1  Introduction

1.1 Evolution of the soils of New Zealand

The New Zealand Soil Classification broadly traces the evolution, or genesis, of New Zealand soils (Hewitt 1998). Figure 1.1 shows major pathways in this evolution. These pathways represent only those likely to have occurred over extensive areas, with many minor pathways or interlinkages omitted for clarity.

In soils from mineral parent materials, Raw Soils develop mostly into Recent Soils. The nature of the parent material strongly determines the subsequent soil genesis pathway. Later, parent material becomes less important (except where upbuilding pedogenesis predominates), and climate and vegetation, along with time, become more important determinants of the character of the soil that evolves (Hewitt 1998; Hewitt et al. 2021).

In terms of abundance in New Zealand, Brown Soils are most common oder (43%), followed by Podzol (13%), Pallic (12%), Pumice (7%), Recent (6%) and Allophanic (5%). The other nine orders account for the remaining 14%, with Oxidic and Anthropic the least common with <1% each.

Soil formation, in its traditionally recognised form, proceeds by modifying a pre-existing parent material according to soil forming factors that dictate a range of processes and their impacts (Simonson 1959). In this situation the soil profile originates via two steps: accumulation (or exhumation) of a fresh parent material at the land surface, followed by modification of the parent material by soil-forming processes and weathering to form soil horizons, leading to a deepening of the soil profile.

However, the outcomes of this classic model of ‘top-down pedogenesis’ are modified when geological deposits (e.g., alluvium, tephra, loess, colluvium) are simultaneously added to the surface, as is commonly the case in New Zealand landscapes. In this situation these two steps occur together (not sequentially) so that the soil profile deepens as the land surface rises. This process, when topdown pedogenesis occurs concomitantly with surface deposition, is called ‘upbuilding pedogenesis’ (Johnson et al. 1990).

The concept of upbuilding pedogenesis was recognised in New Zealand more than 90 years ago (Taylor 1933), but it is only comparatively recently that it has become more fully appreciated (Almond and Tonkin 1999). The deposition at the surface may be incremental (developmental upbuilding pedogenesis), or it may involve the sudden deposition of a relatively thick deposit that buries and isolates the antecedent soil (retardant upbuilding pedogenesis) (Palmer et al. 2025; Alloway et al. 2026).

Upbuilding pedogenesis is captured in the New Zealand Soil Classification mainly through the recognition of lithologic discontinuities and buried horizons (paleosols) in soil profiles, which demand specific notations (O’Brien et al. 2026), often indicated in the text by the term ‘buried’ (e.g., Buried-allophanic Orthic Pumice Soil). Upbuilding pedogenesis is predominant to common in Allophanic, Pumice, Pallic, Raw, Recent, Gley, Podzol and Brown soils, and various other soils also exhibit upbuilding pedogenesis to some degree (Hewitt et al. 2021).

1.2 Objectives

The objectives of the New Zealand Soil Classification are to:

  1. provide a better means of communicating about New Zealand soils and their utilisation
  2. provide an efficient vehicle for soil identification, recognition of soil families and series ( the fourth and fifth levels of the soil classification), correlation of previously defined soil series, and the establishment of soil map legends in soil surveys
  3. enable an efficient stratification of soil database information
  4. draw together knowledge of the properties of New Zealand soils, and important similarities and differences among them.

A discussion of these objectives is given by Hewitt (1984); Hewitt (1987) and the methods and rationale of the classification are also provided by (Hewitt 1993).

1.3 Principles

To accomplish the above objectives, the following principles have guided the development of this proposal (these are explained further by Hewitt (1984)).

  1. The classification should be hierarchical, providing ascending levels of generalisation.
  2. The grouping of soils into classes should be based on similarity of measurable soil properties rather than presumed genesis.
  3. Classes must be designed to allow the greatest number and most precise accessory statements to be made about them, consistent with their level in the hierarchy.
  4. Differentiae should be based on soil properties that can be reproducibly and precisely measured or observed.
  5. Differentiae should, where possible, allow field assignment of soils to classes, either directly or by tested inferences.
  6. The nomenclature of higher categories should be based, where possible, on easily understood English words chosen for their accessibility to non-specialists.
  7. Where possible, continuity with successful parts of the New Zealand Genetic Classification should be maintained.
  8. The soil classification must be valid for the main islands of New Zealand. Classes must be correlated with the Soil Taxonomy (Soil Survey Staff 1999; Soil Survey Staff 2022) to support international extension.

