On the formation of red and black soils in southern India

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Review
Vertisols of tropical Indian environments: Pedology and edaphology
D.K. Pal ⁎, S.P. Wani, K.L. Sahrawat
Resilient Dry land Systems, International Crops Research Institute for the Semiarid Tropics, Patancheru 502 324, Andhra Pradesh, India
abstractarticle info
Article history:
Received 26 September 2011
Received in revised form 17 April 2012
Accepted 28 April 2012
Available online xxxx
Keywords:
Tropical soils
Vertisols
Pedology
Edaphology
Soil classification
Soil resilience
Vertisols in thetropics occur in a range of climates and are used in a range of production systems. This review is a
synthesis of the recent developments in pedology of vertisols achieved via high-resolution micro-morphology,
mineralogy, and age-controldata along with their geomorphologic and climatic history. This knowledge has con-
tributed to our understanding of how the climate change-related pedogenic processes during the Holocene al-
tered soil properties in the presence or absence of soil modifiers (Ca-zeolites and gypsum), calcium carbonate
and palygorskite minerals. These state-of-the-art methods have established an organic link between pedogenic
processes and bulk soil properties; the review also considers the need to modify the classification of vertisols at
the subgroup level. We hope this review will fulfil the need for a handbook on vertisols to facilitate their better
management for optimising their productivity in the 21st century.
© 2012 Elsevier B.V. All rights reserved.
Contents
1. Introduction ...............................................................29
2. Factors in the formation of vertisol ....................................................29
2.1. Parent material ..........................................................30
2.2. Climate ..............................................................30
2.3. Topography ............................................................30
2.4. Vegetation ............................................................32
2.5. Time ...............................................................32
3. Smectite clay minerals in vertisols .....................................................32
3.1. Characteristics of smectites .....................................................32
3.2. Hydroxy-interlayering in smectites and determination of layer charge .................................34
3.3. Genesis of smectites in vertisols ..................................................34
4. Pedogenic processes in vertisols —recent advances ............................................. 34
4.1. Clay illuviation: proanisotropism in vertisols ............................................ 34
4.2. Factors of clay illuviation in vertisols ................................................36
4.3. Relative rapidity of clay illuviation, pedoturbation and slickenside formation . ............................36
4.4. Development of microstructures and vertical cracks in vertisols as controlled by smectite swelling ...................37
4.5. Evolutionary sequences in vertisol formation ............................................38
5. Importance of pedology in the edaphology of vertisols ........................................... 39
5.1. Impact of the close association of non-sodic and sodic shrink–swell soils (vertisols) on crop performance ................39
5.2. Linear distance of cyclic horizons in vertisols and its relevance to agronomic practices ...........................39
5.3. Importance of smectite clay minerals in growing vegetation on weathered basalt and in very shallow cracking clay soils .........41
5.4. Role of smectite, soil modifiers and palygorskite minerals in influencingthedrainageofvertisols ..................... 41
5.5. Climate change, polygenesis, impairment of soil properties and evaluation of vertisols .........................42
5.6. Nature and layer charge of smectite and other minerals in adsorption and desorption of major nutrients ................43
5.6.1. Nitrogen ......................................................... 43
5.6.2. Phosphorus ........................................................ 43
5.6.3. Potassium ........................................................ 44
Geoderma 189-190 (2012) 28–49
⁎Corresponding author. Fax: + 91 40 307 13074.
E-mail addresses: paldilip2001@yahoo.com (D.K. Pal), s.wani@cgiar.org (S.P. Wani), k.sahrawat@cgiar.org (K.L. Sahrawat).
0016-7061/$ –see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.geoderma.2012.04.021
Contents lists available at SciVerse ScienceDirect
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journal homepage: www.elsevier.com/locate/geoderma

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5.7. Role of soil modifiers (zeolites, gypsum and CaCO3) in making vertisols productive in adverse climatic environments .......... 46
5.7.1. Ca-zeolites ........................................................ 46
5.7.2. Gypsum ......................................................... 46
5.7.3. CaCO
3
.......................................................... 46
6. Concluding remarks ........................................................... 46
References .................................................................. 47
1. Introduction
Vertisols have attracted global attention in research, yielding a large
body of data on their properties and management (Coulombe et al.,
1996; Mermut et al., 1996). Although substantial information is available
on vertisols, it remains challenging to optimise their use and manage-
ment (Coulombe et al., 1996; Myers and Pathak, 2001; Syers et al., 2001).
The global area under vertisols is estimated to be approximately
308 M ha, covering nearly 2.23% of the global ice-free land area (USDA-
SCS, 1994); however, the reliability of this estimate remains uncertain
because several countries have not yet been included in the inventory
(Coulombe et al., 1996). In addition, the area under vertisols in a soil sur-
vey area may often be too small to resolve at the scale of map compila-
tion (Table 1). Vertisols and vertic intergrades occur in 80 countries,
but more than 75% of the global vertisol area is contained in only 6 coun-
tries: India (25%), Australia (22%), Sudan (16%), the USA (6%), Chad (5%),
and China (4%; Dudal and Eswaran, 1988; Wilding and Coulombe, 1996).
Vertisols occur in wide climatic zones, from the humid tropics to arid
areas (Ahmad, 1996), but they are most abundant in the tropics and sub-
arid regions. In the tropics, they occupy 60% of the total area; in the sub-
tropics, they cover 30%, while they cover only 10% in cooler regions
(Dudal and Eswaran, 1988; Wilding and Coulombe, 1996). In humid
and sub-humid regions, vertisols occupy 13% of the total land area; in
sub-arid regions, 65%; in arid regions, 18%; and in the Mediterranean
climate, 4% (Coulombe et al., 1996).
Vertisols are an important natural agricultural resource in many coun-
tries including Australia, India, China, the Caribbean Islands and the USA
(Coulombe et al., 1996). Because of their shrink–swell properties and
stickiness, vertisols are known by a number of local regional and vernac-
ular names (Dudal and Eswaran, 1988). They are known in India by at
least 13 different names (Murthy et al., 1982). These names are related
to the characteristic dark colour and/or to aspects of their workability.
Thesesoilsareoftendifficult to cultivate, particularly for small farmers
using handheld or animal-drawn implements. The roots of annual crops
do not penetrate deeply because of poor subsoil porosity and aeration;
therefore, farmers (especially in India) allow these soils to remain fallow
during the rainy season and cultivate them only in the post-rainy season.
Current agricultural land uses (edaphological) demonstrate that al-
though vertisols are a relatively homogeneous soil group, they occur
in a wide range ofclimatic environments globally and also show consid-
erable variability in their uses and crop productivity (Pal et al., 2011a).
Vertisol use is not confined to a single production system. In general,
management of vertisols is site-specific and requires an understanding
of degradation and regeneration processes to optimise management
strategies (Coulombe et al., 1996; Syers et al., 2001). Basic pedological
research is needed to understand some of theunresolved edaphological
aspects of vertisols (Puentes et al., 1988) to develop optimal manage-
ment practices. Thus, a critical review is in order to establish the con-
nection between the pedology and edaphology of vertisols.
MostofthevertisolsinIndialieintheTorridZonebetweentheTropic
of Cancer and the Tropic of Capricorn, where the soils are classed as
“tropical”. As in several parts of the world, vertisols also occur in wider
climatic zones in India (Table 1), in humid tropical (HT), sub-humid
moist (SHM), sub-humid dry (SHD), semi-arid moist (SAM), semi-arid
dry (SAD) and arid dry (AD) climatic environments. In total, they occupy
8.1% of the total geographical area of the Indian sub-continent (Table 1).
Additionally, outside the Deccan basalt region of the peninsula, in the
states of Punjab, Bihar and West Bengal, vertisols and their vertic inter-
grades occur in SHM, SHD and SAM climates (Pal et al., 2010), but they
are not mappable at the 1:250,000 scale. Over the past two decades;
however, the focus of research has shifted from general pedology to min-
eralogical and micro-morphological research. By 2009, a total number of
306 BM (benchmark) vertisols and vertic intergrades had been identified
by the National Bureau of Soil Survey & Land Use Planning (NBSS&LUP;
ICAR), Nagpur, India, which included 112 BM vertisols (Pal et al.,
2009c). They have been indicated (along with their global distribution)
on a 1:1 million-scale map (NBSS&LUP, 2002; Pal et al., 2011a). Although
this review is based on the Indian vertisols, data from other tropical parts
of the world are included where relevant. This review uses state-of-the-
art data on the recent developments in the pedology of vertisols, includ-
ing variation in their morphological, physical, chemical, biological, min-
eralogical and micro-morphological properties. The aim of this review
is to provide a better understanding of vertisols created by the climate
change phenomena of the Holocene, with the goal of optimising their ef-
ficient use and management in tropical India and other tropical regions.
The main objective of the paper is to join pedology and edaphology for
better management of vertisols and to optimise their productivity in
the tropical world during the 21st century.
2. Factors in the formation of vertisol
The soil-forming factors are the most relevant and appropriate
factors explaining vertisol formation. They are interdependent and
highly variable and therefore influence the properties of vertisols in
Table 1
Distribution of vertisols in different states of India under a broad bioclimatic system.
Adapted from Bhattacharyya et al. (2009).
States Bio-climate
a
Area (mha)(%)
b
Uttar Pradesh SAM, SHD 0.41 (0.12)
Punjab SAM
c
Rajasthan AD 0.98 (0.30)
Gujarat AD, SAD, SAM 1.88 (0.57)
Madhya Pradesh SAM, SHD, SHM
d
10.75 (3.27)
Maharashtra SAD, SAM, SHD, SHM
d
5.60 (1.70)
Andhra Pradesh SAD, SAM, SHD 2.24 (0.68)
Karnataka AD, SHD, SHM, H 2.80 (0.85)
Tamil Nadu SAD, SAM, SHD, SHM, H 0.91 (0.28)
Puducherry and Karaikal SHM 0.011 (0.003)
Jharkhand SHM, SHD 0.11 (0.034)
Orissa SHM, SHD, H 0.90 (0.28)
West Bengal SHD, SHM
c
Bihar SHM
c
India 26.62 (8.10)
a
AD: arid dry: 100–500 mm MAR(mean annual rainfall); SAD: semi arid dry: 500–
700 mm MAR; SAM: semi arid moist: 700–1000 mm MAR; SHD : subhumid dry: 1000–
1200 mm MAR; SHM: subhumid moist: 1200–1600 mm MAR; H: Humid: 1600–
2500 mm MAR.
b
Parentheses indicate percent of the total geographical area of the country.
c
In the states of Punjab, Bihar, and West Bengal vertisols and vertic intergrades also
occur in SHM, SHD, and SAM climates (Pal et al., 2010) but th ey are not mappable in
1:250,000.
d
In addition vertisols occur in HT climate (> 2500mm MAR) in Madhya Pradesh and
Maharashtra but they are not mappable in 1:250,000 scale (Bhattacharyya et al., 1993,
2005, 2009; P al et al., 2011a).
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multiple ways (Coulombe et al., 1996). In view of recent develop-
ments in studies of vertisol formation (Bhattacharyya et al., 2005;
Pal et al., 2001a, 2006a, 2006b, 2009a, 2009b, 2009c; Srivastava et
al., 2002), each of these five factors merits discussion.
2.1. Parent material
Several geologic formations provide the basic parent materials in-
volved in vertisol development (Coulombe et al., 1996; Murthy et al.,
1982). Parent materials from inheritance or weathering provide a
large quantity of smectites; however, the distinction between inherited
and newly formed clay minerals is difficult to discern (Coulombe et al.,
1996). A study of the vertisols of the sub-humid, semi-arid and arid cli-
mates of Peninsular India clearly indicates that both plagioclase and
micas are either fresh or weakly to moderately altered, suggesting
that chemical weathering of these minerals has not been substantial
(Fig. 1a,b). These data discount the formation of smectite during the de-
velopment of vertisols (Srivastava et al., 2002) and validate the hypoth-
esis that vertisol formation reflects a positive entropy change (Smeck et
al., 1983). As smectite is solely responsible for the vertic properties of
soils (Shirsath et al., 2000), smectite appears to be the exclusive parent
material of vertisols.
2.2. Climate
The characteristics of vertisols are related to overall climate; how-
ever, other factors such as texture, clay mineralogy, cation saturation,
and the amount of exchangeable sodium equally influence soil mor-
phology (Dudal and Eswaran, 1988; Eswaran et al., 1988).
Large quantities of smectite are required to create shrink–swell
properties in vertisols. Smectite is ephemeral in an HT climate
(Bhattacharyya et al., 1993; Pal et al., 1989). In sub-humid to arid
climates, the weathering of primary minerals contributes very little to
smectite formation (Srivastava et al., 2002). Smectite cannot be formed
or retained in HT vertisols or in sub-humid to arid climatic conditions.
The occurrence of vertisols in the alluvium of weathering Deccan ba-
salt, as well as in HT, SHM, SHD, SAM, SAD, and AD environments inthe
Indian peninsula (Pal et al., 2009c), may suggestthatthe basaltic parent
material influenced soil formation such that similar soils are formed
under different climatic conditions (Mohr et al., 1972). The soils are ver-
tisols (Soil Survey Staff, 2003), but their morphological and chemical
properties differ. Cracks >0.5 cm wide extend down to the zones of
sphenoids and wedge-shaped peds with smooth or slickensided sur-
faces in HT, SHM, SHD and SAM soils, but cracks cut through these
zones in SAD and AD soils (Fig. 2). Soil reactions and the CaCO
3
content
indicate that a reduction inmean annual rainfall (MAR) leads to the for-
mation of calcareous and alkaline soils. Hence, the soils are Typic
Haplusterts in HT, Typic/Udic Haplusterts in SHM, SHD and SAM cli-
mates, and Sodic Haplusterts and Sodic Calciusterts in SAD and AD cli-
mates. Such examples help to define the climatic signatures in soils
that allowresearchers to infer the occurrence of climate change in trop-
ical and subtropical regions of India and elsewhere (Pal et al., 2009c).
