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Fertilizer labels list three numbers; for example, 15-30-15 indicates the relative amounts of nitrogen, phosphorus and potassium, or N-P-K. Choose a fertilizer based on the ratio of these nutrients to each other. For instance, a 15-30-15 fertilizer has a 1:2:1 ratio of N-P-K with one part nitrogen, two parts phosphorus and one part potassium. Flowering plants thrive with a 15-30-15 ratio, while a typical lawn fertilizer may use a ratio of 10-1-1, depending on your soil test. Fertilizers with a 1-1-1 ratio are considered all-purpose.

Nitrogen

Nitrogen is an essential nutrient for numerous plant functions and is one of the elemental components of chlorophyll. Nitrogen fuels vegetative growth, particularly the foliage, stems and branches. Plants can deplete the soil of nitrogen, leaching from the soil faster than any other element. If your plants have yellowing leaves or appear stunted with poor growth, the soil may lack nitrogen. By using a soil test kit, you can determine which element may be missing in your soil. Purchase test kits at most home and garden centers.

Phosphorus

A stimulant for root growth, seed and flower formation, phosphorus is most available to plants when the soil pH is between 5.5 and 7. The pH refers to the acid or alkaline level of the soil, and can range from 0 to 15 with 7 regarded as neutral. Different from nitrogen, phosphorus tends to remain in the soil; add it only for new growth. Root vegetables, such as carrots, use large amounts of phosphorus, particularly during early growth.

Potassium

Potassium promotes root and bud growth and ripening of fruit. It enhances disease resistance as well as tolerance to drought, heat and freezing. This element is essential for all plants to thrive, particularly in changing weather conditions. Potassium also has a tendency to remain in the soil, but is heavily used by growing vegetables, so should be supplemented as needed, based on a soil test.