8
Genesis and Classification of Clay Soils with Vertic Properties in Saskatchewan 1 G. S. DASOG, D. F. ACTON, AND A. R. MERMur 2 ABSTRACT Clayey soils formed in glaciolacustrine sediments in semiarid and subhumid regions of Saskatchewan were studied to understand their genesis and to examine their classification according to the amended definition of Vertisols. Although slickensides were common to all soils, semiarid soils displayed weak horizons, whereas subhumid soils had argillic horizons and prominent mollic epipedons. All the soils had high potential to swell and shrink, and were characterized by seasonal moisture deficits sufficient to induce cracking. Soil dis- placement, as evidenced by slickensides, was attributed to differ- ential wetting in the subsoil. Pedoturbation, as a result of such dis- placement, has been sufficient to prevent strong horizon formation in semiarid soils but insufficient to counter leaching and illuviation in subhumid soils. Blocky and prismatic structure, when dry, and microfabric considerations support a B horizon designation in these soils, which otherwise exhibit indistinct horizons when moist. It is suggested that Vertisols with frigid and cryic soil temperature re- gimes should be grouped into a new suborder, Borerts. It is also suggested that soils that have diagnostic horizons other than mollic be excluded from Vertisols even when accompanied by intersecting slickensides. Additional Index Words: Vertisols, Borerts, Grumusolic soils, soil mechanics model, pedoturbation. Dasog, G.S., D.F. Acton, and A.R. Mermut. 1987. Genesis and clas- sification of clay soils with vertic properties in Saskatchewan. Soil Sci. Soc. Am. J. 51:1243-1250. H EAVY CLAY-TEXTURED SOILS in Saskatchewan oc- cur in glacial lake sediments that extend from semiarid grasslands to humid forests. Their weak de- velopment horizon, cloddy granular surface structure, and atypical nature compared with medium-textured soils were recognized by early workers (Mitchell et al., 1944). Clayton (1963) noted their self-mulching char- acteristics and equated them with Vertic Haplobor- olls. Currently, in Canada, such soils are classified in Rego subgroups of the Chernozemic order (Canada Soil Survey Committee, 1978), where grumic prop- erties are recognized taxonomically at the family level or lower. Recent work by Mermut and St. Arnaud (1983) and Mermut and Acton (1985) suggested that these soils have many characteristics of Vertisols but they failed to meet the climatic requirement of this order (Soil Survey Staff, 1975). Since the temperature criterion for Vertisols was recently waived (Soil Sur- vey Staff, 1982), it was important to establish whether heavy clay soils in Saskatchewan would meet this amended definition. The genesis of highly expansive clay soils in warm temperate and tropical regions is influenced by pe- doturbation and soil instability (Buol et al., 1973; Soil Survey Staff, 1975). Buol et al. (1973) suggested a self- swallowing mechanism wherein the surface soil is ' Contribution no. R508 from Saskatchewan Inst. of Pedology, Univ. of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N OWO; and no. 87-22, Land Resource Research Centre, Agriculture Canada, Ottawa. Research partially funded by NSERC, Canada Grant no. A-0464. Received 18 Aug. 1986. : Graduate Student, Head, Saskatchewan Soil Survey Unit, LRRC, and Adjunct Professor, Dep. of Soil Science, respectively. sloughed off into shrinkage cracks. Upon rewetting, the soil swells but the volume is constrained by the soil in filling the cracks. This results in high pressures, which are relieved by displacing the soil mass to the sides and upwards, forming slickensides, gilgai, and other related features. More recently, Yaalon and Kal- mar (1978) proposed what has been termed a soil me- chanics model (L.P. Wilding, 1984, personal com- munication), whereby unequal wetting from water penetrating to considerable depth along cracks pro- duces horizontal stress in excess of the shear strength of the soil, resulting in failure planes or slickensides. In many Vertisols both processes operate, but the ef- fects on profile development are somewhat different (Ahmad, 1983). Because the swelling pressures due to "self-swallowing" are confined to the depth of profile in which cracking occurs, the vertical component of the swelling pressure might be greater than the hori- zontal component, contributing to heaving and sub- sidence upon wetting and drying, respectively. The objectives of this study were to obtain a greater understanding of the genesis of clay soils in Saskatch- ewan by characterizing morphology, chemical, and physical properties as a basis for evaluating the two most common genetic models, mentioned above, and to examine the classification of these soils in Soil Tax- onomy. MATERIALS AND METHODS Four heavy clay soils belonging to the Sceptre, Regina, Melfort, and Tisdale associations (Mitchell et al., 1944), were selected to represent a gradient in climate and vegetation in Saskatchewan (Fig. 1). They were described in the field, and horizon samples were collected for laboratory characteriza- tion. Also, undisturbed samples were collected in Kubiena- size (8- by 6- by 5-cm) boxes for preparation of thin sections. These samples, after drying, were impregnated with Vesto- pal-150 (Hiils Aktiengesellschaft D-4370, Marl, West Ger- many). Thin sections were described using terminology by Brewer (1976) and Brewer and Pawluk (1975). Argillans, plasma, and masepic fabrics were quantitatively estimated by counting 1000 points at a magnification of 125 X. Illu- viation argillans were counted a second time, and the av- erage of the two counts is reported. Illuviation argillans were distinguished from stress argillans by their sharp boundary with the surrounding matrix. Particle size distribution was determined by the pipette method; liquid and plastic limits by ASTM (1986), and cation exchange capacity (CEC) by BaCU-triethanolamine pH 8. All these methods are de- scribed in Sheldrick (1984). The pH was measured in sat- urated paste and electrical conductivity (EC) in the satura- tion extract. Total C was determined by dry combustion and inorganic C by acid digestion and titration. Organic C was computed as the difference between the total and inorganic C. Coefficient of linear extensibility (COLE) was calculated from clod bulk densities determined when moist (33 kPa) and when oven dry, as outlined in Soil Conservation Service (1972). Width and depth of cracks intercepting a 2-m tape were measured at 20 random locations at the Sceptre and Tisdale sites. Depths were measured by inserting a 2-mm diam steel wire. The average crack spacing was calculated as the quo- tient of the cumulative length of measurements (cm) and the total number of cracks. 1243

