Plants, unlike animals, manufacture their own food using the energy of the sun in a process called photosynthesis. To do this, they need oxygen, carbon dioxide, water and a number of minerals absorbed by the roots from the soil. Annual and perennial flowers grow quickly at the beginning of the season and, in order to reach their full size, need either a soil that contains adequate amounts of these minerals at a pH appropriate for that plant or supplementary fertilization.
The Major Nutrients
The main nutrients, or minerals, needed for growth are nitrogen, phosphorus and potassium. They are called "macronutrients" because they are needed in larger quantities, and yet these are the nutrients most often lacking in soils. Nitrogen (N) regulates the growth of stems and leaves, and when applied as a fertilizer, causes lush, green, leafy growth. Phosphorus (P) helps with growing roots, flowers and fruit. Potassium (K) is also important for flower and fruit growth as well as in assisting with photosynthesis in shady conditions. Fertilizers used on flowers should contain low levels of nitrogen relative to phosphorus and potassium. A 5-10-10 formula (5 percent nitrogen, 10 percent phosphorus, 10 percent potassium) is ideal.
Calcium (Ca), magnesium (Mg) and sulfur (S) are also required by plants in fairly large amounts. Don't worry about supplementing your fertilizer with these nutrients unless a soil test recommends it. Most soils provide adequate amounts for plant growth. Calcium may be in short supply in acid soils, but many plants--blueberries and azaleas among them--prefer these conditions.
Micronutrients are those that are required for plant growth in very small amounts. They are boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), sodium (Na), zinc (Zn), molybdenum (Mo), and nickel (Ni). Two others that may be essential for some plants are silicon (Si) and cobalt (Co).
Shortages of these elements are usually related to a soil that is too acidic or too alkaline for the plant, interfering with its ability to absorb the nutrient from the soil. The most common shortage is that of iron, resulting in a condition called "chlorosis." The leaves turn yellow but keep their green veins. This is usually due to an alkaline soil or excess phosphorus.