What is N-P-K? A Guide to Understanding Fertilizer Ratios

The N-P-K numbers are an essential tool for buying and using the right fertilizers for your lawn or garden. They tell you how much nitrogen, phosphorus, and potassium each fertilizer contains, help you choose the right N-P-K fertilizer, and estimate the amount you need.

What do fertilizer numbers mean?

All commercial fertilizers are legally required to have the three numbers separated by dashes (e.g., 10-0-5, 18-24-8, etc.) on their package. Most people call this the N-P-K ratio, also known as the N-P-K grade, fertilizer grade, or N-P-K code. N-P-K stands for the three essential macronutrients plants need to grow: 

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

Each fertilizer number refers to one of these nutrients and shows its relative amount in the bag.   For example, a fertilizer with an N-P-K ratio of 18-24-8 has: 

• 18% nitrogen (N)
• 24% phosphate – a form of phosphorus (P) plants absorb easily
• 8% potash – a form of soluble potassium (K)

In this case, a 50-pound bag from the garden store would contain 9 pounds of nitrogen, 12 pounds of phosphate, and 4 pounds of potash – 25 pounds of macronutrients in total.

You might wonder what’s in the rest of 25 pounds. Generally, the three primary macronutrients are mixed with an inactive filler (sterile sand or dirt, sawdust, bark, etc.). Some N-P-K fertilizers contain additional macronutrients such as magnesium or sulfur and micronutrients such as iron, zinc, or boron.   

The higher the fertilizer numbers, the more concentrated the nutrients and the less fertilizer you need. For example, to apply 1 pound of nitrogen, you need 5.55 pounds of an 18-24-8 fertilizer (18% nitrogen), but 12.5 pounds if you use an 8-2-6 fertilizer (8% nitrogen).

Note: To calculate the amount of fertilizer containing 1 pound of nitrogen, divide 100 by the nitrogen percentage in the N-P-K. For example, for an 8-4-6 fertilizer, you divide 100 by eight and obtain 12.5 pounds of fertilizer.

The N-P-K values are usually in large font on the front label, but some products have them on the back of the fertilizer package. Their order never changes, whether they are organic fertilizers or synthetic formulas. 

However, some fertilizers have a fourth number. In that case, the fourth nutrient is sulfur, and the N-P-K ratio turns into N-P-K-S or N-P-K(S).  

What is an N-P-K fertilizer?

An N-P-K fertilizer includes all three macronutrients: nitrogen, phosphorus, and potassium. It’s also known as a complete fertilizer (e.g., 30-4-12).

Fertilizers that only have one or two of the three macronutrients are known as incomplete fertilizers. For example, a 0-4-8 ratio shows an incomplete fertilizer that only has phosphorus and potassium (sometimes called a PK fertilizer). A fertilizer with a 0-0-10 ratio only has potassium.

N-P-K ratios in balanced fertilizers

Balanced N-P-K fertilizers contain all three major nutrients in equal or similar amounts. These products often have N-P-K values such as 10-10-10 or 20-20-20, often called a balanced N-P-K ratio. 

Balanced fertilizers, also known as Universal, All-Purpose, or General fertilizers, can be helpful when you don’t know precisely what is missing from your soil. 

Note: The N-P-K ratio doesn’t indicate the nitrogen type. To check if the fertilizer is slow-release or quick-release, read the guaranteed analysis label on the back of the package, which presents all the nutrients in detail.

What do the nutrients in N-P-K do? 

Now that you know what N-P-K numbers stand for, let’s examine what each of the three macronutrients can do for your plants.

Note: Macronutrients are nutrients plants need in large amounts (e.g., nitrogen, phosphorus, potassium, calcium, magnesium). 

Nitrogen (N)

Nitrogen is the “stems and leaves” nutrient. Plants with plenty of nitrogen grow fast, with tall, strong stems and large, sturdy, green leaves. 

