There is no single right answer here, especially without knowing what your native soil is like.
Some native soil is fine as-is (nice loose sandy loam with some decent concentrations of organic matter), and some soil requires quite a bit of effort to prepare (clay).
If what you're trying to do is loosen up some tight clay-like soil, I recommend adding good compost, not potting soil.
Here's what I do to prep an outdoor veggie bed (same routine will work for MJ):
Step 1 - Mix/loosen up the native soil. (Double dig).
Step 2 - Lay down 2-3 inches or so of compost on top of the freshly loosened soil.
Step 3 - Mix this compost into the top foot or so of soil.
Step 4 - Evenly spread some soil amendments over the surface. Something like an organic "all purpose" fertilizer. The Fox Farms Happy Frog "All Purpose" or "Tomato and Vegetable" work well. Follow the recommended rates.
Step 5 - Scratch this fertilizer into the top 3-4 inches of soil.
Step 6 - Add another 3 inches or so of compost on top.
Step 7 - Add a thick layer of mulch. Straw works well outdoors.
Step 8 - Water this down thoroughly.
Then wait... At least several weeks. What you are doing is creating a happy habitat for worms, which will start tunneling up into your rich top layers and bringing nutrient-rich material down to the depths. All this tunneling will result in loose, crumbly, nutrient-rich soil.