Plants create their own food out of water, carbon dioxide and minerals using energy from the sun in a process called photosynthesis. Sometimes, however, there aren't enough minerals in the soil for optimum growth and gardeners need to supplement with fertilizer containing the most important elements, nitrogen, phosphorous and potassium. The percentage of these nutrients is on each package of fertilizer, in that order. For instance, 5-10-10 is 5 percent nitrogen and 10 percent of both phosphorous and potassium. Nitrogen supports green leafy growth and the second two both help with flowering, fruiting and growing strong stems.
Choose your fertilizer based on the type of plant you are feeding. Acid-loving shrubs such as rhododendrons, camellias and kalmias need an acid fertilizer formulated for their soil needs. Roses and other flowers like similar fertilizers, usually a formula low in nitrogen such as a 5-10-10 product.
Choose a fertilizer application type. Slow-release pellets need to be applied only once a season, granular fertilizer needs a few to many applications based on the needs of the shrub or flower and liquid fertilizers may need to be applied as often as every two weeks.
Read the instructions on the package carefully, noting the amount to use and the recommended frequency of fertilization. Different formulations require different amounts of the product to be spread around each plant. The size, type and age of the plant all affect how much fertilizer it needs. Never use more fertilizer than is recommended on the package directions. Excess nutrients may burn the roots and cause brown spots on the leaves.
Measure out the required amount of your fertilizer, spread it around your shrub or flower and mix it into the top half inch of the soil with a small rake or hoe.
Water well to begin the process of making the nutrients available to the roots.