterinthesecond
partofthegrowthseason.Theirrootdevelopmentandconsequently
chancesofrecoveringleachednutrientsarenotimpressive.Theother
cropshaveaslowstartandgrowatahigherroot:shootratio(investing
a largerproportionoftheirdrymatterproductionintherootsystem).
Theseplantsmayprovevaluableinthelongrunwhentheyhaveadeep
rootsystem(Centrosema)orgoodnodulation(Pueraria).ThePsophocarpus
andDesmodiumvarietiestestedareunsuitableunderthese
conditionsbecauseoftheirslowestablishment.Puerariaandtoalesser
extentPsophocarpusarenotsuitableforintercropping (exceptwith
trees)duetotheirwindingstems,whichontheotherhandmakethemwell
suitedforsuppressingweedswhenusedinaplantedfallowsystem.
Liminghadnodramaticeffectonplantgrowth.Above-groundproduction
wasnotsignificantlyaffected,althoughCaandKconcentrationincreased
considerablyandNandPconcentrationslightly.TheincreaseinNconcentrationaswellasareductionofnodulationintheslowstartingspeciesPuerariaandPsophocarpusmaybeattributed to(locally)increasedN
mineralisationbylocaleffectsonsoilpHaroundlimeparticles.Such
localeffectsonpHmaybemoreimportantthaneffectsonbulksoilpH.
ForthefastergrowingspeciesVignaandMucuna(whichtakeupconsiderablymoreNinthefirstperiod),theeffectoflimeonnodulationispositive.Liminginfluencesrootdistributionbydecreasingmaximumroot
PlateIV.Rootdistributionundera2-yearoldPuerariastand;A.andB.
showtworeplicatesamples,A.withandB.withoutathicklayerofstolons;C.showsdetailsofnodulesupto20cmdepth;D.givesadetailof
Puerariarootsinanoldpalmrootchannel;E.picturesademonstration
plotofPuerariainIbadanatUTA'smaincampus,showingamatofstolonsanddecayingleaflitterwithlittleconnectionwiththesoil(the
sameeffectisevenmoreobviousinDesmodiumplots).
43
depthandincreasingtheproportionoftotalrootweightpresentinthe
topsoil.Theoveralleffectoflimeontheefficiencyofnitrogenutilizationprobablydependsonthecropandthedegreeeofsynchronisation
betweencropdemandandsoilsupply.Limeapplicationcanincreaselocal
mineralisation,butthisisonlyusefulinperiodsofhighcropdemand.
Aslimecanbeappliedatanytime,itstimeofapplicationcanbechosen
suchthatNmineralisationtakesplaceintherightperiod.
Therewereconsiderabledifferencesinnodulationamongthespecies
consideredwhenplantedwithoutinoculation.NodulationinCentrosemawas
poor;thisplantmightbenefitfrominoculation,butpotexperimentsby
Otisi (1979)showedthatinoculationofthesoilwithRhizobiumdidnot
improvenodulation(atleastnotfortheRhizobiumstraintested).NodulationstartsslowlyinPuerariaandPhophocarpusatfirstaccompaniedby
yellowingofthe leaves,sotheseplantsmightbenefitintheirinitial
stagesofgrowthfrominoculation.Desmodiumhasalownoduledryweight,
butthelargenumberofsmallnodulesallseemtobe"effective".
Mycorrhizalinfectionlevelsarelow,butduetotheratherhighsoilP
levels,Pnutritionisnotaproblemandmycorrhizaisnotstrictly
necessaryundertheconditionsofOnne.
44
4.UPLANDRICE
4.1.Introduction
Uplandrice(OryzasativaL.)is widelygrowninthehumidtropics.
Richards(1985)discussed thelargenumberofvarieties traditionally
usedinSierraLeoneandS.W.Nigeria,adaptedtoavarietyofgrowing
conditions.Therootsystemofricegrownunderuplandconditionsdiffers
considerably fromtherootsystemunderwet(sawah)conditions.Theroots
ofupland ricecanpenetratequitedeeplyintothesoil.Accordingto
NicouandChopart (1979)therootsystemofuplandriceisintermediate
(inrootingdepthandrootdryweightinthetopsoil)betweenmaizeand
SorghumontheonehandandPennlsetum(millet)ontheother,onsandy
andsandyclayloamsinSenegal.AccordingtoWestphaletal.(1985)the
rootsystemoftraditionaluplandricevarieties,adapted tolowsoil
fertility (especiallynitrogen)extendsdeeperintothesoilthanthatof
"modern"cultivars.
Observationsonrootsofupland ricereportedhere,weremadeinexperimentalplotswhereleachingofwaterandnutrientswasstudiedunder
fieldconditions,withandwithoutNPKfertilisation.Nutrientuseefficiencygenerallydependsonthefertilitylevelofthesoil.Fertilisationcanmodifyboththesupplyofnutrientsinthesoilandthecapacity
foruptakebythecropbyinfluencingrootdevelopment.Athigherfertilitylevelsrootdevelopmentmaybebetterinabsoluteterms-although
root:shootratiowillbelower-thanunderpoorsoilconditons,duetoa
generallybetterplantgrowth.Thegeneralpatternofthisresponsefor
annualcropshasbeendescribedbySchuurman(1983),asshowninfigure
4.1.
Figure4.1 canexplainthecontrastingreportsintheliteratureoneffectsoffertilisationonrootdevelopment:positive,neutralandnegativeeffectscanbeexpected,dependingontheoriginalfertilitylevel
ofthesoil.Althoughthegraphhasbeenderivedfrompotexperimentsundertemperateclimaticconditions,theoverallshapeisprobablyvalid
undertropicalconditionsaswellasitisbasedonelementaryprinciples
ofthefunctionalequilibriumbetweenrootandshootgrowth(Brouwer,
1984).Onlyafewquantitativestudiesundertropicalconditionsare
45
availableintheliterature.Ligthart (1981)foundapositiveeffectof
NPKfertilisation(30kgNperha,25kgP0/ha,30kgK0/ha)onroot
25
2
massofmaizeonsixsoils,andanegativeeffectononesoilinS.E.
Kenya(Kaloleniarea);positiveeffectsonrootdevelopmentweregreatest
inthetopsoil,withaslightimprovementofdeeprootdevelopment.Nutrientuseefficiencyunderconditionsofhighleachingcanbeexpected
todecreasesharplywhenfertilisationexceedsthelevelformaximumroot
growth.Itcanbeexpectedthatnutrientuseefficiency ishighestat
sub-maximumshootgrowth.
shoot /root ratio
(0
weight
6
,,. 12
.3
growth conditions
in the soil
f
•
optimum tor root shoot
agriculture
Figure4.1.Generalréponseofannual(cereal)plantsonincreasingsupplyofwaterandnutrients.Theoptimumforrootgrowthisreachedata
lowersoilfertilitylevelthanthatforshootgrowth.Shoot:rootratio
increasesgraduallyuptotheoptimumforrootgrowthandrisessharply
beyondthispoint.
4.2.Methods
Rootobservationsweremadeinplotsofuplandrice(table4.1)where
soilmoisturecontentandthecompositionofthe soilsolutionwere
monitoredbyM.Weber(LandwirtschaftlicheForschungsanstaltBüntehof,
46
KaliundSalzAG,FRG).Rice(varietyITA307)wassownon27March1985
ataplantspacingof30x 15cm.Amountsof90kgNPK(15:15:15)and100
kgMgSO perhawereappliedintwosplits(3weeksand6weeksafter
4
sowing,respectively).Soilsolutionprobesandtensiometerswereinstalledinthecropfields(infertilisedandunfertilised part)at10cm
depthintervalsformonitoringwaterandnutrientdistributioninthe
soil(datanotyetavailable).
TABLE4.1.Somesoilcharacteristics oftheupland riceplot.
Depth(cm)
0-15
15-25
pH(H20)
476
4.6
pH(KCl)
376
3.65
bulkdensity (g/cm3)
1.37+0.03 (n=4)
1.52+0.01 (n=3)
Duplicaterootsamplesweretakeninfertilisedandunfertilisedplots
usingapinboardof40x60x12cmat3,5,9and14weeksaftersowing.
Eachpinboard samplecontainedtwoplantrows.Afterwashingandphotographingunderstandardconditionseachsamplewascutintosubsamples
per10cmdepthandaccordingtodistancetotheplant (comparefigure
3.1).ForeverysubsamplerootlengthwasmeasuredusingthelineinterceptmethodofNewman(Böhm,1979)anddryweightwassubsequently
determined.PlantsamplesfromthepinboardweredriedandanalysedforN
andP.
4.3.Results
Shootgrowth,NandPuptake
Figure4.2showsdrymatterproductionandNandPuptakeforfertilised
andunfertilised rice.Fiveweeksafteremergence(twoweeksafterthe
firstfertiliserapplication)nodifferencebetweenfertilisedandunfertilisedplotscouldbeseen.Afterfiveweekscleardifferencesingrowth
wereobserved.Nitrogenuptakeoccurredmainlyintheperiodbetweenweek
5andweek9.Afterweek9nonetuptakeofNoccurred (infactNwas
47
lost from the crop)andN concentration (as%of drymatter)decreased
considerably. Unfertilised plots showed the samepattern as fertilised
plots, although at a lower level.Phosphate uptakewas linearwith time
fromweek 5 toweek 14.Betweenweek 9andweek 14P concentration (as%
of drymatter) increased.
P-uptake,(g/m2)
2
3
drymatter,
(g/m2)
1000r
fert.
N-uptake
P-uptake
10
14 9 5
time(weeks)
start flowering 9 -
A
/t>
20
N-uptake,
(g/m2)
A
U
time(weeks)
2
3
P-uptake,(g/m2)
Figure 4.2.Time course of drymatter production and N and Puptake in
fertilised and unfertilised upland rice.
Root observations
The root systems asobtained by pinboard sampling are shown inplateV
and figure4.3.Distribution of rootdryweight is shownin figure 4.4.
and root length infigure 4.5.At 5weeks after emergence (2weeks after
the first NPK split application) the root system on the unfertilised
plotswas slightly better developed.At 9weeksthe root system inthe
fertilised plotswasmore dense in the topsoiland extended deeper into
the soil than the roots intheunfertilized plots.Betweenweek 9and
week 14root development onboth plots continued.On thefertilised plots
48
rootsextendedbeyondadepthof70cm,ontheunfertilisedplotsthey
reached40cm.Aduplicatepinboard sampleconfirmed thedeeprootingbehaviourofthefertilisedplants.Rootlengthdensitiesinthetop10cm
3
ofsoilreachedvaluesof5cm/cm,thedifferencebetweenthetwosubsamplepositions("5-5"and"5-15"cmfromtheplant)decreasedwith
time.Specificrootlength(cm/mg)generallyincreasedwithdepth(figure
4.6.).
Figure4.3.Diagramofricerootdevelopment:s=seminalrootsystem,a
=adventitiousroot,nl,n2,n3=nodalrootsof1st,2ndand3rdwhorl.
PlateV.Rootsystemsofuplandriceonpinboards.
Fertilizer
Fertilizer
9weeks
20rTp.;
40L
U weeks
ftasa&aft
..'.;' :'/.
-iVf''"J '
50
-Fertilizer
+Fertilizer
root weight d e n s i t y (mg/cm 3 ]
,0 0.02
0.06
0.10
0.14
0 0.02
0.06
"I
0.10
0.14
r
5wk
40
Oi
IT
9wk
40
1
-n
r
1 1
!
Uwk
^
uwk
0 - 5 cm from plant
5 -15 cm from plant
70
soildepth
(cm)
average
Figure4.4.Distributionofrootdryweightinuplandrice.
