What do the Three Numbers on Fertilizer Mean

What do the Three Numbers on Fertilizer Mean?

When you buy fertilizer, you’ll notice that the label has three bold numbers printed prominently on the bag. But what do the three numbers on fertilizer mean? How do you know which numbers are best for your lawn?

The ratio of these elements in a fertilizer is called the N-P-K ratio, and in today’s article I’ll tell you everything you need to know about these numbers, and how different types of ratio are best for different times of year and growth stages of your lawn.

Let’s get started!

Trust and Accuracy Information

This article was last updated on by Lawn Chick Owner Sarah Jameson
Article content reviewed for accuracy by Horticulturist Arthur Davidson, A.S.

So, What do the 3 Numbers on Fertilizer Mean?

The three numbers on fertilizer bags stand for N – P – K and signify the ratio of Nitrogen (N), Phosphorus (P), and Potassium (K) contained in the fertilizer.

Each one of these macro-nutrients is used by plants (including grass) in different ways, so the ratio of these three fertilizer numbers can tell you a lot about what type of fertilizer is in the bag, and help you choose the best fertilizer for you.

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Digging Deeper

While these three macro nutrients are the primary nutrients your lawn needs to be healthy, your lawn needs micro nutrients too.


Most of these are already in your soil, but if you want to know exactly what your lawn needs to thrive (and what it doesn’t, so you don’t have to waste your money and time buying and spreading unnecessary products), do a soil test.

I love this one from Amazon and use it every single spring. I take soil samples from around my yard, mix them together, send them off to a lab and can view results in an easy-to-use online dashboard that shows me what (if anything) my lawn is lacking.

It’s the best money I spend each spring, and before you rush off to the garden center and buy hundreds of dollars worth of fertilizer, check to see what your lawn actually needs.

You’ll thank me.

Now let’s take a closer look at what each of these macro nutrients is, what it does, and how your lawn will benefit and utilize each one.

What the Three Numbers on Fertilizer Bags Do for Your Lawn

I’ll go into each of these macro-nutrients in a more in-depth way below, but very briefly this is how Nitrogen, Phosphorous, and Potassium benefit your lawn grass:

How do N-P-K, the three numbers on fertilizer, work for lawn grass and other plants?
  • Nitrogen is the nutrient plants need for leaf growth. By choosing a fertilizer with high levels of nitrogen you’ll help your grass grow a thicker, healthier canopy.
  • Phosphorous helps to generate new plant tissue, and in grass it’s especially good for promoting root growth. This is a vital macro-nutrient if you’re laying sod or planting grass seed as it will help your lawn develop a robust root system that will sustain it through periods of drought.
  • Potassium is the macro-nutrient that will make (and keep) your grass healthy and resistant to diseases. It triggers specific plant enzymes and regulates the uptake of carbon dioxide through leaf pores.

You can see how each macro-nutrient will benefit your grass in different ways, and different ratios will be best for different lawns in different seasons.

Now let’s take a closer look at each one of these.

Why is Nitrogen Important for Your Lawn?

Nitrogen is part of the chlorophyll molecule. This means that it’s necessary for the best possible leaf growth.

What Does Nitrogen Do for Grass - How Nitrogen Benefits Lawn Grass

When you use a high-nitrogen fertilizer (for example a 10-5-5 NPK), you’ll get the benefit of more green growth.

This sometimes leads to less flower production, so keep this in mind before applying a heavy nitrogen fertilizer to a flower or vegetable bed.

You shouldn’t apply a fertilizer rich in nitrogen when your lawn is in the dormant stage. If you do apply this during dormancy, you could find that the weeds in your lawn might be stimulated to grow and spread.

Some of the sources of nitrogen in fertilizer include:

  • Ammonium nitrate
  • Urea nitrogen
  • Sodium nitrate
  • Liquid nitrogen

Why do You Need Phosphate for Your Lawn?

Phosphorus is a necessary nutrient for cell division, as well as for generation of new plant tissue.

It helps to establish strong root growth and is especially useful for the growth of flowers and fruits.

