21  The New Zealand Soil Classification

Classifying a profile according to the first three levels of the NZSC (Hewitt and MWLR Pedology Staff, 2024) requires the following steps:

1. Identify diagnostic features
2. Follow the Key to Soil Orders
3. Follow the Keys to Group and Subgroup.

To extend the classification to levels 4 and 5, the profile form must be identified, key features summarised, and a functional horizon sequence assigned (Webb and Lilburne 2011). This section highlights the elements of the Soil Description Handbook that may be required to complete these tasks.

21.1 Identifying Diagnostic features

The descriptive parameters that help define the NZSC Diagnostic features are outlined in Table 21.1; see Hewitt and MWLR Pedology Staff, (2024) for the full definitions. Any or all of the listed parameters may be required for a confident identification.

Table 21.1: Description requirements for NZSC Diagnostic Features (Hewitt and MWLR Pedology Staff, 2024)

Diagnostic feature	Identification requirements
Horizons, pans and layers
Argillic horizon	Horizon thickness Horizon boundary distinctness Texture (relative to overlying horizon) Coatings or concentrations (lamellae)
Brittle-B horizon	Structure Roots Consistence: Mode of failure
Calcareous horizon	Calcium carbonate concentrations and/or coatings Field test: Calcium carbonate
Cutanic horizon	Coatings
Cutanoxidic horizon	Structure Texture Consistence: Mode of failure, stickiness, plasticity
Densipan	Consistence: Unconfined strength, penetration resistance Field test: Slaking
Distinct topsoil	Horizon thickness Matrix colour (relative to deeper horizon)
Duripan	Pans Field test: Reaction to HCl, slaking
Eluvial horizon	Structure Matrix colour Texture
Fragipan	Structure Roots Consistence: Mode of failure, unconfined strength, penetration resistance Field test: Slaking
Humus-pan	Structure Matrix colour Pans Consistence: Mode of failure, unconfined strength, penetration resistance
Ironstone-pan	Horizon thickness Horizon boundary distinctness (upper) Pans Consistence: Induration
Nodular horizon	Concentrations
Ortstein-pan	Horizon thickness Horizon boundary distinctness (upper) Pans Consistence: Unconfined strength, penetration resistance
Oxidic Horizon	Structure Texture Consistence: Unconfined strength, ped strength, mode of failure, stickiness, plasticity Field test: NaF, slaking
Peaty topsoil	Horizon thickness Texture
Placic horizon	Horizon thickness Horizon boundary distinctness (upper and lower) Matrix colour Consistence: Induration
Podzolic-B horizon	Horizon thickness Structure Matrix colour Coatings and concentrations Consistence: Unconfined strength Field test: NaF
Redox-mottled horizon	Matrix colour Colour patterns
Reductimorphic horizon	Matrix colour Colour patterns Texture
Slowly permeable layer	Structure Texture Consistence: Penetration resistance
Weathered-B horizon	Matrix colour (relative to deeper layers) Structure (relative to deeper layers) Evidence of carbonate loss (relative to deeper layers)
Soil materials and contacts
Allophanic soil material	Field test: NaF Consistence: Unconfined strength, sensitivity, stickiness
Anthropic soil material	Artefacts and/or plastics Recent site disturbances Landform (Human-induced) Colour patterns
Lithic contact	Substrate rock presence Horizon degree of weathering
Organic soil material	Matrix colour Texture Consistence: Mode of failure
Paralithic contact	Substrate rock presence Horizon degree of weathering Roots Colour patterns
Saline soil material	Field test: Electrical conductivity Concentrations and/or coatings
Sulfidic soil material	Field test: pH in water and pH in hydrogen peroxide Matrix colour Colour patterns
Sulfuric soil material	Field test: pH in water Matrix colour Colour patterns Concentrations and/or coatings (especially jarosite)
Tephric soil material	Lithology Mineralogy
Vitric soil material	Rock fragments Texture Lithology Parent material origin Mineralogy
Profile forms
Gley profile form	Horizon depths Matrix colour Colour patterns Field test: Reducing conditions
Mottled profile form	Horizon depths Matrix colour Colour patterns
Soil features
Fluvial features	Horizon thickness Horizon boundary distinctness Rock fragments Texture Consistence: Fluidity
Perch-gley features	Matrix colour Colour patterns Concentrations and/or coatings
Sodic features	Structure Matrix colour Field test: Dispersion
Other differentiae
Childs’ Test	Field test: Reducing conditions
Low-chroma colours	Matrix colour Colour patterns
Reactive-aluminium test	Field test: NaF
Redox segregations	Concentrations

21.2 Following the Key to Orders

In addition to the diagnostics above, successful navigation of the Key may require documentation of the parameters listed in Table 21.2:

Table 21.2: Additional description requirements for NZSC Key to Orders (Hewitt and MWLR Pedology Staff, 2024)

Order	Requirement
Organic Soils	Layers: Horizon thickness
Gley Soils	Layers: Horizon thickness Texture: Texture class (of unconsolidated substrate) Consistence: Fluidity
Ultic Soils	Architecture: Structure Colour: Matrix value (upper B horizon) Texture: Texture group and class, clay content (relative to silt) Secondary features: Coatings Consistence: Unconfined strength under moist conditions Field test: pH Total depth of soil materials
Podzol Soils	Field test: pH
Allophanic Soils	Layers: Horizon thickness
Pumice Soils	Layers: Horizon thickness and lower depths
Melanic Soils	Lithology (fine and coarse mineral fractions) Site surface: Surface cracking Architecture: Structure, rock fragment content and degree of weathering Colour: Matrix value (A horizon, upper B horizon) Texture: Texture class Consistence: Stickiness Field test: Calcium carbonate, pH
Semiarid Soils	Texture: Texture class Field test: Calcium carbonate, pH Laboratory data: Phosphate retention (low) Layers: Horizon thickness
Oxidic Soils	Layers: Horizon lower depth
Granular Soils	Architecture: Structure, rock fragment content and degree of weathering Texture: Texture group
Pallic Soils	Architecture: Structure Colour: Matrix colour (below the A horizon) Secondary features: Coatings Field test: Dispersion Laboratory data: Phosphate retention (low)
Brown Soils	Colour: Matrix colour (below the A horizon) Texture: Texture group Field test: NaF (at least moderate) Laboratory data: Phosphate retention (at least moderate) Layers: Horizon lower depth
Anthropic Soils	Evidence of profile truncation or filling (compare site with surrounding landform(s))
Recent Soils	Structure Consistence: Fluidity
Raw Soils

