Calcification is a pedogenic regime of climates in which evaporation on the average exceeds precipitation. Calcification is associated with a continental climatic regime with low total annual precipitation (middle-latitude steppe climate) and with a tropical wet-dry climatic regime with a short wet season (tropical steppe climate). Rainfall is not enough to leach out the bases, so that calcium and magnesium ions remain in the soil. Grasses, which use these bases, restore them to the soil surface. Colloids remain essentially in place and are not leached out, but are in a dense (flocculated) state and hold the soil into aggregate structures. Calcium carbonate, brought upward by capillary water films and evaporated in dry periods, is precipitated in the B-horizon of the soil in the form of nodules, slabs, and even dense stony layers (caliche). Microbial activity is restricted and humus may be abundantly distributed throughout the A and B-horizons. Humus occurs in progressively smaller amounts as one traces the soil into climate zones of increasing aridity. Calcification is characteristically associated with grasslands-the steppes and semi-deserts.
The pedogenic regime of gleization is characteristic of poorly drained (but not saline) environments under a moist and cool or cold climate. Gleization is thus associated with the climatic regime of polarization (tundra climate) but is also effective in bog environments of continental climates with cold winters. Low temperatures permit heavy accumulations of organic matter to form a surface layer of peaty material. Beneath this is the glei horizon, a thick layer of compact, sticky, structureless clay of bluish-gray colour. The glei horizon lies generally within the zone of ground water saturation; consequently the iron is in a partially reduced condition and imparts the bluish-gray colour.
Finally, there is the pedogenic regime of salinization, or accumulation of highly soluble salts in the soil. Salinization is associated with the desert climatic regime and takes place in poorly drained locations where surface runoff evaporates. Such locations are typically low-lying valley floors, flats, and basins in the continental interiors; and coastal flats in arid climates. Sulphates and chlorides of calcium and sodium are common salts in such soils.
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