CLORPT and soil formation

Landscapes, soils and surface environments - Week 2 Workshop B

Raphael Viscarra Rossel, Lewis Walden

2026-02-24

Workshop B overview

  • Answer the “WHY” question from Block A
  • The CLORPT framework for soil formation
  • Apply CLORPT to WA landscapes (Scarp vs SCP)
  • Understand soil-plant matching and restoration implications

Learning goals

By the end of this block:

  • Understand the 5 factors of soil formation (CLORPT)
  • Use CLORPT to explain scarp vs sandplain differences
  • Describe lateritic vs sandy/podzol profiles
  • Explain Jarrah and Banksia adaptations to soil
  • Discuss soil–plant matching for restoration

Recap Workshop A: What we learned

Workshop A showed us:

  • Soils vary vertically (horizons O–A–E–B–C)
  • Horizons form through additions, losses, transfers, transformations
  • Soil development takes time (years to millions of years)
  • Different environments → different profiles
  • Young soils: thin A over C, weak development
  • Old soils: thick horizons, deep weathering
  • Chronosequences reveal time’s role
  • But we asked: WHY do soils differ?

Today: We answer that question using two WA soil systems as examples

Our study area: Two contrasting systems

Darling Scarp:

  • Deep red-brown laterite
  • Thick, well-developed B horizon
  • Ancient (>100 M yrs)
  • Jarrah forest

Swan Coastal Plain:

  • Pale sandy profile
  • Thin, weakly developed horizons
  • Young (<1 M yrs)
  • Banksia woodland

Both have similar Mediterranean climate (~600–800 mm rainfall)

If climate is similar, WHY such different soils and ecosystems?

Environmental controls of soil formation

We’ve seen HOW soils develop:

  • Processes: additions, losses, transfers, transformations
  • Structure: horizons form over time
  • Time: soil formation is slow

Now we’ll explain WHY soils differ:

  • What controls which processes dominate?
  • What determines how fast development occurs?
  • Why do the same processes create different outcomes?

Important

Five main controlling factors, CL,O,R,P,T

The 5 factors of soil formation

  • Soil formation is controlled by 5 main factors, often remembered by the acronym CLORPT:

  • Can you tell us what each letter stands for?

Important

Vasily Vasilyevich Dokuchaev 1880s; Hans Jenny 1930s

Hint: they are the main drivers of soil formation; why soils vary so much…

CLORPT + A

A – Human activity (added by Hans Jenny (1941), but not in original CLORPT)

CLORPT as a framework for understanding soil formation

  • The purpose of conceptual model of soil formation, S = f(Cl, O, R, P, T), was to identify which independent variables control it

  • The logic was that by holding 4 factors constant and varying 1, we could isolate the effect of each factor on soil properties

Note

This is a conceptual framework for understanding soil-forming processes

Understanding each factor: Climate (Cl)

Climate controls:

  • Temperature (weathering rates)
  • Precipitation (leaching intensity)
  • Seasonality (wet/dry cycles)

Tropical - Kandosol

Arid - Calcarosol

Temperate - Brown Dermosol

  • Tropical: high temp + high rain = deep weathering, nutrient-poor
  • Arid: low rain = minimal weathering, saline/calcareous soils
  • Temperate: moderate = balanced weathering, more fertile

Understanding each factor: Organisms (O)

Organisms influence:

  • Organic matter inputs (quantity + quality)
  • Root depth and activity
  • Aerateion and bioturbation
  • Microbial weathering
  • Nutrient cycling

Note

All plants, microbes, fauna that interact with soil formation processes.

Understanding each factor: Relief (R)

Relief influences:

  • Drainage, runoff, erosion
  • Exposure to sun, wind, and rain
  • Exposure to weathering agents
  • Depth of the soil, and thus horizon development

Important

Controls where water, sediments, and nutrients, which creates catena/toposequence

Understanding each factor: Parent material (P)

Parent material affects:

  • Mineral composition
  • Texture, colour, structure
  • Chemical reactivity, chemical properties
  • Resistance to weathering and erosion

Important

Parent material sets the chemical potential and physical template for soil formation.

Understanding each factor: Time (T)

Time allows processes to act.

Time itself is the “accumulator” of the forming processes, e.g., without time, you just have parent material.

Note

All soil forming factors assert their influence over time.

CLORPT in action: isolating factors

When we see soil differences, we can infer which factor(s) control them by identifying what CHANGED:

  • Same Cl, O, R, P but different T → chronosequence (isolate time effect)
  • Same Cl, O, P, T but different R → catena (isolate relief effect)
  • Same Cl, R, P, T but different O → isolate organism effect
  • Same everything except Cl → isolate climate effect
  • Same everything except P → isolate parent material effect

Next: Apply this thinking to Scarp vs SCP

Applying CLORPT: Two WA soil systems


Scarp SCP
Elevation 200–400 m 0–50 m
Vegetation Jarrah forest Banksia woodland
Soils Laterite Podzolic sands


Both: Mediterranean, ~600–800 mm rainfall


Which factors differ? Which stay the same?

