suspicious minds wrote:
Many leaves, stems, barks, flowers etc contain tannins (tannic acid) which
are leached into the soil therefore lowering its pH.
During the decomposition process the acids are either leached away,
neutralised or broken down.

So, my holly leaves would leached away considerably before being broken
down and therefore it could poison/damage plants around it. Yes? Then
again the holly leaves takes ages and ages to decompose ... Looks like
I also might end up with a couple of tons of the stuff. Holly leaves
anyone?!

Very simply
Woody plants are low in nitrogen when the plants are being broken down the
bacteria need a source of nitrogen and can deplete the soil around them.
When the bacteria die the nitrogen is fixed in soil again and is usable
again.
Also many nitrogen fixing bacteria cannot live in acidic conditions
therefore unusable nitrogen cannot be fixed.

Would you perhaps know how long it takes for the soil to be usable
again?