2.3. Topography
Vertisols generally occur at low elevations, but they also occur at
higher elevations in the Ethiopian plateau or on higher slopes, as in
the West Indies (Coulombe et al., 1996). The majority of the vertisols
in India occur in lower physiographic areas, i.e., in the lower piedmont
plains and valleys (Pal and Deshpande, 1987a; Pal et al., 2009c)orin
micro-depressions (Bhattacharyya et al., 1993; Pillai et al., 1996). Ver-
tisols in micro-depressions are spatially associated with red ferruginous
soils (alfisols) and are seen as distinct entities under similar topograph-
ical conditions on the Deccan basalt plateau in the HT (Bhattacharyya et
a) (muscovite) b) (biotite)
c)
Fig. 1. Representative SEM photographs showing no or very little alteration of micas (a, b) and XRD diagrams of the silt and clay fractions (c) of vertisols of Peninsular India. Adapted
from Pal et al. (2006c).
30 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

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al., 1993)andSAD(Pillai et al., 1996) climates. The red soils are mildly
acidic entisols/inceptisols/alfisols in SAD climates, whereas they are
moderately acidic alfisols in an HT climate. The red soil clays of an HT
climate are composed primarily of smectite–kaolin (Sm–K), whereas
those of the SAD climate contain only small amounts of Sm–K. Sm–K
was formed at the expense of smectite in red HT soils, and in red SAD
soils, it is considered to have originated under a previous, humid climate
regime (Pal, 2003). The genesis of both red soils (alfisols) and vertisols
in the contrasting climate has been explained through the landscape-
reduction process (Bhattacharyya et al., 1993; Pal, 1988), as in similar
soils elsewhere (Beckman et al., 1974). In the initial stage of soil forma-
tion, smectite-rich products of weathering from the hills were deposit-
ed in micro-depressions, as is evident from the lithic/paralithic contacts
of such vertisols (Fig. 3). Over time, these sites gradually flattened, and
internal drainage dominated over surface run-off. After peneplanation,
the red soils (alfisols) of the present (Bhattacharyya et al., 1993)and
the past (Pillai et al., 1996) HT climates on relatively stable surfaces con-
tinued to weather, forming Sm–K. In contrast, vertisols continued to
exist in the micro-depressions (Fig. 4) even in HT climates because of
the continuous supply of bases from Ca-rich zeolites that helps to stabi-
lise the smectite (Bhattacharyya et al., 1993). Because the period of the
HT climate ended during the Plio-Pleistocene transition (Pal et al.,
1989), both smectite and Sm–K in SAD vertisols were preserved to the
present. The SAD climate restricted further leaching in vertisols and
caused calcareousness and the rise in pH (Pillai et al., 1996). Thus, ver-
tisols are not common in residuum on the Deccan basalt of the plateau.
The formation of “gilgai”micro-topography in vertisol areas is not
very well understood (Coulombe et al., 1996). At present, gilgai
micro-topographies are very rare on the Indian sub-continent because
most were obliterated by post-cultural human activities; however, the
depth distribution of soil properties generally differs between the
mounds and depressions of the gilgai topography (Wilding and
Coulombe, 1996; Wilding et al., 1991), resulting in vertical and horizon-
tal spatial variability in vertisols within distances as short as a few me-
tres or less. Such spatial and horizontal variability in SAD vertisols in a
central Indian watershed was observed by Vaidya and Pal (2002).
Watershed vertisols occur in both micro-high (MH) and micro-low
(ML) positions. The distance between these positions is approximately
6 km, and the elevation difference is 0.5–5 m. Vertisols in MH positions
Typic Haplusterts
SHM, MAR 1127mm
Typic Haplusterts
SHD, MAR 1071 mm Udic Haplusterts
SAM, MAR 977 mm
Sodic Haplusterts
SAD, MAR 794 mm
Sodic Calciusterts
AD, MAR 533 mm
Panjri, Nagpur,
Maharashtra.
Nipani, Adilabad,
Andhra Pradesh
Bhatumbra, Bidar,
Karnataka
Paral, Akola,
Maharashtra
Sokhda, Rajkot,
Gujarat
Fig. 2. Cracks are extending beyond the zone of slickensides with increase in aridity (SHM to AD bioclimates). Adapted from Pal et al. (2003a).
Ap
A1
Bss1
Bss2
C1
C2
Fig. 3. A representative vertisol (Typic Haplustert) developed in micro‐depression of a
plateau, showing paralithic contact with the Deccan basalt of central India. Photograph,
courtesy of DKP.
31D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
are strongly alkaline, and those in ML positions are mildly alkaline
(Fig. 5). The formation of sodic soils in MH positions alongside non-
sodic soils in ML positions isa unique phenomenon that occurs because
of micro-topographic differences; these differences result in the non-
uniform distribution of water across the landscape and facilitate greater
penetration of rainwater in ML positions.
2.4. Vegetation
Reports on the influence of vegetation on pedogenesis and distribu-
tion of vertisols are very few (Coulombe et al., 1996). Vertisols that are
not cultivated are associated with native vegetation, such as grasslands
and savannahs (Probert et al., 1987). Vertisols can tilt large trees
(Bhattacharyya et al., 1999b). Not surprisingly, few, if any, commercial
forests are found on vertisols (Buol et al., 1978), but mixed pine and de-
ciduous forests are reported in selected regions of east Texas. At present,
most vertisols are under post-cultural activities that make it difficult to
identify and infer the influence of native vegetation (Coulombe et al.,
1996). Vertisols in India are generally less intensively cultivated
(Bhattacharyya et al., 2007), indicating that management has little
role in their formation and modification. These soils are low in organic
carbon on both the surface and sub-surface layers (b1%), indicating
that biotic factors have no substantive role in the genesis of vertisols(Pal
et al., 2009c).
2.5. Time
Most vertisols are derived from geological rock systems that are
millions of years old (Bardaoui and Bloom, 1990; Yerima, 1986). The
age of the parent material provides only a maximum chronological
point; in reality, millions of years are much older than the true age
of the geomorphic surface or the time required for vertisol formation
(Coulombe et al., 1996). Many researchers suggest that the formation
of slickensides is very rapid and that vertisols are formed on geomor-
phic surfaces in as few as 550 years (Parsons et al., 1973). For exam-
ple, a Vertic Haplustalf formed b100 yr BP (
14
C age, Pal et al., 2006a)in
the alluvium of the central Indian SHD Deccan basalt, which exhibits
pressure faces but lacks slickensides. The occurrence of shrink–swell
soils and slickensides (Singh et al., 1998) in the SHM lower Indo-
Gangetic Plains 500–1500 yr BP (TL ages) indicates that a minimum
of 500 yr is adequate to form vertisols. The vertisols of central and
western peninsular India developed in the Deccan basalt alluvium of
the Upper Cretaceous, mostly during the Holocene period, indicating
a minimum
14
C age of 3390 yr. and a maximum of 10,187 yr. BP (Pal et
al., 2006b 2009c). These data suggest that vertisols in India and else-
where are of Holocene origin.
3. Smectite clay minerals in vertisols
3.1. Characteristics of smectites
Smectite is considered the major mineral in vertisols, with kaolinite
being of secondary importance (Hajek, 1985). In contrast, kaolinite has
been reported to be abundant in some vertisols in El Salvador (Yerima
et al., 1985, 1987) and Sudan (Yousif et al., 1988). In addition, several
non-expanding clay minerals (kaolin, micas, chlorites, palygorskite
and vermiculites) are associated with vertisols and their vertic inter-
grades (Coulombe et al., 1996; Heidari et al., 2008). The shrink–swell
behaviouris primarily governed by the nature of the clay minerals,par-
ticularly their surface properties. Although the soils containing all other
clays shrink and swell with changes in moisture content, changes are
particularly extreme in smectites (Borchardt, 1989). Other minerals,
such as kaolin, micas, chlorites, palygorskite and vermiculite do not ex-
pand on solvation. Bhattacharyya et al. (1997) concluded that the vertic
properties of soils are a function of smectite content. A close examina-
tion of the X-ray diffraction (XRD) diagrams of the fine clays of El
Salvador soils in which shrink–swell processes are related to the fine-
clay kaolin content (Yerima et al., 1985; 1987) indicates the presence
of a smectite peak in the Atiocoyo soils, in which Sm–Kalsodominates.
Thus, the higher value of COLE (0.10–0.12) and the clay CEC value
(62–79 cmol (+)/kg) are compatible with their vertic character. The
presence of expansible minerals might have escaped the notice of re-
searchers in the few shrink–swell soils of the USA (Hajek, 1985)be-
cause the CEC of their clays is >40 (Eswaran et al., 1988).
It was stipulated earlier that the montmorillonitic mineralogy of
soils is associated with vertic properties when smectite exceeds 50%
of the total mineral content in the b2μm clay fraction (Soil Survey
Staff, 1975,1994). A qualitative smectite mineralogy class wasproposed
for the soils that contain more smectite by weight than any other single
clay mineral (Soil Survey Staff, 1998, 1999). This class provided a means
by which smectite can reflect a quantitative dimension of the vertic
properties of soils. Quantitative determination of minerals in the clay
fractions of soils by XRD analysis is difficult, as any attempts in this re-
gard have yielded semi-quantitative estimates (Gjems, 1967). More-
over, such estimation is questionable when minerals are in the
interstratified phase. The presence of Sm–Kinshrink–swell soils is com-
mon in India and elsewhere (Bhattacharyya et al., 1993, 1997; Pal et al.,
1989). Peak-shift analysis (Wilson, 1987)isausefulmethodtodeter-
mine the smectite content in Sm–K. When the smectite component in
Sm–K is highly chloritised, the swelling of smectites on glycolation is
restricted, making the peak-shift analysis ineffective. To circumvent
this problem, the chemical method of Alexiades and Jackson (1965)
has proven effective as a way to quantitatively determine the smectite
content in soil clays, thus establishing the link between bulk soil
properties and clay mineral type (Pal and Durge, 1987). Shirsath et al.
Fig. 4. Schematicdiagram of the pedon site of red soils(alfisols) and blacksoils, (vertisols)
showing the landscape reduction process explaining the formation of spatially associated
red and black soils. Adapted from Pal (2008).
Fig. 5. Juxtaposition of the occurrence of sodic and non-sodic soils (vertisols) on MH
and ML positions in black soil region. Adapted from Pal et al.(2009b).
32 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
(2000) revealed a strong relationship between marked shrink–swell
properties and smectite content in the clay fraction (b2μm). Vertic
properties with a linear extensibility (LE) of 6 in shrink–swell soils cor-
respond to a minimum threshold value of 20% smectite, thus suggesting
that only smectitic soils should be considered shrink–swell soils in the
US Taxonomy.
Smectite species such as montmorillonite, beidellite and nontronite
are reported in vertisols. Of these three, montmorillonite and beidellite
are the most commonly reported, while reports of nontronite are rare
(Coulombe et al., 1996); however, its presence was reported in vertisols
of Upper Volta (Trauth et al., 1967). Coulombe et al. (1996),however,are
of the opinion that although the soil smectites do contain an appreciable
amount of iron, they seldom qualify as nontronite. The occurrence of
iron-rich smectite is related to the ferromagnesian richness of the parent
materials derived from basic igneous rocks. In the Indian sub-continent,
the majority of shrink–swell soils are developed in the alluvium of
weathering Deccan Basalt, which encompasses an area of 500,000 km
2
(Duncan and Pyle, 1988). A review on the mineralogy of shrink–swell
soils (vertisols and vertic intergrades) of India (Ghosh and Kapoor,
1982) indicates that these soils are dominated by beidellite–nontronite
type minerals; the authors compute clay minerals based on smectite;
however, such an approach is not infallible (Sawhney and Jackson,
1958). X-ray diffraction analysis of large numbers of smectite-
dominated fine clays of shrink–swell soils in India (Pal, 2003; Pal et al.,
2000a, 2003a) indicates the presence of small to moderate amounts of
hydroxy-interlayer (HI) material in the smectite interlayers, alongside
a small amount of vermiculite. Hydroxy-interlayers are not easily
detected in the glycolation samples but are discernible during the gradual
heating from 110° to 550 °C of the K-saturated samples that occurs as a
result of the low-angle-side broadening of the 1.0-nm peak at 550 °C
(Wildman et al., 1968;Fig. 6). The presence of vermiculite is also not dis-
cernible in the glycolation samples; it is detected only when the 1.0-nm
peak of mica is reinforced on heating to 110 °C (Pal and Durge, 1987).
Even a small quantity of such impurities (HI materials and vermiculite)
affects the charge and sum relationships using smectite formulae.
The Greene-Kelly test (Greene-Kelley, 1953;Hoffman–Klemen ef-
fect) confirmed the presence of both montmorillonite and beidellite in
the fine-clay fractions of Indian shrink–swell soils in basaltic alluvium,
and the former dominates over the latter (Bhattacharyya et al., 1993;
Kapse et al., 2010; Murthy, 1988; Pal and Deshpande, 1987a); however,
on glycerol vapour treatment (Harward et al., 1969), the clay smectites
expand to approximately 1.9 nm, indicating only the presence of mont-
morillonite (Fig. 6). In other words, fine-clay smectite is nearer to mont-
morillonite in the montmorillonite–beidellite series. Because nontronite
would behave like beidellite in these tests and because clay smectite is
unstable under HCl treatment, consequently releasing considerable
iron in solution, it was concluded that the smectite in vertisols is
nearer to the montmorillonite of the montmorillonite–nontronite series
(Pal and Deshpande, 1987a). Smectite expands beyond 1.4 nm after
glycolation of the K-saturated and heating samples (300 °C; Fig. 6),
indicating its low-layer charge density (as also evidenced by its no
K-selectivity; Pal and Durge, 1987).
