Soils: SOIL GROUP CLASSIFICATION AND NOMENCLATURE

SOIL GROUP CLASSIFICATION AND NOMENCLATURE

Observation of natural categories.

The question of soil classification is a controversial one, but there is no doubt that classification should be based on categories that exist in nature, rather than on arbitrary creations of a classification system.

In a natural classification, groups are recognized and subsequently arranged in a system; the system may change, but the groups will pass almost unchanged from one system to another. Pedologists have approached soil classification in such a manner. Observing that some soils have an ashy surface horizon, resting on another darker in color and richer in fine earth, pedologists called them podsols (ashy soils). Other soils with a deep, dark horizon rich in organic matter and well saturated with calcium were termed chernozems (black soils). Soils of rather undifferentiated profile and of red color were named krasnozems (red soils). In this way, groups of soils have been recognized and arranged in classification systems; although opinions differ as to how these groups should be arranged within a classification system, essentially the same groups exist for all pedologists.

The soil continuum.

Because soils form a continuum, it has been difficult to follow the example of other scientists in categorizing on the basis of clear distinctions between groups. The same processes are involved in the formation of various soils, with the result that a soil may have the essential features of ando soils (amorphous clays) and those of chernozemic soils (deep humic horizon, dark and well saturated with bases) and thus be both ando and chernozemic. Soils may reflect many such heterogeneous conditions--for example, chernozemic (mollic) and gleisolic, gleisolic and podsol, and the like. Accordingly, soil classification must be manifold, recognizing that the same soil may belong to various groups.

Supplementary descriptions.

Various other terms, denoting special features, also may be precisely and simply defined and included in soil nomenclature. For example: clay, loam, sand (referring to soil texture); lithosol (stony, gravely); regosol (sandy); aeolian (formed by wind, dune); hammada (stony desert); and rock outcrop. Acid (H s.), natric (Na s.), magnesic (Mg s.), Ca soil, eutrophic, and dystrophic all refer to absorbed cations. Takyr and lunettes are special kinds of saline soils. Roof clays and sulfate saline clays (cat clays) are special kinds of clays. Gray-brown podsolic, noncalcic brown, graywooded, red-yellow podsolic, and lateritic podsolic are different kinds of lessivé soils; the first three have 2:1 clays, the later two are kaolinitic. Sod and humic are used for soils unusually rich in humus for their groups. Prairie, chestnut (kastanozems), and reddish chernozemic are different kinds of chernozemic soils. Podsolized, leached, or degraded chernozems are types of brunisolic or leached soils or both. Claypan planosols are planosolic clays. Serozems are rendzinas lacking humic horizon. Rubrozems are kaolisols with a deep humic horizon. Red desert are mini-planosols (thin horizons). Mini-planosols, mini-solonetz, and mini-podsols are planosols, solonetz, or podsols with thin horizons. Arctic browns are brunisolic permafrost rankers. Terra rossa is a cinnamonic soil from hard limestone. Terra roxa is an ando-kaolisol intergrade soil. Terre de barre is a eutrophic latosolic kaolisol. Low humic gley is an acid gleisolic soil. Humic gley is a gleisolic chermozemic soil. Desert crust is an accumulation of products of weathering in virtual absence of leaching. Alluvial, desert, forest, forest-steppe, grassland, groundwater, lowland, mangrove, meadow, mountain, paddy, paramo, prairie, tropical, and tundra are environmental, nontaxonomic terms often used in soil nomenclature.