What should I mix up for growing trees in containers? (this is primarily for me indoor meyer lemon tree, but I also have some seedlings that I want to pot up for a while prior to moving them to their new home)

The vermiculite I have is pretty fine. I make most of my garden / pot mixes with some combination of vermiculite, Peat Moss and Compost.

I am afraid that I won't have good enough drainage (for a potted tree) with just a combination of fine vermiculite and compost.

What else can I add to the mix to improve drainage?

Thanks

Jamie