Section 3.2: Soil Horizons

Outcomes

Students will:

  • Investigate the characteristics and formation of surface geology, specifically soil.

  • Differentiate soil profiles across Canada, and understand the mechanical, chemical, and biological processes that contribute to their formation.

Key Terms

Humic Material / Mesic Material / Peat / Permafrost / Plate Tectonics / Soil Horizons

See content or Module Glossary for definitions

What is a Soil Profile?

A soil profile is a vertical cross section of the ground. Soil profiles can show distinct or indistinct layers called soil horizons (see Figure 11). Soil profiles can be used to quickly compare ground characteristics between different locations and to assess whether or not a location is suitable for tasks such as farming.

Figure 11 shows a deep soil profile with a few distinct horizons. Image by Mike Kolman.
Figure 11 shows a deep soil profile with a few distinct horizons. Image by Mike Kolman.
Figure 12 shows distinct soil horizons. Original image by HolgerK.
Figure 12 shows distinct soil horizons. Original image by HolgerK.

What is a Soil Horizon?

Soil horizons form through a variety of physical, biological, and chemical processes that impact varying materials to at different depths within the soil. These processes include weathering, erosion, leaching, eluviation, illuviation, acidification, (de)calcification, (de)salinization, decomposition, mineralization, and many others. A few examples of these processes are listed below.

How do Soil Horizons Form?

Soil horizons form through a variety of physical, biological, and chemical processes that selectively move around materials to various depths within the soil. These processes include weathering, erosion, leaching, eluviation, illuviation, acidification, (de)calcification, (de)salinization, decomposition, mineralization, and many others. A few examples of these processes are listed below.

  • Physical

    • Water flowing down horizons will transport solid and dissolved materials to lower horizons through eluviation and leaching respectively.

    • Weathering and erosion in the soil will break down rocks, altering the texture and appearance.

  • Biological

    • Decomposing materials introduce new nutrients into the topsoil that will eventually make its way into lower horizons.

    • Plants absorb nutrients out from the upper layers of soil, changing the soil chemistry and possibly colour, depending on the nutrients remaining.

  • Chemical

    • Acidic solutions in the water can react with minerals and rocks, altering the appearance, texture, and chemistry of that horizon.

    • Leaching can remove salts and calcium, among many other chemicals and elements, which alters the appearance, texture, and chemistry of the soil.

How do Soil Profiles Compare Across Canada?

Soil profiles from different regions can have similar features but no two profiles will be identical, as no two regions will experience the exact same soil altering conditions year in and year out. Figure 13 shows a general overview of some of the unique regions found across Canada. It is important to note that the lines are not distinct between the various soil profiles. They provide a general sense of where these profiles can be found.

Figure 13 shows the five major regions of Canadian soil. Original image by Wikirictor.
Figure 13 shows the five major regions of Canadian soil. Original image by Wikirictor.

Canadian Shield

The landscape of this profile extends from Newfoundland on the east coast of Canada, to the majority of Ontario and Manitoba, to northern portions of Saskatchewan and Alberta, to much of the Northwest Territories and Nunavut.

  • In the past, the Canadian interior was covered with massive glaciers. About 15,000 years ago these glaciers receded north and they stripped away the top layers of soil, leaving behind very little soil and exposing bedrock, forming the Canadian Shield as we know it.

  • Soil profiles throughout the shield exhibit shallow horizons (if any) as there is very little soil and it has not had enough time to develop.

Mountains and Islands

This profile covers all of British Columbia and the Yukon as well as parts of the Northwest territories and the maritime provinces (excluding Newfoundland). Also known as the Cordilleran and Appalachian Orogens.

  • The Earth beneath our feet is not stationary; in fact, quite the opposite. Huge continental and oceanic plates are constantly moving according to the theory of plate tectonics. Quite frequently (and geologically speaking) they crash into each other, causing one plate to rise up and the other to sink below.

  • The rising side forms mountains. If the conditions at the top of the mountain are just right, soil will erode away, exposing the bedrock at the top, while burying the soil at the bottom. This rising can also reorient the horizons, making them less horizontal.

  • The sinking side forms islands due to water flowing in and eroding the larger landmasses into smaller islands. This can either expose the bedrock below or bury it below a beach.

Prairies

The soil profile for the prairies or the Interior Platform, covers the majority of Alberta, Saskatchewan, Manitoba as well as parts of the Northwest Territories, Nunavut, Ontario, Quebec, and Newfoundland & Labrador.

