Maybe you’ve seen 15-15-15 fertilizer while out shopping for your lawn care supplies. These three numbers refer to its NPK ratio (its ratio of nitrogen, phosphorus, and potassium). 15-15-15 fertilizer is balanced in its content, which seems good … but should you use it? Is 15-15-15 fertilizer good for lawns, or are you better off with a different type of fertilizer for your grass?
Today, I’ll reveal when a balanced 15-15-15 fertilizer is good for your lawn, and when you may want to reach for a different bag.
I’ll discuss the advantages and disadvantages that using this fertilizer ratio offers, and share my best advice about how to understand the ideal type of fertilizer to use on your yard.
Let’s get started!
This article was last updated on by Lawn Chick Owner Sarah Jameson
Article content reviewed for accuracy by Horticulturist Arthur Davidson, A.S.
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What Kind of Sod is Appropriate for 15-15-15 Fertilizer?
15-15-15 fertilizer is so versatile that it’ll work with most kinds of grass (or sod). However, it’s usually most effective with new sod that hasn’t had the chance to take root and get fully established. It can also work well for neglected lawns that haven’t been fertilized in a long time.
The equal balance of nitrogen, phosphorus, and potassium will help the roots get established when laying sod.
It’ll also help your grass stay healthy and grow well.
If your lawn is already established, however, odds are that your yard’s soil already has sufficient levels of some of these macronutrients, and may be deficient in others.
Because of this, you’re often better off with a ratio in your fertilizer that’s tailored to the needs of your soil, and that’s based more on the time of year and the current growth period of your grass.
Also, there are certain grass varieties that don’t need as much nitrogen as others.
For example, St. Augustine grass can get excessively thick if it receives too much nitrogen, which can lead to thatch build up, disease, and pest issues.
How to Feed Grass at Different Times of Year
While there are many types of lawn grass grown in the United States, most are either cool season grasses, or warm season grasses.
Where you live will probably tell you what type of grass you have in your lawn, but those in the transition zone can be guided by when their lawn grows and looks best (spring and fall for cool-season grasses, summer for warm season grasses).
While there’s some nuance to what your grass needs and when, in general:
- At Planting – lawns need extra Phosphorus which promotes healthy root development.
- In Periods of Growth – your lawn needs extra Nitrogen to fuel leaf growth.
- In Periods of Transition and Stress – your lawn needs extra Potassium which fuels overall plant health and resilience.
I recommend choosing a good lawn soil test kit and performing a test so you understand the best
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 are the Benefits of 15-15-15 Fertilizer?
Here are some of the benefits you can expect to see when you use 15-15-15 fertilizer on your lawn:
- A beautiful green color and strong grass blades
- The lawn will be able to get an accurate nutrient uptake
- The whole plant will receive nutrients supporting strong root development and health
Now let’s take a closer look at how the different macro-nutrients in 15-15-15 fertilizer benefits lawns.
Nitrogen (the first number on the bag)
Nitrogen is essential for proper lawn growth and developing lovely green leaves.
Have you noticed any yellowish grass on your lawn? It’s probably because of a nitrogen deficiency.
Another potential sign of nitrogen deficiency is lower-than-average lawn growth during the times of year when your grass type should be pushing as much growth as possible.
Nitrogen is key to grass development above the soil, especially making leaves ready for the process of photosynthesis.
It’s the most important nutrient for your lawn during growth periods (spring and fall for cool-season lawns / summer for warm-season lawns).
Expert Explanation of Why Nitrogen is Essential for Your Lawn
Lakesh K. Sharma and colleagues at the University of Florida, IFAS Extension explains that “Nitrogen is a structural part of chlorophyll that is responsible for photosynthesis.” Furthermore, “Nitrogen is…responsible for stimulating rapid, vigorous growth, and increasing seed and fruit yield.”
Phosphorus (the second number on the bag)
Phosphorus in fertilizer is essential for stimulating grass root growth and helping seed development. The best starter fertilizers on the market will have phosphorus as the highest number in their N-P-K ratio.
Phosphorus deficiency causes growth problems. Your grass may seem weaker or less vigorous.
But phosphorus takes a while to get used up by your lawn, so you don’t want to apply it to heavily.
There are also restrictions in many states about using fertilizers with phosphorus due to its harmful impact on local waterways.
Some states ban its use altogether, and others restrict its use to seeding and sodding projects and restrict its use in every other application.
Potassium (the third number on the bag)
Potassium plays a protective role within your grass. It makes the lawn better able to deal with different kinds of weather, stress from drought, and extreme temperature change.
That’s why my recommendations for the best fall lawn fertilizer contain extra potash – it helps prepare your lawn for frost and freezing temperatures.
Your grass also needs potassium to help ward off disease and develop stronger cell walls, which is why it’s good to make sure you’re including a fertilizer with potassium during the periods of heat and drought that test lawns every summer.
Other Considerations When Choosing a Fertilizer
There’s more to consider when choosing a fertilizer. Here are a few things to keep in mind:
Be Mindful of the Time of Year, Weather, and Lawn Size
High nitrogen content in fertilizer can cause serious issues in areas with severely hot summers. You can even end up scorching your lawn.
