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Minimum threshold value of smectite for vertic properties.

Additional keywords: Mineralogy class, taxonomic rationale.

Introduction

It is well documented that vertic properties of soils are primarily regulated by the nature of clay minerals, particularly their surface properties (Anon. 1975, 1998). Although soils containing all other clays shrink and swell with changes in moisture content, changes are particularly extreme in smectites (Borchardt 1989). Despite this, non-smectitic and mixed mineralogy classes are recognised in Vertisols and their intergrades at the family level in Soil Taxonomy (Anon. 1975, 1994, 1998). This basic contradiction has led soil scientists to express concern about mixed (Smith 1986) and kaolinitic mineralogy classes among shrink-swell soils (Eswaran et al. 1988). It was earlier proposed in Soil Taxonomy (Anon. 1975, 1994) that the montmorillonitic mineralogy class comprised soils with vertic properties when smectite content exceeds 50% of total mineral suite in clay fractions ([is less than] 2 [micro]m). In the revised version, Soil Taxonomy (Anon. 1998) proposed a qualitative smectitic mineralogy class for soils that have more smectite by weight than any other single kind of clay mineral. It thus indicates that there is a scope for a quantitative estimate of smectites for such soils.

From a comprehensive study on the correlation between vertic properties and type of clay minerals, it recently has been indicated (Bhattacharyya et al. 1997) that vertic properties of soils can only be a function of smectite content, even though its content is small, and cannot be induced by kaolinite, despite its presence in large amounts. This indicated a need to determine the minimum threshold value of smectite in the soil control section (SCS) for the manifestation of vertic properties in order to remove the existing ambiguity in the mixed mineralogy class. To work out a minimum threshold value of smectite in shrink-swell soils, qualitative and quantitative mineralogy of the clays of Vertisols and their intergrades spread in different climatic and geographical regions of India have been investigated in the present study.

Materials and methods

Soils

Eight soils representative of Vertisols and vertic intergrades of Inceptisols and Alfisols of India were selected and examined following the standard method (Anon. 1951). These soils occur extensively in the States of Maharashtra (Pedons 1, 2, 3, 4, and 7), Madhya Pradesh (Pedons 6 and 8), and Assam (Pedon 5). Pedons 1 and 3 are Alfisols, Pedons 7 and 8 are Vertisols, and Pedons 3, 4, and 5 are Inceptisols (Table 1).

Table 1. Soil site characteristics of the pedons MAT, mean annual temperature; MAR, mean annual rainfall; MSL, mean sea level
Geology Parent material Physiography Ecological
 region

 Pedon 1--Jambori series--Typic Hapludalf;
 Village, Jambori; Tahsil(A), Ambegaon;
 District, Pune; State, Maharashtra

Deccan basalt Basalt alluvium Plateau top Tropical humid

 Pedon 2 Jambori series Typic Hapludalf: Village,
 Jambori; Tahsil(A), Ambegaon; District,
 Pune; State, Maharashtra

Deccan basalt Basalt-alluvium Plateau top Tropical humid

 Pedon 3--Selu series Vertic Ustochrept:
 Village, Tamasvada; Tahsil(A), Selu; District,
 Wardha; State, Maharashtra

Deccan basalt Basalt-alluvium/ Narrow Tropical
 colluvium entrenched subhumid
 valley

 Pedon 4--Pokhari series--Vertic Eutropept:
 Village, Pokhari; Tahsil(A), Ambegaon;
 District, Pune; State, Maharashtra

Deccan basalt Basalt-alluvium Micro Tropical humid
 depression on
 plateau

 Pedon 5--Sholmarigaon series-Vertic
 Ustochrept: Village, Sholmarigaon; District,
 Morigaon; State, Assam

Alluvium Brahmaputra Flood plain Tropical humid
 alluvium

 Pedon 6--Katur series Vertic Haplustalf:
 Village, Vilayatkalan; District, Katni;
 State, Madhya Pradesh

