I used river silt, a high clay silty soil, which I mineralized, without the muriate of potash or dolomite. I did, as I recall, pour some mulm on a very thin layer of ground peat at the bottom. It was the best substrate for growing plants that I have used. And, it was topped with SMS. I mixed SMS with the river silt for a middle layer, to make the transition less sudden, and topped it with over an inch of SMS.