Rationale for the Key and the Qualifiers of the WRB 2006

When the first official version of the WRB was released at the 17th World Congress of Soil Science at Montpellier in 1998, the fortunate decision was taken to freeze the system for at least eight years in order to allow thorough reflection and field data collection. Since then, the WRB has been tested worldwide and has proven to be very valuable. However, a lot of time was spent discussing the necessity either to add/delete qualifiers to Reference Soil Groups, their ranking, and the ultimate rationale behind the WRB system. This paper summarizes the discussions on the Reference Groups and the qualifiers and provides a rationale for their ranking. It appears that a unified system of ranking rigorously applied to all WRB Reference Groups makes little sense. Therefore, a logical ranking system is implemented that adjusts to the very character of the different Reference Groups while respecting, as much as possible, some general principles for overall ranking.