Chernozemic soils are dominant in the grassland regions of Canada including the great expanse of the Canadian Prairies. In grassland ecosystems the majority of carbon inputs occur below ground through the development of extensive root networks. Ultimately the microbial community in the soil uses the roots as an energy source and release carbon back to the atmosphere; however a small percentage of resistant organic material (or humus) remains in the soils and the amount of humus increases over time. Eventually the carbon gains through root growth approximately equal the carbon loss through microbial decomposition and the humus content of the soil is stable. The amount of carbon held by the soil at this point is primarily controlled by the climate of the site (through its influence on the microbial community) and by the texture of the soil.
The humus additions in the rooting zone create a surface layer where the original mineral parent material is enriched with organic matter. The organic matter causes this Ah horizon to be darker than the underlying mineral horizons and this darker coloured horizon is the source of the Russian word Chernozem (from "chernyi" (black) and zemlya (soil)). The colour contrast is assessed in the field using the Munsell colour system. If the contrast meets specific criteria then the upper horizon is recognized as a Chernozemic A horizon and the soil is placed into the Chernozemic order of soils.
The region dominated by Chernozemic soils has mean annual soil temperatures greater the 0°C but usually less than 6°C and experiences water deficits in most growing seasons. The mean annual water deficits typically range between a low of 6.5 cm in the sub-humid moisture class to a high of 38 cm for the sub-arid class. The great majority of Chernozemic soils are frozen at some point during the winter. The combination of cool to cold temperature conditions and dry soil moisture conditions limit microbial decomposition and allow build up of the humus in the A horizon.
The decomposition of organic matter leads to the release of organic acids, which cause a limited amount of weathering of the minerals in the upper part of the soil. The interaction of the roots and mineral material leads to the creation of a granular soil structure, which is very favourable for air and water movement in the soil and for plant growth. The movement of water through the upper part of the soil also causes the dissolution of readily dissolved (or soluble) minerals such as salts and carbonates. The dissolved salts are usually removed from the soil into the groundwater but the carbonates often re-form (or precipitate) in the upper C horizon (Cca) of the soil. These are termed secondary or pedogenic carbonates to distinguish them from the primary carbonates inherited from the parent material. The layer between the organically enriched A and the layer of carbonate accumulation progressively loses its carbonates until none remain and also undergoes slight structural and colour transformations. This leads to development of the Bm horizon.
Chernozemic soils develop in parent materials ranging from coarse sands through to fine-textured silts and clay loams. If parent materials in the grassland regions are high in expansive clay minerals (clay or heavy clay), the moisture-induced shrinking and swelling of the clays causes mixing of the soil horizons and development of soils of the Vertisolic order. Parent materials that include significant amounts of marine shales are often higher in sodium, and the presence of sodium causes properties associated with the Solonetzic order of soils to develop.
The grassland regions have undergone an almost complete conversion to agricultural production since European settlement began in the 1870s. The pervasive water deficit limits agricultural production to small grains, oilseeds, pulse, and forage crops, and livestock production is also well suited to the region. The grassland soils are also a major reservoir for the storage of soil organic carbon. It has been estimated that 15 to 30% of the original soil carbon was lost after conversion of the native Prairie to agriculture, and adoption of improved soil management can lead to increases in the amount of carbon storage by the soil. This management-related increase in soil organic carbon storage is termed carbon sequestration, and may be an important contributor to reducing total greenhouse gas emissions from Canada.
Chernozemic Great Groups
The four great groups of the Chernozemic order are based on the colour of the A horizon, which reflects the amount of organic matter present in the horizon. Ultimately this is controlled by the climate and its influence on the microbial processes that directly control the amount of organic matter (or humus) present in the surface soil horizon.
The four great groups are the Brown, Dark Brown, Black, and Dark Gray. The criteria for each, and the general climate zones they correspond to, are shown in Table 1.
Table 1: Diagnostic criteria and associated climate and vegetation for the great groups of the Chernozemic order.
|
Brown
|
Dark Brown
|
Black
|
Dark Gray
|
|
|---|---|---|---|---|
| Colour value (dry) |
4.5 to 5.5
|
3.5 to 4.5
|
< 3.5
|
3.5 to 4.5
(Ap 3.5 to 5) |
|
Colour chroma (dry)
|
Usually > 1.5
|
Usually > 1.5
|
Usually "d 1.5
|
Usually "d 1.5
|
| Mean Annual Water deficit |
19 to 38 cm
|
13 to 19 cm
|
6.5 to 13 cm
|
6.5 to 13 cm
|
|
Typical % Soil Organic Matter1
|
2.5 to 3.4
|
3.5 to 5
|
5 to 8.5
|
3.5 to 5.5
|
| Dominant Natural Vegetation |
Short and mid-Grass Prairie
|
Mid-grass Prairie
|
Aspen Parkland
|
Aspen Parkland to Mixedwood Forest Tranisition
|
1; Values shown are for cultivated soils (Rostad et al. 1993. Saskatchewan Institute of Pedology Publication no. M114).
