Open Journal of Soil Science, 2012, 2, 82-90
http://dx.doi.org/10.4236/ojss.2012.22012 Published Online June 2012 (http://www.SciRP.org/journal/ojss)
Review Paper: Challenges and Limitations in Studying
the Shrink-Swell and Crack Dynamics of Vertisol Soils
Takele M. Dinka1, Robert J. Lascano2*
1Department of Soil and Crop Sciences, Texas A&M University, College Station, TX, USA; 2USDA-ARS# Cropping Systems Re-
search Laboratory, Wind Erosion and Water Conservation Research Unit, Lubbock, TX, USA.
Received February 29th, 2012; revised March 26th, 2012; accepted April 11th, 2012
ABSTRACT
The need to study the shrink-swell and crack properties of vertic soils has long been recognized given their dynamics in
time and space, which modifies the physical properties that impact water and air movement in the soil, flow of water
into the subsoil and ground water, and generally alter the hydrology of vertic soils. Measurement of crack properties has
been made by numerous researchers with the purpose to understand and quantify the spatial and temporal dynamics of
shrinking and swelling and the associated formation of cracks. These crack properties, which are important in modify-
ing hydrology of soils are: width, length, depth and orientation of soil’s cracks. To better understand the hydrology of
vertic soils and incorporate crack properties into hydrologic simulation models, several techniques have been developed
to measure crack properties. However, little attention is given to evaluate both the advantages and the limitations asso-
ciated with these techniques. Thus, the purpose of this review is to highlight challenges and limitations that have been
used or might be used to measure cracking in vertic soils.
Keywords: Shrink-Swell; Crack Dynamics; Crack Properties
1. Introduction
According to the USDA soil classification, Vertisols are
clayey soils (>30% clay) that have deep, wide cracks for
some time during the year and have slickensides within
100 cm of the mineral soil surface. Vertisols cover roughly
308 million ha globally [1] and 18 million ha in the USA
[2]. Cover of Vertisols and vertic integrades is esti-
mated to be 320 million ha worldwide [3]. These soils
swell when they are wet, shrink when they are dry and
form cracks during dry seasons. Wide and deep cracks
have a capacity to enhance rapid flow of water and nu-
trients in the subsoil, modifying the hydrology of the
soils [4,5].
Many studies have looked at the dynamics of soil
shrinking and swelling and associated crack formation
for the purpose of improving hydrology models [6-11].
While the literature is clear on understanding the shrink-
ing and swelling of soil cores and attempts are made to
translate some of the knowledge to field observations of
soil shrinkage, a paucity of observations and knowledge
of soil cracking exists. For example, how a soil is ex-
pected to shrink in the field can be modeled; however,
several assumptions must be made to simulate properties
of the cracks, e.g., opening, area density, depth and ori-
entation. The difficulty with modeling actual crack dy-
namics arises from the inability to realistically measure
these crack dimensions in the field and at the appropriate
scale. Moreover, the shrink-swell dynamics and potential
of a soil also varies considerably with time and scale.
In space, shrink-swell properties of Vertisols vary as a
function of soil properties, microclimate, topography, ve-
getation, cropping patterns, and soil management prac-
tices [12-19]. Soil properties that vary in space include
clay content and mineralogy, and water holding capacity
[12,13,15,16,20,21]. Large concentrations of clay, mainly
the fine clay fraction, result in large specific surface area
that helps store water [19]. As a result, when the soil wa-
ter content increases, the surface area of the fine clay
adsorbs water and the volume increases.
*Corresponding author.
#The US Department of Agriculture (USDA) prohibits discrimination in
all its programs and activities on the basis of race, color, national origin,
age, disability, and where applicable, sex, marital status, familial status,
p
arental status, religion, sexual orientation, genetic information, politi-
cal beliefs, reprisal, or because all or part of an individual's income is
derived from any public assistance program.
