Fertilizers come in two varieties: organic and inorganic, or chemical. Organic fertilizers, derived from plant or animal matter, release slowly because they require decomposition by soil micro-organisms before the plant can use them. Inorganic fertilizers are man-made compositions, formulated for various speeds of release.

There are several compelling reasons to use organic fertilizers on your roses. Because they break down slowly, organic fertilizers provide a constant source of nutrition at low concentrations. Their slow breakdown minimizes the chance of burning the plant. Also, they do not leach out of the soil as quickly as chemical fertilizers. In the long run, inorganic fertilizers provide as much or more nutrition to the rose as chemical fertilizers. They also have the added benefit of improving the physical structure of your soil. Your soil, the micro-organisms in it, and your worms need some sort of organic material to keep them healthy, whether it be through fertilizer, soil amendments or mulch. You cannot provide what your soil needs through the use of chemical fertilizers alone.

Because organics work slowly, they must be added to the soil early in the growing season. Also, the soil must be moist and warm enough for soil organisms to be active.

Fertilizers are labeled with NPK numbers, specifying their nitrogen, phosphorus and potassium content. Generally, roses need a higher percentage of phosphorus than the other two ingredients. Good maintenance formulas for roses are 5-10-5, 4-8-4, or 4-12-4. However, throughout the growth cycle, the need for these nutrients varies. Extra nitrogen is needed for early spring growth of stems and foliage, as well as continual moderate supply during the entire growing season. Phosphorus promotes roots and blooms, so a higher phosphorus food should be supplied from 3 weeks before blooming until blooming. Potassium provides health for the plant and is a catalyst for nitrogen and phosphorus. It also builds in hardiness to heat, drought and cold, and is therefore a good supplement just prior to the dormant season.

All-purpose organic blends of fertilizer are easy to use and include trace amounts of beneficial micronutrients. Liquid fish fertilizers supply a quickly available source of nitrogen as well as some other elements. However, some people find their strong, fishy smell objectionable. Bone meal is a valuable source of slow-acting and long-lasting phosphorus. Since it does not move down readily with watering, it should be mixed into the soil--about 1 heaping tablespoon per plant. Blood meal is a source of organic nitrogen. Generally,1 tablespoon applied around each plant is adequate--too much will burn roots. Manure is excellent for a mulch or for incorporating into the soil. Only aged manure (composed) should be used, as there is chance of burn from fresh manure. All types add organic matter and are good soil builders. Composted manure can be spread on the surface to a depth of 2-3 inches.

Soil testing (Cornell Cooperative Extension Service lab) will help you determine what you need to add to your soil. The lab report usually includes recommendations for amounts of nutrients to be applied to the soil, and should also note the soil's pH. Roses prefer a slightly acid soil pH--6 to 6.5. Ground limestone is the best material for adjusting the pH. It supplies the plant with calcium.  If your soil is too acidic, gradually increase the pH by applying 2 tablespoons of ground limestone per bush per year. Both calcium and potassium strengthen the plant's cell membranes, boosting resistance to fungal infection. Sulfur can be added to overly alkaline soil.

In addition to nitrogen, phosphorus and potassium, the soil should be tested for magnesium. This element is present in tiny amounts but nevertheless is very important; it helps improve the structure of the canes and the vigor and height of the plant. If the soil test indicates a magnesium deficiency, scatter Epsom salts over the soil around each plant--about a half a cup for each mature plant.

It is always a good idea to amend your soil with organic matter, such as humus, composed manure for an added source of slow release nutrients. The addition of organic matter will also improve the soil's drainage and nutrient holding capacity. It is recommended that two to four inches of organic matter be added and worked into new beds to a depth of 12 inches.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service