51
+Fertilizer
-Fertilizer
root length density (cm/cm 3 ]
0 I 2 3 L 5
I—'—T
1
r
soildepth
(cm)
Figure4.5.Distributionofrootlengthinuplandrice.
52
root length per
unit dry weight,
(cm/mg)
A
-
40
•
I«
I
-fertilizer +fertilizer
o 5- 5 •
A 5 - 15 A
oaverage •
30
'
20
/B
10 _o
O
|
D ,5
D
\
5 ». \ \
!f*V
A 8
Q
i
i
i J
0-1010-20
3
II *
0-t0 10-20 20-30
5
*'•'>'•'
0-10 10-20 20-30 30-40
9
i
i
i
tïlTlTt
i
0-10 10-2020-30 30-4040-SO50-6060-70
depth.(cm)
U weeks
Figure4.6.Specificrootlength(rootlengthperunitdryweight,cm/mg)
inuplandrice.
Root:shootratio
Figure4.7 showstherelationbetweenrootandshootgrowthwithtime.
Root:shootratiodecreasedcontinuously overtheseasonuptoavalueof
1:7 forunfertilisedplots andtoalmost 1:10forfertilisedplots.When
wecomparethesedatawithfigure4.1,itislikelythatthefertilised
plotsshow themaximumrootdevelopmentthatcanbeexpectedunderthese
conditions.
4.4.Discussion
Therootingdepthobservedherewasbeyondallexpectations,especially
onthefertilisedplots.Thedirecteffectonrootgrowthofthefirst
splitapplicationofNPK,asobservedafter5weeks,wasnegative.It
53
tooklongerbeforealltopsoilwasexploitedbytherootsinthefertilisedascomparedtounfertilised plotsandconsiderablenutrientlosses
mayhaveoccurredinthisperiod.After9weekstherootsystemsonfertilisedandunfertilised plotsweresimilarandinthelastperiodroot
developmentwasgreatestinthefertilisedplots.Therootingdepthof70
cmreachedcan,however,notbeverysignificantforNuptakeasthetotalNcontentdecreased inthisperiod (i.e.Nlossesfromtheplantexceeduptake).Amoredetaileddiscussionwillbepossiblewhentheresultsofthesoilsolutionanalysiswillbeavailable.Thepresentdata
onabove-ground drymatterproductionarenotvery reliableastheyare
basedonsamplesfromsmall areas.
rootdryweight,
(g/cm2)
130r
/r/s=1/2
1
.'//
i /// /
/ 1/ /
;y '
/ r / s=1/5
r/s=1/10
y
'
•+ fertilizer
A-fertilizer
500
1000
shoot dry weight,(g/m 2 )
Figure 4 . 7 . Relation between root and shoot growth of upland r i c e .
54
5.MAIZE/CASSAVA INTERCROPPING
5.1.Introduction
Rootsandintercropping
Intercropping aspractisedbyfarmersinthetropicsforcenturiesisan
intensiveproductionsystem,asitmayincreaselandproductivitythrough
anefficientutilisationofspace,soilmoistureandnutrients,solarradiationandotherenvironmentalgrowthfactors.Advantagestothefarmer
include:highandrelatively stableyields,abetterdistributionoflabourandproductionoverthegrowingseason,reducedadverseeffectsof
pestsanddiseasesandoftenabetterprotectionofthesoilfromerosion.Themaindisadvantageisthelimitedpossibility ofusingfarmmachines.Researchaimedatimprovingintercroppingsystemsbyintroducing
newtechniqueshasbeenslow(Richards,1985)becauseofthecomplicated
natureofintercropping,inwhichpositiveeffectsononecomponentoften
areaccompaniedbynegativeeffectsonothers.Thebasicconceptinan
analysisof intercroppingisthatofreducingcompetition,"nichespecialisation"orcomplementarityofcrops,asitcanbestudiedin"replacementseries"(DeWit,1960).
Theprinciplesinvolvedinthecompetitionbetweenrootsystemsforwaterandnutrientsarelessthoroughlyformulated thanthoseinvolvedin
competitionforlightbyshoots.Combinationsofcropsonthesamefield
maygiveabetterutilizationofsoilresourcesif(andinsofaras)the
rootsystemsofthecomponentcropsarecomplementary inspaceand/or
time,i.e.therootsystemsdifferinsoillayersexploitedand/orinthe
periodofhighestuptakerequirement.Rootresearchofthecomponent
cropsinamonocultureaswellasinmixturesmayhelptounderstandcompetitionforsoilresourcesbythecropsandhelptodevelopplanting
techniquesforthepurposeofreducingcompetitionandincreasingthe
combinednutrientuseefficiency.
LaiandMaurya(1982)studiedtherootsystemofimportantcropsofthe
humidtropicsindeepboxesfilledwithasandytopsoil(figure5.1).
Rootdevelopmentofmostcropsunderthesefavourableconditionswas
deep,withsomeinterestingdifferencesinrootpatternamongcrops.
55
Maizeshowedthecharacteristiccombinationofadeepseminalrootsystem
withnodalrootsextendinghorizontally beforegrowingalmostvertically
intothesoil.Cassavarootstendedtoconcentrateinthetopsoil,extendinghorizontally overconsiderabledistances,althoughdeepersoil
layerswereadequately rootedaswell.Therootdistributionofyamwas
homogeneousthroughoutthebox.Sweetpotatoandcowpeahadmostoftheir
rootsconcentratedneartheplant.Astheauthorsremarked,rootdistributioninthefieldmaydifferconsiderably fromthepatternfoundunder
theirexperimentalconditions,dependingonthesoilprofile.Adequate
descriptionsofrootgrowthinthefieldareveryscarce,however.
cassava 3months
cassava a t matunity
yam at harvest
0 10 20 30 40 50 60 0 10 20 30 CO 50 60 0 10 20 30 CO 50 60
i
1—i
1
1
1-
I
r-
-i
1—i
r-
-1—I—I—I
'bI
//
1
i1 1
//
225
sweet potato
maize Cweeks
0 10 20 30 CO 50 60 0 10 20 30 CO 50
1
1—i
1
1
<—i
i
1
1—i
1 1
1
cowpea Cweeks
0 10 20 30 CO 50
a 6888)300-1000» lO-mg/cm'
i> E ^ 1 00 . 300
c E 2 30 . 100
d EZ3 1 0 . 30
eO
3- 10
fC 2
1- 3
9 E 3 03 -
1
-
h1 1 <03
Figure5.1.Rootdistributioninaboxfilledwithtopsoil(datafrom
LaiandMaurya,1982).
56
Aconsiderableefforthasbeenmaderecentlyinthesemi-aridtropics
todescriberootdevelopmentingrain/legumeintercropping (pearlmillet/
groundnut;GregoryandReddy,1982;Vorasoot,1983).Theyshowedthatin
combinationwitharelativeyield totalofmorethan 1.0forabove-ground
production (1.1-1.3),the"relativeroottotal"alsoexceeded 1.0(1.01.2).Shoot:rootratiosandnutrientuptakeperunitrootlengthappeared
tobeunaffectedbyintercropping.LaiandMaurya(1982)demonstratedthe
existenceofa"relativeroottotal"inexcessof1.0in acowpea/maize
mixture.
Researchoncassava/maizeintercroppingatOnne(vanderHeideetal.,
1985)showedthatcassavadidnotreducemaizeyieldwhenthetwowere
intercropped,butcassavayieldsinintercroppingwerelowerthaninmonoculture,especiallywhenNfertilisationincreasedmaizegrowth.Shadingisobviously importantbutothermicroclimaticaspectscanberelevantaswell.Thehighhumidityoftheairinsideclosedcropcanopies
cangiveproblemsinmaize(fungusdiseases,germinationofseedsinside
thecobs).LocalfarmingpracticearoundOnneistousea plantspacing
whichreducessuchproblems,byplantingmaizeinsmallclustersin-betweencassava,ratherthanasindividualplantsinrowswhichonewould
expecttobethebestwaytofullyinterceptalllight.Plantingmaizein
smallclustersreducesadverseeffectsofrainstormsonyoungplantsearlyintheseasonandresultsinamoreopencropcanopyreducingdisease
problems (theeffectprobablydependsonthescaleoftheplotsandthe
presenceof"windbreaks").Anegativeaspectofsuchaplantingscheme
mightbe,however,thatthesoilisexploitedlessefficientlybyroots
thanbyaregularly spacedcrop.Wedecidedtomakeregularrootobservationsinanexperimentinwhichnitrogenfertilisationwastestedin
threeplantingpatternsasshowninfigure5.2.
5.2.Methods
Rootobservationsweremadeinanexperimentoftheon-goingNmanagement
project (experiment 3), inwhichtheNréponseindifferentplantspacingsistestedinarandomizedblockdesignwith3replicates (twoNlevels:0and60kgN/ha,threemaizespacingsasshowninfigure5.2and
twocassavaspacings:1and2m). Rootobservationsweremadeinoneof
thereplicates,graduallydestroyinghalfoftheplotbydiggingsoil
pits,atacassavaspacingof1m.
57
Thefieldwasinthesecondyearaftermanually clearingthesecondary
forest whenthebulkdensityoftopsoilwasstilllow(1.20+0.03g
3
3
percm (n=4)asmeasured inringsof5870cm ). Allplotsreceived
basaltreatmentsof40kgsinglesuperphosphate,80kgKasmuriateof
potash,20kgMgasmagnesiumsulphateand2kgZnaszincsulphateper
ha.Thesefertilizerswereappliedonedaybeforeplanting (19April).
UreaasNsourcewasappliedintherelevantplotsintwosplitapplications of30kgNperhaeach,4and8weeksafterplanting (17Mayand
14June).
.m
%5 - 5 [ 5 - 1 2 5I 5-5 I 5 - 12.5I 5- 5 I 5- 12 5|s - S|5 -12 5 [ T t
JL
t*
m<jB"c
15-2515-15 | 5 - 5 |S - 15115-25 | IS-25 |5 - 15|5- 5 | 1 5~Ts
- 5 I '5-»I
5
J.5-50|35-A5|25-35|l5-25|5-15|5-5|5-15|15-25|25-35|35-45|t5-50|
-100cm
c= cassava
m= maize
-120
Figure5.2.Schematicpresentationofplantspacingandthepositionof
pinboard sampleandrootmaps.
Fourmaizeseedswereplantedperclusterfor1mmaizespacing,two
seedsforthe0.5mspacingandoneforthe0.25mspacing.Rootswere
58
sampledwithacombinedpinboardof40x 120x 12cmat2,5,8and14
weeksafterplanting.Afterthesamplewaswashedandphotographed,it
wascutper10cmdepthandsubdividedaccordingtothedistancetothe
plant,asshowninfigure5.2.Foreachsubsamplerootdryweightand
rootlengthwererecorded.Beforeapinboardsamplewastakenavertical
rootmapwasmadeofthesoilprofile(almostintheplantrow)toa
depthof50cm.Afterthepinboard samplewastakenahorizontalmapwas
madeat5-10cmdepthfromabout 15to65cmfromtheplantrow.Rootson
themapswerecountedona5x5cmgrid.Fromthefinalharvestsome
rootswerecollectedforcountingmycorrhizalinfectionasbefore.Above2
groundproductiononeachoccasionwasmeasured fromanareaof2-4m;
sampleswereanalysed forNconcentration.
