When is 15-0-15 Fertilizer Best? | How to Use It

If you’re lucky enough to have a beautiful, sprawling lawn around your property, you’ll want to become familiar with 15-0-15 fertilizer. Especially if you’re growing a warm-season, perennial turf variety. If you have just a small patch of grass growing alongside your home, knowledge of this type of fertilizer can contribute to keeping it healthy and lush.

A 15-0-15 feed contains generous amounts of nitrogen and potassium but zero phosphorus. An appropriate dose of this effective combination will promote healthy blade formation and color, a strong root system, and an increased tolerance against disease and environmental stresses. But, it may not be beneficial for every lawn type.

Some turf varieties depend on phosphorus to support healthy growth. As do newly installed sod and freshly germinated grass seeds. Knowing your lawn and its needs will determine the benefits of this high-performing combo. 

Understanding 15-0-15 Fertilizer Label? 

Every fertilizer label is required by law to list its contents in detail for you. Starting with the NPK ratio, followed by any incorporated micronutrients and trace elements, then finishing up with any fillers that are included to support the distribution of the previous two groups.

N-P-K Ratio 

The NPK ratio will indicate the fertilizer’s proportions of nitrogen, phosphorus, and potassium. In this case 15-0-15 means 15% nitrogen, 0% phosphorus and 15% potassium.

With the NPK ratio making up 30% of the total fertilizer, the remaining 70% will consist of other micronutrients. Such as iron, manganese, and zinc. As well as other supportive ingredients that assist with nutrient distribution and absorption. 

But, what are all these nutrients going to do for your lawn? How do they keep it healthy? Let’s take a closer look at each NPK macronutrient. 


Nitrogen is the primary driving force behind healthy blade growth and vibrant color in lawns by stimulating chlorophyll production, an essential component of the photosynthesis process. In which sunlight is converted into food. If a low-nitrogen fertilizer is applied to your lawn, stunted growth and faded color may result. 


In cool-season turfs like Kentucky bluegrass and various fescues, it is phosphorus that strongly supports this all-important photosynthesis process. It also supports robust root establishment in new sod or germinated seedlings. When phosphorus is too high, algae growth occurs. Which can starve your lawn’s roots of nutrients. 


Potassium is what paves the way for healthy water and nutrient intake. While also promoting strong cell wall formation in each blade through the synthesizing of certain proteins and starches. These processes combined are what create a lawn that is strong and tolerant against disease, drought, heat, and cold. 

Why Use a Zero Phosphorus Fertilizer? 

Testing the soil with a pH soil test kit will indicate its current phosphorus content. This is important because high phosphorus levels could lead to your lawn growing poorly or even dying. Too much greatly reduces nutrient absorption and stunts photosynthesis.

This is why a 15-0-15 fertilizer could be the best choice for yours. Another reason is the high risk of contamination in below-ground water tables and nearby lakes, streams, and rivers. 

What is 15-0-15 Fertilizer Used For? 

This dynamic nitrogen plus potassium combo will support your lawn from every angle. Starting way down at the cellular level where these benefits increase strength and vitality to minimize wear and tear and hold up against whatever the weather can throw at it.

Some 15-0-15 feeds are even effective under low-light conditions. Making them the perfect choice for lawns in regions with shorter growing seasons and shorter days. 

Trees and Shrubs 

Trees and shrubs growing in fertilized lawns may eventually show signs of nutrient deficiency as they compete with the lawn for nourishment. In this case, extra doses of a 15-0-15 around the base of each will balance out their needs while avoiding soil and water source contamination. 

Vegetable Gardens 

Broadening your range of application, when spreading a 15-0-15 NPK, to include your veggie patch is a great idea. Some vegetables do benefit from a solid dose of phosphorus at specific stages of growth. But, nitrogen and potassium promote vigorous plant growth and flowering throughout the growing season. 


This is the one area where a 15-0-15 is not recommended. Houseplants require a balanced NPK to thrive, like a 10-10-10 or a 5-10-5. The lack of phosphorus in a 15-0-15 blend could lead to stunted growth, a vulnerability to pests and disease as well as yellowing, wilting leaves. 

Lawn and Grass 

A 15-0-15 fertilizer can benefit any type of grass that is grown in poor soil. But is most successful when applied to warm-season grasses such as Bermuda and St. Augustine. You’ll want to do a little research into the type of grass you have and what it needs before investing in a phosphorus-free blend. 

  • Bermuda – performs best when fertilized in three equally-spaced intervals between May and August 
  • Bluegrass – utilizes below-surface rhizomes to grow and spread. Thus needing more phosphorus to encourage rhizome multiplication and growth. 
  • Buffalo – doesn’t require as much nitrogen as other grass varieties. A more balanced fertilizer would best support its needs.
  • Centipede – as a warm-season grass, this type greatly benefits from a 15-0-15 combination. Phosphorus should only be applied if the soil is completely lacking. 
  • Palmetto – A sub-variety of St. Augustine, this only needs phosphorus when sod is first installed or when new seeds have just germinated. Once established, it will benefit from a 15-0-15 mix.
  • St. Augustine – a warm-season perennial, this type only needs phosphorus when the soil is completely deficient or when new sod is installed.
  • Zoysia – this grass thrives on even higher nitrogen levels than Bermuda. But with the same application schedule.

What Nutrients Does Lawn Need? 

Not all lawns are created equal. Cold-season varieties like bluegrass, ryegrass, and fescues all require a decent balance of nitrogen, phosphorus, and potassium in order to thrive. Warm-season types, on the other hand, don’t require phosphorus. Making a 15-0-15 NPK is the best combo for them. 

