Determination of Reserve Sulfur and Soluble Sulfates in Soils

Bardsley, C. E.; Lancaster, J. D.

Determination of Reserve Sulfur and Soluble Sulfates in Soils

1960

Bardsley, C. E. | Lancaster, J. D.

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.

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