matter typically constitute additional sources of sulfur in most acid sulfate soils. Chromium Reducible Sulfur (SCR or CRS) accurately measures RIS without significant interference from these other sources of sulfur. The chromium reduction method is summarized at p 431 in Soil Chemical Methods:“The Cr reduction method… is based on the conversion of reduced inorganic S to H2S by a hot acidic CrCl2 solution; the evolved gas is trapped quantitatively in a Zn acetate solution as solid ZnS. The ZnS is then treated with HCl to release H2S into solution, which must then be quickly titrated with I2 solution to the blue-coloured end point indicated by the reaction of I2 with starch.”Another common measure of PSA - Peroxide Oxidisable Sulfur (SPOS) - is calculated as the difference between the sulfur determined via peroxide digestion (Sp) and sulfur extracted with a 1 M KCl solution (SKCl). SKCl is a measure of sulfur associated with relativity soluble sulfates (e.g., gyspum and epsomite). Sp is a measure of sulfur associated with both RIS and organic matter. As such, SPOS may overestimate PSA where samples contain even relatively small amounts of organic matter. Therefore, SCR is recommended for samples containing > 0.06% organic carbon. Neither SCR nor SPOS provide a measure of sulfur associated with sparingly soluble sulfate minerals such as jarosite. Chromium Reducible Sulfur v SPOCAS Suspension Peroxide Oxidation Combined Acidity and Sulfur (SPOCAS) method is a suite of tests used to determine the net acidity via the measurement of: Titratable Peroxide Acidity (TPA) (as a measure of PSA); Titratable Actual Acidity (TAA); Excess Acid Neutralising Capacity (ANCE); and Residual Acid Soluble Sulfur (SRAS) or Net Acid-Soluble Sulfur (SNAS) (as measures of retained acidity). As SPOCAS is effectively a self-contained ABA test, it is very useful and popular. However, the use of peroxide digestion for TPA may overestimate PSA where organic matter is present (see discussion on SPOS). Therefore, it is recommended to measure SCR for at least one sample per batch of samples to determine if there is a substantial discrepancy between PSA measured via SCR and TPA. Additionally, note that SRAS does not reliably measure the concentration of jarosite or schwertmannite (the typical sources of retained acidity); and SNAS typically only accounts for 50-60% of jarosite. Consequently, SNAS should be doubled to obtain a more accurate measure of Retained Acidity where jarosite is likely the principal source of Retained Acidity (which is