Looking at fertilizers in the garden store, you’ll usually find a grouping of numbers on the label. The numbers are important, and are required by law in most states. The numbers provide important information about what types of nutrients the fertilizer offers, and how much of each it contains.
What Do the Numbers on Fertilizer Mean?
Typically, there are three numbers on a fertilizer label separated by dashes, representing the percentage of N, P and K. What is NPK in fertilizer? Those three numbers correlate with the value of the three macro-nutrients used by plants: These macro-nutrients are nitrogen (N), phosphorus (P), and potassium (K), or NPK for short. Understanding the meaning of NPK will help you make the right decision for your plants.
The numbers indicate the percentage of these three elements in the fertilizer. The higher the number, the more concentrated that nutrient is in the fertilizer. For example, numbers on fertilizer listed as 20-5-5 means it has four times more nitrogen in it than phosphorus and potassium.
Many plants require a balanced fertilizer. A balanced number of 20-20-20 indicates the fertilizer has the same concentration of all three, and twice as much concentration of one that has 10-10-10.
NPK in Fertilizer
Here’s a brief job description for each of these 3 nutrients:
- Nitrogen (N) – Nitrogen is largely responsible for rapid growth of green leaves.
- Phosphorus (P) – Phosphorus is largely responsible for root growth, as well as flower and fruit development.
- Potassium (K) – Potassium is a nutrient that helps the overall functions of the plant perform correctly and supports the plant’s vitality. (Note that potassium is usually provided in the form of potash, also known as K2O)
The fertilizer numbers’ percentages are calculated by weight. The remainder of NPK fertilizer ingredients are generally sand or ground limestone, and sometimes micronutrients. Occasionally you will also see another number added to the fertilizer numbers. That final number represents the amount of sulfur in the mix.
A fertilizer that contains only one macro-nutrient will have “0” in the other values. For example, if a fertilizer is 10-0-0, then it only contains nitrogen.
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These fertilizer numbers, also referred to as NPK values, should appear on any fertilizer you purchase, whether it is an organic fertilizer or a chemical fertilizer.
How to Choose the Right Fertilizer
The NPK numbers on fertilizer are a way to provide the nutrients that may be missing in your garden soil, which is why the soil test is important as a first step. Your soil may have plenty of one and not enough of the others. If you get your soil tested by an extension service, you'll know which numbers to look for when you buy fertilizer.
Many soils have sufficient phosphorus and potassium for plants, but not enough nitrogen. And plants do not use as much phosphorus as nitrogen and potassium. That means that choosing the right fertilizer may mean picking a fertilizer with a higher first number or three different numbers like 16-8-2.
Knowing the NPK values of a fertilizer will help you select one that’s appropriate for the type of plant you're growing. For example, if you're growing leafy vegetables, you may want to apply a fertilizer that has a higher nitrogen number to encourage leafy growth. If you're growing flowers, you may want to apply a fertilizer that has a higher phosphorus number to encourage more blooms.
How to Calculate for NPK Fertilizer
The next step is to measure your garden and calculate the square footage using the formula length times width. Then purchase enough soil for the area.
The fertilizer numbers can be used to calculate how much of a fertilizer needs to be applied to equal 1 pound (0.45 kg) of the nutrient you are trying to add to the soil. So, if the numbers on the fertilizer are 10-10-10, you can divide 100 by 10 and this will tell you that you need 10 pounds (4.5 kg.) of the fertilizer to add 1 pound (0.45 kg) of the nutrient to the soil. If the fertilizer numbers were 20-20-20, you divide 100 by 20 and you know that it will take 5 pounds (2.2 kg) of the fertilizer to add 1 pound (0.45 kg) of the nutrient to the soil.
Applying Fertilizer
Always apply fertilizer according to directions. It’s important to use the recommended rates or even less. Using too much fertilizer is never a good idea and can burn your plants. If you are a pet parent, have a discussion with your garden center representatives to be sure the fertilizers you choose are pet-friendly.
FAQs
These three numbers form what is called the fertilizer's N-P-K ratio — the proportion of three plant nutrients in order: nitrogen (N), phosphorus (P) and potassium (K).
What do the numbers in NPK mean? ›
The three letters, N, P, and K, correspond with three numbers that indicate the percentage of each nutrient in that particular product. For example, a product marked 10-10-10 contains 10 percent nitrogen, 10 percent phosphorus and 10 percent potassium. A bag marked 20-20-20 containers twice as much of each nutrient.
