Soils of the arctic tundra are so widespread that they may be considered a zonal type along with the podzols, gray-brown forest soils, red-yellow soils, and latosols, but because they are poorly drained they are sometimes classified as intrazonal.
Intensely cold, long winters cause soil moisture to be frozen during any months of the year. Under these cold conditions chemical alteration of the minerals is slow, and much of the parent material of the soil consists of mechanically broken particles. The slow rate of plant decomposition results in the presence of much raw humus, or peat. The tundra soils do not have simple, distinctive soil profiles, but consist of thin layers of sandy clay and raw humus. The surface may be covered with a sod of lichens, mosses, and herbaceous plants.
In the tundra regions of Siberia and North America, the condition of permanently frozen ground, or permafrost. is widespread. Curious lenses, layers, and vertical wedge like bodies of ice are present under the soil.
In parts of central Alaska, principally in the valleys of the Yukon and Tanana rivers, are dark-coloured soils which have been given great-soil group status as thiec arctic brown 'forest soils. The profiles of these soils have a thick, dark A1 horizon rich in organic matter. Downward through lower horizons the soils grade into lighter brown colours and finally to gray in the C-horizon. These soils may have originated in a surface layer of loess (wind-blown silt).
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