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Determination of Reserve Sulfur and Soluble Sulfates in Soils¹

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 2N to HAc and 0.1N to NaH₂PO₄·H₂O and determined turbidimetrically as BaSO₄ 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 0.5N NH₄OAc + 0.25N 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 BaSO₄. 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.

¹ Contribution from the Agricultural Research Service, USDA, and the Mississippi Agr. Exp. Sta., cooperating. Published with the approval of the Director of the Mississippi Agr. Exp. Sta. as Journal Article 800. Presented before Div. II, Soil Science Society of America, Cincinnati, Ohio, Nov. 19, 1959.

² Soil Scientist, Eastern Soil and Water Management Research Branch, SWCRD, ARS, USDA, and Agronomist, Mississippi Agr. Exp. Sta., State College, respectively.

Received for publication November 21, 1959. Accepted for publication March 9, 1960.