Appendix C — Changes in the Fourth Edition

C.0.1 Major

C.0.1.1 Foreword

  • ADD discussion of control sections and how they are used in the classification
  • ADD discussion of lithological discontinuities
  • REMOVE definitions of CEC, ECEC, and P retention

C.0.1.2 Introduction

  • ADD subsection ‘Evolution of the soils of Aotearoa New Zealand’, about the pathways soil development follows in the country
  • ADD subsection ‘The History of the New Zealand Soil Classification’ recounting how the classification has developed from older classifications.
    These two sections replace the previous introductory paragraphs.
  • ADD subsection ‘Soil series, families and siblings’ to explain the history and use of these concepts

C.0.1.3 Diagnostics

  • ADD clear definitions of the phrases ‘in some part’ and ‘in the major part’ to the top of the chapter.
    Rationale these were previously sitting in footnotes within the Allophanic and Brown chapters, making them hard to find.
  • ADD clear direction about how to handle transitional horizons when classifying a profile
  • ADD Anthropic Soil Material as a diagnostic for the Anthropic order.
    Rationale: Anthropic materials were not previously defined, causing confusion during the classification process.
  • ADD Child’s test as a diagnostic for reduction in soils.
    Rationale: The test was previously mentioned at the subgroup level but not defined in detail. The test is also now described in the ‘field tests’ chapter of the Soil Description Handbook (O’Brien et al. 2025).
  • ADD Eluvial horizon as a diagnostic for Pallic and some Brown, Podzol and Ultic soils.
    Rationale: Concepts appearing in the key require a clear diagnostic definition within the classification, independent of the description manual, which was previously implicitly linked.
  • ADD Low-chroma colours as a diagnostic for many soils affected by poor drainage or strong leaching.
    Rationale: The definition of low-chroma colour was previously only available in the definition of a Bg horizon and was therefore hard to find. It is now clearly defined both in this document and in the ‘horizon colour’ chapter of the Soil Description Handbook.
  • ADD Saline Soil Material as a diagnostic.
    Rationale: Elevated soil salinity defines subgroups in the Gley, Semi-Arid, Recent and Raw orders but the concept has not previously been fully explained.
  • ADD Sulfidic Soil Material as a diagnostic.
    Rationale: Sulfidic soils were previously lumped in with less hazardous wet soils and their diagnostic criteria were outdated and only mentioned at subgroup level.
  • ADD Sulfuric Soil Material as a diagnostic.
    Rationale: Sulfuric soils were previously lumped in with less hazardous low-pH soils and their diagnostic criteria were outdated and only mentioned at subgroup level.
  • AMEND Calcareous horizon for clarity, adding a clearer definition of primary vs secondary carbonates and including example horizon notation. Removed extraneous second paragraph. Clarified that both primary and secondary carbonates are diagnostic in the fine fraction, but only secondary carbonates are relevant in the coarse fraction because they show evidence of significant carbonate dissolution, transport and precipitation. Coarse fragments of primary carbonates alone do not necessarily provide evidence that these processes are active.
    Rationale: Removal of the second paragraph is justified on the grounds that the content is better suited to the Methods and Rationale of the NZSC (Hewitt 1993).
  • AMEND Cutanoxidic horizon to clarify that these must also meet the requirements of a cutanic horizon
  • AMEND Densipan to include expected horizon notation, and remove reference to specific types of field testing.
    Rationale: Strength and penetration resistance testing should be undertaken using current best practices, which do not need to be specified in this document.
  • AMEND Fragipan to include expected horizon notation, and remove reference to specific types of field testing. Additionally, clarify that bulk density is expected to be greater than or equal to 1.5 Mg/m3.
  • AMEND Nodular Horizon to have a 5% abundance requirement rather than 15%
    Rationale new soil observations
  • AMEND Peaty topsoil to refer to updated texture class terminology
  • AMEND the table in Reactive Aluminium test to match the new Soil Description Manual and demonstrate more realistic reaction colours.
  • AMEND Tephric material definition to use updated terminology and newer references.
  • REMOVE Crumb structure as a diagnostic.
    Rationale: Updated terms for describing this kind of structure have become available in the new Soil Description Manual.

