I'm a beginner and three months into my first dirted planted tank. Have learned a lot of lessons.
Re miracle gro: I just picked out the big sticks and put it in the tank, then wet it and mixed it with some red clay (iron). No more than 1" is the beginner recommendation to avoid anaerobic conditions.
I capped it with about 3/4" of black diamond blasting sand, medium grade. I've read that gravel you want more like 1-2", but sand should be shallower to allow gas exchange since the particles restrict the flow.
I added trumpet snails to aerate and keep the tank clean (see my thread on to trumpet snail or not- I got lots of input first as to whether to add them or not).
My water was yellow for about the first two months with weekly water changes, 50% for the first few weeks then 25%.
I test the water weekly in between or just before water changes. I have never registered any ammonia or nitrites, not even at the beginning. I have a big sponge filter, and the tank cycled amazingly fast.
I've been advised that with dirt you need to start by planting heavily including some fast growers and floating plants to suck up nutrients.
Happy to answer any specific questions with my teeny tiny bit of experience!