Salix are usually planted straight into the ground, unless its, say, a half or quarter standard Kilmarnock Willow or Flamingo willow. For potting, John Innes No. 3 or a mix of that and a good multi purpose compost.

Hi, welcome to GoY, as Bamboo says best planted directly in the ground, they're often planted in positions which get flooded regularly, because they need a lot of water and help to dry out the area a little, Derek.