How Does Phosphorous Benefit Grass in Lawns

Even though phosphate isn’t as important to lawns as it is to plants that grow flowers or fruits, there should still be some phosphate in the fertilizer you choose to keep your lawn healthy.

And when you start a lawn from seed or buy sod, you’ll want to use a fertilizer that stimulates root growth to help your new lawn survive tough conditions.

Why is Potash Necessary for Your Lawn?

Potash is rich in potassium. That is why it is generally used as the source of potassium in many fertilizers.

Why Lawns Need Potash / Potassium

Potassium is necessary for specific plant enzymes to be triggered. It also helps to regulate the carbon dioxide uptake of a plant by regulating the pores on the surface of a leaf. These pores are called stomata, and gasses pass through them.

You need to apply potassium to your lawn in order to make it as healthy as it can be.

Don’t Forget About Micronutrients like Iron As Well

You need to do a soil test to find out if your soil is deficient in any nutrients, such as iron. Allison Koto, MSU Broadwater County Extension Agent, explains that “alkaline (high pH) soils” often have inadequate iron levels that need to be supplemented. 

Allison points out that “conventional fertilizers generally only contain a small amount of iron.” That means that if your lawn has a significant iron deficiency, you may need to get “a fertilizer such as ferrous iron or iron chelate that will add enough to your soil to make a difference.”

Why You Need to do a Soil Test

As I previously mentioned, you should do a soil test on your lawn every spring. This will give you a baseline for your soil and make it easier for you to understand what your lawn does and doesn’t need.

Get a soil test report in order to find out the characteristics of your soil and the best kind of fertilizer for you.

The best tests (like this one I use every year) will provide a recommended fertilizer grade with specific numbers included in the report. Check this carefully to make sure that you look for the correct fertilizer when you go shopping.

There should also be a management note that gives you information on supplementing nitrogen for your lawn.

In general, the most common grades of fertilizer recommended for lawns include numbers:

  • 5-10-10
  • 5-10-5
  • 10-10-10
  • 8-0-24
  • 6-6-18

Best Overall Lawn Soil Test Kit

The Soil Test Kit I Use & Recommend

There are many options for testing your lawn’s soil, but I prefer a lab-based soil test that provides a detailed analysis of your soil’s nutrients and what’s needed for your lawn to thrive.

I use this one from MySoil every year.

And if you’re interested in taking the guesswork out of what to do next after you get your soil test results, consider Sunday’s subscription lawn-care plan. They test your soil for you and use local weather data to send you exactly what your lawn needs, when it needs it. It’s pretty fool-proof – you can Click Here for Your Instant Lawn Analysis and take 15% off your order with promo code LAWNCHICK2024.

What the Experts Say

Joan Jubela, a Master Gardener at PennState Extension, reminds us how important it is to “follow instructions.” There are potential negative consequences to using too much fertilizer.

One example Joan points out is that too much nitrogen can ultimately make your lawn weaker, even “droopy or spindly,” as well as more “susceptible to pests and disease.” 

Remember the Season

The specific needs of your lawn will change with the temperature and weather. Your goal is always to establish healthy roots and keep them healthy, encourage leaf growth, and boost the disease resistance of your lawn.

Remember to apply starter fertilizer (I use this one from Scotts) during the season in which you plant your grass seed or do overseeding. A quick release fertilizer like this will give your soil the nutrients it needs for grass seed to flourish and grass root systems to establish themselves properly.

Lawns Need Different Ratios of Fertilizer Nutrients at Different Times of the Year

It’s generally recommended to use a heavy fertilizer treatment in the fall and then lightly fertilize in the early part of the spring, with an annual weed pre-emergent included.

If you have a cool-season grass, its primary growing period is in the spring and fall, so feeding your lawn at these times of year is important.

If you have a warm-season grass, this growing period is in the late part of spring and the summer.

You shouldn’t apply fertilizer to your lawn when there is a drought, or when your lawn is about to go dormant for seasonal reasons.

You can check out my annual lawn fertilization schedule for more information.

