Soil Horizons
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. 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)
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. (Elements of the Nature and Properties of Soils, Second Edition, page 12. This citation is obviously not going to be here for the final submission; it's just a handy reference while I construct this page.) 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. 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). (Elements pg. 53) 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 (and the O Horizon, if present) is considered topsoil. Subterranean life (including microfauna, mesofauna, and macrofauna) tend to be the most abundant in this layer. (Add how plants utilize this layer at a later point, perhaps?)
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). (Elements pg. 54) 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.
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. (Elements pg. 53)
C Horizon
The C Horizon is unconsolidated material above bedrock, 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 extend here. 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.
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:
a: Highly decomposed organic matter
b: The soil horizon has been buuried
c: Concretions/Nodules of Fe, Al, Mn, or Ti cement
d: The soil is dense from natural or artificial means, and root access is restricted
e: Moderately decomposed organic matter
f: The soil is frozen
g: Strong gleying/mottling
h: Organic matter was illuviated
i: Slightly decomposed organic matter
j: Jarosite is present
jj: Cryoturbation / Frost churning
k: Carbonate buildup is present
m: Continuous cementation
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 (shiny clay wedges) 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
Brady, Nile C; Weil, Ray R. Elements of the Nature and Properties of Soil. (Second Edition) Pearson Education, Inc. 2004.
Turenne, Jim. Soil Horizons (a Basic Power Point Presentation). http://nesoil.com/properties/horizons/