ῐ
J. Jpn. Soc. Soil Phys.
ῑ῏῎ῒΐῐ
No. 23, p. -ῌ+- ῍,**,῎Maximum Potentially Dispersible and Stabilizable
Clays under Cropping in Soils with Inherent
Textural Di#erences
Velu RASIAH*, ** and Tahei YAMAMOTO** Arid Land Research Center, Tottori University, Hamasaka, Tottori 02*ῌ***+,
Japan (Corresponding address)** Permanent address : Department of Natural Resources, PO Box +*/., Mareeba,
Qld. .22*. Australia
Abstract
Physical and mechanical stresses induced changes in clay dispersion is a major sustainability
and environmental issue, particularly in large-scale intensive agricultural production systems.
However, the dispersed clay may re-stabilize when the stresses are reduced or minimized/
removed. The objectives of this study are to (i) quantify the maximum potentially dispersible(DCmax) and stabilizable clay (SCmax) in soils with textural di#erences when the stresses wereintroduced and reduced, respectively, and (ii) identify the role of inherent soil variable(s) on DCmax
and SCmax. Dispersible clay measurements were conducted at monthly intervals for - years onseven soil types under di#erent cropping treatments. The cropping treatments used in this study
were conventionally tilled continuous corn (CTCC) and forages, which were established in +323 onplot that were previously under CTCC for more than +* years. The CTCC represents stress
imposed and the forages the stress reduced system. The DCmax in the stress imposed system acrosssoils ranged from -., to +0.0 ῌ compared to 0.. to --.2 ῌ, the total clay (TC) content. The DCmax
increased with increasing TC and decreasing soil organic matter (SOM) content. The SCmax in the
stress reduced system across soils ranged from +., to ../ ῌ and it increased with increasing TC and
SOM. Eleven to -1 ῌ of the DCmax was re-stabilized during the --year period under forages, i.e.
stress reduced system. The amount of SOM in the soil at the time of switchover from CTCC
played a signiﬁcant role in the re-stabilization of dispersed clay, particularly in soils with similar
TC. The results show the stabilization of dispersed clay under reduced stresses depended on
DCmax and SOM.
Key words : Dispersible clay, Stabilizable clay, Soil texture, Soil organic matter, Physical andmechanical stresses.

+.

Introduction

Dispersed clays in soils under cropping, canbe a major sustainability and environmental
issue, because of its association with runo# anderosion. In the highly mechanized intensive
agricultural production systems the physicaland mechanical stresses induced by multiple
tillage, heavy machinery wheel tra$c, andwater impact energy from over-head sprinklers
are, at least partially, responsible for increases
in clay dispersion. Number of environmental
factors, such as freezing and thawing (Rasiah
and Kay, +33.) and wetting and drying cycles
also play a role in clay dispersion (Caron andKay, +33,a & b). Weakening and breaking of‘ῌclay῍-ῌsoil organic matter (SOM)῍-ῌclay
῍’ bonds during SOM mineralization, removal

῏

4



of polyvalent cations, particularly Ca,, from‘clay-Ca,-clay’ bonds, and Ca,displacement by Na, under saline condition,
are examples of chemical stress induced clay
dispersion (Baldock and Oades, +33*).
Dispersed clays in soils have impact on soil
physical, chemical and biological processes.
Clay dispersion induced aggregate bondweakening enhances aggregate breakdown
when soils were subjected to further physicaland mechanical stresses (Rasiah and Kay, +33/a; Shanumuganathan and Oades, +32,). Clay dispersion increased sediment load in runo#(Rasiah and Kay, +33/b ; Isensee and Sadeghi,+320 ; Miller and Baharuddin, +320), decreased
water inﬁltration and conductivity in soil proﬁles (Collis-George and Greene, +313), and may
decrease the proportion of habitable pores for
microbes through pore clogging.
The dispersed clay (DC) in a given non-saline
soil under cropping increased with increasing
gravimetric soil water content (q) (Rasiah et al.,+33,). On the other hand, the DC in texturally
di#erent soils increased with increasing total
clay content (TC), soil pH, and q, and decreasing SOM (Rasiah et al., +33,). When q increased
to a potential maximum value, qmax, then potential existed for the dispersed clay to approach apotential maximum value, DCmax (Rasiah, +33.).
In texturally di#erent soils, the DCmax and qmax
depended on TC and SOM (Rasiah, +33.).
Though it is known that under ﬁeld conditions,at least a small proportion of DCmax may restabilize during soil drying, the major proportion would remain dispersed or weakly bonded(Caron and Kay, +33,a & b). The weakly
bonded sites are points of potential failurezones, which are responsible for aggregate
breakdown under physical and mechanicalstresses.
When the physical and mechanical stresses
inducing clay dispersion under conventional
corn were reduced, through switch over toforage-corn rotation, the dispersed clay restabilized during the forage phase (Rasiah andKay, +33.). Furthermore, inclusion of forages

