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

Soil profiles & layer characteristics

• 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

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

• 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