I haven't done this for a tank yet, but whenever I'm planting pond plants and want to keep any silt from flying around, I cap the pots with sand and put a layer of newspaper over the drainage holes at the bottom of the pots to keep soil from leaking out and silting up the water.
For the actual soil, I've actually found the clay soil we have around here works great. Very little organic material and it is actually fairly rich in minerals, etc. for the plants.
My process is pretty simple. First I take the pot, line it with the newspaper (just a single layer or 2 of paper) at the bottom and dig some of the clay soil from somewhere in the yard where it hasn't been augmented with anything else. Once I have the soil, I put in about an inch and tamp it down, then drop in about a teaspoon or so of Ozmocote Plus (a time release fertilizer), then some more soil into the center of the pot, then the plant, draping the roots over the pile of soil. Then I add more soil over the roots until it is about an inch form the top, pat the soil down lightly and then cover with sand until the pot is closer to full.
When I'm ready to add the pot to a container, I do it very slowly, often putting it in a shallow tub of water up to just below the lip of the pot to get it fully soaked through first. That way the water works into it without pushing up air bubbles through the clay soil and disrupting the sand cap. You could use pots without drainage holes, but I think the water slowly seeping up through the bottom and through the newspaper is a better way to wet everything out and without holes you'd be more likely to get air bubbles disturbing the soil.
If I'm careful, there is very little if any haziness to the water from fine escaped clay particles. Now, if I didn't do the newspaper at the bottom and the sand cap on top, it will be a mess. The ozmocote works great for about 6-9 months and I grow various fish in the tubs with no ill effects. Since there is no significant flow through the planting media, I expect it is just used up by the plant with very little if any ever getting into the water column. My pond plants, mostly water lillies, are heavy feeders so they need the ozmocote to thrive and bloom well. For simple aquarium plants, they'd probably be fine without it, especially if you are dosing, but a little will certainly help. Ozmocote is cheaper than using plant tabs, especially if you want to use it for your garden and house plants like I do. For smaller pots you'd only need about a half dozen grains of it or so.
I've been planning to do this with some small pots of crypts in cory tanks where I only have a tiny layer of sand but would like more plants, but just haven't gotten around to it.
I have purchased the top soil you show above and would not use it in a tank. A lot of organics and who knows what is in it. Some stuff might float up and out and I'd just be reluctant to use it without knowing what is in it and what it might leach into my water.
I'm not sure if this helps for what you have planned, but either way good luck and let us know how it goes.