23

,**,

in rotations or as legume covers increased
water inﬁltration (Cassman and Rains, +320),
improved porosity (Disparte, +321), and protected soil surface from rain and sprinkler drop
impact, thereby preventing crust formation(Disparte, +321). Though re-stabilization of dispersed clay might have had several beneﬁcial
e#ects on soil water processes, limited information, however, exists on maximum potentiallystabilizable clay (SCmax) under forages of cornforage rotation. Thus, quantitative information on physical and mechanical stresses induced clay dispersion under stressed systems
and that on stabilization under reduced
stresses are essential. Therefore, the objectivesof this study are to (i) to provide quantitative
information on DCmax and SCmax when the
stresses were imposed and reduced, respectively, in soils with textural di#erences and (ii)identify the role played by inherent soil variable(s) on DCmax and SCmax.
,.

Materials and methods+) General
The data used in this study is from crop
rotation and tillage experiments established in
1 sites (Table +) in Ontario province, Canada, in+323 and continued until +33+. Before +323, theexperimental sites, in farmers’ ﬁeld, were underconventionally tilled continuous corn (Zeamays L.), CTCC, for at least +* years. Conventional tillage involved plowing in the fall,
followed by secondary tillage in the spring.
The CTCC treatment represents the physicaland mechanical stresses imposed system, andthe forage phase in corn-forage rotation the
stress reduced system. Crop rotations at thesites included cereal-forage or soybean-forage
rotation. In this study, however, we will present the data relating to CTCC and corn-forage
rotation only. During the sampling period thecorn-forage rotation plots were in the forage
phase. The forages included in the rotation
were alfalfa (Medicago sativa L.) in soil .,bromegrass (Bromus inermis L.) in soils +, ,, -,and 0, and red clover (Trifolium pratense L.) in῎῍ : Maximum Potentially Dispersible and Stabilizable Clays under Cropping in Soils with Inherent Textural Di#erences

Table +

Soil description and selected properties

Soil description
Soil type

Series name
(US classiﬁcation)

5

Soil properties
Caly

Silt

Soil organic
matter

CaCO-

pH(ῌῌῌῌῌ g/ +** g soil ῌ or ῌ ῌῌῌῌῌ)+Brookston clay
(Typic Haplaquept)

--42

,14-

-4-

*4**

/411

,

Brookston clay
(Typic Haplaquept)

,+4+

.,4-

-4+

*413

14--

-

Tuscola silt loam
(Aquic Hapludalf)

+24/

/043

-43

*4+0

040*

.

Conestoga silt loam
(Aquic Eutrochrept)

+24-

/,4*

-43

+4/*

14,

/

Conestoga silt loam
(Aquic Eutrochrept)

+143

/-4.

-4.

,4*.

14+1

0

Wattford loamy sand(Arenic Hapludalf)

04.

+*4.

,43

*4**

04.*

1

Fox loamy sand(Arenic Hapludalf)

04.

+/41

,4,

*4+*

/42.

soil / and 1. The experimental design at eachsite was a randomized complete block withfour replications. Crop, fertilizer, weed, pest,
and disease managements were carried out according to the recommendations of the Ontario
Ministry of Food and Agriculture (+322a and b).
For further experimental details and other related information, the readers are referred to
Rasiah and Kay (+33. and +33/a, b) and Rasiah
et al. (+33,).
,) Soil sample collection and laboratory
analyses
Soil cores (1 cm diameter and 1 cm length)were collected from the surface * to 1-cm layer
at four randomly selected locations from each
replicate at monthly intervals from May
through September in +323, +33*, and +33+. The
soil cores from each replicate were thoroughly
mixed, and approximately one-quarter of themixed moist soil sieved, on a mechanicalshaker, using a nest of sieves with mesh openings of +*, ,, and + mm, respectively, and ashaking time of one minute. The aggregatesretained on the +-mm sieve, i.e. + to ,-mm aggregates, were used for the dispersible clay (DC)and ﬁeld soil water content (q g/g) determinations. The + to ,-mm aggregates from the +323
soil samples were also used for particle-size,
soil organic matter content, SOM, pH in a + : +soil : water mixture, and calcium carbonate
content (Page, +320) determinations.
The
results of the analyses are reported in Table +.
Dispersible clay measurements on the ﬁeld
moist + to ,-mm aggregates were conductedusing the procedure described by Pojasok andKay (+33*). In brief, three /-g subsamples of theﬁeld moist + to ,-mm aggregates from each
replicate were prewetted on a wetting table at+ cm suction for 3*-min using deionized water.
The prewetted sample was transferred to a /*-ml test tube using .*-ml of deionized water.
The suspension was shaken in a mechanicalshaker for +*-min and then transferred to a ,/*-ml conical ﬂask using another 2*-ml ofdeionized water. After a .*-min settling time
(calculated using Stakes Law), the DC in thesuspension was determined, using a colorimeter previously calibrated for each soil. The DC
is expressed as a percent of total oven-dry soil
mass. The q in the + to ,-mm aggregates was  23  ,**,

