A soil-forming process whereby organic acids (from humus) increase hydrogen ion concentration, as in the transformation of brown-earth soils into acid brown earth, in humid temperate forest regions. For the Model of Acidification of Groundwaters in Catchments (MAGIC), see Cosby (1995) Hydrol. & Earth Sys. Scis 2, 4. See Eidukeviciene et al. (2010) Nat. Hazards Earth Syst. Sci. 10, 1477 on effective soil acidification management.