Scarp CLORPT


Darling Scarp – laterite system

  • Cl: Mediterranean; historically wetter; long-term leaching
  • O: Forest; deep roots; continuous litter
  • R: Elevated, dissected, good drainage
  • P: Granite/gneiss (>2.5 Ga)
  • T: 100+ million years

Laterite formation process

Key driver: High rainfall + deep drainage over geological time

Process:

  1. Intense weathering removes Si, Ca, Mg, K, Na (soluble elements leach downward)
  2. Fe & Al oxides concentrate in upper profile (resistant to leaching)
  3. Ferricrete hardens at top (indurated layer, root barrier)
  4. Mottled zone forms below (Fe redistribution, redox cycles)
  5. Pallid zone develops deeper (clay-rich, low Fe/Al)
  6. Distinct zones form over millions of years

Laterite properties

Physical:

  • Clay-rich B horizon (kaolinite)
  • Hard ferricrete cap (indurated Fe/Al oxides)
  • Poor structure, low permeability in mottled zone
  • Deep profile (often >10 m)

Chemical:

  • Strongly leached (Si, Ca, Mg, K, Na removed)
  • Acidic pH 4–5
  • Very low nutrient availability
  • High Fe/Al, low CEC

Can you think of implications for plant growth in laterite soils?

Jarrah adaptation


How Jarrah copes with laterite:

  • Deep rooting – penetrates to pallid zone
  • Nutrient efficiency – sclerophylly, resorption, mycorrhizae
  • Fire-adapted – nutrient cycling via ash
  • Tolerates acidity and low fertility

SCP CLORPT

Swan Coastal Plain – sand system

  • Cl: Mediterranean (same as scarp)
  • O: Banksia/Proteaceae; shallow roots; organic acids
  • R: Flat; poor drainage; seasonal waterlogging
  • P: Quartz sands (Pleistocene–Holocene)
  • T: <1 million years

Podzol formation process

Process:

  1. Quartz sand parent material (inert, no weatherable minerals)
  2. Organic acids from vegetation (Banksia) leach downward
  3. E horizon forms – bleached, ash-grey (Fe, Al, OM removed)
  4. B horizon accumulates – dark organic-rich band
  5. B horizon below – orange/brown Fe/Al oxides accumulate
  6. Forms relatively quickly (100-1000 yrs) in high-rainfall zones

Key driver: Acidic leaching in nutrient-poor sands

SCP soil properties

Physical:

  • Sand-dominated (>90%)
  • Minimal profile development (A/C or A/E/B)
  • Very low water-holding capacity (<5%)
  • Rapid infiltration, poor retention

Chemical:

  • N, P, K deficient
  • Very low CEC (<2 cmol/kg)
  • Acidic pH 4.5–5.5
  • Quartz-dominated

Can you think of implications for plant growth in these soils?

Banksia adaptation

How Banksia copes with sands:

  • Cluster (proteoid) roots – high surface area
  • Mycorrhizae – critical for P uptake
  • Fire-dependent – post-fire nutrient pulse
  • Water-stress adapted – sclerophylly, shallow dense roots

CLORPT comparison table

Factor Scarp (laterite) SCP (sands)
Cl Mediterranean; historical leaching Mediterranean; vertical leaching
O Jarrah; deep roots; mycorrhizae Banksia; shallow; cluster roots
R Elevated; good drainage Flat; poor lateral drainage
P Granite (weatherable) Quartz (inert)
T 100+ Myr <1 Myr
Result Deep laterite Leached sands

Important

Similar climate → very different soils → different ecosystems

Activity: Connecting horizons to CLORPT (15 min)

Recall from Block A:

  • Scarp: thick red B horizon, deep laterite
  • SCP: thin pale A/E, weak B, sandy

You have a handout with:

  • Profile diagrams (both systems)
  • CLORPT comparison table
  • 3 discussion questions

Work through questions (12 min):

  1. Profile → CLORPT: Which factors explain the horizon differences? (4 min)
  2. Restoration reality: What can you actually change? (4 min)
  3. Climate predictions: Two thought experiments (4 min)

Focus on explaining WHY, not just WHAT

Discussion: Question 1 - Profiles → Factors

Which CLORPT factors explain the specific horizon differences you see?

Scarp profile:


  • Thick red-brown B horizon
  • Clay-rich ferricrete layer
  • Deep mottled/pallid zones
  • Profile depth several meters

SCP profile:


  • Thin pale A/E horizon
  • Weak/absent B horizon
  • Sandy throughout
  • Shallow profile (~1m)

Share: What connections did you make between horizons and CLORPT factors?