6

Table ,The maximum gravimetric soil water content (qmax) and the maximum potentially dispersible
clay (DCmax)
Soil type

Soil property
qmax(g/g soil)
DCmax (g/+** g soil)

Table -

+

,

-

.

/

0

1

*4.0
+040

*4..
+,42

*4.2
04*

*4./
/41

*4.*
142

*4-1
-4,

*4.*
/4.

The dispersible clay at average water content and that at maximum water content expressed
as a fraction of other variables
Dispersible clay type and its relationship with other variables

Soil type

DC (ῌ)

DCmax/TC[DCmaxDC (ῌ)]A

A/DCmax

A/TC

+
,
.
/
0
1

/420
/4+,
,403
,411
-4.*
,4*0
+4,,

*4/*
*40+
*4-*4-,
*4..
*4/+
*42.

+*411
140.
-4,2
,431
.4-2
+4+0
.4+3

*40/
*40*
*4//
*4/,
*4/1
*4-0
*413

*4-,
*4-1
*4+2
*4+1
*4,/
*4+2
*400

DC (ῌ)/DCmax DC (ῌ)/TC
*4-0
*4.+
*4./
*4.3
*4..
*40.
*4,,

*4+2
*4,.
*4+/
*4+0
*4+3
*4-*4+3

DC (ῌ)dispersible clay at average water content (ῌ), DCmaxmaximum potentially dispersible clay, TCtotal clay in soil.

determined gravimetrically and expressed as afraction of the oven-dry soil mass (g/g).
-) Computations
According to Rasiah et al. (+33,) the dispersed clay (DC) across soil types increased
with increasing soil-water content (q),DCipiqi qi

+where the subscript ‘i’ refers to a given soil.
When qi in Eq. [+] approaches a ﬁnite maximum
value (qmax), then clay dispersion is at its potential maximum and this is deﬁned as DCmax. The
DCmax was computed using Eq. [+], for qqmax,
and the computed values are provided in Table,. The qmax used in the DCmax computations arealso provided in Table ,. The values for pi andqi, for each soil, used in the computation of qmax
and DCmax are provided by Rasiah et al. (+33,)
in their table ,.
Rasiah and Kay (+33.) developed a model, topredict the net amount of clay re-stabilized byforages, in corn-forage rotation, and theydeﬁned this as SCnet,
SCnetpq qf+ektqcqcqf ,
where SCnet is the di#erence in DC between
that under CTCC and the corresponding forage
treatment. In Eq. [,] the p, q, k, and qc aremodel parameters, which were estimated using
experimental data. In Eq. [,] q is equal to thedi#erence between q of forage and corn andsimilarly for p. The subscript ‘c’ refers tocorn, ‘f’ to forage, the superscript ‘k’ a rate
constant, and ‘t’ is time, in months, after the
introduction of forages in corn-forage rotation.
The values for the parameters in Eq. [,] areprovided by Rasiah and Kay (+33.) in theirtable -. The amount of dispersed clay that canbe re-stabilized by forages, i.e. under reduced
stresses, as ‘t’ approaches inﬁnity has been
deﬁned by Rasiah and Kay (+33.) as maximum
potentially stabilizable clay SCmax and they῕῔ : Maximum Potentially Dispersible and Stabilizable Clays under Cropping in Soils with Inherent Textural Di#erences

Table .

7

The stabilized clay expressed as a fraction of dispersed clay
Stabilized clay and its relationship with dispersed clay types

Soil type

+
,
.
/
0
1

ῒ

ΐSC (ῌ)ΐSC(ῌ)/TC ΐSC(ῌ)/DC(ῌ) [DCΐSC(ῌ)/ΐSC(ῌ)/DCmax
maxῐDC(ῌ)]

.4/+
-4*,
+411
+42*
,4*3
+4+1
*4//

*4+.
*4+.
*4+*
*4+*
*4+,
*4+3
*4*3

*411
*4/3
*400
*40/
*40+
*4/1
*4.1

*4.,
*4.*
*4/.
*40+
*4.2
+4*+
*4+.

