What would be the differences between potassium sulfate in its granular form versus the water soluble almost-powder form. As it's the same chemical formula, I'm mostly interested in the release time, or delayed release...
Water soluble potassium sulfate is absorbed very quickly by plants (I've noticed effects the same day; it definitely has an effect on tomatoes and peppers within one to three days), and can also be used as a foliar spray. I've used it a lot (the AlphaChemicals brand from Amazon and eBay--you'd probably be better off buying it from AlphaChemicals themselves, though). The water soluble kind is used in hydroponic gardening, too.
Granular potassium sulfate is more likely to be naturally mined, and organic approved, as opposed to synthetic. It is used to amend soil (e.g. to correct deficiencies found in a soil test prior to planting anything); so, I assume it is known for having a long-term impact.
In our clay loam soil, water soluble potassium sulfate doesn't entirely leach through the soil quickly; I mean, if I add a fair amount of it (like one or two tablespoonfuls) in the hole when I transplant a cantaloupe from a foam cup, it feeds the plant for months, and impacts the fruit quality a lot (favorably). I don't know that it would stick around in the soil for years (I imagine the granular kind would); it probably depends on your soil and what chemicals it interacts with in your soil. It's pretty hard to burn a plant with potassium sulfate; I've experimented with it a lot (just the water soluble AlphaChemicals kind), and never actually burned a single plant with it.
FYI: If you need both potassium and phosphorus, monopotassium phosphate is a good alternative to potassium sulfate with a similar potassium impact; there are both water soluble and granular forms of it, too, but I think they're all synthetic. I like Greenway Biotech's monopotassium phosphate (from their website).