Fertilizer Numbers for Lawn

Fertilizer Numbers for Lawn Grass (explained)

So, you’re out shopping for your lawn again, and you notice three numbers on each bag of fertilizer you see on the shelf. In today’s article I’ll unpack what these numbers tell you about each fertilizer product, how each of these three macronutrients can benefit your lawn, seasonal considerations, and much more to choose the best lawn fertilizer for your property … by the numbers. Keep reading to read my guide to fertilizer numbers for lawn grass – what they mean, and how to choose the right fertilizer for your yard to thrive.

Many lawn care newbies won’t know what the numbers on fertilizer bags stand for. Even some seasoned lawn owners may understand the N-P-K ratio, but aren’t sure what the actual number values tell you about how much of each macronutrient is in the product … just which one is the highest.

In this article, I’ll cover everything you need to know about fertilizer numbers for lawn grass.

You’ll learn about what they stand for and how to choose the right fertilizers based on seasons, and why performing a lab-based lawn soil test can transform your lawn by telling you exactly what you need to put down in your yard to unlock its full potential.

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.

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Lawn Fertilizers and their Numbers

Numbers on Fertilizer Bag
On this bag of starter fertilizer, the N-P-K ratio is 12-18-8

Most fertilizer bags have three numbers indicated on the front of the packaging, separated by a dash sign (-).

Most of these bags don’t explain what the numbers stand for, as the manufacturers assume that homeowners already know.

The numbers are a fertilizer buyer’s guide and list information about nutrient provision for your lawn.

Knowing what these numbers indicate and why they’re on the fertilizer bags are key to making your lawn as healthy as possible.

You need the fertilizer that meets your lawn’s specific needs, and your grass needs different nutrients at different times of the year and in different growing conditions to look its best.

So how do you decipher this code and pick the right fertilizer for your lawn?

Here’s what you need to know:

What Do the Numbers on Lawn Fertilizer Mean?

The numbers on a bag of lawn fertilizer show the “fertilizer grade,” also called an “N-P-K” ratio. It represents the nutrient content in each bag of fertilizers, breaking down the amount of:

  • Nitrogen (N)
  • Phosphorus (P)
  • Potassium (K)

The numbers are represented by their symbols on the periodic table of elements, which is why potassium is represented by “K.”

Lawn Fertilizers and Their Numbers

Nitrogen, phosphorus, and potassium are your lawn’s three essential nutrients. They aren’t the only things your lawn needs to perform well, but they are the main ones, the same way protein, fat, and carbohydrates are the macronutrients in our diet.

Different fertilizer bags will contain different balances of each nutrient, the same way different foods we choose on a menu will have different blends of macronutrients.

The varying requirements of your lawn are the reason different numbers are indicated on fertilizer bags.

As the seasons progress, the nutrient requirements of your lawn slowly begin to change. Your lawn might need more of a particular nutrient at some times of year to remain strong and healthy.

What are N-P-K Ratios?

The N-P-K ratio refers to the three numbers written on a fertilizer bag. The ratio is simply the percentages of each nutrient contained in said bag.

For example, a bag labeled 21-3-20 means that the bag contents contain 21% nitrogen, 3% phosphorus, and 20% potassium. These numbers come together to form the fertilizer’s N-P-K ratio.

The Role Each Nutrient plays in Your Lawn

Now let’s talk about each of the three primary nutrients in lawn fertilizer and what they do for grass lawns.

Explaining Nutrients in Fertilizer and How They Benefit Your Lawn Grass

Nitrogen

The first number in the N-P-K ratio always represents nitrogen. This nutrient is responsible for the lush, thick, and green appearance of your grass.

Nitrogen plays a massive role in the creation of chlorophyll, which is the green pigment in grass.

This pigment helps plants photosynthesize and is the compound plants combine with sunlight to produce sugars from water and carbon dioxide.

Nitrogen stimulates leaf growth and helps promote new root growth. High nitrogen fertilizers give the lawn a boost and help turf recover from environmental injury and strain.

Although plants require nitrogen the most, overapplication can lead to excessive top growth, which can tax your lawn grass.

Nitrogen is the nutrient you want to focus on when your grass is actively growing (spring and fall in northern climates, late spring and summer in southern climates).

Phosphorus

The second number on the N-P-K ratio represents phosphorus. This nutrient is essential in the development and strengthening of a healthy root system in turfgrass.

Phosphorus for Lawn Fertilization

While nitrogen alone can create a lush green grass, the lawn won’t be able to withstand drought or irregular watering. That’s where phosphorus comes in.

Phosphorous makes strong roots that keep your lawn healthy throughout the different seasons.

It’s an essential nutrient that forms part of the metabolic processes that ensure energy transmission through the plant.

You must introduce phosphorus at the initial stages of turfgrass growth. It should also be reapplied as the grass progresses in development.

An abundance of phosphorus in the soil allows plants to grow more efficiently.