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Genesis and Classification of Clay Soils with Vertic Properties in Saskatchewan1

G. S. DASOG, D. F. ACTON, AND A. R. MERMur2

ABSTRACTClayey soils formed in glaciolacustrine sediments in semiarid and

subhumid regions of Saskatchewan were studied to understand theirgenesis and to examine their classification according to the amendeddefinition of Vertisols. Although slickensides were common to allsoils, semiarid soils displayed weak horizons, whereas subhumidsoils had argillic horizons and prominent mollic epipedons. All thesoils had high potential to swell and shrink, and were characterizedby seasonal moisture deficits sufficient to induce cracking. Soil dis-placement, as evidenced by slickensides, was attributed to differ-ential wetting in the subsoil. Pedoturbation, as a result of such dis-placement, has been sufficient to prevent strong horizon formationin semiarid soils but insufficient to counter leaching and illuviationin subhumid soils. Blocky and prismatic structure, when dry, andmicrofabric considerations support a B horizon designation in thesesoils, which otherwise exhibit indistinct horizons when moist. It issuggested that Vertisols with frigid and cryic soil temperature re-gimes should be grouped into a new suborder, Borerts. It is alsosuggested that soils that have diagnostic horizons other than mollicbe excluded from Vertisols even when accompanied by intersectingslickensides.

Additional Index Words: Vertisols, Borerts, Grumusolic soils, soilmechanics model, pedoturbation.

Dasog, G.S., D.F. Acton, and A.R. Mermut. 1987. Genesis and clas-sification of clay soils with vertic properties in Saskatchewan. SoilSci. Soc. Am. J. 51:1243-1250.

HEAVY CLAY-TEXTURED SOILS in Saskatchewan oc-cur in glacial lake sediments that extend from

semiarid grasslands to humid forests. Their weak de-velopment horizon, cloddy granular surface structure,and atypical nature compared with medium-texturedsoils were recognized by early workers (Mitchell et al.,1944). Clayton (1963) noted their self-mulching char-acteristics and equated them with Vertic Haplobor-olls. Currently, in Canada, such soils are classified inRego subgroups of the Chernozemic order (CanadaSoil Survey Committee, 1978), where grumic prop-erties are recognized taxonomically at the family levelor lower. Recent work by Mermut and St. Arnaud(1983) and Mermut and Acton (1985) suggested thatthese soils have many characteristics of Vertisols butthey failed to meet the climatic requirement of thisorder (Soil Survey Staff, 1975). Since the temperaturecriterion for Vertisols was recently waived (Soil Sur-vey Staff, 1982), it was important to establish whetherheavy clay soils in Saskatchewan would meet thisamended definition.

The genesis of highly expansive clay soils in warmtemperate and tropical regions is influenced by pe-doturbation and soil instability (Buol et al., 1973; SoilSurvey Staff, 1975). Buol et al. (1973) suggested a self-swallowing mechanism wherein the surface soil is

' Contribution no. R508 from Saskatchewan Inst. of Pedology,Univ. of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7NOWO; and no. 87-22, Land Resource Research Centre, AgricultureCanada, Ottawa. Research partially funded by NSERC, CanadaGrant no. A-0464. Received 18 Aug. 1986.: Graduate Student, Head, Saskatchewan Soil Survey Unit, LRRC,

and Adjunct Professor, Dep. of Soil Science, respectively.

