Syllabus Edition
First teaching 2024
First exams 2026
Components & Structure of Soil (DP IB Environmental Systems & Societies (ESS))
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Soil Systems
Soil components
Soil is made up of a complex mixture of interacting components, including inorganic and organic components, water and air
Inorganic components
Mineral matter:
Rock fragments
Sand
Silt
Clay
These components come from the weathering of parental rock
Organic components
Living organisms:
Bacteria
Fungi
Earthworms
Dead organic matter:
Decaying plants
Animal remains
Animal waste (faeces)
Other components
Water:
Essential for chemical reactions and life
Air:
Oxygen and other gases necessary for organism survival
Soils as systems
Soils are dynamic systems within larger ecosystems
As with any system, soil systems can be simplified by breaking them down into the following components:
Storages
Flows (inputs and outputs)
Transfers (change in location) and transformations (change in chemical nature, state or energy)
Soil System Storages
Storage | Description |
|---|---|
Organic matter | Accumulation of plant and animal matter in various stages of decomposition Provides nutrients, improves soil structure and enhances water-holding capacity |
Organisms | Includes microorganisms, fungi, bacteria, insects and other living organisms present in the soil They play essential roles in nutrient cycling, organic matter decomposition and soil structure formation |
Nutrients | Elements necessary for plant growth, such as nitrogen, phosphorus and potassium Nutrients are stored in the soil and are made available to plants through various biological and chemical processes |
Minerals | Inorganic components of the soil derived from weathering of rocks and minerals Contribute to the physical properties and fertility of the soil |
Air | Pore spaces within the soil are filled with air, allowing oxygen to be available for root respiration and microbial activities |
Water | Soil acts as a reservoir for water, holding it for plant uptake and providing a suitably moist habitat for soil organisms |
Soil System Inputs
Input | Description |
|---|---|
Dead organic matter | Inputs of plant material (e.g. leaf litter) and other organic materials (e.g. dead animal biomass or animal faeces) that contribute to the organic matter content in the soil |
Inorganic matter from rock material | Contributes to the mineral composition of soil, derived from parent materials (e.g. bedrock) and the weathering of exposed rock at the soil surface |
Precipitation | Rainfall or snowfall that provides water (containing dissolved minerals) to the soil system |
Energy | Solar radiation and heat influence soil temperature and biological activities |
Anthropogenic inputs | E.g. compost, fertilisers, agrochemicals, water from irrigation |
Soil System Outputs
Output | Description |
|---|---|
Leaching | Loss of dissolved minerals and nutrients from the soil into streams, rivers, lakes and oceans through water movement |
Uptake by plants | Absorption of minerals and water by plant roots for growth and development |
Soil erosion | Removal of soil particles by water or wind, leading to the loss of topsoil and degradation of soil quality |
Diffusion and evaporation | Diffusion of gases and evaporation of water from soil |
Soil System Transfers
Transfer | Description |
|---|---|
Infiltration | Process by which water enters the soil from the surface |
Percolation | Movement of water through the soil and its layers, typically downward through the soil profile |
Groundwater flow | Movement of water through the subsurface soil layers, often feeding into aquifers and other groundwater reserves |
Biological mixing | Movement of soil particles and materials by soil organisms, including burrowing animals, earthworms and root growth Contributes to the mixing of organic matter and minerals, enhancing soil structure and nutrient distribution |
Aeration | Process by which air is circulated through and mixed with soil |
Erosion | Process by which soil particles are detached and transported by wind or water |
Leaching | Process in which minerals dissolved in water are moved downwards or horizontally through the soil profile Results in the loss of nutrients from the root zone, particularly in areas with high rainfall or excessive irrigation |
Soil System Transformations
Transformation | Description |
Decomposition | The process of organic matter breakdown by microorganisms, results in the release of carbon dioxide, water and nutrients Involves the conversion of complex organic compounds into simpler forms |
Weathering | Physical and chemical processes that break down rocks and minerals into smaller particles, contribute to soil formation Includes physical weathering (mechanical breakdown) and chemical weathering (alteration of minerals through chemical reactions) |
Nutrient cycling | The cycling of nutrients within the soil-plant system involves uptake, assimilation, release and recycling of elements like nitrogen, phosphorus and potassium Ensures the availability and redistribution of essential nutrients for plant growth |
Salinisation | Accumulation of soluble salts in the soil, which can be detrimental to plant growth and soil structure It often results from improper irrigation practices, high evaporation rates, or natural soil mineralisation |
Humification | Process of organic matter transformation into stable humus It involves the accumulation of complex organic compounds, leading to the dark colouration and improved water-holding capacity of soil Contributes to soil fertility and structure |
Examiner Tips and Tricks
It is important you know the definition of processes like infiltration, percolation, decomposition and salinisation. However, make sure you are also clear on whether these processes are transfers or transformations.
If the process involves changing location, it is a transfer. Transformations involve a change in chemical nature, state or energy.
Soil Profiles
Soil profiles develop as a result of long-term interactions within the soil system
These interactions and processes form distinct layers known as horizons
These layers vary in composition and characteristics from the surface downward
This reflects the processes of soil formation over time
Profiles usually transition from organic-rich layers near the surface to more mineral-rich layers deeper down
These lower layers generally contain more inorganic material
The development of soil profiles is influenced by factors such as:
Climate
Vegetation
Parent material
Time
Real-world examples
Tropical rainforests:
Often have thick, organic-rich top soils due to rapid decomposition and high biological activity
Desert regions:
Characterised by shallow, mineral-dominated soils with distinct horizons due to low organic matter input and minimal leaching
Peat soils in boreal forests (e.g. Scandinavia):
Soils characterised by thick layers of partially decomposed organic matter (peat)
This is due to the cold, wet conditions that slow down decomposition rates, resulting in highly acidic and nutrient-poor soils
Prairie soils in the Great Plains, USA:
Soils known for their deep, dark topsoil have developed over millennia
This is due to the accumulation of organic matter from grassland vegetation and the semi-arid climate
Examiner Tips and Tricks
You don't need to learn these specific examples, they are just provided here to demonstrate how different factors can affect the soil profiles of different ecosystems.
Just recall that soils have distinct profiles that are composed of individual horizons.
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