I suspect the stains you described are salts leaching from the soil when you water your pots. Osmocote does container fertilizers that dilute into salt like compounds so I could not promise you that using Osmocote would not avoid this problem. A better solution is to either set the pots on top of saucers and then raise the base of the pots out of any drainage water that collects by sitting them on pot feet or plastic water bottle caps. Another practical solution is to have hanging baskets drain into containers below the hanging pot or place your containers off the deck onto a gravel surface nearby.   You can also group interesting stones or rocks near you potted plants to hide the stains. Enjoy your potted plants and Keep Growing, Marianne Binetti