Solved! How to Read Fertilizer Numbers
Three numbers show the most important information on a package of plant food.
Q: I went to the local nursery to pick up plant food for my garden, and noticed that every fertilizer has three numbers on the front of the package. What do fertilizer numbers mean?
A: Similar to the nutrition information labels on the foods we eat, the three numbers on fertilizer packages that are separated by hyphens are there to show consumers the product’s nutritional value for plants. These numbers, which you’ll see written as 6-4-3, for example, are a fertilizer’s NPK numbers. Although scientists have identified 16 essential plant nutrients, the three most important are nitrogen (N), phosphorus (P), and potassium (K). As such, these “macronutrients” receive prominent placement on fertilizer packaging, often on the front of the label.
Gardeners who understand these numbers and the nutrients they represent are best equipped to select fertilizers that are appropriate for their plants’ needs. Here’s what you need to know about the roles nitrogen, phosphorus, and potassium play in plant growth, and how to choose the right balance of these nutrients for your garden.
First, learn the 411 on N, P, and K.
Nitrogen feeds leafy plant growth. Established lawns use it to grow—and stay green— throughout the year, and thus benefit from high-nitrogen fertilizer. Trees and shrubs need less nitrogen because they only produce one set of leaves per season.
Phosphorus stimulates root, flower, and fruit development. Annual flower beds, fruiting plants such as tomatoes, and newly planted seedlings sometimes need a boost of phosphorus. If the soil already contains a sufficient amount of this element, adding more will cause more harm than good.
Potassium is essential to virtually all plant metabolic processes. It promotes strong stems, well-formed flowers, robust fruits, and healthy roots, and helps plants use water and resist drought. Most soils provide some natural potassium, but it may not be enough for certain plants.
NPK numbers represent the percentage of each element in the fertilizer.
The three NPK numbers on a fertilizer’s label represent the percentage of each element, by weight, inside the package. A package of 24-0-6 fertilizer, in other words, contains 24 percent nitrogen, 0 percent phosphorus, and 6 percent potassium. The balance of the package’s contents is made up of other nutrients (which will be listed somewhere on the label), and/or inert ingredients.
Once you know the percentages of each element in a bag of fertilizer, you can determine the weight of each element that’s inside. This calculation comes in handy if you’ve had your soil tested and have been directed to amend your soil with a specific amount of a particular element. For instance, an 18-pound bag of 24-0-6 contains 24 percent nitrogen, which amounts to about 4.32 pounds of the element (18 x 0.24 = 4.32). If the local extension office tests your soil and suggests that you apply 1 pound of nitrogen per 1,000 square feet of lawn, the 18-pound bag would cover 4,320 square feet (1,000 x 4.32 = 4,320) at the recommended rate.
Synthetic fertilizer numbers are usually higher than those of organic fertilizers.
The NPK number is an excellent way to compare elemental nutrient values of different types of fertilizer. The numbers show how much elemental nitrogen, phosphorus, and potassium there are in the mix—it does not matter whether it is quick release, slow release, or organic. If the sources of each nutrient concern you, it’s a good idea to check the fertilizer’s ingredient list.
Inorganic, or synthetic, fertilizers typically have more concentrated NPK loads—in other words, higher NPK numbers—than organic fertilizers. Gardeners can apply less synthetic than organic fertilizer to deliver an equal amount of nutrients. For this reason, synthetic fertilizers are more cost effective.
Organic fertilizer is derived from natural ingredients that are minimally processed. They take longer to break down and provide your plants with a slow, steady feed of nutrients—sometimes over the course of many months. In the case of accidental spill or overapplication, these fertilizers are less likely to damage plants or soil.
Plants need more than the “big three” to thrive.
As we mentioned earlier, there are 16 nutrients that are essential to plant growth. Nitrogen, phosphorus, and potassium are macronutrients that are contained in most fertilizers. Plants get carbon, hydrogen, and oxygen, through air and water. Calcium, magnesium, and sulfur are considered secondary nutrients, which means that they are important to plants, but plants need less of them than they do N, P, and K.
The remaining 10 essential plant nutrients—boron, calcium, chlorine, copper, iron, magnesium, manganese, molybdenum, sulfur, and zinc—are micronutrients, or trace elements. While trace elements are essential to plant health, they are needed in much lower quantities than the rest. Fertilizers may include some or all of the trace elements, but your garden may or may not need more of them. The best way to find out which elements your soil needs is by conducting a soil test.
Testing your soil will help you select the right fertilizer.
Soil testing is the surest way to know which nutrients are lacking and which are plentiful in your garden soil. The results of the test include both an analysis of your soil as it is, and recommendations for fertilizer applications and rates to safely correct deficiencies. While home test kits are available, the most complete and reliable results come from lab tests.
Every state in the U.S. has a Cooperative Extension Service associated with its university system. Contact your county Extension Agent’s office for directions on how to collect and submit soil samples. The average cost is $15 to $20 per soil test, and it takes about a month to receive the results.