Organic fertilizers will help improve your soil. A fish/kelp liquid fertilizer provides nitrogen and adds the necessary trace minerals roses need. If you have aged manure and composted yard debris or lawn clippings you can add that to the potting soil. It will improve the soil texture and feed your roses.
Nitrogen promotes healthy green leaf growth. Phosphorus helps to build strong roots and aids flower production. Potassium encourages vigorous growth.
Water your roses well the night before you are going to add any fertilizer.