Soil pH — Benchmark Detail - Nature Index

Benchmark Value

4.5 pH (CaCl₂)

Direction: Higher is desirable ↑

Form: MinimumOnly

Scoring Curve

This curve shows how a field measurement for this indicator would score across all available benchmark forms in this context. The scoring engine uses 15 benchmarks together — the OptimalRange form drives the primary score, while 14 guard(s) constrain the result.

Contributing Benchmarks

OptimalRange Primary OptimalRange Guard MaximumOnly Guard MaximumOnly Guard OptimalRange Guard MinimumOnly Guard MaximumOnly Guard MinimumOnly Guard MaximumOnly Guard OptimalRange Guard Point Guard MinimumOnly Guard MaximumOnly Guard OptimalRange Guard MinimumOnly Guard

Evidence & Context

The functional range for ecosystem health is defined by a Lower Critical Threshold of pHCaCl2 4.5, below which toxicity and severe nutrient limitation are likely.

Metric Definition:

Lower Critical Threshold of soil pH measured in 0.01M Calcium Chloride (pHCaCl2)

Benchmark Definition:

A soil pH below 4.5 measured in 0.01M Calcium Chloride indicates a critical threshold where elemental toxicity and nutrient deficiencies impair ecosystem function in the Arid Mountain Ranges & Uplands biome.

Justification:

A soil pH below 4.5 is widely recognized as representing a state of high to extreme acidity where elemental toxicities become a major limiting factor for a wide range of plants.

Sources (1)

Preview of The pH of Australian soils: field results from a national survey - CSIRO Publishing, accessed July 25, 2025,

The pH of Australian soils: field results from a national survey - CSIRO Publishing, accessed July 25, 2025, Journal

Soil pH Explained - Soil Quality Knowledge Base

Supporting Sources (16)

Additional references from the underlying research that informed this benchmark.

Preview of (PDF) The pH of Australian soils: field results from a national survey - ResearchGate, accessed July 25, 2025,

(PDF) The pH of Australian soils: field results from a national survey - ResearchGate, accessed July 25, 2025,

Direct Evidence Journal

Caritat, P. de, & Cooper, M. (2011). The pH of Australian soils: Field results from a national survey. Soil Research, 49, 172-182.

Preview of Al-Beidh, S., & Al-Tawaha, A. R. M. (2023). Basic Properties and Mineralogy of Soils and Their Impact on the Growth of Jarrah Forest in Southwestern Australia. Land, 12(6), 1236.

Al-Beidh, S., & Al-Tawaha, A. R. M. (2023). Basic Properties and Mineralogy of Soils and Their Impact on the Growth of Jarrah Forest in Southwestern Australia. Land, 12(6), 1236.

Contextual Support Journal

Soil Characteristics and Fertility of the Unique Jarrah Forest of Southwestern Australia, with Particular Consideration of Plant Nutrition and Land Rehabilitation - MDPI

Preview of ARID LANDS OF AUSTRALIA - Open Research Repository

ARID LANDS OF AUSTRALIA - Open Research Repository

Contextual Support Journal

ARID LANDS OF AUSTRALIA - Open Research Repository

Preview of ARID LANDS OF AUSTRALIA - Open Research Repository, accessed July 15, 2025,

ARID LANDS OF AUSTRALIA - Open Research Repository, accessed July 15, 2025,

Contextual Support Journal

BioMetric: Vegetation condition benchmarks, short-term review

Preview of Australia - Soils, Climate, Erosion | Britannica

Australia - Soils, Climate, Erosion | Britannica

Contextual Support GreyLiterature

SOIL TESTING

Preview of Land | State of the Environment Report 2011

Land | State of the Environment Report 2011

Contextual Support Journal

Australian dryland soils are acidic and nutrient-depleted, and have unique microbial communities compared with other drylands - PMC, accessed August 28, 2025

Preview of Soil and Landscape Grid National Soil Attribute Maps - pH (Water) (3" resolution) - Release 1

Soil and Landscape Grid National Soil Attribute Maps - pH (Water) (3" resolution) - Release 1

Contextual Support Government

CSIRO. (2021). Soil and Landscape Grid National Soil Attribute Maps - pH - CaCl2. 26

Preview of Soil and Landscape Grid of Australia - CSIRO

Soil and Landscape Grid of Australia - CSIRO

Methodology Source Government

Soil and Landscape Grid National Soil Attribute Maps - pH - Calcium Chloride (3" resolution) - Release 2 - TERN Data Discovery Portal

Preview of Soil and Landscape Grid of Australia - esoil.io, accessed July 13, 2025

Soil and Landscape Grid of Australia - esoil.io, accessed July 13, 2025

Methodology Source Government

UPTAKE OF ARSENIC BY NATIVE PLANTS GROWING ON GOLD TAILINGS IN WESTERN AUSTRALIAN RANGELANDS1

Preview of Soil Constraints in an Arid Environment—Challenges, Prospects, and Implications - MDPI, accessed July 6, 2025,

Soil Constraints in an Arid Environment—Challenges, Prospects, and Implications - MDPI, accessed July 6, 2025,

Contextual Support Journal

Soil Constraints in an Arid Environment—Challenges, Prospects, and Implications - MDPI

Preview of Soil pH | Environment, land and water - Queensland Government, accessed July 30, 2025,

Soil pH | Environment, land and water - Queensland Government, accessed July 30, 2025,

Contextual Support Government

Australian dryland soils are acidic and nutrient‐depleted, and have unique microbial communities compared with other drylands - Arid Ecology Lab

Preview of The pH of Australian soils: field results from a national survey

The pH of Australian soils: field results from a national survey

Contextual Support Journal

The pH of Australian soils: field results from a national survey

Preview of TJORITJA / WEST MACDONNELL NATIONAL PARK - Department of Tourism and Hospitality

TJORITJA / WEST MACDONNELL NATIONAL PARK - Department of Tourism and Hospitality

Contextual Support Government

TJORITJA / WEST MACDONNELL NATIONAL PARK - Department of Tourism and Hospitality

Preview of Tool 6.7 Soil health benchmarks and guidelines for managing problem soils

Tool 6.7 Soil health benchmarks and guidelines for managing problem soils

Contextual Support Journal

Protected Areas - Central Land Council, accessed August 2, 2025

Preview of Vulkathunha-Gammon Ranges National Park

Vulkathunha-Gammon Ranges National Park

Contextual Support Journal

Vulkathunha-Gammon Ranges National Park

Preview of World salinization with emphasis on Australia | Journal of Experimental Botany

World salinization with emphasis on Australia | Journal of Experimental Botany

Contextual Support Government

World salinization with emphasis on Australia | Journal of Experimental Botany

Context

Region Australia
Biome Arid Mountain Ranges & Uplands
Land Use Conservation / Protected Natural Areas
Assessment Pristine Reference
Evidence Type DegradationThreshold

Lifecycle

Status Superseded
Version 1
Effective From 4 Jun 2026
Effective To 4 Jun 2026

Notes

Below pH 4.5, elemental toxicity (e.g., aluminium) and severe nutrient deficiencies are likely to impair fundamental ecosystem functions for adapted native flora.

Related Benchmarks

Other benchmarks in the AUS-AMR-CON-SPH family.