Acidic Worm Bin
This topic is a particular pet peeve of mine. It seems no matter how much I debunk the myth of acidic worm bins this keeps resurfacing. I'm here to tell you now the PH being far off neutral is a load from the southern end of a north facing bull. It can only happen through a lack of knowledge or through carelessness.
PH is a measure of hydrogen (H+) ions vs hydroxide (OH-) ions, an abundance of H+ ions creates a low PH (high acidity) and an excess of OH- ions creates a high PH (highly alkalinity)
Without delving into an advanced chemistry conversation, decomposition of organic matter CAN cause weak acidity due to the respiratory processes of microbes. Carbon dioxide (CO2) released mixes with water (H2O) to form carbonic acid (H2CO3). Other processes going on in a worm bin tend to balance themselves as far as PH (nitrification and denitrification as examples) if however things do go awry and conditions swing toward acidity the simple addition of calcium carbonate (CaCO3) can keep PH fairly close to neutral and avoid sudden swings in PH.
When calcium carbonate reacts with carbonic acid (water must be present, but a worm bin is wet) it reacts to produce water plus calcium bicarbonate (this is what is generally called "hard water") and the PH is fairly neutral.
Where do I get calcium carbonate?
Calcium carbonate is a main component of eggshells, periodically adding some ground eggshell to your bin will stop acidity in its tracks. If you're allergic to eggs or are vegan or have some other reason to not use eggshells garden lime is a suitable source of CaCO3.
What about alkalinity?
I have never yet seen a legitimately alkaline worm bin unless someone added a strong alkaline material like wood ash or builders lime. There are 2 cures for alkaline soil, one is a lot of organic matter and moisture to create carbonic acid through microbial respiration (sounds like a worm bin) the other is elemental sulphur.