What Do the Letters NPK Mean on a Fertilizer?

NPK ratio on a box of fertilizer

The Spruce / Janet Kwan

When choosing a fertilizer for your garden or landscape, you want to make sure your plants are getting the nutrition they need, but there are so many choices when it comes to selecting a fertilizer blend. There are certain rules that all fertilizer makers must follow when they label their products and understanding these rules can make comparing fertilizers much easier.

NPK is one of the primary markers you'll notice on fertilizer bags—it stands for "nitrogen, phosphorus, and potassium," the three nutrients that compose complete fertilizers. While the description of the fertilizer may not expressly say "NPK," you will at least see a series of three numbers, often separated by dashes. These numbers correspond, respectively, to the nitrogen content, phosphorus content, and potassium content of that fertilizer. For example, if you purchase a 10-pound bag fertilizer labeled 5-10-5, it contains 5 percent nitrogen, 10 percent phosphorus, and 5 percent potassium by weight. The remaining 80 percent of the bag's weight is comprised of other minor nutrients or fillers.

Test Your Soil

Having a soil test done before you start adding amendments will tell you what you actually need. If your soil pH is too high or too low, your plants will not be able to access some nutrients, even if they are present in the soil.

The Importance of NPK

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 of how well your soil is meeting the nutritional needs of a plant but still feel the need to feed it at a particular time, you should try one of the following options.

  • Use compost instead of chemical fertilizer.
  • Get your soil tested.
  • Apply a slow-release fertilizer, which is less likely to harm plants to any great degree.

Analysis Numbers

You can get by with a smaller quantity of fertilizers that have high analysis numbers than those with lower numbers. In other words, five pounds of 10-20-10 fertilizer would give you the same nutrient value as 10 pounds of 5-10-5 fertilizer.

Role of Each Plant Nutrient

For practical purposes, here's a brief look at the roles that the contents of NPK play in plant growth.

Nitrogren

Nitrogen, the first number referenced in an NPK sequence, plays a key role in a plant's coloring and chlorophyll production, making it an important factor in leaf development. Fertilizers high in nitrogen are often used for grass or other plants where green foliage growth is more important than flowering. At the opposite end of the spectrum, gardeners sometimes encounter the problem of nitrogen depletion—the yellowing of typically green plants often indicates a nitrogen deficiency.

Phosphorus

The middle number in an NPK series refers to the percentage of phosphorous in the fertilizer product. 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. Phosphorus contributes to many fundamental plant processes, such as rooting and setting flower buds.

Potassium

The final number in the major ingredients listing gives the percentage of potassium in the product. Potassium contributes to the overall health and vigor of plants. It is known to help strengthen plants' ability to resist disease and can be especially important in areas that experience cold or dry weather.

Balanced or Complete Fertilizers

Complete fertilizers or balanced fertilizers are so-called because of their amounts of nitrogen, phosphorus, and potassium. A fertilizer listed as "10-10-10" would be considered a balanced fertilizer because of the equal proportions, while one listed as "10-0-10" would not be considered complete, but referred to as an "incomplete fertilizer."

An incomplete fertilizer is not necessarily inferior to a complete fertilizer. Identifying the right fertilizer for your needs depends on a variety of circumstances. If your soil already has 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—in this case, an incomplete fertilizer may actually be the right choice for you. It's yet another reason why it's so important to have your soil tested. Otherwise, whenever you add anything to your soil, the effect (whether positive or negative) is left to chance.

Other Ingredients

In addition to the major nutrients that are usually noted on the front label, most fertilizers also include additional ingredients that are listed on a side or back label. This may include other nutrients like calcium, magnesium, iron, micronutrients, and even the percentage of organic matter and fillers. Although the minerals and micronutrients are less critical than the major nutrients, a good fertilizer product will include small amounts of other ingredients as well. 

Warning

Avoid over-fertilizing your plants, especially lawns. Nutrients that aren't taken up by plants may run off into sewer systems and rivers, leading to serious pollution problems. 

Organic Fertilizers

Products labeled as organic fertilizers must specify which of the nutrients are organic, and they must be identified as either synthetic and/or natural, by percentage. For example, you might read "20 percent of nitrogen (6 percent synthetic, 14 percent organic)."

Strictly speaking, "organic" material is anything that contains carbon atoms. However, in popular usage, we have come to expect that organic fertilizers, like organic food, are manufactured by natural processes and contain nothing synthetic. That tends to be the case with most commercial products, especially as consumers become more educated, but be sure to read the label before you make a purchase.

Keep in mind, organic fertilizers made from natural ingredients often have lower concentrations of the three major nutrients, so you will need to use larger amounts. However, they do contain many other nutrients that feed both the plant and the soil. If using synthetic fertilizer, you should supplement with some type of organic matter, such as compost or manure, to maintain soil health.