I planted a rhododendron (dwarf rhododendron "baden-baden") in the
alkaline limey soil in my garden

As suggested, a localised area of more acidic soil is probably goinmg to work
better for you, though I have seen pictures of rhodos in the wild growing on
limestone outcrops, so the rules aren't hard and fast.
You may be able to get away with it the way it is, or perhaps with a periodic
Miracid treatment.