We've found some great resources for soil education at these sites:

Soils4teachers by the Soil Science Society- Soil activities and a wealth of knowledge (they also have great Youtube videos), this site has a ton of great activites organized by grade.

Dr. Dirt K-12 Teaching Resources. This innovative teacher shares his activities and lessons.

Underground Adventure. Activities and curriculum created by fieldmuseum.org

Discovery Education: The Dirt on Soil- lesson plan library and activities K-12

BLM: Just for Kids- online interactive site about soil biology created for kid exploration

NRCS Web Soil Survey- students can analyze the soil right where they stand and understand how we use soil type to help us determine how to use the land.