remove munged wrote:

A contribution to the answer to this question was made in Scotland
by a study of what increasing amounts of magnesium do to rhododendron
plants. This species is erroneously believed to require acid soil; it
really requires one of low calcium content.

A lot of research is being done in the UK on this. They have test
gardens for rhododendrons in alkaline soil. They are getting a handle
on it. It requires a lot of chemistry. Sometimes just adjusting pH is
easier.

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