The difference between compost tea and leachate
May 19th, 2010
I realized that I had written this a while ago, but only published it on my newsletter, so I thought I’d post it on the blog for the world to read.
I’ve seen several inaccurate posts/articles about worm composting and in particular the uses for the liquid that often comes out of the bottom of a worm bin. This liquid is called leachate. The US Environmental Protection Agency provides us a handy definition of leachate: “Water that collects contaminants as it trickles through wastes, pesticides or fertilizers. Leaching may occur in farming areas, feedlots, and landfills, and may result in hazardous substances entering surface water, ground water, or soil.” The leachate out of the worm bin is not useful, and can be harmful. I know that some of the people I’ve interviewed have used it on their plants, judiciously. Forest says “I would just dump [it] down the toilet if it was bad. [some of the worm leachate] had sat for a couple weeks and that didn’t have the smell anymore. And that I would mix with water and then water my plants with it.”
But there’s a difference between leachate and compost tea. Compost tea is compost mixed with oxygenated water, creating “a Petri dish: favorable conditions are made for the beneficial soil microbes, already in abundance in worm castings, to reproduce millions of times over in the water culture.” Water running out of the bottom of your worm bin may or not be favorable to these beneficial aerobic microbes. Since it runs through the bottom, it may be collecting microbes that thrive in anaerobic conditions. These microbes, called “obligate anaerobes”, use chemicals other than oxygen to respirate. According to wikipedia, “[t]he most favorable [chemical for respiration] (after oxygen) is sulfate. [Byproducts of which] most of us are familiar with as the rotten egg smell”. This water probably also contains uncomposted organic matter, which is not what you want to apply to plants; otherwise we’d just dump our scraps directly on plants.
In short, what comes out of your worm bin is called leachate and is not compost tea. Don’t use it like compost tea. Dispose of it (I’d probably put it back in the worm bin, myself).
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4 Comments Add your own
1. Mike Baldwin | May 21st, 2010 at 9:10 am
Thanks for your comments on my blog post/questions. I just saved this blog to favorites. How do I receive your newsletter?
Re: Leachate, Bruce at Worm Composting suggests that it be disposed of down a sanitary sewer. As for me, I am trying to avoid having it at all.
Mike
2. vermicomposting | May 21st, 2010 at 9:12 am
Hi Mike,
Glad to help!
To subscribe to the newsletter, go to http://www.mooreds.com/vermicomposting/ and fill out the form.
It’s bi-monthly, but you can see the archives by clicking the ‘newsletter’ link above.
3. Mike Baldwin | May 21st, 2010 at 9:16 am
I’m still confused about compost tea and worm tea.
Besides the worm bin, we have a compost bin in the garden. It also has a spout at the bottom. Is the liquid from this bin beneficial to my garden?
4. vermicomposting | May 21st, 2010 at 10:33 am
Hi Mike,
I’m not a big hot composter, so take this advice with a grain of salt. In my experience, all compost ‘teas’, whether from bacterialogical compost or worm compost, need to be made by submerging the compost in water, and then aerating the water for a period of time. Here’s a good page on making compost tea: http://www.dep.state.pa.us/dep/deputate/airwaste/wm/recycle/tea/tea1.htm
I’m guessing that the water coming out of your compost bin is not all that aerated, so I’m guessing it’s not much help for your garden.
Hope that helps.
Dan
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