The internet is a good source of information on fertilization and it is helpful to familarize oneself with the different materials available and the relationship of the major plant foods, i.e. Nitrogen. Much has been said about fertilizer requirements for roses and most authors feel that they have said all there is about the subject. When I was in the greenhouse business, I did my own soil testing, sent some soil samples to the University, and had foliar analysis done. All tests indicated that the fertility levels were at optimum. But yet, there was an observable difference when different types of plant foods were applied.
Roses require more iron than regular plant foods supply.
Great foliage color and gloss and greatly improved stem length. May. Feed once again about a month later in June. Make another feeding in mid July. Almost all organic fertilizers will raise the pH of the soil. You can monitor pH with an inexpensive test kit available from some garden supply houses.