What is Worm Castings Fertilizer – Is it a Complete Solution?

You have probably heard or read about this black gold magic called worm castings fertilizer. But just how good is it? Does it actually contain good nutrients, and can it be used as a substitute for fertilizer? We’ll answer these questions and more in this article.

What is Worm Castings Fertilizer


  • worm casting is a nutrient-rich natural form of fertilizer
  • worm castings may require amendments when added to pH-sensitive plants
  • worm castings do not have high levels of NPK. However, they work in different ways to benefit plants

The saying ‘you get out what you put in’ applies to gardening and fertilizer.

Using quality, rich fertilizers provide the fundamental building blocks for plants to grow, produce flowers and fruit, and even create the next generation.

So understanding what you are feeding your plants is paramount to being a successful gardener and having a green thumb.

In this article, we will shine a light on worm castings and their role in fertilizing your plants and soil.

So if you are ready, grab a coffee, and let’s get started.

What is Worm Castings Fertilizer?

First, let’s clarify today’s topic – what exactly is worm castings fertilizer?

In short, it is worm poop.

That’s right, you read correctly. We are talking about the excrement of an animal. And we cannot get enough of it!

It is packed full of nutrients and all the good stuff that plants use for fuel to thrive.

If you want to learn more about earthworm castings and how they are formed, check out our in-depth article What are worm castings?

This article, however, focuses on their role as a fertilizer.

Worm Castings Fertilizer: A Closer Look

Now that the cat is out of the bag, we are discussing worm feces. Let’s take a magnifying glass to it and break it down.

worms and worm castings

What NPK Ratios do Worm Castings Have?

If you’ve been around the garden space for a while, you will have come across the acronym NPK before.

For those new to this space, NPK stands for:

  • Nitrogen (N) – for speedy, vigorous growth, leaf structure, and produce production,
  • Phosphorus (P) – to sustain and grow the root system and develop flowers, and
  • Potassium (K) – aids in internal processes like photosynthesis and overall plant health.

When it comes to feeding your plants, gardeners are obsessed with knowing the NPK ratio. It could be 1:1:1, 1:2:1, etc…

To be honest, some gardeners are a little TOO obsessed with the numbers.

Don’t get us wrong. It is helpful to know what you are feeding your plants. However, it isn’t the only thing that you should focus on. Plants require other nutrients and minerals, not to mention the role of the macro and microfauna in soil.

Generally speaking, the amount of NPK in worm casting fertilizer will be less concentrated than commercially made synthetic fertilizers. But that doesn’t mean they are any less effective or inferior.

For those wanting a worm castings NPK ratio, it is roughly 1:0:0.

Worm castings fertilizer has less NPK than commercially made fertilizers from stores.

Worm castings deliver many other critical benefits to the soil that fertilizers do not – not just a nutrient boost.

Let’s now look at the other important aspect of gardening with fertilizers – pH levels (acidity and alkaline).

Worm Castings pH Levels

The pH level is another variable that needs to be considered in the soil. Or in other words, if the soil is acidic or alkaline.

If you are considering adding worm castings to your soil, it is essential to understand how it will impact the pH levels in your soil. And ultimately, your plants health.

testing pH levels of soil

The good news is worm castings generally have a neutral pH. This means it is neither acidic nor alkaline, and the pH reading of 7.0.

Worm castings have a neutral pH reading of 7.0.

If anything, it may have a slightly acidic pH, which means it can measure between 6.5 and 7.0.

We recommend adding some soil amendments if you are adding worm castings to plants that prefer an alkaline soil environment, such as hyacinth, gladioli, and lilac flowers.

Add additives like garden lime, baking soda, or wood ashes to make the soil more alkaline. Or you can use commercial products available on online marketplaces like Amazon.

Are Worm Castings a Complete Fertilizer? – The Big Question

Okay, let’s tackle the elephant in the room – are worm castings a complete fertilizer solution?

While we can bang on about worm castings for days on end, it is NOT a complete substitute for fertilizer in your garden.

We say this because of the concentration (or lack of) of nutrients present in the castings.

Since chemical fertilizers are synthetically produced, they can be manipulated to contain the exact amount of nutrients required for your plants, including higher concentrations. There are apparent benefits of worm castings to be had with this, such as producing tailor-made fertilizers for roses for flower production, etc.

Rather than considering worm castings as a substitute, we consider them a complement to fertilizer for soil health.

This brings us to our next point. Comparing worm castings and fertilizer is not a like-for-like comparison. The purpose of using each is different for your soil and plants.

The primary purpose of fertilizers is to provide the necessary nutrients for a plant.

While worm castings can supply nutrients, we consider their primary purpose to be improving the composition and properties of your soil—improvements like soil aeration, water retention, sustaining the soil’s microfauna, and much more.

Can You Use Worm Castings and Fertilizer Together?

Yes – absolutely!

If it wasn’t evident from the last section, we highly recommend using fertilizer and worm castings together.

From our perspective, they serve different purposes and complement each other nicely.

For feeding, we use fertilizers that provide specific nutrients for our potted plants. At the same time, add worm castings organic fertilizer as a soil top dressing or liquid fertilizer to supplement the core nutrients, such as NPK, iron, sulfur, calcium, and humic acid.

Compare the Results – Before and After Worm Castings

To give you a bit of motivation to start using worm castings in your garden, check out the results of an experiment Reddit user iveo83 did with two containers with the same seeds planted at the same time.

To keep things consistent in the experiment, they used the same soil base and placed it in the same position for light and temperature.

The only difference is 1-2 inches of worm castings mixed into the topsoil.

worm castings vs no fertilizer
credit: reddit

Well, the results speak for themselves. Look at the difference in size and growth rate of the container that used worm castings.

In just two weeks, there is significant growth, with more of the seeds germinating and sprouting.

If we weren’t already enamored with worm castings, we would be after seeing these results.

What is Worm Castings Fertilizer – Frequently Asked Questions

If you are new to the world of fertilizing and worm castings, it is easy to understand you may have some questions. So we’ve included a FAQ section to cover the most common questions.

Can worm castings burn plants?

One of the positives about worm castings not having high concentrations of nutrients is it is safe to use on plant leaves and stems.

This means it is safe to use as an organic matter liquid fertilizer on your plants. This is known as worm tea and is made by mixing worm castings with water. The low nutrient content ensures no excess nutrients remain on foliage, resulting in nasty fertilizer burns.

Can you use too many worm castings?

Another benefit of the low concentration of nutrients means using an excess of worm castings won’t result in nasty burns to your plant root system.

Does it mean you should use 100% worm castings organic material for your soil?

We don’t recommend using pure worm castings for your plants because we mentioned earlier that they might not contain all the essential ingredients required.


References Used

  • pH. (2022, December 1). In Wikipedia. https://en.wikipedia.org/wiki/PH