Thanks to all who replied, especially Stephen -- lots of useful info!

" writes in article . com dated 1 Dec 2005 07:48:56 -0800:
The best remedy for a potted plant is to change the potting soil. What
could you have put in the pot to go pH 9. I don't know of any potting
soils that are more than neutral. I f you have an acid loving plant
requiring a pH below 6. there are specific potting mixtures for that
situation.

The plant has been in the same soil for over a year but the problem is more
recent. All I added was water (through a new hose) and fertilizer. But
it's an undrained pot, maybe that has something to do with the pH going
crazy.

Anyway, last night I added half a cup of Epsom salt (Magnesium Sulfate)
mixed with a gallon of water. Later today I will re-check the pH.

I don't want to mess up the root ball by transplanting it in different soil,
but I might rinse it (overwater and then dump out the excess) if the
chemical/bacterial route doesn't work. That should bring the pH close to 7.

-- spud_demon -at- thundermaker.net
The above may not (yet) represent the opinions of my employer.