Nitrogen is an ingredient in the proteins that build each plant cell and a component of chlorophyll, the pigment that gives leaves their green color and captures sunlight for photosynthesis. 

A deficit in nitrogen leads to slower growth and plants with shorter stems and thinner, smaller leaves. The lower, older leaves turn yellow, eventually drying out and falling off. 

On the other hand, nitrogen excess can delay flowering and fruiting in vegetable gardens and orchards and compromise root growth. It can also burn plants when applied in high amounts in a quick-release form. 

Phosphorus (P)

Phosphorus is the “shoots, roots, and blooms” nutrient. It helps seeds germinate, and seedlings grow faster and with deeper, healthier roots. It is essential in starter fertilizers used when seeding new plants. 

Phosphorus improves crop yield by supporting bigger blooms, timely flowering, and seed formation.  

A phosphorus deficit leads to small seedlings with fewer leaves. Plants also form flower buds late, leading to lower-quality fruits and seeds. Too much phosphorus can prevent the absorption of other essential nutrients, such as iron and zinc. 

Potassium (K)

Potassium is the “growth and resilience” nutrient. This nutrient helps plants better absorb and use water and nutrients, manage their energy and resources, store reserves before winter, and build stronger plant tissue. 

Plants with enough potassium grow healthier and are more resilient to drought, heat, cold, fungal diseases, and insects. Potash also makes fruits and vegetables tastier and better looking.

A potassium deficit can be recognized by slower growth, wilting during hot days, and a higher sensitivity to water stress. Too much potassium in the soil can also harm plants by preventing them from absorbing nutrients such as calcium and magnesium.

Why just those three nutrients?

While plants need about 17 nutrients to grow and thrive, they require these three nutrients in the highest amounts. Nitrogen, phosphorus, and potassium are consumed faster from the soil and require replacement through fertilization more often than other nutrients.

This focus on N-P-K is not new. It goes back to 1840, when German chemist Baron Justus von Liebig, also known as the “Father of the Fertilizer Industry,” framed nitrogen, phosphorus, and potassium as essential nutrients for plant growth for the first time in his paper, “Theory of Mineral Nutrients.”

How to choose the right N-P-K mix for your lawn

There are so many nutrient combinations in stores that it can make your head spin. To figure out which one is the best for your garden or lawn, use these three strategies:

Test the soil. The right N-P-K mix for your plants depends on the soil deficiency you need to correct. Use a test kit or send a sample to the local Extension Office for testing. Also, this fertilizer calculator can help determine what fertilizer mix fits the soil test requirements.

Consider the type of plant. The N-P-K ratio also differs depending on the kind of plant you grow: 

• Leafy plants and turfgrass need lots of nitrogen
• Flowering ornamentals need more phosphorus
• Fruiting crops such as tomatoes and aubergines require more potassium 

Only use fertilizer labeled for the type of plants you grow (turf, flowers, vegetables, cereal crops, etc.).

The season and stage of development also matter. For example, spring turf fertilizers are higher in nitrogen to boost leaf growth, while fall and winter products have more potassium to improve cold hardiness.

Veggies need more nitrogen when young and more phosphorus and potassium when close to blooming and fruiting.

What to read next on this subject: 

• “Fertilizer Basics: What to Look for in Your Fertilizer”
• “What Type of Lawn Fertilizer Do You Need?”
• “Fertilizer 101: How to Fertilize Your Lawn”

Get the Right Fertilizer for Your Lawn

Sometimes, understanding the N-P-K ratio is not enough. If the lawn is wilted, thin, patchy, or refusing to grow, you need a professional hand to help. Find a lawn care pro with LawnLove and enjoy your lawn in its best shape!

Main Photo Credit: Aris Berroya

Sinziana Spiridon

Sinziana Spiridon is an outdoorsy blog writer with a green thumb and a passion for organic gardening. When not writing about weeds, pests, soil, and growing plants, she's tending to her veggie garden and the lovely turf strip in her front yard.

Posts by Sinziana Spiridon