Calluna vulgaris

I'm sorry, Camilla1, I am not a scientist, I just 'know' that it needs acidic soil, just as I 'know' that I can't grow Rhododendrons, Azaleas, Pieris etc. Maybe there's another member who will be able to give you the scientific answer you want. Nature is a puzzle, isn't it. I can only say that the origin of plants - where they grow in the wild - must determine the conditions they need in cultivation in our gardens.

Generally speaking, if an 'acid lover' is located in alkaline or even neutral soil it will gradually become starved of important nutrients which it is unable to access, even though they are there for alkaline-loving plants.

I don't know for sure, either. I always associate calluna with peaty soil, so therefore, acidic, or exposed mountainsides, where the soil gets a lot of rainwater (acidic again). This latter is what I think of with regard to rhododendrons and other Himalayan plants. It is always better to water such plants with collected rainwater, rather than tap water, if you can.