Potential Sulfidic Acidity (PSA) is a function of the concentration of Reduced Inorganic Sulfur (RIS) in a soil sample. If RIS were the only pool of sulfur present in a sample, PSA could be directly estimated from the total concentration of sulfur. In some cases this may possible. However, sulfate minerals (e.g., gypsum, epsomite, and jarosite) and organic matter typically constitute additional sources of sulfur in most sulfidic and sulfuric soils. Chromium Reducible Sulfur (S or CRS) accurately measures RIS without significant interference from these other sources of sulfur. The chromium reduction method is summarized in Soil Chemical Methods:S by a hot acidic CrCl solution; the evolved gas is trapped quantitatively in a Zn acetate solution as solid ZnS. The ZnS is then treated with HCl to release HS into solution, which must then be quickly titrated with I solution to the blue-coloured end point indicated by the reaction of I with starch.) - is calculated as the difference between the sulfur determined via peroxide digestion (S) and sulfur extracted with a 1 M KCl solution (S). 
S is a measure of sulfur associated with relativity soluble sulfates (e.g., gyspum and epsomite). S is a measure of sulfur associated with both RIS and organic matter. As such, S may overestimate PSA where samples contain even relatively small amounts of organic matter. Therefore, S is recommended for samples containing > 0.06% organic carbon.
Neither S nor S provide a measure of sulfur associated with sparingly soluble sulfate minerals such as jarosite.