The nature of soil-water interactions is discussed in the context of both soil formation and drainage water chemistry. The interacting factors regulating drainage water chemistry are then briefly considered, including soil type and parent material, soil atmosphere CO2 concentration, hydrological pathways, soluble salts, topography, climate and biotic effects. Based upon these factors, some sites in NE Scotland thought to be highly susceptible to water acidification are examined, and the low pH of their waters is confirmed. The acidity trends of two superficially similar streams apparently less at risk are compared, and the differences explained in terms of sub-surface soil characteristics. The possible impacts of both afforestation and of acidifying pollutants on water pH are considered briefly where appropriate.