It is important to determine the layer charge of smectite minerals.
Theoretically, this parameter should range between 0.3 and 0.6 elec-
trons per half unit cell in smectites. Tessier and Pedro (1987), howev-
er, reported that high-charge smectite (between 0.45 and 0.60
electrons per half unit cell), is common in soils. Several researchers
(Bardaoui and Bloom, 1990; Chen et al., 1989) also reported the pres-
ence of smectite in vertisols with a layer charge in the range of ver-
miculite (0.6–0.9 electrons per half unit cell). Clay smectite in
selected Indian vertisols also showed a high layer charge (0.28 to
0.78 mol electrons/(SiAl)
4
.O
10
(OH)
2
), and low-charge smectite con-
stitutes >70% in them (Ray et al., 2003); however, a charge >0.6
was attributed to the presence of a small quantity of vermiculite (5–
9%, Pal and Durge, 1987) and to the presence of hydroxy-
interlayering in smectite interlayers (Ray et al., 2003). The layer
charge of clay smectites in Indian vertisols by the alkyl ammonium
method (Lagaly, 1994) showed the presence of monolayer to bilayer
and bilayer to pseudotrilayer transitions, indicating heterogeneity in
the layer-charge density (Ray et al., 2003).
3054 (102-128cm); Fine clay fraction (< 0.2µm); Linga
Fig. 6. Representative X-ray diffractograms of fine clay fractions (b0.2 μm) of the Bss horizons of vertisols of central India; Ca = Ca saturated; Ca–EG = Ca saturated plus ethylene
glycol vapour treated; CaGLV = Ca-saturated plus glycerol vapour treated; Li = Li-saturated and heated to 25 °C, 250 °C (16 h), LiGLV 30-D = Li-saturated and heated at 250 °C plus
glycerol vapour treated and scanned after 30 days;K25/110/300/550 C = K-saturated and heated to 25, 110, 300, 550 °C; K300EG = K-saturated and heated to 300 °C plus ethylene
glycol vapour treated; 6NHCl = 6 N HCl treated fine clays; Sm = smectite, B/N = beidellite/nontr onite; V+ Ch = vermiculite plus chlorite; M = mica; Mt = montmorillonite; K =
kaolinite; F = feldspars. Adapted from Pal et al. (2003a) and Bhople (2010).
33D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
3.2. Hydroxy-interlayering in smectites and determination of layer charge
The presence of low to moderate hydroxy-interlayering in smectites
is more common than its absence in vertisol smectites in peninsular
India (Pal, 2003; Pal and Deshpande, 1987a). Ray et al. (2002) observed
that the higher the tetrahedral charge, the greater the probability that
hydroxy-interlayers will form in fine clay smectites. Hydroxy-Al inter-
layer clays give 2:1 layer phyllosilicate great weathering resistance,
and the interlayered clays are an important component of both moder-
ately weathered and intensively weathered soils. The hydroxy-
interlayers prevent the determination of the layer charge by the
alkylammonium method (Lagaly, 1994) by obstructing the normal in-
trusion of alkylammonium ions into interlayers (Ray et al., 2006a).
Such interlayers result in the formation of relatively more paraffin-
type layers with bi-pseudotrilayer transition, which causes the over-
estimation of the layer charge of core lattice mineral. Ray et al.
(2006a) used several extractants to remove the HI materials from the
fine-clay smectites of Indian vertisols to determine the layer charge of
the cleaned clays. The results were not satisfactory when extractants
other than EDTA were used. The removal of HI materials by 0.25 N
EDTA solution (pH 7.0) was almost complete. Ray et al. (2006a)
obtained the weighted-average layer charge of the pre-treated clays
from 0.40 to 0.46 mol (−)/(Si, Al)
4
O
10
(OH)
2
, and after EDTA treatment,
the charge ranged from 0.27 to 0.33 mol (−)/(Si, Al)
4
O
10
(OH)
2
.These
authors also observed that the Ca-saturated and glycolated fine
clays showed greater intensity than their corresponding Ca-treated cur-
ves. The K-treated curves also showed marked differences when com-
pared with the original fine clays. The removal of HI materials by
EDTA is effective in determining the actual layer charge in soil-clay
smectites.
3.3. Genesis of smectites in vertisols
Smectite, either as a discrete mineral or as a mineral interstratified
with any other layer silicates, remains the most abundant and essential
phyllosilicate in vertisols worldwide. The initiation of vertic properties
at a linear extensibility (LE) of 6 in shrink–swell soils requires a mini-
mum of 20% smectite in their clay fractions (b2μm; Shirsath et al.,
2000). Smectite-clay minerals are ephemeral in the HT climate, where
they are rapidly transformed to kaolin. Therefore, it is difficult to under-
stand the formation of vertisols in HT climates without the presence of
soil modifiers such as Ca-zeolites (Bhattacharyya et al., 1993; Pal et al.,
2006b) that release Ca
2+
ions to prevent the transformation of smectite
to kaolin. The formation and persistence of slightly acidic to acidic Typic
Haplusterts with predominant Sm–K in clay fractions in India
(Bhattacharyya et al., 1999a, 2005; Pal et al., 2009c)andelsewhere
(Ahmad, 1983) is possible in the presence of soil modifiers that main-
tain the base saturation well above 50% (Pal et al., 2003a, 2006b).
Despite the abundance of vertisols in semi-arid regions (Eswaran et
al., 1988), large quantities of dioctahedral smectite cannot form in these
soils, as the primary minerals contribute little towards the formation of
smectites in the prevailing dry climates (Pal et al., 2009c; Srivastava et
al., 2002). XRD analysis of fine clays in Indian SHM, SHD, SAM, SAD
and AD vertisols indicates that dioctahedral smectites are fairly well
crystallised, as they yield sharp basal reflections on glycolation and
show regular higher (though short and broad) reflections. Smectites
show no sign of transformation except for the HI in the smectite inter-
layers (Pal et al., 2009c). Such interlayering was also observed in the
vermiculite in the silt and coarse-clay fractions (HIV) that resulted in
the formation of pseudo- or pedogenic chlorite (PCh; Vaidya and Pal,
2003;Fig. 7). The presence of hydroxy-interlayered dioctahedral smec-
tite (HIS) in the fine-clay fractions, as well as HIV and PCh in the silt and
coarse-clay fractions, indicates that the hydroxy-interlayering in the
vermiculite and smectite occurred when positively charged hydroxy-
interlayer materials (Barnhisel and Bertsch, 1989) entered into the
inter-layer spaces. Moderately acidic conditions are optimal for
hydroxyl-Al interlayering of vermiculite and smectite; the optimum
pH values for interlayering in smectite and vermiculite are 5.0–6.0
and 4.5–5.0, respectively (Rich, 1968), as small hydroxyl ions are most
likely to be produced at low pH (Rich, 1960). The pH of the majority
of vertisols in subhumid to arid climates all over the world is either
near to neutral or well above 8.0 throughout, suggesting that the 2:1
layer silicates suffer congruent dissolution under mildly to moderately
alkaline conditions (Pal, 1985). This finding discounts the hydroxy-
interlayering of smectites after deposition of the basaltic alluvium (Pal
et al., 2011b). The hydroxy-interlayering in vermiculite and smectite
and the subsequent transformation of vermiculite to PCh do not, there-
fore, represent contemporary pedogenesis of vertisols in dry climates.
Indian vertisols contain both NPC (relict Fe–Mn coated carbonate nod-
ules) and PC (pedogenic CaCO
3
;Paletal.,2000b,2009c). Based on
14
C
dates of carbonate nodules, Mermut and Dasog (1986) concluded that
vertisols with Fe–Mn coated CaCO
3
are older than those with PCs that
are formed in soils of dry climate soils (Pal et al., 2000b). Thus, NPCs
were formed in a climate that was much wetter than the present cli-
mate, ensuring adequate water for reduction and oxidation of iron
and manganese to form Fe–Mn coatings. Although the vertisols contain
muscovite and biotite mica in the silt and clay fractions (Pal, 2003),
dioctahedral smectite (DOS) cannot be formed at the expense ofmusco-
vite (dioctahedral mica) because the weathering of muscovite is very
sensitive to potassium levels in soil. Biotite converts to trioctahedral
vermiculite (TOV; Pal, 2003); thus, the simultaneous formation of DOS
and TOV from mica is very unlikely (Pal et al., 1989; Ray et al., 2006b).
Moreover, in the sub-humid and semi-arid climates that facilitate the
formation of CaCO
3
from plagioclase (Pal et al., 2011b), mica may not
yield as much DOS as do vertisols. Thus, the large quantity of DOS
formed under a previous, humid climate regime in the source area as
an alteration product of plagioclase (Pal et al., 1989; Srivastava et al.,
1998). On the other hand, the formation of smectite from biotite is
quite unlikely in a humid climate (Tardy et al., 1973), but vermiculite
could have transformed to HIV, which in turn transforms to PCh
under acidic conditions. The formation of HIS did not continue in the
HT climate, as evidenced from the presence of very small quantities of
clay kaolin (Sm–K). In the event of prolonged weathering of HIS, kaolin
should become dominant (Bhattacharyya et al., 1993). Thus, the HIS in
the vertisols were formed under a previous, more humid climateregime
and its crystallinity, and also the HIV and PCh were preserved in the
non-leaching environment of the latter sub-humid to dry climates
(Pal et al., 2009c, 2011b).
4. Pedogenic processes in vertisols —recent advances
Over the past three decades, excellent reviews have been published
on the formation and pedogenesis of vertisols of the world (Ahmad,
1983, 1996; Blokhuis, 1982; Coulombe et al., 1996; Dudal and Eswaran,
1988; Eswaran et al., 1988; Mermut et al., 1996; Murthy, 1988; Murthy
et al., 1982; Smeck et al., 1983; Wilding and Tessier, 1988; Yaalon,
1983; Yaalon and Kalmar, 1978); however, during the last decade and
a half, the focus of vertisol research has changed qualitatively because
mineralogical, micro-morphological and age-control tools can be used
to measure the relatively subtle processes related to pedology, pal-
aeopedology and edaphology (El-Swaify et al., 1985; Kadu et al., 2003;
Myers and Pathak, 2001; Pal et al., 2009a, 2009b, 2009c, 2011a;
Srivastava et al., 2002; Swindale, 1989; Syers et al., 2001). Hence, a crit-
ical review is in order to place recent research results in the context of
past research; this review is needed to better understand the following
basic issues related to vertisol pedogenesis, with implications for effi-
cient use and management of vertisols for agricultural development.
4.1. Clay illuviation: proanisotropism in vertisols
A review of previous work (Ahmad, 1983;Murthy et al., 1982)indi-
cates that the distribution of clay is uniform throughout vertisols
34 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
because haploidisation within the pedon caused considerable
pedoturbation (Mermut et al., 1996); however, earlier studies reported
that in selected cases, there is a gradual increase in clay content with
depth (Dudal, 1965). It was thought that the increase in clay content
with depth is due not to clay migration but to inheritance from parent
material (Ahmad, 1983).
Studies on vertisols by NBSS&LUP (ICAR) in Nagpur, India, over the
past decade and a half indicated that theclay content of the Bss horizons
ranges from zero to substantially enriched with clay (~20% increase
from the eluvial horizon; Pal et al., 2003a, 2009c). Morphological exam-
ination of the vertisols indicated no sign of stratification in the parent
material and showed no clay skins. However, micro-morphological
investigation of the thin sections indicates the presence of >2% impure
clay pedofeatures (Fig. 8a); these features confirm that the clay is
enriched in the Bss horizons of vertisols by clay illuviation. Therefore,
such vertisols can also have argillic horizons (Pal et al., 2009c). Clay illu-
viation was also identified in clay soils with vertic properties in Canada
(Dasog et al.,1987), Uruguay (Wildingand Tessier, 1988) and Argentina
(Blokhuis, 1982). In the past, it was thought that pedoturbation would
obliterate all evidence of illuviation, except in the lower horizons
(Eswaran et al., 1988; Mermut et al., 1996). Thus, Johnson et al.
(1987) considered this process to be an example of proisotropic
pedoturbation caused by argilli-turbation, which was thought to de-
stroy horizons or soil genetic layers and to make vertisols revert to a
Fig. 7. Representative X-ray diffractograms of coarse clay (a), and silt (b) fractions of vertisols of Peninsular India; Ca = Ca saturated;Ca–EG = Ca saturated plus ethylene glycol vapour
treated; K25/110/300/550 C = K-saturated and heated to 25, 110, 300, 550 °C; 6NHCl = 6 N HCl treated silt fraction; Sm= smectite, V+ Ch = vermiculite plus chlorite; PCh = pseudo
chlorite; K = Kaolin; F = feldspars; Q = quartz. Adapted from Pal et al. (2003a) and Bhople (2010).
35D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
simpler state. The clay-enrichment of Bss horizons via illuviation sug-
gests that the argilli-turbation is not a primary pedogenetic process in
vertisols (Pal et al., 2009c) and represents a proanisotropism in the
soil profile. This finding is further confirmed by a steady decrease in
soil organic carbon and by increases in CaCO
3
, exchangeable magne-
sium percentage (EMP), exchangeable sodium percentage (ESP),
water-dispersible clay (WDC) and carbonate clay (fine earth based)
with depth (Table 2;Pal et al., 2009c). Therefore, pedoturbation in ver-
tisols is a partially functional process that is not able to overshadow the
more important long-termclayilluviation process. Although argillic ho-
rizons are common in vertisols, the B
t
horizon does not get better than
their dominant property (slickensides) because vertisol soil order keys
out before the alfisols according to the US Soil Taxonomy. Thus, at pre-
sent, these soils would still be classed as vertisols (Pal et al., 2009c).
