Soil Orders and Their Forming Processes

Each of the 10 soil orders in the Canadian system of soil classification is characterised by a most dominant soil forming process. At this website you will find videos illustrating some examples of the key soil forming processes found in Brunisols, Luvisols, Organic Soils, Podzols, Regosols and Vertisols.

Human activities such as tillage, excavations, mining, logging, etc. can have profound impact on soil forming processes; and video below illustrates impacts of the climate change on a forest ecosystem in British Columbia, Canada and consequently on soil formation.

Soils classified in the Brunisolic order have undergone only slight development from the parent material, i.e., more so than Regosols. The main soil-formation process in Brunisolic soils is the leaching of soluble salts and carbonates, the formation of iron and aluminum secondary minerals, and structure development in the finer textured materials different from the original structure of the parent material. These processes lead to the formation of a Bm horizon, which is the diagnostic horizon of Brunisolic soils. They can also have Bfj horizon (accumulation of amorphous Al/Fe compounds), or Btj (at least 5 cm thick), or Bf < 10 cm thick or a combination of horizons mentioned above.

Soils of the Luvisolic order are well to imperfectly drained mineral soils that have developed under deciduous, mixed deciduous-coniferous boreal forests, or under mixed forest in the forest-grassland transition zone in mild to cold climates. The dominant soil-forming process in Luvisolic soils is the translocation of clay-sized mineral particles from the A to the Bt horizon.

Organic soils are defined in the Canadian System of Soil Classification as soils that have developed largely from organic deposits. Most Organic soils develop under saturated conditions and are derived dominantly from vegetation that grows in poorly and very poorly drained areas. Organic soils contain > 17% organic C (>30% organic matter) by weight and have a specific depth, degree of decomposition and properties in the middle tier (40 to 120 cm). These soils must have Of (up to 60 cm thick), Om (up to 40 cm thick), or Oh (up to 40 cm thick) organic horizons. Up to 40 cm mineral material is allowed to be present in a soil that is classified as Organic soil as long as organic material is at least 40 cm in thickness.

Podzolic soils are characterized by intense chemical and biological transformations in the upper horizons, resulting in the transformation of the primary minerals and decomposition of organic matter. Soluble organic matter and mobile compounds of Al and Fe are readily leached from the A horizon into the B horizon where they accumulate, or form, a discrete horizon called a podzolic B (Bh, Bhf, Bf). Podzolic soils have podzolic B horizon and have accumulations of organic matter, iron, or aluminum. The podzolic B must be at least 10 cm thick, have a texture coarser than clay, and meet specific requirements for color, organic carbon, iron and aluminum content.

Regosolic soils are weakly developed soils. They do not have a B horizon that is thicker than 5 cm (this is their diagnostic feature) and may even lack an A horizon. The reasons for this can be the following: youthfulness of the material, recent alluvium, nature of parent material (pure quartz sand), dry and cold climate, colluvium on steep slopes, etc.

Vertisolic soils develop in fine-textured soil materials with >60% clay, of which at least half must be montmorillonite. The dominant soil-forming processes in Vertisols are: cracking, argilli-pedoturbation (mixing of clay in the pedon) and mass movement of materials due to shrinkage and swelling of clays during drying/wetting cycles. These soils have two characteristic horizons (either B or C): slickensides (ss = clayey subsurface horizons which have polished and grooved ped surfaces -‘slickensides’, or wedge-shaped structural aggregates ) and vertic (v = horizon with deep and wide cracks when dry, that allow the surface material to fall down the cracks causing the soil to heave). The wetting and drying cycles cause the clays to expand and contract.