Vermicompost Leachate – Will it Harm Plants?

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Robert Pavlis

One of the most hotly debated questions in vermicomposting is this, is vermicompost leachate good or bad for plants? Some say it is great for plants and use it all of the time. Others warn that it should not be used because it contains phytotoxins (plant poisons) and pathogens.

What I find most interesting about this debate is that both sides claim to be correct and yet neither side takes the time to do a search for some scientific evidence to support their position. I guess I approach things differently and the science in this case is easy to find – one Google Scholar search will do it.

Let’s look at the facts surrounding vermicompost leachate.

Vermicompost Leachate - Will it Harm Plants?
Vermicompost Leachate – Will it Harm Plants? Credit: J&J Acres and Deep Green Permaculture

What is Vermicompost Leachate?

There is even confusion about this, but usually from beginners in the hobby. Vermicompost leachate is the liquid that drains out the bottom of the worm farm. This is different than the tea made from finished vermicompost, which is correctly called vermicompost tea, or just compost tea. I have discussed the value of this in Compost Tea – Does it Work?.

To better understand vermicompost and its nutrients, have a look at Vermicompost – Is it Really That Great?

Should a Worm Bin Produce Leachate?

Before we tackle the meat in this topic let’s examine another debated question. Should a worm bin produce leachate?

I see a lot of comments from people who say, if you have leachate you are doing something wrong with your worm farm. That is just another myth – it’s simply not true.

Microbe Science for Gardeners Book, by Robert Pavlis

A worm farm can be running just fine and produce vermicompost leachate. In fact many farm operations and even commercial operations run their worm farm to produce leachate. It’s one of their main goals.

A drier bin that gets drier food will not have leachate. A moist bin that gets wetter food will have leachate. Both are equally good and both grow healthy worms. Being drier may be a good thing if you don’t want to bother emptying the leachate.

The Arguments Against Leachate

There are two main arguments against leachate.

  • Vermicompost leachate contains phytotoxins that harm plants.
  • Vermicompost leachate is anaerobic and therefore contains pathogens.

Let’s look at each claim.

Vermicompost Leachate Contains Phytotoxins

Phytotoxins are chemicals that are harmful to plants. Where would these come from? These chemicals are produced by plants, microorganisms, or by naturally occurring chemical reactions. The term is also used to describe compounds produced by plants to ward off pests.

We can certainly expect vermicompost to contain phytotoxins since they have a lot of bacteria and added plant material.

One key point that discussions seem to ignore is that any phytotoxins in leachate came from the material in the worm bin. If leachate contains them, the vermicompost also contains them, and yet, vermicompost is never described as being toxin to plants. You might think that most of the toxins get washed into the leachate, but what about vermicompost that is made without leachate? It must be full of phytotoxins?

Vermicompost does contain phytotoxins, but the amounts are clearly not enough to cause problems to plants. There is no reason to think leachate is any worse.

Vermicompost leachate improves the growth of corn when diluted by 50% and the phytotoxicity was tested and found not to be a factor. This review summarizes numerous studies that all found improved plant growth from vermicompost leachate.

Phytotoxicity can be tested by exposing germinating seeds to the liquid in question. When leachate is applied to seedlings at full strength it can reduce germination and slow root growth. These effects go away once it is diluted. It is not clear if the toxicity is due to phytotoxins or to the higher nutrients/salt content. What is clear is that vermicompost leachate does not harm plants if it is diluted before use.

Vermicompost Leachate Contains Pathogens

A pathogen is an organism that causes disease. In most cases a human disease organism will not cause disease in plants and vice versa. So, is the concern here about human pathogens or plant pathogens? Comments about pathogens are sometimes followed with examples and E.coli and salmonella are commonly mentioned.

E. coli and salmonella are human pathogens. E. coli does not cause harm to plants and most strains cannot enter a plant. Scientists may have found one strain that can get into the roots of lettuce, but it does not travel to the leaves. Salmonella can infect plants through their leaves, but probably not through the roots. It is unlikely that human pathogens will harm plants.

Some may be concerned about surface contamination of food crops. If that is a concern, then pour the leachate on the soil, not on the plants.

