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Understanding fertilizer: How and when to use fertilizer for your indoor or outdoor garden needs

Ferry-Morse Home Gardening Blog | 2024

Gardener putting fertilizer down in a garden bed.

Gardener putting fertilizer down in a garden bed.

Have you ever found yourself in the home improvement store, staring at a wall of fertilizers, all the packaging colors melting together like a mosaic of crayons left in the sun?

Simply put, fertilizer helps plants grow by adding absent or depleted nutrients to the soil. And in this article, we’ll help you focus on finding the right one for you and your plants!

Specifically, we’ll cover the information you need to fertilize correctly and responsibly in your garden and for your houseplants. Let’s zoom in!

Why do gardeners use fertilizer?

Gardeners use fertilizers to:

  • address soil deficiencies;
  • help newly planted varieties establish; and
  • meet the specific nutrient requirements of different plant varieties

When should gardeners use fertilizer?

Fertilizer is best applied as per the results of a soil test that shows nutrients needed to bring the soil to the nutrient or pH level that meets the needs of plants in a garden.

There are also times of the year when it’s best to apply fertilizer, like early spring, or not to use it, such as during a hot and dry summer when plants are stressed by drought. It makes sense, right?

Gardener preparing to test their soil and determine if it is in need of fertilizer.

Gardener preparing to test their soil and determine if it is in need of fertilizer.

What’s NPK?

Ever look at a package of fertilizer and see three oversized numbers staring back at you?

Here’s what they stand for:

  • Nitrogen (N): Promotes leafy top growth
  • Phosphorus (P): Helps fruit-set and root development
  • Potassium (K): Increases cold hardiness, disease resistance, and durability

NPK represents the primary nutrients in fertilizer, and the numbers show the percentages of each nutrient in the fertilizer, and they are always in the same order as N-P-K.

The essence of using fertilizer correctly is to learn to read the label and then use your soil test results to know which fertilizer your plants need.

For example, fertilizer with a ratio of 10-10-10 fertilizer has ten percent N, ten percent P, and ten percent K, in that order. You might be wondering why the nutrient percentages do not add up to 100 percent. It’s because the fertilizer includes other components, like inert carriers, etc. 

Organic vs. chemical fertilizer

Organic fertilizers consist of things like manure, fish by-products, and blood meal—remains of once-living organisms. Inorganic fertilizers are manufactured from natural materials and chemicals.

But the bottom line is this: Plants only care about receiving their required nutrients, whether they come from organic or chemical fertilizer. So, it boils down to your personal choice.

Most importantly, whatever fertilizer you use, always follow the instructions on the label to avoid damaging your plants and the environment. And that’s it!

What’s the best NPK for my plants?

Find out the best NPK combination for your plants by performing a soil test, which helps you know if you’re adding the right fertilizer to your plants at the right time because it determines nutrient levels in the soil.

Otherwise, you’re kind of just guessing, which can lead to damaged, unproductive, or unattractive plants. For example, applying too much nitrogen at the wrong time can hinder fruit-set in tomatoes and cucumbers, and who wants that?

Close-up of some inorganic granular fertilizer on a table.

Close-up of some inorganic granular fertilizers on a table.

What’s the difference between granular or liquid fertilizer?

Granular and liquid fertilizers differ in the following areas:

CostGranular fertilizers tend to cost less.
ConsistencyLiquid fertilizer’s nutrient composition is evenly distributed in the solution, which is not the same for each granule of dry fertilizer.
MobilityLiquid fertilizer is more mobile in irrigated soil. 
Salt contentGranular fertilizer has a higher tendency to burn plants through improper application.

Decide which one fits your budget and is easiest for you to handle, and the best part is that if you ever have to switch between the two, here’s a handy conversion chart to help.

What’s the best fertilizer for a vegetable, flower, or herb garden?

To keep it simple, here are the general nutrient needs of vegetable, flower, and herb plants:

  • Annual flowers: Medium
  • Herbs: Low to medium
  • Perennial flowers: Low to medium
  • Vegetables: High

It’s also important to remember that fertilizer designed for lawns and turfgrass is usually too potent for vegetable, flower, and herb gardens. So, it’s best to avoid those.


Vegetable fertilizer helps make vegetable plants more productive. Nitrogen boosts leafy green vegetables and lettuce, while phosphorus and potassium enhance plants that produce a vegetable from a pollinated flower, like tomatoes. Learn more by checking out this super-helpful video about choosing the right vegetable fertilizer. There’s good stuff there!


Once established in healthy soil, perennial flowers don’t need much fertilizer. Slow-release fertilizer usually does the trick! If plants show signs of nutrient deficiencies, then a soil test will tell you what you need to correct that. Learn a little more about fertilizing perennials.

On the other hand, annual flowers tend to use up nutrients and often need a boost. A combination of healthy soil, slow-release fertilizer, and regular applications of liquid flower fertilizer should keep them looking vivacious until temperatures become intolerable.

Michigan State University published a fantastic guide on fertilizing annual flowers. Check it out!

Herbs and houseplants

As long as herbs’ soil and growing conditions are favorable in-ground outdoors, they only need light fertilizing, at the most.

Herbs growing in pots and houseplants benefit from slow-release pellets or regular application of water-soluble fertilizers. Here’s great advice about applying fertilizer to plants in containers

How often should I feed my plants?

OK, you might be thinking that plants need regular feedings like pets. Well, not exactly. There’s no straight answer to this question because varieties have different needs, and some don’t even need fertilizer.

Flowers: Fertilize new flowerbeds at the time of planting. Once established, times vary depending on if the flowers are annuals, perennials, flowering bulbs, roses, or wildflowers.

Herbs: Lightly fertilize indoor herbs and outdoor herbs in pots about once every two weeks. Outdoor herbs growing in-ground can do with even less, if any, especially if you plan to consume them, because overdoing herb fertilizer may affect their flavor negatively.

Houseplants: Fertilize during their spring-summer growing season with a houseplant fertilizer, like SUPERthrive.

Vegetables: Vegetables are classified as heavy, medium, and light feeders and should be fertilized accordingly. Please see this guide if you’re interested in raising veggie crops. The main thing is not to misapply nitrogen and phosphorus, resulting in too much foliage, chlorosis, or poor fruit set.


What is SUPERthrive, you ask? SUPERthrive is a family of kelp-based vitamin solutions that can supercharge plant germination and vigorous growth.

And here’s the deal, these products are gentle and organic, offering:

  • Fish emulsion formula to help establish seedling transplants and for regular fertilizing
  • Vitamin solution for an all-purpose liquid fertilizer 
  • Balanced 4-4-4 fertilizer with beneficial fungi for help with nutrient and water uptake
Gardener using superthrive plant vitamin solution.

Gardener using superthrive plant vitamin solution.

Feeding your plants might seem complicated, but it’s not!

Avoid getting lost in the periodic table of it all by focusing exclusively on the varieties you’re growing and considering an all-around safe product like SUPERthrive.

If you’d like to learn more about fertilizers, here are links to resources explaining primary plant nutrients and micronutrients from The Fertilizer Institute and Purdue University, respectively. So please give it a go and keep us posted by leaving a comment below!


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