Is there a reason to use pH 5.5 water to soak Grodan rockwool cubes before first use?
Can I use pH 7.0 water instead? I have a lot more of that on hand since it comes out of the wall for free.
- GeniusLv 41 decade agoFavorite Answer
Rockwool contains lime from the manufacturing process. For hydroponics, this should be washed out or you'll get a pH spike. pH 5.5 is acidic enough to dissolve the lime but not enough to dissolve the wool. Water at pH 7.0 will not dissolve the lime.
I doubt that water coming out of the wall for free is at pH 7.0. Perfectly mineral free water will typically have a pH of around 5.5 because of dissolved carbon dioxide. So you can use distilled or de-ionised water for your washing. You can shake it first in case it was sealed in the bottle and didn't get chance to absorb carbon dioxide.
Tapwater will either contain dissolved salts making it alkaline, usually referred to as hard water, or it will have been made slightly alkaline by the water company to prevent pipe corrosion. Neither is a good choice for hydroponics although you can usually treat it to get the results you want. Softened water is also not a good idea since it will contain sodium salts instead of calcium and magnesium salts. Reverse osmosis softened water is good if you can get enough of it. Mostly though, if you want to do hydroponics, you need to learn what is in your water and then see if there's a way to adjust to so it has what you need in it.