Among all factors, the temporal shrinking and swell-
ing of Vertisols is mainly governed by the amount and
distribution of water in the soil, which is a function of
clay content, weather patterns, landscape positions and
vegetation type. The annual and seasonal variability of

Review Paper: Challenges and Limitations in Studying the Shrink-Swell and Crack Dynamics of Vertisol Soils 83
weather patterns (evapotranspiration) causes a variation
in the amount of soil water and in the temporal and spa-
tial distribution of soil water that all affect the shrink-
swell dynamics of soils. Soil water may be large at lower
positions in the landscape because of sub/surface flow of
water, and/or shallow ground water. In some cases, in-
creased abundance of fine clay particles (large porosity)
from weathering and deposition at the lower positions in
the landscape may also enhance the overall wetness of
the soils, e.g., [22,23]. Root patterns and depths vary by
vegetation type and influence evapotranspiration that
affect the rate of water extraction from the soils and hence
soil shrinking. In contrary, roots may change the size and
pattern of cracks by holding soils together, thus limiting
soil shrinkage [24]. The dependency of shrink-swell and
crack dynamics on several spatially and temporally vari-
able factors makes field and laboratory measurement and
modeling of soil cracks challenging, i.e., from finding the
appropriate measurement technique to the appropriate
experimental design.
Experiments with natural soil clods and small soil
cores have established a well known functional relation
known as the Soil Shrinkage Characteristic Curve (SSCC)
[11,25]. The four soil shrinking phases in SSCC are: 1)
structural; 2) basic; 3) residual; and, 4) zero [25-31].
Each shrinking phase has a different relationship with
change in soil water storage, which further challenges the
understanding and modeling of the shrink-swell pheno-
mena under different scales (Figure 1). In the structural
shrinkage phase, there is no considerable change in bulk
volume because water is lost from large macropores [19,
25]. In the basic shrinkage phase, water is lost from the
inter-particle pores. The amount of water lost from the
soil is equal to the volume of soil shrunken, resulting in a
1:1 relationship between moisture ratio (ratio of volume
of void to volume of solid soil) and void ratio (ratio of
volume of water to volume of solid soil) [5,19]. Basic
shrinkage (normal shrinkage) accounts for the majority
of soil shrinkage and is reported to occur between soil
Basic Structural
Residual Zero
Void R atio

Figure 1. Soil shrinkage characteristic curve and the asso-
ciated shrinkage phases.
water potentials of −33.3 to ≤–1500 kPa. Residual shrin-
kage occurs when the change in volume of the soil is less
than the volume of water lost [28]. In zero shrinkage, the
soil volume does not decrease any further with water loss
and water loss is equal to the increase of air volume in
the soil aggregates [7].
Although the significance of incorporating the crack
dynamics of Vertisols into hydrology models is well un-
derstood, measurements of crack area density, crack depth,
crack volume (capacitance), crack orientations, crack
network and time of crack opening and closing are chal-
lenging. Current estimates of soil cracking usually as-
sume equi-dimensional shrinkage, in which vertical and
horizontal shrinkages are assumed to be equal. Hence, in
the field, vertical movement of soil can be measured us-
ing rods anchored at different depths in a soil, e.g., [6,32,
33]. Measuring horizontal shrinkage is more difficult
because of the irregular geometry and spatial distribution
of soil cracks [34]. Combining the equi-dimensional
shrinkage model with field measurements of soil subsi-
dence is not very helpful in estimating crack characteris-
tics. Many assumptions are made to link the measure-
ments of soil subsidence and soil water to estimate or
calculate crack information. Due to lack of adequate
measurement methods for measuring soil cracking on a
representative spatial scale, too few studies have been
conducted on soil cracking in the field [5,8,9,35].
In the literature, little attention is given to examination
of techniques used to measure or estimate soil cracks.
Thus, the purpose of this review is to discuss challenges
and limitations of techniques used to measure soil crack-
ing. Particularly, the techniques used are evaluated based
on their ability to take frequent and/or continuous meas-
urements, the uncertainty of each measurement, and the
scale the measurement represents. Since water is usually
measured during the crack study, challenges associated
with soil water measurement in a field and the experi-
mental design for the measurement of soil crack and soil
water in a field is also discussed briefly. Our goal for this
review is to stimulate discussion and to encourage those
interested in soil cracking studies to continue our inter-
disciplinary search for better measurement techniques.
