Fertilizer Numbers: How to Use the Fertilizer Numbers to Grow Better

When buying fertilizers for your garden, farm, or container plants, you might have seen three bold numbers or percentages on fertilizer bags or bottles. What do these numbers mean? These numbers—known as fertilizer numbers or NPK numbers—indicate the percentage of three primary macronutrients: Nitrogen (N), Phosphorus (P), and Potassium (K). But how do you know which fertilizer number is best for your garden or backyard? Keep reading to find out.

In this guide, our experts will discuss what these numbers on fertilizer packages indicate, how to choose the right fertilizer number, and how to use them to grow better.

Which Nutrients Plants Do Need

If you enjoy growing plants in your backyard, on a farm, or in a greenhouse, it is essential to feed your plants. Like humans, plants need different nutrients for their growth and development. There are 17 essential nutrients that all plants require for normal growth.

These are two types of nutrients: micronutrients and macronutrients. Nitrogen, Phosphorus, Potassium, Calcium, Hydrogen, Sulfur, and Oxygen are classified as macronutrients, and plants need them in large quantities.

Iron, Copper, Magnesium, Zinc, chlorine, and others are micronutrients required by plants in small percentages. Carbon, oxygen, and hydrogen are provided through air and water. Plants get 14 other nutrients through the soil, but sometimes, you must give extra nutrients through fertilizers or compost. But how can fertilizers help your plants? Learn in the next section.

What is a Plant Fertilizer

Simple saying: fertilizer is plant food. By definition, plant fertilizer is a natural or synthetic material added to the soil to increase its fertility and help plant growth.

Natural fertilizers like compost provide a more balanced mix of nutrients and feed the soil's microbes. Even if you make your compost and add organic matter to your soil, you might still need fertilizers for a healthy plant and maximum harvest.

These commercial fertilizers contain a fixed percentage of essential nutrients that plants need. Your choice of fertilizer should depend on your soil and plant needs in terms of nutrients and uptake speed.

Understanding fertilizer numbers and how to use them to get better results is necessary.

What is a Fertilizer Number

When buying fertilizers, take a moment to read the label carefully. You should know what the fertilizer numbers on the front mean and what ingredients are used to make the fertilizer.

Every plant fertilizer label indicates three numbers that show the ratio of each essential nutrient called the N-P-K ratio. The first number is for nitrogen, which stimulates leaf production. The second number indicates the amount of phosphorus (P), which promotes flowering and fruiting. The third number is for potassium (K), which strengthens plants and helps them stay healthy.

Each number in the N-P-K ratio represents the weight percentage of that nutrient in the fertilizer bag. For example, if the label says 10-10-10 ratio, the fertilizer has 10% nitrogen, 10% phosphorus, and 10% potassium. The remaining 70% consists of filler materials that help distribute the nutrients evenly. Fertilizers with high NPK numbers are more concentrated than fertilizers with low rations.

Why and How N, P, and K in Fertilizers are Important for Plants

Plants need nitrogen (N), phosphorus (P), and potassium (K) for different reasons. Here's how plants use these nutrients:

Nitrogen (N)

Nitrogen is part of chlorophyll and is essential for plant vegetative growth. It helps plants grow leaves and stems. Fertilizers with high nitrogen ratios, like 6-2-1 or 20-10-10, are essential for leafy vegetables like lettuce, spinach, and kale. However, for fruiting and flowering plants like petunias or tomatoes, a high nitrogen ratio can cause high leafy growth at the expense of flowers and fruits.

Phosphorus (P)

Phosphorus helps plants with cell division and tissue growth and promotes strong root development, fruiting, and flowering. Root vegetables, like beets and carrots, and crops like tomatoes and peppers, benefit from fertilizers with high phosphorus ratios.

Potassium (K)

Potassium (K) makes plants disease-resistant, improves the overall quality of fruits and vegetables, and helps regular Carbon dioxide intake through stomata. It is essential for overall plant health and efficient water use. Potassium-rich fertilizers are beneficial for fruit-bearing crops.

