Fertilizing roses means walking a narrow line between too little and too much. The primary nutrients for healthy roses are nitrogen, phosphorus and potassium--three elements identified on the bag of fertilizer by the letters N-P-K. Nitrogen (N) is a component of all proteins and, because water washes it away from the root zone, roses require a consistent supply of it. Nitrogen promotes green growth and good canes but an excess will produce lush, verdant plants with few or no blooms. Conversely, nitrogen-deprived roses will have yellow leaves, no new growth and small pale roses.
Strong root growth and abundant flower production are the results of the right amount of phosphorus (P). A phosphorus deficiency will cause dull foliage, falling leaves, weak flower stems and buds which do not open readily. Phosphorus does not move in the soil so the best time to apply it is in the form of bone meal or superphosphate when the rose is planted.
Potassium (K) or potash encourages vigorous growth but a lack of it will produce yellow leaf margins which turn brown, weak stems and poorly developed buds. It is noteworthy that a deficiency of any or all of these elements impacts the rose leaves which may, at first, seem relatively unimportant. The fact is that 25 to 35 leaves are required to produce one perfect rose bud.
Secondary and trace elements contribute to the health of a rose but often are present in the fertilizers used to transmit the big three, N, P, K. Since an imbalance of essential elements in the soil can have such far-reaching effects, a soil test through your local university extension office is a prudent choice before beginning a fertilizer program. Fertilizers are classified as either organic which includes manures, compost or other plant and animal products, or inorganic. Their nutrient content of organics is low and their cost relatively high, but their soil-improving addition of humus is especially welcome in clay soils. Inorganic fertilizers are manufactured from simple salts which microbial activity, makes available to the plants. They are usually more concentrated and less expensive than the organics, and are available in a variety of forms including liquid and long-term. Rose plants make no distinction between organic or inorganic fertilizers so cost and ease of handling can be a determining factor. Regardless of the fertilizer used, it is important to water deeply so it will enter the soil.
Few rosarians follow the old recommendation of three applications of fertilizers in early spring, early summer and late summer. The preferred schedule now is to fertilize when the plant first leafs out and to continue at six week or even monthly intervals until late summer.