15-0-15 Fertilizer – Ultimate Guide for 15-0-15 Fertilizer 

Trying to decide on the best fertilizer for your plants can be confusing. This guide tells you all about 15-0-15 fertilizer. 

What Is 15-0-15 Fertilizer?  

15-0-15 fertilizer is fifteen percent nitrogen and fifteen percent potassium. It does not contain phosphorus. The numbers on the front of the fertilizer bag are explained in more detail in this article.  

Briefly, nitrogen is a part of every cell in the plant. Without it, the plant can’t grow, have flowers, or have seeds. Too much nitrogen can make a plant grow too much foliage and not bloom or have seeds. An extreme overdose can burn the roots and foliage and kill the plant. 

Potassium is necessary for a healthy, vigorous plant that can resist diseases and pests. Potassium also helps plants endure drought, heat, and cold. A potassium deficiency looks a lot like a nitrogen deficiency. Too much potassium can keep plants from taking magnesium, iron, zinc, and calcium up from the soil. 

What Is 15-0-15 Fertilizer Used For?

15-0-15 is used primarily to fertilize warm-season turf grass.

Warm-season grasses like 15-0-15 photo by Stephanie Suesan Smith

Why Use 15-0-15? 

Use 15-0-15 as a part of your warm-season turf grass fertilization program. Sometimes, phosphorus is too high in the soil, and you need to add nitrogen and potassium, but not phosphorus. I lived in an area where there was so much phosphorus in the soil that the soil lab said not to add any. Too much phosphorus, like too much of anything, can be toxic. 

Types of Fertilizer 

Fertilizer comes in different forms and is used in different ways. Here are the most common forms. 

Liquid Fertilizer 

Some liquids are concentrated and have to be mixed with water before use, while others are ready-to-use right out of the bottle. I prefer buying concentrates because while they are more expensive initially, they are less expensive per use. 

Liquids can be used as a drench, which is poured onto the soil around the plant, or as foliar spray, which is sprayed on the foliage of the plant. Liquids work immediately and last for about ten to fourteen days. 

Water Soluble Fertilizer 

This type of fertilizer is a granule or powder that is designed to be added to water. The fertilizer dissolves in water and is used as a liquid fertilizer.  

Fertilizer for Hydroponics 

Hydroponics systems deliver all the plant’s nutrients in a water solution. Most liquid and water-soluble fertilizers work well in a hydroponics system. 

Granular Fertilizer 

Granular fertilizer is mixed and formed into small rock-type particles. This type of fertilizer is spread around the plant and worked into the top two or three inches of soil. Alternatively, in turf grass, granular fertilizer is spread with a fertilizer spreader that is set to deliver the correct amount of fertilizer for the grass. Granular fertilizer can be fast-release or slow-release, or a combination. Fast-release fertilizer starts working immediately and works for ten to fourteen days. Slow-release fertilizer starts working in about seven days and can work for weeks or even months. Always water after spreading granular fertilizer. 

Organic Fertilizer 

Organic fertilizer has all-natural ingredients on the “allowed in organic gardening” list and has been submitted to a certifying agency to verify the manufacturer’s claims. Fertilizer stating “all-natural” or “natural” is not necessarily made up of ingredients allowed in organic gardening. 

Slow-Release Fertilizer 

Most fertilizer is fast release, with all the nutrients available to your plants immediately. However, that only lasts ten to fourteen days before it has to be reapplied. Slow-release fertilizer has been coated with a substance that gradually erodes when it gets wet. As the coating erodes, the fertilizer releases more nutrients. In this way, slow-release fertilizer can release nutrients for weeks or even months. Slow-release fertilizer starts working in about seven days. 

When and How Do I Apply 15-0-15 Fertilizer?  

Different fertilizers are spread at different rates. While you should always follow the directions on the fertilizer package, here are some general ways the fertilizer is used based on what plants you are fertilizing. 

Houseplants 

Houseplants depend on fertilizer for all their nutrients. Since 15-0-15 does not have any phosphorus, I do not recommend it for houseplants. 

Annual Flowers 

Annual flowers grow fast and bloom, then produce seeds and die. They need phosphorus to trigger their blooms, so I don’t recommend 15-0-15 for them. 

Existing Ornamental Plants 

The only reason to use 15-0-15 with your existing ornamental plants is if the soil test shows your soil is high enough in phosphorus that more would injure your plants. Make sure that any 15-0-15 you use does not include weed killer, or you will kill your plants. Use 1/10 pounds per 100 square feet of landscape bed. 