How to Choose the Right Fertilizer Numbers for Your Garden

Balanced, all-purpose fertilizers with NPK ratios like 5-5-5 or 10-10-10 are best for houseplants. However, by determining your plants' nutritional needs, you can pick a fertilizer with the right combination of numbers.

Here is a checklist to help you choose the right fertilizer numbers to grow your plants better.

Plant Type

First, you need to know what type of plant you're fertilizing—whether it's a vegetable crop, fruit tree, ornamental plant, or another specific plant. This is important as each plant type needs different nutrients.

Soil Testing

Before buying fertilizer, you must get your soil tested. Without soil testing, you're only guessing what your plants need. A soil test will determine the pH level directly affecting how well plants absorb nutrients. Most plants grow in neutral to slightly acidic soil with a pH range of 6 to 7.

For example, suppose your soil is acidic (pH below 6). In that case, you need to use phosphorus-rich fertilizers as phosphorus becomes less available in acidic soil, while calcium and magnesium are more soluble. If your soil test shows no major nutritional deficiencies, our all-purpose and balanced fertilizer, HB-101 Concentrated Fertilizer, is the best choice.

It is best to have your soil tested by a professional lab or buy a DIY soil test kit from a garden store. Once you know your soil condition, you can pick a fertilizer number that addresses any deficiencies.

Fertilizer Analysis

After testing your soil, the next step is fertilizer analysis, which refers to the N-P-K numbers on the fertilizer label. All synthetic fertilizers come with a guaranteed analysis of N, P, and K on the label, while organic fertilizers may have many nutrients but don't always list everything.

Match the fertilizer to your plants according to their nutritional needs. For example, if a 25 lb. fertilizer has NPK numbers 8-4-6, it means that it has 2 lbs. of total nitrogen (0.08 x 25 = 2)

If you can't find an exact N-P-K recommended in your soil test, it is better to match the nitrogen level since nitrogen is often used in larger amounts. However, be careful not to exceed phosphorus levels, as over-fertilizing can harm the plants and environment by leaching into waterways.

Types of Fertilizers

The final thing you should consider is the type of fertilizer, organic or inorganic, and dry or wet.

Dry Vs. Wet Fertilizers

The choice between wet and dry fertilizer is entirely up to you.

Dry or granular fertilizers are commonly used outdoors and in gardens or yards. They are cheaper, have a longer shelf life, and are good for large crops.

On the other hand, wet or liquid fertilizers are suitable for small container plants and indoor plants. Liquid fertilizers allow you to dilute the solution with water, which helps prevent over-fertilizing that can harm or kill the plant. Moreover, these fertilizers can reach the roots easily. You can read the dilution instructions on the label.

Organic Vs. Inorganic Fertilizers

The decision between organic and inorganic (synthetic or commercial) fertilizers depends on your plant's nutrient needs.

Organic fertilizers are made from natural ingredients, are less concentrated, and are not immediately available to plants. It takes time for soil microbes to break down the nutrients and make them usable. They are slow-release fertilizers and provide nutrients over many weeks. The N-P-K numbers on organic fertilizers are often lower (like Archipelago Natural Fertilizer 0–6-0), so they cannot harm your plant.

Synthetic fertilizers are made from inorganic materials and release nutrients quickly. They are more concentrated, fast-acting, and water-soluble. Even the smaller amounts provide the same nutrients, making them cost-effective. Be careful not to use too much, as this can damage your plants.

When to Fertilize Plants

Knowing the right time to fertilize is as important as choosing the right fertilizer. Most perennials, annuals, vegetables, and lawns will grow well if you feed them with balanced granular fertilizer in early spring. However, don't fertilize before heavy spring rains, as the nutrients will wash away.

Annuals benefit from fertilizing three to four times during the growing season with a phosphorus-rich, water-soluble fertilizer. Lawns also do well with a second granular fertilizer application in early fall.

