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Soil classification: Forest & Tundra Soils

Four main soil orders are associated with forested landscapes throughout Canada: Organic, Luvisolic, Brunisolic, and Podzolic.

Organic soils occur where water accumulates to such a degree that decomposition of organic matter by microorganisms is suppressed or essentially eliminated, and layers of organic material (commonly called peat) build up over time. The remaining three orders are found in better drained forest landscapes and at the broadest level their distribution is controlled by the nature of the parent material and by climate.

Soils of the Luvisolic order are dominant in forested landscapes underlain by tills derived from underlying sedimentary rocks or on clayey lacustrine deposits (LUVISOLIC CANADA MAP). Because the glacial materials have a sedimentary rock origin they are relatively high in clay and in base cations such as calcium and magnesium. The diagnostic feature of Luvisolic soils is a textural contrast between the A and the B horizon – the Ae horizon has less clay than the Bt horizon. Because of the high base cation content these soils typically have neutral or alkaline pH values although some acidic Luvisols are found, especially in eastern Canada.

Soils of the Brunisolic and Podzolic orders are found in the same basic parent material type and differ primarily in terms of soil moisture available for soil-forming processes. They are both forested soils found primarily on sandy parent materials in areas underlain by igneous rocks, most prominently on the Canadian Shield, but are also found in other regions on sandy glacio-fluvial deposits. Podzolic soils are dominant on sandy deposits in ecozones or parts of ecozones where the mean annual precipitation is above about 700 mm (PODZOLIC CANADA MAP). At mean annual precipitation levels below this Brunisolic soils are found on the same types of sandy deposits, most notably through NW Ontario and the Canadian Shield in the Prairie provinces (BRUNISOLIC CANADA MAP). Coniferous-dominated plant communities are the major vegetation type found on both types of soils.

Sandy glacial sediments derived from igneous rocks typically have an acidic pH because of the mineralogical composition of the sediments. The acidity of the upper soil is further increased by the organic decomposition products from the coniferous leaf litter. This creates a chemical weathering zone in the upper part of the soil where primary minerals containing aluminum, iron, and other metal ions are weathered and the ions released into the soil solution. In Podzolic soils these metal ions form complexes with the organic decomposition products (also called chelates), and the complexes move with the vertically draining water into the B horizon where they are deposited. In Brunisolic soils on sandy deposits the weathering may occur but formation of the complexes is limited and the distinctive B horizons of the Podzolic order do not form. Because of this Brunisolic soils in these setting are sometimes viewed as juvenile or immature forms of the Podzolic soils.

Further north in Canada the vegetation grades to tundra and ultimately the cold deserts of the High Arctic. In the northern third of Canada, soil-forming processes are dominated by the presence of permanently frozen ground (permafrost) close (1 to 2 m) to the soil surface. In some cases mixing of the horizons due to ice occurs where the soil texture and moisture conditions are conducive to it. In other situations the soil horizons are not mixed together. Again in areas of water accumulation an organic layer may form at the surface of the soil (as with the Organic soils in the forested regions). These soils where permafrost is the major control on soil formation are classified into the Cryosolic Order in the Canadian System of Soil Classification.

 

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