If you are just getting started with gardening, it helps to know what the label on a bag of fertilizer tells you. The three numbers on a fertilizer label, such as 10-20-10, are important to understand before using the fertilizer in your garden.
So, what do fertilizer numbers mean? Fertilizer numbers tell you the NPK ratio: N is nitrogen, P is phosphorus, and K is potassium. Each number gives the percent by weight of a nutrient in the fertilizer. For example, “15-10-5” fertilizer has 15% nitrogen (N) by weight, 10% phosphorus (P) by weight, and 5% potassium (K) by weight.
Of course, 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.
In this article, we’ll take a closer look at NPK ratios and what each of these three nutrients does for plants. We’ll also look at some examples to make the idea clear.
Let’s get started.
(You can also watch my YouTube video on this topic if you like!)
What Do Fertilizer Numbers Mean? (NPK Ratio Explained)
Fertilizer numbers tell us the percent by weight of the three nutrients nitrogen, phosphorus, and potassium in the fertilizer. These 3 numbers are printed on the package.
NPK stands for “nitrogen, phosphorus, and potassium”. To be more precise:
- N is the amount of nitrogen available in the fertilizer, as a percentage by weight..
- P is the amount of phosphate (P2O5) available in the fertilizer, as a percentage by weight.
- K is the amount of potash (K2O) available in the fertilizer, as a percentage by weight.
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.
Example 1: Percentage By Weight For 15-10-5 Fertilizer
Let’s consider a 40-pound bag of fertilizer labelled 15-10-5. With this information, we can find the amount of each major nutrient in the bag.
- Since the bag is 15% nitrogen by weight, it contains 15% x 40 pounds = (0.15)(40) = 6 pounds of nitrogen.
- Since the bag is 10% phosphorus by weight, it contains 10% x 40 pounds = (0.10)(40) = 4 pounds of phosphorus (in the form of phosphate).
- Since the bag is 5% potassium by weight, it contains 5% x 40 pounds = (0.05)(40) = 2 pounds of potassium (in the form of potash).
In total, the 40 pound bag of fertilizer contains 6 + 4 + 2 = 12 pounds of NPK nutrients. The other 28 pounds is mostly filler.
Example 2: Percentage By Weight For 20-0-0 Fertilizer
Let’s consider a 40-pound bag of fertilizer labelled 20-0-0. With this information, we can find the amount of each major nutrient in the bag.
- Since the bag is 20% nitrogen by weight, it contains 20% x 40 pounds = (0.20)(40) = 8 pounds of nitrogen.
- Since the bag is 0% phosphorus by weight, it contains 0% x 40 pounds = (0.00)(40) = 0 pounds of phosphorus (in the form of phosphate).
- Since the bag is 0% potassium by weight, it contains 0% x 40 pounds = (0.00)(40) = 0 pounds of potassium (in the form of potash).
In total, the 40 pound bag of fertilizer contains 8 + 0 + 0 = 8 pounds of NPK nutrients (in fact, 8 pounds of nitrogen). The other 32 pounds is mostly filler.
Why Do Plants Need Nutrients?
Knowing about fertilizer numbers and NPK ratios is great, but why do plants need nutrients? Well, these nutrients help plants to grow, reproduce, and resist diseases or pests.
Each of the “big three” nutrients is important for plants in its own way. Let’s start with nitrogen.
Why Do Plants Need Nitrogen?
Plants need nitrogen for several reasons. One of the most important reasons is that nitrogen is a key ingredient in chlorophyll.
Remember that chlorophyll is the green chemical that is necessary for photosynthesis. If there is no nitrogen, then there is no chlorophyll, no photosynthesis, no energy production, and no growth!
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 and stronger.
In particular, a larger surface area on the leaves of a plant means more sunlight can be absorbed and converted into energy via photosynthesis.
According to the University of Missouri Extension, nitrogen is also found in proteins in the roots and tissues of plants. This helps with water and nutrient uptake.
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. This is because nitrogen is a mobile nutrient, which means the plant can transport it from the lower leaves to the upper leaves.
