What Is NPK? Understanding Garden Fertilizers

All plants require nutrients to grow, but it can be difficult to understand how to apply fertilizers and the necessary nutrients. Learning about the NPK ratio is a great place to start your journey into the fertilizer world. In this article, vegetable farmer Briana Yablonski will explain what NPK means and how you can use it to select the best fertilizer for your garden.

NPK fertilizer. Close-up of a garden trowel filled with NPK fertilizer next to a watermelon plant. Fertilizers are granular and have blue ball-shaped granules.

Contents

Whether you’re a brand-new gardener looking forward to harvesting your first homegrown tomato or have never spent a day without dirt under your fingernails, picking out the correct fertilizer for your garden can be challenging. Considering all the liquid, granular, synthetic, and organic fertilizers available, it can be difficult to know which product to choose. And when you add the different nutrients in each fertilizer, things can become even more confusing!

Looking at the NPK ratio is one way to compare fertilizers and determine which one is best for your crops. All fertilizers must present this information, which states the amount of nitrogen, phosphorus, and potassium in the product. In this article, I’ll cover more details about the NPK ratio and how to use it to select an appropriate fertilizer for your garden.

Before you apply any fertilizers, remember to perform a soil test. This will give you an accurate view of which nutrients are already present in the soil and which are deficient. Adding nutrients to the soil that it doesn’t actually need causes more harm than good.

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What Does NPK Mean?

Close-up of NPK fertilizer. NPK fertilizer, a vital component in plant nutrition, appears as small, granular particles of bright blue color.
NPK ratios indicate essential macronutrients in fertilizers for plant health.

When you browse through fertilizers, one of the first things you’ll see is three numbers in a row separated by dashes. Gardeners and soil health experts refer to these numbers as the NPK ratio since they refer to the amount of nitrogen (N), phosphorus (P), and potassium (K) in the fertilizer. These are the three macronutrients that plants require large amounts of to grow and remain healthy.

It’s okay to think the NPK numbers refer to the amount of nitrogen, phosphorus, and potassium in the fertilizer. But recognize these numbers actually refer to the total amount of nitrogen (N), phosphate (P₂O₅), and potash (K₂O) by weight. For example, a 100-pound bag of fertilizer labeled 15-7-8 contains 15 pounds of nitrogen, seven pounds of phosphate, and eight pounds of potash. The remaining 70 pounds are inert material or plant secondary nutrients or micronutrients.

Since the NPK ratio only tells you about the three macronutrients, you must look closer at the fertilizer package to learn what secondary nutrients and micronutrients the product contains. Most fertilizers list any other present nutrients and include their percentage by weight. For example, a fertilizer package may say it contains 2% sulfur and 0.5% manganese. If a specific element isn’t listed on the package, you can assume it isn’t in the fertilizer.

Understanding Plant Macronutrients

As I mentioned above, the NPK ratio refers to the amount of nitrogen, phosphorus, and potassium in a fertilizer. All plants need large amounts of these nutrients to thrive, so people refer to this trio as macronutrients. However, different types of plants require different amounts of each nutrient.

Learning how plants use the three macronutrients can help you understand how to choose a fertilizer that supplies just the right amount of each nutrient.

Nitrogen

Close-up of a garden trowel full of granular fertilizer on Arachis pintoi leaves. Arachis pintoi leaves are distinguished by their small, oval-shaped leaflets arranged in pairs along delicate, trailing stems. The foliage is dense and lush, forming a low-growing mat that carpets the ground with vibrant greenery. Each leaflet features smooth, glossy surfaces and prominent veins, imparting a fresh and healthy appearance. Granular fertilizers are small blue balls.
A cornerstone of life, nitrogen fuels plant growth and vitality.

Nitrogen, symbolized by the letter N, is a major building block of amino acids. These amino acids join together to form proteins, including structural proteins that make up plant tissue and enzymes that drive processes like respiration and photosynthesis.