sloughed off into shrinkage cracks. Upon rewetting,the soil swells but the volume is constrained by thesoil in filling the cracks. This results in high pressures,which are relieved by displacing the soil mass to thesides and upwards, forming slickensides, gilgai, andother related features. More recently, Yaalon and Kal-mar (1978) proposed what has been termed a soil me-chanics model (L.P. Wilding, 1984, personal com-munication), whereby unequal wetting from waterpenetrating to considerable depth along cracks pro-duces horizontal stress in excess of the shear strengthof the soil, resulting in failure planes or slickensides.In many Vertisols both processes operate, but the ef-fects on profile development are somewhat different(Ahmad, 1983). Because the swelling pressures due to"self-swallowing" are confined to the depth of profilein which cracking occurs, the vertical component ofthe swelling pressure might be greater than the hori-zontal component, contributing to heaving and sub-sidence upon wetting and drying, respectively.

The objectives of this study were to obtain a greaterunderstanding of the genesis of clay soils in Saskatch-ewan by characterizing morphology, chemical, andphysical properties as a basis for evaluating the twomost common genetic models, mentioned above, andto examine the classification of these soils in Soil Tax-onomy.

MATERIALS AND METHODSFour heavy clay soils belonging to the Sceptre, Regina,

Melfort, and Tisdale associations (Mitchell et al., 1944), wereselected to represent a gradient in climate and vegetation inSaskatchewan (Fig. 1). They were described in the field, andhorizon samples were collected for laboratory characteriza-tion. Also, undisturbed samples were collected in Kubiena-size (8- by 6- by 5-cm) boxes for preparation of thin sections.These samples, after drying, were impregnated with Vesto-pal-150 (Hiils Aktiengesellschaft D-4370, Marl, West Ger-many). Thin sections were described using terminology byBrewer (1976) and Brewer and Pawluk (1975). Argillans,plasma, and masepic fabrics were quantitatively estimatedby counting 1000 points at a magnification of 125 X. Illu-viation argillans were counted a second time, and the av-erage of the two counts is reported. Illuviation argillans weredistinguished from stress argillans by their sharp boundarywith the surrounding matrix. Particle size distribution wasdetermined by the pipette method; liquid and plastic limitsby ASTM (1986), and cation exchange capacity (CEC) byBaCU-triethanolamine pH 8. All these methods are de-scribed in Sheldrick (1984). The pH was measured in sat-urated paste and electrical conductivity (EC) in the satura-tion extract. Total C was determined by dry combustion andinorganic C by acid digestion and titration. Organic C wascomputed as the difference between the total and inorganicC. Coefficient of linear extensibility (COLE) was calculatedfrom clod bulk densities determined when moist (33 kPa)and when oven dry, as outlined in Soil Conservation Service(1972).

Width and depth of cracks intercepting a 2-m tape weremeasured at 20 random locations at the Sceptre and Tisdalesites. Depths were measured by inserting a 2-mm diam steelwire. The average crack spacing was calculated as the quo-tient of the cumulative length of measurements (cm) andthe total number of cracks.

1243

1244 SOIL SCI. SOC. AM. J., VOL. 51, 1987

Table 1. Site characteristics of the soils.

Soil

SceptreReginaMelfortTisdale

vocation

Lat

50°53'N50°23'N52°50'N52°52'N

Long

109°29W104°34'W104°31W»103 "52 W/

Mean annual

Precipi-tation

1I1I11

345373

406

PEf

731 1683 /503

Soil temperature at 50 cm

Meanannual

a t\

5.8

Meansummer

\j

13.8

Naturalvegetation

Mixed-grass prairieMixed-grass prairieAspen parklandMixedwood-parkland

transition

Year undercultivation(approx.)

1001008025

Slope

%3222

Approx.agei

yr

14000125001100010000

t PI = potential evapotranspiration. t Taken from Christiansen (1979).

Brown2 Dark Brown3 Black4 Dark Gray5 Gray

North DakotaFig. 1. Map showing the site locations and soil zones in Saskatch-

ewan.

RESULTSEnvironmental Conditions

All the soils are formed in fine-textured glaciola-custrine sediments, largely derived from smectite-richmarine shale. Annual moisture deficits (P — PE), re-ported by Coligado et al. (1968), are approximatelythree times greater in Sceptre and Regina than in Mel-fort and Tisdale areas (Table 1). Mean annual soiltemperatures, reported by Treidl (1979), suggest thatthe temperature regime of the Sceptre and Regina soilsis frigid and that of Melfort and Tisdale soils is cryic(Soil Survey Staff, 1975). All the sites have been cul-tivated for at least 25 yr. The parkland and mixed-wood boreal regions have undergone successions oftrees and grasses prior to cultivation (Ritchie, 1976).Topography is nearly level to gently undulating at all

sites and a closed drainage system prevails. The ageof the geomorphic surfaces is assumed to follow thetrend of deglaciation of the area (Christiansen, 1979).

