From landscapes to soil
Landscapes, soils and surface environments - Week 2 Workshop A
2026-02-24
From horizontal to vertical (variability)
Last week: changes along a hillslope (catena/toposequence)
This week: What happens within a soil profile.
- Why do layers form?
- What creates vertical structure?
- How does landscape position influence soil development (depth)?
Water moves downslope
Water movement controls soil properties and horizon development:
- Infiltration: water enters soil at surface
- Percolation: water moves ↓ through the soil profile
- Lateral flow/Runoff: water moves → slopes
- Accumulation: water and materials accumulate in lower landscape positions
Materials move too
Not only water but materials also move though landscapes and soil profiles:
- Minerals, nutrients move with water
- Finer particles can move downslope
- OM can be accumulate in lower positions
- Sediments can be deposited in toeslopes and valleys
Note
Materials move from some areas (erosion) and are deposited in others (deposition) creating soil variability across landscapes and within soil profiles (horizons).
Movement creates vertical structure
As water and materials move down through soil:
- some material is washed out (leaching/eluviation)
- some material accumulates in lower layers (illuviation)
This creates distinct layers in the soil profile with different properties.
Important
This is how we get soil horizons and vertical structure in soils! and why we need to look at soil profiles, not just surface soils.
Podzol showing leached E horizon and accumulation in B horizon 
What is soil?
Important
Soil IS NOT dirt! It is a dynamic LIVING system that supports life & ecosystem function.
Soil is made up of 5 key components:
- Mineral particles (from weathered rock)
- Organic matter (living and dead)
- Water (the soil solution in pores)
- Air (soil atmosphere in soil pores)
- Organisms (microbes, flora, fauna)
Soil vs. sediment vs. regolith vs rock
Rock: unweathered material
Regolith: unconsolidated material
Sediment: transported and deposited
Soil: weathered material with horizons, supports life
Note
Soil has vertical structure (horizons) and biological activity, while sediment is just loose material without horizons or life. Regolith includes both soil and sediment.
Weathering: breaks rock into regolith and soil
- Physical weathering
- Temperature changes, freeze–thaw, roots
- Chemical weathering
- Dissolution, hydrolysis, oxidation–reduction
- Biological weathering
- Roots, microbes, acids from organisms
These processes create new surfaces, release minerals and nutrients, and increase porosity – the starting point for soil formation.
Soil profiles: what is a soil profile?
Each horizon has different:
- Colour, texture, structure
- Water and nutrient status
Note:
A2 is an older term for E horizon.
Tip
A soil profile shows different (horizons) that have formed through formation processes.
How horizons form. Pedogenesis = soil formation through:
- Weathering of parent material
- Additions – materials added
- Losses – materials removed
- Translocations – materials moved within
- Transformations – materials changed in place
These processes act together, at different rates and depths, creating distinct horizons with different properties, driven by water movement, biological activity, and chemical reactions.
Major horizons designations (O–A–E–B–C)
O – Organic layer (litter, dark)
A – Topsoil (minerals + OM, biologically active)
E – Eluviation zone - not present in all soils.
B – Subsoil (accumulation zone, clay/iron)
C – Weathered parent material
O Horizon - organic layer
- Surface layer of fresh to decomposed organic matter
- Mostly plant litter (leaves, twigs, bark)
- Rich brown to black color
- Common in forests
- May be absent in grasslands or cultivated soils
A horizon - the ‘mixing zone’
- Mineral particles + organic matter
- Dark color from organic matter (humus)
- High biological activity
- Granular structure - best for plant growth
- Where most roots concentrate
- often called ‘topsoil’ (but not always the top layer)
Example soil: Chromosol. 
E horizon (A2 in older terminology) - Eluviation/leaching zone
- Lighter in color (minerals and OM removed)
- Depleted in clay, iron, aluminum
- “E” = eluviation (washing out)
- Sandy or silty texture
- Not present in all soils
Important
Materials percolate from A horizon into B horizon. Creates zone of leaching (eluviation) in the E horizon.
B horizon - accumulation zone (illuviation)
- Accumulation of materials from above
- Clay, iron, aluminum, organic matter
- Often brighter colors (reds, yellows, browns)
- Denser structure
- Root penetration more difficult in this horizon
- Often called ‘subsoil’
Important
Materials percolate from A into B horizon. Creates zone of accumulation (illuviation) in the B horizon.
C horizon - weathered parent material
- Partially weathered rock or unconsolidated material
- Little biological activity
- No horizon development
- Resembles parent material
- Source of new minerals for soil formation
- May be sandy, clayey, or rocky depending on parent material
Profile examples: young vs mature soil
| Property | Young Soils | Old Soils |
|---|---|---|
| Horizon development | Weak | Strong |
| Profile depth | Shallow | Deep |
| Nutrient status | Often high | Often depleted |
| Clay content | Low | High (in B) |
| Weathering | Minimal | Extensive |
Rudosol.
Ferrosol.
Profile examples: contrasting environments
Hydrosol (Wet-Climate)
- Waterlogged with gleyed colours
- High surface OM - slow anaerobic decomposition
- In swamps, wetlands, and coastal lowlands
Calcarosol (Dry-Climate)
- CaCO\(_3\) accumulation throughout the profile
- Alkaline pH, weak structure, CO\(_3\) nodules/pans
- Widespread across arid and semi-arid regions
Reading horizons
Boundaries:
- Abrupt, Clear, Gradual, Diffuse
Texture:
- Sandy, Silty, Clayey, Loamy
Structure:
- Granular, Platy, Blocky, Massive
Time scales of soil formation
| Time | Development |
|---|---|
| Years–decades | OM accumulation |
| Decades-centuries | Distinct A horizon |
| Centuries-millenia | B horizon forms |
| Millenia-millions of years | Deep weathering, laterites |
Important
Around 1 cm of soil takes 100+ years. A non-renewable resource on human timescales.
Soil development stages
- Wet, warm climates, active organisms, develops faster.
- Cold, dry climates, little activity, develops slower.
- Young soils: weak horizons, shallow, often nutrient-rich
- Mature soils: strong horizons, deep, often nutrient-poor
Tip
Development rate depends on climate, parent material, organisms, topography.
Chronosequences (like catenas but with time instead of space)
- Same location, different ages
Different ages but similar:
Parent material
Climate
Vegetation
Topography
Only variable that varies is TIME
Chronosequences used to understand soil development and predict future changes.
Examples of Australian soil profiles
Australian soils are diverse: reflect the wide range of climates, landscapes, and parent materials.
They range from young, shallow soils in arid regions to old, deeply weathered soils in tropical areas.
Soil profiles reveal the history of landscape evolution and environmental change.
Some examples of soil profiles from different regions: 
