When setting up a dirted tank it's vital to add a cap layer, I'll explain...
I normally go the fluorite route when it comes to planted tanks, however in the last 3 years I've gotten more in depth with going the mineralied topsoil route. Mineralized Topsoil is where I'll go with multiple soaking then drying of the topsoil, normally 4-5 times I soak in water for 24-48 hours, then dump the water and then layout the topsoil on a tarp for 3-4 days to completely dry, this method brings out the nutrients to be immediately available to the plants.
So I normally go with 2"-3" of mineralized topsoil and cap it with 1/4" of play sand or gravel depending on the look I'm going for with the tank. The cap stops any dirt from rising in the water, this way you don't end up with a tank of muddy water.
Here's my 30 gallon long tank 30"x12"x16", it's setup using mineralized topsoil with flourite as a cap.