NPK stands for nitrogen, phosphorus and potassium, the three nutrients that compose complete fertilizers. You'll encounter NPK when reading the contents printed on bags of fertilizer. The description of the fertilizer may not expressly say "NPK" (it may simply be implied), but you will at least see a series of three numbers, which correspond, respectively, to the nitrogen content, phosphorus content and potassium content of that fertilizer.
Why is it important to know what NPK means? It's important because chemical fertilizers should not be used indiscriminately. Not all plants have the same nutrient requirements, and you can sometimes do more harm than good when applying chemical fertilizers haphazardly. For example, applying a fertilizer high in nitrogen (indicated by the fact that the first number on the package is high) will cause certain plants to put all their energy into producing foliage, at the expense of flowers.
If you don't know that much about the nutritional needs of your plant (but still feel the need to feed it at the present time), your best bet is probably to:
- Use compost instead of a chemical fertilizer
- And/or have your soil tested
- And/or stick with a chemical fertilizer labeled as being specifically for that particular type of plant
NPK: The Breakdown
In conclusion, let's take a brief look at the constituents of NPK:
Nitrogen promotes leaf development. As Kelly Burke, About.com's Guide to Lawn Care writes, "Nitrogen is a major part of chlorophyll and the green color of plants." As mentioned above, there can be such a thing as "too much nitrogen"; at the opposite end of the spectrum, gardeners sometimes encounter the problem of nitrogen depletion.
Meanwhile, phosphorus plays a key role in the growth of roots, blooming and fruiting. Potassium also plays a part in root growth, as well as in stem development.