4.2. Factors of clay illuviation in vertisols
The majority of calcareous vertisols contain a considerable
amount of WDC, which increases with depth (Table 2). This finding
suggests that the dispersion of clay smectite is possible under slightly
acidic to moderately alkaline pH conditions at a very low electrolyte
concentration (ECe≤1meL
−1
;Table 2) that ensure a pH higher
than the zero point of charge required for a full dispersion of clay
(Eswaran and Sys, 1979). Many researchers have postulated carbon-
ate removal as a pre-requisite for illuviation of clay, as Ca
2+
ions en-
hance the flocculation and immobilisation of colloidal material
(Bartelli and Odell, 1960); however, low quantities of soluble Ca
2+
ions (≪5meL
−1
,Table 2) are not generally sufficient to cause floc-
culation of clay particles in vertisols. Therefore, movement of def-
locculated fine clay smectite (and its subsequent accumulation in
the Bss horizons) is possible in non-calcareous as well as calcareous
vertisols (Pal et al., 2003b).
The primary source of Ca
2+
ions in the soil solution is the dissolu-
tion of NPCs (Srivastava et al., 2002). The depth distribution of EMP,
ESP, carbonate clay, and soluble Na
+
ions in the majority of vertisols
in India (Table 2) suggests that the precipitation of CaCO
3
as PC en-
hances the pH and the relative abundance of Na
+
ions in soil
exchange and in solution. The Na
+
ions in turn cause dispersion of
clay smectites, and the dispersed smectites translocate even in the
presence of CaCO
3
. The formation of PC creates a chemical environ-
ment that facilitates the deflocculation of clay particles and their sub-
sequent movement downward. This finding suggests that PC
formation and clay illuviation are two concurrent and contemporary
pedogenic events that provide examples of pedogenic thresholds in
dry climates (Pal et al., 2003b, 2009c).
4.3. Relative rapidity of clay illuviation, pedoturbation and slickenside
formation
The formation of slickensides has hitherto been considered to be a
very rapid pedogenic process (Parsons et al., 1973; White, 1967;
Yaalon, 1971), as the vertisols are formed on geomorphic surfaces
that are b200 to 550 yr old (Blokhuis, 1982; Parsons et al., 1973). A
Vertic Haplustalf b100 yr age (
14
C age, Pal et al., 2006a) developed
in the alluvium of the central Indian Deccan basalt during the SHD cli-
mate regime, exhibits pressure faces but lacks in slickensides and clay
skins; however, it exhibits weakly oriented clay pedofeatures
(Fig. 8b), undifferentiated clay pedofeatures (Fig. 8c) and poorly sep-
arated plasma (Fig. 9a). Such Vertic Haplustalfs have >8% more clay
in the B horizons than in the Ap horizons, and the fine clay/total
clay ratio in the B horizon is >1.2 times greater than that of the Ap
horizon. The thin sections of the soils did not show any of the
disrupted clay pedofeatures that could be expected in soils with
high COLE (>0.10, Pal et al., 2009a; Eswaran et al., 1988; Mermut et
al., 1996). Thus, the illuviation of clay in the absence of slickensides
suggests that illuviation is a faster pedogenetic process than the for-
mation of slickensides, which does not take place within a 100-year
span. The occurrence of shrink-swell soils over 500–1500 yr (TL
ages) with the illuviated clay features and slickensides of the eastern
lower Indo-Gangetic Plains (IGP) under an SHM climate regime
(Singh et al., 1998) suggests that a minimum time of 500 years is re-
quired to form slickensides in vertisols. Other evidence also supports
a longer time span for vertisol formation (Aslan and Autin, 1998). In-
tensive pedoturbation is therefore not required or important in the
200 µ
µ
m
a) b)
c)
100
µ
m
100
µ
m
Fig. 8. Representative photograph in cross polarised light. (a) Impure clay pedofeatures, (b) weakly oriented clay pedofeatures and (c) undifferentiated clay pedofeatures. Adapted
from Pal et al. (2009a).
36 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
creation of typical morphogenetic characteristics in a vertisol (Yaalon
and Kalmar, 1978).
4.4. Development of microstructures and vertical cracks in vertisols as
controlled by smectite swelling
Huge quantities of smectitic clay induce both horizontal and vertical
stresses in vertisols. Lateral stresses in the upper horizons are not severe
because of low overburden pressure. In addition,cracks also prevent the
development of stress; however, in the sub-surface regions where
sphenoids and/or slickensides are formed, the difference between hor-
izontal stress and verticalstress is quite large (Knight, 1980) when soils
swell. As a result, failure occurs when the vertical stress is confined and
the lateral stress exceeds the shear strength of the soils. Failure occurs
along a grooved shear plane (theoretically 45° to the horizontal;
Wilding and Tessier, 1988). In reality, such shear failure may range
from 10 to 60° (Knight, 1980). The shear failure is manifested as the ap-
pearance of poro/parallel/reticulate/grano-striated plasmic fabric, indi-
cating a prominent surface-oriented plasma separation (Fig. 9b) or
stipple-speckled/mosaic-speckled/crystallitic plasmic fabric related to
Table 2
Physical and chemical properties of Sodic Haplusterts
a
as representative of vertisols of Peninsular India.
(a) Physical properties
Lab.
no.
Hori-
zon
Depth
(cm)
Size class and particle diameter (mm) Fine clay
(%)
Fine clay/total
clay (%)
BD
Mg/m
3
COLE HC
b
cm/h
WDC
(%)
Total
Sand (2–0.05) Silt (0.05–0.002) Clay (b0.002)
(% of b2 mm)
3114 Ap 0–14 0.9 36.7 62.4 26.7 42.8 _ 0.28 1.1 6.6
3115 Bw1 14–40 0.9 34.2 64.9 26.7 41.1 1.5 0.26 2.1 13.9
3116 Bw2 40–59 0.8 33.3 65.9 28.9 43.8 1.6 0.26 1.0 14.8
3117 Bss1 59–91 1.3 35.3 63.4 29.0 45.7 1.5 0.29 0.5 6.4
3118 Bss2 91–125 2.4 37.3 60.3 28.7 47.6 1.5 0.25 0.4 7.6
3119 Bss3 125–150 1.9 38.1 60.0 25.7 42.8 1.6 0.25 0.3 10.0
(b) Moisture at various tensions
Horizon Depth
(cm)
Moisture retention % AWC
33 kPa 100 kPa 300 kPa 500 kPa 800 kPa 1000 kPa 1500 kPa
Ap 0–14 40.1 35.1 30.9 28.3 25.1 22.5 20.3 19.7
Bw1 14–40 41.7 37.3 30.6 28.2 26.2 25.1 19.1 22.7
Bw2 40–59 42.4 40.3 32.2 30.0 26.9 26.8 22.2 20.2
Bss1 59–91 43.9 43.1 33.2 32.6 28.5 27.9 19.8 24.1
Bss2 91–125 43.5 42.7 32.8 32.6 27.8 25.7 19.5 24.1
Bss3 125–150 48.5 42.7 37.5 33.0 29.4 28.3 26.2 22.3
(c) Chemical properties
Depth
(cm)
pH water
(1:2)
CaCO
3
(%)
OC
(%)
Extractable bases CEC
(cmol(p+)kg
−1
)
Clay CEC
(cmol(p+)kg
−1
)
B.S.
(%)
Ca Mg Na K Sum
(cmol(p+)/kg
−1
)
0–14 7.8 9.3 0.81 46.2 14.4 0.6 1.0 62.2 65.2 99 95
14–40 7.9 9.4 0.66 43.4 15.6 2.1 0.7 61.0 61.8 94 98
40–59 8.0 10.7 0.59 42.0 17.8 2.7 0.7 63.0 63.5 95 99
59–91 8.4 11.0 0.61 38.2 20.2 4.2 0.7 63.3 63.5 100 99
91–125 8.5 13.7 0.48 28.9 22.0 5.8 0.6 57.3 62.2 95 92
125–150 8.5 15.6 0.42 25.8 22.4 8.6 1.1 57.9 66.7 96 87
(d) Exch. Ca/Mg, ECP, EMP, ESP and carbonate clay in soil and on fine earth basis (feb)
Depth (cm) Exch. Ca/Mg ECP EMP ESP CO
3
clay (%) CO
3
clay (feb) (%)
0–14 3.2 71 22 1.0 0.4 0.2
14–40 2.8 70 25 2.1 0.4 0.2
40–59 2.3 66 28 4.4 1.2 0.8
59–91 1.9 60 32 6.6 3.2 2.0
91–125 1.3 46 35 9.3 1.8 1.1
125–150 1.1 38 33 12.9 1.9 1.1
(e) Saturation extract analysis
Depth
(cm)
Soluble cations (meq/l) Soluble anions (meq/l) RSC SAR
Sat % ECe Ca Mg Na K Sum CO
3
HCO
3
Cl SO
4
Sum
0–14 72.8 0.3 1.43 0.8 3.2 0.1 5.53 –3.2 0.8 1.53 5.50 0.97 3.0
14–40 70.4 0.3 0.67 0.4 0.8 0.04 1.91 –1.3 0.6 –1.90 0.23 1.1
40–59 73.0 –0.46 0.3 1.1 0.05 1.90 1.0 0.5 0.7 –2.20 0.74 1.8
59–91 77.1 0.4 0.39 0.3 1.7 0.03 2.46 1.0 1.0 0.9 –2.90 1.31 3.0
91–125 63.3 4.7 0.72 0.5 6.0 1.43 8.65 1.0 3.0 0.2 4.45 8.65 2.78 8.0
125–150 85.9 –0.49 0.3 6.3 0.05 7.14 2.0 4.0 0.1 1.04 7.14 5.21 10.0
Adapted from Pal et al. (2003a).
a
As defined by Pal et al. (2006b).
b
9mmh
−1
is the HC (WM) in 0–100 cm depth of soil.
37D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
poor plasma separation in the Bss horizons (Fig. 9c; Kalbande et al.,
1992; Pal et al., 2009c). The presence of sphenoids and/or slickensides
and the dominance of poro/parallel/grano/reticulate-striated plasmic
fabric in Indian vertisols in HT and SHM climates indicate that the
shrink–swell activity of smectites has been extensive. In contrast, the
dominance of stippled/mosaic-speckled plasma in SHD soils, mosaic/
crystallitic plasma in SAM soils, mosaic/stippled-speckled plasma in
SAD soils and crystallitic plasma in AD soils clearly suggests that
shrink–swell is much less significant in the soils of drier climates com-
pared to HT and SHM climates, and it manifests in poor plasma separa-
tion. Therefore, weak swelling of smectite is sufficient for the
development of sphenoids and/or slickensides, but it is not adequate
to cause strong plasma separation (Pal et al., 2001a, 2009c), despite
the fact that the soils have almost identical COLE values and comparable
amounts of expansible clays (Paletal.,2006b,2009c). In these soils,
swelling of smectites, however, has been restricted neither by the pres-
ence of CaCO
3
and calcite crystals, nor bythe decrease in smectite inter-
layer surface area by partial hydroxy-interlayering (Pal et al., 2009c).
The saturated hydraulic conductivity (sHC) of all vertisols is not
identical but decreases rapidly with depth. The decrease is sharper in
SAD and AD soils because of their subsoil sodicity (ESP> 5, Pal et al.,
2009c). The reduced sHC restricts the vertical and lateral movement
of water in the subsoils. As a result, during the very hot summer months
(April to June), there would be less water in SAD and AD subsoils. This
deficit becomes evident from the deep cracks cutting through their
Bss horizons, in contrast to higher MAR soils in which the cracks do
not extend beyond the slickensided horizon at 40–50 cm (Fig. 2). The
lack of adequate soil water during the shrink–swell cycles restricts the
swelling of smectite and results in weaker plasma separation in SAD
and AD soils (Pal et al., 2009c). Thus, the SAM, SAD and AD subsoils re-
main, in general, under less amount of water compared to those of HT,
SHM, and SHD climates during the Holocene period and are modified
in terms of subsoil sodicity, poor plasma separation, and cracks cutting
through theBss horizons due to the accelerated formation of PC. There-
fore, Pal et al. (2001a, 2009c) designated such vertisols as polygenic
soils.
4.5. Evolutionary sequences in vertisol formation
Successive stages of pedogenic evolution in vertisols were con-
ceptualised by Blokhuis (1982), who thought that vertisols would lose
their vertic characters and subsequently convert to non-vertic soils.
Eswaran et al. (1988),however,suggestedthatavertisol(pHb6.5) in
surface horizons would form an argillic horizon as leaching advanced,
and with the accumulation of translocated clay, the soils may qualify
as Vertic Haplustalfs. Both researchers assumed that the original ver-
tisols had no argillic horizon.
Recent studies on the evolution of soils in HT parts of the Western
Ghats (Bhattacharyya et al., 1993, 1999a) and north-eastern
(Bhattacharyya et al., 2000), and southern India (Chandran et al.,
2005) suggest that with time, vertisols in the presence of Ca-zeolites
(Typic Haplusterts) of HT remain as vertisols as long as the zeolites con-
tinue to provide bases by whichto prevent the total formation of kaolin
at the expense of smectites (Fig. 10a).In the absence of zeolites,the soils
would gradually become acidic and kaolinitic and phase towards
ultisols through an intermediate stage of non-vertic alfisols (Fig. 10a).
As silica is insoluble in an acidic environment, the complete transforma-
tion of smectite to kaolinite is improbable; thus, ultisols would remain
unchanged, with Sm–K as the dominant minerals (Chandran et al.,
2005).
