40 



as a 'macronutrient , ' and not as a 'lesser nutrient' as it is cur- 

 rently described " (The Sulphur Institute, 1978, p. 21). 



Sulfur is found in three general forms in the plant: (1) protein, 

 (2) sulf ate-S , and (3) organic S compounds of low molecular weight such 

 as free S-containing amino acids or volatile organic S compounds such 

 as glycosides, mer cap tans, and sulfides. The primary role of S in 

 plant nutrition is as a constituent of the S-containing amino acids, 

 cystine, cysteine, and methionine, and for protein synthesis. Cysteine 

 and methionine may be responsible for more than 90% of the organic S 

 in plants (Thompson et al. , 1970). Plants adequately supplied with S 

 will contain more true protein than plants without adequate S, and the 

 quality of this protein may be higher (Sheldon et al., 1951). Within 

 limits, the amount of methionine present in a protein is a measure of 

 the quality of that protein. Beaton et al. (1971) reviewed literature 

 showing where increasing amounts of added S improved the quality of 

 plant protein as indicated by the increase in methionine content. 



If true protein is not synthesized in plants due to inadequate S, 

 non-protein N such as amides and amino acids or even inorganic nitrates 

 may accumulate and reduce crop quality (Tisdale et al . , 1950; Rendig 

 and McComb, 1956; Coleman, 1957; Dijkshoorn and Van Wijk, 1967; 

 Stewart and Porter, 1969; Cowling and Jones, 1971). When available S 

 is abundant and N is limited in non-legumes, sulf ate-S may accumulate 

 while dry-matter and protein yields are restricted. Metson (19 73) 

 pointed out that in practice, the first situation (excess N in relation 

 to S) is more frequently encountered, particularly in crops regularly 

 fertilized with N. All of the available S is utilized in the 



