‘acid sulfate soil’ is useful for general discussion, but fails to capture nuances required for technical classification of soils. Additional terms such as ‘potential acid sulfate soil’ (PASS), ‘active acid sulfate soil’ (AASS) and ‘post-active acid sulfate soil’ (PAASS) helped researchers distinguish between: non-acidic soils that did not contain substantial quantities of sulfuric acid, but contained the necessary materials to produce it under certain conditions; soils that were acidic due to the presence of sulfuric acid; and soils that were once active acid sulfate soils, but have since weathered or ‘ripened’ to the point they were similar to ‘normal’ sub-aerial soil in terms of their chemical and physical properties (e.g., pH and drainage). As a single soil profile may contain PASS, AASS, and PAASS at different depths, the term ‘material’ was added as a suffix (i.e., PASS-, AASS-, and PAASS-material) to allow for more nuanced descriptions of complex soils. However, these terms are still somewhat clunky and unsuitable for technical classification. Additionally, 'acid sulfate soils' and the variations thereof imply that acid sulfate soils are an altogether different type or class of soil, rather than a feature soils may exhibit under certain conditions. Consequently, there has been a gradual movement away from these terms in favour of more standardised, quantitative definitions. Sulfidic and Sulfuric Material The terms ‘sulfidic material’ and ‘sulfuric material’ were primarily coined by Prof Delvin Fanning (University of Maryland), Prof Martin Rabenhorst (University of Maryland), and Prof Rob Fitzpatrick (University of Adelaide) – and have been incorporated into the Australian Soil Classification (ASC) and World Reference Base (WRB) as diagnostic features of soil. For example, under the ASC, a Sulfuric Extratidal Hydrosol would refer to a soil that is saturated for 2-3 months of the year (i.e., a Hydrosol), located in an extratidal setting, and which contains sulfuric material. Therefore, acid sulfate soils may be described more technically as soils that contain sulfidic and/or sulfuric material. Sulfuric material refers to soil material that has a pH of less than 4 owing to the oxidation of sulfidic material. Sulfidic material refers to “soil materials containing detectable inorganic sulfides (≥0.01% sulfidic sulfur) that can exist as horizons or layers at least 30 mm thick or as surficial features”, and is further divided into 3 classes: hyposulfidic, hypersulfidic, and monosulfidic. Conceptually, hyposulfidic and hypersulfidic materials are used to distinguish between sulfidic material that, respectively, would not and would experience a