An extensive pedogenetic study of vertisols in an Indian
climosequence expands the basic understanding of vertisol evolution
from Typic Haplusterts to Udic/Aridic/Sodic Haplusterts and Sodic
Calciusterts (Pal et al., 2009c,Fig. 10b). These vertisols may remain in
equilibrium with their climatic environments until the climate changes
further, after which another pedogenic threshold is reached. These soils
are of Holocene origin but are the products of polygenic evolution. Due
to subsoil sodicity caused by illuviation of Na-clay smectites in vertisols
of the SAM, SAD and AD climates, the initial impairment of the per-
colative moisture regime would yield a soil system in which gains ex-
ceed losses. This self-terminating process (Yaalon, 1971)wouldlead
to the development of sodic soils in which ESP decreases with depth if
aridity continues (Fig. 11). The soils that exhibit regressive pedogenesis
25 µ
µ
m
100
µ
m
b) c)
a)
Fig. 9. Representative photograph of cross polarised light: poorly separated plasma in VerticHaplustalfs (a), strong parallel plasmic fabric in Typic Haplusterts (b), and mosaic/stippled-
speckled plasmic fabric in Aridic/Sodic Haplusterts of Peninsular India. After Pal et al. (2009a, 2009c).
38 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
(Johnson and Watson-Stegner, 1987) suggest that soil degradation is a
climatically induced, basic and natural process (Pal et al., 2000b).
5. Importance of pedology in the edaphology of vertisols
5.1. Impact of the close association of non-sodic and sodic shrink–swell
soils (vertisols) on crop performance
Natural soil degradation (in terms of PC formation and development
of subsoil sodicity in the vertisols of the Purna Valley of Maharashtra, cen-
tral India; total area ~ 0.6 M ha) is triggered by the semi-arid climate, with
an MAR of 875 mm, a tropustic moisture regime and a hyperthermic
temperature regime (Balpande et al., 1996:Pal et al., 2000b). Such soils
have severe drainage problems, but in the Pedhi Watershed, in the adja-
cent east upland of the Purna valley (area ~45,000 ha), vertisols also have
drainage problems. The area, however, has a higher MAR (975 mm) than
the Purna valley and has similar moisture and temperature regimes to the
Purna Valley. Vertisols are the dominant soil type in the watershed, but as
a result of micro-topographic variation (0.5–5m; Fig. 5), Sodic
Haplusterts occur on micro-high (MH) positions and at a distance of ap-
proximately 6 km, while Aridic Haplusterts occur on micro-low (ML) po-
sitions (Vaidya and Pal, 2002). Cotton performs better in Aridic
Haplusterts (ESP b5; 0.6–1.6 t/ha of seed+lint, yield obtained by the
farmers following typical managements as detailed elsewhere, Kadu et
al., 2003)thaninAridicHaplusterts(ESP>5,b15; 0.6–1.0 t/ha) and
Sodic Haplusterts (ESP≥15; 0.2–0.8 t/ha yield; Table 3;Kadu et al.,
2003). In view of the comparatively poor crop productivity of the Aridic
Haplusterts with the Sodic Haplusterts with an ESP>5 but b15 (having
no soil modifiers; Pal et al., 2006b), Aridic Haplusterts were classified as
Sodic Haplusterts (Balpande et al., 1996; Pal et al., 2000b) because their
resilience could be improved by appropriate management interventions
to enhance crop productivity. Such a close association of Aridic and
Sodic Haplusterts under similar topographical conditions in a relatively
small watershed may be unique from the pedological viewpoint, but
this finding poses a challenge for land resource managers in com-
prehending the differences in the chemical environment between the
Aridic Haplusterts with ESPb5 and the Aridic Haplusterts with ESP> 5
but b15. Thus, for optimised use and management of the latter type of
Aridic Haplusterts, the proposed modifications in their subgroup-level
classification (as per US Soil Taxonomy) are mandatory (Pal et al., 2006b).
5.2. Linear distance of cyclic horizons in vertisols and its relevance to
agronomic practices
Vertisols develop deep, wide shrinkage cracks in the summer. These
cracks close as the soil rewets because of up-thrusting that forces one
Fig. 11. Progressive development of sodicity in vertisols while aridity continues with
time. Adapted from Pal et al. (2011a). ESP = exchangeable sodium percentage.
Fig. 10. Successive stages of pedogenic evolution in vertisols. Adapted from Pal et al. (2009a).
1
Clay illuviation, a requisite for argillic horizon (indicating legacy of Alfisols), although common
does not get better than the slickensides (Vertisol’s dominant p roperty). Since Vertisol soil order keys out before Alfisol in Soil Taxonomy these soils would continue to be grouped as Vertisols.
39D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
swelling clay mass to slip over another, resulting in the formation of
slickensides. The cyclic horizons repeat in the subsoil, the size of which
depends on the length of cycle. One-half of the linear distance (LD) of
the cycle is a measurement of the lateral dimension of a cyclic horizon
(Johnson, 1963). To evaluate subsoil variability and determine LD, it is
necessary to have trenches at least 10 m long with depths of 2 m or
more (Bhattacharjee et al., 1977; William et al., 1996). This fieldwork is
time consuming, laborious and expensive.
A large area of vertisols is used for pastures, and cracks developed
therein may be wide enough to cause dangerous footing for animals
(Buol et al., 1978). Agronomic uses of vertisols vary widely, depending
on the climate. Field moisture conditions, drainage conditions and pat-
terns of vegetation indicate that the maximum oscillation between
wet and dry conditions manifests in micro-depressions that retain
moisture for longer periods and in microknolls, which dry out faster.
Vertisols are capable of tilting large trees, and surprisingly, few if any
commercial orests are cultivated on vertisols (Buol et al., 1978). In addi-
tion, highways, buildings, fences, pipelines, and utility lines are moved
and distorted by the shrinking and swelling of these soils. Prior knowl-
edge about the highs and lows of the cyclic pedons may help the stake-
holders plan their programmes and avoid mishaps. In view of the need
for a method to determine LD, a mathematical equation has been pro-
posed to measure the LD of the cyclic horizons, taking into account
the depth of occurrence of slickensides (Bhattacharyya et al., 1999b).
A standard parabolic equation represented by (y
2
=4ax) was con-
sidered, where ais the focus of the parabola. The concept of cyclic
horizons of vertisols in terms of this parabolic pattern (Fig. 12)cen-
tres around two basic assumptions. The first assumption is that the
depth of the first occurrence of slickensides (b) coincides with the focus
of the parabola. The second assumption is that bholds constant within a
cyclic horizon.
To calculate LD using the equation (LD =MN =2KN =4[a(a+b)]
1/2
(Fig. 12), the values of aand bare needed. The value of b(the depth of the
first occurrence of slickensides) is obtained in the field; however, the
value of acan be obtained only by examining the profile exactly in the
centre of the cyclic horizon. This value is difficult to acquire because cy-
clic horizons in the subsurface cannot be identified from the surface, es-
pecially where microknolls and microdepressions are obliterated. When
the profile is examined away from the centre of the cyclic horizon, the
value of acannot be obtained, and calculation of LD becomes difficult.
Thus, Bhattacharyya et al. (1999b) proposed the following equation.
LD (cm)= 200/Y(2500+ bY)
1/2
,whereY=y
1
+y
2
−2(y
1
y
2
)
1/2
and y
1
and y
2
are the vertical distances (cm) from the first occurrence
of slickensides to the intersecting points of the cyclic pedon such that
y
1
>y
2
and bis the depth of the first occurrence of slickensides. It is al-
ways possible to find the values of y
1
and y
2
if the profile is examined on
either sideof the centre of the cyclic horizon (Fig. 12). This equation can
eliminate some of the need for fieldwork to determine the LD with the
help of three variables, namely y
1
(the length of the slickensided zone
on the right-hand side of the profile wall from the depth of the occur-
rence of slickensides), y
2
(the length of the surface and the slickensided
zone on the left-hand side of the profile wall) and b(the depth of the
occurrence of slickensides). These values can easily be obtained by soil
survey and mapping. The accuracy of the equation proposed by
Bhattacharyya et al. (1999b) is between 81 and 86% in vertisols in
arid to semi-arid climates. The equation provides a new method of locat-
ing micro-depressions and micro-knolls in an effort to better manage
vertisols for agricultural and non-agricultural purposes.
Table 3
Range in values of PC, ESP, sHC and yield of cotton in vertisols of Vidarbha, Central India.
District, Vidarbha Region, Maharashtra, Central India Soil classification PC (%) ESP sHC (mm h
−1
) weighted mean in
the profile (1 m)
Cotton yield (t ha
−1
)
(seed + lint)
Nagpur (MAR —1011 mm) Typic Haplusterts/Typic Calciusterts 3–6 0.5–11 4–18 0.9–1.8
Amravati (MAR —975 mm) (a) Aridic Haplusterts 3–7 0.8–42–19 0.6–1.6
(b) Sodic Haplusterts 3–13 16–24 0.6–9.0 0.2–0.8
Akola (MAR —877 mm) (a) Aridic Haplusterts 3–47–14 3–4 0.6–1.0
(b) Sodic Haplusterts 3–419–20 1–2 0.6
PC = pedogenic CaCO
3
, ESP = exchangeable sodium percentage (sodicity), sHC = saturated hydraulic conductivity. Adapted from Kadu et al. (2003).
Fig. 12. The parabolic path of cyclic pedon where PL, S, UV, O, and KR are latus rectum, focus, directrix, vertex and axis of the parabola (KN = KM, OS = OR, KN = NV, KM = MU,
OJ–OH= y
1
−y
2
= HJ, OJ + OH = y
1
+y
2
, OJ.OH=y
1
.y
2
). Adapted from Bhattacharyya et al. (1999b).
40 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
5.3. Importance of smectite clay minerals in growing vegetation on
weathered basalt and in very shallow cracking clay soils
In general, gabbros and basalts, which are dominated by dark ferro-
magnesian minerals, are more easily weathered than granites and other
light-coloured rocks. The alluvium of the weathering Deccan basalt is
principally responsible for the formation of deep black soils in the Pen-
insular I ndia, as the first weathering product of theDeccan basalt is low-
charge dioctahedral smectite (Pal and Deshpande, 1987a; Pal et al.,
2009c). In the Deccan basalt litho logs, boulders of different sizes adjoin
each other and exhib it spheroid al weathering (Fig. 13a). These boulders
have concentric rings similar to onions (Fig. 13b) that easily come away
under gentle pressure from fingers. The boulders also contain DOS
(Fig. 13c) similar to that of vertisols (Pal and Deshpande, 1987a). Smec-
tites are known for their excellent capacity to hold moisture and nutri-
ents; thus, they help several tree species to anchor on weathered basalt,
even on steep slopes (> 40%). The long-term preservation of tree spe-
cies under national forest management in Maharashtra and Madhya
Pradesh of western and central Peninsular India has become possible
under favourable MAR conditions in HT, SHM, SHD, and SAM climates.
In SAD and AD climates, care is needed at the initial stage of establish-
ment of tree species. The largest Deccan basalt area with the greatest
forest cover lies in the state of Madhya Pradesh, in central India; it is
followed by Maharashtra, in western India. Smectite mineral has been
enormously useful to preserve and sustain thespectacular natural forest
vegetation (Bhattacharyya et al., 2005). The mineral also helps to main-
tain and preserve the forests in lower-MAR areas; however, initial care
is needed during the establishment of tree species. Thus, the natural
abundance of smectite in the Deccan basalt areas of lower MAR can
also prove useful in establishing agri-horticultural crops even on shallow
soils (entisols; depth b50 cm) strewn with small and weathered basalt
rocks.
5.4. Role of smectite, soil modifiers and palygorskite minerals in influencing
the drainage of vertisols
The vertisols of dry climates of peninsular India have poor drainage,
but they show no salt-efflorescence on the soil surface as evidence of
the threat of soil sodicity. These soils also do not qualify as salt-affected
soils per the United States Salinity Laboratory criteria; however, the
sHC of their subsoils is adversely affected by clay dispersion caused by
exchangeable magnesium (Vaidya and Pal, 2002). This point suggests
that the saturation of vertisols, not only with Na
+
ions but also with
Mg
2+
ions, blocks small pores in the soil. In other words, Mg
2+
ions
are less efficient than Ca
2+
ions at flocculating soil colloids, although
the United States Salinity Laboratory (Richards, 1954) grouped Ca
2+
and Mg
2+
together because both improve soil structure. This action is
further impaired by a low level of ESP (>5, b15), which reduces the
sHC to b5 mm/h, causing a >50% reduction in cotton yield (Table 3;
Kadu et al., 2003). This result is due to large amounts of smectite
minerals in vertisols (Typic/Aridic Haplusterts). In general, vertisols de-
veloped in the alluvium of weathering Deccan basalt contain > 95%,
35–40% and 8–10% smectite in the fine-clay (b0.2 μm), coarse-clay (2–
0.2 μm) and silt (50–2μm) fractions, respectively (Pal et al., 2000a),
suggesting that 100 g of vertisols from western and central Peninsular
India may contain 40–50 g of smectite. Therefore, the current lower
limit of a 15 ESP by the United States Salinity Laboratory for all soils is
arbitrary and necessitates the evaluation of a lower ESP limit. To validate
this suggestion, Pal et al. (2006b) undertook an extensive study on
vertisols with and without soil modifiers (Ca-zeolites and gypsum),
Fig. 13. Deccan basaltlitholog showing spheroidal weathering(a), onion like peeling in basalt boulders (b), thin section of basalt boulder showing concentric weathering rinds (c), and
SEM photograph of clay sized dioctahedral smectite as its first weathering product (d). Adapted from Pal et al. (2000a) and photographs (a–d), courtesy of DKP. Adapted from Pal and
Deshpande (1987a) and photogr aphs (a–b), courtesy of DKP.
