Regosolic soils occur in every ecozone but are rarely dominant in the landscape. The characteristic feature of the Regosolic order is that the B horizon is absent or only has limited development (i.e., is less than 5 cm thick). (Orthic Regosol Dune)
Soils of the Regosolic order are most commonly associated with landforms where the land surface is (or has recently been) unstable. Because of the unstable surface, the soil has had little time to develop, and hence soil horizons are very weakly expressed if present at all. The instability could be from either erosion of the landsurface or through deposition of sediment and burial of an earlier surface; in some cases this can occur in different portions of the same landscape. For example, in areas occupied by sand dunes, loss of the soil through wind erosion causes truncation of the surface soil where loss occurs, and where the sand is deposited it can bury any existing soil profiles with fresh sand. (Great Sand Hills) Many of the areas where Regosolic soils occupy greater than 15% of the landscape are areas of active or recently active sand dunes.
River floodplains also commonly have Regosolic soils associated with them. (Orthic Regosol Fluvial) The parent materials in these landscapes are alluvial (fluvial) sediments. In some locations the alluvial sediment is deposited with organic material mixed into it, giving a weakly expressed Ah horizon overlying the C horizon developed in the alluvial sediments; in other locations the C horizon is found at the surface with no organic material mixed in.
Regosolic soils also occur naturally on hillslopes that have high rates of water runoff and where slope processes cause downslope transport of soil. The instability of the surface coupled with low available soil moisture for soil-forming processes limit the amount of soil development. (Orthic Regosol Colluvial)
If the surface (e.g. sand dune, floodplain, or hillslope) remains stable for a sufficient period of time, plants can occupy and stabilize the surface and contribute organic matter above and below the soil surface. Accumulation of organic material on the surface leads to formation of LFH horizons, and the addition of below ground material causes development of Ah horizons. (Orthic Humic Regosol) With time the organic acids from the organic material and the weathering action of water penetrating the layer beneath the A horizon will lead to the development of a weak B horizon and ultimately the soil will be classified into a different soil order.
Regosolic soils are also very commonly found in cultivated landscapes throughout Canada due to the erosional truncation of the soil, especially by tillage erosion. (Orthic Regosol Tillage Erosion) Each time that soils in upper landscape positions (such as the crests of hills) are tilled, a thin layer of surface soil (often 1 mm or less) is dragged away by the tillage implement. Over decades this causes the truncation of the original upper soil horizon and the incorporation of lower soil horizons (B or C horizons) into the surface layer. If the original B horizon is lost and the ploughed layer (Ap horizon) directly overlies the C horizon then the soils are classified as Regosolic. If the C horizon contained calcium carbonate (lime), the calcium carbonate becomes mixed into the A horizon and an Apk horizon is found. The erosional loss of soil can also occur from water and wind erosion processes.
Regosolic Great Groups
The two groups of the Regosolic order are based on the amount of organic additions to the surface soil layer.
Humic Regosol Great Group
Soils of this great group have an Ah or Ap horizon greater than or equal to 10 cm in depth. (Orthic Humic Regosol)
Regosol Great Group
All other Regosolic soils. In these soils the C horizon is found at the surface, or a thin A horizon directly overlies the C horizon or thin B horizon. (Orthic Regosol Sandy)
Both great groups have four subgroups associated with them.
Orthic subgroups of the Regosol great group have either a thin A (Ah, Ap, Apk) horizon (< 10 cm thick) or no A horizon whatsoever. In the Humic great group, the A horizon is greater than or equal to 10 cm in depth. Most commonly the A horizon (if present) directly overlies a C horizon (for example C, Cca, Ck), although a thin B horizon less than 5 cm thick may be present.
Soils of the Cumulic subgroups have C or A/C surface horizons that are underlain by buried A horizons (Ahb) or a series of these buried horizons. These soils occur wherever the landsurface is episodically unstable and previous surface horizons are buried by subsequent deposition. They are common in floodplains, sand dunes, and in slope (colluvial) deposits.
Gleyed and Gleyed Cumulic
These subgroups have the same sequence of horizons as the two subgroups above except that the C horizons have faint to distinct mottles within 50 cm of the mineral surface, creating a Cgj horizon. These mottles indicate that water saturation and reducing conditions have occurred in the layer.