Starter fertilizers for grass that help newly seeded and sodded lawns quickly establish themselves and become resilient to environmental stress contain high levels of Phosphorus. It’s important to note that some states and communities ban or restrict the use of lawn fertilizers with phosphorus, as it can harm local waterways through runoff into storm drains and streams.

Potassium

Potassium is the final number indicated on the N-P-K ratio.

How to Choose a Good Lawn Fertilizer

It is a vital nutrient that improves the overall health of plants because it supports internal plant cell processes such as:

  • Respiration
  • Photosynthesis
  • Protein production
  • Water absorption

Potassium deficiencies result in weak grass, increasing vulnerability to diseases, drought, temperature fluctuation injury, and so much more.

The addition of potassium enables the grass to withstand disease and stress.

It also helps to maintain plant cell pressure, leading to positive outcomes through the development of drought tolerance, disease resistance, and cold hardiness.

You’ll want to use lawn fertilizer with a high Potassium (potash) content during periods of stress. Fertilizing with high-potassium fertilizer at the end of spring and as you enter the heat and drought of summer can help it perform during that difficult season. Fall lawn fertilizers also should have elevated levels of Potassium to prepare lawn for frost and freezing temperatures as it enters dormancy.

Different Seasons and their Varying Fertilizer Ratio Requirements

The ideal N-P-K ratio for lawns changes seasonally.

The number combination you should look for on a bag of fertilizer changes throughout the year. These changes are based on turfgrass requirements for the weather you’re experiencing (and expect to experience in the weeks ahead).

Spring and Summer Lawn Fertilization

Here is a brief guide to fertilizer requirements by season.

The Best Spring Fertilizer Options

In early spring I like to use a balanced fertilizer like the Andersons PGF balanced 10-10-10 with Iron and Micronutrients (take 10% off your order with code LAWNCHICK).

This type of fertilizer sets your lawn up for success for the whole season. It ensures your lawn has a balance of macro and micronutrients available as it greens up and comes out of dormancy.

You’re guaranteed there are sufficient nutrients for every part of your lawn’s needs with a balanced fertilizer like this.

Another solid option is to use a more traditional, slow-release fertilizer. Milorganite (available on Amazon, but I get the best price online at Ace Hardware) is a good choice. It will feed your soil gradually throughout the spring.

The third option is a weed and feed product with pre-emergent like Jonathan Green’s Green-Up Lawn Food with Crabgrass Preventer. This will help you suppress annual weeds while giving a nice big helping of Nitrogen (with a little potash) to help your lawn come to life in the spring.

It’s also phosphorus-free, so it will work for anyone, even if you have local restrictions about phosphorus fertilizer.

Late Spring and Early Summer Fertilizer Options

You should purchase a fertilizer that is high in nitrogen and phosphorus to prepare your lawn for the hot summer months.

However, you should apply it in late spring, not the summer. Applying a high nitrogen fertilizer in the summer can burn your lawn.

DIY Lawn Fertilization Guide

Nitrogen is vital for the stimulation of new root growth. Application and reapplication of nitrogen in the spring will help to fortify turf. It will ensure that your lawn withstands environmental problems such as insects and heat.

If you’re beginning your lawn from seed, especially in the spring, focus on the middle number of the NPK ratio. Fertilizer high in phosphorus will help support the robust root systems that the nitrogen facilitates.

This nutrient will help ensure that the seedlings properly establish themselves.

My Top Fertilizer Picks for Late Spring / Early Summer

Fall and Winter

In the fall and winter, your lawn has a higher potassium requirement.

This nutrient helps your lawn remain nourished and fuels plant health through the milder months of fall. It also boosts the ability of your grass to stay healthy and strong through harsh frosts and the cold winter months.

Potassium for Lawn Fertilization

You want slow-release formulations to keep your turf nourished for the long haul in the winter. This will gradually supply the lawn nutrients without burning its roots.

I have an in-depth guide to buying fall lawn fertilizer here. But my go-to (and what I use on my own lawn) is Jonathan Green’s Winter Survival Lawn Food with a 10-0-20 NPK ratio.

It’s the best fertilizer to use in the fall without question.

Final Thoughts About Fertilizer Numbers for Your Lawn

Fertilizing is an essential aspect of lawn care, and maintaining a healthy, resilient lawn you and your family can enjoy.

The nutrients you provide impact the turf’s ability to recover from stress, improves grass color, and ensures that your lawn stays thick, healthy, and protected from disease and weed invasions.

The information I’ve provided here should help guide you toward choosing the right type of fertilizer for your lawn at different times of year. But I definitely recommend a soil test in the spring to see if your pH levels need correcting. It will help you to understand if your lawn is deficient in any key macro or micro nutrients.

If you’d like additional guidance about what to do when for a show-stopping lawn, grab my free cheat-sheet right here. In it you’ll get a lawn care schedule for the year.

And if you need to know how much fertilizer to order, use my free lawn size calculator to measure your lawn quickly and accurately using satellite imagery:

Demonstration of Measuring a Lawn Using LawnChick.com's Lawn Size Calculator

 

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Sarah’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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