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What's the ground soil like where you're at? Sometimes it's better than store bought stuff. If it's not good and you want to buy soil then don't buy any bullshit like miracle grow or kellogs, it's worth it to spend a little more on something good.
I've never used coco but as far as I know it's just a neutral medium, people use it when they want total control of the amount of fertilizer/nutrients their plants get. I personally wouldn't recommend it for plants in the ground.
Edit: I've never used promix either so I can't comment on that.