Lecture 17

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Chapter 7

Soil Taxonomy


If you tell someone you drive a Ford Ranger, the two words "Ford Ranger" convey considerable information to those who understand pick-up trucks. They will know the vehicle manufacturer, the size and shape of the vehicle, and will be able to make inferences regarding the quality of the vehicle. This is why we name and categorize things, including soils. To the initiated a soil name can convey considerable information. Soil Taxonomy is a system whereby we name, describe, and categorize soils. The system described here is the USDA system. Other systems are in use in various places around the world.


1. Soil Taxonomy is the USDA soil classification system. The smallest classifiable unit of soil is called a pedon. A pedon is a soil individual; it must be at least 1 m2 in area. Smaller units are considered an anomaly or inclusion. A collection of contiguous similar pedons is a mapping unit. Soils are not discreet units like species--horses are distinctly different from cows. Soil taxonomy is more like rock taxonomy. One must establish a line, albeit artificial, to separate one class of rock from another, because in nature there exists a gradation in rocks from high concentrations of a particular element such as silicon to low concentrations of the element.

Soil a soil. A handful of soil is a handful of soil material. But a named soil, such as the Victoria clay, must have all components present to retain its identity. The ear of a horse is not a horse; it is just horse material. Similarly, if one holds a handful of material taken from the Victoria clay, the handful is not Victoria clay, only a sample from the Victoria clay. You might ask, why is this true, and why worry about such things? As an example, perhaps the Victoria clay is different from other clays because of a unique feature in the subsoil. If your handful of Victoria material came only from the topsoil, it contains none of the uniqueness of the Victoria clay.


2. Categories of the system. From most general to most specific, the categories of the taxonomy system are:

great group

Originally, in 1975, there were 10 soil orders. The order now called Andisols was proposed in 1978 and adopted several years later. The twelfth order, Gelisols, was added in 1997. Detail 7-1 in the textbook briefly describes the soil orders. Each order contains in its name a formative element such as "oll" in Mollisols. These letters indicate the order Mollisols in the complete taxonomic name. Designation of the soil orders was more practical than logical. Similar soils were grouped together because of the observation that certain sets of conditions usually appear together. Certain landforms such as ice fields, rugged mountains, and salt flats are not classified and are not soil.

Orders are divided into suborders based on moisture, temperature or other features. Suborders are divided into great groups, great groups are divided into subgroups, subgroups are divided into family, and families are divided into series.


3. Soil series. More than 15,000 series are found in the U.S. The series name is the common name for a soil. All soils within a given series need not be contiguous. The Victoria clay can exist in multiple locations, many miles apart. Series are named for city or geographic feature. Series are sometimes subdivided into phases based on slope, texture or other practical feature; for example the Amarillo is divided into:

Amarillo loamy fine sand, 0-3% slope
Amarillo fine sandy loam, 0-1% slope
Amarillo fine sandy loam, 1-3% slope


4. Moisture regimes. Moisture regimes pertain to the amount of available water in control section of soil. In general, the control section is the layer of soil between where 2.5 cm and 7.5 cm of water would wet a dry soil. For a clay this section is typically the region between 8 and 25 cm deep; for a sand it is typically the region from 20 to 60 cm deep. The moisture regimes are:

Aquic: usually wet, prolonged periods of poor aeration
Udic: enough water throughout the year
Perudic: very wet, much percolation
Ustic: deficient sometime during the year, most rain comes in cropping season
Xeric: deficient during cropping season, most precipitation comes in winter
Aridic or Torric: very deficient, long dry periods, short wet periods


5. Temperature regimes. Temperature regimes are based on the mean annual soil temperature (MAST) at a depth of 50 cm (usually). Typically, the MAST is ~1 0C above mean air temperature. The temperature regimes are:

Pergelic: permafrost is present
Cryic: 0 0C - 8 0C, below 15 0C in summer
Frigid: 0 0C - 8 0C, above 15 0C in summer
Mesic: 8 0C - 15 0C, (47 0F - 59 0F)
Thermic: 15 0C - 22 0C
Hyperthermic: > 22 0C, (>72 0F)

Add the term "iso" if the soil is tropical and therefore temperatures experience minimal seasonal fluctuation. For example a tropical temperature regime with a MAST of 20 0C would be isothermic.


6. Other terms are also used in taxonomy. Various formative elements are shown in Table 7-1 of the textbook. Particle size classes in the subsoil are sometimes used in family names, as are mineralogy classes.


7. Complete taxonomy. The complete taxonomic name for the Wichita soil series is as follows: Wichita fine, mixed, thermic Typic Paleustalf. The series is Wichita; the family is fine, mixed, thermic (meaning: fine textured, mixed mineralogy, thermic temperature regime); the subgroup is Typic (meaning: typical); the great group is Pale (meaning: old); the suborder is ust (meaning: ustic moisture regime); and the order is alf (meaning: Alfisols). Similarly, the Lake Charles soil series is as follows:

Lake Charles fine, montmorillonitic, thermic Typic Pelluderts.


Students are encouraged to look up the following formative elements in Table 7-1 of the textbook, and to browse the following web site.



Web site

Official NRCS USDA soil series descriptions; a database maintained by Iowa State University. URL: www.statlab.iastate.edu/soils/osd/

Nested within the above database is the 1996 edition of Keys to Soil Taxonomy.  URL: www.statlab.iastate.edu/soils/keytax/


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