Soil as a dynamic system

· Soil = a dynamic system within the larger ecosystem, with inputs, outputs, storages and flows.
· Soil is a resource for life and varies widely between ecosystems.
· Soil systems are essential for the water cycle, carbon cycle and nitrogen cycle.
· A soil system can be represented using a systems flow diagram showing flows into, out of and within the soil ecosystem.
· In systems diagrams: storages = boxes; flows = arrows.

This diagram shows soil horizons as layered storages within the soil system. It is useful for linking soil profile structure to inputs, transfers, transformations and outputs in ESS systems thinking. Source

Soil components

· Soil is made of inorganic components, organic components, water and air.
· Inorganic mineral matter = rock fragments, sand, silt and clay, produced by weathering of parent rock.
· Organic components = living organisms and material from the decay of organisms.
· Soil is its own ecosystem, containing interacting soil organisms, microorganisms, fungi and animals.
· Soil properties depend on the balance between mineral particles, organic matter, water content and air spaces.

Soil profiles and horizons

· Soils develop a stable, layered structure called a soil profile over long periods of time.
· A soil profile is made of horizons formed by interactions within the soil system.
· Horizons show a transition from more organic material near the surface to more inorganic mineral material below.
· Key horizons: O horizon = organic layer; A horizon = mixed layer/topsoil; B horizon = mineral soil/subsoil; C horizon = parent material/parent rock.
· O and A horizons are most vulnerable to erosion and degradation.
· A horizon/topsoil is most valuable for plant growth because it has more oxygen, organic matter, microorganisms and nutrient recycling.
· Intensive agricultural systems may lose O and A horizons, leaving mainly B and C horizons, increasing reliance on fertilizers.

Soil inputs, outputs, transfers and transformations

· Inputs include dead organic matter and inorganic minerals.
· Dead organic matter inputs include plant litter, dead animal biomass and manure.
· Inorganic inputs include weathering, deposition, precipitation with dissolved minerals, gases, air, humidity and solar energy.
· In managed soils, anthropogenic inputs include compost, fertilizer, agrochemicals, irrigation and salinization.
· Outputs include loss of organic matter through decomposition, loss of minerals through erosion, plant uptake, leaching, gas diffusion and evaporation.
· Transfers across soil horizons include infiltration, percolation, groundwater flow, biological mixing, aeration, erosion and leaching.
· Transformations within soils include decomposition, weathering, nutrient cycling and salinization.
· Exam tip: distinguish transfer = movement/location change; transformation = chemical/state/system change.

Soil and terrestrial ecosystems

· Soils provide the foundation of terrestrial ecosystems as a medium for plant growth.
· Soils act as a seed bank, water store and store of almost all essential plant nutrients.
· Key nutrients stored in soil: nitrogen (N), phosphorus (P) and potassium (K).
· Carbon is the exception: plants obtain carbon from the atmosphere, not from soil.
· Soil contributes to biodiversity by providing a habitat and niche for many species.
· Soil communities contain high biodiversity, including microorganisms, animals and fungi.

Decomposition and nutrient cycling

· Soils recycle elements as part of biogeochemical cycles.
· Major input = dead organic matter from plants entering the soil.
· Leaf litter is broken down by detritivores such as earthworms.
· Smaller fragments are decomposed by saprotrophs, including fungi and bacteria.
· Decomposition releases nutrients that support plant growth and primary productivity.
· Soil nutrient cycling links strongly to the carbon cycle and nitrogen cycle.

This diagram shows how dead organic matter is transformed by microbes into soil organic matter and soil organic carbon. It is useful for explaining decomposition, nutrient cycling and soil carbon storage. Source

Soil texture

· Soil texture = physical make-up of the mineral soil.
· It depends on the relative proportions of sand, silt, clay and humus.
· Soil texture can be determined using a soil texture key, feel test, or by mixing soil with water and allowing particles to separate into layers.
· Sand = larger particles, usually better drainage and aeration, but lower water retention and nutrient retention.
· Clay = smaller particles, higher water retention and nutrient retention, but more risk of compaction and waterlogging.
· Silt = intermediate particle size and water-holding properties.
· Humus = dark, loose, crumbly material formed by partial decay of dead plant material.
· Humus improves mineral nutrient retention, water retention, soil structure and primary productivity.

This soil texture triangle classifies soils based on the percentage of sand, silt and clay. It is useful for interpreting soil data and linking texture to drainage, aeration, water retention and productivity. Source

Soil texture and primary productivity

· Soil texture affects primary productivity by influencing water retention, drainage, aeration, compaction, waterlogging, leaching and nutrient availability.
· Too much sand can increase leaching and reduce nutrient/water availability.
· Too much clay can reduce aeration and drainage, causing waterlogging and limiting root growth.
· Higher humus content generally improves productivity by increasing nutrient retention and water retention while maintaining a loose structure.
· Productive soils usually balance water availability, oxygen supply, nutrient storage and root penetration.

Soil as a carbon sink, store or source

· Soils can act as carbon sinks, carbon stores or carbon sources.
· This depends on the relative rates of dead organic matter input and decomposition.
· If organic matter input > decomposition, soil acts as a carbon sink.
· If inputs and outputs are balanced, soil acts as a carbon store.
· If decomposition/carbon release > organic input, soil acts as a carbon source.
· Tropical forest soils often store little carbon because warm, moist conditions increase decomposition.
· Tundra, wetlands and temperate grasslands can store large amounts of carbon due to slower decomposition or high below-ground biomass.

This diagram shows how carbon enters soil through plant inputs and moves through microbial decomposition pathways. It supports exam explanations of soils as carbon sinks, stores or sources depending on inputs and decomposition rates. Source

Practical skills and soil analysis

· Be able to compare two B horizon/subsoil samples: one from a garden or field and one from a natural ecosystem.
· Investigate texture, organic matter content, NPK concentrations, aeration, drainage and water retention.
· Determine soil carbon by drying a soil sample, burning off organic matter, and calculating the change in mass.
· Be able to create a systems flow diagram for a soil system.
· Be able to use soil profile diagrams and, at HL, link soil types to biomes.

Exam traps to avoid

· Do not say plants get carbon from soil: plants obtain carbon from the atmosphere.
· Do not confuse leaching with erosion: leaching removes dissolved minerals; erosion removes soil material.
· Do not describe soil as non-living: soil is a living ecosystem with organisms, flows and transformations.
· Do not confuse transfer and transformation in soil systems.
· Do not assume all soils store carbon equally: carbon storage depends on organic matter inputs versus decomposition rates.