Part A - Soil Morphology


Horizon Designations (Chapter 18, pp. 315-321, Keys to Soil Taxonomy, 2010).

Horizon designation will follow standard procedures, including a master, transitional or combination horizon symbol in the "Master" column, and when needed, a lower case symbol in the suffix column, and an Arabic numeral in the "No." column. All B horizons must have a suffix.   Arabic numerals indicating lithologic discontinuities and prime symbols to distinguish otherwise identical designations should be placed in the "Master" column.   If no designation is necessary, contestants may leave a space blank or record a "-" (dash) to indicate no designation.   Only mineral horizons will be described for the contest, and students should be familiar with A, E, B, C and R horizon designations, plus transitional and combination horizons.   Suffix symbols that could be used in the contest area are:   b, c, d, g, k, n, r, ss, t, u, w, and y.  

Description of Soil Horizons

Nomenclature to describe soil horizons

Example = Bt2 horizon

1. Capital letters - master horizons

2. Lower case letters - specific characteristics or subdivisions of the master horizon (see handout)

3. Arabic numerals - further subdivision of horizons with similar features

O horizon

A surface layer dominated by organic materials (> 20% organic carbon)

Oi - slightly decomposed organic matter; can still identify the original plant and animal remains

Oe - intermediately decomposed

Oa - highly decomposed; can not identify the original source of the organic material

A horizon

- mineral horizon (<20% organic C) which forms at the surface or beneath an O horizon

- characterized by a darker color than the rest of the profile due to the accumulation of organic matter; high biological activity

- eluvial horizon (loss of materials such as iron/aluminum oxides and clays)

E horizon

- an intensively leached eluvial horizon in which organic matter along with iron/aluminum oxides and clay have been removed; most commonly found in forest soils.

- typically white or light gray in color due to the lack of coatings on the mineral surfaces

B horizon

- horizon formed beneath an A, E or O horizon and is a zone of accumulation (illuvial horizons). May accumulate clay, iron/aluminum oxides, organic matter, carbonates, etc.

C layer

- a layer of unconsolidated material showing little weathering (alteration) and biological activity (e.g., beach sand, alluvium deposited by rivers, glacial till deposited by glaciers).

R layers

- consolidated rock that can not be dug with a shovel and shows little evidence of weathering (e.g., granite, sandstone).

Transition horizons

- horizons that contain properties of two types of master horizons

Example = AB horizon

- An AB horizon has a dark color due to organic matter (A-like), plus red color due to accumulation of iron (B-like).

- Common transition horizons: AB, BA, BC, CB. The dominant horizon is listed first.

Not every soil contains all of the master horizons.

- O horizons form preferentially under forest vegetation; often absent under grass vegetation

- Soils that have been eroded may be missing their O or A horizon

- A horizon may be missing in some forest soils (e.g., O-E-B-C)

- B horizon may be missing in young soils (e.g., A-C); B horizons take a long time to form.

Additional Terminology

Solum - the zone of active soil formation; comprised of the A, E, and B horizons

Topsoil - the surface layer of the soil – the layer manipulated by tillage; typically the upper 10-25 cm

Subsoil - the soil layers beneath the topsoil (does not include the C horizon)