I do cover the holes in the pot and while the screening (or whatever I have on hand at the moment) works great for the brand of potting soil that I purchase, the soil turns to a very fine silt after the worms are finished with it. It is this almost powdery silty stuff that then pours through. It is not unusual for me to lose a half-pots worth of soil in a 3 or 5 gallon pot over a 6 month period. I use supersoil brand potting mix - not the greatest but readily available where I live.
