Clematis need lime.

It is an old belief that clematis need plenty of lime, and it was often recommended that mortar rubble should be buried under a clematis when planting one. I have also heard people claim a clematis growing in clay soil was only succeeding because it was against brick wall, no doubt ignoring the Rhododendron growing beside it! The fact is clematis aren’t that fussy regarding soil and will happily grow any reasonable garden soil.