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What is NPK Fertilizer? And What Does NPK Do for Plants?

INSIDE: What does NPK stand for? In this comprehensive guide, find out the meaning of NPK, what it does for plants, and how to choose an NPK fertilizer.

Has this ever happened to you? You’re standing in the fertilizer aisle of your local garden center, looking at all the packages, and your eyes glaze over.

There are too many choices. And you’re not sure which one to buy.

When you pick up the first package and glance at the label, your eyebrows flick up as you notice three big numbers on it.

Then you see that ALL the labels have three numbers with dashes separating them, and some have the letters NPK, next to the numbers.

Sighing, you scratch your head and wonder “what is NPK and what are these three numbers on the fertilizer packages?”

Keep reading to find out what NPK means, why it matters, and how much to use.

This post may contain affiliate links. So, I may get a small commission if you buy something after clicking through. I only link to products I would recommend to my best friend.

woman holding npk fertilizer in her hands in the garden
NPK (nitrogen, phosphorus, and potassium) are the main ingredients in most fertilizers.

The letters NPK stand for the three major nutrients that plants need to live and grow – nitrogen, phosphorus, and potassium.

Plants need other nutrients like iron, calcium, and even chlorine, but N, P, and K are the big three.

So what do the 3 fertilizer numbers mean?

The fertilizer numbers, which are always in the same order (N-P-K), tell you how much nitrogen, phosphorus, and potassium are in the fertilizer.

They may be whole numbers or decimals and might be the same number, like 10-10-10, but often are three different numbers.

It’s even possible to have one or two of the numbers be zeros.

  • The numbers are a percentage of how much of each nutrient is in the fertilizer.
  • Since the numbers are a percentage by weight, the higher the number, the more of that nutrient there is in the fertilizer.
  • A 20-20-20 fertilizer has twice as much NPK in it as a 10-10-10 fertilizer.

And why are the numbers important?

These percentages will help you decide which fertilizer to buy and how much to apply to your garden.

And understanding what the numbers mean is one of the secrets to growing a healthy garden.

Knowing how much of each nutrient to apply is important because each one nourishes your plants in different ways.

Take the guesswork out of fertilizing and amending your soil. Get my FREE Soil Tracking chart.

Why is NPK important?

Plants need significant amounts of nitrogen, phosphorus, and potassium to grow and thrive.

Well-fed plants are healthier and more productive.

But your soil may not supply enough nutrients for optimum growth if you don’t add fertilizer.

Fertilizing adds nutrients that your soil lacks and replaces nutrients that plants took up with last year’s growth.

Those beautiful flowers and tasty tomatoes you grew last year used up some of the nutrients in your soil.

Understanding the NPK ratios of fertilizers can help you choose the best one for the type of plants you’re growing.

  • If you’re growing leafy vegetables, you’d want to use a fertilizer that has a higher nitrogen number to encourage leafy growth.
  • Or if you’re growing flowers, and your soil lacks phosphorus, a fertilizer that has a higher phosphorus number can encourage more blooms.

The N in NPK is nitrogen

Nitrogen helps plants make the proteins they need to produce new tissues – especially leafy tissues.

When plants don’t get enough nitrogen, they can turn yellow or pale green.

But on the flip side, if you give a plant too much nitrogen, it may grow tons of foliage but not produce fruit or flowers.

If you’ve ever over-fertilized a tomato plant, you know what I’m talking about – you get a lush plant that grows taller than you, but you get no (or few) tomatoes.

colorful, rainbow chard
Leafy plants like this rainbow chard need nitrogen in order to be productive.

Plants have evolved to take up as much nitrogen as possible because, in nature, nitrogen is often in short supply, so they get it while the getting is good.

But we as gardeners can outsmart mother nature by only giving our plants the nitrogen that they need.

The P in NPK is phosphorus

gardener holding a seedling with its roots exposed
Plants need phosphorus to grow a healthy root system.

Phosphorus stimulates root growth, helps plants set buds and flowers, and produce seeds.

It also helps plants use other nutrients more efficiently, and helps turn energy from the sun into usable energy for your plants.

Pro-tip: An easy way to remember that P stands for phosphorus (and not potassium) is that phosphorus has two Ps in it.

The K in NPK is potassium

Potassium promotes the overall vigor of plants.

  • It helps the plants make carbohydrates and promotes disease resistance.
  • It also helps regulate metabolic activities.
  • Plants low in potassium may be stunted and have lower yields.

What is NPK in complete, balanced, and incomplete fertilizers?

A complete fertilizer is one that has some nitrogen, phosphorus, and potassium in it. For example, a fertilizer with an NPK of 24-16-8 is a complete fertilizer, even though the three numbers aren’t the same.

A “balanced” fertilizer is one that has NPK in equal amounts. An example would be a 10-10-10 fertilizer. A balanced fertilizer isn’t the best choice since plants don’t use nutrients at the same rate, and your soil may not need phosphorus or potassium.

Incomplete fertilizers are missing one or two of the three major nutrients. A fertilizer with 13-0-0 NPK is an incomplete fertilizer that has only nitrogen in it.

So, now that you understand what NPK means, I bet you’re wondering how much fertilizer to use.

Want healthier soil? Grab this free chart to keep track of your fertilizers and amendments and start building healthier soil today!

So, how much NPK should you apply to your soil?

To get an accurate answer to that question, you’ll need to test your soil every few years to determine how much to add.

Soil test kits and tools

Prices last updated on 2023-03-18 at 19:52

When you get your test results back, you may be surprised to learn that you only need to add nitrogen. Most urban and suburban soils in the U.S. have enough phosphorus and potassium. Nitrogen breaks down faster than phosphorus or potassium, so it’s usually lacking in most soils.

And you’ll want to spread your nitrogen applications out over the growing season. Don’t apply it all at once.

If you’re growing plants in containers, you’ll want to use a balanced fertilizer with NPK in a 3:1:2 ratio.

Here are some I recommend.

Prices last updated on 2023-03-18 at 16:19

Download Your Free Soil Tracking Chart

After you get a soil test, fill out the top half of the chart. When you’re ready to amend your soil, use the rest of the sheet to track what you’ve added.

Download the soil amendment log now.

Here’s a sneak peek of your Soil Tracking Chart:

soil tracking chart pdf download

Final thoughts on NPK

Knowing the answer to the question “what is NPK?” can be the difference between growing a healthy garden or growing one that struggles.

Now when you’re shopping for fertilizer, your eyes won’t glaze over.

Instead, you’ll walk through your garden center’s fertilizer aisle with confidence and choose the best fertilizer for your garden because you know what NPK means!

Cheryl Spencer, certified gardener

Hi, I’m Cheryl Spencer, a Certified Gardener.

Born with a plant addiction that has no known cure, I became a Certified Gardener to help ease the symptoms. Now I write articles and create gardening products that help you save time and money in your garden. I believe you can grow your dream garden and still have time to enjoy it. The good news? Anyone can do it. Start here »

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