A plant that lacks nitrogen will also grow slower than a healthy plant. For more information, check out my article on nitrogen deficiency and how to treat it.
One more thing to keep in mind about nitrogen: it is possible to get too much of it. In that case, a plant can be burned by over fertilizing.
If you think this could be a problem, you can learn about low nitrogen fertilizers in my article here.
Why Do Plants Need Phosphorus?
Plants need phosphorus to produce new tissue for growth. According to the University of Nebraska at Lincoln, phosphorus is used to help regulate protein production in plants.
As such, phosphorus helps plants to grow and reproduce. It is vital for the production of roots, flowers, fruit, and seeds.
For more information, check out this article about phosphorus from the Iowa State University Extension.
Along with potassium, phosphorus also keeps plants healthy and free of disease. A lack of phosphorus will cause slow and stunted growth, along with dark green or reddish-purple leaves.
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 important nutrient. Compost or manure can also add some phosphorus to your soil.
Of course, you might also want to check out my article on fertilizers that are high in phosphorus.
On the other hand, excessive phosphorus can also be harmful. According to the Texas A&M University Extension, if you add too much phosphorus to your soil, plants may be unable to absorb iron and zinc.
Too much phosphorus can also pollute water (rivers, streams, lakes, and ponds). It can also cause algae blooms in the water.
If you think this could be a problem, you can learn about low phosphorus fertilizers in my article here.
Why Do Plants Need Potassium?
Plants need potassium because it helps with movement through their tissues. According to the University of Minnesota Extension, 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.
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.
If you have a lack of potassium in your soil, a fertilizer high in potassium (the third number listed on a fertilizer package) can supplement this important nutrient. Compost or manure can also add some potassium to your soil.
You can find out more about high potassium fertilizers in my article here.
On the other hand, excessive potassium in soil can lead to nitrogen deficiency. According to the North Carolina State University Extension, it can also prevent the uptake of other nutrients.
Should I Use Fertilizer In My Vegetable Garden?
You can use fertilizer in your vegetable garden. However, 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.
Organic matter also attracts soil organisms, such as earthworms and beneficial bacteria. The best part about compost is that you can make it yourself with yard waste and kitchen scraps.
For more information, check out my article on how to make your own compost and my article on where to get manure.
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 office will have a soil testing lab.
If you tell them what crops you are growing, they will provide recommendations for soil treatment. To find a soil testing lab near you, check out this map from the USDA, which will show you the agricultural extension offices by state.
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 rainfall, 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 two things: the plant and the 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.)
You can also add manure to your soil, since manure contains a good amount 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!
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 (check the fertilizer package, or your local agricultural extension can help you with this).
The best way to illustrate is with an example.
Example: How Much Fertilizer To Use In Your Garden
First, let’s assume that you got a soil test, and 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 balanced 10-10-10 fertilizer.
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 = 100G / AN
- G is the area of your garden in square feet (A = length times width)
- A is the area to spread the fertilizer over (Example: at X pounds per 1000 square feet, A = 1000)
- N is the first number on the fertilizer label (for a 10-5-5 fertilizer, N is 10).
In this example, G = 1500, A = 1000, and N = 15. So, we get:
- Amount of Fertilizer = 100G / AN
- Amount of Fertilizer = 100(1500) / (1000)(10)
- Amount of Fertilizer = 150000 / 10000
- Amount of Fertilizer = 15 pounds
To give another brief example using the formula: let’s say your garden is 30 feet long by 20 feet wide, so the area G is 600 square feet. Let’s say you are using 19-19-19 fertilizer, so N is 19.
If the area given on the package is A =1000 square feet, then hen the correct amount of fertilizer is:
- Amount of Fertilizer = 100G / AN
- Amount of Fertilizer = 100(600) / (1000)(19)
- Amount of Fertilizer = 60000 / 19000
- Amount of Fertilizer ~ 3.16 pounds
Can You Over Fertilize Your Garden?
It is possible to 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, 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.
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.
You might also want to read my article on whether you need fertilizer for your garden (and when to use them).
I hope you found this article helpful – if so, please share it with someone who can use the information.