Lecture 18

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Chapter 7

Soil Orders


The United States contains many thousands of soil series. However, all soils on earth fit into one of only twelve soil orders. The general characteristics of these twelve orders can be learned and remembered by a normal human mind. This lecture is intended to present the most important characteristics of each of the twelve soil orders. The orders re presented in the following order: first three rather exceptional orders are presented, then the other nine are described in order from youngest to oldest. A student would be wise to review Detail 7-1 in the textbook for a brief review of each order.

1. Gelisols. The syllable indicated Gelisols in the taxonomy name is "el". These soils have permafrost, and are found in Canada, Alaska, and Siberia.


2. Histosols. Histosols are indicated by the syllable "ist". These are organic soils, common in wet, swampy or marshy areas, especially if the climate is cold. These soils have unique properties, including low bulk density, strong acidity, and the tendency to subside over time, especially if drained. Plate 4 in the textbook (between pages 160 and 161) is of the suborder Hemist, a half-recognizable Histosol.


3. Andisols. Andisols are indicated by "and". These are volcanic soils, commonly found along the Pacific Rim. Unique properties of Andisols include low bulk density, the presence of amorphous clays, strong phosphorus retention, and very favorable water-holding properties. Plate 11 in the textbook is an Andisol from Hilo Hawaii.

This concludes the description of the three unique or exceptional orders. The remaining nine orders are presented from youngest to oldest, or from least to most developed.


4. Entisols. Entisols are indicated by "ent". These soils are recent deposits, or are not very developed because perhaps of a climate not conducive to soil development. No pedogenic horizons exist, except for a weak A horizon. These soils have great variability and are widely distributed. They are particulary common in the Rocky Mountains and on barrier barrier islands. Plate 3 in the textbook is of the suborder Psamment (sandy Entisol). Suborders include the following:

Aquents — wet
Arents — Ap horizon
Fluvents — alluvial parent material
Orthents — clay or loam
Psamments — sandy


5. Inceptisols. Inceptisols are indicated by "ept". These soils are weakly developed, and are common in cool or dry climates, or in resistant or new parent material. These soils typically have a recognizable A horizon, but only a weak B horizon. Large regions of these soils are found in China, in the North Atlantic States, and in the Pacific Northwest. Plate 5 in the textbook is a inceptisols of the suborder Ochrepts. Suborders include the following:

Aquepts — wet
Ochrepts — light-colored topsoil
Plaggepts — man-made organic (i.e., manure) horizon
Tropepts — tropical climate
Umbrepts — acid, dark topsoil


6. Aridisols. Aridisols are indicated by "id". These are dry-land soils, and are the most abundant soil type worldwide. Aridisols are light-colored, rich in carbonates, and are typically vegetated with bunchgrasses and shrubs. These soils are often quite productive when irrigated & fertilized. Plate 2 in the textbook shows an Aridisol of the suborder Argids. The suborders of Aridisols are:

Argids — argillic horizon
Calcids — calcic horizon
Cambids — cambic horizon
Cryids — very cold
Durids — duripan
Gypsids — gypsic horizon
Salids — salty


7. Mollisols. Mollisols are indicated by "oll". These are dark-colored prairie soils. This is the most common soil order in United States. These soils are typical of the great plains and of mountain valleys. Characteristics of Mollisols include the deep topsoil with a high base saturation. These soils are usually farmed, but native vegetation, when present, will be grasslands and savannas. Plate 6 in the textbook is a Mollisol from the suborder Udolls. The suborders of Mollisols are:

Albolls — E horizon
Aquolls — wet
Borolls — cold
Rendolls — lime & clay
Udolls — adequate water
Ustolls — dry in winter, moist in summer
Xerolls — dry in summer, moist in winter


8. Vertisols. Vertisols are indicated by "ert". These are self-mixing swelling clays. Vertisols are rather rare on a world scale but are common in India and Texas. Vertisols are fertile but are difficult to farm because of the wide and deep cracks that form when these soils are dry. Vertisols typically have no B horizon because these soils turn over rapidly--i.e., topsoil materials fall in the cracks and end up buried in the horizon. These soils provide poor physical support for roads and buildings because they are so unstable with swelling and shrinking. Native vegetation on Vertisols is grass. Plate 10 in the textbook is a Vertisol from the suborder Usterts.


9. Alfisols. Alfisols are indicated by "alf". These are highly productive forest soils, about equal in productivity to Mollisols. In comparison to Mollisols, Alfisols tend to have somewhat lower fertility because they are more leached, but they are more leached because they are found in regions receiving more rainfall than is typical for Mollisols. Alfisols are common in the American midwest and in eastern Europe. Because of their productivity and abundance, Alfisols are among the most important soils in the world. Alfisols have an argillic horizon. Vegetation is usually hardwood forest, as is typical of Indiana and Ohio. Plate 1 in the textbook is an Alfisol of the suborder Boralf. The suborders of Alfisols are:

Aqualfs — saturated seasonally
Boralfs — cold
Udalfs — adequate water
Ustalfs — dry in winter, moist in summer
Xeralfs — dry in summer, moist in winter


10. Spodosols. Spodosols are indicated by "od". These are sandy conifer forest soils, typical of New England and Scandinavia. These soils form in acid, sandy parent material. Spodosols have an E horizon with a spodic horizon beneath it. These soils are heavily leached, and are usually found in cold climates, although they are also found in Florida. Plate 8 in the textbook is a Spodosol of the suborder Orthod, indicated a typical Spodosol.


11. Ultisols. Ultisols are indicated by "ult". These are subtropical, humid-region soils, common in the southeastern United States and south east Asia. These soils are acidic and infertile, and have an argillic horizon. Ultisols are usually forest or pasture, but are also farmed for high value crops because Ultisols can be highly productive with substantial inputs of fertilizer, lime, etc. Plate 9 is an Ultisol of the suborder Udults, indicating an Ultisol with adequate moisture.


12. Oxisols. Oxisols are indicated by "ox". These are the oldest or most highly weathered soils. They occur in warm, moist tropical regions, and are most common in South America and Africa. These soils are dominated by oxide clays which are very infertile and difficult to manage. Plate 7 in the textbook in an Oxisol from the suborder Torrox, indicating an Oxisol with a relatively dry moisture regime.


Students are encouraged to browse the following web sites.

Web sites

The Zobler World Soils Data Set contains an international view of soil classification based on the FAO system. URL: edcwww.cr.usgs.gov/glis/hyper/guide/world_soil

Soils Online contains perhaps the most extensive array of soils information of any web site. It includes soil taxonomy. URL:hintze-online.com/sos/soils-online.html


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