What are Worm Castings? The Full Scoop on Worm Poop

If you're a home gardener you know that finding the right fertilizers and soil amendments can be a bit of a dirty business - many store-bought fertilizers are high in chemicals and toxic ingredients that harm the environment, leach into the soil and run-off and damage surrounding areas.

In this article, we'll give you the rundown on worm castings (aka worm poop) - the product of 500 million years of evolution and an all-natural, highly effective alternative to chemical fertilizers that have also become an increasingly popular choice among green thumbs.

What are Worm Castings?

Worm castings are the microbial-rich excrement (i.e., poop) of earthworms. Earthworms have highly-specialized digestive tracts that process organic waste and produce worm castings rich in enzymes, minerals, and beneficial bacteria - all things that help plants grow faster and with greater yield. And unlike fertilizers, worm castings improve soil structure, enhance soil water retention, and provide nutrients in a form easily absorbed by plants. 

Worm castings are often called a fertilizer but they are really a natural soil amendment.

This means they add essential elements for a healthy soil and also create conditions that can lead to more beneficial microbial activity in the areas surrounding a plant’s root system.

This microbial activity helps release nutrients and minerals in a form that plants can easily use.

What are Worm Castings: Vermicast vs Vermicompost

Worm castings are often called castings, worm humus, worm compost, vermicompost, and vermicast. 

All of these terms refer to organic matter that has been processed through worms’ highly specialized digestive tract. 

Vermicast, however, usually refers to "pure" worm castings, although the concept of vermicast or “pure” worm castings is very difficult to obtain (or prove!).

Vermicast is usually smaller in texture and more granular in size than other forms of worm castings because the worms have processed all or most of the material through their digestive tract. 

Vermicompost and the other terms usually refer to worm castings mixed with organic matter (e.g., compost) not fully processed by worms. Vermicompost is usually "chunkier" with larger particle sizes than vermicast.

Virtually all worm castings products will have some level of organic matter not yet processed by worms (hence all castings products are really vermicompost at some level).

Here's a quick look at the physical differences between vermicast, vermicompost, and compost:

What do Worm Castings Look Like?

Worm castings’ look and feel will vary based on the bedding and feedstock fed to the worms when they produced the castings. 

Most worm farmers use manure, compost, or peat moss as bedding or feedstock for their worms. 

Castings from manure tend to have finer/smaller particle sizes than castings produced from compost or peat moss. 

Some people think castings look like dark sand or spent coffee grounds - it really just depends on the feedstock used with the worms.

In general, almost all worm castings will be a deep, rich dark brown or black color, and their consistency will be similar to but finer (smaller particle size) than finished compost.

What do Castings Smell Like?

Worm castings have a distinctive earthy smell similar to topsoil. Even in large quantities they in no way resemble the stinky smell you expect from most poop you come across!

What are Worm Castings Good For?

Worm castings’ true superpower is the diverse set of benefits they offer soil and plants. Castings provide minerals to feed plants, millions of microbes that unlock nutrients for plants and also suppress diseases and pests, and structure that improves soil porosity and water retention. This diversity is what truly separates castings from compost and traditional chemical fertilizers.

Let’s run through each of these benefits to learn more about what worms castings are good for and the value they offer plant life.

What are Castings Good For - Minerals and Nutrients for Plants

Most importantly, worm castings add nutrients, minerals, and beneficial microbes to the soil, and these components interact to feed root systems and create healthy conditions for plant growth.

Worm castings contain the "big three" macronutrients nitrogen, phosphorus, and potassium, and they also include a range of other micronutrients and elements, including:

• Calcium, Magnesium, and Sulfur, and 
• Trace Elements like Copper, Zinc, and Iron (which are needed in small amounts by plants)

What are Castings Good For - Beneficial Biology

Worm castings contain beneficial microorganisms like bacteria, fungi, nematodes and protozoa that are introduced during the worms’ digestive process. 