Nitrogen is also an essential component of chlorophyll, the molecule that captures solar energy, which plants use to complete photosynthesis. That’s why plants that are low in nitrogen develop light green or yellow leaves, and those with too much nitrogen have deep green leaves.

If you choose to use organic fertilizers, you can find many nitrogen sources. Some popular, natural, nitrogen-rich fertilizers include blood meal, feather meal, alfalfa meal, and fish fertilizer. Synthetic forms of nitrogen are produced in a lab and include ammonium nitrate, urea, and diammonium phosphate.

The Nitrogen Cycle

Close-up of a blue-gloved gardener's hand applying granular fertilizer to young narcissus plants in a sunny garden. Young narcissus plants emerge from the soil with slender, erect stems. The foliage consists of narrow, strap-like leaves arranged in a tuft at the base of the stem.
Understanding nitrogen’s dynamic movement is crucial for effective fertilization.

Before you apply nitrogen fertilizer, it’s helpful to understand how it moves through the environment. While some nutrients remain in the soil for years on end, nitrogen is one of the most unstable plant nutrients. It’s constantly changing forms and moving throughout the environment, so plant-available nitrogen is always changing.

Nitrogen can escape from the soil by volatilizing into nitrogen gas and escaping into the atmosphere, and nitrogen in the form of nitrate can leach out of the soil and into surrounding waterways. Therefore, it’s important not to over-apply nitrogen fertilizer.

If you’re using organic sources of nitrogen, you should also be aware that plants can’t take up the large nitrogen-containing compounds present in organic matter. First, bacteria must convert these larger compounds into plant-available forms of nitrogen: nitrate and ammonium. Plants can then take up and use the nitrogen. That’s not to say that organic fertilizers are any less effective than synthetic fertilizers, but you should be aware of the importance of beneficial bacteria in making nitrogen available to plants.

Phosphorous

Close-up of a gardener applying bone meal fertilizer to young plants in a raised bed in the garden. This fertilizer is granular, unevenly shaped, brownish-sandy in color. Young plants consist of short stems and small, oval, smooth, dark green leaves.
Vital for growth and energy, phosphorus fuels plant vitality.

Symbolized by the letter P, phosphorus is a vital component of plant genetic material (DNA). It’s also an important component of RNA, the compound that reads DNA and translates the genetic information to the plant.

Phosphorus is also a crucial part of the energy storage and transfer molecule ATP (adenosine triphosphate). Plants produce ATP during photosynthesis and then use the molecules’ stored energy to drive processes, including cell division and carbohydrate formation.

When you put these two functions together, you’ll see that phosphorus is necessary for a wide range of plant processes. It helps with root growth, flower formation, disease resistance, and general plant health. Plants lacking phosphorus may appear stunted, and their older leaves may turn purple.

Some organically approved sources of phosphorus include bone meal and rock phosphate. While these materials are “organic” in the sense that they come from natural sources, they aren’t carbon-containing molecules. However, most of the phosphorus in these organic fertilizers is held in inorganic minerals, where it remains unavailable for plant uptake. These minerals slowly break down over time and increase the amount of soluble, plant-available phosphorus in the soil.

Potassium

Close-up of a gardener wearing black and blue gloves applying Potassium fertilizer to a rose bush in the garden. These fertilizers are granular, unevenly shaped and pale pinkish-orange in color.
A silent messenger, potassium orchestrates plant hydration and nourishment.

Potassium, symbolized by the letter K, is the final plant macronutrient. Unlike nitrogen and phosphorus, potassium is not a part of plant structures. Instead, it acts as a messenger molecule.

It moves water, nutrients, and carbohydrates throughout a plant and also activates enzymes. Since potassium moves water and nutrients to new cells, potassium-deficient plants may develop discolored or curled leaf tips. These plants are also more susceptible to wilting on hot, sunny, and/or windy days.

Organic potassium sources include langbeinite, potassium sulfate, and sunflower hull ash.

What Are the Different Types of NPK Ratios?

