I was filling my containers today and I decided to fill the bottom with the soil from outside in the garden. That is what I use to grow grass from seed for my cat and it works fine.

About one inch, then the rest I fill with bought soil. I'm thinking that most of the roots are not going to get down to the bottom of the container. I'm also thinking that if they do, they'll have to toughen up to get thru the rougher soil, and that is good, cuz they will thus be better prepared for the outside soil when I transplant them.

ellen