There are several possible options. You can fill the boxes with 100% bagged “planting mix.” This medium is available at most garden stores and is composed of composted sawdust and cow manure or “forest products” and “biosolids,” etc. One hundred percent compost is another option. This product is available by the cubic yard from Health Sanitation Services, Sticks and Stones, and other composting facilities. Remember that the above media are nearly 100% organic matter, which means they will decompose over time. Therefore, you will have to replenish (top off) your boxes with a fair amount of new material each year. I prefer to use a mixture of 2/3 to 3/4 loam or sandy soil and mix that with 1/3 to 1/4 of the above planting mixes or compost. This combination gives good drainage, while also providing a fairly stable growing medium, which only needs a small amount of augmentation each year to keep the boxes full.