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WAITE INSTITUTEJt . 2-ø3
LIBRARY
PI¿OPERTIES OT' AGGREGATED SEEDBEDS
A thesis submitted by
MICHAEL VERNO BR¡.TJNACK
B.Ag.Sc. (Hons. ) Adetaide
to
The Universit-y of Adelaide
for the degree of
Doctor of PhilosoPhY
Department of SoiI Science,
lVaite Agricultural Research institute,
The University of Adelaide.
September 1978
.I
TABLE OI¡ CONTENTS
LIST OF TABI,ES
LIST OF FIGURES
LIST OF PLATES
SUMMARY
DECLARATTON
ACKNOVÙTEDGEMENTS
SECTION 1 - I}TTRODUCTION
2.3
Introduc{-ionProperties of different sizecl aggregates2.2.L Chernical ProPerties2.2.2 Physj-cal. and mechanical propertiesCompactabilitY2.3.L Natural aggregates2.3 .2 Arti.ficial aggregatesErodíbilitY and surface crusting2.4.L Effect of water2.4.2 Effect of windAeration2.5.L Inter-aggregate aeration2.5.2 Intra-aggregate aeratj-on!{ater content and temPerattrre2.6
2.7
?later s torageWater movementRate of drYingEvaporationTemperaLure
ent uptakeI'li troget:PhosphorusPoÈassium
2.8 Crop emerçJence and growth2.S.IAffectofaggregatesizeongrowthofmonocotyledons2.8.2 Affect of aggregate size on growth of dicotyledons
2.9 Aggrega,te sizes produced by til'lage2.I0 Summary
SECTION 3 - PHYSICAI AND MECHANICAL PROPERTTES OF AGGREGATBS
AND AGGREGATE BEDS
IntroductionAggregaÈes and thej-r strength
Materials and MethodsResults and Discuss:LonFurther DeveloPments3.2.3.I Materials and Methods3.2.3.2 Results and Discussion
3.2.3-2-i- Devel.opment of a crack theoryapplic.rble to soil
Eeg-e-
xlrl-
I
v
X
xv
xx
xx1
SECTION 2 - REVIEW OF LITERATURE ON TI]E EFFECTS OF AGGREGATE STZE 5
2.L2.2
577
B
910t1I313151515L71818191920202I2L22232324IE¿J
2630
32
2.4
2.5
2.6.L2.6 .22.6.32.6 .42.6.5Nutri2.7.I2-7 -22.7.3
3.13.2
32323233353536
3.2.L3.2.23.2 .3
3.3 Compact-.ion of aggregate berls
3943
].I
Page
3.3.I tr4aterials and Methods3.3.2 Results and Di-scussiou
3.4 Conclusions
5.3.2.3
5 .3.2.4
tr?J. J.
5.3.5.4 Conclusions
434449
4.r4.24.3
SECTTON 4 - EVAPORATION FROM BEDS OF AGGREGATES 50
IntroductionMaterials and Meth<¡dsResults and Discussi-on
Bffect of aggregate sizeEffect of compactionBffect of a soil crustEffect of meteorological factors
4.4 Conclusions
SECTION 5 - SOTL CRUST STRENGTH AND EMERGENCE FORCB OF hTIBAT 66
66107L1t
5.3757578
5054575759606t
4.3.L4.3.24.3.34.3.4
65
5.r5.2
Int-roduct-ionEmergence force of wheaÈ shoots5.2,.I I'¡laterials and Methods5.2.2 Results and DiscussionMeasurement of crust strength, crusting arrct aj.r resístanceof crust5.3.I Materials and Methods5.3.2 ResuÌts and Discussion
5.3'2.I Crust strength as determined by modulusof rupture
Effect of aggregate sizeEffect of comPactionEffect of water Potenti-a1 andtime of sowing
5.3.2.2 Field crust strength at the end of theseason determined with a hand penetrometer (P¡1)
I Effect of aggregate size2 EffecL of comPaction3 Effect of time of sorving
5.3.2.r.r5 .3 .2.L.25.3.2.r.3
5.3.2.2
5.3.2.4.L5.3.2.4.25.3.2.4.3
787882
5.3.2.25.3.2.2
84
858l9Ì92
939394
9596
r00
r00101104r06
108
r08108r08r09l.t0IlI
2.52.6
Field crust strength at the end of emergenceperiod (L97'1) deternríned by hand penetrometer5.3.2.3.I EffecÈ of aggregate size5.3 .2.3 .2 Ef f ect of comPactionAir resistance of soil crust at the end ofthe season
Effect of aggregate sizeEffect of comPactionEffect of time of sowing (f976)and P\IA. treatment (L971)
Rainfall simufationQuanÈi.met resul ts
SECTION 6
6 .1 Introduct-ì-on6.I.I Factors affecting germination and emergence
TernperatureWaterAeration
-SoILTEMPERATUREANDWATERPoTENTIALINAGGREGATESEEDBEDS
6.1.1.16.r.r.26.r.r.3
6.L.2 Effect of soil- structure on soil temperature
6.26.3
6.I.3 Effect of soil- structure on soil water6.L.4 Bffect of meteorological factors un soj-1. i.t:trpe::alure
and water cotrtentMateri-als and MethodsResults and Discussion6 . 3 .1 SoiI wa'ter potential
Effect of aggregate sj-zeEffect of compact.ionBffect of time of sowi-rlg (1976)treatment (1977)
6.3.L.4 Bffect of meteorological factcrs6.3.2 Soil temperature
Effect of aggregate sizeBffect of compactionEffect of t-ime of sowing (1976)treal-:nent (I971)
6 .3.2.4 Ef f ect of me t-eorological fac Lors6.3.3 Soi.l. water potential rarìge and soí1 iemperature
(re77)1 Effect of aggregate size2 Effect of compaction3 Effect of PVA (tc prevent crustj-ngr) treacnient4 Effect of meteorological factors
6.4 Conclusions
SECTION 7 - WITEAT GROI{TII IN BBDS OF AGGREGÄTFS
IntroductionMaterials and Methods7.2.L Gl-asshouse and phytotron preliminary experiments7 .2.2 experiment
.1 Location and layout
.2 Handling of soil
.3 Sieving
.4 Sowing technique
.5 Ðata recorcling7.3 Results and Discussion
7.3.1 Glasshouse and phytotron preli-minary experirrtents7.3.I.1 Effect of aggregate size on MDB and
percentage emergence7.3.L.2 Effect of water content on MDE and
percentage emergence7.3.1.3 Effect of time of sowing on MDE and
percentage emergence7 .3.L.4 Conclusions
7 .3.2 Field trial results7.3.2.L Mean day of emergence in the field
I Effect of aqgi:egate size2 F-f-fect of con.rpaction3 Effect of tj-me of sovr-ing (1976)
anct PVA treatment (L917)
and PVA
arrd PVA
iii.
Pu-gq
7l-2
113ll4t.16116l.L7L22
L23L23130130139
6 .3.I .16 -3.t.26.3.r.3
.3.2.L
.3.2.2-3.2.3
666
.1.40
I4L
range6.3.36.3.36.3.36.3.3
146TT,9
1501511.52.r54
158
158r59159l.6o1(:01.61
161162L64l_6 5
165
L61
L69
I70171r7l"tlLL72173
7.7 5
l-76l.''t7178l''l()'r -70
7.L7.2
Field7 .2.27 .2.2
7 .2.2
I .2.27 .2.2
7 .3.2.r7 .3.2.I7 .3.2.L
7 .3 -2.2
7 .3.2.3
Percentage emergence in the field7.3.2.2.L Effect of a.ggregat-e size7 .3 .2 .2 .2 Ef f ect =f. PVA t¡:eatnrent- ( 1.97f )
Plant grorvth and Yi-eld7 .3.2.3.L PIant heiqht
7 .3 .2. 3 . I .l- Ef f eet of dq-qr- egates i.ze 1.7!ì
7 .3 -2.3.27 .3.2.3 -3
7.3.2.3.1.2 Effect of comPaction7 .3.2.3.1 . 3 Ef fect of time of
sowing (1976) and'PVA treatment (1977)
Tillers Per PlantDry matter Yield and grain Yield7.3.2.3.3.1 Effect of aggregate
síze7.3.2.3.3.2 Effect of Èime of
sowing (1976)
iv.
Page
180
180L82r84
186
r90193
195
195t97200207
2LO
230
8.1a-28.3a.4
7.4 Conclusions
SECTTON 8 - GENERAL DISCUSSION AND CONCLUSIONS
IntroductionConclusionsThe 'idea1' seedbedSuggestions for further work
BTBLTOGRAPHY
APPENDIX
v
TabIeNumtrrer
3.r
TIST OF TABLES
Title
Water content and meau t-ensile yield st'rengthof 9.5-0.5 mm aggregates as a function oftheir initial water Potential.
Tensile yield strength of various aggregatesizes from different soils.
Values of m (equation 3.2) anci crackdistribution in various soils-
Measured values of aggregate dj-ameter and tensileyield strength for the Northfield soil '
Parameters of the equation Loglgl = e-eLogIOãand t-he ,Jerivt:<l value for k- The regressioucoefficient is given bY r.
Va1ues of parameters of Equati.on (3'18)
Void proportions in a bed of aggregates after variousl-evels of comPaction.
A comparison of measured macro-porosity, nL, and
that predícted, rìgr on the assumption of no intra-aggregate .o*pt""ãion during aggregate bed compaction'
Evaporation ratio (Ev) - summary analysis of variancefor experiments I, 2 and 3-
Parameters for eguation (4.If) for each aggregatesj.ze, and surface treatment for experiments I, 2 and 3'
Proceclure for calculation of least squares regressioneguation which characterizes the emergence forceexerted by wheat.
Effect of temperature on median emergence forceand pressure of wheat.
Soil crust strength as determined by MOR : Summary
of analysis of variance.
I"Iean crust strength (kPa) as measured by MOR of crustformecl cn each aggregate size (mm) range'
Effect of aggregate size, compaction and waLerpotential (kPa) on crust strength (kPa) as measured
by MoR.
ru"
34
36
37
4L
43
45
46
47
5B
64
73
74
79
79
BI
3.2
3.3
3.4
3.5
3.6
3.7
3.8
4.t
4.2
5.r
5.2
5.3
5.4
5.5
vl.
P4ge-TableNumber
5.6
5.7
5.8
TitIe
Effeit of aggregate size, compact-iou and tíme ofsowing on crust strength (kPa) as measured by MoR'
Effect of aggregate size, water potential crnd timeof sowing on crust strength (l<L'a) as measured by MOR'
Effect of compaction on crust stretlgth (lcPa) asmeasured by MOR.
Effect of cornpaction and water potential on cruststrength (kPa) as measured bY MoR-
Effect of compactiOn and. sowing date on crusE strength(kPa) as measured bY MoR.
Effect of compaction, v¡ater potential and time ofsowing on crust strength (kPa) as neasured by MoR'
Effect of water potential (kPa) on crust strength(kPa) as measured bY MOR.
Effect of sowing date on crust strength (kPa) as
measured by MOR.
Effect of water potential (kPa) and sowing date on
crust sÈrength (kPa) as measured by l4OR.
Fie1d crust strength at the end of the season -Summary analysis of variance for field trials '
Effect of aggregate size and time of scrwing on cruststrength (kPa) determined with hand penetrometer'
Effect of aggregate size, tj-rne cr sowing ancl compactiorron crust strength (kPa).
Effect of aggregate size, compaction and PVA treatmenton crust strength (kPa) deternrj-ned with handpenetrometer.
Effect of a compaction treatment on crust strength1t<ea) determined rvith hanrl penetrometer'
Effect of compaction and PVA treaLment on cruststrength 1t<ea) determined with hand pene'trometer '
Effect of time of sowi-ng on crust strength (kPa)
determined with hand penetrometer.
Sunmary of analysis of variance of crust strengthat the end of emergence.
B1
82
82
B3
83
a4
84
85
85
B6
89
90
91
92
CI'
92
93
94
5.9
5.r0
5 .II
5.L2
5.r3
5.L4
5 .15
5 .16
5.L7
5.18
5 .I9
5.20
5.2L
5.22
5.23 Effect of compaction on crust st-rength lfra) '
vr-I .
Tabl-eNumber
5.24
5.25
5.26
5.27
5.28
5.29
5 .30
5 .31
5.32
5 .33
5.34
6.I
6.2
6.3
6.4
TitIe
Air resistance of soil crust at the end' of theseason - Summary analysis of variance for fiel-dtrial s .
Effect of aggreqate size, time of sowing and
compaction on crllst air resist-ance (s) '
Effect of aggregate sj-ze, compaction and PVA
t.reatment on crust air resistance (s) '
,Bffect of compaction l-reatment on soil crust airresistance (s) .
Effect of time of sowing on crust air resistance (s) "
Effect of pvA treatrrren! on crust air resistance (s) .
Effect oÍ aggregate size and compaction on time torun-off, volume of run-off and amor¡nt of sedimentin 2OO mI run-off - Sunrmary analysis of variance'
Effect of aggregate size on time to run-off (min) and
volume of run-off (liÈres) from compacted plots'
Effect of aggregate size on time to run-off, volume ofrun-off and sediment in 2OO mI aliguot of run-off -Summary analYsis of vari.ance "
Effect of aggregate size on time to run-off and
volume of run-off from uncompacted plots '
Quantimet 720 results for porosity and horizontal and
ver*.ical projections of soil crusts fornted on
diffe::ent sized aggregates and treatments '
SoiI water potential at the 5 cm depth - s"unmary
o'f L976 data.
SoíI water potential at the 5 cm depth - summary
of L977 data.
Page
96
99.
99
100
r0t
101
LO2
LO2
r03
r03
r05
118
II9
L27
128
I31
134
Parameters for equation (6-7) for aglrregate sizesfor each recorded Period in 1976 '
Parameters for equation (6.'r) for aggregate sizesand surface treatments for each recorded. periocl inL977 .
6.5 Soil temperature at 5 and I0 cm rlepths - summary of1976 data.
Soil temperature at 5 and IO crn depths - sununary of6.61977 data.
TableNumber
6.7
6.8
6.9
6.10
7.L
7.2
7.3
7.4
7-5
7.6
7.7
7.8
7.9
7.r0
7.1r
7.r2
7.r3
7.r4
7 .L5
Title
Values of A for equations (6.24\ to (6.27) .
Mean values of soil temperature range and soil waterpotential range for all treatrnents in the recordedperiods (L977J .
Values of thermal diffusivity for each aggregatesize and treatment in the recorded periods (L977) '
.Parameters for equation (6.33).
Mean day of emergence - sl:rnmary of analysis ofvariance fo:: glasshouse and phytotron experiments '
Effect of aggregate size, water content and time ofsowing on percentage emergence in the glasshouse'
The effect of aggregate size on percentage emergence -Phytotron results.
(Le77) .
(re76).
vrl_r.
Page
r45
L47
14B
153
r65
r66
r68
L74
L75
L76
176
L77
The effect of aggregate size and water conterrt (%) 168
on percentage emergence - Phytotron results '
Effect of aggregate size, water content (%) and 169
time of sowing on percentage of emergence -Glasshouse results.
Effect of sowing date on MDE in the glasshouse' 170
Effect of sowing date and water content on MDE in I70the glasshouse.
Effect of time of sowing on percentage emergence in 171
the glasshouse.
Mean day of emergence - sunmary of analysis of variance L'72
for field trials.
Effect of compaction on MDE (L977) ' 173
Effect of compaction and PVA treatment on MDE
Effect of compaction and time of sowing on MDE
Effect of time of sowing on MDE (L976) '
Effect of PVA treatment (to prevent crusting) on
MDE (L977).
Percentage emergence - surlmary of analysis of datafor L976 and 1977 field trials.
Effect of PVA treatmenton percentage emergence
(to prevent crust formation)(Le7'7) .
7.16 178
TabIeNumber
7.L]
7.18
7.r9
7.20
7.2L
7.22
7.23
7 -24
7.25
8.1
8-2
Title
Final plant height - summary alalysis of variancefor field trials.
Tiller number - sunmary of maximum likelihood'estimates.
Yield data - summary of analysis of variance forfield trials.
-2 )in
l-x.
Page
I8I
r83
r85
r87
188
189
r90
I9I
L92
L96
205
Effect of aggregate size on grain yield (9m
L977.
Effect of aggregate size and compaction on grainyierd (sm-2) in 1976.
Effect of aggregate size compaction and time ofsowing on grain yield (çn-') in 1976.
Effect of sowing date on DM yield (Sm-2) and grainyield (s*-2) in L976.
Effect of compaction and time of sowing on grainyield (stn-2) in 1976 -
References which quote given aggregate size rangesas being optimum for various crops.
Chemical analysis of aggregate size rangescollected from the field-
Predicted aggregate sizes which minimize or maximizevarious seedbed and, plant growth parameters '
x
LIST OF FIGURES
Title
Best fit lines for the relationship LoglgY =A-BLoglgã.
Apparatus for measuri,ng uniaxial compressionbehaviour of aggregate beds.
Compression cul:ve for a bed of 9-5-5-l- rnm
aggregates at -IOkPa v¡ater potential.
Comparison of measured and caLculaLed macroporosil--yin beds of aggregates at various levels of compaction'
Effect of aggregate diameter (d) on evaporation ratio(Ev) through beds of aggregates-
Effect:. of aggregate diameter (d) on evaporation ratio(Ev) from beds of uncompacted and compacted aggreqates'
Effect of aggregate diameter (d) on evaporation ratio(Ev) through beds of uncrusted and crusted aggregates'
Effect of uncrusted and crusted surfaces onuncompacted and compacted beds of aggregates onevaporation ratio, Ev, during Experiment I and 3'
Effect of time on mean evaporation ratio (Ev) for aIItreaLments during Experiments l, 2 and 3.
calibration curve for calculation of median emergenceforce.
Effect of temperature on emergence force and pressureof wheat (triticum aesèivum L- cv. Halberd) shoots'
Effect of aggregate size and compaction treatrnent oncrust strength as determined by modulus of rupture'
Effect of aggregate size and water potential on cruststrength as measured by modulus of rupture'
Effect of aggregaÈe size and sowing clate on cruststrength as determined" by rnod'ulus of rupture'
Effect of aggregate diarneter (d) on crust strength(pH) at the end of the season in 1976 and 1977 '
Effect of aggregate cliameter (d) on crust stre.ngth (PH)
on uncompacted and compacted ireatments at the end ofthe season in l-976 and 1977.
Effect of aggregate diameter (d) on crust strength(Pu) on uncrusted and crusted plots at the end ofthe season in 1977.
3.2
3.1
3.3
3.4
A1
4.2
4.3
4.4
4.5
5.1
5.2
5.3
5.4
5"5
5.6
5.7
5.8
xl_ ,
Fig_ureNumber
7.2
7.3
7.4
Titl-e
5.9 Effect of aggregate size and corresponding waterpotential at 5 cm depth on crust strength at endof emergence deternrined with a hanci peneÈrotneter.
5 .10 Effect of aggregate diameter (ci) c¡n s;oil crul.st arrresist-ance (AR) at the end of the-: se¿rson in l-9'i6 an<l
L971.
5 .1r Effect of aggregate diameter (cl) on soil crust aÍrresistance (AR) at the end of the season on unconpacte<1and compacted treatments.
5 -r2 Effect of aggregate dj ameter (d) on crust air resistance(AR) at the end of the season on early sov/n and lai:esown pIots.
5.13 Effect of aggregate diameter (d) on crust air resistance(AR) at the end of the season on uncrusted a-ncl crustedtreatnients.
5 .14 Variation in crust porosity wit'h dep'Lh in cÏust-s formedon compacted plots of various sized aggregates as
determined by quantimet.
7.L LayouÈ of field experiments in 1976 and L917.
Effect of aggregate size range on mean day of emergencein the glasshouse and PhYtotron.
Effect of aggregate size range anr:l water content onmean day of emergence.
Effect of water conl:ent on mean da-; of emergerrce'
7.5 Effect of water conl:e¡rt olì perceil'-age emerge:nce ofglasshouse and phytol-ron experirnents -
Dry sieve analysis of a.ggregates obtained by rotarysieving into four size ranges.
Bffect of aggregate sjze (d) on mean day of emergence(MDE) in the field in L916 anð. L9'17.
Effect of aggregate diameter (d) oi'r Percentage emergence(Em) in 1976 and 1977 in the field trials '
Effect of aggregate size on plant growth.
Effect of compacLíor¡ tl:eatment on plant growth'
7.6
7.7
7.8
7.9
7.r0
7 .1r Effect of early and late sowing dates on plant Erowthin 1976"
Effect of urrcrusted and crusted soil surface on plantgrowth tn L977.
7.L2
xl_r.
TitIe
Effect of aggregate diameter (d) on dry matter yie'lcl(DM) of wheat in 1976 and- L97'7.
The tidealt seedbecl
7"13
8.1
PIateNumber
3.r
3.2
3.3
4.r
5.1
5.2
><IAI.
LIST OF PLATES
Title
Thin sections of aggregates used to determinethe internal crack patterns of the aggregates(Scale x2) -
Enlargements of thin sections of aggregatesshowing internal cracking patÈerns (Scale x70) '
Internal structure of a bed of 9.5-6.1 mm aggregatesafter various levels of compaction'
Evaporation through beds of aggregates'
A Sealed plastic vial with a coleoptile pushingupa5gglassrod.
B Doubling back of coleoptile in glass tube'
Equipment for air resistance measuremenÈ'
Rainfall simulator in operating position'
Thin sections of surface crust formed under naturalrainfall. collected from the 1976 field experiment afterharvest (Scale x2).
Thín sections of surface crust formed under naturalrair¡falI. Collected from the 1976 field e>perimenÈafter harvest (Scale x2).
Thin sections of soil crust formed under naturalrainfall, from Ejne L977 field e>çeriment, used in the
Quantimet analYsis (Scale x2).
Thin sections of soil crust formed under naturalrainfa1l, from the 1977 field experiment, used in the
Quantimet analYsis (Scale x2).
General layout and appearance of the 1976 fieldexperiment.
Rotary sieve used for obtaining the following aggregatesize ranges: >4' 4-2, 2-L and <l mm'
Framework of plots in excavated area prior to fillingwith aggregates.
Sowing and wetting of PIoÈs.
Seed after placement in plot (4-2 mm aggregates) '
completed plot with tensiomel:er iu place (<I mm aggregates).
5 3
5.4
5.5
5.6
5-7
7.L
7.2
7.3
7.4
7.5
7.6
xrv.
PIateNumber
Title
Emergence on 2-I mm aggregaÈes"
Emergence on <I mm aggregates.
Early sown plots (1976).
Late sown plots (1976).
Appearance of plots after sowing and wetting (1977) '
Plots two weeks after emergence (1977).
Plots six weeks after emergence (1977).
Plots after tillering (L977).
Plots at heading stage (1977).
Plots at harvest (1977).
Appearance of surface of ploÈs after harvest (L977) '
7.7
7.8
7.9
7 .10
7.LT
7.r2
7.L3
7.L4
7 .15
?.L6
7.r7
X\/
SUMMARY
The effect of different aggregate sizes on various properties of
the seedbed and plant growth has been reviewed'
Agqregate strenqth and compaction of aggregate þe<þ
Measurement of aggregate tensile yiel,J strength was made and
related to the toad bearing capacity of aggregated seed̡eds. It was found
that tensile yield strength is dependent on aggregate síze fot a range of
soils, and is highly clependent on water potential' An equation is
developed to d.escribe compaction, of seedbeds, as a function of applied
uniaxial stress and the tensile yield strength of the aggregates. The
implication of the results is that beds of smaller aggTegates are able to
bear greater loads than larger aggregates wi*-hout structural damage ' A
crack theory is developed to explain the size dependence of t-ensiJ-e yield
strength. changes in soil structure during uniaxial compaction l¡¡ere
observed by time-lapse photography and by measurj-ng changes in macr:o-
porosity of wax-impregnated soil blocks after various levels of compaction'
Evaporation from beds of aggregates
Asimplefieldexperimentwasconductedtomeasureevaporative
losses through beds of different sized aggregates, with and without a
surface crust, and with and without a compaction treatment. To enable
comparisons between periods to be mad.e, evaporation is rendered
dimensionless by dividing by the pan evaporat.ion. The 2-I, L-O.5 and
4-2 nm aggregates exhibited the minimr:rn evaporation ratio for the two
uncompacted and the compacted treatments respectively. The presence of a
surface crust reduced the evaporation rati-o by approximately one-half
compared with the uncrusted surface, while a compaction treatrr'ent
resulted in an increase in evaporation ratio over the uncompacted treatment'
xvl
Regression equations relating evaporation ratio and meteorological f actor:s
are developed.
Crust strenqth and emerqence force of wheat
The median emergence force for wheat ( Triticum aestivum L. cv.
Halberd) is determined, using probit analysis, to be 0.5N. This is
similar to the emergence forces reported for ottrer species.
samples of soil surface crust formed under natural rainfall were
collected for strength determinations using a modified modulus of rupture
test. The crust formed on the 4-2 mn aggregates was consistent-ly weaker
than the crust formed on any other aggregate síze. !Ùhen a compaction and
time of sowing treatment was imposed, Erre 4-2 flìIn aggregates still formed
the weakest crust. A compaction treatment and an early sowing date
resulted in stronger crusts being formed than the corresponding uncompactecl
and late sowing treatment. similar results v¡ere obtained in the fj-eld
when crust strength was measured using a hand penetromeÈer at the end of
the field trial.
Crust strength d.epencled on the water potential of the crust,
weaker crusts resulting as the water pctential becomes less negative '
The strength of the surface crust did not impede seedling emergence
in the trials reported.
Regression equations relatj-ng crust strength and aggregate diameter
are developed.
values.
There is good agreement between observed and calculated
The air 'resistance'of soil crusts was measured using a falling-
head permeameter. Air rresistance'is defined as the time taken to force
a volume of air through the crust. The crust formed on the 4-2 mm aggregaÙes
had the Ieast air 'resistance' compared with the crust formed on all other
aggregate sizes. A compaction treatment and. an early sowing date increased
the air tresistancet of soil crusts compared with an uncompacted treatment
xvrL
and Iate sowing date. A PVA trea'Lment (to prevent crusting), however,
redrrced the crust air tresistancet.
Regression equations relating air 'resistance' and aggregate
diameter are developed.
A rainfall simulator was used to measure time to run-off, volume
of run-off and sediment in run-off as measures of crust formation and
erodibility of beds of different-sized aggregates. Beds of large
aggregates (>4 run) withstood a constant rainstorm for a longer period
and with less soil loss than beds of small aggregates (<1 mrn).
Thin impregnated sectiorsof soil crust, formed on differen't-siz()d
aggregates, \^rere analysed on aQuantimet 72O for porosity' pore orientation
and change in porosity with depth beneath the surface. Crusts formed on
large aggregates hTere more porous than those formed on small aggregates.
There \^ras a preferred horizontal pore orientation in crusts formed on
all aggregate sizes, except the <I rûn size. Porosity of the crust
increased with depttr beneath the surface -
SoiI ternperature and water po tential in aggregated bed's
During the field experiments soil temperature and soil water
potential were recorded for the sowing to emergence and emergence to
harvest periods. The compacted plots and uncrusted plots \^iere \^Tarmer
than tϡe uncompacted plots and crusted ptots. Mean temperature varied'
slightly wittr aggregate size wíth larger sizes (>4 run) being $/armer than
the smaller sizes (<I mm).
with soil water potential, however, the uncompacted plots and
crusted ploÈs were wetter than the compacted plots and uncrusted plots '
The smaller and intermediate aggregate sizes (<I and 2-I mm) were wetter
than the larger aggregate sizes (>4 mm) throughout the season.
simple regression equations relating soil temperature and soil
XVIII.
water potential to meteorological facÈors are devefoped.
Wheat qrowth in beds of aggreqates
A two year field experiment was undertal;en Èo assess Lhe effects
of aggregate size, compaction, time of sornting and presence of a surface
crust on the emergence, g::owth and yield of wheat- (Triticum aestivum L-
cv. Halberd).
It was found that coleoptiles emergerl earlier on fíne and
intermediate seedbeds (<I and 2-I rn-rn) than on coarse seeCbeds (>4 mm) '
This however, varied between years. The presence of a surface crust,
a compaction treatment and a late sowing date all delayed emergence.
The percentage emergence was greatest on Lloe 4-2 mm aggregates- A
compaction treatment and time of sowing had no effect on percentage
emergence, but the presence of a surface crust reduced the percentage
emergence compared with an uncrusted surface.
Despite marked differences in. the seedbed., yield differences were
sr:rprisingly small . createst dry-rnatl-er and grai-n yields were obtained
with the <I and 2-I mm aggregates. There v/as, howevel' no significant
difference between the <1 , 2-L and 4-2 nm sizes. A colilpaction treatment
and the presence of a surface crust had no effect on dry matter or grain
yield. However, early sowing date resulted in significantly greater
yields than a late sowing date.
Regression equations relating mean day of emergence' percenÈage
emergence and dry matter yield to aggregate size are developed' Good
agreement between observed resglts and calcul-ated values is obtained'
Aqricultural implications
The effects of seedbed preparation or even the seedbed itself on
final yield are not futly understood at present. This project has shown
that the load-bearj-ng capacity of aggregated seedbeds is a function of
xLx
aggregate size and. '$¡ater potential. crust strength, formed on different-
sized agglegates, varies with aqgregate size, water potenti-al, compaction
and sowing date. The rate of crust formation and erodíbitity of aggregate
seedbeds depends on aggregate size. Evaporation from the seedbed is
affected by aggregate size, compaction and the presence of a crust. soil
temperature and l^rater potential are altered by aggregate size, compaction'
time of sowing and the presence of a surface crust. Although plant growth
and yield is not markedly affected by aggregate size, benefit to plant
establishment is shown by earlier emergence and greatet: percentage elnergence
on intermediate-sized aggregates. These aggregates also tend to form the
weakest crusts, Iose less water and aIe warmer than other sizes' AII
these aspects are important for plant germination and earJ-y growth and
subsequent yield. Thus it appears thaÈ intermediate sized aggregates
form the best seedbed for cereals. Certain reservations must apply to
the above as the seedbeds used in the field experiment are not directly
comparable to those produced by conventional tillage methods"
xx.
Declaration
This ttresis contains no rnaterial which has been accepted
for ttre award of any otlter degree or dJ-p1oma in any University.
To t¡re best of tle authorts knowledge and belief, ttre thesis contains
no material previously pulclished or written by another Person, except
where due reference is made in the text of the thesis.
M.V. B
September, 1978.
xxl.
Acknowl-edgements
I would Iike to express my thanks to Dr' A'R' Dexter'
Senior Lecturer in SoiI Science, for his supervision. guid'ance and
helpful discussion throughout ttre project. Thanks also go to
Dr. J.M. oades, Chairman, Department of SoiI Science, for helpful
discussion during the latter stages of the project'
The assistance of Barbara Hampton, Roslyn Henderson'
,Joy Willis and David Hein at various stages of the field experiments
is gratefully acknowledged.
IamgratefultoTerrySherwinfortakingthecolour
photographs and preparing plates 3-1, 3.2, 5'4,5'5,5'6 and 5'7' and
to Brian Palk for the preparaÈion of plates 3'3 and 5'1
For assistance with computing and sÈatistical analysis
Dr. Peter Baghurst is thanked.
Ðr.JohnHewittassistedinthedevelopmentofthecrack
theory.
The thesis was carefully typed by Pam Anderson'
The financial support of a commonwealth Postgraduate Research
Award is acknowledged.
I would atso like to thank Helen for her understanding'
support and encouragement, and just for being there'
W¡\lïË INSTITUTE
L!B
SECTTON ]. INTRODI.]CTION
The ideal seeclbed should not only provide optimum conditions for'
germinatì.on and emergence, but- also maintain conditions rv-hich
pern¡it opl-imum development of the plant through to harvest. It should
also accept relatively intense rainfall v¿ithout- serj-ous loss of structrrr:e;
it should allow run-off without loss of soil by erosion; and it should
resist crusting and co¡rditions leading to poor aeration. The soil
surface should be maintained in a conditiorr .that is favourable for: the
passaqe of harvesti.ng machinery, but wj'll not be damaged by i-t'
fn most studies on the effect of tillage on crop grorvth,
germination rates and croc establishment data have not been recorded''
the only effect of differenÈ tillage practices measured has been the
final yielct of the crop. some attempt to measure soil- structure produced
by different tillage practices or impJ-ements on plant growth has been
made, but with little success. until 1976 no satisj:ac:to:''ll lr'echod was
available for measuring soil structure i:r siti¡ (DE:xter-, L976) ' The
effects of seedbed preparatiorl or even the see<1bed itself on final yield
are not understood at Present.
Experience in'licates that a certain amount of tillage is
necessary for crop production. In addition to the immedj-ate effect of
tillage on soil structure, the method of tillage may also affect soil
temperature, water storage and nutrient availability. Tillage practice
also affects the incidence of weeds, irrsect pests and prant diseases "
Minimum or reduced tillaqe was developed to proteÇt the land front
erosion, to reduce costs of production, to save time in nultipre cropping
situations and to reduce labour input, particular:J-y at certain tintes'
Generally, minirnum tillage will tend to produce denser soils thr:ough
natural compact:ion and mechanical t-raffic, but the soil type pl-ays a vital
t
part in the effect and in some cases a reverse trend ntay l:e seen. A
comnìon obse::vation iS t-hat early emergence and growth rat-.e seem to be
slo\^rel: on minimunr-tíIled as against conventionally-tilled crops " TÌiis
may þe due to both nuti:ient and soil physical factors. In some instances
root development has been severely restricted by usì.ng a minimum tillage
System" Hov¡ever, good yie1.c1s have been reported once the crop has
adequately establ-ished. Irlinimum tillage may also Çause an increase in
insect pests and plant diseases; however, opinion is divided at this st-age.
See¿bed preparation is a compromise between cost and effectiveness.
The latter depends upon many sometimes conflicting factors. A good seed-
becl consists of agg::egates sufficiently fine to ensure accurate seed
placement, adequate seed-soil contact to allow rapid and unifo::tn- germination,
and good root-soil contact to permit extraction of plant nutrients.
IIowever, aggregates which are too small may be further reduced in size
by rainfall impact to form a surface crust. Such crusts can lead to
water-Iogging and anaerobic conditions or run-off and erosion. They can
also impede plant emerçfence. A fine-seedbed may be unjustifiably
expensive to produce. Therefore it may be more p::ofitable to produce
a compronr-ise seedbed consisting of larger aggreg.rtcs than a fine seedbed
and its associated problems.
There i-s apparently still a wide difference of opinion as to the
sensitivity of crops to seedbed preparation or to the conseo.uent seedbed
conditions. V'Ihen the effect of weed competition is taken into account,
there is a surprising insensitivity of crop gro\^ith to seed.bed preparation.
It has also been noted that differences in early growth, attributed to
different tillage treatments, are not reflectecl in the final yielcl.
Responses to tillage, however, have been demonstrated on soils in poor
physical condition" Most farmers seem to know empiricatly what a good
seeclbed looks l-ike, but little quantitative information is available to
3
def ine what constitutes a good. see,lbed in all conclitions for all crops -
Traditionally, farmers have worked tlhe soil untiL it appears to have a
good tilth. Thi-s may involve up to IO passages by ti11a9e implements
over the land. If ínformation abou.t seedbed conditions on plant growth
and yield were available, perhaps the number of passes could be reduced'
As stated before, the seedbed is made up of vario\ls aggregate
sizes which optimize conditions for germination and emergence- Aggregate
sizes directly influence seed and root contact with the soil and so-i-I-
water, water movement, bulk density, porosity, soil atmosphere, nutrient
uptake, resistance to colnpaction, surface crusting a¡d soil tetnperatui:e"
Thus aggregate size distribution may be a good way of classify'ing seedbeds '
Tillage is one way in which the aggregate size distributic'n nay be al-t-ered '
TiIIage, however, produces a wide ïange of aggreqate sizes which vary
enornìously depending on the type of tillage implement used and on the
initial state of the soil. For this reason, results from experiments in
which the effects of different tiltage implements on crop performance are
compar:ed onJ-y have meaning for the precise conditions at the time and
may not be generally valid. Experiments have bee¡r performed with beds
of sieved aggregates having narrow size distributions. Tt¡ese represent
v¡hat are possibly the only reproducible and' well-defined soif structures
and hence provide the only means by which experiments conducted at'
different times and in different places can be compared quantitatively'
The seedbeds produced by sieving have certain other advantages; they can
be produced so as to be obviousty dífferent and so as to differ in a
graduated manner; extreme conditions of roughness or fineness of tilth
can be attained; and the seedbed.s can be reprod.uced within limits of
experimental error. However' there are several inherent disadvantages'
Thesearethattheplots,ofnecessity,havetobesmallerthandesirable
for accurate assessment of yield, that the seeCbeds are not direcily
4
comparable to titths produced by conventional means, and that- sieving of
the aggregates may have effects other than separation of aggregate sizes.
Examples of the latter are that sieving may redistribute weed seeds' or
may coricentrate Stones in a particular size fraction. However' not
withstanding these disadvantages, by studying t].e effect of seedbed
conditions in this manner a better understanding and appreciation of plant
response to tilths may be achieved.
This thesis embodies the results of an investigal-ion into some
effects of seedbed aggregate size under Australian conditions. In
particular the aims of the project were:
I) to determine how aggregate size is related to aggregate strength
and its effect on the load-bearing capacity of aggregate beds.
2) to exanuine evaporative water losses from aggregated beds - an
important aspect where water conservation is concerned.
3) to determine the emergence force of wheat shoots and to measure
the strengths of crusts formed under natural rainfall - where crusting is
a problem, crust strength and emergence forces of seedlings largely
determine how many plants emerge and hence final yield'
4l to examine how soil temperature and water potential in aggregate<ì
seedbeds are affected by aggregate size and meÈeorological factors -
important considerations for germination, emergence and subsequent growth'
5) to exarn-ine the effects of aggregaÈe size, compaction and surface
crusting on germination, emergence and yield of wheaÈ'
The results of these investigations should aid in the definition
of the ideal seedbed for Australian conditions'
5
SECTTON 2
REVIEW OF LTTERATURE ON THE EFFECTS OF AGGREGATE SIZE
2.I lntroduction
Marshall (1962) defined soil structure as "the arrangement of
the soil parÈicles and of the pore spaces l¡etween them. It inciudes
the size, shape and arrangement of the aggregates formed rvhen primary
particles are clustered together into larger separa-ble units". According
to this definition, there are no structureless soils and soil structure
is altered if the soil is deformed in any way. Thus soils in their
natural condition have at least some of their individual particles
clustered into aggregates, or clods or crumbs and the size distribution
of these aggregates or its converse deternr-ines the soil structure and in
part, the soil tilth.
The concept of soil tilth is still vague, and it involves at
Ieast two separate factors: the coarseness or fineness of the tilth,
which is concerned with the size distribution of aggregates; a¡rd its
mello\^/ness, a property which has not been studied in detail. The
importance of tilth for plant growth oï for ease in lancl management .is
still in doubt. As yet no-one can describe the titth that is required to
give the optimum soil conditions for all phases of plant gro\^tth'
Comparatively little is known about the effect of seedbed conditions on
the germination, emergence and growth of seedlings. For these reasons
at least, it would seem desirable to have some kno\ÀttedEe of the influence
of the seedbed. on all phases of crop development. Largely from practical
experience, farmers have a qualitative idea of r,,rhat the optimum state of
tilth is for a particular crop in a particular area'
Experience indicates that a certaiil amount of tj.Ilage is required
on most soils. However, there is a wide d.ifference of opinion as to the
6
sensitivity of crops in their response i-o seceìl-,ed preparalion
(vüedderspoon, L945¡ Russell , 1945). Data coLlect-ed has shown that
cereals a::e relatively insensitive to the method of seedbed preparation
(Keen, 1930; Russell and Mehta, 1938). Russell (1961) gives soil
conditions which affect plant growth as being the availability of water,
aeration status, the nutrient supply, temperature, presence of toxic
substances, depth of soil and mechanical impedance. However, what are
the characteristics of a good tilth and what soil conditions must it
provirle for the establ-ishment of ttre crop? Sl-ipher (L932) suggests
that the ideal state of the tilih should
offer minimum resistance to shoot and root penetration'
perrnit free intake of rainfall and the retention of adequate wat-er,
provi<le an adequate air suppty wiÈh gas exchange between soi-l- and.
atmosphere,
provide maximum resistance to erosion,
promote microbiological activity, and
provide st¿rlcle traction for implements.
Thus tilt-h is a complex interaction of biological, physj-cal and
engineering factors.
Tilth is a dynamic soil condition. Once formed, tilth begins
to break down, but with proper rnanagement it may be induced to form
again. The question arises as to whether one should continue to use
soil which is in a poor state of tilth or attempt first to improve it.
To improve yietds and reduce the possibiJ-ity of erosion one shoul-d
attempt to improve the tilth of the soíI. Some natural factors that
affect aggregation, which in turn affects til-th are
flocculation and coagulation,
presenÇe of cementing materials, both organic and inorganic,
wetting and dryi¡g, freezíng and thawing of soil water, and organic
7
matÈer and biological activity in the soil.. These factors are slow
in producing any readily visib]e improveme¡rl- in soil tilth. Tillage
operations are a means whereby tilth may be readily altered, although
not aII tillage operations are beneficial.
The final measure of success is the economical production of
high crop yields. To do this it seems that one should determine the
tilth requirements of crop plants on different soils and measure the
effectiveness and suitability of tiltage implements in producing these
rrequired I tj.ltìs.
2.2 Prope rties of different sized agqreqates
fLrere have been very few studies of the chemical and physical
propertÍes of different sizes of soil aggregates. It is important, when
examining this factor, to note always whether the experiments han-e been
done on tilled soil or on untilled soil such as under grass. Vlith a
tilled soil, one would not e>læect very large differences because the
soil is periodically being rearranged and the smaller agglegates are
probably produced by the breaking down of 1a::9e aggregates or clods.
Under grass, however, each aggregate size range may be formed by
different processes and may therefore have different properties.
2.2.I Chemical properties
Tabatabai and Hanway (1968b) sieved surface and subsurface soil
from under grass into the following aggregate size ranges: >9r 9-5r 5-3,
3-2 and 2-1 mm. They found that the percentage of organic carbon increased
as aggregate size decreased for both surface and sub-soils. Wittmus and
Mazurak (f958) found a similar trend in a brunizem soil under grass.
Alderfer and Merkle (1942), using forest soil, found the organic
I
carbon content \¡ras largest in the > 2 mm or 2-1 rnm size and decreased
with a decrease in aggregaÈe size to 1-0.2 mm. A similar trend was also
observed in cultivated soil. carey (1954), using soil from un-tilled
areas, found that organic carbon content increased with a decrease in
aggregate size from >O.42 mm to O.42--0.15 run. Clay content followed a
similar trend. Metzger and Hide (1938), however, found in cropped soil,
tJ-at organic content increased as the size of the aggregates increased.
In a titled soil, Salomon (L962) found a larger organic carbon percentage
in the I-0.25 mm diameter range than in either the >l or the 0.25-0.1 mm
ranges. Craswell et al. (1970) found no difference in the organic
matter content of different aggregate sizes in tilled soils, but a
maximun content in the l-0.5 nm range in virgin soils . These varied
results do not permit of any Eeneral conclusion but serve to iLlustrate
that different soils can behave differently.
2.2.2 Physical and mechanical properties
Effects of size on the physical and mechanical p::operties of
aggr:egates kave also been investigated. Meflzger and llide (1938) and
Tabatabai. and Harrrlay (1968b) found no differerrce in the sand, silt and
clay contents in different size ranges. Hol¡¡ever, Tabatabai and Hanway
did find that the dry bulk density of aggregates decreased with decreasing
aggregate size. This was attributed to the negative correlation between
dry bulk density, g, and organic carbon content, C, which they also
observed:
I = 2.23 - o.23c Q.L)
In other experiments, however, Miller and Mazurak (1958), Wittmus and
Mazurak (1958) and Gumbs and warkentin (1976) fourrd that dry bulk density
increased. with decreasing aggregate size. This discrepancy may be a
consequence of the fact that the two sets of experiments used different
9
soil types and. different aggregate size ranges. Voronin et al. (1976)
also found that as aggregate size decreased from 7-5 mm to 0.5-0.25 mm
the bulk density increased. An inverse relationship between dry bulk
density and aggregate size has been explained by currie (1965). He
pointed out that no aggregate can contain pores approaching it in size'
So each decrease in aggregate size means that that size pore fraction is
eliminated. This is likely to be a significant factor for aggregates
smaller than I mm.
The tensile strength, Y, of aggregates of diameter, d, can be
measured from the force, F, required to crush them between flat parallel
plates (Braunack and Dextet, L978; Dexter, L975; Rogowski, L964¡
Rogowski an'J Kirkham, 1976) ¿
y = ¡ F,/¿m Q.2)
In earlier work (Braunack and Dexter, 1978¡ Dexter, L975; Rogwoski,
l-964l, k was taken as 0.576 but an "improved" value of 0.82I has been
used more recently (Rogowski and Kirkham, L976). If m : 2, Y is
independent of aggregate stze. In tilled soils, Rogowski (1964) found
m = I.5 which is consistent with the presence of large cracks in the
aggregates. The data of MarÈinson and Olmstead (L949') for tilled soils
are consístent with a value of m = L.7 + O.O5 and the data of Braunack
and Dexter (1978) for a tilled soil gave m = I.98 + 0.07. Some data from
ideal, synthetic aggregates (Dexter, Lg75) gave the value m= 2.0 + 0.2.
Values of m = 2 imply that tensile strength is independent of aggregate
size whereas values of m < 2 imply that tensile strength increases with
decreasing aggregate size.
2.3 Compacta-bility
There have been very few experiments in which the effects of
aggregaÈe size on the compactability of aggregate bed.s have been examined-
r0.
2.3.L Natural aqqreqates
Scot-t. Blair (f937) and Scoit Blair and Cashen (1938) investigatecl
the uniaxial compression of tilths of natural aggregates. They plotted
the deformation against the square root of the applied stress. They
found such large differences bet\,reen the compressive behaviour of the
different tilths that they suggested that til.ths could be measured in
this way.
Kuiper:s (1958), using uniaxial compress.ion' performed similar
experiments. He described the d.ecrease in sample height of the aggregal-e
bed, z, causecl by the axial stress, P, bY the equation
z=A- r es)a+bP
where A, a and b are adjustable parameters. soil clay and organic matter
contents were found to be positively correlated with parameter A anrl
negatively correlated with parameter b. The e:ifect c¡f soil wa-Ler content
was also investigated.
Braunack and Dexter (1978) found that compactabiliÈy T¡¡as
independent of size for natural aggregates. It was found that the
compaction equation for aggregate beds
H/uí = 0.4 + o.o expfo .or7 (/-Ð - o.3s rP/vl\f Q.4)
fitted the results for one soil type to within experiment'al error'
Here, H is the height of the aggregate bed and Hi is the initial height.
The uniaxial compacting stress, P, is rendered' dimensionless by dividing
by the aggregate tensile yield strengttr, Y. Equation (2.4) was valid for
beds of aggregates of all sizes and for all water contents dr-ier than
field capacity (.r, tOO cm water suction). However, j.t must be realised
that wetter soils usually may be compacted by smaller levels of applied
Stress, P, because the tensile strengt-hs, Y, c:f the aggregates are smaller'
The results of Martir¡son a¡d Olms'r:'rad (f949) , Rogowski (1964) and
Fogowski and Kirkham (1976) are consistent witli values of m < 2- This
II.
has the implicatiou that berls of smaller aggregates would be less
compactabl.e than beds of J-arger aggregates al- the sane water content.
2.3.2 Artificial aqqreqates
Dexter and Tanner (1973) studied the isotropic compression of
remoulded soil-. They investigated the change in packing density, D,
with apptied isotropic stress, P. D is the proportion of the soil volume
occupied by mineral particles. It was found that the compression
behaviour could be described accur:ately by a double exponential equation:
D = D^ + B exp(-kP) 'l- C exp(-Î,P) (2-5)o
where D is the minimun limiting value of D. and B and C are anounts ofo
the compression due to two separate processes. The value of B was found
to bre independent of water content, w, whereas the value of C increased
Iinearly with waÈer content. Since the wetter sarnpJ.es were seen to
contain moïe aggregates, presr:mab1.y as a result of v¡ater-film cohesion,
the first exponential term was attributed to the rearrangement of the
individual mineral particles in the soil, and the second exponentJ.al term
was attributed to the compression and destruction of the aggregate strucÈure.
In order to investigate the basic mechanics of compression of
aggregated soil, some experiments have been perfoïmed on idealized soils
composed of synthetic aggregates. Davis et qI_. (1973) investigated the
isotropic compression of ideal"j-zed tilths made from aggregates of an oil-
based modellíng clay having a plastic consistency. The tilths were
qornposed of uniform, spherical aggregates arrange<l in the form of regular
"crysta1line" lattices. The compression was measured by the variation of
packi¡g density, D, wíth applied dimensionless isotropic stress, P/y, where
Y is the tensile yield strength of the aggregate material. Since the
geometry of the system was rvell-defined, it was possible to attempt to
L2.
predict the compression behaviour from first principles. The main
assgmption tvas that, as the tilth compressed, flat interfaces developed
between adjacent aggregates and that the aggregates took on the shape of
truncated spheres. Such flat interfaces have been observed in compacted
soils by Day and Holmgren (L952) and McMurdie and Day (1958). The
compression equation developed was
w.H.R. : P/Y Q.6)
where W is a wedge factor which allows for the angles at the edges of
the aggregate interfaces, H is an isotropy factor which allows for the
increasing isotropy of the stresses within the aggregates as the
compression progresses, and R is a p::essure factor on the aggregate
interfaces. The facÈors W, FI and R were all geometrically related to the
packing density, D. Good agreement was obtained between theory and
experiment.
Dexter (1975) extended the investigation of ideal systems to
tilths composed of synthetic brittle aggregates in uniaxial compression-
It \4ras assumed that compactability would be independent of aggregate size
because it was found that m -= 2 ín equation (2.2). When m = 2, a doubling
of aggregate size increases the crushing strength of inclividual aggregates
by a factor of four, but there are only one-quarter as many aggregates
per unit area of sample. Thus the effects cancel out. For: ideal, plastic
aggregates, the implications are the same (Davis et aI. , 1973) - It was
also observed that only about one-half of the aggregates in the compressed
samples had broken and tl-rat the material from thesc had flowed into the
interstices between the remaining, intact aggregates. The packing
density in this work was described by
D = Di - (Di - Dr)[r - "xp(-kP)].Ö F/v) Q-7)
where Di and Dg are the initial and final values of D, the exponent-ial
r3.
term describes the plastic flow of the broken aggregate material, and
O (P/y) is a statistical function describing the proportion of aggregates
broken as a function of dimensionless stress, P/v'
2.4 srodibil itv and surface crustinq
There is an enormous literature on soil crusting and erosiorr but
not much of it compares the effects of different aggregate sizes '
2.4.L Effect of water
Ellison and slater (1945) noted that surface aggregates were
broken down by raindrop impact into smaller size fractions than are
produced by wet-sieving of tire soil . These fragiments are often 'splashed'
from the point of impact and progressively fill the interaggregate pores
until a surface crust or seal is formed. These crusts have a low
permeabitity and their formation usually signals the onset of significant
run-off of surface water. It was found that the greater the percentage
of aqgregates )I mmr the greater vTas the infiltration rate of the soil
(ellison and Slater, 1945). Rose (196I) studied the soil detachrne¡ri:
caused by simulated rainfall on four size ranges of sieved aggregates'
The amount of soil detached was greater wittr smaller aggregates and
was five times as great for <0.5 mm aggregates as for the 7.9-4.4 mm
range. This is in agreement with the finding of Moldenhauer et al' (L967)
that larger soil clods did not break down as easily as smaller clods '
Rai et a-!. (1954) also found that as aggregate size decreased from I mm
to 0.5 mm soil loss increased slightly but a greater soil loss resulted
in decreasing aggregate size to 0.21 inm. Lyles et al. (1969) determined
that as aggregate s-ize range decreased from 76.2-5O.8 mm to 6.4-2'0 mm
the amount of soil erod.ecl increased. Moclenhauer and Koswara (1968) however
L4
found that wash erosion increased as initial aggregate size j-ncreased
from 0.5-2.0 mm to 8-30 mm on soil that had been cropped' A similar
Èrend \^ras observed for pastuïe soil except for the 8-30 nim size range
where losses were considera.bty less. Moldenhauer (1970) however found
that with meadow soils wash erosion decreased as mean aggregate size
increased from 2-4O mm. Mazurak and Mosher (1970) using eleven size
ranges of sieved aggregates, found that maximr.:nt soil splashing occurred
with one of their intermediate aggregate size ranges (2.36-I.68 mm) '
Moldenhauer and Kemper (1969) measured the amount of energy of
rainfall to induce run-off from various sized aggregate beds - they found'
that after the first increment of artificial rain had been applied' there
was a large difference between the intalce rate of the 8-20 mm size range
and that of aII other size ranges. The intake rate of the 8-20 mm size
áeclined rapidly after the first or second incremenÈ of rain' fntake
rate was not consistently related to aggregate sizes less than B mm'
Mol.denhauer (1970) found no significant d'ifference between aggregate slzes
up to 2-40 mm in the rainfall energ)¡ to initiate run-off' As the lower
Iimit of the aggregate size range v¡as increased to I mm, the energy to
initiate run-off increased. The energy required to initj-ate run-off in
the 30-40 mm size range was significantly higher than that- for any other
size range. Aggregate beds from continuous pasture withstood raindrop
action significantly longer than those from continuous maize.