*4,2
*4,.
*4-*
*4-,
*4,1
*4-1
*4++

ῒ
ΐSC (ῌ)ῑstabilized clay at average water content (ῌ), DCmaxῑmaximum
potentially dispersible clay, TCῑtotal clay in soil, and ΐDC (ῌ)ῑstabilized clay at
average water content (ῌ).

showed, through appropriate re-arrangementof Eq. [,], that SCmax can be computed using thefollowing equation,
ΐSCῌq῍ῑῌΐpῐΐq q῍

῎-῏

where q is the average soil water content overthe growing season in a given soil. The ΐSC(q),computed using Eq. [-], for the 1 soils are provided in Table ..
-.

Results and Discussion+) General
The total clay content (TC) in the soils
ranged from 0.. ῌ to --.2 ῌ, the texture fromloamy sand to silty clay, and the soil organic
matter content (SOM) from ,., to -.3 ῌ (Table+). The clay mineralogy is dominated by illite,with subordinate amounts of chlorite, vermiculite, and hydroxy-interlayered vermiculite(Baldock and Kay, +321 ; Evans and Cameron,+32-).
Depending on the length of time (t) after the
introduction of forages, in corn-forage rotation,soil moisture content (q) at sampling, and thesoil type the amount of dispersed clay (DC) inthe soils ranged from *.*/ ῌ to +, ῌ (Rasiah etal., +33, ; and Rasiah and Kay, +33.). In general,the DC was high in soils with high TC, suggesting that source strength played an importantrole on the amount of DC in soil under ﬁeld
conditions. The DC in the 1 soils increased

with increasing TC and q, but decreased withincreasing SOM and soil pH (Rasiah et al. +33,).
The DC in the 1 soils decreased with timeafter the introduction of forages and subsequent to change over from conventional tono-till system (Rasiah and Kay, +33.). Rasiah
and Kay (+33/b) showed that runo# and sediment load, from soil /, increased with increasing DC, regardless of the cropping treatment.
Though runo# simulations were not conducted at the other sites, we anticipate similar
trends from these sites. Indirect evidence forswelling, during wetting, induced structuraldestabilization and reduction in inﬁltration
rate was derived positive associations that existed between DC and runo# (Rasiah and Kay,
+33/b). From the foregoing, it is evident that
DC in the 1 soils played important roles insurface and subsurface soil water ﬂow, and thespatio-temporal changes in DC depended oninherent soil variables, q, and cropping andtillage management practices.
,) Maximum potentially dispersible clay
The maximum potentially dispersible clay
(DCmax) in the soils ranged from -., to +0.0-g
clay/+**-g soil (Table ,). Rasiah (+33.) showedthat DCmax increased with increasing TC anddecreasing SOM, but was determined largelyby the source strength, i.e. TC. The DCmax
estimates, computed using the equation provided by Raisah (+33., Eq. 1) show that +..+-g

8



clay/+**-g soil was potentially dispersible insoil +, at -./ ῍ SOM, and the estimated DCmax
increased to +/.0 g clay +** g+ soil when the
SOM decreased to -.* ῍. This suggests that +*῍ increase in clay dispersion was associatedwith *./ ῍ loss in SOM, possibly through Cmineralization. Similar estimations for soil 0
show that only 0 ῍ increase in clay dispersion
for *./ ῍ reduction in SOM, suggesting theimpact of C-mineralization on clay dispersion
was higher in clay soil than in loamy sand.
The clay dispersed at maximum soil watercontent (qmax) is also potentially available fortransport in surface and subsurface ﬂow (Table,). Because, the clay dispersed at qmax is freefrom soil aggregates, it can be transported in
inﬁltrating water and may clog soil pores
during soil drying. The soil water status favorable for maximum clay dispersion to occur
usually arises after rain or irrigation events.
Thus, in soils with high TC, potential existed
for large quantities of the clay to be dispersed
at qmax and transported in surface and subsurface ﬂow. In soil +, with --.2 ῍ TC, the DCmax
was +0.0 ῍, implying that ῌ /* ῍ of the TC was
potentially available for transport in ﬂow compared to ῌ2. ῍ of the TC in soil 1 (Tables ,and -). Even though the absolute amount of
the DCmax was higher in soil + than in soil 1, alarger proportion of it in relation to TC was
potentially available for transport in runo#
from soil 1. The least amount of DCmax was
found in soil - and . that are characterized byhigh SOM, indicating the importance of SOM
in decreasing clay dispersion at maximum soil
water content.
Even though Rasiah and Kay (+33/) showedthat runo# or sediment load and DC(q) was
positively correlated, in reality though it seems
that DCmax should have been correlated withruno# and sediment load.
During dry-spells that follow rain or irrigation events, the amount of dispersed clay insoils will decrease with decreasing q, i.e. a corollary of the positive empirical relation between
dispersed clay and q (Rasiah et al., +33,). Thus,

