U.S. Soil Taxonomy

	Soil Classification
	Simplified Key to Soil Orders
	Soil Orders: Formative Elements
	Suborders: Formative Elements
	Great Groups: Formative Elements

---

Soil Classification

Six categories are distinguished according U.S. 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 taxa): Suborders are subdivisions of orders that emphasize genetic 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 found 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 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.

 

back to: [Home Page] [Natural Resources]

---

Simplified Key to Soil Orders

  

Table 1. Brief description of Soil Orders.

Soil Order	General Features
Alfisols	Alfisols develop in humid and subhumid climates, have average annual precipitation of 500-1300 mm. They are frequently under forest vegetation. Characteristic features: Clay accumulation in a Bt horizon, thick E horizon, available water much of the growing season, slightly to moderately acid.
Andisols	Andisols are soils with over 60 % volcanic ejecta (ash, cinder, pumice, basalt) with bulk densities below 900 kg/m 3. Characteristic features: Dark A horizon, early-stage secondary minerals (allophane, imogolite, ferrihydrite clays), high adsorption and immobilization of phosphorus, very high cation exchange capacity.
Aridisols	Aridisols exist in dry climates. Characteristic features: horizons of lime or gypsum accumulation, salty layers, and/or A and Bt horizons.
Entisols	Entisols have no profile development except a shallow marginal A horizon. Many recent river floodplains, volcanic ash deposits, unconsolidated deposits with horizons eroded away, and sands are Entisols.
Gelisols	Gelisols are soils that contain within 200 cm of the ground surface the condition known as permafrost.
Histosols	Histosols are organic soils (peat and mucks) consisting of variable depths of accumulated plant remains in bogs, marshes, and swamps.
Inceptisols	Inceptisols, especially in humid regions, have weak to moderated horizon development. Horizon development have been retarded because of cold climate, waterlogged soils, or lack of time for stronger development. Characteristic feature: Texture has to be finer than loamy very fine sand.
Mollisols	Mollisols are frequently under grassland, but with some broadleaf forest-covered soils. Characteristic features: Deep, dark A horizons, they may have B horizons and lime accumulation.
Oxisols	Oxisols are excessively weathered, whereas few original minerals are left unweathered. They develop only in tropical and subtropical climates. Characteristic features: Often Oxisols are over 3 m deep, have low fertility, have dominantly iron and aluminum clays, and are acid.
Spodosols	Spodosols are typically the sandy, leached soils of cold coniferous forests. Characteristic features: O horizons, strongly acid profiles, well-leached E horizons, Bh or Bs horizons of accumulated organic material plus iron and aluminum oxides.
Ultisols	Ultisols are extensively weathered soils of tropical and subtropical climates. Characteristic features: Thick A horizon, clay accumulation in a Bt, strongly acid.
Vertisols	Vertisols exist most in temperate to tropical climate with distinct wet and dry seasons. They have a high content of clays that swell when wetted and show cracks when dry. Characteristic features: Deep self-mixed A horizon , top soil falls into cracks seasonally, gradually mixing the soil to the depth of the cracking.

 

Figure 2. Soil profiles: Vertisol, Spodosol, Alfisol, and Ultisol (Foth, 1984).

 

 

Figure 3. Soil profiles: Oxisol, Mollisols, and Andisol (Foth, 1984).

 

 

Reference

Foth H.D., 1984. Fundamentals of Soil Science. John Wiley & Sons, Inc.

 

back to:  [Home Page] [Natural Resources]

---

Soil Orders: Formative Elements

back to:  [Home Page] [Natural Resources]