The best rate of application for any fertilizer is what is printed on the package so for Osmocote it would be one capful or 3 tablespoons for a two gallon container or 4 square feet of planting space. I would guess your baskets hold about two gallons of soil but you should measure to be sure. You can also measure how much soil your bags hold by using a gallon container to fill them and determine the volume. I suspect you'll end up using one capful of Osmocote per container. Keep growing, Marianne Binetti