Now that you know about the three macronutrients and their importance to plants, let’s look at how people label fertilizers based on their NPK ratios.

Complete Fertilizers

Close-up of a gardener in blue gloves mixing soil and fertilizer with a small garden rake in the garden. Granular fertilizers are round and white.
Essential for balanced soil nutrition, complete fertilizers sustain plant growth.

Complete fertilizers refer to products that contain nitrogen, phosphorus, and potassium. These products don’t have to contain an equal amount of each macronutrient, but they must contain at least some of each.

Complete fertilizers may only contain the three macronutrients, or they may also contain secondary nutrients and micronutrients. Some complete fertilizers consist of a single material like fish emulsion or cottonseed meal, and others are made from a blend of a few different materials.

Complete fertilizers are a good option for your garden if your soils lack nitrogen, phosphorus, and potassium. Just make sure you choose a product with the proper amount of each nutrient. If you’re unsure about how much of each nutrient to apply, conduct a soil test and reach out to your local agricultural extension office for help translating the results.

Balanced Fertilizers

Close-up of a gardener pouring liquid fertilizer from a glass jar onto young cucumber seedlings in the garden. Liquid fertilizers are brownish-orange in color. The seedlings are young and consist of small heart-shaped leaves with finely serrated edges.
For harmonious growth, opt for balanced fertilizers with equal nutrients.

Balanced fertilizers contain equal amounts of nitrogen, phosphorus, and potassium. Some examples of balanced NPK ratios include 3-3-3 and 15-15-15. While these fertilizers all apply the same ratio of macronutrients, they apply different amounts of nutrients. For example, you would have to apply five cups of 3-3-3 fertilizer to supply the same amount of nutrients present in one cup of 15-15-15 fertilizer.

If you don’t know the nutrient composition of your soil, balanced fertilizers are a great option to start with. Organic liquid fertilizers have the lowest impact as well.

Vegetative Fertilizers

Close-up of a girl applying granular fertilizer with a small black spatula to a potted ficus plant on a table indoors. Ficus elastica 'Robusta' boasts a striking appearance with its large, glossy leaves that are dark green and leathery in texture. Each leaf is broad, elliptical, and held on sturdy stems, forming an impressive canopy.
Nourish lush foliage with nitrogen-rich fertilizers during vegetative growth.

If you want to support the vegetative growth of houseplants, greens, and other plants, opt for a fertilizer with more nitrogen than phosphorus and potassium. The extra nitrogen will support plants as they grow and develop new leaves. Fertilizers with NPK ratios of 6-2-2 or 10-3-4 are good options for the vegetative growth stage.

Flowering Fertilizers

Close-up of a gardener's hands in blue gloves applying white granular fertilizer to a blooming daffodil in the garden. Narcissus are charming spring-blooming flowers known for their vibrant colors and distinctive trumpet-shaped corona surrounded by petals. The flower emerges from a slender, erect stem and comes in a yellow and orange color. The foliage consists of narrow, strap-like leaves arranged in tufts at the base of the stems.
Boost blossoms and fruits with phosphorus-rich fertilizers for flowering plants.

When your tomato plants begin producing blooms or your rose plants are ready to flower, switch to a fertilizer with an NPK ratio that encourages flower and fruit production. These fertilizers contain higher amounts of phosphorus. Some NPK ratios that work well for flowering plants include 4-8-5 and 3-6-3.

Final Thoughts

Understanding what NPK means will help put you on the path of choosing a suitable fertilizer. Not only is it important to apply the correct nutrients, but it’s also vital to apply the appropriate ratio. If you’re unsure what NPK is best for your plants, conduct a soil test and consult experts about the results.

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too much fertilizer. Close-up of a gardener's hand with granular fertilizer over a growing cabbage plant in the garden. A young cabbage plant consists of a compact rosette of large, broad, and lobed leaves that emerge from a central stem close to the ground. The leaves are blue-green in color with a waxy texture and slightly jagged edges. Granular fertilizers come in the form of many small, round, orange granules.

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