Evolution of the system
The Revised Legend of the FAO/UNESCO Soil Map of the World (FAO, 1988) was
used as a basis for the development of the WRB in order to take advantage of the
international soil correlation that had already been conducted through this project and
elsewhere. The first edition of the WRB, published in 1998, comprised 30 RSGs; the
second edition published in 2006 and the current (third) edition both have 32 RSGs.