C.0.1.4 Key

  • ADD A note about how to classify restored Anthropic soils to the top of the key.
  • CHANGE the criteria for Recent Soils to allow soils that do not fully meet the criteria for a Distinct Topsoil by only having a ‘structural A’ horizon.
    Rationale: This change helps to account for soils that have only a slight colour difference (e.g. visually perceptible darkening but still within the same Munsell chip, or only a Hue shift) in the A horizon due to tillage, low natural organic carbon content, or similar, but are still clearly too developed and in the wrong landscapes for the Raw order to make sense.
    The definition of Distinct topsoil itself is not proposed to be changed so that the definitions of other orders remain consistent.
  • ADD an additional criteria that Gley soils do not contain a diagnostic amount of allophanic soil material (new criteria 1(e))
    Rationale soils that fail out on this new criteria belong more correctly in the LG Allophanic Gley group, and new criteria directs them there. Gleys that contain tephric soil material that doesn’t meet the requirements for allophanic soil materials (for example, gley soils forming in poorly drained alluvia from mixed sources but including some reworked ash), may still key out as GT Tephric Gleys.
  • ADD a statement to the Allophanic Soils definition covering situations where allophanic soil material is shallow over rock.
    Rationale Without this statement, shallow allophanic soils over rock are most likely to be pushed into the BL Allophanic Brown or RT Recent Tephric subgroups, neither of which may be the most appropriate destination.
  • ADD an additional criteria to Melanic soils specifying the expected dominant soil parent materials.
    Rationale The Melanic order has been occasionally misinterpreted to apply to a wider variety of profiles with dark topsoils than was originally intended.
  • CHANGE criteria 3b of Melanic soils to include some additional possibilities for soils below pH 5.9
    Rationale newly available data
  • CHANGE criteria for Podzol Soils to clarify that the pH limitation applies to the podzolic-B specifically.
  • CHANGE criteria for Oxidic soils to clarify that A horizons with oxidic properties count towards the thickness requirement.
  • CHANGE criteria for Anthropic soils to refer to the new diagnostic soil material and better define truncation.

C.0.1.5 Allophanic soils

  • Update the concept, correlation, occurrence and accessory properties sections to reflect contemporary knowledge.
  • CHANGE the definitions of Perch-Gley and Gley Allophanic Soils to only require signs of poor drainage within 30 cm of the mineral soil surface
    Rationale depth to a drainage limitation should not be dependent on A horizon thickness.
  • ADD subgroup LOBG (Buried-granular Orthic Allophanic Soils)
    Rationale new soil observations

C.0.1.6 Anthropic soils

  • Add discussion about how to relate Anthropic soils to their (probable) pre-altered classification
  • Expand discussion of their occurrence
  • ADD group AO (Māori Anthropic), containing subgroups AOA (Artefact Māori Anthropic), AOO (Organic Māori Anthropic), and AOF (Fill Māori Anthropic).
    REMOVE subgroup AFA (Artifact Fill Anthropic Soils).
    Rationale Māori soils have been previously recognised in the classification at a lower level and with less detail. Adding these groups and subgroups enables clear identification (particularly in mapping) and more effectively distinguishes them from other Anthropic soils.
  • ADD subgroup ARW (Wet Refuse Anthropic Soils)
    Rationale These soils will break down in a different manner to other Refuse Anthropic Soils.
  • ADD subgroups AMI (Inverted-tephric Mixed Anthropic Soils), AMZ (Inverted-impeded Mixed Anthropic Soils) and AMT (Typic Mixed Anthropic Soils).
    Rationale the previous lack of subgroups prevented the classification from being able to represent particular and very distinctive modern land modification practices.
  • ADD subgroup AFU (Treated-sulfidic fill Anthropic Soils)
    Rationale AFU soils have particular management requirements and may be unsuitable for some uses while their chemistry stabilises post-treatment.
  • CHANGE definition of ATX to allow paralithic contacts as well as lithic contacts
  • RENAME AFST (Stony-tailings fill Anthropic Soils) to AFX (Coarse Fill Anthropic Soils).