Oncerun-offcornmenceg,anyisolatedaggregatesareindangerof
being carried a\47ay. Most researchers are in agreement that the erodibility,
E, by surface flow decreases with increasing diameter, d, (Alderman ' L956i
Meyer and Monke, 1965):
E c( d-o'5 Q '8)
15.
2-4.2 Effect of wind
Wind erosion is of great importance in many parts of the world
and has been reviertred by Chepil and lVoodruff (1963) . Less wind erosion
occurs from larger aggregates for two reasons. Firstly, larger eggregates
are less easily moved and secondly. Iarger aggregates provide a rougher
soil surface that reduces the velocity of the turl¡ulent wind above it.
A strong negative correlation v¡as <¡btained by Chepil (1953) between the
amount of soil eroded in v¡j.nd tunnel tests a¡rd the proport.ion of aggregates
>0.84 mm. It is stated in the review (Chepil and Wood.ruff, 1963) that-
tíllage to control rvind er:osion should bring up conpacÈed clods from the
subsoil having diameters in the range 75-100 mm.
Thus, wiL.h one or two exceptj-ons, Iarger surface aggregates appÐar
to resulÈ in less crusting and less erosion by either water or wind.
2.5 Aeration
Adequate soil aeration is essential for the germinaÈion of see<f as
well as for the development of the plant. Oxygen has to be supplied in
sufficient amounts to meeL the requirements for respiration of roots
and of the soil micro-organisms. Similarly, the products of respiration,
such as CO2, have to be able to escape to the atnosphere as fast as they
are being produced in order to prevent the formation of plant-toxic
substances in the anaerobic conditions. It is convenient for most purposes
to divide soil aeration into its inter-aggregate and intra-aggregate
components.
2.5.L lnter-aggregate aeration
The most important mechanj-sm for aeraticn in ncst soil: is
diffusion. This is a molecular kinetic mechanism and depends only on the
air-filled porosity of connected pores in the soil- and not on the sizes of
16.
the pores" In a bed of aggregates, the porosity is independent of
aggregate diameter, ttrer:efore, aeration by diffusion is inclependent- of
aggregate diameter. Vtater films provide an effective barrier to aeration
because the diffusivity of oxygen in water is about 1O-4 of its value in
air.
Recently, the significance of convection as a transport process
in tilled soils has been realised. Holmes et al. (1960) performed
experiments on pots containing different sizes of aggregaÈes. They were
able to control the wind speed and the radiant energy on the soil surfaces.
Smaller thermal diffusivities, as indicated by greater surface temperature
rises, \^7ere found with the finer tit{hs. When white smoke was i-ntroduced
into the air stream over tl:e pots, it was observed to enter and then
empty from the larger pore spaces rvi.Lh a rapid, turbulent motion. This
supports the findings of Wa<ldams (L944') that the flow of heat Èhrough
beds of steel spheres increases greatly when the particle diameter exceeds
5 nm. They concluded that for particle diameters greater than about l0 mm'
pore dimensions become large enough to accommodate eddies which enhance
convective transport (Holmes et aI. ' 1960) .
Effects of air turbulence on soil gas exchalrge have beell
investigated by Kimball and Lemon (1971). They did this by measuring the
flux of heptane vapour through beds of different sizes of particles. They
found heptane fluxes of between three and ten tímes the predicted diffusion
flux through beds of 18 mm diameter particles, and between two and four
times the preclicted diffusion flux through beds of 3 mm diameter particles.
The úange of results in each case v/as positivety correlated with wind
speeds from O-14 km hr-l. Farrell et al. (1966) investigatecl this probl.em
theoretically and concluded tkrat with a wind speerl of 24 km hr-I, surface
air can penetrate coarsely-structured soil- to e depth of several centimet-res.
L] .
!-or particles of l-O run diame.Ler, they predi.cted that the gas flux acl:oss
the soil surface could be as much as IOO ti-mes t-he moLecular diffusion
flux and that 't-he air flow would extend to a depth of about 6 cm'
Yoder (1937), Hagin (I952) and S.Later and Rodrigues (1954)
attributed the poor gror,^/th of plants in beds of small aggregates to
inadeguate aeration. other experiments ha.ve confirmed this- Lemon and
Erickson (Lg52) and Doyle and Maclean (1958) shov¡e,l that soil oxygen
concentrations increased with increasing aggregate sj-ze' llowever, Grable
and siemer (1968) found that aggregate size clid not affect oxygen
concentration at the seeding depth.
2.5.2 Intra-aggregate aeration
currie (1961) showed that the centres of soil aggregates would. be
anaerobic if their diameter exceedecl a value d given by
d - (24t}(- /s)\ Q 's)
where D is the diffusion coefficient for oxygen through the scil, o is
the concentration of dissolved oxygen at the aggregate surface, and' S
is the consumption of oxygen by micro-organisms per unit volume of aggregate"
In an examination of some soil aggregates of l-ow air-filled porosity in
New Zealand, Graclwell (1973) has shown that those of dj-ameter greater
than about IO mm would have anaerobic centres even at the wilting point
and even if they were surrounded by atmospheric air. These regions would
be unsuitabte for root growth.
As aggregate size increased from o.s-to mm Voorhees s! 4' (1966)
found the percerrtage of pores >29 ym increased. The volume fraction of
air at any given water content also increased aS aggregate size increasecl'
This is consistent with the theory of Currie (1965)'described in Section
2 -2.2.
18.
The requirements for: inter- and intra-aggregat-e aeration are
therefore in conflict in that an increase in aggregate size increases
the fortner and reduces the latter. However, provided that aggregates
>IO mm or <I mm are avoided, it is unlikely that aeration will be a
Iiniting factor: for pl-ant growth under most conditions"
2.6 Water content and temperature
V,later content and aeration are closeJ-y refated in non-sweIling
soils because the proportion of the porosity not fill.ed with water is
filled with air. At any given matric vrater potential-, V (in units of
pressure), all pores smaller tharr diameter ô are filled rvith water where
6 = -Aa/Y (2.10)
)
and cl is the surface tension of the \'rateï. At field capacity V= - lOkPa'
ô = 30 Um. Pores of this diameter wiII occur betv¡een aggregates of about
0.2 mm diameter. This, therefore, sets a lower limit for seedbed aggregate
size if inter-aggregate anaerobic conditions are not to prevail at field
capacity.
2.6 .I Vlater st-orage
Several investigations have shown thaÈ the amount of water retained
by aggregates increases with increasing aggregate size (l\lcrol and Palta'
l.97Oi Amemiya, 1965; Tamboli et al . t Lg64r Wittmuss ancl Mazurak' 1958).
The effect is greater at small negative matric potentials (e.9. field
capacity) than at larger negative matric poter:tials (e.g. wilting point).
This trend can be expJ-ained by the 'porosity exclusion principle' of
Currie (1965) referred to earlier. Gumbs and Warkentin (1975), however,
found virtually no difference in water retained at 40-80 cm water suction,
in aggregates ranging in size from 2.3'2, 1.1-0.84 and 0.4-0-2 mm. Chibber
(1964) found that the maximum \dater holding capacity occurre<f with aggregates
in the 1.0-0.2 tnm size range.
t9.
2.6.2 Wal--er movement
Effects of aggregate size cn water movement have also been
invesÈigated. Hubbell (L947) iooked at capillary rise and percolation
through mixtures of aggregaLes and ncrl-aggregated soil. CapilJ-ary rise
was increased only when the proportion of aggregates was restricted to
small sizes (0.6-0"25 mm) and when there was not less than 15 volume
percenb of aggregates -in the soil. With larger sizes (Lo 2.3 mm), the
rate of c¿pillary rise increased only when the volume of aggl:egates i.n
the mixture reached 45-75e". Similar results were obtained v"'ith
percolation rates. Percolation rates v.¡ere greater with Iarger aggregate
sizes. Amem-iya (f965) investigated the influence of aggr:egate size otr
the unsaturated capillary conductivity of aggregate beds. The conductivity
of beds of 1-0.5 rnm aggregates was greater than that of larger aggregates.
It was found that for aggregates larger than I mm, conductivity was
generally a function of water content (or matric potential) and'
independent of aggregate slze. Thrrs, capillary conductivíty could only
be a function of aggregate size if the water content - matric potential-
relationship j-s a function of aggregate size. Benoit (f973), however,
determined that hy<lraulj-c conductivity inc::eased as aggregate size
increased from O-0.8 to L.2-2.0 mm, at maximum water holding capacity and
-0.5 bar matr-i-c potentíal .
2.6.3 Rate of drvi.nq
The effect of aggregate size on the rate of soil drying has
received some attention. In the semi-arid regions of the world it is
particularly important to minimize the rate of drying ancl thus to conserve
soil water. An example of thi.s occurs in Austrafia whe::e wheat is grown
in regions with as little as 250 mm of annual rainfall and where the
potentiat annual evaporation from ari open v¡ater surface or from wet soif
20.
is up to 2O0O mm. In general, smaller aggregates reduce the drying rate
becauSe \Àrater vapour transport by air convection in intra-aggregate
pores is reduced as described earlier (FarreII et al.. , 1966; Ilolmes et al
1960; Kimbatl and Lemon, I97I). Johnson and Buchele (196I) and
Johnson an<l Henry (1964) studied the inftuence of aggregate size and
compaction on soil drying rate and seedling erfler-gence. They found that
as the aggregate size increased, clrying rate increased. CompacÈion
reduced the drying rate but could delay or prevent exnergence of maize
unless the compacted layer 1alas kept moist. They suggested that a
stratified seedbed woul-d be a good compromise to ¡ninimize water I'oss ancl
to maximize germination and emergence. This would cornprise a compacted
Iayer of I mnt aggregates at the 2-5 cm depth with the seed placed 2 cm
below this.
2.6.4 Bvaporation
Hillel and Hadas (1972) deterrnined that, under uniform evaporation
conditions, the minimum water loss occurred through a bed of aggregates
of 1-0.5 mm d.iameter. Under non-isothermal conditions, Hadas (f975)
observed that minimum \¡rater loss occurred v¡ith O.5-2 mm oiameter aggregates.
This is in agreement with results of Holmes et al. (1960) who observed
minimum water losses through beds of 2.5 mm diameter aggregates. KinbaII
(1973) under field conditions also found the minimum evaporation loss
through beds of aggregates of I mm diameter. Feorloroff and Rafi (1963)
found a maxi¡num \^¡ater loss throuqh beds of aggregates of 5-10 nun diameter.
2.6.5 Temperature
There have been very fe:w erxp(:r:irnents to determine how aEgregate
size affects soil temperatur.'e" Vladdams (1944) found that the heat flow
through beds of steel spheres greatly increased when the particte diameter
,
2r.
exceeded 5 rnm. Bhushan et al. (1974) us-ing soil aqgregates in the raIìge
of 4.I2-L2.4 mm, found that seedbeds with the larger aggregates attained
the maxj-mum temperature earlier than seedbeds of the smal.Ier
aggregates. The amplitude of variation in soíI temperature v¡as greatest
in the larger aggregate beds. Holmes eL aI. (1.960) found the surface
temperature to be greater with fine (2.5 mm) aggregates than for other
sizes.
2.7 Nutrient uptake
The majority of plant nutrients occur in the top few centimetres
of the soil profile. In untilled soils. the nutrient coricentrat-ions
decrease r:apidly with depth below the su::face. In till-ed soils, on the
other hand, the regular mixing action of the implements produces a
uniform concentraÈion of nutrients down to the depth of tillage (Willians,
1950). plants get most of their nutrients fron the tilled layer and it
is interesting to look at the effects of the structure of this layer on
nutrient uptalce.
2.7 .L Nitroqen
Hagin (1956) investigated the effect of nitrogen on the yield of
wheat in aggregate size ranges >2, 2-O.5 and <0.5 mm. The aggregates
were aLl produced by the breaking down of Iarge clods. The larger
aggregates produced greater yields Lhan the smaller ones. Addition of
inorganic nitrogen (in the form of NH4NO3) tended to eliminate these
yield differences whereas the addition of organic nitrogen (in the form of
seed meal) enlargerf the yield differences. This difference is presumably
a consequence of the greater mobility oí the inorganic nitrogen compound,
Differences clue to structure ancl due to nitrogen were both statistically
significant, but interactions between structure and. nitrogen were not-
2.2.
!,littmrrs and Mazurak (f958) found total nit-rogen content to l¡e similar
for aII aggregate size ranges. However, it has been shown that the rate
of nitrogen mineralization, which is due to microbial processes,
increases with decreasing aggregate size j-n both the anaerobic (Craswell
et aI. , L}TO) and aerobic (Keresteny et aI., 1963; Seifert, L964) cases.
Similar results were obtained by lfar:ing and Bremner (1964) -
2.7 .2. Phosphorus
In considering available phosphorus under grass, Ialittmus and
Mazurak (1958) found thaÈ the amount increased with decreasing aggregate
size. Tabatabai and Hanway (1968b) found. the availability to be nore
variable: in some soils it increased with decreasing síze whereas in
others the reverse was true. On average, they found little or no effect
of aggregate size on available phosphorus or soil pH. Wiersum (L962) used
pieces of broken porous ceramic material soaked in Hoagland's solution
to simulate soil aggregates. He found that ptants grown on smaller
aggregates had. finer root systems than those gror^/n on large aggregates.
It was found that wheat, sunflower and tomato plants grown'on the smaller
aggregates contained larger amounÈs of phosphorus. The uptake of nitrogen
h¡as not affected by aggregate size. Cornforth (f968) used ryegrass, red
beet and. kale to test the uptaJce of nitrogen and phosphorus from different-
sj-zed natural and artificial aggregates. For the red beet, he found that
tlpterke of nitrogen and phosphorus decreased as the aggregate diameter
increased. For ryegrass, nitrogen upÈake was independent of aggregate
size during the early growth of the plant, whereas phosphorus uptake
decreased witLr increasing aggregate size. With kate, most nitrogen was
taken-up from the 3-I nun aggregates whereas phosphorus uptake decreased
with increasing aggregate size.
23
2.7.3 Potassium
The uptaJce of potassium was investigated by Tabata-bai and. Hanvray
(1968a). They found both the uptake and the dry matter content of the
plants were greater on smaller aggregates during early growtl-r of the
plants. During later growth, there \^ras no significant trend with aggregate
size.
Dexter (1978) has modelled the growth of single root axes through
soils of different structure. The model enables the effects of structure
on the relative uptake of non-mobite nutrients such as phosphorus and
potassium to be estimated. Decreasing aggregate size is predicted to
lead to increased nutrient availa-l¡ility in agreement with the e>çeriment-al
results discussed above. Some plant species are predicted as being more
sensitive to structure than others. In order of increasing sensitivity
to structure, the species are: oats, soybean, triticale, wheatr Pېr
Iucerne, rye, barley and barrel medic.
2.8 Crop emerqence and growth
The seedbed is usually considered to be that layer of soil which
has been tilled to a condition to promote germination of seeds and the
emergence and development of seedlings. Insufficient emphasis has been
placed on the structural features of the soil which favour growth after
emergence. The significance of this can be seen v¡hen one realizes that
the seedbed acts as such for only 7-14 days. The term "rootbed", as
suggcsted by Baver et al. (1972) may be more appropriate since it implies
the structure which influences the whole development of the p1ant. More
than-5OB of the roots of a developed plant remain wittrin the layer of
soil that has been previously tilled (rinney and Knight, 1973).
24-
2.8.I Affect of te size c¡n h of monoco ledons
Dojar:enko (1924) and Kvasnikov (1928) found that optimum yields
of cereals were obtained with beds of 2-I mm and 3-2 mm aggregates
respectively. Hagin (L952) found that coarsely-aggr:egated soil (>2 nrm)
produced better wheat qrowth than finely-aggregated soil (<0.5 nun).
This difference was attributed to restriction of aeration with the finer
aggregates. Jaggi et al- . (lglZ) concluded that a seedbed of 2-I mm
aggregates \^¡ith a clry bulk density of L.2-L.3 g.*-3 woulcl give the best
grain yield of rvheat. They suggested that with large:: aggregate si-zes,
impediment of water movement to the roots may be the factor limiting
plant growth.
The emergence of oats was studied by Thow (1963) in field and
pot experiments. In the field experiments, where the aggregate sizes
vrere not defined., a fine tilth retarded emergence compared wiÈh a coarse
tilth. From the results of pot experiments it was inferred that a
covering tilth above the seed of 25% or more by weight- of 6.35-0.76 mm
aggregates and. not more than 40% of 57-32 nun aggregates would result in
approxirnatellz g6ru emergence. Edwards (1957 a, bi I95B) looked. at the
effect of aggregate size, compaction and sowing date on the eme::gence
and subsequent growth of oats and barley. Seedlir-rgs emerged earlier from
1 mm aggregates than from L2.7-9.5 mm aggregates. The compaction treatmenL
caused the oats, but not the barley, to emerge earlier from the uncompacted
treatment, but did not affect the yield of the crop. The time of sowing
did not appear to have much effect on time to energence or percentage
emergence. The greatest yietd was obtaineC from the <1 rün aggregat-e plot.
This is in agreement with some of the results for oats (Thow, 1963).
Kodama and Suzuki (1955) found that with finer aggregates' surface crusting
reduced emergence. This problem was not encountered by Ed-wards or Thovr-
,rrâïson (1.964) suggested a ñean aggregate diameter of 5 mrn for
25
best resglts in growth of maize. Johnson and Taylor (f960) founcl the
highest rate of maíze emergence resulted when 30e¿ of a soil passed
through a 2.54 mm screen. Anderson ancj Kempel: (L964) reported that
greatest yields were obtained with aggreqates of intermediate stability"
Taylor (L974) also founcl that a seedbed. with a lower size limit of 2 mm
was suitable for early emergence of maize and sorghum.
In consiclering the optimum seeclbed requírentents for sugar canet
Jain and Agrawal (1970) found ttrat the range 6.4-3.2 rnm gave the la::gest:
germinatj-on percentage, an enhanced uptake of nitrogen and consequently
increased the numl¡er of tillers produced and the final cane yiel-d"
Deviation into a finer seedbed than this had a morîe delet.erious effect
than deviation to a coarser seedbed.
2.8.2 Af fect of ec¡ate size on qrowth of dicotyledons
Yoder (1937) est¿rlclished that the emergence of cotton plants was
most rapid when one-hal-f of the aggregates were between 6.2 and 3-2 mnt
and the other half rvere smaller. This proportion also gave the gr:eatest
yieId.
!-or sugar beet and forage beet, Hanmerton (1961 â, bi I963a, b)
found earliest emergence and greatest percentage enlergence with aggregates
of <I mm. Consoliclation gave a slightly earJ.ier emergence, and sowing at
19 mm v¡as superior to sowi.ng at 38 mm. Greater plant dry weights resulting
from more ancl larger leaves both dr.rring development and- at final harvest were
obtained as aggregate size was ïeduced from 9-6 run 1-o <l mm. These trends
\^rere attributed to greater v¡ater availability t-o the seecls and roots
in the finer aggregates. Miller and Mazur¿-l< (1958) aiso found that
greate:: areas of root to soil solrition conLact grfve Ì>etter growth of
roots and shoots of sunflowers, provideci that aeraticn was not limiting.
The best grovrth rvas obtained wi'Lh artificial aggregates in the range
26.
0. 2l-0 .052 mrn.
Mr¡nselise and Hagin (1955) fcrund car:nation rooti.ng Lo be greater
in coarse (>2 mm) than medium (2-0.5 mm) or fine (<0.5 run) aggregates"
This trend was attributed to differences in oxygen supply between the
aggregate sizes. For tomatoes, Doyle and Maclean (f958) found increasi-ng
yields with increasing aggregate size which they correlat-ed with oxygen
diffusÍon rates in the 1oot zone. There was no sígnificant increase in
yie1.d for aggregat-es in ranges Iarger than 5-2 mm. Pollack and Manolo
(1969) reportetl fresh weight of bean seedlings 24% rn]-g]ner when grown in
coarse rather than fine sand. Schuylenborgh (1947) found that v¡hite
mustard yietded best with the 2-1 trun size range. He also found that
deviation towards smaller aggregates was preferable to deviat'ion towards
Iarger aggregates. This latter finding is in contrast to 1:hat of Jair¡
and. Agrawal (1970) given earlier.
Nash and Baligar (L974) reported that soybean emergence occurred
4 days earlier on 2-4 and 2-I run aggregates than in smaller or larger
sizes. They suggested poor aeration in the smallest and poor water
relations in the largest aggregates as beitrg possible causes for delayed
emergence.
2.9 Aqsreqate sizes produced bY tillage
Tillage is often undertaken to control the distribution of
aggregates in the profile. Large aggregates may be placed on the soil
surface as they tend to prevent wind, and water erosion (Lyles arrd woodruff,
Lg62). On the other hand small aggre-qates are desirable in the vicinity
of the seed to provide opti.mum germinatiotr conditions. It has been
determined that all tillage tools have essentialJ-y the same segregatinq
and mixing effect on the soil (Hulburt and Menzel, 1953; I'linkelblech and
Johnson, I964i Ojeniyi, 1978) . Kouwenhoven and Terpstra (1970, 1971)
27.
have examined this effect in more cletail- by using soil aggregates and
glass spheres. The degree of sorting increased with t.he number: of
implement passes, until a¡ equilii¡rj-um was estabfished. Ojeniyi (1978)
also found the aggregate size distributj-on was influenced by a secoird
pass of a di.sc plough, but not by further passes. similar results were
obtained witl-:. one pass of a disc plough follor^¡ed by I to 3 passes of
tines and when using tines onIY.
Col.e (1939) states that the nurürer of tillage operations used in
preparing a seedbed are often a matter of haþ.itua1 prac'Lice, rather than
a considera.tion of the physical conditions to be attairled for a particu1ar
crop. He also mentions the fact that not aII tittage operations are
necessary as thet:e is only a small change in aggregate size distribution
before and. after tillage. The main chanqe is expected in the void' size
distribution, krut there have been hardly any studies of this. The size
distribut-ion of aggregates depends largely on water: content at the time
of tillage. More smaller aggregates and fewer clod's wer:e produced lvhen
tillage was done between 17 and 2Os" water content than when the v¡ater
content was >204.
Using a sieve analysis of the tillecl layer, Nijhawan ä¡ìd Dhirigra
(Ig41) found that hoes and harrows formed more aggregates <3 mm than
mouldboard type ploughs. Previously they had determined that aggregates
3-0.25 mm produced the best growth and yield of bajra, guar, graln and
wheat, contained more organic matter and were more water stable than
aggregates >3 mm.
Singh and pollard (1956) found that the type of tillage i¡rfluenced
the size dist-ribution of agg::egates in the top IO cm of soil. Mouldboard-
tilled plots contained a greater percentage of aggregates >0.5 mm than
did Èine-tilled plots. Johnson and Taylor (1960) determined that three
passes with a disc increased the proportion of aggregates >I9 mm and
28-
reduced the proportion <2.5 mm when compared with a single pass. Greatest
emergellce of maize resulted when 30% of the soil in the top 7.5 crn was
<2.5 mm, Taylor and Johnson (1956) had previously determined that a
fine seedtred. produced rapid emergence of corn, but they did not define
aggregate sizes quantitatively.
The two most important factors affecting structural and surface
conditions produced at the time of tiltage are waÈer content and type of
tillage action. Lyles and Woodruff (J-962) found that the mouldboard
plough produced larger aggregates than either the one-way disc or s'¡b-
surface sweep. Aggregate size varied as the water content at t-he time
of tiltage increased from I to 25%. Using similar tillage implements,
Siddoway (1963) found that a mouldboard plough produced a less varia-ble
aggregate size distrilcution than a sweep. The mouldboard produced a
predominance of non-erodible aggregates (6.4-19 mm) while the sweep
produced a predominance of non-erodible aggregates (>I9 mm) aud erodible
aggregates (<0.84 mm). It must be noÈed, however, these two implements
represent the extremes of tillage: complete soil inversion by the mould-
board and no inversion by the s\^/eep. Ojeniyi (1978) determined that ihe
maximum nurnber of small aggregates and minimum nudber of large voids were
produced by tilling the soil at a water content of approximately 0.9 of
its plastic tinit. Aggregate size was calculated from an intercepted
Iength (Dexter and Hewitt, 1978) and is not a d.iameter as obtained by
sieving.
Larson (1964) attempted to define parameters for evaluating tillage
systems for maize. He suggests an aggregate size of 5 mnt for optimum
seedbed conditions. Allmaras et aI. (1965) assessed the distribution
of aggregates in the row zone of tillage experiments. Different tíllage
treatme¡rts resulted in large differences in the logarithm of geometric
mean diameter (1og c¡aD) of aggregates. The largest difference occurred
)a
between a monldboard (f8.9 mm) and moul.dboard-disc-harrow (7.3 mm)
treatment. A similar aggregate size distri]¡ution, howevelî, was obtained
with the mould.board-disc-harrow (7.3 mm) and mouldl¡oard-rotary tiller
(8.4 mm) treatment. This also varied between years.
Bhushan and Ghildyal (197I) obtained similar resrrlts to Siddoway
(1963) arrd Lyles and V'loodruff (1962), when they found a mouldboard,
produced larger aggregates (19-53 nm) than a disc-harrow (<2 mm). On
ttre basis of aggregate mean weight diameter (M!{D) the implements were
placed in the following order (largest to smallest ¡4WD): mouldboard >
country plough > cultivator > disc-harrow > guntaka (a horizontal bladed
implement). Bhushan and Ghildyal (Lgl2) studied the effect of implement
shape and soil water content on seedbeds. Their results indicate that
as the radius of curvature of an implement increases, the percentage of
small aggregates (<1 nun) decreases while the percentage of large aggregates
(>52 nm) increases. A coarser seedbed was produced when the soil was
dry (5.6%). SmaIIer aggregates were formed under all treatments at the
intermediate rvater content (7.2e"). Bhushan et aI. (f973) also found
significant differences in aggregate size distribution with tillage
treatment. The largest aggregates vlere produced by the disc (19'l->52 rnm),
with smaller aggregates being produced by the mouldboard plough, harrow,
wedge and rotary tiller. Thj.s confirms previous results (GiII and
McCreery, 1960). They also followecl soil changes through a growing
season using wheat as a test crop. Greatest emeïgence was obtained on the
rotary tilled plot (2-6 mm) wíth the least kreing on the disc and mould'board
plots (L2.7-52.8 m¡n) .
Hoyle et aI. (L972) and Hoyle and. Yamada (1975) define a good
seedbed, as havíng 2/3 of the aggregates between 0.5 and lI mm, with the
remaining 1/3 being sma]ler a,nd larEer in size. They achieve this by a
process called "aggresizing" - rototillíng wet soil. they have found that
30
seedling eneïgence \^7as superior on t'aggresized" seedbeds to normal seed-
beds and that the emergence was more uniform, an important consideration
with mechanically-harvested crops.
In comparing rototilling with mouldboard ploughing, Zabashtanskiy
and Brazilevskiy (1975) found. that the rototiller decreased the percentage
of aggregates >IO mm and increased the percentage of aggregates in the
10-0.25 mm and <0.25 mm fractions in relation to the mouldboard. The
rototilled p1ots, however, caused a decrease in sugar beet yield compared
to the moulclboard plots.
2.LO Summary
The debate as to how much and what kind of tillaqe should be
practiced under various conditions has been going on for centuries and no
doubt will continue to do so in the future. In spite of this, the fact
remains that soil aggregate size distribution, as can be modified by
tillage, does influence the physical, mechanical and agronomic properties
of soil. The review has summarLzeð. the results of work in which the
effects of different soil aggregate sizes have been compared with some
reference to the tillage or other soil management practices which produce
them. This should enable a more intelligent discussion cf the 'optimuml
soil structure or aggregate size distrj.bution for the production of crops.
Experiments are often performed with beds of sieved aggreqates
having quite narïow distributions of sizes. These represent what are
possi-lcly the only reprod,ucible and well-defined soil structures and hence
provide the only means by which the results of experiments conducted in
different places and at different titnes c:¿rn be compared quantitatively.
In contrast, real tilths contain wide distríbutions of aggregate sizes and
vary enormously depending on the tlpe of tillage implement used and on the
3I
initial state of the soil, For this reason, the results from experiments
in which the effects of different implements on crop performance are
compared only have meaning for the precise conditions obtaining at
the time, and may not be generally va1id"
Beds of soil aggregates have differenÈ physical and chemical-
properties depen<l.ing on Èhe size of the aggregates. Consequently'
aggregate size influences the suitability of a seedbed as a medium for
the germination of seeds and for the development of roots. Many of the
requirements are conflicting. For example, adequate intra-aqgregate
aeration requires small aggregates wheleas adequate inter-aggregate
aeratíon requires large aggregates. The choice cf any aggregate size
range for a seedbed is a compromise between such conflícting requirements.
some of these conflicts could be largely overcome by having
stratifie<l seedbeds with different aggregate sizes predominati'ng at
different depths. However, the work that has been done on stratified
seedbeds is negligrible, and it is difficult to arrive at any firm
conclusion.
'2')
SECTION 3
PHYSTCAI. AND MECTIANICAL PROPtrRTTES OF AGGRT]GATES AND AGGREGATE BEDS
3.1 Int-roduction
Compaction of agricul-tural soils; is a continual problem and is
becoming increasingly serious with the trend toward larger and. heavier
agricultural machinery. Increased soil bulk density caused by compaction
can result in impeded root grovrth, reduced aeration, reduced water
infiltration rate, and often, in consequence, recluced crop yield. SoiI is
particularly susceptible to compaction when it is in a til-Ied and aggregated
condition.
Severa1 workelîs have investigated the uniaxial conpressj.on of
natural tilths (Scott Blair, L937¡ Scott Blair and Cashen, 1.938; Ku-ipers,
1958) . Day and Holmgren (1952) and lrÍclrlurdíe and Day (I958) oÌ:served the
effect of triaxial compression on l--2 mm aggregates from two soils.
Martinson and Olmstead. (1949) Looked at the strength of various individual
aggregates at various water potentíals. In orde:: to investigate the basic
mechanics of compression of aggregated soil, some workers have used
idealized soil-s composed of synthetic aggregates (Davis et s!., 1973i
Dexter, 1975). For detailed discussion refer to Section 2.3"
Maintenance of soil in an aggregat--ed and aerated condition is
r-Ìecessary for plant growth, and a knowledçJe of the l-oad-bearing capacj-ty
of aggregate beds is desirable if this condition is to be maintained. This
section attempts to extend the previous work to beds of natural aggrega-tes
having different diameters and water contents.
3.2 Agqregates and their strength
3.2.I Materials and Methods
The aggregates used in this study were from the sulface 1.ayer of
33.
the Urrbrae loam soil, which belongs to the Red-Brown Earth group (Stace
et. al . , 1968) . The aggregates were coll-ected by sieving into the for-rr
size ranges 9.5-5.1, 5.I-2.O, 2.0-I.0 and 1.0-0.5 mm. The aggregates
were not spherical, but had a ratio of najor : intermediate : mj.nor
principal axis lengths of 1:0.8:O.6 which was independent of aggregate
mean diameter, d. The aggregates were aír-dried and wetted to saturatior-r
by capi.Ilary action. They were then dried on pressure plates to the range
of seven matric water potentials shovTn in Table 3.I. This procedure
ensured that the aggregates all had a similar recent hi.story-
The tensile yield strength , Y, of the aggregates was measì.lred by
the force. F, required to crush them beLween flat, para1lel plates
(Rogowski , I964i Dexter , L975). The strength was calculated from a form
of equation 2.2 where Y = O.SIO F/a2 (3.I)
The mean diameter, d, was measured separately for each aggregate to
minimize the variations in the values of Y. Twenty aggregates were usecì
for each size range and water potential. In these tests, which were
performed with a O-2 kg top-Ioading balance raised at O.O2I ** "-l ir, -
Ioading frame, the aggregates were mostly ftatter-side-down on the balance
pan in their positions of greatest stability. No account of this effect
was taken here. For this soil, it was surprising that even the wettesÈ
aggregates failed in the brittle mode under axial loading. This illustrat-es
the degree of inter-particle cementing still effective in the 'undisturbed'
wet soíI. Aggregates were taken from the pressure plate in order of
Iargest to smallest for the Y determinations.
3.2.2 Results and Discussion
The values of Y obtained in this way were found to be independent
of aggregate diameter, to within experimental error, but highly dependent
on the soil water cotrtent as shown in Table 3.1.
34
v{ater content and rnean tensile yi-eld strength of 9.5-0.5 mm
agqregates as a function of their initial water potential.TABLE 3 .I
Water Content( % Dry !lt. )
3.r14.8
L7 .9
19 .1
22.7
29.6
34 .1
Matric PotentialkPa
S"E. of Y(kpa)
7
1.5
5
2
6
1.5
o.7
This is in agreement with Martinson and Olmstead (L949) who found a strong
dependence of crushing resistance with water conteni. The va.riability'
V, of the strengths of aggregates was measured by the coefficient of
variation of Y. Th-is Ís defiued as the standard deviation, as a proportion
of the mean va1ue. For the natulal aggregates used here, V = 0.48. This
may be compared with the value of V = 0.2 obtainecl by Dexter (1975) witJl
homogenous, synthetic aggregates.
In order to test more precisely the dependence of Y on d, the
value of m \^/as determined in the equation
F n< dm ß.2)
from the slope of a plot of logr F against 1og d. The value obtained v¡as
m = 1.98 + 0.07. This is very close to the value of m = Zl O.2 obtained
by Dexter (f975). The implication of a value of m = 2.0 ís that the
aggregates do not cc¡ntain any faults. such as cracks, of a size approaching
that of the smallest aggregate used. Rogowski (f964) assumed tirat the
aggregates he usecl contained large cracks and this led to the theoretical
predicticn of nr =. 1"5.
M¿trtir:sorr .tndOln,ste-acl (L949) measured the crushinq strength of
natural aggregates" To provide further informatj-ol1 on vaJues of m, thej.r
-50000
-1500
- 500
-I00-30
-10-0
84
30
33
2,3
27
2L
3.6
lensj.le StrengthY (kPa)
35
d.ata have been interpolated and fitted. as alcove" This yielded the value
m = I.7O + 0.05. This is significantly diffe::ent fr:om that obtainecl aboveo
and is intermed.iate between the present val-ue which implies no cracks, and
the theoretical prediction of Rogowski which implies Iarge cracks '
3.2.3 Further clcvelopments
Further experiments were conducted to determine whether or not
other soils confirmed the above results -
3.2.3.1 Materials and Methods
The aggregates used, in this study were from the surface layers of
the Allora, Bowenville (a variant of the l"lywy-billa group), Waco (from
Jondaryan) and Warwick soils in Queensland - they are al-l self-mulching
black earths - and Georgetown, Mintaro Red-Brown Earth., Mintaro Terla Rossa,
Northfield and Urrbrae loam (Red-Brown Earth Gr:oup) in South Australia
(Stace et a_1=. , 1968) . The aggregates v¡ere collected by sieving into the
five size ranges >9.5, 9.5-6.7, 6.7-4.O, 4.A-2.0 and 2.0-I .0 mm. The
aggregates hrere not spherical-, but had a ratio of major : intermediate i
minor priÏrcì-pal axis lertgÈhs of 1.OO:0.73:0.52 which was independent of
aggregate mean diameter, d. The aggregates vrere v¡etted to saturation h'y
capillary action and were t̡en dried on a pressure plate to -100 kPa matric
water potent-ial. Tilis procedure ensured that the aggregates all had a
similar recent history.
The tensile yielcl strength , !, or' twent.y aggregates of each size
range and soil was determined as above.
In addition to the method describetl ¿ùoove, t-vro soils (the Mintaro
Red-Brown Earth and Urrbrae loam) were tre¿ìt-.ed i1 the following way" The
aggregates were removed from the pressure plate in the order from smallest
to larç¡est for: the Y cteterminations. A fur'cirer replicate was done where
36
the aggregaLes \Àtere removed fr:orn the pressu::e plate anc1 placed' ln air-
tight containers and then use<l for the Y determinatj-ons ' These varj-ations
were employed to determine how differences in exper-imental technigue
affected the values of Y and m.
To investigate the internal crack pattern of aggreEates, three
aggregates (IO run diam. ) from each soil were impregnated with epoxy resin
under vacuum, and thin sections were cut wj.th a diamond saw. The
resulting thin sections were used as negatives to procluce photographs (plate
3.1). Enlargements (lo><) of these were used for the measurement of length
of cracks per unit ar:ea and nurnJ¡er of cracks per unit area. l'he area of
aggregates was determined by cutting and weighing each aggregate outline.
3.2 "3.2 Results and Discussion
The values of Y determined in this manner were found to be
dependent on aggregate diameter as shown in Table 3'2'
TABLE 3.2 Tensile yield strength of various aggregate sizes from
different soifs.
Tensile Yield Strength, Y (kea)
Aggregate Size Range (mrn)
Soil r .0-0 .5
Al-LoralBowenvil.Ie Iwaco I
warwicklGeorgetowulMintaro RBE1
Mintaro Terra RossaNorthfiel-dlUrrb:lae l-oam2Mintaro RBE2Mintaro RBE3Urrbrae l-oanr3urrbrae loaml975
I Samples l-eft on plate and removed in order largest-r'smallest for Y
determination.2- Samp]es left on plarte and removecl in orcler sroallest+largest for Y
d-eterminatiori .3 Sampl-es removecl from plate, placed in a-ir-tight containers' thc:t-t removed
fo:: Y deterinina.tion"
t
33 .86
l-4.497.369 .839.968.928. 14
11 .669.43
L9.40II "21t9 .17
6 .98
24.4816.95L3.44L4.28L6.20
9.O4L2.961.2.94A' E A
tB .5823.38r0 .5629 .95
24.53I5 .68L2.5418 .682L.88I1 .83I8.9325.9021 .4025.6620 "99L] .14
49.OI33.3724.2625.L652.152.O.7022.3L30.5330.5335 .0627 .8923.0336.84
63.8940 .8r52.5369.7465.7350 .4138.9643. B338 .0034.4330.4221 .8028.75
>9 .5 9.5-6.7 4.O-2 "O6.7-4.O 2 .0-r .0
37.
It was noted that the smalLest aggregates, especially of the
Queensland soils, failed in a plastic mode under axial loading. The
aggregates fail-ed with a rapid compression rather than forming distinct
cracks at the point of failure. Previously ít was noted that aggregates
failed in the brittle mode. The variabí]ity, V, of the strength of
aggregates \^ras measured by the coefficient of variation of Y. For ttre
natural aggregates used V = 0'48r the same as before.
To test more precisely tkre dependence of Y on d, the value of m
\^ras determined as in section 3.2.2. The values are presented in Table
3. 3.
TABLE 3.3 Values of m (Equation 3.2) and crack distribution in
various soils.
Soil Number of cracksper unit area (y)
AIlora IBowenville I
Waco IwarwicklGeorgetownlMi.nt-aro RBEIMintaro Te:;:a RossalNorthfieldturrbrae loam2lvlintaro RBE2Mintaro RBE3
urrbrae loam3
0.128o.166o.2460 .13I0 .2830 .0690 .134o.2L2o.L520 .0690.069o.r52
L ,2 '3 Refer Table 3.2
1'he value of m varied greatly between soils and was snall-er than
the previously determined value. The implication of these lower values of
m is that the aggregates contain many large cracks approaching the size of
the smallest aggregates used (2-I mm). The reason may be found in the
theory developed. by Rogowski (Lg64) which may predi-ct an m value that is
too high in relation to soils. The theory applies to relatively un-iform
I.27r.16L.23r.030.990 "96r .36L.23L.25I.43L.76r.63
0 .02650 .03790.0409o.02440.057r0.01400.o3050 .0370o.02470 .01400.01400.0249
m Length of crackper unit area,x (rnrn/mm2)
38
substances, \^/hereas soil is a highly variable medium. The va1ue of m
varied with differing experimental conditions. It is recommended that
in future work the aggregates be removed from the pressure plate and
placed in air-tight containers until required for the tensile yietd
sÈrength determination. Consistent results are obtained using this
technique.
An attempt was made to deveJ-op empirical equations relating m to
the length of cracks per unit area, x, and number of cracks per unit area,
y. The resulting eguations were
and
m = 1.5 - 8.2x,
m=1.5-I.3y
(t2 = 0.19)
(r2 = O.I4)
(3.3)
(3.4)
One would expect thaÈ as the length of crack and nurdber of cracks per
unit area increased, tensile yield strength, Y, would decrease. The
regression coefficients in the above equations are not significant
indicating that perhaps there are cracks which, although not visible on
the photographs (plate 3.I), are affecting Y and hence giving values of
m smaller than the theoretical minimum. In an attempt to resolve this
discrepancy, a small area from each aggregate was selected and enlargements
(x7O) made to try and discern further detail of cracking patterns (Plate
3.Ð. The eueensland. soils and the Georgetown soil show larger numbers of
small cracks, previously invisible, than do the Mintaro soils and the
Urrbrae loam. This may account for the lower than expected values of m.
possibly due to the paucity of the sample used to determine crack length
and munber of cracks in aggregates, an inaccurate result was obtained-
Further replication would be requíred to test this. Also aggregates from
each size range used should be examined, as above, for cracking patterns
to determine if the above resuft is a genuine effect.
\ir/arwick (QId. ) Waco (QId. )
MintaroRed-Brown earth (S.4.)
Bowenvif l-e ( Qld . )
MintaroTerra Rossa (S.A-)
. .:--
Ìi
Allora (Q]d. )
It
ceorgetown (S.4. ) Northfield (S-4,) urrbrae Loam (S-4.)
Plate 3. f Thin sections of aggregates used to determine theinternal crack patterns of the aqgregates (Scate x2) '
I¡larwick (Qld. ) vüaco (Qf d. )
Mintaro Red-Bro\^/n earth (S .4. )
Bowenvilfe (Qld.)
Mintaroterra rossa (S.4. )Arlora (ald.)
Georgetown (S -4") Northfiel,d (S.¿t.) Urrbrae I'¡oarn (S'A')
Enlargements of thin sections o-f aggr'ega'tes showing
internal cracking patterns (Scale x70) 'Plate 3.2
ao
3.2.3.2.L Development of a crack theory applicable
to soil
The following assumptions are made about the aggregate: cracks or
flaws of varying severity are distributed throughout the aggregate.
No interaction of these occurs. Each crack will propagate at some
critical tensile stress, T. Hence if Y is the tensile stress then a
cumulative probability dístribution function F(Y) exists such that F(Y) =
erob[f . Y], 0 < Y < * . This is taken to be the same for aI] aggregates
of the same soil type. The bulk specimen is considered to be composed of
r volume elements of individual volume Vo. Hence the total- vofume is
V = rV (3.5)o
Each volume element is considered to contain a large number of
cracks r and the st¡:engta of the volume element is determined by the
weakest flaw in it, given a uniform tensile strength throughout. Despite
the facÈ that the initial distribution of crack strengths F (Y) , 0 < Y < æ
is not known, the distribuÈion of the strength of the smallest value will
tend to the third asympÈote distribution of extremes (Gumbel, 1960) which
is
Pr (Y) = exp[- (Y/vo) k] ( 3 .6)
Yo, k are positive constants, Y > O and P1(Y) is the probability that the
maximum tensile strength at which fracture of the volume element occurs is
greater than y. For r volume elements in the bulk sample, the strength
of ttre bulk sample is
p (y,v) = prt (v) = exp[- v/vo {Y/vo)
kJ ( 3 . z¡
using (3.5). Hence t-he probal:ility' that the tensile yield strengÈh at
which the aggregat-cr -f r.tctures are less Èhan Y is
O(y) = t-p(y,v) (S.a)
and the distríbution
0 (vl = Õ' (Y)
The mean value of Y, Ï IS
+;,h tflk-lotvr
40.
(3.e)
(3.ro)
(3.rr)
" = ¡i YO (Y) c1Y Yo r (r+r/k)_L/v
where f is the gaÍma function (a constant for k constant). The variance,
ISo2,
2ú= "3,ä,-)Lr (1{-) 2rr{rJ?
k -f
A similar statistical analysis and examples for materials are given in
Freudenthal (1968).
Note that 0(Y) given by Iog(Y) is in fact a probability
distrilcution. That is, for aggregates of the same size a distribution
of fracture strengths occurs. For aggregates of varying sizes n plotting
log y against 1og V will result in a distrilcution of points aborrt the line.
1'ir,osi = loe[vovo!r(1+)] -f,r"sv (3.12)
Frorn (3.I0) and. (3.If) it can be seen tfrat $ is a constant dependentY
on k only, so that the scatter about the line (3.12) is indepenrfent of
the volume V. If k=3.5, which it is approximately for the aggregates
considered, then 9= 0.316. Hence a graph of the following type isY
expected.
Log Y
å\
Log V
4t.
that is logl, fog(l + o) and Iog(V - o) are parallel straight lines.
9epÐel!s--91-gvip9r i:nental errors
It may be that when the crushing force is applied, the tensile
yield strength wifi Ue effectively maximrrm only in part of the aggregate -
Èrue for elongat-ed aggregates certainly" Then V is, sâ!r effectively
ßV, where ß is a constant, O<ß<I. Then
IogßV = Iogß + logv (3.13)
so in fact ß does not af fect Èhe constant k, prov-ì-d.ed ß is the same for
aII aggregates. Similarly with the retaÈion tensile yield strength
y = o.slor/a2 (3"r4, 3.1)
the constant, 0.576, wiII not affect the value of k-
Inte t--ion of results (for the Northfield soil)
The average critical tensile yield strength I is estimated by
averaging twenty measurements for five volume sizes. The volume, V, is
taken to be a constant by the value of the mean diameter,
"ã3
TABLII 3 .4
$=
LogV=Iogc +31ogd (3.15)
Measured values of aggregate diarneter and tensile yield
strength for the Northfield soil.
d (m¡n) oy/v
12. 35
7 .91
5 .01
3 .06
r.76
0 .50
0.35
o.49
0 .48
0 .54
9.43
L2.98
25.9r
30 .53
43.83
4.7I4.60
t2.63L4.69
23.45
Y (kPa) oy (kPa)
o.47
¿.)
A best fit graph of log I against 1o9 d (Fig. 3.1) gives
togtoÇ = 1.88 - 0.806 lovroã (3.16)
so k = 3.72. (Note for 3.2<k<3.7, this distribution (3.9) is very
similar to a normal distribution). This constant k is essentially a
measure of tire dispersion of the strengths of individual- cracks. It
is found that for steel at low Èemperatures k is about 25, f.or ceramics
in the range 5-lO and fibres l-2 (Freudenthal, 1968) . The other
parameter, 1.88, is a measure of both the crack strength,s and frequency
(and any other parameters that may come in as discussed before) -
From (3.10) and (3.If) it is seen that
Z = frtrÉot - r2tréolJà (3.r7)y ro.Ðk
Given 9 = O.al (Table 3.4) , this impliesk = 2.25. This lowervalue occurs becauseY
o is higher than the distribution predicts because of some variation in
volume or effective volume itr each sample of 20 aggregates. Similarly,
if one calculates a 95% confidence interval for log i against tog ã,
then k = 3.72 gives 9 = 0.30. Since I i= irraupendent of V, the 95%YY
confidence interval will essentially consist of stra-ight lines parallel
to the best-fit line of the r,oVa.i - IoørOã graph (FiS. 3.1), and
separated from i'b by togro (1 f hl) , where n is the sample size, thaÈ
is by + 0.054 and -0.061. This range wilt be too narrovr because of the
variation of aggregate sizes in a sample of only twenty. The correct
varues to use wourd be rogro I lTgA where oy is the standard- y'nY
deviation of the sample size, taken as 0.47Í. This gives a 95% confidence
interval of +0.093 and -O"II8. These values are shown in Figure 3.1a.
Values of k, for each soil, and A from equation (3.16) are
given in Table 3.5 .
2-0
t.8
1.6
)o 1.¡f
1.2
lJt
0.8
1.1
1-2
t.0
0.8
a' NORTHFIELD SOIL
r : -0.936k = 3.82
a*rOV = 1.88 -0.S06Loer'ä
r = -0.977k = 3.72
aonr'V = 1.78 -0.784LoOrOd-
\I
0 0.2 0.4 ft6 0.8 1.0 1.2 t 4
b: WACO SOIL
:Y
2.0
1.8
t.6
oEDoJ
.=: ì
0 0-2 0.¿ 0.6 0.8 t.0 1.2 1.4
Loo d-10
Best fit lines for the relationshipf,og191 =[-Blog1gã
Fitted Line95t Confidence Interval.
rig.3.I
43.
TABLE 3"5
SoiI
AIIoraBowenville
Waco
Vùarwick
Georgetown
Mintaro RBE
Mintaro Terra Ros
l¡orthfieldUrrbrae loam
Parameters of the equation LogrOY = J\ - B Logt'd and
the derived value for k. The reqression coefficient
is given by r.
A
4.269
3 .712
3 .828
3.L94
3 .1r9
2.970
4.647
3 -723
8.460
k
Since k is a measure of dispersion of crack strengths and A is a rneasu:r:e
of both crack strengths and frequency of cracks, it can be concluded that-
the aggregates have variable strengths, implying they contain cracks of
varying strengths and of different frequency distributions. The values
of k are similar to those found for ceramics.
3.3 Compaction of aggregate beds
3.3.1 Materials and Methods
Beds of aggregates of each size-range and. water potential were
made by pouring the aggregates into cylindrical compression cells 80 mm
in diameter and 100 mm high. The beds were stressed uniaxially in a
Ioading frame by sÈraining at a constant rate of O.O2I mm s-1. The
stress was measured by a proving ring as shown in Fig. 3.2- Compression
of beds of each size-range and water potential was repeated two or three
times.
The changes in height of the beds were recorded as a function of
r.970L.862
I .783
I .995
2-086
r.899
I.723L.877
I .690
-o.967
-o.942-0 .936
-0.986
-0 .984
-0.975
-0 .988
-o.971
-0.900
o.7027
0 .7953
0.7836
o.9344
o.9618
1 .0100
0.6401
0 .8058
0 .3550
B T
/
ING RING
Apparatus for measuring uníaxial compressionbehaviour of aggregate beds (not to scale).
R
Fig. 3.2
44
axial- stress, P. The height was r:endered diinensio¡rless by using H/Hi,
where H is the heigtrt and Hi is the initia.I he-ight of t-he sample, and
the stress was rendered dimensionless by using tr/l:, rvhere Y is the
tensile yiel.d strength of the individual aggregates. An example of
these measurements is shown in Fig. 3.3.
Beds of aggregates (9.5-6.7 mm) were also made by pouring the
aggregates into a rectangular ceII I53 mm long, 65 nim high and 65 mm wide-
The beds were stressed rrniaxially in a loading frame as above to 0, 10 '
20 and 3Os" of their original height. After each trial, the bed was
impregnated with molten wax. Iemoved. fl:om the compression cell- and
sectioned J-engthways. The distribution of voids and aggregates was
measured according to the method. of Dexter (L976) but modified such
that the structure on the soil sections was analysed at 0.5 mm intervals
ínstead of at 1.0 mm intervals as used previously.
Beds of air-dry aggregates (>4 mm) and aggregates at -10 kPa water
potential were made by pouring aggregates into a glass-sided comp::ession
celt I0O x tOO mm. The beds were stressed uniaxialty as described
previously and the change in appearance recorded on cine film using 1
exposure every 5 seconds.
3.3.2 Results and Discussion
The measured data could not be fia-
by a single exponential equation'
was used:
ed within experimental error
following empirical equation
H/ni = A+8."p[-p/v) 1Ð'f (3.ra)
Here A is the val-ue of'Ht'Hi at P = æ, B = 1-A¡ and a and b are adjustable
paranÌeters describing how rapidJ-y H/Hi decreases with increasing P,/V.
A was almost constant at 0.4. The agreement between eguation (3.I8)
and the experimentat data can be judged from l'ig. 3-3-
H
H¡
1.0
0.8
0.6
o
o
0 40 60 80 100
Compression cu.rve for a bed of 9.5-5.1 mm aggregates at -I0 kPa water potentíal.o Measured- Computed from Equation 3.I8.
120PY
FÍg. 3.3
20
Tab1e 3.6 shows the r¡a}.res of the Parameters a and b computed
45.
-1500 and -500
from the data.