These pathogens originated in the vermicompost, which is considered safe to use. The concern with the leachate is that it becomes anaerobic and may be a better place for the pathogens to grow. Both E. coli and salmonella are facultative organisms, they grow in both aerobic and anaerobic conditions.

What does the science say? This study found no E. coli, slamonella or shigella in leachate. They suggested that the healthy microbes in the bin killed off the pathogens. Another study using leachate on sorghum found no pathogens.

What about the claim that, “some leachate can contain harmful pathogens”. It is true, but it is also true that every plant surface and all soil also contains harmful pathogens. They are even on your hands. That does not mean there is enough of them to cause harm.

Pathogens don’t seem to be an issue.

Uncomposted Bad – Composted Good

This is another weird claim I see. Water running out from the bin, early in the process, is washing “things” off uncomposted material and these things, which are never defined, are harmful. They go on to claim that this is very different from making tea from finished vermicompost because now it is composted and the things have magically disappeared.

If you can’t explain the “thing” – don’t make such illogical comments.

But ….. Some People Used it And Killed Their Plants

Every time this discussion comes up on social media, someone will tell the story of how they used leachate on their plant and killed it. That is proof enough that it is harmful.

There are also lots of cases of people burning their lawn with too much fertilizer – it does not make fertilizer harmful. Vermicompost leachate washes the water soluble nutrients out of the compost. You can expect the concentration to be much higher in it, than in vermicompost. It will also contain higher levels of sodium which is very soluble. Putting too much fertilizer on plants will harm them.

Unless someone tells the story, and also describes the dilution they used, it is not a very useful story for understanding the effects of leachate on plants.

Does Vermicompost Leachate Harm Plants?

Vermicompost leachate does contain phytotoxins and microorganisms but so does the vermicompost. If one is good for soil and plants there is no logical reason to suspect that the other is harmful. Several scientific studies have looked at this issue and clearly show that leachate is good for plants.

It is a good idea to dilute vermicompost leachate by at least 50%, and a 1:10 mixture is even safer. Dilution ensures that the nutrient/sodium level is low enough so it does not harm plants.

Every leachate is different and if you want to be sure that yours won’t harm plants, try the simple test in this video, using leachate instead of water. It is similar to the test scientists use.

YouTube video

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

9 thoughts on “Vermicompost Leachate – Will it Harm Plants?”

  1. Thanks for the article.
    I have tested the use of leachate on our gardens and have to admit it hasn’t been the miracle some people make it out to be. I honestly could not appreciate any difference.
    In the wormfarmers handbook by Rhonda Sherman she recommends to just toss it.
    I’d love to do more experiments but for now I’m just throwing it on our compost piles.

    Reply
  2. After reading all these comments, I believe that I will continue using the eisenia fetid kitchen scraps worm bin leachate in my raised bed bins.

    Reply
  3. Thank you Mr Pavlis, I enjoyed reading your article.
    I really appreciated the links you put in the text so I could read up on the things I didn’t understand fully such as salmonella and E coli.
    I do use the leachate from my worm farm diluted very weak.I was worried because so much information out there says that even though my plants look healthy I may be (or even am ) spreading something around that is dangerous to people. Your arguments are logical to me and the inclusion of the studies is appreciated and convincing. I am more at ease using the lechate as I like to pour water through my worm farms .
    I figured out rains outside on worms. Also I felt my worms would like that, even though it produced more leachate.
    Thanks again for the article ,because I felt like I was doing the wrong thing and you have me feeling more at ease with what I am doing.

    Reply
  4. I really enjoyed this article, thank you! I personally do not use leachate and don’t recommend home growers use it. Here’s why:

    1) While the study on corn maize above found no pathogens in the vermicompost leachate, they tested it beforehand to ensure there were no pathogens. Most non-professional do not have the means to test accordingly.

    2) Non-professional and new vermicomposters often have worm bins that are in less-than-ideal conditions to begin with. I’ve seen all manner of growers write in for support on their worm bins with huge problems and describe foul odors. What’s worse, many of them have used the leachate anyway, used too much, used it incorrectly, etc.

