Worm Compost Leachate, Good or Bad?

Image: Permaculturewiki.

Image: PermaWiki.

In the course of preparing for our worm composting demo last week Kelly and I came across a lot of conflicting information. One of the most contentious issues in worm composting is what to do with the liquid that comes off the worm bin, called leachate.

The controversy stems, in large part, from the debate over aerated compost tea (ACT) vs. non-aerated compost tea. Fans of ACT do not like the fact that worm bin leachate is anaerobic, which they believe encourages the growth of microorganisms unfavorable to plants. They like to point out that worm bin leachate is not ACT.

The ACT debate needs a much longer post, but I did find two peer reviewed studies showing the benefits of un-aerated worm compost leachate: “Vermicomposting Leachate (Worm Tea) as Liquid Fertilizer for Maize and “Vermicompost Leachate Alleviates Deficiency of Phosphorus and Potassium in Tomato Seedlings.” I also found several Extension Service publications touting the use of worm bin leachate.

There are some caveats, however. First, it needs to be diluted–at least 1:1 and maybe, according to some sources, as much as 1:10. And you should probably test it out on a few plants before applying it to your whole garden.

And, from a food safety perspective, I’d avoid applying it to leafy greens and lettuces. I’d also point out that if you have a lot of leachate it might mean that your worm bin has too much moisture in it.

What do you think? Have you used worm bin leachate successfully? What side of the aerated vs. non-aerated debate are you on?

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  1. I think that whether or not the leachate is safe to use depends on what is in the compost bin. I agree that it should be kept off of leafy greens and other vegetables you’re about to eat.

    In the study on “Maize” where the analysis shows that the leachate is safe and beneficial, they’re using cow manure that was hot (thermophilically) composted for two months before adding the worms to it. What they’re analyzing is liquid fertilizer from a well-aged system.

    I wonder if there are any studies out there where they analyze leachate from the slimy raw vegetable waste in a typical worm compost bin?

  2. I’m so glad you’re asking this question, because I’ve been torn with how to deal with my worm leachate (I have other names for it!). I believe if you want to aerate, you have to do it with some kind of pump for 24 hours. I was thinking of maybe not doing that method, getting a whisk, diluting the solution and seeing what happens, rather than going whole hog with the mechanics. I’ll be very interested to see other posts.

  3. The guy who does the LA County composting workshops swears by it as a miracle fertilizer and provides photographs of exceptionally healthy plants to prove his point. He recommends mixing one pint of leachate to one gallon of water and using to water or as a foliar feeder. He did not mention the necessity of aerating it. I have two worm bins and use the leachate as he instructed on my vegetable garden and have been very pleased with the health and productivity of my garden. He said when used as a foliar spray it can also act as a pesticide and if used undiluted can be a weed or grass killer.

  4. My worm bins never had more than a few drops of leachate, which promptly evaporated, although I had plenty of large, healthy worms that produced plenty of moist vermicompost. I don’t live in an especially dry climate either. Not sure how people are getting an ounce, let alone a pint, of liquid off a household worm bin.

    • I get most of my leachate from when I put in moist veggies and fruits without letting them dry out or microwaving them first, like melon rinds and tomatoes that are turning. If you are putting mostly dry veggies or more Browns to your bin then it won’t produce much leachate, but having too dry of a bin is not good either.

  5. I use it. around here we call it “worm pee”. My plants grow really, really well with it. But I use it as part of a “portfolio” of fertility, including conventional compost, crop rotation, and worm castings (=”worm poop” at our garden).

    All the stuff Wade says in Comment, about dilluion, weed killer, etc. I have heard too. Adding to Wade’s comments, I have heard that there is anecdotal support that worm castings & tea act like “systemic” for plants and can prevent pests.

    I use far greater dilution than 1:1 — probably more like 1:10 to 1:20 dilution. I usually dilute with harvested rainwater.

    Mitty, I tend to get more liquid when I haven’t added much “bedding” to my worm bins, just added veggies veggies veggies and ignored the bin.

  6. I don’t typically have much leachate, but then again I live in a very dry climate (Boulder, CO). During my first worm class the instructor said to put it in a gallon jug, mix with water and then every few hours shake it up, no pump needed! Sounds like a weekend project and I would definitely try it!

  7. I have a small bin I have used primarily to dispose of some kitchen wastes. It is a “pre-fab” job, with a catchment at the bottom of several trays for catching excess water. I’ve been able to collect leachate simply by pouring a little extra water in at the top. The system is well-designed so there’s no pooling or puddling up in the layers of bedding.
    I wonder about the question of dilution. In my very small system, the strength of the leachate is partly dependent on how much water I add. Diluting further with a set ratio of additional water seems to be a bit hit-or-miss, but is probably a good idea – you can always use more of the diluted liquid.
    IMHO, using *fresh* leachate on the same families of plants you’re using it feed the worms is probably risky, with much the same caution as is given regarding rotating crops in your vegetable garden. There is always a possibility that a pathogen will find its way through the system from top to bottom and end up on your living plants. My attempted solution has been to put the fresh leachate into a 5-gallon bucket, insert an aquarium aerator stone attached to the smallest aquarium pump I could find, and dilute the solution until the bubbles moved freely to the surface without foaming over the top. I’ve played around a bit with adding blackstrap molasses and even fish emulsion with the intention of feeding whatever microorganisms might be present. One unexpected effect has been the creation in the bucket of a dark solid material very closely resembling humus or finely divided very mature compost. The project is an experiment and so far I haven’t produced enough of the stuff to do more than determine it did not produce any markedly toxic effect to some vegetable seedlings.