Sandstone Alluvium Plain land Tropical
 subhumid

 Pedon 7--Aroli series--Typic Haplustert: Village,
 Aroli; Tahsil(A), Ramtek; District, Nagpur;
 State, Maharashtra

Deccan basalt Basalt-alluvium Piedmont plain Tropical
 subhumid

 Pedon 8--Sarol series--Typic Haplustert:
 Village, Binjalai; Tahsil(A), Indore; District,
 Indore; State, Madhya Pradesh

Deccan basalt Basalt-alluvium Piedmont plain Tropical
 subhumid

Geology MAT MAR MSL
 ([degrees]C) (mm) (m)

 Pedon 1--Jambori series--Typic Hapludalf;
 Village, Jambori; Tahsil(A), Ambegaon;
 District, Pune; State, Maharashtra

Deccan basalt 25.5 >5000 1100

 Pedon 2 Jambori series Typic Hapludalf: Village,
 Jambori; Tahsil(A), Ambegaon; District,
 Pune; State, Maharashtra

Deccan basalt 25.5 >5000 1100

 Pedon 3--Selu series Vertic Ustochrept:
 Village, Tamasvada; Tahsil(A), Selu; District,
 Wardha; State, Maharashtra

Deccan basalt 28.6 982 300

 Pedon 4--Pokhari series--Vertic Eutropept:
 Village, Pokhari; Tahsil(A), Ambegaon;
 District, Pune; State, Maharashtra

Deccan basalt 25.5 >5000 1000

 Pedon 5--Sholmarigaon series-Vertic
 Ustochrept: Village, Sholmarigaon; District,
 Morigaon; State, Assam

Alluvium 24.9 1860 60

 Pedon 6--Katur series Vertic Haplustalf:
 Village, Vilayatkalan; District, Katni;
 State, Madhya Pradesh

Sandstone 25.0 1250 500

 Pedon 7--Aroli series--Typic Haplustert: Village,
 Aroli; Tahsil(A), Ramtek; District, Nagpur;
 State, Maharashtra

Deccan basalt 26.9 1127 340

 Pedon 8--Sarol series--Typic Haplustert:
 Village, Binjalai; Tahsil(A), Indore; District,
 Indore; State, Madhya Pradesh

Deccan basalt 24.4 1050 560

Geology Natural vegetation
 and land use

 Pedon 1--Jambori series--Typic Hapludalf;
 Village, Jambori; Tahsil(A), Ambegaon;
 District, Pune; State, Maharashtra

Deccan basalt Neem (Azadirachta indica),
 mango (Mangifera indica),
 and millets

 Pedon 2 Jambori series Typic Hapludalf: Village,
 Jambori; Tahsil(A), Ambegaon; District,
 Pune; State, Maharashtra

Deccan basalt Neem (Azadirachta indica), mango (Mangifera
 indica), and millets

 Pedon 3--Selu series Vertic Ustochrept:
 Village, Tamasvada; Tahsil(A), Selu; District,
 Wardha; State, Maharashtra

Deccan basalt Babul (Acacia arabica), ber (Ziziphusjujuba),
 palas (Buteafrondosa), sorghum, pigeonpea,
 wheat, gram, and linseed

 Pedon 4--Pokhari series--Vertic Eutropept:
 Village, Pokhari; Tahsil(A), Ambegaon;
 District, Pune; State, Maharashtra

Deccan basalt Neem (Azadirachta indica), mango (Mangifera
 indica), paddy, wheat, and millets

 Pedon 5--Sholmarigaon series-Vertic
 Ustochrept: Village, Sholmarigaon; District,
 Morigaon; State, Assam

Alluvium Sissoo (Dalbergia sissoo), bamboo
 (Dendrocalamas spp), grasses, and paddy

 Pedon 6--Katur series Vertic Haplustalf:
 Village, Vilayatkalan; District, Katni;
 State, Madhya Pradesh

Sandstone Palas (Buteafrondosa), paddy, and millets

 Pedon 7--Aroli series--Typic Haplustert: Village,
 Aroli; Tahsil(A), Ramtek; District, Nagpur;
 State, Maharashtra

Deccan basalt Grasses and shrubs

 Pedon 8--Sarol series--Typic Haplustert:
 Village, Binjalai; Tahsil(A), Indore; District,
 Indore; State, Madhya Pradesh

Deccan basalt Babul (Acacia arabica), ber (Ziziphusjujuba),
 palas (Buteafrondosa), gram, and linseed


(A) Tahsil is equivalent of a district.