The salt frequently found in fertilizers can also burn grass when applied in extreme heat, or when you lay down too much fertilizer (or fertilize with too much frequency).
Use my lawn size calculator to ensure you have an accurate measurement of your lawn’s square footage. Many homeowners estimate their lawn size, and do so inaccurately, meaning they over-apply or under-apply fertilizer.
Test Your Soil and Feed It What it Needs
As I mentioned previously, a soil test of your lawn will tell you what nutrients it needs most, and will also reveal your soil’s pH level. But you don’t have to take my word for it:
What the Experts Say about the Importance of Soil Testing
Soil testing is essential because you don’t want to add more of the nutrients that are already present in the right amounts. Plus, it’s important to be aware of exactly what your soil requires.
As Cale A. Bigelow et al. in the Purdue Agronomy Department explain, “You should only apply the nutrients your lawn needs.” They continue, “Don’t guess the nutrient needs of your soil: you should test your soil to determine what nutrients it needs.”
If you have sandy or coarse-textured soil, Bigelow recommends testing it once each year. On the other hand, if you have a heavy soil with high clay content or fine-textured soil, you can do a soil test once every 2 or 3 years.
As well as the nutrient profile of the soil, the test will also tell you its pH. As we learn from the Purdue Agronomy team, “If the pH is too high or too low, it will limit nutrient availability and turf growth, and will reduce turf health.” That is why he says that “Poor soil pH must be corrected.”
My Take, and What I’ve Learned from Experience
Soil pH is actually super important, because many homeowners have soil that is either too acidic or too alkaline, both of which prevent grass from proper utilizing nutrients.
This means that your grass may not even be able to utilize the nutrients you’re spending money on and spreading in your yard.
I use the MySoil test kit (available on Amazon or directly from the Manufacturer), and love the data I get back which is tailored to homeowners, but your local extension office may offer one as well.
I recommend taking samples from different parts of your property to get the most accurate sense of your lawn’s needs.
Those pine trees in the back yard drop needles which make the soil there more acidic than what you have in your front yard under that maple tree … so it’s worth investing in a couple of kits to equip yourself with the right knowledge to achieve the results you’re after.
When you do a soil test, you get a report telling you what your soil needs the most to be healthier. If you find out that your soil has a deficiency, you should select a fertilizer that has a high content of that specific nutrient.
Your soil test should check for not only the three macro-nutrients but also micronutrients. One of the most well-known micronutrient is iron.
If your soil is deficient in iron, it can lead to major problems with your lawn, including increased risk of lawn disease. Spread iron on your lawn, and you’ll see your grass turn a rich dark green color.
Why Beginners Should Choose Slow-Release Fertilizers
My recommendation for many homeowners (and particularly those new to the lawn care game) is to choose a slow-release fertilizer.
Synthetic, quick-release fertilizers may produce dazzling results in days. But they are also easy to mis-apply and burn or even kill your grass.
A slow-release fertilizer will feed your lawn steadily and keep it looking great for months. They’re also pretty goof-proof. It’s difficult to burn your grass, even if you make a mistake and don’t nail your application.
Another benefit of slow-release fertilizers is that they don’t need to be applied quite as frequently. This saves you time and money.
Slow-release fertilizers will take longer to get results, but those results are sustained and something you’ll be proud of.
These products tend to be natural or organic fertilizers, which are also better for your lawn’s health. They feed the soil rather than simply feeding the grass.
Many of the most heavily marketed synthetic fertilizers are a bit like candy for your grass. They give it a burst of energy, only to make it crash later.
15-15-15 Fertilizer Might be a Great Choice for Your Lawn
As we’ve seen here, 15-15-15 can be a fantastic fertilizer ratio for many lawns. But it isn’t necessarily the right choice for your lawn.
I recommend, testing your soil to find out its levels of macronutrients and micronutrients. You need a fertilizer that addresses your lawn’s specific needs.
What I Use and Recommend
What I personally use when I reach for a balanced fertilizer is The Andersons PGF Balanced 10-10-10 Fertilizer with Iron and Micronutrients.
It is ideal for neglected lawns, feeding new lawns, and for healthy lawns needing a quick green-up at the start of the season.
It is a quick-release fertilizer, but its complete formulation which includes extra iron has all of the micronutrients grass needs to thrive.
This makes it a great choice to correct deficiencies and get your lawn on track (and dark green) in a hurry.
I like to use it in the spring to jump-start my lawn.
You can purchase it online from The Andersons website, and it’s also available on Amazon.
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
Additional Resources
- Understanding Nitrogen in Soils by Fabian G. Fernandez, University of Minnesota Extension (link)
- Plant Essential Nutrients and their Role by Lakesh K. Sharma, J. Mabry McCray, and Kelly Morgan, University of Florida Extension, (link)
- Fertilizing Established Cool-Sesaon Lawns: Maximizing Turf health with Environmentally Responsible Programs by Cale A. Bigelow, Jim J. Camberato, and Aaron J. Patton, Purdue Extension (link)