TABLE 3.6 Values of parameters of equation (3'fB) "
Water Potential (kPa) 0
bParameter
Large(9.5 - Z.O mm)
0.380
Small(2.0-0.5 run)
c.310
Typical S.B. o.o2
For simplicity of presentation, the data have been grouperl into
four di.fferenb water potentials and üvo different size-ranges ' The values
of a and b sholvn are the means of the individual values included -in each
gïoup. The first conclusion to be drawn from Table 3.6 is that there is
no significant difference in the compression behaviour of becl-s of Cifferent-
sized aggregates.
There are differences in the compression behaviour wj-th diffe::ent:
aggregate water potentials. The most notabl-e are the smaller values of a
and b with the saturated aggregates. This rnay be a genuine effect or may
be a consequence of the difficulty of measuring Y with the saturat'ed
aggregates. At. -lo and -30 kPa water potential, the values of a and b appear
to be a maximum. For aggregates drier than -IOO kPa, there aplrears to be
no si.gnificant change in compression beha.viour with water ct>ntent.
fn view of the small changes in a ancl b for unsaturated soil, it'
is possible to write a representative cornpaction ecluation
u/si = 0.4 + 0.6 exp[o .oLle/v) - 0.38 P/Y)'f (3.re, 2.4)
c .0060.0003 c "030.003 o.o20 .001 0.003
0 .0025
0.003r
o.o24
0 " 021.
0.r15
o.L27
0"0r2
o - 01.5
o .490
0 .410
0 .01.0
0 .0190 .350
0.320
ba ba ba a
-10 and -30 -100 and -500
46.
which nay be used as a working equation to describe the uniaxial
compaction of beds of aggregates of any size arìd water content as a
function of dimensionless uniaxial stress.
The results of varying levels of compaction on the internal
structure of a bed of air-d::y aggregaÈes are shov/n in Table 3.7 and
plate 3.3. The results in Tabte 3.7 are the means of those of beds of
sieved aggregates of 6 .7-9.O mm and of beds of aggregates >4 mm as used
in the f-ie1d work (Section 7). These two size ranges gave very similar
results. The porosity and pore-size distribut--ion here only refer to the
inter-aggregate (macro-) porosity in pores larger than 0.5 mm.
TABLE 3.7 Void proporiions in a bed of aggregates after various
Ievels of compaction.
Greater ThanSize (mm)
Void Proportion
? Compaction
30
0.5
I2
4
B
16
Linear PorosityMean AggregateSize (mm)
0.800
0.531
0.193
0 .045
0.002
0.000
0 .120
L2.40
Values are means of 8 lines of measurement from the
impregnated aggregate beds.
It must be remembered. that these void and aggregate sizes are intercepted
lengths and not diametersas obtained by sieving, however, no information
about void size or distribution is obÈained by sieving. lt can be seen
5.70
0. 359
9.63
o -L75
6.67
0 .303
0 .855
o.694
o.469
o.252
0 .078
0.008
0 .783
0 .568
o.299
0 .088
0 .0I70 .002
0.880
0.740
o.476
o.203
o.o42
0.o02
0 20l0
Plate 3.3 Internal structure of a bed of 9.5-6.7 mm
aggregates after various levels of compaction.
From top to bottom 0, 10, 20 and 30% compaction.
(Scale x 0.75)
0t
0z
0l
0
41 .
from plate 3.3 that the voids tend to get smaller as compaction increases.
The aggregate centres move closer together. This can also be seen from
Table 3.7 where the linear porosities decrease with compaction and mean
aggregate size increases because two aggregates pressed together woul-d be
counted as one large aggregate. At any level of compaction the proportion
of small voj-ds is larger than that of large voids. However' as compaction
increases the proportion of large voids decreases more rapidly than the
proportion of smafl voids.
If one assumes that no intra-aggr:egate compression occurs, the
change in porosity can be calculated for each level of compacLion
Oa-b1e 3.8). These can be compared with the porosity measured from the
compacted beds of aggregates @i9 . 3.4) .
TABLE 3.8 A comparison of measured macro-porosityr l¡, and that
predictedr lçr on the assumption of no inÈra-aggregate
compression during aggregate bed compaction.
% Compaction
0
IO
20
30
H/s¡ nc
o.359
0.288
0 .199
0.084
H- = proportion of aggregatesCl
Hv = proportion of voids
rlIJ = measured macro-porosíty
rìc = calculated porosity
The above assumption implies that with increasing compaction the
aggregates remain intact and only the inter-aggregate porosity will
decrease. This is the case as ill-ustrated in Fig. 3.4. If however,
compaction caused a proportionate decrease in intra-aggregate porosity
o.64L
0 .641
0.64L
o.64L0.7
0.8
0.9I.0 0 .359
o.259
0 .159
0 .059
0.359
0.303
0 .175
o.L20
vHHa ll
0.4
0.2
0.1
REFERENCE POINT
'ro%
ozo %
ogo %
0.1 0-2 0.3 0.¿l
MEASURED Qr
Comparison of measured and calculatedmacroporosity in beds of aggregates atvarious leve1s of compaction, assuming
No Aggregate CompressionInter-aggregate Pores and aggregatescompressed equally
. Comparison of measured porosities withcalculated values at 0, 10, 2O and 30tcompaction, assuming no aggregatecompression.
o%
0.3(,
o¡¡¡t-JfC)
(J
Fig.3.4
48.
(the aggregates compressed) then the measured macro-porosity woulcl remaj-n
constant as the total volume of the bed dec::eased. This is not the case,
thus indicating compaction is accounted for almost entirely by eliuuinaticrr
of inter-aggregate porosity. Day and Hohngr:en (1952) and Mcltlurdie and
Day (1958), compressing 1-2 mm aggregate beds triaxj-al.Iy, observed a
similar decrease in pore space. However, they used aggregates wj-th a
higher water content than tllose i.n the present st¡dy" The aqgregates
tended to fail in a plastic mode, whereas those in the present study tend
to remain as separate entities.
Wt¡en beds of wet (Vm = -lOkPa) and dry (Vm = -500001<Pa) aggregates
were stressed uniaxially in the glass-sided ceII , the aggregates \¡¡ere
seen to rotate and reorientate rather than break down into small-er units.
Dry brittle aggregates rotatecl as they moved cfoser togetherr' they cìid
not ::upture completely, as previously assumed, but as compactio¡r increased,
wore progressively by mutual attrition of areas of mutual contact. The
appearance of the bed became more uniform as compaction progressedr however,
ftat interfaces were evident between adjacent aggregates.
With wet aggregates, Iess rotation and reorienbation was evident
as the aggregates moved cfoser together. Areas of mutual contact clid
not wear, as with dry aggregates. Flat interfaces developed more readily-
Aggregates did not appear to fai1, but merely compressed as compaction
progressed. The bed had a uniform appearance, although the flat interfaces
were still evident at the end of the compaction. With both wet and dry
aggregates large poïe spaces decreased in size as the aggregates moved
closer together. Similar interfaces \,sere observed by Day and Holmgren
(Lg52) and llcMurdie and Day (1958). These observations tend to confirm
those of Day and Holmgren (L952) and McMurdie and Day (1958) . They al-so
Iend suppor:t to the results shown in Plate 3.3, where pore space is
reduced with increasing levels of compaction.
49.
3.4 Conclusions
Initial-ly it was forrnd t-.hat aggregate tensile yield strength' Y'
was independent of aggregate diameter. However, further experimentation
has shown y to be depenclent on aggregate diameter for rnany varied soil-s.
As yet no explanation can be offered for thís difference.
The aggregates for the 1975 experiment rvere coll-ectecl from what
was once a vehicle track. Whether or not vehicle co:rrpaction caused
aggregates to form rvhích contained few cracks is yet to be determi.ned.
Flowever, the remaíning aggrec¡ates wel:e collected, from areas either under
crop or pasture. This may have an effect on cracl< number and distribution'
due to che pres;ence and action of plant roots. Thus these different
treatrnr:nts, prior to ter.sil-e yield strength determination, may cause
the diameter dependence, even though precautiolls \^¡ere taken to see all
aggregates had a similar recent history. Further work needs to be done
to elucidate the effects of cracks and their distribution within aggregates
and between different aggregate sizes -
The tensile yield strength of aggregates is highly dependent on
so-i-l water pot-entj-al .
There is no significant difference in the compaction behaviour
of beds of different-sizecl aggregates. Since tensite yield strength is
dependent on diameter, this suggests that becls of small aggregates would
compress less than beds of large aggregates and could support larger
Ioads with less damage.
There are differences in compression behaviour of beds of
different-sized. aggregates r^'ith different water potentials-
pore space decreases and aggregate centres move cl-oser together
with increasing levels of compaction.
50
SECTION 4
EVAPORATION FIìOM BEDS OIl AGGRþ-ìGATES
4"I Introduction
Rohwer (1931) studied the evaporatíon frorn an open water surface
and produced an empirical expression for evaporation rate, E
-'lE = a(b - cB) (d + fu) (es - eU), mm d.y ' (4.1)
where ã, b, c, d and f are constants, u is the wind speed measured at
ground level (t<m clay-I) ancl e= and ed are the saturated vapour pïessu-res
(kPa) of water at the surface and at the dew point respe:tivc,ly. B is
the mean barometric pressure (kPa). The term (eu - .¿) is called the
saturation deficit-.
Penman (1948) also derived an empirical expression for the
evaporation rate, E
-'ls = (itÀ + Ea1)/ (À + y), mm day - @.2)
where H is the net radiant energy available at the surface, Y is the wet
and dry hygrometer constant, ^
= dea/dTa (where eo is the saturated.
vapour pressure of tlre water in the air at temperature Tr) and E¿ =
(.. eU) f (u) (where eU is the saturatecl vapour pressure of v¡ater at the
dew point). Penman's equation is a comJ:ination of the ¡sink. strength'
(Rohwer, 1931) and 'energ-y balance' equations conìmonly used to determine
the evaporation rate, E (Rose, L966) .
The steady-state upward flo'vr cf witci' fron a water-table through
the soil profile is given by Garclner (fg!;B) .
E=-KdYm/dz-K (4"3)
where E is the evaporation rate (mm day-r), K is the hydrauric
conductivity in the same units as E, .td dV*,/d z ís the dimensionless
5r.
matric potential gradient. Equation (4"3) is often wr-itten in the form
E=-D(o)dol¿z-r< (4.4)
where o(0) is the diffusivity
e). Integration of equation
and soil-water properties
(a funct-ion of voh:metric rvater content,
(4.3) gives a relation between soil depth
(4.5)
The solution of equation
dependence of K on Y*.
(4.5) must be based on knowledge of the
Gardner (1958) derj-ved an empj-rical" egua'Lion
r(vm) =v*+b , G-(,,)IM
where a, b and n are adjustable parameters which must be determined for
each soil. By ignoring b, Gardner obtained the function
A.aE = Ø.7')-max dn
where d -is the depth of the water table, a and n are the adjustal¡le
parameters from equation (4.6), A is'a constant which depends on n and
E is the maxirnum rate at which the soil can transmit water from themax
water table to the evaporation zone at the surface.
Evaporaticn rate can be limited by external (nteteo::ol.ogical)
conditions or by the maximum rate at which the soil can transmit water,
whichever is less. Where the water table is near the surface the
water potential at ttre soil surface is low and evaporation rate is
determined by meteorological conditions. Holnlever, as the water table
becomes deeper the water potential at the soil surface increases, the
evaporation rate is limited by soil conductive pro.perties.
Evaporation of soil water after wetting may be characterized by
three stages (Lernon, 1956). The first stage is controlled by meteorofogical-
conditions and lasts as long as the soil profile supplies water to the
evaporaling surface at a rate satisfying the eva.porative potential demand'
'= -J#Tt*,av*
tr .)
As the soil d::ies orrt I waLer cannot be supplied at the potent--ia1
evaporative t:äte and the second stage starts, during which the evapol:at-jol)
rate falls rapidl-y. When evaporation reaches a low, Yet f;rirly cor-ìstant
rate, a third stage may be distinguished.. During the last two stag'es,
the evapor:atiot't process is governed by the soil hydraulic properties.
Theoretical considerations of Hanl<s and Gardner (1965) suggest that the
t-.otal water loss during the first s'Lage of drying may be r:educed if the
water diffusivj.ty in the wet ¡ange is reduced. This can be achieved by'
for example, tillage opei:at"ions. Thc¡ secotrd stage of dr:y-ing may also be
affected. Tþe desired surface and profile conditions must be ¿chieved
before the applicat-ion of water, be it rainfall or irrigation lvater
since, after wetting, no mechanical operations ean be perfor¡ned until the
Suïface dries. by which time ¡1ost of the v¡ater loss has occurrecl"
Till.age causes the formation of an aggregated top-layer of
relatively high porosity overlyi.ng undisturbed subsoil - Field studies
(Atlmaras, l'967) concerning the possif)le effects of tillage on water
loss by evaporation have yielded inconclusive results, due to difficul'ty
in interpretatíon. The field data exhibit interactive effec'ts of site,
infiltration and. non-isothermal conditions in addition to eva¡:cration'
Several workers have overcome this problem to a certain extent by working
with layered and non-Iayered soils under isothermal and non-isothermal
conditions in the Iaboratory (Vfillis, Lg6O; Holmes et aI-., L96O; Hadas
and HiIIel, L972¡ Hillel and Had.as , Ig72; Hadas , Lg75). Kimball- (1973)
has extended this work by looking at the effect of artificj-al aggregates
on evaporation loss in a field situation.
I{illis (1960) and Hadas and IliIIeL (L972) found that evaporaEion
i-s reduced when a coarse-textured soil overlies a fine-textured soif '
In considering specific aggrega'Le size ranges that exhibiL minimrm
evaporative fcsses under bot-h non-isotherr¡al ancl isothermal conci-i'tions,
53"
Holmes et al-" (f960) found a size of 2.5 nrm to be effective, while Hillel
and Hadas (Ig1 2) found the -l-0.5 mm rîange to be effecÈive. Hadas (L915),
similarty, found the 2-0.5 mm range to be effective in reducing evaporative
Iosses.
Hillel and Hadas (1972) found that a clept'h of 3-6 cm of aggregated
soil was necessary to reduce evaporabion. Hadas (1975), however, found
a 5-tO cm depth to be necessary. This may reflect the use of isothernial
and non-isothermal conditions Iespectírrely by these workers. Evaporative
Iosses in the field may be markeclly altered (Al-l-rnaras et. al ., 1977) by
tillage, provided these tillage operations are perforrned prior to wetting
of the soíJ-, and the aggregate size distribution is beLween 0., I-5 mm a.nd
the depth of the aggregates is optimal (10 cm).
Hanl<s and !'Ioodruff (1958) found the evaporation rate, from a wet
soil underneath a dry mulch, increased two to six times when the wind
speed increased from O to 40 km Ìrr-1. Hadas (1975) observed that
cumul-ative evaporation from sieved aggregates increased with t'¡ind >
continuous radiation > intermittent radiation over a thirty day period.
KinbaII (1973), howeve::, found little oï no correlation between wind speed
and evaporation rate. Cary (Lg67) concluded that the most effective
method of reducing evaporative loss depended in part on the vapour pressure
of water in the air above the soil surface. If the vapour pressure is
relatively high, evaporation is best reduced by screening the soil surface
from incoming radiation so that it does not heat to Èhe point where its
vapour pressure ís greater than that of the air at the surface - If,
however, the atmosphere is dry, evaporation control will require a
reduction in the coefficient of transfer of water vapour to the soil
surface. This requires that the soil be modífied to limit vapour transfer
within the profile.
Acharya and Prihar (1969) observed that evaporation through beds of
q¿.
aggregaLes re]-ative to potential evaporation was a linear function of
bed thickness, except where porosiì-y was >0"62 and wind speeds wer:e
greater than 3 m sec I.
It has been suggested that certain sizes of aggregates and
particular thicknesses of aggregate beds are more effective in reducing
evaporation foss than others. However, there has been little work done
on the effect of aggregate size on evaporation l-oss r¡nder field
conditions, the above recommendatj-ons being based largely on laboratory
exper:iments.
This work ,examjnes the effr:ct of aggregate size, surface crusting,
compaction and meteoroiogical factors on evaporative losses t-hrough beds
of aggregates.
4.2 Materiafs ancl Methods
The aggregates used in this study were sieved from the surface
layer (top l0 cm) of the urrbrae loam (stace et al.' 1968). Aggregates
were collected by sieving into the following size ranqes z >4, 4-2, 2-L,
1-0.5, O.5-O.2, O.2-O.1 and O.l--0.007 mm. fhe <I mm treatment contained
equal proportions of l-0.5, O.5-O.2, 0.2-0.1 and 0.I-0.007 mm aggregates'
Ten sintered g]ass funnels (IO cm diam) (P1ate 4.IA) were set up
in the Waite Agricultural Research lnstitute's meteol:oJ-ogical station'
They were at a height of 80 cm above ground, with a 75 cm water: column
(the equivalent of a shallow water table with unlimited v¡ater supply)
(pla.[e 4.I). Experiment l: the following aggregate sizes were pourecl
intc¡ the funnels and levelled with the top of the funnel, )4, 4-2, 2-L,
<1 mm and >4 overlying 2-1 rnm. This gave an aggregate bed height of 9 cm.
'Io extend the investigation further another experiment was set-up to
test the effect of smaller sized aggregates on preventing evaporative
loss under field condii:ions. Experiment 2: tÌle following aggregate sizes
Pl-ate 4 .l Evaporation through beds of aggregates
A Funnel with aggregates
B l,flater reservoir with oil on surface to
prevent evaporat-J-ve losses.
trñ
weïe poured in.Lu the funne|s and l-evelled with the top, l-0.5, O"5-O.2'
O.2-0"L, O.l--0.00'7 ancl <1 mm" A further experimenì- was undertaken to
examine the effect of a compaction treatment on evaporative J-osses.
Experiment 3: the following aggregate sizes were poured into tkre funnefs
and levelred with the top, )4, 4-2, 2-]-,1-0'5 and 0'5-0'2 mm' The beds
were compacted to a height of I cm. This 11s" compaction corresponcls
approximately with the l.Os" compactj.on treat-ment mentioned iu section 3 and
to the B% ccmpaction produceil in the field e>çeriments (Sectio¡i 7) " In
all experj.ruents one-hal-f of 'Lhe aggregate bed:' v¡e::e cr:ustecl b1'spraying
with water from a height of 2Ù cm wi'lh a hand spray. Piastic bags
were placed over the funnels for 24 inr, to all-ov,r equilibration, and then
removecl. No rain was recorded for the experimental periods used. Oil
was fl-oated on the surface of the 250 ml- reservoirs (Plate 4.18) t--o
ensure that aII water loss occurred through the aggregate beds. The
experimental per:iods consisted of 17, 12 and 19 days for ex¡:eriments 1, 2
an<l 3 respectively during January, February, March and April L917.
Evaporation \^ias ïecorded as the volume of water lost durirlg the
preceecling 24 ]¡rs at OgOO each morning of the ex¡ierintental periocl' Water
losl- from t-he reservoir was replaced using a syriuge, after a rea'1i'ng
had h¡een recorded. Air temperature' relative humidity, wind speed an(l
pan evaporation (class A) were recorded at the same tíme'
Recorded evaporation values \^/ere converted from ¡n-illilitres lost
per day to mm auy-I (E) by dividing by the surface area o-f the funnef.
These values were rendereddimensionless by divi,l.Lng l:y llo, the class A
pan evapor-a-Èiorr for the same period., to erlable comparisons !o be made
between different periods " E/Eo was called the "evaporation ratio", Bv'
If meteor:ological conditions affect E and Eo equally, tJren Ey
would be a constant for each aggregate size and treatment cornbinat-ion.
Hov¡ever, it v.,a-s founcl t-hat alt-hough much of the variation in E was
56.
ïemoved by dividing by Eo, the resulting ratie¡ Eyr v¡¿is still. slightly,
but noÈsignificantly, dependent on meteorological conditions. In ord.er
to attempt to obtain an empirical expl:ession for this clependence,
regression equations of the form
Bv=â(1 +bu)(ns-p.) +k (4.8)
were developed. Flere, Ev = E/Eo, â, b, and k are adjustable paralneters'
u is the mean wind speed for the recorded 24 hr period (]<n frr-I), and po
(kPa) and p^ (kPa) are the vapour pressures of water in Èhe soil and
air respectively. Tire major assumption made in calculating ps rvas that
the soil temperature of the aggregate beds would be close to that of
the surrounding air because of the experimental set-up. It is realized
this assumption is untenable on physical grounds, but it enabled an
estimate of E., to be made using simplified meteorological paramet.ers.
The effects of aggregate s:-ze, surface crusting and compaction were
evaluated. The following equation was used to calculate p=, t-he vapour
pressure of water in the soil air
P" = P6exp(-Mvm/'pRT) , kPa (4'9)
hrhere po is the saturated vapour pressuz€': o-f free vater (kPa) at the
same temperature, M is the molecular weight of water (I8 kg), Y* is the
matric potential of the soil water (assumed to be 7.5 kPa), p is the
densiLy of water (I0OO kg m-3¡, T is the absolute temperature (oK, assumed
to be equal to the air temperature), and R is the gas constani (8.314 x tO3,¡
-'l -1K ' kg-mole *).
T'he vapour pressure of the atmospherêr pa, was determined using
the following relation
Pt/po = h
where po iu the saturated vapour pïessure of free water (kPa) at the
(4 . ro)
57.
same temperature a.nd h is the relative humidity. Valtres of po were
obtained from tables in the Handbook of Physi.cs and Chemistry (Weast,
re73) .
4.3 Results and Discussion
Evaporation of v.Tater from the soil is characterizecl by three
stages (Lemon, 1956) . Since soil conditions affected evaporat--ion
(Tabte 4.2), evaporation was probably in the second of Lemon's three
sfages.
A sunmary of analysis of variance of the evaporation ratio (Ev)
for experiments I, 2 and 3 is given in Table 4.1.
Points emerging from Table 4.1 are that aggreç¡ate síze, the
presence of a soil crust and the interaction of aggregaÈe size and crusb
all significantly affect the evaporation ratio (Ev) and hence the
evaporation rate (E : Ey.Eo) in alt three experiments. Íhere is a
significant interaction between aggregate size and vapour pressure
deficit (n, - n.) and aggregate size and wind by vapour pressure deficit
(UPDIF) in experiments I and 2 on]y. The vapour pressure deficit (ns - n¿)
and wind speed (u) did not significantly affect the evaporation ratio.
4.3.I Effect of aggreqate size
The effect of aggregate size on evaporation ratio (Ev) from beds
of aggregaÈes is shown in Fig. 4.I. A minimum evaporation ratio occurs
with the 2-I rnm diameter aggregates. This agrees closely with the range
deterrn-ined by Holmes et aI-. (1960) , Hillel and Hadas (L912) and Hadas
(1975) for isothermal and non-isothermal conditions in the laboratory.
It also agrees wit-h the value determinecl by Kimball (I973), using
artíficial aggregates under field conditions, to limit evaporat-ion'
Aggregates smaller than I mm apparently allowed greater evaporation due to
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.t
l¡J
0
42I 5 62-l
d 1-.¡
Effect of aggregate diameter (d) on evaporation ratio(E") through beds of aggregates.
31-2
7>1>g
2-1-l
Fig. 4.1
TABLE 4.I Evaporation ratio (E,,) - swrunary analysis of variance for experiments 1, 2 and 3-
Treatment
Aggregate Size (Agg.)
CrustAgg. - Crust
vlind (u) -Vapour press .
deficit (Bs-pa)
Vapour press.deficit(r"-n.)Ags. - (ps-pa)
Agg. - (UPDIF)
Crust - (ps-pa)
Crust - (uPorr)
Experiment 3
CompactedLarge & Small
dLeS
% Varn
6L -2
75 -5
80.2
0.5
1.0
80 .5
80.3
80 .4
80.4
('læ
DF
I
4
1
4
1
4
4
II
F
LL7 .9
5.4
9.0
0 .07
4.5
5.7
0.8
0.3
r.8
TeSt
2.4L*+
3.9*t2.4*+
NS
2.4*+
2.4*+
NS
NS
NS
ts Varn
Experiment IUncompacted
Large Aggregates
-0 .5
80.3
82.2
82.5
82.4
-0 .1
73.7
74-4
78 -6
F
1r5.6
25.3
L7.5
1.3
4.5
9.8
0.0
0.6
3.2
Test
2.5*l4 .0*f2.5*+
NS
2.5*7a q*+
NS
NS
NS
% Varn
ExperimenÈ 2
UncompactedSmai-l Aggregates
a.2
90 -9
93.2
93.L
93 .1
2.O
80 .1
83 .6
89 -7
F
74.2
LO1 "6
11.7
2.9
L.7
0.5
2.4
o.2
0.0
Test
2 ¿i.+
4 .0*+
2.4*i
NS
NS
NS
NS
NS
NS
* Significant at p=0.05 t Significant at P=0.01 NS = Not Significant
59.
better water conductance, whereas larger aggregates allowed greater
evaporation, possibJ-y because of better penetration of turbulent air
currents into interaggregate pores (Han]<s and Woodruff, I958; FIoImes
et_ al., 1960; Farrell et al, I 1966; Kimball and Lemon, 1971) "
A stratified aggregate bed (>4 rnm overlying 2-I mm aggregates)
resulted in an evaporation ratio less than that of the 2-l mm aggregates
(nig. 4.1). This tends to confirm the conclusio¡rs of Jchns.;on a¡rd
Buchele (196f) and Johnson and Henry (1964) that a stratifieJ seeclbed
may be the best compromise to ninirn-ize water loss and promote seedlJ.ng
emergence.
4.3.2 Effect of compaction
A compaction treatrnent imposed on each aggregate size results in
an increase in evaporation ratio (Ev) (nig. 4.2) . The nr-inimum
evaporation ratio occurs wit]l the 4-2 mm aggregates, which is slightly
Iarger than that of the uncompacted aggregates. The increase in
evaporation ratio is due to the increased area of contact between the
aggregates. This enables greater conduction of water through the bed
and hence great-er evaporation. It was observed in Section 3.3.2 that
during compression, flat interfaces developed between adjacent aggregates.
Sinilar observations were made by Day and Holmgren (1952) and McMurdie
and Day (1958). Convection of water vapour, however, would become less
important as a transport mechanism. The trend of the compacted graph is
the same as that for the uncompacted aggregates, consequently a sirn-ilar
argument would apply in explaining why aggregates smaller and larger than
the 'optimum' size result in greater ewaporation ratios and hence
evaporative losses.
These results, however, show the opposite trend to those of Johnson
and Buchele (1.961) and Johnson and Henry (f964). They found that a
l¡I
1.1
1.0
0.9
0.8
0.7
0.6
0.5
04
0.3
0.2
0l
0
65421 32-l 4-2
d (--)
Effect of aggregate diameter (d) on evaporation ratio(Ev) from beds of uncompacted (o) and compactecl (o)aggregates.
7<l
Fj-g.4.2
>a
60
compaction treatlnent reduced soil dryi.ng, with the implication that
evaporative -losses woufd al-so be reducerf by thc colnpaction breatment.
This conffict in results may be due tc> the fact that the above workers
were measuritlg drying Lates by rveighing, in a system where water \¡¡as
Iimited, whereas in the present stucly evapo::ation was measured as a
vol-ume of water lost, -ì-n a system wiflr uni-imited water. The above
study was undert.il<en in'bhe openr "\thereas
Johnson and Buchel'e (196I) and
Johnson and Henry (1964) wor:ked under controlled conditions. Johnson
and Br;chele (f96f) and Johnson and Henry (L964) achieved the lar:gest
redrrction in drying rate by using a compacted layer which recluced vapour
transfer and capillary rnovement in a stratified treatment.
If the compaction treatment was applied when the Èggr-egates \.{ere
dry a reduction in evaporation loss or drying rate coul<1 occur" This
would be due to aggregate breakdown and infillíng of pores with broken
material creatj-ng only point contacts bethleell aggregates. This would
effectively red¡ce both convection of water vapour and water conduction
t-Jrrough the bed of aggregates. As mentioned previously sol-1 properties
and meteorological corrditi.ons v¡ilf affect evaporation or d::ying r:ate'
These differences may account for tire opposite effect of the compaction
treatment between the above result and. that of Johnson and Buchele (1961)
and Johnson and Henry (L964).
4.3.3 Effect of a soil crust
vühen a crust was formecl on each aggregate bed, the evaporation
ratio (Ev) vfas reduced to approximately half that of the uncrust-ed bed
(Fis. 4.3). The evaporationr E, from beds of aggregates can exceed that
from a class A pan, Eo. Consequently the ratio E/no : Bv can exceed
I.O as is the case for the u¡rcrusted aggregates (I'iS' 4"3) ' this is
because w.ind convection t-.hi:ough the aggreg;rie bed can give a larger
1.3
1.2
1.1
1.0
0.9
0.8
0.7
r¡¡ o.o
0.5
0.4
0.3
0.2
0l
0
o,t,,,,
IIIIII __-_-- oIIIo
oo
I 2 31-2
54 6 7G1
Fig. 4.3
2-1 >4
d 1,n-¡
Effect of aggregate diameter(d) on evaPoration ratio(Ev) through beds of uncrusted (o-o) and crusted (o---o)aggregates.o uncrusted >4
o crusted lJ Ereaument'
6l_
effective area for: evaporaÈion than the sur:face of the Ì¡ed alone and
also because the surface aggregates would be at a higher temperature
than the pan water surface. Godwilr and spoor (I9fi) observed a similar
result with evaporat.ion of rvater from different soil. til Lhs -
The crust acts as a barrier to vaPour transfer, as a barrier to
air convection currents, ancl also insulates the bed frc¡m incoming solar
radiation. The latter reduces the heat avail-able for evapol:ation,
thereby re<lucing water losses" Bresler and Kemper (1970)rworking with
soil- co¡¡nns in the laboratory, demonsi-.rat-ed sirnil¿:r <1:r-fferellces in drying
rates which were associaÈed v¡ith soil cïusts. The porosity of the crust
is smafler than tha,c of the aggregate bed below (Sect-ion 5.3.2.6¡
Bresler and Kemper, LgTO). Hydraulic conductivi.ty in the crust- would be
greateï than that at the c¡:st-aggregate interface duri-ng the ini"Cíal stage
of drying as the larger pores at the interface dry. consequently, the
crust-aggregate interface would. be unable to transmit sufficient water to
meet evaporatíve demand, causing pores in the crust to empty and dry
rapidly. This results in the crust acting as a dry, physical barrier to
further rapid water losses. Cary and Evans (1975) state, "IÈ appears'
assuming identical initial moisture conditions, soil crusts may have
negligible effects on water loss from the soil under field conditions'
provided there ís no difference in colour and provided the crusts do not
penetrate or crack more than 2 or 3 cm below the soil surface" ' These
conditions are relativel-y narrow constraints which would not be flrlfill-ed
in the field on many occasions, as has been demonstrated by lapendick
e.! e]. (1973) and above.
4.3.4 Ef fect of meteorol.ogícal factors
The effect of crusting, compaction and time on evaporation ratio
(Ev) from beds of aggregates is shown in llig " 4.4. The overlay shows how
'E pr¡E f lunT¡edr(g 'Bpeq e?p6e¡668 (o) palcc&oc pup (o) pr¡ctõ¡octmuo t^el oTlt¡ uollerode^e uo (---) lTcT¡ep a¡nssa¡d rnoden ptre (-) peode puT&i to lca¡Jg
(sÁBP ) lhlll61 8l Ll 91 9t tJ 81 U lt 0l 6 I
P.,qo--o-- e,-ô--qdtÌ"q
o--or!- -O - -9_ _e
L99i8Zl
\
,ta
o\0
I
j
0
0g t It
,\to
a
\\
tIt¡II
,
tI,
tt
J
U
t
It
{
It
a
t \ l
ttt,It
\¡t
¡I t
o t¡II
aa t
,
z001.EI
rtt
7t
t
I oI091 'LII
,,t
t
l,t,t,llUo
I
¡IItt
at a
b
Io
a ,t
1.0
0.9
03
01
0.6
04
03
02
0.1
0
/
r
ìt,tt
tA,\
t,t,t,ttt,
I
\,̂l
tt
II
¡
,I ,I ,
,I
I
.'ltItItI
al, t t,
,TAI
l,\ xL- - -L' ^---^,\
L' \
,ri 0'5 f,
&
x
\Ä
\I \\I I !--^--J A Ät\^/t I
^I
II
II
tII,
tr^-.¿- -
á:\A
1 2 3 4 5 6 7 I I 10 11 12 t3 14 15 16 1t t8 19
TIME ( days )
Fí9. 4 .4 Effect of uncrusted (A) and crusted. (A) surfaces on uncompacted (-) andcompacted (---) ¡eas of aggregates on evaporation raÈio, Ev, duringE:çeriment I and 3.
62.
wind speed (u) and the vapour pressure deficit (ps - Pa) varied for the
same period.
The presence of a surface crust reduces the evaporation ratio to
approximately half that of the uncrusted surface. The effect is greater
when a cornpaction treatment is also applied. The contpaction treatment
resulted in a greater evaporation ratio than the uncompacted beds ' The
ïeasons for these differences have been discussed in section 4'3'2 arrd
4 .3.3.
Although Table 4.I showed that the effects of meteoroloqical
conditions on Eo, were not statistically significant in this e>çeriment'
it was decided to do some further analyses to see what form any dependence
may take. Equation (4.8)was expanded as follows' to
Err=k*a(ps-p-) +ab.u(n=-I>.) (4'tf)
and the parameters determined for each experimental period' The fact
that wind speed (u) and the vapour pressure deficit (1" - n.) did not
significantly affect Er, (Table 4.I) , reflects the over-simpJ-ification made
ín the major assumption in calculating p". The assumptions, however'
enable one to obtain equations relating meteorological factors and 8..' The
same technique would give more reliable and applicable results if soiÌ
temperature and water potential had been measured'
tf Ps - P. is positive, water vapour moves from the soil to the
atmosphere, hovrever, if ps : Pa is negative, water vaPour moves from the
atmosphere to the soil, all ot}rer factors being cor¡stant. In these
experiments þs - Pa) was always positive'
The resultant equations for the three experi-mental periods are
p)dE__ = 0.26 + O.O4 (p"
v
(+0.0s) (+0.o2)
- O .00035u (f,=
(+0 .0001)
- p-)d
(4.r2)
ur, = 0.43 - 0.48(Ps
(+o "11) (ro.ra¡
E., = 0.55 - 0.12(P"
(+0.06) (-ro"1o)
a-0.0033u(p,
(+0.0008)
- 0.000055u(p
(+0.0007)
- p-)d,
-p)"ä
- Pr)
-p)s -a
63.
(4 "t 3)
(4 . r4)
Exper:imenL l, used uncompacted-Iarge aggregates; experinent 2'
r.rncompacted-snral-1 aggregates and experíment 3, used compacted-large and
-small aggregates. Figure 4.5 j.llustrates the fit betv¡een t'he observed
data and calcufated vaiues using equations (4.L2), 4.13) and (4'I4) '
There is good agreemeut betrveen the data and equat:Lcn itl experjrLlent
l(r = 0.53) and experiment 2 (t = o.8o), while there j-s less goodness of
fit in experiment 3 (r = 0.37). The compaction treatment appears to
have introduced some variation which as yet cannot be explained'
l\s the vapour pressule deficit becomes rnore positive (t--he atmosphere
becomi.ng dryer), Eu. increases in equation (4.L2) while it decreases in
equations (4.13) and (4.L4) " If the vapoul pressure deficit were to
become negative (atmosphere becoming twettert), Ev clecreases in equaticln
(4.I2) while j-ncreasing according to equations (4.13) and (4.I4) ' As Lhe
wind speed increases, evaporation rate increases (Hanks ¿¡j [lrtçro<ll:uff '
1958;Hadas,Lg=s)rhoweverrtheevaporationraùio'Ev'je'--:cases
accord.ing to equations (4.Ir) and (4.L4), while it increases accordinq
to eguation (4.r3). when the wind speed decreases the reverse occurs'
A direct comparison to other work is not possiJcle because En, is noÈ a
measure of the actual evaporation rate, but a ratj.o of the evaporation
rate and class A pan evaporation rate '
Equationswerederivedforeachaggregatesizeandforthecrusted
and uncrusted surfaces. The parameters for each equation are presented
in Table 4.2. The onty parameter that changes during an experimental
period is k. AII paramet-ers differ signi.ficantly bel-ween erperimental
oo
tr
1.0
0.8
0.6
0.¡l
02
0tr
o
1.0
0.8
0,6
0.4
0.21.0
EXPERIMENT 2
o
123456789101112TIME (daYs)
EXFER IIYIENT 1
tro tr
1 2 3 4 s 6 7 8 9 10 1l 12 1314 15
TIME (days)
EXPERIIIIENT 3
tr
oootrtro
1234 567 891011121311 151617lE19TtME (days)
0.8
0.6
0.4
0
0.2 trtr
0
16 f7
Effect of time on mean evaporation ratio (Ev) for at1 treaünents duringE:çeriments I, 2 and 3. Curves are plots of Equations 4.I0, 4.Il and 4.I2.rig. 4.5
64.
periods. It j-s noted that k is largest- ft:r the smallest aggregal-e sizer
in each experi-merital period ancl that it also is the smafl-est -{¡or each
agg::egate size that exhibits the lowest evaporation ratio (I:ì") ,
indicating the smallest evaporative water loss ' A similar si1-uation
occurs with tle crusted ancl uncrusted surfaces, k being smal-lest for
crusted surfaces, which exhibit the smallest evaporation rat-io, Evt hence
evaporative foss of \,/ater" The values of k are greater with tl'le cornpacLecl
TABLE 4.2 Parameters for eguation (4.11) for each aggregate stze'
and surface treatmenL for experiments I' 2 and 3'
Parameter
Experiment IUncompacted
Experj-ment 2
Uncompacted
Experiment 3
Compacted
>44-22-L<l,4/ z-t
UncrustedCrusted
t-0 .50.5-0.20.2-0.I
0.r-0.007<I
UncrusteclCnrsted
>44-22-Lr-0 .5
0.5-0.2UncrustedCrusted
-0 .00035-0 .000 35
-0.00035-0 .00c 35
-0 "00035-0 .00035-0 .00035
SE
0 .00010 .00010 " 00010 " 000r0 " 000I0 .00010 .0001
Aggr:egateSize (nm)
0.00330"00330.0033c"00330.00330 .00330.0c33
0.00080 .00080 .00080 .00080 .00080 " 00080 .0008
0 .00070 "00070.00070.00070 .000 7
0 "00070 .0007
treatments g-ran for the unconìpacted treat-ments. This corresponcls wit-h
the p:levious result that a compaction treatment ::esulted in hj gher
evaporation ratios than the uncompacted treatment. consequently the
compacLíon treatment lost more water l-han the uncompacted treat-ment"
0000IIo
56343540l0¿4
96
-o "7.2-o.L2-0.14-o.L2-o.L2-o.L2-o.I2
05^tr050505o4o4
00000
00
0 .100 .100.r00 .1-00 .100 .100.r0
-0 .000055-0 .000055-0 .000055-0.000055-0 .000055-0 .000055-0 .000055
0 .070 .100.31.L.O2o "64o.'12o -57
00U
0000
.L4
.L4
I4
.L4
.L4
.L4
L40U
ô
00U
0
.06
.06
.06
.06
.06
-0 .48-0 .48-0 .48-0 .48-o.48-0 .48-0 .4806
06
0.160.130.r00 .8r0 .100 .I30 .07
0
0
0
00
00
.o4
.04
.o4
.04
.o4
.o4
.o4
0 .02o.o2o.o2o.o20.02o.o2o.o2.
0 .040 .040 .040 .040 .040 .040 .04
SEk a SE ab
65.
4.4 Conclusions
Aggregateslze,presenceofsoilsurfacecrustsandcompaction
all significantly affecl- the evaporation ratio, Ev, hence evaporation
rate.
The meteorological factors considered did not significantly affect
the evaporation ratio, Ev. This shows that the meteorological factors
affect the evaporation from Èhe class A pan and aggregate beds equally'
The2_},I-O.5arrc]4-2nunaqgregatesgavethesmallestevaporation
ratio, Ev, on the two uncompacted and contpacted treatments respectively'
The stratified bed' (>4 overlying 2-1 mm) resulted in the minimum
evaporation ratio for the uncompacted treatments'
The presence of a soj-I surface crust reduces the evaporation ratíot
Eo.r to approximately one-half that on a corresponding uncrusted aggregate
bed.
AcompactiontreaÈmentresultsinanincreaseoftheevaporatioir
ratio, Ev, compared with an uncompacted treatment'
Thevalueofk,apaÏameterfromequation(4.1r)maybeuseful
as an indicato:: of evaporation loss from aggregaÈed beds ' The effect of
aggregate slze, presence of a soil SUrface crust and a compaction treatment
appear to outweigh the effect of meteorologicat factors on E.r'
Further work is necessary to resolve the effect of vapour pressure
deficit on evaporation and the effect shown in equations (4'L2) and
(4.L4) where an increase in winci speed results in a decrease in
evaporation ratio. The assumptio¡r made initially is an over-símplificat'i'on
and may have caused the conflicting results' It did' however' enable an
attempt at estimating the effect of meteorological factors on e\raporabion
ratio, Ev.
66.
SECTION 5
SOTL CRUST STR]INGTH AND EMERGENCE FORC]I Otr \,{ITEAT
5.I Introduction
The formatiOn of a crust on a seedbed can have adverse effects
on soil aeratioD, water infj-ltration and can prevent seeclling emergence
if the crust sets hard, as often happens witlr red-brown eart}rs. Planl-
species, however, vary in the amount of force they can exert in breaJ<ing
through a crust.
Willianrs(1956)measuredthemaxilnumemergenceforceofsma}l-
seeded legumes. He gives median energence forces as: Alfalfa' 0'I5N;
crimson clover, 0.23N; Rose cloVer, 0.24N and s":bt'erraneê.n cl-'>¿er-' 0'59N'
seed weight and maximum emergence force were clos'lly correlatcd (r = 0'99)'
Giffor:d and Thran (1969) developed a special transclucer to measure
emergence force of seedlings. Pregerminated seeds were placecl in the
apparatus, and emergence force recorded. for '7 days. They reported maxim'unt
emergence forces as folrows: Lima beans, 3.04N; Maize, 2-4ON; Cucumbe::,
1.57N; cotton, 0.59N; Radish, 0.42N and TafI wheat grass, 0"06N" These
data alsc illustrate the correlation between seed size and emergeilce
force. Using the same eguipment, Jensen g-t- at' (J9':.2) measur:ed the
emergenceforceofforageseecllings.Again'emergenceforceandseed
weight were closely corretated (r = 0.91) " Prihar anrl Aggarwai (1975) '
however, found no correlation between maximum emergence force and' seed
weightofmaize.TheyreportedforcesrangingfromC'73tol.I7N'
depending on soil density, depth of planting and seed orientation. Tlrese
values are higher than that reported by Badhoria 9t L1-' (L977) for mai-ze
(0 . r7N) .
GarnerandBowen(1966)reportthemaximumemergenceforcefor
cotton being t'9N, which is in agreenrent witlr values of between 1.9 anc]
61 "
2.5N cited h)y Drevr et at. (1971), They also cite a vaLue ai 2'7N for
maize.
Will.iams(1963)reportedtheeffectofte-mperatureOnemerqence
forceoflegumes.Alfalfaproducedamaximumemergenceforceof0.32}Jat
3OoC, while crimson clover gave a maximum of O.6lN at 2OoC' Drew et al.'
(1971), however, found no correlation between seedling thrust and
temperature for cotton.
Jensen et aI. (1g72) determined that' the maxirnum emergence force
for alfalfa an<1 tall- wheatgrass was l-ower when grown at -5ockPa osm'¡t-i<:
potential than at- -'25O ot -O kPa osmotic potential'
Frelicheq-4.(1973)showedthatsixgrassspecies<lifferecj.
greatlyintheirabi.lítytopenetratenonporouswaxcr:ust-s.PuJ¡escent
wheatgrass and smooth bromegrass were most affeclecl by crust h¿rrdness;
taII wheatgrass was least affected. Taylor (L962) aiso ha's -i-ndicated
dif ferences in reaction to crust har.dness. He sh<¡v¡ed that gu;rr was less
affected by crusts than wheat or grain sorghum"
RecenÈreviewsonsoilcrustingbyFar'reiL(1'9.72.),Cary¿.nctEvans
(Lg'|4) and Rao and Bhardwaj (1976) cover aspects of crust fonn'ation and
crust strength.
Many soils with large contents of fine sand and silt are
susceptible to erosion when tilled. This is clue to poor stability of
aggregates when they are wetted rapidly and to raindrop impact on a bare
seedbed. The disruption of soil bonding mech¿*nis'Lns leacs to a r(ìducjtion
in the rnean size of the structural units in the surfacr: l-ayer' fo-]-lowed
byre-sortingandre-pacl<ingbywatermovementinsplash,flor,¿a,nd
sedimentation processes (Bean and \'üeIIs, 1953) " Consequently the surl-ace
tends to be l-evelled and covered with a coni:inuous iayer c;f rela'tively
fine closely-packed particles. Heav)¡ r¿rinfall fol'tolved by a clry periocl
mayleadtotheformati-onronthesoilsurface'ofacrusl-whichis
68.
sufficientllr s¡t.tg to impede seedl-ing elnergence and r^¡hich leads to
increased surface run-off of rainfall and loss of soil by erosion'
several wo1:kÉìrs have usecl the modulus of ruptllre (MOR) as a
measure of crust str:ength (carnes I Ig34; -Richards, 1953; Allison ancl
Moore, 1956; AIIison, 1956). The MoR, S, is given by
_ 3FL / .2 (5.I)S = -'"/ 2bd.2
whereFistheforcerequiredtobreakthecrust'Listhedistance
between suppol:ts, b is the width of the crust, and d is Lhe crust thickness'
carnes (1934) found that crust strength <lepended on water content and the
rate of drying. He suggested that the soil should be compacted below the
seedling, giving it a firm "footing", to enable it to emerge through crusts '
Richards (1953) determined that an increase in MOR from I0'3 to 27'O kPa
reduced emergence of beans from 100 to oå. Allison (1956) usíng maize
as a test crop found the MoR that limited emergence to vary from I20 to
250 kpa. AIIison and Moore (1956) similarly found the MoR that Limited
emergence of sweet corn varied between 130 and 370 kPa' However' when
these soils were treated with a soil conclitioner (vA¡44) the rrloF clf the
crust varied k¡etween 9 and 34 kPa. Hanks and Thorp (1956) fc¡und tha-t a
MOR between 20 and 50 kPa limited the emergence of rvheat' However' they
found that wheat, seed sorghum and soybean exhibited limited emer:gence
at 140 kPa (Hanks and Thorp, Lgs--) ' In a1I cases emergence w'as relatr:ci
to the crust water conLent. Lemos and Lutz (1957) expressed some doubt
as to the applì-cation of laboratory ì4OR measurements to field practices'
No causal relationship between MOR ancl see'fling emergence data is offered
by the above workers.
Usingwaxtosimulatesoilcrusts,Taylor(L962)andFrelichet-al'
(r973)havealsoshownthatemergenceofplantsdecreaseswitl-rincreasing
crust strength and thickness'
69"
Bennett et- aI . (1964) used the force required to pul] fishing
line from beneath a soil cr:ust as ân inclex of crusL si-rel-r91-h. Seecllinç¡
emergence was negativety correlated with crust strength and po:-;j'tiwely
corre]-atedwithwatercontentint.hetopBcmofsoit"I^]hencrust
strength was red¡ced by the addition of soil cc¡nclitj-oners, seedling
emergence increased.
Page and. Ilole (Lg1'7) compared. IIOR| fishing line technique arld a
shear vane technique as measures of crllst strength. They obtained good
correl.ation between techniques when a modified MOR Ineäsure rvas used'
Absolut-e val-ues, hotvever, differed by a factor of t'"ven+-}2' corre-l-ation
between the measures of crust strength and emergence of Lurrrip see<llings
was not good, suggesting that one method was rlo bet.ter than any ot'her
in assessing crust strength.
A nol:e realist-ic app::oach has been suggested by several workers
(Morton and Buche1e, L96O¡ Arndi, 1965a, b; Holder and Bro\^"n ' L9J4¡
chaudhri et al . , Lg76; Had.as amd stibbe, l-g71). This involves f-he use
of probes, forced through crusts fronr belov.t, to simulate emer:ging seedlings'
The method of Mr¡rtoll and Buchele (1960) , however, he'1 no pr'ovis j'an for
producing rainfafl.-i.nduced crusts. Arndt's (I96-ia, b) rn()r-lt<¡cl is r-aboriolrs
and is susceptible to natural rainfalli also only a few measurements rnay
be made in any one area. Holder and Br:own (1975) overcame many of these
objections, although they used a laboratory technigue. They found that
crust strengl-h increased, as rainfall increased: impedance forces of 0'0f6'
o.o2l and.0.oI7N v/ere recorded for rainfall intensities of I'3' 2'5 and
5.1 cm hr-l. Crust impeclance increased initial-1y as the soil dried' and
then decreased as cracking i-rrcreased. Hadas and Stibbe (L917) found' an
increase in crust strength as rainfal-1 intensity increased' However' they
found no correlation bet-ween crust strengttr and wheat enei:gellce (i'n the
fielcl) after a thre-e week periocl . The dj-rect use of seedling emergence as
70
an indicatj-on of crust impedance is unsatisfactory since so many factc¡rs
influence .Lhe ability of each differîent seedling t--c cmerqe. Seedli-ng
variety, spec.ies an,f soil temperature anC water: iratre ber:rl reported to
influence emergencô (Viilliams. 1956; Wiftia¡ns, 1963; Jensen qt g!", 19'72) "
various meb-hods have been proposed to reduce the effect of soil
crusts and to prevent the formatíon of soil crusts. Ttrey may be broadly
divíded into ttrree categories. Firstly, chernícal addj.tives which tend
to make the soil ,,stronger": these include polyacrylics and poly(viny]
alcohols) (De Ment qt al. . 1.955; Iillison, i956; Allison and Moore,
1956¡ Bennett et al ., Lg64; Bl.av-ia et aI .,Ig7L, De tsoodt' L972¡ Oades'
Lg76), seconclly, chemícal a.dd,itives whích tend Èo mal<e the soíl "weaker":
these include soil fracturing agents (Bennett et al- ' L964), phosphoric
acid (Robbins et al. , Lg72) and gypsum (Grierson, lg78t so et g!", l97B) ,
and thirdly, physical amendments v,¡hich also tencl to make I'hre soil "weaker"
and include materials such as asphalt emulsion, sand, perlite (El-Is,
1965), coke (Qashu and Evans , Lg67h asphalÈ, vinyl resins ar:'J black
plastic film (Rennett =!.1. , Lg64), manure (Chadhri et g!" I Lc''i6) acderl
as bands above the seed or as surface bands i ancl rnulches such as hay
(Srlith, 1966) anrl manure (Chaudhri et aI ., 1976) appliecl to the surface as
a whole.
The aim of this section is to determine the emergence force
exerted by wheat and the strength of soil crusts formed under natural
rainfall on beds of different sizes of aggregates. This may enable
recommenclations l-o be made for the optimum seedbed aggregate size which
rn-j-nimizes crust strength and formation ancl erosion losses, while maintaining
an open structure which facilitates seedling emergence'
5.2 EÏrergenc e force of wheai; shoots
seedling emergence is oft-en a critical- st-age .in crop estajcl-ishment'
7L
Germination, the first stage in emergence, is influenced by m<.risture,
oxygen, carbon dioxide, Iight, temperature' soil pFI, mineral nutrition,
dormancy and microorganisms (Hagan , Lg52). In addition a newly germinated
seedling must force its way through the overlyíng soil. Seedbeds are
often rough and excessive planting depth frequently occurs. Moreover'
soil crusts (Hanks and Thorp, 1956, L957) and soil compaction (Morton and
Buchele, 1960; Stout et 41., 196I) often impecle seedling emergence.
5.2.L Materials and Methods
A modified version of the method of !Ùilliams (1956) was used' Seed
(trj.tícum aestivum L. cv. Halberd) presoaked for 12 hr. was placed' in a
glass tube (5 mm I.D.) which was pushe<l into soil (Urrbrae loam, <1 mm
diam.) al 20% water content (dry weight basis) in a plastic vial" The
vials were sealed with lids (plate 5.14) and placecl in the follow-inr;
consÈant temperatures: 25, 20, 15 and 8oC. Close fitting glass :--ods
(5 run diam.) of known weight were carefulty placed on the seed- Additional
weights were added to give weights up to 50 g in 5 g increments. The
position of the glass rod, in relat--ion to the tul¡e' was ma.rked so mcvemenl:
of the rod could be observed. A minimum distance of 2 nuu was arbita.rily
selected to account for further swelling of the seeds prior to emergence
of the coleoptile. !Ùhen the ro<1 had been pushed up a distance of 2 nm oL
more, the seedling was considered to have exerted a force equal to the
weight of the rod. Readings were taken at the end of 7 or 14 days.
Results of seeds that did not germinate were excluded, the othe::s being
scored as successes or failures. Coleoptile diameter was measurecl at the
mid point with a rticrometer.
