After my growing season, I'll cover the boxes with straw from this box.
I am starting to mix my soil with a mixture of compost, peat moss, coarse vermiculite, and soil from the garden as per Mel Bartholomew and Square Foot Gardening. (well, he don't say soil, but I'm using it anyway. In the beginning, we were on a tight budget and we were focused on getting the boxes built and filled. Next year should be better.