Nice to hear from you, barium chloride method one is there using spectrophotometer or titrimethod method based on gravimetric analysis; both the things are in APHA standard methodologies book;
Dr. Kani is right. You can use gravimetric method as given by APHA.
Generally APHA is for water analysis. Form soil you can extract leachate, filter it and then perform analysis. From plant you can crush it (either wet or dry) and then estimate the sulfate.
Sorry Dr. Elsheikh, I do not have any soft copy format for this, this is just my assumption. You can get the procedure for sulfate estimation from internet.
its depend upon the concentration of sulfate , if its high you have to use Barium chloride precipitation method (gravometric method), for the low concentration, the turbidity method used but its not accurate in presence of carbonate