Using Castings and Worm Tea in the Garden

Some interesting questions from Robert:

I’ve been your supporter for years now and you seldom mention how to
use worm castings and worm tea in gardening. So, my questions are:
1] How much soil do you mix with how much castings?
2] How much worm tea should I use on an outdoor garden plant? On an
indoor potted plant?
3] Will too much castings or worm tea burn a plant or grass the way
chemical fertilizer does?

Hi Robert,
You are absolutely RIGHT – I definitely haven’t spent all that much time on these topics. This is likely due to the fact that I don’t actually use worm castings or worm tea all that much in the garden, believe it or not! I typically prefer to employ various in situ vermicomposting systems instead, since these save me from having to do any harvesting/screening etc. A prime example of this is my “Vermicomposting Trench” approach (links to a list of related posts on the blog). As for the worm tea situation – this is actually something I keep meaning to get more serious about, but just haven’t gotten around to quite yet (this is the year, I tell ya!! lol).

Still, I do want to provide you (and others) with a proper response, since this seems to be a topic a lot of people want to learn about.

Let me start by saying that there are a lot of different “grades” of worm castings – I actually prefer the term “vermicompost” for exactly this reason. That is to say that it’s very rare that someone would end up with “pure” worm castings – no matter how much sifting and screening you do, you’re almost certainly going to end up with some other “stuff” in there. These grades of vermicompost I’m referring to are really just (approximate) indicators of the percentage of other stuff (and on the flip side, the percentage of actual worm castings). As you might imagine, the material I would consider high-grade vermicompost – what I might even be tempted to refer to as “worm castings” – would indeed have a very high percentage of actual worm turds, whereas a “coarse grade” vermicompost would be a bit more…uhhh…chunky?

This isn’t helping is it? (haha)
Perhaps some pictures would aid my explanation! In the first two pics you will see vermicompost that’s been screened to 1/4″, while the next two pics show what I mean by “coarse grade” vermicompost – which, as you can see, tends to have a fair amount of other “stuff” in it.

I should point out that, while the 1/4″ screened vermicompost is a really nice material, screening it to 1/8″ would more than likely help you get closer to “pure” worm castings.

As far as putting these materials to good use goes – in a nutshell, the coarser the grade of material, the more I will use. The beauty of really top notch vermicompost is that a small amount goes a long way. My favorite way to use it is to simply drop a small scoopful in the bottom of a planting hole, or around the base of an established plant.

If you happen to be a tomato grower, you may want to check out this article (thanks to Brenda B. for telling me about it):
Instructions on How to Grow Better Tomatoes. They recommend using worm castings (among other interesting suggestions!), and as you’ll see, they agree with my “little goes a long way” philosophy.

[Just so you don’t think I’m making this stuff up, I should point out that this assertion is actually based on scientific research – the team under the guidance of Dr. Clive Edwards at Ohio State University has found repeatedly that even small amounts of vermicompost in soil mixes can offer significant benefits for growing plants – more on that in a minute].

As for the “chunky” stuff, I often prefer to use this as a kind of “vermi-mulch“. My favorite mixes to use are actually those that still contain plenty of worms in them, since (as mentioned) my aim is usually to create some sort of in situ vermicomposting system that will continue to offer vermi-goodness to the plants for the duration of the growing season.

Here are a couple of video posts (relating to vermicompost and in situ vermicomposting systems) you may want to check out as well.

Composting Worms in Raised Beds
When to Harvest Vermicompost

Regarding the use of vermicompost tea – since I am not an expert in this department, I’ll fall back on the experience/knowledge of someone who IS – my good friend, Mike “Strawberry Guy” Wellik. I highly recommend you watch his “Brewing and Using Vermicompost Tea” video below.

OK Robert – perhaps now would be a good time to respond to your ACTUAL questions!

1) I’m going to interpret your first question as basically “what percentage by volume of my soil mix should consist of castings” – hopefully that is close to what you are asking. As mentioned earlier, the great thing about castings is that small amounts (of good quality material) can typically go a LONG way. Atiyeh et al. (2001) found that substituting as little as 5% vermicompost (made from pig manure) in professional-grade horticultural soil mixes resulted in a significant increase in plant growth. What’s really interesting about this is that ALL the plants (including controls) were provided with a normal inorganic fertilizer regimen – thus suggesting that whatever benefits the vermicompost was providing, it went above and beyond basic N-P-K fertilizer value. In that particular study the researchers found that 25-50% substitution had the greatest impact on growth, and this seems fairly consistent with a number of other studies conducted by OSU researchers.

2) Again let me point out that I’m no worm tea expert – but my feeling is that you can’t really add “too much”, assuming it is made properly. The few times I’ve made it, all I did was repeatedly dunk a cloth bag of vermicompost in a big bucket of rain water until the water was dark brown in color. This liquid was then applied liberally in the garden using a watering can.
I would definitely be very careful with non-aerated teas though, especially with potted plants. Various compounds created via anaerobic processes can be phytotoxic (harmful to plants).

3) This is an interesting question, and unlike my hunch on the worm tea front, I’m definitely leaning towards “yes” – there is such a thing as “too much” high-grade vermicompost. I’ve done growth tests with 100% vermicompost and found that plants actually seem to suffer in comparison to those that receive a smaller percentage of the material. This seems to be consistent with what researchers have found, and Atiyeh et al. (2001) in particular suggest this could be due to higher salt concentrations and poorer porosity/aeration. (FYI – in my experience, this is not typically the case with the coarser grade materials)

I would definitely say it’s not quite the same as chemical fertilizer “burns”, however, and I’d actually be surprised if you could add enough to your lawn to cause problems. Nevertheless, this is something I recommend keeping in mind for potted plants.

All this being said – unfortunately one of the really tough things about this topic is the fact that while we have catch-all terms like “worm castings” and “vermicompost”, the material they refer to can vary a GREAT deal from one source to the next. I’ve talked about various “grades” of end product – but really, that’s just ONE factor to consider. Different starting materials, different methods/conditions etc etc etc can impact the properties of the castings/vermicompost.
As such, my recommendation is always to test things out for yourself!

Anyway, Robert – I realize this was a rather long-worded, meandering explanation, but hopefully you (and others) found some value in it! Thanks for writing in.

Atiyeh, R.M., Edwards, C.A., Subler, S., Metzger, J.D., 2001. Pig manure vermicompost as a component of a horticultural bedding plant medium: effects on physicochemical properties and plant growth. Bioresource Technology 78: 11-20.

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    • Anna
    • April 20, 2011

    Great post! I’m wondering if you could expand on this statement: “Different starting materials, different methods/conditions etc etc etc can impact the properties of the castings/vermicompost.” I want to know more :).

    • Bentley
    • April 22, 2011

    I opened up a “can of worms” with that one didn’t I, Anna??
    I think the only way to do that justice is to write an actual post. Will definitely add that topic to the list of RWC “To-Do’s”
    Thanks for the nudge!


    • Anna
    • April 22, 2011

    Haha! I would imagine there are a lot of cans of worms to be opened around the Bentley residence.

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