Soil

The Soil Profile

The soil profile can tell a lot about a soils fertility status. For example, a very fertile soil will have a dark-coloured surface layer owing to its rich organic matter content. By looking at the soil colours at different layers of a soil profile, scientist can characterize a soil by its age, formation process and mineral composition (know more about soil colour). The soil profile is the vertical cross section of soil showing all its layers from its surface to its bedrock.

These layers are referred to as soil horizons. A full view of a soil profile is seen from a soil pit (a hole dug from the soil surface to its bedrock). The soil profile constitutes of regolith this is the name given to all weathered material covering the soil bedrock. The regolith has two components the solum (the upper soil horizons consisting of the most weathered soil) and the saprolite (the lower soil horizons, less weathered and closer to the bedrock).

The soil horizons

The most commonly described soil horizons are the five master horizons designated by upper case letters O, A, E, B, and C (the bedrock (R) is also commonly described). The master horizons are further designated with lower case letters and in some cases numbers. These additional designations give more detailed and definite description of a horizon. For example, soil in the Oe horizon is found in the O horizon and is underlain by incompletely decomposed organic matter.

layers of a soil profile

Furthermore, a horizon can be designated by two upper case letters AE. This means that this layer is found between the A and E horizon and looks like both, but has properties more like A than E (read more about lower case suffix descriptions). It should be noted that not all soils have the complete five horizons.

The O horizon: This is a surface horizon consisting of high organic matter content (at least 20% organic matter). It is prevalent in peat soils and forested soils where anaerobic soil conditions slow down decomposition of organic matter. The outcome is high accumulation of organic matter in this horizon.

The A horizon: This is also a surface horizon. It is the topsoil of many agricultural and grassland soils. It consists of silts and clay particles. It has high organic matter content owing to the deposition of organic materials from plants and living organisms. This layer is prone to many land degradation processes such as erosion, leaching, flooding and landslides.

The E horizon: This is a subsurface horizon with lightly coloured soil. It is characterised by leaching (washing down of nutrients to lower horizons) and has low clay content compared to lower B horizons.

The B horizon: This horizon forms below the A, E or O horizons. It is dominated by illuvial accumulation of silicate clay, aluminium, humus and iron from the upper horizons1. Chemical alterations of clay minerals occur in this horizon.

The C horizon: This horizon contains the least weathered material. It is also called the saprolite. It lacks the properties of the upper soil horizons because the soils here are the least affected by rock weathering processes. Some partially weathered parent materials can be found on this horizon.

The R layer: Finally the R layer underlies all horizons. It is the bedrock of the soil. Rocks such as limestone, granites, and quartzite are found in this layer.

Bibliography and useful links

1. Brady, N (1984) The Nature & Properties of Soils. MacMillian Publishing Company, New York.
2. Providing Soil Information for New England
3. The soil profile

9 Comments

  1. This article is so poorly researched as to be useless, if not misleading to anyone trying to analyze a soil profile. It makes me doubt of the editorial quality of other publications on this website regarding topics in which I’m not an expert and thus not as able to catch errors. This is a disappointing problem I’ve encountered in Permaculture courses, teachers, and published material in the past. Why put out poor quality information if you don’t know what you’re talking about? You are just making the entire concept look bad.

    1. Hi Sebastian Belliard,
      This article is a brief introduction to the soil profile, put in a simple way for the general public. It is not a textbook or a journal publication which is more detailed. You will find more detailed, researched writing in textbooks. Nothing written in this article is technically incorrect- it may be basic information and therefore not additional knowledge to an expert but it is useful information!

  2. I thought soil profile refers to the layers of soil from its surface down to it. We could not easily know how fertile the soil is if we are not going to let analyze its soil content. pH is very important too. I am confused of what this article all about.

    1. Hi Rechel, This article clearly defines the soil profile. ‘The soil profile is the vertical cross section of soil showing all its layers from its surface to its bedrock. To clarify your doubts- yes the soil profile describes the layers of the soil from the surface to its bedrock. Soil pH is important and so is analysing the soil for fertility. There is no disputing these facts but this article is focused on the different layers in the soil profile. It is not clear what your confusions are.

  3. (1984) The Nature & Properties of Soils. I perused this book and I think it’s even older than 1984, might be dated back to 1948. It is in our field and obviously worth contemplation.

    1. Hi Jimmy Fitz, The citation for the book used is Brady, N (1984) The Nature & Properties of Soils. MacMillian Publishing Company, New York. There might be older editions (perhaps the one published in 1948 as you said) but this is the correct citation for the book used in this article

  4. My three year old son and I enjoyed this.
    Can’t wait to hear more about soil!!!:)

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