If you’ve ever picked up a bag of fertilizer, you’ve probably seen a bunch of hyphenated numbers on the packaging. Sometimes they’re all the same number, but usually they’re not. Sometimes they have zeros and decimal points, and you know they all relate to NPK, but what do they mean exactly?
For the longest time, I always assumed (and read multiple times on many supposedly trustworthy gardening sites) that the NPK on a fertilizer label stood for Nitrogen, Phosphorus, and Potassium, but that’s not entirely accurate and the truth is a lot more complicated than that.
What does NPK really mean?
All fertilizers have three numbers on the label that indicate the guaranteed analysis (a list of nutrients in the bag). The analysis lists the percentage (by weight) of each nutrient.
However, contrary to what most references claim, the P and K values do not indicate the percentage of phosphorus and potassium, respectively, in the bag.
The N value indicates the percentage of nitrogen, except the second number represents P2O5 (phosphorus expressed as available phosphate) and the third number represents K20 (potassium in the form of soluble potash).
In simple terms for the general public, fertilizer companies say that these numbers represent nitrogen (N), phosphorus (P) and potassium (K), and that these nutrients are calculated based on the amount that is immediately available to plants.
Why is this important? Because when you get a soil test that has the recommended amounts of fertilizer and you’re trying to figure out how much fertilizer to add to your garden, you need to know the actual nutrient content of the fertilizer you’re adding.
Let’s look at the example of an organic fertilizer with an NPK analysis of 3-9-4.
Reading the label, we see that the bag contains 3 percent nitrogen, 9 percent available phosphate, and 4 percent soluble potash.
For some easy math, imagine that a 100-pound bag of this fertilizer contains 3 pounds of nitrogen, 9 pounds of phosphate, and 4 pounds of potash (the remaining weight is made up of secondary macronutrients, micronutrients, and fillers that give the fertilizer bulk and make it easier to spread).
To calculate the actual P and K content, the following formulas should be used:
P2O5 contains 44 percent of the actual P (so multiply P2O5 by 0.44). K20 contains 83 percent of the actual K (so multiply K20 by 0.83).
Continuing with the 3-9-4 fertilizer example, a 100 pound bag would contain 3 pounds of nitrogen (N), 3.96 pounds of actual phosphorus (P), and 3.32 pounds of actual potassium (K).
I know, it’s not confusing at all, right?!
Why label them this way?
N is expressed on an actual elemental basis, while P and K are expressed on an oxide basis (i.e., they are found in phosphate and potassium compounds).
This misleading labeling dates back to the days when fertilizers were heated with air to convert the phosphate and potassium to their oxides (specifically P2O5 and K20). These values were used in nutrient analysis at the time and continue to be expressed in oxides for historical continuity.
The strange thing is that these compounds are not found in fertilizers, yet they are still used in North America because that is how they have always been (change is hard!).
Consider the ratio
The three numbers on a fertilizer label also represent ratios that indicate the relative proportions of N, P2O5, and K20.
Take for example an NPK ratio of 3-1-2: this fertilizer contains 1.5 times more N than K20 and 3 times more N than P2O5.
Fertilizers with an NPK analysis of 9-3-6 or 18-6-12 also have a ratio of 3-1-2. These fertilizers provide the same amount of nutrients, so it doesn’t matter which fertilizer you use; you just need to use the correct amount.
Pound for pound, 18-6-12 fertilizer contains twice as many nutrients as 9-3-6 fertilizer, so to provide the same amount of nutrients, use half the amount of 18-6-12 fertilizer as 9-3-6 fertilizer.
Whew! Are you still keeping up?
If you find three fertilizers with the same ratios at the store (e.g. 3-1-2, 9-3-6, 18-6-12), buy the one with the higher numbers. From an economic standpoint, you’ll get more bang for your buck because you’ll use less of the 18-6-12.
Why is this important for home gardeners?
So, it only matters if you get a soil test done. Using the formula above, you can calculate the appropriate amount of phosphorus and potassium to add to your garden.
Did you know?
You might be surprised to learn that most standard soil tests don’t measure nitrogen in your soil. This is because nitrogen is constantly being converted into other forms, and levels can change rapidly with weather conditions. By the time you collect a soil sample, send it off, and get the results back from the lab, the reported values are largely meaningless.
So why do soil tests sometimes give nitrogen recommendations? Because they are a best guess based on research trials and general rules for your area regarding crop yields, not the actual levels in your soil.
Soil tests don’t measure nitrogen in the soil, but a complete fertilizer (one that contains N, P and K) is usually recommended, with the recommended amount given per 1,000 square feet.
Read more: What to do if your soil has too much nitrogen
To decide how much fertilizer to apply to your garden, you need to know:
Fertilizer nutritional analysis (so you can calculate correctly) Garden area to be fertilized Recommended application rate
Although soil tests will recommend a specific NPK ratio, you might not be able to find that exact ratio when browsing the fertilizer aisle (especially if you are using organic fertilizers).
Yes, you can. Just choose a ratio that’s close enough, or use a combination of single-ingredient fertilizers (such as blood meal, bone meal, or lambenite) to provide the right amounts of the nutrients your plants need. (Here’s a guide from the University of Georgia Extension for converting inorganic fertilizer recommendations to organic fertilizer.)
RELATED: There’s a right and wrong way to fertilize your garden in the summer
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Recommendation
Single component fertilizer
This post is an updated version of an article originally published on January 16, 2023.
