Watering Plants With PH

This should have been a no-brainer, a duh-moment, a slap-the-forehead thing.
Apparently, I've been suffering from a "Wake up stipid! You're killing your plants!" brain freeze for the last few....forevers.

Do I know that most plants prefer to grow in a certain PH? Ummm, YES. (Is this a trick question?)

Do I know the PH of my rainwater? Well, surprisingly, YES. (I have koi for pete sake!)

Do I know the PH of my groundwater? Ermmm, YUP. (See above mention of koi, but is it a winter or summer measurement you want? Waaaay different!)

Do I know the PH of my yard and garden soil? Well, DUH! (Of course, this is Central New York, so step over 2 feet and it'll change).

Do I know that the PH of the water that I'm watering my plants with (from rainwater, tap or hose) will affect the PH of the soil the plants are growing in? <----That, right there, was my "Wake up stupid!" thought a few weeks ago.

How could I have been so oblivious for so long? I have been gardening for 30-some years. 30-something freaking years! Has this thought ever appeared in my brain? Uh, obviously not. Why not? I dunno!

Our bedrock is composed of -
sediment stone
clay
gypsum
limestone
pumice
travertine
alabaster
granite
slate
sandstone
marble
and even diamonds

Whew!
Anyway, depending on the groundwater pressure and raising or lowering of it (especially in spring), the PH of our well water can change drastically - and fast.
It can sometimes be as high as 9 (though it occasionally tests off the charts) and drop to a 4.5 within the span of a few days.
Top this off with a rain water PH of sometimes 8 (yes 8) and this doesn't make for a good plant watering combination.

This high of a rain water PH makes absolutely no sense to us as we're suppose to be in one of the areas most directly affected by acid rain. Nope, we don't understand it, but that's what we've measured it at on some days.

Am I going to test and adjust the PH every time I water my plants? No freaking way! But does knowing that this is going on make me more adept at dealing with watering them? You bet.

Case in point:

My poor Rhododendron and Azalea seedlings were unhappy. But why, I wondered.
They have excellent soil with adjusted PH at transplant, Hollytone, they're kept moist but not soggy, perfect light amount and I tell them how cute they are (yeah, in baby-toned babble) every day. OK, sometimes a few times a day. And they just weren't those happy, strong seedlings they should be.

So, what the heck is wrong?

Duh-moment: It's the water, stupid!

I tested: OMG! I've been watering them with groundwater at a PH of 9! So, what's happening to their soil? Duh! (yes, I actually slapped my forehead.) It's changing the soil PH.

Have I read to water them with distilled water only? Oh, yes. Many, many times.
Did I want to spend the money on it when I have water flowing from the tap for free? Heh- not this cheapskate.

I gave in. I bought the damn water, flushed their soil, and waited.

What happened?

Well, in less than a month, they've put on weight, gotten taller, stockier, healthier, colored up beautifully and put out many new leaves.

I wish kids were that easy to adjust!

The moral of me almost killing my helpless plants?:

Don't just check your soil PH - pay attention to your water, too!