What Is NPK? Understanding Garden Fertilizers

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.

What Does NPK Mean?

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

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

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

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

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

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

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

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

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.