Soil properties
Landscapes, soils and surface environments - Workshop 2b
Raphael Viscarra Rossel, Lewis Walden
2026-02-25
From minerals to properties
Block a : Weathering creates different clay minerals
Kaolinite (ancient, intense weathering → Scarp laterites)
Smectite (moderate weathering → eastern Vertosols)
Illite (intermediate → some Scarp soils)
But what does this actually mean for how soils behave?
In this section: How mineralogy creates the physical and chemical properties that control ecosystems
This is the bridge. Block 1 told us WHAT minerals form. Now we ask SO WHAT? The clay type determines texture, structure, water holding, CEC, pH — everything that matters for plants and ecosystems. By the end of this block, students should be able to predict soil behaviour from knowing the mineralogy.
Learning goals
By the end of this block you should be able to:
Describe physical properties (texture, structure, PAW)
Explain how OM and clay control CEC
Describe pH and leaching interactions with CEC
Use properties to interpret Scarp vs SCP differences
The thread through all of this: formation → mineralogy → properties → ecosystem function. Each property connects to what came before and explains what comes next.
Soil composition
By volume:
50% pores (water + air)
45% minerals (sand, silt, clay)
5% organic matter
Pore size depends on texture + structure
Bulk density reflects how much of the volume is solid vs pore space.
Soil texture
Sand
Fast
Poor
Low
Loam
Moderate
Good
Moderate
Clay
Slow
High
High
Texture = relative proportions of sand, silt, clay (mineral fraction).
WA examples:
SCP = sand-dominated (Arenosols)Scarp = loam–clay (kaolinitic Ferrosols / Kandosols)
Pore space
Controls permeability (water + air movement)
Soil air vs atmosphere:
O\(_2\)
20.6%
20.9%
CO\(_2\)
0.25%
0.035%
More CO\(_2\) from root + microbial respiration .
Plant-available water (PAW)
Gravitational – drains quickly from large pores, unavailable to plants
Capillary – held in small pores, plant-available
Hygroscopic – bound to particles, unavailable
Quick check (3 min)
Which soil is likely to have the highest plant-available water ?
Coarse sand (90% sand, low clay, low OM)
Loam (around 40% sand, 40% silt, 20% clay, moderate OM)
Heavy cracking clay (50% clay, smectitic, high OM but often waterlogged)
Discuss with a neighbour (2 min), then we’ll take a quick show of hands.
answer; often loam or well-structured clay loam has the best PAW–air balance. reinforce that “more clay” is not always better if aeration becomes limiting.
Soil structure
Soil structure = how particles are arranged into aggregates (peds)
Structure controls: infiltration, aeration, root penetration
Granular/blocky
OM + fungi bind particles
Healthy Scarp soils
Single-grain
No aggregation
SCP sands
Massive
Compacted, no pores
Degraded soils
Bulk density and compaction
Bulk density (BD): mass of dry soil per volume (g/cm³)
Well-structured loam
1.1–1.3
High
Loose sand
1.3–1.6
Moderate
Compacted soil
>1.6
Low
Compacted soil
Loose sand
⬆ BD → fewer pores → poor infiltration, restricted roots, ⬇ OM and biological activity
Organic matter (OM)
Roles:
Promotes aggregation (glues particles together)
Drives nutrient cycling (N, P, S supply)
Adds CEC and buffers pH
Major pool for carbon storage
Food source for soil biota
Important in soils with low clay and CEC (sands, kaolinitic soils).
Typical range: ~0.5–5% in mineral soils
How organic matter cycles nutrients
Inputs: Plant litter, root exudates, microbial biomassDecomposition: Microbes break down OM, releasing nutrientsMineralisation: Organic N, P, S → plant-available forms (NO\(_3^-\) , NH\(_4^+\) , PO\(_4^{3-}\) , SO\(_4^{2-}\) )Immobilisation: Microbes also lock up nutrients in biomass temporarily
Carbon storage:
Soils store ~2–3× more C than atmosphere + vegetation combined
OM decomposition depends on: temp., rain, clay and oxide content , nutrient quality
Association with fine clays and Fe/Al oxides can protect OM → long-term C storage
Connect to mineralogy: kaolinite and oxides in highly weathered soils can both protect OM (micro-aggregation) and fix P, shaping both C storage and nutrient cycling. This sets up the later biogeochemical cycles weeks.
Soil organic C distribution in Australia
Which CLORPT factors control the variation of soil organic carbon in Australia?
Cation exchange capacity (CEC)
CEC = capacity to hold and exchange cations
Holds nutrient cations (Ca²⁺, Mg²⁺, K⁺, NH₄⁺)
Also holds toxic cations H⁺ and Al³⁺ at low pH
Higher CEC → less leaching, better pH buffering, more efficient fertiliser use
Units commonly in cmol(+)/kg or meq/100 g.
