33 



estimating S as sulfate. As previously mentioned, gravimetry, turbi- 



dimetry, titrimetry, and colorimetry are the most common methods for 



measuring sulfate in digested plant tissue, soil extracts, digested 



soil samples, and other aqueous solutions containing sulfate-S. In 



almost all of these techniques, the sulfate anion reacts with the Ba 



cation in solution to form insoluble BaSO,. An extensive review of 



4 



some of these techniques has been made by Beaton et al. (1968) . 



Reduction of S to H S and determination of S as methylene blue 

 has generally been accepted as the most satisfactory method for the 

 colorimetric estimation of traces of S in soils, soil extracts, plant 

 tissue, and tissue extracts (Johnson and Nishita, 1952; Steinbergs et 

 al., 1962; Tabatabai and Bremner, 1970a, 1970b). This procedure was 

 developed by Johnson and Nashita (1952) for soil and plant tissue 

 analysis and has been reviewed by Beaton et al. (1968). This method 

 is very sensitive but quite tedious. Often very small samples are 

 necessary which may lead to sampling difficulties and erroneous 

 results. Aliquots larger than 2 ml cannot be used. A heat source and 

 S-free N are needed for the digestion-distillation apparatus. Each 

 sample must be boiled and refluxed for an hour to reduce all S to H S. 

 The large number of samples handled in this study could not be accomo- 

 dated using the Johnson and Nashita method because of the time involved 

 and the lack of necessary equipment. The author recognizes that some 

 accuracy may have been sacrificed in estimating S by the standard 

 turbidimetric method rather than using the more accurate but tedious 

 methylene blue method. 



