Please check these links.

http://link.springer.com/article/10.1007/BF00010076#page-1

http://www.scopenvironment.org/downloadpubs/scope48/appendix.html

Also please consult the following literature:

Soil Sampling and Methods of Analysis, Second Edition

Edited by M. R. Carter & E. G. Gregorich

Best of Luck

 Soils samples collected from the major arable areas in Britain were used to assess the availability of soil sulphur (S) to spring wheat in a pot experiment. Soils were extracted with various reagents and the extractable inorganic SO4-S and total soluble S(SO4-S plus a fraction of organic S) were determined using ion chromatography (IC) or inductively-coupled plasma atomic emission spectrometry (ICP-AES), respectively.

http://link.springer.com/article/10.1007/BF00010076