These microorganisms aid plant life by feeding on harmful bacteria in the soil, creating an imbalance of “good” bacteria that aids plant development.  

This biology also helps to break down organic materials and release nutrients into the soil, making it easier for plants to uptake nutrients.

What are Castings Good For - Soil Structure

Worm castings help improve the structure of soil, making it more porous and able to retain moisture.

This improves soil aeration (allowing oxygen and water to reach important areas around root systems) and can be particularly useful in areas with poor soil quality or in drought-prone regions.

What are Castings Good For - Pest & Disease Suppression

Another benefit of worm castings is their ability to suppress pests and diseases. The microbes found in worm castings can help to control a variety of pests and diseases, making them a natural and effective alternative to chemical pesticides.

What are the Disadvantages of Worm Castings

Lack of availability and price are the 2 most commonly-cited disadvantages of worm castings. But castings' quality and biological diversity are much more important issues that need to be considered when purchasing worm castings.

Regarding cost and availability, castings are now commonly found in big box retailers and can be purchased online in many places so they are easier to find than ever before.

Castings are generally more expensive than mass-produced compost and chemical fertilizers, but they are also:

• All-natural and safer
• Offer far more benefits than other products, and 
• Are incredibly dense so a little bit goes a long way (when applied correctly)

The real disadvantage of worm castings is that not all worm castings are created equal. 

The quality of worm castings - in nutrient/mineral levels and biological diversity - is related to the inputs (bedding and feedstock) used to feed the worms that produced the castings. 

And much like in a human diet, poor nutrition can lead to a variety of issues in the health of the worm and quality of the worm castings they produce.

One thing you can do before purchasing worm castings is to request a recent biology lab report from the worm castings supplier. These reports outline mineral, bacteria, and biology levels and will give you a better understanding of the quality of the worm castings.

Are Worm Castings Better than Compost

Although worm castings and compost provide similar benefits to soils and plants (and there is a place for both in your garden), worm castings are often seen as a better soil amendment because castings:

• Offer a higher density and concentration of nutrients 
• Have specialized bacteria and enzymes that come from the worms’ highly evolved digestive tract.

The reason worm castings are more nutrient-rich than compost is because as worms break down organic matter, worms are concentrating the nutrients in their bodies and adding important biology to the castings that is not present in compost. 

Worm castings also tend to be more finely textured than compost and offer nutrients in a better water-soluble form that is easier for plants to take up. 

Finally, worm castings are often free of weed seeds and other contaminants that can be present in compost, making them a more consistent and reliable source of plant nutrition.

How to Make Worm Castings

To make worm castings you’ll need earthworms, a worm bin for them to live in, and a recurring source of feedstock like compost or fruit and vegetable scraps.

1 pound of compost worms will process around 1 pound of organic waste per week, and 50-75% of that will be returned in the form of worm castings. So you can expect 1 pound of earthworms to create 3-5 pounds (~1 Gallon) of worm castings every 4-6 weeks.

For more info on starting a worm compost bin, we created a detailed guide that covers bedding, feedstock, watering, and everything you need to do to start a successful worm compost bin at home.

How to Use Worm Castings

Worm castings are usually applied at a rate of 15%-20% castings to 80%-85% planting or grow medium (i.e., a ratio of 1 part castings to 4 parts planting medium).

Worm castings can be used with existing potted plants, gardens, and lawns, or with new plantings and seedlings. The application and use will differ, though, with castings being mixed into the potting medium for new plantings and placed on the surface and gently kneaded into the soil for existing plants.

For a detailed guide with more info on using worm castings, check out our Expert’s Guide to Using Worm Castings.

Summary: What are Worm Castings

Worm castings are literally the end result of 500,000,000 years of evolution at work.

Although many people see earthworms as fish or bird food, earthworms have evolved to play an incredibly important role in plant and soil health. And you can easily “hack” all of their benefits by applying them to houseplants, gardens, and anything else with roots and a stem!