9 



this S was assumed to be organic sulfates that could easily be split 

 from the organic matter. Barrow (1961) deducted from his observations 

 that sulfate released by heat treatment was inorganic because it could 

 be removed with a 0.15% CaCl 9 solution. This extractant does not 

 decompose organic sulfate, and plant uptake of S was closely related 

 to sulfate extracted by this procedure. The fact that organically 

 bound sulfates form a considerable fraction of the total soil S was 

 based on these early reports where NaOH was used to extract most of the 

 total soil S (Williams and Steinbergs, 1959; Freney, 1961). 



Organic soil S is generally separated into the C-bonded fraction 

 and the Hi-reducible or ester-sulfate fraction. The C-bonded fraction 

 is believed to consist largely of S in the form of S-containing amino 

 acids such as methionine and cystine. The Hi-reducible S is the frac- 

 tion which is not bonded directly to C and is reducible to H S by HI. 

 This fraction is believed to consist of S in the form of ester sul- 

 fates, e.g., organic sulfates which contain C-O-S linkages such as 

 choline sulfate, phenolic sulfates, sulfated polysaccharides, etc. 



Prior to the 1960's, only small amounts of cystine and methionine 

 could be detected in soil organic matter (Bremner, 1950; Sowden, 1955, 

 1956; Stevenson, 1956). Due to uncertainties in extraction and analy- 

 sis, the highest values reported for these organic S constituents were 

 only around 10% of the total S content (Freney et al., 1970). 

 DeLong and Lowe (1962) were able to separate this C-bonded fraction by 

 reducing the S to inorganic sulfide with Raney nickel. Carbon-bonded S 

 occupied 47 to 58% of the total S in organic soils and 12 to 35% in 

 mineral soils from Quebec (Lowe and DeLong, 1963) . Data suggested that 

 much of the C-bonded S may have been associated with humic acid 



