Rhododendron prefer a well drained soil but aren’t too particular as long as the soil doesn’t stay wet for long periods.  If you have clay soil, be sure to aments it with plenty of organic matter like composr and also sand to improve drainage.  They also prefer a slightly acidic soil so clay soil might need a little aluminum sulfate…a pH test will help you determine if this is necessary.  Equally important is the location…Rhodos don’t like a spot that exposed to the drying north and west winds so the east, southeast side of the house will be best.