PMDD's again

I finally abtained my Potassium Nitrate and my Potassium 
Sulphate from Eco Enterprises yerterday and am waiting on 
the Microplex.  I'm going to make up a mixture of PMDD's  
to use for now with a liquid iron mix I have.  The question
I have is this.  The potassium Nitrate was in crystal form
and mixed well with the distilled water.  However the potassium
sulphate was a granular form which looked almost like small
gravel about 1/8 in in size.  When I tried to mix it with 
the water it did not want to dissolve.  I stirred it and 
shook it up repeatedly, then I left it set overnight and it
still has dissolved very little.  Is this the right stuff
and if so How do you get it to dissolve?

Thanks again to all you people that have helped me and especially
to Doug who has answered me everytime I have posted a

Paul in Ohio