Q: I have some crusty-looking stuff on the bottom of a clay pot. Can you tell me what it is and how to get rid of it?
A: I think you’re seeing efflorescence. This powdery material is the remains of fertilizer or minerals in the potting soil inside the pot. Water wicks through the clay pot wall and the minerals dissolved in it dry on the outer surface. The white stuff is composed of calcium, magnesium, sulphate, nitrate, phosphate and ammonium salts.
Simply soak the pot for an hour in a bucket of water and scrub off the white powder.