What Do Fertilizer Numbers Mean (NPK Ratio Explained)


If you are just getting started with gardening, the labels on a bag of fertilizer can be confusing.  The three numbers on a fertilizer label, such as 10-10-10, are important to understand before using the fertilizer in your garden.

So, what do fertilizer numbers mean?  The three numbers on a bag of fertilizer, separated by dashes, represent what is called an N-P-K ratio.  Each number represents the percentage by weight of the corresponding nutrient contained in the fertilizer.  For example: a bag of fertilizer labelled “15-10-5” would contain 15% nitrogen (N) by weight, 10% phosphorus (P) by weight, and 5% potassium (K) by weight.

Of course, just knowing what these three numbers mean is not enough to make a good decision about the use of fertilizer.  You should also understand why each of the three nutrients is important.  You should also be aware of the factors that determine what type of fertilizer to use, and how much to use.  Otherwise, you can damage or kill your plants by over fertilizing.

What Do Fertilizer Numbers Mean? (NPK Ratio Explained)

Remember that nitrogen, phosphorus, and potassium (or NPK) are not the only nutrients that plants need for survival and growth.  However, out of all the nutrients, plants will use more NPK (“the big three”, as I like to call them) than any other nutrients.

You might still be wondering what “percentage by weight” means.  Let’s look at an example to clear things up.

Let’s consider the same bag of fertilizer from the example above: it is labelled 15-10-5.  Let’s say that the fertilizer weighs 40 pounds total.

Since the bag is 15% nitrogen by weight, it contains 15% x 40 pounds = 6 pounds of nitrogen.  Similarly, the bag contains 10% x 40 = 4 pounds of phosphorus, and 5% x 40 = 2 pounds of potassium.

In total, the 40 pound bag of fertilizer contains 6 + 4 + 2 = 12 pounds of NPK nutrients.  The other 28 pounds is mostly filler.

Why Do Plants Need Nitrogen?

Having plenty of nitrogen in your soil will encourage green growth on your plants.  This means that the stems, stalks, branches, and leaves will grow larger.

In particular, a larger surface area on the leaves of a plant means more sunlight can be absorbed and converted into energy via photosynthesis.

corn stalk
Plants will produce lots of green growth only if given enough nitrogen.

In addition to using fertilizer that contains nitrogen, you can also use compost or manure to add nitrogen to your soil.  For more information, you can check out my article on how to make compost and my article on how to find manure.

Plants that are deficient in nitrogen will have yellow leaves, starting at the bottom of the plant.  The plant will also grow more slowly than other healthy plants.

iron chlorosis
Plant leaves may turn yellow due to nitrogen deficiency.

For more information, check out my article on nitrogen deficiency and how to treat it.

Why Do Plants Need Phosphorus?

Phosphorus helps plants to produce roots, flowers, fruit, and seeds.  Along with potassium, phosphorus also keeps plants healthy and free of disease.  A lack of phosphorus will cause slow growth and reduced yield in your garden plants.

For more information, check out this article about phosphorus from the Iowa State University Extension.

If you have a lack of phosphorus in your soil (determined by a soil test), a fertilizer high in phosphorus (the second number listed on a fertilizer package) can supplement this nutrient.  Compost or manure can also add some phosphorus to your soil.

If you need to add phosphorus without adding nitrogen, check out my article on low-nitrogen fertilizers.

Why Do Plants Need Potassium?

Potassium allows plants to move water, sugar, and other nutrients through their tissues.  Potassium also helps plants to grow stronger stems.

Along with phosphorus, potassium also keeps plants healthy and free of disease.  For more information, check out this article on potassium from the University of Minnesota Extension.

A lack of potassium will cause slow growth and reduced yield in your garden plants.  For more information, check out my article on potassium deficiency and how to treat it.

Should I Use Fertilizer In My Vegetable Garden?

As a first step, I always recommend using compost and manure to improve the health of your soil.  In addition to providing nutrients, compost and manure add organic material to the soil.

This organic material helps to prevent soil from getting too wet or too dry. For more information, check out my article on how to make your own compost and my article on where to get manure.

compost bin
Compost is a good first step for keeping your soil healthy. Fertilizer should be used as a follow-up to address any remaining nutrient deficiencies.

Before you apply any fertilizer to your garden, you should get a soil test first.  A soil test will tell you whether you have a nutrient deficiency in your soil, and if so, how severe it is.  The results of a soil test will also indicate what type of fertilizer to add, and how much to use.

