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This article in SSSAJ

  1. Vol. 52 No. 4, p. 959-964
     
    Received: July 6, 1987
    Published: July, 1988


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doi:10.2136/sssaj1988.03615995005200040011x

Reduction of Organic Sulfur in Soils with Tin and Phosphoric Acid

  1. H. J. Pirela and
  2. M. A. Tabatabai 
  1. Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

Abstract

Abstract

The chemical nature of organic S in soils is not well understood. In this study, a Sn-H3PO4 reagent was used for reduction of organic S in soils to H2S for subsequent determination as methylene blue. The reduction of soil organic S was compared with that of model S compounds, and with the fractions reduced with HI and with Raney Ni. Results showed that, in general, most of the reduction of organic S in 13 Iowa and 7 Chilean surface soils occurred after distillation for 1 to 3 h, but some soils continued to release H2S for up to 10 h of distillation with this reagent. Expressed as a percentage of total organic S in Iowa surface soils, the amounts of Sn-H3PO4-reducible S ranged from 15 to 64% (avg = 35%) and from 42 to 91% (avg = 60%) for 1 and 10 h of distillation, respectively. The corresponding percentages of the Chilean surface soils were from 8 to 73% (avg = 25%) and from 29 to 100% (avg = 55%). The Sn-H3PO4-reducible S values were significantly correlated with organic S [r = 0.71*** (P < 0.001) and 0.88*** for 1 and 10 h of distillation, respectively]. The amounts of organic S potentially reducible (Sr) with this reagent ranged from 35 to 86% (avg = 61%) of organic S in Iowa surface soils. The corresponding values for the Chilean soils ranged from 33 to 100% (avg = 66%). The times required to distill 50% of Sr ranged from 0.3 to 1.7 h (avg = 0.8 h) for the Iowa soils, and from 0.4 to 9.2 h (avg = 3 h) for the Chilean soils. Studies with five Iowa soil profiles showed that the percentages of organic S reduced with Sn and H3PO4 in 1 and 10 h of distillation varied with depth of sampling and that these values were similar, lower, or greater than those obtained with the HI-reducing mixture.

Journal Paper no. J-12740 of the Iowa Agric. and Home Economics Exp. Stn., Ames, IA. Projects 2112 and 2431.

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