Looking for advise on engineered container soil
I have been lurking on this forum off an on for quite some time but this is my first post so take it easy on my. I am trying to come up with an engineered soil blend to be utilized on a large scale vegetated retaining wall project. These vegetated retaining walls are made from concrete and have an internal volume of about 1 cubic foot. The modules have drip irrigation incorporated in them and will be dosed with collected storm water run off.
I have read many of "Al's" posts on his soil mixes and have incorporated lots of his "philosophy" in what I am trying to accomplish.
Now I am sure some of you are probably thinking just mix up the soil to one of Al's recipes and call it a day, however there are a few requirements which i have which alter some of the ingredients.
I am trying to come up with a mix that is primarily composed of "waste stream" ingredients. I have access to lots and lots of reclaimed crushed red brick which i would like to use as the "turface" portion of the "gritty" mix. Red brick has a very favorable CEC and a slightly basic pH. I also have access to some pine bark fines, and different post consumer composts.
My questions start with the utilization of crushed granite. I understand that it has nice fractured edges to promote good drainage and slowly releases trace minerals over time, but what else is it doing? Why use it instead of just more turface (or in my position more crushed brick)? I am trying to avoid materials which have been mined or that have lots of embodied energy such as heat expanded/fractured materials such as turface and stalite unless they are waste stream.
The other thing about this project is the overall scale. We are talking about many many many of cubic yards (gulp) so the ingredients have to be as economical as possible. (I will be mixing up smaller test batches to get my mix right).
Plant selection is still a bit in the air but will be mostly hardy perennial flowers and grasses.
I look forward to my future participation in this great forum!