Introduction

Version 4.0 of the Zealand Soil Classification is the culmination of a period of development from its initiation in 1983 to wide circulation of versions 1.0 through to 3.0 (Hewitt 1989; Hewitt 2010) for comment and testing. It represents the best attempt, given the current state of knowledge, to classify New Zealand soils. As the knowledge and understanding of New Zealand soils grows, further revisions will be necessary.

The New Zealand Soil Classification is a national soil classification intended to replace the New Zealand Genetic Soil Classification (Taylor 1948; Taylor and Cox 1957; Taylor and Pohlen 1962). The New Zealand Genetic Soil Classification grew out of the need for reconnaissance mapping of the nation’s soil resources. It was successful as a unifying factor in New Zealand soil science, and it played a vital role in the development of pastoral agriculture. However, modern soil surveys and land evaluations required precise definition of classes and keys for their recognition. Furthermore, a new synthesis was needed of the large body of information collected since the 1950s. The present work has grown out of the New Zealand Genetic Soil Classification and, where possible, preserves successful parts of that classification. It has also been influenced by experience in testing the US Soil Taxonomy (Leamy et al. 1983).

Objectives

The objectives of the New Zealand Soil Classification are:

1. to provide a better means of communication about New Zealand soils and their utilisation;
2. to provide an efficient vehicle for soil type identification and correlation, and soil map legend establishment in soil surveys;
3. to enable an efficient stratification of soil database information;
4. to 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).

Principles

To accomplish the 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. Differentia should be based on soil properties that can be reproducibly and precisely measured or observed.
5. Differentia 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 connotative English words chosen for their acceptability to nonspecialists.
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 Soil Taxonomy (Soil Survey Staff 1999) to support international extension.

The Soil Individual

The soil individual is the fundamental unit of soil which is assigned to classes. Cline (1949) defined an individual as “the smallest natural body that can be defined as a thing complete in itself”.

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 Cline’s (1949) or Johnson’s (1963) requirements for a soil individual.

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 Soil Taxonomy (Soil Survey Staff 1999) is recommended as the soil individual for the New Zealand Soil Classification. It is understood that assignments are often made from the examination of volumes of soil smaller than a complete pedon, where they are assumed representative of the pedon.

How to assign a soil to Subgroup level

Normally, a soil pit must be dug of sufficient size to expose the soil horizons to about 1 m depth, or to rock if shallower.

The soil horizons are examined and the assignment is then made by following the key, starting with the Key to Orders. The “Diagnostic Horizons and Other Differentiae” section is consulted as necessary to identify diagnostic horizons and other differentia. For some classes, pH or other chemical measurements must be made. These should be performed on samples taken between the specified depths, and bulked from at least four places in the pit. The characteristics of the soil are compared with the key statements of each soil order, starting with Anthropic Soils and passing down the key to the first soil order that fits them. When a soil order is identified, the chapter concerning that order is consulted and the keys to soil groups and soil subgroups are followed in the same manner to identify the appropriate soil group and subgroup.

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). Figure 2.1 illustrates the relationships between subgroups and groups in the Oxidic Soils order.

Figure 2.1: The hierarchy of the Oxidic Soils as an example of the hierarchical relationships between orders, groups and subgroups. As the diagram suggests, the range of soil properties for each class is related to hierarchical position.

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. Consequently, 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 with responsibility for the national soil classification system, so that appropriate adjustments may 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.

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.

Correlations with other soil classification systems

Classes of the New Zealand Soil Classification do not correspond precisely with classes of other soil classification systems. Despite this, correlations can be made where classes are substantially equivalent. Table 2.1 summarises the correlations of classes of the Zealand Soil Classification with those of the New Zealand Genetic Soil Classification (Taylor and Pohlen 1962) and Soil Taxonomy (Soil Survey Staff 1999), where the reader can also find more information on the technical terms used in the correlation.