Trees and shrubs, especially flowering ones, appreciate fertilizer in spring and fall. Late fall is also a great time to fertilize bulbs, especially if you're planting them for the first time.

Only fertilize well-established plants. Fertilizing seeds or small seedlings can burn them.

How to Fertilize Your Plants

After choosing the right fertilizer number, the next step is to supply fertilizers to your plants. There are two methods of fertilizing the plants: manually and through a drip irrigation system.

DIY Fertilizing

Here are four common DIY fertilizing methods:

• Broadcast Application: This method is best for fertilizing granular fertilizers over large areas like lawns or new garden beds. You can use a drop spreader or hand-rotatory to spread the fertilizer evenly.
• Top-Dress Application: This fertilizing method is used to feed individual plants like perennials, shrubs, or vegetables. It involves sprinkling granular fertilizer by hand around the base of the plant and reaching out to the plant's drip line. Be sure to wear gloves.
• Base Application: This method involves fertilizing plants while watering them. Mix the water-soluble fertilizer with water as directed and water the soil directly at the plant's base using a water can or hose attachment.
• Foliar Application: In this method, you spray fertilizer directly onto the plant leaves. It is useful when plants need to absorb nutrients, like iron, quickly.

Fertilizing through the Drip Irrigation System

DIY fertilizing can be time-consuming and inefficient, especially for larger gardens or crops. That's where fertigation or fertilizing through a drip irrigation system comes in. Fertilizer injectors automatically mix and deliver the right amount of nutrients through your pre-existing drip irrigation system. It is timesaving and ensures even distribution of nutrients.

Fertigation is using irrigation systems to apply fertilizers or soil amendments. In standard drip irrigation systems, tubing and drip emitters supply fresh water to plants. The only difference in fertigation is the addition of fertilizer injectors to deliver nutrients right to the plants' root zone without any fertilizer losses along the way. Though commercial growers most commonly use fertigation to increase yields, home gardeners and small farmers may be interested in trying it out.

Once you know what fertilizer to use for your drip irrigation system and how to insert it into the injector, fertigation is simple. However, if done wrong, it might damage your drip irrigation system.

Here's why: Fertilizers usually contain salts that can clog nozzles and cause pipe buildup, potentially causing your system to fail.

To avoid such problems, use a reliable fertilizer and check the pH of the water before applying it. Also, flush the system regularly to remove any salt or sediment buildup.

Types of Fertilizer Injectors

There are a few varieties of fertilizer injectors available for a drip irrigation system.

EZ FLO fertilizer injectors are the most economical option for most home gardeners and small farms. They are very simple to use and require either dry or concentrated liquid fertilizer to fill the tank. From there, your plants are ready to be fertilized.

Mazzei injectors are commercial grade and do not contain moving parts that could be worn out. Because they work by suction, they require liquid fertilizers. To ensure the best performance, choose a Mazzei injector that matches your irrigation system's flow rate (GPH).

The MixRite is a pump-style fertilizer injector that is very accurate and easy to maintain. The only power it requires is water moving through the system. This fertilizer pump also comes with various options depending on your water flow. It is best for commercial operations because it provides the best portion control.

DripWorks provides various MixRite parts, including new piston kits, plastic springs, and seal kits. You can also find accessories such as wall-mounting brackets to help with installation.

DripWorks also carries the Chapin Hydrofeed fertilizer injector, best for small to medium-sized garden setups. The clear tank allows for easy monitoring and filling.

The Bottom Line

There you go! If you were searching for which fertilizer numbers to buy and how to use them to grow better plants, we hope this guide has been helpful to you. By knowing the roles of nitrogen, phosphorus, and potassium—represented by the N-P-K numbers on fertilizer packages, you can choose the right fertilizer for your plants.

Whether you're growing leafy greens, flowering plants, or fruit-bearing vegetables. The right choice of fertilizer numbers combined with fertilizing through a pre-fixed drip irrigation system will help you achieve healthier plants and maximum harvest.