Nitrogen (N) is listed first, followed by phosphorus (P) and potassium (K). The numbers listed stand for the percentage of each element. A formula of 20-6-12 means the product contains 20% nitrogen, 6% phosphorus, and 12% potassium. Any fertilzer that contains some percentage of each of these three nutrients is called a complete fertilizer. If you see a formula on a label listed as 21-0-0, you know that it is a single purpose fertilizer, in this case nitrogen. For plants that require a great deal of nitrogen (heavy feeders) this is all that is needed to get them through the growing season. A product listed as 3-20-20 with higher amounts of phosphorus and potassium and almost no nitrogen are blossom boosters recommended to improve flowering, bud set, and container grown plants.