4.3 - Soil Profiles and Horizons

Through the interactions of these four soil processes, the soil constituents are reorganized into visibly, chemically, and/or physically distinct layers, referred to as horizons.  There are five soil horizons: O, A, E, B, and C. (R is used to denote bedrock.)

There is no set order for these horizons within a soil.  Some soil profiles have an A-C combination, some have an O-E-B, an O-A-B, or just an O.  Some profiles may have all the horizons, O-A-E-B-C-R.  And some profiles may have multiple varieties of one horizon, such as an A-B-E-B.  There are some generalized concepts of how soil layers develop with time; these are expressed below, but due to the variability of natural processes over geologic time, generalized concepts are sometimes overly general.  Knowing something about the geomorphic history of the area being investigated helps unlock the landscape history the soils show.

  • A: An A horizon is a mineral horizon. This horizon always forms at the surface and is what many people refer to as topsoil.  Natural events, such as flooding, volcanic eruptions, landslides, and dust deposition can bury an A horizon so that it is no longer found at the surface.  A buried A horizon is a clear indication that soil and landscape processes have changed some time in the past.  Compared to other mineral horizons (E, B, or C) in the soil profile, they are rich in organic matter, giving them a darker color.  The A horizon, over time, is also a zone of loss – clays and easily dissolved compounds being leached out – and A horizons are typically more coarse (less clay) compared to underlying horizons (with the exception of an E horizon). Additions and  losses  are the dominant processes of A horizons.
A horizon
  • B: A B horizon is typically a mineral subsurface horizon and is a zone of accumulation, called illuviation. Materials that commonly accumulate are clay, soluble salts, and/or iron. Minerals in the B horizon may be undergoing transformations such as chemical alteration of clay structure. In human modified landscapes, processes such as erosion can sometimes strip away overlying horizons and leave a B horizon at the surface. Such erosion is common in sloping, agricultural landscapes. A bulldozer preparing land for a new subdivision can also leave a B horizon at the surface. The dominant processes in a B horizon are transformations and additions.
B horizon
  • C: A C horizon consists of parent material, such as glacial till or lake sediments that have little to no alteration due to the soil forming processes.  Low intensity processes, such as movement of soluble salts or oxidazation and reduction of iron may occur.  There are no dominant processes in the C horizon; minimal additions and losses of highly soluble material (e.g., salts) may occur.
C horizon
  • O: An O horizon has at least 20%  organic matter by mass. Two main scenarios result in the formation of an O horizon: saturated, anaerobic conditions (wetlands) or high production of leaf litter in forested areas. Anaerobic conditions slow the decomposition process and allow organic material to accumulate. An O horizon can have various stages of decomposed organic matter: highly decomposed, sapric; moderately decomposed, hemic; and minimally decomposed, fibric. In a fibric O layer, plant matter is recognizable (e.g., it is possible to identify a leaf). Sapric material is broken down into much finer matter and is unrecognizable as a plant part. Hemic is in between sapric and fibric, with some barely recognizable plant material present. It is possible to have multiple O horizons stacked upon one another exhibiting different decomposition stages. Because of their organic content, these horizons are typically black or dark brown in color. The dominant processes of the O horizon are additions of organic matter, and transformations from fibric to sapric.
O horizon
  • E: The E horizon appears lighter in color than an associated A horizon (above) or B horizon (below). An E horizon has a lower clay content than an underlying B horizon, and often has a lower clay content than an overlying A horizon, if an A is present.  E horizons are more common in forested areas because forests are in regions with higher precipitation and forest litter is acidic.  However, landscape hydrology, such as perched water tables, can result in the formation of an E horizon in the lower precipitation grasslands, as seen in the profile below. The dominant processes of an E horizon are losses.
E horizon
  • R:  An R layer is bedrock.  When a soil has direct contact with bedrock, especially close to the soil surface, the bedrock becomes a variable when developing land use management plans and its presence is noted in the soil profile description.

Quiz

Question
a yellow box outlines a soil horizon that is at a deeper depth of soil and is markedly darker than horizons above it

The layer at 60-90 cm (2-3 feet; measuring tape is in feet) in this image is darker because it is wicking water up from below, much like a sponge set in a pan of water. The layers above it are drier. Wetting a soil up tends to make it darker. The texture of the indicated layer is sand and there is very little organic matter or soil activity (no clay accumulation, no leaching) going on. What is the likely horizon designation for this layer? 

Looks Good! Correct: The clue is that there is no soil development so the sand is unaltered parent material. This is a C horizon.
Question
A yellow box outlines a horizon close to the surface of the soil.

The soil seen here is very deep formed in alluvial and wind-blown deposits. The area between 30-60 cm (1-2 feet; measuring tape is in feet) has strong prismatic structure due to an accumulation over time of clay from overlying horizons. This accumulation of clay suggests what horizon type? 

Looks Good! Correct: Accumulation of clay represents translocation within the soil profile. The layer in which the clay accumulates would be a B horizon. The development of strong prismatic structure would also support the B horizon designation.
Question
dark soil with worm living in it.

This moist, dark slice of soil came from a pasture and shows the upper 30 cm (12 inches; measuring tape is in inches) of the soil profile. There is strong granular structure and lots of grass roots. There is even an earthworm (at yellow arrow). This layer of soil is mainly sand, silt, and clay, but there is also much more organic matter in this layer than the ones deeper in this profile. The likely soil horizon is:   

Looks Good! Correct: Granular structure and increased organic matter in a mineral (sand, silt, clay) layer is a good indication of an A horizon. Another clue from this picture is the earthworm, these creatures like to be in soil layers that have plenty of food, which for them is organic matter.
Question
a shovel with a sample of soil that has a yellow box around that encompasses the soil just below the grass.

This slice of soil was pulled from a wetland. The first 10 cm (4 inches; in the yellow box) is all decomposing organic matter. The dark gray layer underneath is mostly mineral with some organic matter mixed in. The horizon indicated in the yellow box is a(n) ____ horizon.  

Looks Good! Correct: Horizons that are made up mostly of decomposing organic matter (in the yellow box) are O horizons. The most likely candidate for the horizon under this O horizon is an A horizon, which owes its dark color to the organic matter that coats the surfaces of the sand, silt, and clay particles.