Ever wondered why plants need fertilizer or what the numbers on the label mean? Want to understand why your plant needs nitrogen or when you should fertilize? This post addresses these questions and more!

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What is fertilizer?

Fertilizer is a product that provides nutrients to your plants.

This is necessary for potted plants because the soil in a container has a limited amount of nutrients which becomes depleted over time. Fertilizer adds those nutrients back.

It is possible to buy fertilizers in a number of varying types: liquids, granules, spikes, and more.

How often should a plant be fertilized?

Most houseplant resources recommend only fertilizing once or twice per month at a quarter-strength or a half-strength of the recommended dosage on the bottle.

The recommendation to cut the strength in half or more is because overfertilization can cause serious issues with your plant. Erring on the side of under-fertilization is the safest way to prevent harm.

See the section below on salt buildup and fertilizer burn for more info about the risks of overfertilization.

Is fertilizer a plant’s food?

Nope! Plants use light as their food source. Fertilizer is more like a vitamin or nutrient boost.

That isn’t to say these nutrients aren’t important. They are!! But they are not the plant’s main source of food.

A plant can grow without fertilizer if it has the proper amount of light, but its growth may not be as healthy without fertilizer.

When should a plant be fertilized?

Plants should be fertilized during the growing season or the time of year that plants grow where you live!

For those who have winter, the growing season would be spring, summer and fall.

For those in a tropical or subtropical area, plants can grow all year round!

To learn more about the growing season, check out this resource from the National Geographic.

What do the numbers on a fertilizer label represent?

The three numbers represent the amount of Nitrogen, Phosphorous, and Potassium present in the fertilizer. They are consistently listed in that order.

20-20-20
 N – P – K 

20%20%20%
NitrogenPhosphorousPotassium

What do the numbers mean for fertilizer strength?

Each number is the percentage of each nutrient present in the fertilizer.

If the fertilizer strength is 10-15-10, this would mean that:
10% of the fertilizer is Nitrogen
15% of the fertilizer is Phosphorous
10% of the fertilizer is Potassium

What is the other 65% of the fertilizer then?

Great question! That depends on the fertilizer. Some fertilizers include other nutrients and trace elements. These will be listed on the label as well.

The remaining percentage would be some type of filler.

Does fertilizer strength matter?

Yes! Here’s why:

  • Strength can tell you what a specific fertilizer was created to promote (flowers, foliage, etcetera).
  • Strength should also be used as a reason to proceed with caution.
    • The higher the numbers are in a fertilizer, the easier it is to overfertilize.
  • Strength can help you determine whether a fertilizer is likely to be chemical or organic
    • The higher the numbers, the more likely a product is a chemical fertilizer

Fertilizer causes salt buildup?

Chemical fertilizers contain a lot of salts, which allow the nutrients to be water-soluble.

These salts can build-up in the soil of your plant’s pot over time and cause serious problems for your plant, often referred to as fertilizer burn.

What is fertilizer burn?

High salt concentration in the soil will burn the roots of your plant, preventing the roots from being able to absorb moisture or nutrients. Without the ability to drink or absorb nutrients, a plant will eventually die.

Damaged roots and the problems it causes for the plant above the soil is called fertilizer burn.

Overfertilization and fertilization burn can be visually detected by looking for these symptoms:

  • a crust on your plant’s soil
  • browning leaf tips
  • an increase in leaf loss
  • older, mature leaves wilting
  • stunted or halted growth
  • blackened or browned roots

*** These symptoms can be caused by other issues as well, such as pests or overwatering, so it is important to check your plant for other issues simultaneously.

What does nutrient deficiency look like?

Nutrient deficiency can appear as:

  • lack of growth
  • yellowing between leaf veins
  • leaf wilting
  • brown, crispy leaf tips
  • red or purple-stained leaves
  • yellowing of older, lower leaves

*** Nutrient deficiency can look a lot like symptoms of overwatering or pest problems, so be sure to investigate whether other issues are present before diagnosing a nutrient deficiency.

To see examples of how a plant looks if deficient in specific nutrients, check out this article.

How does a plant use each major nutrient in NPK?

Knowing what each of the major nutrients does for your plant can help you to understand what type of fertilizer to buy.

For example, if your plant is currently forming buds and getting ready to bloom, you may want to use a fertilizer that is higher in Phosphorous to support its blooming.

Use the information about each nutrient below to understand more about your plant’s needs and which fertilizer is right for you and your plant.

Major Nutrient – Nitrogen (N)

Nitrogen promotes:

  • photosynthesis
  • healthy growth
  • dark green, lush foliage

Major Nutrient – Phosphorous (P)

Phosphorous supports:

  • healthy root systems
  • healthy, strong stems/shoots
  • healthy blossoms and bud growth
  • fruit stimulation and growth

Major Nutrient – Potassium (K)

Potassium (also commonly referred to as potash) promotes:

  • general health
  • disease resistance
  • stress tolerance
  • plant strength
  • healthy growth

Organic v. Synthetic Fertilizers

Organic Fertilizers:

  • are formed from natural sources like compost or bone meal
  • often contain a wider range of nutrients, beneficial bacterias, and microbes
  • have low concentrations of nutrients that must be made available to plants through processes done by bacteria and fungi in the soil
  • will not form a crust on the soil
  • feed beneficial organisms in the soil and promote an environment that maintains a healthier, less compacted soil over time
  • are often more expensive because they are less concentrated

Synthetic or Chemical Fertilizers:

  • are formed through manufacturing processes
  • usually contain only a few nutrients
  • are more highly concentrated and in a form that is immediately available to plants
  • may create a crust on the soil if the salts build up too high
  • can cause fertilizer burn from salt buildup
  • are often cheaper because they are more concentrated

Other nutrients/elements

While nitrogen, phosphorous, and potassium are the nutrients deemed to be most important, there are other important nutrients and elements that your plant is dependent on as well.

Other major nutrients your plant needs are calcium, magnesium, and sulfur.

Trace elements your plant needs are boron, copper, iron, chlorine, manganese, zinc, and molybdenum.

Plants also require carbon, oxygen, and hydrogen, which they obtain from air and water.

In total, plants need 16 nutrients and elements to live and thrive.

To learn more about what each of these nutrients and elements do for your plant, check out this link.

Click to read last week’s post: For People Who Want to Understand Botanical Latin



A Natural Curiosity
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