Here are some fun places to explore soil on the web:
*Get the Dirt on Soil. This site has some interactive soil games and a great field guide to soil.
*ISM Geology. This site has a ton of great lessons on geology for all grades. Activities are well thought out and they suggest well-written books as part of their lessons. This lesson is for kindergarten.
*Sorting Rocks. While this is totally about rocks and sort them by size, the process they employ is a good template. There are a number of more advanced lessons on earth science available.