What Do the Letters NPK Mean in a Fertilizer?
Also implied is a percentage symbol after each number because each of the three numbers represents the percentage of that nutrient in the makeup of the fertilizer. Here is an example of how it is used in a sentence: "When I asked at the nursery what the 10-10-10 on the back of the fertilizer bag meant, they said it was the NPK number" (some call it NPK "value" or "ratio").
Why Is It Important to Know What NPK Means?
It is important to understand NPK because chemical fertilizers should not be used indiscriminately. Not all types of plants have the same nutrient requirements, and you can sometimes do more harm than good when applying chemicals 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 do not have a good grasp on how well your soil is meeting the nutritional needs of a plant (but still feel the need to feed it at a particular time), your best bet is probably to:
Use compost instead of a chemical fertilizer
And/or have your soil tested
And/or use a slow-release fertilizer (which is less likely to harm plants to any great degree)
The NPK Breakdown: What Those Plant Nutrients Actually Do
To advance this discussion from the academic to the practical, let's take a brief look at the roles that the constituents of NPK play in plant growth:
Nitrogen promotes leaf development. As Kelly Burke 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, which is why it is an essential nutrient for your plants in spring. Potassium also plays a part in root growth, as well as in stem development.
Take a look at the picture that I have provided on this page. It shows a portion of the back of a fertilizer bag. Specifically, it is a Scotts Turf Builder product intended to help your grass in summer. The NPK value is listed as 28-0-8. That means it contains 28% nitrogen, no phosphorus, and 8% potassium (potash). Why would the company leave phosphorus out in this case? This product used to contain phosphorus. But Scotts explains that they have removed it for environmental reasons. Phosphorus was retained in their "Starter" fertilizer (designed for spring use), however, because "phosphorus is essential to the initial root development of grass plants...."
What Are "Balanced" (or "Complete") Fertilizers?
Complete fertilizers (also known as "balanced fertilizers" or simply "NPK fertilizers") are so called because they contain nitrogen, phosphorus, and potassium -- the Big 3 in fertilizer ingredients. A fertilizer listed as "10-10-10," for instance, would be a complete one. But one listed as "10-0-10" would not be considered complete, the middle zero indicating the absence of phosphorus in the product. This would be referred to as an "incomplete fertilizer."
Do note, however, that an incomplete fertilizer is not necessarily inferior to a complete fertilizer. Which is better really depends on the circumstances. If your soil already had an excess of one of the three nutrients in NPK, you could actually be harming some of your plants by adding more of it to the soil -- which is precisely what you would be doing (inadvertently) by using a complete fertilizer.
This is why it is important to have your soil tested: otherwise, whenever you add anything to your soil, the effect (whether positive or negative) is left to chance.