Even though on the whole, the area may be acidic, the ph depends a lot on what kind of rock your soil was originally made from and what kind of rock you may or may not have in your yard.
I don't bother with the soil testers much anymore, I kind of learned to figure out where I need what and when. Maybe it's a thousand or two years worth of farming in my blood, don't know, I was born a city child.
Anyway, it's possible that you're not using enough of the aluminum sulfate. I looked it up, and the recommended amount for 10 sq.feet in your Ph range to bring it down for 5 to 5.5 for blueberries is 1/2 lb. Maybe 3/4 lb if you have clay soil or down to a 1/4 lb if you have sandy soil.
I have a great clue to my Ph in some areas of my yard,.... wild blueberries growing like weeds.
I use Miracle Grow soil acidifier plant food for my azaleas and one of my rhododendrons in particular. The azaleas edge my lawn and DH is always dumping more lime in the grass, and the rhodie is in a narrow planting area with crushed granite on wone side and pavers in front and concrete steps on the other. The first one we had just couldn't take all that and died on me.

We've never actually tested our soil. The well water tests at 6.5 and the wildflowers growing in the lawn suggest that the ground is just slightly acidic. I think anyone in the foothills with a sandy soil base are going to come out pretty close to that.

Veilchen,

Three years ago we tested our garden extensively with one of those little kits where you add a sample of soil to a test tube, add a measured amount of distilled water, and shake it up with a test strip which turns some color and then you match that color to a master color chart. We got readings ranging from 6.5 to 7.0 at various points in the garden.

Our soil was sandy and some chlorotic seedlings alarmed me that we might have a serious pH problem. It turned out it was just sandy soil lacking in nutrients. We started a foliar feeding program with Miracle-Gro Tomato Formula and our plants instantly became healthy.

I'm trying to build up the organic matter in our sandy soil to increase its natural fertility. In the meantime, or gardening is rather similar to the Mittleider gardening system, which is rather like in-garden hydroponics, it that it places a lot of reliance on using soluble nutrients.

"I cannot grow a blue hydrangea to save my life, they're always pink, despite aluminum sulfate and sulfur. Does anyone know a good amount of either of these two ingredients to really lower the ph?"

Miracle-Gro makes a version of their soluble plant food for acid-loving plants, including hydrangeas. I personally have misgivings about adding large amounts of aluminum sulfate to garden soil, but maybe that is because aluminum has been implicated in Alzheimer's disease. But I forget why. Someone hand me another pickle.

MM

Hi MM!

I've used the Miracid as well. I think my problem is I just need to add more of whatever I'm using.

I am super fortunate to have excellent soil in my yard. In most places there is 18-24" of dark topsoil. Everywhere I garden I amend this with compost, cow manure, and lots of leaf mulch. Therefore I don't need to add any fertilizer, which is why I probably forget to add the aluminum sulfate, etc. often enough--I'm not used to adding pellets, powders, or liquid stuff you have to mix up.

Veilchen,

It'll be a long time before my garden soil gets to be as good as yours. But I've got three compost piles "in work" now and I am adding to them on a weekly basis. They aren't very active and will soon become popsicles as cold weather sets in.

There are other acid forming things you can add to your soil besides aluminum sulfate. Like ammonium sulfate, or mono-potassium phosphate (or phosphoric acid if you can find it), or even ammonium nitrate (garden go boom!).

I notice that Coca Cola contains phosphoric acid. It's fairly acidic, and plants might like it. Maybe pouring some Coke around your plants would be a good idea. Just so they don't get "hooked" on the caffeine like I am. I noticed while sprinkling some coffee grounds on the soil that earthworms are apparently stimulated by the caffeine. I wonder if they could become addicted. I have no idea how earthworms react to Coke.

MM

The bag is out in the shed (and I'm not going out in the cold right now to look at it!), but I may be mixed up--I can't remember if I have aluminum sulfate or ammonium sulfate.

Coke, hmmm.