Morphology and MicromorphologyKey morphological features of the soils are pre-

sented in Table 2. Colors of the Sceptre and Reginasoils are uniform throughout the solum. Surface ho-rizons of the Melfort and Tisdale soils are two unitsdarker in value and one unit lower in chroma thanthe C horizons.

The Ap horizons have a weak to moderate blockystructure, breaking to fine granular. Horizons withprominent slickensides are generally accompanied bycoarse to very coarse angular blocky structure. In theSceptre and Regina soils, the structural units are coarseto very coarse, and the horizon boundaries are diffuseand wavy in the upper solum. It is difficult to recog-nize structure and distinguish horizons of these soilswhen moist. The Melfort and Tisdale soils have finerstructural units that are easier to recognize even whenmoist. A thin E horizon underlain by a Bt horizonwith columnar structure and dark-colored clay skinsis present in the Melfort soil. Mottles in the Ap ho-rizon of the Tisdale soil reflect a wetter moisture re-gime of this soil. The BE horizon has mixed black andbrown colors, indicative of disturbance by land clear-ing and subsequent cultivation.

A high content of plasma with a predominantly por-phyroskelic related distribution pattern typifies mosthorizons (Table 3). A mullgranoidic related distribu-tion, characteristic of mollic epipedons, is evident inthe Ap horizons of the Tisdale and Melfort soils. Themasepic and lattisepic plasmic fabrics, indicative ofstress conditions, are predominant in the sola of theSceptre and Regina soils. Masepic fabric in these twosoils is noticeable even in the Ap horizons (Table 4)and is better expressed in the Bw2 horizons (Fig. 2).Vo-masepic fabric occurs in the Bt2 and BCy horizonsin the Melfort soil. In the Tisdale soil, vo-masepic andlattisepic fabric is moderately well expressed in the BEand strongly expressed in the Bt horizon. Unistrialfabric (Fig. 3), characteristic of the sedimentary parentmaterial (Brewer, 1976), is observed in the C and tran-sitional horizons. Its proportion increases with depthwithin the lower part of the profiles (Fig. 4). Ferri-argillans with strong, continuous orientation are ob-served in the Btl horizon of the Melfort soil (Fig. 5).They are also observed in the BE horizon of the Tis-dale soil; however, there are fewer (Table 4) than inthe Melfort soil and the majority of them are incor-porated into the matrix.

DASOG ET AL.: CLAY SOILS WITH VERTIC PROPERTIES IN SASKATCHEWAN 1245

Table 2. Morphological description of the soil.

HorizonDepth,

cm

Munsell color

Dry Moist Texture Structure ConsistenceBound-

ary Special features

SceptreAp (Apk)tBwl (Bmkl)Bw2 (Bmk2)BC (BC1)BCy (BC2)C (Csk)

0-1010-3535-6060-100

100-115115-150

2.5Y 5/22.5Y 4.5/22.5Y 5/22.5Y 5/22.5Y 4/22.5Y 4/2

and2.5Y 6/2

2.5Y 3.5/22.5Y 3.5/32.5Y 3.4/22.5Y 3/22.5Y 3/22.5Y 3/2; 5/3

and10YR 5/3

cccccc

1 c sbkj2csbk2csbk3 c sbk3csbk2 vc sbk

mfrmfimfidvh, mfidvh, mfidvh, mfi

asdwdwgwgw

Moderate to strong slickensidesAs above

Ap (Apk)Bwl (Bmkl)Bw2 (Bmk2)Bw3 (BmkS)BCyl (BCDBCy2 (BC2)BC (BC)C(Ck)

Ap (Ap)E (Ahe)Btl (Bntl)Bt2 (Bnt2)BCy (BC)C (Csk)

Ap (Ap)BE (BA)

Bt (Btj)BCy (BC)C(Ck)

0-2020-4040-6262-8888-113

113-135135-180180-240

0-1717-2020-3838-6565-105

105-140

0-1010-35

35-8080-130

130-165

2.5Y 5/22.5Y 5/22.5Y 5/22.5Y 5/22.5Y 5/22.5Y 5/22.5Y 5/32.5Y 5/3

5Y3/1ND§10YR 3/1ND10YR 5/2

--

5Y4/12.5Y 3/2

10YR 4.5/12.5Y 4/2ND

2.5Y 4/22.5Y 4/22.5Y 3.5/22.5Y 3/22.5Y 3/22.5Y 3/22.5Y 3/22.5Y 3/2

5Y 2.5/110YR 3/210YR 2/110YR 2.5/110YR 4/210YR 4/2

5Y 2/12.5Y 3/2

2.5Y 3/12.5Y 3/22.5Y 4/2

cccccccc

sicsiclcccc

cc

ccc

Regina2-3 c abk3 vc abk3 c abk3 cabk3 cabk3 cabk1 csbk1 csbk

Melfort2 m-c sbk2f-m sbk2 m-c pr1 c pr2-3 vc abk1-2 vc abk

Tisdale1 c abk2-3 m-c abk

3 vc abk2-3 vc abk1-2 c sbk

dsh-h, mfrdh, mfrdh, mfrdh, mfrdh, mfrdh, mfrdh, mfrdh, mfr

mvfrmvfrdvh, mfrmfrmfidvh, mfr

mfimfi

mfimfimfi

awdwdwdwdwdwdw

gscsdwdwds

gsds

gwgw

Weak-moderate slickensidesProminent slickensidesAs aboveModerate to strong slickensidesWeak slickensides