  • The prairies are safe away from the effects of plate tectonics meaning their soil profiles are more like what was described throughout this section.

  • Soil in this region has had plenty of time to develop relatively undisturbed, making the bedrock much deeper down than in mountainous regions.

Arctic

This profile includes the Innuitian Orogen which is mainly the most northern parts of the Northwest Territories and Nunavut.

  • The arctic region regularly experiences freezing conditions which keeps part of the ground permanently frozen, which is referred to as permafrost (Figure 14).

  • This inhibits the weathering of parent material, making for shallow soil profiles, and making it difficult for leaching to develop horizons.

Figure 14 shows permafrost exposed from erosion of the soil above. Image by Finavon.
Figure 14 shows permafrost exposed from erosion of the soil above. Image by Finavon.

Labelling Soil Horizons

Soil horizons are given alphabetical letters that are shorthand for their characteristics and are separated into two main categories: organic and mineral horizons. Soil horizon labels start with a capital letter indicating which category the horizon belongs to and roughly where the horizon is with respect to the surface layer. This is followed by a suffix that gives a more specific description of the horizon characteristics.

Organic Horizons

  • O - refers to the organic horizon, a layer of grass and moss referred to as peat.

  • Of - refers to fibric or organic materials with easily recognizable origins.

      • Om - refers to mesic material, partially decomposed organic material which contains moderate amounts of moisture.

      • Oh - refers to humic material, highly decomposed organic material that arise from the decomposition and transformation of plant, animal, and microbial residues.

      • Oco - refers to a layer of waste material deposited by aquatic creatures.

  • L - refers to an organic horizon of leaf, twig, and wood litter.

  • F - refers to an organic horizon of partially decomposed litter.

  • H - refers to an organic horizon of highly decomposed litter.

Figure 15 shows the major soil horizons. Image by Wilsonbiggs.
Figure 15 shows the major soil horizons. Image by Wilsonbiggs.

Mineral Horizons

  • A - refers to the top mineral horizon called topsoil. Topsoil is located in the zone of leaching and eluviation, where minerals are washed down to lower horizons, and in the zone of maximum organic matter accumulation, where decomposing material from the O horizon washes down to.

  • B - refers to the horizon directly below the A horizon called subsoil. Subsoil is located in the zone of accumulation, where majority of the minerals are leached or eluviated to through the process of illuviation.

  • C - refers to the horizon directly below the B horizon called parent rock or substratum. This horizon is hardly affected by the processes affecting A and B, and contains a lot of parent material.

  • R - refers to the lowest and hardest soil horizon known as bedrock.

  • W - refers to a layer where water starts to appear (not shown in Figure 15).

It is not uncommon for horizon boundaries to blur together, making them harder to distinguish. As well, it may be difficult to distinguish the boundary between the O and A horizons, making them look like one horizon.

While Sampling

While collecting your soil samples, record the following information on the TREE Sample Form and send it back with your samples:

  • The start and end of the A and B horizons as measured from the soil surface.

      • For example, say the soil horizon starts at 1 cm below the surface and ends at 11 cm, this depth will be recorded as 1-11 cm on the returned information sheet.

  • The total depth of the pit dug to collect your soil samples.

TREE Video for digging a soil pit: http://bit.ly/TREE_SoilPit
OR Learn about soil horizons and how to dig a soil pit from this TREE video: http://bit.ly/TREE_SoilPit

Additional Resources & References for Section 3.2

Resources

  • Canadian System of Soil Classification Handbook: http://sis.agr.gc.ca/cansis/publications/manuals/1998-cssc-ed3/cssc3_manual.pdf
    An in-depth information booklet (1998 edition) on the various soils found within Canada and what comprises the different soil horizons.

  • Synchrotron-Based Techniques in Soils and Sediments: Developments in Soil Science Volume 34 (Textbook), 2006, Edited by Balwant Singh & Markus Grafe, Published by Elsevier
    A textbook that covers the chemistry, physics, and mineralogy of soils and sediments analyzed using synchrotron-based research techniques. Similar techniques will be used when analyzing your soil at the Canadian Light Source. See Module 6 for more information on this!

References

Bastedo, J., James-Abra, E., & Barrett, D. (2017). Canadian shield. Retrieved from https://www.thecanadianencyclopedia.ca/en/article/shield

National Research Council of Canada. (1998). The Canadian system of soil classification (3rd ed.). Ottawa: Research Branch, Canada Dept. of Agriculture.