I have a "dirted" 220 gallon tank. I didn't use Miracle Grow Organic simply because there is a brand of Organic soil I have used for years in my garden and in potted plants in the ponds. I comes in 2 or 3cu. feet bags and I get it at Lowes. What I did is put 1-1/2" inches of organic soil, then capped it with !-1/2 inches of eco-complete. I chose to use the Eco-complete because I like the look of it and it is also an enriched substrate.
After putting in the substrate I had to fill and empty the tank for 7 days for remove alot of the tannins which leech from the soil. Oh, I did not remove any large pieces of anything out of the soil mix. I also ran my Fluval FX5 during this time with Seachem Purgen in it to also help remove tannins. On day 8 I planted, heavily. I still kept Purgen running in the filter for about 3 or 4 weeks, and had very very little tannin color in the water.
I will tell you with a rich soil base you need to plant heavily from the get go. And put in a couple stem plants and maybe Water Sprite to help mop up excess nutrients. I also took a few small water hyacinths from our pond to also help with nutrient removal. You will get diatom algae fairly quickly, just so you know. But my tank cycled in 13 days... honest! I think partically because the eco-complete comes with bacteria in the water it is packed in. Once I put the plants in I really could not believe how quickly it cycled. I tested over and over for a couple days because I could not believe it. So I actually got a BNP and put him in. Also had MTS's and got some Pond Snails in there by accident that came off the water hyacinth out of the pond. Which turned out okay as this multitude of snails with the Pleco really started hitting the diatoms hard and quick. Do watch how long you run your lights so your algae problems don't go crazy in the beginning with all the excess nutrients. Some people use duckweed but that stuff is messy and a bear to totally get out of the tank. After a couple weeks of letting the algae crew do their thing, I added in my three yo-yo loaches to get to work on the pond snail population. The loaches are unable to eat the MT Snails due to their shell configuration.
Anyway, months down the road my plants have grown and filled out amazingly. The two swords I put in grew to the top of the tank, which is 30 inches within 2 weeks. The water sprite started out with 5- 5" tall stems and in a couple weeks was to the tank top and had filled in so much I had to start trimming. Crypts nor any plant ever melted. I am still amazed how big some of the crypts have grown. So the jist of it is... using a dirted tank can be amazing. And as for the loaches digging, I have never had them dig down to the dirt. And two of my guys are about 4" now. They do root, I've seen them root and dig until they actually get under the dwarf baby tears. But also, the organic soil is heavy and I don't think they could root in it even if they could ever get down that far. Sand, maybe but you have many choices to use for a cap over soil.
Sorry I know this was long winded but it might give you an idea of what a dirt tank is like.