11 Soil Subhorizons and Diagnostic Features
Amber Anderson
- Compare and contrast epipedons, subhorizons, and diagnostic features
- Identify common subhorizons, epipedons, endopedons/diagnostic features
As discussed in the previous chapter, horizons can be assigned additional designations for significant features or implications for management found in that layer. They can also have designations for major features found both at the surface and below. We use the terms epipedon, diagnostic subsurface horizon, and subhorizon to describe additional information not captured in the main horizon designation (O, A, E, B, C, R) of a soil.
To get started, note the following definitions:
- Diagnostic horizon = layer of soil whose properties meet certain criteria specified for the purposes of soil classification, i.e. a horizon with key features used to identify the soil type
- Epipedon = one or more contiguous surface soil layer(s) with shared features
- Endopedon = one or more contiguous subsurface soil layers(s) with shared features
- Horizon designation = letters assigned to horizons to denote their properties
Note that any given diagnostic horizon can include multiple horizon designations.
Terms and Examples
Diagnostic Epipedons
Diagnostic epipedons refers to surface layers of a soil with key identifying features. Major diagnostic epipedons include:
- Mollic: Generally thick, dark soil with high base saturation (aka nonacid cation saturation, see cation saturation percentages chapter), indicating significant organic matter accumulation, commonly found in prairie areas or where organic material accumulates
- Composed of one or more A horizons
- Umbric: Similar to mollic, except with a low base saturation
- Composed of one or more A horizons
- Histic: Primarily organic rather than mineral in the surface layer
- Composed of one or more O horizons
- Anthropic: Significant human alteration, like human-transported material found at the surface
- Composed of one or more A horizons
- Ochric: Doesn’t meet other epipedon diagnostic characteristics
- Composed of one or more A horizons
Diagnostic Endopedons or Subsurface horizons
Diagnostic endopedons are groups of one or more adjacent subsurface horizons with distinguishing features. These are other significant features in the soil but below the surface layer. There are many options, but here are a few:
- Argillic: Significant illuvial clay found in this layer
- Composed of one or more B horizons
- Albic: Significantly leached horizon, grey or light color due to loss rather than wetness
- Composed of one or more E horizons
- Calcic: Significant calcium accumulation
- Composed of one or more B horizons
- Glossic: A special form of E and Bt that has ‘tongues’ of the illuvial (E or albic) materials reaching down into the illuvial horizon. The B horizon portion can be argillic, kandic or natric.
- Lithic or paralithic contact: Bedrock (hard or soft) found within profile
- Natric: Significant sodium salt accumulation, with illuvial clay accumulations
- Composed of one or more B horizons
- Spodic: Subsoil layer(s) with accumulations of organic matter and/or dark colored minerals
- Composed of one or more B horizons
- Cambic: Weak development (ex: minimal illuviation); doesn’t meet diagnostic characteristics for other diagnostic subsurface horizons
- Composed of one or more B horizons
Subhorizon
Subhorizons are further indications of processes happening within a layer, usually identified in the field. These may correlate to further diagnostic horizons or features, but may not have the corresponding lab data to support the higher designation. An example would be a Bt vs an argillic horizon: the “t” is a subhorizon meaning illuvial clay, so the described saw clay films when they described the soil. An Argillic means the presence of significant illuvial clay, generally defined by the NRCS as 1.2x the overlying clay for most soils. In most cases, these two would be described in the same zone, but it is possible to have less significant increase, resulting in a Bt not qualifying to be argillic. Similar differences could be an E that doesn’t qualify as albic due to color requirements, or Bk that doesn’t qualify as calcic.
| Subhorizon letter | Usually found on what main horizons | Field indicator |
| p | A | plowed or disturbed, potentially a plow-line, compaction due to tillage or human activity, other indicators possible |
| g | E, B, C | gleyed (grey) color due to wetness |
| w | B | weak development, not clearly falling into one of the other subhorizons |
| t | B | clay films |
| ss | B | slickensides, where shrink-swell clays have slid against each other at intersecting angles |
| k | B | white masses or coats that fizz with HCl |
| y | B | white masses of gypsum crystals, don’t fizz with HCl |
| x | B | fragipan, silica cemented root restrictive layer |
| r | C | appears to be like bedrock, but diggable with hand tools |
| d | C | dense/root restrictive layer |
| Subhorizon | Field indicator required | If significant with lab data, this horizon could be: |
| Bt | clay films | Argillic |
| Bk | white material that fizzes with HCl | Calcic |
| Bn | columnar structure or halophytic vegetation | Natric |
The below soil profile has both argillic and calcic endopedons. Each endopedon has multiple horizons, and the endopedons overlap with each other. The horizon designations for this soil (from top to bottom) are:
- A
- Bt
- Btk
- Bk
The Bt and Btk subhorizon designations are part of the argillic endopedon; the Btk and Bk subhorizon designations are part of the calcic diagnostic horizon.
Note the following definitions:
- Diagnostic horizon = layer of soil whose properties meet certain criteria specified for the purposes of soil classification, i.e. a horizon with key features used to identify the soil type
- Epipedon = one or more contiguous surface soil layer(s) with shared features
- Endopedon = one or more contiguous subsurface soil layers(s) with shared features
- Horizon designation = letters assigned to soil layers to denote their properties
- Diagnostic endopedon = group of one or more adjacent subsurface horizons with distinguishing features
- Diagnostic epipedon = surface layers of a soil with key identifying features
Check with your course instructor to see if you additionally need to know the properties of specific diagnostic horizons, and at what level of detail (this information is included in open-book and/or bonus questions only at Iowa State University and the University of Arizona).