Pedons 1 and 2 are red soils with acidic reaction. Similar soils have been grouped under the kaolinitic mineralogy class (Anon. 1975) according to their clay cation exchange capacity (CEC), even though the clay is dominated by chloritised smectite-kaolin interstratified mineral (Bhattacharyya et al. 1993, 1997). These soils are spatially distributed in the landscape in association with Vertisols and/or vertic intergrades of Inceptisols. Besides, these red soils do not manifest vertic properties in spite of containing little smectite and can, therefore, be used as reference soils to decide the boundary conditions between vertic and non-vertic characteristics.

Pedon 1 (red soil) consists of a 9-cm-thick Ap horizon reddish brown in colour (5YR4/4) with moderate, medium, and subangular blocky structures. The Bt horizon is 98 cm thick, dark reddish brown (5YR3/3), with weak-to-strong medium subangular blocky structures. These soils are clayey and noncalcareous showing clay cutans throughout the Bt horizon.

Pedon 2 (red soil) has an Ap horizon about 14 cm in thickness and a Bt horizon about 132 cm thick. Both the horizons are dark reddish brown (2.5YR3/4), with moderate medium and subangular blocky structure. The soil has clay cutans in the Bt1 horizon from a depth of 14 cm onwards. Texturally, the soils are clayey and noncalcareous.

Pedon 3 (black soil) consists of the Ap horizon (19 cm thick), Bw horizon (46 cm thick), and Bk horizon (25 cm thick). These soils are clayey and dark greyish brown (10YR3/2) with moderate, medium subangular blocky structures showing pressure faces in the Bw2 and Bk horizons. Surface cracks ([is greater than] 5 mm) do not extend beyond the Ap horizon. These soils are moderately alkaline and calcareous throughout the solum depth.

The Ap horizon of Pedon 4 (black soil) is 16 cm thick with a dark brown colour (10YR3/3), and moderate, medium subangular blocky structures. The Bw horizon is about 246 cm thick and is characteristically dark greyish brown to very dark greyish brown (10YR3/2-10YR3/3). These soils have typical subangular and angular blocky structures of black soils showing pressure faces; they are slightly acidic and noncalcareous throughout the solum.

Pedon 5 (black soil) has a 20-cm-thick Ap horizon which is very dark greyish brown (10YR3/2) in the Apl horizon and very dark grey (10YR3/1) in the Ap2 horizon with moderate, medium subangular blocky structure to coarse, strong angular blocky structure. The B horizon is 153 cm thick and is dark grey (10YR4/1) in the Bwg1 and Bwg2 horizons, dark yellowish brown (10YR4/4) in the Bw horizon, and grey (10YR6/1) in the Bwg horizon with moderate, medium subangular blocky structure. Texturally, the Bwg1 horizon is clayey, the Bwg2 and Bw horizons are silty clay, and the Bwg horizon is silty clay loam. The soils are almost neutral in reaction and non-calcareous throughout the solum.

Pedon 6 (black soil) has a 15-cm-thick Ap horizon which is dark brown (10YR4/3), with moderate, medium subangular blocky structure and loamy texture. The Bt horizon is 135 cm thick, dark brown (10YR3/3), with moderate, very coarse columnar structures that break into moderate, medium, subangular blocky structures, and broken and thin clay cutans. These soils are clayey, neutral in reaction, and non-calcareous throughout the solum.