Table 2.1: Correlation of soil groups with the Genetic New Zealand Soil Classification (Taylor and Pohlen 1962) and the US Soil Taxonomy (Soil Survey Staff 1999). The correlations with Soil Taxonomy provide only the nearest equivalents, as criteria differ between the two systems. The lowest category of Soil Taxonomy is given (order, suborder or great group) that can be best related to soil groups of the NZ Soil Classification.

NZ Soil Classification (v. 3)	NZ Genetic Soil Classification	US Soil Taxonomy
ALLOPHANIC SOILS
Perch-Gley Allophanic Soils	gley soils	Aquands
Gley Allophanic Soils	gley soils	Aquands
Impeded Allophanic Soils	YB loams	Cryands and Udands
Orthic Allophanic Soils	YB loams	Cryands and Udands
ANTHROPIC SOILS
Truncated Anthropic Soils	anthropic soils	Arents
Refuse Anthropic Soils	anthropic soils	Arents or Unclassified
Mixed Anthropic Soils	anthropic soils	Arents
Fill Anthropic Soils	anthropic soils	Arents
BROWN SOILS
Allophanic Brown Soils	YB earths (upland & high country)	Dystrochrepts
Sandy Brown Soils	YB sands	Ustochrepts, Dystrochrepts and Psamments
Oxidic Brown Soils	YB earths (northern	Dsytrochrepts
Mafic Brown Soils	BG loams and clays	Dsytrochrepts
Acid Brown Soils	podzolized YB earthsor YB earths	Dsytrochrepts
Firm Brown Soils	YB earths, YB shallow and stony soils	Dystrochrepts and Ustochrepts
Orthic Brown Soils	YB earths, YB shallow and stony soils	Dystrochrepts and Ustochrepts
GLEY SOILS
Sulpuric Gley Soils	gley soils	Sulphaquepts
Sandy Gley Soils	gley soils	Aquepts or Aquents
Acid Gley Soils	gley soils	Aquepts
Oxidic Gley Soils	gley soils	Aquox
Recent Gley Soils	gleyed recent soils	Aquents
Orthic Gley Soils	gleyed recent soils	Aquepts or Aquents
GRANULAR SOILS
Perch-gley Granular Soils	BG loams or BG clays	Aquults
Melanic Granular Soils	BG loams or BG clays	Humults and Udalfs
Oxidic Granular Soils	BG loams or BG clays	Humults
Orthic Granular Soils	BG loams or BG clays	Humults
MELANIC SOILS
Vertic Melanic Soils	BG loams and clays	Ustolls or Vertisols
Perch-gley Melanic Soils	gley soils	Aquolls
Rendzic Melanic Soils	rendzinas	Rendolls
Mafic Melanic Soils	BG loams and clays	Ustochrepts, Eutrochrepts, Ustolls or Udolls
Orthic Melanic Soils	rendzinas and rendzinic intergrades	Ustolls, Udolls or Eutrochrepts
ORGANIC SOILS
Litter Organic Soils	unclassified	Folists or unrecognised
Fibric Organic Soils	organic soils	Fibrists
Mesic Organic Soils	organic soils	Hemists
Humic Organic Soils	organic soils	Saprists
OXIDIC SOILS
Perch-gley Oxidic Soils	gley soils	Aquox
Nodular Oxidic Soils	strongly weathered red loams, brown loams, or BG loams or BG clays	Udox
Orthic Oxidic Soils	strongly weathered red loams, brown loams, or BG loams or BG clays	Udox
PALLIC SOILS
Perch-gley Pallic Soils	yellow-grey earths	Aquepts, Aqualfs
Duric Pallic Soils	yellow-grey earths	Duraqualfs
Fragic Pallic Soils	yellow-grey earths	Fragiudalfs, Fragiochrepts
Laminar Pallic Soils	yellow-grey earths	Haplustalfs, Hapludalfs
Argillic Pallic Soils	yellow-grey earths	Haplustalfs, Hapludalfs
Immature Pallic Soils	yellow-grey earths or recent soils	Eutrochrepts, Ustochrepts
PODZOLS
Densipan Podzols	podzols	Aquods, Orthods
Perch-gley Podzols	gley podzols	Aquods
Groundwater-gley Podzols	gley podzols	Aquods
Pan Podzols	podzols	Orthods
Orthic Podzols	podzols	Orthods
PUMICE SOILS
Perch-gley Pumice Soils	gley soils	Vitraquands
Impeded Pumice Soils	YB pumice soils	Vitrands, Vitricryands
Orthic Pumice Soils	YB pumice soils	Vitrands, Vitricryands
RAW SOILS
Gley Raw Soils	unclassified	Entisols, or not-soil
Hydrothermal Raw Soils	hydrothermal soils	Entisols, or not-soil
Rocky Raw Soils	unclassified	Entisols, or not-soil
Sandy Raw Soils	unclassified	Entisols, or not-soil
Fluvial Raw Soils	unclassified	Entisols, or not-soil
Tephric Raw Soils	unclassified	Entisols, or not-soil
Orthic Raw Soils	unclassified	Entisols, or not-soil
RECENT SOILS
Hydrothermal Recent Soils	recent soils	Aquents, Orthents
Rocky Recent Soils	lithosols	Orthents
Sandy Recent Soils	recent soils	Psamments
Fluvial Recent Soils	recent soils	Fluvents, Ochrepts
Tephric Recent Soils	recent soils	Orthents, Cryands, Udands
Orthic Recent Soils	recent soils	Orthents, Ochrepts
SEMIARID SOILS
Aged-argillic Semiarid Soils	brown-grey earths	Haplargids
Solonetzic Semiarid Soils	solonetz	Natragids
Argillic Semiarid Soils	brown-grey earths	Haplargids,
Immature Semiarid Soils	brown-grey earths	Camborthids
ULTIC SOILS
Densipan Ultic Soils	YB earths or podzols	Aquults
Albic Ultic Soils	YB earths	Aquults, Humults or Udults
Perch-gley Ultic Soils	YB earths	Aquults
Sandy Ultic Soils	YB earths or YB sands	Hapludults
Yellow Ultic Soils	YB earths	Hapludults

