Agriculture Reference
In-Depth Information
pedons of all reference soil groups. The use of these qualiÝers has been tested in WRB Ýeld
correlation tours in Vietnam, China, Sicily, Spain, Georgia, Benin, Hungary, and France.
An issue
in all meetings was the rationale behind the WRB system. Although WSR Number 84 (FAO et al.,
1998) provides a rationale for WRB as a whole, the report is rather scanty on the rationale underlying
the qualiÝer listings of the 30 Reference Soil Groups.
The question of qualiÝer priority ranking is not easy
because there is often a personal
bias in ranking. A road constructor and a farmer will attach different weights to certain soil qualiÝers.
Prominent WRB authors have suggested to do away with priority ranking altogether, and use the
alphabetical order instead. Others maintain that predetermined ranking makes the system easier,
e.g., for students. The following presents the provisional outcome of three years of discussions.
inter alia
General Principles for Distinguishing (Second Level) Soil Units in the WRB
The general rules for differentiating Soil Units are the following:
¤ The second level of the WRB classiÝcation system is deÝned by one or more diagnostic criteria
named ÑqualiÝer.Ò Each qualiÝer has a unique meaning, which, as far as possible, has been applied
to all Reference Soil Groups in which it occurs.
¤ Diagnostic criteria are derived from established deÝnitions of diagnostic horizons, properties, and
soil materials. They may include additional (new) elements, as well as criteria used for phase
deÝnitions in other taxonomies.
¤ Criteria related to climate, parent material, vegetation, or physiographic features such as slope,
geomorphology, or erosion are not considered. Neither are criteria referring to soil-water relation-
ships, such as depth of water table or drainage.
¤ Characteristics/properties of the substratum (below the control section) are not used for the
differentiation of lower level units.
¤ Normally two qualiÝers are sufÝcient to characterize most soils. If additional qualiÝers are needed,
these should be listed after the Reference Soil Group name between brackets, e.g., Geri-Acric
Ferralsol (Humic and Xanthic).
¤DeÝnitions of qualiÝers used may not overlap or conÞict either with each other or with the
Reference Soil Group deÝnitions to which they are attached. For instance, a Dystri-Petric Calcisol
is a contradiction (Dystri clashes with Calcisol), whereas a Eutri-Petric Calcisol is an overlap
because the preÝx ÑEutriÒ adds no information to the Calcisol Reference Group.
¤New units may only be established after being documented by soil proÝle descriptions and
supporting laboratory analyses.
¤
Priority rules for the use of second level soil units. It is recommended that priority rules for the
use of qualiÝers follow the qualiÝer categories and ranking as given below:
¤ Ýrst (if any) one or more Ñstrong expressionÒ qualiÝers
¤ secondly (if any) an intergrade qualiÝer
¤ thirdly (if any) one or more secondary characteristics qualiÝers, directly derived from deÝned
diagnostic horizons, properties, or soil materials
¤ fourthly (if any) one or more secondary characteristics qualiÝers, not directly related to deÝne
diagnostic horizons, properties, or soil materials
¤ Ýfthly (if any) a qualiÝer related to soil color
¤
the ÑHaplicÒ qualiÝer is used for other situations not covered in the foregoing list
THE CHALLENGE OF CAPTURING THE COMPLEXITY OF SOIL SYSTEMS
As Nachtergaele et al. (2000b) rightly point out, much confusion exists about the requirements
that soil classiÝcations and map legends must meet. Soil surveys, and particularly large-scale
surveys, require a measure of detail that cannot be provided by a comprehensive soil classiÝcation
system. It is argued here that the morphogenetic approach to soil classiÝcation, which underlies