Pedon 7 (black soil) has a 16-cm-thick Ap horizon which is very dark greyish brown (10YR3/2), with moderate, medium subangular blocky structures and clayey texture. The AB horizon is 29 cm thick with moderate, coarse prismatic-to-coarse subangular blocky structures. The B horizon is 129 cm thick, of which the first 93 cm is represented by the Bss horizon. The slickensides present in the Bss horizons are sufficiently close to intersect, and are characterised by strong, coarse angular blocky structures. The lower part of the B horizon does not show the development of slickensides. The Bss horizon is 93 cm thick with dark to very dark greyish brown colour (10YR4/2-10YR3/2). This horizon is very dark greyish brown (10YR3/2), with medium-to-strong, coarse-to-medium, subangular-to-angular blocky structures. The soils are clayey, slightly to moderately alkaline, and calcareous.

Pedon 8 (black soil) consists of the Ap horizon (20 cm thick), B horizon (Bw, 30 cm thick; Bss, 130 cm thick), and BC horizon (20 cm thick). Both the Ap and B horizons are very dark greyish brown (2.5Y3/2), with moderate, medium subangular blocky structures; shiny pressure faces are present in the B2 horizon. The Bss horizon is characterised by intersecting slickensides which break into strong, coarse angular blocky structures. The BC horizon is very dark greyish brown (2.5Y3.5/2) with strong, coarse angular blocky structures. These soils are clayey, slightly alkaline, and calcareous throughout the solum.

Analytical techniques

The international pipette method was applied for particle size analysis after the sand, silt, and clay fractions were separated according to the procedure of Jackson (1979). The coefficient of linear extensibility (COLE) was determined according to Schafer and Singer (1976). Linear extensibility (LE) was calculated from the COLE value using the formula (Anon. 1975):

LE = 100 x COLE

The fine earth ([is less than] 2 mm) was analysed for pH, calcium carbonate, and CEC (Richards 1954; Jackson 1973).

Oriented clay fractions ([is less than] 2 [micro]m) were subjected to X-ray diffraction (XRD) analysis using a Philips diffractometer and Ni-filtered Cu K[Alpha] radiation and also Fe-filtered Co K[Alpha] radiation (for Pedons 1 and 2) at a scanning speed of 2 [degrees] 2 [Theta]/min. Samples were saturated with Mg/Ca and K, solvated with ethylene glycol, and heated to 110 [degrees] C, 300 [degrees] C, and 550 [degrees] C. Minerals present in the clay fractions of 8 soils were identified following the criteria of Jackson (1979). The presence of chloritised smectite-kaolin interstratified mineral (Sm/Ka) was identified by a slight shift and tailing of the 0.72 nm peak on glycolation, and gradual reinforcement and/or broadening at the base of the 1.0 nm peak, with a corresponding decrease in the 0.72 nm peak intensity on K saturation and subsequent heating (110-550 [degrees] C) (Bhattacharyya et al. 1993, 1997). Semi-quantitative estimates of clay minerals were made by the method of Gjems (1967). Quantitative estimates of smectite in the clay fractions were performed according to the procedure of Alexiades and Jackson (1965). The values of LE, clay CEC, and smectite content are expressed on the basis of SCS, which is defined by a depth of 25 cm to (i) a lithic contact or paralithic contact if it is within a depth of 1 m; or (ii) a depth of 1 m if the regolith is [is greater than] 1 m thick (Anon. 1975).

Results and discussion

General properties of the soils

Soils with vertic properties had an electrical conductivity [is less than] 1 dS/m, indicating their nonsaline character. The soil reaction ranged from acidic to alkaline in both calcareous and non-calcareous groups of these 8 soil profiles (Table 2). All soils were clayey, with fine clay ([is less than] 0.2 [micro]m) dominating the clay fractions (Table 2). The LE values indicated that, despite the fine textures, these soils do not exhibit shrink-swell phenomena equally. With the exception of the red soils, other soils had LE [is greater than] 6, the limit set (Anon. 1998) for Vertisols and their intergrades.

