43 



poor animal performance when this forage was fed. He considered values 

 of 12 to 17 to be normal. Bremer (1976) also found that the (N:S)t ra- 

 tios widened greatly in bermudagrass as S became deficient. Cowling and 

 Jones (1971) considered 20 the critical ratio for perennial ryegrass. 



Terman et al. (1973) used the (N:S)t ratio at the minimum concen- 

 trations of N and S necessary for continued growth of corn to compute 

 a critical ratio of 16 for this crop. However, they concluded that 

 "... the ratios appear to be meaningful only to indicate the relative 

 amounts of labile N and S present for assimilation by the crop" (Terman 

 et al., 1973, p. 636). The ratios were not closely related to the 

 corresponding dry matter yields. Daigger and Fox (1971) and Kang and 

 Osiname (1976) also failed to relate (N:S)t ratios to yields and S 

 responses in corn. Their poor correlations were associated with 

 late sampling of the ear leaf, low (N:S)t ratios, and variations 

 in the (N:S)t ratio with location. 

 2.3.3 Sulfate Sulfur 



Sulfur in excess of that required for protein synthesis will 

 accumulate in the plant as inorganic sulfate (Rendig, 1956; Walker and 

 Bentley, 1961; Jones, 1962, 1963; O'Connor and Vartha, 1969). Using 

 data from Walker and Bentley (1961), Dijkshcorn and Van Wijk (1967) 

 calculated that a yield response by legumes to applied sulfate could 

 be expected when the sulfate-S content of the dry matter was below 

 0.006 g-atoms per kg (0.0192% S as sulfate). A slow rate of redistri- 

 bution and the localization of metabolic consumption require that this 

 small amount of sulfate must be present within the plant and available 

 for metabolism. This sulfate concentration was proposed as a guide 

 for diagnosing S deficiency since the sulfate concentration in a plant 



