I did a search in the Learning Center and put together a collection of resources geared for middle schoolers on the topic of soil. I also found that there were a few collections already created. I thought the one created by Pam A. was particularly well put together, so I am including it, too. I hope you can find some great ideas within these resources.
Pam A's Collection: Soil Science
My collection: Soil and Middle Schoolers
Check out the Scope on Safety article, too. There are some safety hazards to consider when teaching about soil.
The other soil collection I found is by LeRoy A. It contains a ppt presentation.
I hope this helps.