My soil has a low PH from the get go and I add Lime to the mix to bring it back up. I also add Epsom salt and have used oyster shell along with the lime and salt in my mix. The 3 all do about the same thing, well the oyster shell dose not have mag in it but they all provide calcium and release it at different rates. If you have large enough pots, have it mixed in your soil and your not using pure RO water you can usually go a whole grow with out needing it. I only have to use cal/mag because I use bottled water.