Syllabus Edition
First teaching 2015
Last exams 2025
Soil Systems
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Soil Systems
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)
Soils are highly complex, dynamic systems made up of various storages, flows, transfers and transformations
Soil System Storages
Storage | Description |
Organic matter | Refers to the 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 | Refers to elements necessary for plant growth, such as nitrogen, phosphorus, potassium - nutrients are stored in the soil and are made available to plants through various biological and chemical processes |
Minerals | Refers to the inorganic components of the soil derived from weathering of rocks and minerals - they contribute to the physical properties and fertility of the soil |
Air | The 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 |
Organic matter | Includes inputs of plant material (e.g. leaf litter) and other organic materials (e.g. animal faeces) that contribute to the organic matter content in the soil |
Inorganic matter from rock material | Represents the mineral composition derived from parent materials (e.g. bedrock) and the weathering of exposed rock at the soil surface |
Precipitation | Rainfall or snowfall that provides water to the soil system |
Infiltration | The process by which water enters the soil from the surface |
Energy | Solar radiation and heat influence soil temperature and biological activities |
Soil System Outputs
Output | Description |
Leaching | The loss of dissolved minerals and nutrients from the soil into streams, rivers, lakes and oceans through water movement |
Uptake by plants | The absorption of nutrients and water by plant roots for growth and development |
Mass movement | The downslope movement of soil particles due to gravity, such as landslides or soil creep |
Soil erosion | The removal of soil particles by water or wind, leading to the loss of topsoil and degradation of soil quality |
Soil System Transfers
Transfer | Description |
Biological mixing | The movement of soil particles and materials by soil organisms, including burrowing animals, earthworms, and root growth - it contributes to the mixing of organic matter and minerals, enhancing soil structure and nutrient distribution |
Leaching | The process in which minerals dissolved in water are moved downwards or horizontally through the soil profile - it can result in the loss of nutrients from the root zone, particularly in areas with high rainfall or excessive irrigation |
Eluviation | The removal of fine particles, such as clay and dissolved organic matter, from the upper layers of the soil by the downward movement of water |
Illuviation | The deposition of materials, including minerals and organic matter, in lower soil layers - it occurs as a result of leaching and eluviation processes, leading to the accumulation of materials in specific horizons |
Soil System Transformations
Transformation | Description |
Decomposition | The process of organic matter breakdown by microorganisms, resulting in the release of carbon dioxide, water, and nutrients - it involves the conversion of complex organic compounds into simpler forms |
Weathering | The physical and chemical processes that break down rocks and minerals into smaller particles, contributing to soil formation - it 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, involving uptake, assimilation, release, and recycling of elements like nitrogen, phosphorus, potassium - ensures the availability and redistribution of essential nutrients for plant growth |
Humification | The 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 |
Mineralisation | The decomposition (i.e. oxidation) of the chemical compounds in organic matter, by which the nutrients in those compounds are released in soluble inorganic forms that may be available to plants e.g. the conversion of organic nitrogen compounds into inorganic forms, particularly ammonium (NH4+) and nitrate (NO3-) - it occurs through microbial activity, releasing nitrogen for plant uptake and contributing to the nutrient pool in the soil |
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