[mdjlucan]

What is the best compost to put a lavender plant  in a big big pot

[New shoot]

Lavender needs good drainage and so I would put a decent layer of gravel or broken pots in the base.  For the compost I would use John Innes no. 2 just because that is what I always have on hand.  If it is a mature plant, John Innes no.3 might be better.

John Innes is a soil based compost so can be quite heavy and dense, depending on the soil used as the main ingredient.  To lighten it up, I usually mix some multi-purpose compost in.  It is hard to give a specific amount, but I aim for a mix that still feels like it has some body to it, but also quite easy to dig into with your hands.  Somewhere between 25 and 50 per cent of multi-purpose usually does it.