Agriculture Reference
In-Depth Information
because of the prominence of allophanic soil materials in New Zealand, it was decided to introduce
an academic word to help popularize knowledge of the important soils derived from them.
PRIORITY OF CLASSES OVER KEY
Principle 3 places attention on the function of classes and on class intent. It is useful to separate
in our minds the intention of a class and the keying differentiae. The intention of a class and its
ability to carry accessory information is the most important, and keying differentiae should be less
important. If priority is Ýxed on the keying differentiae, the classiÝcation may force the user to
make inappropriate assignments that violate principle 3.
The introduction to the New Zealand Soil ClassiÝcation states that the classes are the most
important part of the soil classiÝcation. The key is merely a means of assigning soils to these
classes, and by its nature it is imperfect, because only a sample of all the possible soils that might
potentially be assigned were used in its development. Soils will be found that are not assigned to
the appropriate class by the key. This will be apparent when an assigned soil does not conform to
the concept and accessory statements that can normally be made about the class. Because the key
is the servant of the classes, the allocator is justiÝed in placing the soil misÝt into a more appropriate
class. If this is done, however, it must be registered with the person who has responsibility for the
national soil classiÝcation system, so that appropriate adjustments may be made to the key when
the soil classiÝcation is next revised. An assignment contrary to the key must also be noted in any
records or publication of the allocation.
DISTINCTIVE CHARACTER OF NEW ZEALAND SOILS
Many New Zealand soils are representative of major world soils, such as the Allophanic Soils
and Pumice Soils, which were studied during development of the Andisols order of Soil Taxonomy
(ParÝtt and Clayden, 1991). However, some soils and the relationships they display in the landscape
are apparently unique, and are probably related to the following characteristics:
¤New Zealand loess, which is a prominent soil parent material, is predominantly acidic. Calcium
carbonate is not present in the unweathered loess. The loess source rocks have low contents of
calcium, compared to similar loess source basement rocks in North America and Europe (Hewitt,
1997).
¤ Fragipans developed in loess occur in ustic rather than udic moisture regimes. Consequently the
Fragiudepts and Fragiudalfs of Soil Taxonomy are not appropriate great groups for New Zealand;
rather, Fragiustept and Fragiusalf great groups are needed.
¤A gillic horizons do not have the same prominence of expression and widespread occurrence in
New Zealand as they have in other regions of the globe, although they might be expected from
environmental relationships.
¤
Udepts with ochric epipedons and cambic horizons are almost exclusively dystric.
The broad-scale sequence of soils on quartzo-feldpathic parent materials, not inÞuenced by tephra,
on late Pleistocene or early Holocene land surfaces, in mesic and cryic temperature regimes, from aridic
through ustic to udic and to preudic moisture regimes, is different in New Zealand from that found in
North America. The expectation from North America is the sequence AridisolsÏMollisolsÏAlÝsolsÏSpo-
dosols. In New Zealand, however, the sequence is AridisolsÏUsepts/UsalfsÏUdepts/UdalfsÏSpodosols.
The New Zealand ustic and udic soils in this sequence were recognized early in the history of
pedological investigations in New Zealand, and were deÝned, respectively, as the yellow-grey earths
and yellow-brown earths (Taylor and Pohlen, 1968). Following principles 7 and 8, development of
the new New Zealand Soil ClassiÝcation maintained the concepts of the soils that expressed