CLORPT → Horizon connections

What explains the profile differences?

Feature Scarp SCP Key factors
B horizon Thick, clay Weak/absent P (granite vs quartz) + T (100Myr vs <1Myr)
Color Red-brown Pale P (Fe from granite vs none from quartz)
Depth Several meters ~1 meter T + P

Important

Parent material and time dominate the Scarp vs SCP contrast

Discussion: Question 2 - Restoration reality

Restoring cleared SCP land to native Banksia woodland

Part A: Profile goals

What soil features are you trying to restore?

Part B: CLORPT constraints

Which factors can/cannot be changed?

Part C: Species matching

Why plant Banksia, not Jarrah?

How do species requirements connect to soil horizons?

Share: What are your key constraints? What can you actually influence?

CLORPT constraints for restoration (part A)

Part A - Profile goals:

Should identify:

  • Thin A horizon with organic matter
  • Bleached E horizon (from podzolization)
  • Organic-rich Bh band below
  • Sandy texture throughout (this won’t change)

CLORPT constraints for restoration (Part B)

Part B - CLORPT constraints:

Cannot change:

  • P (quartz sand)
  • T (young)
  • R (flat, poor drainage)
  • Cl (Mediterranean 700mm)

Can influence:

  • O (vegetation) - THIS is what restoration focuses on
  • Soil OM - can add but must maintain with right vegetation
  • Nutrients - can add fertilizer but: (1) leaches quickly in sand, (2) native plants adapted to LOW nutrients, excess can be detrimental

Species matching: Why Banksia works, Jarrah doesn’t

Banksia on SCP sands ✓

  • Cluster roots → P uptake in poor soil
  • Shallow roots → suit sandy profile
  • Organic acids → maintain podzol
  • Fire-adapted → nutrient cycling
  • Sandy-adapted mycorrhizae

Jarrah on SCP sands ✗

  • Needs clay B horizon (absent)
  • Needs weatherable minerals (quartz inert)
  • Deep roots need pallid zone (absent)
  • Laterite mycorrhizae (wrong type)

Discussion: Question 3 - Climate predictions

Two thought experiments: Same P and R, different climate

Scenario A: Tropical dunes

Darwin coastal dunes - quartz sand, flat, young

BUT: 1500mm tropical climate

How would the profile differ from SCP podzol?

Scenario B: Arid plateau

Central Australian granite plateau - granite, elevated, ancient

BUT: 300mm arid climate

How would the profile differ from Scarp laterite?

Share: What did you predict for each scenario?

Scenario A: Tropical coastal dunes

Changes from SCP: - 1500mm rain (vs 700mm) + warm year-round + tropical vegetation

Result: - Still sandy, still leached - BUT weaker podzolization than SCP

Why? Warmth → rapid OM decomposition → less organic acid → weaker E/Bh development

Note

More rain ≠ stronger podzolization when climate also affects OM dynamics

Scenario B: Arid granite plateau

Changes from Scarp: - 300mm rain (vs 700mm) + arid conditions + different vegetation

Result: - Red-brown color (Fe still forms) - BUT shallow, not deep laterite - Higher pH, nutrients retained, possible calcrete

Why? Less water → slower/shallower weathering → no deep ferricrete-mottled-pallid sequence

Key: Same P + T doesn’t guarantee laterite when Cl limits weathering intensity

Takeaways for Q3

Changing Cl (climate) on two different systems:

  • Quartz sand + more rain = different podzol (still constrained by quartz)
  • Granite + less rain = different from laterite (climate limits development)
  • P sets the potential (what CAN happen)
  • Cl determines rate/intensity (HOW FAST/DEEP)
  • T allows accumulation (HOW MUCH)
  • O modifies pathways (WHAT process)
  • R controls water movement (WHERE)

Important

CLORPT factors INTERACT, they don’t act independently

Implications for management

Based on your discussion:

  • Soil–plant matching is critical for restoration
  • Formation history constrains what’s possible
  • Climate alone doesn’t determine ecosystems
  • Indigenous management worked WITH these constraints for thousands of years

[Image needed: Scarp forest + SCP BW side-by-side]

Two pathways, two ecosystems

Different CLORPT ➡ different formation ➡ different constraints ➡ different adaptations

Key takeaways

  • CLORPT explains variation even under broadly similar climates
  • Laterite pathway (scarp): ancient, weathered, Jarrah-adapted
  • Podzol pathway (SCP): young, leached sands, Banksia-adapted
  • Soil–plant matching matters for conservation and restoration
  • Next: Mineralogy – the “bridge” between formation and properties

So even on two landscapes with broadly similar climates, different CLORPT histories have sent them down different formation pathways, and it’s those pathways that set the rules for which plants can thrive and how we manage these systems.

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