2. Measurement Techniques
Past studies of soil swelling and shrinking behavior has
involved measurement of the ability of the soil to make
cracks, i.e., soil shrinkage and soil shrink-swell potential,
and actual crack dimensions (length, width, depth and
orientation). These measurements have been done in the
laboratory and/or in the field (in-situ), each requiring di-
fferent techniques. The advantages and limitations of
field and laboratory measurements techniques are dis-
cussed in detail in the following sections.

Review Paper: Challenges and Limitations in Studying the Shrink-Swell and Crack Dynamics of Vertisol Soils 84
2.1. Field Measurement Techniques
The most useful insight on soil cracking is likely to be
gathered by making crack observations in-situ, under
field conditions. The challenge for such measurements is
devising a method that can take measurements at an ade-
quate time and/or spatial scale. Several studies have cha-
racterized and quantified the shrink-swell properties of
soil and associated crack formation among landscapes,
land use types and management systems. Reported me-
thods for measuring shrink-swell dynamics in-situ are
direct crack width and length measurements made by
hand using a ruler [5,9,10,35], measurement of vertical
movement of soils [6,7,32,36], surface photography [34]
and electrical resistivity measurement [37,38].
Direct crack measurements can provide information on
surface crack density and orientation, and help estimate
crack volume. This technique helps measure characteris-
tics of cracks but it is time consuming for measurements
at significant spatial scales. In direct measurement, the
depth, width and length of cracks are measured by hand
using rulers and tape measures. For instance, Kishné et al.
[8,9] measured the surface width, length, and location of
cracks on a 100-m2 area (10 m  10 m) in Texas using a
1 m by 1 m frame with a grid of 0.1-m cell size placed on
the soil surface. Though this method resulted in a de-
tailed 10-year data set on crack area density, the method
is tedious. For example, it took 3 - 4 h to take one mea-
surement from the area during a cracking event (personal
communication, Wes Miller). Because one measurement
takes so long to collect, these measurements only provide
snap-shots, prohibiting continuous monitoring of soil
shrinkage. Additionally, the method only provides sur-
face information with limited depth information. Width
of cracks below the soil surface is difficult to measure
with this technique. The accuracy of a direct measure-
ment is also questionable. Rivera [39] graphed hand-
measured crack volume data vs. the calculated crack vo-
lume based on field-measured soil vertical movement at
two different locations. The results of this comparison
showed coefficient of simple determination (r2) values of
0.12 and 0.10, but more importantly, the magnitude of
the difference was exceptional.
Measurements of vertical shrinkage, made by measur-
ing changes in rod heights anchored at different soil depths,
can provide estimates of crack volume, e.g., [6,7]. To do
so, a monument, which serves as a non-moving reference,
is anchored deep below the surface (~3 m) or into bed-
rock. The heights of the rods relative to the monument
are measured using surveying equipment, such as a digi-
tal level and stadia rod. The changes in heights of the
rods are then used to track the temporal vertical move-
ment of soil layers. Anchoring rods at different depths
provides information about the shrink-swell activity, in-
crementally with depth. Because the measurement is re-
latively quick (compared to hand measurement), multiple
locations can be measured in one day. Therefore, the rod-
based vertical shrinkage measurement allows for multi-
ple locations across a landscape to be monitored. In addi-
tion, installation of the rods is rather quick with the use
of a hydraulic probe. However, taking a measurement re-
quires two people to be present, hence is labor intensive,
and comparison of soil shrinkage layer by layer using
this technique may not provide an adequate result unless
the soil layers move uniformly across a sample area.
Moreover, the use of vertical shrinkage data for hydro-
logy models requires estimation of a crack volume using
assumptions such as equi-dimensional shrinkage. Instead
of rods, though it is not documented yet, magnets along a
soil profile may also be used to monitor vertical move-
ment of soils. A magnet sensor would sense the location
of the installed magnets in the soil profile and help mo-
nitor the temporal variation of the location as influenced
by the vertical movement of the soil. The benefit of use
of the magnets is that the measurement is faster; soil
shrinking at different soil layers can be taken at one posi-
tion and hence comparison of shrinkage based on soil
layer is very easy.
Photography has been used to quantify crack area den-
sity directly. Peng et al. [34] measured soil cracks by di-
gital image analysis and were able to identify crack
changes as small as 1.0 mm2. The digital image analysis
method is advantageous because measurements can be
taken repeatedly, continuously, and non-destructively.
