Fertilizer Numbers – What Do They Mean?

Fertilizer Numbers – What Do They Mean?

Last Updated On: June 7, 2022

Plants need the proper nutrients to survive, just like we do, which often requires the addition of fertilizer to their soil. This can be a tricky task since fertilizers aren’t all identical, containing different types and amounts of nutrients. In most cases, these are displayed using three hyphenated numbers. So, what are these fertilizer numbers and what do they mean in terms of how they feed your plants? Let’s find out.

Fertilizer Numbers

The numbers on a package of fertilizer are referred to as the NPK numbers, which stand for nitrogen (N), phosphorus (P), and potassium (K). These are the macronutrients included in the fertilizer, which provide the plants with the essential nutrients they need to survive. They are displayed using hyphenated numbers, such as 8-16-4. These numbers are always listed in the same order, so you’ll always know which number goes with which element.

Each of those numbers represents the percentage of the particular macronutrient that the package contains. For instance, in the above example, there would be 8% nitrogen, 16% phosphorus, and 4% potassium in that package of fertilizer. Of course, this only adds up to 28% of the contents of the package, with the other 72% made up of other nutrients and inert ingredients.

Why Do The Macronutrients Matter?

The NPK numbers are more important than you may realize since each of those macronutrients offers specific benefits to the plants that they are feeding. By using a fertilizer that has more of a specific element you can encourage different types of growth in your plants. Let’s take a look at what they have to offer.


Nitrogen is essential for the production of chlorophyll, which is the pigment in plants that converts sunlight into food. Chlorophyll also stimulates leafy growth, so for grass and other green plants with a lot of foliage, higher nitrogen levels are a must. Of course, for trees or shrubs that only produce leaves once a year, high nitrogen isn’t as necessary. For flowering plants, less nitrogen is needed since it will actually deter flower or fruit production.


Phosphorus is the macronutrient that stimulates the growth of healthy roots, stems, flowers, and fruit. It is essential for annual flower beds to keep them sprouting every year, as well as seedlings and fruiting plants. A lack of phosphorus can result in weak stems that can’t hold themselves up, buds that don’t turn into flowers, or dropped leaves. For those looking to encourage flowering, using a phosphorus heavy fertilizer can help.


Also called potash, potassium is an essential nutrient that affects all of the metabolic processes of the plant, encouraging healthy development of the roots, stems, flowers, and fruits. It also ensures that the plant uses water and resists drought, and recovers quickly from damage due to weather, insects, or disease.

Nutrient Ratios In Fertilizer

As we mentioned above, the NPK numbers refer to the percentages of each of the three macronutrients in the package of fertilizer. If you are growing flowering plants, a formulation that is higher in phosphorus will help increase flower production, for example.

Those looking for a greener lawn or focusing more on leafy plants in their yard or grown indoors should stick to a fertilizer that is higher in nitrogen for lush growth. There are also balanced fertilizers available, such as 10-10-10 options, which have equal parts of all three of the macronutrients for plants that need a bit of everything.

Since not every type of plant needs the same types of macronutrients to thrive, it is best to check each plant’s specifications and growing instructions before adding any type of fertilizer. Doing so will guarantee that you’re using the right fertilizer for all of the plants in your care.

In most cases, unless you have specific goals, going with a balanced fertilizer is a safe bet. This is generally our recommendation as it gives your plants all the nutrients they need without going too heavy into any particular one.

Different Forms Of Fertilizers

As well as different levels of macronutrients in the fertilizer, there are also a few different forms to choose from. These vary in application method and how the nutrients are released into the soil.

Liquid Fertilizers

Liquid fertilizers are water-soluble and require dilution using a hose or watering can to apply them to the soil. The nutrients are released quickly, so the plant’s health will actually be improved in a matter of days. The downside to liquid fertilizers is that their nutrients only last a few weeks, so you need to add them more frequently to ensure happy plants.

Quick-release Granular Fertilizers

Quick-release fertilizers have a powdery consistency and are water-soluble, so they also need to be mixed with water, though not in the same way. The granules are sprinkled over the surface and then mixed with the topsoil. Then water is applied to decompose the fertilizer and release the nutrients. This form of fertilizer lasts for about 3 to 4 weeks for fewer applications though they take about a week to improve the plant’s condition.

Slow/Controlled-release Fertilizers

Unlike the other two forms of fertilizer, slow- and controlled-release options are not water-soluble. These granules are spread over the surface of the soil and then covered with more soil or compost. At this point, the fertilizers decompose, slowly releasing their nutrients to give the plants the food they need.

This process can last between two and nine months, depending on whether you use slow-release or controlled-release fertilizers. How quickly the nutrients are released for controlled-release nutrients depends on the soil temperature. Slow-release fertilizers also rely on the soil temperature for the decomposition, though moisture and pH levels also factor in.

Plants need about two weeks after fertilization to show any improvement but this does have benefits, like lower risk of burning and longer-lasting nutrients. You likely only need to use this type once or twice during your plant’s growing season, making it the easiest option of the three.

When To Fertilize Your Plants

The best time to fertilize your plants is in the early spring when the plant’s growth cycle begins. Depending on the type of fertilizer you use and the plant you’re adding it to, you may also need to add more fertilizer throughout the spring and summer. In the late fall and winter, when the plant goes into its dormant cycle, you can halt the application of fertilizer until the next growing season.

Can You Over-fertilize Plants?

Over-fertilization is a possibility if you’re adding too much at once or applying it too often. Poor drainage, high soil temperatures, and even excessive moisture can also contribute to over-fertilization. The symptoms could include discolored or limp roots, little to no growth, or yellow, wilted, or dropped leaves. If left untreated, your plants could even die, so keep a close eye on them during fertilization to avoid irreversible damage. Make sure you’re following the instructions on your fertilizer and understand your plant’s needs to avoid over-fertilization.

Fertilizer Numbers

Having an understanding of your fertilizer can help you make better decisions when it comes to your plant. While a balanced fertilizer is a great starting point, you can encourage more robust growth by tweaking the composition of your fertilizer. Armed with this knowledge, you’re be better able to care not only care for your plants, but also to deal with issues as they arise.

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