I use potting soil as a substrate for my planted tanks. I put a layer of rinsed gravel over the top to keep the potting soil in place. If you are willing to just let things settle for quite a long time, you could even end up with mud that would not be easily disturbed without the gravel cover if you started with real garden soil instead of potting soil. I am just not that patient. When I set up a tank I like it to start to look nice in just a few weeks.