Though its use in the field does not measure crack depth,
image analysis helps determine the crack area density at
the surface. The benefit of photography is that a time-
lapse camera could take short-term temporal variation of
cracks and crack opening. Subjectivity on image pro-
cessing [34] and on selection of a representative site has
to be minimized to get a best result. As outdoor cameras
become less expensive, photography becomes a feasible
method for quickly covering a large area. The major im-
passe is the presence of vegetation, which either must be
sparse or removed so that cracks are visible. For instance,
Velde [40] took images of cracks from a cultivated field
where cracks were clearly visible. Vegetation enhances
water loss, so removing vegetation may affect crack dy-
namics and our ability to monitor cracking non-inva-
sively over time.
Electrical resistivity tomography in soils is used to
look at temporal dynamics of wetness and indirectly un-
derstand the crack depth and volume [41], apart from
using it to measure cracks under laboratory conditions
[37,38]. Amidu and Dunbar [41] used this technique to
quantify the effect of gilgai and cracks on soil water con-
tent variability of a Texas Vertisols and found different
soil water regime with depth that occurred due to prefe-
rential flow of water and micro-relief topography. The

Review Paper: Challenges and Limitations in Studying the Shrink-Swell and Crack Dynamics of Vertisol Soils 85
field application of the technique is easy because it is
noninvasive. However, interpretation of resistivity re-
quires separation of changes in water content and crack
development, which both affect resistivity of the soil;
therefore, calibration of the instrument is necessary.
Moreover, the technique is not sensitive to crack orienta-
tion and the isotropic shrinkage property of soils.
Filling a crack with a known volume of sand might
also be applied to estimate a crack volume in a field.
However, we have not seen any report about its field
application. This technique could be labor and resource
intensive and hence cannot be applied to large areas.
Furthermore, the application of sand may also modify the
soil environment and results obtained would be difficult
to interpret.
2.1.1. Experimental Design
Experimental design in a field can be challenging for a
study of shrink-swell and crack dynamics. For instance,
selection of representative soil depth and site(s) to mea-
sure cracks and soil shrinkage requires a thorough inves-
tigation of the spatial variability of the area. While the
study of the dynamics at several depths provides more
data, availability of resources limits its field application.
Studies also suggest that measurement of soil shrinkage
at 0.15 m soil depth would be enough to get total crack
volume of a soil [5,6]. However, cracks up to 1.0 m deep
on a grazed pasture in central Texas, and cracks up to
1.40 m deep were observed in the Gulf Coast Prairie of
Texas [42]. The decision of to what depth to measure soil
shrinkage needs to be based on, but not limited to, the
depth of soils, the degree of soil water content fluctuation
in the soil layer and through the soil profile, the amount
of inorganic carbon, and depth and pattern of plant roots.
Because of a spatial variation in clay, soil water content
and inorganic carbon across a catena, soil shrinkage and
crack formation can exhibit a similar degree of spatial
variability. The variability in degree of changes of water
in space affects the size and depth of cracks. Usually,
cracks become narrower in the subsoil rather than at the
surface because there is less drying in the subsoil. Inor-
ganic carbon reduces soil shrinkage by dilution and/or by
cementing aggregates together [43,44] and hence its va-
riability with depth (generally increases with depth) and
across a landscape affects the choice of measurement
depth and location. Roots depth and pattern also govern
soil water loss through transpiration and hence affect soil
shrinking, therefore, must also be considered in experi-
mental design.
The presence of gilgai (surface micro-topography) and
associated subsurface features make the experimental de-
sign and the measurement of soil vertical shrinkage more
complex [42]. Shape, size, depth and length of gilgai mo-
dify the shrink-swell dynamics of soils across a space
and complicate placement of measurements [9] and two
types of gilgai in the Central Texas were observed: cir-
cular and linear. Absence of clear understanding on the
relationship between microhighs/microlows and cracks
represent an additional challenge. For instance, there are
studies that indicate that microhighs have a greater crack
density than microlows [9,41], while others reported
greater crack density on microlows [45]. Presence of gil-
gai in a field may affect the soil layer continuity, which
also limits comparison of soil subsidence layer-by-layer
across a field.
