What Is NPK Fertilizer? | 2025 Guide

By Amanda Lutz Updated October 24, 2024

The three primary nutrients that plants need to grow and stay healthy are nitrogen (N), phosphorus (P), and potassium (K). An NPK fertilizer is a mix of substances that contain these three macronutrients. Nitrogen helps plants grow leaves, phosphorus contributes to healthy flower and root development, and potassium maintains plant health. This guide dives into the nature of NPK fertilizers and how you or your lawn care professional can use them effectively.

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Breaking Down NPK Ingredients

Breaking down the ingredients in NPK fertilizer can help you understand its importance for your plants’ growth, health, and beauty. Three simple ingredients play a huge role.

Nitrogen

Nitrogen is a vital element for plant growth because it’s a significant part of chlorophyll, which plants need for photosynthesis (the process of using sunlight, water, and carbon dioxide to produce oxygen and energy). Its regular uptake by plants stimulates the production of new cells, which contributes to bigger leaf size and higher stems.

Plants need a continuous supply of this nutrient to grow, but nitrogen deficiency in the soil is common. NPK fertilizers provide plants with an accessible source of this essential nutrient.

Phosphorus

Phosphorus turns the sun’s energy into plant food, fiber, and oil. Similar to nitrogen, it plays an important role in photosynthesis. It also helps plants transfer and store energy, which stimulates proper development.

This plant nutrient promotes healthy shoot growth, improves fruit quality, enhances vegetable and grain crops, and contributes to proper seed formation. Phosphorus also allows your plants to improve their water use efficiency. It helps plants resist diseases, withstand cold temperatures, and handle moisture stress.

Potassium

Of these three nutrients, potassium contributes most to optimal plant growth. It promotes sturdy stem development and enhanced root depth. It also helps flowering plants grow beautiful flowers.

This nutrient regulates the growth of your plants to ensure optimal fruit formation. While most soils supply some potassium, it’s often insufficient. An NPK fertilizer can provide your plants the necessary amount of K.

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What Do Fertilizer Numbers Mean?

You’ll see NPK as a ratio on a fertilizer bag. The numbers are always in the same order. The first number is nitrogen, the second is phosphorus, and the third is potassium.

A label that reads 15-5-10 tells you that the fertilizer contains 15% nitrogen, 5% phosphorus, and 10% potassium. Depending on your plant growing needs, the season, and the soil, you may need a different proportion of NPK nutrients. If you hire lawn care professionals, they can perform a soil test to identify which nutrients your soil needs the most.

For example, if you have flowering plants, you may want higher potassium content in the ratio. A balanced ratio of 10-10-10 can be beneficial if you need a lawn fertilizer. For a vegetable garden, ensure higher nitrogen content since it’s great for leafy growth.

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Calculating Your NPK Fertilizer Needs

Each lawn and garden has different NPK fertilizer needs. To calculate what your plants require, take the following steps. We’ll use nitrogen in this example. 

• Perform a soil test to determine how much nitrogen your soil requires.
• Check the nitrogen ratio on the NPK fertilizer bag.
• To understand how many pounds of nitrogen you have in your bag, multiply the percentage by weight. So, if your NPK ratio is 10-10-10 and you have 50 pounds of fertilizer, nitrogen is 5 pounds. 
• Calculate the area of your lawn (width by length).

Let’s say your lawn is 20 by 50 feet. Its area is 1,000 square feet. You have a 50-pound bag of 10-10-10 NPK fertilizer. The recommendation is one pound of nitrogen per 1,000 square feet. Accordingly, you would spread 1/5 of your 50-pound bag of fertilizer over your lawn.

Determining the necessary amount of NPK fertilizer without a soil test can be hard. Using too much or too little could adversely affect the quality of your vegetation. While you’ll likely hear approximate recommendations about NPK ratio and fertilizer spreading volume, they could hurt your lawn instead of improving it.

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Choosing Fertilizer

If your soil lacks important nutrients, your efforts to grow a lush lawn or beautiful flowers may be in vain. Choosing the right fertilizer is key to meeting your plant-growing needs and bringing your landscaping ideas to life.