Cline MG 1949. Basic principles of soil classification. Soil Science [Internet]. 67(2):81–91. https://doi.org/10.1097/00010694-194902000-00002

Hewitt AE 1982. Decisions in the establishment of soil series. [PhD thesis] [Internet]. Ithaca, New York, USA. https://landcareresearch.on.worldcat.org/oclc/1129896729

Hewitt AE 1984. The NZ soil classification revision: Taylor’s patch reworked. New Zealand Soil News [Internet]. 32(6):220–229. https://doi.org/10.7931/DL30-1984_VOL32_6_ORC

Hewitt AE 1989. New Zealand soil classification (v. 2.0). Dunedin, New Zealand.

Hewitt AE 2010. New zealand soil classification (v. 3.0). [Internet]. 3rd ed. Wellington, New Zealand: Manaaki Whenua Press. https://doi.org/10.7931/DL1-LRSS-1-2010

Johnson WM 1963. The pedon and the polypedon. Soil Science Society of America Journal [Internet]. 27(2):212–215. https://doi.org/10.2136/sssaj1963.03615995002700020034x

Leamy ML, Clayden B, Hewitt AE 1983. Soil classification in the N. Z. Soil Bureau. New Zealand Soil News [Internet]. 31(6):183–185. https://doi.org/10.7931/DL30-1983_VOL31_6_ORC

Soil Survey Staff 1999. Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. [Internet]. 2nd ed. Washington D.C., USA: United States Department of Agriculture. https://www.nrcs.usda.gov/resources/guides-and-instructions/soil-taxonomy

Taylor NH 1948. Soil map of New Zealand. [Internet]. https://doi.org/10.7931/hdbc-9662

Taylor NH, Cox JE 1957. The soil pattern of New Zealand. Wellington, New Zealand.

Taylor NH, Pohlen IJ 1962. Classification of New Zealand soils. [Internet]. In: Wellington, New Zealand: New Zealand Department of Scientific; Industrial Research; p. 15–45. https://doi.org/10.7931/dl1-sbb-026-1