Determination of Reserve Sulfur and Soluble Sulfates in Soils

Abstract

Abstract Organic and reduced inorganic sulfur (reserve sulfur) are converted to sulfates by ignition of a soil‐sodium bicarbonate mixture at 500° C. Converted and soluble sulfates (soil sulfur) are then extracted with a solution that is 2 N to HAc and 0.1 N to NaH 2 PO 4 ·H 2 O and determined turbidimetrically as BaSO 4 after acidifying a portion of the filtrate with 1:1 HCl. Acceptable recovery of sulfur has been obtained from elemental sulfur, plant material, methionine, cystine, and cysteine. Soluble sulfates are extracted from a separate sample with 0.5 N NH 4 OAc + 0.25 N HAc. Activated charcoal is added to decolorize the extract. A portion of the filtrate is acidified with 1:1 HCl containing a “seed” solution of sulfate, and sulfur is determined turbidimetrically as BaSO 4 . Reserve sulfur is the difference between soil sulfur and soluble sulfates. The multiple correlation of reserve and sulfate‐sulfur with total nitrogen in soils was highly significant (R = 0.971). Reserve sulfur was highly correlated with yield of sulfur from three harvests of white clover grown in the greenhouse (R = 0.790). The relation of reserve sulfur and sulfates to the sulfur status of soils is discussed.