23

,**,

the obvious question now is what is the fate ofthis DCmax during soil drying? During soil
drying the DCmax, at least a small proportion of
it may re-stabilize, temporarily, through weak
clay-clay bonding or be deposited on
aggregate surfaces and/or pore walls. During
re-wetting the temporary bonds may break
and release free clay. Evidences for temporary
weak bonding and for the release of free clay
have been provided by Caron and Kay (+33,a).
Thus, we suggest the clay that is potentiallyavailable for re-stabilization through changes
in cropping and tillage practices is that present
at qmax, i.e. DCmax. On the other hand, Rasiah
and Kay (+33.) used the amount clay dispersed
at average ﬁeld water content [DC(q)] as thatavailable for re-stabilization through changes
in cropping and tillage systems.
Because, the qmax soil water status (Table ,) isshort lived, we computed DC(q), for each soil, asa fraction of DCmax [DC(q)/DCmax] and TC [DC(q)/TC] (Table -).
The di#erence between
DCmax and DC(q) is the amount that was dislodged from soil aggregates through saturation and is in a relatively dynamic or in an
un-aggregated state and readily available fortransport. Thirty six to 13 ῍ of the DCmax or +2
to 0/ ῍ of the TC was in dynamic or unaggregated state. Exclusive of the loamy sand
1, /, to 0/ ῍ of the DCmax in silt loam and clay
soils were in un-aggregated or dynamic state
and we suggest that this fraction of the DCmax
need immediate re-stabilization attention.
The DC(q) is the amount of dispersed claythat is in a weakly bonded state due to soil
drying, but may disperse or disaggregate
during saturation. When DC(q) was expressed
as a fraction of DCmax (Table -), it showed that,, to 0. ῍ of the DCmax was DC(q), suggestingthat soil drying played an important role in
reducing the amount of clay available fortransport in water. In clay and silt loam soils,-0 to .3 ῍ of the DCmax was DC(q), The highest
and lowest percents were associated withloamy sands, suggesting inherent soil variables
other TC playing a temporary role in the stabi- : Maximum Potentially Dispersible and Stabilizable Clays under Cropping in Soils with Inherent Textural Di#erences

lization of dispersed clay during soil drying.
The major emphasis on clay dispersion inthis paper is that induced by mechanical andphysical stresses and dynamics of the dispersed clay in relation to soil water status.
However, it is important to recognize here that
dispersion induced by salinity, particularlythat arising from saline water use, may be amajor issue in arid, semi-arid, and tropical environments. Several studies have shown theimpact of salinity on decreases in soil hydraulic conductivity through pore clogging by thedispersed clay (Sumner, +33-). Gypsum application was e#ective in restoring, at least partially, conductivity under saline condition.
This suggests, the pore clogging clay particles
were transported or translocated away frompore walls after gypsum application and were
probably re-stabilized in aggregates. Limited
quantitative information, however, exists on
the fate of dispersed clay under forages inforage-cereal rotations in saline soils.
-) Maximum potentially stabilizable clay
The amount of dispersed clay (DC) underforages in forage-corn rotations decreased withtime, during the - yr period, in the 1 soil types
and the DC under forages were always less thecorresponding continuous conventional corn
treatment (Rasiah and Kay, +33.). The aforementioned provides evidence for the foragephase induced stabilization of clay that was indispersed status under corn. As time approaches inﬁnity, the DC under forages may
decrease to near zero and the amount of clay
stabilized would be at its potential maximum,
SCmax. The SCmax at growing season averagewater content, q, is deﬁned as SC(q). The 
SC(q) is the di#erence in dispersed clay between the corn and forage treatment. During
the --year period, the SC(q) ranged from +.+1
g clay +** g+ soil to ../+ g clay +** g+ soil
(Table .). The SC(q) had been shown to increase with increasing TC, SOM, and soil pH(Rasiah and Kay, +33.).
The clay stabilized by forages ranged from 3
to +3 ῌ of TC (Table .). According Rasiah and9Kay (+33.), the fraction of the dispersed claythat was stabilized by forage is SC(q)/DC(q).
Thus, according to them .1 to 11 ῌ of the dispersed clay was stabilized by forages in - years(Table .).
However, in reality though, asmentioned elsewhere in the text, the total dispersed clay in a given soil is DCmax. Thus, SC(q) was expressed as a fraction of DCmax (Table.). This form of expression revealed that only++ to -1 ῌ of the DCmax was stabilized byforages. We also indicated elsewhere in the
text the di#erence between DCmax and DC(q) is
the amount that was in a relatively free ordynamic state. Therefore, we expressed  SC(q) as a fraction of this di#erence (Table .).
This computation indicated that +. to almost
all the so-called free state clay was stabilized
by forages. Ironically, the highest and lowest
stabilization of the clay in dynamic state
occurred in the loamy sands. The highest stabilization occurred in the loam with the highSOM and the lowest in the loam with low SOM.
This suggests the SOM present at the time of
introduction of forage probably played an important in enhancing the stabilization of theclay in dynamic state.
The maximum amount of clay that can bestabilized by forages depended on the dispersed clay pool and we have shown this poolis DCmax and not DC(q) as indicated by Rasiah
and Kay (+33.). The DCmax is a much larger poolthan DC(q) in a given soil (Tables , and -), andDC(q) is only ,, to 0. ῌ of DCmax. The DCmax
increased with increasing TC and decreasing
SOM (Rasiah, +33.). The SC(q) showed atrend to increase with increasing TC or DCmaxor DC(q) and/or SOM (Fig. + and Table .).
Therefore, we explored SC(q) as a function of
TC and SOM, or DCmax and SOM, or DC(q) and
SOM using the stepwise variable selection procedure. The analyses produced a signiﬁcantrelation only with DCmax and SOM independent
variables,SCq+1/*,/ DCmax*/0 SOM
R,*30 and P **+.