The data r^rere analysed by the method of probits after Bliss (.1 935) '
This facilitates the ca]culation of linear regression equations; v¡hL':ir
índicate the percentage of a seedling pop¡l-atiou able to exert various
Plate 5.1 A Sealed plastic vial with co1-eoptile
pushing up a 59 glass rod.
B. Doubling back of coleoptile in glass
tube.
1
l
\
i¡
fd
I
,i
gV
't)
Ievels of force duríng emel:gence. From t-hese regression equat-i-c-lns the
median emergence force (MBF) can be estimated. This ís defined as the
force which 50% of the populatj-on is too weak to exert'
5.2.2 Results and Discussion
The linear regression eguation may be expressed as
v=l+b (\^7 - X) (5 "2)
where Y is the percentage of E;eedlings' expressed in probits, t-oo weak to
exert a force W, expressed in loga.ritlrms; Í i" tfr. a\zcrage probil-; i i"
the average force in togaritlitns a¡c1 b is the regression coeff-{'cierrt'' It
is necessary to calculate i, Í anct b. The data and apprcrp::j-ate for:mulae
are presented in Tabl-e 5.1. The ]evels of force vJere tr-'ansjlcrnL€d i:o
Iogarit-hms to the base IO ancl 1-he percen'Lage weak seerllings is t:r:ansfor:med
to probit-s from Bliss, Table 1. The pl.ot of Lhese p:r:obj-ts aqainsL -log
force approximates a straight tj-ne (FiS" 5.1). The weighting coefficients
are necessary to take into accollnt the greater accuracy of the prohits as
they approach the median emergence force. they are obtai.ned from RI-iss,
Table III . The weight (,r¡) is obtained by multi¡riyitlg the numbe": of trials
at a given fo¡:ce by the weiglrting cceffj-cient'
The good,ness of fit-- of the observed poinl-s to t-iìe regr:essi-on
eqrration may be calculated by the X2 test fl:r:m the fc¡rtnula given' If the
X2 is smal-ler than the value for p = 0.05, the clal-a tnal' be cotrsj'dered
consistent with the fitted line. The abovt: data i;¡ consistetlt \'/ith the
fitted line.
The median emergenÇe force for t-he variety ô:e 'vrlrr:ab tes'ted is
5l .05 g or 0.49 N. This conpat:es ¡,vith the emergence f-orc:e; rc+L}>tt-ed for
other plant species. l,üilliams (1956) anrf Jensen qt- e¿. Q-c)'l2j report-ed
that emergence force correlal-ecl well vsit-h seecl weight" 1'he regr:essiorr
6
5
4
3
-rv(r,tooÉ,o.
2
1
o
o
1.6 2-00
0.8
LOG|0
0 0.4 1.2
w
Probits vs Log1gWCalíbration curve for calculation ofmedian emergence force.
Fig. 5.I
TASLE
Number ofTrials
42424242424242424242
b
% I{eakSeed1inEs
9.52L6.662I-4223.8033. 3335.7r35.7L38"0947 .6L61 .90
1.403
4.565
¡o910 w
Weightingcoef fi-cient
w
weighÈwX
9.850l8 .43825.23928.93234 .55837 .53039.2334r .58143 -25244 -786
wY
52.O4574.28690.29095.788
113.003LL7 .124Lr7.724L2L.9L5L29.233r39.845
ColeoptileDiam. (mm)
L.492L.649I .650L.623I.61II.6701.681r -684L.756r.946
5.1 procedure for carculation of least sguares regression equation which characterizes theemergence force exerted' by wheat-
W
(s)x Y
Probit g"
lfeak seedlings
5
l_0
1520253035404550
0 .698I .000I.L76I.3011.397L.417I.544L.6021.653r .698
L-424
3 .6884.O294.2074 -2A74.5684.6334.6334.6974.9395 -302
0.336o.4390.51Io.5320 .5890 .6050.6050.618o.623o.628
L4.l-l-218.4382L.46222.34424.'73825.ALO25.4LO25 -95626.l-6626 -376
X
i
À(ryL)t (\^r)
¿_("rY)x (w)
I (wXY) - Xt (r^,Y)
ffi^¡i4- - xx F/x)
2xn_ [r (rv2) - ir ('v) ] - ¡[r (wxy) - ir t'vl ] 3.904
2.)
(fa¡te X'= 15,507) P = 0.05
SubsÈituting values in equation (5.2)
t= Y+¡1w-i) whereY=5and solving for w
= I.708 i.e. I4EF 5I.05 g or 0.49 N
{UJ
(g
't4.
equaÈion for Willj-ams'(1956) clata for small-seeded legumes is
E IL "22 L2.9 , g (5.3)
v¡here E, is the emergence force and z the seed weight in mg' The
average seed weight for wheat (35 mg) was determj-ned. This predicted an
emergerrce force of 380 g or 3.7 N. This is considerably higher than the
experimentally determined vaLue. This discrepancy may have arisen due to
the experimental technique but may have been because cereals ¿1¡çl l¡¡r¡¡çr:s
behave differently. 1rhe coleoptile was not closely restricteo lateralJ-y
withinthe glass tube,thus it cou.ld buckle and grow downwards (Plate 5.18) "
The 'doubling back' of the shoot may be analagous to a similar obser:vation
by Hay (Lg76) with barley roots. This may have resuli-'ed in a smal-le r
emergence force being generat-ed than the coleop'ti]-e was capable of
developing.
The effect of temperature on emergence force and emergence
pressure was also testeO. T'he results are presented in Table 5'2 and
Fig. 5.2.
TABLE 5.2 Effect of temperature on Median Energence For:ce and
Pressure of wheat.
f
TemperatureoC
I15
20
25
tr{edian Emergence PressurekPa
403
234
168
26L
The minimum emergence force is generated at ZOOC while the maximum force
occurs at 8oC. Temperature appears to have a marked effect on the force
generated by seedtings. V,lill.ia¡ns (f963) re,oortecl that crimsorl clover
generated its maximum median emercJence force at 2ooc lvhiLe atfalfa
o.136
0 .506
0 .43Io.637
Medían Emergence ForceN
3000'7
0.8
0.5
0.1
1s0
t 100
\\\\ 350
\
\\ I
I 250\ I\
I
t50
100
50
t0 15 20 25
TEMPERATURE (OC)
Effect of temperature on emergence forcepressure (---) of wheat (Triticr¡m aestivum, L' cv'Halberd) shoots.
NIEz
l¡¡É)u,U't¡JÉÀl¡¡ozl¡,c,Ét¡J
=UJ
zõt¡¡
=
200
Y
\0.6
z
t¡¡oÉ,o
u¡ozUJ(9Ét¡¡
=t¡Jz3ot¡¡
=
5
Fig. 5.2
15.
generated its maximum at 3OoC. The force produced by cri.mson clover
was almost twice that of alfalfa. For wireat the implicat-ion is that-
sowing of the seed should take place when soj.l temperatures are
approximateJ-y 8oC. This worrld enable the m¡rximum emergence force to
be generated after germinat-ion, ancl hence increase the potential for
enìergence. However, this tencls to disagree wit--h the re¡lor:ted optitrtum
temperature for germinat-ion of wheat (10-30oC; Pe¡cival, Lg27-; Wort,
1940) . This nay be explained by the fact that rlj-fferences in varieties,
aeration and water conditions witl affect the optimum temperature.
When t-he soit is v¡et the emergence pressure may be more
important than the emergence force. The emerging seedl inq ma1' pe:netrate
t.l. e aggregates and emerge through them rather than displace the aggregates
as is the case when the soil is drier. In thís case the ¡:'ressure exerted
by the shoot wil.l be more important l-han the force. The measured pressu-res
in Table 5"2 are a bit smaller than those determined by Pfeffer (f893)
of 500-1900 kPa (Avge force 750 kPa) for shoots. The largest pressure
is developed at Boc (403 kPa) and the smallest at 2Ooc (168 kPa).
5.3 Measu,renent <¡f crust strenqth, crustinq and air res-istance
of crust
Probably the most important effect of soil crusLs 1.s orl emergence
and early development of seedlings " Their influence in <lecreasing or
preventing seedling emergence has been described many times (e.g- Hanks
and Thorp , Lg57; Frelich e! aI. , 1973) . SoiI crusts also red-trce water
infiltration with a resulting increase in run-off and erosion (Moldenhauer
and Kemper, 1969).
5.3.I lnlaterial-s al:d Methods
Sect,io¡s of crust were col-lected from plots in the 1976 field
experiment (Section 7) . The crust sections rvere hTetted to saturation hy
'16 "
capillary action and tllen dried to the foll.owing water potentiais; -:l-500,
-500, -I0O ancl *IO kPa. Thj.s ensurecl thaÈ each section hacl a simil.ar
recent history. Crust strengt-h was del-ermined using a liodifi.ed irLodul.us
of rupture (MOIì) test. Sections were placed crust*s-ide-r1orçn on wcoden
supports 22 mm apart. The penetromet-er of Barl-ey e! al . (1965) (with a
6o0 tl-p angle, I.9 mm diam. probe and penetrating downwarcfs al- a rate of
-12 nrm mi.n-r) was used^ to cletermine the- force requ.ired to penel:::ate the
cïust from below" This penetr:atj-on usually occurred by the cracì<ing of
the crust sect-ion. A philips direct reading measuritrç¡ brridge (Irlodel PR9300)
was used to record Lhe for:ce to penetrat-e th,e crust" The thj-r>kness of tbe
sample and total- crack length were recorcled. The l{OR rvas calculated using
equation (5.1.)
s - 3EL/zra2
where s is the MoR, F, the force required to penetrate the crust, L, the
distance between -uhe supports' b, the total length c¡f resulting cracks and
d the sample thickness. Sj.x replicates per ¡¡qr¿'f1¡r)r]t w€r]'e llsec.
Crust str-'ength, was measu::ed on each tr.eai-rne:trt j-rr i:hcl fie:l<i
using a hand penetrometer (Soil test MoCeI CL7OO, flat tip diam" 5 mm) i'n'
1976 and L97'1 aE the end of the s€casori and a.lso al- the end of 1::1le emergence
period in 197'7. The penetrometer was pushed throrrgh Lhe crust at a
constant rate. These measurements differ from the laboratory rneasurements
in that strength \^¡as measured by penetration through the crusi: from above,
whereas the laboratory measurements relate to penetrat-ion from below in an
attempt to simulate an emerging seedling. 8-10 repl-icates per treatment
v¡ere measured.
Measurements of air permeability rvere ma<le at the end of the growing
season. Air permeabil-ity is recognized as an index of soil structure
(Kirkham, :..946¡ Evans and Kirkham, f.g49), an i-ndicator of crust formation
(Grover, 1955) and a measure of compaction (Tanner and Wengei, 1957) ' An
77.
air permeameter, sirnilar to tirat of Tanne:: and We;igef (f957) was cortstructed
(plate 5.2) to measure the air "resistar'ìce" of crusts for:nted on v¿¡rious
treatme-nts in the field. The weight of the float of the air tank was 1.14 kg,
its diameter 16.5 cm and height 18.5 cm. A 5 cm d.iamel--er seamless soil
sampler (plate 5"28) was pushed into the seeclbed. to a dep'Lh of 5 cm. The
permearneter was levelled and not moved for: r:emaining t-.rials. A flexible
hose was connected to 1-he permeameter and soil sample t-ube with minimum
disturbance to the crust. A stop watch was used t-o time the fall of tlte
float can (plate 5.2A,). it being st¿rrtecl simultaneously lvith the release of
float; ten replicates per plot were measored. Air per:meability, K, is
calculated using the following equation
r = V!/ta¡p (5.4)
where V, is the volume of air forced through the sample, L, the time to
force that vol-r.rme through, AP, tJre pressure devel-oped by the air tank, u,
the viscosity of the air and A, a constant. Hence K is proport---i-on¿r.L ts l-/L.
The results discussed are based on faIl times and are not air permeabilities "
The times record.ed as air tresistancest can be used to calculate Kr but it
was thought that the use of the fa-l-I times would not all-er tÌre cot¡clusions
reached., as the relative differences would be the same "
A portable rainfall simutator was used to cietermine time to run-off
and amount of sediment in run-off from plots of different sized aggregates "
These parame'Lers v/ere used as an indication of crust formation and erodibility
of the becls of aggregates. The simulator was calibrated. before iJ:e e>æer.iment
using a collection tray (pl-ate 5.3D) to collect run-off. Simulated rainfali
at the rate of 27 n¡nt hrr-l w.u applied to loeds of aggregates using a raj.nfali
simul-ator (crierson and oades, 1977) (Plate 5.3). Run-off frotn a 0.5 x 0.5 ¡n
plot (plate 5.3E) vras collected in a fl-ume and drawn by suctj-ori into a
calj.brated cylinder (plate 5.3C). Solids in the run-off were rietermined
gravimet::ícally. Run-of f plots \,rere located centrally vri-thin a l- x I rn area
(plate 5.38) direcLly beneath the simulator: nozzle (Plate 5.34) . The .rL:ea
Plate 5.2 Equipment for air resistance tneasurement
A Air tank and water reservoir and air t--ube
B Sample holder.
P]ate 5.3 Rainfall simulator in operating position.
A Motor and splash guard
B Plot surround
C Diaphragm pump and col1ection cylinder
D Calibration tray
E Plot frame
F Power generator
G Ì,Iotor and pump
H water reservoirs.
1A"
around the run-off plot vras covered with plastic, the excess water bei¡g
direc.l-ed away from the col-lection flume, tttus ena-Ìrling the sinulator to
remain in one position for the duration of the experiment. The aggregate
beds were made with a 10 clegree slope toward the collection flume' Three
replicaLes per treatment wel:e fiteasured.
Thin sections of crust, from the 1977 fiel-d trial-¡ welîe prepared as
described in Section 3.2.3.I" These sections were analyse-d for 1'orosity atrd
pore orientation using a Quantimel 72O as described in Murphy eL eL' (L977a) '
The crust was analysed for porosity usi-ng a frame size o1 800 x 500 picLure
points. Vertic¿rl and horizontal projections were neasured to ascertain
orj-entation of pores. Porosity at valious levels wittti¡l the crust was
analysed using a frame size of I x 5OO picture points at 100 pp st-eps from
just below the crust surface to the base of the crust section.
5.3.2 Results and Discussion
5.3.2.I Crust stre nqth as determined by modulus of rupture
From the Tab.le (5.3) it can be seen that there is a significant
effect of aggregat-e size, compacti.on treatment' water potential and time of
sowing on crust strength. These signifj-cant di.ffe::ences are reflected in
the interactions of the various lreatrnents. Crust strengihs cletermi'ned by
MOR are smaller t]- an those determined by hand penetrometer j-n the field
(see Section 5.3.2.2). One would expect crust strength to be higher during
plant emergence because of jamrning of soil plates as mentioned by Arndt
(1965a). Thus one cannot relate MOR measurements to emergence force to get
an assessment of the crust strengths which f-imit emergence.
5.3.2.1"r Effect of aggregate size
The mean value of crust strength for each agg::egate size is givcn
in Table 5.4. The table shorvs that crusts formed on smal-]er aggTegates al:e
significantly stronqer than those formed on larger aggregates, A rninimum
crust strength occurs wj-th the 4-2 mm aggregates " These crust strengths
79.
TABLE 5.3
Treatment
Aggregate size (aSS.)
Compaction (Cornp. ¡
Water potential (Water)
Time of sowing (Time)
Agg. - Comp.
Agg. - Viater
Comp. - Water
Agg. - Tíme
Comp" - Time
Water - Time
Agg. - ComP. - Vüater:
Agg.-ComP.-TimeAgg.-Water-TimeComp. - VJater - Time
SoiI crust strengtJn as def-ermíned by MOR : Summary of
analysis of variance.
F
2.39*+
3"86*+
2.62*+
3.86*+
2.39*+
I .78*+
2.62*+
2 .39*+
3 .86 *+
2.62*+
I.78*+2.39*+
I .78*+
2.62*+
* Significant at P=0.05 + Significant at P=0'ol
TABLE 5.4 Mean crust strength (kPa) as measured by MOR of crust
formed on each aggregate size (nun) railge' (ln:e't¡'rr r>f 9(i
replicates)
<l LSD (e=0.05)
26.4 r.49
¡4m = Mean of >4 and fr' tt"tt*t"'
compare with those found by Richards (1953) to prevent bean emergence '
A compaction treatmenÈ of 2"7 kPa imposed on beds of each aggregate
size increased the strength of the crust formed compared with that of the
equivalent un,compacted plot. This is shown in Fig' 5'3'
Increase in cruSt Strength, due to compaction' results because of
smear:ing of the surface aggregates and aggregates moving cfoser together'
4
I3
I4
L2
3
4
I3
L2
4
L2
3
25.9I457.423
243.524
r27 -865
9.L82
13.880
6 .985
7.678
16 .50 5
rI.3346 .585
6.O20
3.869
9.25L
DF \rT.
26.O 2L.4 2L.9
2-L 4-2 ,'4m
32
30
28
22
20
18
o,tItttItItI
tI
II
tI
IIIIo-
26
24
e-f
Il-c,zt¡JÉt-Ø
Þv,fÊ,()
t
I,
t,
I,
I,
a
-----
I."
<1 2- 1 4-2
AGGREGATE SIZE (mm)
Effect of aggregate size and compaction treatmenton crust strength as determined by modulus of rupture
uncompactedcompacted
74^
Fig. 5.3
Ha---a>4* is mean of >4 and fr' tt."*.rra".
80.
as observed in section 3.3"2 An estimate of the level of cornpaction (II,/u:-)
can be obtained usíng equation 3.1.9 and knowir]g P:2.7 kPa, and Y: 23 kPa
(from table- 3.1) since the water content of the aggregates was approxì-mately
2Oe". The app-lied compaction treat-ment resul-ted- in an eighl- percent
reduction of the height of the aggregate bed. The degree of compaction
would, however, depend on the aggregate size range'being considered. The
4-2 mm aggïegates agaín exhibit a minimum crusi, strength for bot-h the
uncornpacted, and compac'bec1 plots.
As water potential became less negatir-e, (:.t:ust s;Lrength 'ic¡r each
aggregate síz.e decreased. At any water poLential , crust st-::ength decreased
with increasing aggregate s]'zet to the 4-2 mm sj.ze and then increased. This
occurred at aII water potentials except -10 kPa where the rever:se occurrect
and crust strengt-h on the 4-2 mm aggregates was a maximum (FiS. 5.4) .
A time of sowing treatment imposed on aggregate size range :resulted
in weaker crust-s formed on late sowl plots than on early sown plots
(Fig. 5.5). Once again the 4-2 mm aggregates gave a miilimun crust strength
at both sowing dates. The difference between sowing dates is due to a
greater amount of raj-nfall activity on tlte early sown (257 '6 nun) than late
sown (223.2 mm) plots. This would enhance bre¿'rkclov¡n of aggregates anc1
filling in of depressions between the aggregates, thus forniing a thicker
and stronger crust. This is illustrated in Plates 5.4 and 5.5 where a
greater infilting of material between aggregates, can be sejen on thr: earll¡
sown treatments than on the late sown treatments '
The effect of aggregate size and compaction at each r';ater potential
is illustrated in Table 5.5.
There is a significant decrease in crust strength with a decreasc¡ in
water potential on all aggregate sizes, both on the uncolnl)acte'J ancl compact-ed
plots. The difference due to aggregate size is not as markecl as thal- d'ue t'o
\¡/ater potential and compaction"
45
40
35
30
25
20
\II \
ItII
tllto.I
TC)z¡¡¡ÉÞth
FØfÉo
1II ì
o
f="
-ù----¡1\\15oo'
10
5
<1 2-1 4 -2AGGREGATE SIZE (mm)
7 4m
Fig. 5.4 Effect of aggregate size and water potential oncrust strength as measured by modulus of rupture'
..--- - 1500 kPa Vfater Potentialo---o -500 kPa Water PotentialH -IOO kPa Vüater Potentialo---O -IO kPa !{ater Potential
32
30
28
26túo.g¡¡-2_ 24Él-ah
FU'f"" zz
o\ \ \ Ise\\o \\\
20
to______O
<1 2-1 4-2 1 4.AGGREGATE SIZE (mm)
Effect of aggregate size and sowing date on cruststrength as determined by modulus of rupture'
o----{ Early SowingO---O Late Sowing
>4* is the mean of. >4 "rra ft treatments.
18
16
Fis. 5.5
>4!J - EarJ-y
>4C - Early
4-2U - Early
., i l
". iit
{I
>4U - Late
óà¿
>4C - Late
4-2U - Lar-e
.: -.. \ it, j'.
Plate 5.4
4-2C - Early 4-2C - LaLe
Thin sectíons of surface crust formed under natural rainfall.collectecl from the 1976 field experiment after harvest.(Scale x2)U=Uncompacted C=Colnpacted
)ì
2-LU - Ear:ly 2-fU - Late
2-1C - Late
t'i '.- -
2-IC - Early
<lU - Early
tt-t
<lU - Late
<lC - Early
Thin sections of surface crust formed under natural rainfallcollectecl from the l-976 fiel-d experiment after harvest(Scale :r2) -u=Uncomþäcted C:Compacted
<lC - Late
P1ate 5.5
Bl.
TABLE 5.5
Aggregatesize(mm)
<l2-I4-2
>4
Aggregatesize(m¡rù
<I
2-L
4-2
>4
LSD between means 4'2I (P=0'05)
Ttre effect of aggregate sj.ze, conrpaction and time of sowirrg is
summa::ized ín Tabte 5.6.
TABLE 5.6 Effect of aggregate size, compaction and time of sowing
on crust strength (kPa) as measured by MOR'
(Mean of 24 rePlicates)
Compacted
Effect of aggregate size, compaction and water potential
(kPa) on crust strenqth 1ff'a) as measured by MOR'
(Mean of L2 rePlicates)
Compacted
-10
L6.4
L7 .9
r8 .5
l5 .8
Late
20.9
27 .2
22.6
t8 .5
LSD between means 2-98 (e=0'05)
1.,¡.,'e 4-2 mm size and the >4 mm size gave smaller crust strengt-hs
compared with the <l- mm size. Aggregate size has a greater effect on cmsE
strength than time of sowi.ng or compaction'
Asumrnaryoftheeffectofaggregatesize,waterpol-entialancl
38.I29.6
23.4
23.0
34.3
24.8
25.4
28.8
10.2
L2.8
r 3.5
tl.6
2L.2
r8. 3
L6.7
19 "4
30 .5
5r .9
27.3
29.5
36 .0
29.8
25.3
26.8
-500-1500 -l_0-100 -I500 -500 -'l o0
24.4
22.4
2L "O
20.4
Uncompacted
30 .0
24.3
2T.2
20.9
2r.9I8 .5
18. 3
20.5
32.7
33 .5
23.5
27 .7
EarIy Late Eariy
Uncompacted
42.
tirne of sowing is given ín Table 5.7.
TABLE 5.7 Effect of aggregate size, water potential and time of
sowing on crusÈ strength ltcea) as measured by MoR'
(Mean of L2 replicates)
vilater Potential lXPa)
-10
Aggregatesize(mm)
Late
<I
2-L
4-2
>4
11 .5
l3 .4
14 .8
L2.8
LSD between means 4-2I (P=0.05)
In aII cases (except at -IO kPa water potential) the 4-2 nm aggregate
size gives a minimum crust strength.
From the results it appears that the crust formed on 4-2 mm
aggregates is weaker than on any other size. Compaction, time of sowing
and water potential also greatLy affect the general trend of strength of
crust formed on each aggregate size-
5 .3.2.L.2. Ef fect of comPaction
In agreement with the results of Lemos and Lutz (1957) a compaction
treatment gave significantly stronger crusts than an uncompacted treatment'
This is illustrated in Table 5.8-
Effect of compaction on crust strengtJa (kea¡ as measured
by MoR. (Mean of 24O rePlical-es)
Uncompacted LSD (p=0.05)
43.4
45.9
25.2
29.L
43.4
30 .1
27 .5
31 .9
25.2
35.6
25.5
23.4
23.7
22 -2
1.9 .5
2L.6
26.9
24.5
23.2
23.7
2L.9
19 .0
1.8.2
18 .3
15 .0
L7 .4
r7 .2
14.6
EarIy LateEarlyLate EarIy EarlyLate
-1500 -500 -I00
25.3
Compacted.
TABLE 5.8
2L.7 o.94
83.
As water potential decreased crust- strength decreased; however'
a compacted treatment resulted in stronger crusts than an uncompacted
treatment (Tabte 5-9) -
TABLE 5.9 Effect of compaction and water potential on crust
strengÈh (kPa) as measured by MoR' (Mean of 60 replicates)
Water potential (kpa)
-r0
Uncompacted
Compacted
11. .9
16.9
LSD between means I.88 (l=0'O5)
Earlysowingresultedinstrongercrustsbeingformedoncompacted
plots than uncompacted' plots (Tal¡le 5'10) '
TABLE 5.10 Effect of compaction and sowing date on crust strettgth
(kPa) as measured by lrlOR. (Mean of t2O repli-cates)
Time of Sowing
EarIy LaÈe
Uncompacted
Compacted
20.o
2L.6
LSD betrþ¡een means I.33 (P=0 '05)
AsummaryofÈheeffectofcompaction,waterpotentialarrdtime
of sowing is given in Table 5'It'
Thegeneraltrendisthatcompactedplotsformastrongercrust
than uncompacted plots and that in each case crust strength decreases as
water potential becomes }ess negative'
27.4
33.7
28.4
28.9
t9 ,0
21 .8
-1500 -500 -100
23.4
29.O
TABLE 5.II
Uncompacted
Compacted
Effect of compaction, wat-er potential ancl time of
sowing on crust strength (kPa) as measured by MoR'
(Mean of 30 replicates)
vtater Potential (kPa)
-lo
84.
Late
rl .3L4.9
LSD between mearrs 2.67 (P=0.05)
5.3.2.r.3 Effect of water Potential and tíme of sowing
There is a significant decrease in crust strength as water potential
becomes less negative (ra¡te 5.12).
TABLE 5.12 Effect of water potential (kPa) on crust strength (kPa)
as measured by MOR. (Mean of I20 replicates)
Water Potential
-I500 -10
30 .6 l.4.4
LSD betv¡een means 1.33 (P=0-O5)
These results agree with those of Bennett et a1. (L964) and Lemos and Lutz
(1957) who also found that crust strength decreased as water content
increased or \,{ater potential decreased. It is important here to compare
crust strengths with tensile strengths of aggregates measured by crushing
at the same potentials. According to Braunack and Dexter (1978) ' (Tab1e I),
the trend is very similar.
Early sowing of plot-s produced significantly stroncJ€lr crusts that:
plots so\dn at a later date (Table 5.13) -
32.3
36 -1
30 .7
35 .5
25.5
30 .8
18.r25.3
26.2
22.6
t2.618.9
19 .9
r8.4
LateEarly LateEarly LateEarIy EarIy
-1500 -500 -100
28.7 20.4
-500 -100
85-
Ilffect of sowing date on crust strength (kPa) as measured
by IvIoR. (Mean of. 24O replicates)TABLE 5.I3
Sowing Date
EarIy LSD (P=0.05)
26.2 o.94
The combined effect of sowing date and water potential is
summarized in Talble 5.14-
TABLE 5.I4 Effect of \{ater ¡ntential (kPa) and sowilrg date
on crust strength (kPa) as measured by IIOR'
(Mean of 60 rePlicates)
Sowing DaÈe
Vtater potential (kpa) Late
-r500
-500--100
-10
26.7
24.4
19.1
r3.l
LSD between means I.88 (P=0'05)
Crustsonearlysownplotsalestrongerthanonlatesownplotsandcrust
strength decreases as water potential decreases. The difference due to
time of sowing is probably a result, as mentioned previously, of the
effect of greater raindrop action on the sr¡rface of the early sown plots
(257.6 mm) compared with the late sown (22).2 mm) r:ersultinE in thicker
crusts being formed (Plate 5'4 and 5'5)'
5 .3.2.2 Field crust strength a t the end of the season
determined with a hand penetrometer (Ps)
20.8
Late
34.5
33 .0
2L.7
15 .8
Early
A summary of analysis of variance of crust strength at the end of
B6
the season is presented in Table 5.I5'
TABLE 5.15 Fie1d crust strenqth at the end of the season - Summary
Anal-ysis of Variance for field trials '
L977
Treatment F
Agg.
Comp.
Time
Agg.-Comp.
Comp.-Time
Agg. -Time
Agg.-ComP. -Time
PVA
PVA-Agg.
PVA-Comp.
PVA-Agg.-ComP.
2.4L*
3.89*I
r{s
It
t¡
il
}TS
3 .89*
2.4r*2.4L*
* Significant at P=0.05 NS = NoÈ Siqnificant
crust strength was measured at the end of the season as it was
thought that greater differences would be evident when the crust was air
dry.
smaller differences in crust strength were observed at the end of
the emergence period (section 5.3.2.3), when water conÈent of the plots
was high, than when crust strength was determined at the end of the season
(section 5.3"2.2). However, strength at the end of the season has no
effect on percentage emergence whereas strength prior to the end of the
emergence period does.
Again there is a significant effect of aggregate size, compaction
treatment and time of sowing. The poly(vínyl alcohol) (PVA) treatment
which was used to prevent crust formation by stabiliziirg the surface
4
II4
I4
4
I4
I4
29.945
70.552
180 .417
5.L76
0.028
47 .638
1r.86Ill
tt
il
lr
2.4r*3.89*
3 .89*
2.4L*
NS
2.4r*2.4r*
ll
tf
ll
lt
94.l-28
165 .781
3.203
7 .063
8.654
3.672
L.I28
ll
il
ll
I
DF VR F VR
L976
87.
aggregates had no effeet on crust strengÈh. This reflecÈs t-he difficult'y
in spray apptication of aqueous solutions of poly(viny1 alcohof) (PVA) '
since PVA solutions are very viscousr large drops tend to fonn during
spraying. This causes some breakdown of agç¡regates, clue to inrpact, hence
causing stight crusting to occur during application'
5 .3.2 "2.L Ef fect of a te size
Corresponding treatments in 1976 and 1977 will. be discussed first'
with non-corresponding treatments being d.iscussed in the order of 1976 and
1977 results respectivelY.
The effect of aggregate size on crust strength is illustrated in
Fig. 5.6. Minimum crust strength occurred on the 4-2 nm arrd 2-l mm
aggregates in 1976 anð. L97'7 respectively. This difference is due to
differences in climatic conditions between the two years. ToÈal rain
receivecl on the plots in 1976 anô, L977 was 257.6 and 206.9 mm respectivery.
These size ranges, however, agree with the results from the laboratory
study discussed in Section 5.3-2-L.L.
Regression equations h/ere cleveloped of the form
p = a + bd + cd2 (5.5)
where p is the factor being considered (here crust strength) , d, aggregate
diameter and a, b, c adjustable parameters. This form of equation was used
because it has the advanÈage of fitting curves with maxima or minima in
addition to straight lines (if c = 0).
These maxima or minima are given bY
dnluu=b+ 2cd.= O (5'6)
d = -b/2c (5-7)
which gives the aggregate diameter for the "best" soil condition (in
this case minimum crust strengtJr) for the property that one ís considering'
500
400
300
Àf
200
100
o
o o
fse rszz
f se 1e7G
o
GÈl.
oo
o
o
0
12345678ct 2'1 4-2 t4
d ('n'',')
Effect of aggregate diameter (d) on crust strength(e¡¡) at tJle ãnil of the season in 1976 (o) and 1977
(ot. Curves are plots of Equations 5'8 and 5'9'Fig. 5.6
8B
The resulÈing equations are
= 22o - 64d + 7.Ld2, kPa, andPH
PH
= 426 - I14d + I1.Id2, kPa,
(s.8)
(s -e)
(s.rt)
(5.12)
(5.13)
for 1976 and 1977 resPectivelY-
The equations predict a minimum crust strength occurring on 4"5 rnm ald
5.I mm diameter aggregates in 1976 and 1977 respectively' The dj-fference
between observed and calculated values ries in the fact that there were
only four aggregate size classes being considered and the variatíon in the
data was relatively high. However, taki-ng this into consideration, there
appears to be relatively good agreement between the two - to within
experimental error.
The interaction between aggregate size and compaction was significant
in 1976 only. t:ne L97'l data, however, have been plotted to present a
comparison for the previous years results' From Fig' 5'7 it is seen that
minimum crust strength occurs on the 2-I mm and. 4-2 mm aggregates for the
uncompacted and compacted treatment ín L976. In L977 L]ne minimum strength
occurs on the 2-I mm aggregates for both uncompacted and compacted treatments'
on aII aggregate sizes the compacted treatment- results in a stronger crust
than the uncompacted. This agrees with results of Lemos and Lutz (f957)
and those presented in Fig. 5.3. The resulting regression equations
developed for the uncompacted and compacted treatments for L976 are
P = 163.7 - 45.0d + 5.4ð'2, kPar and (s"ro)H
P = 462.7 - 96.5d + 9.IdH
pn = 389.0 - 131 -7ô' + I30.6d2, kPa, and
2Þ =2'18.I-83.2d+8'8d"H
and for 1977 are
, kPa
kPa2
Thesegiveminimumcrustst-rengthsforaggregatediametersof4.Tmmand
Èf
100
0
500
400
300fl.
À.f
200
100
1 31-2
a:1976
se
b:1977
300
200
I
c-f o o
O
O
O
45678,1
o
2ci 2-1
o
OI se
o
oo
0
12345678Gl 2'l 1'2 '
¿
d (--)
Effect of aggregate diameter (d) on crust strength (Pn)
on uncompacted (o) and compacted (o) treatments at theend of the season in 1976 and 1977. Curves are plots ofEquations 5.10-5.13.
Fig.5.7
89
4.2 mm for uncompacted and compacted treatments respectively in L976'
irnL977 the corresponding values are 5'O mm and 5'3 mm' There is
reasonable agreement between years even though <lifferent climatic
conditions prevailed.
In 1976 there \^¡aS a time of sowing treatment, this being replaced
Ln L977 by a PVA treatment with all plots being sown at the same time'
The effect of aggregate size and time of sowing is shown in
Ta-b1e 5 .16 .
TABLE 5.16 Effect of aggregate size and time of sowing on
crust st::ength (kPa) determined with hand penetrometer'
(Mean of 20 replicates)
Time of SowingEarly Late
<l2-l4-2
>4
71.3
r04.579 -O
r14.6
LSD between means 30.o (P=0.05)
onbothearlyandlatesownplotscruststrengthtendstobe
minimal on the 2-1 and 4-2 mm aggregate sizes respectively ' The early
sown plots result in stronger crusts being formed than the late sown'
This has been mentioned previously'
Similarlythe2-Immsizegivesaminimurncruststrengthwhen
considering-the effect of aggregate size, time of sowing and compaction
(Table 5.17).
TheeffectofaggregatesizeandthePVAtreatmentisillustrated
in Fig. 5.8. I'he PVA treatmenù was used to stabilize surface aggregates
against breakdown, thus preventing to a certain extent crust formation'
The 2-t mm size gives the weakest crust for the uncrusterl (PVA treated)
348.0
117 .8
I37 .8
169.1
500
400
300
o
se
IEÀ.!
À1
200
100
o
o
12<l 2-1
45d (--)
o
3t-2
7>l
o
o
0
I6
Effect of aggregate dianeter (d) on crust strength (Pn)
on uncrrusted (o) and crusted (o) ptots at tlle end ofthe season in 1977, curves are plots of Equations
5.I4 and 5.15.
Fis.5.8
90
TABLE 5.17 Effect of aggrega'Ee size,
on crust strength (kPa).
time of sowing and' comPaction
(Mean of 1O rePlicates)
UncompacLed
Compacted
treatment was
Pn = 406 '6 ' L2L'6d + L2'39ð'2
that for the crusted treatment was
Aggregate Size Range (rnm)
kPa, and
>4
Late
19.2
150.0
LSD between means 42-5 (P=0"05)
and crustecl plots. The regression eqr.ration devel-oped for the r:ncrusted
(s.14)
P = 445.1 - IO6.6d + g.84d2, kPa (5.15)H
These give a value of minimum crust strength where aggregate diameter is
4.g and 5.4 mm for uncrusted and. crusted plots respectively. Thus one
expects weaker crusts to form on aggÏegate sizes which tend to ber larger
than the obser.i,'ed result. This discrepancy is probably a consequence of
the use of equation (5.5) which does not provide a perfect fit to the data'
AIso when measuring the plots with larger aggregates one does not know if
anaggregateorcrustisbelowthepenetrometer.Ifanaggregateisbelow'
the resulting strength will be greater than if crust were below' However'
it is not until statistical analysis is performed that any signifícant
differences become apparent - one tends to think of variation as being an
inherent factor in the results. AIIison and Moore (1956) also found the
crust strength to be reduced when the soil was treated with a soil
conditioner. However, the reduction in strength achieved here is not as
275.5
420..5
52.O
90 .5
99 .0
t36 .5
4r .5
L67 .5
r09 .5
166 .0
86 .0
72.O
I58.7r19.5
LateEarIy Early Late EarIy EarlyLate
<I 2-L 4-2
9I
great as thab achieved b1z Allison ancl Moore (f956) '
Table 5"18 summarizes the effect of aggregate sl-ze' compaction
ancl PVA treatment on the resultant crust strengths '
TABI,E 5 .I8 Ef fect of aggregate size r compaction a¡rd' PVA trea'trnent
on crust strength (kPa) determined with hand penetrometer'
(Mean of lO rePlicates)
Aggregate Sj,ze (mm)
Compacted
Uncrusted
Crusted
>4
285.O
217 .O
LSD between means 57.5 (P=0'05)
The2-tÛìIl1aggregatesformtheweakestsurfaceclustonboththe
uncompacted and compacted ptots when a PVA treatmerrt is use<ì' Howeve::'
when a pvA treatment is not applied the weakest crust is formed' on the
>4 mm aggregates whether they are uncompacted or compacted. This tend's to
agree with the aggregate size that gives the minimum strength of crust
predicted by equation (5.15).
5 .3.2.2.2 Effect of comPaction
In both l-976 and L977 a compaction treatment result'ed in st-rouger
crusts being fornred than on uncompacted plots (rabte 5'I9) ' This resull-
alsoagreeswithpreviousresulÈs(Section5.3.2.L.2)andthoseofLemos
and Lutz (1957).
fnLg.l.Tthecompactiontreatrnelrthadasignificanteffectoncrust
strength on the PVA treated plots. These are sulnmatjzeð' in Table 5 '2O '
The compaction of plots results in a strollgel: cr:ust being fonnecì
than noncompaction of plots. The fact, that on the uncompacted-uncrusted
pJ-ots, the cru.st is stronger than on the crusted plots, can be explained as
before,thatsomecrustingoccurredduringtheapplicat.ionoftheDVA.
<l352.0
404.O
2-L
r33 .0
145 .0
4-2
L44.O
L72.O
>4
19r.0
I25 .0
<1
456.O
495 .0
2-L
189 .0
2?,6.O
4-2
266.O
351 .0
Uncompacted
TABLE 5.19
Uncompacted
r13.9
TABLE 5.20
o')
Effect of a compaction treatment on crust strength
(kPa) determined witt¡ hand penetrometer. (Mean of
1O0 replicates in both Years)
t976 L917
LSD (P=0.O5)
t8. r
The effect of compaction and PVA treatnlent on crust
strength (kPa) determined witJ: hand penetrometer'
(Mean of 50 replicates)
Compacted
Uncrusted (rva¡
Crusted.
296
337
LSD between means 25.6 (e=0.05)
HovJever, tkre compaction treatment has a greater effect on crust strength
than the PVA Èreatment.
5.3.2.2.3 Effect of time of sowing
Early sowing or setting up of treatments results in stronger soil
surface crusts being formed than later sowing (ta¡te 5.2L) .
TABLE 5.2]. Effect of time of sowing on crust strength (kPa)
deterrn-íned with hand penetrometer. (Mean of
100 repli-cates)
Early Late
188.4 96.8
L7I.2 13.4 3I7 .0r98 .0
Compacted LSD (p:0.05) Uncompacted. Compacted
202
L94
Uncompacted
LSD between ueans 13.4 (P=0 "05)
93.
This is due to greater rainfall actíon on the early sown (257.6 nm)
pì-ots compared to the late sown plots (223"2 nun) '
5.3.2.3 Field crust strenqth at the end of eme¡:gence
period L9'77) determined bv hand Penetrometer
From Table 5.22 it can be seen that only aggregate size and
compaction treatment significantly affect crust strength at the end of
emergence.
TABLE 5.22 Sununary of Analysis of Variance of Crust Strength
at the end of Emergence.
Treatment F
Agg.
Comp.
PVA
Agg. -Comp.
Agg.-PVA
Comp.-PVA
Agg.-Comp.-PVA
2.52*
4 .00*
NS
NS
NS
NS
NS
Significant at P=0.05 NS = Not Significant
5.3.2.3.1 Effect of ate size
The effect of aggregate size and corresponding water potential of
the plot at the 5 cm depth on crust strengt-h i.si shown in Fig. 5.9. The
2-I and 4-2 nm aggregates give a minimum crust Ltrength- This agrees with
the laboratory measure of crust st::engttr and thaÈ determined in the field
at the end of the season. The crust strength reported here is of the salne
order of magnitude as that which Hanks and Thorp (1956) found to limit the
emergence of wheat.
4
II4
4
I4
3.55 3
37.297
o.726
0.984
0.128
2.058
0.831
DF VR
60
Fig.5.9
50
ú
aa
a'
tJg,40l¡Jt-oo.
tu¡l-
ÉsoctzTl-c,420ÉFv,
ofÉ,u10
O
Â
tI
t,ì
t,
II
I,I
II
I
0
\\\\\\ \ \\
2-1 4-2 - 4AGGREGATE SIZE (mm)
1
Effect of aggregate size and corresponding waterpotential at 5 cm depth on crust strengttt at end
of emergence detentined with a hand Penetrorteter'
.-. Crust Strength
.---o l{ater Potential>Ã
)4m mean of >4 an.l fr treatments.
94
5.3.2.3.2 Effect of Compaction
Courpacted plots had significantty stronger surface ct:usts than
uncompacted plots (Table 5.23).
TABLE 5.23 Effect of compaction on crust strength (kPa).(Mean of 40 replicates)
Uncompacted Compacl-ed
36.0 57 .0
LSD between means 7.0 (P=0.05)
This agrees with previous results reported (Sections 5.3.2.1"I and 5.3.2.2.2)
and with tltose of Lenucs and Lutz (1957).
Gifford ano Thran (1969) reported that maize exerts a maxj-mum
force of 2.4N during emergence. ALlisori (1956) using maize as a test crop'
determined that a crust strength, as determined by MOR, of I20 to 250 kPa
lirrited emergence. If one assumes a direct relationship between emergence
force and crust strength as deterrnined by MOR, a crust strength that will
limit wheat emergence may be estimated.
It has been deterrnined that the maximum emergence force of wheat is
O.5N (Section 5"2.2). If the above data for maize can be appiieci tc ctLrr:r
crops, then each N of emergence force can be impeded b)'a l4()1{ in the r¿rrrge
50-104 kPa. Hence the O.5N maximum emergence force of wheat would be
Iimited by MOR in the range 25-52 kPa. These vafues agree wíth those
determined by l{an1<s and Thorp (1956) to limit wheat emergence. However,
emergence of wheat was not limited by a hand penetrometer resistance of
53 kPa as determined in the field (r'ig. 5.9).
However, ib may not be val-id to relate crust strength, as deter¡nineo
by MOR, directly to maximum emergence force (Arndt, 1965b). The MOR method
estimates crust strength smaller than that experienced by the plant- Ihis
95.
is due to difficutty in measuring crust thickness and Iength of crack
and also because no jamming of crust plates occurs cìuring a MOR test'
Percentage emergence data from Section 7 '3'2'2 was used to
develop an equation relating percentage eme::gence, Em, to crust strengtht
E* : 100-0 - o .24 PH, s" (5'16)
The regression coefficient (r = 0.5) is not significant, thus indicating
the difficulty in relating these two paratneters. A sinrilar view is
expressed by Hadas and Stibbe (1971) who found no correlation between crust
strength and percentage emergence. Royle and Hegarty 1971) have also
developed a regression equation relating percentage emergence, E*, and
peak forcer F¡ required to penetrate a crust from above
E =g2-7.L6Et z (5'17)m
1lk¡e two equations cannot be compared because 5.17 uses force in Newtons
\^rhereas 5.16 uses crusÈ strength in kPa'
The magnitude of the crust strengths reported here do not appear
to prevent emergence, even though they li.e in tire range re¡rorbed by Hank's
and Thorp (1956) to do so.
5 .3.2.4 Air resistance of soi I crust at the end of the season
Soilcrustshavebeenblamedfreguentlyforpooraeration.However,
Van Bave1 (I95I) and Domby and Kohnke (f956) show that crusts have almost
no effect on aeration unless they are completely impermeable or very wet'
Sale (Lg64) found that v¿et crusts greatly reduced diffusion of gas, but
concluded that crusts only inffuence aera.tion when rvet, and then only if
they have few cracks. If adequate routes for gas exchange exist through
most crusts, especially as Èhey crack upon drying, then aeraÈion effects
are usually negligible, compared with other important aspects of soil
96
crusting.
ThissturfywasdonetodeÈermj-neifcrustsformedonvariousseed-
beds wj.th different sizes of aggregates, cliffered in their ability to
transmit air through into the seedbed below'
Table 5.24 presents a summary of analysis of variance of air
resislance (aR) of the soil crusÈ at the end of the season for both 1976
and 1977 fielcl trials-
TABLE5.24AirresistanceofsoilcrustaÈt}reendoftheseason-
Summary Analysis of Variance for field trials'
L976 L977
F
Agg.
Contp.
Time
Agg.-ComP.
Agg.-Time
Comp.-Time
Agg.-ComP "-TimePVA
Agg.-PVA
PVA-ConP.
Agg. -ConP. -PVA
2.39*
3 .86*tt
2.39xtt
tl
ll
3 .86'¡
2.39*
NS
2.39*
* Significant at P=0.05 NS = Not Significant
Aggregate síze, compaction treatmenL, time of sowing and PVA
treatment alt had significatrt effects on air resistance of the soil crust.
5 .3.2.4.L Effect of a re te size
Minimum air resistance occurred with crust formed on 4-2 mm
aggregates in 1976 and >4 nm aggregates in I97l' (FiS' 5'10) ' The
4
II4
4
I4
I4
1
4
34.950
37 .839
L62.932
5.605
3.245
0 .519
4.987il
tl
I
il
2.4r*3 .89*
3 .89*
2.4r*2.4L*
NS
2 -4t*ll
ll
ll
il
L28.989
L24.87L
37 .ALO
334.44I37.4r0
o.914
2.977
tf
lt
ll
tl
D!' VR F VR
7
6
a: 1976
b:1977
9É.5
o
123<l 2-l 1-2
o
12
o
45678>1
678>¿l
4
3
rig. 5.1o
3
2
1
9É.
0
<l 2-145d (..)
31-2
Effect of aggregate díameter (d) on soil crustair resistance (AR) at the end of the season inLg76 (o) and 1977 (o) - Curves are plots ofEquation 5.18 and 5.19.
9't "
regression equations relating air resistance, AR(s), and aggregate diameter,
d (nmr) , for 1976 is
(5.r8)AR
and. that for 1977 is
AR
= 6"3 - 0.57d + O-O6gd2,
= 2-6 - O.2gd + O-O25d2, S (5.le)
predicted minimum crust air resistances are given by aggregates of 4'I mm
and 5.7 mm diameter Í-:or Lg76 and Ig77 respectirzely. These values agree
closely with the observed values. D-ifferences between the two years may
have resulted because of different prevailing climatic conditiorrs ' More
rainfall was r:ecorded in 1976 (257 '6 nm) than in 1977 (206 '9 mm) during
the experimental Period.
Similarly,minimumcrustairresistancesareobserved'on4-2mm
aggregates on both uncompacted and compacted ploÈs in 1976 (Fig' 5'Ila)
andoncompactedplotslrnLSTT(Fis.5.IIb).Uncompactedplots,however,
give a minimum crust air resistance with >4 mm aggregates (FiS' s'ffb)'
The equations relating air resistance and aggregate diameter on uncompacted
ancl compacted plots in 1976 respectively are
AR = 6.0 - O.6Od + 0.080d2, s, and (5'20)
AR = 6.5 - 0.54d + 0.059d2, s' $'2L)
For 1971, the corresponding equations are
AR =2.5-0.28d+0-Ol7d2t st and $'22)
AR --2.6-0.31d+o-O34d2, s' (5'23)
Mininum crust air resistances are calculated to occur on aggregate
diametersof 3.7 mm and 8.0 mm for uncompacted prots in 1976 and 1977'
The corresponding diameter for compacted piots is 4.5 mm for both 1976 and
LITT.Theseresultsagreecloselywiththeobservedresult./\lso,the
compaction treatment gives higher air resistances tharr the uucornpactect
treatment in both years. This agl:ees rvith a statement made by Baver (1956) :
7
6
3
1
a:1976
o
123<1 2.1 1-2
3
2
0
Fig. 5.I1
cIÉ,r
4
456 I7>¡l
b ' 1977
aÉ,
o
7>l
123<f 2.1 1-2
45d (..)
I6
Effect of aggregate diameter (d) on sol-I crust airresistance (AR) at ttre end of the season on
uncompacted (o) and compacted (o) treatments' Curves
are plots of Equations 5 -2O-5.23.
98
,,,Ihe presence of a thin compacted layer in the upper layers may reduce
air: movement through the profile to an exceedingly sl'ovr rate" '
Thetimeofsowingtreat-ment,inLgT6,alsoresultedinminimum
crust air resistances occurring on 4-2 mn aggregates for both early and
late sown prots (ris. 5'I2) ' The regression equation fo:: the earry sown
plots is
AR = 6.5 - 0.34d + O-068d2, s, (5'24)
and that for the late sown Plots is
AR=6.0-O.6Od+O-O72d2, s $'25)
Equation (5.2Ð for early sown plots gi.ves a minimrrm crnst ai:: rasisÈance
wittr an aggregate diameter of 4.0 mm, r,r'hile equation (5'25), for lai-e
sown plots, predicts a minimum occurring on 4'2 nm aggregates' Thus there
is close agreement between observed and calculated values f'cr both the
early ancl late sor^tn treatments.
I.nLgllthePVAtreatmentresultedinminimumcrustairr:esistance
onthe4-2mmaggregates(noPVA)and>4mmaggregates(PVAtreated)
(Fis. 5.r3). The resultant regression eguation for the uncrusted treatment
is
AR = 2.4 - o.25d + o.ol8d2, s, (5'26')
and that for the crusted treatment is
AR=2.8-0.34d+o-o32d,2' s' (5'27)
Aggregate diameters of 5.2 and 6.6 mm, for uncrusted and crusted plots
respectively, are predicted by these eguations to result in minimum crusÈ
air resistance. These agree reasonably wetl with the ob'served minima' The
crusted treatrnent results in higher air resistances than d-oes the uncrusted'
This is consistent with results observed by Grover (1955) '
fheeffectofaggregatesize,timeofsowingandcompactiorron
crust air resistance is summarized in Ta-ble 5'25'
I
7
oø
É.6 oOO
o5
a
oo
12 31-2
456 I<l 2-1
7>4
d (-.)
4
Fig. 5.I2 Effect of aggregate diameter (d) on crust air resistance(AR) at the end of the season on early sown (r) and latesown (o) ptots. Curves are plots of Equation 5'24 and
5 .25.
3
2
1
0
rÍ9. 5.13
O
123<,t 2-1 1-2
o
t
0É.
45d (-.)
I6 7>¡l
Effect of aggregate diameter (d) on crust airresistance (AR) at the end of the season on
uncrusted (o) and crusted (o) treatlents'Curves are plots of Equation 5'26 and 5'27'
oô
TABLE 5.25 Effect of aggregate size, time of sowing and compaction
on crust air resistance (s), (Mean of 10 repficates)
Aggregate Size (m¡n)
>4
Uncompacted
Compacted5
LSD between means O-33 (P=0'05)
Î1lle4.2mmsizedaggregatesgivetheminimumcrustairresistance
with early and late sowing and on uncompacted and compacte<1 plots' crust
air resistance ís significantly greater on tl:le compacted than uncompacted
plots on half of 'bhe time of sowing treatments'
TheeffectofaggregatesizercompactionanrlPVAtreatment'for
Lg77, is summarized in Table 5'26'
TABLE 5.26 Effect of aggregate size, compaction and PVA treatment
on crust air resistance (s). (Mean of 20 replicates)
Uncompacted Compacted
>4
Uncrusted
Crusted
I.82.4
LSD between means 0.14 (P=0'05)
Minimum cr:ust air resistance occurs with aggregates >4 mm on
uncrusted plots, while 4-2 mn aggregates give minimal resistance on
crusted pl-ots. This is observed on both uncompacted and compacÈed plots '
The smaller aggregates (<I mm) give significantly larger crust air
resistances than l.arger aggregates '
Late
I2
5
6.2
5.6
6.5
6.L6.0
5.75.5
4.76
5 0
0
9
0
4
5
6
6.L
Early Late Early Late EarIY EarlyLate
<I 2-L 4-2
2.6
2.4
2
8I2 2.O
r.31.9
r.92.6
2.4
2.4
1.92.I1.9
2-L<I 4-2 >4 <I 4-22-L
r00
The aggregate sizes which gíve minimum air resistances also
gave minitnum crust strengths.