However, simply buying a bag of organic fertilizer and spreading it according to the instructions may not help. To achieve your goals, you need to choose the right product. Here’s how.

Test Your Soil

It’s possible to get a general idea of your soil’s NPK content by performing a DIY test at home. Collect samples from different areas of your lawn or garden to create a representative mix.

Buy a soil test kit online or at the nearest home improvement store. Follow the instructions on the fertilizer labels to test the soil. The result will show the NPK content on a scale (from “surplus” to “depleted,” for example).

Alternatively, you can order a professional soil test from a local agricultural laboratory or through lawn care services such as TruGreen. The results of this test will be much more detailed and provide valuable information about the type of fertilizer it needs.  

Assess Your Plant Types

Begin by identifying the types of plants you have in your garden. Categorize them into flowering plants, vegetables, or lawn grass groups. Next, research the specific nutrient needs of each plant group. Flowering plants may require higher phosphorus levels, while leafy vegetables benefit from higher nitrogen content.

Buy a Preformulated Fertilizer

Once you identify your plants’ needs and know what nutrients are the most beneficial for each plant type, go shopping for a balanced fertilizer. Fertilizer manufacturers offer preformulated products to fit specific plant types and nutrient requirements. For example, a preformulated garden fertilizer’s label would say, “flower and fruit fertilizer 3-3-5.”

While you can find an all-purpose fertilizer, it’s unlikely to have the desired effect. By opting for preformulated products, you can ensure that your plants receive the necessary nutrients for healthy growth. Follow the instructions on the packaging for application rates and frequency to achieve the best results.

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Applying Fertilizer

To apply a fertilizer, you will need the following tools:

• Fertilizer spreader
• Gardening gloves
• Measuring cup or scale
• Sprayer or watering can
• Soil probe or trowel

The frequency of fertilizer application depends on many factors, including the type of plants, soil conditions, and the specific fertilizer used. Generally, you must apply fertilizer every four to six weeks during the growing season.

Follow the instructions the fertilizer manufacturer provides for the specific product you are using. Fertilizers don’t just help your plants grow; they feed weeds, too. To prevent weed growth, apply the fertilizer directly around the plant.

The application method depends on the type of fertilizer you use. Review the main fertilizer types below.

Granular Fertilizer

Granular fertilizer comes in the form of dry pellets or granules. Spread the granular fertilizer evenly over the soil surface, avoiding direct contact with plant stems or leaves. Use a fertilizer spreader or your hands (in gloves) to distribute the granules uniformly.

After you spread the granules, water your lawn. This will help activate the nutrients and make sure they reach the roots. If any fertilizer gets on the plant leaves, wash it off. Follow any additional instructions provided by the fertilizer manufacturer.

Liquid Fertilizer

In most cases, liquid fertilizer needs to be diluted before application. Mix the recommended amount of fertilizer with the appropriate amount of water, and pour it into the sprayer. The mixing instructions should be on the package.

Spray the diluted fertilizer onto the soil around the base of your plants. Avoid spraying or pouring directly onto the leaves since this can damage them. Cover the entire root zone of the plants. After application, water the area lightly to help nutrients get deeper into the soil.

Spike Fertilizer

Spike fertilizer works by releasing nutrients into the soil over time. Before application, ensure the soil is moist but not extremely wet. Using a small gardening tool or your hands, insert the fertilizer spikes into the soil around the base of the plant. Follow the manufacturer’s instructions for the correct spacing and depth.

Gently cover the slow-release fertilizer spikes with soil to ensure they are securely in place and prevent exposure to the air. After inserting the spikes, water the area thoroughly.

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Our Recommendation

NPK fertilizers are an important part of growing healthy and beautiful plants. They contain essential nutrients, large flowers, and tasty fruits that contribute to lush growth. If your soil lacks any of these nutrients, you could face problems with your lawn or garden, no matter how well you care for them. Regularly applying fertilizer can help you achieve your gardening goals.

While you can apply NPK fertilizer independently, consider seeking professional help. Lawn care experts can assist with testing the soil, determining the right amount of fertilizer, identifying the application frequency, and spreading the product efficiently.

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NPK Fertilizer FAQ