  8. Our worm bin was a gift from my brother. It’s just a single plastic bin with a few air holes drilled around the sides. Every time I go to feed them, I feel bad that they might not be getting enough air because of the plastic, lack of air holes, or that leachate can’t drain.

    But there are still a ton of worms and I’m guessing it’s almost time to do a harvest of castings and cull the herd, so it seems like they are at least surviving. Can they go on surviving in this environment? Am I being cruel? Should I at least drill holes in the bottom and add a second bin?

    • It sounds like your bin is working perfectly! Worms ain’t picky. If you feel it is a little too humid in there you could drill a few more side holes. Bottom holes and second bins are not necessary as long as the worm wrangler can keep on top of the moisture problem–and it sounds like you’re doing this just fine. Just keep the contents as moist as a wrung out sponge, and all will be well.

  9. For me leachate is a moot point since I rarely if ever get more than a few drops. I just dont get any drainage and yes I believe those that do have their bin too wet. For those that would say mine is too dry my worm herd is healthy and growing for 3 years now. The only time I might get a little extra drainage would be when I feed a lot of watermelon rinds which my worms love. One of my bin setups is a 3 tier homemade plastic bin. If I leave the lid off it would get too dry and If I leave the lid on it gets too wet and worm go to the top. So I have solved the sweating problem by just leaving the lid cracked 1/2″ or so and then mist the bedding on occasion. By doing this the worms stay down never seeing any on the lid.

  10. I pour about 2 litres of water into my wormery every other day (this appears counter-intuitive but I have been very successful with population growth in the past). It drains overnight from a spigot into a drum. The ground below my wormery is planted with Mondo grass. Where the drum trickles over, the grass has grown twice as high as elsewhere.

  11. There are a few things going on with leachate. Obviously the makeup of leachate varies depending on the worm farm, but typically you would expect leachate to be mostly anaerobic. If you smell your leachate, and it smells bad, one would expect that anaerobes are dominating. On top of this, the pH of the solution will depend on the contents being added to the worm farm, but that’s difficult to assess.

    If you have “fresh” leachate, you will expect there to be a high concentration of microbes still contending for the nutrients available in the oxygen poor environment of the leachate. Old leachate will have settled down a bit, and will smell less bad over time. I would expect the population to have shifted in favour of anaerobes which thrive in less than ideal conditions. Those anaerobes may be less likely to be pathogenic to plant matter, and more pathogenic to other bacteria, but I’m just speculating. The point is, over time the composition of the leachate changes.

    Different soils have different ratios of bacteria to fungi, and likewise different plants prefer different types of soils. Tress generally prefer fungally dominated soils, and grasses, annuals prefer bacterially dominated soils. One difference between these soil types, is the pH of the soil. Fungally dominated soils have a lower (acidic) pH. So if you use some logic here, the Leachate being bacterially dominated is likely to assist plants which prefer a higher pH (alkaline) bacterially dominated soil. In other words.

    Another way to describe this, is imagine walking into a forest. On the outside you find grass, shrubs and annuals. As you walk into the forest, you find less of those plants, and more old trees. Starting from the outside the soil is likely to be bacterially dominated, however deeper in the forest you will mind the soil is fungal dominated. If you measured the pH, it would start low, and get higher deeper in the forest.

    Why is this important? You are feeding your plants bacteria and the nutrients locked up in those cells. Additionally you are adding a solution with a possibly high pH. The plants that like the mixture are likely not to be old growth trees, it’s better for grasses, and annuals etc.

    What I do at home is, leave the leachate in a container somewhere for several months. I then dilute the mix very heavily (1:50 or 1:100), with ordinary tap water. That solution I leave in the sun (in a bucket) for a a few hours while. I then use that mixture on my plants sparingly as they are watered. For trees or other fungal soils, use organic compost, which is fungally dominated.

    • Totally agree. Worm bin leachate haghly depends on how you manage the bin.

      Pastly, i used to process the food with water and paper as CO2 fuel, so i had a lot of leachate. Now i use grounded wood, and almost no leachate.

      But i had 2 galloins of leachate that i wait till from 1 to 3 months to use, as a complement with others fertilizers, organic and not. I cannnot really be sure the participation of the leachate at the total results, but at least i have proof that it doesn’t harm the plant.

      And as said above, the PH of (my) leachate is really alcaline, so i had to dissolute it well (10x) together with PH regulators to reduce it.

      Now i will try to use bubble it togheter with wormcastings (using an aquarium pump), and try on my plants as well.

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