Syllabus Edition
First teaching 2024
First exams 2026
Soil Profiles & Horizons (HL) (DP IB Environmental Systems & Societies (ESS))
Revision Note
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
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
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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