Because of this close relationship to climate the distribution of the great groups of the Chernozemic order show a strong zonal pattern that follows the climate zones in the Canadian Prairies. The Dark Gray zone is the transition between the grassland dominated soils and the Boreal forest to the north, and these soils show evidence of both grass and forest vegetation. The A horizons associated with Dark Gray zone is typically an Ahe horizon.
Chernozemic Subgroups
| Great Groups of the Chernozemic Order | ||||
|---|---|---|---|---|
|
Subgroup
|
Brown | Dark Brown | Black | Dark Gray |
|
Vertic1
|
X | X | X | X |
|
Solonetzic1
|
X | X | X | X |
|
Eluviated1
|
X | X | X | Not applicable |
|
Calcareous1
|
X | X | X | X |
|
Rego1
|
X | X | X | X |
|
Gleyed
|
X | X | X | X |
|
Orthic
|
X | X | X | X |
1: If faint to distinct mottles occur within 50 cm of the mineral soil surface a Gleyed prefix can be added to any of these subgroups (e.g. Gleyed Vertic, Gleyed Eluviated etc.)
Orthic
These are the most widespread of the Chernozemic soils and represent the central concept of the order (or the "true" Chernozem). The A horizons (Ah, Ahe, Ahk, Ap, Apk) meet the criteria for a Chernozemic A horizon. The A is followed by one or more B horizons (Bm, Bmk, Btj, Bnjtj) at least 5 cm thick. The B is underlain by a C horizon containing calcium carbonate - either a Cca or Ck horizon. This subgroup is dominant in mid-slope positions in sloping land surfaces and throughout level landscapes. ( Example of Orthic Dark Brown Chernozem)
Rego
Soils of the Rego subgroup either lack a B horizon or have one which is less than 5 cm thick and are intermediary between the Chernozemic and Regosolic orders. The B horizons are typically Bm or Bmk horizons. Soils of this subgroup occur in several distinct landform positions. They commonly occur in agricultural landscapes on concave upper slope positions that have experienced considerable erosion of the surface soil. They also occur beside ponds where water loss to the atmosphere causes the accumulation of carbonates and salts in the upper portions of the soil profile, including the A (Apk or Ahk) horizons. Finally they are also found in landscapes where the surface soil is unstable, such as in sand dunes or river floodplains.
Calcareous
These soils have a >5-cm-thick Bmk horizon, which contains appreciable primary carbonates or secondary carbonates. This can occur by incomplete weathering of the original carbonates in the soil (in drier landform positions or in soils with very high carbonate parent materials) or by secondary deposition of carbonates adjacent to ponds (sloughs or potholes).
Eluviated
These soils show evidence of weathering of the lower portions of the A horizon and possible translocation of clay from the lower A to the upper B horizon. In the native state these soils must have an Ahe, Ae, or Aej at least 2 cm thick, which would typically be underlain by Bt or Btj. These soils would usually be found in lower slope positions where water accumulation has lead to higher rates of mineral weathering and translocation than elsewhere in the landscape.
Solonetzic
These soils are transitional between the Chernozemic and the Solonetzic orders. They have B horizons (Bnj, Btnj, Btjnj) that contain higher amounts of sodium (relative to calcium) than is typical for Chernozemic soils but that do not meet the Solonetzic criteria of a B horizon where the Ca:Na ratio is 10 or less. In the field the B horizons typically have well developed prismatic structure and hard consistence. They commonly have Ae or AB horizons, and the B horizons may be underlain by saline and carbonated C horizons (Cs, Csa, Csk, Csak).
Vertic
These soils occur on clay or heavy clay parent materials and are transitional to soils of the Vertisolic order. The uppermost horizons may be typical of Orthic soils (i.e., Ah, Ap, Bm horizons) but these horizons are underlain by horizons with slickenslides (Bss, Bkss, Ckss) within 1 m of the mineral soil surface and they may also have a weak vertic (Bvj, BCvj) horizon.
Gleyed
These soils have faint to distinct mottles within 50 cm of the mineral surface, which indicate the occurrence of prolonged periods of water saturation in the upper soil. These horizons are designated with a gj suffix (Aegj, Bmgj, Bgj, Cgj). This can occur due to surface or within-soil water concentration or due to slow vertical drainage of water due to physical conditions in the soil. These soils are transitional to soil of the Gleysolic order. Where these features occur in the other subgroups of the Chernozemic order, a Gleyed prefix is placed before the name of the other subgroup (e.g. Gleyed Rego).