NPK stands for "nitrogen, phosphorus, and potassium," the three nutrients that compose complete fertilizers. You will encounter the letters, NPK when reading the contents printed on bags of fertilizer. The description of the fertilizer may not expressly say "NPK", 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.
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.
Why is it important to know what NPK means?
Not all types of plants have the same nutrient requirements, and you can sometimes do more harm than good when applying chemicals haphazardly. Applying a fertilizer high in nitrogen 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.
- Have your soil tested.
- Use a slow-release fertilizer, which is less likely to harm plants to any great degree.
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.
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.
To exemplify this, the product in the picture provided is a Scotts Turf Builder product intended to help your grass in summer. The NPK value is listed as 28-0-8, meaning 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"/"complete" fertilizers?
Complete fertilizers, or "balanced 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. One listed as "10-0-10" would not be considered complete, referred to as an "incomplete fertilizer."
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.