Page 9 - World Reference Base for Soil Resources of Thailand

Land Development Department Library

8. Many RSGs in the WRB are representative of major soil regionsso as to
provide a comprehensive overview of the world’s soil cover.

9. Definitions and descriptions reflect variations in soil characteristics that occur both
vertically and laterally in the landscape.
10. The term Reference Base is connotative of the common denominator function of
the WRB: its units (RSGs) have sufficient width to facilitate harmonization and correlation with
existing national systems.
11. In addition to serving as a correlation between existing classification systems, the WRB
also serves as a communication tool for compiling global soil databases and for the inventory
and monitoring of the world’s soil resources.
12. The nomenclature used to distinguish soil groups retains terms that have been used

traditionally or that can be introduced easily into current language. They are defined precisely,
in order to avoid the confusion that occurs where names are used with different connotations.

2. Structure
Each RSG of the WRB is provided with a listing of possible principaland
supplementary qualifiers, from which the user can construct the second level

of the classification. The principal qualifiers are given in a priority sequence. The broad
principles that govern the WRB class differentiation are:
 At the first level (RSGs), classes are differentiated mainly according to characteristic
soil features produced by primary pedogenetic process, except where special soil parent materials
are of overriding importance.

 At the second level (RSGs with qualifiers), soils are differentiated according
to soil features resulting from any secondary soil-forming process that has significantly affected

the primary characteristics. In many cases, soil characteristics that have a significant effect on
land use are taken into account.

2.1 First Level: The Reference Soil Groups
Table 2 provides an overview of the RSGs and the rationale for the sequence of
the RSGs in the WRB Key. The RSGs are allocated to groups on the basis of dominant identifiers,
i.e. the soil-forming factors or processes that most clearly condition the soil.
2.2 Second Level: The Reference Soil Groups with their qualifiers

1) Principal qualifiers
The principal qualifiers are ranked and given in an order of importance. The principal
qualifiers are added before the name of the RSG without brackets and without commas.
The sequence is from right to left, i.e. the uppermost qualifier in the list is placed closest to
the name of the RSG.
1) Qualifiers: related to diagnostic horizons, properties and materials
2) Qualifiers: related to chemical properties
3) Qualifiers: related to physical properties

4) Qualifiers: related to mineral properties

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