I grow these gentianas in raised beds with a layer of coarse sand / gravel on top, 10-12 cm, and underneath the gravel I have 20 - 25 cm of garden compost, made from grass, leaves and other plant material from the garden. I mix the compost with some mineral soil. I never measured so I do not know the PH of this mix. But it seems most plants grow very well in it, not only the Gentianas.
The top layer of sand and gravel is there to give good drainage, but also to prevent weed seeds from germinating. I have big problems especially with Stellaria media. I could weed every day and it would still be there.
