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.

When my husband and I moved into our new house, the blank slate backyard became my project.

By spring, I noticed the leaves on some bushes turning yellow. You’d think it’s a nitrogen deficiency, right?

But after some serious digging (pun intended!), I discovered it was iron chlorosis (a lack of available iron) NOT a lack of nitrogen, causing the leaves to turn yellow.

That little adventure taught me a valuable lesson: always get a soil test. It saves you from playing guessing games with your plants.

And when the test results came back, I delved deep into understanding NPK. I was glad I did because this knowledge would soon come in handy.

Walking into the fertilizer aisle of my local garden center, I sighed as I eyed the sea of packages.

With so many choices, it’s hard to decide which one to buy!

If you’ve ever felt that same overwhelm, wondering how to decode those mysterious numbers and letters on fertilizer packages, you’re not alone.

So, if you’re scratching your head about NPK, stick with me. Together, we’ll unlock this gardening mystery.

Heads up: If you buy something after clicking a link in this post, I’ll earn a small commission. I only link to products I’d 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.

NPK Explained: So what do the three 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 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 NPK ratios can really make a difference in your garden. Nitrogen (N), phosphorus (P), and potassium (K) each play an important role in plant growth.

So, when selecting a fertilizer, paying attention to the NPK ratios is like choosing the right ingredients for a recipe.

It’s about giving your plants what they need when needed.

While each nutrient has a main role to play in plant health, it also contributes to various other vital functions.

And they work in tandem with other nutrients to keep our plants thriving.

Now that you understand the pivotal role of NPK in plant growth, let’s delve deeper into each nutrient, starting with nitrogen, the ‘N’ in NPK.

The N in NPK is nitrogen

Nitrogen is key for leaf development, making it essential for young plants and leafy vegetables. It helps give your plants that vibrant green color and robust growth.

  • Nitrogen helps plants make the proteins that 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, on the other hand, is all about the roots, flowers, and seeds.

It supports your plants during their flowering and fruiting stages, ensuring they develop correctly and yield the beautiful blooms or fruits we all want.

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

And then there’s potassium.

It’s not always in the limelight, but it’s working hard behind the scenes to boost your plants’ overall health and vigor, disease resistance, and metabolic processes.

It’s the silent guardian that ensures your plants are strong and resilient.

  • 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?

Wondering what fertilizer has nitrogen, phosphorus, and potassium?

  • A complete fertilizer has all three elements.
    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 N-P-K in equal amounts.
    • An example would be a 10-10-10 fertilizer.
    • A balanced fertilizer isn’t usually a good 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.
    For example, 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 2023-12-07 at 20:02

Side note: 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 2023-12-07 at 21:07

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.

According to the University of Minnesota:

  • Nitrogen is a bit of a wanderer; it moves through the soil and can even lost into the atmosphere or sneak into our groundwater.
  • If you give your plants a big nitrogen feast all at once, especially way before they’re hungry, you have a higher risk of losing a lot of it.
  • Plus, depending on your soil type, dumping all that nitrogen can lead to some not-so-great environmental issues, like water pollution.

So, spread out your nitrogen applications.[1]

Little-known fact: Your test might not have actually measured the nitrogen in your soil.

Why? Since nitrogen levels in soil tend to fluctuate, it’s challenging to get a reliable measure at any given time.[2]

So, the lab may give you a recommended application rate based on their expertise rather than your exact nitrogen levels.

How to choose the right fertilizer based on your soil test

So, you got your soil test results, and you’re scratching your head, wondering what on earth (pun totally intended!) those numbers mean.

As the University of Missouri explains, “Because it is difficult to achieve the exact amount of all recommended nutrients from the garden fertilizer blends available in the market, it is important to match the nitrogen requirement”​

Translation: focus on nitrogen when choosing a fertilizer.

So, if you’re like me and your soil test says you have plenty of phosphorus and potassium, all you need to do is add nitrogen.

  • This means you’ll want to find a fertilizer with low to no P and K.
  • Your best choices are Blood Meal or Miracle Grow Lawn Food.

But what if your soil test shows you also need phosphorus and/or potassium?

Here’s how to calculate the perfect NPK ratio based on your test results. [1]

We’ll use this example: your soil test recommends 2 lb. of nitrogen, 0.6 lb. of phosphate, and 0.5 lb. of potash.

Let’s break it down step by step.

  1. Divide the weight of each nutrient by the smallest number in the bunch. In this case, it’s 0.5 lb.
  2. Calculate the nitrogen: 2 lb. divided by .5  gives you 4.
  3. Calculate the phosphorus: .6 lb. divided by .5 equals 1.2, which rounds down to 1.
  4. Calculate the potassium: .5 lb. divided by .5 is 1.

And voila, you end up with a 4-1-1 ratio. Easy peasy, right?

Even if you can’t find a fertilizer with this exact ratio, you can get something close.

So, you’ll save money and the environment by only using as much fertilizer as your garden needs.

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

Frequently asked questions about fertilizers

What is the best NPK ratio for plants?

The best NPK ratio depends on what your plants need. Here are the common fertilizer ratios to use based on plant needs:
Rooting: 1-2-1.
Flowering and fruiting: 1-2-2.
Leafy growth: 2-1-1 or 3-1-1.

Final thoughts on fertilizers

Remember my little backyard adventure, where yellow leaves led me down a rabbit hole of research?

I never imagined it would lead down such an eye-opening path.

But it did, and it highlighted just how crucial understanding NPK is for gardening.

Our plants need nitrogen to grow lush leaves, phosphorus for robust roots and vibrant flowers, and potassium for their overall well-being.

It feels pretty cool to have that knowledge, doesn’t it?

So, the next time you’re in the fertilizer aisle, you’ll confidently select the best fertilizer for your garden, making decisions rooted in understanding, not just guesses!

Your turn…

Want to keep track of your garden’s NPK levels and what amendments you’ve added when? Download my free soil test tracking chart.

And if you have any questions or thoughts on fertilizing, please leave a comment below.

I’m here to help and would love to hear from you!

4 thoughts on “What is NPK Fertilizer? And What Does NPK Do for Plants?”

    1. Hi Maria,

      I really appreciate your positive feedback! It’s wonderful to know that you found the post informative. 😄 If you ever have any questions or thoughts about NPK or gardening in general, don’t hesitate to share them here.

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