41D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
representing a climo-sequence from an SHM to an AD climate that occurs
in major states in peninsular India. The results of this study indicate that
a precise cause–effect relationship between PC and NPC and exchange-
able Mg, Na and Ca percentages exists in Sodic/Calcic Haplusterts with
and without modifiers. However, the release of Ca
2+
ions from soil mod-
ifiers prevented the rise in pH and ESP and modified sHC (>10 mm/h) in
high ESP, which supports the performance of rain-fed crops. Therefore, a
lower limit of sodicity at an ESP >40 for IGP soils (Abrol and Fireman,
1977), at an ESP>5 but b15 for Indian vertisols (Kadu et al., 2003), at
an ESP 6 for Australian soils, or at an ESP >15 for all soil types (Soil
Survey Staff, 1999) is incompatible with the performance of crops in
highly sodic vertisols that contain soil modifiers, especially Ca-zeolites
(Pal et al., 2006b). sHC impairment in soils, mediated by dispersibility,
is the most important factor in soil degradation (Sumner, 1995). Thus,
the characterisation of sodic soils based on sHC appears to be most ap-
propriate when a 50% reduction in crop yields has been recorded. There-
fore, Pal et al. (2006b) advocated a value of sHC b10 mm h
−1
(weighted
mean in soil 1 m deep) instead of ESP or SAR. Hence, the deciding feature
of soil classification must be the native vegetation because it indicates
the nature of the land much more explicitly and authoritatively than
any other arbitrary definition or nomenclature (Hilgard, 1906).
The dispersibility of clay colloids impairing the sHC of vertisols is gen-
erally an effect of ESP or EMP in the presence or absence of soil modifiers;
however, the sHC of zeolitic vertisols of the Marathwada region, Maha-
rashtra state, in semi-arid western India is reduced to b10 mm/h, al-
though the soils are non-sodic (Typic Haplusterts; Zade, 2007). Such
vertisols have neutral to mildly alkaline pH, ESP b5, but EMP increases
with depth. In some pedons, EMP is greater than ECP (exchangeable cal-
cium percentage) at depths below 50 cm. Mineralogical studies indicate
that palygorskite is found mainly in the silt and coarse-clay fractions
(Zade, 2007). Palygorskite minerals are present in Typic Haplusterts
and also in Sodic Haplusterts/Sodic Calciusterts in association with
Typic Haplusterts in India (Zade, 2007) and elsewhere (Heidari et al.,
2008). This mineral is the most magnesium-rich of the common clay
minerals (Singer, 2002). Neaman et al. (1999) examined the influence
of clay mineralogy on disaggregation in some palygorskite-, smectite-,
and kaolinite-containing soils (ESP b5) of the Jordan and Betshe'an valley
in Israel. Palygorskite was the most disaggregated of the clay minerals,
and its fibre did not associate into aggregates in soils and suspensions
even when the soils were saturated with Ca
2+
ions. Palygorskite parti-
cles thus move downward in the profile preferentially over smectite
and eventually clog the soil pores (Neaman and Singer, 2004). Therefore,
vertisols with palygorskite content have high EMP values, causing dis-
persion of the clay colloids that form a 3D mesh in the soil matrix. This
interaction causes drainage problems when such soils are irrigated,
presenting a predicament for crop production. In view of their poor
drainage conditions and loss of productivity, non-sodic vertisols (Typic
Haplusterts) with palygorskite minerals must be considered naturally
degraded soils. Similar soils may be found elsewhere in the world;
thus, a new initiative to classify them is warranted.
5.5. Climate change, polygenesis, impairment of soil properties and
evaluation of vertisols
Frequent climatic changes occurred during the Quaternary Period
(Ritter, 1996). As a result, soils worldwide were subjected to climatic
fluctuations, especially in the last post-glacial period. Brunner (1970)
reported evidence for tectonic movements during the Plio-Pleistocene
transition, which caused the formation of various relief types. With
the formation of the Western Ghats during the Plio-Pleistocene crustal
movements, the humid climate of the Miocene-Pliocene was replaced
by the semi-arid conditions that still prevail in central and southern
peninsular India. The Arabian Sea flanks the Western Ghats, which
rise precipitously to an average height of 1200 m, the result of a heavy
orographic rainfall all along the west coast. The lee-side towards the
coast receives less than 1000 mm of rainfall and is typically rain-
shadowed (Rajaguru and Korisetter, 1987). The current aridic environ-
ment prevailing in many parts of the world (including India; Eswaran
and van den Berg, 1992) may create adverse physical and chemical
soil environments. This is evident from the occurrence of more alkaline,
calcareous and sodic shrink-swell soils (Sodic Haplusterts/Calciusterts)
of Peninsular India due to a progressive increase in the PC content from
HT to AD climates (Pal et al., 2009c;Fig. 10b), as aridity of the climate is
the main factor in the formation of calcareous sodic soils (Pal et al.,
2000b). The subsoils (SAM Aridic Haplusterts, SAD Sodic Haplusterts
and AD Sodic Calciusterts) remain under less water than those of
Typic Haplusterts in HT, SHM andSAM climates. As a result,the vertisols
in drier regions of India have relatively more PC and ESP, reduced sHC,
(Table 2) and poor micro-structure (Fig. 9c),aswellasdeepcrackscut-
ting through the slickensided zones (Fig. 2). Thus, these modified ver-
tisols qualify as polygenic soils (Pal et al., 2009c). Deep-rooted crops
on Aridic Haplusterts (ESP> 5, b15) and Sodic Haplusterts show poor
productivity (Table 3). Recently, Kadu et al. (2003) attempted to identi-
fy bio-physical factors that limit the yield of deep-rooted crops (cotton)
in 29 basaltic-alluvium vertisols of the Nagpur, Amravati and Akola dis-
tricts in the Vidarbha region in central India. Under rain-fed conditions,
the yield of deep-rooted crops on vertisols depends primarily on the
amount of rain stored at depth in the soil profile and the extent to
which this soil water is released during crop growth. Both the retention
and release of soil water are governed by the nature and content of clay
minerals,as well as by the nature of the exchangeable cations. The AWC
(available water content), calculated based on moisture content, varied
between 33 and 1500 kPa (Table 4), indicating that not only the Typic/
Aridic Haplusterts but also the Sodic Calciusterts can hold sufficient
water; however, a non-significant negative correlation between cotton
yieldandAWC(Table 4) indicates that this water is not released during
the growth of crops. The prevalence of Na
+
ions on exchange sites of
Aridic Haplusterts and Sodic Haplusterts/Calciusterts with ESP> 5
thus amounts to overestimation (Gardner et al., 1984). In fact, moisture
remains at 100 kPa for Typic Haplusterts and Aridic Haplusterts
(ESPb5) after the cessation of rains during June to September, while it
is held at 300 kPa for Sodic Haplusterts (Kadu, 1997) as the movement
of water is governed by sHC, which decreases rapidly with depth, and
the decrease is sharper in Aridic/Sodic Haplusterts (ESP>5, Pal et al.,
2009c). This conclusion is supported by a significant positive relation-
ship between ESP and AWC, and a significant negative correlation be-
tween yield and ESP (Table 4). A significant positive correlation
between yield and exchangeable Ca/Mg (Table 4) indicates that a
Table 4
Co-efficient of correlation among various soil attributes and yield of cotton
a
.
No. Parameter Y Parameter X r
Based on 165 soil horizons samples of 29 vertisols
1 sHC (mm h
−1
) Exch. Ca/Mg 0.51⁎
2 sHC (mm h
−1
) ESP
b
−0.56⁎
3 ESP AWC (%) 0.40⁎
4 ESP Exch. Ca/Mg −0.40⁎
Based on 29 vertisols
5 Yield of cotton (q ha
−1
) AWC (%) WM
c
−0.10
6 Yield of cotton (q ha
−1
) ESP max
a
−0.74⁎
7 Yield of cotton (q ha
−1
) sHC WM
b
0.76⁎
8 Yield of cotton (q ha
−1
) carbonate clay
d
−0.64⁎
10 Yield of cotton (q ha
−1
) Exch. Ca/Mg WM
c
0.50⁎
11 ESP max
a
AWC (%) WM
c
0.30⁎
12 ESP max
a
Exch. Ca/Mg WM
c
−0.55⁎
13 ESP max
a
carbonate clay
d
0.83⁎
AWC, available water content; ESP, exchangeable sodium percentage; sHC, saturated
hydraulic conductivity.
a
Adapted from Kadu et al., 2003.
b
Maximum in pedon.
c
Weighted mean.
d
Fine earth basis.
⁎Significant at 1% level.
42 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
dominance of Ca
2+
ions in the exchange sites of vertisols is required to
improve the hydraulic properties for a favourable growth and final yield
of crops. The development of subsoil sodicity (ESP≥5) replaces Ca
2+
ions in the exchange complex, causing a reduction in the yield of cotton
in Aridic/Sodic Haplusterts (ESP ≥5). A significant negative correlation
between ESP and exchangeable Ca/Mg (Table 4) indicates an impover-
ishment of soils with Ca
2+
ions during sodification by the illuviation of
Na-rich clays. This pedogenic process depletes Ca
2+
ions from the soil
solution in the form of CaCO
3,
with the concomitant increase of ESP
with pedon depth. Thus, these soils contain PC (Pal et al., 2000b), and
carbonate clay, which, on a fine earth basis, increases with depth
(Table 2). This chemical process is evident from the positive correlation
between ESP and carbonate clay (Table 4). A significant positive
correlation between the yield of cotton and carbonate clay (Table 4)
indicates that, like ESP, PC formation also reduces yield and is a more
important soil parameter than total soil CaCO
3
(NBSS&LUP, 1994;
Sys et al., 1993). An accelerated rate of PC formation in dry climates
impairs the hydraulic properties of vertisols, and a significant negative
correlation exists between ESP and sHC (Table 4). The processes
operating in the soils of dry climates also influence the sHC of the
vertisols. A significant positive correlation exists between the yield of
cotton and sHC. In view of the pedogenic relationship among SAT
environments, PC formation, exchangeable Ca/Mg, ESP and sHC, all of
which ultimately impair the drainage of vertisols, the evaluation of
vertisols for deep-rooted crops based on sHC alone may help in plan-
ning and management of soils, not only of vertisols in the Indian SAT
areas, but also of vertisols under similar climatic conditions elsewhere
(Kadu et al., 2003).
5.6. Nature and layer charge of smectite and other minerals in adsorption
and desorption of major nutrients
5.6.1. Nitrogen
One of the forms of mineral nitrogen (N) is fixed NH
4
-N, and several
reports indicate that many tropical soils are endowed with large
amounts of fixed ammonium (Dalal, 1977); however, information on
this important form of N is scarce, especially in the SAT soils (Burford
and Sahrawat, 1989). Thus, realising its overall importance in the N
economy of soils, Sahrawat (1995) determined the fixed NH
4
-N distri-
bution in two of the BM vertisols of Indian SAT, namely from
Kasireddipalli at ICRISAT Center and Patancheru and Barsi in Maharash-
tra state, western India. The amount of fixed NH
4
-N was reported to
have a share of 22 to 59% in the former and 16 to 31% in the latter in
the total soil N, and its percentage was higher in the subsoils. Vermicu-
lites are known to fixNH
4
-N; however, illites and smectites are often
considered able to fixNH
4
-N (Nommik and Vahtras, 1982). Smectites
have no selectivity for non-hydrated monovalent cations such as K+
because of their low-level charge (Brindley, 1966). NH
4
+
, also a non-
hydrated monovalent cation with almost the same ionic radius as K, is
not expected to be fixed in the interlayers of smectites. It is equally dif-
ficult to understand the NH
4
ion-fixing capacity of illites because illites
do not expand on being saturated with divalent cations (Sarma, 1976).
Vertisols developed in the basaltic alluvium of the Deccan basalt of
Peninsular India, are not devoid of vermiculite as reported (Dhillon
and Dhillon, 1991; Mengel and Busch, 1982). Vertisols contain vermic-
ulite in their silt (50–2μm), coarse-clay (2–0.2 μm) and fine-clay
(b0.2 μm) fractions (Pal and Durge, 1987), including the Kasireddipalli
soils (Pal and Deshpande, 1987b). Vermiculite, determined quantita-
tively (Alexiades and Jackson, 1965), constitutes 2.0 to 3.5% of the silt,
3.5 to 10% of the coarse-clay and 5.0 to 9.5% of the fine-clay fractions
(Pal and Durge, 1987). Moreover, the XRD analysis of the total clay frac-
tion (b2μm) of the Kasireddipalli soils indicates that smectite (a low-
charge dioctahedral in nature) is the dominant clay mineral (> 50%,
b70%)andisassociatedwithvermiculite(10–15%; Pal and Deshpande,
1987a). Vermiculite is trioctahedral in nature and is an alteration product
of biotite in the presence of its dioctahedral variety (muscovite; Pal and
Durge, 1987; Pal et al., 2001b).BoththemicasarenotpartoftheDeccan
Basalt, and their presence in vertisols is attributed to erosional and depo-
sitional episodes in the Deccan Basalt areas (Pal and Deshpande, 1987a).