5.3.Results
ShootgrowthandNuptake
Figures5.3and5.4showthetimecourseofdrymatterproductionandN
uptakebymaizeandcassava.Asthedataarebasedonsamplesofonly2
m interpretationiscomplicatedbyalargesamplevariation.InmaizeN
uptakeoccurredintwoperiods,2-5and8-14weeks,withlittleuptake
in-between.Ontheunfertilised plotsinitialuptakeofmineralisedsoil
Nwasfair(uptoatotalcropNcontentofabout10kgNperhaafter5
weeks),butafter5weeksuptakestagnated.After14weeks,thesampled
plantsfromtreatments0.2and0.3wereratherexceptionalinthatthey
hadnocobs(otherplantsintheplothad)andthattheplotfromwhich
the0.1 samplewastaken,wasexceptionallygood(finalyieldofthis
plotwas50%abovetheaverageforthe3replicates).Theplantsampled
fromthe0.1 plotgrewnexttoanoldAnthonatastumpintheplot,which
mayhaveallowedforadeeprootdevelopment (althoughthiswasnotincludedinthepinboard sample).Exceptforthislastplot,theunfertilisedmaizestarted atanNconcentrationofapprox.2%andlater
decreasedtoapprox.1%.Thefertilisedplantsatfirstalsocontained
about2%N.After5weekstheyhadtakenupabout10kgNperhamore
thantheunfertilised plants(fromanNapplicationof30kgNperha).
Betweenweek5andweek8hardlyanynitrogenwastakenupandtheN
concentrationdecreased toabout 1%,apparentlywithouteffectsondry
matterproductionrate whichproceededatabout 75kgDMperhaper
59
day.ThesecondsplitapplicationoffertiliserNafterweek8(30kgN
perha)resulted inareturnoftheinternalNconcentrationto2-2.5%.
TotalNuptakebetweenweek5and8wasappr.35kgNperha.
0.75%
Maize
2.0%
2.5%
Time(weeks)
Figure5.3.Above-grounddrymatterproductionandNuptakebythemaize
componentoftheintercroppingexperiment.
Thegrowthrateofcassavawasmuchlowerthanthatofmaizeinthese
first14weeksandwasonlyapprox.15kgDMperhaperday.TheN
concentrationintheshootswashighatfirst(about5%)butdecreased
laterto1.5-2%.Nuptakeoccuredintwoperiods,justasinthemaize.
Uptakebetweenweek5and8wasverysmall.Fertilisationhadlittle
effectonNuptake.TotalNcontentofthecassavacropafter14weeks
wasabout20kgNperha.
60
Rootgrowth
Thegeneralshapeoftherootsystemsofmaizeandcassavaisshownin
platesVIandVII.Themaizerootsweremainly restricted tothetop10
cmlayer,whilethecassavarootswentdeeperandextendedwellbelow
thoseofthemaizeplants.Aprobleminsamplingcassavarootswitha
pinboard isthatrootsareformedincertainsectorsbutnotinothers,
givingeitherlargeorsmallamountsofrootsinapinboard,butnever
"average"values.Figure5.5 suggeststhatplantingscheme .1gaveabetterutilisationofthesoil,althoughmaizerootsinscheme .3extended
horizontally farenoughtoreachthecassavaplants.
61
Cassava
DM(g/m 2 )
160r
1.5%
o
1%
2%
HO
/
/
/
120
,3%
15kg DM.ha J dag J
100
,4%
80
,5%
60
40
- S ^
H
8
Time(weeks)
2.5%
20
«H
n~ 8
HL
)i
Time(weeks)
2
01o
02û
03o
3
N-uptake
2
(g/m )
.51
A 52
»53
-A^A
Figure5.4.Above-ground drymatterproductionandNuptakebythe
cassavacomponentontheintercroppingexperiment.
PlateVIandVII.Photographsofrootsystemsoftheintercropping
experimentwashedfrompinboards.
LO
m
i—i
t ,„„., j
X.
o oo o
r~
o o
CM CO
CM
o
O'
X
J.
O O:
*-••*
LO >:
site
m
C3
Cw?
r-~
w?
t?
±
C\I 0 0
j
C™?
**4*
O
(NI
O
00
O
O
O
O
<r— (Nj 0 0 "*<?
L_J
L_J
O
O
O O
CM 0 0
63
-100 c m -
i-a 2Scm_i
100cm-
-100cmm.mm.m
50cm.
c= cassave
m=mais
Figure5.5.Schematicshapeoftherootsystemofmaizeandcassavafor
thethreeplantingschemes.
Figure5.6 quantifiesrootlengthdistributionformaizeandcassava.
Detailsofdistributionwithregardtodistancetothenearestplantare
giveninappendix 1foreverylayer.Detailsofrootweightdistribution
aregiveninappendix2and3.Figure5.6 showsthatmaizerootgrowth
almoststoppedafter8weeks;intheperiodbetween8and14weeksthere
washardlyanynetrootgrowth,whilerootsaregraduallybecomingbrown
andstartedtodecay(especiallyonthefertilised plots).Specificroot
length(figure5.7)decreasedwithtimefromappr.8toca2.5cmpermg.
Ineverysoillayerthevaluefirstincreasedwhentherootsbecamemore
branched,butlaterdecreasedagainwhenthefinerootsstartedtodecay.
Fertilisationhadnoconsistenteffectonspecificrootlength,but
increased rootlengthdensityinthetopsoilfrom1-4cmpercm to2-6
3
cmpercm.
64
Maize
8
nr
10
0
2
1
4
i
1
rr
i
2weeks
20
0
i
'
i'
i
10
20
5weeks
\
30
0
!
10 20
average root length per unit
volume of soil(cm/cm 3 )
0
2
4
6
~i
I
'
8weeks
^
30^
40
"~
0
I
1
20
1
10
!
1
1
.j
1
U weeks
40L
soildepth
(cm)
.2
0 fertilizer
5 fertilizer
i
30
Cassave
.i
— i
10 I
average root dry weight
per unit volume of s o i l ( m g / a n ] )
•2
0
. 1 2
.3
1
2 weeks
r-1 • •
20L
~\
5weeks
-i
8 weeks
r
"i—r
j
K weeks
?
50
s o i l depth
(cm)
.1
.3
Figure5.6.Rootlengthdensity (averagedoverpostionsnearandinbetweenplants)ofmaizeasitdevelopsintimeinfertilisedandunfertilisedplots.
1
65
Maize
root length per
unit dry weight
(cm/mg)
2 weeks
8weeks
5weeks
o
01o
• 51
02A
A 52
03D
. 53
• average
•
10X)
A
Uweeks
o ,'
5.0
a
*?-4'
S
•
°
1.0-
TT
0-10
t
10-20
0-10
I
I
10-20
20-30
0-10
I I I
10-20
20-30
aa
J
0-10
,
I
10-20 20-30
soil depth(cm)
Cassava
root length
per unit
dry weight
6|-(cm/mg) <-
0111o
.5111
A5211
• 5311
•kaverage
0211A
8weeks
5weeks
0311 Û
•
U weeks
8 1
8
A
*\"
\
0-1010-20
t I I't't'
0-0 10.20 20-30 40-30
30,40
rrrr
0-10
20-30
10-20
30-40
11111
0-10
20-30 40-50
10-20
30-40
soil depth (cm)
Figure5.7.Specificrootlength(rootlengthperunitdryweight)of
maize(A)andcassava(B)fordifferentsoillayers.
66
Inboth fertilised and unfertilised plots therewas little difference
in root length density between positions near and inbetween plants for
planting scheme 1,while therewas a largedifference for scheme 3and an
intermediate effect in scheme 2 (appendix 1 ) .For cassava, root length
density in the topsoil continued to increase over the 14-week period.
Root length density after 14weeks was still lower than that ofmaizeby
a factor 10 (figure 5.6.B), but root densitywasmore constantover the
top 30-cm layer,with some roots extending to40cmdepth. Inall soil
layers root density decreased with distance from theplant.At 45-55cm
from theplant cassava root length density was about half theaverage for
thewhole soil layer.Fertilisation had no consistent effects on root
distribution, total root length or specific root length incassava. Specific root length decreased over the season from circa 3.5 toappr. 1.5
cm per mg.
JO
5
10
15 20
25 30 350
5
10
15 20
numberofrootsper25cm 2
25 30 35
60
soildepth
(cm)
Figure 5.8.Average root density system seen onvertical rootmaps (number of roots seen per 5x 5cm) .
67
Results for thevertical rootmaps are shown infigures 5.8 and 5.9.
Total root density on thesemaps increased steadily with timeat all
depths. Overall root density on themaps washigher in the fertilised
plots. The general pattern confirms thedata formaize in figure 5.6.A,
asmaizehad by far thehighest root length density.
With N fertilisation
WithoutNfertilization
Noofroots/25cm2
—• week14
lODr
80
o—
-
8
X— —X
••
4
:
2
A— —A
60
\
40
<N
.-°>
Noof roots/25cm
100r
-/V 01
/
51
80
- / n
60
/
40
.o-o-o
20
20
-*,.A
A
. . A - ^ * *
* * ^ * * = * •A--A-*
j
4 maize
cassava
100r
80
cassava
60
.o-o-o-o-o-^
=N?o-o-o
40
20
•^
.-•
,o
60
40
20
52
°vC
2maize
201-
cassava
80
60
*'^Ä--^
0
cassava 2maize
100 r
80
^vy>.
03
1maize
1 maize
\
. ^ - A - A - ^ r A - A . 4-^4
Imaize
cassava
Imaize
j _
20
40
60
60h
/
\
80
Imaize
cassava
j
100fcm)
,A"-X~X--*'
2maize
"O"
,X-X
A
X-i
Imaize
Imaize
cassava
20
40
60
distance
Figure 5.9.Distribution of roots in top soil (0-20cm)onvertical root
maps, according to distance to theplants.
Results for thehorizontal rootmaps are showninfigure 5.10 and 5.11.
After 2weeks more than 50%(average 64%)of the spaceinbetween plant
rows stillwas not covered by roots (0or 1root per 25cm) .
53
^o^
20
Imaize
J
cassava
.--v.^\
/
40
/*
--A-A-'
cassava
'o-o-o
.^-r?»=»^.
cassave 2maize
100h
02
4maize
cassava
100
80
\A-A--A
j
Imaize Imaize
cassava
80
100fcm)
distance
rn
68
2 weeks
2 weeks
¥Z™
(Map 10 Hor 0111}
0
(Map16 Hor5111)
51
«
CM
M
C'M
(Map18 Hor5211)
(Map17 Hor 0 3 2 7)
/
0.3
Legend
•
0 - 1n o / 2 5 c m 2
ZZ2 2- 5
MMMM
c
E53 6-12
13-25
>25
EU no observations
M=maize
C=cassava
5 weeks
MM MM
5 weeks
Map 29 Hor. 0111)
(Map 27 Hor 5111)
Map 31 Hor5211)
IMap32 Hor 0321)
(Map 30 Hor5311)
69
8 weeks
8 weeks
(Mop 41 Hor 5111)
MC
M
M
M
MC
CM
M
M
M
CM
(Map 40 Hor0211)
C
MM
MM
C
Map 42 Hor5311)
(Map41
Hoi-0321
MMMM
1/. weeks
H weeks
(Map 55 Hor 5111)
(Map 54 Hor 0111)
MC
ïï
M
>
M
tf
v
v
M
y/
$K$
///,
Y/
CM
::
'S/--::^/y
(Map 52 Hor 5211)
Wr/2&
<xP/KX>Bi
ÏW f
'M '
'/ /fc 'J
'.•v;-;;:-:;-:v^v;-:rv^:v.: : v;
MM
M
'i
y
YTÀ/
M
(Map53 Hor0211)
y AS
§
M
V/
^ ^ r
f
v%m&
C
MC
CM
^i
4y>fài
\VyJ*
''l
y,
MM
C
Wr-r
/
02
I
C
/ / ^I!^ * ^ * ; v : - * r x x
MM
2
<
WTt^'
^ss&s
i
1
MM
^
',
c
V
7p;
:
3004-'
/I'-*"-
SA
/fe
^
/PWtf>W/J£fV,
lui
MMMM
v///
Figure5.10.Distributionofrootsonhorizontalmapsat5-10cmdepth.