5 .3.2.4.2 Ef fect of compaction
The compaction treatment produced greater crust air resistances
than the uncompacted treatment in both 1976 and 1977 (Table 5.27) '
TABLE 5.27 Effect of compacÈion treatment on soil crust air
resisÈance (s) . (Mean of IOO reps in 1976 and 200
in I977)
L916 l-977
Uncompacted f,SO (P=0.05)
5.4 0.04
These results generally agree witfr BavertS staternent' However'
the compaction treatment has not markedly reduced air resistance to the
extent that aeration would beconre a Iirniting factor for plant growth'
In Section 5.3.2.7..I , it is shown that the compaction treatment
reduces the height of the aggregate berl by 8e"r hence a decrease in large
voids will result. An estimate of this can be marfe by referring to Table
3.7 at the 1O% levef of compaction. Larger pores are reduced in size
hence limiting gas exchange through the soil. continuity of pores may
also be decreased. This will also result in increased times for gas
exchange on the cornpacted plots compared with the uncompacted plots '
5.3.2.4.3 Effect of time of sowing ( 1976) and PVA
treatment (L977\
Timeofsowinghadasignifj.canteffectoncrustairresistance.
Higher air resistance at the end of the growing season occurred on early
5.8 0.09 I.9 2)
Compacted LSD (P=0.05) UncomPacted Compacted
WAITE INSTITUTE
LTBRîBI
qt
Late
sown than late sown plots (Table 5-28) "
TABLE 5.28 Effect of ti.me of sowing on crusÈ air resistance (s) '
(Mean of 100 rePlicates)
Early LSD (l-:=0 . 05 )
5.9 0 .09
The difference probably reflects the time difference the two sets of plots
were exposed to incident rainfall during the season. The early sown plots
received 251.6 mm rainfatl while the late sown plots received,223.2 mm.
This difference is also reflected in crust strength (Section 5.3"2'2'3) '
The PVA treatment also had a significant effect on crust air
reistance. Crusted plots resulted i¡ higher air resistances than uncrusi-ed'
plots (Table 5.2g), despite the presence of a slight crust due to PVA
application.
TABLE 5.29 Effect of PVA treatment on crusÈ air resistance (s)'
(Mean of. 2OO rePlicates)
Uncrusted LSD (P=0.05)
r.8 o .04
These resul-ts agree with those observed by Grover (1955) for uncrusted
and crusted soil surfaces.
5.3.2.5 Rainfall Simul-atlon
,Ihe nulnber of replicates was limited by the availability of sieved
aggregates, thus only the >4 mm and 4-2 rt;rr- aggregates \^lere used to test
for differences due to compaction, in time to run-off, volume of run-off
and amount of sedíment in 2oo ml aliguot of run-off. sufficient sieved
aggregates of all sizes were available to test these differences when beds
aa
Crusted
LO2.
were not compacted.
The result of analysis of variance for the conrpacted treatmenÈ
is summarized in Table 5.30-
TABLE 5.30 Effect of aggregate size and compaction on time to
run-off, volume of run-off and amount of sediment
in 200 mI run-off - Summary of analysis of variance'
Time to Run-off Sediment
Treatment VR F
Agg.
Comp.
Agg. -conp.
o.42L
0.000
1.684
NS
NS
NS
* Significant at P=0.05 NS = Not Significant
only aggregate size has a significant effect on time to run-off and
volume of run-off . This is illustrated in the foll-ovring(rable 5'31) '
TABLE 5.31 Effect of aggregate size on time to run-oft (min) and
volume of run-off (titres) from compacted plots'(Mean of 7.2 replicates)
Time to Run-off (nrin) LSD (p=0.05)
4-2
o.25
The >4 aggregates can withstand the applied rainfall (27 mm tt-1)
for a greater time than L:ne 4-2 nun aggregates before a crust forms ' Thus
these larger aggregates are less susceptible to erosion than the smaller
ones. Moldenhauer and Kemper (1969), Moldenhauer et aI. (1967) and' Rose
(196I) observecJ a similar result when they applied water at a given rat-e to
I5
87 .805
0. 378
I .56I
III
5 "32*NS
NS
L76.O95
0.857
4.661
tr ît*
t{s
NS
DF VR VRF F
Volume Run-off
25 2.5 2.8 L.4
>4 4-2 >4
LSD (P=0.05) Volur'.re of Run-off (litres)
103
various sizeil aggïegates " since the amount of sediment in t-he aliquot of
run-off is similar for each aggregate sj,ze, more soil would be lost from
the smaller aggregates, due to the increased. volume of run-off, than from
the }arger aggregates. A similar result was Observed by Oades (1976) for
PVA treated and untreated Plots-
Asummaryofanalysisofvariancefortheuncompactedplotsis
presented in Ta-ble 5.32.
TABLE 5.32 Effect of aggregate size on time to run-off, volume of
run-off and sediment in 200 mI aliquot of run-off -
Summary of analYsis of variance.
Time to Run-off Sediment
Treatment DF
* Significant at P=O'05 NS = NoÈ Significant
Aggregatesizehasasignificanteffectontimetorun-offand
volume of run-off. Trhe data are summarized. in Table 5.33.
TABLE 5.33 Effect of aggregate size on Èime to run-off and volume
of run-off from uncompacted plots (Mean of 3 replicates).
LSD (p=0.05)
F
NSAgg
Time to Run-off (min)
Volume of Run-off (litre)
3.5
0.5
The>4mmaggregatesizemaint.ainsanopensurfacestructurefor
a longer period of time than any of the other aggregates used' Consequentl'y
the amcunt of run-off is approximately half that from t-he other aggregate
sizes. since the amount of sediment in a 2oo ml.aliquot is the same for
3 4.O7*23.894 L7 .70L 2.5334.O7*
VR F VR F VR
Volume of Run-off
I82.3
15
2.9
I62.2
26
L.2
<I 2-L 4-2 >4
Loô,
each aggr:egate size range greater soil losses occur with finer aggregaLes
tha¡r coarse aggregates. Similar results were observed by Moldenhauer
and Kemper (1969), Lyles g! al" (1969) and Moldenhauer (1970).
5.3.2.6 Qua ntimet result-.s
No statistical analyses of quantimet data were calrj-ed out, because
only one Section of cr:ust was measured and only one set of measurements
were rnade on each section. This was due to the lirnited time ar¡aifable on
the Quantimet 720 situated at Flinders universit'y, south Australia'
photographs of the sections measured are presented in Plates 5'6 and 5'7
to enable comparison between tabulated r:esults (Tal¡'Le 5 ' 34) and aç¡gregate
arrangement and PorositY -
Porosity <lecreases v¡ith decreasing aggregate size within all
treatment groups except- the >4 mm crust-cornpacted treatment' This is not
as evident when the corresponding treatments are compared j-n Ptat'e 5'6 and
5.7 . Horizontal and vertical projections were measured in an attempt to
estimate pore orientation. These vlere used in a similar manner t:o
horizonÈal and vertical Feret diameters in Murphy et aI. (I97la) ' If
there was a preferred horizontal orientation of pores the vertical
projection woul-d be greater than the horizontal projection ancl vice versa
for vertically orientated. pores. If , however, there \^/as no preferred
orientation both horizontal and vertical projections would be similar'
There appears to be a slight preference for horizontal pores in all
treatrnentsr êspecially the non PvA-treatmentsr except the <I treatrnents
(plate 5.6) . This agrees with observatj.ons by oades Ãa16) foI. crust':rf
soils and Murphy et aI. (Lgl7b) for compacted soils'
Evans and BuoI (1968) have afso examined thi-n sections of soj'I
crusts. They noted some orientation of soil particles in some sections '
which may be anal-agous to the por:e orientation oJ:served in the sections '
>4U - Crusted >4C - Crusted
>4rJ - Uncrusted >4C -'Uncrusted
4-2U - Crusted 4-2C - Crusted
4-2v - UncrusLed 4-ZC - Uncrusted
Thin sections of soil crust formed under natural raínfall,L]ne L97'7 field ex¡teriment, used in the Quantimet analysis-(Scale x2)U=Uncompacted C=Compacted
i:'r- .' q
Pl-ate 5 .6 from
2-IU - Crusted 2-lC - Crusted
<lC - Crusted
<lC - Uncrrlst-ed
I
iì
2-LU - Uncrusted
<lU - Crusted
<IU - Uncrusted
2-IC - Uncrusted
;a;
Thin sections of soil crust formed under natural rainfall,L:ne L977 field experiment, used in the Quantimet analysis(Scale x2)U:Uncompacted C=Compacted
Plate 5.7 from
TABLE 5.34
Porosity (%)
Quantimet 720 results for porosity and horizontal and vertical projections
of soil crusts formed on different sized' aggregates and treatments '
Uncrusted Crusted
CompacÈedUncompacted
HorizontalProjection
vertical (Py)Projection
(PvlPH - r)
(PH)
>4
7.2
3r0
346
0.116
ts(¡
0 .096
430
392
4.L
<1 4-2z-I
1ii
15 .6
668
L7.8
284
27360
o .109 0.040
>4
18 .5
0 .087
768
706
2-L<l I
42r
469
13.6
619
649
0 .114 0.048
4.8
4-2
15.9
542
0 .040
52L
>4
Compacted
L7.5
o.097
533
585
<1
o.o22
737
72L
6.9
2-L
L3.7
0.073
570
6L2
4-2
19 -5
579
607
c.048
>4
Uncompacted
19 .5
o.L32
746
659
<I
o -oo2
729
73ll
5.3
2-r 4-2
578
63
620
649
LL.7 2L.7
0.095 0 .046
106.
Their observations, however, \^rere only visual and gualitative with ¡:o
measurements being made.
Fig. 5,I4 shows variation in porosity of the crust with depth
from the surface for variorrs treatments rneasured.. The crusts formed on
dj.fferent aggregate sizes have a low porosity just below the surface and
then an increasing porosity with increasing depth. One cannot make further
comparisons because the crusts measured were from clifferent treatments-
Generally, though, one can conclude that a small porosity is exhibited
just beneath the crust surface and that porosity increases with increasing
depth from the surface down to the base of the crust section.
5.4 Conclusions
The emergence force for wheat (Triticum aestivum L. cv. Halberd)
was determined to be 0.5N, which compares with reported emergence forces
of other plant species. The corresponding emergence pressure for this
variety of wheat is 230 kPa. Temperature has an effect on emergence force
and pressure.
Aggregate size, compaction, water potential and time of sowing
all have a significant effect on soil crust strength. The PVA tr:eatment
had. no effect on soil crust strength.
Soil crusts formed on the 4-2 rnm aggregate size range exhibited
minimum strength.
Compaction treatments resulted in stronger crusts being formed
than non-compacted treatments with aII aggregate size ranges. However,
Lt¡e 4-2 mm sj-ze range resulted. in the minimum crust strength.
As water potential decreased soil crust strength decreased, with
the crust on the 4-2 mm size range being weakest-
Stronger soil crusts were formed on early sown than late sown plots t
t}¡e 4-2 mm aggregates forming the weakest crust a.t- both sowing dates.
40
35
30
15
10
Fig.5.I4
Ë. \ .f.lf
8.7SURFACE ( mm)
I,,t,t
,,,,,
,,t,,,,tt
J\
25
\\\\IttIItI
III
fì- zoøoÉoo.
t\t
tt,,
t,
T
,tt
t,ttlt.t.
lt
tr'<lc PVA
a 1-2c
| 2-t c PVA
)4c
I
-t'
I
2.9DISTANCE
5.8FROM
J
'l/.5
.iI
0 11.6 14.5
Variation in crust porosity with depth in crustsformed on compacted (c) plots of various sizedaggregates as deterrnined by quantimet.
107.
soil crust air resistance is significantly influenced by
aggregate síze, compaction, time of sowing and a PVA treatment'
The crust formed on 4-2 mm an<l >4 mm aggregates gave the minimum
air resistance in L976 and I977 respectively'
Minimumairresistanceoccursthroughacrustformedon4-2mm
aggregates when a compaction treatment is applied' a time of sowing
treatment imposed and when PVA is applied to the aggregates '
Crustsformed'oncompactedplots,earlysownplotsandplotswith
no pVA applied exhibit hiç¡her air resistances than the correspondinq
uncompacted pJ-ots, l.ate sown plots and' plots with PVA applied'
Good. agreement was obtained betv¡een measured crust strengEh and
air resistances and calculated values, using regression equaF-ions relating
crust strength and air resistance to aggregate diameter '
crust strength at the end of the entergence period did not impe<le
emergence of wheat seedlings-
Large aggregates resist crust formation for a longer peri-od of
time tl-an small aggregates. consequently less run-off and erosion occurs
from these large aggregates than from sma-:i- agg::egates '
compaction does not affect the time to run-off or amourrt of run-cff
on a particular aggregate size.
Porosityofasoilcrustincreaseswittrdepthbeneaththesurface.
Crustsformedonlargeaggregatestendstobemoreporousthan
those formed on small aggregates'
Incrustsformedundernaturalrainfallt]:ereisapreferred
horizonÈal orientation of pores on all aggregate sizes except the <1 mm
aggregates, where there is no preferred orientation of pores'
IOB.
SECTION 6
SOTI, TEMPERATURE AND WATER POTENTTAL TN AGGRtrGATB SEEDBEDS
6.1 Introduction
Temperature, water supply, aeration and mechanical irnpedance seem
to be the main environmental factors that affect germination and emergence
of seedlings. However, there have been few field studies investigating the
effect of aggregate size on seedbed temperature and water potential.
6.I.I Factol:s affectinq germination and emergence
6 .I.I.1 Tetnperature
Each plant has a minimum and maximum temperature at which
seeds will germinate, and an optimum temperature at which germ-ination wiII
be greatest. These temperatures, however, are only approximations because
external conditions, exposure time and treatment history rvill affect them'
Results of germination studies, made at constant temper:atures, do not
necessarily reflect field germination where soil temperature at the seed
depth may fluctuate widelY.
Theoptimumtemperatureforgerminationofwheatisreportedas
being in the range IO-3Ooc (percival, Lg2I; Harrington, L923¡ Vflilson,
]92A¡ Tang, 1931 ; V,IorÈ, 1940). Differences in varieties, aeration and
water conditíons aII affect the optimum. Kanemasu et aI' (1975) reported
the optimum germination temperature for sorghum to be 23oC- This is
slightly lower than t]ìe 25oC value reported by Singh and Dhaliwal (L972) '
They also reported that wheat, peas and turnips germinated and emerged
at 5oC, but the time to emergence increased and percentage emergence was
Iow. This is in general agreement with Taylor and McCalr (1936) who
found time for germination of wheat increased at low temperatures (l2oC) '
A similar result was observed by Lind.strom -q!.r. (1976). Various optimal
temperatures have been reported for germination of various clops: maize'
I0f, "
32oC (Allmaras _g! È" , Lg64); soybean, 33-36oC (Edv¡ards, 1934) ; cotton,
33-¡4oC (Camp and r[alker, L927¡ Arndt, Lg32); tobacco, 24oC (Kincaid,
1935); sugar beet, Z5-3OoC (Leach, L947); cabbage and spinach, 8-1-loc;
capliflower and parsley, lf-IBoC; asparagus, carrot, lettuce, onionr pêâs,
radish, Èomato, l-B-25oC; cucr:mber, lima bean, 25-3OoC (Kot-owski, Lg26) .
Harrington (I923) tested the germination of many species at both
constant and alternating temperatures, He found that carrot, parsley'
timothy, awnless bromegrass, Perennial and Italian ryegrass and meadow
fescue germinatefl as weIl with constant temperatures as with alternating
temperaLures. However, redtop, parsnip, celery, orchard grass, K.entucky
bluegrass, Bermuda grass and Johnson grass gerrninated better with alternating
rather than consLant temperátures. Thompson et aI. (L977) also relnrted
that with many English native herbacious species, germinaLion response
is enhanced by temperature fluctuations of as little as IoC- Singh et a1."
(1973) reported that rice germinated better at a cons'tant temperature of
3OoC than with an alternation between 20 and 3OoC. Chaudhary and Ghildya1
(1969) observed a s j.milar resul-t.
6 .L.L.2 Water
collis-George and associates (1959 | L962, 1966), Sedgley (1963)
and Manohar and Heyclecker (f964) showed thrat the rate of germination
decr:eased as soil- water potential became more negative, until germination
ceased at -IOOO kpa. They ascribed the dec::ease in germination raLe to a
reduction in the area of contact between the seed and soil water and to
the reduced hydraulic conducti-vity of the soil. Pawloski and Shaykewich
(Lg1-2) found the rate of wheat germination greatly reduced at- -1500 kPa
water po.bential . Ashraf and Abu-Shakra (1978) observed a sj.mjl.ar resul.t.
Hadas (l-969), however, found that the imbibition state is little affected
by the total contact area and total water stress. These fact-ors mayt
r10
however, be decisive during the "triggering" stage that foflows imbibition"
Doneen and MacGiltivray (1943) examiued. many vegetable species and
found that t-he gerrnination time was shorter at a high soil water content
than at a low soil water content. Good germination of atl species lvas
obtained beLween field capacity and wilting point" IÌanks and Thor:p (1956'
1957) reported similar resufts for rvheat, sorghurn and' soybeans' AIso the
percentage germination was higher at high soil water content (18%) than at
low soil water content (1:ø). Feddes (L972) detennined the l-imits for
optimum radish germination to be betrveeu -9"8 and -49 kPa soil water
potentì-al. Hunter and Erickson (Lg52) determinec the Iimítinq soil water
potentials for the germination of maizer rice, soybeans an<l suqa= beet'
to be -I25O, -800, -660 and -350 kPa respectively'
6.1.I .3 ee-fgligl-
oxygenreguirementsforgermirrationappeartovaryconsiderably._a _2 -I
Critical minimum values for wheat range from 0.4 lo L-7 x IO - g cm s t
(Hanl<s and Thorp , 1956; Ilutchins, Lg26) ' The upper value corresponds
Èo an air space of about 16% in a silty clay loam and 25% in a fine sandy
l-oam, at water contents in the range 25% Lo 1OO% of tot-al available wate::'
These are somervhat higher than the values of 1o to 15% quoted for optimum
growth of wheat and oats in a soit at or near field capacity (Baver' 1956) '
oxygen concentration in the soil atmosphere is usually less than 20'6s"
(sto]zy, Lgl4), but the germination of wheat is severely retarded only if
the oxygen concentration is below 5%, as may be the case in saturated soils'
Concentrations of 0.5s" vüiIl completely inhibit germination (Kaak ancl
Kristensen, l-967). Carbon dioxide does not inhibit germination at
concentrat-ions of l-ess than 20s", and this concentration is not expected
under the usuar conditions of growth (Thornton, Lg44) - singh ancl Ghildyar
-B -') -](Igl7) reported a minimum oxygen supply rate of 0.6 x 1O-" g cm "s - for
LL2.
to be higher wiLh 2.5 mm aggregates than any other: size used. Bhr¡shan
et aI . (1974) using soil aggregates i¡r the Ïange 4-I2 mm, found tha'b the
larger aggreEates attained the maximum temperature earlier than the sma]ler
aggregates. The amplitude of variation in soil temperature was greatest-
in the larger agg::egate beds. Depending on other conditions, soil
temperature may or may not limit gerrnination' If aggregate sizr:
d.istribution, as rnodified by tillage, results in scif temperatr.rres being
close to the optimum for germination, increased germination and ea::Iy
seedling growth wilI resul-t.
6.1.3 Effect of so iI structure ol'ì soil rvater
several- workers have shown that the amount of water retained by
aggregates increases with increasing size (Rbro1 and Palta ' L9'7O;
Amemiya, 1965; Tamboli et al . , 1964; I¡littmus and }Iazurak, 1958) ' rlo\4'ever'
Gumbs and I,larkentin (1975) found virtually no difference in water retained
at 4O-BO cm water suction as aggregate size increased from 0.2 Lo 2'3 mm"
Capiltaryriseand.capill.aryconductivít.ywerefoundtoincrease
as the proportion of small aggregates increased (HubbeLl, 1947) and the
size of the aggregates decreased (Amerniya, 1965). Benoit (1973), however,
determined that hydraulic conductivity in<:reased as aggreEate size increased
from 0.8 to 2.0 mm, both at maximum water holding capacity and -o'5 bar
matric potential.
The effect of aggreqate size on the rate of soil drying has also
received attention. fn general, smaller aggregates reduce the drying ral-e
because water vapour transport by air con'¿ection in intra-aggregate pores
is reduced (Kimball and Lemon I Lg'lL; Farrell et aI - , 1966; Holmes et al ' ,
1960). Johnson and BucheÌe (1961) and Johnson and Henry (1964) found'
that as aggregate size irrcreased the clrying rate increased' compaction
reduccd the drying rat,e, but could. delay or prevent emergence of roaize
113 .
unless the cornpacted layer was kept moist. It has been denronstr:ated that
th.e ger:mination rate clecreases as the soil v¡ater pot-entj-al- becomes more
negative (¡lanohar and Heydecker, 1964), hence the amount of available
water in the seedbed is important, as it ì;arÇely deLermines the success
of germination and subsequent seeclling development'
6.L.4 Effect of meteoro Ioqical factors on soil- temperattrre
and water content
Meteorological factors, as well as aggregat-e sizes (soil structure) ,
influence the temperature an'J water content of soil ' Various cornbinations
of radiant energy, air temperatu::e, relative hr:midity aud wind speed have
been simulated to exanrine the rel-ative importance of these factors ' In
most instarlces these experimeuts have ]ceen conducted under isothermal and
steady-state conditions .
Hanks ar-rd l¡loodruff (1958) found the evaporation rate, from a wet
soil beneath a dry mulch, increased two to six times when the windspeed
increased from 0 to 40 km hr-I. Hadas (1975) observed that cumuiaLive
evaporation, from sieved aggregates, increa.sed with wind > continu¡us
radiation > inte::mittent radiation over a thirty day period. cary (L967)
concluded that the most effective method of reducing evaporative water:
Ioss depended in part on the vapor-rr pressure of water in the air above the
soil surface. rf the vapour pressure is relatively high' evaporation is
best reduced by screening the surface from incoming radiation, so that
it does not heat to the point v/here its vapcur pressule is greater t--han
that of the air at the surface. If, however, lhe atmosphere is dry'
evaporation controt wiII require a reduction in the coefficient of transfer
of waÈer vapour to the soil sur:face '
Keen and Russefl (l-921) examined the factors affecting soil
temperature. They found positive correlations between soil temperature ancl
II4 "
air temperature, solar radiation and hours of srrnshine. The::e wcret
however r excepti'ons ' Negati-ve correlat-ion \^¡as o]¡tained with rainf all
while wind speed had Little or no effect on soil temperature ' Similar
results were obtainecl by Balasubramanium (1966) with the exception of
wind speed, with which he found a negative correlation on soil temperature
at the 6 cm dePth.
The aim of this section is to examine how soil temperatu::e and
water potential, in aggregated seed"beds, are influenced by aggregate size,
time of sowing, presence of a surface crust and meteorological factors '
It may he possible to determine an aggregat-e size raÏIge which optinrizes
soil tenrperature and water potential , while re'lucing watel: losses' hence
increasing gerrnination and. post-emergence survival of seedlings.
6.2 Materials and Methods
Soil temper:ature and soil water- potent,ì-al were recorded. aS close
as possible, at three hourly intervals during the daylighÈ hours in 1976'
In L977, however, they were recorded at three hourly íntervals around the
clock. Soil temperature and water potential vrere recorded for the sowing
to emergence period inboth years. After a break of 2-3 weeks, a further
7 day peri-od ín 1976, and a 5 day period ín 1977 \'Ùere recorded' This was
repeated at regular intervals through the season until a total of 29 days
and 28 days were recorded for both the early and Iate sown plots in 1976
and all plots in L977.
Previously calibrated thermistors were used in conjunction with a
digital multimeter (Marconi instrument-s ' model TE261O) to measure the soil
temperature at both the 5 and IO cm depths in the seedbed' Thermistors have
a large temperature coefficient of resistance. The resistance' R' varies as
' ^ ^tt/r (6.1)R=Ae '
lls.
\^/here A and B are ad justable pa::ameters and T is the absofute temperature.
Solving for tempera-tt:re, T (oc),
in the tensiometer bodY.
Equations relating soil temperature'
(6.2)
Ts (oK) , soil water Potential,
BT = LogJR/Ð - 273.2
B is the slope when Log.R is plotted again sl I/T, and Log"A is the
intercept. A typical calibration curve is shown in Fig. 6.I" The values
of A and B were determined by calibration in a water bath using the
temperature range O-4OoC in 5oC increments " A computer program was writ'ten
to calculate values of A and B for each thermistor used in the field.
soil water potential was measured at the 5 cm dept-h ortly, using
previously calibrated Soíltest soil moisture cells (MC3loA) (Ilughes, L966)
in conjunction with a soil moistru'e and temperature bridge (Natiotral
instruments, sydney, model 2OOA) . The moisture cel-ls rvère cal j-hrated j-n
E}le >4 mln aggregates, wet to a range of water potentials (-I0, -30, -70 and
-IO0 kpa) at a range of constant temperatures (15, 20 and :OoC) . The
calibration was unaffected by temperature, so a mean calibration curve
was used for atl moisture cells (fig. 6.2). Because of the linear responsef
the calibration curve was extrapolated to estimate water potentials at the
dry end of the scale.
Both recording instrumenLs were unaffected by tem¡:erature fluctuation¡
soiltest tensiorneters (so. 2IOO) were used j-n 1976, but- under the
prevailing conditions were foun<l to be unreliabl-e due to bubble formation
V(kpa), and meteorological factors were deveJ-oped, for the 5 crn depth only,
Ts=â+b\Y+cTa+du+eh, (6"3)
for compacted plots in 1976 and unconçacted plots LnL917- FIeIe â, b, c, d
and e are ad-iustabl-e parameters and, \" the soil water potential (kPa), Ta,
oK
- 0.9
- 1.3
ØEIo
Ér" - 1.7oJ
-21
- 2.5
3.2 3.3 34
v,3.5
x'10-336
K31
o
\pical thermistor calibration curve for thedetermination of parameters in eguation 6'2'
Fig.6.1
túo-Jl
roo
0-L
-9tt ..iú4'u3o(¡,
3.0
1.0
oÐoJ
30 3.2 3.4 3.6 3.8 4.0 48 50 s.2 54
OHMS
42 4.4 4.6
ResistanceLoo"10
Fíg. 6.2 Calibration curve (o--o) for soil moisture cells.
rl6
the air temperature
humidity (%).
Equations '
(oc), u, the wind speed (rmtr-l) and h, the relative
after Rohwer (1931), relating soil water potential,
V (kPa), and meteorological factors were developed for the 5 cm depth only'
V=a(I+bu)(n5-n¿)*k, kPa (6.4)
for the compacted ancl uncompacted plots in 1976 and 1977 respectively'
Here a, b and k are adjustable parameters, u, the wind speed (kf,hr-I) and
(p" - pr) (kPa) the vapour pressure deficit, with Ps, being the equilibrium
vapour pressure of water in the soil air and pa, the vapour pressure of
water in ttre atmosphere. ps was determinecl using the following relati-onship
ps = poexp (-MVm,/pRT) , kPa (6 '5 ' 4 '7)
where p6 is the saturated vapour pressure of free water at the same
temperature, M, the molecular weight of water (I8 kg), Y*, the water potential
(pa) , e, the density of water (IOOO kg,*-3) , T, the absolute temperature (of)
and R, the gas constant (8.314 x lo3 ,rç-I xg-*orJ5. The vapour pressure
of water in the atmospherêr Par was determined using the following refation
n./no = n (6"6, 4.8)
v\rhere po is the saturated vapour pressure of free vrater at the Same
temperature and h ís the relative humidity. Values of Po were obtained from
tables in the Handbook of Physics and Chemistry (Weast, L9'73) '
values for air temperature, T¿, wind speed, u, at 2 m height' and
relative humidity, h, were obtained from meteorological records of the
lrlaite Agricultural Research Institute.
6.3 Results and Discussion
6.3.I Soil water Potential
comparisons between :-976 and 1977 data are not possfüle, because
lt.7 "
1976 data rú/ere recorded dur-ing the daylight irorrrs only, wher:cas \97'/ data
were recorde.d arouncl the clock"
The 1976 data have been summarized into tv¡o periods; firstly,
from sowi.ng to emergence and, secondly, from emergenÇe to the fj-nal reading'
This was done for both early and late sown plots. The suûllF.ary is sholn
in Table 6.I.
The data lor 19'17 (uncompacted plots only) have been summarized into
the four nreasuring periods, which can be conveniently considereo as being
the period from sor,r,ing to emergence (6/7-L6/1) and then the period fr:om
emergence to the final reading (8/8-L2/8¡ 5/9-9/9t 3/LO-7/LO) ' The
summary is shown in Tabl-e 6.2.
Statistical analysis of the data was not possiJole due to non-
replication of measurements. Also there was a strong clependence of the
data on time (FiS. 6.3), consequently if normal statistical anal'ysis had
been carried out, âtry significant differences present would have been
eliminated due to the analYsis.
6.3.1-.1 Effect of aggregate size
The effect of aggregate size on soil water potential, fo::. the clay-
Iight hours during the recorded periods in L976, is shown in T¿rlcle 6"I'
The early sown plots were wetter and remained wetter through the
season than tl.e late sown plots. This w<>ul-d be d-ue to the greater amount
of rainfal-I which fell on the early sown plots (25'i.6 nm,) than on the late
sown plots (223.2 mm). With the early sown plots the smalL and interlnediate
sized (<I and 2-1 mm) aggregates hrere the wettest. The intermecliate sized
aggregates correspond to those which exhibit the minimum evaporation l-oss
(Hadas, Lg75; section 4). It sjmj-.l.ar situation occurs on the late sown
plots also. On any aggregate síze, the water potential did not reach the
level reportecl to limit or inhibit germination (-1000 kPa) (coll-is-George
o
Soi
l Wat
er P
oten
tial
kPa
.L o oI t\) o
¡ À èI Ct) o
I æ oI ^Ðo
è o
I IF
{ P'
Q Oì (,
¡9 o oq- æ
\r__
_
, ,À
¡ 5 o ê À¡ e o JS o o
€. q¡
{ =m e D¡
an
O,'Ú
trl
o H
r-h
boF
nrt
rrO
5r) O
rto,
Ffr
do h5t+
rP
.O !, 0,
rt
rO þ
l-'
aEts
o\0
0-¡
r¡O
\,1
5.Þ
HO tso ts
.rt
PfJ o{ A
, Ê
Jrt
ct
'1 (D
õ r-
t
ctb
ooct
t-h
ooÞ fJ
rt P.
rt Ê
,!'P o x1 ol ol o.
iF
t PrÞ o5 o,o
nóo
oo l-l
H.
H.H
o Þrt o
orË
ct lj
d ort
l'1
5pr
oct É
ul l-
l oo -t
I :
, t
)
t ,
ls e o N è e o
€- d, ¿
,
t )
t t , t
f9 è êl! o ê
EË
æ
N è ê o
l\¡ o¿ (r
lo
cnoc
Soi
l Tem
pera
ture
o
TABLE 6.r soil water potentiar at the 5 cm depÈh (kPa) - sulnmary of l-976 data'
Meteorological data( for measured Period)
Treatment
Early sownSowing-Emergence(Mean of 12 daYs
Ða t MeanEarly sownEmergence-FinaI(Mean of 17 daLate sownSowing-Emergence(Mean of 23 days)
Da I ht MeanLate scwnEmergence-Final(Mean of 6 days)
Rain(mm)
22-6
39.2
27.8
¿r.zDa t I'iean1
.L
PH
oepth
5
5
5
5
r700r50
0900I 200
090012001500I700
<I
4
0r700 -30 -
0900 -28-I0 .0-r0.
-9.2-a ')
-II.3
i54.3-133.I-L39 -4-L64 -6
-)q )
L41 .8
2
2
1
1
090c -83.8
I 7 l15c0 E'1200 -59.
2-I
-19-0
-L4.8-I9.8-20.9-20.8
-26.5
-22.8-30.3
-71 .2
-80.2-65.4-73.6-89 .6
-74.9
109 .l--69.9-53.2-67 .5
4-2
-L^ )
-44 -O
-L9.92 I
-L2 -6-r9 .0-25.4
-205.9-l-65.2-158.7-18 3 .8
-45 .1
-L22.7-18.6-40.6-49.3
-I18.4
-72 -8
>4
-L3.4-23.7-t( ?
-53.8
-50 .0-57 .7
-2L.4
-rAL.7
-L43 -4-L28 -9-r35 -0-159.8
-I'c7.4
-239 -9-169 -5-L26.L-1 38. 3
2-I>4
-32.66-2
-20.2-33.1-4L -2
-94 -5
-91 .5-91 .5
-ou. /
-60.4-54.4-59.3-69 .0
-44.L-5¿. t
-23.5-29.'s-?? 6
Unconpacted
-2J.5-30.2
-L4 -3
57
2
I
-8.-15.-I8.
-26.8
Agqregate Size (mm)
-26 -9a.4
-19 .5-26.3-31 .5
Compacted
-14 .L
-15.8-72 -4
-1r9 .9-82.O-59.2-69 -2
.L2T.L
-I28 .8-l-Cg.4-rL3.2-133.3
ôa E-ô¿ -J
10 .814 .0L4 ¿6
Air Temp.(oc)
1r .0
l_0 .0L2.T
l? I
l3 .0
IOl3L4i4
52
5
0
13 -4
91
2
0
1r.L4.L4-T2
10.1lr .llr.6
r0.6L3.7l3 .6
oo
1r.6
lr.4r_I .9
ro -2
1^ 1
62
4
4
l_0.15.i5.1EaJ
LL.9
105452
(e")h
64585255
62
6559
58
52.5
]L595862
51
TABLE 6.2 soil water potential at t].e 5 cm depth (kPa) - slurunary of L977 ð'aEa (Uncompacted'
plots only).
Meteorological data(for measured Period)
Rain(mm)
2-A
0"0
TreatmenÈ
6/7-L6/1Sowing-Emergence
Mean8/8-L2/8Emergence on
Depth
5
5
Time
0300060009001200I500180021002400
030006000900r2001500180021002400
<I
-2A.O
-29.8-30.I-26.3-22.7-25.6-29.6-29.E-30.4
-30.8-3L.4-27 -4-2L.O-22.9-27 -8-3C.4-31.3
2-L
-30.5-30.9-28.7-22.L-20 -9-24.4-28.3-29 -8-26.9
-21 .9 -34 -6
-38 -2-37 .8-?q ?
-27 -l-.28. 3
-34.4-37.6-37.8
4-2
-r9 .3
-2r.5-2L.4-19 .0-t5 .0-L5.2-19 .3-20 -9-2L.6
-54 -8-54 -9-52.3-38 .8-40.2-48.8-53.0-54 -L-49.6
>4
-30.6-3L.4-25.4-L7.I-L8 -2-23.4-27.O-28 -5-25.2
-83 -2
-88.6-88 .7-19.8-55 .3-t¿-z-90 .0-96.9-94.L
>4
¿-L
-tt.¿
-80.0-77 -O
-64 -2-61 .L-'76.4-88 .8-83.1-81 .0
-12.L-74.r-65.4-48 .8-55 .9-73 -9-15.4-74.5-61 .1
UncrustedPVA
-3r.4-3L.2-2'7 -8-2L -7_2L.L-25.9-29.2-30 -4-27 .3-6s .3-64.8-60 .9-43.7-49 -L-
-62.6-68.6-61 .B
Aggregate Size (mm)
-43 -4
-45 -6-45.3-37.6-35 .9-4T -4-48.5-46.4-46.L
Crusted
-60.4 -44.8
-48 -5-49.9-43.2-33.0-38.4-47 .3-49.3-49 -O
AirTemp(oc)
L2 -O
II10 .7
1010l1L415L2
.6()
.J
^.5t
.0
15.5
L4 -2L4.OL4.1L7.619.t15 .915 .313 .9
Windspeed(kmh-r)
t0.61
LO.2LLO.229.64
L2.87LI.22r0 .3810.03r0 .37
15 .16
13"94L] .23L4.99L5 -'2!L
23.48IL.24L2 -74L2.44
ReI.Hum.(%)
AA
50504538464853
oz
686665565l6I6769
41Mean
. ../continued P-\Ð
TABLE 6.2 continued
Treatment
s/e-e/eEmergence on
Mean3/L0-'1/LoFinal Reading
Depthr
5
5
0 30006000900I2001500I8002I002400
80021002400
0 30006000900L2001500
Time <I
-L66 -L
-160 .5-160.4-l-58 . 3
-L79.4-193.9-L79.8-r53.6-l-42.7
-291 .L-267.2-4 08 .9-53'7 .2-549.6-525.7-370.9-589 .9-443.3
2-I
-60 .6
-6L.2-61 .O-67.3-56.7-51 .2-60 .8-58 .8-56 .1
-94.6-r00 .5-r34.5-I27 .e-L24.7-13r.2-L29.6-r34 -4
4-2
-9A.7-95.4
-105 .9-r13.6-107 .5-L06.2-] ^o o
-87 .5-10 3 .1
>4
-r35 -6
-r33. I-LL5 -2-L46.2-L25.L-145.I-L36.2-148 .0-L43 -9
-225.1-220.8-34L.2-463 -3-5L2 "3-387.1-260.I-375 .8
-325.O-27 3 -'7
-686.9-500 .8-564 -O
-355.r-265 -2403.3
-42L.7la 2 z 2 3 Ào 3
2-L>4
-L27.L
9
3
92
ö4
-r37 -2
-119.-1 )1
-I03.-116.-133.-L26 -
-L28.
-583.9
-395 -6-368.8-675.8-459.6-671 -1-618 -7-1L3.6-lor.2
UncrusPVA
-tr9 .8
-LL9 -4-11 3 .0-rl5 .9-1r5.0-131.3-L2L.4-L22 "2-L20 -4
-46'o -L
-330.3-277 -4-RO] )
-514.O-6 30 .1-5L2.5-334.4-508 .9
Crusted
Aggregate Size (rnm)
-tL9.4-Lr3 -2-116.8-L2L.4-L23.7-I22 -4-LL7.5-106.6-tr7 -6
-301.6
-205 .0-2L5 -O
-307 .7-29L -4-34L.2-318 .7-361 .3-372.9
AirTemp.(oc)
9.0
0o
5l43
5
3
7
ll011ll
9Il
II.6L2.215 .811 a
L8 -4t6 .313.913.7r4 -9
L3.47
l3 .99t2.9915 -2415 .9914.14rl .09LL.74L2.O4
II .9I
9.94]0.34r5.311? LL
L3.7L10 .9LL.4610.96
wrnd'soeed
' -l(knh -)
Rel-.Hurn "
(%)
60
72705348L960686-l
51
64645l4439465549
Meteorological data(for measured Period)
Rain(mm)
r9 .6
Ht\)o
Meanr.4
L2L.
and sands , l'962i Pa\^7fosl<i an<l shaykewich, I972¡ Ashraf and Abu-shakra,
1978) . This hras true for both the early and late sown ploÈs.
Duringthedaylighthoursfortheear^Iysownplotstherewasa
general trend on al-I aggregate sizes of water potential becoming more
negat-ive from o90o hr to l-500 hr and then 1.ess negative. on the fate
sown plots, hov¡ever, the reverse occurred. No explanation can be offered
for: this discrepancy. on certain days this may occur due to the surface
heating and vapour moving down the profile and condensing as it becomes
coofer, but: these conditions would not be expectecl to p::evail for long
periods of time. Possibly, plant growth may affect water potential later
in the season, by shading thereby reducing the incident radiaf ion on the
surface of the PIot.
The early so\^7n aggregate beds dried out during the season, vrhereas in
the late sown plots \,^rater potential became less negative on aII plots,
except the >4 mm size. This may result from the small number of recorded'
days after emergence, for the late sown plots compared with ihe early
sown plots. Also it is possibly a response to rainfa]l over the l-6 anc
18 days , for the late and early sown pl-ots respectiveJ-y, prior to the
measuring period, which v/as greater for the late sown (37'2 nun) plots than
the early sown (13.6 nm) plots. Consequently the Iate sown plots would
appeartogetwetterduríngtheseason,whiletheearlysownplotsappear
to dry.
Theeffectofaggrega[:esizeonwaterpotcntialj-nL9TTisshown
in Table 6.2.
water potential is affected by aggregate sIze, with the il-rtermediat'e
sizes being the wettest throughout each rneasured, period and the season :ls
a whole. This is in agreement with the 1976 resu-].Ls and it alsc coi:respcnds
with the aggregate size that exhibits the minimum evaporation l-oss (Hadas'
L22.
L975¡ Section 4). During the sorving to emergence period the water
potential did not reach a Level \^7here it was likel.y to limit or prevelÌt
germination of seeds (Ashi:af ancl Abu-Shal<ra, 1978) -
During the first two recording periods, the water potential. became
less negative from 0600-l-200 hr and. then more negative to 2400 hr- This
is the f:everse of what is expected (Rose, 1968), where the soil dries
during the day and then becomes wetter overniqht. Durinq the last two
r:ecordinq periods, however, the expecl-ed trend in water potenl--.ial' or
content is foJl-owed (Fiq. 6.3) .
The water potential of the seedbeds becoirres moro negative as t-he
season progresses and as the seedbed dries (¡'ig. 6.4) . This ir¡ si¡r'tj-Jar
to the early sowrr plots in 1976.
There is a marked difference in wate:: potential at- corresponcling
times during the day between the two years. This reflects the difference
in rainfall between the two years with 257.6 mm being recorded in L976
and only 206.9 mm in 1977 during the experimental period's '
6.3)-.2 Effect of Compaction
The water potential was monitored only on the uncompacted pJ"ots
during 1977 (Table 6.2) . However , rn L976, both uncompacted and compacted
treatnents were monitored for the early sown plots and the compacted
treatment for the late sown plots only (fabl-e 6.f). Discussion will be
confined to the L976 data.
On the early sown plots the compacted treatntent is drier than the
uncompacted treat:nent. This is consistent with results in Section 4 where
the compacted aggregates lost more water, by evaporation, than the
uncompacted aggregates. Thi.s is also tJce case when a soil surface crust
is present. AIl plots in 1976 were crusted over (Plate 7.9 and 7.1-0) " The
Iate sown-compacted plots were consi<lerably clrier than the coLresporrding
6h -16/7
rig. 6.4
t/a -tzlt s/s - s/s sfio - z/to3 12
0
-100
- 200
-300
- 400
500
\\\
\
lÉo-i<
J
l-zl¡¡l-oo-
É,l¡J
k3
\\ \\ \\
VJater poÈential at the 5 cm depth during the day an{ through the season for l-977
(uncompacted Plots only) -
potenÈials are means of all treatments and days in the stated periods'
123
early sown plots. Bowever, the difference between the two sowing dates
became smal-l-er toward the end of the season-
6.3.1.3 Effect of tj-me of sowincl (1976) anil PVA
treatment (L977)
Earl-y sown pl-ots \^/ere considerabl)z wetter thair late sorvn plots
in the sowing to errÌergence period and elnergence to fínal readinq period
(Table 6.f). This reflects the. difference in rainfa.l-t between the earJ-y
(2.5'1 .6 mm) and }ate sowing date (223-2 mm) . The t-rend is the same on all
aggregate sizes ancl with the uncompacted and cornpacte<1 treaÈmenL--s '
In :-977 from sowing to emergence (6/7-16/7) the uncrusterf treatment
is wetter than the crusted treatrnent (FiS. 6"5). This may be due to less
infiltration of rainfall on the "rrr"tua plots than ot-r the uncrusted plots.
However, as the Season progresses, the uncrusted treatment tends to be
drier than the crusted treatment (FiS. 6.5). This is consis.Lent with the
resuLt in Section 4, where the presence of a surface crust- reduced
evaporat-ion losses to appr:oximat-ely one-half that of the uncrusted surface '
The same general trend j.s shown with uncrusted and crustecl surfaces
as with aggregates (compare Fig. 6.4 and 6.5), where the beds are drying as
the season progresses. The uncrusted surface, however, results in a
greater dryíng than the crusted surface, especially at the end of the
season. This is when the plant may suffer water stress ancl final yield
may be affected. This may not be important as the plant would ì:e extracting
\^rater from greater depths than 5 cm (where water potential was monitored)
at the end of the season.
6.3.L.4 Ef fect of meteorological factors
Equation (6.4) was expanded as follows
v = k + a(Ps - pa) * ab.u(ps - pa), kPa (6.7)
aft - rclt als - tzls s/s - s/s slro -tlro3 12
12
o-o' 'o'o- o'
12
O- 9. o o-o-o
I
o
otI¡IIt¡to
ol!o-.j<
J
trzt¡J
bo,
É.u¡
k=
0
-100
-200
- 300
-400
-600
.qb'
'o
water potential at the 5 cm depth during the day and through Ètre season fot L977
on uncrusted. (r--o) and crusted (o o) beds. Potentials are means of aII times
for atl days and tie"t*.ttts during Èhe recorded period'
- 500
Fig. 6.5
r24.
and the values of the adjustable parameters, k, a and ab determined for
the sowing to emergence and emergence to final reading periods fcrr the
early and l-ate sowing dates in 1976. e similar analysis was performed
for the sowing to emergence (6/7-L6/1) and emergence to final reading
periods (8/8-12/8, 5/9-g/g and 3/IO-7/LO) in L977 '
The regression equations developed for L976 pertain to the compacted
treatment, while those for L9'77 pertain to the uncompacted trea'Lrncnt'
Theregressionequationforthesowingtoemergencepe::iocl(1976)
with the early sowing date is(6 .8)kPa,
and that for the late sowing date
Y = -92.6 - L27.3 (ps - pa)(+ 2.5) (+ 8.3)
Y = -IO5 .7 - 64.8 (ps - Pa)(+ r.9) (+ 3.4)
V = -1I5.9 - 52.9(ps - pa)(+ 2.7',) (+ 5 .2)
y = -IOO .2 - Io5.2 (ps - Pa)(+ 4.0) (+ 4.7)
+ 3.8u(ps - pa)(+ o.8)
- O.O67u(ps - pa)(+ o.24)
The regression equations for the emergence to final readiug period
kPa (6.e)
(6 .10)
(6.rr)
(1976) are
+ 0.77u(p= - pu.) , kPa(+ o.29)
+ l.OOu(p" - Po) , kPa(.{- 0 .31)
for the early and late sowing dates resl>ectively. The values in brackets
are the standard errors of the parameters, Equations (6'B), (6'9) ' (6'f0)
and(6.II)accountfor83.6>",66.6%,4L.3%and84.5%ofthevariationof
data respectively. This indicates that meteorological factors can have a
considerable influence on t]-e soil water potential.
When(ps-pa)ispositive,watervapourmovesfromthesoiltothe
atmospher:e (the soit dries out). However, when (ps - pa) is negative'
water vapour moves from the atmosphere to the soil (atmosphere dries) '
During the periods of measurement (p" - p.) was always positive " As
(ps-pa)increases,thewaterpot-enti.albecomesmorenegativeaccordinqto
r25.
equations (6 . B) , (6 . 9) , (6 " l-0) and (6 .1.1) . The reverse is true whert
(ps - pa) decreases " It can be seen from the equations that the vapour
pressure deficit (ps - pa) term has a larger effect on soil water potential
than the wind speed (u). Equations (6.8), (6.r0) and (6.rI) show that as
the nind speed increases the soil water potenti-al becomes Iess negati've "
one woul<l expecL that as the wincl speed increased the water vapour would be
r:emoved thus increasing the vapour pressure deficit and:lesulting in the
soil dryinq. This is the case as described in equation (6.9) " No
explanation for this ptrenomena in equatiorrs (6-8) , (6'I0) and (6"11-) can
be forv¡arded. Ït may depend orr whether the wind is constant or ciusl:y and
whether or not the moving air is saturated or dry. Al-so these eguations
only apply to daylight readings. This may introduce soriìe unintentional
bias to the resurts. rn the emergence to finaL reading perioc one would'
expect the influence of wind on soil water potential to be minima-l, due
to the presence of the crop increasing the thickness of the boundary layer '
There wou]d be very little air movement close to the soil surface, hence
the vapour pressure def icit woul-d exert mor:e inf lu4jrrce on slr> Ll wate::
potenti.al than the rvind. The vapour pressure deficiL is inílu::rced by t-'he
relative humidity which in turn is affecte<l by air temperature and solar
radiatioll. Thus if any of these factors change markedly, the vapour
pressure deficit will- be altered. Consequently the soil water potential is
influenced indirectly through the factors which influence the vapour pressure
rleficit. Equations of the form of equation (€'.1) were developed because the
meteorological factors can be summarized within the vapour deficit term"
Theregressioneguationsdeveloped-forthefourrecorded¡:eriods
during L977 are similar to those for L976. This is surprising corrsideringt
Etle L97'l data was col]ected throughout the whole day and th'rt in 1976 during
daylight hours onlY.
Forthesowitrgtoemergenceperiod(1.977)theequationrelating
126.
soil water potential' V, and meteorological factors is,
Y = -86.3 - 9l.7(ps - pa) - 0.60u(ps - Pa), kPa(+0.7r) (+3.7) (o.2e)
An equation \^/as developed for each measured period after emergence
(Lg17) " These can be summarized as follows for 8/8 Lo l2/8,
(b.t2)
(6.r3)
(6 "r4)
(6.r5)
V = -85.7(+r.r)
- 77 "6 (ps - pa)(+4.0)
- 0.54u(p, - p.) , kPa(+0.r7)
r.or 5/9 to 9/9,
Y = -78 .4 - 66.2 (ps - pa)(+o . e7) G4 .7)
and for 3/LO to 1/LO,
Y:-89.5-90.S(ps-Pa)(È . 3) (!2 -5)
- o'82u(P= - Pt) ' kPa(+0.28)
- o"33u(p= - no) , kPa(+0.r7)
Eguations (6.12) to (6.I5) account Írot 84.3, 84-3, 84.2 anð' 92'1 petcent of
the variation respectively. The (ns - na) term was always positive as
during Lg76. According to equations (6.I2) through (6.15), as (p= - p.)
increases the water potential becomes more negative ' The reverse occurs
as (n" - na) decreases. The vapour pressure deficit (ps - pa) term has a
larger effect on soil water potential than the wind speed (u).
The eguations for 1977 show that as Ùhe wind' speed increases the
soil water potential becomes more negative. consequently as the wind speed
increases the soil dries, and vice versa. This is the reverse of what is
predicted in all equations, except equation (6'9) , Lor 1976'
Equations were derived for each aggregate size in 1976 and 1977 and
for the uncrusted and crusted surfaces in Lg7'7, for each recorcled period'
The parameters for each equation are presented in Table 6.3 and Table 6'4'
For both the 1976 and l-977 equations (Table 6.3 ancl 6-4) the onJ.y
parameter to change within a measured period j-s k'
It is observed that the value of k is smallest for the 4-2 mm (L977)
aggregate size j.n all measured periods. This may j-ndicate that this s-i ze
TABLE 6.3
Period
L976 EarLY Sown
Sowing to Emerg.
1976 Late sownSowing to Emerg.
1976 Early SownEmerge + Final
1976 Late Sown
Emerge + Final
parameters for equation (6.7) for aggregate sizes for each recorded'
period Ln L976-
Parameter
ab SÐ
0.830.830 .830 .830 .83
o.24o.24o.24o.24o.24
o.29ñ)qo -29o.29o -29
o.32a.32o -32o.32o.32 ts
NJ\i
Aggregatesize(mm)
>4
4-22-L<l
>412_L
>4
4-22-I
<1
'4/z-t
>4
^_12-L<I
'4/z't
>4
4-22-r<I
'4/z-t
k
-92.4-93 .5-92.4-93.1-92.3
-107 .1-I04 .8-109 .1-r05 .0-l.O4.9
-I15 .0-LL9.6-Ir1 -9-119 . t-1r3 .5
-100.1-100 .4-100.4-r0r .3
-99 -7
car
3.23.23.2J.Z
3.2
2-O2.62.62-62-O
3.23.2J.Z?)3-2
a
-L27 -2-L21 .2-L27 .2-L27.2-L27.2
-64.3-64.3-64.3-64.3-64.3
-5L.2-5L -2-5L.2-5L.2-5L -2
5.05.05.05.05.0
-I05 - I-105.1-105.r-105 . r-r05.1
SE
OA
8-4AL
8.48.4
3.5'E?q3.53.5
5-2q)\)5-25.2
-0.05-0.05-0 .05-0.05-0.05
+3 .9+3 .912O
+3 -9+3.9
+0 .7E+c .78+0.78+0 .78+0 .78
4.84-84-84.84-8
+I .0+1.C+1 .0+l .0+f .0
t2.8.