The identification of vermiculite by XRD analysis in different soil size
fractionsisfraughtwithsomedifficulty in the ubiquitous presence of
chlorite. Its presence is resolved by following the progressive reinforce-
ment of the 1.0 nm peak of mica while heating the K-saturated samples
at 25, 110, 300 and 550 °C (Pal and Deshpande, 1987a; Pal and Durge,
1987), and its quantity is estimated semi-quantitatively following the
method of Gjems (1967). Thus, it would be prudent to attribute the ob-
served NH
4
-N fixation in vertisols by Sahrawat (1995) to the presence of
vermiculite only. Both Kasireddipalli and Barsi vertisols have an almost
uniform distribution of sand, indicating any lack of lithological disconti-
nuity in the profile, and amidst this, the clay increases >8% in the sub-
soils than in the surface layer (Sahrawat, 1995). Thus the clay-enriched
subsoils have been the result of clay illuviation (Pal et al., 2009c). During
this pedogenetic process, the fine clay fractions containing not only the
Na-saturated smectite but also vermiculite could translocate downward
in the profile. The translocation of clay-vermiculite might have enriched
the subsoils with vermiculite that may possibly explain the observed
increasing fixation of NH
4
-N with soil depth (Sahrawat, 1995). Such a
basic understanding is essential to include fixed NH
4
-N in assessing the
potentiality of N available in tropical soils in general and the vertisols
in particular.
5.6.2. Phosphorus
Among the soil properties that influence phosphorus (P) adsorp-
tion by soil minerals are the nature and amount of soil components
such as clay, organic matter, and hydrous oxides of iron and alumin-
ium (Sanyal and DeDatta, 1991). These authors critically analysed
the findings of several researchers, which indicated a significant cor-
relation of P sorption parameters with clay content, and they pro-
posed that this is a mere reflection of the effect of specific surface
area on P adsorption. In soils, hydrous oxides of iron and aluminium
occur as fine coatings on the surfaces of clay minerals (Haynes,
1983), and these coatings have large specific surface areas that can
adsorb large amounts of added P. This characteristic suggests that
crystalline aluminosilicate minerals merely play a secondary role in
P adsorption (Ryden and Pratt, 1980). Fine clay smectites of vertisols
of HT, SHM, SHD, SAM, SAD, and AD of Peninsular India are partially
hydroxy interlayered (Pal et al., 2009c). The hydroxy interlayering
in smectite interlayers is the not a contemporary pedogenic process
because in the prevailing mild to moderately alkaline pH conditions,
the hydroxides of iron and aluminium cannot remain as positively
charged cations to enter the negative environment of the interlayers
of smectites (Pal et al., 2011b). The presence of HIS in the fine clay
fractions indicates that the hydroxy-interlayering in the smectite in-
terlayers did occur when positively charged hydroxy interlayer mate-
rials entered into the interlayer spaces at a pH far below 8.3 (Jackson,
1964). Moderately acidic conditions are optimal for the hydroxy-Al
interlayering of smectite, and the optimum pH for interlayering in
smectite is 5.0–6.0 (Rich, 1968). The pH of the vertisols is mildly to
moderately alkaline. Under such a chemical environment, 2:1 layer
silicates suffer congruent dissolution (Pal, 1985). This scenario
discounts the hydroxy-interlayering of smectites during the forma-
tion of vertisols in the Holocene period (Pal et al., 2011b) and the cre-
ation of any positively charged hydroxides that can fix added P, as in
highly weathered acidic soils. Therefore, the highest surface area of
smectite and/or hydroxides of iron and aluminium with no positive
sites plays a small role in the adsorption of added negatively charged
phosphate ions in vertisols. This supports the ICRISAT's classical ex-
perimental observations on P adsorption and desorption on vertisols
(Kasireddipalli soils), which clearly indicate that P adsorption is
not a major problem in the vertisols and that all the adsorbed P is
easily exchangeable by P
32
and a small amount of P is adsorbed in
the non-exchangeable form (Sahrawat and Warren, 1989; Shailaja
43D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
and Sahrawat, 1994; Warren and Sahrawat, 1993). ICRISAT (1988)
envisaged that CaCO
3
could adsorb P because the effective sorption
by CaCO
3
is not well understood, and P adsorption is not always relat-
ed to CaCO
3
content (Goswami and Sahrawat, 1982); perhaps the
critical factor here is the quality of the CaCO
3
. The vertisols of Penin-
sular India under a SAT environment contain CaCO
3
of both non-
pedogenic (NPC) and pedogenic (PC) origin, and both of them effer-
vesce with HCl and cannot be distinguished without examining the
soil thin sections under a microscope (Pal et al., 2000b). During the
formation of vertisols in the SAT environment, NPCs (pedorelict) dis-
solve, and the soluble Ca
2+
ions released from NPCs become precipi-
tated as PC at a pH of approximately 8.2 and may also react with
phosphate ions to form Ca-P. Both PC and Ca-P may have the least sol-
ubility in the prevailing mild to moderately alkaline pH conditions of
vertisols. This may be the reason why Ca-P in the vertisols of SAT has
a dominant share among the other soil P compounds, such as Fe–P
and Al–P, causing a very low level of soluble extractable P
(b5mgkg
−1
soil) by Olsen's method (ICRISAT, 1988). It is interesting
to note that grain sorghum grown on vertisols responds little to ap-
plied P unless the level of Olsen's P was b2.5 mg kg
−1
soil (ICRISAT,
1988). Additionally, some leguminous crops such as chickpea and
pigeonpea are less responsive to fertiliser P than sorghum and pearl
millet (ICRISAT, 1981). The root systems of chickpea exude organic
acids (malic or citric) (ICRISAT, 1988) and those of pigeon pea produce
piscidic acid (Ae et al., 1990), which can dissolve Ca–PandFe–P, making
more P available to the plants. The root exudates containingsuch organ-
ic acids and the rootlets in the soil through which rainwater passes, or
other sources of CO
2
, can cause an increase in the solubility of PC and
Ca–P. The improved management (including pigeonpea) in the long-
term heritage watershed experiment at the ICRISAT Center, Patancheru,
under rain‐fed conditions (Wani et al., 2003, 2007) indicates that dur-
ing the last 24 years, the rate of dissolution of CaCO
3
was 21 mg yr
−1
in the first 100 cm of the Kasireddipalli soils, which caused a slight in-
crease in exchangeable Ca/Mg and a decrease in pH (Pal et al., 2011a).
The rate of dissolution of Ca–P under the present improved manage-
ment system is sufficient, as it does not warrant the application of a
high dose of added P fertiliser to produce an incremental grain yield
of 82 kg ha
−1
yr
−1
. However, predicting a time scale when soils will
be devoid of Ca–Pisdifficult unless a new research initiative in this di-
rection is taken up.
5.6.3. Potassium
Vertisols are stated to be adequately supplied with potassium (K),
and therefore, responses to applied K are generally not obtained
(Finck and Venkateswarlu, 1982). Extensive research on K behaviour
in Indian vertisols for the last two-and-a-half decades may be a good
example for understanding the basic issues of K adsorption and de-
sorption (Pal, 2003). As the Deccan basalt does not contain micas
(Pal and Deshpande, 1987a), the vertisols derived from its alluvium
are not expected to be micaceous. The small amounts of micas in ver-
tisols are concentrated mainly in their silt and coarse clay fractions
(Pal and Durge, 1987; Pal et al., 2001b), and their parental legacy is
ascribed to erosional and depositional episodes experienced by the
Deccan basalt areas during the post-Plio-Pleistocene transition period
(Pal and Deshpande, 1987a). Petrographic and scanning electron mi-
croscope (SEM) examinations of the muscovites and biotites of the
vertisols of Peninsular India indicate little or no alteration (Fig. 1a,b)
(Pal et al., 2001b; Srivastava et al., 2002). Therefore, highly available
K status of vertisols appears to be related to the retention of elemen-
tary layers of the micas, which favours the release of K
+
even though
its content is low. The precise nature of soil mica in the silt and clay
fractions was determined on the basis of the X-ray intensity ratio of
peak heights of 001 and 002 basal reflections of mica (Pal et al.,
2001b). The ratio is generally greater than unity in the silt but is
close to unity in the clay fraction (Fig. 1c). The ratio>1 suggests the
presence of muscovite and biotite minerals. If muscovite minerals
were present alone, the ratio would have been close to unity (Tan,
1982). In the event of a mixture of these two micas, both will contrib-
ute to the intensity of the 1.0 nm reflections, whereas the contribu-
tion of biotite to the 0.5 nm reflection would be nil or negligible,
thus giving a higher value to the intensity ratio of these reflections
(Kapoor, 1972). Thus, the silt fractions of the soils contain both mus-
covite and biotite, whereas the clay fractions are more muscovitic in
character (Fig. 1c). The enrichment of soils with muscovite is not
favourable so far as the K release is concerned. This is evidenced by
the reduced rate of K release in the vertisols compared with the
much higher rate of K release from the biotite-enriched soils of the
IGP (Pal et al., 2001b) when they were subjected to repeated batch-
type Ba–K exchange under identical experimental conditions (Pal
and Durge, 1989). Muscovite and biotite micas co-exist in soil envi-
ronments. The weathering of muscovite in the presence of biotite is
improbable. Therefore, the quantity of muscovite cannot be used as
an index of K reserve in soils (Pal et al., 2001b). For this reason, Pal
et al. (2001b) felt it was necessary to provide a selective quantifica-
tion of biotite mica in the common situation in which soils contain
mixtures of biotite and muscovite. The contents of biotite in vertisols
and their size fractions were estimated through a rigorous and ex-
haustive Ba-K exchange reaction. The cumulative amount of K re-
leased at the end of final extraction by the soil's size fractions when
the release of K nearly ceased was considered as mainly coming
from biotite (Pal et al., 2006c)(Fig. 14). The amount of clay biotite,
silt biotite and sand biotite in the representative vertisols of central
India ranged from 1.0 to 1.6, 0.2 to 0.3 and 0.2 to 0.4%, respectively,
constituting 7–19, 2–3 and 2–5% of the total mica in the respective
size fractions. In the b2mm fine earth fraction, the biotite quantity
does not exceed 1%, which constitutes approximately 6–8% of the
total mica. For any size fraction, the cumulative amount of K released
on a biotite weight basis follows the order>cumulative amount of K
released on the entire mica weight basis >cumulative amount of K re-
leased on the weight basis of the size fraction (Table 5). The signifi-
cant positive correlation between the cumulative K release of soils
and their size fractions is mainly from biotite and is established
from the statistical analysis of bivariate data sets of several parame-
ters that directly or indirectly influence K release. The significant pos-
itive correlations between cumulative K release from sand, silt and
clay and their corresponding total K contents, respectively (Table 6),
indicate that the K release is a function of total K content in micas
and feldspars. However, the positive correlations between total K
contents in sand, silt, clay and soil and their mica contents (Table 6)
indicate the predominant influence of mica to supply K to the plants
grown in vertisols. Furthermore, significant positive correlations be-
tween the cumulative K release of sand, silt, clay and soil and their re-
spective mica contents (Table 6) indicate that the K release from
either the soils or different size fractions are controlled mainly by
Fig. 14. Relationship between numbers of extractions and cumulative K release
(mg/100 g
−1
mica) of micas in various size fractions of a vertisol. Adapted from
Pal et al. (2006c).
44 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
mica. However, better correlations than those between the cumula-
tive K release of sand, silt, clay and soil and their biotite contents
(Table 6) provide incontrovertible evidence that the K release in
soils is primarily controlled by biotite mica. This further supports
the earlier observations on the inertness of muscovite mica in the re-
lease of K in the presence of biotite (Pal et al., 2001b). The released
amount of K from sand-, silt- and clay-sized biotite (Table 6) is in con-
trast to the relationships observed between cumulative K release and
particles of specimen biotite by earlier researchers (Pal, 1985; Pal et
al., 2001b; Reichenbach, 1972). This indicates that large-sized biotite
particles have a lower K selectivity than finer particles. Comparable
cumulative amounts of K released from sand and silt biotite
(Table 6) indicate that not only the sand-sized but also some portion
of silt-sized biotite of the vertisols have a greater number of elemen-
tary layers along with little weathered biotite. It is observed that dur-
ing the formation of vertisols since the Holocene (Pal et al., 2006b),
there has been no substantial weathering of biotite under the SAT en-
vironments. This validates the earlier hypothesis (Srivastava et al.,
2002) that the formation of vertisols reflects a positive entropy
change due to a lack of any substantial weathering of primary min-
erals. The relevance of the almost-unaltered biotites (Fig. 1b) is that
both sand and silt biotites have highly favourable K release potential,
which is reflected in the medium to highly available K status of the
vertisols of India and elsewhere (Finck and Venkateswarlu, 1982).
Agronomic experiments on the vertisols of central India have indicat-
ed crop response to K fertilisers after two years of cropping with hy-
brid cotton (Pal and Durge, 1987). Therefore, the present available K
status may not be sustainable over a longer term because the contents
of sand and silt biotites are low. This information helps dispel the
myth that the vertisols are rich in available K and that they may not
warrant the application of K fertilisers.