What is the NPK ratio for fertilizer? ›
A NPK ratio of 4:2:1 (N:P2O5:K2O) is generally considered ideal and accepted for macro-level monitoring of consumption of plant nutrients for the country as a whole. However, it is difficult to trace the genesis of this NPK ratio.
What is the NPK level of fertilizer? ›
The three numbers that you see on a fertilizer label, such as 5-5-5, tell you what proportion of each macronutrient the fertilizer contains. The first number is always nitrogen (N), the second is phosphorus (P) and the third is potassium (K).
What is 20-20-20 fertilizer used for? ›
NPK 20 20 20 is a highly concentrated, balanced plant fertiliser. It contains equal parts nitrogen, phosphorus, and potassium. It is ideal for growing plants in poor quality soils as it provides a high amount of each of the three essential plant macronutrients.
What is the best NPK ratio for vegetables? ›
A common recommendation for vegetables is to apply 1 pound of a 10-10-10 fertilizer or 2 pounds of a 5-10-5 (or 5-10-10) fertilizer per 100 feet of row.
How do you choose NPK ratio? ›
For instance, to encourage plants to root, you need to choose a fertilizer that is higher in phosphorous than in nitrogen or potassium, i.e. one with a 1-2-1 ratio. The following are the main fertilizer ratios used by gardeners: Rooting: 1-2-1. Flowering and fruiting: 1-1-2, 1-2-2, 2-1-2.
What is the best NPK fertilizer for all plants? ›
NPK 15:15:15 ratio represents a balanced fertilizer, with equal amounts of Nitrogen, Phosphorus, and Potassium. This type of fertilizer is often a good choice for general gardening purposes where a variety of plants are involved.
What is the most commonly used fertilizer? ›
Urea is the world's most common and widely used nitrogen fertilizer in the agricultural sector for improved plant growth and yield.
What happens to plants if there is too much fertilizer? ›
Too much fertilizer (applied too often or too much) can burn or desiccate roots, ultimately killing the plant. Too much fertilizer can also be harmful to the environment. But, fertilizers do a lot of good by providing macro and micronutrients that enhance the health and performance of plants.
In the gardening world, nitrogen, phosphorous, and potassium (N-P-K) are known as primary or macronutrients, because they are required in larger quantities than other elements for plant growth and survival. As a result, these nutrients are usually depleted from the growing media first.
How to understand fertilizer numbers? ›
It's important to note that the Nitrogen, Phosphorus, and Potassium numbers on your fertilizer are percentages. For instance, a bag labeled with “23-3-8” will contain 23% Nitrogen, 3% phosphorus, and 8% Potassium. Together, these percentages show the fertilizer's “N-P-K ratio”.
How many times should I use NPK fertilizer? ›
As a general guideline, apply NPK 19:19:19 fertilizer every 2 to 3 weeks during the growing season. However, it's important to follow the instructions provided on the fertilizer packaging, as different brands may have specific recommendations.
What's the difference between 10-10-10 and 20-20-20 fertilizer? ›
A 20-20-20 fertilizer has 10% more nutrients that the 10-10-10 so it's better for plants that are already established.
What is the N-P-K of Miracle-Gro? ›
Miracle-Gro 3000992 Dry Plant Food, 8 oz Box, Solid, 24-8-16 N-P-K Ratio.
What can you use 10-10-10 fertilizer on? ›
10-10-10 is called an all-purpose fertilizer because it can be used on almost any type of outdoor plant, including vegetables, shrubs, trees, flowers, lawns, and houseplants. Generally, it can be applied to any plant that doesn't need one of the three macronutrients in higher amounts.
What does NPK 20-20-20 mean? ›
For example, a 20-20-20 blend means that the fertilizer contains 20% nitrogen, 20% phosphorus, and 20% potassium. Another common blend is 16-20-18, which refers to 16% nitrogen, 20% phosphorus, and 18% potassium.
What does 10-10-10 mean in fertilizer? ›
The numbers 10-10-10 represent the amounts of Nitrogen (N), Phosphorus(P) and Potassium(K) - or as they're commonly known, NPK. That means there's about a full ten percent each for all three nutrients which are considered primary ingredients needed by plants to survive!
Can NPK be too high? ›
Phosphorus and potassium don't move readily in the soil while nitrogen does, so every year that you add 10-10-10 in the appropriate amount for your plants needs for nitrogen you're adding too much phosphorus and potassium.
What is 19 19 19 fertilizer used for? ›
The use of 19:19:19 fertilizer aids in addressing nutrient deficiencies, enhancing biomass production, and promoting healthy plant growth. Its balanced nutrient content provides crops with the necessary elements to thrive and increase yield.
