Worm Castings 101: Introduction to Gardening with Worm Castings

Ever wonder why some gardens have plants that look lush and healthy while your plants struggle to thrive? The answer may be down to a garden super ingredient; worm castings. Discover everything you need to know in our beginner’s guide to ‘What are Worm Castings?’

What are worm castings


  • Worm castings are produced as a by-product of worms as they work their way through soil and organic matter (aka worm poo)
  • Castings contain many beneficial nutrients and minerals that plants can use during all stages of development.
  • Worm Castings have many applications for indoor and outdoor gardens.

Welcome to the wonderful world of worm poo!

Yes, that’s correct. At the Garden Bench Top, we love worms and their excrement!

And by the end of this guide, our goal is that you will love the stuff, too, and understand why we just cannot get enough of it!

credit: tenor

So grab your gloves, and let’s get our hands dirty!

What Are Worm Castings?

So why do we get so excited about something that comes out the wrong end of an equally off-putting creature?

Because plants love worm castings (or worm poo), anything that improves our plants’ health or productivity is automatically in our good books.

Worms form part of nature’s army of macrofauna that helps break down organic soil materials into usable nutrients and minerals.

As worms eat and munch their way through the ground, they consume unprocessed materials, like leaves from trees and plants or food scraps, and convert them into worm castings. To put it differently, Worms essentially process raw, complex nutrients into a form that can be absorbed into a plant’s root system.

Other Name(s) for Worm Castings

Before we move further, we thought it would be wise to identify the other names (or synonyms) that other gardeners may use for worm castings.

Worm castings are also known as:

  • vermicast/vermicastings
  • earthworm castings
  • black gold

We particularly like the last nickname because it is true! Worm castings are like striking gold for a gardener! And well, the black element of the nickname provides the perfect segue into the next section – worm castings appearance.

What do Worm Castings Look Like?

Okay, the black gold nickname may be slightly misleading. While worm castings’ color is dark, it is not strictly black.

Worm poo is more of a dark brown color and will often match the color of the soil the worm has been consuming.

Fresh worm castings look like small spherical-shaped bits of soil. It almost looks grainy from afar.

Although it may be clumped in parts, the consistency of the castings will be even. This makes sense because the organic materials and soil must pass through tiny thin worms.

Actually, you may have unknowingly seen worm castings before on your travels through parks or fields. Have you ever seen strange clumps of soil on the surface of a lawn or field?

lawn with worm castings
lawn with organic worm castings

These balled-up clumps of dirt are the by-product of earthworms as they tunnel down into the soil, also known as worm castings!

What Do Worm Castings Do for Soil?

As mentioned, worm castings are processed soil and organic materials into usable minerals and nutrients. The building blocks essential for healthy growth.

So by adding pure organic worm castings to garden soil, you are providing an instant natural boost of nutrients to your soil.

It is like a light natural fertilizer, which is one of the primary purposes we incorporate worm castings into our garden regimen.

But did you know worm castings also change the properties of soil?

Adding worm castings to soil:

  1. Increases the water-retaining properties of the soil. This is particularly important for outdoor garden soil that may become dry and hydrophobic and begins to repel water.
  2. Aerates the soil – the simple act of mixing the worm castings into the soil will aerate the soil. However, worm castings also generally contain worm eggs that will hatch and tunnel their way around the soil. This creates airways in the ground, allowing oxygen to reach the roots of the plants.
  3. Food source – we mentioned worms were a part of the macrofauna in the soil. But did you know their castings provide a valuable food source for the microfauna that also lives side-by-side with the worms? The role of microfauna is the same – they are an integral part of the soil that helps to break down nutrients into usable forms.
soil with worm castings
soil with worm castings

What Do Worm Castings Do for Plants?

If it isn’t apparent yet, worm castings are a valuable source of plant vitamins and minerals. They feed and sustain plants, providing the foundational building blocks of plant growth.

Worm castings supply nutrients and minerals for plants to grow healthy and strong.

Because the castings are an all-natural fertilizer (versus a manufactured commercial fertilizer full of chemicals), they are safe for seedlings.

Plus, you can turn it into a liquid fertilizer (known as worm wee – who comes up with these names?!) and spray it all over your plants without fear that it will leave chemical burns.

What Are Worm Castings Good For?

Okay – so full disclosure.

If you want an unbiased answer to this question, you may be in the wrong place.

We love worm castings and use them every opportunity we get. And if you cannot tell by the tone of this article – worm castings get us excited!

credit: giphy

How to Use Worm Castings

Some examples of practical applications for worm farms include:

  • Liquid fertilizer for your outdoor plants – mix it into your watering container and apply it to your plants while watering.
  • Bottom feeding your houseplants – include some worm wee in distilled water you use when bottom watering your indoor plants.
  • Rejuvenate old potting mix – give tired potting mix a lift by adding worm castings. Not only will it provide an immediate nutrient boost, but it will also aerate and increase the water retention of the soil.
  • Seedling starter mix – worm castings are a mild form of fertilizer. This is perfect for starting seedlings, which can be sensitive to commercial fertilizers, that may be too strong.

These are just a few ways to use worm castings in your garden. Your imagination only limits your options.