Converting the genetic-based national soil classification to the World Reference Base: a challenge for updating soil mapping in Portugal

The diversity of soil classification systems makes correlation between them challenging. The World Reference Base for Soil Resources (WRB) stands as a reference framework for international correlation of soils. In Portugal, soil mapping remains heterogeneous, as 55.3% of the mainland was mapped at a scale of 1:25,000 using the Classification of the Soils of Portugal (CSP), while the remaining area was mapped at a scale of 1:100,000 using the WRB system. Converting these legacy maps into a unified national 1:100,000 soil map in the WRB system requires the reinterpretation and remapping of the outdated 1:25,000 soil map. The present study discusses the categorical levels of the genetic-based CSP dating from 1974, their correspondence with WRB, and the relevance of updating this classification. The majority of CSP categorical levels show limited correspondence with WRB Reference Soil Groups, since CSP definitions lack the quantitative diagnostic criteria and thresholds required by the WRB. Exceptions include the “ Barros ” order (83% Vertisols), and the suborders Colluvial soils (100% Regosols) and Humic Litholic soils (78% Umbrisols). Overall correspondence improves when considering soil-forming factors such as lithology, relief, and climate, allowing for a better estimation of dominant and codominant RSGs within mapping units, as well as improved assignment of principal and supplementary WRB qualifiers. The proposed methodology supports the updating and harmonization of soil mapping in Portugal, but, to be successful, it requires the collection of new soil data to refine mapping units. New soil data would also be of great usefulness for revising and modernizing the CSP and improving the use of the WRB system.