Leachate vs Castings Tea Revisited

A question from Jae:

I have a question about whether I should really use the worm leachate and worm castings that have been gathering in my farms. A lot of online articles say worm leachate can be damaging for plants while a lot of other people say leachate is actually quite good. Additionally for worm tea, which I gather is made by diluting worm castings with water has to be used immediately. Is worm tea also something that could work for plants and seeing as we have a lot of castings on our hand at the moment, is there a way to store the worm tea without using it immediately?

Hi Jae,
This is something a LOT of people wonder about – largely because of all the conflicting information out there. It is also a prime example of one of those vermicomposting topics where “it depends” is likely the most accurate answer (there are lots of topics like that in vermicomposting – haha).

Let’s start with what I feel is “bad advice” – particularly bothersome since it gets spread around by those who have quite a lot of influence. It’s this idea – largely promoted by stacking bin manufacturers it seems – that you need a “tea” reservoir below your worm bin, and that all liquids draining down into this reservoir are fair game for use as a liquid fertilizer (usually referred to as “worm tea”).

I think in some cases people are even being advised to actually run water through the system periodically to literally create this run-off “tea”.

The potential problems with this are: 1) the fact that a lot of what drains down – especially early on – is liquid released from decomposing food wastes, NOT stabilized, beneficial compounds from finished castings; 2) this liquid then sits in the reservoir, goes anaerobic – usually becoming foul (potential for formation of plant-harming compounds), and 3) in cases where you are running water through the system, you are just lowering the quality of your future vermicompost (and potentially hampering the process by making everything super soggy).

All that being said, I do want to point out that leachate isn’t always going to be “bad” (as some people seem to suggest). My advice is to always treat it with caution…but I will admit that there are likely going to be situations where it is perfectly fine, beneficial even. The key factor in my mind is the maturity of the system – leachate from a brand new bin versus leachate from a 4 month old (well managed) bin, for example, are completely different!

The more finished vermicompost there is, the better quality (and more stable) of “tea” you’ll end up with. As I discovered during my “Non-Aerated Castings Tea” experiment, basic water extracts made from good quality castings are very stable and can sit for months (maybe even years) without any putrification.

I also wanted to mention that even the unstable, smelly leachates can be used in some way or another. If you aerate them and/or mix with a lot of rain water (or aged tap water) before use, and then use them in the garden you should be totally fine (maybe not in smaller pots, or with plants that are a bit more sensitive).

Getting back to your specific questions…

I can see why you might question the use of leachates (and my explanation above should help in that department), but I am a little unsure about your mention of castings (wondering if you should use them). Even when you end up with wet, skunky material, it’s usually just a matter of letting it sit exposed to good air flow for a period of time (helps to mix it during this process as well). That should help to “finish” it off.

My bottom-line recommendation in general is to focus primarily on producing high quality castings, and then to make quality tea from that. If you add various “microbe foods” during the brewing process, make sure to use the tea right away. If you make basic water extracts, the liquid should be totally fine to sit for extended periods.

Hope this helps!

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    • BioMike
    • March 19, 2019

    I like this post and the question that it tries to answer. The fact that there is a lot of different opinions (has this actually been studied in detail?), opens room for false information.

    Based on my knowledge as a biologist I would like to add some comments. If you would look at what the leachate could contain it would be the following: water (duh), salts, stable organic compounds and microorganisms. Let’s break that down:

    Water: Most of it, adds close to nothing benefitial in itself.

    Salts: The effect of salts could have a big effect on the use of the leachate as it infuences everything a lot. Too much salt could harm the plants and microorganisms in the leachate. One question is thus how salt leachate actually is. Various sources always say to dilute the leachate before use for this reason. If the amount of salts is not too high, it is actually beneficial to plants as it contains trace elements and provides a (indirect) nitrogen source for take up.

    Stable organis compounds: Like said before above, older bins have most likely more of these compounds than young bins. These are the same compounds that could be found in forrest soil and compost. Simple organic compounds, like sugars, will most likely not be present in leachate, as microorganisms quickly absorb them. These compounds have most likely little infuence on plants, but could improve the microbial soil community (which indirect would be beneficial for plants).

    Microorganisms: Expect them to be present in the leachate, but I doubt they will do much in the leachate. The reason for this is that carbon is hard for them to get; no sugars and the stable organic compounds are much harder to degrade for carbon and energy. A high salt concentration also inhibits growth of most soil microorganisms. If the collection container is closed (and an anearobic environment is created), growth of microorganisms will be inhibited as well. Anearobic microorganisms will be able to grow but will run into growth problems as well for the reasons mentioned above.

    So, in perspective. I think the longer the leachate is sitting in a closed bottle, the less benefitial it will be. I doubt it will be really damaging to plants, unless the salt (mostly sodium and potassium) concentration runs through the roof. The best proof of the beneficial properties of vermicompost to plants are the trench examples on this site.

    • Jae
    • March 19, 2019

    Fantastic response! Thank you so much clearing up a lot of misunderstanding about vermicompost.

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