Classifying Soils | Soils - Part 1: The Origin and Development of Soil (How Soil Gets a Life and a Name)

Classification systems are essential for us to be able to study and communicate information about soils. While classification systems for plants and other organisms are quite old, a comprehensive soil classification system was only recently developed. (Table 1.1).

Table 1.1. Classification scheme for soils (Soil Survey Staff, 1975)
Taxonomic Level	Number of Classes	Examples (Abbreviations)
Order	10	Alfisol (alf) — deciduous forest Aridisol (id) — dry/arid, desert Entisol (ent) — beginning: no horizon Histosol (ist) — organic; boggy Inceptisol (ept) — young; few horizons Mollisol (oll) — soft; grasslands Oxisol (ox) — oxide; tropical Spodosol (od) — mineral; coniferous Ultisol (ult) — last; hot, humid Vertisol (vert) — invert; swelling
Suborder	47	Saprist — sphagnum Humod — organic Andept — volcanic
Great Group	185	Borosaprist — frigid Cryohumod — colder than frigid Dystradept — low base saturation
Subgroup	970	Fluvaquentic — flood plain Borosaprist — wetness Lithic — weathered Rock Cryohumod Aquic — wetness Dystradept
Family	4500	Coarse-silty — texture, mineralogy Mixed, mesic — temperature regime
Series Texture	10,500	McCook loam — surface Soil

This system may appear to be relatively complex. However, it is much like giving names to people that reflect their heritage or which describe them in a way that makes them different from everyone else. For instance, we might identify a person as Nancy Marie Wilson Johnson. We could also describe her as having blonde hair, blue eyes, a high forehead and being long-waisted, round-faced and short. Classifying soils is similar to using these descriptions to identify Nancy Johnson.

For our purposes we need to establish some general concepts which allow us to use and understand soil classification at a practical level.

Soils are classified according to the number of horizons in the soil profile and the soil properties of each horizon. Well developed soils are old and contain all three master horizons (A, B, and C) as well as several subdivisions of the master horizons. Young soils, or soils that have not developed extensively because of the lack of formation factors (climate, topography, vegetation, parent material, time), may have only two horizons — the surface soil or A horizon and the parent materials or C horizon. The presence of a B horizon indicates an older mature soil.