50 

 yield; this index was unaffected by plant age, N level, or plant part. 

 Their data indicated that wheat plants with more than 10% of their S 

 in sulfate form were adequately supplied with S. 



Recently, Spencer and Freney (1980) reported a critical value for 

 sulfate S (as a percent of total S) of 13% for field-grown wheat. This 

 value was least affected by the age of the plant or N supply and was 

 recommended as a convenient index of the S fertility status of wheat. 

 2.4 EVALUATING THE SULFUR FERTILITY STATUS OF SOILS 

 2.4.1 Extraction Techniques 



Soil testing for S to evaluate S available to growing crops is not 

 as developed nor as precise as it is for P, K, Ca, Mg , and even some of 

 the micronutrients. The complicated and relatively unknown nature of 

 soil S and the multiplicity of sources of S to plants makes the rapid 

 assessment of plant-available S even more difficult. We know that soil- 

 solution sulfate and adsorbed sulfate are readily available sources of 

 S. Other sources include soil organic matter, precipitation and irri- 

 gation water, atmospheric S, and fertilizers and pesticides. Therefore, 

 crop response to S is frequently as dependent upon management practices 

 and location as it is upon available soil 5. 



Excellent reviews of available techniques to assess the S fer- 

 tility status of soils have been written by Reisenauer et al. (1973) 

 and Beaton et al. (1968) . Few new developments have been made in 

 recent years to improve analytical techniques for S determination or 

 the soil test calibration necessary to apply these techniques to crop 

 response in the field. Some soil S extractants that have been used are 

 listed in Table 7. Reisenauer et al. (1973) pointed out that no one 



