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Department of Soil Science
University of Saskatchewan
51 Campus Drive, Saskatoon SK S7N 5A8

Orders: Chernozemic

Chernozemic soils are dominant in the grassland regions of Canada including the great expanse of the Canadian Prairies (CHERNOZEM CANADA MAP). 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 0C but usually less than 6C 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.

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