10

Fig. +

23,**,

The relationship between maximum potentially stabilizable, [ῌSC (ῌ)], or dispersible, [ῌDC(ῌ)], and total clay (TC).

Equation [.] indicates that SC(q) increased
with increasing DCmax and SOM. However, therole played by DCmax is di#erent from that ofSOM.
The amount of clay that can bestabilized by forages depends largely on thedispersed clay pool. This pool could be either
TC or DCmax or DC(q). The aforementioned
analyses, i.e. using TC or DC(q) or DCmax as
the clay pool, indicate that DCmax is the poolthat determined SC(q) and not TC or DC(q).
Equation [.] also indicates that SOM enhanced
clay stabilization when the dispersed pool was
not a limiting factor. For example, in the
loamy sands (soils 0 and 1) with 0.. ῌ clay
content, the inﬂuence of SOM on clay stabilization is higher in soil 0, with ,.3 ῌ SOM, than insoil 1, with ,., ῌ SOM. The SOM clay stabilization enhancement role was signiﬁcant only
when DCmax was used as the dispersed clay pooland not TC or DC(q). This provides further
support to our hypothesis that DCmax is poolthat determined the maximum potentiallystabilizable clay.
The enhanced e#ectiveness of SOM in clay
stabilization under forages, suggests the binding materials arising from short-term forages
were more e#ective under high initial SOM
and is consistent with the results reported byother workers (Nadler and Letey, +323 ; Swift,+33+). The results are also compatible with theobservation made by Baldock and Oades (+33*)
that Ca, addition during straw decomposition
resulted in increased structural stabilization
through synergistic e#ects.
We suggest that in soils with high DCmax theorganic binding materials produced by shortterm forages were more e#ectively utilized
during clay stabilization. This probably led tothe immobilization of the organic materials inthe stabilized aggregates, thereby leading togradual increases in SOM. The aforementioned
hypothesis is consistent with the ﬁndings of by
Dormaar and Foster (+33+).
As mentioned elsewhere in the text, quantitative information on the stabilization of dispersed clay in saline soils under forages is
scarce. Recently, however, Mitchell et al. (,***)
showed that soil aggregation under legume
winter cover crops, followed by summer
cereals using saline water for irrigation, washigher than without cover crop treatment.
Further, the stability of soil with gypsum treatment and saline water use was less than covercrop. However, surface soil salinity was higher
under winter cover than the gypsum treatment. Unfortunately, the changes in clay dispersion after saline water irrigation and wintercover crop treatment were not measured in῍ῌ : Maximum Potentially Dispersible and Stabilizable Clays under Cropping in Soils with Inherent Textural Di#erences

this study. Nevertheless, the results indirectly
show that clay dispersion might decreased
after the introduction of winter cover crops
according to Rasiah et al. (+33,), who showednegative correlations existed between aggregate stability and dispersed clay (Rasiah et al.
+33,).
Even though the forage phase in forage-cornrotation was e#ective in stabilizing dispersed
clay, the practical applicability of this rotation
is limited, because, such rotations may not be
economically feasible or acceptable. Further, if
animal husbandry is not incorporated in thefarming systems then e$cient utilization of
forages is limited. Thus, it seems winter covercropping or under-seeding green manurelegumes in cereals, particularly under corn,more appropriate than forage-corn rotations.
..