Kishné et al. [9] and Knight [46] reported a relatively
greater amount of soil water content in the circular mi-
crolow than in the microhigh in Vertisols of Central
Texas [9] and Victoria, Australia [46], which makes soils
in the microlows wetter than microhighs, and hence less
cracks in the microlow. Nonetheless, because of the wet-
ness, more vegetation is common in microlows than in
the microhighs. The presence of vegetation could in-
crease cracking by enhancing loss of water through eva-
potranspiration [46] and at the same time slowdown soil
cracking by keeping soils together with their roots [24].
Therefore, the crack variability associated with micro-
highs and microlows must be considered at that scale.
In addition to the measurement techniques used, the
type of study (whether it is in a laboratory or field) af-
fects the result and interpretation of the shrinkage pro-
perties of soils. Bronswijk [7] developed equations that
relate vertical soil subsidence and-crack volume to change
in soil water storage based on the assumption of isotropic
shrinkage and used in the SWAT hydrology model [6].
However, there is evidence suggesting that soils do not
always shrink equally from all sides. Peng et al. [34]
found that vertical and horizontal soil shrinkages are
anisotropic and Cabidoche and Ozier-Lafontaine [47]
found a vertical soil movement slightly greater than
horizontal movement, claiming that soils slide along
slickensides during shrinking that limits the opening of
the cracks. However, in small cores where slickensides
are less observed, shrinkage of small soil cores is isotro-
pic [30,33]. This lack of consensus on the shrinkage
property of soils could be due to differences in the mea-
surement scale in addition to the measuring techniques.
2.1.2. Measuring So i l Water Con tent in Verti s o l s
Since soil water is the main governing factor for shrink-
ing and swelling of soils and formation of cracking, thus
measuring soil water content is equally important as
measuring cracking. Usually, the neutron attenuation me-
thod is used to measure soil water content in cracking
soils [32,48] because it minimizes soil disturbances, and
is non-destructive [49], and quicker than gravimetric
sampling. Use of other soil water measuring methods,
such as time domain reflectometry and other electromag-

Review Paper: Challenges and Limitations in Studying the Shrink-Swell and Crack Dynamics of Vertisol Soils 86
netic sensors is of limited use in Vertisols because cracks
separate the soil from the sensors and or a limited sphere
of measurement by the sensors and hence our discussion
focuses on the neutron attenuation method. The large
sphere of influence of a neutron meter reduces the effect
of cracking on the soil water measurement and does not
require the sensor to be in direct contact with the soil. A
neutron meter can sense a soil volume that ranges from
900 cm3 to 4.2 m3 (a diameter of 97 mm to 1.6 m) of
soils at very wet and dry conditions, respectively [50]. A
relationship between soil water content and soil subsi-
dence measurements, therefore, can be developed with
confidence, provided a calibration between neutron counts
vs. volumetric soil water content is established. A good
calibration is achieved when all the variables, which may
vary in time and space and affect volumetric water con-
tent of soils, are addressed and considered during the
calibration process.
Establishing a calibration of a neutron meter in a Ver-
tisol is critical and it can be different from calibrations in
other soils, because Vertisols change volume upon dry-
ing. Literature on calibration of neutron meters is abun-
dant [49,51-56]. Usually the calibration of the neutron
meter is done in a field but if it is made on a packed bar-
rel, a subsequent adjustment of the calibration based on
soil volume change is suggested [48], though the tech-
nique is tedious. Because of its simplicity, a field calibra-
tion for Vertisols at dry and wet soil conditions is pre-
ferred. A field calibration in dry conditions automatically
accounts for volumetric changes. More than one calibra-
tion site may be needed depending on the objective of the
research. If it is to do a long-term measurement over a
wide area, the calibration model needs to address the
spatial variability of soil properties such as clay content
and inorganic carbon [57] that could affect the reading of
soil water content.
