around plants would be expected to increase SO absorption. Hill 

 (1971) found that absorption of SO by vegetation increased with wind 

 velocity above the plants, height of the canopy, and temperature. 

 2.3 EVALUATING THE SULFUR REQUIREMENT OF CROPS 



2.3.1 Critical Concentrations 



The S requirement of a plant has been defined as the ". . . mini- 

 mum uptake of this nutrient associated with maximum yield of dry matter" 

 (Stanford and Jordan, 1966, p. 258). The concentration of the nutrient 

 present in a plant when this condition is met is usually referred to as 

 the critical concentration or the critical percentage (Ulrich, 1952; 

 Thompson et al., 1970). As with any nutrient, the critical concentra- 

 tion of S in plant tissue can vary with species, the type of tissue 

 (leaves, stems, roots, grain, etc.), and the age of the tissue. Some 

 values for critical S concentrations in different crops are reported 

 in Table 6. 



Since the protein contents of plants vary widely with species, 

 age, environment, and nutrition, the wide variability in S content of 

 plants is not surprising; the S requirement will depend on the amount 

 of S associated with plant protein. The protein content of the vege- 

 tative part of a plant decreases with age as a result of growth dilu- 

 tion of the protein by carbohydrates, hemicellulose, lignin, etc. 



2.3.2 N:S Ratios 



The relationship of S to plant protein and the corresponding 

 relationship of N to protein has led to the realization that the ratio 

 of N to S in the plant may be a more reliable measure of the S require- 

 ment than is the absolute level of S. Genetics determine the sequence 

 and number of amino acids in the polypeptide chains of a specific pro- 

 tein, but nutritional and environmental factors may influence the 