Conclusions

In previous studies, the dispersed clay at average ﬁeld water content [DC(q)] was considered amount that was available for stabilization when forages were introduced (unstressed
system) in forage-corn rotation. In this study,
we show the maximum potentially dispersible
clay (DCmax) is the pool that was available forstabilization when the forages were introduced. The DCmax is much larger than DC(q),which ranged from ,, to .3 ῌ of DCmax acrosssoil types. The DCmax under conventional corn(stressed system) ranged from -, to 2. ῌ of thetotal clay (TC) across soil types. This indicatesthat substantial quantities of TC were in dispersed state in the stressed system. When theimpact of the stresses were reduced, throughthe introduction of forages, the dispersed clay
re-stabilized and the amount stabilized ranged
from ++ to -1 ῌ of the DCmax across soil types.
The sources strength, the total clay content(TC), determined DCmax pool and consequently
SCmax. Soil organic matter (SOM) had opposing
e#ects on SCmax and DCmax. The SCmax increased
with increasing SOM in a given soil and acrosssoil types. Thus, we suggest SOM conservationand management not only enhances clay stabi

11

lization, it may contribute towards C-sequestration in soil, particularly under reduced tillage systems. C-sequestration in soil may help
in reducing the concentration of green housegases in atmosphere.
/.

Acknowledgment

The authors gratefully acknowledge theﬁnancial support provided by the Japanese
Ministry of Education, Culture, Sports, Scienceand Technology to the ﬁrst author as a visiting
Professor to conduct research for one year at
the Arid Land Research Center of the Tottori
University, Japan.
References
Baldock, J.A. and Oades, J.M. (+33*) : Structuralamelioration of a South Australian red earth
using calcium and organic amendments. In
Soil Structure Research in Eastern Canada.
Proc. Eastern Canada Soil Structure Workshop, Guelph. pp. +-+ῌ+.+.
Baldock, J.A. and Kay, B.D. (+321) : Inﬂuence ofcropping history and chemical pretreatment
on the water stable aggregation of a silt loam
soil, Can. J. Soil Sci, 01 : /*+ῌ/++.
Caron, J. and Kay, B.D. (+33,a) : Short term decrease in soil structural stability following
bromegrass establishment on a clay loam soil,
Plant Soil., +./ : +,+ῌ+-*.
Caron, J. and Kay, B.D. (+33,b) : Rate of response ofstructural stability to a change in water content : Inﬂuence of cropping history, Soil Tillage Res., ,/ : +01-+2/.
Cassman, K.G. and Rains, D.W. (+320) : Cropping
systems approach to salinity management in
California, Amer. J. Alt. Agric., + : ++/ῌ+,+.
Collis-George, N. and Greene, R.S.B. (+313) : Thee#ect of aggregate size on the inﬁltration behavior of a slaking soil and its relevance toponded irrigation, Aust. J. Soil Res., +1 : 0/ῌ1-.
Disparate, A.A. (+321) : E#ects of root mass density
on inﬁltration among four Mediterranean dryland forages and two irrigated forage legumes.
MS Thesis. University of California Press, Riverside, USA.
Dormaar, J.F. and Foster, R.C. (+33+) : Nascent aggregates in the rhizosphere of perennial ryegrass, Can. J. Soil Sci., 1+ : .0/ῌ.1..
Evans, L. J. and Cameron, B.H. (+32-) : The Brookston series in Southwestern Ontario : Characteristics, classiﬁcation, and problems in deﬁn-12ΐῐ῎῔῕ῑ ῒ 23 ῏ ῌ,**,῍ing a soil series, Can. J. Soil Sci., 0- : --3ῌ-/,.
Isensee, A.R. and Sadeghi, A.M. (+33-) : Impact oftillage practices on runo# and pesticide transport, J. Soil Water Conserv., .2 : /,-ῌ/,1.
Miller, W.P. and Baharuddin, M.K. (+320) : Relationship of soil dispersibility to inﬁltration anderosion of Southeastern soils, Soil Sci., +., : ,-/ῌ,.*.
Micthell, J.P., Shennan, C. Singer, M. J. Peters, D.W.
Miller, R.O. Prichard, T. Grattan, S.R. Rhoades,
J.D. May, D.M. and Munk, D.S. (,***) : Impactsof gypsum and winter crops on soil physicalproperties and crop productivity when irrigated with saline water, Agric. Water
Mangt., ./ : //ῌ1+.
Nadler, A. and Letey, J. (+323) : Adsorption isotherm of polymers on soils using tritium
labeled compounds, Soil Sci. Soc. Am. J., /- :+-1/ῌ+-12.
Ontario Ministry of Agriculture and Food. (+322a) :Field crop recommendations. Publication No.
,30. OMAF, Toronto.
Ontario Ministry of Agriculture and Food. (+322b) :Guide to weed control. Publication No. 1/0.
OMAF, Toronto.
Page, A.L. (ed). (+320) : Methods of Soil AnalysisPart ,. ASA and SSSA, Madison, WI, USA.
Pojasok, T. and Kay, B.D. (+33*) : Assessment of a
combination of wet sieving and turbidimetry
to characterize the structural stability of moist
aggregates, Can. J. Soil Sci., 1* : --ῌ.,.
Rasiah, V. and Kay, B.D. (+33/a) : Characterizing
rate of wetting : Impact on structuraldestabilization, Soil Sci., +0* : +10ῌ+2,.
Rasiah, V. and Kay, B.D. (+33/b) : Runo# and soil
loss as inﬂuenced by stability parameters,
cropping, and tillage practices, Geoderma., 02 :-,+ῌ-,3.
Rasiah, V. (+33.) : Equations to predict measures ofstructural stability at minimum strength, Soil
Sci., +/2 : +1*ῌ+1-.
Rasiah, V. and Kay, B.D. (+33.) : Quantifying thechanges in clay stabilization after the introduction of forages, Soil Sci., +/1 : -+2ῌ-,0.
Rasiah, V., Kay, B.D. and Martin, T. (+33,) : Variation of structural stability with water content: Inﬂuence of selected soil properties, Soil Sci.
Soc. Am. J., /0 : +0*.ῌ+0*3.
Shanmuganathan, R. J. and Oades, J.M. (+32,) :E#ect of dispersible clay on the physical properties of B horizon of a red-brown earth, Aust.
J. Soil Res., ,* : -+/ῌ-,..
Shennan, C. (+33,) : Cover crops, nitrogen cycling
and soil properties in semi-arid irrigated vegetable production systems, Hort Sci., ,1 : 1.3ῌ
1/..
Stivers, L. J., Shennan, C. Jackson, L.E. van
Bruggen, A.H.C. Miller, P.R. and Gri$n, C. J.
(+33*) : Winter cover cropping in vegetableproduction systems. In : Abstracts of the International symposium on Agroecology and conservation issues in temperate and tropicalregions. ,0 Sept. +33*. University of Padova,
Italy.
Sumner, M. E. (+33-) : Sodic soils : New perspectives, Aust. J. Soil Res., -+ : 02-ῌ1/*.
Swift, R. S. (+33+) : E#ects of humic substances andpolysaccharides on soil aggregation. In Advances in soil organic matter research : The
impact on agriculture and environment. W.S.
Wilson (ed). The Royal Soc. Chem., Cambridge,
UK. pp. +/-ῌ+0,.
Wyland, L. J., Chaney, L.E. Klonsky, W.E. Koike, S.
T. and Kimple, K. (+330) : Winter cover crops ina vegetable cropping system : Impacts on nitrate leaching, soil water, crop yield, and management cost, Agric. Ecosystems Environ., /3 :+ῌ+1.