You can do your own soil test by buying a kit online or from a local garden center.  However, you will get more detailed and accurate results if you send a soil sample to be tested at a lab.

Your local agricultural extension will have a soil testing lab, and they will provide recommendations for soil treatment if you tell them what crops you are growing.  To find a soil testing lab near you, check out this map from the USDA, which will show you the agricultural extension offices near you.

If you do choose to use fertilizer, one important thing to remember is to water adequately after fertilizing.  Don’t apply it during a drought – instead, apply it before it rains, or water it in yourself.

Ideally, you will fertilize the soil before planting anything in your garden.  Otherwise, you may burn your plants (more on this later).

What Is The Best NPK Ratio For Vegetables?

I hate to give the cop-out answer, but: it depends.  The best NPK ratio for vegetables (or fruit, or anything you are growing) depends on the plant, and also on your soil.

For example, let’s say your soil is a bit low in nitrogen, and you want to grow lettuce, which requires lots of nitrogen.  In that case, you will want to use a fertilizer that is high in nitrogen – that is, the first of the three numbers (the “N” in “NPK) should be high.

green lettuce
Lettuce needs lots of nitrogen to grow all of that beautiful green foliage!

You can also add manure to your soil, since manure contains high levels of nitrogen.  For instance, chicken manure contains about 1.5% nitrogen by weight, according to Wikipedia.

It is important to remember that more is not always better with fertilizer.  If you use too much of a fertilizer that is high in nitrogen, it can cause excessive green growth at the expense of flower and fruit development.

This would mean that you would get very tall, green, leafy tomato or pepper plants with not much fruit at all! For more information, check out my article explaining NPK ratios in fertilizers.

Before adding fertilizer to your garden, always get a soil test to determine where your nutrient levels stand.  For more information, check out my article on how to do a soil test.

How Much Fertilizer Should I Use In My Garden?

Once you decide on which type of fertilizer to use, you will need to calculate the area you want to fertilize.  You will also need to know the application rate (your local agricultural extension can help you with this).  The best way to illustrate is with an example.

First, let’s assume that your local agricultural extension recommends that you add 1 pound of nitrogen per 1000 square feet in your garden.  Also, let’s also assume that you are using a 10-10-10 fertilizer for your garden.  Finally, let’s assume that you measure the dimensions of your garden at 50 feet long by 30 feet wide.

First, we calculate the area of your garden, which is the length times the width for a standard rectangular garden.  In this case, the area is 50 feet x 30 feet = 1500 square feet.

Next, we want to calculate the weight of nitrogen you will need.  Since you need 1 pound of nitrogen for 1000 square feet, you will need 1500 / 1000 = 1.5 pounds of nitrogen for your garden.

Finally, we want to calculate the amount of fertilizer to use.  Since the fertilizer we chose is 10-10-10, that means it is 10% nitrogen by weight.  Since 10 pounds of fertilizer would give us 1 pound of nitrogen (10% of 10 pounds is 1 pound of nitrogen), we will need 15 pounds of fertilizer (10% of 15 pounds is 1.5 pounds of nitrogen).

So, we need to spread 15 pounds of fertilizer over the entire 1500 square feet of garden.  If you want a shortcut formula, here it is:

amount of fertilizer = A / 10N

where A is the area of your garden in square feet (A = length x width) and N is the first number on the fertilizer label (for a 15-10-5 fertilizer, N is 15).

To give another brief example using the formula: let’s say your garden is 30 feet long by 20 feet wide, so the area is 600 square feet.  Let’s say you are using 19-19-19 fertilizer, so N is 19.  Then the correct amount of fertilizer is A / 10N = 600 / 10*19 = 600 / 190, or about 3.16 pounds.

Can You Over Fertilize Your Garden?

Yes – you can certainly over fertilize your garden.  If you do, your plants will end up with fertilizer burn.  This can cause the leaves to turn yellow or brown, and the plant may eventually die.

fertilizer burn on leaf
These blueberry leaves have fertilizer burn, caused by an excessive amount of nitrogen.

Fertilizer burn, or over fertilization of plants, is more likely if you use a fertilizer that has a high concentration of nutrients (in the form of salts). For more information, check out my article on over fertilizing plants.

Conclusion

By now, you have a better idea of what the three numbers on a bag of fertilizer are telling you about the nutrient content.  You also know what factors to consider when deciding which fertilizers to use, and how much to use.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions or advice of your own about fertilizer, please leave a comment below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

Recent Content