Many, continuous, thin clay skinsAs above. Moderate to strong slickensidesProminent slickensides

Common, medium, distinct mottlesHorizon mixed brown and black; common thin

clay cutans; moderate to strong slickensidesProminent slickensidesAs above

f Horizon designations in parenthesis according to Canada Soil Survey Committee (1978).t Abbreviations according to Soil Survey Staff (1951).§ ND = not determined.

Table 3. Related distribution, plasmic fabrics, and pedological features of the soils.

Horizon Depth, cm Related distribution Plasmic fabric Pedological features

Ap 0-10 PorphyroskelicBwl 10-35 As aboveBw2 35-60 As aboveBC 60-100 As aboveBCy 100-115 As aboveC 115-150 As above

Ap 0-20 PorphyroskelicBwl 20-40 As aboveBw2 40-62 As aboveBCyl 88-113 As aboveBC 135-180 As aboveC 180-240 As above

Ap 0-17 MullgranoidicE + Btl 17-38 MuUgranoidic-PorphyroskelicBtl 20-38 PorphyroskelicBt2 38-65 As aboveBCy 65-105 As aboveC 105-140 As above

Ap 0-10 MullgranoidicBE 10-35 Mullgranoidic-PorphyroskelicBt 35-80 PorphyroskelicBCy 80-130 As aboveC 130-165 As above

SceptreMa-skel-lattisepicLatti-skel-masepicMasepic and lattisepic and some unistrialIn-masepic with considerable unistrialUnistrial and mosepicPredominantly unistrial

ReginaIn-masepicMasepic and lattisepicMasepic and lattisepicUnistrial and weak ma-insepicAs abovePredominantly unistrial

MelfortInsepicInsepicInsepicVo-masepicAs abovePredominantly unistrial

TisdaleInsepic and weak lattisepicLattisepic and masepicVo-masepicUnistrial and vo-masepicPredominantly unistrial

Comment gypsum crystallariaAs above

Common gypsum crystallaria

Common soil nodulesChannel and planar void ferriargillansAs aboveVery few ferriargillansCommon gypsum crystallariaAs above

Common to many soil nodulesFerriargillans predominantly within matrixFerriargillans rare. Few soil nodulesFew gypsum crystallariaGypsum rare

fFew = <2%; common = 2-20%; many = >20%.

1246 SOIL SCI. SOC. AM. J., VOL. 51, 1987

Table 4. Point-count estimates of illuviation argillans, plasma,and masepic plasmic fabric in selected horizons •

of the soils studied.

Horizon

ApBw2

ApBwlBw2

ApE + BtlBtlBt2

BEBt

Depth

cm

0-1035-60

0-2020-4040-62

0-17

20-3838-65

10-3535-80

Illuviationargillans t

SceptreNPNP

ReginaNPNPNP

MelfortNP1.83.6tr

Tisdale1.0tr

PlasmaJ

6979

888887

7660ND81

8480

Masepicfabricj

815

71010

00

ND10

415

t NP = not present; tr = trace; and ND = not determined.t Expressed as percent plasma.

Physical and Chemical PropertiesPhysical and chemical properties of the soils are

presented in Table 5. Except for some surface hori-zons, the soils contain >50% clay. Increasing claycontent with depth in all except the Regina soils istypical of lacustrine sediments. Sand content is no-tably low. The COLE values are high for all soils. Theyare particularly high for the Regina soil and relativelylow for surface horizons in the Melfort and Tisdalesoils. Plasticity index values are also high and mosthorizons classify as highly plastic inorganic clays (CH)in the unified soil classification system used by engi-neers. Both COLE and plasticity index closely followthe trend in fine clay.

