49 



depends on the supply of mineral ions and varies to a much greater 

 extent than that of organic S. 



The ability of the species to accumulate sulfate in excess of 

 metabolic requirements must also be considered. Grasses are vigorous 

 sulfate accumulators, whereas the legumes rarely accumulate high con- 

 centrations of sulfate (Metson, 1973). Dijkshoorn et al. (1960) 

 reported a critical level of sulfate in ryegrass herbage of 0.01 

 g-atoms S per kg dry weight (0.032%). Some critical sulfate-S levels 

 in selected crops from the literature are given in Table 6. 



Metson (1973) compiled an excellent review of the literature 

 relating to total S, sulfate-S, and N:S ratios and the S fertility of 

 grasses and legumes. He found that sulfate-S varied in a similar 

 fashion to total S and concluded that the preferred diagnostic tech- 

 nique might be whichever property is the more conveniently determined. 



Freney et al. (1978) studied total S, sulfate-S, (N:S)t, amide N, 

 and sulfate as a percentage of total S as indices for diagnosing the 

 S status of wheat in Australia. They found that each of the indices 

 was strongly related to dry matter yield. Critical, total S values 

 varied considerably with type of the tissue and age (Table 6) . 

 Sulfate-S changed only slightly with age and N supply but failed to 

 provide any discrimination between plants suffering from different 

 degrees of S deficiency; only "deficient" and "adequate" categories 

 could be identified. The (N:S)t was most appropriate for young wheat 

 plants only. Amide N provided the greatest relative change in values 

 between S-deficient and S-adequate plants, but could be influenced by 

 the Zn and Fe supply to the plant. They concluded that sulfate-S ex- 

 pressed as a percentage of the total S provided the best correlation with 