1.4 The soil individual

The ‘soil individual’ is the fundamental unit of soil assigned to classes. Cline (1949) defined an individual as ‘the smallest natural body that can be defined as a thing complete in itself’. The Soil Taxonomy (Soil Survey Staff 1999) regards the polypedon as the soil individual. This is rejected here because, as discussed by Hewitt (1982), it does not fulfil the requirements for a soil individual by Cline (1949) or Johnson (1963).

In New Zealand the soil individual has traditionally been the soil profile. Usually conceived as a two-dimensional section exposed by a soil pit, it is in fact a three-dimensional slice sufficiently thick to sample and examine hand specimens. It should therefore be termed a ‘soil profile slice’. With the realisation that soils should be examined in successive horizontal sections as well as the vertical profile, there is increasing acceptance that a volume of soil the size of the pedon Soil Survey Staff (1999) represents a better soil individual than the soil profile slice.

Accordingly, the pedon, as defined in the Soil Taxonomy (Soil Survey Staff 1975) and referred to as ‘a unit of sampling’ by (Soil Survey Staff 1999), is recommended as the soil individual for the New Zealand Soil Classification. Assignments are often made from the examination of volumes of soil smaller than a complete pedon where they are assumed to be representative of the pedon.

Notwithstanding this definition, an alternative concept of the pedon was proposed by Holmgren (1988), which now forms in part the basis for digital soil mapping and thus also applies to parts of New Zealand’s soil survey online database, S-map: ‘A pedon is the possibility for soil observation in respect to a geographic point location. It can be realized by a set of observational propositions, each spatially and temporally specified in relation to that location’.

1.5 How to assign a soil to subgroup level

Normally, dig a soil pit of sufficient size to expose the soil horizons to about 1 m depth, or to rock if shallower. Examine the soil horizons and assign the soil by following the key, starting with the Key to Orders. Consult “Diagnostic Horizons and Other Differentiae”, to identify diagnostic horizons and other differentiae, which should be applied according to a specified control section. For some classes, pH or other chemical measurements must be made.

Compare the characteristics of the soil with the key statements for each soil order, starting with Organic Soils and passing down the key to the first soil order that matches. When you have identified the soil order, consult the relevant section on that order, and follow the keys to soil groups and soil subgroups in the same way to identify the appropriate soil group and subgroup.

Note that in the keys to the groups and subgroups, the soils, following usage in the Soil Taxonomy, are generally listed from most to least problematic in terms of agricultural or horticultural land uses. For example, in Allophanic Soils, Perch-gley Allophanic Soils are listed first in the group keys, and Ironstone Perch-gley Allophanic Soils head the list in the subgroup keys, with subsequent taxa in each section having decreasing limitations for agriculture.

Also, note that the word ‘typic’ does not necessarily mean the most extensive or typical; rather, it is a taxon representing soils without any of the characteristics defined for other taxa in the same class, (i.e. with no aberrant properties). Being placed last in the keys, it is in effect a default class. Similarly, the word ‘orthic’ (meaning normal, conventional) describes a soil with no special qualities worthy of separate taxon status. It is not necessarily the most common taxon.

The name given to a soil assigned to a subgroup is made up of three elements in the sequence: subgroup, group, and order (for example, Nodular Perch-gley Oxidic Soils). Note that ‘Perch-gley’, being hyphenated, is one word, thereby conforming to the three-element nomenclatural rule. Figure 1.2 illustrates the relationships between subgroups and groups in the Oxidic Soils order.

1.6 Classifying to the fourth and fifth levels of the New Zealand Soil Classification

It is possible to classify soils to a more detailed level than the subgroup. The definition and criteria of the fourth and fifth levels of soil classification are described in detail by Webb and Lilburne (2011).

Historically, these more detailed soil classes were called soil series and originally constituted a grouping of soils with similar modal profiles, similar temperature and moisture regimes, and the same or very similar parent materials and associated landforms (Taylor and Pohlen 1979). Because series were primarily used to describe natural soil-landscape units, the within-series variability could be significant and encompassed soil properties including texture, stoniness, and depth to bedrock (Taylor and Pohlen 1979). As a result, pedons that traditionally belonged to the same soil series may today be classified into different subgroups. Identified by a geographical name, various subdivisions of a series, including soil type (used commonly as a map unit) would carry the same name.