How to Spread Fertilizer

It may not seem important right away, but spreading your fertilizer correctly is important for your lawn to look beautiful.

Incorrect spreading of fertilizers can lead to a lawn with stripes in different colors – dark green where more fertilizer dropped, lighter green where coverage was light.

Spread the fertilizer evenly to ensure that the growth and color of your lawn will be uniform.

You should use a high-quality fertilizer spreader to spread your fertilizer. The most common kinds available are cyclone or broadcast spreaders and drop spreaders.

Spreading Fertilizer with a Broadcast Spreader

Broadcast spreaders are usually the most effective and you can cover more ground with them, faster. I use the Scotts Elite spreader (Amazon link), but there are lots of options.

You can drive yourself nuts reading reviews for something like this, but my advice is to look for something without a wheel cavity where seed or fertilizer can get caught and then fall out in clumps. That’s the primary reason people get poor results with these spreaders.

As you spread the fertilizer on your lawn, you must overlap the pattern of your spread. So spread half of the material by traveling north to south across the entire lawn, then spread the other half moving east to west.

A lot of people get stripes in their lawn because they only move back and forth and don’t use this technique, which allows for an even spread.

Use the right technique with the right amount of fertilizer with any decent spreader and you’ll get great results.

Look for a Natural Fertilizer

It’s best to use fertilizers that use natural ingredients instead of synthetic chemicals made in a lab.

And I’m not saying that because I’m some hippy … the difference is in how these two different types of fertilizer work.

The TLDR version of why this is true is:

  • Organic Fertilizers feed the soil.
  • Synthetic Fertilizers feed the plant.

Over time synthetic fertilizers that give you great results make your lawn an addict. Your soil is depleted so your grass will depend upon your use of these products.

Organic or Synthetic Fertilizer

I always recommend you create healthy soil (the grass will follow).

More About Organic Fertilizers

A natural fertilizer gives your lawn the nutrition it needs by feeding the living organisms in the soil.

When you put down a natural fertilizer, the microscopic organisms (including bacteria and fungi) in the soil process the fertilizer. As this happens, the fertilizer breaks down, then plants can use those nutrients.

With natural fertilizers, you will probably find that they have smaller N-P-K ratios. Examples may include 1-1-6 or 2-3-2.

The reason why natural fertilizer has smaller numbers is because of the level of nutrients immediately available to your lawn as soon as it’s applied.

While it takes soil microbes longer to process nutrients may seem like a drawback, the reality is quite different. Instead, the nutrients release more slowly over a longer time period. The natural fertilizer acts as a slow-release fertilizer, meaning you won’t have to fertilize as frequently.

You can read about my favorite organic lawn fertilizers here.

And if you have access to screened compost, top dressing your lawn with that is pretty much the best thing you can do for your yard.

Fertilizer is an Important Part of Good Lawn Care!

As we’ve seen here, fertilizing your lawn is a critical part of taking care of your property. But you have to use the right fertilizer, in the right concentration, at the right time.

What do the 3 Numbers on Fertilizer Mean?

If you want the greenest and healthiest lawn possible, keep all the information about fertilizer I’ve shared here in mind the next time you go shopping for lawn care products.

And test your soil, so you’re not buying nutrients that are already plentiful in your yard!

At Lawn Chick, I am committed to publishing accurate, useful, and trustworthy resources for my readers. As part of this commitment, I’ve invited subject matter experts to review our articles for accuracy. I invite you to read our editorial policy and publishing standards which outlines in detail how every article on this site is sourced, edited, fact-checked, and vetted.



Sarah Jameson’s blog, Lawn Chick, is read by over 2 million homeowners each year and she is regularly cited as an expert source of lawn care knowledge by major publications. Her goal is to meet you where you are, and help you achieve a yard you’ll be proud of. Ready to take the next step toward improving your lawn? Grab her free lawn care cheat-sheet: What to Do When - Take the Guesswork Out of Lawn Care, or upgrade your garage by browsing her favorite DIY lawn care products.

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