Lime is cheap. It is also used for other things, including as an animal feed supplement. I buy mine in 40 lb bags and if I remember correctly the cost last time was around $20. All you'll need is a smaller bag which should cost about $5.
Ions are just charged particles; cations have a net positive charge, anions have a net negative charge. Opposites attract, so materials with a cation exchange capacity will hold onto cations, tightly enough that they wont leach from the soil, but they remain plant available. The hydrogen ion is a cation (H+), and also responsible for acidity.
You can get Dolomite lime at any home depot or gardening store.
I am a moron when it comes to science. The shit just makes no sense to me. Calculus, statistics - no problem. Start talking ions and I am lost. So while interesting, no comprendo.
I make my own mix. Cheap potting soil (only nutes are from the compost, no time released stuff), Peat Moss, Plain Perlite, Plain Vermiculite, and some dolomite lime. I use Dyna Gro for my nutes.
As stated, I can not argue the science. My water is tap thru a Pur filter and then sits out. So speaking purely from my experience, which is about 8 grows so not much, I have found that when I get lazy with PHing, the plants suffer. When I am religious about it, they don't. Maybe coincidental. Maybe circumstantial. But that has been my experience.
Where I am in the growth cycle and thus the nutes being administered makes a large impact on the PH. When feeding only water, I generally need to add about .5ml of PH down. In early veg with minimal nutes, that is about .3ml. In full flower, it is generally at 6.3 on its own.
I use tap thru a PUR filter and it is about 7.8. Really, I think it depends on your water supply, not the filter.
You should try not pHing anything from the start. Try it with one plant. You mentioned using pH down when giving just water... do you also use pH Up when adding larger doses of nutrients?
This is the chasing the tail scenario I was referring to. It sounds like you have tap water with a decent mineral content, considering it is that alkaline. Liquid Up and Down aren't long term solutions like dolomitic limestone is (dolomitic is preferred for the Mg as well as Ca). With peat moss, you may need to re-lime at some point in the future (every few weeks or month depending)... typically I always just make sure I add enough during each transplant and especially the pre-bloom transplant.
Now, if you're using straight sphagnum peat from a bale, it needs even more lime. Again, peat is quite acidic (pH 4). Potting mixes typically come pre-limed to some extent but straight sphagnum bales most likely do not. So, I would recommend the 2 tbsp (heaping) per gallon in this case. Test before using and make damn sure the pH is near 7. I'd also recommend replacing at least 40% or your sphagnum with coir.
Otherwise, generally materials lower in CEC need less lime applied more frequently. Materials with greater CEC need more lime applied less frequently. Also, even if pH does drop after applying nutrients, it should rebound.
