What are the 3 fertilizer numbers and how to understand them

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  • The three digits on fertilizers are the NPK ratio, which stands for nitrogen, phosphorus, and potassium.
  • These three nutrients have distinct functions and are essential for the growth and health of a plant.
  • Fertilizers can be organic or synthetic and come in dry or liquid form.
  • Visit Insider’s Home and Kitchen Reference Library for more stories.

As most plant parents know, feeding your plants is an essential part of their care routine. Plant fertilizers are intended to increase the fertility of a plant and promote its growth. Typically, they are either organic (plant or animal based) or inorganic (man-made chemicals), and some are water soluble (water soluble) while others are dry (granules). ).

Choosing the right fertilizer can seem overwhelming, especially when it comes to deciphering the numbers on the label and knowing what type of fertilizer will work best with different types of plants. Erin Marino, Editorial Manager and Plant Expert at The Sill, shares her expertise on plant fertilizers so you can feel confident and informed the next time your plants need to be fed.

What does each number mean?

Plant fertilizers represent three numbers on the label indicating the ratio of each nutrient, or the NPK ratio. The first number is nitrogen (N), which increases leaf production. The second is for phosphorus (P), which promotes fruiting and flowering, and the third is for potassium (K), which promotes hardiness and is essential for the metabolic process. These three elements are essential to support the health of any plant.

“You can have an all-balanced fertilizer, for example you would see 10-10-10 for houseplants, or choose 15-30-15 depending on what you are trying to facilitate for your plant,” says Marino.

Each number represents the percentage by weight of each nutrient in the package; for example, if a vegetable fertilizer contains a ratio of 10-15-10, it means that it contains 10 percent nitrogen, 15 percent phosphorus, and 10 percent potassium. The remaining 65 percent is a combination of other micronutrients such as iron,


zinc

, boron and copper.

A higher concentration of any of the three macronutrients will impact a plant’s needs in correlation with the function of the macronutrient. In other words, a high nitrogen content will lead to greater leaf growth, a high phosphorus content will lead to more fruiting or flowering, and a high potassium content will lead to greater hardiness.

“Nitrogen, phosphorus and potassium are the main macronutrients your plant needs,” says Marino. “Fertilizers with higher ratios are more concentrated than those with lower ratios, so if you see high numbers you’ll want to dilute with more water before you fertilize your plants.”

Which fertilizer numbers are right for your plants?

Overhead shot of brightly colored bromeliads in a field.

Flowering plants like bromeliads prefer fertilizers that are high in phosphorus.

Haruo Amano / EyeEm / Getty Images


While balanced, all-purpose fertilizers with a 10-10-10 or 5-5-5 ratio are best for most houseplants, reducing the plant’s needs will allow you to choose a fertilizer with the number that best meets these needs. .

“If you’re new to using fertilizer, it’s best to look for one that meets your specific needs,” says Marino. “Ratio is determined by goal, so looking for the goal first will help you find the right gear.”

If the plant is green and leafy, a higher nitrogen content is better. If your plant is a flowering or fruiting plant, a higher phosphorus level is preferable. If you are growing vegetables, high potassium content is best.

If you don’t already know that flowering plants like anthurium and bromeliads will benefit from a fertilizer with a higher phosphorus content, or leafy plants like fiddle leaf figs and monsteras will benefit from fertilizer. higher nitrogen content, check with your local nursery or a simple Internet search on your plant’s fertilizer needs can help.

“All three nutrients are essential, but some plants may benefit more than others,” says Marino. “For example, the popular houseplant pothos needs all three nutrients, but it will benefit more from a higher nitrogen ratio because it is a fast-growing creeper that produces many leaves. than a fertilizer with a higher phosphorus ratio, because this is not the case. bear fruit or flowers. “

Types of fertilizers

Once you understand the meaning of the numbers on plant fertilizer, the next step is to differentiate between organic and inorganic, and dry (granular) versus wet (liquid).

Wet versus dry

While Marino says choosing between wet and dry is more of a preference, there are pros and cons to both.

“Granular fertilizers tend to be used more outdoors and are generally less expensive, so they can be used for large crops and have a better shelf life,” says Marino. “For houseplants or small potted plants outdoors, I recommend using a liquid fertilizer that allows you to dilute the fertilizer with water (over-fertilization can kill a plant) and can go down to where the roots are. “

Most water soluble plant fertilizers have label instructions on how to dilute the fertilizer, but as a general rule a half-quarter to quarter dilution is recommended, which is approximately half a teaspoon per gallon of water.

Organic vs inorganic

The choice between organic and inorganic, or synthetic, fertilizers may also be preferred. But it can also depend on the nutrient and growth needs of your plants, as organic fertilizers work in a slow and natural way, while synthetic fertilizers work quickly.

“Synthetic fertilizers tend to be more concentrated, so if you buy them in liquid form and dilute them, it can be more profitable. You can use less and get the same amount of nutrients, ”says Marino. “Organic fertilizers are made from natural ingredients, so they tend to be less concentrated and break down longer when applied. This helps prevent over-fertilization and provides stable nutrients over a longer period of time. “

When to fertilize

Someone picking a radish in a garden.

Different plants will have different fertilizing needs, like edible crops, which should be fertilized before planting in the spring.

Emily Suzanne McDonald / Getty Images


The advantage of most fertilizers – whether organic, synthetic, wet, or dry – is that they have back label instructions on how and how often to fertilize. For houseplants, the rule of thumb is to fertilize every four to six weeks and about every two to three weeks during their growing seasons in the spring and summer.

For gardens with edible crops, a granular or dry fertilizer should be mixed with the soil before planting in the spring. For perennial flowering gardens, fertilize before the spring growing season and do not fertilize during a frost.

Over-fertilizing plants is usually worse than picking the wrong numbers, as over-fertilizing can cause plants to burn and weaken them. If your plants’ needs are different from what is listed on the fertilizer numbers, the plants will usually show signs.

For example, choosing a fertilizer rich in phosphorus even if the plant is not flowering will not produce green leaves, thus proving the need for a fertilizer rich in nitrogen which increases leaf growth. Knowing your plant is the best way to determine how many fertilizers to prioritize, according to Marino.

Insider’s takeaway

Choosing the right fertilizer doesn’t have to be a daunting process anymore with the right knowledge at your fingertips. Understanding the NPK ratio, the plant’s needs, and the pros and cons of different types of fertilizer can help you make the right decision and help nourish your plants to keep them healthy throughout the season.

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