Hi Gang, I have used styrofoam slabs in the past with good effect to: 1. make the pot lighter, 2. work with deep growing roots, and 3. cool the soil. I used styrofoam logs about 1 or 2 inches on a side and about 12-18 inches long. I set them vertically in the pot and filled in between them with regular soil mix. The vertical arrangement allowed a deep root run, but the styrofoam helped make the large pot noticeably lighter. The best part was that by strategically placing the styrofoam I could limit some soil heating, not a lot but enough that I could measure a few degrees difference (compared to non-styrofoam--containing pots). The soil in the back side of the pot (away from the sun) did benefit--I presume it was just simple insulation effect due to styrofoam. Perhaps there is no difference for plants if I should use styrofoam peanuts, or styrofoam logs, in terms of possible toxic effects. But, for sure, the logs did not seem to provide much in the way of air exchange as I hope to obtain with the peanuts. I'm hoping for great things from the peanuts-as-soil-amendment experiment. But, like so many things in gardening, what we hope for and what we get are often different entities. As I think about the styrofoam logs I suppose any cooling benefit would not be felt if I had better shade for my pots, or in areas without hot, hot summer sun. So much about soils, especially potting soils, seems to be climate or season specific. Cordially, Conroe Joe