"symplastless" wrote:

The element sulfur would lower pH. However it keeps reverting back.

Actually elemental sulfur is very slow acting and lasts quite a while,
but should be built up slightly every couple years in smaller doses.
Its advantage is that it can provide a very strong effect if needed by
applying more.

You could add pine needles.

Pine needles and oak leaves work, but they rot and need to be
replenished annually. They are rather weak and can't be used to make
too large a change.
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