[TABULAR DATA 2 NOT REPRODUCIBLE IN ASCII]

Mineralogy of the soils

XRD examination of the silt fractions (50-2 [micro]g) of the 8 soils indicated the presence of kaolin, mica, chlorite, and vermiculite. Smectite was not detected in these fractions (XRD patterns not shown).

XRD examination of the clay fractions ([is less than] 2 [micro]g) of red soils (Pedons 1 and 2) indicated that Sm/Ka was dominant in the clay, together with mica, with minor amounts of chloritised smectite, quartz, and feldspar (Fig. 1). Although these soils were dominated by Sm/Ka ([is greater than] 50%), the clay CEC data in the SCS [[is less than or equal to] 24 cmol(+)/kg; Table 2] were indicative of the kaolinitic mineralogy class (Smith 1986) and subactive CEC activity class (Anon. 1998). Despite the presence of smectite as discrete mineral, and also as a component in Sm/Ka (Fig. 1), these soils did not manifest vertic properties (LE [is less than] 6; Table 2).

[Figure 1 ILLUSTRATION OMITTED]

Clay minerals of black soils developed on Deccan basalts (Pedons 3, 7, and 8) were composed primarily of smectite ([is greater than] 50%), with small amounts of kaolin, chlorite, vermiculite, mica, and quartz (Fig. 2). The smectite was reasonably ordered as it yielded a sharp basal reflection on glycolation and displayed a regular series of short and broad higher order reflections (Fig. 2). The clay CECs of these soils in the SCS [[is greater than] 85 cmol(+)/kg; Table 2] were indicative of the superactive class (Anon. 1998) and thus the clays were part of the montmorillonitic mineralogy class (Smith 1986). The presence of the smectite mineral thus justified very high LE values (LE [is greater than] 15; Table 2).

[Figure 2 ILLUSTRATION OMITTED]

The clay fractions of other black soils (Pedons 4, 5, and 6) were dominated by Sm/Ka and mica ([is greater than] 50%), with subordinate amounts of chloritised smectite, vermiculite, and quartz (Fig. 3). The peak value of Sm/Ka (0.7 nm) was sharper and not as broad at the base compared with Sm/K peaks of Pedons 1 and 2. The clay CECs in the SCS [[is greater than] 40 cmol(+)/kg; Table 2] indicated that these soils were part of the active CEC class (Anon. 1998) and mixed mineralogy class (Smith 1986). Thus, the soils showed moderate LE values ([is greater than or equal to] 7), which were sufficient for them to qualify as vertic. This apparently may justify the provision of mixed mineralogy class for shrink-swell soils (Anon. 1975). However, this is not tenable in view of the data obtained and discussed later.

[Figure 3 ILLUSTRATION OMITTED]

Quantitative determination of smectite

Quantitative determination of minerals present in the clay fractions of soils by XRD analysis is difficult to perform with comparable precision for all mineral components simultaneously. Any attempt in this regard has always yielded semi-quantitative estimates (Gjems 1967). Even such estimation becomes questionable for a mineral when it is interstratified with other minerals.

The presence of smectite-kaolin interstratified minerals (Sm/Ka) in shrink-swell soils is common in India (Pal et al. 1989; Bhattacharyya et al. 1993, 1997) and elsewhere (Wilson and Cradwick 1972; Herbillon et al. 1981; Norrish and Pickering 1983; Churchman et al. 1994; Delvaux and Herbillon 1995). The peak shift analysis (Wilson 1987) has been found to be a useful method to determine the smectite content in Sm/Ka (Bhattacharyya et al. 1993, 1997). When the smectite component in Sm/Ka is highly chloritised, the swelling of smectites on glycolation is restricted, making peak shift analysis ineffective. The chemical method of Alexiades and Jackson (1965) for the quantitative determination of smectite in soil clays has been found to be effective in circumventing this problem, particularly when attempting to establish a link between bulk soil properties and clay mineral type (Pal and Durge 1987, 1989). In view of the objective of the present study to establish a link between vertic properties and the amount of swelling mineral content, soil clays of both red and black soils were assessed by the chemical method (Alexiades and Jackson 1965) to obtain the quantitative value of smectite (Table 2).