52
sSSrO)--
(Map 56 Hor5311)
70
percentage of topsoil
50
100
weeks
113 - 25
> 25 no per 2 5 c m 2
Figure5.11.Frequencydistributionofrootdensityonhorizontalroot
maps.
After5weeksontheaverage25%oftheareastillhadnotbeenreached
byroots.Theresultswererathervariable,possiblybecausetherecanbe
a sharpdecreaseofrootdensityintherangeofdepthsinwhichthemap
wasmade(5-10cmdepth).Inthelastobservationnofixedcriterion
wasusedwhetherornottoincludedecayingmaizeroots.Noconsistent
effectsofplantingpatternorfertilisationemerges,althoughespecially
after5weeksthe.1plantingschemegavethemostevendistributionof
roots.
Figure5.12showscassavarootdistributionatharvest time,attheend
ofthedryseason,plantedinthepreviousyear.Compared totheobservationsinmaize/cassavaafter14weeks,therootdensityinthetopsoil
waslower,whilerootdensityinthesubsoilwasslightlyhigher(roots
penetratebelow60cm,especiallyinoldtreerootchannels,compare
plate I).Apparentlycassavarootdensityinthetopsoilgradually
increasedafterthemaizeharvest,whilerootdevelopmentinthesubsoil
71
continued slowly.Nfertilisationhadlittleeffectoncassavarootdistribution(deeprootdevelopmentwasslightlybetterinthefertilised
plots).Horizontaldistributionofrootsinbothtopsoilandsubsoilwas
fairlyhomogeneous.
2
0
number per 25cm 2
6 6 10 12 H
U
10
20
àrti.
LI
L-'
30
A with N-fertilisation
No horizontal map
A no
Ü0
H/}
50
depth
(cm)60L
number per 25cm2
15r
A-A
10
horizontal distribution
top soil0-20cm
A v**•^S
A'
A' A - A '
V
5|-
10
50
30
A-*-
70
4r
...A-A.
A-Ä
A-A-*'
^ A
A--A-A
"Sfr
subsoil 20-60cm
^j**-4"-**^
j _
'f10 30
cassava
50
110cm
70
j
90 MÎOcm
cassava
Figure5.12.Resultsofverticalrootmapforoneyearoldcassava.
Mycorrhizalinfection
Mycorrhizalinfection(table5.1)ofbothmaizeandcassavawashighcompared totheresultsforcovercrops.Nfertilisationhadanegativeeffectonmycorrhizalinfectioninthetopsoil,butapositiveeffectin
72
thesubsoil.
TABLE5.1.Mycorrhizalinfection(%ofrootlength)14weeksafterplanting.
Depth
Maize
ON
Cassava
60N
ON
60N
0-15 cm
42.7
379
31.8
3379
15-30cm
19.2
32.1
53.3
32.9
5.4. Discussion
Therootdistributionofmaizeandcassavaappearstobecomplementaryin
bothspaceandtime,whichmakespositiveresultsinintercroppingunderstandable.Cassavarootsextendsomewhatdeeperintothesoilanddevelop
laterintheseason,givingthemaizetheopportunity toexploitthetopsoilandthemineralisationflushatthebeginningoftherains.Theroot
densityofcassavainthetopsoilremainedlowuntilaftertheharvest
timeofthemaize.Thispatternmayexplaintherelativelyhighrecovery
offertiliserNincassavafoundinfertilisationexperiments (Vander
Heideetal., 1985).
Althoughaveragerootdensityinthetopsoilwashigh,considerable
"gaps"without rootsexistedinthehorizontalplane.Itispossiblethat
rootswerepresentinthelayerjustaboveorjustbelowthehorizontal
rootmap,butsofarthegapsprobably indicatethatnitrogenmineralised
inthisvolumeofsoilwillnotbeinterceptedandmayleachtothe
subsoil.Arootdensityof2-5per25cm ontherootmap(roughly
equivalenttoarootlengthdensityof0.16-0.4 cmpercm)canbejust
sufficienttotakeupallavailablenitrogenwhenitispresentinthe
ammoniumform(DeWilligen,1985).
Thethreeplantingschemestestedinthisexperimentshowedsomedifferencesinrootingpattern,butlesssothanexpected.Horizontalspread
ofmaizerootsfromtheclusterofplantsinscheme.3allowedexploitationofalargepartofthetopsoil,althoughsomewhatlaterthanin
scheme.1.
73
Fertilisationincreased rootdevelopmentinthetopsoil,buthadlittle
effectonrootingdepth.ThetimecourseofNuptakebythecropsuggests
thatthesecond splitapplicationofNcamejustintimetopreventseriousshortageofNinthecrop.Thefactorsinvolvedinthegoodgrowthof
unfertilisedmaizenearoldAnthonatastumps,deservefurtherstudy.
TABLE5.2.Cassavaandmaizeyieldsonexperiment3(averageforthree
replicateplotspertreatment)in1985(Cassavaplantedin1984,maize
sownin1985)(unpublisheddatakindly suppliedbyIr.A.C.B.M.vander
Kruijs).
Cassava'84/'85
tuberfreshweight,
ton/ha
Maize'85
grain,
kg/ha
N-fertilisation(kg/ha):
0
60
0
60
cassavaspacing1in
maize25cm
50cm
100cm
average
29.5
32.8
32.2
31.5
30.2
31.9
28.2
30.1
643
546
540
576
1893
1708
1555
1719
cassavaspacing2m
maize25cm
50cm
100cm
average
24.0
28.6
24.9
25.8
25.1
29.2
25.7
26.7
542
771
648
654
1845
1825
1835
1835
0.78
0.84
1.14
1.07
cassavaat2m
cassavaat1m
Dataofabove-ground productioninexperiment3(table5.2)showthat
neithertheplantingschemenorNfertilisationhadeffectoncassava
>ields(1984planting);halvingplantdensitydecreasedcassavayieldsby
22%onunfertilisedand 16%onfertilisedplots.In1985Nfertilisation
wasnecessarytoobtainreasonablemaizeyieldsinexperiment3.The
plantingschemeprobablyhadsomeinfluenceonmaizeyieldsforthe1m
spacingofcassava,butnotforthe2mspacing.Maizeyieldswere14and
7%higherwhencassavaplantdensitywashalvedonunfertilisedandfertilisedplotsrespectively.Thefactthattheinterferencebetweenmaize
andcassavadecreaseswithNfertilisationsuggeststhatcompetitionfor
soilNratherthanotherfactorssuchascompetitionforlightisthe
74
mainprocessonunfertilisedplots(competitionforlightbecomingmore
importantbothinabsoluteandinrelativesensewhenfertiliserisappliedandtotalproductionlevels rise).Furtherdiscussionwillbepossiblewhenyieldandlightinterceptiondatafortheexperimentasa
wholewillbeavailable.
75
6.ALLEYCROPPING
6.1.Introduction
Rootsystemsofalleycroppingtrees
Huck(1983)gaveasummaryofrootdistributioninrelationtoagroforestryandcommented thatlittleinformationisavailableonfunctionalroot
parameterssuchasrootlengthorrootsurfacearea.Nairetal.(1984)
reviewed thesuitabilityofalargenumberoftreespeciesforagroforestryindifferentclimaticzones,butgavelittleinformationonrootdevelopment.SomedatacanbefoundintheworkofCoster(1932,1933a,b,
c.)Theopeningsentenceofoneofhisarticlesstillseemstobevalid:
"Untilthirtyyearsagoresearchonthemutualinfluenceofplantsina
community (forest,arableland,meadowsetc.)hasmainly,ifnotexclusively,beendirectedatabove-groundorgans;mutualshadingwasstudied
especially.Duringthelasttenyearsmanyresearchershavebecomeaware
thatbelow-ground organsareatleastasimportantinthisrespectas
above-ground organs."Thisstatementcanbecompared toHoward's(1924)
conclusionthat"itisremarkable ...thatnodetailedinformationofthe
distributionofrootactivityduringtheyearisavailable.Nevertheless
itisclearly essentialintheecologicalstudiesofthefuture."Howard
described rootsystemsofseveraltropical(Indian)fruittrees,ina
deeplyweathered soilas consistingofbothadeeptaprootsystemanda
superficialbranchrootsystems.
Costerstudiedthesuitabilityoflargenumbersoftreesasgreenmanuresforintegrationintoteakplantations.Hisfirstpublication(1932)
dealswiththerootsystemaftersixmonthsofgrowthonadeep,homogeneous,red-brownvolcanicsoil.WemayclassifytherootsystemsdescribedbyCosterintothreegroups:I.Deeppenetration,fewsuperficial
branchroots,II.Deeppenetration,manysuperficialbranchrootsand
III.Shallowrootsystemswithalargehorizontalspread.Figure6.1
showstwoexamplesofeachtype.Ascanbeseenfromtable6.1,groupI
containsonlyrelativelyslowgrowingtrees,witharoot:shootratioof
2.5:1 to1:2.5.GroupIIcontainsmanyfastergrowingtrees,witha
root:shootratioof1:2to1:6.Byfarthedeepestrootpenetrationand
76
TABLE6.1.Classificationof treespecies according totheshapeof their root system after 6months (based ondata ofCoster
1932,nomenclature and spelling of local namesnotmodernised).