TABLE 6.4
Period
6/7-16/7Sowing toEmergence
B/B-L2/BEmergenceon
s/e-e/eEmergenceon
3/LO-1 /LoEmergenceon
Parameters for equation (6-7) for: agg¡regate sizes and
surface treatmetrts for each recorcled period in L911 '
Parameter
-0.33-0.33-0.33-0.33-0.33-o.33-0.33
SE
o.29o.29o.29o.29o.29o.29o.29
0 .070.r7o.Ll0.t7o.r10.r70 .17
o.28o .28o.2ao .280 .28o.2a0 .28
0.i70 .170.r7o.L-i0 "170 .17o.L]
I045
62
I
-89.-88.-90.-89._oo
-90.-89.
>44-22-L<I
'4/z-tUncrustedCrusted
)12.5
2.52.52"52.5
2.L2.L2.r2.L2.LL.4r.4
-90 .0-90 .0-90 .0-90 .0-90 .0-90.3-90 .3
-78.1-15.7-78 "7-79.O-78.1-79.4-17.1
>4
4-22-I<I
'4 / z-lUncrustedCrusted
-64.5-64.5-64.5-64.5-64.5-65.7-65.7
L.4I.t,L.4L.4r.41.01.0
-0.87-0 .87-0.87-o.81-0 .87-0 .84-0 .84
4-1
4.14.14.14.74.1
>4
4-22-L
<l_>4 ¡ z_7.
UncrustedCrusted
I.81.81.81.8I.8L.2L.2
-85.7-84 .0-84 .5-85 -1-85 .9-86 "6-85.2
-76.5-76 .5-76 "5-76.5-76.5-71 .o-71 .o
-0 .56-0 .56-0 .56-0 .56-0 .56-0 .55-0 .55
4.r4.r4.L4.L4.r4.r4.L
>4
4-22-L<l
,4/z-:-UncrustedCrusted
-86.2-86 .0-86 .3-86.2-86.2-86.3-86.2
3.73.t3.7J" /
3.15. t3.7
IIIII00
2
2))2
B2o1
-9L.7-9L.7-9L.'ì-9r.7-9L.7-9L.1-9L.7I
I
-0.60-0 .60-0 .60-0 .60-0.60-0 .60-0.60
AggregateSize(ntn)
SE1- SEa ab
L29.
range results in the l.east negative \n/ater potential wit-hin the seeclbed '
This sitrral:ion is simil-ar tc¡ that ir-i Section 4 where a snall value of k
occurred on the size range rvhich had the lowest evaporatj-on ratio hence
Iost less water. The valr-re of l< is approxinatety the same for each
aggregate size. consequently it worrl-d. be possible to derive one ecluation
for each measured period". It is obvious that- the aggregate sizes used <1o
not markedly affect soiI vtater potential (no one aggregate size appear-'s
to be better tl-ran any o1:her with regard to optimizinq scil water pctent:al) '
soil water potential appe,ars to be influenced rnore b)' neteor:ological
conditions than aggregate sizes. The lack of sígnificance of aggregate
size may be due to tJ¡e fineness of the aggrega,t-e sizes used, s''ren of the
Iargest size used. The inrportance of convective transport in chanqing or
modifying soil- water content/poLential has been appreciated recently
(Far:rell et al . , L966; Ojeniyi, l-978) . These workers, however, stress bhe
importance of pores of around I mm diameter as being the minimum size
which allow ef fective collvective fl.ow through t--i].lerl soil ' It is noted
from Table 3.7 that pores of this size occur: on-1,y in beds of aggregates
>4 mm ancl then they form l.ess than l-oø" of the macropores present'
consequently, changes due to convection are limited to the large aggregar-es'
Another factor also j.s that the aggr:egate beds were crusl-ed over in 1976
and in Lg77. This would further reduce the effect of convection as a
mechanism for changing soil water potential '
ojeniyi (197S) found positive relations between soil structure and
some meteorological factors. However, he worked with coarser til-ths
than those used in this study and also used a different for:m of regression
equation"
Changesintheparameterkbetweenperiodswouldbeinresponseto
charrges in meteorological conditions as the season prcrgresses '
130 .
Eguation (6.7) takes no account of the fact t-haL the v¡ater
potential at any time, t, is usually heavily dependent on the water
potential at time, t-1. In future study or analysis of results this
factor should be considered.
6.3.2 Soil Temperature
Direct comparisons between 1976 and 1977 data are not possible,
because the 1976 data were recorded during the daylight hours only, whereas
the I977 data were recorded around the clock-
The 1976 data have been summartzed into two periods. Firstly'
from sowing to emergence and secondly, from emergence to the final reacÌitlg-
This was done for both the early an<l late sown plots. The suntnary is
presented in Table 6.5.
The I977 data have been summarized into the four measu.::ilg periods,
which may be conveniently divided into the sowing to emergence period
(6/7-L6/7) and the emergence to finat reading period ß/e12/8¡ 5/9-9/9¡
3/IO-7/LO). The summary is shown in Table 6.6.
statistical analysis of the data was not possible due to non-
replication of measurements. There was, however, less dependence- of the
data on time (Fig. 6.3), compared wittr water potentí.als. i-E n¡rma.t
statistical analysis was performed any differences present dr:e tcr
treatments would have been efindnated by the analysis'
6.3.2.L Effect of aggregate size
Table 6.5 shows a summary of the effect of aggregate sj.ze on soil
temperature during L976. These are daylight temperatures only.
At any depth on any sowing date there is very little <lifference j-n
soil temperature between aggregate sizes. In general though the larger
aggregates tend to be warmer than the smaller aggregates at both the 5 cm
TABLE 6.5 Soil temperature at 5 and 10 cm depths (oC) - stlÍrmary of' L976 ¿ata'
Treatment
Early sownSowing+Brnergencellean of 12 daYs
Da ri t Mean
Da 1i t MeanGrand i',IeanEarly scwnEmergence+FinalMean cf 17 daYsDa 1ita t Mea.n
Da ri t Mean
Meteorological data
ReI.Hum.(eó)
Kal_n(mn)
22.6
?o,
PUJF
5
nepth
IO
5
Time
0900I2001500I70C
0900t 200r500t 700
0900r700
i rzooro I ogoo
<l
q^)
L3.2Lt3.4ILL. !)LL.94
L\.62
I .651) ¿.411 aAIJ. J9
L2.A5
L¿. z\)L2-24
2-L
1r-54
9.4812.7312.61II .28
11.78i li-69
9 .1012. a91) t1 C
't f ?o
II.OJ
-LU ..f úI3-82 \2.9C
L2.28 jl-i -99
I
IIl.
I
LO.9213.65
l_.r .:! 5t_2 83
IL.92iI.86
4-2
1r .96
9-8313.48L3.28IL.27
tt-93t-l .90
9 .0513 .841? ¿q
LL.27
L2.2L
11.31L3.T2
f a ?oL2 -51
LI.2713.88
9.51L3 -29r3 .00I1.I7
>4
10 .30L6.L2r .) ot
9 .89
LI.]4
12 -O2
r a a^
l_1 - 84
LI.L2L2.57
r? 1q1) L^
1) a7
12 -45
Aggregate Size (mm)
>42-L
9.26L2.8C13.08TI.64II .69
''l 1 qLL¿. ZV
9.L614.09a 2 0-
11-69
LA.4612 q'7
T2.IL.
l0 -6313"66
it .71
11 0,
Uncompacted
LO.82
8.7010 .7012 "IOli-78
9.251 1 CC
r3.4011-25
ii.391l-.97
f 1 tr?LL.JI
I0 -00r3.14
1) )L
1l-1513.34
1 1 0'1
Compaction Treat:nent
Compacted
L2.72
10.3015 .5014.c511.06
L2.44
1I.98L2.9L
Aír TemP-(oc)
L3.2
(]
0
6
6
1^
L4l3
IO
1i "0
'laì ô
L2.L
wlnoSpeed{kú--)
LO -2
101I1lI
II6aJ
'l't ^
IIi1
49
58
É. /1
5258
6559
G::and Mean
.r'continued
TABLE 6-5 continued'
Treatment
Late sownSowing+EmergenceMean of 23 daYs
DA Ti t Mean
D ii ht MeanGrand MeanLate sownEmergence+FinalMean of 6 d.aYs
Da Ii t Mean
l_ t Mean
Depth
5
l0
5
IO
0900L200I5001700
0900I 200l5 001700
0900I 200I500r700
0900L2001 500I 700
Time <II
I
L2 -58
9.9813 -621A 6L
t2 .10
il.14
I .951I .081) ¿.9
12 -a7
16-06
L2.74r'7 -51I O ??
15 .56
t4 -51
L1,.461t 1î,
I 6 r-,o
1"5.60
2-L
LO -49I ¿ ??
15 .09T2.L2
1l-96rr - 861 L2 -48
15.36
1) 6A
L] -38T1 .2LL4.22
i1.5813 .96'tq ?q
L4.59
r_5.31i14"60l3 .85
4-2
13. OO i r¡. Zr
10 .19L3 -7414.5314 -40
12.LL11 .01
9.26tl-39L2.58].4.62
I-91IO .8IL2.331'r oo
15 -52
't, trô
I1.0414 .6814 .85II .5I
8.98lr-131? q?
L2.OO
l-3.o2
12.o911 .16
.l_5 .75
13.36L8.2111 ?L
14.L4
12 -61 i
L7 -L7 i
i7.sì i
ra--75 \
L3.61 ì 13"84
-ÉL!.¿)
L3.ol15-1114 -61
lr .3614 .01-f E tôLJ . J >
i4 .60
1 À ?ô
Aggregate Size (mm)
>4>42-I
I
12.55
10 -1973.4i14.30ì I îtrL'. ¿J
L2.O2TT.L.9
9 .3r.ì1 LtL2 -96
l-4.96
14.64r4.33
LL.641L ¿.q
L6 -3614.83
Uncompacted
L2 "6L
t0-r8L3.16L4.54l1 .98
I-9610-91L2 -43iI-98
1C ?q.
1^ ^\
L4 -16
L2.4I16 .68LO-lZ
L4.35
L2 -2411 .841I .07
t5 "o4
14.4'713 .90
11"3313.9615.61i4 -70
Compaction Treatrnent
Compacted
L2 "89
L2.T311"37
9.20It-3512.7312.20
16.o2
13 -0618"02f ? ôo
15.o2
r5.ll1A 2",
lr .591 n ?ô
15 -84i5 "o2
Air Temp-(oc)
13 .0
r0"5714 - l_8
L4.821r .99
r0 "513 -2L4 -514 .0
LJ.4
ll .9L4 -7L4 -2L2.8
wanoSpeed(kr,h-1)
ll .9
r0.6L3.1IJ.O
oo
L4.L
Éa
64tro
5255
o¿
10.6i5l_5
l5
2IL
^
1Lqc
5A62
Meteorological Data
I{CI "
Huin.(%)
Rat_n(nm)
21 .8
2L -2
Pl-ùi\)
Grand Itlean
a')4,
and l0 cm dept-hs- No one aggregate size appeörs to be markedly warmer
or cocl-er, during the day, than any obher size"
on afl aggregate sizes and. at both depths the late sowt-¡ plots
are vrarmer than the early sown plots, except for the early sown pÌots
at IO cm in the sowing to emergence period whj-cl] are r¡tarner than the
corresponding tate sown plots. Air temperatures tend t'o be warmer later
in the season a1so.
onatlaggregatesizesandonbothsowingdatesthe5cmd'epth
generally is warrner than the 10 cm dept-h during the daylight bours"
The maximum temperature, on each aggregate si-ze' occurred at
either 1200 or l5oo hrs, rvith Èhe late sown plots tending to reach a
maxinum at the l-atter time-
ft should be noted that the temperatures on all- aggi:egate sizes,
during the day, fall within the range generally accepted' as being optimal
for germination of wheat (Percival, L92L; Wort, f940) '
The optimum temperature for root development of temperate cereals
Iies j-n the range LZ-2soc (IIagan, Lg52). Tn 1976, at the I0 cm d'epth,
all aggregate sizes with the early sowj.ng date were very close or within
this range tlrroughout the season. The aggreEates in the late sown plots,
however, only reached thi.s temperature at the l-0 cm depth, after emergence'
At the 5 cm depth the larger aggregates (>4 Ûun) generally reached
the maximum temperature earlier than the small aggregates (<l mm) ' This
agrees with the result of Bhushan et aI. (1974) '
In L9':-7 soiJ- temperatures \^rere recorded around the clock at both
the 5 cm (so,øi-ng) and IO cm depths (table 6.6). Throughout the season
rdne 4-2 mm aggregates were the coolest at the 5 cm depth and warmest at' the
IO cm depth compared v/ith aII c¡ther aggre-gate sizes. Tn the f i r'si- two
recorded pe::iods the larger aggregates (>4 m¡:) were warmer than the sma-l'Ier
aggregates (<I m¡n) at the 5 cm depth, with the reverse occurri'ng cluring the
TABLE 6.6 Soil temperature at 5 and I0 cm depths (oC) - surunary of' L977 data'
Meteorologica-I Data
Treatment
6/t-L6/7Sowing-+¡¡¡ergence
IuIean
MeanGrand Mean
68666556516I6769
Râ]-N(nm)
2-O
P(,,è
D
5
l0
Time
0300060009001200i_500I80021002400
<l
9 .81
6 .806 -529 .00
15 .0515 .57LO -258.r37 .L7
10 .09
0 30006000900L200150018002LOO
2400
8.648 -248-5r
11.4113.r3LL.6210.02
9 -L4
qqq
2-L
9.95
6.81
9.L2r5.31L6.O2I0.39
o 1<
1-2r
6.5
'ì^ ll
8 -19I .40B .6i
It. t8L2 -82rI -61-1ñ 1L
9. 30
9-10qqÃ
9.539 -66
L2.L213 .88I¿. lO
r1.31io.L,7tt_ - 21
9 .91
5.995 -7r8 -29
L4 -^_t15 .039.62a ao
Á?q
6 -596 -399.]L
1q. o.7
16.III0.05
7 .976.93
t-0.05i0,03 l0 .1510 .14
o
ö
11I3ll
99
q)
.13_55.88
tr1
.519)
-03
Aggregate Size (mn)
,_ >42-L\/
9.96
6.926.66f .i8
L5.L2L5.6610 .45
8 .311 -34
I .808 .408.68
l1 .40l.3.20Il .6010.15
9 .31
t0 .07l_0 - 19
Uncompacted.
6.426.r'7I .89
15 .06L5 -57
9 -997 -786 "18otra
9 .38I .989.49
i1.4311 1e
L2.LOl0-7r
9 -89
l0 .08
-i ô 6q
Cornpaction Treatment
Compacted.
o o?
6.826.519 -23
L5 -28L5 -19r0. 31
8 -20t -24
10-it
8.50I .108.44
LL.16L3 -471t .57
q q'l
9 .01-
ro.c2
Uncrusted
l0 .2I
6.996.129.33
l-5.1716 -47l0 -65I .40??ô
10.30
o7Q
8. 378.65
r1"6913.53].L.87lc .21_
9.3r
LO.25
PVA Treatment
9.30
6.266.O28.78
14 .5814 .88
9 -66t -)ó6 _63
9 .10B .71a .9b
tl .50l_3.r0ll .80l0 .4t9.60
9 -85
1õ LCI
Crust AirTemp.(oc)
12.0
10.610 .5ll.3L4.615 .51) )
1r .010"7
10 .6t
LU.¿L1^ 2)9.64
L2.877L.2210.3810.03l-0.37
wanoSpeedf
-L*it-f l
Rel.Hum.(%)
ôz
. . ./continued
TABLE 6.6 continued
Treatir'.ent
8/8-L2/8
Mean
MeanGrand, Mean
D
10
Time
I
0200 I
0600 |
0900L20015001800210 02400
5
c300060009001.200I500l-80021002104
I1.OIl0 .9I
.7 1C)
9 -O4L4.25L5.24]t .04I .848 .08I .84 I .05
9.64 öLI.]2 LL.2415.5I I5 .9814 -8 15 .00
43
8.2-70
9.8
1.1.o
7.4
9.53o'to
9 .51L2 -26r3 .82L2 -65I0 .99i0.r7
9.6t-q ?q
9 -4611.87I ? tr.')
L2.52r0 .8810.06
I0 .82 r0. 35
LO.62l0 .90 r2.03
l0 .6510. 3510"6612.1614.51t3.68L2.L911.43
9.r9I .919 -36
L2.L5L3.75L2.2I10 .54
9 .80
l0 .8910.r3
ll .08
7 -85
10 .17t5.921A ¿-?
Il. 36a)L8-54
t-o
r0 .81IO-74
Aggregate Size (mm)
<I 4- >42-L
2- >4
I.r57 .849.83
15 .06L5.94LL.749.51I .80
I9 .839 .509 .76
1) )Lr3-94L2 -'78a 1 aELL.ZJ
l0 .48
0 .86
lt .04LL.22
1 .597 .449 .55
L4.9115"69LT.24I .908.27
t_r.28
i0 -03q f.g
9.90L2.IOl3 .68L2.86l1 .3810 .6i
LO.46
I0.87
Compaction Treatment
UncompactedI Compacted.
8 .037 .169 -75
r5 .03L5.94lr-609.37I -64
ro.92l_l - 08
8.29I.r1ooo
1\ ¿.2
16 .55IL.999.639 .00
lr .05lr-01
9 .50q lg
9 .601) ¿.1
L4.L4L2 -6810 .96l0-r7
9 -55Cì ??
9 -561-2 -LL13.901) 6?r0.96i0-i6
LL.I21o.71
Uncruste Crusted
1))
7 -499.3C
14 .5815.09i0.85I "641 õ')
9 .989 .659 -94
1) ^t\
L3 -92L2.90il .38LO.62
I 0 .10
LO -72ir .35
ArrTemp.¡'Oñ \
15 .5
L4 "214 .0L4 -7L7 .6i9 .Ii5 .915 .313.9
1) õÀ
L7.231 A OÕ
L5.24a1 ,4Q
LL.24L2.74L2 -4415.16 4''Ì
49505045tÔJO
464853
PVA Treatment Ir{eteoroloEical Data
(icrnn- I )
wanoSpeed.
Re1Hr.rm
('o)
Ra]-n(n"¡'.)
0 -0
Hc*lL¡
. ,./continued
TABLE 6.6 continued
Aggregate Size (nun) Compaction Treatment Meteorologícal data
Treatment
5/e-e/e
Mean
MeanGrand Mean
Rain(nm)
L9 -6
9.7L (l2Ã
H(¡J
IO
D
06000
1500I8002i-002400
090L20
030006000 900L200150018002 10024AO
6.556.99o.L\
L2 -1913.10t0 .698.8r7 -66
i0 .02
8. 398. 3rI .98
t1 .34L2 -29LL.49lO.II
q ?î
9 .5t
9.16
<1 2-
a^^
1 .866 -40
5 .18(Q^
9 -47L3 -6213.7rLA.25
7
7o
t0L2II
.75
-66-56.2I
9 -16I .69
L227
9 -219.51
8 -46
5
5o
1IL2
9l6
.731)
-66
.56
.99?Ã
-60.10
9q?o "1o ??
LL"64L2 -72T2.T3r1 .03l_0 "32IO.BO
9.38
65
a
5
13 .59l-3.54r0 .40
8.407 .08
tr
6.o
t0 .05
8-259 .03
LT.62L2.56''r 'l qL
l0 .079 -r4
e.63i e.tL
,l
I
I !
4- >42-L>4
I .50
5.535.9r8.23
LL.9412.37
9.747 -756.59
r0 .10
8 .508 -449 ,01
l-L.29L2.33t1 .59l.O.2.7
9 .38
9 .30
8.90
5 ,5r6.O49 .00
L2.E7r3.0610 .09
7 .946.69
o(¿r0.I8
8.64I .50ooo
LL.24L2 -40ll-7310.43
o r,q
UncomPacted Compacted
9-05
5.946.449.06
L2 -54t2.84LC.238.281 .06
l0 .019.53
Uncrusted
q¿q
6.L66.629.49
f 2 ?O
L3 -72LC.668.517.30
o
o
9
IIL21I
10
at
.00¿.)
.40^o
8.19o^ô
8.11LI.32L2.48I1.53r0 .06
on?l0 .06
9 -16
PVA Treatrnent
Crusted.
8.46
5.295 .878.57
L2.O3L2.L89.677 .656 -46
8.738.66() ')')
LI.34L2.33l1 .68LO.44
9 -64LO.26oo¿
AirTemp(oc)
9.0
Wl.NCl
Speed(knùì-r)
7-O7.O
10 .5IL.7'l 1 ú-
o?
8.511
13 "99L2.991\ 2L1 É ôO
L4 -741't nq
LL.7412.O4
¡
II
i3.41
ReI.Hum.(%)
60
72705348AO
606867
/continued
TABLE 6.6 continued
Treatment
3/La-7/rc
Mean
lúrean
Grand lfean
D
5
l0
Time
0300060009c0L200150018002r002400
'r< )L15.15
15 .07 15.81
l5 -45
<I 2- 4-I
L4.L420.L92A.L2r6 .58r3.7IL2.81
LL,5211-02L4.2623.5211 a)
L7 -21I3 .56L2.52
I0 .88I0 .4012 -86t8.i61 0 Áô
1 E îfLJ.¿!
1) ¿q
1t .7C
11 .7LL.2
>4
t3-80 ]-4.76
20 -7619 -4815 .8313 .09L2 -33
11.3611 .08L4.L
'ì tr' Á,? 14 - 9rJ i4.59L6 "L2 1L !.)
Aggregate Size (nm)
>4
2-I
r( 10
LI.92It .56L3.8820.3620.54L6.1413.69I2.80
c 300060009c0l2cr015 00r8002 I0024CO
L2.89L3 -6216-95La.52L] "L415.I7L4 "25
l3 .41L2.75l-3.25l7 -101ô ,4 1
t7.19I5-5CL4.^-Q
a ^
aa
l¿.1 5
r4 .58i7"00r o c\]
13.44 1A to
1A 4A t5 .3016-05 17.I117 a,? LO-Zl
16.36 t6 .5s
ì1 10 13 .051) i¿. r3.53
',!l-'tl
L2.54 i13"55
ÌA-)/,!5 .44
L5 -241q ))
Uncompacted
14 -76
L5.49
Compaction TreatmenÈ
Conrpacted
r1.17r0 .75l_3 .581^ 1')
i6 .19r3 .04T2.L6
11.78I1.37r4 .1320.4820 -L6l-6.46r3 .5712.1315 .09
l5 .09
13.81]-3.26L3.49L6.4218.66
L3 "21L2.151t tr?
L6.77L8.13L6.92L4 -98'r4'ìo
I7 .50t5.7Cl¿+ . oo
T5 "L2 i5.c9
l5 -61
l5-39i5.18
r3.25L2 "66t3.I3Ll -A2LP..94I7 "T5i5 .1,6r4'Ìn
PVA Treatment
11 .801I.401L )q)) ñq
2),.63L6.97i3.8rL2.86
Il.15r0 .37l-3.42i9-lr19 .03l5 -68r2 .80L2 -O3
t_4 -82r5 .40
t3 .8313-3413.88r6 .561A 1Áì ? 10
L5.52t4.66
At_rTemp.(oc)
Uncrus Crusted wr-nc.
Speed-1
(kr,rh-')
1L )L l-r . :r-L
rt.61) )
15 .8L7 -318.416 .313.9]-3.1
9.941n ?4
15.3rt3.44L3.7IIO.T9LL.46'ln qÁ
Dal
ilum.(z)
6464E]JL
4439465549
Meteorological Data
.b.ar-n(nm)
5l L.4
F,.'{
I38
last two recorded periods" This may influetrce tinte to emergence, as it
is shown in Sectíon 7 that the seedfings enre:'ged earlier on Lhe silaller
aggregates than the large aggreç1ates.
tf one considers mean plot temperatures for the aggregate si-zes
used (mean of the 5 and IO cm temperature) the temperature difference
between aggregate sizes j-s consiclerably recJuced (i-e" at the 5 c-m rlepth
in the sowiug to etnergence period a difference of O.8oC exisLs between the
highest ancl lowest temperatures; ho1./ever, on a lvþole plot basis thj-s
difference is only O.2oC) . tthus there is rrery Iittle clifference in
temperat-gi:e between aggregate sizes on a whoie plot basis' Howevert
there is a larger dj-fference when a particular: clepth is considered.
In the sowing to emergence period tjre ternperat--ures on all aggregate
sizes fall into the range or are slightly brelow the range corlsidered to
be optimum for germination ancl emergence (Percival, L92I; VJort, 1940).
Later in the season the shading effect of the crop would modify soil
temperature, although the soil temperature i¡creases throuEh the season
(l-ig. 6.6) "
Lt is observed that at all times cluring the season the I0 cm
depth meern temperature ís higher than that at Lhe 5 cm depl-h. These
temperatures also faII into the range considered optimum for root growth
(Hagan, L952).
No one aggregate size is markedly warmer or cocl-er than any other
size usecl . Ifoweverr further repJ-ication of temperature measurements woulcl
have been desirable. ft appears that during the sowing to emergence period
a l-ower temperature (8-9oC) is desirable fo:: early emergence, but later in
the season a slightly higher temperature ltSoc) is desirable for rnaximunr
root grovrth and yield. (Hagan, 1952). These condj-tions are fulfilled w-i-th
the <f mm a¡:cl 2-I mm aggregates respectiveJ-y and correspond t-o those sizes
which resurt<-:d in the earliest emergence ar-rd- highest grain yielcls (section 7)
22
20
II
Àtt,t
It,
pur 15É,3kÉ,l¡J0-Et¡Jl-
õ10U)
I,,t
tII
âIt,,I I t
II t
,,
I I
12
I IIt
,,
\I \\
I \
21 3 12 215312
617-1Fig. 6.6
243617
Effect of time on
12213ale - nla
soil temperature at the 5 cm
5/e -e/e(-) and l0 cm (---) depths
s/l0 -zlto
tt
Ittt
Mean of all aggregate sizes and treatments for all times during the recorded Period.
oo
20
^15¡¡JÉ.ft-É.l¡Jo-Eu¡l-Jf0oØ
4,,
3 12 21 12 u312
alz-rolt21 3 12 24
tlt -rzlt s/g-glg sfro -z/ro
îLg. 6.7 Effect of time on mean soil temperature in uncompacted
35
Mean of all aggregate sizes and both depths for all times during the recorded Period.
t39
AggregaLe sizes can al-ter the temperatur:e rvithin the se,edbed as
do tj-Ilage practì-ces (Allmaras et al-., Lg'|2). Howevel:' larger differences
are experienced in a fielcl situation where coal:se tilths are Produced by
primary tillage which ex¡rerience greater changes due to convective processes
(ojeniyi, 1978), compared \^'íth the experimental study undertaken here using
finer ti1ths r.r'hj-ch are more typical of seedbeds '
6.3.2.2 Effect of Compaction
soil temperature was rnonitored on both the uncompact-ec1 ancl
compacted treatments in 1976 and 1977. The 1976 rea-dings co::r:esponcl to
daylight measurements only, while those tn L9l7 correspond to clay and night
measuremen'bs.
Duringthedaylighthours,of.lgT6,thecompactecl-plotswerefr:om
O.2Bo to l.90oc rvarmer than the uncompacted plots. Presunably the
compaction treatment results in better conduction of radiant heat within
the plot during the day. This would be due to the increased area of
contact bet-*een aggregates as a result of compaction (Sec'bic¡n 3; Day and
Holmgren, :952¡ Mciulurdie and Day, 1958). Iladas (L917) has shown tha'c the
therma-I concluctivity of aggregate beds decreases as ttre size of the
aggregates increase and nurnber of points of contact decrease.
The late so\1rn compacted plots are warmer than the corresponding
early sown plol,s. This is due to the fact that the air temperatur:e, Iater:
i¡r the season, tends to be v\¡armer a-Iso'
DuringLSTTL]neuncompactedplotsvleres}ightlywarmer(mean
temperature) than the compacted plots (Fig. 6.7). This is the reverse o-f
the 1g76 results. However, the 1976 data are biased towa,rds day]-iqht
hours and consequently on]-y experiencecl the heating effech- during the dery
and no cooling during tl-re nJ-ght"
140.
At the 5 cm depr:h, however, Lhe compacLed plots were rvarmer than
the ¡ncornpacted, r^rith the posì-tj.on beì-ng l:ever:serf at the I0 cm clepth-
The la,::gest difference at t--he 5 cm depth being in the o:cde:r of 0^35oC while
that at the 1O crn <lepth being O"4goC" The compaction tl:eatrneut would
decrease the air-space within the seedbed and increase the 1-herrnal
conductivity and volumetric heat capacity, which wor:ld tend to decrease
temperature variations in'the surface layers. This could cause an increase
in surface layer tenperature"
It ap¡tear:s that a compaction treatnìent fiay significan'tly al-ter t--he
soil temperature to an extent where -i-t j s optimal -for germination a¡ld
emergence of seedlings, other things being constant or non-limiting"
6.3.2.3 Effect of time of sowinq (L976) and PVA treatment (L9i7)
tn the sowing to ernergence period (1976) the early sorvn plots
were cooler at the 5 cm d,epth and warmer at the 10 cm depth than the
corresponding late sown plots " fn the emergence to final reading period
(l-g76), however, the early sown plots were cooler than the corr:esponding
late sown plot--s at both the 5 and IO cm <1epths. This rt'oüId ltrrgel)¡ reflec:+-
the clifference i-n ai-r temperature exper:iertc:r-:<f by the plots during this
period (ta¡te 6"5) .
It is Shown (Section 7) that an early sowing date resr:fts in
earlier emergence than a late sowing date. The slightly higher temperature
at the sowing clepth (5 cm) in the late sorvn plots may cause delayed
germination, hence emeï'gçince. It is shown (section 5) that temperature
also affects the shoot enrel:gence force, with larger forces being exerted
at J.orver tenrperatures. Thi.s may also reflect in the time to emergence
results, wi.L.h the early sown plots beinq at a cooler temper:attire a.nd shoots
emerging earlier: than the late sov¡n pl ots. An earl.y sow-ing date tna-y be an
advantage irr tha,t- Iower temperatures are experiencerl for germinatiou and
141..
emergence. The bemperatultes experienced at Ì:oth sowing dat-es l.i.e in the
range of those suggested as being opLirnal for gerrnirlation and emerqence
(percival-, f.g2L). Condj.tions and inte:ractions -in t-he field wou]cl vary
consiclerably nore than conclj-tions in the fabor:atory used t--o establ-ish
these optimum temPeratu::es .
A PVA treatment was applied in 1977 to prevent- crust formation "
A slight cr:ust, however, forrnecl durinq application, but did not adversely
affect the emergence of seedlings (Section 7) '
The crusted plots (no PVA) were cooler than the rtncrusted plots
(PVA applied) l-hi:oughout the wìrol.e season (mean pJ-ot- temperature) (nig"
6.8). This is presumably Cue to the insulating effect that the surface
crust has on the seedbed.
At the 5 cm depth the crusted plots are cooLer than the uncrusted
plots by 1.37oc to o.gooc. However, at the l-O cm clepth the crusted pÌots
are warmer than the uncrusted plots by O.34oc to O.l-Ooc. one would' expect
the uncrusted plots to be r,^/armer at the 5 cm depth, because the PVA
treatment resulted in these plots being darker in colour than t-he cnrsted
plots (plates'7.LL, -7.72,7.I1). Consequently these plots would absorb
greater amounts of radiant heat-
The hj-gher temperatures in the uncrusted plots (at 5 cm) is
reflected ín earlier emergence of seedlings (Section 7) compared with
the crusted plots. The crust also provides a mechanicaf barrier which
would delaY emergence.
6.3.2.4 Effect of met-eoro loqical factors
Regression equations for the daylight readings in I976 relating
soil ternperature {T", oK) to soil water potential (V, kPa) and the
meteorological factor:s, air temperature (Ta, oC), wind speed 1u, kmh-l)
and relative húmiCib.y (h, %) were developed. An equation \^/as derîived for
22
20
ttp
t¡JÉ,fkÉ,l¡JÀEl¡Jl-
15¿t
a
^t
ItJ
oØ10
¡I
\
3 12
e/r -tz/s24 3 12 21 3 12 21
12 24 3
ah -rclt ¡lg - e/g eJto - z/toEffect of time on mean soil temperature in uncrusted (-) and crusted (---) plotsMean of aII aggregate sizes and bottr depths for aII times during the recorded period'
J\
tt
t
5
rig. 6.8
144 .
rises and relative hunúdity J-ncreases' t-he soil temper:ature j¡cr:eases
(equations (6.20) to (6.23)). Vlith an increase in v¡ind speed, the so-i1
temperal-r.rre increases accorcling to equations (6.20) and (6'23) r^'hile it
decreases according to equations (6.21.) and (6-22) " No explanat'ion can
be offered for this d,iscrepancy in the effect of wind speed.
The ef:tect of changes in relai-ive humidity on changes in soil
temperature are not sign-ì-fican'E in al-I equations. vlind speed changes
arso seem to be of rittle significance ín changíng soil temperature. The
effects of air temperature and soil- water potential are considel:ably larger
than those of any of the other factors '
rn the sowing t-o emergence period ("q.r.ti,r. (6.2o)), air t'emperature
has a negligibte effect on sc¡il temperature. The soil water potential and
wind speecl have a greater j-nfl,uence on soil tempera'Lure than any of the
other factors during this period. This may be due to the fact that the
experimental site was relatively opeu (plate 7'3 and 7'II) initì-a1ly' so
the wind was abl-e to blow across the plo'Ls, unrestricted by any barrier.
As the season progresses the surroundiDg crop acts as a wind break, thus
reducing the effect of wind and perhaps causing air temperature to becotne
more iniportant as a factor in altering soil temperature '
Ingeneraltlreseresu].tsagreewiththoseofKeenandRussell
(r92I), Decker (1957) and Bal-asubramanium (l-966) who found that soil
temperature was positively correlated with air temperature and negatively
correlated (or had very Iittle effect) v¡ith wind speed' solar rad.iation was
not included in this stud.y because it was felt that the three hourly
varial-ion would be greater than any three hour change in soil temperatu:re
(the variation in solar radiation was far greater than any respolrse of soil
temperature) .
The effect of soil struc bure and meteorological condit-ions on soilL
tenrperature i.s assessed by the following equatíons. For the sowing
to emergence Period,oK (6 "24)
r45 "
oK (6.25)
ovI\ (6 "26)
T = 268"'16 + A - 0.11Y + O-OI3Ta + 0'O2lu -1. 0"002h'(10.21) (+o"oo7) (+0-orl) (+0.006) (+0'0018)
and for each recorded period" after emergence the eguat-ions can be
summarized for 8/8-L2/8,
S
TS
Lo1 5/9-9/9
= 267.'76(+o.22)
: 2.68.83 -r- A - O"IOV + O.O37Ta - O'002u + 0'008h'(j0.29) (+0.0ooe) (+o.or4) ({-O-OO4) (+0'002)
T
T
+ A - 0.11Y r- O.i3ATa - O.O55u + 0'006h'(+o"oor) (+o.or?) (+0.007) (+0'00r)
and for 3/IO-7/]-O,
S
oK (6.27)= 2J2.62 + A - 0.075Y + O.lOTTa + 0'029u + 0'0C9h'(+0.44) (aO.OOO8) (+O.Or6) (+0.009) (10'004)
where A is the aggregate effect' The values of A' fo:l each period'
presented in Tabl-e 6-7"
TABLE 6"7 Values of A for equations (6'24) to (6'21) '
are
A
Aggregate Size(mm)
Sowing to Emergence
e/8-L2/8s/e-e/e3/Lo-7 /ro
<l)Ãt- / ')--L
0.000
0.000
0 .000
0 .000
The large Y aggregate sizes tend to be negatj-vely correlated with
soiltemperature.The4-2mnaggregateshavethegreatesteffectonsoil
temperature. It is noted in Section 6.3.2-I that this size range tends to
be cooler than all the other Size ranges, which confir¡rs the above result'
Smaller aggregate sizes tend to increase soil temperature whereas the
larger aggregate sj-zes decr:ease it' This is probably a function of the
0 .028
-0.075
-o.o42
-o -26L
-0 "009
0.025
0 .108
0 .082
-0 .399
-o.367
-o.265
-0 .698
-0 . 117
-0 . r17
-0 . o23
-0.073
4-22-L >4
L46.
nufüber and. area of points of conta<:t, which would be large:: with small
aggregates. bltus increasing the importance of conduct-ion as a mechanism
of increasing the temperat-ure within the aqgregate bed '
The interaction between aggregate size (soil structure), soil
water potential and meteorological factors is a complex one" The simplistic
equations used in this stucly probabl! over-sirnptify the situat'ion, but are
useful as basic precìictors of the li]cety effect any one f-actor is li-kely
to have o' soil- temperature and even soir water potenl-ial " They certainly
indicate which aggr:egat-e size has the most effect on soil temperature and
hence the sizes one sìrould. aj-m for or avoid in preparation of a seedbed.
6 .3. 3 SoiI watei: po tential- range and soil temperature
ranse (L911)
The soil water potential range (for the r:ncompacted plots only)
and soil ternperature range for each aggregate size and treatment were
calculated for the 1977 data only (Tabte 6'8) '
The ranges lvere calcufated as means of the maximum minus the
minirnum daity values for the factors being considered. Again no statistical
analys.is of the data was performed, due to rlon-replicai:ion and the time
dependence of the data.
Thermal diffusivity o{m2s-r) is defined as follows
, -- o":- rc.2e)2
where * :2r/períod (where a period - 84,6OOs for dail-y variation) and
z is the ,,damping depth" (cm), which is the depth of soil which causes
the temperature ïange to drop by a factor oi l/e. Since the teinperature
rarlge varies as
R0,e-x/z (6.2e)
the reductionand if the range at 5 cm = R5 and the range at l0 cm = RtO
r48.
ratio is
u = R1o7n5 (6.30)
Then the damping depth -is calcul-ated as
z = *2-t\ (6 " 3r)InF
where x, and xI are the two depths one is considering (the 10 cm and 5 cm
depths in this case). Pl:esumably, on the compacted pl-ots, the distance
between the thermistors was 4.63 cm due to the reduction in heigì]t of the
bed. Hence in this case the dampi.ng depth was calculated as
z - 4'63 /tna rc -32)
As an examplerif F = 0.5, then Z = J.2L3 cm ancl D = r.93 x l0-7 *2=-r.
Values of thermal diffusj.vity are presentcrcl for each aggregat-e size atrd'
treaÈment for the measured periods in Table 6'9'
TABLE 6.9 Values of thermal diffusivity fo:: each aggregate srze
ancl treatment in the recorded periods (L971) '
Períod Dxf0 -7 (m2-L
Agg. Size(nur)
6/1-L6/78/8-r2/es/e-e/e3/Lo-] /Lo
Crust
1..53
2.04
2.30
2.30
A val-ue o¡ p = ].93 * lO-7*2=-1 i= a vafue typical of dry
unstructurect soir, v;hile a wet unstructure<l soil- is typically 6 .'7 x 1-o"1*2u-l
(De Vries, 1963) . The consequence of a lower therrnal diffusivity is that
Iess heat wil"l be transferred thr:ough the seedbed in a given time-
The ther:mal diffusivity of the aggregate beds increases throughout
the season. This is probably due to a settling of the bed (by rai-nfa1'I
impact) or an infiJ-Iing of pore space (due to aggregate l-¡reakdown under
2.O4
3 -1_2
3.55
3.33
r.37r.822.O4
2.30
1.45
r.532.44
2.O4
I .93
2.L7
2.93
2.59
2 "04
2.L]2.44
2.59
))2
2.5L
2.85
2.64
1.00
L.24
I.E6L.75
I Õ?
2.L7
2.93
2.16
<I 4-22.-I '4 / z-1.>4 Uncoml:. Uncr.Comp "
149
rainfall) thus increasing the area of contact between agg::egates. The
<l mm aggregate beds have the largest thermal diffusivity compared with
the othor aggregate sizes.
A compaction treatment increases the thermal diffusivity considerably
compared wiÈh the uncompacted treatment. This is due to the increased
area of contact between adjacent aggregat-es. This is also refl-ected in
the reduced soj.l temperature range in the compacted beds at the 5 cm depth,
thus indicating t-hat the incoming heat is being conducted to greater depths
in the aggregate bed.
The presence of a surface crust slightly reduces the thermal
diffusivity compared with an uncrusted surface. The surface crust may be
considered as a dry mulch which insulates the seedbed bel-ow from incoming
radiation.
Thermal diffusivity is affected by aggregate size' a compaction
treatment and the presence of a surface crust. These changes in diffusiviLy
alter the temperature range experienced under the above treatments. llowever'
the change is not a marked one with any particular aggregate size or
treatrneni- being tbettert o:: tworset than any other'
6.3.3.1 Effect of aqgregate size
soil temperature range or fluctuation has recently been recognised
as being important in inducing germination (Thompson et aI ' , 1-9'77) and' in
increasing plant growth (vlatker, I97O) . In a field situation soil
temperatures fluctuate in a diurnaf pattern, however, there is little
information on how this is affected by wefl defined soil structure (aggregated
seedbeds) .
The daily fluctuation or range in soil water potential rn'otLld be
more important in the sowing to emergence period than in the post-emerqence
150.
period, bec--ause aft-er emergence the plant roots would be extr:actj-ng l'rater
from the profile belorv the tO cm depth. Again tbere is little quantitatS-ve
information with regard to v¡atcr potentiat range anrl wel-l cle:Eined soil
struct-ural condibions. Ho\^rever, it is possible that the actual- value of
soil water potential is nore itnportant than the range'
The temperature range is greater on larger aqgregaLes (>4 mm) than
fine aggregates (<l- nrm) at both the 5 cm and Io cm depths. This is
Çonsistent with the results of Bhushan et aI-" (1974) " The temper:ature
range at- the lo cm depth is appi:oximately half that at tbe 5 cm depth.
This is observed for aII aggregate sizes and a1l- periods of recor<ling'
The2-ITrmaggregatestend.toexhibitthelargesttemperature
range, and it is noted that they also have the smallest soil water pot-ent-ial-
range.
The aggregate size (2-I mm) which exhibits the smallest wat-er
potential range, also corresponds to the size range that exhibited the
Iowest evaporation ratio (Section 4) and remained the wettest through the
season.
cn al.I aggre-gate sizes the rvater po+:cntial range increases a's the
plots dry out through Lhe season. The layerecl aggregated seedbed does not
appear to be any more effective in reducing temperature a-nd water potential
ranges than any other aggregated seedbecl'
6.3.3.2 Effect of Cc¡rnpactiotl
soil water potential was not monitored on the compacted plots, ancl
conseguentty discussion is confined to soil temperature. range only'
Thetemperaturerangeisgreaterontheuncompact'edptotsthanthe
compacted plots for all period.s of measui:ement. The temperature range at
the I0 cm depth is approxima'bely half that at the 5 cm depth, a similar
r5t.
situat-.-ion to specif ic agqr:egate sizes. Since compaction would .irlcrease
the volrrmetrj,c heat capacj-ty and also the thermal conductivity of t.he soil,
by increasing area of contacl- bet-ween aggregates, the temperature range
wouLd k¡e less on the compacted treaLments. This ís because rtoi:e heat
would Jre conducted to deeper layers during the day, consequently the
temperature range at l-ov/er profile depths may be increased. It- is
observed that the soil temperature range at the t0 cm depth in the
compacted. ptots is higher than the uncompacted plots '
6.3.3.3 llffect of PVA (t-o prevent crust-irlq) treatment
The presence of a soil- crust reduces the soil- temperature range"
The crusted pl.ots exhijcit a lower temperature range than the uncrusted
plots" îhe crust acts as an insulator.to incoming radiat-ion thus re<1uci-n'g
temperature variation within the seedbed. The temperature range at the
I0 cm depth is approximately half that at the 5 cm depth. within the
crusted plots the temperature range is less than the uncrusted plots at the
L0 cm depth also.
In a simi,¡ar manner the presence of a surfacer c:-'t¡st redÌ¡ces the
water potential range compared with the uncrusted surface, except in the
sowing to emergence period. This corresponcls with the result that the
presence of a crust reduced the evaporation ratio (Section 4) and the fact
that crusted beds remained wetter through the season. If the seedbed is
wetter there wj-]l be less var:j.ation in soil water potential range " However,
as the aggrega.te beds dry out cluring the season, the water potential range
incr:eases more on the uncrusted plots than the crusted. It is observed
that the range in the crusted plots is approximately half that of the
uncrusl-ed p1ots, except for the first two recorded periods. Thjs r:educt'ion
in range clue
between the 5
to crusting is similar to that of the soil temperature ranges
an<1 lO cm clepths on all treatments '
r52.
Temperature fl-u.ctuations as small as 1oc (Thcmpson et g] - , Jg77)
have been found to enhance germination in cerLain species. The 't-emperature:
ranges observed a¡e consicleraìrly larger than this and would appear thus
not to l.i:nit germination of wheat under the experj-mental conditions "
blalker (1970) found that the grovrth of corn was enhanced when the soil
temperature range was 6oC about a mean of 234C. The range observed at the:
5 cm depth was greate:: than this, but the ra.nge a'h the l-0 cm depth was
sJ-igh'tly lower than 6oC. It wouf d appear that the ranges in tempera'ture
observed in the field compal:e with ihose usecl in laboratory situations to
determine optiurum temperature ranges -
6.3.3.4 Effect of meteorological- factors
Equations were developed for eacl-¡ recording periocl , fot each aggr-egate
size and the urrcrusted (pva treated) and crusted sur:faces, in an at--tempt
to determine v¡hich of the meteorologicat factors had the greatest
inf luence on soil temperature lîange and soil water potent-i-al range '
Sirnple linear regression equations of the form
p=a*bM (6.33)
where P = T", or Yo- (the soil temperature range (oc) or soil rvater
potential ratlge (kPa) ), M: T. or u or h (the air tempe-rature (oC) , wind
_tspeed (kmh-t) or rel-ative humidity (%) respectively) ' and a and b are
adjustable parameters .
The coefficients for the regression equations are tabulated in
Table 6.I0. The reg::ession coefficients for aII interactions are very low,
except in one or two cases. This indicates that periraps a non-linear
moCel would be more appropriate. However. the linear model used does
indicate which of the met-eorological factors seem to be important in their
effect on soil temperature range ancl water potential range r even thou<¡h the
interactj.on is not significant. Perhaps better agreement would be obtained
by using t--he ranges of the met-eor:ol-ogical factors instead of meatls.
TABLE 6 .IO Paraneterîs for equation (6 .33) '
T"t
Àgg.Size(mm)
>4
4-22-L<I
>4
2-rUncr.Cr.
>4
4-22-L
,ã,L2-L
Uncr.Cr.
RecordPeriod
6/7-L6/7
8/8-L2/8
Hult¡
a
6.255 .88
6.r96.L76.t55 .965 ,88
6.O75 .867.9L5.285.465 -826 .41
b
0.35o.32
0.380.330.340.32o.32
o.230 .I80.r30 .I8o.2Lo.220.15
T2
Ta
00000
24T718I920
o.220.r7o.230.r30.I70.r40.15o.22o.rl
a
L6.2L15 .61
15 .90L6.57l-6.67L5.2215.19
r8 .59L7.94L6.O2r7.5319.0717.33I8. 32
b
-0 .54-0 .55
-o.49-0.6r-0.6r-0.5r-0.51
-0 .58-0.60-0.39-0.6r-0.67-o.52-o.62
u
12
00000
00
1624232020
2L20
o.490 .540 .570 .590.550.45o.64
a
16 .3416.08L6.24L5.2614 .90
L6.4L15.17
11.501r.88t0 .91L2.O2L2.39II. 36
r3 .05
b
-0 .09-0 .09-0 .09-0 .08
-0 .09-0 .08
-0.08
-0 .06-0.05-0 .04-0 .05-0 .06
-0.06-0.05
Í2
h
0.32o.25
o.270.300 .280.290.26
0.450.330 .380 .340 .360.450 .30
a
2.77-8.53
-7 .2Lr .38
-r1.345.59
-2.80
-234.7-180.1-79.4-83 .7
-239.6-L78.2-L49.7
-L.49-L.75
-I.16-0.95-0 .30-L.62-4 .08
b
6.97
98.66.6r.o4.II.o4
9I3
4III
Ta
t2
0 .530.r7
0 .09o.470.050.610.26
0 .390 .690 .57o.740.580.560 .58
a
-22.6-27 .7
-26.5-II .8-2L -3-1r.6-53.0
101.937 .7LO.216.279.254.543.5
000
-0o
52I860I9I5
o.72-0.15
b
-r1.9-5.4-2.L-2.4-9 .5-7 .O-5 .5
t2
u
0.0080.0060.070 .004I .5xIO-4
0.050 .00r00000
00
2009o709l5I5l3
a
-36 .8-58 .7
-56 .8-23.9-20.9-3r.5-I05 .6
26.444.LL2.522.L51 .33r .53r .5
b
0 .350.47
0.570.220 .090.280 .87
-2.2-r.8-o.7-0 .8-2.4-L.7-r.5
Y"t
x2
h
00
6026
0.480 .540.090.40o.24
00000
00
4677588769
6566
...r/continued
TABLE 6.10 continued
Tsr
Uncr. = Uncrusted
Ysr
Agg.Size(uun)
>4
4-22-L
,ãt2-L
Uncr.Cr.
>4
4-22-L
rãr2-L
Uncr.Cr.
RecordPeriod
s/e-e/e
3/Lo-7 /L
a
16 .9613.0223.L613.45r8 .1720 .81r3 .09
5 .307.78
r1.359.708.20
IO.146.79
b
-o.94-o.67-r.53-0 .7r-L.20-r.40-0.630.320.06o.L2
-0 .0I0 .090 .070 .16
Ta
t2
0 .03o.o20 .09o.o20 .070 .080.0r000
4 o-40 .0r0.0070 .03
.II
.008
.0r4xI
a
2L2L2720
.75
.47
.7L
.0I
22.4725.60L9.75r8 .8413.5820.64L2.9415 .59r7.0315.61
-0 .98-r.05-1.38-r .00-1.r3-1.30-o.92
b
-Q.72-0.39-o.62-0.28-o.49-o.47-0.53
r2
u
0 .060 .08o.L20 .080.rlo.L20 .06o.420 .20o.230.r2o -22o.2Lo.28
a
18. 3514 .08L8.79L4.7LL6.3217.2315.6624.5219 .0627 .9518.092L.7323.O42L -49
-0.16.O.II-0 .15-o -L2-0.15-0.15-0.r3
b
-o.27-0 .19-0.28-0 .16-o.23-o.22-o.23
12
h
0 .130 .08o.L20 .I00 .15o.L20.r00.550.45o.440 .37o.440.430 .50
a
-509.0239.4L24.5
-67L.3354.3
-519.0337.0
-1490 .8-754.O-40 .3
-746.7-18r.9-1195 .0
-88 .7
b
42.9-33 .3-18 .0
56.2-51.7
44.L-46.0
63.727 .4-2.520.L
-32.742.8
-L2.6
12
Ld
0.r0o.260.3r0.070.r90.r00.320 .090 .040.050.010 .010 .040 .0r
a
-847.L-83.9-3r.1
-L457 .860.7
-901.3-39.3
-542.O-447 .3-2L5.7-492.3
-1459.9-764.O-500 .3
b
54.rL.42
-0.6396.4-L3.258.2-3.2
r8 .5
-0.8.
II.3.
65.17.
0864I7
4
î2
u
o.280.00I0 .0010 .36o.o2o.290.003I0030
00
.9xIf4
.03
.005
.01
.4x10-
.003
.83
a
-2360.8-L746.0
'99:9-2325.5-48L7.8-27t7 .4-1821.5
-703.9-196.8-L29.L-97L.9-523.3-698.2-3I0.4
b
9.62.LL.4
13.36.69.53.7
994LI7
56
3426
03679
4L29
00000
00
69L42753406I27
12
h
00000
00
20260l30422643
t2 = ,"gtession coefficient.
Cr. = Crusted
L54.
Air temperai-u-re has the greatest infl.ttence on soil- temperature
range a.nc1 soil wat-er potential. l:ange compared with eit-her rvind speed or
relaÈir¡c humidity. Both the rvind speed and relative hurni-dity are negatively
correlated with soil temperahure range while they are both variably
correfated with soil water pot,entì.al- range. (Some cases negatively while
others are positively correlated). Air temperature is positively
correlated with both soil temperature anrL soil water poiential range in
more inst-ances bhan it is negatively correlated. It appears tl-rat rrtore
measurements are required t-o resol-ve the paraclox in correlations.
6.4 Concl-usions
Aggregate size, a compaction treatment, the presence of a surface
crust and meteoroloqical factors al-I affect soil temperature, soil water
potential and the daily variation of both. The interactions of these
factors is complex and the regression equations developed tend to over-
simplify the situation. However, they do enable one to determine which
factors are important and the factors whicl-r can be mr¡dified in the see<fbed
by tillage. Further field studj.es wit-h greatcr reptication of rneasure¡nents
are required to resol-ve apparent contradictions predi.cted by the regression
equations. Measuremen'ts in seedbeds produced by conventional tillage
seems to be a logj-cal extension of the measurements ma<1e in the experimental
seed.l¡eds, and they would enable the val-iclity of the equat:ions to be fu::ther
tested.
