I've often wondered why there were little white balls that looked like styrofoam in potting soil. It just seemed out of character. After all, there are not little white balls that look like styrofoam in regular soil, not even organic. My research began and ended on the Internet. It turns out that those things are something called perlite. Perlite is a soft, volcanic glass, which I find odd since it doesn't really resemble glass. However, its properties make it perfect for potting soil. Perlite holds water relatively well so when it is mixed in potting soil it keeps the soil relatively moist. It also prevents compaction of the soil.