    3) There simply aren’t near enough studies on vermicomposting leachate to know for a fact that for the majority of the time the potential for pathogens is minimal. That doesn’t mean it’s bad, just that my preference is to err on the side of caution until we have enough scientific data. And let’s face it, comparisons in soil and plant science is already extremely complex because of the massive amounts of variables. Add the novice aspect to that and I’d say adhering to ‘If you don’t know, don’t go’ is best.

    Here’s an example of how plants pathogens may be more prevalent in leachate than we might think. Many plant pathogens are spread through water. So imagine you’re a grower and you’re composting your plants (versus many home vermicomposters who are composting food scraps from the kitchen). You put some diseased tomato leaves in there for the worms to consume. The bin is a bit dry so you moisten it up a bit. Water droplets drip down and fall over the diseased leaves and into the bin. A few days later, you use it on your plants. Did the scientist use leachate from newbie vermicomposter in the suburbs of Michigan for their test? Probably not.

    4) Nigel above made this point also: if the leachate sits there and becomes stagnant, you can’t expect good things from that. How many vermicomposters leave the leachate in the bin too long.

    So to avoid writing a novella (too late?), I think the concept of whether leachate is bad or good vastly depends on the practices of the vermicomposter and their ability to keep a healthy bin.

    All that being said, before reading this article I was way more vehement about people not using vermicomposting leachate prior to this article and feel like an exception needs to be added to my agument against using leachate of, ‘how experienced are you at keeping a healthy envionment for your worms and how much do you understand the process of creating a healthy harvest of leachate?’

    Reply
  5. Regarding your comment at the beginning of this post, not only in my opinion are you unfortunately correct that people want to have an opinion (belief) that somethings are true without bothering to consult the scientific literature about growing plants, but about life generally. Few, it seems, want to consider the evidence of anything, as opposed to just having an opinion (belief) because one feels like it.

    Reply
  6. As a grower and supplier of worms for over 23 years (previously nurserymen) this is a question that often crops up, from our experience the leachate is not harmful but is not really that beneficial especially when diluted at the often quoted rates, some customers actually want to buy a wormery specifically for the production of worm “tea” as they often describe it, we have also had customers who are concerned that their wormery is not producing any “tea” and what can they do about it, our stance on this is to manage the wormery in the correct manner, by all means use the leachate as part of the normal watering regime and if doing this, allow the leachate to freely drain into an open vessel and to use/drain on a regular basis and not allow it to collect in the sump of the wormery until it becomes stagnant and anaerobic also, don’t expect miracles!

    Reply
  7. I grow native plants, and have noticed, that, close to the house, where the lawn was, even when I don’t mow, and remove all the non-native (possibly allelopathic) plants, that trees and native seeds don’t germinate…yet, across the street, in a much more native area where the soil has had minimal disturbance and I find few earthworms, about 30 species of native plants grow. I have seeded native plants in this location in case there are simply more birds in the area across the street. The other day, I dug a 2x10x3″ deep plot in the “ex-lawn” the other day, collected, and counted the worms…286. Pretty impressive, and few other organisms were present, none in anything like this number. I plan next year (at the same time, if I can find a spot in similar light) to dig up a similar area across the street. I know these interactions are only “just” beginning to be examined. One question I had was is, can I make fertilizer out of the worms themselves (assuming they are made of the nutrients from the soil and I would like those nutrients to be “put back”)? Do the worms themselves drive allelopathy or are they allelopathic?

    I would really like to find a way to exclude the worms from an area to compare growth. What I am interested in is returning soil heavily dominated by human activity right around the house to “native” state. I cannot grow ephemerals here, even though I have the right light, moisture, and soil conditions. Slugs are a HUGE detriment, but don’t seem to be the only issue. I often find earthworms hanging out beneath the non-native plants and not so much under native ones. I wish I were young again and/or somewhere I could take an advanced degree (oh and had the resources, lol).

    Sooo many questions.

    Reply
    • “can I make fertilizer out of the worms” – anything living contains nutrients that are returned to soil when it decomposes.
      “Are worms themselves allelopathic” – not that I know of.

      Reply

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