Family
No Scientific name
shoot
height
2
shoot
dry
weight
g
3
root:
shoot
(dryweight)
4
main
root
length
0.34
1.9
2.5
0.70
0.83
0.45
1.9
1.1
1.9
3.0
2.6
1.9
2.3
2.6
2.2
2.3
3.5
2.0
2.1
1.9
1.8
2.0
2.1
2.0
0.55
0.27
0.39
0.25
0.54
0.25
0.27
0.15
0.15
0.22
0.18
0.56
0.27
0.41
0.54
0.33
0.24
0.70
2.7
2.4
3.2
1.8
2.4
2.4
3.4
1.9
2.9
2.9
4.4
3.2
2.7
2.8
2.6
2.2
2.6
2.7
3
9
7
8
4
4
6
7
2
8
15
12
8
2
7
1
7
4
3.0
8.2
3.3
7.3
3.1
5.3
4.8
8.9
2.8
5.6
6.7
11
6.6
3.5
4.5
7.2
5.4
4.8
0.26
0.19
0.58
0.10
0.14
0.03
0.15
0.24
0.14
0.11
0.21
0.62
0.21
0.48
0.40
0.24
0.36
0.16
0.11
0.51
0.37
1.6
1.3
1.6
3
1
1
3
4.3
3.5
4.5
5
no.of
branch
roots
> 1m
6
branch
:main
root
length
7
root
system
at later
stage
Group I.Deep root system, fewsuperficial roots» relatively slowgrowth
Moraceaea
Leguminosae
Euphorbiaceae
Anacardiaceae
Sapindaceae
5Artocarpus intégra
8 Albizzialebbeck
9 A. lebbeckioides
12Acacia leucophlaea
17Tamarindus indica
20Bauhinia purpurea
21 Cassia fistula
35Dalbergia latifolia
36D. sissoo
37 Pterocarpus indica
42Swietenia humilis
46S.macrophylla
46Azaridachta indica
47Hevea brasiliensis
48Gluta renghas
50Schleichera oleosa
Nangka
Tekik
Kedinding
Pilang
Asem
Tajoeman
Trenggoeli
Sonokling
Sono sissoo
Sonokembang
Grootblad mahony
Mimba
Rubber
Rengas
Kesambi
1.6
0.8
0.5
0.8
0.5
1.3
0.6
0.5
1.4
1.1
0.3
0.3
0.8
1.7
1.2
0.2
35
14
8.3
11
16
35
18
17
36
42
31
22
25
74
42
12
2.3
2.6
1.5
2.6
0.38
0.36
1.0
3.1
0.54
2.0
1.1
1.6
Group II.Deep root system,many superficial roots,relatively fast growth
Leguminosae
Meliaceae
Sterculiaceae
Lythraceae
Myrtaceae
Verbenaceae
6
7
10
11
14
15
25
29
31
32
33
45
55
56
58
61
64
69
Group III.Shallow
Leguminosae
Anacardiaceae
Malvaceae
Bombacaceae
Bixaceae
Flacourtiaceae
Myrtaceae
Verbenaceae
Enterolobiumsaman
Albizzia falcata
A. procera
Acacia villosa
Leucaena glauca
Adenanthera microsperma
Cassia stamea
Indigofera galegoides
Tephrosia maxima
T.vogelii
Sesbania sesban
Melia azadarach
Melochiaumbellata
Guazumaulmifolia
Kleinhoviahospita
Lagerstroemiaspeciosa
Eucalyptus alba
Vitex pubescens
Regenboom
Sengonlaoet
Weroe
Kemlandingan
Segawe
Djohar
Marmojo
Djajanti
Mindi
Senoe
Djati londo
Ketimono
Woengoe
-
Laban
root system,many superficial roots,VJ
13 Acacia oraria
16Adenanthera pavonina
19Bauhiniamalabarica
22Cassia leschenaultiana
23C. occidentalis
24C.pumila
26C. tora
27Crotalaria anagyroides
30Tephrosia Candida
34Aeschynomeneamericana
39Flemingiastrobilifera
49Semecarpus heterophylla
52Thespesia lampas
53Ceiba pentandra
54Ochroma lagopus
59Bixa orellana
60Homaliumtornentosurn
63Tristania conferta
66Lantanacamara
67 Tectonagrandis
68T.hamiltoniana
Kendajahan
Sentlng
Entjeng2
Katepeng
Opo 2kebo
Ingastjelik
Kemien
Kapok
Balsa
Kasoemba keling
Delingsem
Temblekan
Djati
-
67
450
130
120
56
69
240
68
190
500
2400
560
210
68
56
67
210
18
II
II
I/II
I/H
II
III
III
various growthrates
Segawe sabrang
-
1.0
2.2
1.2
2.0
0.9
1.0
0.8
1.3
0.6
1.2
3.6
1.7
1.0
0.7
0.8
1.1
1.5
0.3
0.5
1.0
0.7
0.9
1.2
0.9
0.4
1.7
1.5
1.6
0.9
0.5
1.1
0.7
0.8
0.6
0.8
0.6
1.2
0.3
0.5
12
48
16
130
120
410
32
490
580
56
200
15
470
18
110
47
100
37
640
45
12
1.7
1.0
1.2
1.0
1.6
1.7
1.3
1.5
1.6
1.3
1.6
1.4
1.3
1.4
1.4
1.5
-1
2
8
7
6
10
4
11
2
1
2
13
2
-
.3.6
5.0
5.6
6.9
7.2
.8.1
2.2
9.3
4.7
18
3.4
8.2
4.3
17
6.8
2.8
I
III
III
77
thehighest shootproductionwasfoundisSesbaniasesban.GroupIII
containsseveralshrubsandgreenmanures,somewithaconsiderableshoot
production,withalowroot:shootratio(1:2to1:30).
Themostdesirablerootdistributionforinclusioninmixedcropping,
typeI,seemstooccuronlyinplantswhichestablishthemselvesrather
slowly.Figure6.2 showstherelationshipbetweenlengthofthemainroot
(whichmayserveasanindicationofrootingdepth),numberofbranch
rootsofmorethan1minlength(whichisanindicationofhorizontal
rootextensioninthetopsoil)andshootdrymatterproductionforall
individualtreesdescribedbyCoster (1932)."Ideal"trees,opensymbols
intheupperrighthandcorner,apparentlydonotexist.
Laterrootdevelopment,asdescribedbyCoster (1933a),maybesomewhat
different:sometreesofgroupIdevelopanextensivehorizontalroot
systemaswell(Albizzialebbeckioides,Swieteniamacrophylla);some
treesofgroupIIcanlaterbeclassifiedasbelongingtogroupI,asthe
main,deeprootsbecomethedominantaspect (LeucaenaandtoalesserextentAcaciavillosa);sometreesofgroupIIdevelopstillmoresuperficialroots(Lagerstroemia, Vitex)andonetreeinitiallyingroupIII
formsatypeIrootsysteminlaterstages (Bixa).Earlyrootdevelopment
apparently cannotbeconsidered indicativeofthatinlaterstages,althoughsomecorrelationremains.AccordingtoCosterLeucaenaisthemost
suitabletreeformixedcropping,duetoitsdeeprootdevelopment,especiallyatlaterstages.HeconsideredGliricidiasepiun(notincludedin
thisstudy)tobeasuitabletreeaswell.Hisobservationswerein
agreementwithastudyofKeuchenius(1927)oftreesusedasshadetrees
inteaplantations.DataofKeucheniushavebeensummarised infigure
6.3.
Formosttreespecies50%oftherootmassisfoundinthetop50cm
and70%inthetop100cm.ForLeucaenathisis30and50%,respectively.
ApartfromapaperbyDijkman(1950)thesetreesseemnottohavebeen
studiedafterthesecondWorldWaruntillthe1970's,wheninterestin
agroforestrywasrevivedandnewLeucaenavarietieswithahighgrowth
rateandN-bindingcapacitywereselected (NationalResearchCouncil,
1984).
78
deep
1.Albizzia lebbeck
2. Schleichera oleosa
mixed 3. Leucaena leucocephala
l*. Albizzia falcata
shallow 5 Lantana camara
6. Tectona grandis
G)
so
.-VS' >:
®
«• too
em
0
50
MO
IM
200
Figure6.1.Exemplesoftreerootsystemsafter6monthsbyCoster
(1932):groupI.1.Albizzialebbeck,2.Schleichersoleosa,GroupII:3.
Leucanea"leucocephala,4.Albizziafalcata,GroupIII.5.Lantanacamara,
6.Tectonagrandis (teak).
79
shoot dry matter (g)
2000
1000
•
•
o•
••
100
oo •
o
o- :
° °
oo
10
o
°
^ ^oJ oo
1.0
:*
co
° o o°
°o °
oco
oo°
no of branch roots>1m
2.0
o0or1
•2ormore
I
i
3.0
40
mainrootlength(m)
Figure6.2.Relationbetweenrootingdepth,horizontalspreadofroots
and above-grounddrymatterproductionoftreesafter6monthsbyCoster
(1932).
Agboolaetal.(1982)havedescribedhowGliricidiasepium,whichhad
beenintroduced intoNigeriaasafence(hedge)species,wasdiscovered
byunknownNigerianfarmersasasuitablespeciestobeincludedinmixed
cropping,especiallyasacompaniontoyam,whichcanclimbitsstem.
Agboolaetal.commented thattheleguminous Gliricidiaimprovessoil
fertilityaswell.Cole(1983)investigated rootdevelopmentofagroforestrytreesattheU T AmainstationatIbadan.Hisdataaresummarisedinfigure6.4.
LeucaenaandGliricidiaaredeep-rooted,whileAcioaandespecially
Alchorneahaverathershallowrootsystemsandextendhorizontallywell
intotheareareservedforgrowingarablecrops.ThefactthatAlchornea,
whichisacommonpioneertreeinsecondarysuccessionofbushfallows,
hasaverysuperficialrootsystemseemstoconformtoageneralpattern
ascanbederivedfromKahn(1978)(figure6.5).
80
cumulative percentage root d r y weight
20
40
60
80
100
**"""-•*. O«*. xv^
100-
1501-
D
\
> s °^\
\
^Crotalariaanagyroides N
2oo|_ ADerrismicrophylla
vTephrosiaVogelii
•Erythrina lithosperma
2501-aCrotalariausaramoensis
•Indigofera endecaphylla
300[- vTephrosiaCandida
oAlbizziafalcata
•Leucaenaglauca
350h
«^A
400
450L
depth
(cm)
Figure6.3.Rootdistributionofleguminoustreesinteaplantationsin
Indonesia(datafromKeuchenius,1927).
Thiseffectisalsoreflectedintheoveralldistributionoftreeroots
undersecondaryforest (figure6.6),althoughtherootdistribution
apparentlydoesnotchangeafter5years.
Kushwahetal. (1973)andNelliatetal.(1974)discussedhowknowledge
oftheshapeofcoconutpalmrootsystemscanbeusedforconstructing
plantingschemesforagroforestrywithlittleinterferenceamongcrops.
Rootsystemsofoilpalmsaresimilartothoseofcoconutpalms(Tan,
1979).TinkerandZiboh(1959)describedsoilprofilesofoilpalmplantationsinSouthernNigeria,mostlyonacidsandysoils.Generallya
heavyrootmatisfoundinthetop5cmwithfewrootspenetratingbelow
40cm,althoughinsomeprofilespalmrootsmaybefoundatadepthof
150cm.
81
Leucaena
,0
stem (cm)
20 40 60 80 100
Gliricidia
fine root density
(mg/cm3)
0-30
stem (cm)
0 20 40 60 80 100
EZ3 3-10
I W 1 1- 3
^
s /\
Alchornea
„0 20 40 60 80 100
"m////MA
0
10
v/
V7\ 0.3 - 1
V
/
Y„
C 3 0.1 - 0.3
[~71 0.03- 0 1
Acioa
Wl
0.01. 003
0 20 40 60 80100
I
I <0.01
d r y weight of roots
per 20cmdepth
100
1000(g)
20
40
60
80
100L
depth
(cm)
w
o
o
o
o
â
total
E kg/plant
Leucaena o — • 1.37 + 0.26
Gliricidia A — • 0.35+0.48
c
A l c h o r n e a a — • 0.32 +0.22
Acioa
v T 0.24 + 0 2 8
Figure6.4.Rootdistributionofalleycroppingtrees(Leucaena
leucocephala,Gliricidiasepium,ÂlchorneacordifoliaandÀcioabarter!)
atIbadan(datafromCole,1983;calculationoftotalrootdryweight
assumingradialsymmetryaccordingtoVanNoordwijketal., 1985).
AlleycroppingatOnne
FollowingthesuccessfuldevelopmentofalleycroppingtechniquesonneutralalphisolsatIbadan(Kangetal., 1985),similarexperimentswere
startedatOnneaswell.Astreespeciesfourintroduced specieswere
tested (3Leguminosae:Leucaenaleucocephala,Gliricidiasepiumand
82
Flemlngiacongestaanawellknownforestplantationtree(relatedto
teak,Tectona)Gmelinaarborea(verbenaceae)andtwolocalspecies
(Alchorneacordifolla(Euphorbiaceae)andAnthonatamacrophulla
(Caesalpinioideae-Leguminosae),whicharedominantspeciesinthebush
fallowinSENigeria.Establishmentofthetreerowsprovedtobedifficult,especiallyforAnthonata,whichisremarkableasitformsamajor
componentofthesurroundingbush,regrowingfrommanyoldstumps
coppicedinthecroppingperiod.