C.0.1.7 Brown soils

  • ADD subgroup BLP (Placic Allophanic Brown Soils)
    Rationale new soil observations
  • ADD subgroup BXM (Mottled Oxidic Brown Soils)
    Rationale new soil observations
  • ADD Subgroup BFF (Placic Firm Brown Soils)
    Rationale new soil observations
  • ADD Subgroup BOMP (Mottled-pallic Orthic Brown Soils)
    Rationale new soil observations
  • REMOVE requirement for absence of a placic horizon in group BS (Sandy Brown Soils)
    Rationale requirement was inconsistent with the existence of BSMP (Mottled-placic Sandy Brown Soils) within the group.
  • REMOVE subgroup BLAM (Acidic-mafic Allophanic Brown)
    Rationale: With the additions made to the Mafic Melanic and Mafic Brown groups, this subgroup has become superfluous. Linking allophanic subgroups directly to mafic groups (EM, BM) will better account for the mechanisms behind the allophanic characteristics in these soils that are likely different to that of soils with allophanic characteristics from tephra and more felsic lithology.
  • REMOVE subgroup BFAC (Acidic-cemented Firm Brown Soils)
    Rationale redundant
  • REMOVE subgroup BFAL (Acidic-allophanic Firm Brown Soils)
    Rationale redundant
  • MOVE BAX (Pan Acid Brown Soils) up one position in the key and rename to BAC (Cemented Acid Brown Soils).
    Rationale this matches the order seen in the Pan Podzols and allows for a wider range of iron-pan types.
  • MOVE BFMA (Mottled-acidic Firm Brown Soils) from first to fourth position in the BF key and BFMW (Mottled-weathered Firm Brown Soils) from third to first position
    Rationale consistency with other parts of the classification

C.0.1.8 Gley soils

  • CHANGE the criteria for GU (Sulfuric Gley Soils) group to refer to the new sulfuric soil materials diagnostic and extend the control section from top 60 cm to top 90 cm
    Rationale environmental hazard from these soils does not decrease with depth so the maximum control section should apply
  • ADD Subgroups GSOU (Peaty-sulfidic Sandy Gley Soils) and GSU (Sulfidic Sandy Gley Soils) to the top of the GS group key.
    Rationale Sulfidic soil materials can form in some coastal Sandy Gley Soils if the conditions are right. These subgroups can potentially be encountered in low dune-swales and poorly drained coastal sandplains.
  • ADD Subgroups GROU (Peaty-sulfidic Recent Gley Soils) and GRU (Sulfidic Recent Gley Soils) to the top of the GR group key.
    Rationale Sulfidic soil materials can form in some Recent Gley Soils if the conditions are right. These subgroups can potentially be encountered adjacent to low-flow drainage lines and water bodies in coastal floodplains.
  • ADD Subgroups GOOU (Peaty-sulfidic Orthic Gley Soils) and GOU (Sulfidic Orthic Gley Soils) to the top of the GR group key.
    Rationale Sulfidic soil materials can form in some Orthic Gley Soils if the conditions are right. These subgroups can potentially be encountered in dips, prior stream channels, and other low-lying areas of coastal plains.

C.0.1.9 Melanic soils

  • ADD subgroup ERJ (Argillic Rendzic Melanic Soils)
    Rationale new soil observations
  • ADD subgroups EML (Allophanic Mafic Melanic), EMA (Acidic Mafic Melanic).
    Rationale Acidification, high P retention and presence of allophanic material in Melanic soils from mafic rock types (e.g., basalt) as observed under higher precipitation (>800–1000 mm) does not necessarily lead to the loss of the typical characteristics of Melanic soils, such as the dominance of mafic parent material, dark-coloured Fe/Mg/Ti-rich minerals, smectitic clays, a well-developed pedality and higher organic carbon contents. Despite their lower soil pH and exchangeable bases, they are more closely related to soils in the existing EM group than they are to their former groups (BL, BM) in the Brown Soil order.

