In a given area over the period of soil formation, environmental conditions cause a certain set of soil processes to occur, which leads to a distinctive set of soil horizons at the time we observe the soil. These soil horizons are the basis for classifying the soil in the Canadian System of Soil Classification. The Canadian System is a rigorous taxonomic system (taxonomy is the science of classification). The System is a comprehensive method for assigning pedons to the soil classes; if the system is correctly used, a pedon can only be assigned to one specific class. The System is also a hierarchical system: each group occupies a distinct position within the overall system. The major levels used in the classification include: Order, Great Group, and Subgroup.
First class within the System is the Order. Which Order a given pedon is assigned to is based on properties of the pedon that reflect the nature of the overall soil environment and the effects of the dominant soil-forming process. There are ten Orders in the Canadian System of Soil Classification (Table below). The system has changed through time, and the last major addition to the system occurred in 1996, when the Vertisolic Order was added to the system.
Table: Summary of the Soil Orders in the Canadian System of Soil Classification
| Order | Diagnostic Horizon | Comments |
|---|---|---|
| Chernozemic | Ah, Ap, Ahe | A grassland soil whose diagnostic horizon is formed by high levels of organic matter additions from the roots of grasses. |
| Solonetzic | Bn or Bnt | A grassland soil with high sodium levels in the B horizon; usually associated with a clay-rich B horizon and often with saline C horizon material. |
| Podzolic | Bf or Bh | A forest soil normally associated with coniferous vegetation on igneous-rock derived parent materials. High acidity in the A horizon results in formation of a bleached Ae horizon and deposition of iron and aluminum in the B horizon. |
| Luvisolic | Bt | A forest soil found in areas with parent materials derived from sedimentary rocks. Dominant process is eluviation of clay from the Ae horizon and its deposition in the Bt horizon. |
| Brunisolic | Bm | A forest soil whose properties are not strongly enough developed to meet the criteria for the Luvisolic or Podzolic Orders. |
| Gleysolic | Bg, Cg | Found throughout Canada wherever temporary or permanent water saturation cause formation of gleyed features in the profile. |
| Regosolic | No B horizon | Found throughout Canada wherever pedogenic conditions prevent the formation of B horizons (unstable slopes, sand dunes, floodplains etc.). |
| Vertisolic | Bss, or Css and Bv | Associated with high clay glacio-lacustrine landscapes; characterized by shrinking and swelling of clays. |
| Cryosolic | By, Cy, Cz | A soil of arctic and tundra regions; characterized by presence of permafrost. |
| Organic | O horizon | Organic soils are associated with the accumulation of organic materials (peat) in water-saturated conditions. They are most commonly associated with Boreal Forest soils. |
Great Groups are sub-divisions of each Order. The Great Groups reflect differences in the strengths of the major processes or a major contribution of a process in addition to the major one. For example, if someone describes a Prairie soil as a Dark Brown Chernozem, the Order is Chernozemic (i.e., soils with rich topsoil that developed under grassland vegetation) and the Great Group is Dark Brown, which refers to the surface colour of the soil and reflects an intermediate level of soil organic matter (SOM) additions relative to the lower SOM levels to the south and the higher SOM levels to the north.
Subgroups are subdivisions of Great Groups. They are differentiated on: how closely they correspond to the central concept of the Great Group; intergrading towards soils of another Order; or additional special features within the profile. For example, an Orthic Dark Brown Chernozem reflects the central concept of the Dark Brown Great Group within the Chernozemic Order. In contrast, an Eluviated Dark Brown Chernozem has evidence of translocation of materials between horizons via eluviation and illuviation, but does not meet the classification criteria of the Luvisolic Order.