Methods to increase the speed of dissolving potassium sulfate in water apart from heating, grinding or shaking I bought potassium sulfate to be used as a fertilizer. The fertilizer comes in the form of small pebbles 1-2mm in diameter. Wikipedia states that the solubility is around 111g/liter at 20°C. 

I tested several concentrations

100g / liter

50g  / liter

25g  / liter

at room temperature (22° C). After 2 hours there are still solids remaining in the solution. The water here is hard (EC= 0.6ms / cm) which hinders the dissolution. What can I do to improve the solubility of this fertilizer in water apart from heating, using reverse osmosis, grinding the pebbles or vigorous shaking of the solution (which worked) but is very tedious.
Since you're really asking about speeding dissolution, not increasing solubility, use a mixer; either:


From above, e.g., a drill attachment for mixing paint, or
From below, e.g., a magnetic mixer/stirrer.


The links are just as examples; you could devise your own mixer.