C.0.1.10 Organic soils

C.0.1.11 Oxidic soils

C.0.1.12 Pallic soils

C.0.1.13 Podzol soils

C.0.1.14 Pumice soils

C.0.1.15 Raw soils

C.0.1.16 Recent soils

C.0.1.17 Appendices

  • ADD Cryic Soil order as an appendix for use in the Ross Sea region, with eight additional order-specific diagnostic criteria, three groups, and 15 subgroups.

C.0.2 Minor

C.0.2.1 Foreword

  • Expand the list of key technical references to include recent publications e.g. Soils of Aotearoa New Zealand

C.0.2.2 Introduction

  • UPDATE subsection ‘Objectives’ to include a reference to the Methods and Rationale
  • UPDATE Figure 1.2 (new nodular and rocky subgroups)
  • ADD Table 1.1 summarising estimated occurrence of Soil Taxonomy orders in New Zealand

C.0.2.3 Correlations

  • Discussions of correlations between the NZSC and other systems have been updated to reflect changes to both them and the NZSC.

C.0.2.4 Terminology

  • AMEND the definition of Cutanoxidic horizon to put all requirements in the numbered criteria list.
  • AMEND the definition of Densipan to put all requirements in the numbered criteria list.
  • AMEND the definition of Duripan to include a horizon notation example.
  • AMEND the definition of Ironstone-pan to include a horizon notation example.
  • AMEND the definition of Tephric soil materials to simplify the terminology list.
  • AMEND the definition of Truncated Anthropic soils for clarity.
  • AMEND the definition of Mixed Anthropic soils for clarity.
  • AMEND the definition of Refuse Anthropic soils to specifically include plastics in ‘manufactured organic material’ and remove the requirement to attempt ignition of suspected methane.
  • AMEND the definition of Firm Brown Soils to be more specific about the strength requirements.
  • AMEND subgroup ZPOZ (Peaty-silt-mantled Perch-gley Podzols); in requirement 1(b), ‘pedality’ was used where ‘structure’ should have been.
  • AMEND group ZX (Pan Podzols) to be more specific about the strength requirements.
  • AMEND group ZO (Orthic Podzols) to be more specific about excluded horizons.
  • AMEND the definition of MOI (Immature Orthic Pumice Soils) to correct colour requirements.
  • RECODE AFE (Earthy Fill Anthropic Soils) to AFT (Typic Fill Anthropic Soils).
  • RECODE BFAC (Acidic-cemented Firm Brown Soils) to BFCA.
  • RECODE BFAL (Acidic-allophanic Firm Brown Soils) to BFLA.
  • RECODE PXJM (Argillic-mottled Fragic Pallic Soils) to PXMJ.
  • RENAME BAMP (Mottled-placic Acid Brown Soils) to BAMP (Mottled-impeded Acid Brown Soils).
  • REPLACE ‘short-range’ and similar terms with ‘nanocrystalline’ when discussing allophane
  • REPLACE various terms for in-profile stone > 2 mm (e.g. clasts, gravel) with ‘coarse rock fragments’.
  • REPLACE qualitative terms for ped size with numeric ranges/limits
  • REPLACE use of the term ‘acid’ with ‘acidic’ when used as a descriptor.
  • REPLACE references to E horizons with cross-references to the new eluvial horizon diagnostic.
  • REPLACE references to salinity cutoffs with cross-references to the new Saline soil materials diagnostic.
  • REMOVE the phrase ‘(or some subhorizon of the B)’ from the definition of the Podzolic-B horizon (superfluous).
  • REMOVE redundant reference to depth requirements in part 2 of the LOA (Acidic Orthic Allophanic) soil definition.
  • REMOVE use of ‘ripened/unripened’ - dated terminology

C.0.2.5 Consistency

  • REMOVE reference to ‘stony brown’ soil group in the Brown soils accessory properties list, point 8. This concept was retired between v2.0 and v3.0.
  • AMEND all units to SI standards (e.g. CEC in cmolc/kg)
  • Refer consistently to ‘B or BC’ horizons where appropriate.
  • Numbered major headings
  • Consistent formatting for criteria lists

C.0.2.6 References

  • References have been updated where new editions or versions of certain documents have become available. New contextual references have been added in a number of places.