What is the best, most cost effective soil acidifier for acid loving plants.

I use holly tone for my hydrangeas, but I know it’s not the cheapest.

Elemental sulfur - not to be confused with “sulfate”. Slow reacting, but acidifies soil over a long time period.

Aluminum sulfate (alum) can be used for a quick-rapid drop in soil or water pH but doesn’t have a long-term residual effect. It’s the hydrolysis of aluminum that results in the acidity (pH decrease), not the sulfate.

Sulfur

quote:

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coffee grounds


Fresh are acidic, used are pretty neutral I think due to brewing process

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Correct. Used coffee grounds are not doing anything to change soil pH.

I use acidified cotton burr compost

Using unused, 33% sulfuric acid (battery acid) to lower blueberry soil pH to 4.5–5.5 is technically possible and used in commercial applications, but it is dangerous and risky. While it quickly acidifies, improper use can easily burn plant roots, kill soil microbes, or cause chemical burns