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

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Classifying Soils

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.

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