The formal definition and correlation of new soil series ceased in the early 1990s, but the existing series are still widely known and used by scientists, administrators and land users. By this time though, soil series were defined according to three main criteria: the nature of the parent material or substrate, the particle-size characteristics, and the permeability profile (Hewitt 1992). This definition was formally introduced as the fourth level of the New Zealand Soil Classification in 1994, termed the soilform (Clayden and Webb 1994). In 2011 the soilform was superceded by soil families. A fifth level, the soil sibling (roughly equivalent to soil type), was also introduced to further refine the description of the physical attributes within a family. Together, soil family and sibling form the soil entity (i.e., map unit) depicted in S-map.

1.7 Misclassification

The classes are the most important part of the soil classification. The key is merely a means of allocating soils to these classes, and by its nature is imperfect because only a sample of all the possible soils that might potentially be allocated were used in developing the key. As a result, soils will be found that are not allocated to the appropriate class by the key. This will be apparent when a soil allocated to a class does not conform to the concept and accessory statements that can normally be made about that class.

Because the key is the servant of the classes, the allocator is justified in placing the soil misfit into a more appropriate class. If this is done, however, it must be registered with the person(s) with responsibility for the national soil classification system, so that appropriate adjustments can be made to the key when the soil classification is next revised. An allocation contrary to the key must also be noted in any records or publication of the allocation.

1.8 Justification of new subgroups

Justification for new subgroups may be made in two ways. First, if a soil is judged to be misclassified and a more appropriate class is not available, then a new subgroup may be justifiable. Second, an existing subgroup may encompass a set of soils with properties that are too wide in range. The old subgroup could be split into two new ones. Splitting may be justified if it will significantly increase the number and precision of accessory statements that can be made about both of the new classes.

1.9 Correlations with other soil classification systems

Classes of the New Zealand Soil Classification do not correspond precisely to classes of other soil classification systems. Despite this, correlations can be made where classes are substantially equivalent. It is likely that all orders of the Soil Taxonomy are represented in New Zealand, albeit some uncommonly or rarely, although Gelisols have not yet been formally identified in the New Zealand archipelago.

Table 1.1 and Table 1.2 summarise the correlations of classes of the New Zealand Soil Classification with those of the New Zealand Genetic Soil Classification (Taylor and Pohlen 1968) and the Soil Taxonomy (Soil Survey Staff 1999; Soil Survey Staff 2022). Further correlation with the World Reference Base (IUSS Working Group WRB 2022) are presented in Hewitt et al. (2021).

