I use about a 1.5" soil layer and cover it with 1-2" of sand or gravel,
I add about 15% compost to my soil and about .5-1% dolomite. I add the dolomite because I had a nutrient deficiency in some Ludwigia arcuata and I injected some Dolomite into the soil and it cleared up.
My soil has a calcium or magnesium deficiency. if you try soil the soil you use may not need the additives.
Brian