The int-ermediate sized agglregates (2-1 and 4-2 mm) remained the
wettest throughou,t ttre season. These aggregates also tended to be the
coolest throughout the season. It is shown that the best yielrl of grain
was obtained from these sizes also (L917 resul-t-'s, Section 7) ' The larqer
(>4 mm) and small-er (<l- run) aggregates tended to be drier and w¿:rmer: through
155.
the seasor-ì compareri wj-th the intermediate sizes.
A comltact|on treatment resul.ted in tÌre aggregat-.e seedbeds being
cli: j er and rvarmer than the corresponding uncompacted treatment (1.976
results) .
The presence of a surface crust resulted in the uncrusted plots
bej-ng drier and warmer than the corresponding crusted plots (L917 results) -
On aII aqgrega't--e sizes and treatments 'l-he soj-Ì water potenti-al and
soil temperature weïe veïy close to or within the ranges quoted as being
optimum for çJerrnination and subseguent root growth-
No one aggregate sj-ze or treatment was outstandJ-ng in optJ-inízing or
prodçcj-ng the opt-ional soil water potential or: soil ternpe:ratnre for cereal
growth. Greater differences would proba-bly result if coarser: seedbeds v/ere
utilized, due to the increased importance of convection cornpared with
conduction as rnechanisms for inducing changes. However, the resul-ting
conditions would probably not be so favourable, and with coarse seedbeds
it is difficuÌt to maintain a uniform sowing depth, an important consideration
if even emer.gence and maturity of the crop is to be achieved.
Regress-iori equations were cleveloped ::elating soif v¿at'er potential
and. soil temperature to rneteorological factors. Some inconsistencies
-in the relations were observed. fn L976 and 1977, as the vapour pressure
deficit increased, the soiJ- water potential became more negative (the soil
dried) . vfith respect to wind speed however, ín 19'76 as the wind speed
increased the soj.l- water potential became less negative (soil- becoming
wetter), while Ln ;-977 as the wind speed increased the soj.l water potential
became more negative (soil dried out). This discrepancy may have resulted
because of recordings bei-ng ma<1e during the daylight hours in 1976 and
around the clock Ln L971. Further measurements need to be made to resolve
this apparent contradiction in results. The effect of aggregate size and
presence of a surface crust seems to have little effect on soil water
156.
potential compared wit-h Lhe vapour pressrire deficit (Tables 6'3 ¿nd 6"4) "
Soil ternpeLature increases as the soj-l water potential becotnes
tììoïe ltegative, and air tcmpcrature and reJative humidì-ty increase' The
effecÈ of wincl speecL was variabfe. An increase in wind speed resulted in
soil temperature increasj-ng in some instances, whi.le in others the
re-verse occurred" The effect of wind speed and ::efative humidity, however'
was almost negì-igibJ-e compared with the effec--t of soil water potential and
ai.r temperat-ure.
Soil- strncture (aggregate size) has an appreciable effect" on soil
temperature (Tabl-e 6.1) . The J,arge atrd. intern.rediate s-ized aggregat-es
resuft in cooler soil temperatures than fine agqregates '
Althoughtheequatjonsdevelopedaresimple(Iinear),theyindicate
which factors are important i.n moclifying soil temperature ancl soil water
potential. In particular they indicate that Èlle 4-2 trw aggregate size
results in a soil temperature and soil water potential which are relat-ively
close to the guoted optima for germination and emergence of temperat-e cereals '
Thetenperature].a.ngeatihe5cmdepthisapproximatell'twicethat
at the l-0 cm c1e¡:tli in alf per:ioc1s of measurelnent. This is equivalent to a'n
effectíve thermal diffusivity of l-.93 x 10-7m2s-L in the seedbed" The
larger aggregate sízes have a greater temperature range 't-han the smaller
sizes. A sinilar situation also occurs with so:i-I water potential ranges'
The presence of a surfaoe crust reduces both the soi-I temperal-ure
and soil water Potential ranges.
A compaction treatment reduces the soil temperature range compa::ed
with an uncompact-ed treatmenÈ.
The ranges in temperature observec were considerahly larger than
the repoi:ted. minirnum of IoC for the initiation of germitration in certain
species.
157"
Air temperature exerts a greater influence on soil temperatrrre
range and soil \^rater potential range than either wind speed or rel.ative
humidity.
r58.
SECT]ON 7
WHEAT GRO\^I'I'II TN BEDS OI¡ AGGRBGATES
7.I Introrlucl--ion
On most soils a certain amount of tillage is necessary for crop
production. Flowever, the debate as to how much and what kind of tillage
conbinues. In spite of .Lhis, the fact r:ernains that soil aggregate s-ize
distri]:ut-ion, asi can be modified by ti.Ilage, does influence the physiczrJ' ,
mecha-nical and açlronomic properties of the soil . /\:ì.sort:here is a
diffe::ence of opinion as to the sensitivity of crops to seedbed preparaticn
(I^ied<lerspoon, 7-915¡ Russell-, Lg45) " Differences observed in early growth
often are not reflected in final yields.
Experiments have been perfon.ned, using beds of siev,e<l aggregates
wil-h narrow sj,ze-range distributions (Edv¡ards, L957; Hammerton, L96L¡
Jain and Agrawal , Lg'|O; Tayfor , L974), Lo test the effect of aggregate
size on germination, emergenÇe and yield of various crops- These proba'bly
represent what are the only reprocìucibl-e a-¡rd wetl. defined soil structures
and hence provicle the oniy means r,rhereby experiments conductecl at different
places and at cliffer:ent tj.mes can be ccnìpared quanti'catively. Ot"her:
advantages associated with sieved beds are thal- one can produce seedbeds
which are obvi.ousty different and <1if fer in a g::aded manner, ext'reme
conditions of fineness or roughness can be attai¡'lecl and the seedbecls can
be reproduced within linits of experimental error. However, there are
several inherent disadvantages. These being that the plots, of necessity,
have t-o l¡e smaller than desj.rable for accurate yield êssesn:-rrlent¡ tÌ,at t'he
seedbeds aïe not directly comparable to tilths produced by tillage, and
that sier¡ing may have effects other than separatj-on of aggregate sizes '
In contrast, ti11age produces a wide range òf aggregate sizes whj'ch
vary enormously depending o¡r the type of tillage implement used and on the
r59.
ínitial state of ì:he soif , Ilor t-his reason, resr.t.Lts from experi-rnents i'n
whicl.r t-tre ef{ects of dif:Eerent tillarge Lreatments on crop performance are
compared only have meanj-nq for the pr:<-:cise conclil-ions at the time and mary
nc¡t be generally valid. In nrost studies on the effecL of tillage on crop
growth, gerrn-ination rates and crop estabtishment data have ¡oL been'
rec:orcled; tire onJ-y effect of d-iffere¡t til]age practices measured has
been the final crop yiel<l" Some a.ttempt to measure soil structure produced
by clifferent-- tj-Ij-age pr.act-ices or i.mpì-<lrnents on pJ-ant growth has been trtacle,
but with líttle success. lJntil 1976 Do satisfactory method was availal¡l-e
for measuring sr.ril Structure in situ (Dexter, 1976). However, as stated
previously, tilths produced hry tillage are not reproducible and- only
represent conditions at that particu]ar time. The effects of seed'bed'
preparation or even the seedbed itself on final yield are not understood
at present.
The aim of this section is to examine the effects of nggreqate size,
compaction, time of sowirrg and Surface crusting on emergence and yield' of
wheat. This should aict in ciefining more clearly seedbed requiretnen'ts for
wheat under Au.str:alian conditions -
I .2 Materials ancl Methods
7 "2.L Glasshouse air<l Phytotro np rel.iminary experiments
A preliminary glasshouse expe::J-ment v¡as carried out to examine the
effects of aggr:egate size, water content and Sowing date orr wheai- seedling
emergence. surface soil of urrbrae loam was sieved j'nto the following size
ranges." )4, 4-2, 2-1 and <l rnm, Fíve sOiI treatments, three water
contents and two sowing clates were used. AggÏeg¿tes were dumped into l0 cm
plasti-c pots. to a depth of 8 cm and levell-ed. Fj-ve presoaked seecls (B hr)
of wheat ( Trj-1.--j.cum aesti-vum L. cv. Halberd) were placed in each pot' The
160 "
seeds \^/ere covered with a I cm layer of t-l-re appropr:iate agcl::eg.:.b.e size
rarlge. Fitte¡: paper was placed or.r the surface, to prevent crr-rstjng, and
water ad<led to give water contents of 15, 20 ancì 30% (OD wt. basj-s).
Foil. was placed over the pots, to prevent water loss, and tlìe poi:s wer:e
pJ.aced in a glasshouse" Each tr:eat-ment was repj-ica'ted six times" Pots
were examine,J daily for: emerging coleoptiles " The sowirrg cì.ates were
early, I8/3/76 and l-ate, 2/4/76.
A sj:nitar e>:¡;er-irneut to above was carried ottt concr-r::rentÌy in a,
phytotron un-it, wj.th a corrstant temperature of 2OoC and usi.trg only one
sowing date"
7 .2.2. l-j.el-d Experinent-
In most i.nvestigations on the growth of cereals on different seed-
Lreds, Iittle attention has been paid to the emergence of Lhe plants apart
from comparison between the final stands. It seems probable that e:;act
information on the rate of emergence and per:cetrtage emergence in the field
is sparse because of difficulty in estimati.ng tìre numl¡er of seecìs scrvn when
a conven¡ional drill is used. The technique descri.bed he:re itivo'l.r¡ed +¿he
hand planting of a known number of seeds in pJ-ots so that the ex;rct
percentage emergence ancl sowing-ernerEence interval could be deterrninerl .
7.2.2.L Location and laYout
The experiments were locatcrl in the field (w-ro) at the waite
Aqricuftur:al Research Institute, Sc¡uth Australia. In l9'76, wheat (Tri L.icum
aestivur¡l L" cv. Hafberd) was sohrn in severrty-t.wo plots (a completely
randomized bl-ock design) incorporaLing the fol-lowing treatments: two
sowing dates, 22nd, June and 26th Jul1z; four aggregat-e size ralrges; ancl
plots made of coarse aggregates overlying fine aggregates, ancl two -ler'¡eis
of cotnpaction (zero conipaction and 2.7 kPa appl-ì-erì stl:ess). Each c<>v¡irtr;
1.6I "
date was allocat.ecl to a whoLe block, with aggreqaLe sizes and t-.he
compacl-ion treat:men'Ì: being allocat.ed randomly to ploÈs with:Ln each block.
Three r:eplicates of eaci-r 'Lre-atment gave thirty plots per sow.ing <3,ate
with three guar:cl pl ot-s at either end of the block "
In L9'71 a similar experiment-al layout vras used. However, only
one sowing date (lst JuIy) was used aÐcl a poty(vinyl) alcohol {PVA)
[uowiol 40/88 ]Ieuchst, Àust,] treatment was incorporated. trourfold
replication of each treatment gave a t-otal of eighty plots. Bxperirnental
Iayout for 1976 and 1977 i-s illust-rated in F-ì-9. 7.L. P-l-ate 7.I shcws
experin".ental layout and plant growt-Ìr in 1976.
The choj.ce of layout was governed by tvro factors. First1y, the
quantity of soil- which couid be han<ll-erl and sieved in the availak¡l.e time
was limited.. This controll-ed the size and number of plots. Secondly, the
plots had to be accessible for transport of soil to the sieve and for
subseguent plant measurements .
7.2.2.2 Handling of soil
For the 1976 experinrent two a::eas of 7 m x 2 nr \\¡ere excevated. to
a depth of 10 cm. The soiL was piJ-ed alougsic-i,e the are-;rs for sieving.
For the 1977 experiment an area of 15 m x 2 m was excavated to a <1epth of
I0 cm. The soil was treaterf sinrilarJ-y in both years. A framework of
vertical pi-eces of slotted maisonite of 50 cm x 50 cm x 10 cm, was placed
in the excavated area (Plate 7.3) to sepa::at.e the plots. This method
achieved ccmplete root separation to the Ì:ase of the experimental- seedberls.
7.2.2.3 Sieving
Sieving commencerf wkren the soil was dry enorrqh to pass through thc
smallest screen without ctogging. A rotary sieve 'i,vas used to separate the
required aggregate size ranges (Plate 7.2) " 'Ihe sier¡e corisists of a series
>4
U
<1
U
>4
U
2-L
U
>4
c
>4
2-LU
4-2
U
<I
U
<I
c
<I
c
2-L
c
>4
U
>42-L
U
>4
2-Lc
>4
2-Lc
>4)-1c
<I
c
2-L
U
<I
U
>4
U
<lU
>4
c
4-2
U
4-2
c
>4
U
2-IU
2-L
c
2-L
c
>42-L
U
4-2
U
4-2 4-2
Uc
4-2
c
>4
c
2-L
U
4-2
U
I97 6 22 June - Sowing Date 26 JuIy - Sowing Date Block IIBlock f
L977 lst July - Sowing Date
<I
U
>4
U
<l
U
4-2
c
>4
U
>.4
2-LU
<l
c
<I
2-L
>4
U
4-2
c
4-2
c
4-2
U
2-IU
>42-Lc
2-L
c
>4
c
4-2
U
>4
c
>42-L
U
<I
c
<I
U
4-2
U
<I
U
>4
2-Lc
<I
U
>4
U
>42-L
U
2-rc
2-IU
2-tU
>42-rc
>4
c
4-2
U
2-L
U
4-2
U
>4
C PVA
<1
U
<I
c
>42-L
U
2-L
U PVA
4-2
U
>4
2-L
U PVA
2-l
C PVA
>4
4-2
U
>42-L
U PVA
4-2
c
4-2
C PVA
>4
U PVA
>4
2-L
U PVA
>4
2-rc
>4
2-L
C PVA
<l
C PVA
<l
U PVA
4-2
U
>4
U PVA
<I
U
>4
C PVA
2-L
c
<I
u
>4
2-L
c
>4
C PVA
<I
C PVA
4-2
C PVA
2-L
U PVA
>4
2-L
C PVA
<1
C PVA
>4
2-L
c
4-2
<I
U PVA
2-L
U
>4
2-L
U
>4
U
<1
U PVA
>4
c
2-L
C PVA
>4
2-L
C PVA
Fig. 7.L Layout of field experiments in 1976 and L977.
..continued
Fig. 7.L continued
L977
>4
2-L
U PVA
4-2
c
>4
2-rU
2-L
U
2-r
U
<l
U
2-L
C PVA
<I
U PVA
4-2
U PVA
>4
2-L
c
2-L
c
4-2
C PVA
2-L
c
<l
c
2-L
C PVA
4-2
c Pv?\
>4
c
>4
U
>4
U
2-L
U PVA
4-2
U PVA
>4
C PVA
>4
U
<I
c
4-2
U PVA
4-2
U P\A
>4
2-L
U
4-2
U
>4
2-L
C PVA
2-L
c
2-L
U PVA
<1
U
2-L
U
>4
U PVA
>4
U PVA
>4
c
Empty
<1
C PVA
4-2
c
Plate 7. I General layout and appearance of the 1976 field
experiment.
Foreground: EarlY sown Plots.
Background: Late sown Plots.
Differences in growth were maintained throughout
the season.
Plate 7.2
Plate 7.3
Rotary sieve used for obtaining the following
aggregate size ranges: >4, 4-2, 2-L and <1 mm.
Framework of plots in exacavated area prior to
fitling with aggregates.
L62.
of conceltric cylinders of ',{oven \¡/ire screetls in order of J-argest to smal.lest-
screen opening,from the centre outwards. 'fhe screen sizes used \^/ere 4' 2
and l mmr giving nomina] size ranges of >4, 4-2,2-1 ancl <l mm" Tlre
screens s19pec1 al.: arì crngle of 9(). This could be altcred to vary tlie rate
at which t-he soil passed through. 'Ihe screens rotated at 30 r"p.rn. Soil
was fed with a shovel into the centra} screen through a chute" T'he
separated aggregates were collectcd using trays pì-aced uncler apprcpriate
chutes at the delivery end of the sieve"
The choj-ce of screen sizes t\¡as deterrnined by the nature of the
soil anci by the requirement to produce a series of graded seed̡eds varyinq
from coerse to fine. Screen sizes smaller than I mm \^Iere impract-icaI
because the rate of separation rvas slow and sufficj-ent soil coufd nol- be
sieved in the time available. The upper limit was determined by the fact
that it is not possible to maintain a unifornr sowing depLh with seedbeds
consísting of large aggregates. The separated aggregates rvere placed in
air-tight plastic buckets for storaqe (Plate 7.44). Water contents were
deLermined prj.or to placement of the aggregates into the p1ots. The
proportj.cn oÍ rrario¡s sized aggregates in eacb sieved size range \^Ias
determi.neo by h;lnct sieving a bucket ôf ac¿gtegates through a nest-
of sj.eves (Fig" 7.6) .
7.2.2.4 Sowing Technique
The sieving operation produced four aggregate size ranges which,
together with a 'stratified' seedbed (ccarse overlying fine aggreEates)
rnade up the five soil treatments. These were allocated to plots, in l-976
and 1977, according to the experimental Ìayout ín Fig. 1.L. Each plot t'vas
fill-ed to a depth of 5 cm with the appropriate aggregaLe size rarrqe and
levelfed. The seed usecl was tested for germination capac-ì-ty, whhch on
no occasion feII below 96%. Sowing was f¿rcil-itated by placing a close-
fitting board, with 5 rows of evenly spacecl hofes, giving a total of 85 holes'
163
into the pl.ot (l.Iate I .4C) . One ser:d rvas droppe<1 through each hol-e onto
the soil- Plal-c: '1 "4 and'l "5. In L97'1 , 90 seeds per plot were so!¡rt.
Precalíbratecl thermj.sLors, resistance bfocks and tensiometers v/ere placed
in the apprc,priate tre;rtilerrts. Tlterrtr-LsLors we:le pla-ced at bottr the I0 cm
(base of seeclbed) a¡rd 5 cm (sowing depth) depths, while resistance blocks
and tensiometers vrere placed at the 5 q¡ (sowing) dept"h only. These
readings were nct ::eplicated clue to unavailabiliLy of adequate nurnbers of
thermistors aricl resistance blocks. llalf of the atnounL of water required
to bring aII pl o't-s Eo 2Oea v¡ater content (OD basis) was applied with a
knapsack spray" ln L9-/7 inal-f. of the treatments were s¡lrayed with a 2g¿
aqueotrs PVA solution irrstead of v¡.rter. The sown seeds; were then uovere<l
with a 5 cm layer of aggregates, the surface being carrefully level.l-ed.
The remainder of the water, or PVA solution ín L9'77, was then applied. To
prevent excess water being applied to adjacenL plots a wooden shield which
fitted over the plot was used to prevent splash and drift (Plate 7 -4D) .
In 1976 the application of water caused a certain amount of crust format-ion.
A similar result was al-so observed in 19'17. This was due to the method of
applicati.on: the d:r:o¡>s from the knal>sa-ck were large hence causing cr:ust
formatj-on, even on PVA plots.
The compaction treatment rvas applj-ed to hal-f of the plots as
follows. The sowing board was placed on the approprJ-ate p1ot, three hours
afÈer wetting Lo 20% water content, ancl an opera.tor stood on it. Än even
application pressure \^¡as achieved, itr this manner, to the whole p-lot.
Compactiorl pressure was 2.7 kPa. Coniple'bed ptots are sliown in Plates 7-48,
7.6 anð, 7.I1.
The ai:ea around the experimental plots v¡as sohrn with wheat
(Triticr:m aesti-vuin, L. cv. Gabo) tn 1-9'71. This r.^¡as done in an attempL to
preve¡t bird cla¡rage which occurred on the plots in 1976, causing yi'eld losses
Plate 7.4 Sowing and wetting of plots
A Buckets used. for storing aggregates
B PIot framework
C Sowing board
D Shield to prevent drift while wetting
with knapsack spray
E Completed plot.
Plate 7.5 Seed after placement in pJ-ot (4-2 mm aggregates).
Plate 7.6 Completed. plot wi.th tensiometer in place
(<1 mm aggregates).
tf
Ii¡I
lI
I
i
I.t
II
L
1F{
ii
t64
7 .2"2-5 Data lìecolrling
The c<¡leoptil.es wh.ich emel:ged v/ere coìrnted daily olzer t-he entire
perioct of etìlergerrÇe. This yielded ::esuits on two aspects of elrlergence
the time j-nLe¡rval bet--wecr¡r sowi¡g and Èrnergeflce, and the perccntagc emefgelÌce-
The niean day emergence as defilieci i:y Edwards (1957a) was used to
<leterrni.ne the tine per:'-od. beLween sor^Iir-ìg artrl emergence. Briefly, it is
calculated as fol-lows: thc cJarys v¡ere ntu¡bi'-r:ed, sowing day beì-ng day 1,
the next day 2 ¿urcl so or-r. 'l'l-re nunr]¡er of ccleoptiles emerging on a plot itr
any day was multj-plied by the d.ay nr¡¡rlcer and t-he products summed over
the whole enel:qence peri.o<l . This to.tal clivided by the total nu¡iÌ:er of
coleoptiles ernerging on the plot, gave t-he mean d-ay of emergence (MDE) for
that. plot. This represenf-s the mean inte::vaf betr¿een sowing and enìergence'
but the MDE tet:m has bee¡ usecl in its literal sens€, throuqhout.
The total number of coleoptil.es emerging expressed as a percentage
of the number solvn has been call-ed the percentage emergence.
plant heights were recorded each week, usitrg the same ten plants
per plot, throughout the ç¡rowing season. Tiller coun'¿s were recorded for
these plants also.
Soil t-.emperature and soil v¡ate¡: potential vtere recorded at-, as
close as possible, three hourly intervals during da-y-Iight hours in 1976
and around the clock in 7.977. Temperature was recìorded with a digital
multimeter (I.Iarconi instruments, Model IE 2610) and vrater potenti.al vrith
a soil moisture and ternperature bridge (Nationat instrurnents Mcdel 2004) "
Dur:ing 1976, measul:ements were recorded for the sowing to emergence
interval and 1--hen after a break for periods of 6-10 days untj-f a t-otal of
29 days was recorcled through the season for both early anit IaLe sov¿n ¡ll.ots'
Duri¡g J,977, measurements; were record.ed for the sowing to enrergence 1:eri:J '
AfLer a three week break, a period of five days of recordings were made,
this being repeated to give a total of 28 days of measui:ement through t-he
165
season. ¡IeasuremenLs v¡ere made in one pì-ot of each aggrecfat-.e size and
combinat-ion of Lreatments .
1\t harvest, aII plants \^/ere cut at ground level rvith shears and
placed in bags. It was thought that, due to the small size of the plots,
border effects would be minimal and if plants were cliscarded, yield
measurements would suffer. Total dry maÈter (DM) was recorcled by weighing.
The graì-n v¿as thr:eshed using a bench-top thresher" The grain yield was
recorded by weighing.
7.3 Results and Discussion
7.3.I Gfasshouse and phybo tron preliminary experiments
These experirnents were carried out init.ially to ascertain vrhether
aggregate sLze, water content arrd time of sowing had a significant effect
on mean day of emergence and percetrtage emergence.
From Table 7.I it is seen ÈhaÈ agç¡regate size, water conLent and
time of sowing aII significantly affect mean day of emergence-
TÄBT,E 7 " I ivlean day of Emergence - sunìmary of analysis of variance
for glasshouse and phytotron experir¡ents "
Glasshouse Phytot-ron
FTreatment
Aggregate size (aSS.)
Water content (Vlater)
Time of sowing (time)
Agg. - Vùater
Agg. - Time
Irüater - Tirne
ASg.-Water-Time
2.96*
3.26*
2.36*
fl
il
2.6tl-l_1. .186
25.060
2.016
L.394
4.618
L.209
4
2
II4
2
I
4.L66
r0 .733
6 .803
ft
il
il
tt
6
J
2
2.43*
3.06*
3 .91*
2.00*
NS
3.06*
NS
DFFVRDF VR
* Significant at P=0.05 NS = Not Significanl:
il
166.
Since the percentage enrergence data were bínomial-ly distributed,
the following model was postulated to enable the results to be analysed.
eU+agg. effect+water effect*time effectProbabiLity of emergence = U+agg. effect*water effect+time effect (7.1)
1+e
This quantity always lies between 0 and t. Estirna'bes of Uraggregate effect,
water effect and time effect \^rere obtained using an iterative maxj-mum
Iikelihood technique. The test statistic ís distributed as X2. This
Linear logistic model, however, is inappropriate for the'phytotrorr data,
since |OO% emergence was observed for luhe 4-2 run aggregaLes. The ma.ximurn
Iikelihood estjmate of probability for emergence in this class is therefore
1, an ínfj.nite argument for the exponential. To overcome this the binornial
variance was calculated and comparisons between classes made-
The percentage emergence data could have been transformed to degrees,
as did Edwards (I957a). However, the resultant distrjlcution is only very
approximately normal. The maximum likelihood estimate technique used is a
more rigorous analysis of the data. Table 7.2 summari-zes the percentage
emergence for the glasshorrse experiment.
TABLE 7.2 Effect of aggregate size, water content and time of
sowing on percentage emergence in the glasshouse.
TeSt
EffecL of Wa'Ler
Effect of Aggregate Size
Effect of Time of Sowing
Vüater/Agg. Size
Vlater/Time
Agg. Size,/Time
Water/Agg. Size/rime
( ooìk
NS
3.84*
NS
NS
NS
t5 .5r*
54 "9
7.3
5.1
I0 .5
4.2
3.0
22.O
X2
* Significant at P:O.05 NS = Not Significant
1.67 "
Water co¡tent and titne c)f sowing have a significant effect on
percentage enìergerlce.
7"3"I.1 Effect of asqreclat.e size on MDE ancì Percerttage
emergence
Tire effect of aggregate size on MDE is shown in F:'q" 7"2a ancl Ì¡
fot glasshouse and phytotron l:espectively.
seedlíngs in the glasshouse emerged significantly earJ-j-er on the
smal.Ie:: (<l rnm) and large (>4 mm) aggregates than on the i-ntermediate
sized aggregates. Seedlingsr growrl at a constant temperature (ZÙoC) in
the phytotron, emerged significantly earlier on l:he finer (<I mm) aggregates
tþan the coarser (>4 nrn) agg-regates. A trend of earf ier through to later
etnergence is e¡,zident with increasing aggregate size front <l mm to >4 mm.
The MDE in the phytotron are lower than those in the glasshouse ' This rnay
be due Èo temperature effect. These results agree with those of Edwards
(1957a) who de.l-ermined that emergence of barley and oats was earlier on
fine aggregates than on coarse aggregates. Taylor (L974) obser:ved a simi]ar
result for maize and sorghum.
the effect of aggregate size arrc water content o1'ì NDE is shown in
Fig. 7.3a and b for glasshouse and phytotrorl experiments respectively.
Generally, on each aggregate size range,l5s" water content delays emergence'
30% water content pïomotes ear'Iy emergence with the 20s" vtater content
occupying an intermedj.ate position. Again, earlier emergence is exhibited
on finer aggregates than coarse aggregates '
Percerrtage emergence $/as significantly affected by aggregate size in
the phytotron experiment and then only by the <I nm size (Table 7.3) '
Percentage emergence is significant.ly l.ess on the small agqregates
than on the large aggregates. There is no significant difference bet-ween
the other three size cl-asses.
I a: Glasshouse
se
1 2-1 4-2 > 4,42-1
b ' Phytotron
se
<1 2,1 4-2 > 4AGGREGATE slzg (mm)
Effect of aggregate size range on mean day ofemergence in the glasshouse and phytotron.
I
I
17rú3l¡¡C)zu¡c,trb=l¡¡l!oào5z¡¡lE
4
3
7
6
I
å3r¡r 5(Jzl¡¡c,É.¡¡¡
=¡¡¡
ä4ozË3
2
Fig. 7.2
I
2-1AGGREGATE
a, Glasshouse
se
>-L
2-1
b' Phytotron
T
I se
>4
ja,
å73t¡Jo2¡¡¡19E-6Et¡l
,\o
¡
a
o
àô5zt¡¡E
I
<1 2-1 4-2 > 4
4
3
7
T\
^6ollt
3t¡¡()ãb(9É.t¡¡El¡¡
oIozUJ
= 3
t-
2
Effect of aggregate size range and water contenton mean day of emergence.
.------! 30t Water Contentl---l 2gg Water Contento----o I5t Water Content
1 4-2SIZE mm
Fig. 7.3
168.
TABLE 7.3 The effect of aggregate size on perceutage emergence -
Phlztotron results.
93. 3
z
6t .64
100 .0
0
<l 2-L 4-2
Àggregat-e size (mm)
% Emergence
Binornial- Variance
>4
95 .0
2
A combinatio¡ of small aggregates (<I nm) wiÈh ej-ther: hígh and low
water contents consiclerably ::educes the percen'tage einergeDce (Table 7 -4).
TABLE 7.4 The ef fect of agigregate size and w¿rter contenL (s3)
on percent.age entergel'ìce - Phytotron resuJ-ts.
Water Content %
Aggregate size (nm) Fi¡romi¡rI Va.ri¡rnce
<1
2-l4-2
>4
At high wateï cont-.ents v¡ith smal.l. a.ggregates, lack of aerat-ion malz
have limitecl germination aud emergence' '¡¡hile rvi'bh a fc¡w water content
mechanical impedanÇe may have -t-imite<l enLer:gence. It was noted that in the
<l mm and l5ea rvater treatmenL Èhe seed Ìrad germi.nated, but díd nc¡t emerge-
The 4-2 mm aggregates appear to be supe::ior: tr: any other size itr percentage
emergence. The inter:mediate rvater conf;ent of. 2Oga r:esulted in the greatest
percentage emergence on all aggreç¡al-es . Consequently, it was clecidecl to
bring all- field plots to an initial water co¡rtenl: of. 2o>..
A similar result was ok;served -'i-n thts qlõrssltr¡use experiment wjth a
time of sowing treat-Inent imposed (Table 7.1-t) "
4
1
o
2
100.0
r00 .0
I00 .0
90 .0
65"0
95 .0
r00 .0
9s .0
20.0
85 "0
r00 .0
L00 .0
15 20 30
TABLE 1.5
169.
Effect of aggregate size, water content (e¿) ¿rnd 'time
of sowing on percen'Eage of emergence - Glasshouse results.
36 .6
70 .0
83.3
90 .0
56 .6
60 .0
86 .6
83.3
100 .0
70 .0
86 .6
90 .0
90 .0
90 .0
90 .0
86 .6
93.3
96.6
96.6
83.3
90.0
93.3
96.6
86.6
96.6
Time of Sowing
15Ear:Iy
20 30 t5Late
20 30
23.3
86 .6
76.6
83.3
76.6
<1
2-L
4-2
>4
>42-I
The intermediate water content resulted in the greatest percentage
emergence ou all aggregate sizes except the >4 íun aggregates '
7.3.L.2 Effect of water content on t4DE and percentage
emergence
Water content had an appreciabte effect on MDE in both the glass-
house and phytotron experiments. The MDE decreased with increasing water
content (nig. 7.4a, l>). l'he constant temperature of the phytotron al-so had
t.l:e effect of reducing time to emergence when one conpares MDE with that in
the glasshouse.
percentage erergence v/as greatest with the intermed'iate water
contenÈ in both the glasshouse and phytotron experiments (!-iS. 7"5) '
Greater percentage emergences were observed for all water contentsr except
L5%, in the phytotron than glasshouse. This may be due to a temperature
effect.
It is interesting to note that this water content (2Oe") fo¡l maximum
emergence is the same as the optimum water content (2Oe") for tillage
deterrnined for this soil (ojeniyi and Dexter, 1978) ' At that water content'
tillage with a set of tines produced the minimum proportion of aggregates
a: Glasshouse
se
4
5101520253035
b: Phytotron
se
5101520253035wATER conrrnr 1/¡
76
ftt!,
;oZõtoÉt¡¡
=t¡¡
oà5ô2l¡¡
=
I
I
6o.úIl¡¡()zR,t¡¡ r,c,É¡¡¡
=l¡¡¡!o
o2u¡E
3
Fig. 7.4 Effect of water content on mean day of emergence.
100
90
^80"-\
¡¡J(,zl¡¡
P70l¡JE¡¡J
¡¡¡I
ñ60()É,t¡Jo.
50
40
30
5 10152025 3035wArER coNrENr (%)
Effect of hrater content on percentage emergenceof glasshouse (o---r) and phytotron (o---o)experiments.
Fig. 7.5
I70 .
larger than 4 mm.
7.3.1"3 Effect of time of sov'ting on MDE aud percent-age
emergence
Seedlings emerged I"2 days earlier from late sown pots than early
sown pots (Table 7.61 .
TABLE 7"6 Effect of sowing date on MDE in the glasshouse
(¡tean of 90 pots) .
Early LSD (P=0.05)
6.9 o .45
Early emergence of seedlings also resultecl from late soil/n pots at a high
water content, wj-th time to emergence increasing as water content decreased
(sa¡te 7 .7) -
TABLE 7.7 Effect of sowing date and water content on MDE rn
t-he gÌasshouse (Mean of 30 Pots) .
we" Late
I5
20
30
5.9
5.7
5.4
LSD between means 0.79 (P=0.05)
Time of sowing also had a significant effect on the percentage of emergence '
Late sown lnts had a greater percentage emergence than early sown pots
(rabte 7.8) .
5.7
Late
5.7
6.9
8.0
EarIy
t7r.
TABLE 7.8 Effect of t-ime of sowing on percentage emerglence
in the glasshouse.
Late
% Emergence 84 -4
7.3"L.4 Conclusions
The results of these preliminary experiments gave an indication
of the reacbion of wheat to var:ious aggregate sizes, water contents and
sowing dates. It was decided to extend these treatnents to the field and
to wet aII aggregate seedbeds to a 20% water content, as ihis had'
consistently given the greatest percentage of emergence.
7.3.2 FíeId trial" results
The percentage of aggregates within each size Íange of sieved
aggregates is shown in Fig. 7.6. Generally, there lvas not less than 75%
of the specified size range present in each aggregate size range collected
from the field.
7 .3.2.I Mean day of emerqence in the fiel-d
The analysis of variance for mean day of emergence for the field
trials of L976 and. L977 are summarized in Tabl-e 7.9.
The salient point which emergesfrom Ta.ble 7"9 is that- the
aggregate size had a significant effect on I4DE in both experiments.
Compaction treatnents significantly affected the 1977 experiment,
but not .bhe 1976 experiment. A compaction and time of sorving treatment was
significant in 19?6 while a compaction and PVA treatment was significant in
L977 .
78.8
Early
f00100
8080
a) < lmm b)1'0 - 2'0mm
60
ñvlt¡¡
(,l¡JÉ.c,(9
lrot¡¡c,
z¡¡¡(JÉ.¡¡¡À
40
20
100
20
<1.0t.0
I2.O
60
40
20
r00
60
40
20
<1.0
0
>9.5
c) 2'0- 4'0mm
t.0I
2.0I
9'5>9.5
1'0 2-0 5'1
,lo rlt ,tu
2.0I
5.1
5'lI
9.5>9.5
8080
d) )4mm
t'0 5.1I
9.5>9.5
60
10
dØl¡¡F(9l¡¡É,o(,
lÀot¡¡c,Fzu¡(JÉ,UJÂ.
0
<t.0 <1.0
AGGREGATE SIZE RANGE (mm ) AGGREGATE SIZE RANGE ( mm)
Dry sieve analysis of aggregates obtained by rotarysieving into four size ranges.
2.OI
5.12'O
2.0I
5.1
t I
Fig. 7.6
L72.
TABLE 7.9
Aggregate size (ASS.)
Compaction (Comp")
Tj-me of sowing (Tine)
Agg. - Conp.
Agg. - Tirne
Conrp " - Time
Agg.-Comp.-TimePVA
Agg. - PVA
Comp. - PVA
Agg.-Comp"-PVA
Mean day of emergence - summary of analysis of var:iance
for field trial-s.
r976 L977
2.52*
4 "00*
F
NS
I
4"00*
I\IS
4 .00'k
NS
tl
tl
* Significant at P=0"05 NS = Not Significant
As might be expected the time inì-erval between sowing and emergence
was significantly affected by date of sowing.
In Lg77 the presence of a soil crust also siqn-i.ficantly affected'
the mean day of emergence.
7.3.2.L.L Effect of aggreqate srze
As expected from the preliminary experiments (sect-ion 7.3.L) ,
aggregate size significantly affected the MDE. In 1976 earliest emergence
of seedlings occurrecl wíth the 2-l nìm aggregates and then the time to
emergence increased with increasing aggregate size (FiS. 7.7a) - In I977
a similar trend r.= ob=u"ved. with the earliest emergence occurri.ng with
the fí¡est aggregates and latest emergence rvith l-he coarse aggregates
(FiS. 7.7b). Ed.wards (1957a) observed similar results r^¡ith the emergence
of barley and oats. Thow (1963) also observ'ed earlier emergence of oats
4
T
I4
4
I4
I4
I4
2.6L*NS
4.08*
NS
NS
4.08*
I{S
lf
il
il
It
6.187
o.643
LL.763
I .801
r .304
4.493
I,.090il
il
il
tt
22.277
5 .413
37 . r37
L.406
4.142
0 .840
o.772il
il
il
VRDF \rRF
a:197616
15
13
12
13 o
o
o
41
oÈE
u¡o=
o
o
cl 2-13
1-2 >1 >112 45678
2-1
16
15
otn
3r¿t¡Jo=
b: 1977
o
o
12
<l 2-11 2345678910
1-2 >t >!2-l
d (t'-)
Effect of aggregate size (d) on mean day of emergence(!DE) in the fietd in 1976 (o) and 1977 (o) . Curvesare plots of Equations 7.2 and 7.3.
Fig. 7.7
l_73-
from fine seedbeds t-han coarse seedloeds "
Regression equartions of the form descr:Lbed in Section 5 we-r:e
developed, for both 1976 and 1977, Lo relate MDB to aggregate diarnete:l, d.
The result-.ant equations are showÌl below : for 19'16,
MDE = 14.35 - O.3cl + 0.054d2,
MDB = L2.57 + O.6d + O.O3d2,
days, and
days
(7 .2)
(7.3)
for L97'7.
Equ-ation (7.2) pr:edicts a minimum MDE occurrinq wj-th 2.7 mm aggregal-es
in 1976, while equation (7.3) predicts an increasing MDE with increasi.ng
aggregate dianteter, d, for L9'77. Fresumùly, this is because smaller
aggregates aïe better in drought corrditions. Emergence from coa::sely
aggregated beds can be delayed in two main ways: the contact area of seed
and moist soit is small and seed slippage between aggregates causes
emergence from greater-than-optimum depths. Ear:ly emergence of cereals
is desirable to ïeduce competition between the cereal and weeds (Russel.l '
1961), which ultimately affects final crop yie1d" However, other factors
must be cons-idered - fine aggregaÈes are more prolle to crusting than
coarse aggregates (Section 5.3.2.5) whj-ch, depending on climatic conditions,
may prevent emergence altogether even though a rninirourn MDE results with
fine aggregates in the absence of an impenetrable crust'
7.3.2.L.2 Effect of compaction
The compaction treatment had a sigrnificant effect on MDE ín 1977
only (Table 7.I0).
TABLB 7.IO Effect of cornpaction on MDE (L977) (Mean 40 replicates).
Uncompacted LSD (P=0.C5)
L4.62
CompacÈed
T4.L4 o.4
I74.
Energence was 0.5 day earlier on uncompacted plots. Royle and
Heqarty (L971) also found that compaction delayed enìergence of cal-abrese.
Edwarcls (1957), however, observed earlier emergence on compa(:ted plots
than on uncompacterl plots for oat-s. The r:educed tinrc to cmerqenÇe reported
here for the compacted plots may have resulted because of mechanical
impedance in the plot. Another possibitit¡¡ may be the presence of a
surface crust, which would delay ernergence. A PVA treatment combíned with
compaction reducecl the time Lo etnergence (Table 7.Il-) "
TABLE 7"II Effect of compactj-on and P\/A treaLment on MDE (1.977)
(Mean of 20 replicates).
Crusted
Uncompacted
Compac tecl
14 .55
15 .48
LSD between means 0.6 (l=0.05)
Emergence j.s 1.7 days earlier on tlre uncrusted-compacted plots
compaïed to the corresponding crusted-compacted plots" Êdwar<ls (1957a),
holever, experienced no problems rvith so-il crusting, whereas the Urrbrae
l.oam used in Lhís stucly is susceptible to crust,ing. In l-976, compaction
did not have a significanL effect on MDE. This may have resulted due to a
difference in seasonal cond.itions compared Lo 1977. Howeverra conpaction-
time of sowing treatment had a significant effect on ÙlDE. Early sowr¡-
compacted plots emerged I"4 days earlier than the corresponding late sown
plots (Tab]e '1 .I2) .
Edwarcls (L957a) suggested thaL this ís pbssibly due to a reduction
in t'reight of overlying soil that the plant has to emerge through. In
Section 5"3.2.1-"I it was shown that the compaction treatment resulted in an
eight per:cent hei-ght redr:ction of the seedbed. This means the height of soj-l
l-3.14
13.17
Ur:crusted(PVA)
L75.
TA]]LB 7 . 12 Bffect of compaction and tj.me of sowing on .lvIDE
(1976) (¡1ean of 15 replicat-es) "
Time of Sowing
Late
Uncompacted
Compact-ed
L4.59
15 .32
LSD betrveen means 0 .7 (P=0 .05)
above the seeds would har¡e been reducecl from 50 mm to 46 riun, the sowing
clepth on the compacted p]ot-s would l:ave been 4 mm less than that on the
uncompacted ploLs. Since the grovrtt rate of vJheat coleoptiles, under
fj-eld con<litions, is 10 ** day-I (Coombe, personaf communication, Ig'78)
and assurning they grow at- this rate on all treaLments, this implies that
the coleoptiles would emerge 0.4 days earlier on the compacted treatment.
However, conditions not being optimum on all treatments resulted in the
coleoptiles emerging 1"4 days earlíer on the earJ-y sown-compacted plots'
Thus the suggesti-on of Edwards (1957a) may .in fact be correcL fcr the
early sor'm plots as they are wetter th¿in the fate sown pJ-ots t-hroughout
the season (Section 6). Hov¿ever, the compaction treatmetrt rttay also cause
more intimate seed-soil contact, thus improving imbibition of wat-er, hence
resulting in early emergence. With late sowing, however, the seedbed
was d.rier (Section 6) , due to less rainfall after sov'ring, thus delaying
emergence by 1.4 days.
7 "3 .2.I .3 Effect of tírne of scwins (f976) and P\4\ t.r:eatment
(Le71)
As inclicated i¡r bhe preliralnar:y experi.ment (Section 7 - 3 .1 - 3) time
of sowing has a significant effect on MDE (Table 7.13) '
l-4.2'7
13 .94
EarIy
Ll6 "
TABLE 7.13 Effect of time of sowing on MDE (1976) (Mean of
30 replicates) .
Time of Sowing
Early LSD (P=0.05)
t4.1r 0.5
Late sowing delayed emergent:e by 0.85 days compared with early sowing.
A similar result for barley and oats was obtained by Ecìwards (f957a).
The presence of a soil surface crust sir¡nificantly delayecl
emergence (Table 7.l.4) .
TABLE 7.I4 Effect of PVA treatment (to prevent cmsting) on MDE
(1911) (Mean of 40 replicates).
Uncrusted(eva¡
LSD (P=0.05
13.75 o.4
Bmergence was delayed by 1"3 ciays due to the presence of a crust"
This may result in the plant suffering water stress later in the season
and affecting final yieJ,d (Aspinall e! aI. , L964) .
7.3.2.2 Percentage emerqence in the field
The percentage emergence data for 1976 and 1977 were analysed using
the maximum likelihood technique (Section 7.3.1). This technique was used
because the data were not normally distrijcuted as requi-red for applying the
normal sta'tistical tests, and is amo::erigorous test than transforming the
data to approximate a nornìal distribution.
A summary of percentage emergence da'ha analysis for l-976 and L971
is presented in Table 7.15.
14 .96
Late
15 .0r
Crusted
r71 .
TABTE 7.I5
Effect of Aggregate Size
Effect of Compacti-on
Effect of Tj.me of Sowing
Effect of PVA treaLrnent
Percentage emergence - summary of analysis of data
for 1976 and ]977 field trials.
L976 r977
Tes L
9.48*
I.{S
3 "84*
ll
* Significant at F=0.05 NS = lJot Siqnificant
Agg-çegate size had a significant effect on percentage enel:(lence in
1976 and 1971. A PVA treatment also hacl a significant effect or¡ percentage
emergence in L977. The time of sowing treatment (f976) \das nearly
significant,as the tesf- X2, was 3.8. Edwards (f957a) observed a s.imilar
result for barley and oats. Interactions between treatments were not
significant.
7 .3.2.2 "L Ef fect of aqqreqai¡: s-Lze
Fig. 7"8 illustrat--es the effect of aggregate size on pel:Çentage
emergence fo:: both years of the fjeld trial. The maximum percentage
emergence \^/as observed on l-he 4-2 nun aggregates in bothr 1976 and 1977 -
This, however, is not reflected in ùhe final yietd measurements (Section
7.3.2.3.3) where the maximum DM and grain yielcìs were observed with 2-I mm
and <I nm aggregates in 1976 and 1977 respectively-
Regression equatì-ons relatíng aggregate diameter, d, and percentage
emergence, Em, were developed. The equation fot L976 being:
E¡ = 83.4 +- 1.5d - o.2d'2, z, and (1 '4)
for L977 t
E¡ = 83.5 + 3.5d - O.4d2, z. (7'5)
44.2
0.6
3.6ll 30 .3
tt
37 .0
0.6
9.48*
NS
NS
TestX2
X2
94
90
86
82
78
o
o o
O
12145678<f 2-l 1-2 >4
d (n- )
o
o
8E
t¡J
74
o
>¡l2-l
Effect of aggregate diameter (d) on Percentage emergence(Em) in 1976 (o) and 19?7 (o) Ín the field trials. curvesare plots of Equations 7.4 and 7.5.
Fig. 7.8
l7B "
Equation (7 -4) predict-s a maximum percentage emergence occulîring with
4.I run diameter aggregates i¡r L976, while equat-.ion (7.5) predicts 4.6 mm
dianeter agg.regates as being the optimum size for maximum ¡rercentage
emergence i-n L977. These values agree closely with the observed resul'b"
Aggregat-e size also affected the appearance of the coleoptiles as
they emerged. This is illustrated in Plates '1 .7 anð,7.8. The fine
aggregat-.es resul.t-ed in uneven emergencc: and a whitj.sh appearance of
coleoptiles, h/hile coarser aggregates resulted in more even emergence aud
a greener coleoptile.
The results present-ed above for wheat, which shc¡w that greatest
perceutage emergence occurs with an intermediate aggregate size I'ange I are
comparable wi'bh those for sugar cane (Jain and Agrawal , 1.970) and cotton (Yode
Lg37). Slightly different results have been obtaiued by researchers working
with barfey and oats (Edwards, 1957a). oats (Thow, 1963), and mai-ze and
sorghum (faylor:, L9'74). They found that finei: tilths gave greater percentage
emergences.
7.3.2.2.2 Effect of PVA treatment (1977)
The presence of a soil surface crust significantly recluc:ed tLre
percentage emergence. This is despite the fact that sJ-ight crtrsting
occurred, during application of the PVI\ on those plots which were meant to
have no crusts (Section 5). Percentage emer:gence was significantly grea.ter
on the uncrusted than crusted plots (fabte 1.L6) -
TABLE 7.16 The effect of PVA treatment (to prevent crust forma.tion)
on percentage emergence (L977).
Crusted
Em (%) B1 .5
0 .95
91 .5
0 .70
Uncrusted(PVA)
sll
Plate 7.7
P1ate 7.8
Emergence on 2-1 mm aggregates. Note green
colour of coleoptiles (1976).
Emergence on <1 mm aggregates. Note white
colour of coleoptiles (L976).
r19.
The crusb acts as a physical L:arrier to etrrergence of t-he seedlings,
thus if seedlings are unalrle to generate sufficient :fiorce they wì-Il not
emerge (Section 5.1). The seedlings'abiliLy l-o generate sufficierlt force
depends on many factors, such as Lenper:ature (Sectj-on 5 .2.2; I¡tril-liams '
1963) , water potential (Jensen et q!. , 1.9'12) and variety of speci-es (idilf iams,
1956). The crust also delayed emergence (Section 1.3.2.I.3) by I"3 days
compared to the uncrust-ed. surface. This clelay may have affected the
abj-lit1z of some secdlings to emerge, since the crust would be dry-i-ug out and
becoming strongeç thus preventing emergence (Hanks, 1960). Han-ks ;rnd Thorp
(1956) ha.¿e shown 'Lhat pe::centage emergence of wheat decreases wíth
i¡lcreasing crust sti:ength and that ultimate emeïgence depends on the water
potential of the crust.
7.3.2.3 P1ant qrowth and yield
7 .3.2.3.L Plant tleiqht
plant growth through the season was recorded as plant height at
weekly intervals Ln L911 and in 1976 until the l.2th week after sowing, and
then after a 3 week interval, wíth a final neasurement being recorcl-ed at
harvest. Leaf area iudex measurements were impractical . 'Ihe two exì:r:emes
of aggr:egate size, the >4 mm and <I mm, are ptotted for 1976 and L977 ín
Fig. 7 "g. All curves follow a similar trend. In 1976 the order of growtl:
curves was >4, 2-L, 4-2 and <I nrrn wh-ile in L9'/7 it was >4, 4-2, 2-I and
<I mm.
1.3.2.3. 1.I Effect of aqqregate size
In both years plants grew faster on the larger aggregates than on
the smaller aggregates (FiS. 1.9). The difference in growth, in 1976r ilôY
be due to differences in water and tempel:a'ture conditions within the plots "
Growth differences between years are due to seasonal conditions ' 257 '6 mm
100
90
80 -----l
70
3ooF
Ëso¡¡¡I
,405^go
20
10
4¿ t'
o6/
-V;-- a¿
to¿ o'
-E-
0
23456789 10 11 12 13 14
TIME AFTER SOWING
15 't6 17 18 19 20 21 22 23 24 25
(wks )
Fig. 7.9 Effect of aggregate size on plant growth
o>4mmo<Imm¡>4 mm
O --- El <1 mm
L977
L976
r80.
of rain fel-I on the early sown plots while 223.2 mm fell on the late sown
plots in 1976 (a difference of 34"4 mm) and 206.9 mm of v:ain fell in 1977
during the experimental period"
7 .3.2. 3.L.2 Bffect of compaction
The effect of conrpaction -rs showu in Fig. 7.10. very littl-e
difference occurs between t}¡e uncompacted and compactecl treatments" Howevel:,
lcetween II and 12 weeks after sowing in both yeat:s, the compactecl Lreatmelìt
"CTOSSeS Over"and grows at a faster rate than the uncontpacted.
The cross-oveï pc'int corresponds Èo a ra.infall period. In 1976 all
plots viere crusted over, thus infil-tration into the seedl)ecl woul-d be
approximately the same. În 1977 infittration would have been greater on
the PVA treated (reduced crust fo::mation) and larger sized aggregates
(Section 5). In Section 6 it is shown that the compacted plots are d'rier
than the uncompacted plots, however, better root-soil contact would exist
within the compacted plots enabling ptants to extract water more readily
(tqitter and Mazurak, 1958) and this may expJ-ain the diffe::ence in growth
at this time.
In Section 4.3 it was shovrn that evaporative \^Iater loss was
greatest from compacted piots. These losses were detelrnined using bare
soil. In the field, however, plant cover reduces the incident racliation
reaching the soil surface and hence evaporation loss " During periods of
high temperature plant transpiration is reduced (Salisbury and Ross' 1969)
and due to the shading effect water losses from the so-il are further
reduced.
7.3.2.3.1"3 Effect of tj-me of sowing (L9-16) and
PVA treatment (1971)
Time of sowing had an effect on plant growth (Fig' 7'I1)' The l'ate
90
80
?70.:¿
,ï oot¡J
50z
o.- 40
30
20
100
10
l-o-
2ttt--¡-'.
0
2345678910
f-_!O-<Þ -.oI--{
11 l2 13 14
AFTER SOWING
t5 16 17 18 19 2 21 2223 24 25
(wks )TIME
Fig. 7.IO Effect of compaction treatment on plant growth.
