Accurate and harmonised soil maps are essential for sustainable land management and achieving the Sustainable Development Goals (SDGs). In Portugal, existing soil maps remain inconsistent in scales (1:25,000 vs. 1:100,000), classification systems (national vs. WRB), and methodologies. Based on combinations of soil-forming factors (climate, lithology, and relief), the present study develops a fully operational workflow to update and harmonise soil mapping at a 1:100,000 scale using the World Reference Base for Soil Resources (WRB). Following the delineation of provisional land units, the methodological approach included: (1) compiling, validating and harmonising legacy soil profiles to support classification conversion and methodology validation; (2) converting the Soil Classification of Portugal (SCP) into WRB Reference Soil Groups (RSGs) and key qualifiers; (3) refining provisional LUs to improve their internal consistency; (4) defining Soil Mapping Units (SMUs) through identification of dominant, co-dominant, and associated soil units within different LUs; and (5) quantitatively validating SMUs against 165 legacy soil profiles using a presence/absence scoring method. The SMUs achieved 76–82% correspondence with legacy profiles under two validation scenarios. Despite legacy data constraints, the defined SMUs clearly differentiate WRB RSGs and qualifiers (principal and supplementary) across a Mediterranean landscape, enabling the identification of key soil functional properties often absent from legacy maps. The present approach provides a reproducible, scalable, operational framework for harmonised soil mapping and land evaluation. It also supports evidence-based soil management and can be readily adapted to other regions with comparable legacy data and environmental conditions. • Soil-forming factors drive WRB RSGs and qualifiers distribution in Mediterranean areas. • Updated soil mapping units show high agreement (76–82%) with legacy soil profiles. • Success of soil map updating depends on the accuracy of national-WRB conversion. • Reliable soil mapping with limited legacy data is enabled by land units' delineation. • The developed approach supports soil mapping harmonisation and land evaluation.
Guerreiro et al. (2026) studied this question.