If ever I needed proof that good potting mix matters to plants, I got it last weekend while repotting chestnut seedlings. These two seedlings were grown in the same size pot, in the same lighting conditions, and with the same watering and fertilizing regimen. Yet you can see the vast difference in root development. Plus, the chestnut on the right is substantially taller, as you’ll see in the photo below.
I remember what happened. I had run out of potting mix, so I substituted topsoil for a small number of seedlings. Even after “cutting” it with peat moss, the topsoil was too heavy and thick to be used as a container medium.
Moral of the story: use a good potting mix (here’s one I use) and beef it up with compost (and sand, if you’re working with woody plants). Avoid using topsoil in containers even if it’s amended. Your plants will be larger and stronger. And the more extensive root system will help plants deal with drought and neglect.