ΐ¼ : Maximum Potentially Dispersible and Stabilizable Clays under Cropping in Soils with Inherent Textural Di#erences

13

 
Velu Rasiah* **ῌ*
* 

**  !

"#$%&'()*῎+,-῏.ῌ/0.1234)56789:ῌ.1;<=>?)@A
B81CD.EFGH.IJKL,M NON17P"#&$%327QRNST)1,OUVWXYZ8[\+,]^_7`AM &a.3 bic 2de&῍f&gXhX)@AB i1,#_&#j)@k,$%_"# bDCmaxc Z[\_"# bSCmaxc &]^Sῐl\ῐ+,mZ biic
DCmax @nF SCmax )op+,q῍#j'r bSc lst,mZKL,M "#$%&u\3vw&*-xyz)@AB 0 vῑ&#_&#jl῎AB - {|&|}~XYM)῎APXY*-3

.)*WX ῒ*z&  bCTCCc ZKL +323 {)\WXYM gm3 +* { CTCC xyz)LYM CTCC 3delY& 3delQPNY&lLP+M "#ῐ bTCc 7 0..--.2 ῍)B delYZ8#j& DCmax 3 -.,+0.0 ῍&KLYM DCmax 3 TC & #j&῍/- bSOMc &QR)B891YM delQPN
YZ8#j& SCmax 3 +.,../ ῍&KL TC SOM pH &)B891YM DCmax &++-1 ῍3&K27QRNYZ8 - {|[\NBAYM CTCC OP ¡¢,£ +1¤27QRW¥PX,£)¦§+, SOM &ῐ3¨©ª& TC l«¬#j­&$%"#&[\()81®¯l°±YM m&²³32QR´&$%"#&[\_7$%"#µῐZ
SOM )¶·WX,mZl¸NBA,M!"#"$ : $%_"# [\_"# #_ #j῍/- -῏./0.2
¹º{~» : ,**+ { - ~ -* »¹῏{~» : ,**+ { 2 ~ ,* »