56 

 result of mineralization of organic matter; this change could explain 

 some of the variable correlations obtained with chemical extractants. 



Robertson and Yuan (1973) studied S availability on two Florida 



35 

 soils using the "relative specific activity" (RSA) ratios of S in 



plant tissue at two rates of S fertilization. Their calculations were 

 similar to S "A" values. Increased S availability would be indicated 

 by a narrower ratio of RSA at two rates of S. They found a ratio of 

 2.6 in soybean tissue for an Orangeburg fine sandy loam (Typic 

 Paleudult) and a ratio of 2.9 for a Lakeland fine sand (Typic 

 Quartzipsamment) . A significant yield increase with soybeans was ob- 

 served in the greenhouse on the Lakeland soil when 45 ppm S was 

 applied. No response was observed at the low S rate (15 ppm S) . 

 Plants did not respond to applied S on the Orangeburg soil. 



Beaton et al. (1968) list several other "biological" methods 

 which have been used to evaluate available soil S. Since all of these 

 involve either a bioassay or incubation procedure, they are time- 

 consuming and are not conveniently adapted to most soil chemistry 

 laboratories. These techniques are listed below: 



"a" value obtained by extrapolation of yield of nutrient 



curves, and is closely related to "A" values. 



algae growth of algae indicates soil S status. 



Aspergillus growth of Aspergillus niger indicates soil S 



status . 



incubation soil incubated to measure biological conver- 

 sion of organic S to inorganic sulfate. 



Neubauer seedlings of spring barley used to extract 



soil S. 



