These are the nutrients that you will see commonly identified on the fertilizer package. Nitrogen, Phosphorous, and Potassium are labeled as a sequence of three numbers (10-10-10) or (20-20-20).
Nitrogen promotes healthy vegetative, green growth. Phosphorous is vital to promote strong root growth and flower production.
Potassium makes sure all is in good working order somewhat like a vitamin. I call it the plants immune system booster as it helps the plant through stressful times such as disease/insect damage, drought and cold temperatures.
Balanced fertilizers have an equal representation of the three major nutrients, for example, 10-10-10. Fertilizers formulated for strong green growth will have a larger percentage of nitrogen. Fertilizers for evergreen trees or turf grasses will have a large percentage of nitrogen, for example, 30-10-10. Fertilizers developed for flower production normally has a high phosphorous content, for example, 10-20-10.In general, stay away from high nitrogen content fertilizers for use with roses.
How you fertilize your roses depends on what stage of the growing process your roses are at. If you are fertilizing newly planted roses you should only use bone meal or super phosphate (very small amount) in the planting hole at the time of planting. You certainly do not want to promote a lot of green growth at the expense of a good root system.
Your basic rose feeding program should start out with a balanced fertilizer (10-10-10), or one with a high phosphorous content (5-10-5, 10-20-10). Even better are rose fertilizers, which include the other macro and micronutrients.
The secondary macronutrients are sulphur, calcium, and magnesium. Use of a high quality rose fertilizer will ensure that these macronutrients are made available to the roses.
Of these secondary macronutrients, magnesium is of substantial interest to the rose gardener. Magnesium sulfate, provided in the form of Epsom salts, is a time-honored secret for intensifying flower color, increasing flower production, and flushing harmful salts through the soil.