Worm Castings: A (Very) Natural Plant Fertilizer & Booster

Worm castings, or in other words… worm poop.

They make not be the most glamorous of substrate additives, but this potent material can do wonders for your plants.

As with many things in this world, sometimes nature simply does it best. 

Sure, we can synthesize all sorts of interesting fertilizers, substances and supplements these days – which certainly have their place – but your growing supplies don’t all need to look like a pharmacy counter.

It’s time to get our hands dirty (metaphorically) because we’re about to dig in to worm castings 101!

worm castings

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What are Worm Castings?

Worm castings are as natural as they come.

The literal waste of worms, it’s a lot more interesting than it sounds. 

Kind of like that expensive coffee that’s made from civet cat dung (called kopi lawak if you’re interested). The civet eats the coffee cherries and its digestive system somehow produces a superior product full of depth and flavour.

That’s essentially what we have here too with worm castings!

Or at least with nutrients, hopefully not flavour…

worm castings
This is what the worm casting process looks like.

The idea is that the worms digest the soil, processing and refining it, and eventually leaving behind a more much more nutrient rich organic material (with some extra beneficial components).

And before you ask, no – it doesn’t smell like poop.

Lets get that out of the way early.

People often say they have a consistency of fine coffee grounds and I’m inclined to agree. They essentially look and smell like earth.

worm castings/earthworm castings
The final product of Earthworm castings look like any old pile of earth to me…

See worm castings on Etsy.

Benefits of Worm Castings

1. They provide a slow-release fertilizing solution – the digestive tract of the worms coats the material and effectively bundles it up into tiny long-lasting fertilizer granules. Handy!

2. The nutrients are far more bioavailable – worm castings reportedly come with a variety of beneficial microorganisms, and their unique microbial activity helps plants to absorb and utilize the available nutrients.

3. They provide a diverse range of micro-nutrients – worm castings are a complex blend of nutrients. Bringing a lot of other useful elements to a healthy soil mix that regular liquid fertilizers don’t, e.g. iron, sulfur and calcium.

4. They increase water retention and improve soil structure – even without its fertilizing ability, worm castings are an excellent soil additive that can help boost soil aeration and keep your plants fed and healthy.

5. They’re naturally pest resistant – worm castings have been shown to be effective at increasing plant resistance to a variety of pests, including; aphids, spider mites, mealy bugs, and caterpillars.

6. They won’t cause nitrogen burn – some fertilizers can harm plants with excessive nitrogen levels if you’re not careful. Thankfully, worm castings are never going to do this.

worm castings/earthworm castings
Worm castings are quite the wonder material in my books.

Disadvantages of Worm Castings

1. It’s not a quick fix – the way the nutrients are “packaged” in worm castings means they release slowly. So, if your plants are bigger and/or starving for nutrients, you may want to give them a liquid fertilizer boost too.

2. They’re more expensive (than standard chemical fertilizer) – though by no means a particularly pricey product – especially amongst organic fertilizers in general – you’ll get more bang for your buck if you buy cheap liquid fertilizer vs organic worm castings.

3. Quality varies – Both the nutrient diversity and density are going to be primarily driven by the species of worms and what they’re consuming. So the quality can vary a lot.

4. Low NPK values – you’ll find worm castings are often labelled with low NPK values, which are the typical macro-nutrient values that fertilizers are characterized by – nitrogen, phosphorous and potassium.

Though, due to the high bio-availability and diversity of nutrients in worm castings, the NPK numbers really aren’t representative of their true capacity. Earthworm castings are proven to provide a huge boost to plant growth and health.

That being said, if you’re growing leafy foliage plants that need high nitrogen (e.g. Philodendron) then you’re probably going to want to use a combination of worm castings and a general plant fertilizer for best results.

How to Use Worm Castings in Potted Plants

1. As Part of Your Mix

The easiest way to use earthworm castings is to incorporate them right from the beginning.

I like to use them in all my tropical substrate blends.

They’re particularly important if you’re using coco coir as a base (as I tend to do) as you need to add some organic components in there.

Generally, I look for worm castings to make up about 10% of the overall mix. Though others will recommend as high as 20%.

It’s not an exact science but you don’t want to go much more than that – especially in terrariums where you want to tightly control the amount of organic material that’s available to break down.

Tropical terrarium substrate
One of the chunkier tropical substrate mixes I use, with about 10% worm castings.

2. Sprinkle on Top

If you already have your plants potted up and they’re not ready for a substrate refresh, you can simply sprinkle some earthworm castings on top.

That way, some of the nutrients will dissolve in the liquid whenever you water the plant, and will then be pulled down into the substrate as it passes through.

Remember, you tend to lose some soil/substrate out the drainage hole every time you water a potted plant anyway, so this way you’re just topping it back up again.

worm castings/earthworm castings
Just a light sprinkle of worm castings every now and again can help.

3. Make a Worm Tea

Compost tea has been a horticultural staple for years, and worm tea is the natural extension of that.

It’s a fairly straightforward process of adding your worm castings to a cheesecloth sac or similar, then steeping your new “compost tea bag” in purified water.

I’ve personally never tried this, but it”s this versatility that makes worm castings such an interesting natural fertilizer solution.

Worm Castings for Sale

Arguably the biggest challenge with worm castings is finding the best quality sources.

It’s not a highly-regulated product, and without using the typical NPK score to gauge it’s composition there’s always a bit of guesswork/faith involved.

That’s why it’s best to do a little research first.

Earthworm Castings / Vermicast / Vermicompost

The majority of products I see (and use) are earthworm castings.

Whether it’s because earthworms produce the best material or just because they’re everywhere – I don’t know. But, I can personally sing the praises of the stuff.

It’s often sold under the names “Vermicast” and “Vermicompost”.

From what I can tell, vermicast is supposed to be the name for a refined product of castings, and vermicompost the more rough and ready mix of earthworm castings and partially decomposed organic waste – just like a typical compost bin with food waste.

If vermicast is the triple-filtered Russian vodka, then I guess vermicompost is the bathtub moonshine.

But then, they seem to be sold under the names interchangeably so who’s to say?

See worm castings on Etsy.

Alternatively, Make Your Own Homemade Worm Castings

The most efficient way to get high quality worm castings is doing it yourself.

Now, getting your own composting worms may not be to everyone’s taste (and like me, not everyone has a garden) but modern worm bins seem to be only a little more work than a typical compost pile solution.

If you need a way to process organic garden waste products, and you want rich soil with improvements in plant growth, soil quality and essential nutrients – why not?

Frequently Asked Questions

Are worm castings a complete fertilizer?

Worm castings are a fantastic all-round fertilizer and soil conditioner, however due to their low NPK score they can’t be considered a complete fertilizer for all plants.

Can worm castings burn plants?

Worm castings contain low nitrogen levels, and so cannot burn plants.

What is NPK of worm castings?

The NPK of worm castings are low, usually around 1-0-0. Though, it varies a lot between products and isn’t representative of its strong nutrient profile.

What do earthworm castings look like?

Earthworm castings look a lot like coffee grounds. Small black pellets that are soft to the touch.

How much worm castings to add to soil?

The amount of worm castings to add to soil will depend on what you’re using it for, but somewhere in the range of 10-20% is usually best for a balanced mix.

What is the pH of earthworm castings?

Earthworm castings have a neutral pH of 7, making them a versatile solution that can also help to balance the pH of the soil or substrate they’re added to.

Are worm castings poop?

Worm castings are another name for worm poop, though they also tend to include a mix of bedding materials and other organic waste materials.

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