Ministry of Environment

Soils

2.3 The Canadian System of Soil and Soil Climate Classification

L.M. Lavkulich and K.W.G. Valentine

A classification system is an arrangement of things made by man to help him remember them and to arrange his knowledge of them into some logical order. A system is needed because a large number of unordered objects cannot be remembered individually and the logical arrangement is needed for reference. A system is also required so that relationships between the various objects can be deduced or inferred, and so that predictions can be made about the objects' properties and behaviour.

Soil Classification

The Canadian System of Soil Classification which has developed gradually over more than 60 years, attempts to arrange the soils of Canada according to their properties. It is, therefore, a natural or taxonomic system. The different categories are based as much as possible on properties that can be seen in the field. Where this is not possible, or where definite limits of field properties must be set, laboratory measurements are made. For instance the percent carbon content of the A horizon and the percent iron plus aluminum content of the B horizon are both used as criteria for particular soils within the classification. But even where laboratory tests are ultimately necessary, there is usually a field property that can be used as an indication. In the above two cases the blackness of the A horizon would give a clue to the carbon content and the redness of the B horizon can indicate the iron plus aluminum content.

The particular properties that are chosen to differentiate categories are for the most part connected with particular groups of processes because this is the way we think about soils. In effect, therefore, soils are grouped according to how they were formed, that is according to their genesis, because it is the different balances of soil-forming processes that produce the different properties. Although soils are defined on their inherent properties, various groups of soils throughout the system will also be associated with particular environments. Such a taxonomic classification is essential in understanding soil systems as well as in making resource inventories.

Soil varies in thickness over the earth's surface from a few millimetres to several metres, and different soils grade imperceptibly into one another. The minimum depth of soil material that is classified is 10 cm and the smallest three-dimensional unit at the surface of the earth that is considered as a soil is called a "pedon". By definition a pedon is between 1 and 2 m deep and its lateral extension is between 1 and 3.5 m. The actual dimensions depend on the lateral and vertical variability of the soil horizons and upon some other factors such as the depth to bedrock.

The System of Soil Classification for Canada has six levels of generalization or categories as outlined below:

Order - This is the highest level of generalization. The pedons are grouped according to their properties that reflect the nature of the soil forming processes. For example, the total effect of soil processes in the soils of the Chernozemic order leads mainly to the accumulation of organic matter in the topsoil under a grassland vegetation. The number of specific statements that can be made about this category are few; only general statements are possible. Nine orders are recognized, seven for mineral soils, one for organic soils, and one which can contain both.

Great Group - This class is a subdivision of the order and is, therefore, more specific and more precise statements can be made about it. Great groups are based on properties that reflect differences in the strength of the dominant processes or a major contribution of a process in addition to the dominant one. In the Chernozemic order the great groups are differentiated on the basis of the colour of the topsoil. This is an indication of the amount of organic matter that has accumulated. The Brown soils in the driest areas have the least and the Black soils in the coolest and wettest areas have the most. The Dark Gray soils occur near the forest-grassland transition where a secondary process of the leaching of organic matter and clay from the topsoil becomes important. It is the great group that will be used to describe the soil landscapes of British Columbia in Part 3. The classification system down to this level is as follows:

Order Great Group
Brunisolic Melanic Brunisol
  Eutric Brunisol
  Sombric Brunisol
  Dystric Brunisol
Chernozemic Brown
  Dark Brown
  Black
  Dark Gray
Cryosolic Turbic Cryosol
  Static Cryosol
  Organic Cryosol
Gleysolic Humic Gleysol
  Gleysol
  Luvic Gleysol
Luvisolic Gray Brown Luvisol
  Gray Luvisol
Organic Fibrosol
  Mesisol
  Humisol
  Folisol
Podzolic Humic Podzol
  Ferro-Humic Podzol
  Humo-Ferric Podzol
Regosolic Regosol
  Humic Regosol
Solonetzic Solonetz
  Solodized Solonetz
  Solod

Below these first two levels there are three more subdivisions as follows:

Subgroup - A subdivision of the great group that defines the central concept of the great group (Orthic) or certain variations from that concept which are grading towards other orders, e.g. a Gleyed Brown Chernozemic or a Solonetzic Brown Chernozemic, where the normal features of a Brown Chernozemic soil are being modified by the occurrence of excessive water or soluble salts respectively. Special features of the soil profile such as thin iron pans (Placic) or repeated organic layers (Cumulic) are also used to classify subgroups. The subgroup is visualized as having a particular assemblage of horizons and the Canadian System of Soil Classification specifies the horizons that each subgroup category must have, along with others that it may have.

Family - A subdivision of the subgroup which identifies a group of soils that are relatively homogeneous in mineralogy, texture and soil climate as well as genetic soil horizons. Because of the more precise nature of this class many statements can be made about the use and management of the soils in relation to plant growth, hydrology and engineering characteristics.

Series - A subdivision of the family, this is a group of pedons (a polypedon) with horizons whose features such as colour, texture, structure and thickness fall within a narrow range. The pedons have all been developed from similar parent material. This is probably the most important category of soils for consideration of use and management since it applies to real soil bodies - polypedons.

There used to be a further category called a Type based on the texture of the topsoil. This has now been dropped but may be found in some early reports.

The higher classes of the Soil Classification (order, great group, subgroup and family) are especially important in studying genetic relationships, drawing inferences and allowing correlations which are often required for regional studies. The two lower categories are more suited to local and site specific studies and to land utilization programs.

