Gardening Methods

Miracle-Grow Garden Soil Question?

Hey I just got home from lowes where I bought a bag of Miracle-Grow Garden Soil which I intened to use for containers. I just saw on the back of the bag its says Not for use in pots or containers. Why? Can I mix it with regular potting soil and then use it in containers? HELP!
I called up miracle grow and they said the problem is if you dont mix the garden soil with potting soil there wont be adequate drainage. he told me to mix =)

Perhaps it contains a substance that expands when wet?

NoMoreGrasshoppers.com How to: Create a Soil Mix for Container Gardens and Raised Beds