The degree of acidity and alkalinity is measured on a scale of 0-14, with a pH of 7 neutral, 0-7 acidic, and 7-14 alkaline.
You can test your garden soil with cider vinegar. Add a couple of drops to a soil sample- if it Fizzes, the soil is Alkaline.
Reduce Alkalinity
To Reduce Alkalinity in the soil you could add sphagnum peat, elemental sulfur, aluminum sulfate, iron sulfate, acidifying nitrogen, or organic mulches.
Reduce Acidity
To Reduce Acidity in the soil you can add Limestone powder. Both types (calcium carbonate and dolomitic limestone- which will also add magnesium) work equally as well at raising the pH. To reduce the acidity organically, add 1/2″ of wood ash over the soil and then mix it in.
This will require repeated applications over the course of 2 or more years, but it’s a great way to recycle woodstove ashes while benefiting the garden.

additional information is available at the
Clemson Coop Extension.
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