21.3 Completing the classification to Subgroup level

At the Group and Subgroup levels of the classification, requirements become more uniform. For example, the majority of Orders need pH data to identify acidic subgroups. Additional requirements are listed in Table 21.3 below.

Table 21.3: Additional potential description requirements for NZSC Groups and Subgroups, by Order (Hewitt and MWLR Pedology Staff, 2024)

Order	Requirement
Organic Soils	Texture: Organic matter decomposition state Field test: pH
Gley Soils	Texture: Texture group Architecture: Structure, rock fragment content Secondary features: Concentrations, coatings Consistence: Stickiness Field test: pH Buried horizons
Ultic Soils	Architecture: Rock fragment content and lithology Matrix colour Secondary features: Coatings Field test: pH Laboratory data: Exchangeable cations
Podzol Soils	Architecture: Structure Texture: Texture class Consistence: Unconfined strength, penetration resistance
Allophanic Soils
Pumice Soils	Architecture: Roots, rock fragment content and lithology Matrix colour Consistence: Unconfined strength, packing, mode of failure
Melanic Soils	Laboratory data: Exchangeable cations, phosphate retention
Semiarid Soils	Matrix colour Architecture: Structure, rock fragment content and degree of weathering Field test: pH
Oxidic Soils	Texture: Texture class Architecture: Rock fragment content
Granular Soils	Field test: pH
Pallic Soils	Architecture: Structure, rock fragment content and degree of weathering Secondary features: Induration Field test: pH
Brown Soils	Architecture: Structure, rock fragment content and degree of weathering Secondary features: Induration Field test: pH Laboratory data: Bulk density, exchangeable cations
Anthropic Soils	Land use history (Truncated and Mixed Groups) Laboratory data: Bulk density (Compacted Subgroup)
Recent Soils	Architecture: Rock fragment content and lithology Texture: Texture class Field test: pH Soil temperature (mean annual) Evidence of past burial events (e.g. flood deposition)
Raw Soils	Architecture: Rock fragment content and lithology Texture: Texture class Consistence: Fluidity Soil temperature (mean annual) Surface water Parent material origin

21.4 Completing the classification to Sibling level

Detailed requirements for classifying the Family and Sibling are contained in Webb and Lilburne (2011).

21.4.1 Family requirements

Table 21.4: Parameters required to identify the NZSC Family

Parameter	Requirements
Soil Profile Material	Rock fragment content (either texture code or percentage estimate) Rock fragment roundedness Depth to regolithic or saprolithic material, if encountered Depth to lithic or paralithic contact, if encountered Thickness of organic soil material, if encountered
Rock class of profile stone and bedrock	Lithology of rock fragments (if encountered) Lithology of substrate non-soil materials (if encountered)
Rock class of fines	Originating lithology of soil materials <2 mm
Texture group class	Texture class of all horizons to 60 cm depth or the limit of soil material, if shallower
Permeability class	Profile permeability class

21.4.2 Sibling requirements

Table 21.5: Parameters required to identify the NZSC Sibling

Parameter	Requirements
Soil depth (easily diggable depth)	Horizon thicknesses Rock fragment content per horizon Secondary features: Pans Consistence: Unconfined strength under field conditions Substrate: Paralithic or lithic material
Topsoil stoniness	Rock fragment content as percentage volume within the top 20 cm of the profile
Texture class	Texture class of all horizons to 100 cm from the soil surface or to the limit of soil material, if shallower
Drainage	Profile drainage class
Rooting barriers	Presence of any rooting depth restrictions within 100 cm of the soil surface, type and depth encountered

21.4.3 Functional horizon requirements

Table 21.6: Parameters required to identify NZSC Functional Horizons

Parameter	Requirements
Horizon notation	Presence of any A, O, or L horizons (for identification of topsoil)
Texture	Texture group Pan types, if present
Firmness	Unconfined strength under field conditions
Tephric materials	Parent material origin (specifically Tephra) Originating lithology of soil materials <2 mm
Stone content	Rock fragment content (either from texture code or percentage estimate)
Structure	Structure degree of development (where units are of pedogenic origin) If horizon is pedal, ped shape and size
Consistence	For cohesive soils, unconfined strength under field conditions For non-cohesive soils, particle packing
Permeability	Horizon permeability class

Hewitt AE, MWLR Pedology Staff, 2024. New Zealand soil classification (draft 4th edition). [Internet]. 4th ed. Wellington, New Zealand: Manaaki Whenua Landcare Research. https://soils.landcareresearch.co.nz/topics/soil-classification/nzsc/nzsc-wg

Webb TH, Lilburne LR 2011. Criteria for defining the soil family and soil sibling : The fourth and fifth categories of the New Zealand Soil Classification. [Internet]. 2nd ed. Christchurch, New Zealand: Manaaki Whenua Press. https://doi.org/10.7931/DL1-LRSS-3-2011