Knowledgeofrootgrowthofthetree,especially rootdepthanddegree
ofinterferencewithcroproots,wasconsidered tobeusefulinevaluatingthesuitability ofthevarioustreespecies.
_/&?,.ÎM
o so 100cm Macaranga
hurifolia
Figure6.5.Rootsystemoftypicalpioneertreesinsecondaryforest
succession(Macaranga,Musanga)andtreesoflatersuccessionalstages
(Tarleta,Diospyros,Parinari,Memecylon),accordingtoKahn(1978).
83
0
O
r e l a t i v e dry weight of roots(cumulative%)
20
40
60
80
100
fl-150
Figure6.6.Root distributionofforest fallowvegetation under high
rainfall conditionsatKade (Ghana)andYangambi (Zaira.)(data from
GreenlandandKowal, 1960).
6.2. Methods
Ontwooccasions,inAprilandJune,vertical rootmaps weremadeinsoil
pits bordering thealley cropping experiment.Rootsweremapped overa
distanceof1m (i.e.tothemidpoint betweentworowsoftrees (2m
apart). Sampleswere collected toobservemycorrhizal infection.Asthe
remarkably slow establishment ofAnthonata from seedlings mightberelatedtopoor establishmentofaycorrhiza, toanumberofseedlings grown
in plastic bags some soilwasadded, collected nearAnthonata trees that
were growing well.
6.3.Results
Rootmapsofthealley cropping plots contained rootsoftrees, cassava
and weeds. Only forthethicker roots identificationwaspossible.Figure
6.7 shows thedistributionofthemain tree roots.Alchorneaand
Gliricidiahardly penetrated intothesubsoilandextended itsroots
horizontally into thetopsoil.AnthonataandFlemlngiagavea slightly
better penetration,butmainly extended horizontally.Leucaenaand
84
Gmelinabothcoveredthewholespaceoftherootmap;Gmelinarootscould
befoundat1mdepthat1.5mdistancefromthetrunk.Leucaenashowed
bothdeeppenetrationandhorizontalextension(PlateVIII).Figure6.8
showsthetotalcountsontherootmaps.TheLeucaena,Gmelinaand
Flemingiaplotshadthebestrootdevelopmentbothintopsoilandsubsoil.
April 1985
0
20
June1985
d i s t a n c e to tree(cm)
Q 20 40 60 £ 80 100120 0 20 40 60 80 100120
i
1 1 i L j
rsy-i
1 1 1
• l
Alchornea
40
60
0
1
•^—i
1
1
1
1 i—f
-i
1
1
r
20
Gliricidia
40600
-i
1
20
Anthonata
40
60L0
20
-1
1
••
1
1
r
T
I
i
I
I
•
Flemingia • * .
40
• •
*t
60
0 -^
1
20 *
:.
1
1
1
1
1 (H
1
i
i
n^n
1
Leucaena
40
60
0
-[
1
1
20
40Hv
60
.
• •
1—-si | i r y n
•
••••• •
• ••
Gmelina"* * * •
*
••
••• • •
1
1
1—I
70U
depth
(cm)
Figure 6 . 7 . D i s t r i b u t i o n of woody roots on root maps of s i x a l l e y cropping t r e e s .
85
PlateVIII'.Detailsofrootsinalleycropping:A.Gharcoaledoldtree
stumpunderploughed layerinricefield,B.Alchornearootsrestricted
tothetopsoil,C.Leucaena/cassavarootsintermingled inthetopsoil,D,
E.detailofLeucaenanoduleindirectcontactwithcassavatuber.
86
8
number p e r 2 5 c m 2
30
number per 25 c m 2
10 12 H 16 18 20
• — • Leucaena/cassava
o — o Gmelina /
x — x Flemingia/
o a Alchornea/
A—AGIiricidia/
• — • Anthonata/
30
ffif>
A0
w
• — • Leucaena
o — o Gmelina
x — x Flemingia
a aAlchornea
A—AGIiricidia
• — AAnthonata
50^
depth(cm)
60A
o
numberper25cm2
50r
depth(cm)
number per 2 5 c m 2
t o p s o i l 0 - 20 cm
20
15
a —-•..
\
'\ \f„\
0-20cmdepth
Q^
25
D—-D—a--Z
r£-^
•-^>«
10-
A-A ..
A' /
4>&AA
1
20-60cmdepth
*
subsoil 20-60cm
\,-&
o—o.b
!><K3E
A-^
10
^ ^ ^ ^ W A A t t A A ^
30
50
tree
100
cassava
Figure6.8.Distributionoffinerootsinalleycroppingplots:A.tree+
cassava+grassrootsinApril,B.tree+grassrootsinJune.
Mycorrhizalinfectionwasfoundtobelowinthetopsoil(2-5%)forall
trees,exceptAlchornea(28%)andGliricidia (13%).Thesmallexperiment
70
90c 90cm
87
onÂnthonataseedlingsgavenegativeresults:themycorrhizainfection
levelwasquitehighwithoutadding"Anthonata"soiltotheplasticbags.
Thetreatment testedevenseemedtohavehadanegativeeffecton
mycorrhizalinfection,withoutaneffectonseedlinggrowth.Theslow
establishmentofAnthonataprobably isnotduetotheabsenceofasuitablemycorrhizalsymbiont.
TABLE6.1.Mycorrhiza (% rootlengthinfected).
Soildepth:
April
0-15
15-30
June
0-15
_
Anthonata
seedlings-infection
seedlings+infection
Alchornea
Gliricidia
Leucaena
Flemingia
Gmelina
_ .
2.5
28.3
12.5
3.5
1.6
3.8
3.3
6.4.Discussion
Thesixtreespeciesobservedshowedmarkeddifferencesinrootdistributionwhichprobablyaffecttheirsuitabilityforalleycropping.Notree
speciesfitstheidealpatternoffastgrowth,deeprootpenetrationand
norootsinthetopsoilbetweenthecrops.AlchorneaandGliricidia
scarcelypenetrateintothesubsoil.Anthonatahasaslowestablishment
phasebutproducesareasonablydeeprootsystemlater.Flemingiaand
Leucaenaformrootsbothinthetopsoilandinthesubsoil.Thedeepest
rootdevelopmentaswellasthefastestgrowthabovegroundisfoundin
Gmelina,butthistreealsoformsmanyrootsinthetopsoilanditsshoot
growthisratherwildanddifficulttocontrol.
Althoughthealleycroppingtrialwassetuptogetexperiencewithlocalperformanceofthevarioustreespecies,nottomakedetailed,statistically reliablecomparisonsofyieldlevels,someindicationsmay
neverthelessbederivedfromthe1985harvestofcassava(1984planting)
(figure6.9).Cassavagrowthpersurfaceareagenerallydecxreasedwith
decreasingdistancebetweenthealleys,exceptfortheAnthonataplots
32.3
10.8
4.3
25.0
2.6
2.3
0.0
88
whichmayserveasacontrol,asthesetreesdidnotgrowwelluntilthe
endof1984.Gmelina,LeucaenaandAlchorneagavethelowestcassavatuberyields.Ascassavayieldgenerally isinsensitivetoNsupply(in
Onne),negativeeffectsoftreerowsshadingthecropprobablyarepredominantinthecaseofcassava.
rel.yield(%)
100r
^-\
x».
.A-V
*->C
80
60
no.of tubers/plant
tuber fresh weight
_i
i
<L
i
100
r
80
A
\
•Leucaena
oGmelina
xFlemingia
•Alchornea
AGIiricidia
AAnthonata
60
40
<
leaves fresh
weight
stems freslv
weight
<.
8
4m
8
6
4m
distance between
alleys
vrf'ft
Figure6.9.Relativeyieldofcassava (harvestApril 1985)fromalleycroppingtrial,at3distancesbetweenthealleys.
Negativeeffectsoftreerowsaremorepronounced onabove-groundorgans
(leavesandstems)thanontotaltuberfreshweight.TheFlemingiaplots
yieldedthehighestnumberoftubers/plant,butgaveanintermediateto-
89
taltuberyield.Apparently thegrowthrhythmofthecropsdiffersunder
theinfluenceofaccompanying trees(differenttimesofshadingorN
flushfrommineralisationoftreebranchesusedasmulch?).
Onemaywonderwhetherornotitispossibletofindsuitabletree
speciesinthelocalvegetationwhichcouldbeusedforalleycropping.
Suchtreesshouldnotbesoughtinthepioneerstagesoftheforest
(Alchorneatypeofrootsystem),norintheclimaxforest(slowestablishment).Intermediatesuccessionalstagesmaycontainthebestavailablecompromisebetween"ideal"rootdistributionand"ideal"fastestablishment.Anthonatamayserveastheexample.
Alleycroppingcanberegardedasalinearlyorganisedvariantofthe
localAnthonata-coppicefarmingsystem(withtheadvantageofpotential
mechanisation).Moreresearchintothefunctioningofthelatterunder
localfarmconditionsisimportanttolearntheessentialaspectstobe
includedinalleycropping.Observationsdescribedhere(chapter2and5)
ofthepresenceof"oldtreerootchannels"andtheirpossibleimportance
intheuseofsubsoilresourcesbythecropcanberelevantinthis
respect.Thelocalfarmingpracticeofplantingcertaincropsnearold
Anthonatastumpsshouldbeevaluated.
90
7.GENERALDISCUSSION
Theobservations onrootdistributioninthesoilatOnnegavesome
surprisingresults.Conventionalwisdomattheresearchstationstated
thatnocroprootspenetratebeyond 15cmdepth.Ourobservationsshowed
thatthisstatementisnottrue.Arootingdepthof50-100cmwasfound
foruplandrice,cassava,Centrosema,GmelinaandLeucaena.
Rootresearchintheinitialphaseofaresearchprogramcanbehelpful
inselectingsuitablecropsforintercropping.Ourobservationsindicate
thatAlchorneaandDesnodiuaarenotverysuitable,whilethemaize/cassavamixturecanbeexpectedtohaveadvantagesovermonocultures.For
alleycroppingtreesitistooearlytochoose suitablespecies,asnone
ofthetreespeciestestedfulfillsallrequirements (fastgrowth,deep
rootdevelopment,nointerferencewithcroproots).
RootresearchshouldbecombinedwithobservationsonNuptakeduring
thegrowingseasontounderstand thedynamicsofsoilnitrogenunderfarmingconditions.OnthebasisofN-uptakecurvessuitabletimingforadditionalmeasures(fertilisationorpossiblysomemanipulationofmineralisationrates)canbeselected.Suchmeasurescansubsequentlybetestedonwhetherornottheyincreasenutrientuseefficiency.Thepresent
researchproject,startingwiththestatisticalevaluationoftheinteractionbetweenvariousintercroppingsystemsandfertiliserrates,may
notbethemostefficientwaytoimprovelocalfarmingpractices,unless
itiscombinedwithstudiesofrelevantprocessesinsoilsandcropsunderlocalfarmingconditionsasoutlinedhere.
Thepresentobservationssuggestthataneedforadditionalresearch
existsondetailsoftheleachingprocessinthefield,combinedwith
rootresearch(comparabletotheuplandriceexperiment)andfurther
studiesoftheroleofoldAnthonatastumpsandoldtreerootchannelsin
facilitatingdeepcroprootdevelopment.Thetechniqueofhorizontalroot
mappinginstudiesofintercroppingmayyieldusefulinformation(providedthatsometechnicalproblemsofvariabledepthofobservationwillbe
solved).
