Agriculture Reference
In-Depth Information
keying-order decisions. The last class in any key is usually the class closest to the central concept
of the classes in that category. For this reason, many of the Ýnal groups to be keyed out are named
Ñorthic,Ò in the sense of ordinary or central. Similarly, many of the Ýnal subgroups to be keyed out
are named Ñtypic.Ò
The soil individual is the fundamental unit of soil that is assigned to classes, and the population
of soil individuals may be seen as the foundation of the hierarchy. Cline (1949) deÝned an individual
as Ñthe smallest natural body that can be deÝned as a thing complete in itself.Ò Soil Taxonomy
(Soil Survey Staff, 1999) regards the polypedon as the soil individual. This deÝnition was rejected
in the New Zealand Soil ClassiÝcation because, as argued by Hewitt (1982), it does not fulÝll
ClineÔs (1949) or JohnsonÔs (1963) requirements for a soil individual.
In New Zealand, the soil individual has traditionally been the soil proÝle. Usually conceived
as a two-dimensional section exposed by a soil pit, it is in fact a three-dimensional slice sufÝciently
thick to sample and examine hand specimens. With the realization that soils should be examined
in successive horizontal sections as well as the vertical proÝle, there is increasing acceptance in
New Zealand that a volume of soil the size of the pedon (Soil Survey Staff, 1975) represents a
better soil individual than the soil proÝle slice.
Accordingly, the pedon, as deÝned in Soil Taxonomy (Soil Survey Staff, 1975), is applied as
the soil individual for the New Zealand Soil ClassiÝcation. It is understood, however, that assign-
ments are often made from the examination of volumes of soil smaller than a complete pedon,
where they are assumed representative of the pedon.
HIERARCHICAL POSITION OF THE HYDROMORPHIC SOILS
The operation of principles 2 and 3 is illustrated by treatment of soils with evidence of wetness
and biochemical reduction, identiÝed as Gley or Mottled classes. Gley classes may be deÝned in
terms of either measures of water saturation, reduction morphology, chemical measures of reduction,
or some combination of these. Problems in the use of water saturation as the basis for identiÝcation
have been discussed by Childs and Clayden (1986). Probably the most convincing argument against
reliance upon water saturation differentiae is that the required data are seldom available, and where
they are available, extrapolations must be made to other soils by use of the reduction morphology.
Emphasis in the New Zealand soil classiÝcation has been placed on reduction morphology as the
alternative that enables assignment based on direct Ýeld observations in any season.
The New Zealand Soil ClassiÝcation recognizes two kinds of Gley soil classes. Where features
associated with strong gleying occur together with the distinct marks of other soil-forming processes
of soil in deÝned orders, then the soils are recognized as either Perch-gley or Groundwater-gley
groups of these orders. The Gley Soils order, however, was deÝned for soils in which such distinct
marks of other soil-forming processes were absent. Gley Soils therefore comprise soils that are
either too young to display the distinct marks of other soil-forming processes, or in which the
marks of those soil-forming processes have been retarded or destroyed by gleying.
Two other means of classifying gley soils were considered. All gley soils could have been
collected into the Gley Soils order, as in European soil classiÝcations (e.g., Avery, 1980), or
distributed among other orders, as in Soil Taxonomy. Although either approach would have produced
a more symmetrical classiÝcation, they both had disadvantages for the New Zealand Soil Classi-
Ýcation.
The collection of all gley soils into one order would have defeated principle 3 of the New
Zealand Soil ClassiÝcation. That is, classes must be designed to allow the greatest number and
most precise accessory statements to be made about them. Such an order would have incorporated
a very diverse set of soils for which the only possible accessory statements would concern wetness.
It should be noted that other soil classiÝcations that recognize gley soils at order level do not
incorporate all gley soils into that one class. The Soil ClassiÝcation for England and Wales (Avery,
