5.1 SoilIB Environmental Systems and Societies HL: Revision notes

Soil is a dynamic system within the larger ecosystem, with its own inputs, outputs, storages and flows. It is made 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 minerals, loss of energy as heat.
• Transfers occur across horizons and into and out of soils; transformations change the components or the whole soil system.

Systems flow diagrams show these flows. Soils develop a stable layered profile of horizons over long periods.

Soils are the foundation of terrestrial ecosystems: a medium for plant growth (a seed bank, a store of water and almost all essential nutrients; carbon is the exception, obtained from the atmosphere). They provide habitat and niches that contribute to biodiversity and are central to recycling elements in biogeochemical cycles.

Soil texture depends on the relative proportions of sand, silt, clay and humus, and affects primary productivity through their differing influence on water, air and nutrients.

Soils act as carbon sinks, stores or sources depending on the relative rates of input of dead organic matter and decomposition. Input above decomposition: sink. Equal: store. Decomposition above input: source.

(HL) Soils are classified and mapped by the appearance of the whole soil profile. Horizons are horizontal strata that are distinctive to the soil type. The key horizons are:

• O horizon: organic layer of litter and humus.
• A horizon: mixed layer or topsoil just beneath the O horizon; rich in organic matter, the most valuable for plant growth.
• B horizon: mineral soil.
• C horizon: weathered parent rock.

The A and O horizons are the most vulnerable to erosion and degradation, which has implications for sustainable management.

(HL) Five factors influence soil formation: climate, organisms, geomorphology (landscape), geology (parent material) and time.

Climate affects weathering, leaching and decomposition; organisms add and mix organic matter; slope affects drainage and erosion; parent material supplies minerals; and soil takes very long to form.

(HL) Soils rich in sand, silt or clay differ in particle size and chemical properties: sand has large particles and drains fast; clay has very small particles and holds water and nutrients.

Soil properties can be determined by analysing sand, silt and clay percentages, percentage organic matter, percentage water, infiltration, bulk density, colour and pH.

(HL) Carbon is released from soils as methane or carbon dioxide.