Multimedia Gallery - Master Horizons and Layers
ID # 12
Master Horizons and Layers
Soil Horizons and description.
STEM Standard addressed: ESS2A - Earth Materials and Systems
Appropriate Grade Level(s)
Materials are best used for
General Course Areas
- Classroom Lectures
- Distance Education Classes
- Extension Presentations
- Website Information
- Introduction to Soil Science
Soil horizons are approximately parallel to the surface and have formed in place as a result of the soil processes. You can easily see soil horizons in excavations or road cuts. A well-developed or older soil may have many horizons whereas a young or poorly developed soil may have only two visible horizons. In areas of significant soil or land disturbance, such as urban areas, where erosion is severe, or where landslides have occurred, some horizons will be missing. A soil profile is a vertical cross-section of all the soil horizons at a particular location and forms the basis for understanding and communicating all the soil properties.
Soil horizons are described using a series of letters and numbers. Capital letters designate master horizons: O, A, E, B, C, and R horizons. The thickness of each layer varies with location.
The O horizon (organic horizon), though not always present, is generally the uppermost layer of the soil and is made up of organic material. It consists of accumulations of organic matter in various stages of decay. This horizon is often found in undisturbed areas such as wetlands and forests. It is not present in agricultural fields or in suburban or urban settings where the soil has been disturbed by plowing or development.
The A horizon is commonly called topsoil and typically has friable consistence and a granular structure. It is usually darker than the lower layers because of its higher organic matter content, and is often more fertile when compared with underlying horizons.
The E horizon, or eluvial horizon, is characterized by a light color or bleached appearance and is a zone of removal, or eluviation (materials exiting). Dissolved minerals, nutrients, and clay move out of the horizon carried by water as it percolates downward in the profile. The primary feature of this horizon is loss of clay, iron, aluminum, and/or organic matter leaving a concentration of more inert sand and silt particles. It is commonly found in well-developed soils or soils that have undergone extreme translocation or leaching.
The B horizon is also referred to as the illuvial horizon (zone of accumulation), or subsoil. This horizon is usually higher value (lighter) than the A horizon due to its lower organic matter content. It is the zone of accumulation for materials eluviated from the A and E horizons. B horizons always include a subordinate horizon distinction. In well-developed soils the B horizon commonly has the highest clay content.
The C horizon is referred to as the parent material or the soil material similar to the parent material. It is less weathered than the upper horizons and does not have soil structure. It may be massive, massive-rock controlled fabric, or single grained. Often it contains partially disintegrated or weathered parent material transported by gravity, wind, and water or from the underlying bedrock. The R horizon is bedrock. Bedrock can be within a few inches of the surface or many feet below. It is often too deep to observe in a soil pit.
Peer Review: Yes
Credit this item to: Know Soil Know Life, SSSA
Media Date: 2012-12-01
Provided By: Susan Chapman
* Know Soil Know Life
Please login to submit a comment.
Log In to your account
Already a member, certified, or existing customer?*
* Cookies must be accepted to log in.
Not sure if you have an account?
Check Your Email
Connect with members and access the information you need.
Ready to Join?
If you have an account, login on the left. Not sure if you have an account or need to create one? Check your email with the link above. We look forward to welcoming you.