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:
- Irregular shaped, randomly oriented, intrusions of displaced materials within the solum.
- 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:
- The surface horizon (Ah), when dry, has massive structure and is hard.
- No other undisturbed A and B horizons common to Chernozemic soils are present, other than the Ap or Ah horizon.
- 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).
|Diagnostic horizons are underlined|
|Color value of A horizon||≥3.5 (dry)||<3.5 (dry)|
|Chroma of A horizon||usually >1.5 (dry)||usually ≤1.5 (dry)|
|A horizon||not easily distinguishable from rest of solum||easily distinguishable from rest of solum|
|Gleysolic Vertisol||Gleysolic Humic Vertisol|
|Diagnostic horizons are underlined|
|Ah horizon||<10 cm thick||≥10 cm thick|
|Vertisol||Orthic Vertisol O.V|
|Gleyed Vertisol GL.V|
|Gleysolic Vertisol GLC.V|
|Humic Vertisol||Orthic Humic Vertisol O.HV|
|Gleyed Humic Vertisol GL.HV|
|Gleysolic Humic Vertisol GLC.HV|
Source: The Canadian System of Soil Classification (Third Edition)