Syllabus Edition

First teaching 2024

First exams 2026

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Soil Profiles & Horizons (HL) (DP IB Environmental Systems & Societies (ESS))

Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Jacque Cartwright

Soil Profiles

  • Soils develop a stable, layered structure known as a profile

  • A soil profile is a vertical section of soil

    • It shows distinct layers from the surface down to the underlying rock

  • Soil profiles help in classifying soils by examining their composition, colour, texture, and structure

    • Soil profiles can provide information about the soil’s history, nutrients, and suitability for various uses

Cross-section diagram of soil layers labeled from top to bottom: humus/organic layer, top soil, sub soil, parent material, and bedrock, with plant roots.
Soil profile diagram
Close-up of an exposed riverbank showing distinct soil layers, with visible roots and varied soil textures, suggesting erosion and stratification.
Soil profile photograph (Richard Webb, CC BY-SA 4.0)

Layer characteristics

  • Organic material:

    • The uppermost part of the soil is often rich in decomposed plants and animal material

    • This supports plant growth

  • Mineral content:

    • Different layers show varying mineral levels, often influenced by water movement

    • Some layers have nutrients while others are nutrient-poor due to leaching

  • Texture and colour:

    • Soil layers differ in colour and texture, with colours often indicating the presence of certain minerals (e.g., red soils may have high iron content)

    • Texture varies from sandy to clay-rich, affecting water retention

  • Rock fragments and weathered material:

    • Deeper layers contain larger pieces of rock and weathered parent material, providing a foundation for the soil layers above

Using soil profiles to understand soil processes

Transfer processes

  • Movement of water and nutrients:

    • Water carries nutrients downward (leaching) or upwards (capillary action), affecting the nutrient availability in each layer

  • Organic matter movement:

    • Decomposed organic material often moves downward, enriching the upper soil layers

Transformation processes

  • Decomposition:

    • Organic matter breaks down, enriching soil with nutrients essential for plants

  • Weathering:

    • Rock and parent material slowly break down, creating finer particles that contribute to soil formation

Classification of soil types by profile

  • Different soil types have distinct profiles, reflecting the climate, vegetation, and biome in which they are found:

    • Brown earth soils:

      • Common in temperate deciduous forests

      • These soils are fertile with nutrient-rich upper layers, supporting diverse plant growth

    • Oxisols:

      • Found in tropical rainforests

      • These soils have deep, red-coloured profiles rich in iron and aluminium, formed through intense weathering and significant nutrient leaching

    • Podzols:

      • Associated with coniferous forests

      • These soils are acidic with lighter layers, as nutrients are heavily leached out, leading to reduced fertility

    • Aridisols:

      • Common in desert biomes

      • These soils contain minimal organic material, with shallow profiles and limited nutrients, often sandy or rocky in texture

Soil Horizons

  • Soil profiles are made up of different horizons (layers)

  • Over a long period of time, interactions and processes within the soil system produce these horizons

  • Soil profile diagrams provide a visual representation of the horizons present in a soil system

    • There are six horizons

O horizon

  • The uppermost layer is the O horizon, also known as the organic horizon

  • It is composed mainly of organic matter such as leaf litter, decaying plant material and organic debris

  • Dark in colour due to high organic content

  • It is rich in nutrients

  • It serves as a site for nutrient cycling and organic material decomposition

  • Provides nutrients for plant growth and habitat for microorganisms

  • Easily eroded by wind and water, especially in disturbed areas such as farmland

A horizon

  • The A horizon, also called the topsoil or mixed layer

  • It is a mixed mineral and organic horizon

  • Also dark in colour due to the accumulation of organic matter

  • Essential for plant growth as it contains high levels of organic material, microorganisms, and nutrients

  • It provides a favourable environment for root development

  • Intensive farming can deplete or remove this layer, increasing the need for fertilisers to maintain soil fertility

E horizon

  • The E horizon, also known as the eluvial or leached horizon

  • Due to the downward movement of water, it is characterised by the leaching or removal of minerals and nutrients

  • It often appears lighter in colour than the surrounding horizons

B horizon

  • The B horizon, also called the illuvial or deposited horizon (sometimes called subsoil or mineral soil)

  • It is the layer where minerals and nutrients leached from the upper horizons accumulate

  • It often exhibits different colours, textures, or chemical properties compared to the horizons above and below it

  • Higher in clay, iron, and aluminium compounds compared to the upper layers

  • Holds water and nutrients that can be accessed by deeper plant roots, but is less fertile than topsoil

C horizon

  • The C horizon represents the weathered parent material from which the soil has formed

  • It is composed of partially weathered rock fragments and may contain limited organic matter

  • The properties of the C horizon influence the development and characteristics of the upper horizons

R horizon

  • The R horizon, also known as bedrock

  • It is the underlying solid rock that forms the base of the soil profile

  • It is often unweathered

  • It is relatively unaffected by biological activity and represents the original geological material from which the soil formed

Illustration of a soil profile with labeled layers: O horizon (loose decayed organic matter), A horizon (mineral matter with humus), E horizon (eluviation), B horizon (accumulation), C horizon (altered parent material), R horizon (unweathered parent material).
A soil profile is a representation of the different horizons present in a soil system
  • The distinctive horizons in a soil profile show a transition from more organic components in the upper surface to more inorganic components in the layers below

  • These layered horizons provide information about the soil's

    • Composition

    • Nutrient content

    • Water-holding capacity

    • Drainage characteristics

  • They help scientists, farmers, and land managers understand the properties and fertility of soils

    • This helps them to make informed decisions regarding land use, crop selection and soil conservation practices

Soil horizons in natural vs. agricultural systems

Natural systems

  • In undisturbed soils, all horizons (O, A, B, and C) are typically present

  • This allows rich, stable ecosystems to develop with diverse plant and animal life

Agricultural systems

  • In areas with intensive agriculture, only B and C horizons may remain intact due to erosion and loss of topsoil

  • Impact on fertility:

    • Loss of the O and A horizons reduces soil fertility and requires significant fertiliser use

  • Sustainability concerns:

    • Removing the organic-rich upper layers leads to long-term soil degradation, requiring careful management to restore soil health

  • Crop rotation and cover crops:

    • Using cover crops or rotating crops can reduce soil erosion and maintain soil fertility in agricultural systems

  • Reduced tillage:

    • Minimising tilling helps to maintain soil structure, preserving the O and A horizons and supporting long-term soil health

Examiner Tips and Tricks

The key horizons you should be aware of are: organic layer, mixed layer, mineral soil and parent rock (O, A, B and C horizons). Make sure you are clear on the differences between these horizons, particularly the organic richness of O and A compared to the mineral-rich B and C.

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.