Worm Castings


I usually use various types of compost (especially shrimp and seaweed) and use different parts of the garden to compost in place when space becomes available. There are earthworms all over the place because of the huge amount of organic material I add to the surface of the soil. This is a no dig garden and it is very successful.

I am considering adding a thick layer of worm castings on top of the soil at the end of the growing season this year and covering it up with mulches. But I don’t want to apply something my resident earthworms would not find ‘edible’. Their job is to turn the soil for me.

I know that worm castings are good for the plants but are they good for the resident earthworms?


Thank you for contacting Toronto Master Gardeners.

It sounds like you take great care with your soil. Placing things on top rather than digging down is a great approach as it preserves the soil structure the worms have worked so hard to create. As I am sure you are aware, do make sure the compost is well broken down before adding it to the garden to avoid issues with rodents etc.

Soil needs to exist in a balance where too much of any one thing can have a negative effect on your plants. Soil needs to be a balance of clay silt and sand. There is an optimal level of Nitrogen, Phosphorus and Potassium and  many smaller nutrients that also make up the soil. The balance of all of these elements will determine the pH of the soil and the accessibility of these nutrients to the plant. It is possible to have too little and too much of any one ingredient.

When working with soil amendments it is important to test the soil to ensure you are adding the correct things and not overloading the soil with other nutrients. With the composting you are doing it would be of great help to do a soil test to ensure you have a good balance in the soil before you add more to the soil and before adding another new product. If you end up adding too much of something and the balance goes off it can take many years to recuperate. If you have a lot of worms, as it sounds you do, adding worm castings is probably not necessary. I can not find any science research on bringing outside worm castings into an environment will effect the local population of worms.

There are many places to have your soil tested. It is important to follow the directions carefully and have samples from different parts of your garden. Once you have those results you can see what nutrients are lacking and which nutrients you need to be careful to avoid. More is not always better and testing will help you get the most out of your garden.

Here is the link for the testing centre at Guelph University: https://afl.uoguelph.ca/soil-testing-services