New Zealand Soil Classification (NZSC)

The NZSC has a hierarchical structure with five levels:

1. Order: The 15 soil orders are the highest, most generalised level of the classification and provide the national overview of New Zealand soils.
2. Group: The orders are divided into 74 soil groups based on variation in factors such as drainage status, parent material, chemical and physical properties.
3. Subgroup: The soil groups are divided into 299 subgroups that provide more detail about the range of soils included in each soil group.
4. Family: The soil subgroups are subdivided into family criteria that provide more detail about the soil parent materials, rock class, texture, and permeability.
5. Sibling: The soil family are subdivided into sibling criteria that provide more detail about soil depth, stone content, upper and lower textures, and drainage. The family also contains functional horizon attributes detailing horizon stone content, texture, structure size, and consistence.

Soil siblings are described by a sequence of up to six functional horizons. Functional horizons are soil 'horizons' (layers of soil material), occurring in soil profiles, characterised by clearly defined soil morphological parameters. They are 'functional' horizons because they are used to predict soil characteristics such as profile available water and hydraulic conductivity.

Example of a NZSC Hierarchical Structure

To find the correct soil class for a soil we examine the soil horizons (or layers) exposed in the side of a freshly dug soil pit, usually to the depth of about 1 m (unless rock is found at a shallower depth). We then assign the soil to the correct soil class, by using the keys to orders, groups, and subgroups in Hewitt (2010) and family and sibling criteria in Webb and Lilburne (2011). The simplified key below provides a guide to the correct soil order. The keys for assigning a soil to the correct soil class use diagnostic horizons and other defined soil features.

Definitions of the diagnostic horizons or diagnostic features are more restrictive than definitions of horizon notations (for example, thickness limits are imposed on many diagnostic horizons). The relationships shown here are therefore only approximate.

Complete lists and definitions of soil horizon notations are given in Milne et al. (1995), and diagnostic horizons, soil materials and soil features are given in Hewitt (2010).

The soil classification can be used to trace the formation, or evolution, of New Zealand soils through time.

The diagram illustrates major pathways in the evolution of New Zealand soils.

In (I) Mineral Parent Materials, (II) Raw Soils develop into (III) Recent Soils. The nature of the parent material then strongly determines the subsequent soil formation pathway (IV). Later, parent material becomes less important, and climate and vegetation become more important controls over the character of the soil that evolves (V).

Soils will only proceed the whole way down a soil formation pathway on relatively stable sites.

Erosion or additions of fresh sediment may impede, halt or even reverse soil evolution. Many (II) Raw Soils may never develop into (III) Recent Soils or other soils because they occur in environments with continual cycles of erosion or additions of fresh sediment.

The pathways shown represent only those likely to have occurred over extensive areas. Other pathways are omitted for clarity. For example, Allophanic Soils may also be formed in quartz, feldspar, and mica parent material, and Ultic Soils may also be formed in mafic or tephra parent materials.

Major pathways in the evolution of New Zealand soils (click to enlarge). Source: Hewitt (1998)