Soil classification, map legends and GIS logic: experiences from converting the legend of the soil map of Belgium into WRB

A systematic soil survey of Belgium was conducted from 1948 to 1991. The purpose was
to have a soil information base adequate for boosting agricultural production after the
Second World War. Field surveys were done at the detailed scale of 1:5000, while the
maps were published at a 1:20,000 scale. To enable soil surveyors to identify soils in the
field, an original soil classification was developed based on readily observable physical
and morphogenetic characteristics. Within the European Union, the World Reference Base
(WRB) for Soil Resources (IUSS Working Group WRB, 2007), has been adopted as the
common classification system. As soil surveys in most European countries were
conducted independently, the challenge now is to convert the national legends into a
common WRB legend. In Belgium, such a conversion is being implemented and we
present some lessons learned in relation to classification and map legends.

The legend of the soil map of Belgium is based primarily on soil texture, drainage status
and profile development. “Soil series” are defined in an open and non-hierarchical
structure by combining these three categorical variables, and to which modifiers can be added such as parent material, stoniness or depth to a substratum. The WRB-2007
classification is based on diagnostic features defined by morphological, physical and
chemical properties. The conversion of the legend of the soil map of Belgium to WRB is
based on insights gained from classifying, so far, more than 360 legacy soil profiles in
combination with field observations. From these insights, heuristic rules have been
deduced regrouping soil series into Reference Soil Groups (RSG) and for which some
qualifiers could also be identified. Other qualifiers were determined by relying on
databases of the legacy soil profiles. To take regional variability into account, the
conversion is done by physiographic zone.

Converting the legend of the soil map of Belgium to WRB actually leads to a regrouping
of soil series into broader WRB categories and which can neatly be represented on
1:50,000 scale maps. Hence, it does not imply substituting one classification with another
one. Users, who would need the detailed information, can still refer to the detailed symbols
of the soil series. The regrouping of soil series has been made possible thanks to the
flexibility of WRB for combining various qualifiers. However, as the WRB-2007
classification leads to a varying number of qualifiers it is less practical for constructing
map legends. Therefore, the WRB-2010 guidelines (IUSS Working Group
WRB, 2010) propose to organise qualifiers in main and optional ones with priority rules
for the main qualifiers. As illustrated in Table 1, this approach may highlight, or hide,
some qualifiers inconsistently. When for example only two qualifiers are retained in a
map legend, the qualifier Endogleyic of stony, poorly drained Albic Podzols will not be
indicated, while it will be indicated when such soils are not stony (Table 1). Moreover, the
proposed hierarchy is sometimes also inconsistent when compared across Reference Soil
Groups (RSG) as illustrated for the Arenosols and Regosols (Table 1). These drawbacks
could be avoided if qualifiers would be organised into thematic groups ― such as profile
development, texture, drainage, depth of substratum and fertility ― rather than by ranking
them. Grouping qualifiers thematically would have the advantage to give more flexibility
to the map user working with GIS, and indeed, it would render WRB a more “GIS logic”
classification system.