While collecting soil water measurement over time
with a neutron meter, the distance between the soil sur-
face and the top of the neutron access tube varies because
the soil height changes. During soil drying or wetting,
measurements taken at a constant distance from the top
of the access tube will be at different depths in the soil,
depending on the magnitude of the soil vertical move-
ment [48]. Based on our observation, the distance can
vary up to 50 mm from its origin. Placing a neutron me-
ter on a stand with a constant height above soil surface
avoids this problem [48,57]. Presence of cracks near the
access tube can affect soil water measurement. If a rain
falls when cracks are open, runoff may accumulate be-
tween the outer side of the neutron access tube and the
soil wall, which can result in overestimation of soil water
content in the soil profile. Moreover, since we observed
an access tube with free water inside, the access tubes
have to be checked for wetness before taking a measure-
ment or have to be sealed at the bottom during installa-
tion. Assessing free water inside and around the access
tube before measurement is, therefore, important to re-
duce a measurement error.
2.2. Laboratory Measurement Techniques
Soil shrink-swell potential along with water content mea-
surements is a common way to estimate soil cracks in
hydrology models. The relationship between soil shrink-
swell potential and soil physical and chemical properties
has been studied [19,30,58-62]. Major soil properties
correlated with soil shrink-swell potential are fine clay
and total clay content, water holding capacity, dry and
wet bulk density, inorganic and organic carbon, specific
surface area and exchangeable cations. Researchers have
reported similar and contradicting results about shrink-
swell potential and soil properties as discussed below.
Clays in Vertisols have a large surface area and miner-
als (mainly smectitic) that make them have large shrink-
swell potential and most studies show a positive and
strong correlation between soil shrink-swell potential and
total clay content [16,19,58,61,62]. In contrast, Yule and
Ritchie [30] and Gray and Allbrook [59] found no rela-
tionship between clay content and soil shrinking potential
in their studies. However, Gray and Allbrook [59] had
allophone in the clay-size fraction and once allophone
was removed, a better relationship between clay content
and shrink-swell potential were found. The soils studied
by Yule and Ritchie [30] ranged in clay content from 45
to 70%. At this range, likely other soil properties domi-
nated the variability in shrink-swell potential. Regardless,
McCormack and Wilding [60] concluded that clay con-
tent is a reliable source to calculate and predict shrink-
swell potential.
The Coefficient of Linear Extensibility (COLE) of soil
is used to describe the shrink-swell potential of natural
soil clods and gives the relative potential of soils to
shrink and swell from a water potential of 33.3 kPa, i.e.,
field capacity, to oven dry [7,21,61,63]. The COLE is
usually calculated by using the difference in volume of a
soil clod measured at 33.3 kPa (Vm) and measured after
oven dried (Vd), as follows:


1/3 1/3
1/3
md
d
VV
COLE V


The soil water content, especially oven dry, does not
accurately represent field wetness. Therefore, measuring
the shrinkage limit of soil clods at an oven dry condition
may overestimate the soil shrinkage potential expected to
occur under field conditions. In the calculation of COLE,
swelling of soils beyond 33.3 kPa is also ignored. Be-
cause Vertisols are high in clay content (>30%), the field

Review Paper: Challenges and Limitations in Studying the Shrink-Swell and Crack Dynamics of Vertisol Soils 87
capacity of the soil may go beyond the commonly as-
sumed soil water potential of 33.3 kPa; therefore, we
might expect the soil to swell at soil water wetter than
33.3 kPa. To account for this discrepancy in clayey
materials a value of 10 kPa, instead of 33.3 kPa, has
been used to represent the soil water potential at field
capacity [64].
In most studies, the volume of soil clods is measured
based on Archimedes’s law, i.e., the clod is immersed in
water, and the volume of water displaced by the clod is
the same as the volume of the clod. To avoid penetration
of water into clods during immersion, clods are usually
coated with Saran or a paraffin wax. The advantages of
Saran coating clods are that soil samples are intact (natu-
ral fabric) and the measurement error is low [25]. It has
been reported that the classical Saran method limits wa-
ter penetration during immersion, requires a correction
for the volume of Saran coatings, may limit swelling, and
may not properly shrink with the clod [65,66]. Moreover,
Saran-coated clods may not be reused for other analysis,
like chemical analysis or volume measurement at multi-
ple water contents. As a result, distinct samples are re-
quired for each analysis [25,65], which makes the me-
thod resource demanding. The COLE measurements can
also vary depending on the water content a soil clod is
coated and studies show that wetting of clods before
coating results in greater COLE values [58,66]. As an
alternative to the Saran method, use of a three-dimen-
sional (3D) image scanner that measures volume of soil
clods, allows volume measurement of a single clod at
multiple soil water potentials. The disadvantage of the
3D scanner is that it is time-consuming (>1 h per clod)
and image processing is prone to subjectivity.
