Horizons and Layers

Designations for Horizons and Layers

Horizon Designations (Chapter 18, Keys to Soil Taxonomy, Twelfth Edition 2014 – available online).

Horizon designations will follow standard procedures, including a master, transitional or combination horizon symbol to be recorded in the “Master” column, and when needed, a lower case symbol in the suffix/subordinate distinction column labeled “Sub”, and an Arabic numeral in the “No.” column.  Arabic numerals should be used in the “Prefix” column preceding the “Master” column to indicate lithologic discontinuities.  Prime symbols to distinguish otherwise identical designations should be placed in the “Master” column.  All B horizons must have a suffix/subordinate distinction.  If no designation is necessary for a cell, contestants may leave the cell blank or record a dash (-) to indicate no designation.  Only mineral horizons will be described for the contest, and students should be especially familiar with the master horizons A, E, B, C, and R layer designations, and the appropriate application of transitional and combination horizons.  Contestants are expected to be familiar with suffix symbols, vertical subdivisions and discontinuities. Special clarification will be communicated to your coach/instructor concerning special guidance when appropriate, or will be added as an addendum if necessary.

Thumbnail Descriptiond 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 layer

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)

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