Porous Plant Pots become stained and impregnated with mineral deposits from soil Leaching over time. A stiff brush, some Household Bleach, and a little "Elbow Grease" will clean them up in short order.
1 cup of Household Bleach
9 cups of water
Mix the bleach and water thoroughly. Scrub the inside and outside of the pots to remove the deposits. It may be necessary to soak badly stained pots in the solution before you scrub them. It is best to clean pots after ever use, not only to remove stains, but also to kill any bacteria that may attack tender young plants. Rubber Gloves will protect your hands from pot chips and bleach.