91
8.ACKNOWLEDGEMENTS
ThanksareduetotheNetherlandsGovernmentandU T A forforgrantinga
9-monthstraineeshipatHarenandatIbadan/Onne,aspartoftheproject
"Nitrogenmanagementinthehumid tropics".Dr.B.T.Kang,Ir.Janvan
derHeideandIr.AdvanderKruijsgaveadviceandsupervisionand
helpedorganisingallfieldwork.Mr.M.Weberallowedustomakeroot
observationsinhisexperimentalplot.Mr.EmanuelStephansprovedtobe
avaluableassistantinthefieldwork,whileMr.GerardBrouwerandMr.
JohanFlorisgaveassistanceinHaren.Dr.Ir.AntondeJager,Ir.Peter
deWilligenandIng.MarjoleinHanegraafcritically readthemanuscript.
92
9.SUMMARY
Inexperimentalfieldsaimedatimprovingnitrogenmanagementinthehumidtropics,rootstudiesweremadeatUTA'shighrainfallsubstationat
Onne(S.E.Nigeria).Deeprootdevelopmentcanbeimportantforrecoveringleachednutrientsincludingnitrogen,because(andinasfaras)
supplyofnutrientsthoughmineralisationanddemandbythecroparenot
completely synchronised.Rootdevelopmentonacidsoilscanberestricted
bytoxiclevelsofAl.InOnneinseveralcropsdeeprootdevelopmentwas
observedinsubsoilchannels,formedbyremainsoftreeroots.Thesoil
inthesechannelswasloose,butmoreacidthanthesurroundingsubsoil.
Soilcompactionduetomechanisedfarmingprobablyisasimportantin
restrictingrootgrowthassoilacidityundertheseconditions.
Rootobservationsweremadeinanexperimentwithsixleguminouscover
crops:Centrosemanacrocarpum,Desaodiumovalifolium,Pueraria
phaseleoides,Psophocarpuspalustris,MucunautilisandVigna
unguiculata.Acomparisonwasmadebetweenlimed(1tonperha)andnonlimedplots.Rootsystemsweresampledwithpinboards.Thesixcropsdifferedingrowthpatternandabove-groundproductionaswellasinroot
development.Althoughthebasicshapeoftherootsystemwasthesamefor
allspecies,i.e.averticaltaprootandabundantbranchrootsinthe
topsoil,rootingdepthvariedfrom70cminCentros
enato40cmin
Desaodium.VignaandMucunagrewfastandagedquickly,coupledwitha
lowroot:shootratioandarelativelyhighNconcentration.Pueraria
establisheditselfrelativelyslowlybutshowedthebestnodulation.
PsophocarpusandDesaodiuaareunsuitableundertheseconditionsasthey
starttooslowlyandhavetobeweededfrequently.Liminghadnodramatic
effectonplantgrowth,butincreasednodulationinthefastgrowing
VignaandMucuna.Rootdistributionwasinfluencedbyliminginallspecies:maximumrootdepthdecreasedandrootweightdensityinthetopsoil
increased.Mycorrhizalinfectionlevelswerelow.
Therootsystemofuplandricewasstudiedinanexperimentalplot
whereleachingofwaterandnutrientsisstudiedunderfieldconditions,
withandwithoutNPKfertilisation.Fiveweeksafterplanting(twoweeks
afterthefirstfertiliserapplication)fertilizerhadamarkedeffect
ondrymatterproduction,butrootsontheunfertilisedplotswere
93
slightlybetterdeveloped.Later,rootsonthefertilisedplotsextended
toadepthof70cm,whileontheunfertilisedplottheyonlyreached40
cmdepth.Thisrootextensionoccurredafterthemainperiodofnitrogen
andphosphorusuptakehadpassed.Root:shootratiodecreased continuously
overtheseasontoavalueof1:7forunfertilisedandalmost1:10for
thefertilisedplots.
Root observationsweremadeinamaize/cassavaintercroppingexperimentwhichispartoftheon-goingNmanagementproject(experiment3),
inwhichtheNresponseindifferentplantspacingsiscomparedwithtwo
levelsofNfertilization(0and60kgNperha).Threemaizespacings
areused:plantsingroupsof4spaced 1mapart,plantsingroupsof2
spaced50cmapartorsingleplantsataspacingof25cm;cassavais
plantedatdistancesof1m.Rootinvestigationsweremadewithpinboards,verticalrootmapsandhorizontalrootmapsinthetopsoil.
Maizerootsweremainly restricted tothetop10cmofthesoil,while
cassavarootswentdeeperandextendedwellbelowthemaizeplants,althoughatamuchlowerrootlengthdensity.Themaizespacingof25cm
gaveaslightlyfasterutilisationofthetotaltopsoilvolume.Fertilisationincreasedrootdevelopment ofmaize,buthadnoeffectoncassava.Nitrogenuptakebythemaizeoccuredintwopeaks,immediately
followingthetwofertiliserapplications.
Someobservationsweremadeonthe rootdevelopmentofsixtreespecieswhicharebeingtestedinanalleycroppingexperiment (Anthonata
uacrophylla,Fleaingiacongesta,Gliricidiasepiua,Leucaena
leucocephala,Gmelinaarborea,Alchorneacordifolia).Verticalrootmaps
weremade,andthedistributionofwoodyrootswasrecordedseparately.
Gmelina,LeucaenaandFleaingiaallshowedagoodpenetrationofthesubsoilwiththeirroots.AlchorneaandGliricidiarootsweremainlyrestrictedtothetopsoil.Anthonata,whichformsamajorcomponentofthe
localbushfallowagriculture,hasaveryslowestablishmentphase,but
givesreasonablepenetrationofthesubsoil.
94
10.SAMENVATTING
Onderzoeknaardebewortelingvandiversegewassenwerduitgevoerdop
proefveldenwaargetrachtwordtdestikstofhuishouding indenattetropen
teverbeterenophetsub-stationvanhetU T A inOnne(Z.O.Nigeria).
Diepebewortelingkanvanbelangzijnvoorhetopnemenvannutriënten
(incl.stikstof)dieuitdebovenlaaguitspoelen,aangezien(envoorzover)aanboddoormineralisatieenopnamebehoeftevanhetgewasniet
volledigsynchroonverlopen.Bewortelingopzuregrondenkanbeperktzijn
doortoxischeAl-concentraties.InOnneblekendiversegewassendiepe
wortelstevormeningangenstelselsindeondergrond,gevormddoorvroegereboomwortels.Grondindezegangenstelselswaslos,maarhadeenlagerepHdandeomringendeondergrond.Derhalvelijktonderdezeomstandighedenbodemverdichtingdoorgemechaniseerdelandbouweenzekerzo
belangrijkefactordietotbeperkingvandebewortelingkanleidenalsde
zuurgraadvandebodem.
Bewortelingsonderzoekwerduitgevoerdineenproefmetzesvlinderbloemigegroenbemesters:Centroseaamacrocarpum,Desmodiumovalifolium,
Puerariaphaseoloides,Psophocarpuspalustris,MucunautilisenVigna
unguiculata.Bekalkte(1tonperha)enniet-bekalkteveldjeswerdenvergeleken.Wortelstelselswerdenmetnaaldenplankenbemonsterd.Dezes
groenbemestersverschildeningroei-ritmeenboven-zowelalsondergrondseproduktie.Bijallesoortenhadhetwortelstelseldezelfdegrondvorm,
eenverticalepenwortelenrijkezijwortelvormingindebovengrond,maar
debewortelingsdieptevarieerdevan70cmbijCentrosematot40cmbij
Desmodium.VignaenMucunavertoondeneensnellegroei,maarookeen
snelleveroudering,bijeenlagewortel/spruit-verhouding enrelatief
hoogN-gehalte.Puerariavertoondeeenlangzamevestiging,maarvormdede
meestewortelknolletjes.PsophocarpusenDesmodiumblekenongeschiktvoor
dezeomstandigheden,aangezienzetetraagaandegroeikwamenenvaak
wiedenderhalvenoodzakelijkwas.Bekalkinghadgeeningrijpendegevolgen
voordeplantengroei,maarstimuleerdedewortelknolletjesvormingbijde
snelgroeiendeVignaenMucuna.Dewortelverdelingwerdbijallegewassen
beïnvloeddoorbekalking:debewortelingsdieptenamafendebewortelingsintensiteitvandebovengrondnamtoe.Dewortelsvertoonden
weinigmycorrhiza.
95
Debewortelingvan"drogerijst"werdbestudeerdineenproefveldwaar
uitspoelingvanwaterennutriëntenwerdgevolgdonderveldomstandigheden,bijalofnietbemestenmetNPK.Vijfwekennazaaien(tweewekenna
deeerstekunsmestgift)haddebemestingeenduidelijkeffectopdebovengrondsegroei,maaropdeonbemesteveldjeswasdewortelontwikkeling
ietsbeter.Laterblekenopdebemesteveldjesdewortelstoteendiepte
van70cmdoortedringen,terwijlopdeonbemesteveldjesslechts40cm
werdbereikt.Dezeuitbreidingvandebewortelingvondechterplaatsna
deperiodevandevoornaamsteN-(enP-)opname.Dewortel/spruit-verhoudingdaaldegedurendehetgehelegroeiseizoentoteenwaardevan1:7op
deonbemesteenbijna1:10opdebemesteveldjes.
Debewortelingondereenmengteeltvanmaisencassavewerdonderzocht
ineenexperimentvanhetlopendeN-huishoudingsproject (experiment3 ) ,
waarindestikstofreactievandiverseplantverbandenwordtonderzocht
(bij0en60kgN/ha).Drieplantschema'svoormaiswordenvergeleken:
planteningroepjesvan4,met 1m tussenruimte,ingroepjesvan2met50
cmtussenruimteenafzonderlijkeplantenmet25cmtussenruimte;decassavehadsteeds1m tussenruimte.Bewortelingsonderzoekwerduitgevoerd
metnaaldenplanken,verticaleenhorizontale"wortel-kaarten".Debewortelingvandemais wasinhoofdzaakbeperkttotdebovenste10cmvande
grond,terwijldecassavedieperworteldeendegeheleruimteonderde
maisopvulde,zijhetmetveelgeringerebewortelingsintensiteitdande
maisvertoonde.Waardemaisomde25cmgeplantwerd,wasdegehelebovengrondietssnellerbewortelddanbijdegegroepeerdemaishetgeval
was.Bijbemestingmet60kgNperhanamdemaisbeworteling toe,maarde
cassavebewortelingbleefonveranderd.N-opnamedoordemaisvondplaats
intweepieken,directvolgendopbeidebemestingen.
