What Are Worm Castings? It’s More Than Just Worm Poop!

Worm castings are the richest natural fertilizer known to humans.

Worm castings are a by-product of vermicomposting. After worms digest organic materials, they release waste from their bodies called castings. To answer the question “what are worm castings”, the simple answer is worm poop (or worm poo if you’re from Australia like me)!

It’s not just any ordinary poop though. It’s odorless for a start. More importantly worm castings are regarded as the richest natural fertilizer known to humans.

The appearance of worm castings resemble little football-shaped particles. Of which are a rich, black soil color.

What are Worm Castings? – In More Detail

As worms digests organic food, they reduce nutrients to their most usable form. Worms also remove heavy metals like copper, lead, and cadmium from soil. This makes worm castings metal free.

Worm castings contain more nutrients and bacteria than any other fertilizer. This includes minerals such as nitrates, phosphorus, magnesium, potassium and calcium. It also contains manganese, copper, zinc, cobalt, borax, iron, carbon and nitrogen. Worm castings also contain enzymes and humus. The finished product is neutral pH of 7.0. Plants get their nutrients and water from the soil. A single tablespoon of worm castings could feed a small pot plant for over 2 months. Worm castings increases garden yields by as much as 25%.

Some of the bacteria form the earthworms digestive system makes it way into the castings. Worm castings contain a unique combination of helpful enzymes and good bacteria. This microbial activity is up to 20 times higher than in the soil and organic matter that they digested. This helps to stimulate plant growth.

Soil is the foundation for healthy plants. Worm castings make a great soil conditioner. It improves soil structure, water retention and drainage. Worm castings are extremely water absorbent, able to hold 2-3 times it’s own weight in moisture. It is also far richer in nutrients than soil itself. With 2 times more calcium, 5 times more nitrogen, and 7 times more phosphorus and potassium.

Benefits of Worm Casting

Here are 11 key benefits for using worm castings  in your garden:

  1. Packed with nutrients which helps to make plants grow faster
  2. All nutrients are water-soluble, making it an immediate plant food
  3. Extracts toxins, harmful fungi and bacteria from the soil. This helps prevent plant disease
  4. Repels pests that feed on plants such as aphids and spider mites (a natural pesticide)
  5. Contains beneficial microbes and bacteria which helps protects plants from disease
  6. Contains humic acid which aids in plant nutrient absorption and stimulates plant growth
  7. pH neutral – this offsets acidic conditions in soil
  8. Improves the condition of soil including stability, aerationdrainage and water retention
  9. Increases garden yield by as much as 25%
  10. Worms remove toxic heavy metals from the soil. This means their castings are heavy metal free
  11. Cannot burn plants as the nitrogen is slow releasing and it does not contain any salts

How to Start a Worm Farm

You might want to check out this “ultimate” guide on how to start a worm farm.

Worm Castings vs. Chemical Fertilizers

Chemical fertilizers are a quick win solution which help plants grow fast and strong. They contain all the essential nutrients for plant growth. This include nitrogen, potassium and phosphorus. But it also damage soil, increase acidity (lowers pH), encourages plant disease and contaminates ground water.

The acids contained in chemical fertilizers destroy soil structure. This makes it more compact and less penetrable. And the increase in acidity kills beneficial microbes which protects plants from disease. We all know how important earthworms are to have for a healthy garden. Earthworms cannot survive the acidic conditions as well. Chemical fertilizers are high in nitrogen in relation to phosphate. This makes plants more susceptible to disease.

Worm castings are pesticide free, contain no pathogens and are an alternative to chemical fertilizers. It’s fact that organic fruit and vegetables are superior in nutritional value compared with chemically aided fruit and vegetables. And there’s no contest for taste either – organic wins every time!

Chemical Fertilizers Worm Castings
Contains pesticides Free of pesticides
Nutrients must be broken down before the plants can absorb them All nutrients are water-soluble, making it an immediate plant food
Can burn plants if applied incorrectly Cannot burn plants since the nitrogen is slow releasing and it does not contain any salts
Damages the condition of soil Improves the condition of soil
Lowers pH pH neutral
Kills beneficial microbes which protects plants from diseases Prevents plant diseases by extracting toxins, harmful fungi and bacteria from the soil
High in nitrogen which makes plants more susceptible to diseases Nitrogen is slow releasing

How to Use Worm Castings

Worm castings help germinate new seedling by supplying extra nutrients and improving water absorption.


So now when someone asks you “what are worm castings”, you can tell them it’s the most incredible poop in the world. The power of worm castings should not be understated, it’s the ultimate organic fertilizer and soil conditioner for your garden. This is one of many reasons why you should start a worm farm.