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Guiding questions

• How do soils play a role in sustaining natural systems?
• How are human activities affecting the stability of soil systems?

SL and HL knowledge statements

5.1.1 Soil is a dynamic system within the larger ecosystem that has its own inputs, outputs, storages and flows.

5.1.2 Soil is made up of inorganic and organic components, water and air.

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

5.1.4 Soil system inputs include those from dead organic matter and inorganic minerals.

5.1.5 Soil system outputs include losses of dead organic matter due to decomposition, losses of mineral components and loss of energy due to heat loss.

5.1.6 Transfers occur across soil horizons, into and out of soils.

5.1.7 Transformations within soils can change the components or the whole soil system.

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

5.1.9 Soils provide the foundation of terrestrial ecosystems as a medium for plant growth (a seed bank, a store of water and almost all essential plant nutrients). Carbon is an exception; it is obtained by plants from the atmosphere.

5.1.10 Soils contribute to biodiversity by providing a habitat and a niche for many species.

5.1.11 Soils have an important role in the recycling of elements as a part of biogeochemical cycles.

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

5.1.13 Soil texture affects primary productivity through the differing influences of sand, silt, clay and dead organic matter, including humus.

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

Knowledge statements for HL only

5.1.15 Soils are classified and mapped by appearance of the whole soil profile.

5.1.16 Horizons are horizontal strata that are distinctive to the soil type. The key horizons are organic layer, mixed layer, mineral soil and parent rock (O, A, B and C horizons).

5.1.17 The A horizon is the layer of soil just beneath the uppermost organic humus layer, where present. It is rich in organic matter and is also known as the mixed layer or topsoil. This is the most valuable for plant growth but, along with the O horizon, is also the most vulnerable to erosion and degradation, with implications for sustainable management of soil.

5.1.18 Factors that influence soil formation include climate, organisms, geomorphology (landscape), geology (parent material) and time.

5.1.19 Differences between soils rich in sand, silt or clay include particle size and chemical properties.

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

5.1.21 Carbon is released from soils as methane or carbon dioxide.

Practical activities

1. 5.1.3 Application of skills
   • Sample two soils from the subsoil (B horizon): one from a local garden or field, and one from a natural ecosystem.
   • Investigate texture, organic matter content, nitrogen, phosphorus, and potassium (NPK) concentrations, aeration, drainage and water retention.
   • Determine the amount of carbon in a dry soil sample by burning off the organic matter and calculating the change in mass.
2. 5.1.8 Application of skills: Create a systems flow diagram representing the soil system.
3. 5.1.15 Application of skills: Use soil profile diagrams to classify examples of soils that can be linked to the biomes studied, for example, brown earths to temperate deciduous forests, or oxisols to rainforests.
5. Add from existing PSOW booklet.

Possible engagement opportunities

• from official syllabus.
• Turn into CAS suggestions connected to UN SDGs.

Happy learning!