Smectite content of soil clays of Pedon 1 varied from 13% to 22% in the solum and 19% in the SCS, whereas smectite content of soil clays of Pedon 2 varied from 12% to 16% in the solum and 15% in the SCS (Table 2). Smectite content in soil clays of black soils ranged from 26% to 70% in SCSs (Table 2).

Establishment of a minimum threshold value of smectite

In the present study, a highly significant positive correlation (r = 0.98 at P = 0.01) was obtained between smectite content and LE (Fig. 4a) when values of each soil horizon of 8 pedons were considered. This indicated that the magnitude of the shrink-swell phenomena is primarily regulated by the smectitic clay mineral. Furthermore, the correlation between these 2 parameters on a SCS basis was also highly significant (r = 0.99 at P = 0.01). The regression equation (Fig. 4b) yielded a value of 20% for smectite content (LE = 6) in the SCS. It is stipulated in Soil Taxonomy (Anon. 1998) that the minimum value of LE is 6 for soils to be classified as vertic. This means that soils with vertic properties must have a smectite content [is greater than or equal to] 20% in SCSs. The soils with vertic properties studied here, particularly those with vertic intergrades (Pedons 4, 5, and 6), do have a smectite content [is greater than or equal to] 20% in SCSs. These soils had a mixed mineralogy class based on both XRD and clay CEC data. If the mineralogy class for these soils is considered to be mixed, the inherent relationship between truly expanding minerals and shrink-swell properties that essentially reflects the genesis of vertic properties (Fig. 4a and b) will be highly undermined. Moreover, if non-expanding types of minerals such as kaolins, micas, chlorites, and vermiculites are considered to be minerals that do not expand on solvation, it is difficult to reconcile their non-expanding characteristic with the shrink-swell properties. Usually, shrink-swell phenomena are positively correlated with the content of expansible mineral (Franzmeier and Ross 1968; Karathanasis and Hajek 1985; Smith et al. 1985), as indicated by high COLE values and clay contents dominated by minerals of the smectite group. Bhattacharyya et al. (1997) suggested that deviation from this fundamental fact may still occur if a minimum threshold value of smectite content in the SCS is not established. It was proposed in Soil Taxonomy (Anon. 1994) that soils with vertic properties in which smectite content (semi-quantitative data) was [is greater than] 50% were part of the montmorillonitic mineralogy class. Later, this criterion was revised in Soil Taxonomy (Anon. 1998) and it was proposed that soils with a higher smectite content (by weight) than any other single type of clay mineral were part of a qualitative smectitic mineralogy class. The present study, however, provides a quantitative amount of smectite in the SCS as a minimum threshold value for manifestation of vertic properties in soils. Therefore, for shrink-swell soils, the mineralogy class should be only smectitic. A realistic mineralogy class for Oxisols has already been recognised in Soil Taxonomy (Anon. 1975, 1998) in view of their highly advanced stage of pedogenic development.

[Figure 4 ILLUSTRATION OMITTED]

Conclusions

The results of the present study demonstrated that the vertic properties were manifested in soils from a group of 8 soils from different soil taxonomic orders in India when they had a smectite content of 20% (as defined by Alexiades and Jackson 1965) in the SCSs. The correlation between smectite content and LE showed that LE [is greater than] 6 corresponded to a smectite content [is greater than] 20%. Thus, the smectitic mineralogy class appears to be a realistic proposition for shrink-swell soils only.

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Manuscript received 14 May 1999, accepted 17 September 1999

S. K. Shirsath(A), T. Bhattacharyya(A), and D. K. Pal(AB)

(A) Division of Soil Resource Studies, National Bureau of Soil Survey and Land Use Planning, Amravati Road, Nagpur 440 010, India.

(B) Corresponding author; email: dkpal@nbsslup.mah.nic.in
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Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Date:Jan 1, 2000
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