Distribution patterns of World Reference Base soil groups relative to soil forming factors

Relationships between World Reference Base (WRB) soil groups and the soil forming factors of climate, parent material and topography are examined using the ISRIC WISE Global database (2002 and 2008 versions). For each of 70 different environmental regimes, being combinations of the above three variables, the dominant WRB soil groups were determined and presented with the aid of pie charts. Soil qualifiers associated with the 1988 FAO soil classification scheme are also presented for each regime. Results are brought together into two summary “star charts” that give an overview of the broad distribution patterns of these soil groups relative to the three variables.The charts may provide a useful first approximation of WRB soil groups likely to occur under different environmental conditions and form a basis for regional modelling of WRB soil distribution. They have a potential for widespread application in conventional and possibly also quantitative soil mapping programs as they are based on a global soil database and do not require sophisticated data sources or technologies. Testing with 100 independent samples revealed a moderate predictive success rate, with 58% of test samples matching one of the top three predictions, and 74% matching one of the top five predictions. The charts appear to be effective in suggesting a range of likely WRB soil groups in a given environment, but could not be relied upon to predict a single specific soil group.The World Reference Base for Soil Resources classification scheme is shown to be at least moderately guided by the soil forming factors of climate, parent material and topography. This suggests that genetic factors are an important guiding principle behind the classification scheme. It may represent a scheme with an appropriate balance between soil management and soil genesis factors in its underlying principles.

Research Highlights► WRB soil group occurrence revealed in 70 climate-parent material-topographic regimes ► Results presented in pie charts and two summary overview charts ► Results will aid regional modelling of WRB soil distribution ► WRB scheme is shown to at least partially reflect soil genesis factors