Agriculture Reference
In-Depth Information
recognized, three general texture classes (Ýne, medium, and coarse) and three general slope classes
(with slopes less than 8%, 8 to 30%, and more than 30%). Most soil mapping units were in fact
soil associations, the composition of which was indicated at the back of each paper map sheet. The
dominant soil unit gave its name (and appropriate color) to the mapping unit, followed by a number
unique to the associated soils and inclusions. Texture (1, 2, and 3) and slope symbols (a, b, and c)
were included in the mapping unit symbol.
In 1984, during the initial Agro-ecological Zones study (FAO, 1978), the mapping composition
was formalized, and the proportional distribution of soil units and their corresponding soil phases
(if any), textures, and slopes in the mapping unit could be expressed as a percentage. The rules
used to do this were logical (a function of the number of soil units associated and included), but
obviously resulted only in a very rough estimate of the proportions of each unit present in the
mapping unit.
Although initially developed as a Legend for a speciÝc map, not a soil classiÝcation system,
the FAO Legend found quick acceptance as an international soil correlation system, and was used,
for instance, as a basis for national soil classiÝcations (Bangladesh, Brammer et al., 1988; Kenya,
Sombroek et al., 1982), and regional soil inventories as in the soil map of the European Union
(CEC, 1985).
With the applications as a soil classiÝcation, numerous comments and suggestions were received
to improve the coherence of the system. In fact, some combinations of diagnostic horizons could
not be classiÝed, while one soil unit identiÝed in the key (gelic Planosols) did not occur in the
map. The revision effort undertaken in the 1980s Ýnally resulted in the publication of the Revised
Legend of the FAO/UNESCO Soil Map of the World (FAO/UNESCO/ISRIC, 1988). This revised
legend was applied to the World Soil Resources Map at 1:25,000,000 scale, accompanied by a
report (FAO, 1993) and presented at the Kyoto ISSS Congress.
The revised legend retained many features of the original FAO legend. The number of Great
Soil groupings increased from 26 to 28: the Rankers and Rendzinas were grouped with the
Leptosols, the ÑaridicÒ Yermosols and Xerosols disappeared, and new Great Soil groups of Calcisols
and Gypsisols were created. The Luvisols (AlÝsols)ÏAcrisols (Ultisols) division was further divided
according to the activity of the clay fraction resulting in four symmetric groups (Luvisols, high
base saturation, high-activity clays; Acrisols, low base saturation, low-activity clays; Lixisols, high
base saturation, low-activity clays; and Alisols with low base saturation and high-activity clays).
The revised legend also created at the highest level the Anthrosols, grouping soils strongly inÞu-
enced by human activities. The number of soil units increased from 106 to 152. Texture and slope
classes remained unchanged, but were not represented on the map produced for obvious reasons
of scale. A start was made with the development of a set of names at the third level. The latter
were further elaborated in a comprehensive set of third level ÑqualiÝersÒ by Nachtergaele et al.
(1994) and presented at the 15
ISSS Congress in Acapulco.
As had happened with the Legend, the Revised Legend was used as a basis for the national
map legends as in Botswana (Remmelzwaal and Verbeek, 1990).
In a parallel development, a working group of the ISSS had been active in the development of
an internationally acceptable soil classiÝcation system (Ýrst meeting in SoÝa, 1982). In 1992, at a
meeting of the Working Group RB (Reference Base), the strong recommendation was made that
rather than developing a fully new soil classiÝcation system, the Working Group should consider
the FAO Revised Legend as a base and give it more scientiÝc depth and coherence. This principle
was accepted, and the Ýrst draft of the World Reference Base (WRB) appeared in 1994
(ISSS/ISRIC/FAO, 1994), still showing large similarities with the FAO revised legend.
WRB aims to satisfy at the same time two very different kinds of users of soil information:
one, the occasional interested user, who should be able to differentiate the 30 reference soil groups,
and second, the professional soil scientist who needs a universal nomenclature for soils in a simple
system that enables him to communicate about the soils classiÝed in his national soil classiÝcation.
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