Brunisolic soils are one of three soil orders for forested soils in Canada (the other two are the Podzolic and the Luvisolic orders). Soils of the latter two orders have diagnostic horizons (e.g. Bt, Bf, Bh) which have clear criteria that must be met for placement into these orders to occur. The Brunisolic order was created for those soils that don't meet the criteria of the other forested soil orders. Brunisolic soils can be viewed as a stage in an evolutionary sequence that begins with an unweathered parent material (Regosolic soils) and ends with development of a mature forested soil of the Podzolic or Luvisolic orders. The Brunisolic stage may, however, last for many thousands of years into the future.
The greatest extent of the Brunisolic soils occurs in a broad sweep from the southern Yukon through to NW Ontario and again through the northern Quebec. In the western portion of the Boreal Shield and the northern Taiga Shield ecozones, the Brunisolic soils occupy the same parent materials (sandy glacio-fluvial and till sediments) as the Podzolic soils elsewhere in Canada. Brunisolic soils generally are found in portions of these ecozones where the mean annual precipitation is less than 700 mm. The drier conditions coupled with the cool temperatures (mean annual temperatures a few degrees warmer or colder than 0°C) may limit the biological processes responsible for formation of Podzolic soils. Often the Brunisolic soils in these ecozones have the same visual appearance as a Podzolic soil (Eluviated Dystric Brunisol) but fail to meet the rigorous criteria for a Podzolic B horizon. The parent materials are derived from igneous rocks and typically have moderately or highly acidic pH values and support a coniferous-dominated forest cover. At the northern extent of these ecozones the Brunisolic soils are interspersed with soils of the Cryosolic order.
Brunisolic soils are also found developed in sandy parent materials in regions underlain by base-rich sedimentary rocks. These soils will often have a slightly acidic or basic pH and may have a Mixedwood (deciduous and coniferous) tree cover. This type of Brunisolic soil is found in the Mixedwood Plains ecozone of southern Ontario and Québec, in the Boreal Plains ecozone, and throughout the northern portion of the Boreal Cordillera ecozones in the southern Yukon. In the Montane Cordillera ecoregion along the Rocky Mountain front and in southern B.C., both the acidic and base-rich parent materials can occur in close proximity to each other. In southern B.C. ecoregions such as the Thompson-Okanagan Plateau, Brunisolic soils are intermixed with Podzolic soils at higher elevations, with Luvisolic soils on sedimentary derived parent materials, and with Chernozemic soils in the valleys.
The main horizon associated with the Brunisolic order is the Bm horizon – a horizon that has undergone minimal pedogenic alteration. Bm horizons typically have redder colours and more developed soil structure than the C horizon. If the horizon is very similar to a Bf or Bt horizon but does not meet the specific diagnostic thresholds required, the pedologist often assigns a j (for juvenile) suffix to the Bf or Bt (ie, Bfj, Bhjfj, Btj). This indicates to the user that the soil may have the same basic properties as the mature soil but the rules of the system requires it be placed in a separate taxonomic order.
Brunisolic Great Groups
The four great groups of the Brunisolic order are based on two factors. The first, pH, distinguishes between moderately to highly acidic, primarily igneous rock-derived parent materials and slightly acidic or basic, sedimentary rock parent materials. The second, presence of an Ah (or equivalent Ap) horizon, distinguishes between regions where earthworms and other soil fauna mix forest leaf litter into the upper mineral horizon and those regions where such mixing does not occur. Brunisols with an Ah horizon would primarily be found in the Mixedwood Plains ecozone of southern Ontario and Québec. Where these soils are cultivated, mixing by cultivation creates an Ap horizon, and the Ap must be at least 10 cm thick and have a colour value <4 to meet the A horizon criteria.
The four great groups are the four combinations possible for the two factors:
| A horizon |
Ah or Ap ≥ 10 cm
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Ah or Ap < 10 cm or absent
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Ah or Ap ≥ 10 cm
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Ah or Ap < 10 cm or absent
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| pH |
≥ 5.5
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≥ 5.5
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< 5.5
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< 5.5
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Brunisolic Subgroups
The subgroups of the Brunisolic order are based on a) presence of a Duric horizon, b) presence of an Ae horizon or slight gleying and c) soils lacking either a) or b).
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Subgroup
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Great Group
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Melanic
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Eutric
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Sombric
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Dystric
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Orthic
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X
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X
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X
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X
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Eluviated
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X
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X
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X
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X
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Gleyed
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X
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X
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X
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X
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Gleyed Eluviated
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X
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X
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X
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X
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Duric
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Not applicable
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Not applicable
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Not applicable
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Not applicable
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Orthic subgroup
Soils of this subgroup have a Bm (or Bfj or Btj) at least 5 cm thick and do not have any of the other horizons discussed below. These soils will typically have a reddish B horizon that immediately underlies the surface LFH layer.
Eluviated subgroup
These soils have an eluvial Ae (or Aej) at least 2 cm thick.
Gleyed subgroup
These soils have faint to distinct mottles within 50 cm of the surface (Bgj) or prominent mottles within 100 cm of the surface, indicating moderate levels of water saturation in the profile. This prefix can also be added to the Eluviated subgroup.
Duric subgroup
These soils have a cemented Duric horizon (Bc, BCc) within the upper 1 m of the soil.