Though the labels on fertilizer may seem a bit overwhelming at first, the explanation of the numbers is relatively straightforward. The three main numbers on any fertilizer label will be the percentage of nitrogen (N), phosphorous (P) and potassium (K) in the product. These elements may also be expressed by the abbreviation N-P-K. Each of these nutrients has a slightly different effect on the plant, and choosing the proper mixture depends on the plant and soil conditions.
Nitrogen is one of the most common elements on the planet and makes up 78 percent of the air you breathe. Nitrogen is also an important plant food and is always listed first on the fertilizer label. It helps in the formation of chlorophyll, the key chemical that makes photosynthesis possible and gives plants their distinctive green colors. Without chlorophyll, most plants could not produce the sugars they need not only to grow, but also to survive.
As one of the most abundant chemicals on Earth, phosphorous is the second chemical mentioned on a fertilizer label. Phosphorous is most common in a granular form but is also in liquid form for liquid fertilizers. The chemical is especially useful in helping to create the sugars and starches, and promotes blooming, so it is often an important part of fertilizer for flowers and other blooming ornamentals.
Though potassium is not normally a nutrient that is lacking in soil, it is very important for plant growth. Generally, plants consume nitrogen more than any other nutrient, but potassium is the second most-consumed nutrient. It is important in building protein and is vital in producing good quality fruit in many cases. Therefore, it may be an important component for those growing fruit trees. It also helps grow strong cell walls for stalk development.
Depending on the fertilizer you choose, other nutrients may also be given a value. Even if the fertilizer contains no nitrogen, phosphorous or potassium, they will be listed in a prominent place on the label with the number zero. Other nutrients that may be listed on the label include calcium, magnesium, sulfur and many others.
In order to choose the most effective fertilizer, it is important to understand what your specific plant needs. While fruiting plants tend to need a significant level of potassium, and blooming plants may need more phosphorous, nearly all plants need at least some nitrogen. Still, some of these nutrients may already be present in the soil, so often the best thing to do is have your soil tested to find out what nutrients may need to be replaced.