Idk.but I think tjatha just what u buy from the grow as pH up..I don't think the plants benefits from the hydroxide....I use potassium bicarbonate.? That's just me I have no tech.info to back any of it..lol
It shouldn't matter much as long as you keep it mild. You might have to adjust your nutrients slightly to compensate for the extra Potassium that will be available. Lye is pretty caustic and I would be skittish about using too much of it to change soil Ph. Bicarbonate is less reactive. I use wood ashes myself, which can add micronutrients as well as help to buffer the soil and raising the Ph.
The solution I have is maybe a teaspoon of lye added to half a liter of water. 2 ml of that to 2.5 ltr of water results in about pH of 6.5. So it's not much... might look into using baking soda though, less toxic to my skin if nothing else :)
PH up is most commonly just that - potassium hydroxide(giving potassium) and PH down is either nitric acid(giving nitrogen) or phosphoric acid(giving phosphorus). You can use as much as you need from either of those. They give nutrients the plants need and use.