What do the NPK numbers on my fertilizer package mean?
These numbers indicate the levels of the major nutrients in the fertilizer mixture. Each of the ingredients is listed as percent by weight in the package. Of all the nutrients necessary to plant health, only these three are unlikely to be available in adequate quantity in the soil or environment of the plant (though some secondary or trace nutrients may be deficient in some situations).
N signifies the amount of nitrogen. Nitrogen is essential for all plant growth as it is a component of most of the components of living cells. e.g. proteins, DNA, RNA, chlorophyll, etc. Plants grown in low nitrogen conditions do not flourish and their leaves turn a pale green.
P signifies the amount of phosphorus. Phosphorus is also an important constituent of many cell components, e.g. DNA, RNA, and is involved in many of the photosynthesis processes. For plants, it is important for root development, strong stalks and stems. Most importantly it promotes flower, fruit and seed development.
K signifies the amount of potassium. Potassium is involved in many of the biosynthetic and energy producing activities of cells. In plants, potassium plays an important role in photosynthesis. It also increases root growth and improves drought and disease resistance. If potassium is deficient then leaves will appear wilted and even necrotic.
Different packages of fertilizer will have varying proportions of the three components. As mentioned above the proportion of N, P and K is listed as a percentage of each component in the package by weight. Thus, a listing of 10-10-10 would indicate a balanced mixture of the three components with each one comprising 10% of the weight of the package. Some fertilizers used in gardening have only one or two components, for example Superphosphate could have an N:P:K ratio of 0-17-0.
Some fertilizer formulas are designed to stimulate specific growth, such as abundant fruit or thicker leafiness, and the relative presence of the major elements reflects that design. While plants do not care about the origin of each of the nutrients, some sources are ready for uptake by the plant more quickly. Organic fertilizers tend to require more breakdown by microorganisms before the nutrients are available and offer nutrients in lower concentrations than synthetic sources. Those factors may be an advantage or a disadvantage depending upon the situation.
Reading the label:
A typical label might read:
Although, as discussed above, the N:P:K values are usually regarded as the percent weights of the three elements this is not strictly true. Although the nitrogen value is the percent by weigh of nitrogen in the mixture this is not the case for the phosphorus and potassium values. These are actually given as percentages of phosphoric oxide and soluble potash respectively.
The label may also tell you the period of time over which the nutrients will be released, the source of the various nutrients, the potential acidity or alkalinity of the fertilizer, the recommended application rate and the micronutrients present in the formula.
Courtesy of the NYBG Plant Information Service