UncompactedCompactedUncompactedCompacted
L977
L976
100
o
o
a
90
80
3toÞ60T9iso
3nÀ
30
20
10
0
234 5 6 7 I910 11 12 13 14 'r5 16 17 18 19 2021 22232425T tME AFTER sowtNG ( wks)
Effect of early (o) and late (o) sowing dates on plant growth in 1976.rig. 7.1I
I8I.
sovrn plots çJreh¡ faster, initial-Iy, than the ear1.y sown plots" Thi-s may
be due to higher soil temperatures in the plots (Section 6) . HovJ(ìver, as
the season progressed, the growth rate of the early sown plots \^7as greater
than tha-t of the Iate sown. Final heights reflect tire differe¡rce i..n
length of the g::crwing season between the two sowing dates. There was a
four weel< dífference bet-.v¡eel: the earlv and fate sowrr pi-ots.
The PVÄ, treatment had very little effect on plant growth (Fig. 7.L2) .
Iloth uncrusted ancl crusted plots appeared to gr:ow at the satne rate, with
the only difference between the trvo being plant. h.eights. The presence of a
crust appears to onJ-y affect MDE (Section 7.3.2.f .3) where emergence is
delayed by I.3 days.
A sunurary analysis of va::iance for fína.l plant height is presented
in Table 7.L'7 f.or 1976 and 1977 respectively.
TABLE 7"I7 Final plant height - summary anal.ysis of variance forfield trials.
L976 L977
Aggregate size (aSS.)
Compaction (Comp.¡
Time of Sowing (Time)
Àgg. - Comp.
Agg. - Time
Comp. - Time
Agg.-Comp"-TimePVA
Agg" - PVA
Comp. - FVA
Agg.-Comp.-PVA
F
Ìqs
NS
NS
NS
NS
NS
NS
3.722
2.076
11.305
L.235
2.O40
o.290
o -747u
il
il
il
4
II4
4
t4
I4
I4
2.6L*
NS
4.08*
NS
NS
NS
NS
o.678
il
tl
ll
il
il
tf
tt
2 "346
1.r90o.263
o .880
r.624
o .073
VRDÌ- VRF
* Si.gnificant at P=0.C15 l'IS =' Not Significant
90
80
^70cIr- 60fIi50
â¿oJÀ
100
30
20
10
0
23456 7 8 I 10 11 12 13 14 15 16 17 18 19 20 21 2223 A 25
TIME AFTER SOWING (wks )
Fig. 7.L2 Effect of uncrusted (r) and crusted (Å) soil sr¡rface on plant growttr in 1977.
LB2 "
The salient poirrts fi:om this are: that aggi:egate size and date of
sowj-ng had a signif icant effect on final plant height in l-97t5 only.
TalLer plants were produced from larger aggregate sizes and the early sowing
date. This is evident from t-ig. 1.9 and 7"I1. ÈIagj-n (1952) also t-ound
taller plants grew on coarse agqregates than on fine aggregates - No other
treatrnent hacl any signi-ficant-. effect on plant height" Pfate 7"9 shows
differences between early sor,vn pJ-ots at tilleri.ng and Plate 7"1-0 shows the
late sown plots at the same tirne. Differences in height are evídent and
also the effect due to aggregate size i.s notj"ceable. Pl-ate 7"f also
illustrates differeìtces i.n plant height and growth in 1976. These differences
were maintained throughout the season. Plates 7 -LL-7.16 illu'sLrat'e a small
portion of the 1977 field trial. At any stage Èhroughout the season there
is no noticeable difference in plant heights between plots. The clifference
shown in the central plots was due to incomplete ernergence.
7 -3.2.3.2 Till-ers pe:: pl.ant
Sj.nce tiller numbers do not follow a norrnal distribution, it was
ass\uned they fotlowed a truncat-ecl poisson distribution, whích is an
approximation of a binomial clistribution. This assunption was macle because
the parameter being investigated, tiller number, was small and a large
population was being sampled. A maximu¡n likelihood estimate tectrnique was
used to estimate the parameter À in the following model,
-ìe ¡xProb(X = x tillers) (7.6)
-À 'xlI-e
where the test stat-istic isl d-istributecl as X2'
Table 7"lB summarizes the effect of the expe::iment-al treatmenÈs
on til-ler nurnber:.
Plate 7.9 Early sown plots (1976).
Plants at tillering stage.
Differences between plots were maintained
throughout the season - from emergence to
harvest.
top to bottom: Iooking from south to north
(refer Fig. 7.I).
>a u
a-2 c
>ac
a-t u
-t u
<tc
<t u
)¡u
<ru
a-2 c
<tu
)t u
l-r u
>a u
<t c 2-r u
>ac
).u
2-r c
tg2.1
U
>* " ì+,a-2 u <tc
2-t u
2-r c
)¡^2-t-
>f,"
a-2 u
2-t U ì+,a-t c
a-t u )¡c r a-l u 2.r c
Plate 7.10 Late sown plots (1976).
Plots 6 weeks after emergence.
Differences between plots were maintained
throughout the season - from emergence to
harvest.
top to bottom: looking from south to north
(refer Fig. 7.I).
a-2 c
)au
<tu
"{u<tu
a-l c
r.2 U
<iu
r.l U
2-r u
<tc
<tc
2-r c
>ac
t.r u
>a¡¡u
<tG
t.r c
>ac
t-2 u
a-t c
>au
elU
rau
a-2 u
2-r u
>.l-r
?*"
<t u>at-t G t.t c
u>ai:r'
1.2 U )¡c 2.r u )t u
f {'
.'11'\t\'
tt +iÞ.
trl
f
l. '.
+
.1
.F
i¡.lË
{I
-L
Plate 7.II
Plate 7.12
Appearance of plots after sowing and
wetting (L977).
Dark plots - treated with PVA.
Light plots - no PVA treatment.
Plots two weeks after emergence (L911) -
PVA treated plots still obvious.
r-l G
a-r G
a-t u
a-l u
>ac
>al-t
a-t u
<tu
>.C ?YA
.tu
<!G
¡t C
ffurve ffu >.c?yA
fiueve r-rcryA r-ruryA
U 'YA
>a;¡u
ff u rve t-rc m
raG
Èru7n
a-t u
Plate 7.13
Plate 7.14
Plots six weeks after emergence (1977).
Note difference in central plots - due
to crust formation when PVA solution
was applied.
Plots after titlering (1977) .
Note central plots with little or no growl-h.
l-l U>al-l UryA
t-r c rYA t-l u ?Y <t u
>ac a-t u
>al-t
tju >.c ?YA
fiut"^
.-l C
r-l U
<tc
>at-t U
'YAU >. C ?YA
>al-l u ?YA t-t c ?YA t-t u rya
>ac l-l U
<tu
r-l C <tc
Plate 7. 15
Plate 7.16
Plots at heading stage (L971).
Note ptots with little or no growth-
Plots at harvest (1971).
Some lodging has occurred.
.-l c
a-2 u
Dat:i
D'C a-2 U
"C?YA
-tc
'$ u re 'r$u
'$u eve t-tG wA t't u ñn ¡l u
a-2 u
UryA l-tCryA t.tUryA .tU
'*ru* '$u D.ctryA
a-lc Dac a.l U ¡lG
183.
TABLE 7.1-B tiller number - sufirmary of maximum l.il<elihood estimates.
rg't6 L977
Aggr:egate sizeCompaction
Time of Sowing
PVA 3.84*
* Significant at P=0.05 NS = Not Significant
Time of sowing was the only factor to have a significant effect
on tilJ.er number in 1976, An early sowing date (2.l0till-ers per plant)
produced significantly more tillers per plant than a late sowing date
(1.91 tillers per plant). This difference is reflected in the yield of
dry matter (DM) and grain (Section 7.3.2.3.2). Tiller survival in the
field is an important consideration in determining yield (Forster and
Vasey, 1931). It is also well established that tilter profusion is
affected by the space available to the plant. Consequently plots with
fewer pla.nts may have more tillers per plant, thus obscuring alrty t::eatment
effect.
In Section 7.3.2.L.3 it is shown that late sowing delayed emergence
by 0.9 days, this in turn affected the total percentage of emergence - the
later sowing date resulting in a lower percentage emergence. Consequently
there were fewer plants on the late sown plots than the early sown plots,
which would explain the greater number of tillers per plant on the late
sown plots due Èo tillers utilizing greater available space on the late
sown p1ots.
The PVA treatment was the only factor to have a significa¡rt effect
on tíller nurnber tn L977. There were significantly fewer till-ers on
uncrustecl (I.39 tillers per plant) than on crusted (1.98 tille::s per plant)
Test:
NS
NS
ll
NS
NS
3.84*
2.6
0 .56
7 .00I 47.L
il
27
o.4
tLTestx2
184.
plots" In Sectj-on 7.3.2.1.3 it- j-s also shown that the preserlce of a crust
delayed emergence lcy 1.3 days " Again tl-ris is re.flected in percent-.ege
cmergence, wj-th a greater number of plants enLerging on bhe uncrusLed than
crusted 1>1.ots. This would reflect the great-er number of tillers on the
crusted plol;s' as the plants tend to "compensate" for missing plants by
tillerj.ng and thus utilizing available space.
This situatíon rnay also be analagous with ther obser:va'Li-on that
roots, whett they are impeded, tend to brancll laterally (Abdalla, Hettiaratchi
and Reece, 1969). In this case, as the shool- is impeded by the ci:ust,
more tillers niay be induced to form. Ilowever, this similarj.ty may be
purely coi.ncidental .
Edwards (1957b) observed a decrease in tiller numJ¡er per plant with
incr:easing aggregate size and with earlier sowing date for barley and oats.
The differences, howe.ver, were small " Although the d.ifferences \dere
not significant, the l-ate sown plots tended to have more tiflers per plant
than early sown plots. This result is the reverse of that obtained above
and may be due to the difference in crops used, soils used or prevailing
clima L.ic conrl.itio¡rs .
7.3.2..3"3 Dry matter yield and qrain yie-'-d
Because of the constraints of plot size, the yield estimates are,
of necessity, not as precise as other measurements. However, this loss
of precision may be balanc:ed to a certain extent by control over variation
in soil conditions, sowing depth and seed rate due to this small plot
technj-que. Va.riation due to pests and disease was reduced. by spraying for
control. Coilection of yield data wiII not be as accurabe as other factors
examined, but general t¡:ends wil-I remairr the same.
The analysis of variance of the yj-e.ì-d data fo:: both years is
summarized in Table 7.19.
185.
Tì\BIJI 7.19 Yield Data - sumnary of analysis of variance forfield triafs.
L976 l-971
Grain Yield
) \)*NS
F
Asgregat-e size (ASS.)
Compaction (Comp.;
Tíme of Sowing (Time)
Agg. -Comp .
Agg. -Tirne
Comp . -Time
Agg. --Comp " -Tj-me
PVA
Agg. -PVA
Comp.-PVA
Agg. -Comp. -PVA
il
NS
NS
NS
NS
NS
* Significant at P--0.05 NS : Not Sígn.ificant
Table 7.19 sb.ows that aggregate sj-ze had a significant effect on
Dry l'{al-ter (D¡4) yieJ.d in 1976 and 1977. However, it significantly affected
the grain yiel-d in 1977 only. Aggregate size combined wit-h a compactJ-on
treat-ment significantly affected grain yield in 1976, but noL in 1977.
Dry matter yield and grain yield are significantly affected by the sowing
date (L916). Grain yield is also affected by a time of sowing and
compaction treatment (1976) and by the interaction betwoerr aggregate size,
compaction and, sowing date (1976). The PVA treatment had no effect on
any of the yield parameters considered, rvhich is surprising in view of
the fact tha.t the crusted plots had sj-gnificantly more tillers per plant
than the unc::usted pJ-ots.
NS
NS
4 .08*
2.61.x
I{S
4.08
2.6r*0 .250
0 .458
0 .751
o "756
o.951
tl
tl
il
It
ll
ll
o.462
2.7932.36r1.348
68 .684
4.626
0"3r05.145
5.202tl
ll
il
I
2 "6f.*NS
4 .08*ÀTC
Nì.)
NS
NS
2.809
0.003
2L "8632.57L
0 .067
r.073
1..o72
U
I
4
I1
4
4
I4
I4
I4
0 .165
o "6420 .601
0 .590
0 .973NS
NS
I\5
NS
NS
lt
ll
tt
2 .868
0"686
2 "52*NS
FVR VRFF VRVRDF
Yie-ldGrain Y:i-eldDM Yield
186.
7.3.2.3.3.1 Effect- of aggregate size
The effect of aggregate size on DItl yis-1¿ is illustrated in
Fig. 7.13. creater DM yields were obtained in 1976 than in I9'7-/ due to
differences irt rainfall between the Èwo years " The plots receivecl a
total of 257.6 mm ¿¡8 223.2 mm on early anrl late sown plots respectively
in I976u while only receiving 206.9 mm in L971. It is weII estaloLished
that yietd depends on the amount of rainfall received during the season
(Donald and Puckridge, 1915).
The 2-l- mm aggregates realized Èhe greatest DVr yiel-d in 1976, vrith
the <l nìm aggregates real:-zi-ng the greatest DM yield in 19i7.
The re<tression equat-ion developed, re-1aiing DM y.ield, DM, and
aggregate size, d (mm) for 1976 is
DM = 700.a + L42.36d - rs.o2d2, 9^2, (7.7)
and that for 1977 is
DM = 895.5 - r8.3d, g -2, (t2 = 0.89) (7.8)
Equation (7.1) predicts a maximum DM yield with aggregates 4.7 mrn diameter"
Vùith eqtration (7.8) an optimum size of aggregates cannot be estilnated.
However:. the impl-ication is that with a drier year greater yields may be
expected from small-er aggregate sizes.
In the rdry' year (L977) it is seen that the intermediate sized
aggregate bed.s (2-1 and 4-2 mm) remained wet'ter than the large or small
aggregate beds through the season (Section 6). These sizes also correspond
to those which exhibit minimum evaporative water loss (Section 4).
Consequently greater yields may be expected from these intermedíate and
slight--ty smaller sized aggregate beds unrler drought conditions, as plants
woulcl be less likely to suffer from water stress at any stage through the
seasorr. The resul t-s fron 1976 irnply that with a weLter year greater yields
may be expected, from beds with larger aggregates. With small aggregates
o
1100
1000
900
800
700
NtE
I
=o
a: 1976
o
o
o
123456600
900
800
700
600
7>4
I<t 2-l 4-2
12345678>¿l
>g2-l
o
>¡lF¡
b: 1977o
oÂl¡E
3
=o
<t 2-1 1-2
d (-'n)
Effect of aggregate diameter (d) on dry matter yield (DM)
of wheat in 1976 (r) and 1977 (o). Curves are plots ofEquations 7.7 and 7.8.
Fig. 7.13
187.
in such conditiorrs, wa+,er logging rnay occur'. which will in<luce anaerobic
conditions. Hagin (1952) observed greater DM yields of wheat on coarse
aggregates (>2 mm) than on any other size used. Titis tlisagrees rçitl-r the
c¡bserved DM yields i¡r L976 ancl 1977. Taylor (L9-1 4), however, usiug maize
and sorghurn, fouud gre;rter DM yieJ-<ls on finer aggregates than coarse
aggregates. This agrees with the observed DM yields in L977, but not
those in 1976" The experiment of Taylor (l-91 4) rvas noL a fiel-d t-rial , but
a controlled pot experiment. SirnilarlyJaggiet al. (L972) found greater
DM yields of wheat on finer aggregates than coarse aggregates. Edr¡ards
(f958) also founcl the greatest yield of sl-raw for barley an<1 oa1-s on fine
aggregates.
Aggregate size significantly affected grain yieJ-d in L977 only.
The 1976 results did not reach significance because of grain losses due
to lodging and some bird attack. The grain yielJ in L917 reflects the
trend exhibited by the DM yield (FiS. 7.f3b) in that yield increases with
decreasing aggregate size (rable '1.20) .
TABLE 7"20 Effect- of aggr:egate size on grain yield(Mean of 16 replicates)
_')(sm ) in 1977.
'4 / z-1.<I
362.4 304 .0
LSD between means 42.1 (P:0.05)
The result presented above, which shows greatest grain yietd occurs with
an intermediate size range, is comparable to that of Jaggi et aI. (L972)
who also used v¿heat. A slightly different conclusion has been obtained
with barl-ey and oats (Edwards, I95B) and sugar beet (Hammerton, 1963a).
They found that finr:i: tilths gave greater yields.'
A cotnpaction treatment imposed on each agg::eg.rte size significanr-Iy
333.6 308 .0348.4
2-L 4-2 >4
188 "
affected gi:ain yield in 1976 olrJ-y (Tabl-e 7 "2L) .
TABLE 7 " 2T Effect. of aggregate size arlcl (:c)mpactior; cln grain yield
(9*-2) in l-976. (Meatr of 6 ::eplj,c,: bes)
>42-1.
Uncompacted
Compacted
r59 .6
22.4 "O
LSD between means 87.8 (P=0"05)
The 4-2 nulr aggregates gave the greal-est-- yield on the uncornpacted
plots whereas the 2-l rnm aggregartes gave the grea-tesL yield on t:he compacted
plots. The compaction treatment significantly reducecl the yield on the
<l mm agqregates, while it significantly increased it on the 2-l and >4 nm
aggregates.
The effect of the compaction treatment is discussed in Sect-ion
5.3.2.LI, wheïe it is shown the aggregate bed \^Ias compacted 8e"- In the
coarse aggregate beds this would tend to improve soil-root corrt'acl-', while
in the fi.ner aggregates it v¡ould have the effect of increasing the
mechanical- impedance of the krecl . thus making it more difficul-t -for the
roots to peneÈrate. Compaction is known to reduce root gr:ovrth (Jamison
et al., l-g52; Taylor et al., Lg64). However, its effect on crop yields
varies; increasing it in some cases (Passioura and Leeper, 1963) while
reducing it in others (Phi.l.l jps and Kirkant, L962i. Edrvards (l-958) found
that a compaction treatment increased the yield of oats compared with an
uncompacted treatment" Compaction, however, had no effect orr the yield of
barley.
Grain yield (I976) was also signífJ-cantly affected by the interaction
between aggrega'ce size, compaction and time of sowing (Ta]¡le 7 "22) '
r90 .4
12.4
200"0
I-12.O
171.6
270 "4
154 .0
245.2.
<1 2-l 4-? >4
r90 "
inducing anaerobic condiÈions (a form of r¡¡ater stress (Kramer' 1959)) '
which is not conducive to plant grorvth. I¡later stress during anthesis
greatly affects the final grain yield (Aspinall .! aI. ' 1964; Fischer and
Kohn, 1966).
7.3.2.3.3.2 Effect- of time of sowing (1976)
DM yield and grain yield were both signi-ficantly greater with the
earlier sowing date than with the late sowiirg date in 1976 (Table 7.23) .
TABLE 7.23 Effect of sowing date on DM yield (gm-2¡ and qrain
yield (s*-2) in 1976. (Mean of 30 repì.-ì-cates)
LSD (p=0 .05 )
DM yieldGrain yield
154.6
39.2
The total amount of rainfall received by the early sown plots $las 257.6 mm
whiLe the late sown plots received 223"2 mm. As mentioned previcusly,
yield is dependent on rainfall received. The di-fference -in yield between
sowing dates confirms this. Eclwards (1958) also noted a decrease in
straw yietcl anC grain yield of bartey and oats, \^Iith later sowi-ng date.
The effect of sowíng dates (L976) can be appreciated when Plates
7.9, 7.10 are examined. There are obvious differences due to aggregate
size and compaction. These differences were maintained through the season
and are reflected in the yield results.
The differences dure to aggregate síze, compaction and PVA (L917) are
not as obvious as those during the previous yearb experi.ment (Plates 7.L2-
7.L6). The lack of obvious difference, apart from the three central plots
where emergence was incomplete due to crt¡st formation during PVA
application, is also ref.l-ected in the lack of siqnificance in the yield
7I6 .8
IO8.8
1074 .8
264.O
EarIy Late
191.
resul-ts of t-reart-mel1ts .
Grainyieldotllywassign:i-fJ'cantJ.yaf.fectedwhenacompactj'on
treat-nrcnt was imposed on the s;owing rlat-es. Earl-y sown-colllpacted plots
yielded siqni-fican'tly more gr:ai-n than any other tr:eatment (Table 7 '24) "
TAPLE' 7.24 Effect of eornpactiotr and time of sowing on Erain
yield (g"n-2) in 1976 " (Mean of L5 repJ-icates)
Late
Uncompacted
Contpacted
230.4
291 .2
1.t"9 "6
98.0
LSD betrveen means 55 .5 (P=0 .05)
This resul.t is consistent \¡rith the other results above' As shown ea::Iier
the early sown plots received 34"4 mnt more rain than the l'ate sown plots
(Section 7.3"2.3.1.1), consequently the compaction treatment may have
improved root-soil. contact and water avaifability in the early plots
compared to the late plots. In the lat--e sown plots, less watel: woulcl be
available anl¡v'7ay, due to less raj.nfafl, hence the plants possiJrly suffered
water stress, at- Some sLage of growth, thus causing a reductj-on 'in yield'
The appearance of the plots in L91'7, after harvest, is shown in Plate 7 ')-7 '
It can be seen that the pvA treated plots rnaintainecl a more porous sur:L'ace
structure, whereas the untreated. plots crusted over.
Ingeneralrthereappearstobegoodagr:eementontheoptimum
aggregate size range for the seedbed of nany crops (Table 7 '25i ' The
I-4 mm size rarige is the most frequently quoted, as being the opt'irtlum for
germination, eïnergence and subseqrrent grow'ch of the crop' The conc-l-usions
obtained here fall- into this category also. Thus even when soil i-s prone
to crusting, as is the Recl-'Brown-Earth, the aggregate size ranqe for optimum
germination, errrergence ancl growth is consistent wit-h the values reporterl in
EarJ-y
Plate 7.I7 Appearance of surface of plots after ha::vest
(Le71) .
PVA plots have maintained surface structure'
Plots with no PVA treatment have formed a
surface crust.
a-2 u ì* ,r* >fi u ta c Pl/A
2-t U PVA ¡tU
a-t u
>tä u PvA 2-t C PìrA
a-2 c rac elG
AggregaÈeÐiameter
Range(mm)
<I
2-L
4-2
4-8
I?f..Lg7"25 References which quote given aggregate size ranges as being optimum for various crops.
Cotton
Yoder (!937)
P\oNJ
Cereals
Edwards (1957)Thow (f963)
Dojarenko (L924)Jaggi et al. (L972)This work (L917)
Kvasnikov (L92e)Hagin (1932)This work (L976)
Sugar-cane
Jain çAgrawal(re70)
Jatn ò.
Agrawal(le70)
SugarBeet
HarunerÈon( 1e61 )
Maize &
Sorghum
Johnson &
Taylor (1960)Taylor (L974)
Larson (1964)
soybean
Nash &
Baligar(Le74)
Nash g
Baligar(L974
Sunflower
Mi11er &
Mazurak(le58)
Lemon &Erickson (L952)
Doyle sMclean (1958)Lemon & Erickson(Le52)
Tomatoes
193.
tl-re literature.
7.4 Conclusions
It is concl-uded that on the soit used, under the condiiions
described, all seedbeds produced sufficient plants to establj-sh a crop.
This agrees with previous conclusions 'that cereals are relatively
insensitive to seedbed conditions (Keen, 1930; Russell and Mehta, 1938;
Edwards I958) . Ne-verthel-ess, there was approxirnately a -l-0% d-ifference
between the best and worst percentage emergence on. different aggregate
sizes.
The mean day of emergetìce was invariably earlier on the fine
(<l ancl 2-1 mm) seedbeds than the coarse seedbeds.
A compact-ion treatment delayed emergence by half a day compared
wiLh an uncompacted treatment"
An early sowing date resulted in earlier emergence of coleoptiles,
by 0.85 days over a late sowing date.
The presence of a soil surface crust deraye<l emergence of
coleoptiles by l- " 3 days.
The greaLest percentage emergence occurred on the 4-2 tw¡ size
range.
A compaction and tj-me of sowing treatment had no effect on the
percentage emergence.
percentage emergence of coleoptiles was greater on uncrusted than
crusted p1ots.
The differences in yield obtaine<l on the various sieved aggregate
size ranges \^¡ere surprisingly small. Although differences in seedbed
conditions v¡ere extreme, plants produced satisfactory yiel<1s on all size
ranges. This also confirms the lack of sensitivity of cereals to seedl¡ecl
conditions. This result may aJ-so explaín why small differences in yield
194
are obtained when different 1--illage treatments have been appljed (Keen,
1930; Grierson and French, I975).
T'he greatest dry matter yields \^/eÌ:e obtained with the <f nrm and
2-I mm aggregates, which agrees with the mean day of emergence resrrlt.
Grain yielcl v¡as greaÈest wit]r the <I mm and 2-I mm aggregates also-
There was, however, no significant difference between the <I, 2-I and
4-2 mm size ranges,
A compaction treatment had no effect- on dry matter or grain yield.
A similar result was observed with the PVA treatment.
Time of sowing significantly affected cìry matter ancl. grain yields -
Early sowing resu1ted in significantly higher <1ry matter and grain yields
than did late sowing,
The fact that the crusted (non-PVÄ-treated) plots containe<l more
titlers per plant than uncrusted plots (PVA-treated), did not significantly
affect final yields, as the PVA treatment had no effect on any yield
parameter measured.
The influence of climatic variation has J:r:en mentioned where
tentative explanati.ons of certain ptìenomena l-lave been advancecl" It should
be recognized that the effect of rainfall, temperature, solar radj-ation
and other cl-j.maEic variables are complex and with the available data most
relationships between these factors and plant grovrth is conjectural and
useful, not as proven observations, but as indicators toward frrther
experimental endeavour in this field.
r95.
SBCÎTON 8
GBNEIìAI, DTSCUSSION AND CONCLUSIONS
B "1 Introduction
The qeneral objective of this investigation has been 'to increase
the spzrrse knowl-edge about the consequences of using different siz<-:s c¡f
aggregates in seedbeds, so that the ideal seedbe<l can be more nearly
specified. More specificalJ-y, the investigation deternined the conseqìlences
of us-ing different s.ized aggregates on several physical and agronomic
factc¡rs i¡cluding compõrctability of seedbed.s, vrheal'. gerl'linaf-ion, energence
and yield, crust formation by raindrop impac't, crust strength, soil
temperaLure, soil- water potential and evaporation losses-
The results of the investigation were used in an attempt to
define the ideal- seedbed for cereal growth under Australian conditions.
Experimental beds of sieved aggregates, having narro\^I size distributions,
were used because they have wel.l--defined structures and are reproducible -
This provides a means whereby this work can be compared with that carried
out at clif f erent locations arrd times. Other reasons for us j-n,ri s j-eved l¡eds
of aggr:egates have been elucidated in Sections I and 7. It w¿rs found that
the sieving techniqr-re did, not resul-t in sj-gnificantly <l.ifferent nutrient
status in the different aggregate size ranges (table I '1) .
Since compaction may reduce emergence and hence final crop yielclt
an idea of the load-bearing capacity of seedbeds was gained, beJ-ng based
on knowledge of the tensite yield strength of individual aggregates
(Section 3).
Fallowing is a common practice in the wheat growing areas of
southern Australia (Schultz , L97I , I972; Grierson and l'rench' l-975) '
Fallowing is defined as "the practice of ploughing the soil. in winter
(aug.- Sept") and keeping it bare over the summer period (Dec. - Feb.) and
196 "
TABLB B.I Chemicai analysis of aggregal--e size ranqesi col.lected
from the field.
0 .1460.153 0.15r
r "44 I .511 .54
300340 3I2
4-22-Ite Size (mm)
<1 >4
312
L.44
0 .148
Total I (ppm)(Hydrazine Sulphate Method)
Total C (e")
(Fisher Induction Method)
Total- N ( e¿)
(I.{icro-Kj eJ-dahl- l4ethod)
then sowing a crop in the autumn (ltlay - June) ". The idea is to conserve
the water from winter rains over the summer months for the following crop.
It is, however, a relatively inefficient process, since fess than 30% of
the winter rain is conserved (Schultz, L97I, L972). Evaporation l-osses
may be minimized by using certain aggregate sizes as described in
Section 4.
previous studies examini-ng the effect of different aggregate sizes
on plant growth have tended to use naturally stable aggregettes (Edwards '
I957a) or aggTegates procluced by crushíng large cl-ods (Haqin I 1952; .1'aggi
et al., L91O¡ Taylor, Lg74), and consequently there were no problems
assocj-atecl with the presence of a surface crust. Littte or no mention
has been mad.e regar<1ing the problern of soil crusting and strength of crusts
formed on different sized aggregates. The aggregates used in the seedk¡eds
in this stucly were collected from Urrbrae Ìoam (a red-brown earth)
(Stace et al., 1968), whictr is a soil prone to for:m strong surface crusts
(Piper. 1938). Soils of this type make up approximately 6% of the land
area util-ized for wheat-grorving in Australia (McGarity, L9'75) . Depending
on seaso¡al cpnditions, the strength of the soil crust and emergence force
L9l.
of the seedl.i.ngs cleterrrine the success of eniergence and crop establishment"
The effect of aggregate size on rate of c:rust format-i<¡n, strength of the
crusL formed and the air rresistancet of the crust was determir-red along
with the emergence force of the cereal used in the study (Section 5) '
Soit temperature, water supply, aeratJ-on and mechanical. impedance
are the main environmental factors affecting germinal-ion and emergence of
seedlings. These facLors ar:e in turn affected by the p::evailirrg meteo-
rological conditions. There have been very ferv stud.ies of the ef fect-. of
different aggregate sizes (wel-l defined structure) in the seedbed on
any of Lhese f¿ctor:s. The effect of different aggregat-e sizes i-n the
seedbed on soi-'l- temperature aird soil watei: potential was studied- In
conjurrction with this, the effect of meteorological factc>rs on soil
temperature and soil water potential was also examj-ned (Section 6) '
As stated earlier, there has been littte or no attempt to define
more clearly the ideal- seedbed for cereal (wheat) growl-h under Australian
conditions. In experi-ments using different impJ-ements for seedbed
preparation, smatl or zero differences in yield have been demonstrated
and no seedbed structural measurements have been made (Gri-erson and
French, lg75) . Experimen'bal seedbeds of sieved aggregates were userf to
determine the field response of v¡heat to graded, well defined differences
in seedbed structure (Section 7).
8.2 Conclusions
The main conclusions of the investi.gation may be stated as folfows:
that the tensile yield strength, Y, of aggregates depend.s on a.ggregate
diameter, d, for many soils. The value of Y is highly dependent on soil
water: potential-. There is, however, no <lifference in compaction behaviour
of beds of clifferent sized aggregates, but there is a difference in
l9B
cotüpact--j-on behavioulî due to wat-er potentiaJ- dif f e::ences ' Pore space
decreases as the level of conpaction incr:easesl"
Aggregate sLze, the presence of a surface crust and a compactÍon
treatment alJ. significantly affe,ct t-he evaporation ratio, Ev, hence
evaporation losses from aggregated beds. Met-eorological factors did not
significantly affect the evaporation ratio. For the two uncotrpactecl and
one cotÌìpacted treatment used, aggregates r¡f 2-l , 0.5-I and 4-2 mm gave
the lowest evaporation r:atio, hence evaporat--ion foss, respectively. The
presence of a surface crust recfuced the evaporation ratio to approximately
one-ha1f that ofì a correspondì-ng uncrus+.ed surface" A compaction treatment,
however, resulted in an increase in the evaporation ratio compared wì-th
an uncompacted treatment"
The emergence force of the wheat used in the fiel-d experiments
was detel:rnined to be 0.5N. Emergence force was affected by temperaturet
with higher forces bei.ng exerted at low temperatures (8oC). Aggregate
síze, a compaction treatment, time of sowing and v¡ater pot-ential all
significantly affectecl crust strength. A PVA treatment (to prevent crust
formation), however, had no effect on crust stren<7th. The weake:st crust
was formed on the 4-2 wn aggregat-es. A conrpaction treatment and an
early sowì-ng date resulted in stronger crusts being formecl compared wit'h
the uncompacted treatment and the late sowing date. Crust strength
decreased as the water potential became less negative. Crust air 'resistance
was significantly affected by aggregate sj-ze' a comPacl-ion treatment, time
of sowing and a PVA treatment. Crusts fo::med on the 4-2 rnm and >4 mnt
aggregates had the least air Lesistance'. Crusts formed on compac'Led
plots, earl-y sown plots and plots with no PVA applied (crusted) exhibit
greater air tresistancest than the corresponcling uncompacted plots. Iate
sown plots and plots with PVA applied (urrcrusted) . c::usts formecl under
natural rainfall- j.n the field clid not imped-e or inhibit ernergence of
r99 .
seedl-ings. Large aggreqates (>4 nìn) re-qj.st crust forrnation for lonqer
periods than smaller aggr:eEates (<4 nm). Consequer-rtly there is qreater
infil-tration and less erosion loss of soj-J. from larger aqgregal--es than
smaller aggregates " Compaction had no effect on lle rate of crust-
formation with respect to the aggr:egate sizes examined. Pores within a
crust appear to have a preferred hori.zont:aI orjenta'Lj-on, with uo continuc>us
pores opening at t-.he surface.
Scil temper:ature and water poten'tial and thei:: ranges are affect-ed
by aggregate size, a compaction treatment, time of sowing and the preseÌice
of a surface crust. Meteorologi-cal factors also i.nf-tuenceC soil t-emperature,
soil- water pot--entiaf and their range. However, the effect was v¿lr:iable.
The 2-l anð. 4-2 nìm aggregat.es were the r,rettest and coolest through the
season. The largest (>4 mm) and snal,lest (<f mm) aggregates were warmer
and drier throughout the season than the intermediate sizes. A compactio:r
treatment resulted in plots being drier and warmer than uncotlpacted plots.
The uncrusted plots were also drier and warmer thran the crustecl p1ots.
Regression equations relating soil ternperatr:::e and water pot-eniial to
meteorological factors were developed. The etluations resulted in some
irrconsistencies in the effect cf wínd speed on soil temperatur:e and water
potential . Meteorological f actors appear: to have a greater j.nf-lueirce on
soil water potential than does aggregate size. Hov¡ever, the reverse is
true when considering soil temperature. Large aggregate sizes have greater
soil temperature and soil water: potential ranges than small aggregate sizes.
A conìpaction treatme.nt and the presence of a surface crust reduce both
soil tempeïature anrf water potential range within tlle seedbed.
AlI aggregate sizes userf in the investigation produce<J suffic.ient
plants to establish a crop. The percentage emergence and the percentage
of survivinq piants on no occasion fel-I below 50ea of the total- sown. Irt
particula:: the mean dery of emergence v¡as earlier on fine anrl interrnediate
200 "
aggregat,e sizes tharr coarser sizes. A conìpactj-on treaì:ment, t.he presence
of a sr:rface crust- and a late sow-Lng date clelayeO emergence of the
seedlings compaLed r+ith the corresponding urrcc;nrpacted, uncrtlsLecf anci early
sown plots. percentage emergence of seedlings was greates't on the 4-2 mm
aggregates conìpared w-ith the other sizes, and on the uncrusted plots
compared with the crusted plots. A col¡paction and time of sowir-rg
treatment had no effect on percentage emergence of seedl-ings. The
differences in yieJ.d obtained on t-he various aggregat-e seedbeds were
su::prisingl-y smal-I. GeneralJ-y, the plants produced satisfactory )¡j-e1ds on
all aggregate sizes, considering the extrenes in condj,tj-ons. 'Ihe greatest
dry ma-t-ter ancl grain yj-eJ-ds v/ere obtaineo with the fine ancl j-nt-ermediate
aggregat-e sizes. There were, however, differences due to seasonal
conditíons. Afso there was no sj-gnificant difference between the 2-l- and
4-2 mm size ranges. An early sowing date resulted in higher dry matter
and grarin yields tha-n a 1a1-,e sowing cLate. A compaction treatment- and the
PVA treatment both had no significant effect on any yield parameter
recorded.
B .3 The 'ideal' see<fbed
The ideal seedbed is difficult to define, sínce the seedbed is a
compromise between many and, at times, conflicting requirements' Attempts
at defining the seedbed have involved using a sieve analysis of the tilted
layer (CoIe, Lg3g; Bhushan alld Ghildyat, L91 2; Hoyle and Yamacla, 1975)
and more recently the impregnatiou of the tilth in situ with mol-ten wax
(Dexter, l-976) and in terms of shear strength (Cottis-Ceorge anrl Lloyd,
1978). Sieve anal.ysis only gives information about aggregate size
distribut-ion arrd is subject to sampling error" The rvax impregnation
method gives information about pore size distributions and one can calculat'e
taggregatet sizes. However, these taggreqate' sizes are measured l-engths'
201 .
an¿ not diameters as obl-ained by sieving. The sJrear strengtl-r techrrique
is time-consuming and does not give any informatic-'n r:egarditlg aggregate
or pore size distr:ibution. The above techniques are, holuever, atr
attempt to measure the seedbed coriditions at or ¡rfter ti1lage, but the
effects of tillage are not as reproduc.ible as are the experimental
seedbecls used here. Thus it is ext::emely difficul-t to define or even
deterrnine seedbed conditions produced by tillage" Consequeut-ly it is
useful to study the ef fects of well def ined, narro$/ size distrj-but j-ons of
aggregates in the seedbed and the response of plants 1-o these aggregate
sizes. T.'he one disadvantage in using these exçerimental seedbeds is
that they are not directly comparaÌ:Ie to tilths p::oduced by conventiona-l
farming practices, but they do enable one to determine pJ-ant r:esponse to
different sized aggregates and the unclerlying principles would be the same,
This may indicate which sizes should be produced in the field and may
suggest which size ranges ti1-tage implements should produce for optimum
germination and emergence of the crop-
It has been suggested that the optirrum size for aggregates' in the
seedbed, is 1-5 mm (Rrissell, 196l; Greenl-and, 1971). However, oLher:
aggregate sizes have also been suggeste<1 as being optimal: <I mrn (Edwai:c1s,
1958; Hammerton, 1.963) a¡d 4-8 mm (Yoder, L937; Jain and Agrarval-, 1970) '
The conditions which a seedbed should provide have been elucidated in
Section I anrl need no repetition here-
The concl-usion from the presenÈ study is that seedbeds should
consist of intermediate sized aggregates (1-4 mm diameter). This agrees
with previously suggested sizes which provide opt-imum conditions for
germination, emergence and subsequent growth. Ilowever, many other fact'ors
are involved, such as the ability to withstand rainfall (i"e. r:esi-st crusL
formal-ion), provide adequate hrater sl-orager maintain aerobic co¡rditions'
202.
prevent a build up of excess tenperature, and prevent water logging.
Early and even emergel-ice is desirable to recluce weed corLrpetition
with the crop ancl Lo achieve unifornr maturing of the crop for mechanical
harvestinq. Ltepen,ling on seasonal conditions, it was sl-rown that early
emergence resull*ed from the <1 mm aggregates in a tdry' year and the
2-] nrm aggregates in a twettert year. These sizes havinc¡ t-.he least
negati-ve water potent-i.al l-hroughorrt the seasorr :-tl t-...1,: i:e spective years of
the fiel<l experimerrt. There was, however, l-ittle or no signif-ican1-
differe-nr:e between the 2-I and 4-2 mm s j"zed aggregates. These sizes also
correspond.ed to those that had the lowest evaporation r:atio, hence lost
the Ieast arnount of water by evaporation. A general r:egressio¡-l equation
was developecl relating evaporation ratio, Ev, to aggregate diaineter, d(mm) '
0.2d + 0.03c12 (8.r)
This equat-ion predicts a minirnum evaporation ratio with aggregates of 4 mm
diameter. This is higher than the observed value, but withj-n the range
suggested as for¡n-ing the optimurn seedbed. These sizes also maintain
tempel:atures close to t-.hose cousidered as being optimal fo:: germination a.ncì
root grow'Lh, However, it was also shov¡n tliat larger aggregate sizes
(>4 m-rn) resist crust formation for longer pe::ioCs c¡f cirne than smaller
sizes (<4 mm). Thus if surface conditions are to remaín open and favourable
for eurergerrcer larger aggregates are desirable on the surface. Ïn conjunction
with this it- was also demonstratetf that the 4-2 mm aggregates formed the
weakest surface crust compared with any other size used, for alf treatments-
Emergence of seedlings, however, was not restricted or limited by the crust
formed on any aç¡gi:egat.e size, except in two instances where crust formecl
and was stabilized by PVA application (Plate 7.L2-1.16). The strength of the
crust depencled largely on water potent-ial. Thus, depending on seasonal-
condit.ions the crust may f.imit or everÌ prevent seedling ger:minatíon and
: 0.5BV
203.
emergerlÇe (l4clntyre, Ì955). The soil usecl in tkLe fiel.d exper:irrents
was a red-brown eart"h which is susceptible to crust fornrat-ion, however,
crust forrna-t--ion was not of grea't importance in del-enuinì-ng crop
establishment" Albhough the presence of a crust d.elayed emergence and
reduced water loss, it is an undesirable feature of the seedbed in that
crusts cên prevent emerç¡ence if they dry ancl harden. It was also shown
that- compact-ion of a-qgregate beds did not depend on aggregate size, but
on water potential " Also. the strength of individual aggregates depend.ed
on sj-ze and v¡ater potential-.
In agricul,t-ural. sítuations, soil is usually coml:a-cted t'tot l:y
pure uniaxiaL stress but a corrJ¡ination of uniaxial and shear sLresses.
Nevertheless, some -idea of the compaction that woulcl be expected under
given circumstances can be estimated from a l<nowledge of the vertical
stress acting on the surface of a tilled soif. In the fiel-d. stresses
of 1.60 kPa can be produced by horses and cattle, 95 kPa by man and 60 kPa
by sheep (LulI, 1959) , The pneumatic wheels of vehicl-es can be inflaterl
to 100 kPa for tractors ancl 200 kPa for trailers, and may proCuce local
stresses twice these values (VandenBerg ancl GilI, 1962) .
For any given farming operation, tJterefore, it is ¡:,ossible to
estimate the vertj-cal stress being applied to the til-l-ed soii" Also,
some arbitary maximum compaction for aggregate beds can be chosen that
wil-I not lead to excessive darnage to soil structure. For exarnple
H/Hi = 0.7 may Jre set as the limit (equation 3.1-8, 3.1.9). Then from a
knowledge of the varia'b.ion of tensile yield strength, Y, with wa-ter
content it is possjJcle to estimate the maximu¡r soil water conte;rt above
which cer:tain farming operations should not be carried out.
Consicler the case of a tractor and trailer with trailer rvheels
exerting 200 l<Pa onto the surface. If H/Hí = 0.7 ancl Y varies wj.th water
204
content as shown in Tabl-e 3"1r-chen usitlg the working equati.on (3.I9) it
can be shov¡r1 that the 'Lractor and trailer shouid not be used al- wat-.er
contents greater than abouL L2e".
Another alterrrat.ive is to increase the tensile yield strength,
Y, of the aggregates by the use of a soit corrditioner. This woulcl have
the effect of increasing the l-oad-bearing capacity of t-he soj-I ¡ or
increasing tÌ're maxj-nrurr water content at which given oper:ations could be
performed. The only drawback of Lhis, however, is the prohibitive cost
of soil conditioners, wnich make it impracticable for large scal-e use"
Tt was obs;erved, initially, that crushing s'trengi-h, F' was
proport.ional to c12, ancl hence that Y is independent- of aqg::egate d j-ameter,
the impl.ication of this being that. the load-bearing capacity is
independent of aggregate diameter. This being the case, agricultural
tiltJls could be produced with aggregate sízes which optirnize other soil
physical conditions as requi::ed, without regard to load-bearing
considerations. However, later experiments showed that Y is not
independent of aggregate cj,iameter. This impl-ies that beds of smal-l-er
aggregates wc¡ul-d have a greater l-oad-bear:inq capacity than beds of larger
aggregates. conseqrrenLly the cptj.mum ¿..gr::l-cultuL:a-l til-th worrld h¿rve to
take into consideration load-Ì:earing capercity as wel.l as oLher factors
which optimize soil physical fact--ors for plant growth "
The idea of beds of smaller a.ggregates having a greater load-
bearing capac;ity than becls of larger aggregates agrees wefl with the
proposed si-ze range to provide optimal conditions for germj-nation,
emergence and growth. The smafler aggregates would withstan,l the passage
of farm machinery and maintairr "optimìlm'r con(litions for plant qrowth
better than larger aggregates' This, hov'¡ever' woul'd d'epend 1'argery on
the water conl-ent at the l-irne.
utilizing the equations re-l-ating a99regate diameter (d, mm) to
2.O5.
varior.rs parameters (crust strength, crlust air tresistance', evaporat-i-ou
ratio, mean <lay of emergence, percentage emergence and yiel.d). an idea of
the ideal seedbed aggregate size can l¡e obtained for both a rwetr year
and a 'dryr year (Ta-ble 8.2).
TABLE 8.2 Predicted aggregate sizes which minimize or maximize
various seedbed and plant growtll parameters.
Par:ameter Predícted optimumte Size (nm)
MinimumCrust Strength
Minimum CrustAir 'Resistancet
MinimumEvaporation Ratio
4.O
4"55 .1.
4.r5.7
MinimumMDE
MaximumPercentaqe Emergence
Maximum DM &
Grain Yield
2.7Increasing MDE with in-
creasing agg. size
4.7increasing yield with de-
creasing agg" size
4.L4.6
L976 - 257.6 mm rain ('wet') L971 - 206.9 mm rain ('dry')
There is general agreement between both years with respect to
aggr:egate sizes that optimize (nr-inimize or maxinr-ize) certain conditions.
Generally they are slightly larger than the suggested range to optimize
seedbed. conditions. This may be a response to differences in clirnatic
conditions experienced in Australia, and also the fact that the soil used
is prone to crusting. One would expect in a wetter year that larger
aggregate sizes woufd be preferred to smal-Ier aggregate sizes and vice
versa in a dry year " This is because small aggregates would tend to crust
', '7
7.4L976L977
7-47.5
r916r977
1.27.3
L976L977
8.1
r9"16L917
5 .1_B
5 .19
5.85.9
r9l6I977
Equation No.Year
206.
over and become water-1.ogged, hence an.r<.:rcìric, more read:lly than larger
aggregates in a tv/ett year. In a tdryt year: Lhe beds of stnall-<':¡:
aggregates woufd ret-ai¡r greater amounLs of available waLer t-han beds of
Iarger aggregates.
The stratified seedbed used (>4 mm overlying 2-I m:m) did not
appear t-o be any bettel: or worse than any other seedbed, with regard to
emergence, ar.:cì growtJr and final- yietd. However, this seedbecl dj-d result
in a lower evaporat-ion ratio, hence water loss, than any other seedbed.
Temperature aud water potentials in the str:atified seedbed di<l not rliffer
greatl-y from the uniforn', seedbeds.
A compactíon treatmen't ancl the presence of a s¡rr:face crust had
unfavourable consequences o¡.r seedbed properties examj-necf . Compactj-on
resul-ted in stronger surface crusts, delayed eme.rgence and resul-ted ín
\,l.rarmer and drier seedbeds compared wi-th the uncompacted plots. The
presence of a surface crust, while prevenÈing excess evaporation losses
delayed emergence of seedlings. These two treatments, however, had no
effect on pl.ant yield. The delay in emergence of seedlìngs may be
sufficient fol: weed specj.es to gain a competitive adrzautage, thus reducing
avail-able space and water for the growing cro.pr hence reducing yields.
This, however, was not a problern in the experimental seedbeds as they
were kept weed-free by hand weeding.
The eviclence suggests that the 'ideaf' seedbed for Australian
conditions (when consídering a soil that is prone to fornring stronq
surface crusts) should consist of larqe aggreqates (>4 run) on the surface,
say to a depth of 3 cm (fig.8.1). Under these aggregates is a layer
(3-IO crn) of intermediate sized agqregates (1-4 mm) into which the ser:d
ís placed (fi.g. 8.I) . Fine aggregates seem to be unclesiral:l.e because
they crust readily and may be anaerohic at field capacity.
The large surface aggregates withstand rainfafl aud prevent surjlàÇe
) 4mm
1-4mm
OO
Fig. 8 .I The 'ideal I seed.bed .
SURFAçE
-
0
3cm
sowrNGDEPTH
BASE OFSEEDBED
5cm
l0 cm
207 .
cïusts f::om reaclily fornring, wirile allor'¡i-¡rg adequate irrfilLration and
preventing erosion losses. The intermediat,e aggregates provide good soil-
seed contact. and soil--root contact resulting in even germination and elnergence
of the crop. Although atl" til-l-age implements sort 1-l're soil aggregates
during tillage, maybe nore controf of the aggregate sizes produced and
clegree of sorting should be considerecl with a view of i:educing the number
of implement passes to produce t-he required tilth. A-Lthough the 'ideal'
seedbed for Australian conditiorrs does noE differ: marl<edly from whab has
been suggestecl before, Ít is an attempt to base the design on quantitative
data rather than qualitative ol-¡servations as previously has beetl done"
8.4 Sugqes'tions for further work
Future studies should endeavour to t est the applicability of
equation (3.19) to different soil t1pes. A vaLuable contribution to the
subject of aggregat-e bed cornpa"t:-on would be the development of an equation
that is applicable in all- situatj-ons. Such an equation would incorporate
separete terms for all the principal physical processes that- occur, shoulcl
be easy to use anci should be based on a small number of simple mechanical
tests. Futu::e studies should al so investiga-te the effects on aggregate
beds of combinations of uniaxial and shear stresses as occur under
tractor whee1s and in other agricultural- situations. l'urther investigation
is also required into the relation of tensile yield strength, Y, and
aggregate size. In conjunction with this, the effects of sLze, number and
distribution of cracks within aggreqates should be examined. This would
lead to a more complete unCerstanding of the mechanics of aggregate
strength arrd seedbecl compaction.
More inforrnation is needed on emergence forces of plant species and
some atLempt made to relate these to cr:ust si:rength under variot:s conditions-
This would enable one to preclict whet-her o:¡ not a particular plant species
208.
would be able to emerge under a particular set of conclitions"
Fur:ther r^¡orl< is necessary to resolve apparent cont-ratlictj.orls in the
effect of meteorological factors on soif tentperature, water ¡rotential and
evaporation losses - Further replì.cation of the measurements in this study
by automatic recording devices is necessary. In analysi¡g the effect
of meteorological fact-ors on seedJ:ed conditions,f a time-series analysis
seems more appropriate than the simplistic linear moclel.s used in the
study. This is because the data was heavily t-ime-depenclent" The number
of degree-<1a1's j-ir each aggregate seedbed from soling to emel:qence above
say, L0oc should be cletermined. Th-is would cletermirre whethe:: or not
any one agg::egate size pr:ovicles better temperatur:e conditions for
germination and growth than any oLher size"
The measurements of soil temperature and water potential could,
in the future, be extended to seedbeds in the field produced by
conventional tillage methods. In conjunction with these measuretrenl:s, an
assessment of structure should al-so be made preferabJ-y using an in sitrr
technique (e.g" Dexter, l-976). In this manner t-he structu.re of seedbeds
procluced by various tillage implenients and managelnent practices could l:e
assessed for suitability as seedbeds.
Stratified seedbeds requíre further study as there is potential
for reducing evaporation losses (conserving water) and the effect of
compaction. There are a great number of possible combj.nations of
stratification that one could assess.
Different crops need to be assessed for response to seedbed
conditions. One would expect that the seedbed requirements for small-
seeded crops (l-ucerne, Medicago spp.) woul<l differ from those of Iarge
seedecl varieties (maize , Zge mays L. ) .
The response Lo addecl fertilizers in d,ifferent sized a-ggregate
seedbed.s v¡as not studied. Plants may respond dif f erent-Iy r:n dif ferent
209
sized aggregates in a field situation. I.Ir.rtrients may be utilized more
efficientJ-y from smaller-sized aggregates as predic:ted in Dexterrs
(1978) model. This requires further fiel-d testing-
Investigation into methods of reducing 'Ehe number of passes of
tillage implements, and subsequent compaction problems, should. be under-
taken. These could take the form of assessing the tseedbedr strtlcture
under conditions of minimum tillage or to investigate the effect of
,mellowi-ng' as a means of using the weather and hence reduce the number
of tillage passes to produce a seedbed. AII these should resul-t in the
til1age process becoming more economical- ancl the production of op'timum
seedbeC conditions for: ger:mination, emergence a-nd subsec4uent growbh of
the crop.
As stated before, the definition of the 'ideal' seeCbed is very
difficrrlt and. in all probability impossible, because of the ntany inter-
actions occurring in a fie-ld situation. It certainly would be impossible
to define for atl possible conditions, but by using experimental seedbeds,
a better understanding of crop response is gained and an icl-ea of what
constitutes an tidealt seedbed becomes more apparent.
2t0
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230
APPENDIX I
Compaction of aggregate beds
by
M.V. Braunack and A'R. Dexter.
Braunack, M. V. & Dexter, A. R. (1978). Compaction of aggregate beds. In W. W. Emerson, R. D. Bond & A. R. Dexter (Eds.), Modification of soil structure, (pp. 119-126). London, Wiley.
NOTE:
This publication is included in the print copy of the thesis held in the University of Adelaide Library.
