32 



reviewed some of the problems with the Leco method and suggested that 

 the problems may be related to (1) the size of sample and the mesh- 

 size, (2) the amount and type of combustion accelerator, (3) the method 

 of instrument calibration, and (4) the incomplete sample combustion 

 or the release of S0_ as well as S0 9 . They concluded that the Leco 

 method was "... unsatisfactory for research requiring accurate and 

 precise determination of total sulfur in soils" (Tabatabai and Bremner, 

 1970b, p. 419). However, the Leco method has proven to be of value in 

 plant tissue analysis (Jones and Issac, 1972; F. Adams, personal com- 

 munication) . 



The Mg(N0_)„/HN0 digestion resulted in slightly higher levels of 

 S than the Bardsley and Lancaster method. This was probably due to 

 more effective digestion of the soil minerals because of the prediges- 

 tion step on the hotplate. The better contact of the soil particles 

 with the Mg(N0 ) /HNO may have resulted in less loss of S through 

 volatilization. Therefore, the Mg(N0_) o /HN0 digestion procedure was 

 adopted as a suitable alternative to either HC10,/HN0 digestion or 

 NaHCO digestion with acid extraction. 



6. ANALYTICAL TECHNIQUES FOR DETERMINING SULFUR 



Sulfate S in soil extracts, in digested-soil extracts, and in 

 digested plant tissue was determined by a turbidimetric procedure 

 (Massoumi and Cornfield, 1963; Chaudry and Cornfield, 1966) or by the 

 indirect method of Hue and Adams (1979). Because of difficulties 

 associated with S analyses, these techniques and others are discussed 

 below. 



Most analytical techniques for determining S first involve the 

 conversion cf various forms of S to the sulfate ion and quantitatively 



