Soil horizons

Amber Anderson

Learning Objectives
  • Match soil horizons with processes occurring within the zone
  • Identify horizons given characteristics
  • Predict potential management or use challenges based upon given horizon sequence
Keywords: Horizons, development, subhorizons

Horizon overview

A visual chart displaying the relationships of color, clay content, structure, and organic matter content for O, A, E, B, C, and R horizons. Soil color tends to get lighter as the soil profile gets deeper. Clay content decreases in the E horizon and increases in the B horizon. Structure becomes coarser as you go down the horizons. Organic matter exponentially decreases as the soil depth increases.
Illustration by Madeline Schill. 2021 in Soils-Iowa’s Nature Series. 

Watch the overview video at: https://www.youtube.com/watch?v=BUt9gAvb_1o

Transition horizons

Sometimes a layer is not clearly one process or another, but rather where two are combining. These are called transitional horizons, and indicated by two capital letters like AB or BA. The first of the two is the more dominant of the two processes. A similar-looking notation but with an added /, like E/B or B/E mean that there are distinct areas of each in the layer rather than a smooth transition.

Check it out!
Visit the NRCS Official soil series description page to find a soil series description and the horizons found in that soil. Hint: see if your name or home town/favorite town in the US have their own soil series.[1]

Subhorizons

Additional lowercase letters are used to further differentiate horizons.

Watch the overview video at: https://youtu.be/XrN68MEAy4M

Check it out!
Descriptions of all horizons and subhorizons used in NRCS classifications can be found on page 46–51 in this reference.[2]

 

Numbering

In order to distinguish one horizon from another, numbers at the end indicate multiple of the same zone, split by other differences like structure, redox features, or color.

Numbers at the beginning of the horizon indicate it is part of a different deposit or parent material. Because we might not know how many exist or be able to dig down far enough to find all parent materials, we start numbering from the surface even though the older deposit is on the bottom.

 

Key Takeaways
  • Horizons are general concepts used to describe the major process(es) happening in the layer
  • Not all horizons are found in every soil, sometimes multiple of the same horizon are found in one profile
  1. https://soilseries.sc.egov.usda.gov/osdname.aspx ↵
  2. https://www.nrcs.usda.gov/resources/guides-and-instructions/field-book-for-describing-and-sampling-soils ↵

License

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Introduction to Soil Science Copyright © 2023 by Amber Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.