The nomenclature used for the four higher categories is descriptive of either the soil morphology or the kinds of environments in which the soils are found. The soil series, because of its more local character and precise nature, is named after a geographic or cultural feature near the site where the soil was first observed.

Soil Climate Classification

Climate is an important factor in soil formation as well as being important for biological growth. Climate involves temperature, energy and moisture relationships. It is dynamic with respect to place and time. Most classifications of climate have emphasized the aerial portion of the earth and are interpretations of air temperatures and precipitation distributions. A climate classification is needed that shows the interaction between aerial climate and soil climate. The latter affects root development, plant growth, soil structure, soil behaviour, and the environment for soil flora and fauna. Soil climate is affected by water content, soil depth, surface cover, position on the landscape and the effects of man.

A soil climate classification for Canada has been produced based on temperature and moisture conditions for periods which are significant to plant growth. The distribution of soil temperature classes and soil moisture regimes in British Columbia is shown in Figure 2.3.1. Soil temperature classes are based mainly on mean annual soil temperatures, the length of the growing season and accumulated degree days. The classes recognized in British Columbia are given in Table 2.3.1.

Table 2.3.1
Soil Temperature Classes

Class
Mean Annual Soil Temp. (ºC)
Growing Season Days Over 5ºC
Degree - days Over 5ºC
Very Cold SUBARCTIC -7 to less than 2 less than 120 less than 555
Cold CRYOBOREAL 2 to less than 8 120 to 180 555 to 1110
Moderately Cold CRYOBOREAL 2 to less than 8 less than 220 1110 to less than 1250
Cool BOREAL 5 to less than 8 over 170 1250 to less than 1388
Moderately Cool BOREAL 5 to less than 8 less than 220 1388 to less than 1720
Mild MESIC 8 to less than 15 200 to 240 1720 to 2220

Figure 2.3.1

Moisture classes are recognized on the basis of whether the soil is saturated or not for long periods. For aquic regimes subclasses are separated on the length of time the soil is saturated during the growing season. For moist and submoist regimes the subclasses are based on calculations of intensity and degree of water deficits during the growing season. For a more complete description of soil climates the publication Soils of Canada should be consulted (Clayton et al. 1977 in Further Reading).

Soil Classification and Soil Mapping

One of the principal uses to which the soil classification has been put is the mapping of soils in the field. The classification system is used to define and label the types of soils that are found in the landscape, and then lines are put on the map to show the boundaries between these different soil landscapes.

Maps can be made to show the geographical distribution of soils classified at any level of the system from order to series. The higher categories will be used on small scale maps where generalizations about large areas are necessary. The lower categories will be more applicable to large scale maps where it will be possible to draw lines round areas which contain soils with a narrow range of properties. In fact, the great group, subgroup and the series have been the categories of classification most often used in mapping. In addition, a special mapping unit called an "association" has also been commonly used.

An example of a small scale map of the soils of British Columbia is that portion of the Soils of Canada map published at a scale of 1:5 million or the map shown in Figure 3.2.1 for British Columbia. Map units delineate areas within which the predominant soils would belong to one great group. Special symbols are overprinted to show areas where excessive water or stones and shallow bedrock cause modifications in the profile typical of that great group.

Formerly, more detailed soil survey maps were published at a scale of 1 or 2 inches to the mile (1:63,360 or 1:31,680). Soil series were used as the mapping unit either singly or in two- or three-fold combination. More recently, soil surveys have been published regularly at a scale of 1:126,720 or 1:125,000 and a special mapping unit that is not a classification category has been used. This is the soil association, made up of a group of soils that occur together in the landscape. Each of these soils will belong to a series but they will differ due to slightly different combinations of soil forming factors. For example, in a rolling landscape there may be a shallow, dry soil on the ridge crests, a well drained, moist soil on the mid-slopes, and a deep, wet soil in the hollows. These would be three series but they could be mapped together as a single soil association. To be grouped as an association, a soil series must have developed from the same parent material under the same climatic regime and usually within the same vegetation zone.

The distinction between a classification category and a mapping unit is important and often causes confusion. A soil association is a mapping unit not a taxonomic category. Great groups, subgroups and series on the other hand are taxonomic categories which may be used as specific mapping units or as components. As formal classification categories they are concepts with specific definitions and limits to their properties. As mapping units they are used to label the range of properties that the predominant soils will have in that area. But it must be remembered that some soils in the map unit will not fit the particular great group or series classification used. The soil mapper or surveyor, will usually try to keep these different soils down to about 15 or 20 percent of the map unit area.

A discussion in greater detail of the types of soil maps and reports that have been produced in British Columbia is included in Part 4.

Further Reading

  • Canada Soil Survey Committee, 1978. The Canadian System of Soil Classification, Agriculture Canada, Queen's Printer, Ottawa.

    A complete description of the System of Soil Classification as used in Canada. The publication describes the system as well as giving illustrations of the major soils.

  • Clayton, J.S., W.A. Ehrlich, D.B. Cann, J.H. Day, and I.B. Marshall, 1977. Soils of Canada. Research Branch, Agriculture Canada, Ottawa.

    An account of the major soils in Canada, their characteristics, distribution and extent. Included are descriptions of soil climate as well as maps of Soils of Canada and Soil Climates of Canada.