[dice]

Compost tends toward neutral pH. Regardless of whether the inputs
that you are composting start off acidic or alkaline, if you compost
them long enough the result is neutral pH compost. (Still good plant
food and growing medium, of course, and food for all kinds of
bacteria and fungi in the soil.)

One way to lower the pH of container mix is to add some peat
moss. Peat moss is harder to get to soak up water after it has
completely dried out than compost, so you do not want to over
do it. Once it has soaked up water, though, it holds water well.

(Peat moss is not really a sustainable resource, in that it grows
so slowly that it takes forever to grow back once it is harvested.
Coir is a good substitute in terms of water holding capacity and
texture, but it does not have the same effect on pH that peat
moss has.)

Another thing that will lower pH is cottonseed meal (a plant food),
although one probably does not use enough of it at one time
to make any drastic change in pH. I have heard that oak leaves
will reduce pH as they decay, but I do not know if that is true
or not. (I did not hear it from a scientist that was testing pH
as it decayed.)

If one is buying fertilizers like Tomato Tone, Plant Tone, etc,
one can mix in half Holly Tone. Holly trees like acid soils, and
Holly Tone fertilizer is probably formulated with that in mind.