What Worm Castings Are & How They Help Plant Growth

This blog post is a subsection of our Worm Castings 101 - Everything You Need To Know guide. In the 101 guide we cover multiple topics about worm castings, so if you'd like a more thorough walk through of what worm castings are and what they have to offer then click through to read more!


Worm castings have quickly gone from being a virtually unknown substance to becoming a go-to organic fertilizer in a gardener’s toolbox. Despite this rise in popularity there isn’t a large repository of information regarding what worm castings are and how they help a plant in its lifecycle. I’d like to help provide some useful information on what exactly worm castings are and how they work in the garden!

What are Worm Castings?

Worm castings are the end result of a worm eating and digesting organic matter. In other words, it’s worm poop! Don’t worry - they’re nothing to be queasy about as they’re non odorous (seriously!) and packed with tons of goodness for plants. 

If you want to learn about the benefits of worm castings then we go through a host of those benefits in our blog post, The Benefits of Worm Castings.

Across the internet worm castings have also been called vermicompost and vermicast, almost interchangeably. However, there is a tiny difference between them. Technically, vermicast is pure worm castings / worm poop while vermicompost will include other organic matter. Like I said though, the difference is minimal so use whatever feels right!

Worm castings being sprinkled into a raised bed

How Do Worm Castings Help Plant Growth?

As you’ll read in our The Benefits of Worm Castings blog post, there are many advantages to using worm castings in your garden, such as:

  • Increase plant yield
  • Protect your plants from diseases and pests
  • Increase soil water retention
  • Increase nutrient uptake

But how does this happen? Let’s break things down a bit further because it is a fascinating look into how plants grow and how worm castings actually improve this process. First, let’s list off exactly how worm castings improve your plant’s growth and then dive into each one:

  1. Increases the microbes in the soil around your plant
  2. Increases the soil’s water retention
  3. Slow-release of nutrients

Let’s look at the first two at the same time because they work in unison to improve a plant’s nutrient uptake. But before we do I’ll break down how nutrients are taken in by your plants.

Plant Nutrients

Like all living things, plants need nutrients to grow and thrive. There are three levels of nutrients: primary nutrients, secondary nutrients, and micronutrients. All of these nutrients are essential to a plant’s growth.

  • Primary nutrients are usually required in the largest amounts. Examples include nitrogen, phosphorus, potassium, carbon, hydrogen, and oxygen.
  • Secondary nutrients are usually needed in moderate amounts compared to the primary essential nutrients. Examples include calcium, magnesium, and sulfur.
  • Micronutrients are required in much smaller amounts. Examples include boron, manganese, and iron.

Microbes and Plant Growth

Microbes are found in massive numbers in worm castings, and it’s the core difference between regular old compost and worm castings. So much so, a red wiggler's intestinal tract contains approximately 1,000 times more microbial life than the food it consumes. The question though is why is it important to have microbes in your soil? And the answer to that is because microbes are what break down decomposed organic matter found within the soil into the form nutrients are able to be absorbed by the plant. Without microbes, some nutrients in the soil would not be broken down enough to be used by plants.

Water Retention and Absorption

Aside from carbon, all of a plant’s nutrients are taken in through its root system. There are two distinct methods of nutrient uptake through a plant’s root system: root interception and mass flow.

One way nutrients are absorbed is through a process called root interception. Root interception refers to when roots grow and extend further into the soil. As the roots grow and come into contact with new soil, the roots then take in the nutrients found within the newly contacted soil.

The second method of nutrient uptake is called mass flow, which refers to a plant absorbing water for the transpiration process. The transpiration process sounds fancy, but all it is is the absorption of water into the plant, which is then transported up through the plant where it then evaporates through the leaves. The evaporation of water through its leaves is what cools the plant down. It’s akin to humans sweating. Admittedly that is a very simplified explanation of the process, however, it’s important to know because the vast majority of a plant’s nutrient uptake is absorbed through the water that is taken in for the transpiration process.

Transpiration Process of a Plant
Transpiration is when water is absorbed through a plants roots and eventually evaporates through a plant's leaves.

While both methods provide nutrients to the plant, the vast majority of nutrients are taken in through the mass flow process described above.

Plant nutrient chart

Table from Texas A&M Agrilife Extension - Essential Nutrients for Plants.

This is why having water in the soil is important. Thanks to worm castings’ ability to retain up to three times its weight, your soil will have more water to draw from.

Worm Castings “Slow Release”

A big benefit to worm castings is its microbes breaking nutrients in the soil down into the form necessary for plant absorption. However, there are micronutrients found within worm castings, and just because micronutrients are needed in much smaller quantities than the primary and secondary nutrients does not mean they are not necessary for a plant’s growth. Quite the opposite - the are just as important as the primary and secondary nutrients.

What’s wonderful about worm castings is that the nutrients found within them are slow-release. This means that its nutrients are not released all at once. The mucus that cover the castings (which happens inside the worm’s digestive tract) is what allows this “time release” effect. This means that by adding worm castings to your garden you will ensure continual uptake of these essential micronutrients.

Worm Castings Are a Powerhouse in the Garden

Thanks for taking a bit of a deeper dive into what worm castings are and how they can be a huge help to your plants. Plant health and growth are a fascinating process and I hope this breakdown helped you better understand plant growth and built up your fascination as well.

Worm castings are unique to the gardening world thanks to the unique characteristics they have due to traversing the worm’s digestive system. As worm castings continue to gain traction we will continue to learn more about this precious gardening resource, so stay tuned as we’ll be paying close attention to any new learnings and studies that come out!

1 comment

  • Keep me posted very interesting.


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