It really isn't K2O. The potassium content is expressed as if it were K2O. Don't ask me why. It sounds deliberately obscure to me, but the practice is rooted in ancient tradition going all the way back to alchemy, probably. Murate of potash is really potassium chloride, KCl.
To add SO4, the best chemical to get is magnesium sulfate, also known as Epsom Salt. It is cheap, and supplies both magnesium and sulfur, both essential macronutrients.
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