New Zealand Soil Classification updated and extended
A number of new soils have been recognised in New Zealand and are published in a new edition of the Soil Classification in New Zealand (Hewitt 2010).
The new soils are mainly new subgroups, for example, a weathered subgroup has been added to Pallic and Brown soils that have underlying strongly weathered gravels (this material will have significant effects on deep percolation) and a tephric subgroup is recognised in Sandy Recent Soils. The previous fourth level of the classification (Soilform; Clayden & Webb 1994) has been modified and replaced with two new categories – the fourth and fifth levels, called the family and sibling respectively (Webb & Lilburne 2011).
The family is the category below Subgroup, and is designed to identify the dominant lithological composition of soil profiles. Families are identified by soil material, type of rock, texture, and the permeability classes in 100-cm depth. Recognition (or identification) of families follows the soil classification principles summarised by Hewitt (2010), particularly the following:
- The grouping of soils into classes should be based on similarity of soil properties rather than presumed genesis
- Differentiae should be based on soil properties that can be reproducibly and precisely measured or observed
- Differentiae should allow field assignment of soils to classes.
Soil families are given a geographic name and are also identified with a 4- or 5-character abbreviated name. Geographic names, also used to name soil series, have been retained because the users of soils are familiar with this usage. The family name is suffixed with an italicised ‘f’ to distinguish the family from the soil series. Soil families are more precisely and objectively defined than the old soil series.
The fifth category (sibling) is designated by a number and further refines the description of the physical attributes.
The sibling-level criteria are soil depth, topsoil stoniness, soil texture-profile, natural soil drainage, and a unique sequence of up to six functional horizons. The functional horizon codes indicate stone content, texture, structure size, and consistence. One small change that may be of interest to pedologists is the refinement of the imperfectly drained class, where pedons with mottled horizons above 30 cm and gleyed horizons below 60 cm are now classified as imperfectly drained soils rather than poorly drained soils. These soils commonly occur in Pallic environments that have a slowly permeable horizon in the upper subsoil but are not poorly drained (very few people would artificially drain these soils).
Application of the soil family and sibling criteria subdivides a number of the former soil series and merges others. Figure 1 gives an example of this. However, the new classification now correlates the same soil with the same name, wherever it occurs, rather than having different names within different survey areas. This means some soil names have changed and some South Island names will now occur in the North Island, and vice versa. All the pedons in the National Soil Database have now been classified at the sibling level and each measured horizon has been classified in terms Of the functional horizons. This greatly facilitates the grouping of similar soils or similar horizons and increases our ability to estimate a wide range of derived information for soils throughout New Zealand. See the factsheets on S-map Online for examples of the derived information.
|Figure 1. Illustration of the revised classification of four National Soil Database profiles from one soil series (Taihape) into three families. Each profile is labelled with its family name and sibling number. Each horizon is labelled with its functional horizon code. The Taihape family is now more precisely defined as being Typic Orthic Gley, moderately deep on rock, silty, moderate over slow permeability. The shallower, better drained part of the Taihape series is now classified as either the older Pahiatua family or the younger Mahoenui family.|
Clayden B, Webb TH 1994. Criteria for defining the soilform – the fourth category of the New Zealand soil classification, Landcare Research Science Series 3. 36 p.
Hewitt AE 2010. New Zealand soil classification. 3rd ed. Lincoln, New Zealand, Manaaki Whenua Press. 136 p.
Webb TH, Lilburne LR 2011. Criteria for defining the soil family and soil sibling: the fourth and fifth categories of the New Zealand Soil Classification. 2nd ed. Lincoln, New Zealand, Manaaki Whenua Press.
Linda Lilburne, Trevor Webb & Allan Hewitt