Gardenia with leaves turning yellow

We have three gardenias in front of the house which get direct sunlight
until 2-3ish.  Then they are in the shade the rest of the day.  We
recently
replaced one because its leaves turned yellow and fell off.  This was two
weeks ago.  Now the leaves on the new one have started to turn yellow
around
the bottom (all the way from the bottom to probably 1/4 the way up).  The
other two gardenias have always been fine.  All three recently just
bloomed
too.  I was suprised the new one bloomed but it did.  But now we are
worried
that this new one will loose its leaves eventually too.  I read on the
internet last night to add some epsom salt, so I did that.  It is to
combat
magnesium deficiency if I remember right.  I'm thinking that particular
gardenia is getting too much water.  It is on an automatic dripline and it
goes every other day for about 17 minutes.  That particular gardenia is on
the end of the dripline but the dripline wraps around the bush to tie off
the line.  I turned the dripline off yesterday to see if that will help
any.
Anyone else have any ideas on why this one gardenia could be turning
yellow?
Poor drainage maybe?  I understand some leaves will turn yellow and just
fall off, part of being a plant but the whole bottom prortion should not
be
turning yellow :(

Thank you!