55 

 organic fractions. The Johnson and Nishita (1952) reduction technique 

 and S determination as methylene blue must be used with alkaline 

 extracts. Color due to organic matter and cations does not interfere 

 with this method as it may with BaSCy precipitation in the turbidimetric 

 determination of sulfate (Beaton et al. , 1968). 

 2.4.2 Biological Techniques 



Several biological methods have been used for measuring available 



35 

 S in soils. These include "A" and "L" values where radioactive S is 



used to determine availability of soil sulfate (Nearpass et al., 1961; 

 Bettany et al., 1974). "A" values were developed for measuring avail- 

 able macronutrients , particularly P (Fried and Dean, 1952), but have 



35 

 been adapted for S measurements. A known amount of S is applied to a 



35 

 soil and the subsequent proportion of S in the plant allows calcula- 

 tion of an "A" (availability) value. Nearpass et al. (1961) found "A" 

 values from 9.8 to 42 ppm in 30 soils from the southeastern United 

 States by growing cotton plants at six levels of applied S. These 

 values were significantly correlated with S concentration in the 

 plants (r = 0.91) and S uptake (r = 0.96). 



Bettany et al. (1974) used a modification of the "A" value which 

 they called the "L" (labile) value (Fried, 1964; Larsen, 1967). The 

 following equation was used to calculate the availability index: 



35 35 32 3? 



Sadded to soil ( Splant - ~Sseed) - "Sadded to soil 



35 



Splant 



They found that "L" values increased with subsequent harvests of 

 alfalfa and concluded that this increase in the availability index was 

 a direct result of isotopic dilution of the added S due to mineraliza- 

 tion of native soil S. Labile S apparently changes with time as a 



