5.1 SoilIB Environmental Systems and Societies SL: Revision notes

Soil is a dynamic system within the larger ecosystem, with its own inputs, outputs, storages and flows. It is made up of inorganic and organic components, water and air.

• Inputs: dead organic matter and inorganic minerals.
• Outputs: loss of dead organic matter by decomposition, loss of mineral components, and loss of energy as heat.
• Transfers occur across soil horizons and into and out of soils.
• Transformations within soils change the components or the whole soil system.

Systems flow diagrams show the flows into, out of and within the soil ecosystem.

Soils develop a stable, layered structure called a profile, made up of several horizons. It is produced by interactions within the system over long periods of time.

Soils are the foundation of terrestrial ecosystems.

• A medium for plant growth: a seed bank, a store of water and almost all essential plant nutrients. Carbon is the exception; plants obtain it from the atmosphere.
• They contribute to biodiversity by providing habitat and niches for many species.
• They are central to the recycling of elements in biogeochemical cycles.

Soil texture is the physical make-up of the mineral soil. It depends on the relative proportions of sand, silt, clay and humus.

Texture affects primary productivity because sand, silt, clay and dead organic matter (including humus) influence water retention, drainage, aeration and nutrient supply differently. Sand-rich soils drain fast and hold little water or nutrients; clay-rich soils hold water and nutrients but may waterlog; humus improves retention and structure. A balanced mix (loam) with humus is typically most productive.

Soils can act as carbon sinks, stores or sources, depending on the relative rates of input of dead organic matter and decomposition.

• Input greater than decomposition: sink.
• Input equals decomposition: store.
• Decomposition greater than input: source.