Changes in the Fourth Edition

Major

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 Anthropic Soil Material as a diagnostic for the Anthropic order.
  Rationale: Anthropic materials were not previously defined, causing confusion during the classification process.
• ADD Eluvial horizon as a diagnostic for Pallic and some Brown soils.
  Rationale: Concepts appearing in the key require a clear diagnostic definition within the classification, independant of the description manual, which was previously implicitly linked.
• 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 been fully explained.
• AMEND Calcareous horizon for clarity and brevity, adding a clearer definition of primary vs secondary carbonates and including example horizon notation. Removed extraneous second paragraph.
  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 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 the table in Reactive Aluminium test to match the new Soil Description Manual draft and demonstrate more realistic reaction colours.

Key

• MOVE Anthropic soils to the top of the key.
  Rationale: Removing Anthropic Soils from contention first can reduce confusion when applying the rest of the key, and keeps the overall classification structure relatively simple. The Australian Soil Classification with its Anthroposol order and the Japanese Soil Classification with its Developed/Reclaimed soils (造成土大群) make a similar choice. The World Reference base is effectively the same - it places its two equivalent orders (Anthrosols and Technosols, for agricultural and urban/industrial situations respectively) in second and third place after the Histosols. This reflects the rapidity with which organic soil materials can accumulate on the land surface. The Histosol definition itself allows for the presence of anthropic features like artefacts, so all human-modified profiles are effectively classified out early in the WRB key.
  By contrast, US Soil Taxonomy uses the Anthric and Plaggic Epipedon concepts to modify other orders. This has the advantage of keeping human-influenced soils conceptually close to their more natural neighbours, but adds a large number of subgroups to the classification, making it potentially more difficult to use. These subgroups will continue to proliferate as humans discover new and interesting ways to modify their environment. Other systems either ignore human-modified profiles or deliberately exclude them from consideration, neither of which practice is considered useful.
• 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

Anthropic soils

• ADD group AO (Māori Anthropic), containing subgroups AOA (Artefact Māori Anthropic), AOB (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.

Brown soils

• ADD subgroup BML (Allophanic Mafic Brown Soils)
  Rationale see comments on Melanic soils below
• ADD Subgroup BFF (Placic Firm Brown Soils)
  Rationale new soil observations
• ADD Subgroup BFWA (Acidic-weathered Firm Brown Soils)
  Rationale new soil observations
• ADD Subgroup BFPA (Acidic-pallic Firm Brown Soils)
  Rationale new soil observations
• ADD Subgroup BOMP (Mottled-pallic Orthic Brown Soils)
  Rationale new soil observations
• ADD Subgroup BOWA (Acidic Weathered Orthic Brown Soils)
  Rationale new soil observations
• REMOVE requirement for no 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.
• AMEND Mottled subgroups in the Brown Soils (BLM, BSMP, BSM, BMMG, BMM, BAMP, BAM, BFMA, BFMC, BFMW, BFMP, BFM, BOMA, BOM) to be defined by the presence of a mottled profile form or perch-gley features. This also applies to the new subgroup BOMP.
  Rationale: Perch-gley features can induce a profile form that is in part reduced, but not necessarily as strongly as is required for a gley soil. The reduction is often seasonal rather than year-round. These soils are usually considered imperfectly rather than poorly drained. For further discussion, see Hewitt et al. (2021), pp. 76-78.
• MOVE BAX (Pan Acid Brown Soils) up one position in the key.
  Rationale this matches the order seen in the Pan Podzols.

Melanic soils

• 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.

Raw soils

• ADD group WW (Hydric Raw Soils)
  Rationale These soils are usually under shallow water (e.g. lower tidal areas, lake shores).

• ADD subgroups WST (Tephric Sandy Raw Soils) and WSS (Typic Sandy Raw Soils).
  Rationale new soil observations

Recent soils

• ADD subgroups RXMA (Mottled-acidic Rocky Recent Soils) and RXM (Mottled Rocky Recent Soils).
  Rationale new soil observations

Minor

Correlations

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

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 simpify 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 OrthicPumiceSoils) 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 use of the term ‘calcium/magnesium ratio’ with ‘calcium/magnesium molar ratio’
• REPLACE various terms for in-profile stone > 2 mm (e.g. clasts, gravel) with ‘rock fragments’.
• 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.

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 cmol_c/kg)
• Refer consistently to ‘B or BC’ horizons where appropriate.

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.
• A Glossary has been added.

Hewitt AE 1993. Methods and rationale of the New Zealand soil classification. [Internet]. Wellington, New Zealand: Manaaki Whenua Press. https://doi.org/10.7931/DL1-LRSS-2

Hewitt AE, Balks MR, Lowe DJ 2021. The soils of Aotearoa New Zealand. [Internet]. 1st ed. Switzerland: Springer Nature Switzerland. https://doi.org/10.1007/978-3-030-64763-6