Chapter 13: Vertisolic Order

A diagrammatic representation of profiles of some subgroups of the Vertisolic order is given in Figure 40. The subgroups include soils that may have horizon sequences different from those indicated. In the description of each subgroup, presented later in this chapter, a common horizon sequence is given; diagnostic horizons are underlined and some other commonly occurring horizons are listed.

Soils of the Vertisolic order occur in heavy textured materials (≥60% clay of which at least half is smectite) and have shrink-swell characteristics that are diagnostic of Vertisolic soils. However, they lack the degree or kind of horizon development diagnostic of soils of other orders, except as noted below. The central concept of the order is that of soils in which cracking, argillipedoturbation, and mass movement, as evidenced by the presence of slickensides and severe disruption within the control section, are the dominant soil-forming processes. These disturbances, caused by shrinking and swelling, the result of wetting and drying cycles, are strong enough that horizons diagnostic of other soil orders have either been prevented from forming or have been severely disrupted. However, in terms of color and organic matter content, they may have a Chernozemic-like A horizon at the surface, or they may have features diagnostic of the Gleysolic order. The major areas of Vertisolic soils occur in the cool, subarid to subhumid, grassland portion of the Interior Plains of western Canada. Minor areas of Vertisolic soils may occur in valleys of the South Cordilleran Region, in parts of the South Boreal Region, and in the Cool Temperate Regions of central Canada. (Brierley et al. 1996)

Soils of the Vertisolic order are defined based on the occurrence of BOTH a slickenside horizon and a vertic horizon within the control section. A slickenside horizon (ss) is one in which several (more than two) slickensides, which may be intersecting, can be recognized. A vertic horizon (v) is one that has been strongly affected by argillipedoturbation, as a result of shrinking and swelling, and is characterized by the presence of:

1. Irregular shaped, randomly oriented, intrusions of displaced materials within the solum.
2. Vertical cracks, often containing sloughed-in surficial materials.

The disruption within this horizon, resulting from the shrinking and swelling process, is either strong enough to prevent horizons diagnostic of other orders from forming or, if diagnostic horizons are present, they are disrupted to the extent that they are no longer continuous throughout the pedon and their orientation has been severely changed.

Distinguishing Vertisolic Soils from Soils of Other Orders

Vertisolic soils are distinguished from soils of other orders by having both a vertic and slickenside horizon. They also have no undisturbed horizons that are diagnostic of other orders, except that they may have a Chernozemic-like A horizon at the surface or may have features diagnostic of the Gleysolic order, or both.

Guidelines for distinguishing Vertisolic soils from soils of the Chernozemic and Gleysolic orders follow.

Chernozemic Vertisolic soils are distinguished from soils of the Chernozemic order based on the following:

1. The surface horizon (Ah), when dry, has massive structure and is hard.
2. No other undisturbed A and B horizons common to Chernozemic soils are present, other than the Ap or Ah horizon.
3. Both a slickenside horizon (ss), the upper boundary of which occurs within 1 m of the mineral surface, and a vertic horizon (v) are present. The occurrence of these horizons together takes precedence over the occurrence of a Chernozemic-like A horizon.

Gleysolic Vertisolic soils are distinguished from Gleysolic soils on the basis that both a slickenside horizon (ss), the upper boundary of which occurs within 1 m of the mineral surface, and a vertic horizon (v) are present in Vertisolic soils. The occurrence of these horizons together takes precedence over the occurrence of gley features diagnostic of the Gleysolic order.

Vertisolic soils are divided into two great groups: Vertisol and Humic Vertisol, mainly based on the color of the A horizon as indicated in the Vertisolic order chart. The color of the A horizon reflects differences in the nature and amount of organic matter incorporated with the mineral material as a result of differences in climate and vegetation.

Subgroups are separated based on the kind and sequence of horizons indicating conformity with the central concept of the great group. Most of the subgroups within the Vertisolic order were formerly recognized taxonomically at the family or series level as Grumic (Agriculture Canada Expert Committee on Soil Survey 1987).

Vertisol

These are soils that generally occur in the most arid portion of the climatic range of Vertisolic soils. They are primarily associated with a native vegetation of xerophytic and mesophytic grasses and forbs. They have both a vertic and a slickenside horizon as specified for the Vertisolic order. Except for the poorly drained members, they also have brownish-colored A horizons (dry color value of 3.5 or lighter, chroma usually greater than 1.5 dry, and organic C usually 3% or less) similar to associated Chernozemic soils. However, they differ from Chernozemic soils in that it is often difficult to separate the A horizon from the rest of the solum based on color alone, and the Ah horizon is massive and hard when dry. They also lack other horizons, in the upper solum, common to Chernozemic or other soils.

