I use a combination of topsoil, peat moss (moisture retention), and a cow manure compost bagged mix available here in PA at Lowes. It's in a white bag and goes for like $2.60 a 40 pound bag. I've been using this for several years now and have great results. The Night Crawlers love the stuff and so do the plants
Depending on depth of the bed, mine are 10 inch high sides (4'x8' beds), filling the bed can cost some money early on. Spend the money early and just top off a little each year to refresh. I think you will be glad you did. Best of luck