When to Use 15-0-15 Fertilizer 

When applied at the proper time and with the proper dosage, a 15-0-15 fertilizer can considerably improve your lawn’s performance and vitality. Applying too much, not enough, or at the wrong time may actually cause damage. Let’s break down some recommended application times by season. 


In both cold and warm climates, spring is the best time to apply a 15-0-15 fertilizer. As your grass wakes from winter dormancy, it will then be the most receptive to added nourishment and soil conditioning. March or April in warm climates is ideal. In colder regions, April – May is best.  


Bermuda, Zoysia, Centipede grass, along with other warm-season grasses will typically grow more vigorously in summer. Consuming all the nitrogen and potassium naturally present in the soil and from spring fertilizing to support that. To continue healthy, vibrant growth into autumn, a second application may be necessary in July or August. 


nitrogen-heavy fertilizers should be applied no later than August because, in most regions, lawns will go at least partially dormant toward the middle of autumn. Meaning their internal processes for nutrient absorption and distribution slow way down or stop altogether as outside temperatures drop. Excessive nitrogen applied within its dormancy stage may cause irreparable damage to the roots. 


Fertilizing is not necessary for winter, no matter what your climate. Even in warmer areas, outside temperatures still drop triggering a partial dormancy state. It’s a simple survival mechanism that maintains your lawn’s “vital signs” until the weather warms up. At which time, your fertilizing regimen can begin again. 

How to Use 15-0-15 Fertilizer for Lawns 

15-0-15 blends come in both slow-release, dry formulas that are distributed across your lawn using a push spreader or liquid concentrates that are easily applied using a hose attachment.

Each brand will come with its own unique set of instructions based on formulation and application method. It’s important to follow these directions for optimum results.  I’ll be sharing two of the best liquid performers I’ve tested in a moment.

Application Rate for Lawns 

Application rates will vary based on formulation. Slow-release formulas may only recommend one application for the whole season as nutrients are conveyed to the roots with every watering or period of rainfall. Liquid formulas will need to be applied 2-3 times per season for consistent results. 

How Often to Use 15-0-15 

When using a liquid concentrate or premix, application times should be evenly spaced so as not to dry out or damage your lawn. Once in spring, again in mid-summer and in poor soils, a third may be applied no later than the end of August. 

Best 15-0-15 Fertilizers 

I know that product testing and review research takes time, effort, and money. So, I’ve done some extensive testing of my own, including consumer feedback from across the lawn care spectrum. For your convenience, here are the 2 liquid fertilizers that came out on top in terms of performance and ease of use and are specifically for lush, beautiful warm-season lawn varieties.  

Simple Lawn Solutions | Nitrogen & Potash 15-0-15 

Simple Lawn Solutions | Nitrogen & Potash

This ready-to-use option comes pre-diluted with added sulfur to lower pH levels, if needed, enabling better nutrient absorption. I simply attached this to my hose and walked back and forth, spraying evenly. Just as you would when watering.

This formula had a surprising effect on some emerging weed patches I had, too. Weeds don’t like high levels of nitrogen so those went away rather quickly.

One bottle covers 1600 sq ft, which took care of my entire front lawn. Which I found to be pretty economical. It starts working within 8 hours but it may take up to a week for full effects to be visible. 

How To Use: Apply this in the cooler hours of the morning so as not to burn grass blades and refrain from watering your lawn again for a couple of days.  Also, wait to apply any kind of herbicide right after fertilizing with this. Doing so may have a detrimental effect. 

Customer Reviews: High marks from consumers trying to grow warm-season lawns in poor conditions. Improved soil quality transformed their lawns from mud pit to lush, green sod to become the envy of their neighbors.


  • Added sulfur to balance pH levels
  • High nitrogen content acts as a secondary herbicide


  • Not recommended for new sod or newly germinated seedlings

PetraTools | Fertilizer for Lawns, Grass and Plants 15-0-15 

PetraTools | Fertilizer for Lawns, Grass and Plants 15-0-15 fertilizer

This maximum-strength liquid concentrate offers an NPK that’s free of phosphorus yet fortified with humic acid for robust growth and root formation in all warm-season types of turf. I can see how this would be ideal for harsh southern summers but it has also proven to work really well in our northern climate too. One jug covered 4 acres of our grassy spaces and best yet, this product is safe enough for all the kid and pet activity we can throw at it.

How To Use: Mix 5oz of this easy-to-use lawn booster into 4 liters (1 gallon) of water for maintenance. Or 10oz per 4 liters (1 gallon) for application in more challenging areas. Repeat every 14 days throughout the growing season.

Customer Reviews:  Homeowners are sharing how pleased they are with the great results they get paired with a fair product price. Especially those trying to grow lawns in desert regions.


  • Added humic acid, essential for healthy growth
  • Great value for money


  • Not for use in nitrogen-rich soil

Final Thoughts on 15-0-15 Fertilizer 

In my experience, there’s no better fertilizer to use on a warm-season lawn variety than a 15-0-15 NPK blend. With either a slow-release method or one of the two liquid options I recommend, you’ll be providing the nitrogen and potassium that your lawn craves without the risk of burn or water source contamination that often comes with excessive phosphorus use.

Even if you live in a harsh winter climate, summers can still get quite warm. Enabling you to grow a Bermuda, St. Augustine, or even a centipede lawn. For success, a 15-0-15 is your best bet.