Soil quality — Acid-base accounting procedure for acid sulfate soils — Part 2: Chromium reducible sulfur (CRS) methodology

ISO 14388-2:2014 specifies a suite of methods used to determine the net acidity in acid sulfate soils. ISO 14388-2:2014 specifies a method for measuring chromium reducible sulfur (S_CR) by iodimetric titration of distilled hydrogen sulfide trapped as zinc sulfide, following acidic chromous chloride digestion. This method determines inorganic sulfides (e.g. pyrite, marcasite, greigite, mackinawite) and elemental sulfur in acid sulfate soil without interferences from organic sulfur and oxidized forms of sulfur such as sulfate.

On a separate test portion of soil, the pH in a 1 mol/l KCl soil suspension (pH_KCl) is determined. When pH_KCl is < 6,5, titratable actual acidity (TAA) is then determined. Subsequently, potassium chloride extractable sulfur (S_KCl), calcium (Ca_KCl), and magnesium (Mg_KCl) can also be determined. Where jarosite is identified in the soil (or where pH_KCl is < 4,5), net acid-soluble sulfur (S_NAS) is determined by the difference between hydrochloric acid extractable sulfur (S_HCl) and potassium chloride extractable sulfur. On samples where pH_KCl is < 6,5, acid neutralizing capacity is determined by measuring either inorganic carbon (C_IN) by combustion furnace, or ANC_BT (ANC measured by back-titration of acid remaining following an acid digest).