Historical developments in the understanding of acid sulfate soils

•Background information for papers in Geoderma special issue on acid sulfate soils•Sulfidization and sulfuricization processes explained.•Potential, active and post-active acid sulfate soils concepts, problems, and uses•Genesis of acid sulfate soils related to sea and water table level fluctuations.•Classification/taxonomy of acid sulfate soils

Literature about the historical and current recognition of kinds, names and classification for, overall processes (sulfidization and sulfuricization) that form and conditions that induce the formation of potential, active and post-active acid sulfate soils are reviewed to set the stage for papers presented elsewhere in this special issue of Geoderma that contains some of the papers presented at the Eighth International Acid Sulfate Soils Conference held at the University of Maryland, College Park, MD, USA, July 17-23, 2016. Mention is made and examples cited of environmental problems such as AMD (acid mine drainage) and ARD (acid rock drainage), fish kills in waters receiving drainage from acid sulfate soils, even from ones considered post-active, and special land reclamation and management practices to produce crops on land disturbed (often deeply) by humans (including engineers poorly educated about acid sulfate soils) or by natural causes (sea level rise or fall, or extreme drought, such as the recent “Millennium Drought” in the Murray-Darling Basin of Australia. The global distribution of acid sulfate soils is considered, recognizing that post-active ones are extensive, with many likely currently unrecognized, although features in some of them, such as silcrete and ferricrete, likely owe their origin to acid sulfate weathering phenomena. Most coastal subaqueous soils are influenced by sulfidization and constitute sulfidic materials that if dredged and deposited in upland disposal areas become exposed to aerobic conditions and are likely to give rise to active acid sulfate soils with sulfuric horizons.