Even if you get some clumps into the water, it'll sit at the bottom, but eventually dissolve. Whenever I make a white mix (KNO3, KH2PO4, GH Booster, and sometimes KSO4), I shake it a whole lot and then put it down. And whenever I pick it up, I shake it a bunch. Eventually, all the solids dissolve. But in the meantime don't worry about adding too much to the tank at once (due to clumps) - in the long run, assuming you do regular water changes, it all evens out over time.