My son tested types of bagged soil for germination and seedling growth. He used plain topsoil, Scott's brand potting soil, and Miracle-Gro brand potting soil. Both the Scott's and MG have built in fertilizer. He planted broccoli seeds.
Anyway, the seedlings in the Miracle-Gro did absolutely horrible. Fewer seedlings emerged, and ALL of the ones that did where (and continue to be) withered, yellow, and stunted. The new hypothesis is that the Miracle-Gro simply has too much nutrients (probably nitrogen) for the young seedlings.
I've used the stuff before to start seeds, and I remember having bad results. In my mind, all of the bagged "soils" were about the same. Now I know better.
Edit: Final results: Germination was highest in the Scott's, and initial growth was the same in the Scott's and regular top soil. Now that true leaves have formed, the plants in the Scott's are growing better than the ones top soil. The plants in Miracle-Gro are still pathetic.