Erwerdenookenkelewaarnemingenverrichtaandebewortelingvanzes
boomsoortendiegebruiktwordenvoor"haag-teelt"(alleycropping)
(Ânthonatamacrophylla,Fleaingiacongesta,Gliricidiasepium,Leucaena
leucocephala,GaelinaarboreaenAlchorneacordifolia).Erwerdenverticalewortelkaartengemaaktendeverdelingvanhoutigewortelswerdapart
geregistreerd.Gnelina,LeucaenaenFleaingiablekenalledriegoedinde
ondergronddoortedringenmethunwortels,maarookindebovengrond,
tussendegewassen,wortelstevormen.AlchorneaenGliricidiablekentot
debovengrondbeperktmethunwortels.Anthonata,eenbelangrijkecomponentvanhetplaatselijke"stobben-braak"landbouwsysteem,blijktzich
slechtszeerlangzaamtekunnenvestigen,maarwelgoedmetwortelsinde
96
11.RINGKASAN
Penelitianinidiselenggarakansehubungandenganadanyaperbaikan
managemenNitrogendidaerahhumid tropisIITA-Onnesubstaslunyang
bercurahhujantinggi(Nigeriatenggara).Perkembanganakaryangdalam
sangatpentingartinyauntukmengatasikehilangannutrisimelalui
pencuciansepertiNitrogen,dikarenakantidakadanyasinkronisasiantara
penambahannutrisimelaluimineralisasidankebutuhantanamanakan
nutrisi.Perkembanganakarpadatanahasamsangatdibatasiolehadanya
kandunganAlyangterlalutinggi.DidaerahOnneperkembanganakaryang
dalamdaribeberapatanamanbanyakdijumpaididalamsaluranakarpada
lapisanbawahyangterbentukdarisisa-sisaakarpohontua.Struktur
tanahdidalamsaluraninigembur,akantetapibereaksilebihasam
daripadatanahsekelilingnya.Padasituasiini,pemadatantanahyang
disebabkanolehmekanisasipertanianjugamerupakanfaktorpentingdi
dalammembatasipertumbuhanakarsepertihalnyakemasamantanah.
Suatupenelitianakardilakukandenganmengunakan6macamtanaman
kacang-kacanganpenutuptanahyaitu,Centroseaamacrocarpum,Desmodium
ovalifoli.ua,Puerariaphaseoloides,Psophocarpuspalustris,Mucuna
utilisdanVignaunguiculata.Dalampercobaaninimembandingkan2
perlakuanyaitu,pengapuran(1ton/ha)dantanpapengapuran.Pengambilan
contohperakarandilakukandenganmenggunakanmetodepinboard.Keenam
tanamanmenunjukanperbedaandalampolapertumbuhandanproduksibagian
tanamandiatastanah,maupundalamperkembanganakarnya.Walaupun
demikianpoladasardarisistimperakarannyaadaiahsamauntuksemua
speciesseperti"verticaltaproot"nyadanpercabanganakaryangbanyak
terdapatpadalapisanatas,sedangkankedalamanperakaranyangberbeda
adaiahpadaCentroseaasedalam70cmdanDesmodiumsedalam40cm.Vigna
danMucunamenunjukhanpertumbuhanyangcepatdanpenurunanpertumbuhan
yangcepatpula,keduanyamempunyainisbahakar:bagiantanamandiatas
tanah(root:shootratio)yangrendahdanmempunyaikonsentrasiNitrogen
yangrelatiftinggi.Puerariamempunyaitingkatpertumbuhanyangrelatif
lambat,tetapinampaknyamempunyaibintilakaryangterbaik.Psophocarpus
danDesaodiuatidakcocoktumbuhpadadaerahini,karenakeduatanaman
initumbuhsangatlambatsehinggaseringkaliharusdilakukanpenyiangan.
Pengapurantidakmempunyaipengaruhyangkuatterhadappertumbuhan
97
tanaman,akantetapimeningkatkanpembentukanbintilakarpadaVignadan
Mucunayangmemilikitingkatpertumbuhancepat.Distribusiakardari
semuaspeciesdipengaruhiolehpengapuranyaitu,maksimumkedalamanakar
menurundanberatkeringakardilapisanatasmeningkat.Adapunlevel
infeksidarimycorrhizaeadalahrendah.
Penelitiansistimperakaranpadi(uplandrice)dilakukanpadaplot
percobaanpencucianairdannutrisipadakondisilapang,denganperlakuan
pemupukandantanpapemupukanNPK.Padasaattanamanberumur5minggu(2
minggusetelahpemupukanpertama)pemupukanmempunyaipengaruhyangjelas
terhadapproduksibahankering,tetapiakartanamanpadaplottanpa
pemupukanmenuniukkanperkembanganyanglebihbaik.Namunpadaakhirnya
akartanamanpadaplotpemupukanberkembangsedalam70cm,sedangpada
plottanpapemupukanhanyamencapaikedalaman40cm.Pengembanganakar
initerjadisetelahperiodeutamadaripenyerapannitrogen.Nisbahakar:
bagiantanamandiatastanahmenurunsecarateraturhinggamencapainilai
1:7untukperlakuantanpapemupukandanmendekati1:10untukperlakuan
pemupukan.
Penelitiansistimperakaranlainnyadilakukanpadaplotpercobaan
jagung/ketelapohonintercropping,yangmanamerupakanbagiandari
projekpenelitianmanagemennitrogen(percobaan3)yaitusuatupercobaan
mengenairesponNpadabeberapajaraktanamdenganperlakuan2level
pemupukan(0dan60kgN/ha).Tigamacamjaraktanamjagungyang
digunakanadalah,4tanamandalam1grupdenganjaraktanam100cm,2
tanamandalam1grupdenganjaraktanam50cm,dan1tanamandalam1grup
denganjaraktanam25cmantarasatudanlainnya;ketelapohonditanam
denganjaraktanam100cm.Metodapengamatanakaryangdigunakanadalah
metodapinboard,metodapemetaanakarvertikal,danpemetaanakar
horisontalpadalapisantanahatas.Akar-akarutamajagungterbatas10cm
dilapisanatas,sedangakarketelapohontumbuhlebihdalamdan
berkembangdenganbaikdibawahakartanamanjagung,walaupunpanjang
akarnyajauhlebihrendah.Akarjagungdenganjaraktanam25cmnampaknya
dengancepatmendudukiseluruhlapisanatas.Pemupukanmeningkatkan
perkembanganakarjagung,akantetapitidakmempunyaipengaruhyang
berartiterhadapketelapohon.UptakeNolehtanamanjagungterjadidalam
2titikpuncakyangsecaralangsungmengikutipolapenambahanpupuk.
Beberapapengamatanakardilakukanpada6speciespohonyangsedang
ditelitipadaplotpercobaanalleycropping(Anthonatamacrophylla,
Fleningiacongesta,Gliricidlasepiua,Leucaenaleucocephala,Gaelina
98
arborea,Alchorneacojrdifolia).Pemetaanakarvertikaldandistribusi
akaryangtelahberkayudilaposkansecaraterpisah.Gmelina,Leucaenadan
Flemingiasemuanyamenunjukkanpenyebaranyangbaikpadalapisanbawah.
AlchorneadanGliricidiaterbataspadalapisanatas.Anthonatayang
merupakankomponenutamadaritanamansemaksetempat,menunjukkan
kelambatandalamphasetertumbuhan,akantetapimenunjukkanpenetrasi
akaryangcukupbaikpadalapisanbawah.
99
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110
1 1 . APPENDICES
112
APPENDIXI.Rootlengthdistributionofmaizeandcassava(legendssee
AppendixIII).
Maize
0211
0111
0
o ! !
r
5
10
1
0
i
i
•
5
10
i
i
2weeks
—
0
10
20
•h
30
J
5weeks
0
u
10
20
30
r
40
L
Tî
8weeks
0
10
root lengthlcm/cm 3 l
5
10
1
1
i
10
20
20
0311
I"
30«•
soil depth
(cm)
•>n~
K weeks
113
APPENDIX I , c o n t i n u e d .
Maize
5111
J
o
10
5
5311
5211
10
)
5
10
1
i
•
0
20
I
l
2weeks
20
10
root length (cm/cm 3 )
5
10
c
1
~ T
r
5weeks
I
30
o
'
t?i!
10
w
20
8weeks
30
;
•!
*
i\:'
40
10 20
Ji
30-
Uweeks
I
il
40 50so if depth
(cm)
fH!i
114
APPENDIXI,continued
Cassava
0211
0111
.1
)
o
1
*
•
10
.2
1
1
1
!
i
1
.3
1
0321
.4
1
.2
1
20L
2weeks
1 1
i?
II
i
i
— i
!
m
5weeks
0
. !'
10
•t, I ',
20
wr
J
8weeks
30
40
o
rr
D
10
20
i_L
TTT
i'l.
Hweeks
30
L
40
s o i l depth
(cm)
.4
1
r o o t length (cm/cm 3 )
.5
.6
.7
1
1
1
115
APPENDIX I , continued
Cassava
5211
5111
.2
20
30
r
'
ii
y1
*
ITT
•*
10|—H-JI
20
3°W
40
50
soiIdepth (cm)
-it
TT
!
-|
1
M
;
40>-
1
4 '
: f;
II
ia
5weeks
M i l l
• f t
i l
8weeks
!» J 'I
I
1
T
1
1
i
U'
' i l l
>
i ;
T •
i li •
!;
sH
TT
'
2<
I !: !
10
30
'
Ï T\
t
If
2weeks
IS!
D
0
20
.1
i!
D
20
10
]
+• :
10
0
5311
.3
T
*•
*
14weeks
i
A'UL• U
.2
1
.3
1
root length 1cm/cm')
.4
.5
.6
1
1
1
116
APPENDIX I I . Root weights i n m a i z e / c a s s a v a i n t e r c r o p p i n g .
Maize
o
_J
1
-|
1
2
1
0
1
1
~i
2
1
1
average root w e i g h t
p e r u n i t volume o f soil (mg/cm 3 )
0
1
2
1
1
1
1
ÏÏ
10
2weeks
•20L
T
0
'
1
T
1
TT
1
1
1
1
1
10
5 weeks
20
30
0
i
r
-i
r
I
r
i
20
I
1
10
I1
8 weeks
30P
0 fertilizer
5
40
"1
i
1
r
—?
10 |--r
20f,
1/»weeks
30 i
.2
40L
soil depth
(cm)
.3
Cassava
i
I
i
2 weeks
:
i
average root length per unit
volume ofsoil (cm/cm')
0
.1
.2
.3
T
-1
1
J
i
i
i
5weeks
!
•
>
•
J
8weeks
: i
40
soildepth
(cm)
If
'
Kweeks
r
"i
i
iH
0 fertilizer
5 fertilizer
.2
,3
i—"
-]
1
118
APPENDIXIII.Rootweightdistributionofmaizeandcassava.
Maize
0211
0111
oft-
5
T"
0321
10
— I
10 l
r
2weeks
20•
0
10
5 weeks
20
30
t
20-
¥.
8 weeks
30 -
1
10
T
20*
soiidepth
(cm)
K weeks
average
5-5
5.15
........15.25
25-35
-"•'•• 35-45
- x - , 45-55
—— 55.65
D
root density (mg/cm 3 )
5
10
1
1
119
APPENDIX I I I , c o n t i n u e d .
Maize
5211
5111
5311
root density(mg'cm3)
5
0
"T
r
10 '••
•10
—I
r
10
—i
10
—»
2 weeks
20 •
30-
OHIO J-
5weeks
20 30 -
o
'iI
10
i'
8 weeks
20
.
30
10
u
B
-i—
I!
20 »
30 soil depth
(cm)
1 U weeks
3L
1
!•
i
120
APPENDIXIII,continued
Cassava
0211
0111
0321
.2
.3
root density(mg/cm3)
X
.5
.6
2weeks
20l
10 rM
5weeks
20 h--
I
30
40
o
10
8weeks
20
30
"Ci
n
"B
40Ui
soil depth
(cm)
tfr
f
"1
j
U weeks
r»
1
1—!
121
APPENDIXIII,continued
CQSSQVQ
5111
5211
5311
.3
ID
2 weeks
0
10
71
20
I $If 5 weeks
30
40
50L
0
10
20
30
.1
D
8 weeks
E
40"-
o
10
*i
20
30
40
... •< A
i
L
50
soil depth
(cm)
MLl.
0
U weeks
ILL
Iffi
root density (mg/cm 3 )
.4
.5
-6
T