The Sceptre and Regina soils are mildly to moder-ately alkaline throughout the solum and parent ma-terial, but the Melfort and Tisdale soils are moderatelyacid at the surface and near neutral in the rest of thesolum. A sharp increase in salinity occurs in about 1m in the Sceptre and Regina soils but is shallower in

the Melfort and Tisdale soils. Salinity is particularlyhigh in the Melfort soil. Organic C is lower and de-creases gradually with depth in the Sceptre and Reginasoils but it is higher and decreases abruptly betweenAp and Bt horizons in the Melfort and Ap and BEhorizons of the Tisdale soils. The Sceptre and Reginasoils are calcareous throughout and show weak accu-mulation of carbonates in the Bw and BC horizons.Cation exchange capacity decreases with depth in allbut the Sceptre soils. The narrow ratio of exchangeableCa to Mg is attributed to higher Mg in the soil solutioneither from dolomitic limestones common in the areaand/or Mg replacement from layer silicates duringpyrite oxidation, as proposed by Mermut and Arshad(1987). Magnesium is the dominant exchangeable cat-ion in the Melfort soil, a common characteristic ofclay soils with Solonetzic morphology in Manitoba(Ellis and Caldwell, 1935). Exchangeable Na is negli-gible in the upper solum of all soils, but exchangeablesodium percentage (ESP) ranges from 7 to 10% in thelower solum of the Sceptre, Melfort, and Tisdale soils.

CrackingCracks opened to the surface in cropped fields be-

tween the last week of June and the first week of Julyin the Sceptre and Regina soils, and during the lastweek of July to the first week of August for Tisdalesoil in 1984 and 1985. Cracks were not evident in theMelfort soil during these 2 yr. The measurements dur-ing 1985 are summarized in Table 6. In the subhumidTisdale soil, cracks were slightly narrower and werespaced farther apart than in the semiarid Sceptre soil,despite the fact that annual crops were grown contin-uously at the Tisdale site, but were alternated with afallow year at the Sceptre site. There was no significantdifference in the depth of cracks, however.

DISCUSSIONSoil Genesis

All the soils are characterized by seasonal moisturedeficits and cracking; however, the magnitude and fre-quency of seasonal deficit is much greater for theSceptre and Regina than for the Melfort and Tisdale

Fig. 2. Masepic fabric in the Bw2 horizon (42-62 cm) of the Reginasoil (crossed polarizers).

Fig. 3. Unistrial sedimentary fabric in the C horizon (180-240 cm)of the Regina soil (crossed polarizers).

DASOG ET AL.: CLAY SOILS WITH VERTIC PROPERTIES IN SASKATCHEWAN 1247

Table 5. Physical and chemical properties of the soils, t

Hori-zon Depth

Totalsand

Totalsilt

Totalclay

Fineclay

(< 0.2 urn) COLE LL PI EC

Or-ganic CaCOj

C equiv

Exchangeable

CEC Ca Mg Na

dS — g kg-' — - cmolc kg"1 -

ApBwlBw2BCBCyC

ApBwlBw2Bw3BCylBCy2BCC

ApEBtlBt2BCyC

ApBEBtBCyC

0-1010-3535-6060-100

100-115115-150

0-2020-4040-6262-8888-113

113-135135-180180-240

0-1717-2020-3838-6565-105

105-140

0-1010-3535-8080-130

130-165

1287210

12122111

775321

84610

423938312833

2423232727242323

435934311917

3622252422

465356677167

7575757171757676

503461667982

5674697578

243337323129

3450494450403534

291742414737

3349464136

0.0960.1110.0970.075NDND

0.1040.1680.1470.1470.1490.137NDND

0.063ND

0.1030.1020.1260.097

0.0660.1200.1160.1120.091

4956627272ND

626772696868NDND

52ND59727377

5562657573

2431384542ND

Regina303542434138NDND

Melfort21ND32454848

Tisdale2531394845

7.77.88.08.07.87.7

7.77.98.08.07.77.77.77.6

5.56.16.67.57.67.6

5.85.26.57.47.4

0.50.50.61.45.35.3

1.00.40.50.73.24.24.24.1

0.4ND0.72.57.07.4

0.50.70.75.45.6

15.88.36.76.67.06.6

16.012.39.88.06.26.76.86.7

50.720.817.810.97.96.0

54.017.715.28.29.1

168175614042

4159746773666461

4ND

7164845

702

6551

30.629.129.735.135.833.1

49.549.643.937.535.036.337.336.9

41.1ND37.235.833.435.4

45.642.638.833.534.0

23.922.119.019.622.821.4

36.232.928.525.724.926.427.423.4

18.6ND11.713.716.416.0

28.317.819.117.316.4

8.014.218.019.817.817.0

12.918.520.518.114.715.113.612.5

12.4ND27.026.426.025.8

10.416.319.519.620.1

0.10.51.62.82.42.0

0.00.00.81.61.21.51.71.6

0.0ND1.01.63.21.8

0.31.11.92.32.0

t COLE = coefficient of linear extensibility; LL = liquid limit; PI = plasticity index; CEC = cation exchange capacity; and ND = not determined.

soils. Cracks promote bypass flow of water, which in-duces differential wetting of subsoils. Such wetting ismost likely to occur during periods of heavy rain inlate summer or fall when soils are dry from the pre-ceding crop. In years when fall precipitation is low,the soil may enter the winter in a dry and crackedstate. Moisture present in the soil would migratetoward cracks in response to lower temperatures inthese pore spaces. Pressure created from ice lenses

Table 6. Width, depth, and crack spacing in the Sceptreand Tisdale soils.