CEC sources: clay mineralogy + OM
1. Clay mineralogy:
Kaolinite
1:1
Low
~5–15
Illite
2:1, K-fixed
Moderate
~20–40
Smectite
2:1, expanding
High
~80–150
2. Organic matter:
OM can contribute 30–50% of total CEC in some soils
Especially important in sandy and kaolinitic soils
Total CEC = Clay CEC + OM CEC
Total CEC is the sum of the contributions from clay minerals and organic matter. This combined capacity determines the soil’s ability to retain essential nutrient cations and buffer against pH changes.
Why OM is critical on sandy soils
Total CEC = Clay CEC + OM CEC
Scarp (Ferrosol)
~8
~4
~12
SCP (Arenosol)
<0.5
~1
<1.5
On the SCP, OM provides most of the CEC
Lose OM → lose almost all nutrient retention
Remove the OM input → CEC collapses → nutrients leach → very difficult to restore
CEC in Australian soils
Arenosol
Minimal clay (quartz sand)
<1.5
Very low
Ferrosol
Kaolinite + Fe/Al oxides
~12
Low
Chromosol
Illite / mixed clays
~35
Moderate
Vertosol
Smectite (cracking clay)
~105
High
Soil pH
Controlled by:
Weathering and leaching (removes bases)
Organic matter (produces weak acids)
CEC (capacity to retain bases)
Fertilisers and plant uptake
Balance of acidic (H⁺/Al³⁺) vs base cations (Ca²⁺, Mg²⁺, K⁺)
pH and nutrient availability
Very acidic (<5): Al toxicity, P fixation
Optimal (6–7): Most nutrients available
Alkaline (>7.5): Fe, Mn, Zn deficiency
Soil organisms are also pH-sensitive
Al toxicity: Below pH ~4.5, Al³⁺ dissolves from clay edges → damages root tips → stunted growth
How soils acidify
Rain (slightly acidic) dissolves minerals
Releases base cations (Ca²⁺, Mg²⁺, K⁺) into solution
Water leaches bases downward and out of root zone
H⁺ and Al³⁺ dominate exchange sites → acidic
Acidification rate depends on: rainfall × CEC × time x management
Which CLORPT factors control pH?
Soil pH in Australia
CEC–pH link
Calcareous
Variable
CaCO₃
7.5–10
Smectitic
High
Base cations
6–7
Illitic
Moderate
Some retention
5–6
Kaolinitic
Low
Weak
~4–5
Sandy
Very low
Almost none
~3–4
Low CEC soils acidify faster
⬆ acidity → more Al³⁺ → more toxicity
Electrical conductivity (EC) – a salinity indicator
EC = how well soil solution conducts electricity
Higher EC → more soluble salts → salinity indicator
Why it matters:
High salinity → osmotic stress (plants can’t take up water)
Common units: dS/m
<2
Low
No effect
2–4
Moderate
Sensitive crops affected
4–8
High
Most crops affected
$>$8
Very high
Only tolerant species
We’ll return to EC when we cover hydrology and salinity in later weeks
Scarp vs SCP properties
Texture
Loam–clay
Sand (>90%)
Structure
Granular/blocky
Single-grain
PAW
Moderate
Low
OM
Low (2–3%)
Very low (<0.5%)
CEC
Low (~12)
Very low (<1.5)
pH
4.5–5.5
4–6
Fertiliser loss risk
High (leaching + P fixation)
Very high (leaching)
Vegetation
Jarrah (deep roots)
Banksia (shallow)
Why properties differ
Scarp:
Ancient granite + long-term leaching
→ Kaolinite + Fe/Al oxides
→ Low CEC, acidic, strong P fixation, moderate PAW
SCP:
Young quartz sands
→ Minimal clay, almost no Fe/Al oxides
→ Very low CEC, rapid drainage, low PAW, strong leaching of nutrients
Plants adapt to constraints, they don’t “fix” soils
Activity: Predicting behaviour (20 min)
Stage 1 — Scarp vs SCP (5 min):
Using your handout reference table:
Which soil loses applied fertiliser faster? Why?
Which soil is more drought-prone ? Why?
Deep-rooted crop — which soil is more suitable? Why?
A farmer limes to raise pH — which soil holds the effect longer? Why?
Stage 2 — Chromosol challenge (15 min):
New soil, new CLORPT inputs — can you predict its properties?
Work through the property chain on your handout
Properties control function
Texture → pore size distribution, drainage, PAW
Structure + OM → aggregation, infiltration, rooting, bulk density
CEC (clay type + OM) → nutrient retention, buffering
pH (CEC + leaching + inputs) → nutrient availability, toxicity
Together, these create environmental constraints that shape ecosystems
Key takeaways
Formation → mineralogy → properties → ecosystems Properties flow from mineralogy Its all connected : You can’t change one property without affecting others (e.g., adding OM improves structure, CEC, and pH buffering simultaneously)Low CEC = vulnerable soils : Kaolinitic/sandy soils lose nutrients faster, acidify faster, and require more intensive management (Scarp and SCP are both low-CEC, just for different reasons)Scarp vs SCP synthesis : Similar climate, different parent material + time → different minerals → different properties → different ecosystems and management needs
Next (Week 3): Measuring, mapping, monitoring spatial variation
