I have Miracle grow organic in 3 tanks.. all capped with black sand blast sand.20/40 grit size . Plants grow great,, need at least 1 " of cap or it will leach. I found it best to screen out the big chunks so all you are using is the fines. Also wet it to the point of saturation before adding the cap, press down lightly to remove all air pockets.
It works just as well if you don't screen it, but it is messier when you pull a plant out,, lots of floaters and pieces all over the tank.. I often just cut the roots around the plant to reduce the mess..
Make sure you know the correct placement of your plants.. My first tank, almost 3 yrs ago is a jungle,, nothing is where it should be, and I would have to do a complete tear down to scape it correctly.. Plants grow great, fish love the arrangement, but it could look better...