In addition to the Saran coating technique, pedo-trans-
fer functions have been used to estimate COLE. Inter-
pretations of the relationship between soil properties and
shrink-swell potential of a soil requires a clear mechanis-
tic explanation of how soil properties determine shrink-
swell potential. The relationships between COLE and soil
properties have been successfully implemented; but re-
sults are not consistent. For instance, McCormack and
Wilding [60] calculated a multiple regression to relate
COLE to soil properties including fine, coarse and total
clay, and soil water content at –1500 kPa. Surprisingly,
none of the variables was significant when total clay
content was included. Anderson et al. [58] and Smith et
al. [16] also found a positive correlation between COLE
and exchangeable sodium percentage, while Gray and
Allbrook [59] found no significant relationship because
the soils had low concentrations of exchangeable cations.
Organic matter improves soil structure and increases
soil porosity; therefore, if a soil holds more water, it may
shrink more upon drying. Conversely, if the water is held
in larger pores, the effect of organic matter on shrink-
swell could be to reduce shrink-swell potential because
the water loss is structural (Figure 1). For this reason,
the relationship between organic matter and soil shrink-
age is not simple [59] and contrary results have been
found and reported. For instance, a positive correlation
was reported between organic carbon and shrinkage in
both topsoil and subsoil by Reeve et al. [61], while Smith
et al. [16] reported no significant relationship. Moreover,
Davidson and Page [12] reported that removal of organic
matter increases swelling capacity of soils, claiming that
adsorption of organic matter on soil clays modifies the
swelling property of the clay.
3. Summary
A mechanistic understanding of soil crack formation and
geometry for in-situ conditions is not well known. Clearly,
exploration of soil cracking is very limited by the diffi-
culty in observing the shrink-swell phenomena, particu-
larly observations that are minimally invasive, allowing
monitoring of cracking with time. Current techniques of
measuring soil cracking and the shrink-swell dynamics of
Vertisols are far from providing complete information for
understanding their impact on large and small-scale hy-
drological processes [67]. Information needed includes
crack area density, depth, orientation and network, open-
ing and closing time, and pattern of formation. No single
technique of those reviewed can provide this information
continuously, nondestructively and with a reasonable cer-
tainty in the field.
While we can currently measure soil subsidence in the
field, the relationship between soil subsidence and crack-
ing is poorly developed. Understanding and quantifying
the in-situ relationship between change in soil water
storage and the mechanisms of cracking, which are ver-
tical cracking, surface and subsurface horizontal cracking
and diagonal cracking, is currently a challenge to im-
prove our understanding of how water moves through
these cracks. Improving the accuracy and efficiency of
promising technologies such as surface photography,
electrical resistivity measurement, and use of magnets
provide opportunities to collect better information on soil
cracking. Particularly when used together or with subsi-
dence information measurements. Laboratory studies are
used to measure and model soil shrink-swell potential so
that the information can be transferred to hydrology mo-
dels and applied on landscape and watershed scales. The
COLE helps estimate maximum soil shrinkage in a field,
and can be converted to crack volume with certain as-
sumptions. However, COLE does not account for tempo-
ral variability of soil water change that mainly governs
crack formation. Therefore, to estimate the apparent oc-
currence and volume of cracks, use of COLE should be
supported with the temporal change of soil water storage.
A combined use of field and laboratory techniques, as-

Review Paper: Challenges and Limitations in Studying the Shrink-Swell and Crack Dynamics of Vertisol Soils 88
sisted by models, may help get all the necessary informa-
tion. Acquiring crack information will fill the gap in hy-
drology models that are applied on shrink-swell soils.
Developing advanced techniques (both software and
hardware) that address the spatial and temporal dynamics
of soil shrinkage and crack formation is needed.
4. Acknowledgements
This work was partially supported by the Texas AgriLife
Research and a Cooperative Agreement with the USDA
NRCS Texas Soil Survey and a Grant No. EAR 0911317
from the National Science Foundation.
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