I have some landscape posts in my garage and I am about to make a planter box from there. It really is nothing more than to hide the empty space between the ground and a small shed I have in the backyard. I think I will end up making it 8-10" tall, 3' and about 12' long. Under the boxes in clay where currently there are two Brown's Yews growing. I haven't decided what exactly I want to put in there but butterfly bushes are a possibility as well as lilac bushes.

In any case, I was wondering about the soil requirements. I had thought of buying some soil from a local place that sells a mixture of topsoil, compost and shredded bark but I was wondering if I could dig the soil myself from the woods.

We have a heavy red clay here but in the woods is at least 6 inches of dark and rich looking soil. Obviously from natures compost pile. Would that topsoil dug from the woods meet most of the nutritional requirements for plants like butterfly bushes or lilacs? What I need to add anything to it?

Azaleas were also a candidate at one point but I have backed off of that a bit since my planter boxes were going to need to be around 18' deep and their soil requirements are the opposite of what our limestone filled clay would provide.

Thanks in advance for the help