• Soil is often not enough to grow rich, brilliant flowers.good planting soil image by tomcat2170 from Fotolia.com
  
  There are three essential ingredients in plant fertilizer: nitrogen, phosphorus and potassium. Each of these elements helps the plant grow and all are crucial for the plant to survive and prosper. On a fertilizer bag, the ratio of these three elements will be listed. The ratio lists three numbers. The first number is always nitrogen; the second, phosphorus; and the third, potassium. For example, a number that reads 12-6-10 has 12 percent nitrogen, 6 percent phosphorus and 10 percent potassium. The rest of the fertilizer is filler, such as sawdust, and inert ingredients.
  
  

Nitrogen

• Nitrogen, like the other two main active ingredients in fertilizer, is missing from soil because plants consume it when they grow. The fertilizer, therefore, has what the soil doesn't. Why is nitrogen important? First, it's used in chlorophyll, the part of the leaf that performs photosynthesis, a process that converts sunlight into energy for the plant. Second, nitrogen is used in all cells, not just chlorophyll, in the form of enzymes and protein. Because of this, nitrogen promotes plant growth, and for seeds or young plants, fertilizers high in nitrogen content will help them the most.
  
  

Phosphorus

• Like nitrogen, phosphorus is a macronutrient involved in photosynthesis. Phosphorus helps root growth, lowers the stress of the plant to diseases and shocking weather conditions and is part of the formation of starches, sugars and oils. It also encourages the growth and maturation of fruits and flowers. For this reason, a plant that is already established should receive fertilizer high in phosphorus.
  
  

Potassium

• Potassium is another macronutrient essential for plant health. Through the action of the sodium-potassium pump, potassium helps plants move liquids like sugar water, as well as other nutrients, through the tissues of the roots, stems, leaves and fruit. When the plant isn't getting enough potassium, the leaves turn yellow on their edges. Unlike phosphorus and nitrogen, potassium is plentiful in soil. However, the form it's in isn't available to plants to absorb, so fertilizers are needed to supplement the plant's potassium diet.