Average

Observation date Width Depth Crack spacing

Unistrial fabric (%)

20 July 1985

29 Aug. 1985

Sceptre Site2.0 40.1

(0.5-6.3)t (12.4-79.2)Tisdale Site

20 60 100 20 60 100 20 60 IOO 20 6O IOO

1.6(0.7-3.1)

36.5(11.1-76.4)

111

154

t Figures in parenthesis denote range.

Fig. 4. Occurrence of unistrial fabric in the soils studied, estimatedby point counts.

Fig. 5. Illuviation ferriargillans in the Bntl horizon (20-38 cm) ofthe Melfort soil (crossed polarizers).

1248 SOIL SCI. SOC. AM. J., VOL. 51, 1987

would be exerted on the soil, but owing to high shearstrength in the frozen state, shearing is unlikely. How-ever, during the spring, differential wetting of soilwould occur due to bypass flow of meltwater. Ripley(1973) attributed rapid warming of a Sceptre soil toinfiltration of snpwmelt through cracks. Snowmelt maybe particularly important for the Sceptre and Reginasoils because the average fall precipitation is <20 mmand, hence, insufficient to replenish water used duringthe previous growing season prior to freezing.

Reports relating COLE (Franzmeier and Ross, 1968)and plasticity index (Nayak and Christensen, 1971) toswelling pressure suggest that all soils studied have thepotential to generate high swelling pressures. Swellingpressures of 400 to 1000 kPa and shear strengths of20 to 40 kPa observed when Regina soil materials wereextensively moistened (D.J. Fredlund, 1985, personalcommunication) clearly indicate that the potential forshearing exists in these soils. As explained by Yaalonand Kalmar (1978), differential wetting of subsoil cre-ates sufficient difference in horizontal and verticalstress to result in displacement and, hence, slicken-sides. This model essentially explains the occurrenceof slickensides in all soils of this study.

Slickensides may also form from crack filling, asillustrated by Boul et al. (1973). Fine granular surfacestructure typifies these soils, especially in native andcultivated grasslands. Such material can be readilytransported via cracks and obviously infill the crackby gravity. Soil displacement occurs due to volumeadjustment to account for this increase in material inthe subsoil upon rewetting. However, the zones ofslickensides (Table 1) extend below the depths of opencracks (Table 6), suggesting that this mechanism isonly partly responsible for soil displacement. Consid-ering the effect of these two mechanisms on soil for-mation, elaborated by Ahmad (1983), the lack of gilgaimicrorelief in these soils and in those under nativegrasslands suggests that the self-swallowing mecha-nism is of lesser importance.

In spite of the similarity in the nature of slickensidesin the subsoils of these soils, there are striking mor-phological differences in the upper part of the solum.The Melfort and Tisdale soils have columnar and pris-matic structures and clay skins in readily discernibleB horizons. It appears that slickensides may form insubsoils where the difference between horizontal andvertical stresses is large and the upper solum lies in arelatively stable zone where low overburden pressureand cracks would prevent the development of highlateral stress, as reported by Yaalon and Kalmar (1978)and Knight (1980). Under these conditions, when othersoil-forming processes such as clay peptization, leach-ing, and lessivage are relatively strong, processes ofdisplacement do not effectively counter surface-ori-ented processes. Much lower concentration of argil-lans in the Bt2 than in the Btl horizon of the Melfortand in the Bt than in the BE horizon of the Tisdaleattests to the validity of this reasoning.

The distribution of masepic fabric appears to be re-lated to the intensity of pedoturbation. Experimentalstudies by Dalrymple and Jim (1984) and others haveshown that mere wetting and drying of soils, howeveruneven, will not result in masepic fabric. Despite highCOLE, the absence of masepic fabric in the Btl ho-

rizon of the Melfort and the inconspicuous nature ofit in the BE horizon of the Tisdale soil support theseobservations. Further, the extension of masepic fabricto surface horizons in the Sceptre and Regina soils andits confinement to horizons where slickensides areprominent in the Melfort and Tisdale soils supportthe contention presented above, that the upper partof the solum of the Melfort and Tisdale soils is notstrongly influenced by displacement forces generatedbelow. These observations also suggest that either theinfluence of subsoil displacement on the upper solumis greater in the Sceptre and Regina soils, or the off-setting influence of other soil-forming processes areless under the semiarid environment of soil forma-tion. Probably both mechanisms serve to explain theweak morphology and the observed distribution ofmasepic fabric in the Sceptre and Regina soils.

