How do you plan on starting your plants in the ground? By seeds or seedlings? If you are going to do seeds I would recommend putting a inch or two of topsoil or compost where you want to grow. Then plant in to that. If you are going the seedling route dig a small hole about twice the size of the seedlings roots. Put topsoil or compost in the hole with the seedling and you should be good to go. I have even planted potatoes straight in to material like you are describing in your post. Good luck.