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There are six categories in the Soil Taxonomy:
Order (11 taxa): This category is based largely on soil forming processes as indicated by the presence or absence of major diagnostic horizons. A given order includes soils whose properties suggest that they are not dissimilar in their genesis. They are thought to have been formed by the same general genetic processes.
Suborder (60 number od taxa): Suborders are subdivisions of orders that empasize gentic homogeneity . The presence or absence of properties associated with wetness, climatic environment, major parent material, and vegetation.
Great Group (approximate 303): Great groups are subdivisions of suborders according to similar kind, arrangement, and diagnostic horizons. The emphasis is on the presence or absence of specific diagnostic features, base status, soil temperature, and soil moisture regimes.
Subgroup (> 1,200): Subgroups are subdivisions of the great groups. The central concept of a great group makes up one group (Typic). Other subgroups may have characteristics that are intergrades between those of the central concept and those of the orders, suborders, or great groups. Extragradation is used to identify critical properties common in soils in several orders, suborders, and great groups.
Family: Families are sound in soils with a subgroup having similar physical and chemical properties affecting their response to management and especially to the penetration of plant roots. Differences in texture, mineralogy, temperature, and soil depth are bases for family differentiation.
Series (approximate 17,000 in the U.S.): Its differentiating characteristics are based primarily on the kind an arrangement of horizons, color, texture, structure, consistence, reaction of horizons, chemical, and mineralogical properties of the horizons.
(Phase: technically not a class in Soil Taxonomy but used in field surveying)
Figure 11.1.1. Orders, suborders, great groups, subgroups, families, and series - U.S. Soil Taxonomy.
Soil Taxonomy is based on the properties of
soils as they are found in the landscape. One objective of the system is
to group soils similar in genesis, but the specific criteria used to place
soils in these groups are those of soil properties. Because Soil Taxonomy
is a hierarchical system each soil is grouped first in the broadest
category first. When more details are added lower categories are defined.
Differentiating characteristics are not uniformly applied to all soils at
a given categorical level, because soils have an enormous complexity.
Therefore, in Soil Taxonomy certain types of differentiating
characteristics are applied only to certain taxa (of the level above which
one is considering) to produce the desired taxa at the level with which
one is dealing.
Table 11.2.1. Formative elements of soil orders.
Table 11.2.2. Brief description of Soil Orders.
Figure 11.2.1. Soil profiles of a Vertisol, Spodosol, Alfisol, and an Ultisol (Foth, 1984).
Figure 11.2.2. Soil profiles of an Oxisol, Mollisols, and an Andisol (Foth, 1984).