Table 1.1: Areal representation (%) of soil orders in New Zealand based on the Soil Taxonomy (from Hewitt et al. (2021))
Very common Common Less common Rare
Inceptisols (47.4) Alfisols (9.9) Mollisols (1.2) Oxisols (0.2)
Spodosols (13.1) Entosols (7.4) Histosols (0.9) Vertisols (0.1)
Andisols (12.9) Ultisols (4.2) Aridisols (0.9) Gelisols (<0.1)1
1 Small areas of Gelisols, underlain by contemporary permafrost in debris-mantled slopes, may occur in proximity to glaciers and rock glaciers above c. 2,000 m elevation in alpine areas of the South Island. Support for such occurrences is provided by topoclimate modelling (Sattler et al. 2016), along with abundant geomorphological evidence (e.g., Soons and Price (1990)), but actual ‘cryic’ soil profiles are yet to be observed. The Gelisols, provisionally represented as ‘Cryic Raw Soils’ in the New Zealand Soil Classification, therefore require further evaluation.
Table 1.2: Correlation of soil groups with the New Zealand Genetic Soil Classification (Taylor and Pohlen 1962) and the Soil Taxonomy (Soil Survey Staff 1999; Soil Survey Staff 2022).
NZ Soil Classification (v. 3) NZ Genetic Soil Classification US Soil Taxonomy
Allophanic Soils
Perch-Gley gley soils Aquands
Gley gley soils Aquands
Impeded yellow-brown (YB) loams Cryands and Udands
Orthic YB loams Cryands and Udands
Anthropic Soils
Truncated unclassified unclassified
Māori unclassified unclassified
Refuse unclassified unclassified
Mixed unclassified unclassified
Fill unclassified unclassified
Brown Soils
Allophanic YB earths (upland & high country) Dystrudepts
Sandy YB sands Dystrustepts, Dystrudepts, Haplustepts, and Psamments
Oxidic YB earths (northern) Dystrudepts
Mafic brown granular (BG) loams and clays Dystrudepts
Acid podzolized YB earths or YB earths Dystrudepts
Firm YB earths, YB shallow and stony soils Dystrudepts and Dystrustepts
Orthic YB earths, YB shallow and stony soils Dystrudepts and Dystrustepts
Gley Soils
Sulfuric gley soils Sulphaquepts, Sulfaquents
Tephric gley soils Aquands
Sandy gley soils Aquepts or Aquents
Oxidic gley soils Aquox
Recent gleyed recent soils Aquents
Acid gley soils Aquepts
Orthic gleyed recent soils Aquepts and Aquents
Granular Soils
Perch-gley BG loams and BG clays Aquults
Melanic BG loams and BG clays Humults, Udults and Udalfs
Oxidic BG loams and BG clays Humults and Udults
Orthic BG loams and BG clays Humults and Udults
Melanic Soils
Vertic BG loams and BG clays Ustolls and Usterts
Perch-gley gley soils Aquolls and Aquerts
Rendzic rendzinas Rendolls
Mafic BG loams and BG clays Haplustepts, Ustolls and Udolls
Orthic rendzinas and rendzic intergrades Ustolls, Udolls, Haplustepts and Calciustepts
Organic Soils
Litter unclassified Folists
Fibric organic soils Fibrists
Mesic organic soils Hemists
Humic organic soils Saprists
Oxidic Soils
Perch-gley gley soils Aquox and Udox
Nodular strongly weathered red loams, brown loams, BG loams and BG clays Udox
Orthic strongly weathered red loams, brown loams, BG loams and BG clays Udox
Pallic Soils
Perch-gley yellow grey earths Aquepts, Aqualfs, Ustepts and Ustalfs
Duric yellow grey earths Duraqualfs, Durustalfs and Durustepts
Fragic yellow grey earths Fragiochrepts, Haplustalfs and Fragiustalfs
Laminar yellow grey earths Haplustalfs
Argillic yellow grey earths Haplustalfs, Hapludalfs and Natrustalfs
Immature yellow grey earths or recent soils Haplustepts
Podzol Soils
Densipan podzols Aquods and Orthods
Perch-gley gley podzols Aquods and Orthods
Groundwater-gley gley podzols Aquods
Pan podzols Orthods and Humods
Orthic podzols Orthods and Humods
Pumice Soils
Perch-gley gley soils Vitraquands
Impeded YB pumice soils Udivitrands and Vitricryands
Orthic YB pumice soils Udivitrands
Raw Soils
Hydric unclassified Aquents, Wassents and not soil
Gley unclassified Entisols, Aquents and not soil
Hydrothermal hydrothermal soils Entisols, Orthents and not soil
Cryic unclassified Turbels and Orthels and not soil
Rocky unclassified Entisols, Orthels and not soil
Sandy unclassified Entisols, Psamments and not soil
Fluvial unclassified Entisols, Fluvents and not soil
Tephric unclassified Entisols, Orthents and not soil
Orthic unclassified Entisols, Orthents and not soil
Recent Soils
Hydrothermal recent soils Aquents, Orthents
Rocky lithosols Orthents
Sandy recent soils Psamments
Fluvial recent soils Fluvents, Udepts and Ustepts
Tephric recent soils Orthents, Cryands and Udands
Orthic recent soils Orthents, Udepts and Ustepts
Semiarid Soils
Aged-argillic brown-grey earths Haplargids and Paleargids
Solonetzic solonetz Natrargids
Argillic brown-grey earths Haplargids and Calciargids
Immature brown-grey earths Haplocambids and Aquicambids
Ultic Soils
Densipan YB earths and podzols Aquults (Albiaquults, Epiaquults)
Albic YB earths Aquults, Humults and Udults
Perch-gley YB earths Aquults
Sandy YB earths and YB sands Hapludults and Haplohumults
Yellow YB earths Hapludults
Note: The correlations with the Soil Taxonomy provide only the nearest equivalents because criteria differ between the two systems. The lowest category of the Soil Taxonomy is given (order, suborder or great group) that can be best related to soil groups of the New Zealand Soil Classification.