Soil Horizons

A basic diagram of the most common Master Horizons of a soil profile

Soil Horizons are the distinct layers of a soil profile. They are divided into these layers, referred to as "Master Horizons" (from top to bottom): O Horizon, A Horizon, E Horizon, B Horizon, C Horizon, and R Horizon. There are also an H Horizon, F Horizon, and an L Horizon that revolve around organic material, similar to the O Horizon, but with more specific qualities and generally more obscure. The amount of horizons can vary with different soils; the most well-developed soils have all of these layers, and the least-developed soils might only have an A and a D horizon. (This Page is under construction by Jason Kaczmarczyk)

Main Master Horizons

Master Horizons are the main layers of a soil profile, described below.

O Horizon

The O Horizon is composed of organic material that has accumulated and been modified (physically and chemically) over time, typically from the remains of plant and animals [1]. This horizon is most easily observed in soils that are rarely if ever disturbed and with plenty of foliage and/or organisms nearby to contribute to its development, such as forests; in more barren locations such as grasslands, an O Horizon is usually rarer. [1] Due to its presence being determined by external factors (outside of the original parent material that form soils), it is the only layer not dominated by mineral substances. This layer has three well-accepted subordinate horizons: Oi (slightly decomposed organic matter), Oe (moderately decomposed organic matter), and Oa (highly decomposed organic matter). [1] Microbial activity is high in this layer, utilizing the abundance of organic matter and decomposing it in ways that allow it to contribute to the soil profile.

A Horizon

The A Horizon is a well-weathered and fertile layer dominated by mineral particles but still rich in organic matter, especially if covered by an O Horizon, which can leach decomposed organic matter into the A Horizon. This is a much thicker layer than the O Horizon, dominated by highly weathered mineral particles (the most highly weathered from the parent material of the soil), and typically darker and coarser than other Soil Horizons. (Elements pg. 53) The A Horizon is considered topsoil. If this layer has properties of both an A and an E Horizon, it is considered an A Horizon if it is dominated by humidified organic matter. [4] Subterranean life (including microfauna, mesofauna, and macrofauna) tends to be the most abundant in this layer.

E Horizon

The E in "E Horizon" stands for eluviation, another word for leaching. This name is appropriate because in this layer clay, iron, and aluminum oxides leach into the lower layers (mostly the B Horizon). [1] Like the O Horizon, this layer is not always present, but when it is, it's usually in forested areas and rarely in grasslands. Because of the loss of material through eluviation, it tends to be noticeably lighter than the layers above and below it. [1]

B Horizon

The B Horizon is also known as the subsoil. B Horizons are often greatly composed of material illuviated (washed in from) layers above it, mostly clay, iron, aluminum oxides (deposited by elluviated water), and minerals that formed in the layer. [1]

C Horizon

The C Horizon, also known as the substratum is unconsolidated material above bedrock. [2] It is insufficiently weathered to be considered soil, but still considered a layer of a soil profile. Subterranean life is far scarcer in this layer, and plant roots do not usually extend here, although it is usually soft enough for root penetration. [4] It is essentially a transitional layer from bedrock to soil.

R Horizon

This layer is simply bedrock with minimal to no weathering visible. It is composed of the parent material that would eventually be transformed into soil. Excavating this horizon generally requires specialized equipment, and roots are usually unable to take advantage of what cracks may be in this layer. [2]

Other Master Horizons

These master horizons are dominated by plant-based organic matter in well-drained soils, occurring most commonly in forests. [5] These layers are generally more obscure than the previously mentioned Soil Horizons due to these specialized circumstances.

L Horizon

The L Horizon stands for "Litter Horizon" and is dominated by plant material with minimal to no visible decomposition, with plant elements easy to identify. [5]

F Horizon

The F Horizon stands for "Fermentation Horizon" and is composed of moderately decomposed plant material, but the plant origins are still distinguishable. [5]

H Horizon

The H Horizon stands for "Humic Horizon" and is composed of material that is well humified and decomposed by water, and identifying plant material is difficult. [5]

Transitional Horizons

Soil Horizons do not always form distinct bands with unique and easily identified properties. Often Soil Horizons form Transitional Horizons, which have two forms. [3] The first is when a horizon has dominant properties of one Soil Horizon and subordinate properties of another; these Transitional Layers are designated by putting the dominant horizon properties letter first, followed by the subordinate horizon; an example would be a BC horizon, with properties more like a B Horizon but still demonstrating sufficient similarities to a C Horizon. [3] The second form of a Transitional Horizon is when the properties of both horizons are very comparable in representation; these have the letters separated with a "/", such as a B/C horizon, which is almost equally a B and a C Horizon. [3]

Subordinate Horizons

In order to more accurately describe the characteristics of the master horizons, lowercase letters from the Latin Alphabet are added. depending on the characteristics of the soil. Almost all letters are used, with the exception of l and u. Instead, there are jj and ss distinctions. Subordinate horizon symbols include the following: [3]

a: Highly decomposed organic matter is present

b: The soil horizon has been buried

c: Concretions/Nodules of Fe, Al, Mn, or Ti cement is present

d: The soil is dense from natural or artificial means, and root access is restricted

e: Moderately decomposed organic matter is present

f: The soil is frozen

g: Strong gleying/mottling is present

h: Organic matter was illuviated

i: Slightly decomposed organic matter is present

j: Jarosite is present

jj: Cryoturbation / Frost churning is present

k: Carbonate buildup is present

m: Continuous cementation is present

n: Sodium buildup is present

o: Iron and Aluminum oxides buildup is present

p: The soil has been heavily disturbed, typically by tillage

q: Silica buildup is present

r: Bedrock is weathered or soft

s: Organic matter and Iron and Aluminum Oxides were illuviated (not to be confused with h and o, which are only organic matter and Iron and Aluminum Oxides, respectively)

ss: Slickensides are present

t: Buildup of silicate clays is present

v: Pilinthe is present

x: Fragipan is present

y: Buildup of gypsum is present

z: Buildup with salts more soluble than gypsum is present

References

[1] Brady, Nile C; Weil, Ray R. Elements of the Nature and Properties of Soil. (Second Edition) Pearson Education, Inc. 2004. pg 53-55.

[2] Turenne, Jim. Soil Horizons (a Basic Power Point Presentation). http://nesoil.com/properties/horizons/

[3] https://www.soils.org/publications/glossary/appendix/

[4] http://www.fao.org/docrep/W8594E/w8594e0g.htm

[5] http://forestfloor.soilweb.ca/definitions/soil-horizons/