Soil Taxonomy with 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 square m 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.

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.

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

  • order
  • suborder
  • great group
  • subgroup
  • family
  • series

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. 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.


Source: Chapter 7: Soil Taxonomy. ENV 320 Lecture Topics. Northern Arizona University. Flagstaff, AZ. Available at (verified 19 August 2004).