Horizon DesignationHorizon designation in these clayey soils is difficult

due to lack of color contrast, poor expression of struc-ture when moist, and the common occurrence of car-bonates to the surface. The first question is whetherto separate the solum and the C horizon. Tradition-ally, the horizon below the Ap, if massive and calcar-eous, has been designated as a C horizon (Ayres et al.,1985). Inasmuch as the C horizon is least affected bypedogenic processes and lacks properties of O, A, andB horizons; horizons that retain the majority of theoriginal unistrial sedimentary fabric (Fig. 2b) shouldbe designated as C horizons. All the horizons desig-nated as C in the field had >75% unistrial fabric (Fig.3) and lacked slickensides. This suggests that smallfluctuations in moisture content at this depth, as re-ported by de Jong and McDonald (1975), are not suf-ficient to reorganize the clay fabric. Horizons with asignificant proportion (25-75%) of unistrial fabric re-flect long-term fluctuations in the lower limit of sea-sonal wetting and drying in these soils.

The second question, then, must address the appro-priate designation of horizons within the solum. InMelfort and Tisdale soils, the B horizon is readily ap-parent, as reflected by changes in color, removal ofcarbonates, and translocation of clay accompanied byalteration of sedimentary structure. In the Sceptre andRegina soils, the changes are more subtle. They havecoarse prismatic structure breaking to angular blockystructure when dry; however, this is not obvious whenthe soil is moist. As White and Bonestell (1960) notedin some gilgaied soils of South Dakota, the structureof these soils needs to be described both when dry andmoist. Subsoil horizons that show blocky or prismaticstructure are considered to be B horizons (Canada SoilSurvey Committee, 1978; Guthrie and Witty, 1982).Therefore, a case for B horizons can be argued onstructural considerations alone, and these B horizonsshould be designated Bw (Bmk in the Canadian sys-tem).

Soil ClassificationVertisols have marks of processes that mix the soil

and prevent the development of diagnostic horizonsthat one might otherwise expect to find (Soil SurveyStaff, 1975). The Sceptre and Regina soils conform to

DASOG ET AL.: CLAY SOILS WITH VERTIC PROPERTIES IN SASKATCHEWAN 1249

this concept, and now that the te.mperature criterionis waived, they meet the requirements of the Vertisolorder. In addition to mollic epipedons, the Melfortand Tisdale soils have features such as prismatic orcolumnar structure and illuvial argillans, which sug-gest that the influence of soil movement and insta-bility is less than that in Vertisols. The presence ofthese features signifies stability in the upper solum andshould receive greater consideration in classifying asoil than the slickensides in the lower solum. There-fore, such soils should be excluded from Vertisols evenwhen accompanied by slickensides. The Melfort andTisdale soils should appropriately be grouped as verticintergrades (Argic Vertic Cryoborolls). These soils arecomparable to Pelosols of Germany (and France),which have slickensides accompanied by evidence ofleaching and illuviation (Schlichting, 1968).

The Sceptre and Regina soils do not meet the re-quirements of any suborders of Vertisol listed in SoilTaxonomy. They cannot be Xererts because cracksmay open and close more than once each year. Inaddition, the moisture regime is not that of a Medi-terranean climate. They cannot be Usterts, since cracksare unlikely to remain closed for 60 consecutive d whenthe soil temperature is continuously >8°C. This re-quirement suggests that the definition of Usterts wasintended for soils of warmer regions with ustic mois-ture regimes. Therefore, a new suborder, Borerts, whichcorresponds with Borolls and Boralfs, seems appro-priate for Vertisols of cool and cold continental cli-

• mates. There are an estimated 14 563 km2 of Sceptre,Regina, and similar soils in the Brown and Dark Brownsoil zones in Canada alone (Clayton et al., 1977), andappreciable areas within the Black soil zone of Sas-katchewan (Mermut and St. Arnaud, 1983) and Man-itoba that may meet the definition of the proposedBorerts. Clay soils in the Red River Valley of NorthDakota, the Grumusols of Montana (Hogan et al.,1967), and gilgaied soils in South Dakota (White andBonestell, 1960) would also have to be considered forinclusion in this group.

Although more documentation of these soils is re-quired in subhumid regions, it is proposed that or-ganic C content may best serve as a criterion to sub-divide Borerts at the great group level. Two greatgroups, Haploborerts (organic C of <2% in the upper15 cm) and Humiborerts (organic C of >2°/o in theupper 15 cm) are possible separations, based on theproperties of Saskatchewan soils. If subgroup criteriawere based on the moisture regime, aquic, ustic, andudic subgroups would occur in Saskatchewan. Thesecriteria can be improved when more data on the pro-posed Borerts become available.

ACKNOWLEDGMENTThe financial assistance from the Association of Univer-

sities and Colleges of Canada in the form of a Common-wealth Scholarship to the senior author is gratefully ac-knowledged.

1250 SOIL SCI. SOC. AM. J., VOL. 51, 1987