Bulbs and Tubers 

I would not use 15-0-15 for bulbs. They need phosphorous to trigger their flowers. 

Vegetable Gardens 

I would only use 15-0-15 in my garden if my soil test said I did not need any phosphorus. In that case, I would use one cup of granules per ten feet of row. I would work the fertilizer into the top two to three inches of the soil and water the area where I spread the fertilizer. 

Trees 

I would not use 15-0-15 on trees unless the soil test said not to add any phosphorus. I would use a pound per 100 square feet and spread it along the dripline of the tree. Water the tree after fertilizing. 

Lawns 

15-0-15 is ideal for warm-season grasses such as Bermuda, centipede, St. Augustine, and zoysia grass. These grasses are sensitive to too much phosphorus, so the 15-0-15 doesn’t include it. Generally, this fertilizer would be used in the early spring and the fall.  Apply four pounds of fertilizer per 1,000 square feet each time. 

Where Do I Buy 15-0-15 Fertilizer?  

You can find 15-0-15 fertilizer in Walmart and nurseries as well as online vendors such as Amazon

What Is the Best 15-0-15 Fertilizer?  

Here are my picks for 15-0-15 fertilizers. 

Best Liquid 

Superior Nitrogen & Potash 15-0-15 NPK- Lawn Food Quality Liquid Fertilizer is my pick for the best liquid fertilizer. In addition to nitrogen and potassium, this fertilizer has humic acid to promote rapid green-up and good growth. Humic acid is a human-grade ingredient, and this is a quality product. One gallon of fertilizer treats 12,800 square feet. That allows you to treat the average lawn several times a year with just one gallon of fertilizer.  Superior Nitrogen & Potash 15-0-15 NPK- Lawn Food Quality Liquid Fertilizer is good for all grass types, including Bermuda, St. Augustine, Zoysia, Buffalo, Centipede, Palmetto & Bluegrass. You can use a pump sprayer, a hose-end applicator, or a small tank sprayer.  

Best Granular Fertilizer 

Fertilome (10767) Centipede Lawn Fertilizer 15-0-15 is an excellent fertilizer. Fertilome is a well-respected company that makes good fertilizers. Fertilome (10767) Centipede Lawn Fertilizer 15-0-15 helps your grass green up quickly in the spring. It also helps turn yellow summer grass into green fall grass. It also contains copper, zinc, iron, and manganese. These trace elements help the grass grow good roots and healthy, green grass. The package has the settings you should put on your fertilizer spreader to put four pounds of fertilizer per 1,000 square feet. 

What Are the Ingredients in 15-0-15 Fertilizer?  

A 15-0-15 fertilizer has fifteen percent nitrogen and fifteen percent potassium. Sources of nitrogen vary by manufacturer but usually include urea. Potassium is usually some form of potash. The remaining seventy percent of ingredients include proprietary ingredients to make the nutrients more available to the lawn, fillers to make the fertilizer easier to spread, and ingredients to retard spoilage. 

Frequently Asked Questions 

How do I decide if you need 16-4-8 or 15-0-15 fertilizer? 

If your soil test shows sufficient phosphorus in the soil, choose 15-0-15. If your soil is low in phosphorus, then 16-4-8 fertilizer would be better. 

Is 15 0 15 fertilizers ok for southern lawns? 

Yes, it works on warm-season grasses, which is what grows best in the south. 

How do I fertilize centipede grass with 15-0-15? 

Most fertilizers tell you to put down four pounds of fertilizer per 1,000 square feet of lawn in the early spring and fall. 

In conclusion, 15-0-15 is best used for warm-season turf grasses in the early spring and fall. If you want to use a liquid fertilizer, Superior Nitrogen & Potash 15-0-15 NPK- Lawn Food Quality Liquid Fertilizer is a good fertilizer to use. If you prefer granular fertilizer, Fertilome (10767) Centipede Lawn Fertilizer 15-0-15 is a good product. 15-0-15 is not used for most other applications unless your soil has so much phosphorus your soil lab recommends not adding any more. If you use 15-0-15 for anything other than turf grass, make sure you do not get a product with weed killer in it, or the product will kill your plants. 

Photo of author

Stephanie Suesan Smith

Stephanie Suesan Smith has a Ph.D. in psychology that she mainly uses to train her dog. She has been a freelance writer since 1991. She has been writing for the web since 2010. Dr. Smith has been a master gardener since 2001 and writes extensively on gardening. She has advanced training in vegetables and entomology but learned to garden from her father.

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