The potting mix used in Dooley's pot may have been originally neutral or alkaline sot eh roots may be in a location where the soil remains little affected by the soil acidifier. Continue following the instructions on your acidifier label and apply as often as recommended. Do not exceed the amounts listed by much as excess sulphur can burn the plant roots. Just keep at it and regularly check the soil pH level (monthly is fine) to notice how the soil acidity is changing.

Be aware that amending the soil will be something you need to do forever because garden soil has a tendency to revert back to its original soil pH if you stop amending it. I would recommend that you also try watering sometimes with rainwater because the tap water in some cities is alkaline.

Of the various amendments out there, iron-chelated liquid compounds acidify the soil faster and are useful when your plant shows signs of iron chlorosis but, for use on a long term basis, I would probably try non-liquids like aluminum sulfate (keep it away from azaleas and rhodies), iron sulfate, garden Sulphur or green sand. Those do not have to be re-applied as often as liquids.

Aluminum is what turns the blooms blue and hydrangeas have an easier time processing aluminum when the soil is acidic. If a soil test indicates that your soil has aluminum, you may not need to add more but would benefit from acidifying the soil so the plants can absorb the one that your soil already has.

I should also point out that controlling the soil pH levels is easier when the plant is in a pot. When the shrub is planted in the ground, it is more difficult.

Luis

Thank you Luis!

My bag says Sulfur 30% - 18% free sulfur, 12% combined sulfur. 12 lbs per 100 sq. ft., increase rate to 15 lbs in heavy clay soils, which I think I have (hard as nails right now, came up in huge solid clumps when wet). 1 tbs per 4" of pot diameter.

So I should hunt down some aluminum sulfate granules? Dooley is already in the ground, so I am going to deal with it that way. We ARE on municipal water, but we're looking at reclaiming the roof water via tanks, hopefully in the fall before the rain hits, and that is destined for the garden in it's entirety. We're not in town, we're out in the redwood trees, in the boondocks, but I'm sure our water is on a par with urban water. Don't think our water comes from the river, but I could be wrong in that.

If your soil already contained naturally occuring aluminum, the contents of your bag would be enough but since you do not know, try to find some aluminum sulfate. A soil test is the only way to know if your soil currently has aluminum.

I obtain some rainwater from wine barrels. Had to replace two this year due to leaks though. The new ones are made of hard plastic but allow me to store more water than the rain barrels did. Hope your roof/water project works well for you!

Found the aluminum sulfate granules at little nursery down the road. Didn't have the money on me at that moment, but at least now I know where to go, since my local HD does not carry it. Next newbie question: Do I spread it around and then scratch it into the surface and then water, or should I poke deep/skinny holes in the dirt and fill them (I could use a long 1/2" thick knitting needle)?

Oh, and on my way down the road today, somebody has a BLUE hydrangea with several dozen brilliant blue flower heads, deep in the shade under a tree. Likely to be a redwood tree, given where we live, but I'll have to look again (cautiously) as I go by on the highway and check. Sigh. I have SUCH blue lust.....