Potassium adsorption/fixation in vertisols does not appear to be suf-
ficiently severe to conclude that K becomes completely unavailable to
plants (Finck and Venkateswarlu, 1982). Bajwa (1980) reported that
soil clay beidellites can fix more K than vermiculite, being nearly 80%
against added K, whereas clay montmorillonites can fix only approxi-
mately 18%, much less than by the vermiculite clays. The study by Pal
and Durge (1987) on K adsorption by the vertisols of Peninsular India
indicates that fine clay smectites adsorbed 50–60% of added K,
amounting 25–30 mg K/100 g clay. This apparently suggests that the
fine clay smectites of Indian vertisols are close to beidellite (Bajwa,
1980). Through a series of diagnostic methods to characterise the fine
clay smectites, Pal and Deshpande (1987a) confirmed that they are
nearer to the montmorillonite of the montmorillonite–nontronite se-
ries. Because smectites can have no K selectivity (Brindley, 1966), fur-
ther characterisation of fine clay smectites (Pal and Durge, 1987a)
indicated the presence of vermiculites, whichare generally not detected
on the glycolation of Ca-saturated samples but can be detected by a pro-
gressive reinforcement of the 1.0 nm peak of mica while heating the K-
saturated samples from 25 to 550 °C (Fig. 6). Similar results were
reported by Ruhlicke (1985) while reporting a K adsorption of
60 mg K/100 g in bentonite (montmorillonite) deposits. The content
of vermiculite was quantified following the method of Alexiades and
Jackson (1965) by Pal and Durge (1987), and the vermiculite content
ranged from 5 to 9% in the fine clay of vertisols. Pal and Durge (1987)
concluded that the observed K adsorption by the silt and clay fractions
is due to the presence of vermiculite and not to smectite. The smectites
of vertisol clays belong to the low-charge dioctahedral type, and thus,
they expand beyond 1.0 to 1.4 nm with the glycolation of K- saturation
and heating the samples at 300 °C (Fig. 6). These smectites, when treat-
ed according to the alkylammonium method (Lagaly, 1994), showed
the presence of both monolayer to bilayer and bilayer to pseudotrilayer
transitions. The layer charge of the half-unit cell of smectite ranges from
0.28 to 0.78 mol (−)/(SiAl)
4
O
10
(OH)
2
, and the low-charge smectite
constitutes >70% in them (Ray et al., 2003). The position of the higher
charge with 0.78 units or lower appears to be due to the presence of
small amounts of vermiculite as determined quantitatively by Pal and
Durge (1987). The limited leaching in vertisols and small amount of ver-
miculitewould lessen the rate of addedK-fertilisers whenrequired. If K-
fertilisers are added as a basal dose, the K
+
ions would be fixed in the
interlayer of vermiculite, which would make the NH
4
ions from N-
fertilisers more labile for ready assimilation by growing plants if not
added as a basal dose. In addition, ammonium retention by low charge
smectites is expected to be low, and thus, the addition of K may not
cause a reduction in crop yield, as experienced elsewhere with high-
charge smectite (Chen et al., 1989).
Table 5
Cumulative K release from a representative Vertisol and its size fractions.
Horizon depth (cm) Fine earth (b2 mm)
cumulative K release in
75 extractions
Sand (2–0.05 mm)
cumulative K release in
10 extractions
Silt (0.05–0.002 mm)
cumulative K release in
35 extractions
Clay (b0.002 mm) cumulative
K release in 60 extractions
SF
a
MB BB SF MB BB SF MB BB SF MB BB
mg K 100g
−1
Ap 0–15 69 429 6059 20 272 7000 16 191 7004 114 561 6990
Bw1 15–41 41 277 4230 12 162 7006 15 195 7009 92 509 6998
Bw2 41–70 39 267 4097 23 297 6997 13 184 7011 88 502 6999
Bss1 70–95 45 261 4638 15 191 6986 14 161 6990 91 433 7000
Bss2 95–135 49 286 4793 24 334 6991 15 162 6991 92 462 6999
Bss3 135–155 37 235 3849 13 147 6907 16 184 7008 94 471 6984
Adapted from Pal et al. (2006c).
a
SF = on the basis of size fraction; MB = on the basis of mica content; BB = on the basis of biotite content.
Table 6
Co-efficient of correlation among various soil characteristics.
Parameter r
Cumulative K of sand Total K in sand 0.635⁎⁎
Cumulative K of silt Total K in silt 0.771⁎⁎
Cumulative K of clay Total K in clay 0.822⁎⁎
Total K in sand Sand mica 0.933⁎⁎
Total K in silt Silt mica 0.766⁎⁎
Total K in clay Clay mica 0.981⁎⁎
Total K in soil Soil mica 0.979⁎⁎
Cumulative K of sand Sand mica 0.524⁎
Cumulative K of silt Silt mica 0.694⁎⁎
Cumulative K of clay Clay mica 0.851⁎⁎
Cumulative K of soil Soil mica 0.429⁎
Cumulative K of sand mica Sand biotite 0.894⁎⁎
Cumulative K of silt mica Silt biotite 0.917⁎⁎
Cumulative K of clay mica Clay biotite 0.978⁎⁎
Cumulative K of soil mica Soil biotite 0.435⁎
Adapted from Pal et al. (2006c).
⁎Significant at 0.05 level.
⁎⁎ Significant at 0.01 level.
45D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
5.7. Role of soil modifiers (zeolites, gypsum and CaCO3) in making
vertisols productive in adverse climatic environments
5.7.1. Ca-zeolites
During the last 3 decades, zeolite minerals have been recognised with
increasing regularity as common constituents of Cenozoic volcanogenic
sedimentary rocks and altered pyroclastic rocks (Ming and Mumpton,
1989). Zeolites have also been reported as secondary minerals in the
Deccan flood basalt of the Western Ghats in the state of Maharashtra,
India (Jeffery et al., 1988; Sabale and Vishwakarma, 1996). Among the
commonly occurring species of zeolites, heulandites have a wide occur-
rence both in time and space (Sabale and Vishwakarma, 1996). Zeolites
have the ability to hydrate and dehydrate reversibly and to exchange
some of their constituent cations. Consequently, they can influence the
pedochemical environment during the formation of soils. The signifi-
cance of zeolites has recently been realised in the formation and persis-
tence of slightly acidic to acidic vertisols (Typic Haplusterts) in HT
climatic environments, not only in central and western India
(Bhattacharyya et al., 2005), but elsewhere (Ahmad, 1983). Zeolites
can indeed provide sufficient bases to prevent the transformation of
smectite to kaolinite, thus making the formation and persistence of ver-
tisolspossibleeveninahumidtropicalclimate(Bhattacharyya et al.,
2005), as zeolites can maintain the base saturation of soils well above
50% (Bhattacharyya et al., 1993; Pal et al., 2006b). The formation and per-
sistence of vertisols in the Western Ghats over millions of years
(Bhattacharyya et al., 1993; Pal et al., 2009c) has provided a unique ex-
ample that in an open system such as soil, the existence of a steady
state appears to be a more meaningful concept than equilibrium, in a rig-
orous thermodynamic sense (Smeck et al., 1983). The knowledge gained
on the role of zeolites in the persistence of vertisols not only provides a
deductive check on inductive reasoning regarding the formation of soil
in the humid tropical c limate (Chesworth, 1973, 1980),butitalsothrows
light on the role of these minerals in preventing the loss of soil productiv-
ity even in an intense leaching environment. This indeed may be the rea-
son why crops do not show a response to liming in the acid soils of the
tropical Western Ghats (Kadrekar, 1979).
Many productive vertisols under rain‐fed conditions have been ren-
dered unproductive for agriculture under irrigated conditions in the
longer-term. However, some zeolitic vertisols of the SAD parts of west-
ern India have been irrigatedthrough canals for the last twenty years to
produce sugarcane. These soils lack salt-efflorescence on the surface
and are not waterlogged at present, suggesting that these soils are not
degraded due to their better drainage. However, these soils are now
Sodic Haplusterts in view of their pH, ECe and ESP values, but they
have sHC>10 mm hr
−1
(weighted mean in the 0–100 cm, Pal et al.,
2011). A constant supply of Ca
2+
ions from Ca-zeolites in these soils
most likely helps maintain a better drainage system. Because of such
natural endowment with a soil modifier, no ill effects of high ESP
(>15) in crop production in the vertisols of Gezira in Sudan (El
Abedine et al., 1969; Robinson, 1971) and in Tanzania (Ahmad, 1996)
were observed. In addition,some vertisols of the AD climate of western
India produce deeply rooted crops such as cotton under rain fed condi-
tions comparable to those of the Typic Haplusterts of the SAM climate of
central India. The sHC (weighted mean, 0–100 cm) of these soils is
>15mm hr
−1
, despite being Sodic Calciusterts(Pal et al., 2009c). How-
ever, the sustainability of crop productivity in the dry climate depends
on the solubility and supply of Ca
2+
ions from zeolites such that it is suf-
ficient to overcome the ill effects of the pedogenic threshold of dry cli-
mates (Pal et al., 2003b, 2009c). Such situations are unique in nature
and pose a great challenge to soil mappers to classify them as per the
US Soil Taxonomy when they have good productive potential despite
being sodic in nature.
5.7.2. Gypsum
Arid and semi-arid environments trigger natural soil degradation
processes in terms of the precipitation of CaCO
3
and the concomitant
development of sodicity (Pal et al., 2000b). Despite this possibility, se-
lected vertisols of the SAD climate in southern India are non-sodic and
support the growth of crops such as cotton, pigeonpea and sorghum.
The development of sodicity has been prevented by the presence of
gypsum in these soils, but the soils are calcareous in nature. The soils
have an sHC >30 mm hr
−1
despite the rapid formation of PC, unlike
in the zeolitic vertisols of the SAD climate (Pal et al., 2009c). This can
be attributed to the greater solubility of gypsum (30 me/L) than that
of Ca-zeolites (b0.1 me/L) in distilled water (Pal et al., 2006b). The gyp-
sum in such soils is antagonistic to the formation of more soluble salts in
soils, as it prevents clay dispersion. Although the sustainability of crop
productivity in these soils depends on the gypsum stock, the present
poor productivity of cotton (approximately 2 t ha
−1
)maybeenhanced
by irrigation because the gypsum present would prevent water logging
because of better drainage (Pal et al., 2009b).
5.7.3. CaCO
3
The subsoil sodicity impairs the hydraulic properties of the vertisols
of SAT environments, and this leads to the formation of sodic soils with
ESP decreasing with depth. These soils are impoverished in organic car-
bon but have become enriched with CaCO
3
with poor sHC (Pal et al.,
2009c). However, such soils show enough resilience under the im-
proved management (IM) (catchment management followed by
adopting legume-based crop rotation, improved nutrient management
and without any chemical amendments) system of ICRISAT,
implemented in Patancheru, India. Through the implementation of
such practices, a substantial increase in soil organic carbon (SOC)
stock was observed (Wani et al., 2003). The resilience of such soils has
been maintained by implementing IM practices in vertisols. The in-
crease in SOC is, however, related to chemical changes after the speci-
fied management interventions. In vertisols (Sodic Haplusterts, Pal et
al., 2011a) after 30 years of IM, the weighted mean (WM) of sHC in
the first 100 cm of the profile increased by almost 2.5 times due to the
reduction of ESP through the dissolution of CaCO
3
, making the soils
more permeable to air and water. In the last 24 years (since 1977),
the rate of dissolution of CaCO
3
has been 21 mg yr
−1
in the first
100 cm of the profile. Dissolved Ca
2+
ions improve the Ca/Mg ratio on
the exchange complex of soils under IM compared with those under
traditional management (Pal et al., 2011a).
The changes in soil properties, as stated above, suggest that CaCO
3
is
dissolved through the cations of acidic root exudates and carbonic acid
that formed due to evolved CO
2
fromtherootrespirationinanaqueous
solution, resulting in the formation of Ca (HCO
3
)
2
.ThesolubleCa
(HCO
3
)
2
, therefore, helps restore both the soluble and exchangeable Ca
ions in the soils. The ESP decreases and the soil structure improves; as
a result, the hydraulic properties of soils are improved. This improve-
ment in soil properties highlights the role of CaCO
3
, which remains
chemically inert (Pal et al., 2000b)duringitssequestration(Sahrawat,
2003) but acts as a soil modifier during the amelioration of degraded
soils. The improvements in soil properties are also reflected in the classi-
fication of vertisols. The original Kasireddipalli soils (Sodic Haplusterts)
now qualify as Typic Haplusterts (Pal et al., 2011a).
6. Concluding remarks
Vertisols are used for the production of various agricultural crops
under both irrigation and rain-fed conditions. Agronomic practices for
growing crops under irrigation do not cause soil degradation in HT, and
crops grow well in SAD and AD climates due to the presence of soil mod-
ifiers. Without soil modifiers, however, the soils degrade under irrigation
in SAT environments and lose their productivity. Non-sodic vertisols
(Typic Haplusterts) with minerals such as palygorskite have severe
drainage problems, like the non-zeolitic Aridic Haplusterts, even with
an ESP≥5butb15. Zeolitic Sodic Haplusterts have no drainage problem
and are productive like Typic Haplusterts at present. These agricultural
land uses clearly highlight that even though vertisols make up a
46 D.K. Pal et al. / Geoderma 189-190 (2012) 28–49

Author's personal copy
relatively homogeneous major soil group, they show a considerable var-
iability in their land use and crop productivity. It is therefore important
to understand the factors that cause the variability in their properties.
A synthesis of recent developments in the pedology of vertisols achieved
through the use of high resolution micro-morphology, mineralogy, and
age control data along with their geomorphologic and climatic history,
has helped us better understand the effects of pedogenetic processes
due to changes in climate during the Holocene in modifying the soil
properties in the presence or absence of soil modifiers (Ca-zeolites and
gypsum), CaCO
3
and palygorskite minerals. The state-of-the-art informa-
tion developed through this review has helped establish an organic link
between pedogenetic processes and bulk soil properties and has provid-
ed a better understanding of many pedological and edaphological issues
related to vertisols. It is hoped that this review will serve as a handbook
to assess the health and quality of vertisols while developing suitable
management practices to enhance and sustain their productivity. How-
ever, much of the success of the management interventions still depends
on the proper classification of vertisols at the subgroup level, identifying
the impairment of drainage in Aridic Haplusterts (ESP ≥5, b15), Typic
Haplusterts (with palygorskite) and the improvement of drainage in
Sodic Haplusterts/Sodic Calciusterts with soil modifiers. At present, the
vertisols of the SAT environments are less intensively cultivated because
of their inherent limitations, despite that they represent a productive re-
source under improved management. Therefore, areas dominated by
vertisols require immediate national attention so that they can be used
judiciously to produce more food required for the populous Indian sub-
continent and other countries in the developing world.
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