N = Nitrogen promotes healthy leaf growth by stimulating the production of chlorophyll 
P = Phosphorus supports the vigorous development of roots, stems, blossoms, and fruits.
K = Potassium plays a key role in helping plants digest and manufacture their foods. Helps flower formation

Water before and after fertilizing: A plant stressed from lack of water is more likely to be burned by nitrogen fertilizers, so make sure that the soil around the plant is wet before you add fertilizer. Watering after fertilizing helps to move nutrients into the root zone.

Start fertilizing in early spring and stop in late summer or fall: Make your first application about four to six weeks before growth begins in spring or, in areas where winters are cold, about the time you take off your winter protection.

Nitrogen: Nitrogen fuels a rosebush’s growth, and you must add it to the soil regularly. This element stimulates dark green, healthy foliage growth; because a plant’s energy to make flowers is manufactured in its leaves, healthy leaves mean more flowers. Most rose foods have several times more nitrogen than phosphorous and potassium. Don’t worry about the numbers too much. Just don’t buy one of those “bloom” foods that has no nitrogen at all.

Phosphorus and potassium: Phosphorus and potassium are called macronutrients because roses need them in larger supplies than other nutrients. Some soils already contain enough phosphorus and potassium for healthy rose growth; adding more to them does little good. If your soil is short on phosphorus, add some directly to the planting hole when you put in your roses, so that it gets where it needs to go.

How to Fertilize Your Roses - dummies

Links:
How to Apply Fertilizers to Your Garden | Old Farmer's Almanac