Since potting soil is inorganic substance, the way I fertilize my potted roses is to apply Osmocote (or any slow-release fertilizer) after spring pruning, add water-soluble fertilizer after two sets of leaves have grown, and then add water-soluble fertilizer every time after massive deadheading/late summer pruning. It's better to water the roses well before fertilizing. It doesn't matter what kind of fertilizer you use, just be aware that the younger the plant, the lighter the fertilizer you apply. Happy gardening!
