Crusty White Stuff on Pots 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.