The poorly drained members occur commonly in depressional positions associated with better-drained Vertisolic soils. Besides the presence of both a slickenside and a vertic horizon, they have the general characteristics specified for the Gleysolic order. As well, under native conditions they lack a mineral-organic surface (Ah) horizon 10 cm or more in thickness. If cultivated the Ap horizon has one of the following properties:

1. Less than 2% organic C.
2. A rubbed color value greater than 3.5 (moist) or greater than 5.0 (dry).
3. Little contrast in color value with the underlying layer (a difference of less than 1.5 units, if the value of the underlying layer is 4 or more, or a difference of less than 1 unit, if that value is less than 4).

Orthic Vertisol

Common horizon sequence: Ah, Bv or Bvk, Bss or Bssk or Ckss, Ck

These soils have the general properties specified for the Vertisolic order and those members of the Vertisol great group other than the poorly drained members. They have a vertic horizon (Bv or Bvk) and a slickenside horizon (ss), the upper boundary of which occurs within 1 m of the mineral surface. The slickenside horizon may be confined to the B horizon (Bss or Bssk), may form part of the C horizon (Ckss), or may form part of both.

Orthic Vertisols, and all other subgroups of Vertisolic soils, may have saline features. These features are separated taxonomically at the series level, or as a phase of any taxonomic level above the family.

Gleyed Vertisol (GL.V)

Common horizon sequence: Ah, Bvgj or Bvkgj, Bssgj or Bsskgj or Ckssgj, Ckgj or Ckg

These soils have the general properties specified for the Vertisolic order and those members of the Vertisol great group other than the poorly drained members. They differ from Orthic Vertisols by having faint to distinct mottles that indicate gleying within 50 cm of the mineral surface.

Gleysolic Vertisol (GLC.V)

Common horizon sequence: LFH or O, Bvg, Bssg or Cgss, Cg

These soils have the general properties specified for the Vertisolic order and the poorly drained members of the Vertisol great group. They differ from Orthic and Gleyed Vertisols mainly by having, within 50 cm of the surface, either colors that indicate poor drainage and periodic reduction (diagnostic of the Gleysolic order) or prominent mottles that indicate gleying, or both.

Humic Vertisol

These Vertisolic soils occur in the more moist portion of the climatic range of Vertisolic soils and are primarily associated with a native vegetation of mesophytic grasses and forbs or with mixed grass, forb, and tree cover. They have an A horizon that is darker in color than soils of the Vertisol great group. Except for the poorly drained members, the surface horizon has a dry color value of less than 3.5, chroma usually 1.5 or less dry, and organic C usually greater than 3%. The A horizon is easily distinguishable from lower horizons based on color alone. However, the boundary between the A horizon and lower horizons is usually very irregular. These soils have both a vertic and a slickenside horizon as specified for the Vertisolic order. They lack other horizons in the upper solum, common to other soils, except they may have a Chernozemic-like A horizon. The poorly drained members occur commonly in depressional positions associated with better-drained Vertisolic soils. Besides the presence of both a slickenside and a vertic horizon, they have the general characteristics specified for the Gleysolic order. As well, under native conditions they have a mineral-organic surface (Ah) horizon 10 cm or more in thickness. If cultivated, the Ap horizon has all the following properties:

1. At least 2% organic C.
2. A rubbed color value of 3.5 or less (moist) or 5.0 or less (dry).
3. At least 1.5 units of color value (moist) lower than that of the next underlying horizon, if the color value (moist) of that horizon is 4 or more, or 1 unit of color value lower than that of the underlying horizon, if its color value is less than 4.

Orthic Humic Vertisol (O.HV)

Common horizon sequence: Ah, Bv or Bvk, Bss or Bssk or Ckss, Ck

These soils have the general properties specified for the Vertisolic order and those members of the Humic Vertisol great group other than the poorly drained members. They have a vertic horizon (Bv or Bvk) and a slickenside horizon (ss), the upper boundary of which occurs within 1 m of the mineral surface. The slickenside horizon may be confined to the B horizon (Bss or Bssk), may form part of the C horizon (Ckss), or may form part of both.

Gleyed Humic Vertisol (GL.HV)

Common horizon sequence: Ah, Bvgj or Bvkgj, Bssgj or Bsskgj or Ckssgj, Ckgj or Ckg

These soils have the general properties specified for the Vertisolic order and those members of the Humic Vertisol great group other than the poorly drained members. They differ from Orthic Humic Vertisols by having faint to distinct mottles that indicate gleying within 50 cm of the mineral surface.

Gleysolic Humic Vertisol (GLC.HV)

Common horizon sequence: LFH or O, Ah, Bvg, Bssg or Cgss, Cg

These soils have the general properties specified for the Vertisolic order and the poorly drained members of the Humic Vertisol great group. They differ from Orthic and Gleyed Humic Vertisols mainly by having either colors that indicate poor drainage and periodic reduction, diagnostic of the Gleysolic order or prominent mottles that indicate gleying within 50 cm of the mineral surface, or both.