Basically, Barium Chloride reacts with Sulfate to form Barium Sulfate, which comes in the form of a precepitate.
From there, Mass of Sulfate/ Mass of Barium Sulfate * Amount of Barium Sulfate = Mass of Sulfate. From there, you can find the percentage of Sulfate in the Fertiliser.
I just did an assignment on this and mainly used Shah's and other reports.
Here's how I set it out.
Consider how much sulphate SO4 is in Barium Sulfate, so mm(SO4)/mm(BaSO4) = ??%
Therefore the mass of SO4 is found through ??% x Mass (Barium Sulfate)
Then mass of SO4 / mass of fertiliser x 100 = percentage of sulphate in fertiliser.