80 

 The Mg(N0 ) /HNO digestion procedure was compared to the diges- 

 tion procedure of Bardsley and Lancaster (1960) and to total S esti- 



2 

 mation using a Leco Sulfur Analyzer. 



Bardsley and Lancaster mixed finely ground soil with NaHCO in a 



porcelain crucible and ignited the mixture at 500 C for 3 hours in a 



muffle furnace. The oxidized mixture was then extracted with NaH„P0, * 



2 4 



HO in 2 N HOAc. Sulfate-S was determined turbidimetrically . 



The Leco Sulfur Analyzer used in this study ignites a soil (or 

 plant) sample in a high frequency induction furnace. A 0.5-g sample of 

 finely-ground soil was treated in a ceramic crucible with Fe , Sn, and 

 Cu accelerators. The crucible was covered with a porous cap and heated 

 to a very high temperature (ca. 1,600 C) in a stream of oxygen. Sulfur 

 is converted to gaseous SO which is trapped in dilute HC1 containing 

 KI, starch, and a trace amount of KIO solution. The reactions 

 involved are indicated below (Bremner and Tabatabai, 1971): 

 KIO + 5KI = 6HC1 -> 3I„ + 6KC1 + 3H ? 



S0„ + I + 2H ->■ H„S0, + 2HI . 

 2 2 2 2 4 



The results of these comparisons are presented in Table 11. The 

 soils studied and treatments were the same as those used in the com- 

 parison of different extraction methods (Table 9). 



The Leco method was unsatisfactory for the determination of total 

 S at the levels present in these soils. Tabatabai and Bremner (1970b) 

 and Bremner and Tabatabai (1971) also experienced poor recovery of 

 soil S and low precision using this technique for mineral soils. They 



9 



Laboratory Equipment Corp., St. Joseph, Michigan. 



