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Orders: Brunisolic

Brunisolic soils are one of three soil orders for forested soils in Canada (the other two are the Podzolic and the Luvisolic orders)(Eluviated Eutric Brunisol and site). 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 quite 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 (BRUNISOLIC CANADA MAP). 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 (PODZOLIC CANADA MAP). 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 (BRUNISOLIC CANADA MAP). 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.

As befits an order that exists to “catch” soils that don’t quite make the criteria for other orders, there are several horizon types that are diagnostic for the Brunisolic order (although all must be at least 5 cm thick). The main horizon associated with the Brunisolic order is the Bm horizon – a horizon that has undergone minimal pedogenic alteration. If the horizon is very similar to a Bf or Bt horizon but does not meet the specific diagnostic thresholds required, the pedologist may assign 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.

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